Table of contents

The non-linear optical properties of semiconductors are reviewed. Examples are given of optical non-linearities which are connected with real or virtual excitations in the electronic system of a semiconductor, and others which are due to photothermal effects. Also included are some aspects of electro-optics, and the authors show how the various types of optical non-linearities can be used to build optically bistable memories, modulators, switches or oscillators. The materials they consider are bulk samples, quantum wells and quantum dots of II-VI and III-V compounds with a brief mention of organic dyes.

The authors report theoretical results pertaining to the electronic properties of Ga(In)As-based quantum wells and superlattices. The focus is on a comparison between the Ga_0.47In_0.53As-InP and Ga_0.47In_0.53As-Al_0.48In_0.52As systems where the well-acting material is the same, the barrier-acting materials are very similar but where the conduction band offsets are very different ( approximately=0.24 and 0.44 eV respectively). Features of electron and hole energy levels as well as scattering lifetimes are discussed.

Unlike bulk semiconductors, quantum well (QW) structures exhibit unique optical features, because their band structure is completely different. In this paper, the authors review the applications of multi-quantum well (MQW) structures to AlGaAs/GaAs distributed feedback (DFB) and distributed Bragg reflector (DBR) lasers. Theoretical and experimental results on the lasing characteristics of MQW DFB lasers are described, mainly focusing on linewidth. Reduction in the chirp and the alpha -parameter were measured, and a linewidth as narrow as 1.5 MHz was obtained. Next they review various characteristics of MQW DBR lasers. In these lasers, MQW structures were used both as an active layer and as a low loss passive waveguide. Highly efficient surface emission from the second-order grating was observed. They also measured the fast bistable switching and beam switching characteristics. In the last part they review the fabrication and characterisation of AlGaAs/GaAs quantum well wires grown on corrugated substrates.

MBE grown quantum devices for the detection of infrared light are introduced. These structures are based on photon-assisted resonant tunnelling and will have a projected tuning range of 5 to 30 mu m. Two structures are discussed: a single quantum well device with large initial population for high adsorption, and a double quantum well structure with promise of sharper detection linewidth and wider tuning range. Optical and electrical measurements of material properties are presented, and their implications on the devices discussed.

1.3 mu m GaInAsP/InP GRIN SCH MQW laser diodes grown by low-pressure MOCVD were operated without mode hopping in a single longitudinal mode over a wide temperature range Delta T (5-45 degrees C). The temperature coefficient of the wavelength in this range was 0.8 AA deg^-1, which was as small as that of the distributed feedback laser. At the extremes of this temperature range mode hopping occurred. The wavelength did not shift into the nearest neighbouring mode but into one which was separated by several mode spacings. An interference between the fundamental and higher-order transverse modes might be the underlying mechanism.

The introduction of carriers into a quantum well quenches the excitonic absorption in a manner which is suitable for optical modulation. In this work selective doping of a single quantum well provides a large electron population which is successfully controlled using the reverse bias field of a semiconductor junction. Quenching and recovery of the excitonic absorption is observed using spectral photocurrent techniques and a corresponding optical modulation is demonstrated in a device which incorporates an optical waveguide. Narrow quantum wells are shown to have the most desirable optical characteristics and an analysis of observed waveguide modulation performance indicates that contrast ratios of >10dB are readily available. A simple model is used to predict the bias-dependent behaviour of the device and the agreement found with experiment suggests that this is a useful tool for design purposes.

The structure and characteristics of low threshold long wavelength ( lambda >1.07 mu m) strained-layer InGaAs-GaAs-AlGaAs quantum well broad area lasers and, in particular, buried heterostructure strained-layer lasers formed by wet chemical etching and a two-step metal-organic chemical vapour deposition growth process are reviewed. Low aluminium composition confining layers allow for high-quality regrowth interfaces and effective use of a silicon dioxide mask for selective epitaxy. The buried heterostructure lasers reported have narrow active region stripe widths, low threshold currents, high output powers, and operate on a stable fundamental lateral mode.

A 5*5 monolithic InGaAs-InP multiple quantum well two-dimensional spatial light modulator is reported for the first time. Silicon oxynitride passivation was employed to produce array pixels on a 400 mu m pitch. Spatially uniform modulation of 3.28 dB (2.15:1 contrast ratio) with an inter-pixel standard deviation of only +or-0.10 dB was obtained from eight active pixels at 1585 nm.

The authors model the room temperature electroabsorption of coupled double quantum wells using a transfer matrix technique to calculate carrier wavefunctions and their overlaps. Their behaviour with an electric field is compared with uncoupled multiple quantum wells with optical modulation in mind. Consideration of wavefunctions and energy levels allows them to choose a coupled well structure with good optical modulation at a low electric field. By fitting three parameters once, they predict absorption spectra. These are used to assess coupled quantum wells within an asymmetric Fabry-Perot reflection modulator. Very low voltage operating characteristics were calculated allowing for a total fluctuation in the coupling barrier width of one monolayer: a contrast ratio of 2.1:1, an insertion loss of 3.3 dB and an applied voltage of only 0.7 V; alternatively, a very high contrast with an insertion loss of 9.4 dB operating at 2.2 V. However, these predictions may be difficult to fulfil in devices grown at present because of the built-in field within the intrinsic region of a p-i-n diode.

The performance of long-wavelength asymmetric Fabry-Perot modulator (AFPM) devices has been modelled to establish whether InGaAs/InP multiple quantum well (MQW) material can yield the high contrast (in-state:off-state) of 100:1 observed at shorter wavelengths using GaAs/AlGaAs. The absorption and electroabsorption spectra for MQWS with 70 AA barriers were predicted using photocurrent data from single 45, 78 or 110 AA InGaAs wells. Using these spectra the thickness of intrinsic MQW material required to obtain the necessary absorbance changes was calculated. The widest wells lead to the best modulation for a given bias voltage. However, the modulation obtained using 78 AA wells is only slightly worse and it occurs at an ideal wavelength of 1.55 mu . Narrow 45 AA wells never yield very good performance. A high contrast can be obtained with a reasonable insertion loss and a low biasing voltage if 1.4 mu m of 78 AA wells or 1.7 mu m of 110 AA quantum wells are used. The spectral performance of these absorbers in an AFPM cavity has been modelled over a 100 nm wavelength range. A contrast of 164:1 (22 dB) for an insertion loss of only 3.95 dB is predicted at 14 V bias. The performance of AFPM devices at very low biasing voltages around 5 V is also detailed.

p-n junctions have been formed by anodic oxidation of HgCdTe, HgZnTe and HgMnTe followed by a low-temperature anneal. The dependence of junction depth on anneal time, temperature, concentration of native acceptors and material composition has been investigated. The results are interpreted as a diffusion of mercury into the underlying bulk from a finite source formed during anodisation at the interface between the oxide and the bulk of mercury-based semiconductor alloys. The converted layers exhibit high recommendation times and the method can be used for fabrication of photoconductors and photodiodes.

Observations are reported of nuclear magnetic resonance (NMR) for nuclei in single GaAs/AlGaAs quantum wells, using an optical pumping and detection scheme. The technique gives signals only from nuclei in contact with the quantum-confined electron wavefunction and therefore discriminates against substrate and barrier nuclei. Some general factors which affect the strength of signals are discussed and preliminary results are presented which show the potential of NMR for measuring strain in pseudomorphic systems and for detecting and identifying impurities.

The low-temperature photocurrent, steady-state and time-resolved photoluminescence of a double-barrier diode under operation is studied. The observation of luminescence, with exciting light above (as well as below) the quantum well absorption edge, indicates that the tunnelling of holes plays an important role. Furthermore the authors have seen evidence of resonant tunnelling of minority holes when the double-barrier diode is illuminated. These holes are provided by a quantised level on the collector side. They recombine with collector electrons, which provides a signature of this level. The strong Stark shift of this new crossed transition gives information on the potential distribution in the diode.

The non-linear optical properties of CdSe microcrystallites exhibiting three-dimensional quantum confinement effects are investigated using (i) nanosecond pump-probe spectroscopic techniques and (ii) single beam saturation experiments to study the dependence on particle size. The experimental data show unique non-linear behaviour arising from state filling and Coulomb effects as well as trap-related phenomena exceedingly different from other non-linear optical materials.

Clear evidence for resonant tunnelling of electrons in a p-i-n quantum well modulator is provided by picosecond pump-and-probe electro-absorption measurements. The temperature-independent escape times show a drastic reduction when an electrical field is applied perpendicular to the wells, with a pronounced minimum at the field corresponding to the resonance between the n=1 electron level in one quantum well, and the n=2 electron level in the adjacent one. The authors' calculated field dependence of the electron tunnelling times proves that this behaviour is the signature of resonant tunnelling.

Intervalence band absorption is discussed as a phenomenon that affects the threshold current of a semiconductor laser. However, calculated values of the absorption coefficient are too small to yield a remarkable effect. The situation is changed if phonon-assisted intervalence band absorption is taken into account. Then the absorption coefficient is considerably enlarged, for instance by more than an order of magnitude in In_0.72Ga_0.28As_0.6P_0.4 with a laser wavelength of 1.3 mu m.

The effect of As pressure of the disordering effect of Zn diffusion into GaAs/AlAs superlattices has been studied. It has been found that the amount of disordering is increased when no excess As is added to the ampoule. In previous work, it was found that dislocation loops form near the Zn-diffusion front. The present results confirm these observations, except for the case where the Zn diffusion was performed in the absence of excess As. Moreover, the Zn penetration was found to be the greatest when there was no excess As added to the diffusion ampoule. Three suggestions are presented to explain the observations.

Junction space charge measurements were used to study a mid-gap level in chemical vapour deposition (CVDS) grown Si_1-xGe_x alloys of three different compositions. Evidence is given for the thermal and optical capacitance transients being fairly exponential. Absolute values of emission rates and capture cross sections for electrons, as well as enthalpies and entropies, are presented. The results suggest that this defect exhibits only a small lattice relaxation effect.

The standard theory of Fermi-Thomas screening of remote impurities in quantised inversion layers uses the assumption that the difference between the energy of the ground state and the average potential energy in the layer is not altered by the presence of a remote impurity. Analysis shows that this assumption is not correct and that, as a consequence, the screening factor is smaller than is usually assumed. The results are illustrated for the case of GaAs inversion layers.

Charge transport calculations carried out for thin plate-shaped off-axis n-Si samples have pointed to the likelihood that a longitudinal electric field induces space charge layers at the sample surfaces. Current-voltage measurements on such samples provide evidence of excess current flow attributable to conduction in the surface space charge layers predicted by theory.

The authors report here a study of the conduction-band offset and differential conductance of a 20 nm wide, In_0.53Ga_0.47As/InP, single-barrier structure grown by atmospheric-pressure MOCVD. For processed samples with 50 or 100 mu m diameter mesas, comparison between experiment and numerical calculations shows that the conductance at 77 K, normalised with respect to the mesa area, is governed by the intrinsic thermionic emission and normal tunnelling transport mechanisms. The conduction-band offset for these samples is measured to be 180-190 meV. For processed samples with mesa diameters greater than about 50 or 100 mu m, the normalised conductance at zero bias is enhanced by several orders of magnitude compared with that of the smaller, well behaved mesas. This is attributed to parallel conduction via structural defects such as pin-holes or point/extended defects within the barrier which are spatially distributed on the scale of 50-100 mu m.

The authors report the results of an investigation of persistent photoconductivity (PPC) in Al_xGa_1-xAs/GaAs HEMT structures grown by MOVPE, and the effects of different dopants and Al_xGa_1-xAs compositions. Temperature- and time-dependent Hall measurements for combinations of selenium-/silicon-doped, x=0.32/x=0.45 Al_xGa_1-xAs/GaAs layers are presented, allowing comparison of the factors influencing PPC. From their results they conclude that the amount of PPC exhibited by MOVPE grown HEMT structures is significantly reduced by using Se as the dopant, as opposed to Si. The PPC of the Se-doped samples also shows a virtual independence to Al_xGa_1-xAs composition. The relative stability of these Se-doped Al_xGa_1-xAs/GaAs structures at cryogenic temperatures should correspond to a significant reduction in the amount of drain-current collapse seen in HEMTS at low temperatures.

N-type modulation-doped Ga_xIn_1-xAs/Al_yIn_1-yAs heterostructures grown by molecular beam epitaxy on InP substrate have been characterised by double-crystal X-ray diffraction and X-ray topography, temperature-dependent Hall measurements, and photoluminescence and absorption spectroscopy. Both the constituent Ga_xIn_1-xAs (x=0.453) and the Al_yIn_1-yAs (y=0.421) layers are strained. X-ray Pendellosung fringes from the Ga_xIn_1-xAs as well as the Al_yIn_1-yAs epilayers are observed, indicating an almost perfect interface smoothness and homogeneity of the layer composition. Despite the highly strained Al_yIn_1-yAs the electron mobility reaches 9.0*10³, 6.5*10⁴, and 8.9*10⁴ cm² V^-1 s^-1 at 300, 77 and 4 K, respectively, thus indicating only a minor influence of the strain of the Al_yIn_1-yAs upon the 2D electron mobility. In photoluminescence the excellent material quality manifests itself in the narrow bound exciton peak from the Ga_xIn_1-xAs layer and in the absence of any transition related to the 2D electrons and holes trapped near the heterointerface.

Ohmic contacts to n-GaAs (Si doped at 2.7*10¹⁵ cm^-3 and 4.5*10¹⁸ cm^-3) have been formed using a Ge/Pd/GaAs metallisation and annealing for short times yielding specific contact resistivities ( rho _c) of 2.27*10^-4 Omega cm² for low-doped epilayers and 1.25*10^-6 Omega cm² for high-doped epilayers. The resulting contacts display a smooth surface and sidewalls. Contact has also been made to two modulation doped structures with specific contact resistivities of 4.22*10^-4 and 1.09*10^-4 Omega cm² using the same metal system.

The current plotted against voltage for a resonant tunnelling device having one rough interface has been calculated with first-order perturbation theory. The roughness gives rise to a small asymmetry in the characteristics for the two possible directions of the current. Other possible reasons for such observed asymmetries are also discussed.

It is shown that the linear Thomas-Fermi screening for an electron gas in a triangular well subjected to a quantising magnetic field always breaks down at sufficiently low temperatures. This is not the case for an ideal 2DEG. The Thomas-Fermi parameter Q approaches a constant value in the low-temperature limit, whereas for an ideal 2DEG it varies as T^-1. Further it is shown that a lateral confined electron system is surrounded by broad depletion regions caused by electrons trapped in surface states at the lithographic boundaries. The widths of the depletion regions are estimated from the difference in energy between the conduction band level and the level of the surface states. As an example of nonlinear screening the authors consider the QHE breakdown in a narrow channel and show that the observed linear dependence between the critical voltage for breakdown and B-B_i can be explained if the electrons in the channel are considered as a non-metallic region. Here B_i corresponds to i filled Landau levels, i being an integer. A reasonable agreement with experimental results is obtained.

Schottky diodes were formed on n-GaAs surfaces which had previously undergone a heat treatment of 450 degrees C for 120 S, which is similar to those typically used for ohmic contact alloying. The Schottky diodes formed on heat treated GaAs wafers were found to exhibit up to a factor of 30 times higher reverse-bias leakage current than diodes fabricated on substrates which were not heat treated. Controlled etching of heat-treated GaAs substrates results in a significant improvement in the performance of Schottky diodes subsequently formed thereon. GaAs substrates that were capped by a spin-on glass layer during heat treatment exhibit much less surface degradation when compared with uncapped GaAs surfaces. It is found that the diode parameters of heat-treated samples can be completely recovered by controlled etching of the top GaAs active layer prior to Schottky gate deposition.

The total iron and the Fe³⁺ concentration are determined by means of SIMS and EPR measurements. The deduced electron concentration is in good agreement with the results given by Hall effect measurements.

The room-temperature, band-edge photoluminescence spectra of degenerately doped (n=1, 2, 3, 8, 10*10¹⁹ cm^-3) and compensated (K=0.05, 0.1, 0.2, 0.3, 0.5) samples have been investigated for identification of the radiative recombination mechanism and to analyse the possibility for a quantitative estimation of the Burstein-Moss shift. The measured spectra consist of two emission bands. The high-energy band (band-edge luminescence) predominates in intensity over the impurity-associated emission band. The spectral shape is discussed in accordance with theoretical ideas that it follows the free electron energy distribution. The calculated curves (over the full range of electron concentration) are in good agreement with the experimental spectra.

The phosphorus content of doped layers approximately 180 nm thick at the base of trenches 0.7 mu m diameter and 4 mu m deep has been determined for two wafer samples. The analysis has been carried out by a combination of taper sectioning and electron microprobe analysis (EMPS) under carefully controlled conditions. Average phosphorus concentrations of 1.6 and 1.0 wt.% have been obtained for the two types of wafer, compared with 3.35 and 2.02 wt.% respectively obtained for the layers on the upper surfaces of the wafers.

The velocity-field characteristics of holes in strained p-modulation doped In_xGa_1-xAs quantum wells (x=0.17 and 0.13), grown on GaAs have been measured at 77 K. Two samples at each indium fraction were studied with different doping concentrations. Saturation is almost complete at a field of 4 kV cm^-1, with drift velocities of around 4.9*10⁶ cm s^-1 for the low doped samples and 3.3*10⁶ cm s^-1 for the high doped. The latter are likely to be typical of structures used for p-type field-effect transistors (FETS). The authors believe that the performance of such devices will be limited by this very low saturation velocity.

The electroabsorptive properties of semiconductor quantum dots in glass are observed for macroscopic field strengths up to 8*10⁵ V cm^-1. The quadratic dependence of the induced absorption change on the applied field turns over to a linear dependence at the highest fields, and a tentative explanation for this behaviour is suggested.

The authors have studied the temperature dependence of side wall depletion in shallow mesa-etched GaAs-AlGaAs wires with and without illumination in the temperature range from 4.2 to 300 K. In the dark, the edge depletion width monotonically increases as the temperature is decreased, whereas under illumination it is reduced at low temperature where persistent photoconduction occurs and the electron density increases in the channel.

Metal-organic molecular beam epitaxy of high-quality InP and GaInAs layers is presented. A T=4 K exciton-dominated photoluminescence, together with T=77 K mobilities exceeding 50000 cm² V^-1 s^-1 have been obtained on InP epilayers, with an n-type carrier concentration in the low 10¹⁵ cm^-3 range. Ga_xIn_1-xAs ternary compounds have been grown on 2" InP wafers with excellent uniformity and reproducibility. Heterostructures such as single heterojunctions and quantum wells have been made with good electrical and optical properties. GaInAs/InP quantum wells as thin as one monolayer with energy shifts as much as 519 meV higher than the bulk have been grown. A two-dimensional electron gas with mu (300 K)=10000 cm² V^-1 s^-1 and mu (4 K)=80000 cm² V^-1 s^-1 has been measured.