Two-Dimensional Growth Mode and Reduction of Dislocation Density in Nitride Layers
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1040-Q07-01
Two-Dimensional Growth Mode and Reduction of Dislocation Density in Nitride Layers Krzysztof Pakula1, Jacek M. Baranowski1, and Jolanta Borysiuk2 1 Institute of Experimental Physics, University of Warsaw, Hoza 69, Warsaw, 00-681, Poland 2 Institute of Electronic Materials Technology, Wolczynska 133, Warsaw, 01-919, Poland ABSTRACT Heteroepitaxial two-dimensional (2D) and three-dimensional (3D) MOVPE growth of nitride layers on sapphire substrates using low temperature (LT) AlN nucleation layer are discussed. It is shown by Atomic Force Microscopy and Transmission Electron Microscopy that the 2D or 3D growth mode of successive AlGaN layers depends predominantly on the growth conditions of the underneath LT AlN nucleation layer. Modifications of growth conditions of LT AlN nucleation layer in respect of the growth in the conventional way, leads to a drastic change in properties of the successively grown at high temperature AlGaN layer. Main modification is connected with a drastic reduction of the growth rate of LT AlN nucleation layer at higher rector pressure. In this case the growth at high temperature starts evenly on the whole surface, retaining atomic flatness, and occurs in the 2D mode. Therefore, it is possible to grow even a very thin, continuous AlGaN layers directly on top of LT nucleation layer. The 2D growth mode leads to significant change in extended defect distribution. Edge dislocation density being close to 1010 cm-2, results in the effective relaxation of the internal stress. On the other hand mix dislocations density are close to 108 cm-2. All observed mix dislocations occur in pairs with dislocations with opposite sign. This suggests that all mix dislocations originate from dislocation half-loops. Both types of dislocations, edge and half loops can be effectively reduce in-situ, during the growth using annealing in the presence of silane . This process and precise control of the growth conditions leads to decreasing of mix dislocations density to 2·106 cm-2 and edge dislocations density to 5·108 cm-2. The newly developed two-dimensional growth mode of nitride layer opens up a new way to reduce strain, decrease density of dislocations and improvement of crystallographic properties. INTRODUCTION History of nitrides technology begins in 1970's, but most important step was accomplished at late 1980's by group led by Isamu Akasaki at Nagoya University. Using MOVPE technology they developed the method of so-called “low temperature buffer layer”. According to original paper [1] it can be distinguish four stages in GaN layer formation process. First, the thin, low temperature (LT) AlN is grown at about 700ºC, giving polycrystalline layer thoroughly covering whole sapphire substrate. After increasing temperature above 1000ºC, the growth of main, GaN layer is started. This secondary nucleation has place only in distinctly separated sites leading to formation of three-dimensional (3D) islands. Through the lateral growth islands can coalesce and form continuous layer. From that moment growth can
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