Adrian Powell

SiC materials-progress, status, and potential roadblocks

SiC materials are currently metamorphosing from research and development into a market driven manufacturing product. SiC substrates are currently used as the base for a large fraction of the world production of green, blue, and ultraviolet light-emitting diodes (LEDs). Emerging markets for SiC homoepitaxy include high-power switching devices and microwave devices for S and X band. Applications for heteroepitaxial GaN-based structures on SiC substrates include LEDs and microwave devices. In this paper we review the properties of SiC, assess the current status of substrate and epitaxial growth, and outline our expectations for SiC in the future.

Recent progress in SiC DMOSFETs and JBS diodes at Cree

This paper discusses the recent progress in large area silicon carbide (SiC) DMOSFETs and junction barrier Schottky (JBS) diodes. 1.2 kV and 10 kV SiC DMOSFETs have been produced with die areas greater than 0.64 cm2. SiC JBS diode dies also rated at 1.2 kV and 10 kV have been produced with die areas exceeding 1.5 cm2. These results demonstrate that SiC power devices provide a significant leap forward in performance for industrial electronics applications. At 1.2 kV, SiC DMOSFETs offer a reduction of power loss of greater than 50 % with dies less than half the size when compared to silicon (Si) IGBTs. The SiC JBS diodes offer significant reductions in reverse recovery losses. At 10 kV, there are no Si devices that can compete with SiC on a single device basis. Data on 1.2 kV and 10 kV devices are presented along with future trends.

Performance and Stability of Large-Area 4H-SiC 10-kV Junction Barrier Schottky Rectifiers

The forward and reverse bias dc characteristics, the long-term stability under forward and reverse bias, and the reverse recovery performance of 4H-SiC junction barrier Schottky (JBS) diodes that are capable of blocking in excess of 10 kV with forward conduction of up to 10 A at a forward voltage of less than 3.5 V (at 25degC) are described. The diodes show a positive temperature coefficient of resistance and a stable Schottky barrier height of up to 200degC. The diodes show stable operation under continuous forward current injection at 20 A/cm2 and under continuous reverse bias of 8 kV at 125degC. When switched from a 10-A forward current to a blocking voltage of 3 kV at a current rate-of-fall of 30 A/mus, the reverse recovery time and the reverse recovery charge are nearly constant at 300 ns and 425 nC, respectively, over the entire temperature range of 25degC-175degC.

Drift-Free, 50 A, 10 kV 4H-SiC PiN Diodes with Improved Device Yields

The path to commericializing a 4H-SiC power PiN diode has faced many difficult challenges. In this work, we report a 50 A, 10 kV 4H-SiC PiN diode technology where good crystalline quality and high carrier lifetime of the material has enabled a high yielding process with VF as low as 3.9 V @ 100 A/cm2. Furthermore, incorporation of two independent basal plane dislocation reduction processes (LBPD 1 and LBPD 2) have produced a large number of devices that exhibit a high degree of forward voltage stability with encouraging reverse blocking capability. This results in a total yield (forward, 10 kV blocking, and drift) of >20% for 8.7 mm x 8.7 mm power PiN diode chips—the largest SiC chip reported to date.

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 Area (up to 1.5 cm2) 4H-SiC 10 kV Junction Barrier Schottky Rectifiers

DC characteristics and reverse recovery performance of 4H-SiC Junction Barrier Schottky (JBS) diodes capable of blocking in excess of 10 kV with forward conduction of 20 A at a forward voltage of less than 4 V are described. Performance comparisons are made to a similarly rated 10 kV 4H-SiC PiN diode. The JBS diodes show a significant improvement in reverse recovery stored charge as compared to PiN diodes, showing half of the stored charge at 25°C and a quarter of the stored charge at 125°C when switched to 3 kV blocking. These large area JBS diodes were also employed to demonstrate the tremendous advances that have recently been made in 4H-SiC substrate quality.

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 .

State of the Art 10 kV NMOS Transistors

Rapidly improving 4 H-SiC material quality and a maturing MOS process/design have enabled the development of the largest 10 kV MOSFET to date and the first 10 kV n-IGBT capable of flowing 10 A and 4 A, respectively, with very low on- resistances. With 20 V on the gate, both devices have aVp~ 5V with a positive temperature coefficient for on-resistance that facilitates their use in a parallel configuration. Each device has its own advantages. The conductivity modulated n-IGBT offers higher current density operation (up to 100 A/cm ) while the majority carrier MOSFET offers extremely fast 5 kV switching with only 140 nsec of turn-off time and a manageable 160 W/cm of dissipated power at 20 kHz. These exciting results indicate that the 10 kV SiC NMOS switches may potentially revolutionize emerging high voltage, high frequency power electronics.