Study on Chemical Treatment and High Temperature Nitridation of Sapphire for III-Nitride Heteroepitaxial Growth
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Study on Chemical Treatment and High Temperature Nitridation of Sapphire for III-Nitride Heteroepitaxial Growth F. Dwikusuma1, D. Saulys2, and T. F. Kuech1 1 Department of Chemical Engineering, University of Wisconsin, Madison, WI 53706 2 Materials Research Science and Engineering Center, University of Wisconsin, Madison, WI 53706 ABSTRACT We have systematically studied the effects of wet chemical etching and high temperature nitridation on the resulting sapphire surface morphology and chemical transformation. The etching of c-plane sapphire substrates using H2SO4, H3PO4, and a 3:1 H2SO4 :H3PO4 mixture as a function of temperature and etching time was studied and compared with H2 etching at 1100°C and air-annealing at 1400°C. The surface nitridation using NH3 and N2 at 1100oC was studied as a function of NH3 concentration, nitridation time, and surface pretreatment. Atomic force microscopy and x-ray photoelectron spectroscopy were used to study the surface morphology and chemical composition. The detailed surface morphology after chemical etching was a function of the chemical composition and the specific time and temperatures. The smoothest, pitfree sapphire surface was obtained by etching in pure H2SO4 at 300oC for 30 min. Sulfuric acid etching at higher temperatures or for longer periods generated an insoluble mixture of Al2(SO4)3 and Al2(SO4)3·17H2O crystalline deposits on the surface. Phosphoric acid and the 3:1 H2SO4:H3PO4 mixture etched the sapphire preferentially at defect sites and resulted in pits formation on the surface. The high temperature sapphire nitridation resulted in nitrogen incorporation into the surface. The nitrogen content of nitridation layer depends on NH3 concentration, nitridation time, and surface pretreatment. The nitrogen contents of sapphire treated with H2SO4 and 3:1 H2SO4:H3PO4 are about the same as the as-received sapphire. While the nitrogen content of the air-annealed sapphire is ~1.6 times higher then the nitrogen content of the as-received sapphire.
INTRODUCTION Sapphire (α-Al2O3) has been the most widely used substrate for GaN growth despite of having ~16% lattice-mismatch and ~34% thermal expansion coefficient mismatch with GaN. Prior to GaN growth, the chemical treatment of sapphire has been used to remove residual polishing damages1 and surface nitridation has been used to improve film nucleation.2 A routinely employed chemical treatment is etching using a 3:1 H2SO4:H3PO4 mixture and H2 at ~1100oC. Another approach to sapphire surface preparation is to anneal the sapphire in air at high temperature (≥1000oC).3 The chemical and physical states of sapphire surface after these chemical treatments have not been studied in detail despite their widespread use. It is now well established that nitridation of sapphire surface prior to GaN growth can improve the material properties.2 Although there has been a general consensus on all sapphire nitridation studies that nitrogen was incorporated into the sapphire surface layer, the compound containing nitrogen formed upon nitri
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