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Surface Science in an MOCVD Environment: Arsenic on Vicinal Ge(100)

Description: Scanning tunneling microscope (STM) images of arsine-exposed vicinal Ge(100) surfaces show that most As/Ge steps are reconstructed, and that a variety of different step structures exist. The entire family of reconstructed As/Ge steps can be divided into two types, which we have chosen to call ''single-row'' steps and ''double-row'' steps. In this paper we propose a model for a double-row step created by annealing a vicinal Ge(100) substrate under an arsine flux in a metal-organic chemical vapor deposition (MOCVD) chamber.
Date: November 1, 1998
Creator: McMahon, W. E. & Olson, J. M.
Partner: UNT Libraries Government Documents Department

Using Measurements of Fill Factor at High Irradiance to Deduce Heterobarrier Band Offsets: Preprint

Description: Using a 2D device simulation tool, we examine the high irradiance behavior of a single junction, GaAs concentrator cell as a function of the doping in the back surface confinement layer. The confinement layer is designed to be a barrier for both holes and electrons in the base of the solar cell. For a p-type base we show that the FF of the cell at high concentrations is a strong function of both the magnitude of the valence band offset and the doping level in the barrier. In short, for a given valence band offset (VBO), there is a critical barrier doping, below which the FF drops rapidly with lower doping. This behavior is confirmed experimentally for a GaInP/GaAs double heterostructure solar cell where the critical doping concentration (at 500 suns) in the back surface confinement layer is ~1e18 cm-3 for a VBO of 300 meV.
Date: July 1, 2011
Creator: Olson, J. M.; Steiner, M. A. & Kanevce, A.
Partner: UNT Libraries Government Documents Department

Effect of Sb on the Properties of GaInP Top Cells (Presentation)

Description: The summary of this report is that: (1) Sb can be used to increase V{sub oc} of a GaInP top cell; (2) the photovoltaic quality of GaInP is relatively unaffected by the presence of Sb; and (3) Sb-doped GaInP/GaAs tandem cells show promise for achieving efficiencies over 32%.
Date: May 1, 2006
Creator: Olson, J. M.; McMahon, W. E. & Kurtz, S.
Partner: UNT Libraries Government Documents Department

Effect on Sb on the Properties of GaInP Top Cells: Preprint

Description: It is well known that the efficiency of GaInP/GaAs tandem solar cells is limited by the band gap of the GaInP top cell, which, in turn, is determined by the degree of compositional ordering in GaInP base layer. Attempts to raise the band gap by the addition of Al to the top cell have met with limited success due to the strong affinity between Al and oxygen. Here we investigate a different approach. It has been shown that the presence of antimony on the surface of GaInP during its growth suppresses the ordering process and increases the band gap. In this paper, we study the effects of Sb on the properties of GaInP top cells. We show that, in addition to raising the band gap of GaInP, it also increases the incorporation of Zn and changes the relative incorporation of Ga and In. These effects depend strongly on the substrate orientation, growth temperature and rate, and the Sb/P ratio in the gas phase. We show that the band gap of the GaInP top cell (and the Voc) can be increased without reducing the minority carrier collection efficiency. The implications of these results are presented and discussed.
Date: May 1, 2006
Creator: Olson, J. M.; McMahon, W. E. & Kurtz, S.
Partner: UNT Libraries Government Documents Department

New Materials for Future Generations of III-V Solar Cells

Description: Three- and four-junction III-V devices are proposed for ultrahigh-efficiency solar cells using a new 1-eV material lattice-matched to GaAs, namely, GaInNAs. We demonstrate working prototypes of a GaInNAs-based solar cell lattice-matched to GaAs with photoresponse down to 1 eV. Under the AM1.5 direct spectrum with all the light higher in energy than the GaAs band gap filtered out, the prototypes grown with base doping of about 10{sup 17} cm-3 have open-circuit voltages ranging from 0.35 to 0.44 V, short-circuit current densities of 1.8 mA/cm2, and fill factors from 61% to 66%. To improve on the current record-efficiency tandem GaInP/GaAs solar cell by adding a GaInNAs junction, the short-circuit current density of this 1-eV cell must be significantly increased. Because these low short-circuit current densities are due to short diffusion lengths, we have demonstrated a depletion-width-enhanced variation of one of the prototype devices that trades off decreased voltage for increased photocurrent, with a short-circuit current density of 7.4 mA/cm2 and an open-circuit voltage of 0.28 V.
Date: October 6, 1998
Creator: Geisz, J. F.; Friedman, D. J.; Olson, J. M.; Kramer, C.; Kibbler, A. & Kurtz, S. R.
Partner: UNT Libraries Government Documents Department

Electron Microscopy Studies of GaP(N,As) Grown on Si

Description: The objective of this work is to perform transmission electron microscopy (TEM) studies of GaP(N,As) alloys grown by metal-organic chemical vapor deposition (MOCVD) on Si substrates. These alloys are of interest for the fabrication of high-efficiency tandem solar cells based on Si. The results indicated that the nucleation and growth conditions used are critical for obtaining planar epitaxial layers with a low defect density. In particular, antiphase domains are eliminated using a low growth temperature. TEM studies of these alloy layers, which contain only a few percent N, revealed no phase separation. However, electron diffraction studies revealed the first evidence of CuPt-type atomic ordering in these P-rich, dilute nitride alloy layers.
Date: January 1, 2005
Creator: Norman, A. G.; Geisz, J. F.; Olson, J. M.; Jones, K. M. & Al-Jassim, M. M.
Partner: UNT Libraries Government Documents Department

Examination of 1D Solar Cell Model Limitations Using 3D SPICE Modeling: Preprint

Description: To examine the limitations of one-dimensional (1D) solar cell modeling, 3D SPICE-based modeling is used to examine in detail the validity of the 1D assumptions as a function of sheet resistance for a model cell. The internal voltages and current densities produced by this modeling give additional insight into the differences between the 1D and 3D models.
Date: June 1, 2012
Creator: McMahon, W. E.; Olson, J. M.; Geisz, J. F. & Friedman, D. J.
Partner: UNT Libraries Government Documents Department

Optical Investigation of GaNAs

Description: A systematic study of the energy and time-resolved photoluminescence of GaInP/GaNxAs1-x double heterostructures has been performed for 0=x=1.3%. A large temperature-dependent optical-bowing coefficient (about 20-25 eV) is observed and the bandgap variation with temperature is found to depend on the nitrogen content. Finally, the minority-carrier lifetime is not simply related to the nitrogen content. Instead, the recombination rate is proportional to the majority-carrier concentration for x=0.3% and the carbon concentration for x=0.3%.
Date: September 28, 1998
Creator: Keyes, B. M.; Geisz, J. F.; Dippo, P. C.; Reedy, R.; Kramer, C.; Friedman, D. J. et al.
Partner: UNT Libraries Government Documents Department

Phase Separation and Facet Formation during the Growth of (GaAs)1-x(Ge2)x Alloy Layers by Metal Organic Vapour Phase Epitaxy

Description: Metal organic vapour phase epitaxy (GaAs)1-x(Ge2)x alloy layers, 0<x<0.22, were grown at temperatures between 640 and 690 C, on vicinal (001) GaAs substrates. Phase separation occurred in all the layers. The phase-separated microstructure changed with alloy composition, growth temperature, and substrate orientation. In x {approx} 0.1 layers grown at 640 C, Ge segregation occurred on {l_brace}115{r_brace}B planes associated with a {l_brace}115{r_brace}B surface faceting. Increase in growth temperature led to the formation of large, (001)-oriented, irregular-shaped platelets of Ge-rich material. Growth on {l_brace}115{r_brace}B substrates resulted in a ''natural superlattice'' of GaAs/Ge along the growth direction.
Date: September 13, 1999
Creator: Norman, A. G.; Olson, J. M.; Geisz, J. F.; Moutinho, H. R.; Mason, A.; Al-Jassim, M. M. et al.
Partner: UNT Libraries Government Documents Department

Passivation of Interfaces in High-Efficiency Photovoltaic Devices

Description: Solar cells made from III-V materials have achieved efficiencies greater than 30%. Effectively ideal passivation plays an important role in achieving these high efficiencies. Standard modeling techniques are applied to Ga0.5In0.5P solar cells to show the effects of passivation. Accurate knowledge of the absorption coefficient is essential (see appendix). Although ultralow (<2 cm/s) interface recombination velocities have been reported, in practice, it is difficult to achieve such low recombination velocities in solar cells because the doping levels are high and because of accidental incorporation of impurities and dopant diffusion. Examples are given of how dopant diffusion can both help and hinder interface passivation, and of how incorporation of oxygen or hydrogen can cause problems.
Date: May 13, 1999
Creator: Kurtz, S. R.; Olson, J. M.; Friedman, D. J.; Geisz, J. F.; Kibbler, A. E. (National Renewable Energy Laboratory) & Bertness, K. A. (NIST, Boulder, Colorado)
Partner: UNT Libraries Government Documents Department

CuPt-B ordered microstructures in GaInP and GaInAs films

Description: The authors examine CuPt-B atomic sublattice ordering in Ga{sub 0.51}In{sub 0.49}P (GaInP) and Ga{sub 0.47}In{sub 0.53}As (GaInAs) III-V alloy films grown by atmospheric- and low-pressure metalorganic chemical vapor deposition on singular and vicinal (001) substrates. The influences of growth conditions and substrate miscut on double- and single-variant ordered microstructures are investigated using transmission electron microscopy (TEM). Relatively thick (>1-2 {micro}m) double-variant ordered GaInP and GaInAs films show complementary superdomain formation. Single-variant ordered films on <111>B-miscut substrates contain single-phase domains, separated by antiphase boundaries (APBs). The appearance of APBs in TEM dark-field images is anticipated from electron diffraction theory.
Date: March 1, 2000
Creator: Ahrenkiel, S. P.; Jones, K. M.; Matson, R. J.; Al-Jassim, M. M.; Zhang, Y.; Mascarenhas, A. et al.
Partner: UNT Libraries Government Documents Department

Nitrogen-Induced Modification of the Electronic Structure of Group III-N-V Alloys: Preprint

Description: Incorporation of nitrogen in conventional III-V compound semiconductors to form III-N-V alloys leads to a splitting of the conduction band into two nonparabolic sub-bands. The splitting can be described in terms of an anticrossing interaction between a narrow band of localized nitrogen states and the extended conduction-band states of the semiconductor matrix. The downward shift of the lower sub-band edge is responsible for the N-induced reduction of the fundamental band-gap energy. The modification of the conduction-band structure profoundly affects the optical and electrical properties of the III-N-V alloys.
Date: April 1, 1999
Creator: Walukiewicz, W.; Shan, W.; Ager III, J. W.; Chamberlin, D. R.; Haller, E. E. (Lawrence Berkeley National Laboratory); Geisz, J. F. et al.
Partner: UNT Libraries Government Documents Department

Local Structures and Interface Morphology of InGaAsN Thin Films Grown on GaAs

Description: The compound semiconductor system InGaAsN exhibits many intriguing properties which are particularly useful for the development of innovative high efficiency thin film solar cells and long wavelength lasers. The bandgap in these semiconductors can be varied by controlling the content of N and In and the thin films can yet be lattice-matched to GaAs. In the present work, x-ray absorption fine structure (XAFS) and grazing incidence x-ray scattering (GIXS) techniques have been employed to probe the local environment surrounding both N and In atoms as well as the interface morphology of InGaAsN thin films epitaxially grown on GaAs. The soft x-ray XAFS results around nitrogen K-edge reveal that N is in the sp{sup 3} hybridized bonding configuration in InGaAsN and GaAsN, suggesting that N impurities most likely substitute for As sites in these two compounds. The results of In K-edge XAFS suggest a possible trend of a slightly larger coordination number of As nearest neighbors around In atoms in InGaAsN samples with a narrower bandgap whereas the In-As interatomic distance remains practically the same as in InAs within the experimental uncertainties. These results combined suggest that N-substitution of the As sites plays an important role of bandgap-narrowing while in the meantime counteracting the compressive strain caused by In-doping. Grazing incidence x-ray scattering (GIXS) experiments verify that InGaAsN thin films can indeed form very smooth interfaces with GaAs yielding an average interfacial roughness of 5-20{angstrom}.
Date: February 23, 1999
Creator: Allerman, A.A.; Chen, J.G.; Geisz, J.F.; Huang, S.; Hulbert, S.L.; Jones, E.D. et al.
Partner: UNT Libraries Government Documents Department

Inverted GaInP/(In)GaAs/InGaAs Triple-Junction Solar Cells with Low-Stress Metamorphic Bottom Junctions: Preprint

Description: We demonstrate high efficiency performance in two ultra-thin, Ge-free III-V semiconductor triple-junction solar cell device designs grown in an inverted configuration. Low-stress metamorphic junctions were engineered to achieve excellent photovoltaic performance with less than 3 x 106 cm-2 threading dislocations. The first design with band gaps of 1.83/1.40/1.00 eV, containing a single metamorphic junction, achieved 33.8% and 39.2% efficiencies under the standard one-sun global spectrum and concentrated direct spectrum at 131 suns, respectively. The second design with band gaps of 1.83/1.34/0.89 eV, containing two metamorphic junctions achieved 33.2% and 40.1% efficiencies under the standard one-sun global spectrum and concentrated direct spectrum at 143 suns, respectively.
Date: May 1, 2008
Creator: Geisz, J. F.; Kurtz, S. R.; Wanlass, M. W.; Ward, J. S.; Duda, A.; Friedman, D. J. et al.
Partner: UNT Libraries Government Documents Department