Specific features of the surface morphology of modified AlN/sapphire substrates fabricated by thermochemical nitridation
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ACE AND THIN FILMS
Specific Features of the Surface Morphology of Modified AlN/Sapphire Substrates Fabricated by Thermochemical Nitridation S. I. Kryvonogova, A. A. Krukhmaleva, S. V. Nizhankovskyia, N. S. Sidelnikovaa, E. A. Vovka, A. T. Budnikova, G. T. Adonkina, and A. E. Muslimovb a
Institute for Single Crystals, National Academy of Sciences of Ukraine, Kharkov, 61001 Ukraine email: [email protected] b Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskii pr. 59, Moscow, 119333 Russia Received April 25, 2014
Abstract—The influence of the reducing annealing of sapphire substrate during the thermochemical nitrida tion of sapphire on the structure of the nitride layer in the AlN/sapphire template has been investigated. Iso thermal annealings of (0001) and (10 12) sapphire substrates have been performed at the nitridation temper ature (1450°C) and different reducing potentials of the gas medium. It is found that the initial terracestep structure of substrates is transformed during annealing as a result of the diffusion and thermochemical etching of sapphire substrates; this process is accompanied by the fragmentation and merging of steps and by an increase in their width and height. The degree of stepstructure distortion increases with an increase in the reducing potential of the gas medium and the annealing time. Hexagonal etching figures are observed for (0001) sapphire substrates. It is established that the AlN/sapphire template surface relief is formed mainly in the initial stage during thermochemical etching of the stating substrate, which precedes the formation of a nitride layer. The sapphire–nitride transformation does not lead to a further increase in the surface roughness. DOI: 10.1134/S1063774515010125
INTRODUCTION Sapphire is one of the most widespread substrate materials for the mass production of devices based on nitrides of thirdgroup metals (GaN, InN, and AlN) and their solid solutions [1–4]. However, the hete roepitaxy of nitrides on sapphire substrates leads to a high density of dislocations and causes other structural defects because of the lattice mismatch and difference in thermalexpansion coefficients [1, 4, 5]. This cir cumstance significantly complicates the fabrication of heterostructures with high functional characteristics. Since the preparation of substrates for homoepitaxial growth is expensive for mass production, the interest of researchers to modified sapphire substrates with a buffer nitride layer (GaN/sapphire or AlN/sapphire templates), which play a role of intrinsic nitride quasi substrate, constantly increases [6–8]. We developed a new method for preparing AlN/sapphire templates by nitridizing sapphire in a gas mixture of N2, CO, СО2, Н2, and Н2О with low СО2 and Н2О concentrations [9]. This method makes it possible to obtain polar (AlN(( 0001) //Al2O3 ( 0001) , AlN( 0001) //Al2O3 (11 20) ), semipolar (AlN(10 13) //Al2O3 ( 0001) ), and nonpolar (AlN(11 20) //Al2O3 (10 12) ) AlN crystalline layers with thicknesses from 20 nm to several mic
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