Undoped and doped GaN thin films deposited on high-temperature monocrystalline AlN buffer layers on vicinal and on-axis
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Undoped and doped GaN thin films deposited on high-temperature monocrystalline AlN buffer layers on vicinal and on-axis a (6H)–SiC(0001) substrates via organometallic vapor phase epitaxy T. Warren Weeks, Jr., Michael D. Bremser, K. Shawn Ailey, Eric Carlson, William G. Perry, Edwin L. Piner, Nadia A. El-Masry, and Robert F. Davis Department of Materials Science and Engineering, North Carolina State University, Box 7907, Raleigh, North Carolina 27695-7907 (Received 10 July 1995; accepted 3 January 1996)
Monocrystalline GaN(0001) thin films have been grown at 950 ±C on hightemperature, ø100 nm thick, monocrystalline AlN(0001) buffer layers predeposited at 1100 ±C on a(6H)–SiC(0001) Si substrates via OMVPE in a cold-wall, vertical, pancake-style reactor. These films were free of low-angle grain boundaries and the associated oriented domain microstructure. The PL spectra of the GaN films deposited on both vicinal and on-axis substrates revealed strong bound excitonic emission with a FWHM value of 4 meV. The near band-edge emission from films on the vicinal substrates was shifted slightly to a lower energy, indicative of films containing residual tensile stresses. A peak attributed to free excitonic emission was also clearly observed in the on-axis spectrum. Undoped films were too resistive for accurate Hall-effect measurements. Controlled n-type, Si-doping in GaN was achieved for net carrier concentrations ranging from approximately 1 3 1017 cm 23 to 1 3 1020 cm 23 . Mg-doped, p-type GaN was achieved with nA –nD ø 3 3 1017 cm23 , r ø 7 V ? cm, and m ø 3 cm2yV ? s. Double-crystal x-ray rocking curve measurements for simultaneously deposited 1.4 mm GaN films revealed FWHM values of 58 and 151 arcsec for deposition on on-axis and off-axis 6H–SiC(0001)Si substrates, respectively. The corresponding FWHM values for the AlN buffer layers were approximately 200 and 400 arcsec, respectively.
I. INTRODUCTION
Recent research regarding II-VI compound semiconductors and device structures has culminated in the successful fabrication of the first blue-green1 and blue2 injection laser diodes (LD’s) and high-efficiency blue3 light-emitting diodes (LED’s). Comparatively, the direct band gap III-V nitrides possess greater physical hardness, much larger heterojunction offsets, higher melting temperatures, and higher thermal conductivities.4 GaN (wurtzite structure), the most studied of the III-V nitrides, has a room temperature band gap of 3.39 eV and forms continuous solid solutions with both AlN (6.28 eV) and InN (1.95 eV). As such, materials with engineered band gaps are feasible for optoelectronic devices tunable in wavelength from the visible (600 nm) to the deep UV (200 nm). The relatively strong atomic bonding and wide band gaps of these materials also point to their potential use in high-power and high-temperature microelectronic devices. Specific applications for these wide band gap semiconductors include UV, blue and blue-green light emitting diodes, UV photodetectors, short-wavele
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