Grain Expansion and Subsequent Seeded Growth of A1N Single Crystals
- PDF / 387,842 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 7 Downloads / 238 Views
E11.1.1
Grain Expansion and Subsequent Seeded Growth of AlN Single Crystals Dejin Zhuang, Raoul Schlesser and Zlatko Sitar Department of Materials Science and Engineering, North Carolina State University Raleigh, NC 27695-7907, U.S.A. ABSTRACT Ongoing efforts of growing large AlN single crystals at NCSU using an induction-heated, high-temperature reactor are based on (1) engineered expansion of single crystalline grains with increasing boule length, as well as (2) the development of a growth process that enables seeded growth on AlN surfaces previously exposed to air. The growth process is based on physical vapor transport (PVT), where AlN powder is sublimed in a high purity nitrogen atmosphere. The growth temperature was typically in the range of 2250 to 2300 °C. In this study, tungsten crucibles were used in combination with graphite insulation and were found to be durable for AlN growth. Boule growth was interrupted several times in order to refill the AlN powder source and the growth surface was subjected to surface preparation to facilitate epitaxial re-growth. Grain expansion was studied as a function of process parameters. Crystalline quality of large single crystalline grains was correlated with their surface morphology. INTRODUCTION The development of III-nitride based high-power, high-frequency electronic devices, as well as short-wavelength optoelectronic devices is currently hampered by thermal stresses and a high density of defects in AlGaN heterostructures grown on non lattice-matched substrates. Single crystalline AlN is a promising substrate for high quality AlGaN epitaxy, as its close thermal and lattice match drastically reduces defects in the heterostructures [1] and therefore enhances device performance and reliability [2, 3]. In addition, the high electrical resistivity and excellent thermal conductivity of AlN, combined with its wide bandgap (6.2 eV), make AlN substrates particularly suitable for high power microwave devices and UV emitter/detector fabrication. To date, AlN crystals have been mainly grown by physical vapor transport (PVT) technique, in which AlN source material located in the hotter part of a crucible sublimes at elevated temperature and transports to the relatively cooler end of the crucible, where it condenses as a single crystal on an appropriate growth surface. The growth of AlN crystals may be either unseeded or seeded; seeds may be either foreign substrates, such as SiC or sapphire, or, ideally, previously prepared AlN single crystals. To obtain commercially viable growth rates (>100 µm/hr), high process temperatures (>2100 °C) are required in AlN PVT growth [4, 5]. At such extreme temperatures, foreign substrates may decompose and contaminate the grown boules. Furthermore, the thermal expansion coefficient mismatch between AlN and foreign substrates generally leads to severe stresses and cracks, which are difficult to eliminate [6]. Therefore, the development of a self-seeding process, in which single crystalline grains expand as the growth proceeds, remains critical fo
Data Loading...
