Surface preparation and homoepitaxial deposition of AlN on (0001)-oriented AlN substrates by metalorganic chemical vapor deposition

Abstract

Chemical surface treatments were conducted on mechanically polished (MP) and chemomechanically polished (CMP) (0001)-oriented single crystalline aluminum nitride (AlN) substrates to determine a surface preparation procedure for the homoepitaxial deposition of AlN epitaxial layers by metalorganic chemical vapor deposition. MP AlN substrates characterized by atomic force microscopy exhibited 0.5 nm rms roughness and polishing scratches, while CMP AlN substrates exhibited 0.1 nm rms roughness and were scratch-free. X-ray photoelectron spectroscopy analysis of MP and CMP AlN substrates indicated the presence of a surface hydroxide layer composed of mixed aluminum oxide hydroxide and aluminum trihydroxide. Wet etching with sulfuric and phosphoric acid mixtures reduced the amount of surface hydroxide. Ammonia annealing at 1250 °C converted the substrate hydroxide layer to AlN and increased the rms roughness of MP and CMP AlN substrates to 2.2 nm and 0.2 nm, respectively. AlN epitaxial layers were deposited at 1100–1250 °C under 20 Torr total pressure with a V/III ratio of 180–300 in either N2 or H2 diluent. High-resolution x-ray diffraction measurements revealed that AlN epitaxial layers deposited on MP substrates were strained due to nucleation and coalescence of AlN grains on the mechanically damaged surfaces. AlN deposited on CMP substrates was epitaxial and strain-free. Thermodynamic models for nitridation and AlN deposition were also proposed and evaluated.

Progress on n‐type doping of AlGaN alloys on AlN single crystal substrates for UV optoelectronic applications

“…Improved surface processing will be critical for achieving high quality epitaxy on AlN. A recent study reported by Rice et al describes a procedure by which high quality AlN homoepitaxy can be obtained on properly CMP substrates [6]. Furthermore, studies of strain relaxation in AlGaN alloys grown on AlN are necessary to guide device design and ensure low defect densities are preserved in active heterostructure layers.…”

“…The selected precursors flow yielded a growth rate of 500 nm/hr with no observable pits on the surface. The structural quality of these films have been described elsewhere [6]. The AlGaN was deposited on that layer at a temperature of 1100 ºC by introducing the Ga precursor into the above described gas flow.…”

“…As these native substrates became available, studies of AlN homoepitaxy [3][4][5][6] and AlGaN heteroepitaxy [7] have been reported. AlN possesses high hardness and chemical stability, and forms a stable native hydroxide surface layer upon exposure to ambient air or water, necessitating the development of proper surface polishing and preparation processes in order to enable high quality epitaxy [8].…”

Low-threshold stimulated emission at 249 nm and 256 nm from AlGaN-based multiple-quantum-well lasers grown on sapphire substrates

“…3 The root-mean-square (RMS) surface roughness of the AlN template is less than 0.10 nm and 0.12 nm, respectively, as determined by 1 Â 1 lm 2 and 5 Â 5 lm 2 AFM measurements, which are comparable to bulk AlN substrates. 16 Thus, the AlN template layer used herein provided a smooth surface for subsequent growth of AlGaN-based laser structures.…”

Phonon pressure coefficients and deformation potentials of wurtzite AlN determined by uniaxial pressure-dependent Raman measurements

“…As a result, high structural defect concentrations are commonly observed in heteroepitaxially grown films, seriously degrading the material quality [1,5]. AlN substrates appear as most promising alternative for the continuous evolution of nitride based optical devices towards the deep UV spectral range [6][7][8]. Consequently, not only the growth of AlGaN films with a high AlN concentration becomes feasible, but also ultimately homoepitaxial growth of AlN for device applications comes within reach.…”