Understanding the Pyramidal Growth of GaN
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our knowledge, they have never been really understood. Thanks to a combination of TEM techniques we were able to determine the structure of these macroscopic pyramids and were able to understand their growth mechanism. EXPERIMENT We investigated GaN layers grown by MOCVD on (0001) A1203 substrates at temperature varying from 900°C to 1150'C, with or without a buffer layer of AIN or GaN [3]. In this communication, we mainly focus on the TEM results of samples that exhibit macroscopic hexagonal pyramids at their surface (fig. la). Such samples were obtained when we tried to optimise the buffer layer. For these samples, the average full width at half maximum (FWHM) for (0004) beam of X-ray rocking curves is about 18 minutes. Specimens for TEM were prepared using the standard techniques: mechanical polishing and Argon ion milling. Conventional and HREM TEM observations were realised on a JEOL4000EX electron microscope, specially equipped for HREM (Scherzer resolution about 0.17nm). CBED patterns were obtained with a JEOL2010 field emission gun microscope. CBED simulations were made with the EMS software [4]. RESULTS State of the art GaN multilayers contain a large density of defects, which are mostly dislocations [5]. However, the characteristic of layers containing pyramids is that they contain both linear defects such as dislocations and planar defects (fig. 1). Such planar defects have 393 Mat. Res. Soc. Symp. Proc. Vol. 395 @1996 Materials Research Society
Fig. 1: a) Image of the surface of a GaN layer (Tgrowth= 1150 0 C, GaN buffer layer) made by an optical microscope. Hexagonal pyramids are clearly visible. b) Two-beam image of a GaN layer containing the same kind of pyramids (Tgrowth=1000*C, no buffer layer) : the layers mainly contains dislocations and b*-planar defects (the planar defects on the b*= plane form in fact hexagonal chimneys) c) Two beam image of the previous sample (viewing direction: b*=[0,1,-1,0]). The top part of the GaN layer is seen. Here we have the chance to cut an hexagonal macroscopic pyramid at its apex. The centre of the macroscopic pyramid contains an hexagonal chimney which has an hexagonal nucleus in top of it. been commonly reported [6][7]. The most frequent ones are on the c-basal plane and on the {0,1,- 1,0} planes (denoted b*-planar defects as the normal of the defect plane is parallel to the basis vector b*=[0,1,- ,0] of the reciprocal space), but some defects on the 12,-4,-4,01 planes (denoted in a similar way, a-planar defects) have also been observed and resolved [6]. As the aplanar defects have been observed only on few samples we refer to reference [6] for a better presentation of these defects. Here, we focus our observations on the b*-planar defect and its relation to the pyramids. Observation of these samples in three perpendicular directions (fig. 1c, 2a, 2b) enables to determine that the b*-planar defects, associated by group of six, form tiny hexagonal chimneys (up to 75nm in "diameter"). They should be no confusion with nanotubes that people have reported [8] : our
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