Ejiro Emorhokpor

Status of Large Diameter SiC Single Crystals

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.

Advanced PVT Growth of 2 & 3-Inch Diameter 6H SiC Crystals

Abstract. The Wide Bandgap Materials Group of II-VI Inc., develops, manufactures and markets n+ and semi-insulating (SI) 6H SiC crystals, including vanadium-compensated and V-free. The PVT growth process is tuned to produce high-quality semi-insulating 6H SiC boules with micropipe densities below 15 cm (for 2-inch wafers) and below 70 cm (for 3-inch wafers). Room temperature resistivity for 2-inch and 3-inch SI V-doped wafers is greater than 10 Ω·cm and 10 Ω·cm, respectively. A novel synthesis process is used for the production of high-purity polycrystalline SiC source, yielding a material in which most impurities are below their GDMS detection limits. An advanced PVT process (APVT) has been developed for the growth of V-free SI 6H SiC crystals. These APVT SiC crystals contain boron below 6.2·10cm, nitrogen below 4.0·10cm and demonstrate semi-insulating behavior with ρ between 10 and 10 Ω·cm. Photoluminescence and EPR of V-free 6H SiC has been studied and EPR data have been assigned to native point defects (C vacancy, Si antisite and CVAC-CSi pair).

Growth and Characterization of Large Diameter 6H and 4H SiC Single Crystals

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.

Growth of Undoped (Vanadium-Free) Semi-Insulating 6H-SiC Single Crystals

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.

Growth of Large Diameter 6H SI and 4H n+ SiC Single Crystals

SiC single crystals are grown at II-VI by the seeded sublimation technique. The process has been scaled up and optimized to support commercial production of high-quality 100 mm diameter, Semi-Insulating (SI) 6H substrates and 100 mm 4H n+ substrates. The growth process incorporates special elements aimed at achieving uniform sublimation of the source, steady growth rate, uniform doping and reduced presence of background impurities. Semi-insulating 6H substrates are produced using precise vanadium compensation. Vanadium doping is optimized to yield SI material with very high resistivity and low capacitance. Crystal quality of the substrates is evaluated using a wide variety of techniques. Specific defects, their interaction and evolution during growth are described with emphasis on micropipes and dislocations. The current quality of the 6H SI and 4H n+ crystals grown at II-VI is summarized.

Dislocation in 4H n+ SiC Substrates and their Relationship with Epilayer Defects

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.

Characterization of SiC Substrates Using X-Ray Rocking Curve Mapping

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.

Comparison between measurement techniques used for determination of the micropipe density in SiC substrates

Micropipe density (MPD) is a crucial parameter for silicon carbide (SiC) substrates that determines the quality, stability and yield of the semiconductor devices built on these substrates. The importance of MPD is underscored by the fact that all existing specifications for 6H- and 4H-SiC substrates set upper limits for it. Several methods for measuring the MPD are known, however, their reliability and applicability to various types of substrates (e.g. semiinsulating, conducting, etc.) has not been systematically studied. The subject of this paper is a comparative study of various techniques used for the MPD measurement accompanied by statistical analysis of the results. The study was initiated by several organizations working in the immediate field of silicon carbide or in closely related fields and included SiC substrate manufacturers, substrate consumers, equipment manufacturers and universities. The study represented a round robin experiment in which MPD was measured on thirty SiC wafers of various pedigrees. The values of MPD have been determined using both destructive and non-destructive techniques. The repeatability of each technique is analyzed and compared with that of other techniques.

Growth of Large Diameter SiC Crystals by Advanced Physical Vapor Transport

Semi-insulating 6H SiC substrates, 2”, 3” and 100mm in diameter, and n+ 4H SiC substrates, 2” and 3” in diameter, are grown at II-VI using the Advanced Physical Vapor Transport (APVT) technique [1]. The process utilizes high-purity SiC source and employs special measures aimed at the reduction of background contamination. Semi-insulating properties are achieved by precise vanadium compensation, which yields substrates with stable and uniform electrical resistivity reaching ~ 1011 Ω-cm and higher. Conductive n+ 4H SiC crystals with the spatially uniform resistivity of 0.02 W-cm are grown using nitrogen doping. Crystal quality of the substrates, their electrical properties and low temperature photoluminescence are discussed.

Automated Mapping of Micropipes in SiC Wafers Using Polarized-Light Microscope

We have developed a process that is able to detect, count, and map micropipes on SiC substrates. This process uses a polarized light microscope to scan the wafer. The pictures taken are analyzed with a program that produces a micropipe map as well as numerical defect distribution data in a text file. The results of the process were validated with x-ray topography measurement. The repeatability of this process is also studied and reported.