Richard Siergiej

Greater than 20% radiant heat conversion efficiency of a thermophotovoltaic radiator/module system using reflective spectral control

An InGaAs monolithic interconnected module (MIM) using reflective spectral control has been fabricated and measured in a thermophotovoltaic radiator/module system (radiator, optical cavity, and thermophotovoltaic module). Results showed that at a radiator and module temperature of 1039/spl deg/C and 25/spl deg/C, respectively, 23.6% thermophotovoltaic radiator/module system radiant heat conversion efficiency and 0.79W/cm/sup 2/ maximum thermophotovoltaic radiator/module system power density were obtained. The use of reflective spectral control increased the spectral efficiency and thus the thermophotovoltaic radiator/module system radiant heat conversion efficiency by /spl sim/16% (relative). However, the amount of useful radiation reaching the MIM decreased by /spl sim/7% (relative) compared to using transmissive spectral control. Also, the thermophotovoltaic system radiant heat conversion efficiency and maximum power density using either transmissive or reflective spectral control decreased as the MIM temperature increased. The MIM using reflective spectral control was found to be more sensitive to changes in the MIM temperature than the MIM using transmissive spectral control.

MOCVD growth of lattice-matched and mismatched InGaAs materials for thermophotovoltaic energy conversion

The details of MOCVD growth of lattice-matched (0.74 eV) and lattice-mismatched (0.55 eV and 0.6 eV) InGaAs-based thermophotovoltaic (TPV) devices on InP substrates are discussed. The optimization of growth conditions, structural parameters and run-to-run consistency have played a key role in the development of high quality TPV devices, particularly in the development of lattice-mismatched materials.

Thermophotovoltaic system testing

To illustrate the variety and complexity of thermophotovoltaic (TPV) system designs, the present status of selected systems, ranging in power output from 50 W to 2 kW, is reviewed. The design and analysis of each of these power systems is complex due to the interactions among the radiator, photonic cavity and filter/semiconductor device elements. To draw meaningful conclusions and aid the development of new power systems, a methodology for measuring and predicting the performance of system designs is required. To first order, this includes an understanding of the semiconductor diode characteristics, emitter/filter spectra and radiative properties of the system components.

Investigating thin film stresses in stacked silicon dioxide/silicon nitride structures and quantifying their effects on frequency response

We have investigated the frequency shift caused by thin film stresses in silicon nitride (Si3N4) and silicon dioxide (SiO2) layers used during the fabrication of lead zirconium titanate (PZT) microelectromechanical (MEMS) devices. The films are deposited via thermal oxidation and low pressure chemical vapor deposition onto bare silicon wafers. Although previous research has reported stresses in these films, this paper introduces approaches to measure, model and predict the effects of these stresses on the frequency response of MEMS devices. Thin film stresses in each layer of a PZT stack are measured using a curvature measurement system, and these measurements are correlated to finite element analysis results and experimental vibration data. These comparisons illustrate the significance of the thin film stresses and their effect on the mechanical behavior of MEMS devices, in particular that as the membrane structures get thinner the frequency shift becomes much larger.

InGaAsP/InGaAs Tandem TPV Device

Power conversion in a thermophotovoltaic (TPV) system can be accomplished with single p‐n junctions which are interconnected on‐wafer (monolithic interconnected module (MIM) approach) or off‐wafer (individual chip module approach) to form large area arrays. Using a MIM architecture, 0.6 eV InGaAs diodes grown lattice mismatched to InP have produced a power conversion efficiency of 23% and a power density of 0.65 W/cm2 at cell and radiator temperatures of 25°C and 1000°C, respectively. A shortcoming of a single p‐n junction is inefficient use of the incident spectrum due to over‐excitation losses from high, above bandgap energy photons. In order to overcome these losses, tandem TPV devices have been proposed which comprise two or more p‐n junctions of differing bandgaps. In this work we report on the growth, fabrication and characterization of InGaAsP/InGaAs (0.72/0.60 eV) tandem TPV diodes. Electrical measurements using a grey body source reveal an open circuit voltage of 0.504 V/cell and a fill factor of...

Advanced Thermophotovoltaic Devices for Space Nuclear Power Systems

Advanced thermophotovoltaic (TPV) modules capable of producing > 0.3 W/cm2 at an efficiency > 22% while operating at a converter radiator and module temperature of 1228 K and 325 K, respectively, have been made. These advanced TPV modules are projected to produce > 0.9 W/cm2 at an efficiency > 24% while operating at a converter radiator and module temperature of 1373 K and 325 K, respectively. Radioisotope and nuclear (fission) powered space systems utilizing these advanced TPV modules have been evaluated. For a 100 We radioisotope TPV system, systems utilizing as low as 2 general purpose heat source (GPHS) units are feasible, where the specific power for the 2 and 3 GPHS unit systems operating in a 200 K environment is as large as ∼ 16 We/kg and ∼ 14 We/kg, respectively. For a 100 kWe nuclear powered (as was entertained for the thermoelectric SP‐100 program) TPV system, the minimum system radiator area and mass is ∼ 640 m2 and ∼ 1150 kg, respectively, for a converter radiator, system radiator and environ...

Pilot-Production Yield of Indium Phosphide-Based Thermophotovoltaic Monolithically Interconnected Modules

Yield data from a pilot-production run of thermophotovoltaic (TPV) devices are presented. A single lattice-mismatched 0.6 eV InGaAs epilayer device design was grown on 166 3-inch InP wafers by metalorganic vapor phase epitaxy (MOVPE) in a commercial reactor using standard chemical precursors. Epiwafers were processed in batch-style as 30-junction monolithically interconnected modules (MIMs) using standard proximity photolithography, wet chemical etching and plasma-enhanced chemical vapor deposition (PECVD). It is understood that yield is the product of several loss parameters. This work has shown that process consistency can be maintained and a reduction of losses at crystal growth can be achieved with the implementation of appropriate characterization techniques such as surfscan, photoreflectance, triple-axis X-ray diffraction (TAXRD) and in-situ temperature monitoring. In addition, InP as a substrate material has often been associated with an undesirably high incidence of wafer breakage. However, this work has shown that with some care in wafer handling, mechanical yield issues are not necessarily worse than in a standard GaAs fabrication line

Lattice-Mismatched InGaAsP and AlGaInAs Quaternary Materials for Thermophotovoltaic Applications

Experimental results are presented for lattice-mismatched (LMM) In 0.8Ga0.2As0.76P0.24 and Al 0.07Ga0.255In0.675As quaternary materials used in thermophotovoltaic (TPV) devices. Epistructures were grown on 3-inch diameter InP substrates by metalorganic vapor phase epitaxy (MOVPE) and processed as monolithically interconnected modules (MIMs) using conventional photolithography and chemical wet etching. Electrical characterization of single-bandgap quaternary devices revealed that the two have roughly equivalent electrical performance. Comparable X-ray diffraction data, high voltage factors and fill factors indicate the materials are of high crystalline and electrical quality, and perform comparably to expectations set by long experience with 0.6 eV lattice-mismatched InGaAs devices of similar device design. Integration of the high and low bandgaps into tandem devices has proven problematic, however, suggesting unresolved materials and processing issues

Effect of Metal Coverage on the Performance of 0.6‐eV InGaAs Monolithic Interconnected Modules

With the device performance of 0.6eV InGaAs monolithic interconnected modules (MIMs) reaching open circuit voltages of 400 mV/junction and achieving excellent quantum efficiency, the next step to improve performance focuses on controlling the parasitic optical absorption in these MIMs. With an integrated spectral control approach, the design of grid finger and interconnect metallization affects both the output power and the optical absorption of the MIM. The effect of metal coverage on the optical and electrical performance of MIMs processed in a multi‐wafer environment is presented.