Threading Dislocation Density Reduction in GaN/Sapphire Heterostructures
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INTRODUCTION GaN and other wide band group III-nitride semiconductors have been recently recognized as very important materials for the fabrication of optoelectronic devices emitting in the green - ultraviolet range. Moreover these materials have a large potential for electronic devices capable of operation under high-power, high- frequency and high-temperature conditions due to their superior physical properties. Important efforts have been made to improve the growth of these materials. The large differences in lattice parameters (16%) and thermal expansion coefficients between GaN and sapphire are the common reason for generation of very high concentration extended defects. Threading dislocations are the most common line defects observed for the films grown on hexagonal sapphire and SiC (0001) substrates. These dislocations are to found to originate at the substrate interface and caused by the inversion domain boundaries, stacking mismatch boundaries and double positioning boundaries [1,2]. Although GaN provides highly efficient devices the concentration of threading dislocations is usually very high (109-1010 cm-2). The lowest densities of these
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defects lie in the mid 108 cm-2 range [3,4]. It is necessary to decrease the concentration of threading dislocations for further improvement of laser diode. I addition to relaxation of lattice structure, the generation of planar type of extended defects may terminate threading dislocations and prevent their propagation through active layers. It is well known that the quality of final structure strongly depends on initial stages of growth. Several initial MOCVD growth conditions commonly used for GaN layer grown on $O2O3 substrate include: (1) high temperature nitridation of the sapphire surface; (2) deposition of low- temperature (600 oC) buffer layer followed by a high temperature (10501080 oC) epilayer growth; (3) low temperature AlN buffer growth followed by the high temperature GaN growth. The optimal growth temperature for the case of pulsed layer deposition of GaN is lower (720- 800 oC) owing to higher energy of grown species. The high average kinetic energy along with pulsed mode of laser ablation plasma is responsible for high quality films at lower substrate temperature. In the case of laser ablation, it is in principle possible to transfer the composition from the stoichiometric target to the substrate. In general, raising the growth temperature is effective in improving the crystallinity of epitaxial layers unless it causes thermal decomposition of the crystal [5]. In this investigation we have adopted two types of growth conditions: (1) pulsed layer deposition of "high- temperature" (850 oC) buffer layer and (2) "high- temperature" (850 o C) AlN buffer growth followed by the low temperature growth (750 oC) of GaN. The growth at high-temperature results in the formation of stacking faults that can buffer threading dislocations generated at optimized (stoichiometric) low temperature growth. We have investigated defect reduction as a function
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