Only a few years ago, an account of degradation of silicon carbide high-voltage p-i-n diodes was presented at the European Conference on Silicon Carbide and Related Compounds (Kloster Banz, Germany, 2000). This report was followed by the intense effort of multiple groups utilizing varied approaches and subsequent progress in both fundamental understanding of this phenomenon and its elimination. The degradation of SiC p-i-n junctions is now well documented to be due to the expansion of Shockley-type stacking faults in the part of the devices reached by the electron-hole plasma. The faults can gradually cover most of the junction area, impeding current flow and, as a result, increasing the on-state resistance. While in most semiconductors stacking faults are electrically inactive, in hexagonal silicon carbide polytypes (4H- and 6H-SiC) they form quantum-well-like electron states observed in luminescence and confirmed by first-principles calculations. The stacking-fault expansion occurs via motion of 30° silicon-core partial dislocations. The Si–Si bond along the dislocation line induces a deep level in the SiC band gap. This state serves as both a radiative and a nonradiative recombination center and converts the electron-hole recombination energy into activation energy for the dislocation motion. Dislocation motion is typically caused by shear stress, but in the case of SiC diodes, the driving force appears to be intrinsic to the material or to the fault itself, i.e., the fault expansion appears to lower the energy of the system. Stable devices can be fabricated by eliminating stacking-fault nucleation sites. The dominant type of such preexisting defects is the segment of basal plane dislocations dissociated into partials. The density of such defects can be reduced to below 1cm−2 by conversion of all basal plane dislocations propagating from the substrate into threading ones in the epitaxial layer. Remarkable progress in fabrication of low basal plane dislocation density material offers hope of bipolar SiC devices being available commercially in the near future.

We identify two categories of reconstructions occurring on wurtzite GaN surfaces, the first associated with the N-face, (000), and the second associated with the Ga-face, (0001). Not only do these two categories of reconstructions have completely different symmetries, but they also have different temperature dependence. It is thus demonstrated that surface reconstructions can be used to identify lattice polarity. Confirmation of the polarity assignment is provided by polarity-selective wet chemical etching of these surfaces. The potential applications for blue light emitting devices continue to drive research efforts to understand the growth of GaN. In the fabrication of most nitride-based devices, epitaxial growth occurs on the c-plane of wurtzite GaN. A key characteristic of wurtzite GaN is its polarity. No symmetry operation of the crystal relates the [0001] to the [000 ] direction, and so the (0001) and (000) surfaces are inequivalent. The former surface is known as the Ga-face, and the latter as the N-face. While the atomistic details of surface structure are known to govern growth kinetics, little has been understood, until recently, about the surface structures of wurtzite GaN. Remarkably, a few groups have reported the inability to observe any surface reconstructions at all on wurtzite GaN other than a 1×1.[1,2] At the same time, a number of other groups have reported a variety of reflection high energy electron diffraction (RHEED) patterns, including 1×1, 2×1, 2×2, 2×3, 3×2, 3×3, 4×4, and 5×5.[3-9] However, the polarities of the surfaces which gave these diffraction patterns were unknown.

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Microstructure of α-GaN films grown by organometallic vapor phase epitaxy on sapphire substrates using low temperature AlN (or GaN) buffer layers has been studied by transmission electron microscopy. The defects which penetrate the GaN films are predominantly perfect edge dislocations with Burgers vectors of the 1/3〈112̄0〉 type, lying along the [0001] growth direction. The main sources of threading dislocations are the low angle grain boundaries, formed during coalescence of islands at the initial stages of GaN growth. The grain sizes range from 50 to 500 nm, with in-plane misorientations of less than 3°. The nature of these threading dislocations suggests that the defect density would not likely decrease appreciably at increasing film thickness, and the suppression of these dislocations could be more difficult.

Structural investigations of organometallic vapor phase epitaxy grown ␣-GaN films using high-resolution transmission electron microscopy and scanning force microscopy have revealed the presence of tunnel-like defects with 35-500 Å radii that are aligned along the growth direction of the crystal and penetrate the entire epilayer. These defects, which are termed ''nanopipes,'' terminate on the free surface of the film at the centers of hexagonal growth hillocks and form craters with 600-1000 Å radii. Either one or two pairs of monolayer-height spiral steps were observed to emerge from the surface craters which allowed us to conclude that nanopipes are the open cores of screw dislocations. The measured dimensions of the defects are compared to Frank's theory for the open-core dislocation. © 1995 American Institute of Physics.Gallium nitride and its related alloys ͑AlGaN and InGaN͒ are important wide band-gap semiconductors that have potential applications in both short-wavelength optoelectronic and high power/high frequency devices. 1 It is widely accepted that both the external efficiency and reliability of light emitting devices are sensitive to the type and density of extended defects in the material. Nitride films deposited on sapphire, which is poorly matched to GaN both in terms of lattice parameter and thermal expansion coefficient, typically exhibit dislocation densities in the 10 10 cm Ϫ2 range. 2,3 Most of these dislocations are of pure edge type forming low angle ''twist'' boundaries. Recently, we have reported observations of another type of defect which could have a profound impact on characteristics of high voltage power devices. 4 These defects, referred to as nanopipes, are long, faceted empty pipes which thread through the entire thickness of the GaN epilayer. The radii of the pipes are in the 35-500 Å range and they appear to propagate along the c axis of the film. A similar defect is frequently observed in another wide band-gap semiconductor with a hexagonal crystal structure, namely, silicon carbide. 5 The present study, which combines high-resolution transmission electron microscopy ͑HRTEM͒ and scanning force microscopy ͑SFM͒, provides evidence that the nanopipes occur at the cores of screw dislocations. SFM images show that spiral steps emerge from the crater formed where the screw dislocations intersect the surface. These spiral steps create hexagonal growth hillocks which eventually lead to a nonplanar surface morphology. Although we believe that these observations can be understood in terms of Frank's theory for open-core dislocations, we note several quantitative discrepancies. 6 The 2.8-m-thick ␣-GaN epilayers described here were grown at 1040°C on ␣-Al 2 O 3 (0001)substrates in an inductively heated, water cooled, vertical organometallic vapor phase epitaxy ͑OMVPE͒ reactor. 7 An AlN buffer layer was first deposited at 450-500°C using 1.5 mol/min triethylaluminum, 2.5 standard liters per minute ͑slm͒ NH 3 and 3.5 slm H 2 flows. After annealing in 2.5 slm NH 3 and 3.5 slm H 2 for 10 min at ...