Monte Carlo analysis of a monolithic interconnected module with a back surface reflector

Recently, the photon Monte Carlo code, RACER-X, was modified to include wave-length dependent absorption coefficients and indices of refraction. This work was done in an effort to increase the code`s capabilities to be more applicable to a wider range of problems. These new features make RACER-X useful for analyzing devices like monolithic interconnected modules (MIMs) which have etched surface features and incorporates a back surface reflector (BSR) for spectral control. A series of calculations were performed on various MIM structures to determine the impact that surface features and component reflectivities have on spectral utilization. The traditional concern of cavity photonics is replaced with intra-cell photonics in the MIM design. Like the cavity photonic problems previously discussed, small changes in optical properties and/or geometry can lead to large changes in spectral utilization. The calculations show that seemingly innocuous surface features (e.g., trenches and grid lines) can significantly reduce the spectral utilization due to the non-normal incident photon flux. Photons that enter the device through a trench edge are refracted onto a trajectory where they will not escape. This leads to a reduction in the number of reflected below bandgap photons that return to the radiator and reduce the spectral utilization. In addition, trenches expose a lateral conduction layer in this particular series of calculations which increase the absorption of above bandgap photons in inactive material

Frequency Selective Surfaces as Near Infrared Electro-Magnetic Filters for Thermophotovoltaic Spectral Control

Frequency selective surfaces (FSS) effectively filter electromagnetic radiation in the microwave band (1 mm to 100 mm). Interest exists in extending this technology to the near infrared (1 {micro}m to 10 {micro}m) for use as a filter of thermal radiation in thermophotovoltaic (TPV) direct energy conversion. This paper assesses the ability of FSS to meet the strict spectral performance requirements of a TPV system. Inherent parasitic absorption, which is the result of the induced currents in the FSS metallization, is identified as a significant obstacle to achieving high spectral performance

Recent progress in InGaAsSb/GaSb TPV devices

AstroPower is developing InGaAsSb thermophotovoltaic (TPV) devices. This photovoltaic cell is a two-layer epitaxial InGaAsSb structure formed by liquid-phase epitaxy on a GaSb substrate. The (direct) bandgap of the In{sub 1{minus}x}Ga{sub x}As{sub 1{minus}y}Sb{sub y} alloy is 0.50 to 0.55 eV, depending on its exact alloy composition (x,y); and is closely lattice-matched to the GaSb substrate. The use of the quaternary alloy, as opposed to a ternary alloy--such as, for example InGaAs/InP--permits low bandgap devices optimized for 1,000 to 1,500 C thermal sources with, at the same time, near-exact lattice matching to the GaSb substrate. Lattice matching is important since even a small degree of lattice mismatch degrades device performance and reliability and increases processing complexity. Internal quantum efficiencies as high as 95% have been measured at a wavelength of 2 microns. At 1 micron wavelengths, internal quantum efficiencies of 55% have been observed. The open-circuit voltage at currents of 0.3 A/cm{sup 2} is 0.220 volts and 0.280 V for current densities of 2 A/cm{sup 2}. Fill factors of 56% have been measured at 60 mA/cm{sup 2}. However, as current density increases there is some decrease in fill factor. The results to date show that the GaSb-based quaternary compounds provide a viable and high performance energy conversion solution for thermophotovoltaic systems operating with 1,000 to 1,500 C source temperatures

Optical properties of thin semiconductor device structures with reflective back-surface layers

Ultrathin semiconductor device structures incorporating reflective internal or back surface layers have been investigated recently as a means of improving photon recuperation, eliminating losses associated with free carrier absorption in conductive substrates and increasing the above bandgap optical thickness of thermophotovoltaic device structures. However, optical losses in the form of resonance absorptions in these ultrathin devices have been observed. This behavior in cells incorporating epitaxially grown FeAl layers and in devices that lack a substrate but have a back-surface reflector (BSR) at the rear of the active layers has been studied experimentally and modeled effectively. For thermophotovoltaic devices, these resonances represent a significant loss mechanism since the wavelengths at which they occur are defined by the active TPV cell thickness of {approximately} 2--5 microns and are in a spectral range of significant energy content for thermal radiators. This study demonstrates that ultrathin semiconductor structures that are clad by such highly reflective layers or by films with largely different indices of refraction display resonance absorptions that can only be overcome through the implementation of some external spectral control strategy. Effective broadband, below-bandgap spectral control using a back-surface reflector is only achievable using a large separation between the TPV active layers and the back-surface reflector

Recent progress in GaInAsSb thermophotovoltaics grown by organometallic vapor-phase epitaxy

Studies on the materials development of Ga{sub 1{minus}x}In{sub x}As{sub y}Sb{sub 1{minus}y} alloys for thermophotovoltaic (TPV) devices are reviewed. Ga{sub 1{minus}x}In{sub x}As{sub y}Sb{sub 1{minus}y} epilayers were grown lattice matched to GaSb substrates by organometallic vapor phase epitaxy (OMVPE) using all organometallic precursors including triethylgallium, trimethylindium, tertiarybutylarsine, and trimethylantimony with diethyltellurium and dimethylzinc as the n- and p-type dopants, respectively. The overall material quality of these alloys depends on growth temperature, In content, V/III ratio, substrate misorientation, and to a lesser extent, growth rate. A mirror-like surface morphology and room temperature photoluminescence (PL) are obtained for GaInAsSb layers with peak emission in the wavelength range between 2 and 2.5 {micro}m. The crystal quality improves for growth temperature decreasing from 575 to 525 C, and with decreasing In content, as based on epilayer surface morphology and low temperature PL spectra. A trend of smaller full width at half-maximum for low temperature PL spectra is observed as the growth rate is increased from 1.5 to 2.5 and 5 {micro}m/h. In general, GaInAsSb layers grown on (100) GaSb substrates with a 6{degree} toward (111)B misorientation exhibited overall better material quality than layers grown on the more standard substrate (100)2{degree} toward (110). Consistent growth of high performance lattice-matched GaInAsSb TPV devices is also demonstrated

Step structure of GaInAsSb grown by OMVPE (Organometallic Vapor Phase Epitaxy)

The microscopic surface morphology of GaInAsSb grown by organometallic vapor phase epitaxy (OMVPE)on GaSb substrates has been studied by atomic force microscopy. The effects of growth temperature, alloy composition, and substrate misorientation on the step structure were investigated. The competition between thermodynamically driven phase separation and kinetically limited surface diffusion is discussed

OMVPE growth of GaInAsSb in the 2 to 2.4 {micro}m range

Ga{sub 1{minus}x}In{sub x}As{sub y}Sb{sub 1{minus}y} epilayers were grown lattice matched to GaSb substrates by organometallic vapor phase epitaxy using all organometallic precursors, which include triethylgallium, trimethylindium, tertiarybutylarsine, and trimethylantimony. Layers were grown over a temperature range between 525 and 575 C, a V/III ratio range between 0.9 and 1.7, x < 0.2 and y < 0.2, and on (100) GaSb substrates with 2{degree} toward (100) or 6{degree} toward (111)B. The overall material quality of these alloys depends on growth temperature, In content, V/III ratio, and substrate misorientation. A mirror-like surface morphology and room temperature photoluminescence (PL) could be obtained for GaInAsSb layers with peak emission in the wavelength range between 2 and 2.4 {micro}m. Based on epilayer surface morphology and low temperature PL spectra, the crystal quality improves for growth temperature decreasing from 575 to 525 C, and with decreasing In content. In general, GaInAsSb layers grown on (100) GaSb substrates with a 6{degree} toward (111)B misorientation exhibited smoother surfaces and narrower full width at half-maximum values of 4 K PL spectra than layers grown on the more standard substrate 9100 2{degree} toward (110). Nominally undoped GaInAsSb layers grown at 550 C are p-type with 300 K hole concentration of {approximately} 5 {times} 10{sup 15} cm{sup {minus}3} and hole mobility of {approximately} 430 to 560 cm{sup 2}/V-s. The n- and p-type doping of GaInAsSb with diethyltellurium and dimethylzinc, respectively, are also reported

InGaAsSb thermophotovoltaic diode physics evaluation

The hotside operating temperatures for many projected thermophotovoltaic (TPV) conversion system applications are approximately 1,000 C, which sets an upper limit on the TPV diode bandgap of 0.6 eV from efficiency and power density considerations. This bandgap requirement has necessitated the development of new diode material systems, never previously considered for energy generation. To date, InGaAsSb quaternary diodes grown lattice-matched on GaSb substrates have achieved the highest performance. This report relates observed diode performance to electro-optic properties such as minority carrier lifetime, diffusion length and mobility and provides initial links to microstructural properties. This analysis has bounded potential diode performance improvements. For the 0.52 eV InGaAsSb diodes used in this analysis the measured dark current is 2 {times} 10{sup {minus}5} A/cm{sup 2}, versus a potential Auger limit 1 {times} 10{sup {minus}5} A/cm{sup 2}, a radiative limit of 2 {times} 10{sup {minus}6} A/cm{sup 2} (no photon recycling), and an absolute thermodynamic limit of 1.4 {times} 10{sup {minus}7} A/cm{sup 2}. These dark currents are equivalent to open circuit voltage gains of 20 mV (7%), 60 mV (20%) and 140 mV (45%), respectively

Lapped substrate for enhanced backsurface reflectivity in a thermophotovoltaic energy conversion system

A method is described for fabricating a thermophotovoltaic energy conversion cell including a thin semiconductor wafer substrate having a thickness ({beta}) calculated to decrease the free carrier absorption on a heavily doped substrate; wherein the top surface of the semiconductor wafer substrate is provided with a thermophotovoltaic device, a metallized grid and optionally an antireflective (AR) overcoating; and, the bottom surface (10 ft) of the semiconductor wafer substrate is provided with a highly reflecting coating which may comprise a metal coating or a combined dielectric/metal coating

Spectral ellipsometry of GaSb and GaInAsSb: Experiment and modeling

The optical constants {epsilon}(E)[={epsilon}{sub 1}(E)+i{epsilon}{sub 2}(E)] of single-crystal GaSb at 300K have been measured using spectral ellipsometry in the range of 0.3-5.3 eV. The {epsilon}(E) spectra displayed distinct structures associated with critical points (CPs) at E{sub 0} (direct gap), spin-orbit split E{sub 0}+{Delta}{sub 0} component, spin-orbit split (E{sub 1}, E{sub 1}+{Delta}{sub 1}) and (E{sub 0}{prime}, E{sub 0}{prime}+{Delta}{sub 0}{prime}) doublets, as well as E{sub 2}. The experimental data over the entire measured spectral range (after oxide removal) has been fit using the Holden model dielectric function based on the electronic energy-band structure near these CPs plus excitonic and band-to-band Coulomb enhancement effects at E{sub 0}, E{sub 0}+{Delta}{sub 0} and the E{sub 1}, E{sub 1}+{Delta}{sub 1} doublet. In addition to evaluating the energies of these various band-to-band CPs, information about the binding energy (R{sub 1}) of the two-dimensional exciton related to the E{sub 1}, E{sub 1}+{Delta}{sub 1} CPs was obtained. The value of R{sub 1} was in good agreement with effective mass/k{sup {rightharpoonup}}{center_dot}p{sup {rightharpoonup}} theory. The ability to evaluate R{sub 1} has important ramifications for recent first-principles band structure calculations which include exciton effects at E{sub 0}, E{sub 1}, and E{sub 2}. The experimental results were compared to other evaluations of the optical constants of GaSb