Evidence of the Meyer-Neldel Rule in InGaAsN Alloys: Consequences for Photovoltaic Materials
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Evidence of the Meyer-Neldel Rule in InGaAsN Alloys: Consequences for Photovoltaic Materials Steven W. Johnston and Richard S. Crandall National Renewable Energy Laboratory Golden, CO 80401, U.S.A. ABSTRACT We present data showing the potential adverse effects on photovoltaic device performance of all traps in InGaAsN. Deep-level transient spectroscopy measurements were performed on InGaAsN samples grown by both metal-organic chemical vapor deposition and RF plasmaassisted molecular-beam epitaxy. For each growth technique, we studied samples with varying nitrogen composition ranging from 0% to 2.2%. A deep hole trap with activation energy ranging between 0.5 and 0.8 eV is observed in all samples. These data clearly obey the Meyer-Neldel rule, which states that all traps have the same emission rate at the isokinetic temperature. A fit of our trap data gives an isokinetic temperature of 350 K. We find that the emission time for all deep hole traps is on the order of milliseconds at room temperature. This means that both deep and shallow traps emit slowly at the operating temperature of solar cells—thus, the traps can be recombination centers. INTRODUCTION Adding nitrogen to GaAs initially reduces the bandgap [1-3]. This alloy can be grown epitaxially on GaAs, and lattice-matching can be improved by also adding In [4]. With about 2% N and 6% In, a 1-eV bandgap material lattice-matched to GaAs can be grown [4,5]. These properties are advantageous for developing a four-junction high-efficiency solar cell, consisting of GaInP, GaAs, InGaAsN, and Ge. Such a structure has an ideal AM0 efficiency of 41% [6], but to date, poor minority-carrier properties have limited the material's usefulness in multijunction cells [5,7]. Deep-level transient spectroscopy (DLTS) is a powerful technique for characterizing material defects and providing information to identify lifetime-killing defects that degrade device performance. DLTS data have been reported on both metal-organic chemical-vapor-deposited (MOCVD) and RF plasma-assisted molecular-beam epitaxy (MBE) InGaAsN alloys. Krispin et al. report several hole traps in MBE-grown material having activation energies of 0.16 to 0.17, 0.35 to 0.36, 0.39, 0.55, and 0.69 eV [8]. The larger activation-energy traps (0.55 and 0.69 eV) appear in the largest concentrations. They assign the 0.55-eV level to an FeGa (Fe on a Ga site) substitutional defect and the 0.69-eV level to a GaAs-/2- (charge-state change) anti-site defect [9]. Chen et al. [10] report a hole trap in N-implanted GaAs and suggest that this level at 0.545 eV is a nitrogen-related acceptor defect. Kwon et al. [11] and Kaplar et al. [12] have reported hole traps with activation energies of 0.10, 0.23, and 0.48 eV, along with a broad peak corresponding to ~0.5 eV in MOCVD-grown material. Kaplar et al. [13] also report activation energies of 0.37, 0.51, and 0.71 eV in MBE material. Johnston [14] reported a deep hole-trap ranging in activation energy from 0.61 to 0.79 eV in MOCVD samples of varying In and N composition; using t
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