Table of contents

Using selectivity doped GaAs-AlGaAs heterostructures we provide conclusive evidence that correlation among the DX^--d⁺ charged donors strongly enhances the mobility of a two-dimensional electron gas (2DEG) residing 10 nm away from the charged donors. This is accomplished by changing the extent of correlation without affecting the total number of charged donors and density of electrons in the 2DEG. The experiments are used to prove that the DX donors are negatively charged in their ground state. A theory treating correlated DX^--d⁺ charges can approximately account for these results.

The effect of subthreshold ultrasound treatment on the dislocation-related photoluminescence (D bands) and EBIC contrast is studied in n-Ge_xS_1-x-Si heterostructures. It is shown that ultrasound treatment causes both an increase in the intensity of the D1 and D2 bands and changes in their linear polarization. The proposed mechanisms for the phenomena include (i) ultrasound-stimulated dislocation gettering of the impurities and (ii) the reorientation of the impurities in order to reduce strain near the dislocation.

The effect of high-intensity ultrasonic vibration on the spectrum of deep electron traps in bulk n-type GaAs has been studied by means of deep-level transient spectroscopy. The ultrasonic treatment results in a drastic reduction of the EL6 trap concentration and a generation of three other traps, suggesting an ultrasound-driven transformation of defects associated with the traps. It is argued that the traps EL6, EL5 and EL18 are associated with the following native defects: As_Ga-V_As, V_Ga-V_As and V_Ga respectively. The defect transformations are described by two reactions involving emission of arsenic interstitials.

A theoretical study of the expected photoluminescence line shifts for an interdiffused GaAs/AlAs quantum well has been performed. The interdiffusion coefficient used was assumed to vary exponentially with the Al molar fraction. A comparison of the variation of photoluminescence energy shift with time indicates that this temporal variation is relatively insensitive to the molar variation of the diffusion coefficient. Further studies show that the photoluminescence energy shifts of different well widths may be used to investigate the molar dependence of the diffusion coefficient.

Hot electron kinetics and electric breakdown in a semiconductor with a band of delocalized impurity states are investigated theoretically for the first time. We consider the case when the impurity band is separated from two allowed bands of the crystal by gaps of forbidden states. Contributions to the kinetics from all three groups of carriers in the conduction, valence and impurity bands are taken into account including changes in carrier concentrations due to impact ionization in a high field. It is shown that a new combined overheating mechanism of S-type negative differential conductivity can take place in the system which is connected with simultaneous changes in the carrier concentrations and mobilities. It is predicted that in some cases the current-voltage characteristics of the system can have two S-shaped parts. The process of breakdown in the system combines features of impurity and interband breakdown.

In the present paper self-diffusion of indium and antimony is studied in polycrystalline InSb films by the neutron activation tracer scanning method. Grain boundary diffusion parameters are evaluated in the temperature range 200 to 400 degrees C. The pre-exponential factor and activation energy for indium self-diffusion are 1.17*10^-10 m² s^-1 and 0.84 eV respectively. For antimony self-diffusion these parameters are 1.32*10^-8 m² s^-1 and 1.11 eV. Both indium and antimony atoms diffuse via grain boundaries in the temperature range studied. The grain boundary energy and its temperature dependence are also reported.

Different ZnSe/GaAs epilayers prepared by low-pressure metal organic vapour-phase epitaxy under the conditions given by Nurmikko and Gunshor (1993 Physica B 185 16) are studied through their low-temperature transmission, reflection and photoluminescence spectra. Several samples are etched out from their GaAs substrate and the properties of as-grown surfaces and etched surfaces are compared. The etched samples seem to be free of in-plane strain and to behave like bulk ZnSe. The free-exciton excited state (n=2) is clearly observed in transmission, reflection and luminescence spectra. We also studied the thermal quenching of the photoluminescence in order to confirm the origin of some bound-exciton centres.

The phenomenon of 1/f noise in SiC has been studied in detail for the first time. Spectral noise density dependences for n-type 6H-SiC samples with doping level N_d-N_a approximately=10¹⁷ cm^-3 were measured at 300-600 K over the frequency range from 20 Hz to 20 kHz. The value of the Hooge parameter alpha for 1/f noise is in the range from 5*10^-5 to 10^-3. A low value of a indicates a rather high degree of structural perfection of the material. The value of the 1/f noise depends rather weakly on temperature in the range 300-450 K. The sources of the 1/f noise are distributed uniformly in the channel of SiC JFETs used for studying low-frequency noise in 6H-SiC. For all samples, two local levels can be plainly observed from noise measurements. A frequency-dependent decrease of the 1/f noise was observed on the injection of minority carriers (holes) with very small concentration. Holes have been injected into the n-channel of FETs due to the forward bias on the gate p⁺-n junction. The change in the low-frequency noise due to the appearance of the holes is qualitatively similar to the illumination-induced modification of 1/f noise in n-GaAs and n-Si observed and explained theoretically earlier. It is supposed that bulk 1/f noise in n-SiC is due to fluctuations in the occupancy of the levels that form the density-of-states tail near the conduction band edge.

The reactions at the interface between CdTe and the contact metal platinum have been studied using various approaches including theoretical and experimental ternary phase diagram determination, bulk and thin film diffusion couples and electrical characterization of elemental and compound contacts on p- and n-Cd_0.95Zn_0.05Te (CZT). At the interface of Pt/CdTe diffusion couples a non-planar reaction layer of the intermetallics PtCd and PtTe is formed. This is consistent with the studied ternary phase diagram. Additional compounds which are stable in direct contact to CdTe can also be identified using these data. The contact barrier heights of pure Pt and of alloys with Cd or Te on CZT substrates and their variation after various annealing procedures were determined both on etched (Te-rich) and on virtually stoichiometric CZT surfaces. All the Pt-based contacts on the etched CZT surface showed a similar behaviour, determined by the Te excess and the ensuing formation of PtTe in direct contact to CZT. In contrast to that, a wide range of electrical properties can be achieved on the stoichiometric surface by deposition of Pt with or without additional Cd or Te interlayers due to the variety of phases forming in direct contact to CZT after deposition and annealing.

The purpose of this paper is to investigate the effect of annealing on the series resistance R_s and the shunt resistance R_sh of a p/i/n type solar cell made of a-Si:H thin films in a SnO₂/a-SiC:H(p-type)/a-Si:H/a-Si:H(n-type)/Al sandwich structure. The series resistance and the shunt resistance were determined from DC current-voltage (I-V) characteristics by means of Warashina's method. Thermal annealing carried out in the temperature range 70-175 degrees C showed that the series resistance R_s decreased and the shunt resistance R_sh increased with increasing annealing temperature.

Photovoltage measurements and internal photoemission measurements have been performed for Au/n-GaAs and Ag/n-GaAs contacts within the temperature range of 7 to 300 K. Reliable Schottky barrier heights (SBH) are determined from internal photoemission measurements which show that the SBH in both the contacts is independent of temperature, implying the interfacial Fermi level pinning relative to the conduction band minimum. It is found that the current transport processes such as thermionic emission, tunnelling and leakage current cannot be responsible for the observed photocurrent-photovoltage relationship for large photocurrent in the photovoltage measurements. The activation energies found for the current transport are actually the same in both the contacts, which are almost equal to half of the bandgap of GaAs, indicating that the recombination current is the dominant process. In addition, our results indicate that the equation currently used for the recombination current in Schottky contacts is generally over-simplified.

Atomic layer epitaxy (ALE) was used to grow GaInP/GaAs MESFET structures, where control of the thickness of the GaInP layer is critical. GaInP/GaAs Schottky diodes showed higher breakdown voltages than GaAs Schottky diodes. MESFETs with GaInP/GaAs Schottky gates showed a good saturation and pinch-off behaviour of the drain-source current.

Spatially resolved measurements of the photoresistance of a 2D electron gas in a quantizing magnetic field have been performed using visible light. The influence of illumination on the resistance is a maximum when the light spot illuminates an edge of the sample. The sign of response is determined by the directions of both bias current and magnetic field as well as by the edge which is under illumination. Using this technique we are able to measure the characteristic length scale of the confining potential at the edge and we obtain a value of approximately=20 mu m for the sample used.

We have calculated the current response of a sub-micron GaAs optical switch to light pulses of two different intensities and of wavelengths 799 nm, 734 nm and 633 nm using the Monte Carlo method. We found that the effect of increasing the energy of the incident radiation was to cause a significant lengthening and broadening of the initial transient current response, whilst the effect of increasing the intensity of incident radiation was to delay the rate at which the device could be cleared of charge carriers. We have examined effects of displacement current in addition to the carrier current. The net current shows interesting structure which may have been observed experimentally.

We have performed Monte Carlo simulations of InP MESFETs of lengths 0.6, 0.9 and 1.2 mu m and compared these results with those on GaAs MESFETs of the same dimensions. The direct current IV characteristics of the two materials were found to be similar though the GaAs characteristics were on the whole superior, reaching their operating point at lower drain voltages and possessing higher gains. However, oscillations in the drain current caused by changes in drain voltage in the GaAs devices were not present to the same degree in the InP devices. This difference is caused partially by the onset of the negative differential regime in InP at a higher electric field than in GaAs but the primary cause is the longer ballistic transport times in InP. This suggests that InP MESFETs may prove to have superior frequency response characteristics than GaAs MESFETs.

Tin sulphide (Sn_xS_y) thin films have been prepared on Pyrex glass substrates by the spray pyrolysis technique using tin chloride (SnCl₂) and n, n-dimethylthiourea (CH₃NHCSNHCH₃) as starting materials. The depositions were carried out in the range of substrate temperatures from 320 to 450 degrees C. From X-ray diffraction measurements and SEM micrographs, it is found that the values of x and y, which determine the type of deposited compound, depend on the substrate temperature. At lower substrate temperatures (320-360 degrees C), mixed phases such as Sn₂S₃ and gamma -Sn₂S₃ are present. At intermediate substrate temperatures (370-390 degrees C), the SnS phase is predominant. For substrate temperatures higher than 390 degrees C the deposited material is mainly SnO₂. The optical reflectance and transmittance of SnS were used in an iterative method to obtain the refractive index (n) and the extinction coefficient (k). These values of n and k were used to calculate the absorption coefficient ( alpha ) and the bandgap (E_G), giving the result E_G=1.27 eV. Measurements of the dark conductance ( sigma ) as a function of T were made and an activation energy of 0.54 eV was determined from the slope of a plot of ln sigma versus 1/T.

Quantum well (QW) intermixing has been performed using low-energy broad-area ion implantation to increase the bandgap energy in a spatially selective manner. There is a maximum single dose beyond which further intermixing of the QWS is impeded by damage to the semiconductor surface. We demonstrate that this problem can be overcome by using a series of implants and rapid thermal anneals, with each rapid thermal anneal repairing the crystal surface. Using this technique we have observed shifts in optical bandgap for multiple implants greater than 2.5 times that observed for a single implant.

The effects of Si surface conditions on the susceptibility of Si precipitation at the AlSi (1 wt%)/Si interface have been studied. Conditions of the Si surface on which AlSi films are to be sputtered were varied by two wet cleanings, using an ammonium peroxide mixture (APM) and diluted hydrofluoric acid (HF). For AlSi/Si interfaces, Si precipitation after annealing at 400 degrees C for 2 h and that after AlSi sputtering were observed by scanning electron microscopy after removing AlSi films and by cross-sectional transmission electron microscopy (TEM) respectively. 1.0 mu m diameter contact resistance (R_c) was measured using Kelvin patterns. Complete suppression of Si precipitation at the AlSi/Si interface was observed by SEM for the APM-cleaned contacts resulting in a low and uniform R_c in a wafer, while large precipitates were observed resulting in high and non-uniform R_c for the HF-cleaned contacts. Furthermore, while Si precipitates on the substrate for the HF-cleaned AlSi/Si observed by TEM were epitaxially grown on the substrate, those precipitates for the APM-cleaned one were not grown and scarcely observed in the TEM image. Oxygen atoms formed by the APM cleaning were detected by energy dispersive X-ray spectroscopy at the AlSi/Si interface -even after annealing at 400 degrees C. A new model of the thin silicon oxide layer to suppress Si precipitation is proposed.

The vibrational spectroscopy of NNO defects in Si introduced by ¹⁶O, ¹⁴N and ¹⁵N ion implantation is studied, and especially the N-isotopic shifts of the localized vibrational modes. These investigations show that the local modes of the three impurity atoms comprising the defect are only weakly coupled dynamically. Ab initio cluster calculations of the local mode frequencies of the defect are performed. Several models are investigated, and the model consisting of a bridging O atom adjacent to the N pair defect accounts for its dynamic properties.

Bandgap tuning by impurity-free vacancy disordering was investigated on carbon and zinc p-doped laser structures. Zinc diffused during annealing and consequently only carbon-doped material lased. After annealing, threshold current densities rose from 225 to 255 A cm^-2 due to an increase in the transparency current; the gain constant of the quantum wells remained constant.