P. G. Neudeck

The mechanism of micropipe nucleation at inclusions in silicon carbide

A model is presented for a possible mechanism of screw dislocation (including micropipe) nucleation in silicon carbide. The model is based on the observation of micropipe nucleation at the sites of foreign material inclusions using synchrotron white beam x-ray topography and transmission optical microscopy. It is shown that incorporation of the inclusion into the growing crystal can lead to deformation of the protruding ledge which constitutes the overgrowing layer. Accommodation of this deformation into the crystal lattice leads to the production of pairs of opposite sign screw dislocations which then propagate with the growing crystal. Evidence for the existence of such pairs of dislocations is presented.A model is presented for a possible mechanism of screw dislocation (including micropipe) nucleation in silicon carbide. The model is based on the observation of micropipe nucleation at the sites of foreign material inclusions using synchrotron white beam x-ray topography and transmission optical microscopy. It is shown that incorporation of the inclusion into the growing crystal can lead to deformation of the protruding ledge which constitutes the overgrowing layer. Accommodation of this deformation into the crystal lattice leads to the production of pairs of opposite sign screw dislocations which then propagate with the growing crystal. Evidence for the existence of such pairs of dislocations is presented.

Electron channeling contrast imaging of atomic steps and threading dislocations in 4H-SiC

Direct imaging of atomic step morphologies and individual threading dislocations in on-axis epitaxial 4H-SiC surfaces is presented. Topographically sensitive electron images of the crystalline surfaces were obtained through forescattered electron detection inside a conventional scanning electron microscope. This technique, termed electron channeling contrast imaging (ECCI), has been utilized to reveal the configuration of highly stepped, homoepitaxial 4H-SiC films grown on 4H-SiC mesa structures. Individual threading dislocations have been consistently imaged at the core of spiral atomic step morphologies located on the 4H-SiC surfaces. The ability of ECCI to image atomic steps was verified by atomic force microscopy.

Nondestructive analysis of threading dislocations in GaN by electron channeling contrast imaging

Threading dislocations in metal-organic chemical-vapor grown GaN films were imaged nondestructively by the electron channeling contrast imaging (ECCI) technique. Comparisons between ECCI and cross-sectional transmission electron microscopy indicated that pure edge dislocations can be imaged in GaN by ECCI. Total threading dislocation densities were measured by ECCI for various GaN films on engineered 4H-SiC surfaces and ranged from 107to109cm−2. A comparison between the ultraviolet electroluminescent output measured at 380nm and the total dislocation density as measured by ECCI revealed an inverse logarithmic dependence.

Lowered dislocation densities in uniform GaN layers grown on step-free (0001) 4H-SiC mesa surfaces

We report that very low threading dislocation densities (8×107∕cm2) were achieved in uniform GaN layers grown by metalorganic chemical vapor deposition on (0001) 4H-SiC mesa surfaces 50μm×50μm in area that were completely free of steps. Transmission electron microscopy (TEM) indicated that all observable GaN film threading dislocations were of edge type. TEM analysis of the defect structure of the nucleation layer (aluminum nitride, AlN) revealed a lack of c-component dislocations, and the clean annihilation of lateral, a-type dislocations within the first 200 nm of growth, with no lateral dislocations developing threading arms. These results indicate that the elimination of steps on the initial (0001) 4H-SiC growth surface may play an important role in the removal of mixed and c-type dislocations in subsequently grown AlN and GaN heteroepitaxial layers.

Polytype identification in heteroepitaxial 3C-SiC grown on 4H-SiC mesas using synchrotron white beam X-ray topography

Abstract Synchrotron white beam X-ray topography (SWBXT), post-oxidation color mapping, atomic force microscopy (AFM) and scanning electron microscopy (SEM) have been used to study the defect and polytype distribution in a thin film grown on mesas structures on part of a 4H-SiC crystal subjected to a non-optimal version of the previously reported [Appl. Phys. Lett. 77 (2000) 1449] procedure for achieving atomic flatness. This combination of techniques was used to provide confirmation that atomic flatness could only be achieved on mesas containing no screw dislocations. It was also found that the vast majority of the atomically flat mesas were of small size as expected since the probability of a screw dislocation threading a mesa is proportional to its area. For these small mesas, either no 3C-SiC nucleation or complete coverage by one or other of the 3C variants was observed. In mesas containing screw dislocations, mostly those of larger size, either no 3C nucleation or partial coverage by single or double phase 3C was observed. In contrast to our previous observations of on-axis mesa samples grown under different conditions in which polytypes such as 15R were observed [J. Phys. D 28 (1995) A56; in: L. Terminello, N. Shinn, G. Ice, K. D’Amico, D. Perry (Eds.), Applications of Synchrotron Radiation Techniques to Materials Science, Mater. Res. Soc. Symp. Proc. 375 (1995) 327] no polytypes other than 4H, 3C(I) and 3C(II) were detected anywhere on this wafer. In the absence of the stacking sequence template provided by the risers of steps (whether from screw dislocations or offcut angle), this data indicates that no other polytype forms but 3C under these growth conditions. This study indicates that combined use of SWBXT in back-reflection and forward-reflection geometry, post-oxidation color mapping, AFM and SEM, is ideally suited for detailed microstructural and structural mapping in SiC.

Improved ultraviolet emission from reduced defect gallium nitride homojunctions grown on step-free 4H-SiC mesas

We previously reported 100-fold reductions in III-N heterofilm threading dislocation density achieved via growth on top of (0001) 4H-SiC mesas completely free of atomic scale steps. This letter compares the electroluminescent (EL) output of GaN pn junctions grown on top of 4H-SiC mesas with and without such steps. An average of 49% enhancement of the ultraviolet luminescence (380nm) was observed in step-free mesas over comparable “stepped” counterparts. Despite the intense EL from the step-free devices, significant leakage was observed through the periphery of the device, possibly due to the lack of GaN junction isolation processing.

Microstructure of heteroepitaxial GaN grown on mesa-patterned 4H-SiC substrates

Cross-sectional transmission electron microscopy and atomic force microscopy have been used to study the microstructure of a thin heteroeptiaxial GaN film grown on (0001) 4H-SiC mesa surfaces with and without atomic scale steps. Analysis of a mesa that was completely free of atomic-scale surface steps prior to III–N film deposition showed that these GaN layers had a wide variation in island height (1–3μm) and included the presence of pit-like defects on the film surface. This sample had a low dislocation density (5×108∕cm2) as compared to conventionally grown samples on unpatterned (0001) on-axis 4H-SiC (2×109∕cm2), coupled with a 3–5 times increase in grain size. A comparison of a GaN film on the step-free 4H-SiC mesa region with a GaN film on a stepped 4H-SiC mesa region on the same substrate showed that the presence of surface steps reduced the overall grain size of the film from 7–10μm to a grain size of about 2–3μm. Since the GaN films grow via a Volmer–Weber mechanism, a decrease in the number of he...

Growth and characterization of 3C?SiC and 2H?AlN/GaN films and devices produced on step-free 4H?SiC mesa substrates

While previously published experimental results have shown that the step-free (0?0?0?1) 4H?SiC mesa growth surface uniquely enables radical improvement of 3C?SiC and 2H?AlN/GaN heteroepitaxial film quality (>100-fold reduction in extended defect densities), important aspects of the step-free mesa heterofilm growth processes and resulting electronic device benefits remain to be more fully elucidated. This paper reviews and updates recent ongoing studies of 3C?SiC and 2H?AlN/GaN heteroepilayers grown on top of 4H?SiC mesas. For both 3C?SiC and AlN/GaN films nucleated on 4H?SiC mesas rendered completely free of atomic-scale surface steps, TEM studies reveal that relaxation of heterofilm strain arising from in-plane film/substrate lattice constant mismatch occurs in a remarkably benign manner that avoids formation of threading dislocations in the heteroepilayer. In particular, relaxation appears to occur via nucleation and inward lateral glide of near-interfacial dislocation half-loops from the mesa sidewalls. Preliminary studies of homojunction diodes implemented in 3C-SiC and AlN/GaN heterolayers demonstrate improved electrical performance compared with much more defective heterofilms grown on neighbouring stepped 4H?SiC mesas. Recombination-enhanced dislocation motion known to degrade forward-biased 4H?SiC bipolar diodes has been completely absent from our initial studies of 3C?SiC diodes, including diodes implemented on defective 3C?SiC heterolayers grown on stepped 4H?SiC mesas.

Strain relief and dislocation motion in III-nitride films grown on stepped and step-free 4H‐SiC mesas

The impetus for dislocation motion in thin films is generally understood in terms of Peach-Koehler forces. For the case of III-nitride films grown on step-free 4H‐SiC mesas, however, it is the gradient of the strain energy from the mesa edge that is capable of driving misfit dislocations. Using the strain profile as a function of the distance from the mesa edge and the line tension of the c-plane threading arms, we have calculated the excess stress driving the half loop from the mesa edge into the mesa interior. We have also compared the half-loop excess stress with the excess stress driving the tilt of threading edge dislocations, which has been proposed as one of the principal strain relief mechanisms in III-nitride films. The excess stress driving c-plane half loops ranges from a few 1000MPa at the mesa edge to few 100MPa towards the mesa interior, while the excess stress driving the tilt of threading edge dislocations is in excess of 20000MPa. The greater excess stress driving dislocation tilt, howeve...

Valence band structure and band offset of 3C- and 4H-SiC studied by ballistic hole emission microscopy

p-type Schottky barriers in Pt∕3C-SiC contacts have been measured using ballistic hole emission microscopy (BHEM) and estimated to be ∼0.06eV higher than identically prepared Pt∕p-type 4H-SiC contacts. This indicates the 3C-SiC valence band maximum (VBM) is ∼0.06eV below the 4H-SiC VBM, consistent with the calculated ∼0.05eV type-II valence band offset between these polytypes. We also observe no evidence of an additional VBM in 3C-SiC, which supports the proposal that the second VBM observed in BHEM spectra on 4H-SiC is a crystal-field split VBM located ∼110meV below the highest VBM.