Table of contents

Recently there has been greatly expended interest in narrow bandgap materials. Modern epitaxial techniques and the growing interest in nanostructures have provided areas of application for some of the unique properties of the narrow bandgap material. As always, one of the primary sources of interest is the small bandgap which makes them the material of choice for many applications in the infrared. However, in recent years their other unique properties have been the basis for a broader set of interests in narrow bandgap semiconductors. The type II band offsets (InAs/GaSb) have been the basis for novel tunnel devices and infrared superlattices. The very small effective masses inherent in small bandgap materials make them the obvious candidates in which to observe quantum confinement effects at larger dimensions than in materials of larger effective mass and wider gap. The ease of making electrical contact to some of the materials (ohmic contact to n-InAs) has made them the material of choice for electrical nanostructures. The ability to put in large amounts of magnetic ions to make magnetic semiconductors has led to a number of novel properties. The technical importance of a narrow bandgap and the unique applications promised by some of the other properties of these materials bode well for substantial research in narrow bandgap semiconductors well into the next decade.

Short-period strained-layer alpha -Sn/Ge superlattices have been synthesized recently by a low temperature molecular beam epitaxy technique which allows a large variation of substrate temperature. Thin, tetragonally distorted alpha Sn layers h ave been stabilized on Ge substrates by growth conditions far way from thermal equilibrium. The fundamental bandgap of Sn/Ge superlattices is shifted towards lower energies with increasing average Sn concentration, as expected from theory.

Structural studies have been performed using transmission electron microscopy and diffraction on III-V ternary and quaternary alloy epitaxial layers. The results revealed that 'natural superlattice' structures had spontaneously formed in some of the layers by phase separation and/or atomic ordering which occurred at the surface during growth. The structure of these natural superlattices and the growth conditions for their occurrence are described and their effect on the electrical and optical properties of the layers, e.g. bandgap narrowing, and possible mechanisms of formation are briefly discussed.

In the ternary semimagnetic compound Hg_1-xFe_xSe some of the Hg ions are statistically substituted by Fe ions. The most interesting features of this material are related to the donor properties of the Fe ion, whose energy level lies about 210 meV above the band edge degenerate with the quasi-free conduction band states for sufficiently large x>or=0.0003 the Fermi energy gets pinned to the donor level. In this mixed-valence regime a multitude of interesting phenomena are observed: pronounced reduction of the microscopic scattering rate as expressed in the mobility and Dingle temperature, fluctuations between delocalized and localized states giving rise to a three-dimensional analogue of the quantum Hall effect, and spin fluctuations. These effects are demonstrated in data from numerous magnetospectroscopy experiments (transport, magnetization, IR and FIR spectroscopy) in DC magnetic fields up to 18 T and pulsed fields up to 100 T. The magneto-optical spectra are characteristically structured by nonlocal interaction between electromagnetic radiation and charge carriers. Different theoretical approaches ranging from an effective Wigner condensation over short-range correlation are discussed in relation to the experimental data. Due to the pronounced reduction of scattering, HgSe(Fe) proves to be an ideal matrix to study the semimagnetic behaviour of different paramagnetic substitutes as in the quaternary compound Hg_1-x-yFe_xMn_ySe. Particular attention is given to possible quasi two-dimensional modifications of the compounds in heterostructures and superlattices.

The authors study the dependence on the molar fraction x of the band structure and the electron scattering mechanisms in H_1-xFe_xSe. By carrying out a careful analysis of the available data they determine the values of linear coefficients describing the conduction and valance band-edge variation as functions of the molar fraction x. The crossover from the zero-gap band to the open bandgap configuration at x approximately=0.08 is predicted. They show that is is possible to explain, in a consistent way, the drop of the electron mobility for x>0.002 as being due to the alloy scattering mechanism. Clear evidence of the enhancement of the mobility related to the spatial correlation of the scattering is observed. The enhancement, however, is shown to be smaller than expected on the grounds of a simple short-range correlation model. They ascribe this fact to the broadening of the Fe d level whose width becomes greater as the molar fraction x increases.

The authors investigate the electron paramagnetic resonance (EPR) linewidth and amplitude of the Fe³⁺ ions in semimetallic HgSe:Fe. An effective spin relaxation mechanism caused by RKKY coupling between Fe³⁺ and Fe²⁺ ions is proposed to explain the experimental data. They conclude that the strongly nonuniform relaxation rate originating from the nonuniformity of the local environment of Fe³⁺, leads to a decrease of the EPR amplitude as observed in experiment.

The possible influence of the hybridization-mediated spin-dependent p-d interaction on the crystal field split Fe²⁺ levels in HgSe:Fe was studied by infrared transmission measurements at helium temperatures. Simultaneous high doping of HgSe with Fe and Ga makes it possible for the first time to investigate the Fe²⁺ crystal-field optical transitions (E⁵ to ⁵T₂) in this semiconductor. The obtained absorption spectra consists of a strong peak corresponding to the non-split zero-phonon transitions and a broad bump caused by phonon-assisted transition. The recently theoretically predicted energy broadening of the Fe²⁺ excited state (⁵T₂) in HgSe, resulting from a strong p-d hybridization, was confirmed and estimated to be equal to about 20-30 cm^-1.

The authors present results of measurements of electron concentration and mobility in mixed crystals of Hg_1-xZn_xSe (0<or=x<or=0.07) doped with resonant Fe donors (0<or=n_Fe<or=5e10¹⁹ cm^-3) from liquid helium to room temperature. The data indicate that the presence of Fe considerably improves the electrical properties of the material. The analysis of the mobility in terms of the scattering from ionized centres (with possible spatial correlation of impurity charges taken into account) and alloy scattering leads to an agreement between the measured data and the calculated values.

(Cd_1-xMn_x)₃As₂ (CMA) is a semimagnetic n-type semiconductor with E_g approximately=-0.095 eV. Magnetic susceptibility measurements in single crystals of CMA show that in weak applied fields (H<or=300 Oe for x=0.10) values of chi _dc(T) for samples cooled in zero magnetic field and in a field are different below T_f approximately=200 K. This behaviour is observed in compounds with x>or=0.04 and is attributed to presence of large magnetic clusters in the sample. From the data of the thermoremanent magnetization it is suggested that some of them can remain up to temperatures much higher than T_f.

Under conditions of a fractional occupancy of impurity states, it is possible for the charges localized on the impurities to correlate their mutual positions. The electron mobility in such samples exhibits greatly enhanced values. The effect has been previously observed in HgSe:Fe. The authors study the case of HgSe doped with Fe (resonant donor n_Fe=2*10¹⁹ cm^-3) and Ga ('shallow' donor 0<or=n_Ga<or=1*10¹⁹ cm^-3, i.e. up to complete filling of Fe-donor states). By keeping the concentration of Fe constant and changing the concentration of Ga in the sample they can vary the parameter that rules the degree of spatial correlation, i.e. the ratio of the number of unoccupied states to the total number of donors. As the alternative method of tuning the discussed ratio, the hydrostatic pressure is also employed. The results of the conductivity and Hall effect measurements are compared with the short-range correlation model modified appropriately in order to account for the presence of randomly distributed Ga ions. A quantitative agreement is observed. Intuitively, additional donors, when fully ionized, should lead to a drop in the mobility. They show that conditions exist when co-doping with Ga can lead to a surprising enhancement of the mobility.

The magnetic properties (susceptibility, high-field torque and magnetization) of cubic HgFeSe are analysed. The Van Vleck magnetism of Fe²⁺ is well evidenced. The energy splitting of the Fe levels in HgSe is shown to differ considerably from that in other selenides, leading to a lower than usually observed value of the magnetic susceptibility.

Recent observations of novel electronic processes in semimetallic HgTe-CdTe superlattices are reviewed, in the context of distinctive features in the superlattice band structures. It is demonstrated that many of the observed phenomena are either directly or indirectly related to an anticrossing of the HH1 band, which is nearly dispersionless in the growth direction k_z, and the E1 band, which has strong dispersion in k_z. For example, it has recently been demonstrated that there is a critical magnetic field above which one can magnetically activate a bipolar plasma whose density increases linearly with B-B_crit. A second example is the observation of broad quantum Hall plateaux in superlattices with strong 3D dispersion, which contradicts the previous theoretical understanding of how dimensionality influences the quantum Hall effect. Other topics include recent technological advances towards more sophisticated structures (e.g. controlled modulation doping, set-back doping and submicron lateral patterning), comprehensive new theoretical descriptions of magnetotransport and magneto-absorption in HgTe-CdTe superlattices, and novel processes predicted to occur in HgTe-CdTe quantum wires.

The authors have investigated the pressure dependence of the infrared photoluminescence (PL) of HgTe/CdTe superlattices (SLS) at liquid nitrogen temperature up to 30 kbar. They observed several PL peaks with energies ranging from approximately 130 meV to approximately 700 meV. The most prominent peak at approximately 130 meV, which has been attributed to the recombination across the SL bandgap, moves higher in energy with a pressure coefficient of <or approximately=1 meV kbar^-1. Other peaks at higher energies have similar pressure coefficients of 0-2 meV kbar^-1. A calculation based on the envelope-function approximation gives a pressure coefficient of at least approximately 6.5 meV kbar^-1, which is far outside the error bars of the measured pressure dependence of the main peak, <or approximately=1 meV kbar^-1. Possible explanations, including a modification of the current model of HgTe/CdTe SL bands and a reinterpretation of the PL peaks, are examined.

Hg_1-xCd_xTe is an important material for infrared device technology. Despite a variety of investigations on molecular beam epitaxially (MBE) grown Hg-based superlattices and modulation-doped structures, little attention has been paid to the study of the photoluminescence properties of the constituent materials. The authors report on a systematic study of infrared photoluminescene on Hg_1-xCd_xTe layers grown by MBE. The Fourier transform photoluminescence of Hg_1-xCd_xTe layers with 0.25<x<0.93 grown on (110) and (211) GaAs was investigated in the spectral range from 01 eV to 1.5 eV. Emission lines were observed with a full width at half maximum down to 15 meV. Photoluminescence and transmission properties are compared over the temperature range from 4.2 K to 300 K. The possible influence of Burstein-Moss shift, impurity bands and exponential band tails are discussed. Photoluminescence at high temperatures originates from band-to-band transitions. However, low temperature photoluminescence can be attributed either to impurity bands or to additional states due to band tails.

Open-gap n-type Hg_1-x-yCd_xMn_yTe crystals have been successfully grown on CdTe substrates by the LPE method from Hg-rich solutions. Using the high-quality crystals with y=0.01-0.02 grown by this method, the authors have succeeded in observing the magnetophonon resonance oscillations and the magnetic quantum oscillations due to the Auger recombination process. These oscillations are analysed based on the modified Pidgeon-Brown model. In both types of the oscillations, the temperature dependence of the observed peak positions is found to be stronger than those predicted by the model which includes the temperature dependence of the energy gap E_g. The origin of their anomaly is discussed.

The electrical properties of the anodic-Hg_0.833Zn_0.167Te interface were studied by means of capacitance-voltage measurements of metal-insulator-semiconductor capacitors. Hg_0.78Cd_0.22Te test samples were simultaneously processed for comparison. The fixed insulator charge density was lower in HgZnTe than in HgCdTe, but still large enough to invert the p-type material surface at 0 V. The HgZnTe interface was more stable under thermal treatments than the HgCdTe interface. Hall-effect measurements at variable temperature were used to determine the bulk electrical properties. A two-band model with empirical relations for the energy gap and intrinsic carrier concentration yielded an excellent fit to the experimental data.

The authors report for the first time on Fourier transform photoluminescence excitation spectroscopy in the mid-infrared covering the wavelength range between 1 and 5.5 mu m. By studying a medium-bandgap Hg_0.66Cd_0.34Te bulk crystal and InSb bulk material, direct evidence for the excitonic nature of the luminescence transitions has been found in both cases. In addition, the excitation spectra on MBE-grown Hg_1-xCd_xTe superlattices show the step-like density of states of the confined two-dimensional system.

There is still considerable controversy in the field of recombination mechanisms in Cd_xHg_1-xTe (CMT), particularly with regard to those operating in p-type material. It is vital to understand this behaviour if improvements in device performance are to be made. This paper reports on lifetime measurements made on both liquid phase epitaxial (LPE) and metal-organic vapour phase epitaxial layers. Factors studied include the influence of surfaces, substrate/layer interfaces and the dependence of lifetime on carrier concentration and temperature. The effect of different post-growth annealing schedules has also been considered and comparisons made between MOVPE material doped with acceptor impurities and that controlled by metal vacancies. As transient photoconductive decay results did not produce the trends expected from device results at 192 K, measurements have been made under steady-state conditions for representative samples. These steady-state lifetimes are the first reported in MOVPE material and are augmented by estimates of capture rates and densities of traps.

The phonon spectra of mixed-crystal Hg_1-xCd_xTe with composition x=0.18 to 0.45 have been investigated by means of far-infrared transmission and reflection spectroscopy at temperatures from 4.5 to 300 K. In addition to low-frequency TA mode absorption and two-phonon combination absorption in transmission spectroscopy the fine structures of the CdTe-like optical phonon reflection band and the complex structure of the HgTe-like band, as well as the plasmon-LO phonon coupling effect have been observed and investigated both experimentally and theoretically.

Deep level transient spectroscopy (DLTS) can be used to characterize deep levels in Hg_1-xCd_xTe if the method is adapted to the narrow bandgap semiconductor case. The presence of the free carrier tail and the Poole-Frenkel effect may significantly influence the emission rate, and must be considered. The trap energy level extracted using DLTS is its entropy, which is equal to the trap activation energy only if the trap level follows the majority carrier band edge. If the trap capture cross section is measured as a function of temperature, the temperature dependence of the trap energy may be found. An n-type MIS capacitor sample was characterized as an example. The emission rate for the electron trap measured showed strong free carrier tail and Poole-Frenkel effects. The true trap energy was found to be a linear function of temperature.

A naturally occurring two-dimensional electron system-an inversion layer adjacent to a single grain boundary in narrow-gap diluted magnetic semiconductor Hg_0.79Cd_0.19Mn_0.02Te (E_g<or=200 meV)-was studied by means of quantum magnetotransport in magnetic fields up to 20 T and at temperatures down to 100 mK. Precise Hall quantization, plateau flatness down to 5*10^-5, as well as nearly vanishing longitudinal resistance minima, were observed. Detailed analysis of the data provided information on the bypass resistance and the homogeneity in the electron distribution.

On the basis of Kane's band model the authors apply the envelope-function method to calculate the superlattice states. Inserting these results into Kubo's formula the dynamical conductivity for interband transitions is derived. Adding the lattice contribution of the dielectric function they obtain the complex refractive index. Their results are compared with experimentally determined absorption coefficients, measured with Fourier transform spectroscopy on MBE-grown superlattices fabricated on a Riber 2300 system at the authors institute.

Results of tight-binding and eight-band k.p calculations of the electronic band structure of long wavelength Ga_1-xIn_xSb/InAs superlattices are compared with experimental energy gap and absorption coefficient data. The effective masses, band splittings and absorption coefficients observed in this system illustrate the potential of these structures for application in focal plane array systems demanding high detectivities or relaxed cooling requirements. Comparisons with Hg_1-xCd_xTe, the industry standard, are particularly favourable at longer wavelengths (8-12 mu m and beyond), due to both a substantial reduction in tunnel currents and a suppression of impact ionization noise processes. The authors also find that the InSb- or Ga_1-xIn_xAs-like nature of the interfaces should affect the energy gap of a Ga_1-xIn_xSb/InAs superlattice, and that substantially larger optical absorption coefficients are to be expected in structures with InSb-like interfaces. Their calculations are in agreement with experimental absorption spectra and with observed dependences of energy gaps on interfacial chemistry, measured in samples in which the nature of the interfaces was controlled through appropriate shuttering sequences and use of interrupts during growth by molecular beam epitaxy.

The effect of interface bond configuration on the structural and optical properties of short-period ( approximately=50 AA) GaSb/InAs superlattices has been examined. Structures consisting of eight monolayers of GaSb and seven monolayers of InAs with either 'GaAs-like' or 'InSb-like' interface bonds were grown by MBE. Evidence for differences in the structural properties, vibrational properties and band structure of the two types of material was obtained using X-ray diffraction, Raman scattering, interband magneto-absorption and photoconductivity measurements.

InAs/Ga_1-xIn_xSb superlattices have been investigated by magneto-optical and magnetotransport techniques. Bandgaps, determined from interband magneto-optical measurements and from the temperature dependence of the intrinsic carrier concentration, are in the long-wavelength infrared region (8.3 to 12.4 mu m) and are in good agreement with gaps calculated from a two-band model. Both electron and hole effective masses were measured by cyclotron resonance and the electron effective mass is found to be a factor of 4-5 times larger than in HgCdTe (the industry-standard IR material). This is necessary for reduced dark currents and good optical absorption coefficients in this material. Finally, the first observation of higher-order electron and hole subbands (by interband magneto-optics) in this material system is reported.

In this paper the authors provide the rationale, and a preliminary experimental investigation of, the Sb/GaSb system as a potential indirect narrow-gap superlattice, where spatial quantization effects are proposed to induce a positive valence-conduction band energy gap in the Sb semimetal layers. The experimental study has investigated growth of Sb/GaSb single heterojunctions and elementary multilayer structures using molecular beam epitaxy. Although Sb is rhombohedral and GaSb has the cubic zinc blende structure, the biatomic planar structure of Sb mimics the (111) plane in the zinc blende structure, and registry between the zinc blende and rhombohedral lattice can be preserved if epitaxy is achieved along the (111) direction. The authors show that high-quality Sb layers can be grown epitaxially on GaSb(111) homoepitaxial films, and that GaSb can be subsequently grown on Sb. The epilayer growth and heterojunction structure have been characterized by in situ reflection high-energy electron diffraction, X-ray diffraction and X-ray photoelectron spectroscopy. Electrical properties of Sb epilayers with thicknesses in the range 150-2500 AA have been determined by temperature-dependent Hall measurements.

The authors have investigated the interface of InAs/GaSb and InAs/AlSb grown by migration-enhanced epitaxy by photoluminescence and Raman spectroscopy. The significant effects of interface bonds, InSb or GaAs (AlAs), on the luminescence as well as phonon spectra are discussed. The confined phonon modes in both systems, InAs/GaSb and InAs/AlSb, are also compared to study the characteristic properties of InAs heterostructure.

The authors report on an experimental study of a novel three-terminal negative differential conductance device. Interband tunnelling from an electron inversion layer through the depletion barrier into resonant states of the bulk valence band is controlled by the gate voltage V_g at an InAs or InGaAs metal-insulator-semiconductor field-effect structure. Upon increasing V_g from the threshold voltage by up to 2 V the peak current can be varied by a factor of 5000 (500) with a peak-to-valley current ratio in excess of 2(16) for InAs (InGaAs). The effective tunnelling electric field and an areal electron density, as determined by measurements under high magnetic fields, are found to be only weakly dependent on V_g and on the tunnelling voltage. The mechanism of current control is discussed in terms of a variation of effective tunnelling area.

The authors have studied the cyclotron resonance (CR) of 2D electrons in strained InAs single quantum wells, which are grown on either a GaSb or an AlSb buffer layer in order to vary the degree of strain in InAs. For samples with a GaSb buffer layer, the measured CR mass, depending upon the quantum well thickness, varies from 0.0339m₀ to 0.0468m₀, which is heavier than that of bulk InAs at the conduction band edge, 0.0234m₀. This finding is quantitatively explained by the non-parabolicity effect in InAs and the mass renormalization effect originating from the penetration of the electron wavefunction into the barriers. On the other hand, for samples with an AlSb buffer layer, the CR mass, depending upon the carrier concentration, varies from 0.0320m₀ to 0.0412m₀, and is lighter than that of equal well thickness but with a GaSb buffer. This reduction in mass is accounted for by the stronger misfit strain in InAs when an AlSb buffer layer is used.

Electron cyclotron resonance is studied in very deep quantum wells consisting of InAs between AlSb barriers. High electron mobility and strong conduction band non-parabolicity together with the small effective mass and the large effective g factor of this material enables the authors to observe simultaneous cyclotron transitions between adjacent sets of spin-split Landau states. Their experiments resolve spin-conserving transitions involving two or three different Landau levels depending on the filling factor. The results are compatible with a single-particle model.

Measuring photoconductivity at low temperatures, the authors investigate the recently observed bipolar behaviour of the persistent photoeffect in InAs/AlSb quantum wells. Depending upon the incident wavelength they observe either a persistent increase or a persistent decrease of the carrier density in the well. They discuss their experimental findings in terms of a simple model based on the band structure as known to date and the growth parameters of the heterostructure.

Mesoscopic phenomena in microstructures that incorporate narrow-gap diluted magnetic semiconductors may exhibit a number of novel features driven by spin-orbit and spin-exchange interactions. The influence of these interactions upon quantum interference effects, such as universal conductance fluctuations, low-frequency noise and quantum localization, is described. Results of recent millikelvin magnetoconductivity measurements, carried out for microstructures of Hg_1-x-yCd_yMn_xTe bicrystals and Pb_1-xMn_cTe epilayers are presented.

Diluted magnetic IV-VI semiconductors like Pb_1-xMn_xTe or Pb_1-xMn_xSe exhibit a strong dependence of the effective g-factors of the free carriers on temperature and magnetic field, caused by an exchange interaction between the free carriers and the magnetic moments of the paramagnetic ions. In quantum wells or superlattices the strength of the exchange interaction depends on the probability of finding the carriers within the diluted magnetic component. With various PbSe/Pb_1-xMn_xSe superlattices the authors performed coherent anti-Stokes Raman scattering (CARS) experiments in the mid-infrared using CO₂ lasers to obtain very precise data on the spin splittings of electrons and holes. The analysis of these CARS data in the framework of an envelope-function approximation yields a type-l' alignment with electrons confined in the Pb_1-xMn_xSe layers and holes in the PbSe layers.

In the small-gap semiconductor Pb_1-x-ySn_yMn_xTe the magnetic phase transition from a ferromagnetic to a spin-glass state at high carrier concentrations is studied. The authors present new data on magnetization, AC susceptibility and specific heat for various carrier concentrations showing the transition. They also present frequency-dependent AC susceptibility measurements. For the spin-glass sample there is a strong dependence on the measuring frequency, which follows the Vogel-Fulcher law. Furthermore, they calculate the phase boundary in the x-p magnetic phase diagram using the mean random field model. There is a satisfactory agreement between the experimental data and the calculations.

The authors report transport and far-infrared magneto-optical measurements in n-type HgZnTe-CdTe and HgTe-CdTe superlattices over a large temperature range (1.5-300 K). The data show unambiguously that the HgZnTe-CdTe superlattices are semimetallic at low temperature and are degenerate intrinsic semiconductors for T>100 K, in good agreement with the theory which predicts a semimetal-semiconductor transition induced by temperature. Influence of magnetoplasma effects is finally discussed.

Zero-gap semiconductor HgTe in the presence of crossed magnetic and electric fields is considered theoretically. An external magnetic field opens an energy gap between the Landau levels in the conduction and the heavy-hole valence bands. An electric field in the crossed-field configuration closes this gap, inducing an insulator-metal transition at low temperatures. The structure of Landau levels in HgTe in crossed fields has been calculated. In order to decide whether the gap really closes, the authors have also calculated the k_z dependence of the Landau subbands. For B=2 T the insulator-metal transition occurs at E approximately=0.98 kV cm^-1. It is shown that the insulator-metal transition is preceded by tunnelling effects.

The striking anomalies of the magnetoresistance of samples of zero bandgap p-type Hg_1-xMn_xTe (x=0.03 and x=0.065), including a step negative magnetoresistance associated with the Hall mobility minimum and an anomalous SdH oscillation, have been observed at temperatures from 0.3 to 4.2 K and in magnetic fields up to 7 T. The heavy-hole intraband scattering and threshold scattering from the conduction band to the valence band for two samples are discussed.

The Hall effect and the Shubnikov-de Haas effect of a quasi-two-dimensional inversion layer adjacent to grain boundaries in InSb and Hg_0.23Cd_0.77Te bicrystals are investigated in magnetic fields up to 14.5 T, temperatures down to 0.4 K and at different angles between the grain boundary plane and the magnetic field. For magnetic fields perpendicular to the grain boundary interface a complicated pattern of Shubnikov-de Haas oscillations and well developed quantum Hall plateaux with the quantum numbers j=7 and j=9 are observed at an HgCdTe sample. A cosine dependence of the Hall coefficient and the oscillation period of the ground subband in tilted fields was found for both InSb and HgCdTe bicrystals, whereas the first excited subband showed remarkable deviations from the two-dimensional behaviour. From these deviations, an estimate of the spatial extension of the wavefunction is given. The diamagnetic Shubnikov-de Haas effect yields depopulation fields which can be explained within the Reisinger-Koch model. First results of cyclotron resonance measurements show well separated transmission dips for different subband masses at HgCdTe bicrystals.

Photovoltaic oscillations due to interband magneto-optical transitions have been observed on the quaternary open-gap semimagnetic alloys Hg_1-x-yCd_xMn_yTe. In addition to the conventional transitions, phonon-assisted interband excitations have been observed for the first time on a semiconductor. It is argued theoretically that the resonant phonon-assisted interband transitions must originate from the camel-back shaped Landau subbands. Consequently, the authors observations confirm the existence of such subbands in the Gamma ₈ valence bands of semimagnetic materials. The determined optic phonon energies correspond to the longitudinal HgTe-related mode.

The results of an experimental study of the optical and photoelectrical properties of Eu-containing lead chalcogenides and superlattices are presented. From both sets of experiments band offset data for the lead chalcogenide-europium chalcogenide heterojunction are derived. The results are compared with the data obtained from a linear interpolation of the band structures between those of the binaries.

The authors report on magnetotransport measurements on the solid solution Cd₃(As_xP_1-x)₂ in stationary fields up to 17.3 T at liquid helium temperatures. The measurements are compared with those of Arushanov et al. (1982 Sov. J. Quantum Electron. 12 1256) on samples with electron concentrations about ten times higher and discussed with respect to possible models of the energy band structure. So at low carrier concentrations a four-ellipsoid model is proposed.

In two recent review papers on metal organic vapour phase epitaxy (MOVPE) of cadmium mercury telluride (CMT) particular emphasis was placed on the crucial importance of doping studies to the realization of future device structures Irvine et al. (1991) and Tribowet (1991). If the full potential of MOVPE growth of CMT is to be realized then extrinsic doping of heterostructures is required. If the doping and composition junctions can be grown with the correct degree of grading then this will create the potential for the production of device structures leading to either improved performance and/or increased operating temperatures. This papers reviews published doping studies and also presents some recent results on both acceptor and donor doping studies carried out by the authors. In the latter studies, interdiffused multilayer process (IMP) growth of CMT has been performed at approximately=360 degrees C using dimethyl cadmium (DMC) and di-isopropyl tellurium (DIPT) as the MO precursors while the Hg overpressure was provided by a heated elemental source. Alternative acceptor doping sources to arsine have been investigated including phosphine, triphenyl arsenic, and phenyl arsenic of which the latter appears to be most suitable. Iodine has continued to show the donor dopant potential in CMT that it exhibited with higher-temperature ( approximately=400 degrees C) MOVPE growth using di-ethyl tellurium (DET). Characterization of fully doped structures is described.

Narrow CdTe/CdHgTe/CdTe structures have been prepared taking advantage of interdiffusion between CdTe and HgTe elemental layers grown by MOCVD at 365 degrees C. Numerical simulation of interdiffusion by the finite difference method shows how the shape of the graded wells obtained by interdiffusion depends on the thermal cycle of preparation. Photoluminescence transitions are observed which allow one to probe the composition of the alloys at the centre of the wells.

The authors report the influence of growth conditions and substrate type on the quality of Hg_1-xMn_xTe (MMT) epitaxial layers. MMT epitaxial layers were grown using di-isopropyl telluride (DIPTe) tricarbonyl methylcyclopentadienyl manganese (TCMn) and elemental Hg in a horizontal atmospheric-pressure MOVPE reactor. Mn concentration depends strongly on both growth temperature and position of the substrate on the susceptor. This is primarily due to the large difference in pyrolysis characteristics between the DIPTe and TCMn precursors. Epitaxial growth was possible up to x approximately 0.8 but high growth temperature led to polycrystalline deposition. Investigation of the morphology and crystallinity of the epitaxial layers on GaAs (100), (Cd, Zn)Te (100) 2 degrees off to nearest (110), (211)B and (111)B substrates revealed that (Cd, Zn)Te (100) 2 degrees off to nearest (110) substrate was the most suitable of all tested substrates. The room-temperature reflectance and transmission spectra over the wavelength range 0.3-30 mu m were taken. Thicknesses of the films have been estimated from the interference fringe peaks near 20 mu m. The optical transition Lambda _4.5v- Lambda _6c (the E₁ peak) and its doublet (the E₁+ Delta ₁ peak), the absorption edge in transmission, and the behaviour of the interference extrema with Mn composition have been examined.

A modified approach for epitaxial growth of HgCdTe (MCT) by isothermal vapour phase epitaxy (ISOVPE) under high hydrogen pressure is reported. The growth and anneal are carried out in a re-usable and demountable internally heated high-pressure furnace. The technique offers a low-cost, easily implemented alternative to other ISOVPE growth techniques which include sealed quartz ampoule and open-tube growth. By controlling the hydrogen pressure in the 600-1200 psi range (4.1-8.2 MPa) during a Hg anneal in the 450-550 degrees C temperature range, the gradient in the MCT composition can be controlled and tailored. Using this approach the authors report MCT epilayer growth on (111)B CdTe and CdZnTe with thickness uniformity to within 1.5% over a 6 cm² area. Compositional uniformity as measured by 300 K FTIR transmittance indicates a cut-off wavelength of 6.3 mu m to within 0.1 mu m measured over the same area. Undoped n-type epilayer mobilities in the range of (2-7)*10⁴ V^-1 s^-1, carrier concentrations of (2-8)*10¹⁴ cm^-3 and excess carrier lifetimes as high as 1 mu s at 80 K are reported. In addition, photoconductive detectors fabricated from ISOVPE epilayer material have yielded 80 K responsivity >8*10⁴VW^-1 and a D* value of 1.5*10¹⁰ cm Hz^0.5 W^-1 measured over the 8-11 mu m spectral band.

Liquid phase epitaxy (LPE) is currently the most widely used thin-film technology for (CdHg)Te IR photodiode arrays. This paper discusses several key materials parameters and how they have been improved The main areas addressed are: (i) A reduction in background impurities following an extensive chemical analysis programme, leading to improved LPE charge preparation, LPE boat cleaning and epitaxial layer annealing. Typical n-type background electrical properties are n₇₇=6*10¹³-1*10¹⁴ cm^-3, mu ₇₇=(2.2-2.4)*10⁵ cm² V^-1 s^-1 (x=0.21). (ii) Donor doping with In and Al and acceptor doping with Cu and Rb to provide n- and p-type carrier concentration control using impurities rather than native defects and residual impurities. In doping is shown to give good control in the range n=10¹⁴-10¹⁸ cm^-3 with approximately 100% activity after annealing. Cu doping gives approximately 100% activity in the range mid-10¹⁷ to mid-10¹⁸ cm^-3 in the as-grown state, but after annealing to remove Hg vacancies, most of the Cu diffuses to the surface. Initial work using Rb doping has shown that the Rb rapidly diffuses to the surface and the substrate-layer interface. (iii) A novel design of LPE graphite sliding boat featuring integral HgTe sources for Hg overpressure control and very low loss-rates of Hg, giving excellent compositional reproducibility ( Delta x+or-0.0025, Lambda lambda _RT+or-0.2 mu m) at x=0.21.

The molecular beam epitaxial growth of (100) Hg_0.8Cd_0.2Te epilayers has been systematically investigated in order to optimize the growth parameters and in particular the Hg/Te flux ratio. Both the pyramidal hillock density and the mobility depend on the Hg/Te flux ratio. A minimum in the hillock density correlates well with the largest mobilities. As is well known, electrical properties are strongly influenced by structural defects. Structural defects due to non-stoichiometry, i.e. vacancies and antisites, can be largely reduced by optimizing the Hg/Te flux ratio. It is shown that an optimum Hg/Te flux ratio exists within a narrow range between 270 and 360 at the growth temperature of 180 degrees C, in contrast with the larger range of the Hg/Te flux ratio over which single-crystal growth could be maintained. The x values of the Hg_1-xCd_xTe epilayers were determined from the E₀ gap dependence as well as from the E₁ gap dependence. The difficulties encountered in determining the x value from normal transmission curves, i.e. from E₀(x), due to the Burstein-Moss shift are discussed.

The electron minority carrier mobility is an important parameter because of its effect on the diffusion length for determining the performance of semiconductor infrared detectors. The suppression of the diffusion current of a photodiode with magnetic field has been used to determine the electron mobility in epitaxially grown p-type Hg_1-xCd_xTe (CMT). This technique enables the mobility to be measured in the active region of a photodiode. Results have been obtained as a function of temperature for material covering a wide range of acceptor concentrations from 2*10¹⁵ cm^-3 to 2*10¹⁷ cm^-3. The mobility in the more highly doped samples is considerably lower than is found in low-doped n-type material and this can largely be accounted for by ionized impurity scattering. A semi-empirical formula is derived and is shown to give reasonable agreement with the experimental results. The deduced mobility can be combined with a measurement of the diffusion length to calculate the effective minority carrier lifetime in the active region of the diode. This has been compared with the lifetime measured by photoconductive decay in separate samples of the same material.

The authors show that Hall effect measurements versus magnetic field R_H(B) at 77 K permits one to identify extended defects in HgCdTe and to evaluate their basic parameters. Three different groups of samples can be singled out. First, those with the classical R_H(B), consisting of two plateaux in weak and strong magnetic fields with a transition which conforms to the theoretical value of the Hall factor in n-HgCdTe, corresponding to the most perfect crystals. The distinctive feature of the second group is a monotonic decrease of R_H, which reaches saturation only in very high fields. It is attributed to the presence of a second set of electrons appearing when the constituent dislocation density on the subgrain boundaries exceeds, at a certain misorientation across the boundary, some critical level corresponding to the appearance of a percolation level. For the third group, with R_H(B) between those plateaux, a minimum appears. This is connected with the influence of isolated inhomogeneities with conductivity higher than the conductivity of the surrounding medium. This is presumably due to tellurium-rich precipitates, transformed during anneal in the HgTe inclusions.

The authors have developed a new approach to fabricate coherent InAs insertion in a crystalline GaAs matrix by conventional MBE. The 7% lattice mismatch between InAs and GaAs requires specific strategies to individually control the formation of the InAs/GaAs and GaAs/InAs interfaces, which is governed by a unique balance of surface and interface energy as well as kinetic processes. The interfaces of the strained InAs insertions are then in register with the surrounding GaAs matrix, as determined by high-resolution X-ray diffraction and transmission electron microscopy. This high level of growth control allows one to fabricate a novel class of artificial materials by using either exactly (001)-oriented substrates or vicinal substrates tilted towards (100). The controlled incorporation of fractions of InAs monolayers into the GaAs matrix offers the unique possibility of manipulating the translational motion of excitons in the crystal on an atomic scale and hence to tune the optical response. Moreover, the authors are able to obtain isolated InAs quantum dots in a crystalline GaAs matrix on vicinal (001) GaAs substrates. The absence of any surface and interface states makes this system promising for the study of the optical response of ultrasmall zero-dimensional systems.

InAs/Al_xGa_0.48-xIn_0.52As single and multiple quantum wells (QWS) grown in InP by modified solid source molecular beam epitaxy are investigated by photoluminescence (PL) spectroscopy. By adjusting the QW thickness between 2 and 16 monolayers (ML) and the barrier composition between Al_0.48In_0.52As and Ga_0.47In_0.53As, the PL emission can be tuned from 1.0 to 2.2 mu m. The excitonic nature of the low-temperature recombinations and the observations of luminescence at room temperature demonstrate the high quality of the samples and the potential of the InAs/Al_xGa_0.48-xIn_0.52As system for optoelectronic device applications.

Magneto-optical properties of quantum dots in semiconductors have been considered using the model of hard-wall confinement. The eigenenergies of the system have been computed. Taking into account the real band structure of InSb-type materials (narrow energy gap and strong spin-orbit interaction) the authors have been able to describe very well the existing magneto-optical data on the quantum dots in InSb.

Subband states in n-inversion layers on HgCdTe are resonances due to the coupling to the bulk valence-band continuum. The width of the resonances increases with decreasing gap energy. Thus the existence of subband states is questionable if the gap energy is zero. The authors study the subband problem for MIS structures on narrow-gap semiconductors starting from Kane's k.p Hamiltonian. The analytical approach with no specified surface potential shows that for E_gap=0 the free-electron term, which is usually neglected for narrow-gap systems, becomes crucial, because it changes the order of the subband differential equation. The authors also solve the subband probe for an analytical surface potential and calculate the density of states. The results demonstrate the existence of a subband resonance even for vanishing gap energy provided that the free-electron term is not neglected.

A new type of electron and hole confinement in semiconductor double heterostructures is discussed. The confinement occurs when the effective mass of charge carriers in the central region is higher than that in the outside regions and it results from a 'kinetic well' produced by the transverse free motion. The calculated density of electron states is similar (but not identical) to that of the potentially confined 2D systems. The effect of an external magnetic field on the system is considered; it is shown that the energies of the kinetically confined states are strongly nonlinear functions of field intensity. Specific structures are described in which the kinetic confinement of electrons or holes would be possible. The kinetic confinement due to band non-parabolicity is also considered and its observation in InAs accumulation layers is reviewed.

The authors study the energy spectrum of a single spherical quantum dot of narrow-gap IV-VI material embedded within a host IV-VI material of different bandgap. The calculations of the energy levels as a function of the dot radius are performed. When the anisotropy is strong, the energy spectrum is investigated using Fal'kovskii's adiabatic approximation for multiband systems. When the quantum dot material has an inverted bandgap with respect to the host, low-energy edge states are shown to exist.

The authors report their latest results on the fabrication and successful operation of HgCdTe infrared diode lasers. The stripe-geometry double-heterostructure lasers were grown by molecular beam epitaxy (MBE). The active layer thickness ranges between 0.9 and 1.4 mu m, and the p⁺ and n⁺ confinement layers were in situ doped up to 10¹⁸ cm^-3 with arsenic and indium, respectively. Five double heterostructures were grown, all of which produced working lasers. The devices were operated under pulsed currents at temperatures between 40 and 90 K. The 77 K stimulated emission wavelengths for these lasers were 2.9, 3.4, 3.9 and 4.4 mu m. Operation at 5.3 mu m was demonstrated at 60 K. The lowest 77 K threshold current density was 419 A cm^-2 which is very close to the prediction of a numerical calculation. Characterization of the devices, including, for example, temperature dependence of the threshold currents and spectral analysis, was performed and showed the characteristics of well-behaved, stable devices that operated without failure while being tested.

The spectroscopic properties of CdHgTe/CdTe heterostructures are studied using photoluminescence absorption and for the first time, Fourier transform photoluminescence excitation spectroscopy. The analysis of these data shows that the line position can be well predicted from the confined carrier levels. The absorption spectrum is fitted with a theoretical calculation up to the second transition energy. The linewidth is dominated, in thin wells, by a well width fluctuation of the order of +or-1 monolayer, while in wide wells it is due to alloy fluctuations and defect-induced broadening mechanisms.

The process of p to n conversion in Cd_xHg_1-xTe (CMT) is important to both photodiode and photoconductor device fabrication. It is the purpose of this work to investigate type conversion of CMT as a result of annealing in Hg vapour. p-type CMT grown by liquid phase epitaxy (LPE) and with x=0.22+or-0.02, has been annealed in an open-flow furnace at 200 degrees C. A Hall and strip technique has been used to identify the resultant p/n junctions, formed by Hg diffusing into the CMT and reducing the concentration of metal vacancies. Junction depths are found to be dependent upon the square root of anneal time, the initial acceptor concentration, N_a, and background donor concentration, N_d. These findings are compared with other published data, and the mechanism of mercury diffusion is discussed with regard to N_a and N_d, as well as Te precipitates.

The authors have fabricated p-on-n LWIR HgCdTe diodes by extrinsically doping a double-layer heterostructure grown by metalorganic chemical vapour deposition (MOCVD) using the interdiffused multilayer process (IMP) on GaAs substrates. The p-side of the junction was obtained by a fast As diffusion, from an ion-implanted source, ahead of the heterointerface when the annealing was performed at very low partial pressure of Hg. Examples of photodiodes fabricated by this process are given. Thus, for devices processed by As implant/diffusion, an R₀A of 24.2 Omega cm² for a cut-off wavelength of 11.7 mu m at 77 K and a quantum efficiency of 51% were obtained for a mesa unpassivated structure, and an R₀A of 91 Omega cm² for a cut-off wavelength of 10.3 at 77 K and a quantum efficiency of 50% were obtained for a planar structure. At 77 K the transport mechanisms limiting the device performance near zero bias have appeared to be thermal processes (ideality factor n between 1 and 2). However, other mechanisms evident in reverse bias such as shunt and tunnelling, may also operate.

Experiments have been designed to measure interdiffusion coefficients in CdTe/HgTe (001) superlattices grown by molecular beam epitaxy. The interdiffusion coefficients are deduced from double crystal X-ray diffraction on post-growth-annealed samples. The annealings are realized in the 200-240 degrees C range under Hg saturated pressure. The results indicate that interdiffusion is dependent on concentration. The authors calculate diffused profiles from an expression on D(c,T), and simulate the corresponding X-ray spectra to fit them with the experimental ones.

Doped homo- and heterostructures of Cd_xHg_1-xTe (CMT) are required for some types of new-generation devices. These structures have been grown by metal organic vapour phase epitaxy (MOVPE) at approximately 360 degrees C onto both CdTe and GaAs (buffered with CdTe) substrates. The structures were produced using both acceptor and donor doping with arsenic and iodine respectively, and the junctions formed following annealing in mercury to eliminate metal vacancies. This paper summarizes progress in characterizing these junctions both in terms of x and electrical behaviour.

Separate confinement heterostructure (SCH) lasers with different active layer were grown by molecular beam epitaxy for photopumped emission around 3.4 mu m. They show that a multiquantum well gain medium has a superior threshold variation characteristic temperature, though low-temperature thresholds are higher than with a thick active layer. A SCH with a single quantum well as active layer does not show any stimulated emission, because of poor carrier transfer from the barriers to the well.

The performance of p⁺-n HgCdTe photodiodes is analysed in comparison with n⁺-p ones. The authors consider the 40-77 K temperature region and 7-15 mu m cut-off wavelengths. The effects of doping profiles on the photodiode parameters (R₀A product, quantum efficiency) are elucidated by forward-condition steady-state analysis. The influence of different junction current compositions on the R₀A product is considered. It is clearly shown that the R₀A product for p⁺-n junctions is higher, especially for long-wavelength photodiodes. Results of calculations are compared with experimental data reported by other authors.

X-ray photoelectron and Auger electron spectroscopy as well as reflection high energy electron diffraction have been used to investigate the influence of heat treatment and sputtering on the oxygen and carbon contamination of the (100) Cd_0.96Zn_0.04Te surface. It was found that oxygen contamination can be completely eliminated from the surface at 300 degrees C in an ultra-high vacuum but not carbon. Heating at still higher temperatures decreases the carbon contamination only slightly while increasing the Zn to Cd ratio on the surface considerably. This affects the Cd_0.96Zn_0.04Te surface structure and hence subsequent molecular beam epitaxial growth. The authors have found that sputtering with argon ions at an incident angle ranging between 20 degrees and 60 degrees at temperatures between 200 and 250 degrees C can remove nearly all carbon contamination from the surface without destroying the (2*1) surface reconstruction.

The authors consider interband and intersubband absorption in HgCdTe/CdTe single quantum well structures. By measuring the transmission in an internal reflection geometry with and without a metallized surface and polarized light, they study the dependence of the infrared absorption on the polarization of the exciting electric field.

Using a newly developed picosecond mid-infrared double-resonance pump-probe spectrometer carrier recombination and intraband scattering have been studied in a range of high-quality InAs epilayers grown by MBE. In as-grown samples recombination rates are typically dominated by recombination at the InAs/GaAs heterointerface with an interfacial recombination velocity of approximately=2.7*10⁴ cm s^-1, but Auger recombination (characterized by an Auger coefficient of 1.1*10^-26 cm⁶ s^-1) dominates in the thicker samples at higher excitation levels. At short pump-probe delays dynamic spectral hole burning in the absorption continuum has been seen for the first time. Spectral holes (with typical widths of approximately=300 cm^-1) are created at pump pulse intensities of approximately=10⁷ W cm^-2 and photon energies 32-57 meV above the band edge. The spectral holes track with the pump intensity and photon energy, and an analysis of their widths and depths gives an estimate of 30-50 fs for the carrier intraband scattering time.

Picosecond recombination processes of photoexcited electron-hole plasma in lead chalcogenides are directly monitored via time-resolved pump-probe experiments at wavelengths from 3 to 10 mu m. Bleaching of the interband absorption due to band filling gives rise to a large change in the refractive index. The time-resolved change of absorption of PbSe at T=70 K reveals a rapid recombination within 100 ps for excitation densities above 7*10¹⁷ cm^-3. Comparative measurements of mid-infrared luminescence spectra demonstrate that this behaviour is due to recombination by stimulated emission. At T=300 K, Auger recombination on a time scale of 2 ns is observed below the threshold of stimulated emission. The analysis of the time-resolved data gives an Auger coefficient of 1.1+or-0.3*10^-28 cm⁶ s^-1 for a carrier density of 3*10¹⁸ cm^-3.

The conduction electron spin resonance (CESR) in n-type InSb is shifted by the spin polarization of the nuclear and electron spins (Overhauser shift). The authors measured this shift by optically detecting the magnetic CESR using coherent spin-flip Raman scattering of two CO lasers in the infrared region. This method is very useful for studying the hyperfine interaction in narrow-gap semiconductors with high effective g-factors. By using a combined CESR/NMR technique, the effective g-factor in InSb was determined at different magnetic fields with the high precision available from NMR experiments. The Overhauser shift is directly proportional to the probability mod Psi (0) mod ² of finding an electron at the nuclear site of the ¹¹⁵In or the ¹²¹Sb isotope, respectively. In thermal equilibrium, when the spin polarization of the nuclei is known, measurements of the Overhauser shift directly yield information on mod Psi (0) mod ² of the conduction electrons at the respective nuclear site. Observing the shift while either building up dynamic nuclear polarization or studying the relaxation of the nuclei from high spin polarization to thermal equilibrium yields information on the nuclear spin-lattice relaxation time T₁ of each of the isotopes.

The authors have performed magneto-absorption measurements in p-Ge under intense crossed electric and magnetic fields. The results are interpreted with the coupled-band (Pidgeon and Brown) model. By comparison with the parabolic (Luttinger) model they show that nonparabolic effects, caused by the influence of the spin-orbit split-off band and the conduction band on the valence bands, have to be taken into account to explain the observed spectra.

Resonant impact ionization (RII) spectroscopy, a new technique for studying low concentrations of trap levels in narrow-gap Hg_1-xCd_xTe has been used to investigate impurity and defect levels in both bulk and liquid phase epitaxy (LPE) crystals of this ternary material. Mercury interstitials, deliberately introduced into bulk samples with x approximately=0.22 and x approximately=0.24, provide direct evidence for the formation of trap levels near 45 and 61 meV above the valence band edge for these x-value samples. The 61 meV level in the x approximately=0.24 (E_g=121 meV at 5 K) sample is seen to enhance the transition strength of resonant two-photon magneto-absorption (TPMA) observed at high fields by acting as a near-resonant intermediate state for the two-photon process. In the LPE material, samples which have received just a post-anneal in an Hg-saturated atmosphere show strong RII structure due to defect levels at approximately=60 meV above the valence band, whereas samples which are both doped with indium and post-annealed show only intrinsic one-photon magneto-absorption structure.

An approach for characterization of some narrow-gap semiconductors from the minimum of the bulk reflectivity spectra in the plasma reflectivity region is presented. Determination of the concentration, effective mass, relaxation time and optical mobility of free carriers of IV-VI laser materials is given. A simple empirical method for estimation of the bulk reflectivity minimum in the case of interference spectra is proposed.

A new model has been developed for the interpretation of photo-Hall measurements in semiconductor samples of arbitrary shape and inhomogeneous carrier distributions. The model has been tested for samples of n-type indium antimonide and has yielded new results for the carrier recombination time which was found to increase from 350 ns to 850 ns as the photogenerated population increased from 6*10¹¹ to 4*10¹³ cm^-3. The results are interpreted in terms of a single mid-gap trap.

(Hg,Cd)Te was the first mixed crystal which allowed the study of the transition from a wide gap semiconductor (CdTE) to a narrow-gap material without change of the optical transitions forming the interband edge. Now with (Pb,Sr)Se a material with rocksalt structure is available offering the same possibility. As an example, a critical re-examination of interband and intraband contributions to the refractive index is given. In the narrow-gap lead salts a maximum of n(E) near E_g was found earlier and attributed to the rapid change of absorption coefficient at the interband absorption edge. On the basis of measurements of the real absorption edge slope, as determined both by band structure and by broadening, a study was made connecting for the first time the experimental absorption edge parameters with the n(E) maximum.

Buried heterostructure (BH) lasers with active layers of PbSe, PbEuSe, PbSrSe and PbSnSe were made using molecular beam epitaxy (MBE). After growing the active layer the growth was interrupted. By use of photolithography and Ar⁺ ion beam etching, mesa stripes were defined and overgrown by a second MBE step. The overgrowth was investigated employing scanning electron microscopy (SEM). A selective chemical etch was used to resolve areas of different composition. The SEM pictures indicate the crucial role of the mesa shape and surface quality for the overgrowth process. Sharp edges result in deep cracks or grooves beside a well grown ridge. The authors successfully employed a chemi-mechanical polish after the ion milling process. The obtained smooth mesa shape results in a better overgrowth. As an alternative approach for fabrication of BH lasers shadow-masked growth of the active mesa stripe was investigated. Masks with slits in the 15 mu m range and sharp edges were obtained by anisotropic etching of (100)Si BH lasers with an active stripe width of 20 mu m were made by this technique and showed a threshold current comparable to double heterostructure (DH) lasers. Lateral structuring improved BH laser far-field distributions compared with DH lasers. The observed far-fields can be explained by a waveguide model.

Narrow-gap IV-VI lead chalcogenide layers grown on Si substrates with the aid of an intermediate CaF₂/BaF₂ buffer relax the thermal-mismatch-induced strain by misfit dislocations which glide on the primary (100) glide planes inclined 54.7 degrees with respect to the (111) surface. Macroscopic strain measurements show near complete mismatch strain relaxation down to cryogenic temperatures even after multiple cycling for binary PbSe, while ternary PbSe_1-xTe_x (x=0.4) strains elastically below room temperature. The morphology of the resulting dislocation steps is revealed by scanning tunnelling microscopy for the IV-VIs and atomic force microscopy for the fluoride buffer, and the number of dislocation step lines agrees with the calculated thermal strain relaxation.

Scanning tunnelling microscopy (STM) has been used to produce images of InSb(100) prepared by in situ treatment of several cycles of low-energy argon ion bombardment and annealing. Electron diffraction studies, following annealing to 625 K, show the c(8*2) pattern associated with the In-rich reconstruction also observed during MBE growth. These STM images demonstrate the improved surface ordering with successive cycles resulting in atomically flat terraces over areas of the sample in excess of 1000 AA*1000 AA. The observed reduction of macroscopic morphological defects, such as In droplets, is discussed as a function of surface treatment.

Photoconductivity spectrum and kinetics measurements of strained Pb_1-xSn_xTe/PbTe/BaF₂ superlattices (SL) and multiquantum well (MQW) structures show that this heterojunction is of the staggered SL (type 1'). A deep resonance level was found that does not occur on single epilayers.

The authors present a new class of infrared photodetectors based on the led-tin tellurides doped with group-III impurities. The persistent photoconductivity effect appearing in these materials provides an internal signal integration resulting in a considerable increase in signal-to-noise ratio. The techniques of photomemory quenching are discussed. In some quenching regimes the effect of giant quantum efficiency stimulation has been observed. The possibility of generation of the photoinduced spatially non-equilibrium states provides physical evidence for the construction of an integrated 'continuous' focal-plane matrix based on Pb_1-xSn_xTe(In).

The authors report on the high-field (H<400 kOe) magnetoresistance of initially dielectric Pb_0.75Sn_0.25Te(In). The data taken at 1.4-4.2 K show an increase in magnetoresistance with time (localization) for a relatively low concentration of nonequilibrium electrons n. The characteristic time for localization tau depends exponentially on H and linearly on T. The possible origins of the effect are discussed.

The characterization of semiconductive TlBiSe₂ thin films by the use of electron microscopy and infrared spectroscopy is presented. It was determined that the deposition conditions profoundly influence the microstructure and surface quality of the as-grown films, consequently affecting their electronic properties such as the free-carrier mobility, the amount of band edge tailing and the value of the optical energy gap. The results show that these two techniques are a very useful combination in material characterization.

The current mechanism of Pb_1-xEu_xSe diodes prepared by the MBE technique was studied by measuring R₀A of diodes with five different europium contents as a function of temperature. Good agreement was obtained between theoretical results and experimental data at high temperatures. At low temperatures, R₀A is not dominated by band-to-band tunnelling, but by excess tunnelling. The authors favour the explanation of trap-assisted tunnelling since R₀A is an exponential function of the energy gap. From the theoretical model, a minority carrier lifetime of the order of a nanosecond was derived.

The interface energy spectrum in real band-inverted PbTe/SnTe heterojunctions formed in both the (111) and (001) planes is calculated. It is shown that, even if the valence band of SnTe lies above the conduction band minimum of PbTe, the interface midgap states may still exist due to the strain effect on the band gaps.

Magnetotransport and optical measurements have been carried out on (001) and piezoelectrically active (111) InGaSb/GaSb and InAs/GaSb structures. In accordance with theory, these comparative studies reveal enhanced carrier density and mobility for growth in the (111) direction. The observed Stark shifts of up to 100 meV, in the (111) oriented InGaSb/GaSb structures are well accounted for by the estimated built-in piezoelectric field, of the order of 1*10⁵ V cm^-1. The absorption coefficient of (111) oriented InAs/GaSb superlattices shows significant enhancement.

Doping superlattices ('nipi' crystals) have been grown (by MBE on GaAs) and characterized in narrow-gap binary InAs for the first time. Low-temperature absorption measurements demonstrate effective optical bandgap lowering by >25%, giving long-wavelength thresholds of approximately=4.2 mu m in cooled trial photoconductive detectors. Long minority carrier lifetimes (100-400 mu s) and low power nonlinear optical saturation results confirm the character of the nipi potential and measured data agree well with an envelope-function calculation of the electronic subband structure. Intersubband transitions are detected and modelled in asymmetric InAs nipi samples and indicate the possibility of improved 10 mu m detectors in this material.

The growth, by MOVPE, of a range of antimonide-based material systems suitable for providing devices responsive to 2-4 mu m wavelength radiation is reported. Photodetectors with external quantum efficiencies of 60% at 2.2 mu m have been fabricated from an InGaSb homojunction. In order to examine the possibility of tuning the wavelength of emission or detection by using a strained single quantum well (SSQW) of InGaSb/GaSb this has been grown in the depletion region of a GaSb homojunction. These novel heterostructures have been grown to produce devices without the need for conventional doping. Using the crossed-gap alignment of InAs/GaSb the authors can form a diode-like structure. The most promising devices have a turn-on voltage V_TO of 0.7 V (1mA), and a typical reverse voltage V_R=-12 V (0.1 mA) and a best V_R of -12 V (10 mu A) for a 100 mu m diameter device with some evidence of avalanche breakdown in the structure. Abrupt doping junctions have been formed between GaSb and GaAs:Si substrates. The mismatch between the layers is ameliorated by using a low-temperature buffer layer to improve the interface. Avalanche breakdown starts at -4.5 V in these structures and reverse bias currents of <10 mu A at up to 2 V from unintentionally doped GaSb (uGaSb) with a carrier concentration of 10¹⁶ cm^-3 p. Schottky barriers are an alternative to p/n junctions, but they cannot be made at all on uGaSb because of the low barrier height, which is of the order of 0.2 eV. We have overcome this problem using a thin capping layer of highly dislocated GaAs. Surprisingly this has been successful for both regrowth on older structures and contiguous in situ growth.

The authors describe the properties of heterostructure InSb/In_1-xAl_xSb diodes grown by molecular beam epitaxy. The leakage current in a homostructure InSb p⁺ pi n⁺ diode at room temperature is dominated by an electron current which is generated in the p⁺ material. They show that a thin strained layer of In_1-xAl_xSb grown between the p⁺ and pi regions produces a barrier in the conduction band which reduces substantially the diffusion of electrons from the p⁺ layer to the pi region, and hence to the diode junction, leading to an improvement in performance. They present experimental results which show that the saturation leakage current reduces by over an order of magnitude as the aluminum composition in the barrier is increased, provided that it remains strained. The effect on device performance of exceeding the critical thickness for strain relief is also demonstrated. The experimental data are compared with analytic and numerical models, and factors which determine the optimum barrier composition, thickness and doping are discussed.

Investigations of carrier exclusion as a possible way to considerably disturb the electron-hole plasma in narrow-gap semiconductors have enabled one to reveal and to use for practical purposes such nontrivial effects as the negative differential conductivity, the suppression of Auger generation of carriers, the negative luminescence and radiative cooling. The physical nature of these effects and the principles of their use in new semiconductor devices are discussed.

A combination of surface-sensitive techniques and electron transport measurements have been used to characterize the effect of argon ion bombardment and annealing on a series of InSb(100) samples. Ex situ electrical conductivity and magnetoresistance measurements at 4.2 K, and in situ high-resolution electron energy loss spectroscopy (HREELS) carried out at 300 K, indicate that all the samples studied exhibit enhanced n-type behaviour after the surface cleaning procedure. This effect is most pronounced after annealing to between 450 and 500 K and arises from the formation of a high-density electron gas with a sheet carrier concentration of approximately (6.5-9.0)*10¹² cm^-2. The carrier concentration is significantly reduced on annealing to higher temperatures up to a maximum of 700 K. Electron-energy-dependent HREELS measurements of the plasmon energy and intensity, in conjunction with model calculations based on dielectric theory, indicate that the n-type layer is approximately 500 AA thick and located approximately 175 AA below a surface depletion layer. The occupancy of the electronic subbands has been obtained by Shubnikov-de Haas measurements and self-consistent calculations. These show that the positive charge which confines the electrons is spread over approximately 300 AA with a best fit being provided by a Gaussian-like potential profile. The calculations demonstrate that the corresponding wavefunction spread for the i=0 subband, which contains approximately 40% of the total carriers induced, has a spatial dimension of approximately 500 AA in good agreement with the HREELS results.

The author presents a detailed study of a wide variety of quantum well infrared photodetectors, which were chosen to have large differences in their optical and transport properties. Intersubband transitions based on photoexcitation from bound-to-bound, bound-to-quasicontinuum and bound-to-continuum quantum well states were investigated. The measurements and theoretical analysis included optical absorption, responsivity, dark current, current noise, optical gain, hot-carrier mean free path, net quantum efficiency, quantum well escape probability, quantum well escape time, as well as detectivity.

The use of the optical matrix method for computing the energy states and transmission probability in the bound-to-miniband (BTM) and step-bound-to-miniband (SBTM) transition III-V quantum well infrared photodetectors (QWIPs) is described. The effect of exchange energy due to electron-electron interaction was taken into account in the calculation of the Fermi level in the quantum well. Numerical simulations of the dark current as a function of temperature and bias voltage have been performed for both BTM and SBTM QWIPs. It is shown that dark current in these miniband transport QWIPs is dominated by the thermionic-assisted tunnelling conduction via the miniband for T>or=60 K, whereas resonant tunnelling conduction prevails for T<or=40 K. The results show that a significant reduction in dark current can be achieved for the BTM QWIP as compared with that of a conventional GaAs QWIP.

Responsivity and thermionic dark current are studied in asymmetric stepped GaAs/AlGaAs multiquantum well infrared detectors with a single bound state. Although attributed to different transport properties, both the photoexcited and the thermally emitted carriers exhibit asymmetric transport properties with respect to the voltage polarity. The experimental characteristics are explained by the minibands-quasi continuum energy structure above the barrier.

Quantum aspects of stimulated emission have been revealed in bulk narrow-gap semiconductors such as InSb and Hg_1-xCd_xTe with x=0.211, being excited at the quantum limit by the electric field E_y* induced by passing a current density J(//x) through the sample subjected to a transverse high magnetic field H(//z). The phenomenon has been demonstrated to be a useful tool for the spectroscopy of the band parameters, such as the energy gap epsilon _g, the electron effective mass m₁*, and the effective g-factor g₁, being respectively determined as 0.18 eV m₁*(0)=0.015m₀ and g(0)=-47 for InSb at room temperature, in addition to such direct applications as a tunable infrared laser. The critical current density J_c for lasing has been observed to be 16-40 A cm² for H=7 T in InSb at 80 K. The particularly low value of J_c can be explained from the extremely high value of the gain of the stimulated emission at the band edge at the quantum limit, which diverges to infinity in the high-field limit.

Narrow-gap semiconductors have been used for decades in the fabrication of magnetic field sensors, such as magnetoresistors and Hall sensors. Magnetic field sensors are, in turn, used in conjunction with permanent magnets to make contactless potentiometers and rotary encoders. This sensing technology offers the most reliable way to convert a mechanical movement into an electrical signal, and is widespread in automotive applications. Recent developments in the growth of thin epitaxial layers of InAs and InSb on semiinsulating GaAs or InP substrates have resulted in the development of magnetoresistors with excellent sensitivity and operating temperatures up to 285 degrees C. Magnetoresistors and Hall sensors require a very thin active semiconductor region, a high carrier density and a high room-temperature mobility. The best materials are narrow-gap III-V compounds. 2DEG layers in InSb and InAs would be ideally suited for these devices. The accumulation layer at the surface of InAs has been used to make magnetoresistors, Hall sensors and magnetotransistors. n-type doped thin InSb films are used to make magnetoresistors that outperform Si-based Hall sensors, even with integrated amplification. The authors describe device design criteria, materials requirements and a direct comparison of the three types of galvanomagnetic devices, magnetoresistors, Hall sensors and magnetotransistors, made from the same material. They compare the output of different magnetic field sensing technologies, such as Si and GaAs Hall sensors, and NiFe-based magnetoresistors, with InSb magnetoresistors.

Nb/n-type InAs/Nb superconducting devices with oxide-free interfaces are fabricated to study the interface effect on the superconducting characteristics of the device. The maximum supercurrent as well as the maximum supercurrent-normal resistance product is enhanced by RF sputter cleaning of the InAs surface. Using a device where Nb is deposited on InAs without breaking the vacuum, the carrier concentration dependence of the proximity-effect-induced pair potential is obtained. This dependence is consistent with the theory if some assumptions are made.

The basic properties of nipi structures are discussed in terms of their potential interest for photon detection. A comparison of different semiconductors (GaAs, InSb, InAs, PbTe) for the realization of either short-period nipis with strong overlap of the e-h wavefunctions or long-period nipis without overlap is presented. The advantages of long-period nipis are demonstrated with PbTe p-n-p and p-i-n-i-p structures. Barrier leakage is shown to be of substantial importance for gaining the detector response in selectively contacted structures which are based on narrow-gap semiconductors.

Mercury cadmium telluride (MCT) versus quantum well structures as infrared detection materials are discussed. While MCT has been developed as a superior infrared detector material for more than two decades, the rapid progress in the last few years in quantum well infrared photodetectors (QWIP) is very impressive. Competition and comparison between the two material systems are expected to last for a long period in view of the fundamental advantages and limitations of the materials.

Current-voltage characteristics (CVC) were measured in PbTe doped with 0.1-0.4 at.% of Ga at high electric fields E<or=3000 V cm^-1 and temperatures T=4.2-170 K. CVC of PbTe(Ga) differ drastically from CVC of pure PbTe: the pronounced linear region is observed only at T>or=70 K and for very weak electric fields E<or=1 V cm^-1; at higher fields CVC become superlinear and fulfil the Poole-Frenkel law. The final switching to a high conductance state takes place at E>or=500-1000 V cm^-1. In liquid helium the relaxation time of the conductance induced by a high electric field exceeds 10⁴ s. At higher temperatures the relaxation time drops drastically and becomes 10^-2-10^-3 s at 77 K. On switching, the increase in conductivity can exceed seven orders of magnitude and CVC can change and obtain long linear and sublinear regions characteristic of pure PbTe.