Experimental Characterization and Modeling of InP-based Microcoolers
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Experimental Characterization and Modeling of InP-based Microcoolers Rajeev Singh1, Daryoosh Vashaee1, Yan Zhang1, Million Negassi1, Ali Shakouri1* Yae Okuno2, Gehong Zeng2, Chris LaBounty2, John Bowers2 1 Jack Baskin School of Engineering, University of California, Santa Cruz, CA, 95064 2 Electrical and Computer Engineering, University of California, Santa Barbara, CA, 93106 *[email protected] ABSTRACT We present experimental and theoretical characterization of InP-based heterostructure integrated thermionic (HIT) coolers. In particular, the effect of doping on overall device performance is characterized. Several thin-film cooler devices have been fabricated and analyzed. The coolers consist of a 1µm thick superlattice structure composed of 25 periods of InGaAs well and InGaAsP (λgap ≈ 1.3µm) barrier layers 10 and 30nm thick, respectively. The superlattice is surrounded by highly-doped InGaAs layers that serve as the cathode and anode. All layers are lattice-matched to the n-type InP substrate. N-type doping of the well layers varies from 1.5×1018cm-3 to 8×1018cm-3 between devices, while the barrier layers are undoped. Device cooling performance was measured at room-temperature. Device current-versus-voltage relationships were measured from 45K to room-temperature. Detailed models of electron transport in superlattice structures were used to simulate device performance. Experimental results indicate that low-temperature electron transport is a strong function of well layer doping and that maximum cooling will decrease as this doping is increased. Theoretical models of both I-V curves and maximum cooling agree well with experimental results. The findings indicate that low-temperature electron transport is useful to characterize potential barriers and energy filtering in HIT coolers. INTRODUCTION Modern electronic and optoelectronic components require active temperature stabilization for optimum performance. Most optoelectronic devices used in communication systems are composed of InP-based materials. Conventional thermoelectric (TE) coolers can satisfy cooling requirements, but they are prohibitively large and expensive due to difficulties in integration with the devices to be cooled [1]. Heterostructure integrated thermionic (HIT) microcoolers have been proposed to be monolithically integrated with optoelectronic devices, thereby considerably reducing size and cost constraints [2, 3]. TE coolers utilize the Peltier effect for heat transfer. In addition to exploiting the Peltier effect for cooling, HIT microcoolers employ thermionic (TI) emission to increase the cooling power of standard TE coolers. In TI emission, electrons are emitted from the cathode over a potential barrier to the anode. This emission process is useful in heat transfer because electrons with higher energies (hot electrons) are able to traverse the potential barrier to the anode while electrons with lower energies (cold electrons) remain confined by the potential barrier to the cathode. This is an evaporative cooling of the electron gas a
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