Electrical Properties of Phosphorus-Doped and Boron-Doped Nanocrystalline Germanium Thin-Films for p-i-n Devices
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Electrical Properties of Phosphorus-Doped and Boron-Doped Nanocrystalline Germanium Thin-Films for p-i-n Devices William B. Jordan and Sigurd Wagner Department of Electrical Engineering, Princeton University, Princeton, NJ 08544 ABSTRACT Nanocrystalline germanium thin-films deposited on glass by plasma-enhanced chemical vapor deposition from germane and hydrogen were doped with phosphorus and boron. We report some electrical transport and structural properties of Ge films as a function of dopant species and doping levels. The dark conductivities of the phosphorus- and boron-doped films are approximately three to four orders of magnitude higher than the intrinsic nanocrystalline germanium. In the solid phase, phosphorus comprised about 1 atomic percent in the Ge bulk over the range of source gas ratios used, and the conductivity remained fairly constant, indicating saturated conditions. Boron comprised about 10 atomic percent in Ge at the highest dark conductivity, while increased doping turned the films amorphous. To test the doped layers for device applications, an all-nanocrystalline germanium p-i-n diode was constructed and showed rectification when measured in the dark at room temperature. INTRODUCTION Nanocrystalline germanium (nc-Ge:H) has recently attracted interest for possible use in p-i-n devices such as low gap solar cells, long-wavelength photodetectors, thermophotovoltaics, and thin-film diodes due to its high optical absorption coefficient and conductivity, and its low optical gap. Similar to µc-Si:H, nc-Ge:H will likely not exhibit the light-induced degradation of amorphous material. A working prototype nc-Ge:H p-i-n photodetector has been demonstrated [1], but which used µc-Si:H and a-Si:H for the p-type and n-type doped layers, respectively. Consequently, the intrinsic nc-Ge:H layer was graded from Ge to Si at each of the contact layers to avoid an abrupt interface. There have been no studies to date reporting either n-type or p-type doping of nc-Ge:H. It has been shown [2] that at high hydrogen dilutions Ge films as thin as 10 nm can be substantially nanocrystalline, indicating that very thin layers of doped nc-Ge:H could be achieved. Studies on growing doped c-Ge thin films by molecular beam epitaxy have shown that phosphorus and boron are excellent n-type and p-type dopants, respectively, in Ge. Previous studies [3-5] have shown that the crystalline structure of intrinsic nc-Ge:H is a strongly dependent upon the combination of deposition temperature and hydrogen dilution. A sequence of four phases a-nc-a-nc is found as deposition temperature is increased from room temperature to 350oC, and the temperatures at which the transitions occur are dependent upon the amount of hydrogen dilution. Dark conductivity of the intrinsic nc-Ge:H films deposited between 150-200oC was approximately 1×10-4 S/cm [5], while the dark conductivities of the nand p-doped films reported here are approximately three to four orders of magnitude higher. In addition, at deposition temperatures above 310oC, intrinsical
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