Table of contents

The study uses photocurrent spectroscopy and luminescence techniques to investigate the effect of interdiffusion on a GaAs/AlGaAs system with four quantum wells. The wells are non-identical in that the two central wells have equal (symmetric) Al barriers, while the outer two have unequal (asymmetric) ones. This results in the magnitude of the spectral blue-shift induced by the interdiffusion being different for the two well configurations. Investigations are carried out into the response of the two well types to differing levels of interdiffusion. The interdiffusion is brought about by capping with SiO₂ followed by annealing, and we show that the extent of the interdiffusion can be controlled by the thickness of the encapsulant, and that the effect saturates for thicker caps.

Some factors affecting the MOVPE growth of GaSb and InSb and structures based on these materials and their alloys are investigated. Trimethylantimony (TMSb) and tertiarybutyldimethylantimony (TBDMSb) have been synthesized and assessed by growing bulk layers of GaSb and InSb. The use of TBDMSb has allowed the growth of InSb at 400 degrees C as compared with 450 degrees C using TMSb. all the alkyls used have had their pyrolysis kinetics determined under real growth conditions using in situ ultraviolet spectroscopy. A thorough study of the initial nucleation of GaSb onto GaAs substrates has been performed using atomic force microscopy (AFM), TEM (cross-sectional and plan view) and quasi-elastic light scattering (QLS). Bulk layers of GaSb have been grown using optimized and non-optimized buffer layers to assess the effect on optical, structural and electrical properties of the grown layers.

A spectroscopic investigation of the electron energy distributions and spectral photosensitivities of the photoemission from field-assisted caesiated metal-semiconductor structures has been performed. Strong field heating of the electron distribution in the semiconductor crystal due to the applied electric field was directly observed and an increase in the photoemission sensitivity by a factor of approximately=30 was seen upon the application of an applied electric field of approximately=2.7*10⁷ V m^-1. From an analysis of the variation of the photoemission current with reverse bias an estimate of 5.2+or-1.3 nm was obtained for the hot electron mean free path in Al_0.35Ga_0.65As. The energy distribution curves of the emitted photoelectrons were measured and were modelled to extract electron temperatures as a function of field in the high-field regime. Effective temperatures in excess of 5000 K were observed for fields of approximately=2*10⁷ V m^-1.

We investigate tunnelling transport in an asymmetric double-barrier structure with heavily doped contacts by use of a quantizing magnetic field applied perpendicular to the layers. We find that for bias voltages in the region of the resonance, the current-voltage (I-V) characteristics exhibit step-like features that shift to higher biases with increasing magnetic field. Our experimental results clearly show two distinct regions in the I-V curves where tunnelling is controlled by Landau 'ladder' formation in either the well or the barriers. It is shown that electrons with relatively large in-plane energy tunnel conserving their in-plane energy rather than their in-plane momentum.

We propose a transfer matrix formalism of the triple-band effective-mass equation to describe the transport process of carriers through an arbitrary structure with layers of different materials. We use the coherent tunnelling approach in the flat band approximation. Special attention is paid to the boundary conditions. The triple-band matrix formalism is applied to AlAs/GaAs resonant tunnelling diodes and n- and p-type InAs/AlSb/GaSb resonant interband tunnelling diodes. We compare the results of the single- and double-band effective-mass equation with the triple-band effective-mass equation. We show that the influence of the non-parabolicity of the bands on the position of the energy levels in the quantum well for the simple resonant tunnelling structure is of the same order as the shifts induced by the charging of the quantum well. In the case of the p-type resonant interband tunnelling diode, we calculate that the effect of the split-off band is not negligible.

The transport properties of delta -GaAs (Si) at high temperatures were studied using the 'mobility spectrum' technique. Strong changes in the subband occupancies were observed in a low-doped sample (N_D=1.5*10¹² cm^-2) as the temperature increased from 77 K to 300 K while in a high-doped sample (N_D=5.0*10¹² cm^-2) these changes were small. For an explanation of the observed behaviour with temperature Poisson's and Schrodinger's equations were solved self-consistently. The mobility of the lowest subband (i=0) was found to be relatively independent of temperature while the mobilities of the higher subbands (i>0) decreased with increasing temperature in both low- and high-doped samples. At close to room temperature, the conductivity of the delta -structures can be described well by a two-carrier model where the first group consists of the electrons of all the higher subbands having approximately equal mobilities.

We present experimental results on hot electron noise in untreated and hydrogenated samples of silicon-doped GaAs. Short n⁺-n-n⁺ structures are used to resolve noise sources arising from different mechanisms which dominate electron scattering. Passivation of centres of impurity scattering by hydrogen and the resultant effect on noise temperature and spectral density of current fluctuations are clearly observed at intermediate hydrogen doses (10¹⁷ cm^-2): low-field mobility of electrons increases, electron heating by electric field is favoured, noise sources due to Gamma -L and Gamma -X intervalley transfer and resonant scattering of hot electrons are better resolved, especially at low temperatures. The effects at a hydrogen dose of 10¹⁸ cm^-2 suggest that the impurity passivation is accompanied by formation of new scattering centres.

The well known geometrical magnetoresistance effect was applied to metal-semiconductor-metal and GaAs Gunn device-type structures as angle-dependent magnetoresistance measurement and it has been proved to be an effective tool. Using samples rotated in magnetic field, the method makes possible the simultaneous determination of the electron mobility in an epitaxial layer grown on a conductive substrate and the resistance of the contacts, as has been demonstrated by a great number of experimental results. Questions which arose during the experiments have been studied by computer simulations for a wide range of sample parameters. Model calculations have shown the requirements on the samples, on the conditions of the measurement and the limitations to the correctness of the fitted and resultant data. In practice limitations are not very severe for different real metal-semiconductor-metal structures.

A new theory for the junction capacitance in the case of free electron freeze-out into dopant levels is presented. It is shown that drastic changes in the capacitance properties at low temperatures are due to the sharp increase of the width of the particular region where the modulation of the space charge by a small high-frequency AC signal takes place (the so-called Debye tail). On the basis of an analysis of the exact Poisson equation we derive simple and accurate expressions for high- and low-frequency capacitance which properly take into account the Debye tail effects. It is shown that the anomalous capacitance properties in Al_xGa_1-xAs epitaxial layers governed by the DX centres can consistently be described over a wide temperature range on the basis of our analysis.

The electrical and thermal characteristics of three ohmic contact structures: (i) in situ deposited epitaxial Al on delta -doped GaAs, (ii) evaporated Au-Ge on delta -doped GaAs (iii) evaporated Au-Ge on GaAs were compared with each other. The I-V characteristics of the non-alloyed Au-Ge and the epitaxial Al, as-deposited, on delta -doped GaAs were linear, whereas the non-alloyed Au-Ge contact on GaAs showed a nonlinear I-V characteristic, as expected. The specific contact resistivity ( rho _c) of the non-alloyed Au-Ge contacts on delta -doped GaAs was 1.4*10^-6 Omega cm², whereas it was 9*10^-7 Omega cm² for the epitaxial Al contacts on delta -doped GaAs, which is the lowest ever reported for a pure metal on delta -doped GaAs. No value of the specific contact resistivity ( rho _c) could be obtained for the Au-Ge contacts on GaAs because of the nonlinearity of the I-V characteristics. After an alloying process at 450 degrees C all samples showed linear I-V characteristics. The specific contact resistivity of all samples varied inversely with the n⁺ doping concentration up to 5*10¹⁷ cm^-3. Above this concentration the specific contact resistivity of the Au-Ge and epitaxial Al contacts on delta -doped GaAs was constant and hardly changed after alloying. Our results confirm that the well known double-barrier (metal-n⁺-n) model is valid for the ohmic contact on delta -doped GaAs.

Current-voltage characteristics of thin buried oxide SOI material are shown to exhibit Fowler-Nordheim type conduction that is affected by field intensification arising from asperities at the interfaces. The conduction is modelled to yield parameters that can be used to deduce information concerning the quality of the interface and the likely form of the asperities. For the SOI material used in the experiments it is determined that the asperities at the body/buried oxide interface are larger and smoother than those at the substrate/buried oxide interface. It is also demonstrated that under some circumstances the effect of the breakdown can be used to estimate the size of the asperities.

A new line spectrum at about 860 meV has been observed in silicon co-doped with iron and boron. The isochronal annealing behaviour of this line spectrum and the FeB centre was studied by transmission spectroscopy and electron paramagnetic resonance (EPR), respectively. The good agreement between the two measurements strongly suggests that the line spectrum originates from excitations at the trigonal FeB pair complex, well known from previous EPR studies. The positions of the FeB donor and acceptor levels were estimated to be at E_v+110 meV and E_c-275 meV respectively, in close agreement with previously reported values for the FeB pair. Furthermore, the energy difference between the ground and first excited states of this centre was determined to be 1.09 meV, which compares favourably with the 0.75 meV EPR value previously obtained.

The magnetic circular dichroism of the absorption (MCDA) of V_Si⁴⁺ in 6H-silicon carbide shows a strong temperature dependence. Within a crystal field approach the MCDA and g-factors of the hexagonal site defect could be explained, whereas the situation is more complicated for both quasi-cubic site defects owing to a dynamical Jahn-Teller effect.

We report on the magneto-optical properties of large CdS_0.4Se_0.6 nanocrystals in a glass matrix. Optical pumping in the two high-energy bands of the photoluminescence spectrum is studied with resonant excitation and in the presence of a magnetic field. We also investigate magnetic field-induced polarization of luminescence for non-polarized excitation. We show that the higher energy band is most likely to be due to excitonic recombination. The Lande g-factor is found to be anisotropic. Its maximum value is estimated from the study of magnetic field-induced circular polarization. The second band is attributed to donor-hole recombination. We show that electrons on the donor level are partly optically oriented by circularly polarized light and that the transition may be excited either from the A or from the B valance band. The maximum degree of circular polarization is calculated in the case of optical pumping with resonant excitation of the A exciton for an assembly of wurtzite nanocrystals with randomly oriented c axes. It is found to be equal to 5/7 instead of 1 in the bulk material. In the case of magnetic field-induced polarization, the effective Lande factors for the different luminescence bands are calculated and compared with the experimental results.

Electroreflectance (ER) spectra of InGaAs/GaAs asymmetric step quantum wells (ASQWS) have been investigated under an applied electric field up to 130 kV cm^-1 at 120 and 300 K. From the ER lineshape analysis the field dependences of the electron and heavy-hole ground-state excitonic transition energies, their intensities and the linewidth broadening have been determined. Some weak features due to the transitions involving excited quantum states have also been observed. The spectroscopic data have been compared with calculations performed by the transfer matrix method. The Stark shift of approximately 40 meV at a field of 100 kV cm^-1 for the ground-state exciton transition is found to exceed significantly that in the square quantum well. The considerable change in ER lineshape and the rapid rise in linewidth broadening with increasing electric field are shown to be mainly caused by the effects of exciton quenching and tunnelling of carriers out of the well, which occur in the ASQWS studied here in high electric fields.

The electronic states and optical transitions are studied in a strongly coupled asymmetric triple quantum well structure. The photoluminescence (PL) spectra are shown to be a strong function of the external electric field applied perpendicular to the heterointerface. The dramatic change is attributed to the mixing or the resonance and/or anticrossing of three electronic states.

The partial intermixing of well and barrier materials offers unique opportunities to shift locally the bandgap of quantum well (QW) structures. In combination with the quantum confined Stark effect it can be used for many applications in integrated optoelectronics. Here we investigate theoretically the optical and electro-optical properties of partially intermixed GaAs-GaAlAs QW structures. We calculate the dependences of energy shifts and absorption spectra as a function of the well width, the interdiffusion length and the applied electric field. We show in particular that large blue shifting by interdiffusion and efficient electro-optical modulation can be achieved only for a narrow range of QW widths.

Microwave enhancement of magnetically confined discharges is shown to produce significant increases in etch rates of III-V semiconductors and in the emission intensity from the active species over conventional radio-frequency (RF) plasmas. These electron cyclotron resonance (ECR) discharges require some alteration of the gas chemistries commonly used for reactive ion etching, especially polymer-forming chemistries, and generally perform best using pure gases. Additional RF-biasing of the sample position is used to obtain practical etch rates, and the requisite anisotropy and the source-sample distance must be minimized to preserve the high active species density produced in the microwave discharge.

Microwave-enhanced SF₆/O₂ discharges with low ion energies (<or=100 eV) were examined for dry etching of W, SiN_x and SiO₂ films on GaAs. The etch rates of W and SiN_x peak at 10% O₂ flow rates, which produce higher atomic fluorine densities in the discharges. Undercutting of W features is minimized when using Ti masks because of a sidewall passivant mechanism. Tungsten films may also be anisotropically etched in BCl₃/Ar discharges at somewhat lower rates than with the SF₆-based chemistry. Fluorine residues on the GaAs surface after dry etching were removed by NH₄OH/H₂O wet chemical cleaning, while an H₂ plasma treatment at room temperature was also effective in removing dry etch residues.

Thick (<or=4 mu m) tri-level resist schemes are increasingly common in processing of III-V devices such as heterojunction bipolar transistors (HBTS). Under the type of low-pressure, low-bias dry etching conditions necessary during processing of these damage-sensitive devices, conventional reactive ion etching produces impractically low etch rates. These etch rates can be substantially enhanced (sixfold) at the same pressure and bias by employing a hybrid microwave (2.45 GHz)-RF (13.56 MHz) reactor, which separates the control of the plasma density and the ion energies. Examples are given of the anisotropic etching of a tri-level resist scheme containing a PECVD SiN_x transfer layer and a thick resist base layer, a typical scheme used in the processing of self-aligned HBTS.

Boron-doped silicon ((B) approximately 10¹⁷ cm^-3) was heated in H₂ gas at a temperature in the range 900<or=T<or=1300 degrees C and quenched to room temperature. Some of the dissolved hydrogen formed H-B pairs and the remainder (H_h), which was infrared inactive was released during anneals at T<or=200 degrees C leading to an increase in (H-B). The total hydrogen content, consistent with secondary-ion mass spectrometry, yielded a solubility given by S_H=9.1*10²¹ exp(-1.80 eV/kT) cm^-3. 2 MeV electron irradiation at room temperature converted H_h into defects incorporating two hydrogen atoms, suggesting that H_h may be present as H₂ molecules.

The density of states of cylindrical quantum wires is calculated in the presence of charged impurities. A multiple-scattering approach is used in order to describe impurity bands for small impurity densities and band-tails for large impurity densities. Excited impurity bands are obtained within our approach. An analytical expression is given for the disorder-induced renormalized band-edge in the band-tail regime.