Can Laterally Overgrown GaN Layers be free of Structural Defects?
- PDF / 1,134,233 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 49 Downloads / 232 Views
Can Laterally Overgrown GaN Layers be free of Structural Defects? D. Cherns and Z. Liliental-Weber* H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK *permanent address: Lawrence Berkeley Laboratory, Materials Science Division, Berkeley, CA 94720, USA Abstract Transmission electron microscopy has been used to examine dislocations present in an epitaxial laterally overgrown (ELOG) sample of GaN grown on (0001)sapphire. Studies of both plan-view and cross-sectional samples revealed arrays of dislocations present in the (11-20) boundary between the seed and the wing (overgrown) material and at the meeting front between adjacent wings, as well as dislocations in the form of half-loops extending into the wing regions. Both the boundary and half-loop dislocations had 1/3 Burgers vectors which were either perpendicular (boundary dislocations) or at 30° (half-loops) to the boundary plane. Large angle convergent beam electron diffraction was used to show that the boundary dislocations and halfloops correlated respectively with tilts and twists of the wing material about (11-20). A model is proposed whereby the half-loops are generated from threading dislocations by shear stresses acting along the stripe direction. The origin, and elimination, of these stresses is discussed. Introduction The epitaxial lateral overgrowth (ELOG) method was introduced in order to reduce the density of threading dislocations generated in (0001)GaN films grown on highly latticemismatched substrates such as (0001)sapphire. In the ELOG method, a mask, such as SiO2, is deposited on a part-grown GaN layer. On resuming the growth, GaN does not nucleate on the mask material. Growth then proceeds laterally from seed columns of GaN which grow through gaps etched into the mask. Eventually, the GaN “wings” meet to produce a continuous film. Provided threading dislocations continue to propagate in the seed material close to the c-axis as in conventional GaN growth, the wing regions should be substantially dislocation-free. It is now clear that this simple model of ELOG growth does hold entirely, with significant, if lower, densities of dislocations reported for the wing material. Estimates vary widely, from, for example, below 106 cm-2 [1] to nearly 108 cm-2 [2], with dislocation densities in the seed material in the range 109 – 1010 cm-2. In addition, some observations show significant bending of the wings, which may lead to the generation of new defects [3]. Such behaviour may result from the high stresses generated by thermal mismatch between the GaN and the mask material [4]. In this paper, we use transmission electron microscopy (TEM) and large angle convergent beam electron diffraction (LACBED) to correlate the presence of dislocations in the wing regions of an ELOG sample with measurements of the local strain. A new mechanism of dislocation propagation in the wing material is proposed which is driven by shear stresses and involves both glide and climb. The implications of the results for reducing dislocation den
Data Loading...
