Papers by Author: Ilya Zwieback

Abstract: We investigated selective etching of SiC in molten KOH + NaOH + Na₂O₂ mixtures in application to defect analysis. Etch rate was measured as a function of etchant composition, temperature and other process variables. Optimal etching conditions were established for reliable differentiation between TSDs, TEDs, BPDs and stacking faults (SF).

Abstract: Large-diameter SiC single crystals are grown at II-VI by the sublimation technique. 100mm substrates of semi-insulating 6H SiC and n-type 4H SiC are produced as commercial products; in development, diameter expansion to 125mm has been achieved. Over the last two years, significant improvements have been made in crystal quality. The values of FWHM of x-ray rocking curves are typically 20-40 arc-seconds for 6H SI wafers and 12-30 arc-seconds for 4H n+ wafers. Micropipe density is less than 3 cm-2, and less than 0.1 cm-2 in best substrates. Electrical resistivity of SI substrates is, typically, of 10¹¹ Ω•cm or above. For 4H n+ substrates, the typical dislocation density is about 9×10³ cm^-2 and the typical BPD density is less than 1×10³ cm^-2.

Abstract: Knoop microhardness assessments were conducted on a variety of 6H- and 4H-SiC substrates to assess any appreciable differences that may need to be considered in wafer manufacture and general application. Nitrogen-doped, vanadium-doped and unintentionally doped (UID) substrates with both on-axis and 8° off-axis orientations were assessed. In general, the Knoop hardness values fell in the 2000 to 2500 kg/mm2 range (equivalent to approximately 20 to 25 GPa). Hardness values measured in the <1100> crystal direction were significantly higher than in the <11-20> direction. Undoped and vanadium-doped samples were harder than nitrogen-doped samples. For both 6H and 4H nitrogen-doped samples, the hardness was as much as 10% higher for 8° offcut wafers than for on-axis.

Abstract: X-ray rocking curve characterization is a relatively fast and nondestructive technique that can be utilized to evaluate the crystal quality of SiC substrates. The contribution of lattice curvature to rocking curve broadening is estimated, and shown to be the major contribution to the measured broadening (FWHM). The feedback on lattice quality is used to optimize our SiC growth process. In the optimized growth runs, the typical variation in rocking curve sample angle Ω across the entire 3” diameter wafer is about 0.2 degrees. Possible mechanisms leading to changes in the lattice curvature are discussed.

Abstract: Etching of 4H-SiC wafers in molten KOH as a method for micropipe and dislocation density analysis was investigated. The obtained results were correlated with those of the synchrotron white beam x-ray topography. Heavily nitrogen-doped SiC shows a significantly different etching behavior in comparison with the low-doped material. This complicates identification of different types of threading defects. In particular, it is difficult to separate Threading Screw Dislocations (TSD) from Threading Edge Dislocations (TED). Depending on the level of doping and thermal history of the crystal, some of the etch pits emerging due to the 1c screw dislocations can be as large as those due to the micropipes.

Abstract: II-VI is developing large-diameter SiC crystals to be used as lattice-matched, high thermal conductivity substrates for new generation GaN-based and SiC-based semiconductor devices. Large-diameter 6H SiC single crystals are grown at II-VI using our Advanced PVT sublimation growth process. Stable SI properties are achieved by compensation with vanadium, which results in high and spatially uniform resistivity, on the order of 1011 Ohm-cm. The quality of the presently grown 100 mm 6H SI substrates has been dramatically improved [1], and they are free of edge defects. Micropipe density in the 100 mm 6H SI substrates ranges from 2 to 8 cm-2 and dislocation density from 3·104 to 6·104 cm-2. X-ray rocking curves measured on as-sawn 100 mm 6H wafers showed edge-to-edge lattice curvature () between 0.1° and 0.3° and FWHM of the rocking curve between 50 and 100 arc-seconds

Abstract: Several morphological defects in 4H SiC epitaxial wafers, including Comets and Triangles, may significantly impact on the yield and reliability of SiC devices. The formation of these epilayer defects is closely related to the substrate quality. This paper focuses on the study of the substrate quality and its relationship with defects in the epilayers. The crystalline quality of 4H n+ substrates has been characterized by x-ray diffraction, and the distribution of dislocations has been determined using etching in molten KOH. The relationship between Comet and Triangle epilayer defects and the dislocations has been established. A 10-fold reduction in the overall dislocation density in the 4H SiC substrates was achieved through technological improvements. The improvement was validated by the reduction in the number of the epilayer defects.

Abstract: SiC substrates produced at II-VI, Inc. have been characterized using x-ray rocking curve mapping (topography). The rocking curves have been measured in the
-scan mode for the (0006) Bragg reflection of 6H and the (0004) reflection of 4H SiC substrates. The maps contain information extracted from the rocking curves, such as the peak angle (
) and the rocking curve broadening (FWHM). In the case when lattice distortion is present due to the elastic or plastic deformation, the peak angle (
) changes gradually upon scanning, with the d
/dx gradient proportional to the lattice curvature in the plane of diffraction. Multi-peak reflections and/or sharp change in the value of
indicate the presence of misoriented grains. X-ray rocking curve mapping of SiC substrates yields excellent measures of crystalline quality that contain important information on the lattice strain and sub-grain misorientation.

Abstract: Over the past year, II-VI has transitioned from 2” to 3” commercial SiC substrates. Large-diameter semi-insulating 6H-SiC and n-type 4H-SiC single crystals are grown using the Advanced PVT growth process. Expansion of boule diameter from 2 to 3 and up to 4.25 inches has been carried out using a specially designed growth technique. Stable semi-insulating properties in 6H-SiC are achieved by precise vanadium compensation. The technique of compensation is optimized to produce a controlled and spatially uniform distribution of vanadium and high and spatially uniform electrical resistivity reaching 10 10 – 1011
·cm. N-type 3-inch 4H-SiC crystals are grown using doping with nitrogen, and 3-inch 4H-SiC substrates show uniform resistivity of about 0.018
·cm. The best quality semiinsulating (SI) 3” 6H-SiC substrates demonstrate micropipe density of 3 cm-2, and n-type 3” 4H-SiC substrates - about 1 cm-2. X-ray rocking curve topography of the produced 3” SiC substrates is used for evaluation of their crystal quality.

Abstract: II-VI has developed an Advanced PVT (APVT) process for the growth of nominally undoped (vanadium-free) semi-insulating 2” and 3” diameter 6H-SiC crystals with room temperature resistivity up to 1010 W·cm. The process utilizes high-purity SiC source and employs special measures aimed at the reduction of the impurity background. The APVT-grown material demonstrates concentrations of B and N reduced to about 2·1015cm-3. Wafer resistivity has been studied and correlated with Schottky barrier capacitance, yielding the density of deep compensating centers in 6H-SiC in the low 1015 cm-3 range for both ntype and p-type material. The nearly equal density of deep donors and deep acceptors ndicates that the centers responsible for the intrinsic compensation can be amphoteric. TheEPR density of spins from free carbon vacancies is about 1014 cm-3. It is also hypothesized that impurity-vacancy complexes can be present in the undoped material and participate in compensation.