Table of contents

The major topics discussed at past conferences on narrow-gap semiconductors are briefly reviewed. After some remarks on the history of the discovery of the band structure of grey tin, the probable band structures of Sn-Ge alloys are discussed. This is followed by an account of MBE fabrication of films of alpha -Sn and alpha -Sn-Ge alloys which are stabilised by deposition on nearly lattice-matched InSb and CdTe, and of recent band structure calculations.

Several III-V quantum well and superlattice structures provide mechanisms for obtaining long-wavelength band-to-band absorption at low temperatures. These structures may provide useful alternatives to the II-VI HgCdTe alloy system for long-wavelength detector applications if they can be designed to provide adequate detector performance. The multi-quantum well and superlattice physics which influences detector performance is reviewed, along with the advantages and disadvantages of each approach. The present status of these III-V approaches for long-wavelength detectors is also reviewed, with particular emphasis on the experimental results for InAsSb strained-layer superlattice detectors and AlGaAs/GaAs intersub-band detectors.

Lasers made of lead salt compounds have proven to be best suited for coverage of the mid-infrared region from 3 to 30 mu m. Nowadays double-heterostructure (DH) lasers are the standard, using as active layers PbEuSSe for the short-wavelength region below 4 mu m, PbEuTeSe or PbEuSe for the 4-8 mu m range and PbSnTe or PbSnSe for wavelengths beyond 8 mu m. Originally liquid phase epitaxy but now molecular beam and hot wall epitaxy techniques are most commonly used to produce a wide variety of laser structures. While simple DH lasers with contact or mesa stripes for lateral confinement prevail in commercial devices, highly sophisticated prototypes with buried layer, corrugated distributed Bragg reflector (DBR) and distributed feedback (DFB) and single- and multi-quantum-well structures have been successfully manufactured. Gas spectroscopy has remained the main field of application where mode quality and tuning properties of the emitted radiation are most important. The dominant development goal has been the increase of the operating temperature to make the use of simple cooling equipment possible. Theoretical models for threshold current calculations have been established; however, the reliability of the results has been impaired by the fact that basic material properties of the ternary and quaternary compounds are not well know. In the course of this paper the state of the art and the development trends of lead salt lasers are described. The various fields of applications are summarised. Two examples-one dealing with car exhaust monitoring, the other with clinical isotopic ratio measurements of the exhaled breath-are discussed in some detail.

The authors describe the observation of infrared emission stemming from intersub-band transitions in quantum wells and superlattices. Two methods of electron excitation into higher energy levels are employed: in-plane carrier heating and vertical injection. For both methods, considerations for the design of efficient emitters are given.

The authors report on InAs_1-xSb_x/InSb (x=0.83-0.81) strained-layer superlattice infrared detectors grown by metalorganic chemical vapour deposition. The long-wavelength photoresponse of these detectors demonstrates potential far-infrared applications for III-V superlattices. Examples of both photovoltaic and photoconductive detectors are described.

Semiconductor single-quantum-well double-heterostructure lasers with a Pb_0.95Sn_0.05Se active region from L_z=400 to 2000 AA were fabricated by hot wall molecular epitaxy. For L_z<or=500 AA two different laser emission lines were observed at energies larger than the energy gap of Pb_0.95Sn_0.05Se. These lines correspond to transitions between localised states in the quantum well with n=1 and n=2, with the selection rule Delta n=0. The anomalous temperature dependence of the threshold current of the two lines reflects the step form of the density of states for the quasi-2D system of the QW active region. For L_z>1000 AA the laser emission corresponds to the energy gap of the material in the active region. The lowest threshold current (230 A cm^-3 at 77 K) and the highest operating temperature (218 K) were obtained for the laser with L_z=2000 AA.

Research work is reviewed which uses the phenomena of minority carrier extraction and minority carrier exclusion to reduce the carrier densities in narrow-gap Hg_1-xCd_xTe alloys to near-extrinsic values at temperatures where the materials are normally intrinsic. This is of particular interest for the suppression of Auger noise which otherwise severely limits the performance achievable from infrared detectors at near-ambient operating temperatures. Improved performance has been obtained from excluding photoconductive detectors at modulation frequencies above 10 kHz. Theoretical work predicts improved performance from heterojunction extracting diodes. Early experiments on 'proximity-extracting' diode structures show large reductions in leakage current at intermediate and ambient temperatures compared to non-extracted diodes, but the residual current is higher than that predicted from Shockley-Read generation. Bipolar transistor action is observed at temperatures where the material is near-intrinsic in equilibrium. The transistor characteristics are strongly influenced by electron impact ionisation.

Auger suppression has been demonstrated as a means of reducing the noise in narrow-gap semiconductor photon detectors at near-ambient temperatures. To achieve this low-noise state, residual Shockley-Read (SR) generation processes must be negligible. Calculations have been made for a near-ambient-temperature n- pi -p CMT photodiode operated in reverse bias. At low bias this is a leaky, low-resistance device, but at high bias it exhibits Auger suppression and negative resistance. The authors determine the effect of residual SR centres, taking into account partial trap occupancy, degeneracy, generation-recombination mechanisms, compensation, large deviations from equilibrium and graded interfaces.

The contribution of the graded region of implanted p-n junctions is analysed using an exponential profile. Though previously neglected, the authors have recently shown that this contribution to the saturation current of HgCdTe diodes is significant. Assuming a dominant Auger recombination, an analytical solution to the continuity equation is obtained. An expression for the current generated by the graded region is presented for both ohmic and reflecting boundary conditions. A revised condition for a 'wide' region is derived. When the region is 'narrow', the current differs drastically from that of the zero-gradient case. The effects of the junction depth and the substrate and surface concentrations on the current are investigated. It is shown that the reverse current does not saturate.

Low-leakage high-performance photovoltaic detectors were fabricated from long-wavelength infrared (LWIR) HgCdTe epitaxial material grown by metal-organic chemical vapour deposition (MOCVD) on GaAs substrates. Layers were grown by two different MOCVD techniques, a conventional alloy growth and an interdiffused multiple-layer growth. MOCVD HgCdTe layers were characterised by background electron concentrations of (1-3)*10¹⁵ cm^-3 with electron mobilities up to 200000 cm² V^-1 s^-1 at 77 K. Surface and cross-section dislocation densities were 10⁶-10⁷ cm^-2 and occasionally (4-5)*10⁵ cm^-2. GaAs substrates were (100) and (111B) misoriented towards 110. The layer structure in which the devices were fabricated consisted of an absorbing LWIR HgCdTe layer grown on a buffer of CdTe. A thin layer (1-2 mu m) of wide-band-gap HgCdTe was grown last. Devices were fabricated in a single as well as double layers using a mesa geometry with a native oxide passivation. Junctions were formed at 2-3 mu m depth by a low-energy arsenic implant ( approximately 100 keV) which behaved as a finite diffusion source during the post-implant anneal. The best measured zero-bias resistance-area products (R₀A) at 77 K were 1.67 Omega cm² for the alloy layers for a cut-off wavelength of lambda _c=14.94 mu m and 7.22 Omega cm² for the interdiffused multiple layers for lambda _c=15.92 mu m at 77 K. The results indicated a definite device performance improvement in double-layer compared to single-layer structures. Analysis of the transport properties for a device fabricated in a single layer with a cut-off wavelength of lambda _c=8.35 mu m at 77 K and 9.1 mu m at 40 K suggested that the deviation from thermal processes occurs at temperatures of approximately 40 K.

Epitaxial IV-VI lead chalcogenide layers are grown on Si substrates with the aid of MBE-deposited stacked BaF₂-CaF₂ buffer films. The epitaxial buffer helps to overcome the large lattice and thermal expansion mismatch between IV-VIs and Si. Linear arrays of photovoltaic IR sensors fabricated in these layers cover cut-off wavelengths ranging from 3 mu m (PbS and Pb_1-xEu_xSe). The temperature coefficient of the band gap (cut-off wavelengths of the sensors) of PbSnSe-on-Si is smaller than in the bulk. This is explained by residual mechanical strain in the layers at cryogenic temperatures, while most of the strain due to the thermal expansion mismatch is relaxed at room temperature.

Detailed theoretical and experimental investigations of mercury-based long-wavelength narrow-band-gap semiconductor photoconductors operating in the 200-300 K temperature range are reported. A generalized doping-dependent figure of merit of the semiconductor, which determines the ultimate performance of the high-temperature photoconductor, is proposed. This figure of merit has been calculated for various temperatures as a function of composition and doping. High-temperature long-wavelength photoconductors from both HgCdTe epilayers and bulk HgZnTe crystals have been fabricated and characterised. It is shown that optimised uncooled 10.6 mu m photoconductors can achieve a detectivity higher than 10⁸ cm Hz^1/2 W^-1, approaching the performance of slow thermal detectors.

The theory of free-carrier-induced optical non-linearity is reviewed. Examples of such mechanisms in narrow gap semiconductors are discussed. HgTe, HgMnTe and zero-gap HgCdTe are shown to have record, picosecond speed, optical non-linearities at 10.6 mu m. The largest occurs in Hg_0.84Cd_0.16Te, whose chi ⁽³⁾=2*10^-3 ESU at 80 K. The theoretical model suggests that these non-linearities are caused by laser-induced carrier temperature modulation, which produces large carrier density variations in zero-gap materials. The thermal processes have saturation power densities in the 100 kW cm^-2-1 MW cm^-2 range. At such intensities, the dielectric constant of HgTe is modulated by about 10%.

The authors have measured and analyzed the Raman-scattering and far-infrared spectra from unoriented n- and p-type Hg_1-xCd_xTe with x approximately=0.21. Measurements were made in the 80-180 cm^-1 frequency region at temperatures of 10 and 80 K. Comparison of the n- and p-type spectra leads to the conclusion that the electronic contribution to the Raman spectra is small. In particular, the data show that the 135 cm^-1 feature originates in lattice effects and may be related to the Te-3Hg-Cd combination as previously suggested. The weak but distinct fine structure present in the CdTe-like reststrahlen band at approximately 153 cm^-1 is tentatively assigned to be a result of localised oscillations of Cd ions in the alloy lattice. A study of the spectral changes between 10 and 80 K clearly indicates a softening of the HgTe-like TO phonon.

Information on both the alloy composition and the lattice properties of Hg_1-xCd_xTe for x-values between 0.20 and 0.31 have been obtained by resonance Raman spectroscopy with laser photon energies between 2.35 and 2.7 eV. In addition to the HgTe-like TO and LO modes and the CdTe-like LO mode, the authors have studied the resonance behaviour of the mode at 133 cm^-1, which has been identified as originating from the preferential clustering of 3Hg and 1Cd about the Te. They find that the intensity of this peak for various bulk and epitaxially grown samples is unusually large only near the E₁ resonance. Pulsed laser annealing with an Nd:YAG-pumped dye laser strongly suppresses this mode in all samples, suggesting that extremely rapid epitaxial regrowth may inhibit the 3:1 cluster formation. Detailed comparisons are presented for Hg_1-xCd_xTe films grown by liquid phase epitaxy (LPE), molecular beam epitaxy (MBE) and metal-organic chemical vapour deposition (MOCVD) procedures.

The authors have studied resonance Raman scattering in two samples of (110) Hg_1-xCd_xTe (nominal compositions x=0.2 and 0.3) in the region of the E₁ optical feature. The maximum enhancement for the 'clustering', 'HgTe-like' LO and 'CdTe-like' LO and TO modes occurs at the energy of the E₁ electronic transition of Hg_1-xCd_xTe for their samples. The 'CdTe-like' LO and TO mode is thus identified as a propagating phonon. This is the marked contrast to a similar mode in InAs_0.8P_0.2 which is localised at the P sites. Although the resonant behaviour of the 'clustering' mode suggests that it is an alloy mode, their experiment does not provide the basis for a conclusive assignment. The polarisation behaviour of the symmetry-forbidden 'HgTe-like' LO mode as well as the nature of its resonant enhancement indicate that the mechanism of the forbidden LO scattering is either the intraband Frolich interaction or impurity-induced scattering, not morphic effects as proposed in prior works.

Self-consistent tight-binding supercell calculations of localised defect levels and nearest-neighbour relaxations are presented for Cd, Hg and In interstitials at tetrahedral and hexagonal sites in CdTe and HgTe. The mechanisms driving the relaxation and the resulting character of the lattice distortion about various interstitials are insensitive to the alloy composition and should remain valid for interstitials in Hg-rich HgCdTe.

Vacancies are known to play an important role in the Hg-based narrow-gap alloys HgCdTe and HgZnTe. In this paper the authors summarise their recent calculations of the vacancy formation energies in HgCdTe and HgZnTe. They find that the vacancy formation energy in these alloys varies non-linearly with alloy concentration, resulting in higher vacancy densities than those predicted on the basis of a linear variation of the vacancy energies. Surface segregation in these alloys is driven by bond strength and bond length differences. They review their recent calculations which show that the chemical terms dominate in HgCdTe and result in Hg-rich surfaces, while in HgZnTe the strain terms contribute as well and result in a less Hg-rich surface, suggesting that HgZnTe surfaces may be more amenable to surface processes such as passivation.

The present understanding of band edge and free carrier properties in Hg-based superlattices such as HgTe-CdTe is reviewed. The authors emphasise the relation between distinctive aspects of the superlattice band structures obtained theoretically and corresponding features in the magneto-transport, magneto-optical and optical data. Theory predicts a high sensitivity of the free carrier properties to the magnitude of the valence band offset. One finds that nearly all of the main experimental results (excepting low-temperature negative differential resistance) are qualitatively consistent with a large offset, but are difficult to explain if the offset is small. The valence band offset controversy thus appears to have been largely resolved.

The band structures of HgTe, CdTe, ZnTe, the alloys Hg_1-xCd_xTe (HCT) and Hg_1-xZn_xTe (HZT) and several small-gap superlattices (SL) are calculated using a tight-binding model. The authors calculations show a nearly linear dependence of the energy gap on the concentration in HCT, but a strong non-linear variation in HZT. The electron mass as a function of the band gap is found to be the same in HCT and HZT in the small-gap region. Their calculated SL bands, in fair agreement with the most recent experiments and theories, support the assumption of a large valence band offset (350 meV) between HgTe and CdTe.

Far-infrared magneto-transmission and magneto-transport measurements were performed on two HgTe/Hg_0.15Cd_0.85Te superlattices over a wide temperature region. Cyclotron resonances of holes and electrons associated with the HH₁ band and of thermally excited electrons in the E₁ band are identified. The electron effective masses obtained from the cyclotron resonance data display a strong temperature dependence. An attempt is made to correlate the observed mass behaviour with the temperature-induced transition from the zero-gap to the open-gap regime of the superlattices.

Far-infrared (10-250 cm^-1) reflectivity measurements have been made on three MBE-grown HgTe-CdTe superlattices. Analysis of plasmon effects with a superlattice dielectric function gives the effective mass parallel to the superlattice planes. The low-temperature masses are near those of the Hg_1-xCd_xTe alloy with the same energy gap, suggesting that the superlattice conduction band is like that of the alloy. Comparison of the effective mass values with those calculated from the multiband tight-binding model gives a valence band offset near 350 meV. The infrared data also give accurate values of the layer thickness and composition.

It has previously been shown that the surface layer capacitance of a narrow-gap semiconductor is significantly affected by quantisation of the surface carrier states. The sub-band energies, the spatial distribution of the sub-band charges and their density of states can be obtained by fitting the measured capacitance-voltage relation to a model description of the sub-band structure. Here the authors employ capacitance to determine E₀, the ground state sub-band energy in the inversion layer potential at the threshold voltage. E₀ is studied as a function of the doping N_A-N_D and the pressure-dependent gap E_g. They show that resonant defect level energies can be obtained from the capacitance when E₀ is large enough.

Recent work has shown that the electronic sub-bands of HgCdTe in an inversion layer potential are split for k/sub /// not=0 by spin-orbit coupling into a pair of distinct bands E₀^+or-(k/sub ///). Parameters related to the splitting were evaluated in the approximative terms of the Bychkov-Rashba model. The authors calculate the splitting with reference to a microscopic model to uncover the explicit dependence on k/sub /// and N_s. They show that the relevant quantity for the coupling is not the mean electric field averaged over the interface potential, (E_z), but the expectation value of the product of E_z and a material-specific coupling parameter alpha ₄₆ which is z-dependent at the surface.

The authors report the observation of an incoherent tunnelling phenomenon in Al/ZnS/p-Hg_1-xCd_xTe metal-insulator-semiconductor (MIS) capacitors in the temperature range 15<or=T<or=110 K. Under thermal equilibrium, both capacitance-voltage and conductance-voltage characteristics show strong oscillations when the MIS capacitor is biased into strong inversion. The data can be understood as being due to electron tunnelling between the three-dimensional (3D) valence band of p-type Hg_1-xCd_xTe and the 2D inversion layers at the interface of Hg_1-xCd_xTe and ZnS. This process, which is inevitably momentum-non-conserving in a non-degenerate semiconductor, is made possible via intermediate gap states. The temperature dependence of the oscillation amplitude can be well explained by the positive temperature coefficient of the band gap and the carrier freeze-out effect.

The authors review the experimental investigations carried out on IV-VI artificially structured materials are summarised. The problems related to the determination of band offsets are discussed and special emphasis is given to strain effects and the consequences of interdiffusion. Deviations from the bulk magnetic behaviour reported in dilute magnetic IV-VI quantum well structures are presented. The unique structural properties of layered PbGeTe/PbTe which exhibit a stabilisation of the cubic phase are discussed.

It is shown that for the diluted magnetic semiconductors Sn_1-xMn_xTe a critical carrier density (p_c approximately=3*10²⁰ cm^-3) can be found above which ferromagnetic behaviour is displayed. This behaviour can be explained with a two-band RKKY model. Preliminary experiments on the nature of the low-temperature phase are presented.

The magnetisation of IV-VI diluted magnetic semiconductor systems has been measured at 4.2 K using the sample extraction method in steady magnetic fields up to 30 T. Measurements at lower temperatures have been made using pulsed magnetic fields. The results have been fitted to an expression containing a Brillouin function, representing isolated magnetic ions, plus a term representing pair interactions. The fitting parameters are: x₁, the effective occupation probability of a cation site by an Mn²⁺ ion; T₀, representing the exchange interaction between Mn²⁺ ions; x₂, representing the number of Mn²⁺ ions in pairs; and J_p, a pair exchange parameter. Reasonable agreement of these high-field parameters with parameters obtained from the low-field susceptibility was found. Fits including the nearest-neighbour pair exchange function confirm the dominance of nearest-neighbour pairs in the antiferromagnetic exchange interaction.

Low-temperature measurements of the magnetic susceptibility, plasma reflectivity and Hall effect as a function of carrier-concentration were performed on Pb_1-x-ySn_xMn_yTe samples with x=0.72 and y=0.03, 0.06, 0.08. The dependences of the paramagnetic Curie temperature and effective mass of holes on manganese content and carrier concentration were determined. The analysis of the experimental data has been performed assuming that two valence bands are relevant in the case of PbSnMnTe: the non-parabolic light-hole L band and the parabolic heavy-hole Sigma band. Theoretical calculations of the paramagnetic Curie temperature dependences on manganese content and carrier concentration were also performed on the basis of the band structure model mentioned above and assuming the RKKY exchange interaction between Mn spins. It was found that the compositional evolution of the band structure determined the carrier concentration dependence of the magnetic properties of PbSnMnTe.

Among various non-linear Raman techniques coherent anti-Stokes Raman spectroscopy (CARS) has become most popular because of its strong, easily detectable coherent signal. For the spectroscopy of narrow-gap semiconductors which are only transparent for infrared light this advantage is of special importance. Using Maxwell's theory the process can be described by a third-order non-linear susceptibility chi ⁽³⁾. Its strongest Raman-like resonances in a semiconductor exposed to a magnetic field are the spin-flip resonances from which the effective g factors of electrons and holes can be deduced. The interference of resonant and non-resonant contributions to the susceptibility in some cases causes complicated line shapes which have to be analysed carefully to find the correct resonance positions. A short review of the theoretical description of the process and the experimental apparatus suitable for the investigation of narrow-gap semiconductors in a magnetic field is given. Then the analysis of the line shapes is described. Results are discussed for several III-V compounds, II-VI narrow-gap materials and IV-VI epitaxial films.

The authors performed coherent Raman spectroscopic investigations of spin-flip transitions in Pb_1-xMn_xTe and Pb_1-xEu_xSe in magnetic fields down to 500 G. In order to find a consistent set of band parameters for the Mitchell-Wallis k.p model, magneto-optical interband absorption measurements were also performed. In the Raman spectrum of Pb_1-xEu_xSe with x>1% a completely new resonance with high oscillator strength appears. Although the levels involved have not been identified unambiguously up to now, the experimental findings indicate the existence of a magnetic phase transition in Pb_1-xEu_xSe, which they observed by optical methods.

Anomalous temperature dependence has been observed in the magneto-transport of PbTe thin films on BaF₂. A factor-of-10 increase in the longitudinal magnetoresistance rho _zz(B) at high fields is seen for decreasing temperature in the range 12<T<0.3 K, while the Hall coefficient changes by less than 20%. These observations suggest that the localisation of carriers on impurities or magnetic freeze-out is not occurring, contrary to the case of HgCdTe. Far-infrared transmission using lambda _L=70.6 and 118.8 mu m in magnetic fields up to 13 T exhibits temperature dependence in cyclotron resonance, dielectric anomaly and spin resonance transitions. The temperature dependence in n-type PbTe is due primarily to the transfer of carriers from the lowest-energy strain-split (111) valley to the higher-energy (111) valleys as the temperature is increased. The transfer of carriers is not the origin of the anomalous magneto-transport, however, since below T approximately=2 K no transfer of carriers occurs, while the transport temperature dependence continues for another order of magnitude in temperature.

The authors have measured the wavelength dependence of persistent photoconductivity in films of indium-doped Pb_0.75Sn_0.25Te using a cryogenically cooled spectrometer to avoid saturation of the photoresponse from the room-temperature thermal background. Using only the 300 K blackbody radiation incident on the entrance slit of the spectrometer, they observed a 2% drop in the sample resistivity when the grating was rotated to pass a 0.1 mu m band width centred at lambda =14 mu m. At longer wavelengths the resistivity drop increased, reaching 30% at 50 mu m. They interpret these results in terms of a configuration coordinate model of the lattice relaxation around the indium impurity site. They estimate the quantum efficiency of this material to be about 0.3% before the photoresponse saturates. The large photoconductive gain ( approximately 10⁷) which results from the long persistence may be useful in detectors for some slow imaging applications.

The author reviews far-infrared studies of spin resonance (SR) in narrow-gap semiconductors, with emphasis on SR of the conduction band. While the SR transition is normally electric-dipole-forbidden, it is well known that there exist mechanisms (e.g. 'non-parabolicity' or inversion asymmetry) which relax these selection rules. The author discusses these mechanisms and their relative importance. Although the author concentrates in this review on the Gamma ₆-like conduction band of InSb, the author also reviews experimental results on other materials (e.g. HgSe, HgTe, Hg_1-xMn_xSe). The author discusses the anisotropy of the spin resonance and compares the g factors of free and donor-bound electrons. Furthermore, the author describes certain 'external' effects which influence the observation of SR in semiconductors, e.g. spin-spin exchange interaction in diluted magnetic semiconductors, plasma-shifted SR and uniaxial stress. Special attention is given to the interference of the electric dipole and magnetic dipole matrix elements, which provides a unique opportunity for the determination of the inversion asymmetry parameter (including its sign) in zincblende narrow-gap semiconductors.

The authors have observed and described new optical transitions between magneto-donor states in InSb assisted by optic phonon emission. The phonon-assisted transitions provide a unique opportunity to investigate high excited states of the magneto-Coulomb system. High-resolution data reveal the presence of excited magneto-donor states belonging to the same Landau sub-band.

The authors detect the Overhauser shift of the conduction electron spin resonance (CESR) in n-type InSb via coherent spin-flip Raman scattering. In thermal equilibrium this shift is proportional to the probability mod psi (r_k) mod ² of the conduction electron at the nuclear position r_k. By partially saturating the CESR, the nuclear spin polarisation is dynamically increased (DNP) and the Overhauser shift increases in proportion. The time dependence of the Overhauser shift yields directly the nuclear spin-lattice relaxation times for both In and Sb. The optical method results in very high CESR sensitivity. Narrow-gap semiconductors not accessible by classical double-resonance techniques can be investigated.

The transient infrared absorption of hot electrons in n-doped InAs is studied in the wavelength range around 7 mu m. After heating the electron gas by a picosecond infrared pulse, the free carrier absorption shows a fast rise by several tens of per cent which relaxes within 100 ps. Excess LO phonons created by the cooling process are essential for the observed change of the free carrier absorption. The studies presented give valuable information on the distribution function of hot phonons.

Conduction electrons in GaAs are described theoretically using a five-level k.p model and including effects of the electron-optic phonon interaction. The dispersion relation E(k) in the band and in the forbidden gap, the energy dependence of the effective mass and the spin g-value, as well as the resulting peculiarities of the cyclotron resonance are calculated and compared with experimental data of various authors. It is shown that the electrons in GaAs exhibit typical narrow-gap properties.

The authors report on the study of quantum wires and dots near the surface of InSb that contain quasi-one-dimensional and zero-dimensional, i.e. discrete, electron systems. Both types of samples are derived from metal-oxide-semiconductor devices and make it possible to tune the electron number by a gate voltage. The laterally confining potential leads to the formation of one-dimensional sub-bands and to discrete levels respectively. Both are examined by far-infrared resonance spectroscopy. In order to attain a sufficiently high absorptance they fabricated the wires and dots by holographic lithography in arrays on macroscopic areas. Resonance energies of about 10 meV due to the lateral quantisation are found to be comparatively large because of the low conduction band mass of InSb. In particular, the influence of the narrow-gap band structure of InSb and of the electron-electron interaction on the quantised levels is discussed.

Electronic properties of spike-doped quantum wells in InSb are calculated and the effects of a uniform background p-doping are studied. The calculation is self-consistent and includes non-parabolic corrections. In this many-sub-band system, electrons in high- and low-energy sub-bands are found to respond differently to the overall transfer of charge from the well to acceptor states.

The light holes in InSb are characterised using a Shubnikov-de Haas technique by exploiting the type II band offset of an InAs_0.15Sb_0.85/InSb strained-layer superlattice. The data are analysed to determine the effective masses and g factors as a function of carrier concentration.

Recent experiments show that narrow-gap semiconductors have significant advantages for potential device applications of electron tunnelling. As a result of the small electron effective mass in InAs, the authors can observe resonant tunnelling through a 24 nm quantum well, the longest coherence distance reported for double-barrier heterostructures. Another novel finding is that single-barrier tunnelling devices of InAs/AlGaSb/InAs exhibit negative differential conductance at room temperature owing to the favourable band alignment. The high electron mobility in InAs will lead to lower parasitic device resistances and improved high-frequency performance compared to devices based on GaAs. To compare the experimental results with theory, the two-band model of a narrow-gap semiconductor has been incorporated in the standard calculation of tunnelling current-voltage characteristics.

Hall effect and electrical resistivity measurements were carried out on InAs epilayers grown by molecular beam epitaxy (MBE) from a dimeric As₂ source. As evidenced from the measurements, the electrical transport coefficients depend on the epilayer thickness, flux ratio and substrate temperature. In particular, a strained-layer superlattice (SLS) as the InAs/substrate interface was found to be important in reducing the defect density. For layers grown at optimum conditions, the carrier density decreases with thickness d while the Hall mobility reaches a certain saturation ( approximately 5*10⁴ cm² V^-1 s^-1) for d>3.0 mu m. The mobility peaks at around liquid nitrogen temperature for thick samples and at higher temperatures for thin layers, which the author explains in terms of impurity scattering. In addition to ionised impurity and optical phonon scattering, possible mechanisms limiting mobility are discussed in these compensated layers. No carrier freeze-out was observed; instead, an apparent increase in carrier density due to impurity conduction was found.

Heterojunctions between semiconductors with mutually inverted bands contain massless spin-non-degenerate interface electron states independent of their profile. The universality of these states is due to the specific symmetry (the supersymmetry) of the effective Hamiltonian. The giant Landau splitting of interface states and selection rules for optical transitions in a magnetic field are predicted. Other types of 'supersymmetric' heterostructures-ferroelectric and antiferromagnetic domain walls-also possess interface states with unique properties.

Infrared photoluminescence has been studied in novel multilayer structures of the binaries PbS and PbSe grown on (111)-oriented BaF₂ substrates by hot wall epitaxy. The photoluminescence spectra are consistent with radiative recombination of two-dimensional electrons and holes confined in the PbSe wells. In contrast to the PbTe-based multiquantum well structures on (111)BaF₂ substrates, and as a consequence of the smaller ellipsoidal mass anisotropy of PbS and PbSe, transitions between the lowest electron and hole sub-bands of the (111) transverse valley and also of the (111) oblique valleys are observed at low temperatures. While the photoluminescence spectra of the PbS-PbSe structures investigated are consistent with type I band alignment, possible elastic strains from lattice mismatch between the constituents are found to have a potentially significant effect on the band offsets of this heterostructure.

The electrical behaviour of phosphorus in bulk Hg_0.8Cd_0.2Te crystals is compared with that in Hg_0.7Cd_0.3Te epitaxial films grown from Te-rich solutions. The mass action constants associated with the process of site transfer of phosphorus atoms from Hg to Te or interstitial lattice sites have been deduced for the bulk crystals and the epitaxial films. Additionally, the mass action constants for the lattice site transfer of As and Sb have also been deduced. The values of these constants indicate that the lack of site transfer from Hg to Te or interstitial lattice sites in the case of Sb and Bi, inferred experimentally in epitaxial films, is ascribed to a lower diffusivity of the Sb and Bi atoms in (Hg, Cd)Te. The implications of the amphoteric behaviour of the group V dopants on device fabrication are also discussed.

The authors have performed transport experiments under hydrostatic pressure on samples of two different narrow-gap mercury telluride mixed compounds: Hg_1-xCd_xTe (x=0.27, 0.28, 0.30) and Hg_1-xZn_xTe (x=0.15). The investigated samples of HgCdTe are n-type samples grown by the molecular beam epitaxy technique on GaAs/CdTe substrates and doped with In during the growth. The experiments, performed at liquid helium temperature in the magnetic field range B=0-5 T and under hydrostatic pressure up to P=11 kbar, allow them to conclude that this doping process does not induce any resonant impurity level as previously observed for boron-implanted samples. They used the same experimental techniques (0<B<19 T, 0<P<9 kbar, 1.68<T<300 K) to study HgZnTe samples grown by the travelling heating method. This open-gap material (x=0.15) is a high-mobility semiconductor (n=6.8*10¹⁴ cm^-3 and mu =1.8*10⁶ cm² V^-1 s^-1). Nevertheless, its pressure behaviour in zero magnetic field and in the high-magnetic-field regime shows the existence of two impurity levels.

The authors compare the material properties of conventional- and interdiffused-grown layers of Hg_1-xCd_xTe/GaAs grown by metalorganic chemical vapour deposition (MOCVD). These results are also compared with those of state-of-the-art LPE HgCdTe/CdZnTe grown from Te-rich solutions. Data are presented on surface morphology, compositional uniformity, double-crystal X-ray diffraction, chemical defect etching, Hall effect, minority carrier lifetime and laser-beam-induced current (LBIC) characterisation on (111)B, (221) and (100) orientations. The compositional uniformity is 3.3% for 3 in diameter HgCdTe/GaAs. The interdiffused growth method results in better compositional uniformity than the conventional method, while the structural quality of layers grown by both techniques is similar. Etch pit densities of (1-2)*10⁶ cm^-2 are routinely obtained for (111)B and (221) orientations, while values as low as 5*10⁵ cm^-2 have been occasionally observed. Initial data are also presented for HgCdTe layers grown on (100)GaAs/Si substrates.

InSb films having 77 K mobilities above 100000 cm² V^-1 s^-1, 77 K carrier concentrations less than 5*10¹⁵ cm^-3 and X-ray rocking curve widths of 100 arcsec have been grown on GaAs using molecular beam epitaxy (MBE). A thin (300 AA) InSb layer was grown at 300 degrees C using an atomic layer epitaxy (ALE) technique prior to the main InSb growth to reduce defects. Transmission electron microscopy, Nomarski microscopy, X-ray rocking curve widths and channelling Rutherford-backscattering measurements all confirmed the improved quality of these films. The influence of other growth parameters such as substrate temperature, ALE layer thickness and Sb over-pressure on the film properties is discussed. Electron cyclotron resonance (ECR) has been used to characterise the InSb films. Collectively, the results show that the best InSb epilayers have properties approaching bulk InSb.

Superconducting infrared detectors can be classified as thermal or photon, depending upon the detection mechanism. Superconducting transition edge microbolometers, which are thermal detectors, exploit the steep change in resistance with temperature at the transition temperature. Superconducting photon detectors include those based upon the non-equilibrium (Testardi) effect and those based upon photon-assisted tunnelling. Performance calculations for all three effects based upon the use of YBaCuO suggest high responsivity and detectivity; measurements are either lacking or else do not support the predictions. The principal application for the YBaCuO superconducting microbolometer is as a staring array in a thermal imaging (FLIR) system. The principal application for the non-equilibrium and photon-assisted tunnelling detectors is for very-long-wavelength detection of cold bodies in space.

The electronic structure and interface properties of epitaxial alpha -Sn and alpha -Sn_1-xGe_x alloy films grown by molecular beam epitaxy on GaSb, CdTe, InSb and Ge substrates were studied by synchrotron radiation photoemission spectroscopy and X-ray diffraction. Deposition at temperatures below approximately 150 degrees C results in amorphous alloy films. At higher substrate temperatures GaSb and CdTe partially react with the overlayer to form precipitates containing either GeSb or SnTe in the interface region. Homogeneous single-crystal alloy films of 200-300 AA thickness were grown on (100)Ge at approximately 400 degrees C. Angle-resolved valence band photoemission spectra show the opening of the band gap by alloying Ge to alpha -Sn. For an alloy with x=0.52 the Gamma ₈ point is shifted by approximately 0.16 eV below E_F.

The authors report molecular beam epitaxy growth of grey tin films on (001)CdTe substrates. The transverse magnetoresistance and Hall effect have been studied. Films thinner than 400 AA are p-type with carrier densities of (1-2)*10¹⁹ cm^-3; thicker films are n-type with carrier densities as low as 2*10¹⁷ cm^-3 Shubnikov-de Haas (SDH) oscillations are observed in samples having Hall mobilities >or=10⁴ cm² V^-1 s^-1 at low temperatures. Beat patterns are observed in the SDH spectra, which they ascribe to either inhomogeneous doping, arising from the diffusion of Cd and Te from the substrate, or quantisation of the motion in the direction parallel to the film normal. For field directions at 45 degrees and 90 degrees to the film plane normal, clear SDH oscillations are also observed at fields much lower than the expected size effect cut-off field, implying that specular reflection at the film boundaries plays a significant role.

Far-infrared cyclotron resonance and plasma-shifted cyclotron resonance were observed in thin layers of alpha -Sn grown by molecular beam epitaxy on CdTe substrates, and were used to determine the effective mass m* of electrons at the Gamma point and to estimate the dielectric constant epsilon . In these experiments the plasma-shifted cyclotron resonance was observed for the first time at frequencies below the plasma edge, omega < omega _p. The data give m*=0.0333+or-0.0002, the matrix element E_p=24.0+or-0.5 and an estimate of epsilon approximately=23 for a sample with an electron concentration n approximately=3*10¹⁷ cm^-3, in good agreement with earlier results obtained on bulk samples.

The authors report the first experimental and theoretical study of nonlinear optical mechanisms in alpha -Sn and alpha -Sn_1-xGe_x grown by MBE. Non-degenerate four-wave mixing has been employed to measured third-order nonlinear susceptibilities at 10.6 mu m as a function of temperature, laser intensity and difference frequency. There is good agreement between experiment and theoretical calculations based on free carrier contributions to the susceptibility. Comparison is made with optical nonlinearities in Hg-based materials grown by MBE.

Describes growth of the first thin ( approximately 1 mu m) epitaxial films of pure bismuth-antimony alloys using molecular beam epitaxy techniques. These structures were grown at elevated temperatures on single-crystal barium fluoride substrates of (111) orientation. Electron microscope observations show the films to be featureless and defect-free on the scale of 0.1 mu m. The films grow with their trigonal axis parallel to the (111) axis of the substrate, and Laue-backscattering pictures show that they are epitaxial. Mobilities of alloys with x=0 are of the order of 2 m² V^-1 s^-1 at room temperature and increase to over 10 at 20 K and 100 at liquid helium temperatures. These values are far superior to those of other bismuth films grown to data, and approach mobilities observed in single-crystal bismuth. The dependence of energy gap and c axis lattice constant on x is different from that in bulk alloys, which may be due to the effects of strain arising from the 3.6% lattice mismatch between sample and substrate.

Substitutional iron in HgSe forms a resonant donor state whose energy is superimposed on the conduction band continuum. Above a certain doping level the system of Fe donors is only partially occupied by electrons, i.e. two charge states, Fe³⁺ and Fe²⁺, coexist, corresponding to the inhomogeneous mixed valence regime. Under these conditions the Coulomb repulsion between the donor electrons tends to keep them apart, leading to a correlation of their positions. The existence of the correlation results in a dramatic reduction of the rate of scattering by ionised impurity potentials. The same Coulomb interactions are responsible for the formation of the Coulomb gap in the one-particle density of impurity states, which in turn suppresses (otherwise very efficient) resonant scattering. As a result, the mobility and the Dingle temperature in HgSe:Fe can exceed those in HgSe doped with Ga to similar doping levels. A review is given of relevant experimental results obtained in HgSe:Fe, Hg_1-xMn_xSe:Fe and HgSe_1-xTe_x:Fe by means of the Shubnikov-de Haas effect, resistivity and Hall voltage studies. Experimental findings are compared with results of numerical simulations and with a simple model calculation which accounts for short-range correlations.

The temperature and concentration dependence of the EPR absorption due to the Fe³⁺ state in HgSe is shown to be consistent with a Coulomb gap in the defect density of states. The latter is a consequence of inter-ion Coulomb interaction which renders a correlated ground state with short-range order of the partially ionised resonant donors. Possible mechanisms for a decrease of both the mobility and the EPR intensity for very high iron concentrations in this ordered, glass-like state are discussed.

The specific heat of II-IV group Fe-based DMS has been investigated at temperatures below 20 K. All zero- and open-gap compounds have a magnetically inactive singlet ground state, well separated from the excited states. Within a crystal field model one is in principle able to understand this behaviour in all cases. The Fe²⁺-Fe²⁺ interaction and the role of the Fe levels in the band structure will be discussed.

The authors analyse the high-field magneto-transport data on the semimagnetic semiconductor Hg_1-xMn_xSe:Fe using DC, non-destructive pulsed and destructive pulsed magnetic fields up to 100 T. The concept of the 'localised zero-scattering potential' is introduced as an alternative model to the 'space charge superlattice'.

Hg_1-x-yCd_xMn_yTe crystals have been successfully grown on CdTe substrates for the first time by the LPE method from Hg-rich solutions (x<or=0.4, y<or=0.1). LPE layers exhibit high electron Hall mobilities and excellent compositional uniformity both in the plane and along the growth direction. Such a fact has not been reported for Hg_1-xCd_xTe. Using the high-quality crystals grown by this method, a new type of magnetophonon effect associated with a recombination process with the emission of two TA phonons was studied in Hg_1-x-yCd_xMn_yTe. From the analysis of experimental results of magnetophonon resonance (MPR) recombination, the band parameters near the band edge are determined precisely. Furthermore, the MPR recombination transitions associated with the impurity level were also detected. An energy gap dependence of the resonant acceptor state has been deduced for a wide range of zero-gap regions.

The kinetics of spin polarisation of free charge carriers and magnetic ions in semimagnetic semiconductors irradiated by polarised light is studied. The magnetisation arising in the sample under polarised light optical pumping turns out to be anomalously small. It is shown that the dominating relaxation mechanism of magnetic ions in such materials is the exchange scattering of holes with magnetic impurity ions.

Far-infrared magneto-optical studies of zero-gap p-type Hg_1-xMn_xTe (x approximately=0.07) for temperatures down to 1.6 K and magnetic fields up to 9 T are presented. Transitions of the resonant acceptor state to the uppermost heavy-hole level are identified by the selection rules and the temperature-dependent exchange effect together with k.p Landau level calculations. The zero-field binding energy of the resonant acceptor is found to increase rapidly as the temperature is lowered. Taking N₀ beta =0.6 eV the authors find that the resonant acceptor energy decreases initially with increasing field and then increases. These observations indicate the formation of resonant acceptor bound magnetic polarons (RABMPS) in HgMnTe at low temperature and low external magnetic field. The zero-field binding energies of RABMPS are found to be 5.0(5.4) meV in the limit of zero temperature for x=0.065(0.070).

Low-field transport studies of HgTe/CdTe superlattices grown by laser-assisted molecular beam epitaxy show weak localisation effects for H<30 Oe and T<30 K. The authors have observed small-amplitude fluctuations (dR/R approximately 10^-5) which are superimposed on the anti-weak localisation background. Field modulation techniques have been used to enhance the structure and study its temperature and field dependence. They suggest that these small-amplitude fluctuations in their macroscopic sample ( approximately 4 mm²) originate from quantum interference effects which are enhanced by the relatively large coherence length in this type III superlattice system.

Far-infrared (FIR) magnetotransmission was investigated in a series of p-type Hg_1-xMn_xTe crystals (0.10<or=x<or=0.15) with excess shallow acceptor concentrations ranging from 6*10¹⁵ to 3*10¹⁶ cm^-3. The FIR transmission displays a precipitous drop with increasing magnetic field. This magnetic-field-induced absorption is strongly dependent on temperature and is strongly anisotropic, i.e. for polarisation E perpendicular to B the absorption is always much higher than for E//B. The behaviour is similar to the anisotropy of DC hopping conductivity and leads the authors to the conclusion that, for energies lower than 4 meV, the observed FIR magnetoabsorption originates from optically induced hopping between neutral and ionized states. At higher photon energies, magnetotransmission in the Faraday geometry displays a striking asymmetry with respect to circular polarisation, the origin of which is not presently understood.

A theory of resonant electron-optic phonon interaction in a zero-gap semiconductor in the presence of an external magnetic field has been developed. Three distinctly different cases have been considered: (1) interband spin-conserving case (initial and final electron states in the conduction and heavy-hole bands with the same spin orientation); (2) interband spin-flip case (initial and final states in the conduction and heavy-hole valence bands with different spin orientations); (3) intraband spin-conserving case (initial and final states in the conduction band).

In order to shed some light on the controversy concerning the nature of the field-induced localisation in narrow-gap n-Hg_1-xCd_xTe, the authors have performed magnetoresistance and Hall effect measurements on n-Hg_1-xMn_xTe up to 140 kOe and down to 30 mK. For magnetic fields H below 50 kOe the role of the quantum corrections to the conductivity tensor components is pointed out. The strong and temperature-dependent positive magnetoresistance which is observed for H>50 kOe can be interpreted as the field-induced Anderson localisation, driven presumably by disorder-modified electron-electron interaction in the spin-polarised band. In still higher fields, however, the magnetoresistance changes sign, a behaviour that is not well understood but may follow from the presence of accumulation layers at small-angle grain boundaries.

A number of Hg_1-xCd_xTe samples of different composition (x=0.18-0.50) have been investigated at low temperatures of 0.3-4.2 K in magnetic fields of 0-7 T. A group of non-linear I-V relations have been observed. The experimental results show that the electron concentration, mobility, composition and crystal quality all influence the hot electron effect.

The transport and optical properties of new quaternary semimagnetic Hg_1-x-yCd_xMn_ySe (x=0.1, 0<y<or=0.1) solid solutions grown by the modified Bridgman method were investigated. The values of E_g and delta E_g/ delta T as a function of Mn content were determined and a comparison of experimental and calculated data of absorption coefficients was made. Three-mode character of the compositional transformation of the phonon spectrum is ascertained. The effects of plasmon-phonon interactions were analysed taking account of the contribution of free charge carriers and interband transitions to the crystal dielectric permittivity.

The authors report studies into the molecular beam epitaxial growth and characterisation of InSb/CdTe heterojunction and multilayer structures. It is shown that the use of a Cd/Te flux ratio of 3/1 (J_Cd/J_Te=3) during the growth of CdTe results in the formation of a high-quality InSb/CdTe interface as assessed by transmission electron microscopy and magneto-transport studies. The quantum Hall effect is observed in single heterostructures formed by the growth of CdTe under J_Cd/J_Te=3 flux conditions on InSb at a substrate temperature of 200 degrees C. The electrical results are repeatable with mobilities in the region of 24000 cm² V^-1 s^-1 at 4.2 K. Growth of CdTe under J_Cd/J_Te=3 flux conditions has allowed growth on InSb at a substrate temperature of 300 degrees C and resulted in the successful growth of InSb/CdTe superlattices at substrate temperatures compatible with the growth of electrically active InSb.

Heteroepitaxial (100)CdTe//(100)InSb structures with chemically abrupt and morphologically smooth interfaces have been grown on (100)GaAs by a combination of metalorganic magnetron sputtering for InSb and low-energy bias sputtering for CdTe. The range of growth conditions under which high-quality heterostructures can be fabricated, however, seems to be quite restricted.

Preliminary experimental spectral photoconductivity, time-resolved photoconductivity and non-linear optical absorption data are presented which demonstrate that NIPI structures have been produced in InSb for the first time. The measured band gap reduction and lifetime enhancement effects are in excellent agreement with the predictions of the authors' theoretical model and indicate a clear potential for these structures as high-performanceIR detectors.

The author reviews developments and trends in: narrow-gap semiconductors, magnetic semiconductors, magnetooptics, magneto-transport effects and semiconductor superlattices.

Brief reviews of research on nine materials (or families of materials) from the subject group are presented, focusing on the two decades prior to the first (1964) conference of the series, but also including some significant events from earlier times. The materials are the IV-VI semiconductors, the column-V semimetals, Bi₂Te₃, Te, Mg₂Sn, graphite, grey Sn, InSb and HgCdTe. Some general remarks on early semiconductor research are also included.

Pb_1-xSr_xS films and Pb_1-xSr_xS/PbS double-heterostructure lasers were prepared using a hot wall epitaxy technique. Pb_1-xSr_xS films with energy band gaps up to 1.1 eV (x=0.15) were obtained. The band gap increased very rapidly with the SrS content as dE_g/dx=7.5 eV (x<0.04), while the lattice constant increased as da/dx=0.034a_PbS. Impurity dopings were performed and electrical properties were measured for the Pb_1-xSr_xS films. Films with large p- or n-type concentrations were obtained by doping with Tl or Bi impurities. Pb_1-xSr_xS/PbS double-hetero diode lasers with broad area and stripe contacts were fabricated. Laser operations were obtained up to 210 K pulsed for the broad-area laser and 245 K pulsed (174 K cw) for the stripe contact laser around 3 mu m.

Molecular beam epitaxy is used to prepare high-mobility films of InSb and InAs either homoepitaxially or heteroepitaxially on GaAs substrates. Silicon donors and beryllium acceptors can be introduced at high concentrations ( approximately 10¹⁹ cm^-3), although low-temperature growth (<or=300 degrees C) must be employed in the case of silicon in InSb to avoid compensating amphoteric behaviour. Atomic plane doping of these impurities is studied by quantum transport measurements. Up to five sub-bands are occupied at high doping levels. Little or no diffusion of silicon away from the doping plane is found provided that the growth temperatures are kept low.

Non-resonant four-wave mixing experiments were performed for a determination of the free carrier intraband energy relaxation time in semiconductors. The signal intensity was measured as a function of the pump beam detuning Delta omega . For PbSe and PbSnSe this line shape showed an unexpected strong asymmetry; that is, the Stokes signal dependence on Delta omega was different from that of the anti-Stokes signal. The authors explain this asymmetry by an interference with resonant scattering due to a shallow impurity with binding energy of the order of a few cm^-1.