Thermal Conductivity of GaN Grown on Silicon Substrates
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Y10.35.1
Thermal Conductivity of GaN Grown on Silicon Substrates C. Mion1, Y.C. Chang1, J. F. Muth1, P. Rajagopal, J. D. Brown2 1 ECE Dept Box 7911, North Carolina State University, Raleigh , NC, 27695 USA. 2 Nitronex Corporation, 628 Hutton Street Suite 106, Raleigh, NC, 27606 USA.
Abstract One of the principle problems of high power electronic devices is the extraction of heat from the active region of the device1. The thermal conductivity of the substrate is a crucial parameter affecting the thermal dissipation capability of a device. In this study we investigated the thermal conductivity at room temperature of five GaN samples grown on silicon, employing the 3 omega thermal conductivity method. The thickness of the GaN layers varied from 0 to 700 nm. The effective thermal conductivity for the GaN layers was found to range from 130 to 140 W/m.K which is comparable to thermal conductivity of the silicon substrate. This result indicates that the heat transfer of GaN on silicon is as good as either GaN or silicon alone, and that no substantial thermal degradation attributable to the GaN/Silicon interface is observed.
Introduction High power transistors operate with high current densities in small geometries that generate significant amounts of heat. Elevated junction temperatures limit device performance. This can lead to device failure, or unreliable performance resulting from changes in device characteristics over time. A great deal of effort is spent in understanding the best way to remove heat from the junction through better device design and efficient packaging. Fundamentally the efficiency of heat extraction is determined by the thermal conductivity of device layers and substrate. Due to the lack of a native bulk GaN or AlN substrates, GaN has been grown on a wide variety of materials. The two dominate substrate materials are sapphire and silicon carbide. Silicon carbide has excellent thermal conductivity of 270 W/m.K2, but is relatively expensive and is typically limited to 2 or 4 inch wafers. The thermal conductivity of sapphire at room temperature is relatively low at 35 W/m.K.2 Recently there has been success in using silicon as a substrate for GaN transistors, LEDs and other devices. The use of silicon substrate potentially has several advantages including low cost, scalability to large wafer sizes, and a reasonable thermal conductivity of 145 W/m.K.2 The growth of GaN on silicon carbide and sapphire is typically initiated with a nitridization step followed by a low temperature step that results in island growth. The temperature is then raised, islands coalesce and two dimensional growths follows. A variety of detailed studies have been carried out to understand this process. In the case of GaN on silicon the detail of the formation of a buffer or interfacial layers is relatively unknown. From a heat transfer point of view the presence of a low thermal conductivity interfacial layer can significantly impede removal of heat from the junction. In this paper we investigate the thermal conductivity of the
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