Microstructural Study of GaN Grown on Sapphire by Mocvd
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quality. In this study we have grown samples with a 200 A AIN buffer and observed that good quality single crystal GaN is still obtained. The quality of the material was determined by cross-section Transmission Electron Microscopy (TEM). Crystallographic orientation relationships between GaN, AIN, and sapphire were determined by electron diffraction patterns. RESULTS Figure 1 is a bright field micrograph taken at the GaN zone axis [0110] with the [0002] reflecting plane vector. The AIN thickness is measured to be 200 A and the GaN thickness is 2.2 pm. The complete structure is visible.
Figure 1 Bright field micrograph taken at the zone axis [01101. The complete structure is seen with a 200 A AIN buffer and the 2.2 gm GaN layer. A number of threading dislocations are evident with line directions of [00011.
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There are a large number of threading dislocations with line directions of [0001]. The threading dislocation density as determined from four different areas is -108 #/cm 2 . These threading dislocations originate at the interface between AIN and sapphire and thread across the GaN cap layer. The invisibility criteria (g.b=0, zero contrast) was not satisfied for these threading dislocations for the reflecting plane vectors of [0002], [2110], and [2112]. Hence, the Burger's vector cannot be ascertained in this zone axis. The sapphire gives a darker contrast because it is thicker compared to GaN. Dislocation densities are comparable to those with reported 500 A buffers (estimated from published micrographsl). Figure 2 a is an electron diffraction pattern taken at the AIN region. The thickness of AIN was only 200 A, therefore it was expected that spots would be evident from all three regions i.e. the sapphire substrate, AIN buffer and the GaN cap layer. None of the observed spots could be attributed solely to AIN. The basal lattice constant of AIN (aAIN= 3 .1114 A) is very close to the basal lattice constant of GaN (aGaN= 3 .18 6 A). If GaN and AIN unit cells are conformal i.e. all the corresponding planes and directions are parallel, the spots from AIN in the electron diffraction pattern would be expected to be very close to the spots from GaN. Therefore the pattern is indexed with the following crystallographic orientation relationship between sapphire, AIN and GaN: (000 2 )GaN // (0002)A1N // (000 6 )Sapphire [01I_0]GaN // [0110]AIN // [1210]Sapphire
This crystallographic relationship is illustrated in Fig. 3, with the resultant 30* twist of the GaN hexagonal unit cell with respect to the sapphire unit cell. Figure 2 b is a pattern generated by "Diffract® software". The orientation used was the same as above and the pattern generated (Fig. 2 b) matches the one obtained (Fig. 2 a) by TEM, confirming the orientation relationship between GaN and sapphire. Figure 4 is a high resolution micrograph of the interface between AIN and Sapphire in which the lattice fringes are readily visible. Note that differentiating between GaN and AIN is very difficult since their lattice constants and symmetry are very similar.
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