Growth of Large AlN Single Crystals Along the [0001] Direction
- PDF / 172,198 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 61 Downloads / 212 Views
0892-FF21-01.1
Growth of Large AlN Single Crystals Along the [0001] Direction Ziad G. Herro, Dejin Zhuang, Raoul Schlesser, Ramon Collazo and Zlatko Sitar Department of Materials Science and Engineering, North Carolina Sate University 1001 Capability Dr., Raleigh, NC 27695, USA ABSTRACT We have demonstrated growth of large AlN single crystals using (0001)-oriented AlN seeds. Boules with a diameter of 15 mm and length up to 12 mm were obtained from 5 mm seeds. Step flow growth was observed on both Al and N-polar surfaces. N-polar face was suitable for growth within a large window of growth parameters while the Al-polar seeds yielded high-quality crystals only at low supersaturation. INTRODUCTION The known properties of AlN indicate that it has a great potential as a substrate for III-nitride epitaxial growth due to its reduced lattice mismatch with AlGaN, as compared to SiC and sapphire. AlN’s large bandgap of 6.2 eV enables blue and ultra violet, opto-electronic applications, while its high thermal conductivity is most desirable for the fabrication of highpower/high-temperature electronic devices. The preparation of single crystals of industrially relevant size is therefore an important task. Due to the high AlN melting point of 2800 °C at 100 atm pressure of N2 [1], the physical vapor transport (PVT) method seems to be the most promising technique to grow AlN. However, there are three major difficulties hindering the reproducible production of large AlN single crystals: (i) PVT growth is affected by contamination issues, as particularly carbon and oxygen have high solubilities in the Al sublattice [1]. These compounds form Al4C3(s) and Al2O3(s), which deposit on the seed surface during the initial growth stages and result in deterioration of crystal quality. When O and C are both present in the growth environment several gas species may form at growth temperature, and both C and O can be incorporated into the growing crystal [1]. Therefore, it is crucial to reduce the C and O concentrations present in the gas phase. The main source of impurities typically is the AlN powder source. One way to reduce these impurities is to sinter the source at high temperature prior to growth. Using this technique AlN crystals containing C and O on the 100 ppm level were reported in the literature [2, 3]. (ii) It is difficult to tailor the thermal gradient inside of the growth chamber. This is due to the need of a desired axial temperature gradient to reduce the amount of gas species arriving at the crystal surface and to allow surface diffusion to take place. Based on Al(g) and N2(g) partial pressures calculation, Dryburgh reported growth rates ranging from 400 µ/h to 1.5 cm/h in the range of 2000-2400 °C at moderate temperature gradients (10-20 °C difference in temperature between source and seed) [4]. Porous graphite is a good candidate as a thermal insulation material as it is easily machined, but cannot be used in direct contact with W crucibles due to the formation of less stable tungsten carbide. Furthermore, N2(g) reacts wit
Data Loading...
