M. Skowronski

Microstructural characterization of α‐GaN films grown on sapphire by organometallic vapor phase epitaxy

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〈1120〉 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.

Determination of wurtzite GaN lattice polarity based on surface reconstruction

We identify two categories of reconstructions occurring on wurtzite GaN surfaces, the first associated with the N face, (0001), 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.

Degradation of hexagonal silicon-carbide-based bipolar devices

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° sil...

Open‐core screw dislocations in GaN epilayers observed by scanning force microscopy and high‐resolution transmission electron microscopy

Structural investigations of organometallic vapor phase epitaxy grown α‐GaN films using high‐resolution transmission electron microscopy and scanning force microscopy have revealed the presence of tunnel‐like defects with 35–500 A 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 A 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.

Reconstructions of GaN(0001) and (0001̄) surfaces: Ga-rich metallic structures

Reconstructions of GaN(0001) and (0001) surfaces are studied by scanning tunneling microscopy and spectroscopy, by electron diffraction, by Auger electron spectroscopy, and using first-principles theory. Attention is focused on Ga-rich reconstructions for each surface, which are found to have a metallic character involving significant overlap between Ga valence electrons. The electron counting rule is thus violated for these surfaces, but they nonetheless form minimum energy structures.

Dislocation conversion in 4H silicon carbide epitaxy

Abstract The propagation of basal plane dislocations from off-axis 4H silicon carbide substrates into the homo-epitaxial layers has been investigated using chemical etching, optical microscopy, and transmission electron microscopy (TEM). The etch pit densities of threading edge and basal plane dislocations changed significantly across the epilayer/substrate interface. We have observed conversion of basal plane dislocations in the substrates into threading edge dislocations in the epilayers. TEM observation revealed that the threading dislocations in the epilayers are inclined from the c-axis toward the down-step direction. The conversion is interpreted as a result of the image force in the epilayers between flowing growth steps and basal plane dislocations. This effect can lead to an apparent improvement of the structural quality of epilayers compared to that of substrates.

Lifetime-limiting defects in n− 4H-SiC epilayers

Low-injection minority carrier lifetimes (MCLs) and deep trap spectra have been investigated in n− 4H-SiC epilayers of varying layer thicknesses, in order to enable the separation of bulk lifetimes from surface recombination effects. From the linear dependence of the inverse bulk MCL on the concentration of Z1∕Z2 defects and from the behavior of the deep trap spectra in 4H-SiC p-i-n diodes under forward bias, we conclude that it is Z1∕Z2 alone that controls the MCL in this material.

Semi‐insulating 6H–SiC grown by physical vapor transport

Semi‐insulating 6H–SiC crystals have been achieved by using controlled doping with deep‐level vanadium impurities. High resistivity undoped and semi‐insulating vanadium‐doped single‐crystals with diameters up to 50 mm were grown by physical vapor transport using an induction‐heated, cold‐wall system in which high purity graphite materials constituted the hot zone of the furnace. Undoped crystals were p‐type due to the presence of residual acceptor impurities, mainly boron, and exhibited resistivities ranging up to 3000 Ω cm. The semi‐insulating behavior of the vanadium‐doped crystals is attributed to compensation of residual acceptors by the deep‐level vanadium V4+(3d1) donor located near the middle of the band gap.

Structure of recombination-induced stacking faults in high-voltage SiC p–n junctions

The structure of stacking faults formed in forward-biased 4H- and 6H-SiC p–n− diodes was determined using conventional and high-resolution transmission electron microscopy. Typical fault densities were between 103 and 104 cm−1. All observed faults were isolated single-layer Shockley faults bound by partial dislocations with Burgers vector of a/3〈1–100〉-type.

Intracenter transitions in the dominant deep level (EL2) in GaAs

Intracenter transitions in the major deep level EL2 in GaAs were identified for the first time by superimposing photocurrent measurements on those of optical absorption. These transitions were found to be responsible for the characteristic EL2 absorption band between 1.0 and 1.3 eV. At low temperatures (<60 K) intracenter absorption exhibits a fine structure involving the zero phonon line and replicas at energies close to those of transverse acoustic phonons (TA). This coupling with TA phonons is a strong indication that EL2 is an extrinsic self‐trapping center.