Gallium nitride (GaN) crystals of the best structural quality are grown by ammonothermal method in a supercritical ammonia solution inside high pressure autoclaves. This lecture will focus on the basic ammonothermal growth. The growth mechanism in different crystallographic directions, growth morphology and structural quality of GaN crystals, will be discussed. Structural properties and shape of the seeds will be shown. The influence of the crystallization run parameters, the internal configuration of the autoclave and the structural quality of the obtained GaN will be discussed. The path from bulk crystal to finished substrate of GaN will be presented. All challenges and difficulties to grown bulk GaN from ammonothermal method will be demonstrated. Scenarios for the further development of bulk GaN crystallization will be presented.
Implantation of Ga ions into ammonothermal GaN crystals is proposed as a method of controlling the concentration of gallium vacancies. Ultra-high pressure annealing (UHPA) is expected to facilitate the diffusion of the implanted Ga ions into the entire volume of the crystal. Gallium vacancies are expected to be replaced by the Ga ions. Since Ga vacancies act as acceptors in GaN, reducing their content will result in a higher free electron concentration in the crystal.
Gallium ion implantation and UHPA processes will be presented in detail. Values of the main parameters of UHPA allowing for the complete replacement of the Ga vacancies with Ga ions in GaN crystals will be determined. The morphology as well as structural, optical, and electrical properties will be compared for samples without any treatment and those implanted and annealed. The ultimate goal is to obtain highly conductive GaN crystals with the lowest possible Ga vacancy concentration.
In this paper we would like to report the recent progress in basic ammonothermal GaN crystal growth. Growth on ammonothermal GaN seeds is presented. Structural properties of the seeds are shown and discussed. A new shape of seed crystals is proposed. It allows to eliminate the appearance of cracks in the growing GaN. The unwanted lateral growth is restricted and controlled. A higher growth rate, both in lateral and vertical directions, is obtained. Generally, the high structural quality of native seeds is maintained. In case of crystallization in lateral directions the structural quality of the seeds can even be improved. The influence of the crystallization run parameters, growth direction and configuration on the structural quality of the obtained GaN will be discussed.
Recent progress in bulk GaN growth technology will be presented. New results of basic ammonothermal GaN crystallization and halide vapor phase epitaxy (HVPE) of GaN will be shown and analyzed. The advantages, disadvantages and challenges of both methods will be discussed. An influence of lateral growth on critical thicknesses and structural quality of crystallized GaN layers by both methods will be demonstrated. Reduction of lateral crystallization and growth only in one crystallographic direction will be shown.
Gallium Nitride (GaN) substrates gained much interest because of their usefulness in making devices for microwave high power electronics, as well as electronic devices of high operating voltage (above 1000V). Among many techniques of GaN crystallization, ammonothermal method is regarded as a key technology of bulk GaN production. In this method, GaN is deposited on native seeds in supercritical ammonia solution at high pressure (4 kbar) and temperature (600 °C) conditions. Intentional doping allows to obtain crystals of wide spectrum of electric properties (n-type, p-type, highly resistive). In case of the latter ones, compensation of unintentional oxygen donors by shallow and deep acceptors is used. It enables to fabricate 2-inch highly resistive semi-insulating substrates of thermally stable resistivity of at least 1011 Ω cm at room temperature. Those substrates are characterized by outstanding structural quality (flat crystal lattice, dislocation density of 5x104 cm-2). Semi-insulating substrates were successfully used in fabrication of high electron mobility transistors (HEMT), operating at 22 GHz..
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