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One of the critical components in a thermophotovoltaic (TPV) system is the infrared-sensitive photovoltaic (PV) semiconductor device that converts the emitted radiation to electricity. Currently, several semiconductor material systems are under development by various workers in the field. The most common are InGaAs/InP, GaSb, and InGaSbAs/GaSb. These devices normally have electronic energy bandgap values in the range of 0.50-0.74 eV. In addition, the design and structure of these devices fall into two distinct formats: conventional planar and monolithic interconnected module (MIM). The conventional planar devices normally have one semiconductor junction and are high-current/low-voltage devices. In a MIM, small area PV cells are connected in series monolithically, on a semi-insulating substrate. This results in the formation of a single high-voltage/low-current module. Electrical and optical performance results for MIMs with electronic energy bandgaps of 0.60 and 0.74 eV will be presented.

The demand for spacecraft power has dramatically increased recently. Higher efficiency, multi-junction devices are being developed to satisfy the demand. The multi-junction cells presently being developed and flown do not employ optimized bandgap combinations for ultimate efficiency due to the traditional constraint of maintaining lattice match to available substrates. We are developing a new approach to optimize the bandgap combination and improve the device performance that is based on relaxing the condition of maintaining lattice match to the substrate. We have designed cells based on this approach, fabricated single junction components cells and tested their performance. We will report on our progress toward achieving beginning-of-life AMO multi-junction device conversion efficiencies above 30%.

A monolithic interconnected module (MIM) structure has been developed for thermophotovoltaic (TPV) applications. The MIM consists of many individual InGaAs cells series-connected on a single semi-insulating (S.I.) InP substrate. An infrared (IR) back surface reflector (BSR), placed on the rear surface of the substrate, returns the unused portion of the TPV radiator output spectrum back to the radiator for recuperation, thereby providing for high system efficiencies. MIMs were fabricated with an active area of 0.9×l cm, and with 15 cells monolithically connected in series. Both lattice-matched and lattice-mismatched InGaAs/InP devices were fabricated, with bandgaps of 0.74 and 0.55 eV, respectively. The 0.74 eV MIMs demonstrated an open-circuit voltage (Voc) of 6.16 V and a fill factor of 74.2% at a short-circuit current (Jsc) of 0.84 A/cm2, under flashlamp testing. The 0.55 eV MIMs demonstrated a Voc of 4.85 V and a fill factor of 57.87percnt; at a Jsc of 3.87 A/cm2 Electrical performance results for these MIMs are presented.

The bandgap of InP is near ideal for the high theoretical conversion efficiency of the air mass zero (AM0) light spectrum experienced in space. This combined with the well known radiation resistance makes InP an ideal candidate for space solar cell use. The p/n configuration has several advantages over the n/p configuration, however, the conversion efficiency of p/n InP cells has been low compared to the n/p configuration, mostly do to poor performance of the Zn doped emitter. We have recently achieved a 17.6% AMO conversion efficiency for a p/n InP cell on an InP substrate and have achieved this record efficiency through understanding of the role of Zn interstitial defects and the hydrogen interactions with these defects. The effect of the Zn defect on device performance will be described.

A series of metals was examined for suitability for the Wheel Abrasion Experiment, one of ten microrover experiments of the Mars Pathfinder Mission. The seven candidate metals were: Ag, Al, Au, Cu, Ni, Pt, and W. Thin films of candidate metals from 0.1 to 1.0 micrometer thick were deposited on black anodized aluminum coupons by e-beam and resistive evaporation and chemical vapor deposition. Optical, corrosion, abrasion, and adhesion criteria were used to select Al, Ni, and Pt. A description is given of the deposition and testing of thin films, followed by a presentation of experimental data and a brief discussion of follow-on testing and flight qualification.

The metallurgical and electrical behavior of Au/Zn contacting metallization onp-type InP was investigated as a function of the Zn content in the metallization. It was found that ohmic behavior can be achieved with Zn concentrations as small as 0.05 atomic percent Zn. For Zn concentrations between 0.1 and 36 at.%, the contact resistivity ρc was found to be independent of the Zn content For low Zn concentrations the realization of ohmic behavior was found to require the growth of the compound Au2P3 at the metal-InP interface. The magnitude of ρc is shown to be very sensitive to the growth rate of the interfacial Au2P3 layer. The possibility of exploiting this sensitivity to provide low resistance contacts while avoiding the semiconductor structural damage that is normally attendant to contact formation is discussed.

Near-theoretical-minimum values of specific contact resistivity, ρc (in the mid-to-low E-8 Ω-cm2 range) have been achieved for Ni-based contacts to moderately doped (2E18 cm−3) n-type InP. In each case these values are an order of magnitude lower than those previously achieved. These ultra-low resistivities are shown to result when the metallurgical interaction rate between the contact metal and the semiconductor is sufficiently reduced. Several methods of reducing the metal-InP reaction rate and thus achieving lowered resistivity values are demonstrated. We show, for instance, that the introduction of a thin (100Å) Au layer at the metal-InP interface retards metal-semiconductor intermixing during sintering and results in a ten-fold reduction in pc. Another method consists of ensuring the perfection of the near-surface InP lattice prior to and during contact deposition process. Use of this technique has enabled us to fabricate, for the first time, Ni-only contacts with pc values in the low E-8 Ω-cm2 range. We present an explanation for these observations that is based upon the magnitude of the In-to-P atomic ratio at the metal-InP interface.

A unique process for texturing InP (100) wafers by anisotropic etching has been developed. The process produces irregular V-grooves on the surface, which reduce the surface reflectivity. The process does not require photolithography or masking. The etching characteristics depend on doping, with etching tending to proceed more rapidly on the more heavily doped samples. Reduced reflectivity surfaces formed using this process can be applied to solar cells, photodetectors, and other optoelectronic devices.

We have investigated the electrical and metallurgical behavior of Ni, Au-Ni, and Au-Ge-Ni contacts on n-InP. We have found that very low values of contact resistivity (pc) in the E-7 Ω-cm2 range are obtained with Ni-only contacts. We show that the addition of Au to Ni contact metallization effects an additional order of magnitude reduction in pc. Ultra-low contact resistivities in the E-8 Ω-cm2 range are obtained with both the Au-Ni and the Au-Ge-Ni systems, effectively eliminating the need for the presence of Ge in the Au-Ge-Ni system. The formation of various nickel phosphides at the metal-InP interface is shown to be responsible for the observed pc values in the Ni and the Au-Ni systems. We show, finally, that the order in which the constituents of Au-Ni and Au-Ge-Ni contacts are deposited has a significant bearing on the composition of the reaction products formed at the metal-InP interface and therefore on the contact resistivity at that interface.

We have investigated the contact spreading phenomenon observed when small area Au contacts on InP are annealed at temperatures above about 400 C. We have found that the rapid lateral expansion of the contact metallization which consumes large quantities of InP during growth is closely related to the third stage in the series of solid state reactions that occur between InP and Au, i.e., to the AU3In-to-AugIn4 transition. We present detailed descriptions of both the spreading process and the Au3In-to-AugIn4 transition along with arguments that the two processes are manifestations of the same basic phenomenon.