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Ultrafast time-resolved measurements were conducted to investigate long-wavelength acoustic phonon scattering and velocity reduction in Bi2Te3/Sb2Te3 superlattices. We show that both these phenomena suppress heat transfer process, with the phonon scattering contributing more in differentiating the lattice thermal conductivities among films with different periods. Measurements of reduction in the acoustic phonon amplitudes support the decrease in the thermal conductivity for certain superlattice periods, which is not predicted by acoustic mismatch theory. This study is a direct measurement of coherent acoustic phonons in superlattices which is of significant interest to thermoelectrics.

SiGe based Focal Plane Arrays offer a low cost alternative for developing visible- NIR focal plane arrays that will cover the spectral band from 0.4 to 1.6 microns. The attractive features of SiGe based IRFPA's will take advantage of Silicon based technology, that promises small feature size, low dark current and compatibility with the low power silicon CMOS circuits for signal processing. This paper discusses performance comparison for the SiGe based VIS-NIR Sensor with performance characteristics of InGaAs, InSb, and HgCdTe based IRFPA's. Various approaches including device designs are discussed for reducing the dark current in SiGe detector arrays; these include Superlattice, Quantum dot and Buried junction designs that have the potential of reducing the dark current by several orders of magnitude. The paper also discusses approaches to reduce the leakage current for small detector size and fabrication techniques. In addition several innovative approaches that have the potential of increasing the spectral response to 1.8 microns and beyond.

The team of RTI International (RTI) and D-STAR Engineering (D-STAR) and our government partner US Army CERDEC (CERDEC) are pleased to provide SERDP this technical report for the program entitled Efficient Thermoelectric Power Conversion of Waste Heat for Deployed Forces (SI-1651) . This report will focus on the state of the waste heat conversion system at the end of the first proposed phase of the three-phase program and look forward to designs that could address issues encountered in the first phase. This report will first build an appropriate background based on data to show the evolution of the expected performance of the waste heat recovery system over phase 1. What will be shown are the changes in expected thermoelectric device efficiency and power output based on the actual available T from the specific 3kW TQG that the team has procured for this program. This analysis will culminate in an estimation of the maximum power output with current technology for thermoelectric conversion and heat exchangers. Following this analysis, the actual performance of the phase 1 system will be discussed including power output and system level integration issues encountered and the lessons learned in the program. Finally, this report will show initial designs for future work that could address many of the issues encountered in phase 1 that could be used for future success on such a program. Data was gathered on the generator set during the course of phase 1, which the team had to react to in the design and expectations for the heat recovery system. The most important piece of data was the lower available T that was measured which caused the team to lower expectations for efficiency of the thermoelectric (TE) convertor. Also, some changes in power output targets for each phase are requested after analysis of the generator exhaust heat content has shown that the ability to reach the original targets may not be possible., Prepared in collaboration with D-STAR Engineering, Seymour, CT, and RTI International, Research Triangle Park, NC.

Femtosecond pulses are used to excite A(1g) optical phonons in Bi2Te3, Sb2Te3, and Bi2Te3/Sb2Te3 superlattice. Time-resolved reflectivity measurements show both the low-frequency and high-frequency components of A(1g) phonon modes. By comparing the phonon lifetime, it is found that the scattering rate (inverse of lifetime) in superlattice is significantly higher than those in Bi2Te3 and Sb2Te3. This represents the direct measurement of coherent phonon lifetime reduction in superlattice structures, consistent with the observed reduction in thermal conductivity in superlattices. (c) 2008 American Institute of Physics.