Chris LaBounty

Thermal conductivity of Si/SiGe and SiGe/SiGe superlattices

The cross-plane thermal conductivity of four Si/Si0.7Ge0.3 superlattices and three Si0.84Ge0.16/Si0.76Ge0.24 superlattices, with periods ranging from 45 to 300 and from 100 to 200 A, respectively, were measured over a temperature range of 50 to 320 K. For the Si/Si0.7Ge0.3 superlattices, the thermal conductivity was found to decrease with a decrease in period thickness and, at a period thickness of 45 A, it approached the alloy limit. For the Si0.84Ge0.16/Si0.76Ge0.24 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/Si0.7Ge0.3 than for Si0.84Ge0.16/Si0.76Ge0.24. The thermal conductivity increased slightly up to about 200 K, but was relatively independent of temperature from 200 to 320 K.

SiGeC/Si superlattice microcoolers

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 3omega method. SiGeC/Si superlattice microcoolers with dimensions as small as 40×40 µm^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.

Thermionic emission cooling in single barrier heterostructures

Nonisothermal transport in InGaAsP-based heterostructure integrated thermionic coolers is investigated experimentally. Cooling on the order of a degree over 1 μm thick barriers has been observed. This method can be used to enhance thermoelectric properties of semiconductors beyond what can be achieved with the conventional Peltier effect.

Enhanced Thermionic Emission Cooling in High Barrier Superlattice Heterostructures

Abstract : Thermionic emission current in heterostructures can be used to enhance thermoelectric properties beyond what can be achieved with conventional bulk materials. The Bandgap discontinuity at the junction between two materials is used to selectively emit hot electrons over a barrier layer from cathode to anode. This evaporative cooling can be optimized at various temperatures by adjusting the barrier height and thickness.

Monolithic integration of thin-film coolers with optoelectronic devices

Active refrigeration of optoelectronic components through the use of monolithically grown thin-film solid-state coolers based on III-V materials is proposed and investigated. Enhanced cooling power compared to the thermoelectric effect of the bulk material is achieved through thermionic emission of hot electrons over a heterostructure barrier layer. These heterostructures can be monolithically integrated with other devices made from similar materials. Experimental analysis of an InP pin diode monolithically integrated with a heterostructure thermionic cooler is performed. Cooling performance is investigated for various device sizes and ambient temperatures. Several important nonideal effects are determined, such as contact resistance, heat generation and conduction in the wire bonds, and the finite thermal resistance of the substrate. These nonideal effects are studied both experimentally and analytically, and the limitations induced on performance are considered. Heterostructure integrated thermionic cooling is demonstrated to provide cooling power densities of several hundred W/cm2. These microrefrigerators can provide control over threshold current, power output, wavelength, and maximum operating temperature in diode lasers.

MODELING AND OPTIMIZATION OF SINGLE-ELEMENT BULK SiGe THIN-FILM COOLERS

Abstract Modeling and optimization of bulk SiGe thin-film coolers are described. Thin-film coolers can provide large cooling power densities compared to commercial thermoelectrics. Thin-film SiGe coolers have been demonstrated with maximum cooling of 4°C at room temperature and with cooling power density exceeding 500 W/cm2. Important parameters in the design of such coolers are investigated theoretically and are compared with experimental data. Thermoelectric cooling, joule heating, and heat conduction are included in the model as well as non-ideal effects such as contact resistance, geometrical effects, and three-dimensional thermal and electrical spreading resistance of the substrate. Simulations exhibit good agreement with experimental results for bulk Si and SiGe thin-film coolers. It turned out that in many spot cooling applications using two n- and p-elements electrically in series and thermally in parallel does not give significant improvement over single leg elements. This is in contrast to conventional thermoelectric modules and is due to the aspect ratio and special geometry of thin film coolers. With optimization of SiGe thin-film cooler, simulations predict it can provide over 16°C with cooling power density of over 2000 W/cm2.

Integrated cooling for Si-based microelectronics

Thin film thermoelectric coolers are advantageous for their high cooling power density and their potential integrated applications. Si/sub 1-x/Ge/sub x/ is a good thermoelectric material at high temperatures and superlattice structures can further enhance the device performance. Si/sub 1-x/Ge/sub x/ and Si/sub 1-x/Ge/sub x//Si superlattice structures were grown on Si substrates using molecule beam epitaxy. Si/sub 1-x/Ge/sub x/ and Si/sub 1-x/Ge/sub x//Si superlattice thin film microcoolers with film thickness of the order of several microns were fabricated using integrated circuit processing technology. Micro thermocouples and integrated thermistor sensors were used to characterize these coolers. Maximum cooling power density on the order of hundreds of watts per square centimeter was measured at room temperature. It is possible to monolithically integrate these coolers with Si-based microelectronic devices for localized cooling and temperature stabilization.

Thermoreflectance imaging of superlattice micro refrigerators

High resolution thermal images of semiconductor micro refrigerators are presented. Using the thermoreflectance method and a high dynamic range PIN array camera, thermal images with 50 mK temperature resolution and high spatial resolution are presented. This general method can be applied to any integrated circuit, and can be used as a tool for identifying fabrication failures. With further optimization of the experimental set-up, we expect to obtain thermal images with sub-micron spatial resolution.

Cooling Power Density of SiGe/Si Superlattice Micro Refrigerators

Experiments were carried out to determine the cooling power density of SiGe/Si superlattice microcoolers by integrating thin film metal resistor heaters on the cooling surface. By evaluating the maximum cooling of the device under different heat load conditions, the cooling power density was directly measured. Both micro thermocouple probes and the resistance of thin film heaters were used to get an accurate measurement of temperature on top of the device. 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. Various device sizes were characterized. The maximum cooling and the cooling power density had different dependences on the micro refrigerator size. Net cooling over 4.1 K below ambient and cooling power density of 598 W/cm 2 for 40 × 40 µm 2 devices were measured at room temperature.

THERMAL CONDUCTIVITY OF INDIUM PHOSPHIDE-BASED SUPERLATTICES

Semiconductor superlattice structures have shown promise as thermoelectric materials for their high power factor and low thermal conductivity. While the power factor of a superlattice can be controlled through band gap engineering and doping, prediction and control of thermal conductivity has remained a challenge. The thermal conductivity of three different InP/InGaAs superlattices was measured to be between 4 and 9 W/m-K from 77-320 K using the 3! method. Although the thermal conductivity of InP is an order of magnitude higher than that of InGaAs, we report the intriguing observation that as the fraction of InP is increased in InP/InGaAs superlattices, the thermal conductivity decreases. For one superlattice, the thermal conductivity was even below that of InGaAs. These observations are contrary to predictions of effective thermal conductivity by the Fourier law.Semiconductor superlattice structures have shown promise as thermoelectric materials for their high power factor and low thermal conductivity. While the power factor of a superlattice can be controlled through band gap engineering and doping, prediction and control of thermal conductivity has remained a challenge. The thermal conductivity of three different InP/InGaAs superlattices was measured to be between 4 and 9 W/m-K from 77-320 K using the 3! method. Although the thermal conductivity of InP is an order of magnitude higher than that of InGaAs, we report the intriguing observation that as the fraction of InP is increased in InP/InGaAs superlattices, the thermal conductivity decreases. For one superlattice, the thermal conductivity was even below that of InGaAs. These observations are contrary to predictions of effective thermal conductivity by the Fourier law.