GaAs is an attractive material for space solar cells due to it possessing the highest photovoltaic efficiency among semiconductors. However, material vulnerability to radiation damage is a limiting factor, which should be resolved. This can be done through the optimization of solar cell geometry. Specifically, ultra-thin GaAs solar cells possess increased radiation hardness in combination with high specific power (W/kg) making them desirable for space applications. In this research, modeling of single-junction thin GaAs based solar cells with an active device thickness <; 1.6 μm is performed to evaluate cell performance at high irradiation levels (1 MeV electrons with a fluence of 1E15 e - /cm 2 ) associated with high energy orbits. The radiation tolerance of two configurations is evaluated; conventional and rear junction (thick emitter) with a backside gold reflective coating. In addition, solar cell structures utilizing n- and p-type base are simulated. The Hovel model, modified to include the photon recycling effect, is used for modeling. The effect of radiation on GaAs electronic properties is counted through degradation of minority carrier lifetime and changes in the carrier concentration. It's found that conventional n-emitter/p-base and the rear junction with p-type thick emitter configurations with a heavily doped absorber ~5E18 [cm -3 ] and thickness below 800 nm have increased radiation hardness with a degradation rate <; 7%.

Betavoltaic converters are attractive power sources due to their long duration, continuous discharge and highenergy densities in comparison with other modes of energy harvesting. The main obstacle to widen the application of betavoltaics is their low power output. 2D Monte Carlo numerical simulations coupled with drift-diffusionmodeling were conducted to evaluate commercially available and emerging wideband gap semiconductors as potential absorbers with widely available tritium (H-3) isotope. Results show that power enhancement can be achieved by using emerging ultra wide bandgap material such as diamond. Using those materials, >40% increase in the power density in comparison with current 4H-SiC technology can be achieved.

To improve the performance of photodiodes based on narrow-bandgap InAs/GaSb type-II strained layer superlattices (T2SLs), knowledge of the vertical minority carrier transport is necessary. For this purpose, the key parameters influencing vertical minority-carrier electron transport in an nBp MWIR detector were studied: diffusion length, lifetime, mobility. The detectors were designed with p-type, 10/10 ML, InAs/GaSb T2SL absorbers, targeting a 50% cutoff wavelength of 5.0 mu m at 80 K. The nBp structure is attractive because the junction field predominately drops across a relatively wide-gap InAs/AlSb SL barrier, which reduces the expected generation-recombination dark current. Measurements of the electron-beam-induced current (EBIC), combined with minority carrier lifetime results from microwave reflectance measurements, enabled the determination of the minority carrier diffusion length (L-e) and mobility in the growth direction as a function of temperature. The L-e was extracted at each temperature by fitting the EBIC data to analytical expressions for carrier collection efficiency. The EBIC measurements were also repeated at different electronbeam energies to vary the distribution of minority carriers near the surface to gauge the surface recombination velocity. Microwave reflectance allowed for accurate measurement of the minority carrier lifetime over a large range of excess carrier concentrations, enabling a separation of recombination mechanisms. The lifetime and extracted diffusion length data were then used to estimate the diffusion coefficient and mobility versus temperature by applying the Einstein diffusion relationship.

An ion beam sputtering process was used to grow polycrystalline Ge films at low temperatures (200-400 °C). The effect of transition metals (Sn, Al) and ion assisted beam was evaluated to enhance grain growth. It was found that the presence of the seed metals facilitates the grain growth even at low temperatures, below eutectic point. Modification of the microstructure was found impinging the growth of Ge films with a 100 eV ion assisted beam in the energy dose range 0-7 eV/atom. Ge layers deposited on Sn with ~4eV/atom doses had the largest grain size. X-ray diffraction (XRD) and Atomic Force Microscopy (AFM) were employed to characterize the film microstructure. The grown poly-Ge films were used to grow GaAs structures.

Low temperature (≤450°C) ion beam deposition was employed to grow polycrystalline Germanium (Ge) films on Si (100) crystal, polymer film, and glass. The effects of deposition temperatures and energies of sputtered atoms on the crystal quality of Ge films were studied. XRD characterization of the material showed that at above 300°C, Ge films had a phase transition from amorphous to polycrystalline structure. Samples deposited at 1000eV beam energy had the best crystallinity Successful results have been achieved by growing up to 600 nm thick poly-Ge film layers at 450°C substrate temperature on plastic and glass substrates.

Solar cells utilizing multi-quantum well (MQW) structures are considered promising candidate materials for space applications. An open question is how well these structures can resist the impact of particle irradiation. The aim of this work is to provide feedback about the radiation response of In0.01Ga0.99As solar cells grown on Ge with MQWs incorporated within the i-region of the device. In particular, the local electronic transport properties of the MQW i-regions of solar cells subjected to electron and proton irradiation were evaluated experimentally using the electron beam induced current (EBIC) technique. The change in carrier collection distribution across the MQW i-region was analyzed using a 2D EBIC diffusion model in conjunction with numerical modeling of the electrical field distribution. Both experimental and simulated findings show carrier removal and type conversion from n- to p-type in MQW i-region at a displacement damage dose as low as ∼6.06–9.88 × 109 MeV/g. This leads to a redistribution of the electric field and significant degradation in charge carrier collection.

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In this paper, a complex analysis of the radiation response of GaAs solar cells with multi quantum wells (MQW) incorporated in the i-region of the device is presented. Electronic transport properties of the MQW i-region were assessed experimentally by the electron beam induced current (EBIC) technique. A 2-D EBIC diffusion model was applied to simulate EBIC line scans across device structure for different radiation doses. The results are interpreted using numerical modeling of the electrical field distribution at different radiation levels. Type conversion from n- to p-type was found in MQW i-region at displacement damage dose as low as low as ~9.88E9 MeV/g. This is supported by experimental and simulated EBIC and electric field distribution results.

Experimental results on triple-junction solar cells irradiated by 3 MeV proton irradiation to very high damage levels are presented. The minority carrier transport properties were obtained through quantum efficiency and EBIC measurements and an analytical drift-diffusion solver was used in understanding the results for different degradation levels where multiple damage mechanisms are evident.

Reported is the characterization of irradiated InGaP 2 /GaAs/Ge multijunction (MJ) solar cells using the cathodoluminescence (CL) imaging/spectroscopy and electron beam induced current (EBIC) modes of scanning electron microscopy (SEM). These techniques were applied to verify the influence of radiation damage on the optoelectronic properties of each subcell in the monolithic triple junction structure and correlate them with the illuminated (AM0, 1 sun, 25°C) current-voltage (IV) and quantum efficiency (QE) characteristics.

The effect of various types of in-grown stacking faults and threading screw/edge type dislocations on carrier lifetime and diffusion lengths in 4H-SiC epitaxial films was investigated through cathodoluminescence decays and charge collection efficiencies of electron beam induced current signals at specific defects sites. Most stacking faults yielded ~40% reduction in the carrier lifetime. Moreover, drastic lifetime reductions were observed in regions containing surface triangular defects and bulk 3C polytype inclusions. Dislocations of both types serve as efficient recombination centers, though stronger reduction in diffusion lengths was observed in the vicinity of screw type dislocations.

We report the results of the characterization of irradiated InGaP 2 /GaAs/Ge multijunction (MJ) solar cells using the cathodoluminescence (CL) imaging/spectroscopy and electron beam induced current (EBIC) modes of scanning electron microscopy (SEM). These techniques were applied to verify the influence of irradiation damage on the optoelectronic properties of each subcell in the monolithic MJ structure and correlate them with the illuminated (AM0, 1 sun, 25°C) current-voltage ( I - V ) and quantum efficiency (QE) measurements. Minority carrier lifetime degradation data from CL measurements confirm that the GaAs subcell dominates the overall degradation of the 3J device. Also, a carrier removal mechanism in the GaAs subcell was revealed from the EBIC/CL measurements.

Local recombination properties of threading screw and edge dislocations in 4H–SiC epitaxial layers have been studied using electron beam induced current (EBIC). The minority carrier diffusion length in the vicinity of dislocations was found to vary with dislocation type. Screw dislocations had a more pronounced impact on diffusion length than the edge dislocations, evidencing stronger recombination activity. Temperature dependence of EBIC contrast of dislocations suggests that their recombination activity is controlled by deep energy levels in the vicinity of dislocation cores. This paper shows that the type of dislocation (screw or edge) can be identified from analysis of EBIC contrast.

We report the results of the characterization of irradiated InGaP2/GaAs/Ge multijunction (MJ) solar cells using the cathodoluminescence (CL) imaging/spectroscopy and electron beam induced current (EBIC) modes of scanning electron microscopy (SEM). These techniques were applied to verify the influence of irradiation damage on the optoelectronic properties of each subcell triple junction structure and correlate illuminated (AM0, 1 sun, 25°C) current-voltage (IV) and quantum efficiency (QE) characteristics.

Much has been accomplished in the last few years in advancing the performance of type-II superlattice (T2SL) based infrared photodiodes, largely by focusing on device and heterostructure design. Quantum efficiency (QE) has increased to 50% and higher by using thicker absorbing layers and making use of internal reflections, and dark currents have been reduced by over a factor of ten by using bandstructure engineering to suppress tunneling and generation-recombination (G-R) currents associated with the junction. With performance levels of LWIR T2SL photodiodes now within an order of magnitude of that of HgCdTe (MCT) based technology, however, there is renewed interest in understanding fundamental materials issues. This is needed both to move performance toward the theoretical Auger limit, and to facilitate the task of transitioning T2SL growth from laboratories to commercial institutions. Here we discuss recent continuing efforts at NRL to develop new device structures for enhanced detector performance, and to further our understanding of this material system using advanced structural and electronic probes. Results from electron beam induced current (EBIC) imaging and analysis of point defects in T2SL photodiodes will be presented, showing differentiated behavior of bulk defect structures. We will also describe a study comparing intended vs. as-grown T2SL photodiode structures by crosssectional scanning microscopy (XSTM). Using parameters extracted from the XSTM images, we obtain detailed knowledge of the composition and layer structures through simulation of the x-ray diffraction spectra.

The influence of postgrowth high-temperature anneals between 1400°C and 2400°C on the behavior of the D1 center in semi-insulating 4H-SiC was studied by photoluminescence. The optical signature of D1 was observed up to 2400°C with intensity maxima at 1700°C and 2200°C. It was also found that changes in the postannealing cooling rate drastically influence the behavior of the D1 center and the concentrations of the VC, VSi, VC–VSi, and VC–CSi lattice defects observed from electron paramagnetic resonance experiments. The change in intensity of the D1 defect has some correlation with the intensity change of the VC–VSi pair defect at temperatures above 1900°C. In addition, infrared photoluminescence spectroscopy studies showed that changes in the intensity of the D1 defect at 2100°C to 2400°C annealing temperatures and variable cool-down rates have close correlation with intensity changes of the UD2 defect.

Cathodoluminescence real-color imaging and spectroscopy were employed to study the properties of Ga(2)O(3) nanowires grown with different Sn/Ga ratios. The structures grown under Sn-rich conditions show large spectral emission variation, ranging from blue to red, with a green transition zone. Spectral emission changes correlate with changes in the chemical composition and structure found by energy dispersive spectroscopy and electron diffraction. A sharp transition from green to red emission correlates with a phase transition of beta-Ga(2)O(3) to polycrystalline SnO(2). The origin of the green emission band is discussed based on ab initio calculation results.

The behavior of the D1 center in semi-insulating 4H-SiC substrates revealed by low-temperature photoluminescence was investigated after post-growth high temperature anneals between 1400 and 2400oC. The influence of different post-anneal cooling rates was also studied. The optical signature of D1 was observed up to 2400oC with intensity maxima at 1700 and 2200oC. We propose that the peak at 1700°C can be related to the formation and subsequent dissociation of SiC native defects. It was found that changes in the post-annealing cooling rate drastically influence the behavior of the D1 center and the concentrations of the VC, VSi, VC-VSi and VC-CSi lattice defects.

A novel erbium chloride seeded growth process was developed for the growth and fabrication of erbium doped GaN nanocrystals. This erbium chloride seeded technique simplifies the delivery of erbium into a metal organic chemical vapor deposition system. Additionally, this selective growth of the GaN nanocrystal only occurred in the presence of the ErCl3 seed. Strong green emission, distinctive of the Er3+ ion, was observed in the GaN nanocrystals.