Mg related Defect Formation during MOVPE Growth of GaN based Films studied by Transmission Electron Microscopy
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Mg related Defect Formation during MOVPE Growth of GaN based Films studied by Transmission Electron Microscopy Roland Kröger, Stephan Figge, Tim Böttcher, Peter L. Ryder, and Detlef Hommel Institute of Solid State Physics, University of Bremen Kufsteiner Str., 28334 Bremen, Germany
ABSTRACT P-type active incorporation of magnesium into the gallium nitride lattice is still a challenge for the realization of wide band gap light emitters. This work presents a detailed microstructural study of phenomena connected with the problem of Mg doping of films grown by metal organic vapor phase epitaxy. Transmission Electron Microscopy both in conventional and in high resolution mode are used to obtain a better understanding of the Mg related defect formation processes. It is shown that the defect formation never occurs instantaneously at the beginning of the doping but a defect free zone always precedes the onset of the defect formation. This effect cannot solely be addressed to the memory effect of the reaction chamber since it is found that the defect density oscillates during growth. A more detailed investigation reveals that segregation of Mg plays a crucial role in the formation of defects. The observations emphasize the fact that a critical surface concentration is necessary for the defects to form. Beside the known processes of inversion boundary formation it is found that small nitrogen polar GaN grains in GaN films are gradually increasing on the expense of Ga polar grains if a sufficiently high Mg coverage and film thickness is realized. This process is found to occur in alternating steps along the {1123} and {0001} planes.
INTRODUCTION The developing capability of using GaN based heterostructures for light emitting devices requires a significant optimization of the p-type doping. The most suitable candidate for this purpose is magnesium. But Mg is found to form a number of defects, which are important obstacles for its application in opto-electronic systems. It is known that Mg leads to the formation of inversion domain boundaries (IDB) by switching the polarity from Ga- to N- polar if a surface coverage of about one monolayer is reached [1]. Furthermore it is known that Mg is not always continuously incorporated into the lattice but tends to form pyramidal defects and inversion domain boundaries [2]. These defects passivate a large amount of Mg and are possibly responsible for the blue photoluminescence transition detected for films with high Mg content [3]. It is already necessary to incorporate two orders of magnitude higher Mg concentrations into the film compared to the actual hole concentration [4]. Thus, it is crucial to fully understand the mechanism of the Mg related defect formation. It seems that there is only a small parameter window in which the pyramidal defect formation can be avoided. This restricts the maximum room temperature free hole carrier concentration at the time being to about 1018 cm-3 [4]. Hence, a deeper understanding of the Mg-incorporation is required to control the electrical properti
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