Timur Valerevich Malin

The aim of the present work is to investigate in detail the effect of SiN coating on the electrophysical properties of high electron mobility transistors and to compare the obtained results of the conventional AlGaN barrier coating with the GaN-cap layer. Both of them were fabricated on identical AlGaN/GaN heterostructures. In this work, pulse measurements of current–voltage characteristics from different quiescent points revealed a 4.5-fold reduction in the gate-lag effect on the sample with the SiN-cap, compared to the GaN-cap coated sample. A simple technique for analyzing the behavior of the channel resistance as a function of the gate voltage is proposed to determine the threshold voltage stability and to measure the resistance of the channel regions apart from the under-gate region. When the AlGaN/GaN HEMT surface is passivated by a SiN-cap layer, the threshold voltage instability is practically not detectible for gate-lag measurements, in contrast to surfaces with a GaN-cap coating. It is shown that the nature of the traps involved in the gate-lag effect is different from the nature of the traps involved in the drain-lag phenomenon.

The chemical kinetics of the formation of two-dimensional (2D) crystalline and amorphous silicon nitride (SiN) layers on the Si(111) surface during nitridation has been studied as a function of ammonia flux using the reflection high-energy electron diffraction technique at a fixed substrate temperature. It was experimentally determined that the formation rate of both crystalline and amorphous silicon nitride phases increases with ammonia flux growth. The formation rate of the crystalline silicon nitride phase is characterized by a linear dependence, while the dependence of the formation rate of the amorphous phase on the ammonia flux is sub-linear and two orders of magnitude slower than that of the 2D crystalline phase due to the higher activation barrier formation of amorphous SiN. The processes on the Si(111) surface are described in the framework of a kinetic model of chemical reactions including the interaction between top-active Si atoms on the surface and chemisorbed NH2 particles. The experimentally determined SiN formation rates as a function of ammonia pressure are successfully described by a simple set of kinetic equations.

The origin of the contrast appearing in STM images at the boundary between diverse ordered structures is studied using the example of two structures, (7 × 7)_N and (8 × 8), formed in the system of a two-dimensional silicon nitride layer on the Si(111) surface during ammonia nitridation. A significant dependence of the contrast between these structures on the voltage applied to the tunnel gap was found and studied both experimentally and theoretically. Variations in the contrast were quantitatively studied in the range from −3 V to +3 V, and they were studied in more detail for the positive biases on the sample from +1 V to +2.5 V, where the contrast was changed more than 2 times. Within the one-dimensional Wentzel–Kramers–Brillouin (WKB) model for the tunnel current, a comparatively simple procedure is proposed for the correction of the experimental STS-spectra of differential conductivity to identify the adequate (feasible) density of electron states (DOS). It is shown that the (8 × 8) structure DOS corresponds to a graphene-like layer of silicon nitride structure. The proposed correction procedure of the empirical differential conductivity spectra measured by STS will be useful for the quantitative determination of the DOS of new two-dimensional materials and surface structures.

Based on theoretical predictions, it has been demonstrated that it is possible to purposefully change the elemental composition and position of the adsorbed particles on the GaN surface, thereby controlling the surface energy and morphology of GaN. Comparison of experimental data obtained by reflected high-energy electron diffraction and the calculated concentration of ammonia fragments on the GaN surface, and surface energy showed that the movement of adsorbed ammonia fragments into strongly bound states is an effective mechanism to control the GaN morphology. The minimum value of equivalent NH₃ beam pressure at different temperatures to prevent the conversion of the two-dimensional (2D) GaN layer to three-dimensional (3D) islands has been established. It was shown that the boundary between the 2D and 3D states on the surface is defined by the elemental composition of adsorbed particles on the surface and the temperature dependence of the surface energy of the facets of islands.