Table of contents

The authors demonstrate the effectiveness of both X-ray diffraction and X-ray reflectivity in the structural characterization of semiconductor structures. By combining information from both techniques the abruptness of the interfaces for Si_1-xGe_x structures, with x=0.1-0.57, may be determined. For superlattice structures with x<0.3 both types of interface were found to have a root mean square (RMS) roughness of 0.5+or-0.3 nm. For a Si/Si_0.45Ge_0.55 superlattice structure the interfaces are found to have differing roughnesses. For the SiGe-on-Si interface the RMS roughness is found to be 0.5+or-0.2 nm; however, the Si-on-SiGe interface has a larger value of roughness, 1.0+or-0.3 nm. This roughness at the Si-on-SiGe interface is found to be dependent on the Ge content of the layer and it is shown by transmission electron microscopy analysis to be long ranged (about 70 nm) and wavy at the interface.

The damage introduction rates of BF₂⁺, B⁺ and F⁺ ions implanted into (100) bulk silicon have been investigated over the energy range 3 to 13 keV amu^-1 at temperatures of 77 K and 300 K. The samples have been analysed using 2 MeV He⁺ Rutherford backscattering and channelling, and it is found that the damage created by BF₂⁺ molecular ions is significantly greater than that created by an equivalent dose of the atomic species B⁺ and F⁺. This damage enhancement by the BF₂⁺ ions can be separated into bulk effects, which are due to displacement spikes, and surface effects, caused by multiple collision sequences involving the constituent particles (B and F) after dissociation of the incident BF₂⁺ ion. Similar surface damage enhancements of 1.8 and 1.9 are found at 77 K and 300 K respectively, for particle energies of 3 keV amu^-1. The energy dependence of the surface enhancement factor shows a strong dependence upon ion energy reaching a maximum of 3.5 at 8 keV amu^-l but approaches unity at higher energies.

The results of studies on the morphological features and chemical composition of the prominent types of oval defects present on GaAs and GaAlAs epitaxial layers grown by molecular beam epitaxy (MBE) are reported. Based on these studies, a possible mechanism for the formation of cored oval defects is proposed. The influence of this defect on the performance of MODFETs is also discussed.

The surface acoustic wave (SAW) transmission has been measured at 4.2 K, as a function of magnetic field up to 5 T, for a GaAs wafer on which a heterojunction has been grown by MBE. Frequencies up to 1920 MHz were generated by interdigital SAW transducers deposited directly on the GaAs surface. At all frequencies, oscillations in the SAW attenuation, analogous to the Shubnikov-de Haas oscillations in sigma _xx, can be observed due to piezoelectric coupling to the non-localized electrons of the two-dimensional electron gas (2DEG) of the heterojunction. The authors find, at frequencies of about 900 MHz and above, an additional attenuation at all magnetic fields, which increases with magnetic field. At half-integer filling factor, the increase in attenuation is initially linear with increase of field and then, at higher fields, the attenuation increases as the square root of the field. They interpret this as due to deformation potential coupling to the localized electrons of the 2DEG. A small SAW attenuation is observed at zero magnetic field which may be due to a very low density of three-dimensional electrons in the GaAs buffer layer or remaining in the doped AlGaAs layer.

The local empirical pseudopotential theory, implemented via the S-matrix layer method has been used to study the conduction band state ordering in (GaAs)_n(AlAs)_n superlattices grown in the [001] and [110] directions. An oscillatory dependence of the energies on the layer thickness n is found. It is shown that the Gamma -state becomes the lowest energy state for n>or=8 in the [001] case, and for n>or=12 in the [110] case.

Under an inversion bias and in response to a linear voltage sweep, the capacitance voltage (C-V) and current-voltage (C-V) characteristics of metal-insulator-semiconductor (MIS) capacitors are described and discussed. An improved model for the generation width is applied, in which the effect of inversion minority carriers on the minority quasi-Fermi level has been taken into account. A differential equation is developed with which to describe the change of the depletion region under the linear voltage sweep. The high-frequency MIS capacitance and gate current are related to the depletion region width. In this way the non-equilibrium characteristics of linear-sweep MIS capacitors are described.

A buried-channel CCD is presented, suitable for integration in a high-energy ion-implanted CMOS process. The BCCD channel is high-energy ion-implanted and the gate structure is non-overlapping. The required submicron spacings between adjacent gates are created by a sequence of processing steps. No demands are imposed on the lithography used. SEM photographs show a well-defined gate structure with straight spacings exhibiting minor width variation. The parasitic potential well, associated with the presence of spacing between the gates, has little influence on charge transport performance. Delay lines have been operated with transfer inefficiency of 10^-5 and less.

A theoretical description of a single non-compensated delta -layer with an additional gate electrode is presented. For layers with varying impurity density, the energy level positions, electron concentrations and mobilities in different subbands as well as the net layer conductivity have been calculated as functions of applied depleting voltage phi ₀. Changing the number of the energy levels in a quantum well and transforming the electron wavefunctions, the voltage produces a specific transconduction versus phi ₀ dependence in the structure.

The investigation of deep levels of argon-implanted LEC-grown semi-insulating GaAs with implantation dosages ranging from 1*10¹¹ to 1*10¹⁵ cm^-2 has been performed. Using a photoinduced transient-current spectroscopy (PITCS) it was demonstrated that, for implantation dosages below 1*10¹³ cm^-2, a negative peak or negative transient current (NTC) was observed in the temperature range from 330 to 350 K. The magnitude of this negative peak increased with dosage up to a level of 1*10¹² cm^-2, beyond which it decreased with dosage. The dosage dependence of the EL3 peak height and the resistance of the specimen have also been investigated. It was observed that the variation of the EL3 peak height with dosage was similar to the variation of the magnitude of the negative peak, that is the EL3 peak height likewise increased with dosage up to 1*10¹² cm^-2, and then decreased. The resistance of the original high-resistivity specimen dropped abruptly when the dosage reached 1*10¹² cm^-2. This critical dosage (1*10¹² cm^-2) was found to be a threshold for the generation of a highly disordered state.

Vertical transport in a GaAs/(Ga,Al)As superlattice has been studied using photoluminescence and magnetophotoluminescence measurements with in-plane fields of up to 25 T at temperatures in the range 4-80 K. Vertical transport at 4 K (which is determined by exciton transport) has been found to be less efficient than free-carrier-like transport at high temperatures (47 and 80 K). Application of a high, in-plane magnetic field tends to reduce the hole transport considerably. In addition, a drastic enhancement of the total photoluminescence yield with in-plane field has been observed.

Optical properties of highly excited II-VI multiple quantum-well structures are investigated by means of photoluminescence, reflectivity and Raman scattering techniques. Experimental evidence of exciton-electron and exciton-exciton scattering processes in the range between I0 and 100 K is reported.

Improved etching uniformity of III-V semiconductors under conditions of minimal electrolyte circulation was achieved by pulsed anodizing at 50 V (constant current) in an electrolyte containing both oxide-forming and oxide-dissolving components. For carrier concentration profiling, the process was modified to operate at 10 V (constant voltage). Profiling of multilayer systems embracing a range of binary, ternary and quaternary compounds indicated that the 10 V process was capable of improved junction resolution over greater etch depths than previously possible with the conventional low-voltage process.

A method using oxygen plasma treatment to remove successively-electron-exposed negative resist is proposed. The change in resist thickness with plasma treatment time depends on the different doses and the method of exposure. The resulting curves are very similar to the dissolution curves of positive resist. The etching rates in the depth of the exposed resist show that the top edge of the resist is less sensitive to dose changes than the bottom edge. The theoretical models for energy distribution in the resist film support such assumptions. The experimental data could be useful for more correct modelling and profile simulation of electron-exposed negative resists.

Schottky diode characteristics and deep levels of Si-doped n-type GaAs after hydrogen plasma exposure have been investigated as a function of substrate temperature during hydrogenation. The barrier height and reverse breakdown voltage of Au Schottky diodes on Si-doped GaAs increased during hydrogen plasma exposure. The ideality factor of the hydrogenated samples was lowered to 1.11 at substrate temperatures between 150 and 250 degrees C, and deep levels in these samples were effectively passivated at these temperatures. These properties of n-GaAs can be related to hydrogen redistribution in an electrically passivated surface layer. Good diode characteristics and the most effective passivation of deep levels in GaAs could be achieved at a substrate temperature of about 200 degrees C and a power density of 0.06 W cm^-2.