Table of contents

Diffusion experiments of Zn and Cd in VPE InGaAs, lattice matched to InP, are reported. A closed ampoule technique is used. For the simultaneous diffusion of Zn and Cd from a Zn₃As₂/Cd₃As₂ source an increasing amount of Cd₃As₂ results in a decreasing p-n junction depth whereas a high hole concentration (more than 10²⁰ cm^-3 at T_diff=600 degrees C) fixed by the Zn acceptor concentration level can be realised. The Cd atoms whose interstitial diffusion coefficient is small compared with that of the Zn interstitials penetrate into the crystal as deep as the Zn atoms. These experimental results are in accordance with numerical simulations of the simultaneous diffusion in III-V compounds starting from the concept of an interstitial-substitutional diffusion mechanism. The diffusion method presented is suitable for the generation of shallow- and high-doped p-type regions during controllable diffusion times.

Optical transmission, photosensitivity, magnetoresistivity, I-V and C-V characteristics of 'thick' (period D>or approximately=500 AA) PbTe Pb_1-xSn_xTe multiple quantum wells (MQWS) were studied. Investigation of the electrical characteristics showed quasi two-dimensional transport properties. Theoretical analysis of the experimental optical transmission spectra showed that for MQWS grown on BaF₂ substrates the main contribution to the optical absorption coefficient involves transition in three (111) valleys oblique to the axis of the MQWS. The optical absorption in such MQWS for polarised light must be anisotropic in spite of the cubic lattices of the constituents. The values of the R₀A product for MQWS investigated are much higher than for heterojunctions.

Two MBE-grown GaAs/AlGaAs heterostructure wafers have been modified by helium ion implantation. Since implantation essentially only changes electron mobility, low-temperature experiments allowed the authors to study the quantum corrections to the electrical conductivity as a function of mobility. The first heterostructure wafer only had one electronic subband at the GaAs/AlGaAs interface populated. Weak localisation magnetoresistance was interpreted by a theory valid in a regime with only a few elastic scatterings during a time of phase coherence. At magnetic fields of the order of the inverse electron mobility, a quadratic magnetoresistance shows up in the experiments. Whereas they find that the temperature dependence of this conductivity correction is well in agreement with predicted effects of electron-electron interaction, the dependence on mobility is larger than any existing theory predicts, yet still in the region of the conductance quantum. The second heterostructure wafer had an electron density, which corresponded to a slight population of the second subband. The weak localisation magnetoresistance has for these samples a very unusual shape with 3 maxima. The authors interpret this behaviour as a result of a spin-orbit effect on the weak localisation in the upper subband induced by the interband scattering.

The authors have performed self-consistent density functional calculations using ab initio pseudopotentials on (InAs)_n/(GaSb)_n superlattices for n=2, 3, 6. From these, they have determined the band offsets for the three different (001) interface bonding configurations possible. They have also studied the effect of relaxation due to the disparate bonding strengths. Finally they also present results for the (110) interface which show that the valence band offset is orientation dependent.

The modulation of the potential barrier of an isotype heterojunction by the incorporation during epitaxial growth of thin, heavily doped n and p regions either side of the heterointerface has been investigated by computer modelling. The resulting device structure, known as a doping interface dipole or DID, was first proposed by Capasso and co-workers (1985) as a means of introducing a bias-dependent pseudo-band offset in heterojunction devices. The results presented here predict how this bias dependence varies with the doping dipole parameters and the material properties of the host heterojunction, chosen here to be a pseudomorphic Si_1-xGe_x/Si p-isotype heterojunction.

A new technique, using the absolute amplitude of the low-field Shubnikov-de Haas oscillations, is presented which allows the direct measurement of inter-subband scattering in 2D heterojunction structures. When applied to a heterojunction with two subbands occupied the technique is used to show that inter-subband scattering is independent of temperature between 1 and 4 K. In agreement with previous reports frequency and amplitude intermodulation, which increases with temperature, is also observed. This is attributed to mixing, by thermal or impurity damping, between the two sets of oscillations in the Fermi energy. This explanation is supported by a model calculation which reproduces the features of the experimental results.

The electric field distribution, the velocity distribution, the effective saturation velocity, and the maximum operating frequency are evaluated for an InGaAs/AlGaAs MODFET by taking into account the electric-field-induced mobility degradation, which becomes important when the electric field in the channel exceeds in *=v_th/2 mu _o=1.7 kV cm^-1, where v_th=3.9*10⁷ cm s^-1 is the thermal velocity of an electron. In the limit of small gate-length, the effective electron velocity is shown to approach the limiting value v_th, the internal transconductance reaches the limit C_ov_th, and the saturation current is /_Dsat=n_sDqv_thW, W being the gate width. The ratio of the carrier density at the drain end of the channel to that at the source end, at the onset of saturation, is found to be n_sD/n_so=1-(V_c/V'_g)((1+V'_g/V_c)¹2/-1), which approaches unity in the short-channel limit and V'_g/2V_c in the long-channel limit, in direct contrast to the well known pinch-off behaviour, where n_sD vanishes. Here, V_c=v_thL/ mu _o, where L is the gate length. An analysis of the effective saturation velocity, and maximum frequency of the MODFET as a function of the gate length is presented.

The authors present reflectance and total photoelectric yield measurements of commercially available silicon wafers. The measurements have been performed at the XUV reflectometer station at HASYLAB(DESY) in the photon energy range of between 50 and 900 eV. Fitting of the reflectance as a function of angle of incidence using Fresnel's equations yielded values for the thickness of the native oxide layer and optical constants of the Si wafer. The surface roughness of Si wafers could also be determined. The optical constants derived from angular dependent total photoelectric yield measurements are in good agreement with those obtained from reflectance measurements on the same Si wafer. Some of the Si wafers investigated were cleaned by a combination of chemical etching and in situ thermal treatment in order to reduce the oxide layer. Si wafers which were solely etched revealed an oxide layer clearly thinner than that of unetched samples as well as a drastic decrease in their surface roughness. Subsequent in situ thermal treatment resulted in oxide-free Si wafers. This treatment, however, always brought about a strong increase in surface roughness.

High quality films of zinc oxide were prepared by an activated reactive evaporation technique onto 7059 Corning glass substrates. Film resistivities of approximately 10^-3 Omega cm and optical transparency of more than 88% in the wavelength range 300-800 nm were prepared by adjusting depositing parameters. The electron carrier densities in the range 2-12*10¹⁹ cm^-3 and mobilities of 10-45 cm² V^-1 s^-1 were observed. The energy gap of zinc oxide film was evaluated as 3.3 eV from optical absorption measurements.

LPE-made AlGaAs double heterostructure laser diodes were investigated by deep level transient spectroscopy. In the Sn-doped n-type confinement layers two electron traps named DX1 and DX2 here were always found. Under non-saturation conditions a significant change of the low-temperature part of DLTS spectra curves was observed after applying forward voltage for a short time. Relaxation of DLTS signal and sample capacitance at 80 K took several hours. Only the DX2 trap contributed to this persistent photoconductivity effect.

Experimental data concerning high-field parallel transport in GaAs/GaAlAs quantum well structures are reported. The results strongly suggest that (i) in modulation doped GaAs quantum wells the non-equilibrium LO phonons (hot phonons) are non-drifting, and (ii) in samples with similar 2D electron densities hot phonon effects increase with reduced dimensionality, and hence with increasing 3D electron concentration. Both observations are in excellent agreement with a recent model of high-field transport involving hot phonons.

The AC surface photovoltage (SPV) is observed in n-type silicon (Si) wafers rinsed by an alkaline solution composed of ammonium hydroxide, hydrogen peroxide and water (modified RCA rinse) in a quartz container. The SPV increases in an almost stepwise manner at first and then saturates as the native oxide (SiO_x) grows in the clean room ambient. When a small quantity of aluminium (Al) is added to the alkaline solution, the saturated value of the SPV becomes dramatically high. This is possibly because AlSiO in the native oxide acts as a negative charge inducing high surface potential, which contributes to the high SPV, in the n-type Si wafers.

Far-infrared Fourier transform spectroscopy and photocurrent (PPC) in semi-insulating bulk GaAs after illumination at low temperatures with either white light or 1.1 eV light. The samples can be classified into two categories; one which does not exhibit PPC and one which does. Electronic energy levels of shallow acceptors (carbon and zinc) are observed only in samples from the first category after photoquenching EL2. Both shallow energy levels and PPC are thermally quenched at 80 K and recovered after photo-illumination at T<or=10 K while EL2 is still in its metastable state. Thus, an unidentified defect complex is responsible for these processes. The absence of PPC and infrared absorption spectra of shallow acceptors from samples in the first category suggests that defects other than normal EL2 are involved in compensating carbon and zinc after photoquenching.

CdS-CdSe multilayers form the only known example of a wurtzite, type II superlattice in which lattice strain generates giant piezoelectric fields which modify the band structure. The exciton luminescence band, ascribed to recombination of electrons in CdS layers with holes in CdSe layers, broadens and shifts to lower energy with increasing delay after excitation of the superlattice with a short pulse of light. These effects are interpreted in terms of dynamic band restructuring due to screening of the piezoelectric fields by photogenerated carriers.

Metal-organic reactive ion etching of GaAs and related III-V semiconductor materials, using an RF plasma containing a mixture of C₃H₈:H₂ is described. The kinetic properties of the process are compared with CH₄:H₂ and C₂H₆:H₂ etching of GaAs. Results indicate that when the H₂ flow dependency is optimised, C₃H₈:H₂ produces between 20-38 nm min^-1 GaAs etch rates, with a reactivity of 2.5 greater than CH₄:H₂ RF plasmas.

Mg implanted into GaAs is annealed in an arsine-nitrogen atmosphere using rapid thermal annealing. This atmosphere allows the application of high annealing temperatures up to 1050^oC without surface degradation. Consequently only short annealing times are required. Sharp doping profiles are obtained. A 20% decrease in the minimum sheet resistance due to reduced outdiffusion of Mg is found when the temperature ramps in the annealing cycles are increased from 50Ks^-1 to 300Ks^-1.