Aaron J. Ptak

Dilute nitride GaInNAs and GaInNAsSb solar cells by molecular beam epitaxy

Dilute nitride films with a roughly 1 eV band gap can be lattice-matched to gallium arsenide and germanium, and therefore could become a critical component in next-generation multijunction solar cells. To date most dilute nitride solar cells have been plagued with poor efficiency, due in large part to short diffusion lengths. This study focuses on two techniques aimed at improving the quality of dilute nitride films grown by molecular beam epitaxy: the utilization of biased deflection plates installed in front of the nitrogen plasma source, and the introduction of antimony during growth. Results from GaInNAs cells grown with and without deflection plates, and GaInNAsSb solar cells are reported. The use of biased deflection plates during GaInNAs growth improved every aspect of solar cell performance. For the GaInNAs devices grown with deflection plates, the dark current density, open-circuit voltage, and fill factor were the best of the devices studied. The GaInNAsSb cells had the highest quantum efficienc...

A comparison of MBE- and MOCVD-grown GaInNAs

We have been able to discount three point defects as the factor limiting GaInNAs material quality by comparing samples grown by two different growth techniques. Samples with vastly different concentrations of hydrogen and carbon have very similar properties in terms of deep levels, mobilities, and minority-carrier lifetimes. In addition, growth of hydrogen-free samples and corresponding measurements of vacancies provide strong evidence that gallium vacancies have an effect, but are not a limiting defect.

Low -acceptor-concentration GaInNAs grown by molecular-beam epitaxy for high-current p-i-n solar cell applications

We report GaInNAs grown by solid-source molecular-beam epitaxy (MBE) with background acceptor concentrations less than 1014cm−3, yielding depletion widths in excess of 3μm. GaInNAs p-i-n solar cells fabricated from this low-acceptor-concentration material show greatly increased photocurrents and internal quantum efficiencies close to unity for band gaps as low as 1.15eV. The low acceptor concentrations may be due to low levels of background impurities, such as hydrogen and carbon, in the MBE-grown layers. We discuss the dependence of the acceptor concentration on the substrate temperature used for GaInNAs growth.

Electron Hall Mobility in GaAsBi

We present measurements of the electron Hall mobility in n-type GaAs1−xBix epilayers. We observed no significant degradation in the electron mobility with Bi incorporation in GaAs, up to a concentration of 1.2%. At higher Bi concentration (≥1.6%) some degradation of the electron mobility was observed, although there is no apparent trend. Temperature dependent Hall measurements of the electron mobility suggest neutral impurity scattering to be the dominant scattering mechanism.

Monolithic, Ultra-Thin GaInP/GaAs/GaInAs Tandem Solar Cells

We present here a new approach to tandem cell design that offers near-optimum subcell bandgaps, as well as other special advantages related to cell fabrication, operation, and cost reduction. Monolithic, ultra-thin GaInP/GaAs/GaInAs triple-bandgap tandem solar cells use this new approach, which involves inverted epitaxial growth, handle mounting, and parent substrate removal. The optimal ~1-eV bottom subcell in the tandem affords an -300 mV increase in the tandem voltage output when compared to conventional Ge-based, triple-junction tandem cells, leading to a potential relative performance improvement of 10-12% over the current state of the art. Recent performance results and advanced design options are discussed

Effect of growth rate and gallium source on GaAsN

GaAs1−xNx with x=0.2% is grown by metal–organic chemical vapor deposition with growth rates between 2 and 7 μm/h and with two gallium sources. The GaAsN grown with trimethylgallium at high growth rates shows increased carbon contamination (>1017 cm−3), low photoluminescent lifetimes (∼0.2 ns), and high background acceptor concentrations (>1017 cm−3). The GaAsN is improved if it is grown with a lower growth rate or if triethylgallium is used, resulting in lower carbon contamination (∼1016 cm−3), longer photoluminescent lifetimes (2–9 ns), and slightly lower background acceptor concentrations (<1017 cm−3). The lifetime decreases with carbon concentration, implying that the low lifetimes in this sample set may be caused by nonradiative recombination at a center containing both N and C.

Dominant recombination centers in Ga(In)NAs alloys: Ga interstitials

Optically detected magnetic resonance measurements are carried out to study formation of Ga interstitial-related defects in Ga(In)NAs alloys. The defects, which are among dominant nonradiative recombination centers that control carrier lifetime in Ga(In)NAs, are unambiguously proven to be common grown-in defects in these alloys independent of the employed growth methods. The defects formation is suggested to become thermodynamically favorable because of the presence of nitrogen, possibly due to local strain compensation.

Effects of temperature, nitrogen ions, and antimony on wide depletion width GaInNAs

GaInNAs is a promising candidate material to increase the conversion efficiency of triple junction solar cells, but the dilute nitrides suffer from low-to-nonexistent minority-carrier diffusion lengths. The use of molecular beam epitaxy grown p-i-n structures with wide depletion widths can achieve high photocurrents in dilute nitrides, but this requires background doping below 2×1014cm−3 in the i layer. Here, the authors report on a number of factors that increase the net background acceptor concentration, hindering the effects to realize wide depletion widths, including high substrate temperatures, ions from the rf plasma source used to provide active nitrogen, and the addition of Sb. In addition, low substrate temperatures lead to an increase in n-type conductivity. Solar cell results that show the deleterious effects of Sb on GaInNAs devices are presented, including decreased open-circuit voltage and fill factor.

GaInNAsSb Solar Cells Grown by Molecular Beam Epitaxy

The first GaInNAsSb solar cells are reported. The dilute nitride antimonide material, grown by molecular beam epitaxy, has a bandgap of 0.92 eV and maintains excellent carrier collection efficiency. Internal quantum efficiency of nearly 80% at maximum is obtained in the narrow bandgap GaInNAsSb cells. The short-circuit current density produced by the GaInNAsSb cells underneath a GaAs sub-cell in a multijunction stack, determined from the overlap of the quantum efficiency and the low-AOD spectrum, is 14.8 mA/cm2. This is sufficient to current match the GaInNAsSb sub-cell to the other sub-cells in a GaInP/GaAs/GaInNAsSb solar cell. However, the open-circuit voltage and fill factor of the antimonide devices, 0.28 V and 0.61, are somewhat reduced when compared to GaInNAs devices with 1.03 eV bandgaps. The GaInNAsSb devices had wider depletion regions, which improves the collection efficiency but adversely affects the fill-factor and dark current by increasing depletion region recombination

Observed trapping of minority-carrier electrons in p-type GaAsN during deep-level transient spectroscopy measurement

Deep-level transient spectroscopy measurements on a reverse-biased p-type GaAsN Schottky diode grown by metalorganic chemical vapor deposition show a minority-carrier trap signal corresponding to an electron trap having an activation energy of about 0.2eV. The proportion of trapped electrons agrees with that of modeled defect states near the Schottky-barrier metal interface whose occupation is affected by changing reverse-bias conditions. Estimates of thermionic emission provide adequate filling of the traps with electrons from the metal. The inclusion of a GaAs layer between the metal and GaAsN layer reduces the electron-trapping signal.