High Spatial Resolution Thermal Conductivity investigation of SiC Wafers
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High Spatial Resolution Thermal Conductivity investigation of SiC Wafers
D.I. FLORESCU*, FRED H. POLLAK*, G.R. BRANDES**, B.E. LANDINI**, and A.D. SALANT** * Physics Department and New York State Center for Advanced Technology in Ultrafast Photonic Materials and Applications, Brooklyn College of CUNY, Brooklyn, NY 11210 ** ATMI, Inc., 7 Commerce Drive, Danbury, CT 06810
ABSTRACT Silicon carbide (SiC) is a material with very attractive properties for high power/high temperature electronic devices. Its mechanical strength, high thermal conductivity (κ), large bandgap, and extreme chemical inertness are a few of the characteristics making SiC interesting for semiconductor electronics. Due to the significant head load generated over large areas in high power devices, it is desirable for the thermal properties of the substrate to be uniform and optimal. Scanning thermal microscopy (SThM), which provides nondestructive, absolute measurements of the thermal conductivity with a spatial/depth resolution in the 2-3 µm range, was used to examine the room temperature κ as a function of position of four 2” diameter SiC wafers. Wafers of 4H and 6H polytype were fabricated with carrier concentrations in the (1-3)x1018 cm-3 and (6-9)x1017 cm-3 ranges, respectively. A radial distribution of the thermal conductivity was determined for all the investigated samples. For a radius r < r1 (r1 ~ 0.3”) and r > r2 (r2 ~ 0.7”) highest thermal conductivity values were measured in the range of (3.8-3.9) W/cm-K, comparable to the highest κ reported for this material [D. Morelli et al., Inst. Phys. Conf. Ser. 137, 313 (1993); E.A. Burgemeister et al., J. Appl. Phys. 50, 5790 (1979)]. For r1 < r < r2 the thermal conductivity drops to about (2.85-3.25) W/cm-K interval. Atomic force microscopy (AFM) investigation reveals that the influence of surface roughness effects on κ is negligible. The κ dip may arise from a higher basal plane defect density in this region that could be associated with the presence of super screw dislocations, or “micropipes” [M. Dudley et al., J. Phys. D: Appl. Phys. 28, A63 (1995)]. The implications of these findings for device applications and design are considered.
INTRODUCTION Recent interest in developing advanced electronic devices that operate at high temperature and/or high power has brought into attention many new challenges for E2.4.1
semiconductor materials and the related processing technology [1]. Some of the compound semiconductors, such as SiC and group III-nitrides have significant advantages for device applications because of their wider band gaps (higher operating temperatures), larger breakdown fields (higher operating voltage), higher electron saturated drift velocity (higher operating current and faster switching), and better thermal conductivity (κ) coefficient (higher power density) [2]. Since most of these applications are related to high power applications it is essential to understand the thermal characteristics of SiC. The thermal conductivity of solid materials consists of both lattice and ele
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