Microcrystalline Germanium Photodetectors
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Microcrystalline Germanium Photodetectors M. Krause, H. Stiebig, R. Carius, H. Wagner Forschungszentrum Jülich GmbH, IPV, 52425 Jülich, Germany ABSTRACT For sensor applications in the detection of near infrared light we have prepared µc-Ge:H by plasma enhanced chemical vapor deposition (PECVD) with a mixture of germane and hydrogen, investigated its structural and electronic properties and incorporated it into thin pin diodes. In order to ensure microcrystalline growth we had to use high hydrogen dilution. However, only the material prepared with a ratio of germane to hydrogen of 0.2% shows high crystallinity. The optical absorption is remarkably different from c-Ge and exhibits no indication of a direct gap at 0.8eV. When this material is implemented as part of a 110nm thin absorber, a short circuit current of 20mA/cm² and a quantum efficiency of 15% at a wavelength of 1.1µm are achieved. Higher germane concentrations in hydrogen lead to poor electronic properties due to an increase of the amorphous phase and the short circuit current of the devices deteriorates. As for crystalline germanium photodiodes cooling of the devices is used to overcome the restriction originating from the high free carrier concentration. INTRODUCTION For the detection of near infrared (NIR) light, both SiGe alloys as well as bulk Ge are used as absorber material in the crystalline semiconductor technology. As an alternative way, utilization of microcrystalline germanium prepared with thin film technology provides the opportunity to combine a low temperature process and an easy way of device integration. Since for NIR detection low band gap material is required, µc-Si:H and µc-Si1-xGex:H alloys are promising candidates due to their enhanced absorption in the long wavelength region. The material was prepared by plasma-enhanced chemical vapor deposition and intensively investigated due to the fruitful synergism with applications in photovoltaics [1-5]. The important role of hydrogen dilution on material quality and device performance was pointed out in earlier studies [1,5,6]. With decreasing hydrogen dilution the microstructure changes from microcrystalline to amorphous. Additionally, the transition region between microcrystalline and amorphous growth was found at higher hydrogen dilution when more germane was added in relation to silane. Unfortunately, alloying silicon with germanium was accompanied by a decrease in material quality likely due to the increasing defect density. This was also reflected in the performance of devices prepared with µc-Si:H and µc-Si1-xGex:H absorber layer [3,6]. The best diodes were achieved near the transition region between microcrystalline and amorphous growth. However, with increasing germanium content not only the NIR spectral response increases, but the performance deteriorates due to the poor material quality. In order to find the limits of these alloys i-layer of microcrystalline germanium were incorporated in pin diodes with p- and n-doped silicon layers using composition buffer layers. EXPERIMENTAL DE
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