Impurity Effects in the Growth of 4H-SiC Crystals by Physical Vapor Transport
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ABSTRACT SiC is an important wide bandgap semiconductor material for high temperature and high power electronic device applications. Purity improvements in the growth environment has resulted in a two-fold benefit during growth: (a) minimized inconsistencies in the background doping resulting in high resistivity (>5000 ohm-cm) wafer yield increase from 10-15% to 7085%, and (b) decrease in micropipe formation. Growth parameters play an important role in determining the perfection and properties of the SiC crystals, and are extremely critical in the growth of large diameter crystals. Several aspects of growth are vital in obtaining highly perfect, large diameter crystals, such as: (i) optimized furnace design, (ii) high purity growth environment, and (iii) carefully controlled growth conditions. Although significant reduction in micropipe density has been achieved by improvements in the growth process, more stringent device requirements mandate further reduction in the defect density. In-depth understanding of the mechanisms of micropipe formation is essential in order to devise approaches to eliminate them. Experiments have been performed to understand the role of growth conditions and ambient purity on crystal perfection by intentionally introducing arrays of impurity sites on one half of the growth surface. Results clearly suggest that presence of impurities or second phase inclusions during start or during growth can result in the nucleation of micropipes. Insights obtained from these studies were instrumental in the growth of ultra-low micropipe density (less than 2 micropipes cm-2 ) in 1.5 inch diameter boules. INTRODUCTION To effectively compete as a viable semiconductor material suitable for commercial and military technologies, SiC crystal perfection and diameter of SiC substrates must approach at least GaAs-like specifications. Silicon Carbide (SiC) crystals are of special interest compared to other contemporary high frequency semiconductors, such as Si and GaAs, because of its unique properties for high power density microwave generation, for high power switching, and for high temperature and radiation tolerant operation. As shown in Table 1,SiC exhibits a higher thermal conductivity (kT), a higher critical breakdown field (E), and a saturated velocity (V,) equal to Table I -- Comparison of fundamental properties of Si, GaAs, and SiC for device applications E,(eV) E,,(MV/cm) V.,(00'cm/s) K,(W/cmK) e Si GaAs 6H-SiC 14H-SiC
1.12 1.42 3.02 3.26
0.6 0.6 3.2 3.0
1.0 2.0 2.0 2.0
1.5 0.5 3.0* 3.0*
245 Mat. Res. Soc. Symp. Proc. Vol. 572 ©1999 Materials Research Society
11.8 11.8 10.0 9.7
GaAs at high fields desirable for high power devices. As a consequence, solid-state SiC electronics are having a pervasive impact on advanced DOD systems, as well as on commercial applications, ranging from passively cooled high power and high voltage switches, satellite communications, and cellular phones, to nuclear instrumentation, where high temperature and radiation tolerant SiC properties offer major advantages over
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