The effects of the CdCl 2 passivation treatment on thin-film CdTe photovoltaic films and devices have been extensively studied. Recently, with an addition of CdSeTe layer at the front of the absorber layer, device conversion efficiencies in excess of 19% have been demonstrated. The effects of the CdCl 2 passivation treatment for devices using CdSeTe has not been studied previously. This is the first reported study of the effect of the treatment on the microstructure of the CdSeTe /CdTe absorber. The device efficiency is <1% for the as-deposited device but this is dramatically increased by the CdCl 2 treatment. Using Scanning Transmission Electron Microscopy (STEM), we show that the CdCl 2 passivation of CdSeTe/CdTe films results in the removal of high densities of stacking faults and increase and reorientation of grains. The CdCl 2 treatment leads to grading of the absorber CdSeTe/CdTe films by diffusion of Se between the CdSeTe and CdTe regions. Chlorine decorates the CdSeTe and CdTe grain boundaries leading to their passivation. Direct evidence for these effects is presented using STEM and Energy Dispersive X-ray Analysis (EDX) on device cross-sections prepared using focused ion beam etching. The grading of the Se in the device is quantified using EDX line scans. The comparison of CdSeTe/CdTe device microstructure and composition before and after the CdCl 2 treatment provides insights into the important effects of the process and points the way to further improvements that can be made.
Epitaxial GaAs1−xNx alloy layers, nominally 200-nm-thick, with x up to 0.0375 were grown on GaAs(001) at temperatures Ts varying from 500 to 650 °C to investigate nitrogen incorporation and lattice parameter variations during organometallic vapor phase epitaxy from trimethylgallium, tertiarybutylarsine, and 1,1-dimethylhydrazine. Quantitative secondary ion mass spectrometry measurements (SIMS) indicate that N incorporation decreases systematically with increasing Ts to become almost negligible at 650 °C. All films are coherent with the substrate as judged by high-resolution x-ray reciprocal lattice mapping although atomic force microscopy and cross-sectional transmission electron microscopy reveal the presence of cracks in films with x>0.02. High-resolution x-ray diffraction measurements combined with SIMS analyses indicate that the lattice constant decreases linearly with increasing x following closely the predictions of Vegard’s rule for x<0.03. At higher concentrations, the lattice constant decreases more rapidly as a significant fraction of N atoms becomes incorporated in nonsubstitutional sites as demonstrated by nuclear reaction analysis.
We have measured the near-band-gap absorption of GaAs 1−x N x thin films with x Ͻ 0.012. The spectra were analyzed with a model which allows a precise determination of the band gap and of the width of the optical transitions; the latter is found to increase, for x Ͼ 0.002, well beyond what is expected in a III-V alloy. Ab initio calculations were performed within the generalized-gradient approximation with the exchange-correlation functional of Engel and Vosko [Phys. Rev. B 47, 13164 (1993)]. They reveal that the band gap depends markedly on nitrogen atomic configuration. The anomalous broadening of the intrinsic optical transitions thus gives strong evidence for configuration-induced band-gap fluctuations.
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