P. Pirouz

Growth defects in GaN films on sapphire: The probable origin of threading dislocations

Single crystal GaN films with a wurtzite structure were grown on the basal plane of sapphire. A high density of threading dislocations parallel to the {ital c}-axis crossed the film from the interface to the film surface. They were found to have a predominantly edge character with a 1/3{l_angle}11{bar 2}0{r_angle} Burgers vector. In addition, dislocation half-loops, elongated along the {ital c}-axis of GaN, was also found on the prism planes. These dislocations had a mostly screw character with a [0001] Burgers vector. Substrate surface steps with a height of 1/6{ital c}{sub Al{sub 2}O{sub 3}}, were found to be accommodated by localized elastic bending of GaN (0001){sub GaN} planes in the vicinity of the film/substrate interface. Observations show that the region of the film, with a thickness of {approximately}100 nm, adjacent to the interface is highly defective. This region is thought to correspond to the low-temperature GaN {open_quote}{open_quote}buffer{close_quote}{close_quote} layer which is initially grown on the sapphire substrate. Based on the experimental observations, a model for the formation of the majority threading dislocations in the film is proposed. The analysis of the results leads us to conclude that the film is under residual biaxial compression. {copyright} {ital 1996 Materials Research Society.}

Epitaxial growth of 3C–SiC films on 4 in. diam (100) silicon wafers by atmospheric pressure chemical vapor deposition

Silicon carbide (SiC) films have been grown on 4 in. diam (100) silicon wafers by atmospheric pressure chemical vapor deposition, using propane, silane, and hydrogen. X‐ray photoelectron spectroscopy data confirm that the films are stoichiometric SiC, with no major impurities. X‐ray diffraction and transmission electron microscopy (TEM) data indicate that the films are single‐crystalline cubic polytype (3C) across the 4 in. substrates. With the exception of slip lines near the edge of the wafers, the films appear featureless when observed optically. The nitrogen concentration, as determined by secondary ion mass spectroscopy, is 4×1018 cm3. Cross‐sectional TEM images show a fairly rough, void‐free interface.

The microstructure of SCS-6 SiC fiber

A thorough investigation of the microstructure of single SCS-6 SiC fibers widely used as reinforcements in metal-matrix and ceramic-matrix composites has been made. Various techniques of electron microscopy (EM) including scanning (SEM), conventional transmission (TEM), high resolution (HREM), parallel electron energy loss spectroscopy (PEELS), and scanning Auger microscopy (SAM) have been used to analyze and characterize the microstructure. The fiber is a complicated composite consisting of many different layers of SiC deposited on a carbon core and different carbonaceous coatings covering the SiC layers. The structural and chemical aspects of each layer are characterized and discussed.

Polytypic transformations in SiC: the role of TEM

Abstract In this paper, the structure of SiC is briefly reviewed and a recent dislocation model for polytypic transformation is explained. The model is based on the asymmetry in the mobility of partial dislocations in SiC. For this reason, the nature of dislocations in SiC is also considered and the asymmetry in dislocation velocity is attributed to the different core structure of the two partial dislocations. Following the description of the polytypic transformation model, some experiments in its support that employ transmission electron microscopy are presented.

Chemomechanical polishing of silicon carbide

In an effort to improve silicon carbide (SiC) substrates surfaces prior to epitaxial growth, two chemomechanical polishing (CMP) techniques were investigated and the results were compared with a mechanical polishing procedure involving various grades of diamond paste. This work focused on silicon-terminated (0001) SiC surfaces. The two CMP techniques utilized (i) chromium oxide(III) abrasives and (ii) colloidal silica polishing slurry. The best surfaces were obtained after colloidal silica polishing under conditions that combined elevated temperatures ({approximately}55 C) with a high slurry alkalinity (pH > 10) and a high solute content. Cross-sectional transmission electron microscopy showed no observable subsurface damage, and atomic force microscopy showed a significant reduction in roughness compared to commercial diamond-polished wafers. Growth experiments following colloidal silica polishing yielded a much improved film surface morphology.

Recombination-enhanced extension of stacking faults in 4H-SiC p-i-n diodes under forward bias

The extension of stacking faults in a forward-biased 4H-SiC PiN diodes by the recombination-enhanced motion of leading partial dislocations has been investigated by the technique of optical emission microscopy. From the temperature dependence of the measured velocity of the partials, an activation energy of 0.27 eV is obtained. Based on this and analysis of the emission spectra, a radiative recombination level of 2.8 eV for the stacking fault, and two energy levels for the partial dislocation, a radiative one at 1.8 eV and a nonradiative at 2.2 eV, have been determined.

Observation of 4H–SiC to 3C–SiC polytypic transformation during oxidation

We have observed the formation of single and multiple stacking faults that sometimes give rise to 3C–SiC bands in a highly doped n-type 4H–SiC epilayer following dry thermal oxidation. Transmission electron microscopy following oxidation revealed single stacking faults and bands of 3C–SiC in a 4H–SiC matrix within the 4H–SiC epilayer. These bands, parallel to the (0001) basal plane, were not detected in unoxidized control samples. In addition to the 3.22 eV peak of 4H–SiC, Cathodoluminescence spectroscopy at 300 K after oxidation revealed a spectral peak at 2.5 eV photon energy that was not present in the sample prior to oxidation. The polytypic transformation is tentatively attributed to the motion of Shockley partial dislocations on parallel (0001) slip planes. The generation and motion of these partials may have been induced by stresses caused either by the heavy doping of the epilayer or nucleation from defect.

Controlled growth of 3C-SiC and 6H-SiC films on low-tilt-angle vicinal (0001) 6H-SiC wafers

We have found that, with proper pregrowth surface treatment, 6H‐SiC single‐crystal films can be grown by chemical vapor deposition (CVD) at 1450 °C on vicinal (0001) 6H‐SiC with tilt angles as small as 0.1°. Previously, tilt angles of greater than 1.5° were required to achieve 6H on 6H at this growth temperature. In addition, 3C‐SiC could be induced to grow within selected regions on the 6H substrate. The 3C regions contained few (or zero) double‐positioning boundaries and a low density of stacking faults. A new growth model is proposed to explain the control of SiC polytype in this epitaxial film growth process.We have found that, with proper pregrowth surface treatment, 6H‐SiC single‐crystal films can be grown by chemical vapor deposition (CVD) at 1450 °C on vicinal (0001) 6H‐SiC with tilt angles as small as 0.1°. Previously, tilt angles of greater than 1.5° were required to achieve 6H on 6H at this growth temperature. In addition, 3C‐SiC could be induced to grow within selected regions on the 6H substrate. The 3C regions contained few (or zero) double‐positioning boundaries and a low density of stacking faults. A new growth model is proposed to explain the control of SiC polytype in this epitaxial film growth process.

Stacking fault energy of 6H-SiC and 4H-SiC single crystals

Abstract Single crystal 4H and 6H polytypes of SiC have been deformed in compression at 1300°C. All the deformation-induced dislocations were found to be dissociated into two partials bounding a ribbon of intrinsic stacking fault. Using two-beam bright-field and weak-beam dark-field techniques of transmission electron microscopy, the stacking fault energy of these two SiC polytypes has been determined from the separation width of the two partials of dissociated dislocations. The stacking fault energy of 4H-SiC is determined to be 14.7±2.5mJm−2, and that of 6H-SiC to be 2.9±0.6mJm−2. As a verification, the stacking fault energy of 4H-SiC has been determined also from the minimum radius of curvature of extended nodes. This latter method gave a value of 12.2±1.1mJm−2 which is within the range determined from measurement of partial dislocation separations. The experimental values of stacking fault energy for 4H- and 6H-SiC have been compared with estimates obtained from a generalized axial next-nearest-neighbour Ising (ANNNI) spin model. It is found that the theoretical models predict the lower stacking fault energy of 6H-SiC compared with that of 4H-SiC, and the predicted energies are, respectively, within 5% and 40% of the experimental values.

Growth of high quality 6H‐SiC epitaxial films on vicinal (0001) 6H‐SiC wafers

Previously reported growth of SiC films on SiC by chemical vapor deposition (CVD) used Acheson and Lely α‐SiC crystal substrates. We report the CVD growth and evaluation of high quality 6H‐SiC films on 6H‐SiC wafers cut from large boules grown by the modified‐sublimation process. The single‐crystal 6H‐SiC films were grown on wafers oriented 3° to 4° off the (0001) plane toward the 〈1120〉 direction. The films, up to 12 μm thick, had surfaces that were smooth and featureless. The high quality of the films was demonstrated by optical and electron microscopy, and low‐temperature photoluminescence.