Gehong Zeng

A comparison of thin film microrefrigerators based on Si/SiGe superlattice and bulk SiGe

Most of the conventional thermal management techniques can be used to cool the whole chip. Since thermal design requirements are mostly driven by the peak temperatures, reducing or eliminating hot spots could alleviate the design requirements for the whole package. Monolithic solid-state microcoolers offer an attractive way to eliminate hot spots. In this paper, we review theoretical and experimental cooling performance of silicon-based microrefrigerators on a chip. Both Si/SiGe superlattice and also bulk SiGe thin film devices have been fabricated and characterized. Direct measurement of the cooling along with material characterization allows us to extract the key factors limiting the performance of these microrefrigerators. Although Si/SiGe superlattice has larger thermoelectric power factor, the maximum cooling of thin film refrigerators based on SiGe alloys are comparable to that of superlattices. This is due to the fact that the superlattice thermal conductivity is larger than bulk SiGe alloy by about 30%.

Study of thermomechanical properties of Si∕SiGe superlattices using femtosecond transient thermoreflectance technique

Using a Femtosecond Transient Thermoreflectance (FTT) technique, we studied the thermomechanical properties of two Si∕SiGe superlattices. A theoretical model is presented which agrees well with the experimental results and allows us to determine the cross-plan thermal conductivity of the superlattices at room temperature. We also show that, from the experimental curve, we can extract the thickness of the metallic film, the longitudinal sound velocity and the refractive index of the superlattice using acoustic echoes and Brillouin oscillation data.

Short time transient thermal behavior of solid-state microrefrigerators

We present detailed experimental and theoretical studies of the short time transient thermal behavior of SiGe superlattice microrefrigerators on a chip. Transient temperature profiles of microrefrigerator devices of different sizes are obtained using thermoreflectance technique. Thermal imaging with submicron spatial resolution, 0.1 K temperature resolution, and 100 ns temporal resolution is achieved. The dynamic behavior of the microrefrigerators shows an interplay between Peltier and Joule effects. Peltier cooling appears first with a time constant of about 10–30 μs, then Joule heating in the device starts taking over with a time constant of about 50–150 μs. The experimental results agree very well with the theoretical predictions based on thermal quadruple method. The difference in the two time constants can be explained considering the three-dimensional thermal resistances and capacitances of the microrefrigerator. In addition this shows that the Joule heating at the top metal/semiconductor interface ...

Thermal characterization of thin film superlattice micro refrigerators

Micro refrigerators based on thin film SiGe/Si superlattices are investigated. Cooling performance of 2.7 degrees at room temperature and 10.8 degrees at 200 C has been measured. Cooling characterization was done using micro thermocouples, and also with the use of visible wavelength thermoreflectance method. Reflectance images of the temperature distribution are presented with spatial resolution better than traditional infrared cameras.

Optimization of doping concentration for three-dimensional bulk silicon microrefrigerators

We designed and fabricated a three-dimensional (3D) silicon microrefrigerator, which demonstrates a cooling power density over 200W/cm with only ~1degC cooling. The high cooling power density is mainly due to the high thermal conductivity and heat spreading effects. These devices have potential application in hot-spots management to reduce the chip peak temperature and realize on chip thermal management. A finite element model is developed to study and optimize these 3D devices. The simulation results showed that the optimized doping concentration to achieve the maximum cooling for these 3D silicon microrefrigerators (5e18 cm-3) is different from the conventional ID device, where S2sigma achieves the maximum at the doping of 5e19 cm-3. At its optimized doping concentration, these silicon microrefrigerators could reach a maximum cooling of 3degC. Further studies prove that this deviation is due to the nonidea factors inherent within the device, e.g. semiconductor-metal contact resistance, Joule-heating from probe contact resistance etc. Thus to optimize the real device, it is necessary to chose a full model considering all the nonideal factors

6 Watt Segmented Power Generator Modules using Bi2Te3 and (InGaAs)1-x(InAlAs)x Elements Embedded with ErAs Nanoparticles.

We report the fabrication and characterization of segmented element power generator modules of 16 x 16 thermoelectric elements consisting of 0.8 mm thick Bi2Te3 and 50 μm thick ErAs:(InGaAs)1-x(InAlAs)x with 0.6% ErAs by volume. Erbium Arsenide metallic nanoparticles are incorporated to create scattering centers for middle and long wavelength phonons, and to form local potential barriers for electron filtering. The thermoelectric properties of ErAs:(InGaAs)1-x(InAlAs)x were characterized in terms of electrical conductivity and Seebeck coefficient from 300 K up to 830 K. Generator modules of Bi2Te3 and ErAs:(InGaAs)1-x(InAlAs)x segmented elements were fabricated and an output power of 6.3 W was measured. 3D finite modeling shows that the performance of thermoelectric generator modules can further be enhanced by the improvement of the thermoelectric properties of the element materials, and reducing the electrical and thermal parasitic losses.

Segmented Power Generator Modules of Bi 2 Te 3 and ErAs:InGaAlAs Embedded with ErAs Nanoparticles

We report the fabrication and characterization of segmented element power generator modules of 254 thermoelectric elements. The element is 1 mm × 1 mm in area, which consists of 300 µm thickness Bi 2 Te 3 and 50 µm thickness ErAs:(InGaAs) 1-x (InAlAs) x , so that each segment can work at different temperature ranges. Erbium arsenide metallic nanoparticles are incorporated to create scattering centers for middle and long wavelength phonons, provide charge carriers, and form local Schottky barriers for electron filtering. The thermoelectric properties of ErAs:InGaAlAs were characterized by variable temperature measurements of thermal conductivity, electrical conductivity and Seebeck coefficient from 300 K to 600 K. Generator modules of Bi 2 Te 3 and ErAs:InGaAlAs segmented elements were fabricated and an output power over 5.5 W was measured. The performance of the thermoelectric generator modules can further be improved by improving the thermoelectric properties of the element material, and reducing the electrical and thermal parasitic losses.

Superlattice microrefrigerators flip-chip bonded with optoelectronic devices

A 3D electrothermal model was developed to study the InP-based thin film In/sub 0.53/Ga/sub 0.47/As/In/sub 0.52/Al/sub 0.48/As superlattice microrefrigerators for various device sizes, ranging from 40/spl times/40/spl mu/m/sup 2/ to 120/spl times/120/spl mu/m/sup 2/. We discussed maximum cooling and cooling power densities for current devices, analyzed the non-idealities of current devices and proposed an optimized structure. The simulation results demonstrated a maximum cooling of 30/spl deg/C with cooling power density over 300 W/cm/sup 2/ with an optimized structure based on the current device geometry. Furthermore, we also demonstrated that a maximum cooling, over 10/spl deg/C with power density over 900 W/cm/sup 2/, could be possible when the current figure of merit of InGaAs/InAlAs superlattice is enhanced five times with the non-conserved lateral momentum. Besides monolithic integration, we also propose a flip-chip bonded solution to integrate these microrefrigerator with the optoelectronic chips. Preliminary 3D electrothermal simulation will be present to analyze its cooling effects for this 2-chip integration model.