Stephan G. Müller

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.

Approaches to Stabilizing the Forward Voltage of Bipolar SiC Devices

We identify several promising approaches to PiN diode fabrication, which greatly reduce forward voltage (Vf) drift in PiN diodes fabricated on standard 8° off-axis 4H-SiC substrates. Our best results require thick buffer layers and growing the entirety of the active device structure without interruption. We address the roles of buffer and drift layer thickness, continuous growth, processing variations and alternative substrate preparation, including ) 0 2 11 ( substrates, on Vf drift. Lastly we report on progress made to reduce the density of stacking fault nucleation sites in PiN diodes.

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 .

Large Diameter 4H-SiC Substrates for Commercial Power Applications

The SiC power device market is predicted to grow exponentially in the next few years. In the development of substrates for this emerging commercial market, it is imperative to develop the product to meet the needs of the targeted application. In this paper we will discuss the status and requirements for SiC substrates for power devices such as Schottky and PiN diodes. For example, for the SiC Schottky device where current production is approaching 50 amp devices, there are several substrate material aspects that are key. These include: wafer diameter (3-inch and 100 mm), micropipe density (<1cm -2 for 3-inch substrates and as low as 30cm -2 for 100-mm substrates), dislocation density, and wafer cost.

Large Area SiC Epitaxial Layer Growth in a Warm-Wall Planetary VPE Reactor

Experimental results are presented for SiC epitaxial layer growths employing a largearea, 7x3-inch, warm-wall planetary SiC-VPE reactor. This high-throughput reactor has been optimized for the growth of uniform 0.01 to 30-micron thick, specular, device-quality SiC epitaxial layers with background doping concentrations of <1x1014 cm-3. Multi-layer device profiles such as Schottky, MESFETs, SITs, and BJTs with n-type doping from ~1x1015 cm-3 to >1x1019 cm-3, p-type doping from ~3x1015 cm-3 to >1x1020 cm-3, and abrupt doping transitions (~1 decade/nm) are regularly grown in continuous growth runs. Intrawafer layer thickness and n-type doping uniformities of <1% and <5% s/mean have been achieved. Within a run, wafer-to-wafer thickness and doping variation are ~±1% and ~±5% respectively. Long term run-to-run variations while under process control are approximately ~3% s/mean for thickness and ~5% s/mean for doping. Latest results from an even larger 6x4-inch (100-mm) reactor are also presented.

GROWTH OF SiC SUBSTRATES

In recent years SiC has metamorphisized from an R&D based materials system to emerge as a key substrate technology for a significant fraction of the world production of green, blue and ultraviolet LEDs. Emerging markets for SiC homoepitaxy include high-power switching devices and microwave devices. Applications for heteroepitaxial GaN-based structures on SiC substrates include lasers 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.