High Cooling Power Density of SiGe/Si Superlattice Microcoolers
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High Cooling Power Density of SiGe/Si Superlattice Microcoolers Gehong Zeng, Xiaofeng Fan, Chris LaBounty, John E. Bowers, Edward Croke1, James Christofferson 2, Daryoosh Vashaee 2, Yan Zhang 2, and Ali Shakouri2* Department of Electrical and Computer Engineering, University of California, Santa Barbara, California 93106 1 HRL Laboratories, LLC, Malibu, California, 90265 2 Baskin School of Engineering, University of California, Santa Cruz, CA 95064 ABSTRACT Fabrication and characterization of SiGe/Si superlattice microcoolers integrated with thin film resistors are described. Superlattice structures were used to enhance the device performance by reducing the thermal conductivity, and by providing selective emission of hot carriers through thermionic emission. Thin film metal resistors were integrated on top of the cooler devices and they were used as heat load for cooling power density measurement. Various device sizes were characterized. Net cooling over 4.1 K and a cooling power density of 598 W/cm2 for 40 × 40 µm2 devices were measured at room temperature. INTRODUCTION With the rapid development of VLSI technology, heat generation and thermal management are becoming one of the barriers to further increase clock speeds and decrease feature sizes. There has been an increasing demand for localized cooling and temperature stabilization of optoelectronic devices. Thermoelectric (TE) coolers based on bulk Bi2Te3 are commonly used for electronic and optoelectronic device cooling, but they cannot be directly integrated with the IC fabrication process. Recently p-type BiTe/SbTe thin film coolers have been demonstrated with high thermoelectric figure-of-merit and cooling power density1. Si-based microcoolers are attractive for their potential monolithic integration with Si microelectronics. SiGe is a good thermoelectric material especially for high temperature applications2,3, and superlattice structures can further enhance the cooler performance by reducing the thermal conductivity between the hot and the cold junctions, and by selective emission of hot carriers above the barrier layers in the thermionic emission process4-19. SiGe/Si superlattice structures were grown on Si substrates using molecular beam epitaxy (MBE). Thin film resisters were integrated with the cooler devices. It shows the possibility to monolithically integrate these coolers with Si-based microelectronic devices for localized cooling and temperature stabilization. EXPERIMENTAL DETAIS The structure of the microcooler samples consisted of a 3 µm thick 200 × (5nm Si0.7Ge0.3/10nm Si) superlattice grown symmetrically strained on a buffer layer designed so that the in-plane lattice constant was approximately that of relaxed Si0.8Ge0.2. The doping level is 5×1019 cm-3 for both the superlattice and the buffer layer. A 0.5 µm Si0.9Ge0.1 cap layer was grown on the superlattice with the top 0.25 µm doped to 2 × 1020 cm-3 for device ohmic contact. This Si0.7Ge0.3/Si superlattice has a valance band offset of about 0.2 eV, and hot holes over this barrier produce th
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