(Invited) Technology of Gallium Nitride Crystal Growth - Challenges and Perspectives

Abstract

It is well known that gallium nitride (GaN) is the basic material for fabricating optoelectronic devices as Light Emitting Diodes (LEDs) and Laser Diodes (LDs). GaN is also an attractive choice for electronic high-frequency and high-power devices. This is due to its wide band gap, high electron velocity, high breakdown field strength and high thermal conductivity. Most of the mentioned GaN-based devices are built on foreign substrates (sapphire, silicon, or silicon carbide). For many years, there was no mass production of native substrates. This happened because GaN crystal growth is quite a challenging process. The compound melts congruently at high temperature and extremely high nitrogen pressure (>6 GPa). Thus, today, it is impossible to grow GaN from the melt. The crystals should be obtained by other techniques requiring lower pressure and temperature. Crystallization from gas phase, solution, or any combination thereof must be included. For more than three decades, crystallization of bulk GaN has remained a big challenge for the nitride community. Work on developing bulk GaN has mainly been motivated by the LD market. All commercially available LD epi-structures are built on GaN wafers. Today, however, not only the LDs, but also the electronic industry and the demand for vertical high power transistors and diodes seem to be the driving force for crystallizing bulk GaN. Thus, GaN substrates of relatively high quality have recently become available. Today, GaN is crystallized by three main methods: i/ sodium flux (Na-flux); ii/ halide vapor phase epitaxy (HVPE); and iii/ basic or acidic ammonothermal growth. Perspectives of all the mentioned above technologies will be presented in this lecture. Special attention will be paid to the basic ammonothermal method and HVPE. It will be shown that growth from the vapor phase needs a new philosophy. The crystallization processes on foreign seeds (sapphire and gallium arsenide) reached their maximum possibilities. Despite many tricks during crystal growth runs the structural quality as well as yield of the obtained GaN crystals and wafers are very low and cannot be improved. The only way is GaN-on-GaN crystallization. Recently, significant progress has been presented in the basic ammonothermal method. High structural quality crystals of lateral size larger than 2 inches and thickness exceeding 6 mm were obtained in one crystal growth process. Use of ammonothermally grown GaN as seed for HVPE will be presented. All challenges and difficulties to grow bulk GaN from the vapor phase will be demonstrated. Scenarios for the further development of bulk GaN crystallization will be shown. Important parameters of GaN substrates for epitaxy of optoelectronic and electronic device structures will be discussed. A new way of better controlling the electrical properties of GaN crystals and wafers will be proposed.