V.D. Heydemann

Transmission electron microscopy analysis of mechanical polishing-related damage in silicon carbide wafers

The subsurface damage generated by mechanical polishing of silicon carbide wafers was investigated and quantified by plan view transmission electron microscopy (TEM) and atomic force microscopy (AFM). Damage generated during polishing using diamond abrasives with 0.5 µm particle size consists of dislocation loops with length up to 400 nm from the scratches. The total dislocation density was estimated at 5 × 1010 dislocations cm−2. TEM analysis of the Burgers vectors indicates that the initial perfect dislocations have a Burgers vector of b = a/3 11–20-type with many dislocation dissociated into two partials with b = a/3 1–100. The depth of damage was estimated to be up to 50 nm. 4H–SiC homoepitaxial layers grown on mechanically polished substrates without further surface treatment exhibit threading dislocation density along scratches in the order of 105 cm−1.

Chemi-Mechanical Polishing of On-Axis Semi-Insulating SiC Substrates

Conventional colloidal silica (CS) chemi-mechanical polishing (CMP) of Si-face SiC substrates typically results in a low material removal rate (MRR<0.1μm/h). In this study, the chemical surface oxidation and mechanical removal of the oxide layer during CMP of on-axis semi-insulating (SI) 6H SiC substrates, and the effect on material removal rate and surface roughness were investigated separately by addition of (a) compatible oxidizers, (b) abrasives and (c) both oxidizers and abrasives to the CS slurry. Neither oxidizer nor soft abrasive addition individually resulted in a significant MRR increase, yet both increased the substrate surface roughness. The addition of nano-size diamond abrasive (0.1μm grain size) to the CS slurry resulted in a MRR of 0.60μm/h, a 10x increase over CS CMP, and produced substrates with a Ra surface roughness of 5.5Å. The addition of 0.1μm diamond abrasive and sodium hypochlorite oxidizer to the CS slurry resulted in a MRR increase to 0.92μm/h and produced substrates with a Ra surface roughness of 5.2Å.

Structural properties of SrO thin films grown by molecular beam epitaxy on LaAlO3 substrates

SrO films were grown on LaAlO3 substrates by molecular beam epitaxy and characterized using reflection high-energy electron diffraction (RHEED) and x-ray diffraction (XRD). The evolution of the RHEED pattern is discussed as a function of film thickness. 500A thick SrO films were relaxed and exhibited RHEED patterns indicative of an atomically smooth surface having uniform terrace heights. Films had the epitaxial relationship (001)SrO‖(001)LaAlO3; [010]SrO‖[110]LaAlO3. This 45° in-plane rotation minimizes mismatch and leads to films of high crystalline quality, as verified by Kikuchi lines in the RHEED patterns and narrow rocking curves of the (002) XRD peak.

Growth of GaN films on GaAs substrates in an As-free environment

We investigated growth of GaN films on [001] GaAs substrates by plasma-assisted molecular beam epitaxy in an As-free chamber. The crystalline quality and the surface morphology of the films were studied with x-ray diffraction and transmission electron, scanning electron, and atomic force microscopes. We determined that direct GaN deposition on the thermally desorbed substrate resulted in the growth of a polycrystalline film containing misoriented grains and inclusions. We achieved a significant improvement of the film quality by adopting a procedure consisting of a substrate nitridation, deposition of a low-temperature buffer layer, and a high-temperature overgrowth.

Selectivity and Residual Damage of Colloidal Silica Chemi-Mechanical Polishing of Silicon Carbide

The selectivity, material removal rate, and the residual subsurface damage of colloidal silica (CS) chemi-mechanical polishing (CMP) of silicon carbide substrates was investigated using atomic force microscopy (AFM) and plan view transmission electron microscopy (TEM). Silica CMP, in most process conditions, was selective. In the damage region surrounding remnant scratches, the vertical material removal rate exceeded the planar material removal rate, which resulted in an enhancement of the scratches over the duration of the polishing process. The material removal rate was low, about 20 nm / hr. In addition, the selectivity leads to a slow removal of residual subsurface damage from mechanical polishing. The silica CMP polished surface exhibits significant subsurface damage observed by plan view TEM even after prolonged polishing of 16 hours.

Spatially Resolved Photoluminescence and Thermally Stimulated Luminescence in Semi-Insulating SiC Wafers

We report on non-contact and non-destructive spatially resolved characterization of traps and luminescence centers in vanadium-free semi-insulating 6H-SiC. Two optical techniques were employed: photoluminescence (PL) mapping and thermally stimulated luminescence (TSL) imaging on SiC wafers. PL and TSL topography reveal inhomogeneity at the periphery regions of the wafers. Low-temperature PL spectra show broad bands with the maxima at 1.75eV and 1.2eV, including a sharp zero-phonon line at 1.344eV. The TSL glow curves at T>80K show different peaks in the visible and infrared bands. The luminescence spectrum of the 105K TSL peak replicates 1.75eV band, while the 120K peak corresponds to the 1.2eV band. Additionally, the high temperature TSL peak at 210K shows an excellent match with 1.344eV zero phonon line. The trap energies of different peaks are calculated. We discuss a model of complex defects composed of closely spaced electron (hole) trap and UD3 defect.

Preparation and evaluation of damage free surfaces on silicon carbide

A new chemical mechanical polishing process (ACMP) has been developed by the Penn State University Electro-Optics Center for producing damage free surfaces on silicon carbide substrates. This process is applicable to the silicon face of semi-insulating, conductive, 4H, 6H, onaxis and off-axis substrates. The process has been optimized to eliminate polishing induced selectivity and to obtain material removal rates in excess of 150nm/hour. The wafer surfaces and resultant subsurface damage generated by the process were evaluated by white light interferometery, Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM), and epitaxial layer growth. Residual surface damage induced by the polishing process that propagates into the epitaxial layer has been significantly reduced. Total dislocation densities measured on the ACMP processed wafers are on the order of the densities reported for the best as grown silicon carbide crystals [1]. Characterization of high electron mobility transistors (HEMTs) grown on these substrates indicates that the electrical performance of the substrates met or exceeded current requirements [2].

Photoluminescence and Thermally Stimulated Luminescence in Semi-Insulating SiC

We performed non-contact and non-destructive spatially resolved characterization of traps and recombination centers in semi-insulat i g SiC wafers using thermally stimulated luminescence (TSL) and scanning photoluminesce ce (PL). In Vfree samples, we observe a broad PL band with a maximum at 1.18eV ac companied with a sharp zero-phonon line at 1.34 eV. Intense TSL in the visible and IR spectral regions yield a glow curve with the maximum at 110K attributed to nitrogen traps. The TSL spectrum closely resembles the PL spectrum of the 1.18 eV PL ba nd. Comparison of TSL in V-doped and V-free samples is performed. TSL images are c or lated with PL maps.