Table of contents

Langmuir-type evaporation of (100) oriented epilayers has been investigated. An activation energy of 1.98+or-0.18 eV for both Cd and Te₂ desorption has been determined. A quantitative analysis of the excess Cd desorption in the beginning of the CdTe evaporation from initially stoichiometric CdTe has been carried out yielding an upper limit for the Cd deficit in the epilayer of 2-5%.

The authors present results of electronic structure calculations for complex defect centres-chalcogen impurity pairs-in silicon: O₂, S₂ and SO, in different geometrical configurations. Treatment of many-electron interactions is included through configuration-interaction, and is shown to be relevant to the correct prediction of ground-state properties. They also report their results for excited states and hyperfine terms.

The authors present the results of a pseudopotential many-band calculation for electron tunnelling through alternate layered structures of GaAs/AlAs for both zero field and applied electric fields. The build-up of superlattice properties and Stark ladder states are clearly seen. The results are illustrated within an analytic tight-binding model.

Analytical equations for obtaining surface recombination velocity are proposed. AC surface photovoltages in strongly inverted silicon wafers are governed by the frequency response of the strong inversion layer, which is characterised by an inversion time constant. Based on the assumption that the strong inversion layer communicates only with the interface traps, the inversion time constant should give the surface recombination velocity when the inversion layer charge density is known. Trials of the present method have shown reasonable results when applied to previous experiments.

A method is described for the self-consistent computation of electronic states of semiconductor superlattices and MQWS with arbitrary compositional and doping profiles within effective mass theory and the envelope function approximation. Non-parabolicity of bulk bands is accounted for by means of the Hellmann-Feynman theorem. Many-body effects are included within local density functional theory; the exchange-correlation potentials obtained using the Tanatar-Ceperley and Ceperley-Alder correlation energies for the two-dimensional and three-dimensional electron gas, respectively, are compared.

Transitions from the ground state to excited states of the Be-acceptor confined in GaAs/AlGaAs QWs (50<L_z<140 AA) have been observed via two independent spectroscopic techniques: two-hole transitions of the bound exciton observed in selective photoluminescence and resonant Raman scattering.

The authors have systematically investigated the absorption and derivative absorption of a series of GaInAs/InP multi-quantum well structures (MQWS) grown by low-pressure (LP) metal-organic vapour-phase epitaxy (MOVPE). Excitonic transitions between n>or=1 confined states (electron-heavy holes and electron-light holes) have been resolved, and are seen clearly even at room temperature. Comparison with photoluminescence (PL) data shows Stokes shifts as weak as 4 meV, indicating minor potential fluctuations. These results confirm LP-MOVPE as an attractive approach for growing sophisticated structures over large substrates for optoelectronic device production. Each intrinsic excitonic transition energy was carefully determined by performing theoretical fits in the light of the two-dimensional theory of direct allowed excitonic transitions. Comparing with recent theoretical results, they find a satisfactory agreement but, in some cases, finite departures of the alloy compositions or quantum well widths with respect of the nominal values have been found. This results in sizable shifts of the series of excitonic transitions with respect to the expected positions, which establishes excitonic absorption as a quantitative and non-destructive tool for checking the wells' homogeneity, composition and thickness.

The damage, due to reactive ion etching in CF₄+O₂ plasma, has been investigated by I-V and C-V techniques using silicon devices such as Au/n-Si Schottky contacts, p⁺-n diodes and MOS structures. The forward characteristics of a Schottky diode and an exposed junction diode are significantly degraded due to RIE. The amount of damage increases with increasing RF voltage. Considerable recovery occurred when various post RIE treatments were used for the lower RF voltage RIE etching. Almost no recovery upon annealing is found for the higher voltage etching, which suggests the formation of more complex, defect sites at the higher etching voltages. MOS capacitors as well as MOSFETS were used to investigate the damage effects of RIE in Si-SiO₂ systems.

In order to investigate the electrical damage induced in Si-doped GaAs by CH₄/H₂ RIE, electrical characterisations (I-V, C-V) of Schottky diodes fabricated on the dry-etched surface were performed. After RIE carried out at 1.3 W cm^-2, the Schottky barrier height decreases from 0.79 V to 0.53 V. Successive wet chemical etchings of the dry-etched surface show that the in-depth extent of this damage is at least 50 nm. Doping profiles, derived from conventional C-V analysis show a decrease in free-carrier concentration in the surface after RIE. The extent of this passivation is inversely dependent on the initial doping level. This effect is attributed to a chemical neutralisation of the doping centres by atomic hydrogen. The reactivation energy for recovery of the donor activity is around 1.75 eV.

A new method of hydrogen passivation is described in which a flow of molecular hydrogen is passed through dense microwave plasma across a magnetic field. This magnetic field serves to protect the sample surface from being bombarded by charged particles. Hence the crossed beam (CB) method makes it possible to avoid plasma etching and heavy ion bombardment that are so detrimental for many semiconductor devices. The positive effect of CB treatment on n-GaAs Schottky diodes is described. The effect consists of shallow donor passivation and improvement of I-V characteristics.

(For pt.1, see Phys. Status Solidi, vol.149, p.465 (1988)). Electronic and/or optical excitation at surfaces is assumed to modify the cohesive properties of localised sites. This leads to change of the activation energy and entropy for surface-assisted mechanisms (adsorption, desorption and reactions). These are introduced into the models leading to the Langmuir and BET isotherms. It is also shown how the Eley-Rideal and Langmuir-Hinshelwood mechanisms are modified upon electronic excitation. In order to understand how electronic excitation may modify reaction rates, a reaction is considered in which a normally non-volatile intermediate product becomes volatile upon electronic excitation. This is discussed in the case of doping-assisted reaction and laser-assisted reaction. In this last case, by taking into account the effects of laser irradiation on localised states and on the temperature of the irradiated zone, one may describe situations where either rapid saturation or slow increase, or a sharp threshold for reaction takes place.

Ab initio LDF calculations are carried out on 75 atom H-terminated clusters of Si containing 4 O atoms surrounding a Si, Al or B core. The clusters represent models of NL10 and NL8 thermal defects in Si. If the Si and O atoms are arranged in the normal zig-zag sequence along (011) then there are no gap levels. Such a cluster possesses a large strain energy. An alternative arrangement of atoms gives a strain-free cluster containing over-coordinated pairs of O atoms. This is stable and possesses donor levels. A single pair of these defects gives a midgap level whereas two pairs give, in addition, a donor level in the upper half of the gap. The wavefunctions of these complexes are described and appear to be consistent with ENDOR data. The relative stability of these complexes over those with normal O bonding is greatest for Al and Si cores, less so for B and least for C. This can account for the effect of Al, B and C on the production of thermal defects. The model also implies a much greater diffusion rate for di-oxygen complexes than for a single interstitial O atom.

A novel special experimental procedure, where a helium gas pressure cell is employed to vary the hydrostatic pressure at low temperatures, is used to study the freeze-out of carriers on the metastable states of Si donors in GaAs. This technique separates the modifications of the mobility originating from alterations in the band structure from those related to the electron transfer to DX centres. The procedure allows a more precise evaluation of the ability of high-pressure experiments to distinguish between models of positive and negative U for the DX centre. The mobility increase with pressure is theoretically shown to occur for both repulsive (positive U) and attractive (negative U) on-site electron-electron interactions, provided that the inter-site Coulomb interactions are taken into account.

Clean hydrogen-terminated amorphous silicon surfaces have been generated by etching in various solutions based on hydrofluoric acid. The cleaned surfaces were characterised by X-ray photoelectron spectroscopy which showed the residual carbon and oxygen contaminations to be extremely low. It was also observed that the use of ethanol-based solutions led to an improved surface. Finally, it is shown that the contamination levels depend critically on the purity of the starting chemicals.

The nature of the lowest conduction band in thin (GaAs)_n(AlAs)_n(110) superlattices has been investigated using a self-consistent local pseudopotential method and an effective mass-Kronig-Penney method. The pseudopotential method predicts that the lowest conduction band is confined to GaAs layers and its character is L-like, X-like and Gamma -like for ultrathin, thin and thick superlattices, respectively. The Kronig-Penney method has been used to investigate differences in the nature of the lowest conduction band in GaAs/AlAs superlattices along the (001) and (110) directions of growth.

The authors report the first fabrication of Ga_0.49In_0.51P/Ga_xIn_1-xAs n- and p-type lattice matched (x=1) and strained (x-0.85) heterostructure insulated gate field effect transistor (HIGFET) grown by low-pressure metal-organic chemical vapour deposition (LP MOCVD). The growth conditions of Ga_0.49In_0.51P have been optimised to produce a high-purity insulator material. The n-type devices show good pinch-off characteristics, with a small variation of threshold voltage with temperature consecutive to the low trap density in that system. High transconductance values have been obtained on p-type devices, due to the large valence band discontinuity between GaAs and Ga_0.49In_0.51P.

The authors report the fabrication of high-quality GaAs/GaInP n-p-n heterojunction bipolar transistors grown by low-pressure metal-organic chemical vapour deposition on semi-insulating substrates. Various structures with homogeneous and graded bases have been fabricated. Doping profiles together with X-ray double diffraction patterns demonstrate the excellent control of growth parameters such as thicknesses, doping levels and interface quality. The static characteristics of the devices show current gains as high as 400, which is the highest value reported in that system.

The dependence of GaAs etch rates on cathode temperature has been investigated in the range of 285-365 K for a CH₄:H₂ RF plasma. Results indicate that the apparent activation energy E_a, is strongly influenced by the CH₄ partial pressure. GaAs etch rates have an activated temperature dependency with measured values of E_a, varying from approximately=0.7 eV in H₂ down to approximately=6.5 meV under high CH₄ partial pressures. Thus GaAs etch rates become progressively independent of temperature with increasing CH₄ partial pressure. Consequently etch rate stability is enhanced at high CH₄ partial pressures, thereby enabling reproducible deep mesa etching.