Growth of Thick AlN Layer by Hydride Vapor Phase Epitaxy Yu-Huai LIU  , Tomoaki TANABE, Hideto MIYAKE, Kazumasa HIRAMATSU, Tomohiko SHIBATA 1 , Mutsuhiro TANAKA 1 and Yoshihiko MASA 2 Department of Electrical and Electronic Engineering, Mie University, 1515 Kamihama, Tsu 514-8507, Japan 1 NGK Insulators, Ltd., 2-56 Suda-cho, Mizuho-ku, Nagoya, Aichi 467-8507, Japan 2 Sumitomo Metal Mining Co. Ltd., 6-1 Suehiro-cho 1-chome, Ome, Tokyo 198-8601, Japan (Received March 10, 2005; accepted March 23, 2005; published April 8, 2005) Thick AlN crystals were grown by conventional hydride vapor phase epitaxy (HVPE) on AlN/sapphire templates under low pressure (15 Torr) at high temperature (1100  C–1200  C). Colorless, mirror-like AlN films were obtained at the growth rates of up to 20.6 mm/h. The best root mean square (RMS) value of atomic force microscope (AFM) observations for the AlN surface was 2.34 nm. The typical values of full width half maximum (FWHM) of X-ray rocking curves for (0002) and (10  12) diffraction of AlN films were 173–314 arcsec and 1574–1905 arcsec, respectively. We also investigated the influences of carrier gas, growth temperature and growth rate on the crystal quality. [DOI: 10.1143/JJAP.44.L505] KEYWORDS: AlN, hydride vapor phase epitaxy, HVPE, carrier gas, growth temperature AlN is a promising substrate material for fabricating deep UV emitters and power devices due to its wide bandgap, high thermal conductivity, and excellent insulating proper- ties. Although thin AlN films have been grown on sapphire substrate by metal organic vapor phase expitaxy (MOVPE), 1,2) it is not practical to grow thick AlN films by this method due to the low growth rate. As recently reported, 3) sublimation method can be used to make bulk AlN crystals, but it is difficult to control the size and the shape of AlN crystals. Also, with liquid phase epitaxy, the growth of AlN crystals is hindered by the low nitrogen solubility in Al-melt. 4) Since hydride vapor phase epitaxy (HVPE) has already been proven to be a highly efficient method for producing bulk GaN crystals, 5) it should be feasible to grow AlN by HVPE. Moreover, a thermodynamic simulation and growth experiment of HVPE with a source zone temperature at 500  C produced mainly AlCl 3 , not AlCl, a chemical that is dangerous to quartz reactor and chamber. 6) However, only a few groups have studied AlN growth by HVPE, 6–8) and in these cases, sapphire and SiC are mainly used as a substrate. In addition, the effects of growth parameters, such as temperature and growth rate, have not been fully inves- tigated. In this letter, we will describe our HVPE experi- ments on the growth of AlN crystals on AlN/sapphire templates. Smooth, transparent, and colorless AlN crystals with a mirror-like surface were obtained. We will also describe our findings on the crystal quality for various carrier gas compositions, growth temperatures, and growth rates. AlN epitaxial films were grown using conventional HVPE with a horizontal hot-wall quartz flow reactor. AlN/sapphire templates with a 1-mm thick MOVPE-grown AlN layer were used as substrates. 1) The source materials include Al metal, HCl and NH 3 . The HCl line (20% HCl þ 80% H 2 ) flow rate was varied from 2 to 50 sccm and the NH 3 flow rate was fixed at 10 sccm. The carrier gas was H 2 and/or N 2 with flow rate of 200 sccm for HCl line and 1.5 slm for NH 3 , respectively. The growth pressure was 15 Torr and the growth temperature was varied from 1100 to 1200  C. The growth rate was adjusted from 3.6 mm/hr to 31.6 mm/hr by controlling the flow rate of HCl line. For all runs, the growth time was 30 min. To characterize the as-grown AlN films, we use scanning electron microscopy (SEM), an optical microscope, an atomic force microscope (AFM), and X-ray rocking curve (XRC). Using our HVPE system, we determined the growth conditions for clear, transparent AlN films with mirror-like surface and for opaque AlN films. These conditions are listed in Fig. 1, where hollow marks represent conditions for transparent films while solid marks illustrate conditions for opaque films. Different mark shapes correspond to various cases of carrier gas composition: case 1) N 2 (circles), case 2) N 2 +H 2 (spades), and case 3) H 2 (a cross). It should be noted that in case 1), the N 2 compositions are 97.7–99.9% since HCl line was diluted with 80% H 2 . From Fig. 1, it can be seen that AlN films with mirror-like surface can be obtained at the higher growth temperatures even with a fast growth rate using case 1) or 2), for example, 20.6 mm/h at 1150  C, which is approximately 2 times the growth rate in ref. 8 and 5 times that for mirror-like AlN by chemical vapor reaction process reported recently. 9) From microscope 1100 1125 1150 1175 1200 0 5 10 15 20 25 30 35 , :N 2 , :N 2 +H 2 :H 2 Hollow: Transparent Solid: Opaque Growth Rate (μm/h) Growth Temperature (°C) Fig. 1. Growth conditions for transparent AlN films (solid marks) and for opaque AlN films (hollow marks), with different cases of carrier gas compositions. case 1) N 2 carrier gas (circles), case 2) N 2 þ H 2 carrier gas (spades), and case 3) H 2 carrier gas (a cross).  E-mail address: yhliu@elec.mie-u.ac.jp Japanese Journal of Applied Physics Vol. 44, No. 17, 2005, pp. L 505–L 507 #2005 The Japan Society of Applied Physics L 505 JJAP Express Letter