Material and device characterization toward high-efficiency GaAs solar cells on optical-grade polycrystalline Ge substrates

In this work, the authors present a detailed characterization of the material and device properties of GaAs materials grown on optical-grade poly-Ge substrates. Although the minority-carrier lifetime of the starting optical-grade polycrystalline Ge substrate is about a factor of 8 less than that measured in single-crystal electronic-grade Ge, the minority carrier lifetime in GaAs-AlGaAs double-hetero (DH) structures grown on these two substrates were about comparable. C-V measurements on poly-GaAs p⁺n junctions indicate negligible role of grain-boundaries in majority-carrier trapping and also that no compensating deep levels were introduced into the n-GaAs active layers from the optical-grade substrates. The polycrystalline GaAs p⁺-n junctions were evaluated by dark In I-V measurements and the authors observed that there is a considerable variation of the saturation dark current density (within a factor of ten) of diodes located in various grains. Electron-beam induced current scan on a p⁺-n GaAs junction on poly-Ge indicates a base diffusion length of ~1.2 μm near the depletion-layer but a much longer diffusion length away from the junction-region. The performance of the poly p⁺-n GaAs cells is improved by the introduction of an undoped spacer in the p⁺-n junction. Diode I-V data of p⁺-n GaAs junctions, grown with this spacer, show a factor of near 100 reduction in diode saturation dark-current density. The reduction in dark current is believed to be associated with the reduction of tunneling currents in the depletion-layer of the p⁺ -n junction in polycrystalline materials. Since the series resistance of the lightly-doped substrate is presently limiting the efficiency of large-area cells, efforts are underway to develop GaAs solar cells on more heavily-doped poly-Ge substrates