The Effects of Atmosphere, Temperature, and Bandgap on the Annealing of GaInNAs for Solar Cell Applications
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The Effects of Atmosphere, Temperature, and Bandgap on the Annealing of GaInNAs for Solar Cell Applications A.J. Ptak, Sarah Kurtz, M. Young and C. Kramer National Center for Photovoltaics, National Renewable Energy Laboratory Golden, CO 80401, U.S.A. ABSTRACT The effects of thermal annealing on the minority-carrier diffusion lengths and depletion widths of GaInNAs are studied. We find that diffusion lengths in as-grown, Be- and Si-doped GaInNAs are limited to less than about 0.2 µm for samples with low concentrations of nitrogen. For higher concentrations of nitrogen, the diffusion lengths are not measurable. Annealing under a variety of temperatures and atmospheres typically makes the diffusion lengths even shorter. These short diffusion lengths are not yet long enough for GaInNAs to be useful in a nextgeneration, four-junction structure. Using undoped GaInNAs in a p–i–n structure is a promising approach to increase device performance. Currently, however, the depletion widths are too small at the bandgaps necessary for solar cells, and annealing does not appear to improve them. INTRODUCTION The Ga1-xInxNyAs1-y (hereafter, GaInNAs) material system is potentially useful in longwavelength lasers [1], heterojunction bipolar transistors [2], and multijunction solar cells [3]. This alloy system, however, suffers from a number of defects that limit the overall performance of these devices, including arsenic antisites [4,5], nitrogen split interstitials [6-8], and gallium vacancies [7,9,10]. Thermal annealing is commonly used to improve the quality of the GaInNAs alloys and is reported to decrease the concentrations of these defects [6,7,11]. In addition, an increase of the photoluminescence (PL) intensity over the as-grown samples is observed for annealed samples grown by several growth techniques [12-14]. There are some complications due to annealing, however, including a blue shift of the emission wavelength [12,13,15,16] and a change in the composition of quantum well samples [13,17]. The background carrier concentration in the material is also affected, increasing or decreasing depending on the type of anneal [6,14,18,19], and an anomalous change from p-type to n-type conductivity can be observed for certain anneal conditions [20-22]. All of these issues make device design complicated, especially when a GaInNAs layer is “unintentionally” annealed due to the growth of subsequent layers in a multi-layer device. Although majority-carrier devices like lasers will benefit from an increase in the PL intensity due to annealing, it is unclear how these changes will affect a minority-carrier device such as a solar cell. With this in mind, we have studied the effects of different annealing temperatures and atmospheres on the minority-carrier diffusion length of molecular-beam epitaxy (MBE)-grown GaInNAs. As-grown (unannealed) GaInNAs doped with either Be or Si often showed small but measurable diffusion lengths (about 0.2 µm or less) for bandgaps above ~1.2 eV. These values are much lower than necessary to generate ade
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