David Phillip Malta

Sublimation-Grown Semi-Insulating SiC for High Frequency Devices

In this paper we show the progression in the development of semi-insula ti g SiC grown by the sublimation technique from extrinsically doped material to hig h purity semi-insulating (HPSI) 4H-SiC bulk crystals of 2-inch and 3-inch diameter without re sorting to the intentional introduction of elemental deep level dopants, such as vanadium. Secondary ion m ass spectrometry, optical absorption, deep level transient spectroscopy and electron parama gnetic resonance data suggest that the semi-insulating behavior in HPSI material orig inates from deep levels associated with intrinsic point defects. While high temperature resistivity measurements on different high purity 4H-SiC samples indicate activation energies ranging from 0.9 to 1.6 eV, HPSI wafers with homogeneous activation energies near mid-gap are demonstrated. The roomtemperature thermal conductivity of this material approaches the theoretical maximum of ~ 5 W/cmK. Additionally, HPSI substrates exhibit micropipe densities as low as 8 cm -2 over the full diameter of a 3-inch wafer. MESFETs produced on HPSI wafers are free of backgating effects and have resulted in the best combination of power density and efficiency reported to date for SiC M ESFETs of 5.2 W/mm and 63% power added efficiency (PAE) at 3.5 GHz.

100 mm 4HN-SiC Wafers with Zero Micropipe Density

Recent advances in PVT c-axis growth process have shown a path for eliminating micropipes in 4HN-SiC, leading to the demonstration of zero micropipe density 100 mm 4HN-SiC wafers. Combined techniques of KOH etching and cross-polarizer inspections were used to confirm the absence of micropipes. Crystal growth studies for 3-inch material with similar processes have demonstrated a 1c screw dislocation median density of 175 cm-2, compared to typical densities of 2x103 to 4x103 cm-2 in current production wafers. These values were obtained through optical scanning analyzer methods and verified by x-ray topography.

Development of Large Diameter High-Purity Semi-Insulating 4H-SiC Wafers for Microwave Devices

The next generation of wireless infrastructure will rely heavily upon wide band gap semiconductors owing to their unique materials properties, including: their large bandgap, high thermal conductivity, and high breakdown field. To facilitate implementation of this next generation, a significant effort is required to make SiC MESFET and GaN HEMT microwave devices more suitable for widespread application. Currently, the interest in high-purity semiinsulating (HPSI) 4H-SiC is critically tied to its influence on microwave devices, whether performance or affordability. To address these issues, we have developed high-purity 3-inch and 100 mm 4H-SiC substrates with low micropipe densities (as low as 1.4 cm -2 in 3-inch and <60 cm -2 in 100 mm) and uniform semi-insulating properties (>10 9 Ωcm) over the full wafer diameter. These wafers possess typical residual shallow level contamination less than 1x10 16 cm -3 (5x10 15 nitrogen and 3x10 15 boron) with best nitrogen values of 3x10 14 . In this paper, we will report on the development of our HPSI growth process focusing on the specific areas of the assessment of semiinsulating character and device applicability.

Silicon Carbide Crystal and Substrate Technology: A Survey of Recent Advances

The quest of driving SiC toward the realization of its full potential as a semiconductor material continues in many organizations world-wide. R&D and manufacturing efforts continue to address issues of scale-up of wafer size, improvements in wafer shape and surface characteristics, reduction of background impurities in bulk crystals, controlled uniformity of electrical properties, and reduction and control of crystalline defects. Significant progress has been made in several key areas. Increased manufacturing activity in the production of 3-inch diameter crystals has led to substrates with micropipes densities <30 cm -2 in n-type and <80 cm -2 in semi-insulating material, and R&D demonstrations of substrates exhibiting micropipe densities <0.5 cm -2 in n-type and <5 cm -2 in semi-insulating wafers. Developmental 100-mm diameter substrates exhibiting micropipe densities <60 cm -2 in both n-type and semi-insulating materials have now been demonstrated. Significant improvement in bulk crystal purity has been achieved with reduction of impurity concentrations below 5 x 10 15 cm -3 .

Defect Status in SiC Manufacturing

Availability of high-quality, large diameter SiC wafers in quantity has bolstered the commercial application of and interest in both SiC- and nitride-based device technologies. Successful development of SiC devices requires low defect densities, which have been achieved only through significant advances in substrate and epitaxial layer quality. Cree has established viable materials technologies to attain these qualities on production wafers and further developments are imminent. Zero micropipe (ZMP) 100 mm 4HN-SiC substrates are commercially available and 1c dislocations densities were reduced to values as low as 175 cm-2. On these low defect substrates we have achieved repeatable production of thick epitaxial layers with defect densities of less than 1 cm-2 and as low as 0.2 cm-2. These accomplishments rely on precise monitoring of both material and manufacturing induced defects. Selective etch techniques and an optical surface analyzer is used to inspect these defects on our wafers. Results were verified by optical microscopy and x-ray topography.

Enhanced Carrier Lifetime in Bulk-Grown 4H-SiC Substrates

To devise a means of circumventing the cost of thick SiC epitaxy to generate drift layers in PiN diodes for >10kV operation, we have endeavored to enhance the minority carrier lifetimes in bulk-grown substrates. In this paper, we discuss the results of a process that has been developed to enhance minority carrier lifetimes to in excess of 30 μs in bulk-grown 4H-SiC substrates. Measurement of lifetimes was principally conducted using microwave-photoconductive decay (MPCD). Confirmation of the MPCD lifetime result was obtained by electron beam induced current (EBIC) measurements. Additionally, deep level transient spectroscopic analysis of samples subjected to this process suggests that a significant reduction of deep level defects in general and of Z1/Z2, specifically, may account for the significantly enhanced lifetimes. Finally, a study of operational performance in devices employing drift layers fabricated from substrates produced by this process confirmed ambipolar lifetimes in the microsecond range.