Influence of Doping Concentration and Ambient Temperature on the Cross-Plane Seebeck Coefficient of InGaAs/InAlAs superlattices

We have developed thin film heaters/sensors that can be integrated on top of superlattice microcoolers to measure the Seebeck coefficient perpendicular to the layer. In this paper, we discuss the Seebeck coefficients of InGaAs/InAlAs superlattices grown with Molecular Beam Epitaxy (MBE) that have different doping concentrations, varying between 2e18, 4e18, and 8e18 to 3e19 cm. It was interesting to find out that -contrary to the behavior in bulk material -the Seebeck coefficient did not decrease monotonically with doping concentration. A preliminary theory of thermoelectric transport in superlattices in the regime of miniband formation has been developed to fit the experimental results. The miniband formation could enhance the thermoelectric power factor (Seebeck coefficient square times electrical conductivity) and thereby improve the Figure of merit, ZT. With this improvement, InGaAs/InAlAs superlattice microcooler become a promising candidate for on-chip temperature control. Introduction Lasers and Optoelectronic devices are very sensitive to chip temperatures. Heating has various detrimental effects on the device’s performance. For example, a typical distributed Bragg reflector laser (DBR) [1] exhibits wavelength changes due to temperature fluctuations as pronounced as 0.28nm/C, however, the channel spacing for a Wavelength Division Multiplexing (WDM) system is only about 0.2~0.4nm. Thus, one or two degree temperature changes will result in crosstalk between neighboring channels. Furthermore, with increasing temperatures, the threshold current density, output power, and the spectral linewidth of optoelectronic devices will also change. In all, increasing temperatures have been the bottleneck for optoeletronics, preventing improved speed, bandwidth, and stability. The most commonly used thermoelectric coolers are made of Bi2Te3. Although the absolute cooling of these commercial devices can reach 70C, however, its large device size, its leg length of at least a few mm, its low Carnot efficiency 6~8%, and the bulk fabrication technology, make it incompatible with the micro-sized optoelectronic devices. Intensive studies looking for monolithically grown micro-coolers based on III-V materials in order to realize the goal of onchip temperature stabilization have been done . However, the bulk InP and GaAs are very inefficient thermoelectronic materials. Finding an answer to the question, “how can the figure of merit for InP and GaAs be improved?”, is the key to success. Since 1993 Hicks, L.D. and Dresslhaus, M.S. showed that the thermoelectric properties could be enhanced via low dimensional structures, like superlattices and nanowires. [7,8] Since the improvement due to electron confinement in low dimensions is mainly in the plane of the multiquantum well structure, integration with optoelectronic devices and reducing the parasitic effect of the substrate has remained a key challenge. Some researchers have been focused on crosssection superlattice microcoolers and achieved noticeable success: R. Venkatasubramanian et.al. [9] demonstrated absolute cooling of 32K at room temperature for P-type, Bi2Te3/Sb2Te3 superlattice with a ZT of 2.4; T.C. Harman et. al. showed 43.7K cooling for PbSeTe/PbTe quantum dots superlattice with a ZT of 1.6; JiZhi Zhang et. al. [6] recently experimentally demonstrated AlGaAs superlattice microcooler cools 0.8C at 25C and 2C at 100C; UCSC/UCSB collaboration also demonstrated an N-type InP substrate based InGaAs/InP superlattice microcooler with a maximum net cooling of 2.5C . It is likely that the enhanced cooling by the superlattice structure is a result of: (i) thermal conductivity reduction; (ii) thermionic transport by electron filtering effects. Both factors increase the figure of merit,