R.W. Hoffman

Hydrogen passivation of dislocations in InP on GaAs heterostructures

The effects of hydrogenation on the properties of Zn‐doped InP/GaAs heterostructures grown by metalorganic chemical vapor deposition were studied by current‐voltage (I‐V), deep level transient spectroscopy (DLTS), and photoluminescence. Significant improvements in leakage current and breakdown voltage in InP diodes on GaAs were observed after a 2 h hydrogen plasma exposure at 250u2009°C. DLTS indicated a corresponding reduction in total trap concentration from ∼6×1014 to ∼3×1012 cm−3 at a depth of ∼1.5 μm below the surface. The Zn dopants were completely reactivated by a subsequent 5 min 400u2009°C anneal without degradation of the reverse current or reactivation of the deep levels. Anneals in excess of 580u2009°C were necessary to reactivate the deep levels and degrade the leakage current to their original values, indicating the passivation of threading dislocations by hydrogen, and the existence of a wide temperature window for post‐passivation processing.

Monolithically interconnected InGaAs TPV module development

This paper describes the status of development of an indium gallium arsenide (InGaAs) monolithically-interconnected module (MIM) for thermophotovoltaic (TPV) energy conversion applications. The MIM structure features series interconnected InGaAs sub-cells on an insulating indium phosphide (InP) substrate, with a rear-surface infrared (IR) reflector. Motivations for developing the MIM structure include: reduced resistive losses; higher output power density; improved thermal coupling; and, ultimately, higher system efficiency. An optical model has been developed, free carrier absorption coefficients have been measured and a prototype MIM device has been demonstrated. A rear surface IR reflector has been developed with /spl sim/98% reflectance in the sub-bandgap (>1.7 /spl mu/m) region.

Hydrogen passivation of n+p and p+n heteroepitaxial InP solar cell structures

High-efficiency, heteroepitaxial (HE) InP solar cells, grown on GaAs, Si or Ge substrates, are desirable for their mechanically strong, light-weight and radiation-hard properties. However, dislocations, caused by lattice mismatch, currently limit the performance of the HE cells. This occurs through shunting paths across the active photovoltaic junction and by the formation of deep levels. In previous work we have demonstrated that plasma hydrogenation is an effective and stable means to passivate the electrical activity of dislocations in specially designed HE InP test structures. In this work, we present the first report of successful hydrogen passivation in actual InP cell structures grown on GaAs substrates by metalorganic chemical vapor deposition (MOCVD). We have found that a 2 hour exposure to a 13.56 MHz hydrogen plasma at 275 C reduces the deep level concentration in HE n+n InP cell structures from as-grown values of approximately 10(exp 15)/cm(exp -3), down to 1-2 x 10(exp 13)/cm(exp -3). The deep levels in the p-type base region of the cell structure match those of our earlier p-type test structures, which were attributed to dislocations or related point defect complexes. All dopants were successfully reactivated by a 400 C, 5 minute anneal with no detectable activation of deep levels. I-V analysis indicated a subsequent approximately 10 fold decrease in reverse leakage current at -1 volt reverse bias, and no change in the forward biased series resistance of the cell structure which indicates complete reactivation of the n+ emitter. Furthermore, electrochemical C-V profiling indicates greatly enhanced passivation depth, and hence hydrogen diffusion, for heteroepitaxial structures when compared with identically processed homoepitaxial n+p InP structures. An analysis of hydrogen diffusion in dislocated InP will be discussed, along with comparisons of passivation effectiveness for n+p versus p+n heteroepitaxial cell configurations. Preliminary hydrogen-passivated HE InP cell results will also be presented.

Improved performance of p/n InP solar cells

The high electrical conversion performance and radiation resistance of InP solar cells were discovered during the last decade. The combination of these two characteristics make InP a very attractive material for space solar cells. To date, the best performance results for both homo-epitaxial and hetero-epitaxial InP solar cells were achieved using a n/p configuration. The p/n configuration is desirable for hetero-epitaxial growth on inexpensive, strong, light weight, group IV substrates such as Si and Ge. We have succeeded in developing p/n configuration homo-epitaxy InP solar cells with begining-of-life AM0 efficiency values exceeding 17.6%. The high efficiency values resulted from improved emitter performance due to a reduction of Zn interstitial defects in the p-type emitter. Preliminary 3 MeV proton irradiation resistance data are presented which show the high efficiency p/n cells retain 75% of begining-of-life power after 7/spl times/10/sup 11/ protons/cm/sup 2/ fluence.

High beginning-of-life efficiency p/n InP solar cells

The high electrical conversion performance and radiation resistance of InP solar cells was discovered during the last decade. The combination of these two characteristics makes InP a very attractive material for space solar cells. To date, the best performance results for both homo-epitaxial and hetero-epitaxial InP solar cells were achieved using an n/p configuration. The p/n configuration is desirable for hetero-epitaxial growth on inexpensive, strong, lightweight group IV substrates such as Si and Ge. Furthermore, diffused-junction p/n cells demonstrated a higher radiation resistance than the n/p configuration cells; however, the testing of the p/n configuration was limited to lower quality cells as judged by their beginning-of-life (BOL) efficiencies. We have succeeded in developing p/n configuration homo-epitaxy InP solar cells with BOL efficiency values exceeding 17.6p tested under air mass zero (AM0) conditions. The high efficiency values obtained from our cells resulted from improved emitter performance, which was due to better control of the growth of Zn-doped p-type InP.

Hydrogen-interstitial Zn defect complexes and their effects on the device characteristics of heteroepitaxial p/sup +/n InP cell structures

Hydrogen passivation of Zn interstitial defects (Zn/sub I/) is shown to enhance the turn-on voltage (V/sub TO/) of heteroepitaxial p/sup +/n InP/GaAs cells. By using a combination of photoluminescence (PL), electrochemical C-V dopant profiling, secondary ion mass spectroscopy and current-voltage (I-V) measurements we demonstrate that the mechanism for this improvement results from reduction in recombination-generation and shunt losses due to hydrogen deactivation of Zn/sub I/ defect complexes whose high concentration is due to the presence of dislocations. The deactivation of Zn/sub I/ deep donors also results in an increase in the effective emitter acceptor concentration by /spl sim/50% due to the elimination of the compensation effect introduced by active Zn/sub I/ donors.

Hydrogen passivation of interstitial Zn defects in hetero‐epitaxial InP cell structures and influence on device characteristics

Hydrogen passivation of hetero-epitaxial InP solar cells is of interest for deactivation of dislocations and other defects caused by the cell/substrate lattice mismatch that currently limits the photovoltaic performance of these devices. Here we show that in addition to passivation of dislocations, hydrogen deactivates interstitial Zn donor defects present within the Zn-doped emitter of metal organic chemical vapor deposition (MOCVD)-grown p+n hetero-epitaxial InP devices. Zn interstitial passivation increases the forward bias turn-on voltage of hetero-epitaxial InP diodes by as much as 280 mV over the non-hydrogenated value, reaching a value of 960 mV which is close to that obtained for homo-epitaxial diodes. The increase is reproducible and is not observed for either n+p structures or homo-epitaxial p+n structures. Through a combination of photoluminescence, C–V profiling, SIMS and I–V measurements we explain that the source of the voltage enhancement is a combination of decreased acceptor compensation in the emitter and decreased current losses due to depletion region recombination and shunting paths associated with the high concentration of Zn interstitials in Zn-doped hetero-epitaxial InP.

The Effect of The Zn Interstitial Defect on the Performance of p/n InP Solar Cells

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. The authors have recently achieved a 17.6% AM0 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 comparison of hydrogen passivation in heteroepitaxial n/sup +/p and p/sup +/n solar cells

Hydrogen passivation of MOCVD-grown heteroepitaxial InP space solar cell structures with both n/sup +/p and p/sup +/n configurations are compared and the different passivation characteristics are investigated. A 2 hour exposure to a 13.56 MHz hydrogen plasma at 250/spl deg/C reduces the deep level concentration in the base regions of both n/sup +/p and p/sup +/n heteroepitaxial InP cell structures from as-grown values near 1/spl times/10 cm/sup -3/, down to the 2-5/spl times/10/sup 12/ cm/sup -3/ range, resulting in significant reductions in reverse leakage current for both configurations. All dopants were successfully reactivated by a 400/spl deg/C, 5 minute anneal with no detectable activation of deep levels. Passivation of both structures are stable to >500/spl deg/C. I-V characteristics demonstrate significant differences in the voltage behavior of the two cell configurations, with a /spl sim/280 mV increase in built-in voltage for p/sup +/n cells and no corresponding change for n/sup +/p cells. This enhancement in V/sub bi/ is attributed to complex interactions between Zn, H and extended defects within the heavily doped emitter.

Ultra-low resistance, nondestructive contact system for InP/InGaAs/InP double heterostructure TPV devices

A contact system for use on the n/sup +/ InP window layer of an InP/InGaAs/InP thermophotovoltaic (TPV) cell is described. The contact system, composed of an Au-7 at.% Ge mixture, exhibits very low contact resistivity values on both lattice matched and lattice mismatched heterostructures without the need for contact sintering. Specific contact resistivity values in the low 10/sup -6/ /spl Omega/-cm/sup 2/ range are achieved (without sintering) for the lattice matched devices. For the lattice mismatched devices resistivity values approaching the theoretical minimum value in the mid 10/sup -8/ /spl Omega/-cm/sup 2/ range are achieved, again without sintering.