Recent progress in SiC single crystal wafer technology

This paper reviews recent developments in silicon carbide (SiC) single crystal wafer technology. The developments include the attainment of wafer diameters up to 100 mm and micropipes with densities less than 1 cm on 4H-SiC wafers with a diameter of 100 mm. Furthermore, high-quality SiC homoepitaxial thin film growth has been achieved, and owing to the availability of large high-quality epitaxial wafers, SiC power devices have begun to show performance levels that largely exceed those produced from Si. In this paper, the problems faced in the development of large high-quality SiC wafers are overviewed in the light of recent achievements. INTRODUCTION Global warming and the remarkable rise in oil prices have made the development in energy-saving technologies imperative. In particular, the efficient use of electricity is of paramount importance since it is the most widely used energy source at home and in the industry. In this respect, an improvement in the performance of power semiconductor devices is critical. Today, these devices are used in nearly everything – from home appliances to automobiles, and therefore, it is necessary to enhance their efficiency. For this reason, the limitations of the material properties of present-day Si power devices are currently being debated, and wide band gap semiconductors such as silicon carbide (SiC) and gallium nitride (GaN) have attracted considerable attention. The technological potential of SiC for power device applications stems from its outstanding physical and electronic properties [1]. A large band gap (roughly three times that of Si) enables device operation at an increased temperature (600°C for transistors) and with low leakage currents. SiC can withstand high electric fields before breakdown, ten times greater than what Si can endure, and also high current densities. The high electron saturation velocity enables SiC to generate high power at high frequencies. SiC has excellent thermal conductivity, larger than that of copper at room temperature, which makes it suitable for high power operation. In the last decade, SiC crystal growth technology has achieved significant progress and enabled the growth of high-quality, large-diameter SiC crystals. The availability of large-diameter SiC single crystals has resulted in the rapid progress in SiC thin film epitaxy and device fabrication. Several device prototypes have already been demonstrated, which include PiN rectifiers with a blocking voltage of 19 kV [2], Schottky barrier diodes (SBDs) with an extremely low recovery current [3], MOSFETs with a blocking voltage of 700 V at a specific on-resistance of 1.8 m cm [4], and SiC commutated gate turn-off thyristors that operate at a current rating of 150 A, with which 180 kVA inverter has been successfully operated [5]. Since 2001, SiC SBDs have been commercially available and applied to several power electronic systems such as switched-mode power supplies and industrial frequency inverters. In this paper, I will overview the recent developments in SiC single crystal wafer technology. In particular, I will describe the issues pertaining to the enlargement of the active area of SiC devices to allow high-current operation. The technology has shown rapid progress, and further developments are expected in high efficiency SiC power devices and systems. SiC CRYSTAL GROWTH SiC bulk crystals are almost always produced by a physical vapor transport (PVT) growth process; in this process SiC source powder sublimes and is recrystallized on a slightly cooled seed crystal at uncommonly high temperatures (> 2400°C). The growth is generally conducted on a SiC {0001} platelet or wafer, resulting in a crystal growth direction being parallel to the <0001> c-axis. The extremely high process temperature for SiC bulk crystal growth gives rise to increased difficulty in growing high-quality crystals as the crystal diameter increases, and the successful development of the diameter-enlargement process has been a key issue for the SiC bulk crystal growth technology. The growth of high-purity SiC homoepitaxial thin films on SiC single crystal wafers is required to obtain n/n wafer structures for power device applications. For the epitaxial growth of SiC, chemical vapor deposition (CVD) is advantageous in that the epitaxial layer thickness and impurity doping can be precisely controlled and made uniform. Several trials of SiC CVD were performed in early days, but a serious problem of 3C inclusions hindered the successful deposition of high-quality SiC thin films. In the late 1980s, a group at Kyoto University developed a new technique called “step-controlled epitaxy” [6], in which surface steps introduced by vicinal {0001} substrates allowed the deposited thin films to succeed the stacking sequence (polytype) of the underlying The 5th International Symposium on Advanced Science and Technology of Silicon Materials (JSPS Si Symposium), Nov. 10-14, 2008, Kona, Hawaii, USA