Kazuya Takada

Nitridation behavior of sapphire using a carbon-saturated N2–CO gas mixture

The authors previously developed a sapphire nitridation method using carbon-saturated N2–CO gas mixture to form a high-quality AlN film for III-nitride-based optoelectronic devices. In this study, the nitridation behavior of (0001) (c) plane and (112¯0) (a) plane sapphire was studied to elucidate and optimize the process at temperatures of 1823 and 1873 K. The AlN film thickness, surface morphology, crystal quality, and interfacial phenomena were investigated as functions of nitridation time and temperature. Fundamentally, the AlN film grows as a result of the diffusion process that occurs in the AlN film. The voids found at the AlN/sapphire interface indicate that the Al2O3 dissociates into Al3+ and O2− ions, and that the ions diffuse in the AlN film. However, the growth rate of AlN film does not obey the simple diffusion model. The AlN film thickness has a maximum and decreases slightly with time, which indicates that the thermal decomposition of AlN film must be considered when comprehensively describing the nitridation process.The authors previously developed a sapphire nitridation method using carbon-saturated N2–CO gas mixture to form a high-quality AlN film for III-nitride-based optoelectronic devices. In this study, the nitridation behavior of (0001) (c) plane and (112¯0) (a) plane sapphire was studied to elucidate and optimize the process at temperatures of 1823 and 1873 K. The AlN film thickness, surface morphology, crystal quality, and interfacial phenomena were investigated as functions of nitridation time and temperature. Fundamentally, the AlN film grows as a result of the diffusion process that occurs in the AlN film. The voids found at the AlN/sapphire interface indicate that the Al2O3 dissociates into Al3+ and O2− ions, and that the ions diffuse in the AlN film. However, the growth rate of AlN film does not obey the simple diffusion model. The AlN film thickness has a maximum and decreases slightly with time, which indicates that the thermal decomposition of AlN film must be considered when comprehensively describi...

Self-Separation of a Thick AlN Layer from a Sapphire Substrate via Interfacial Voids Formed by the Decomposition of Sapphire

A technique for separating a thick AlN layer grown by hydride vapor phase epitaxy (HVPE) on a (0001) sapphire substrate was developed. By heat treatment at 1450 °C in a gas flow containing H2 and NH3, many voids could be formed at the interface between a thin (100 nm) AlN layer grown at 1065 °C and the sapphire substrate due to the preferential decomposition of sapphire. During the cooling process after the subsequent growth of a thick (85 µm) AlN layer, the thick AlN layer separated from the sapphire substrate with the aid of the interfacial voids. The freestanding AlN substrate thus obtained had a smooth (0001) surface, a dislocation density of 1.1×109 cm-2, and an optical transparency for wavelengths above 208.1 nm.

Carrier Gas Dependence at Initial Processes for a-Plane AlN Growth on r-Plane Sapphire Substrates by Hydride Vapor Phase Epitaxy

The influence of the carrier gas used during the thermal cleaning of r-plane sapphire substrates and the subsequent first AlN layer growth at 1050 °C on two-step growth of a-plane AlN layers by hydride vapor phase epitaxy (HVPE) was investigated. When hydrogen (H2) was used as the carrier gas, the decomposition of r-plane sapphire occurred during the thermal cleaning, and unintentional nitridation of the sapphire surface occurred at the beginning of the growth of the first AlN layer, which resulted in the occurrence of misoriented AlN grains in the second AlN layer grown at 1450 °C. When a mixture of H2 and nitrogen (N2) was used as the carrier gas, nitridation of the sapphire surface occurred during the thermal cleaning, which also resulted in the occurrence of misoriented AlN grains. A single-crystalline a-plane AlN layer free of misoriented grains could be obtained by using only N2 as the carrier gas during the thermal cleaning and the growth of the first AlN layer to prevent nitridation of the sapphire surface.

Formation mechanism of AlN whiskers on sapphire surfaces heat-treated in a mixed flow of H2 and N2

The formation mechanism of AlN whiskers on sapphire substrates during heat treatment in a mixed flow of H2 and N2 was investigated in the temperature range of 980–1380 °C. AlN whiskers grew above 1030 °C after covering the sapphire surface with a thin AlN layer. The existence of pits on the sapphire surface beneath the thin AlN layer was observed. Both AlN whisker and pit densities of samples were on the same order of 108 cm−2. These results suggested the following mechanism. First, the sapphire surface reacts with H2, and the generated Al gas reacts with N2 to form a thin AlN layer on sapphire. Then, the sapphire surface reacts with H2 diffusing to the AlN/sapphire interface. The Al gas escapes through dislocations in the AlN layer to leave pits on the sapphire surface, and finally reacts with N2 to form AlN whiskers on the top surface.