The atomic structure of extended defects in GaN
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RODUCTION Due to their direct bandgap, III-V nitrides are excellent candidates for optoelectronic application from red to ultra-violet (1.89 eV : InN, 6.2 eV : AlN). However the fabrication of devices is encountering intrinsic problems which are now motivating a world wide research effort. This has been exponentially increasing for the last ten years since the discovery of Mg for p doping[1]. Light emitting diodes were available from 1993 and laser emission was demonstrated at the end of 1995, all in layers containing up to 1010 cm-2 extended defects[2,3]. The large majority of the extended defects are threading dislocations which originate from the interface with the substrate and mostly cross the whole epitaxial layer[4-6]. Planar boundaries on the { 11 20 } prismatic planes have also been investigated and have led to some controversial reports. They have been characterized as double positioning boundaries[7] stacking mismatch boundaries[8] and inversion domain boundaries[9], as well as stacking faults[10-12]. In this report, we discuss our results on the atomic structure of the extended defects obtained in (Ga, Al) N layers grown by molecular been epitaxy on SiC and sapphire in the light of recent literature. EXPERIMENTAL DETAILS The investigated GaN layers were grown on the (0001) sapphire or 6H-SiC surfaces by electron cyclotron resonance (ECR) or NH3 gas source MBE. HREM experiments were carried out along the [1120] and [0001] GaN directions on a Topcon 002B microscope with a resolution of 0.18 nm (Cs = 0.4 mm).
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CRYSTALLOGRAPHIC CONSIDERATIONS Along the c axis, this misfit is quite small in the case of 6H-SiC, whereas it is close to 20 % on sapphire. Whereas wurtzite is a stacking of two interpenetrating hcp lattices, the 6H packing of SiC is more complex as it is deduced from fcc by twinning every three sequences. Recent work has shown that steps at the (0001) 6HSiC connect terraces by a displacement vector which is either zero, or equal to one of the three stacking fault vectors of the hexagonal lattice[10]. When this vector has a component along [0001], an equivalent translation will exist between islands grown on the adjacent terraces. For growth on (0001) sapphire, the situation is more complex. A simple geometrical analysis shows that even on a flat surface, there are eight possibilities for the growth of a GaN layer. They are either related by displacement vectors and/or an inversion operation[13]. Using 5c GaN ∼ 2c sapphire, were identified four types of steps which lead to residual translations upon growth on adjacent terraces (table 1). The induced translation is better minimized by the formation of inversion domains on top of hexagonal terraces rather than prismatic stacking faults[14,15]. Steps A-A or B-B A-B A-A or B-B A-B
h/c Al
2O 3
unit
nc (n: integer) 1/6, 5/6, 7/6 ,11/6 1/3, 2/3 1/2, 3/2
TR/ cGaN unit
TR/nm
0 ∼ 1/12 ∼ 1/6 ∼ 1/4
0 ∼ 0.0432 ∼0.0863 ∼ 0.1295
Table 1: The 4 values of the residual translation TR due to steps between oxygen terraces of the (0001) surface of sapph
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