The cross-plane thermal conductivity of four Si/Si 0.7 Ge 0.3 superlattices and three Si 0.84 Ge 0.16 /Si 0.76 Ge 0.24 superlattices, with periods ranging from 45 to 300 and from 100 to 200 Å, respectively, were measured over a temperature range of 50 to 320 K. For the Si/Si 0.7 Ge 0.3 superlattices, the thermal conductivity was found to decrease with a decrease in period thickness and, at a period thickness of 45 Å, it approached the alloy limit. For the Si 0.84 Ge 0.16 /Si 0.76 Ge 0.24 samples, no dependence on period thickness was found and all the data collapsed to the alloy value, indicating the dominance of alloy scattering. This difference in thermal conductivity behavior between the two superlattices was attributed to interfacial acoustic impedance mismatch, which is much larger for Si/Si 0.7 Ge 0.3 than for Si 0.84 Ge 0.16 /Si 0.76 Ge 0.24. The thermal conductivity increased slightly up to about 200 K, but was relatively independent of temperature from 200 to 320 K.

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Monolithically integrated active cooling is an attractive way for thermal management and temperature stabilization of microelectronic and optoelectronic devices. SiGeC can be lattice matched to Si and is a promising material for integrated coolers. SiGeC/Si superlattice structures were grown on Si substrates by molecular beam epitaxy. Thermal conductivity was measured by the 3 method. SiGeC/Si superlattice microcoolers with dimensions as small as 40ϫ40 m 2 were fabricated and characterized. Cooling by as much as 2.8 and 6.9 K was measured at 25°C and 100°C, respectively, corresponding to maximum spot cooling power densities on the order of 1000 W/cm 2 . © 2001 American Institute of Physics. ͓DOI: 10.1063/1.1356455͔Thermoelectric ͑TE͒ refrigeration in a solid-state active cooling method with high reliability. Bi 2 Te 3 -based TE coolers are widely used for cooling and temperature stabilization of microelectronic and optoelectronic devices, but their processing is a bulk technology and is incompatible with integrated circuit fabrication process. Solid-state coolers monolithically integrated with microelectronic and optoelectronic devices are an attractive way to achieve compact and efficient cooling. It can lower the cost of fabrication and packaging, and can selectively cool individual key devices instead of the whole chip. However, the thermoelectric figure of merit ͑ZT͒ is quite low for most of the semiconductors used in microelectronics and optoelectronics. This makes it difficult to get high cooling performance. Recently heterostructure thermionic and superlattice coolers have been proposed, and theoretical calculations show that large improvements in ZT can be achieved and efficient refrigeration becomes possible with coolers made of conventional semiconductor materials.

We have grown composite epitaxial materials that consist of semimetallic ErAs nanoparticles embedded in a semiconducting In 0.53 Ga 0.47 As matrix both as superlattices and randomly distributed throughout the matrix. The presence of these particles increases the free electron concentration in the material while providing scattering centers for phonons. We measure electron concentration, mobility, and Seebeck coefficient of these materials and discuss their potential for use in thermoelectric power generators.

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We studied the cross-plane lattice and electronic thermal conductivities of superlattices made of InGaAlAs and InGaAs films, with the latter containing embedded ErAs nanoparticles ͑denoted as ErAs:InGaAs͒. Measurements of total thermal conductivity at four doping levels and a theoretical analysis were used to estimate the cross-plane electronic thermal conductivity of the superlattices. The results show that the lattice and electronic thermal conductivities have marginal dependence on doping levels. This suggests that there is lateral conservation of electronic momentum during thermionic emission in the superlattices, which limits the fraction of available electrons for thermionic emission, thereby affecting the performance of thermoelectric devices.

We characterize cross-plane and in-plane Seebeck coefficients for ErAs: InGaAs/ InGaAlAs superlattices with different carrier concentrations using test patterns integrated with microheaters. The microheater creates a local temperature difference, and the cross-plane Seebeck coefficients of the superlattices are determined by a combination of experimental measurements and finite element simulations. The cross-plane Seebeck coefficients are compared to the in-plane Seebeck coefficients and a significant increase in the cross-plane Seebeck coefficient over the in-plane Seebeck coefficient is observed. Differences between cross-plane and in-plane Seebeck coefficients decrease as the carrier concentration increases, which is indicative of heterostructure thermionic emission in the cross-plane direction.

Low dimensional and nanostructured materials have shown great potential to achieve much higher thermoelectric figure of merits than their bulk counterparts. Here, we study the thermoelectric properties of superlattices in the cross-plane direction using the Boltzmann transport equation and taking into account multiple minibands. Poisson equation is solved self-consistently to include the effect of charge transfer and band bending in the potential profile. The model is verified with the experimental data of cross-plane Seebeck coefficient for a superlattice structure with different doping concentrations. The simulations show that thermoelectric properties of superlattices are quite different from those of bulk materials because the electronic band structure is modified by the periodic potential. The Lorenz numbers of superlattices are surprisingly large at low carrier concentrations and deviate far away from the Wiedemann-Franz law for bulk materials. Under some conditions, the Lorenz number could be reduced by 50% compared to the bulk value. Most significantly, the Seebeck coefficient and the Lorenz number of superlattices do not change monotonically with doping concentration. An oscillatory behavior is observed. The effects of temperature and well and barrier thicknesses on the cross-plane Seebeck coefficient and Lorenz number are also investigated.