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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 ...

Device performance and defects in AlGaN/GaN high-electron mobility transistors (HEMTs) have been correlated. Surface depressions and threading dislocations, revealed by optical-defect mapping and atomic force microscopy (AFM), compromised the effectiveness of the SiN x surface-passivation effect as evidenced by the gate-lag measurements. The residual carriers in the GaNbuffer layer observed from the capacitance-voltage depth profile have been attributed to the point defects and threading dislocations either acting as donors or causing local charge accumulations. Deep-level transient-spectroscopy measurements showed the existence of several traps corresponding to surface states and bulk-dislocation defects. The formation of electron-accumulation regions on the surface or (and) in the GaN-buffer layer was confirmed by currentvoltage measurements. This second, virtual gate formed by electron accumulations can deplete the channel and cause a large-signal gain collapse leading to degraded output power. A good correlation was established between the device performance and defects in AlGaN/GaN HEMT structure.