Anisotropic Transport in Microcrystalline P-I-N Devices
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'A. MA(ARICO, I. MART INS, 'P. LoURO AND R. SCIHWARZ 'Electronics and Communications Department, ISEL, Lisboa, Portugal 2 FCT!UNL, Monte da Caparica, Portugal ' Physics Department, IST, Lisboa, Portugal 'M. VIEIRA, A. FANTONI, 'M. FERNANDES,
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ABSTRACT Entirely gc-Si:H p-i-n structures presenting an enhanced sensitivity to the near infrared region and a positive spectral response under forward bias higher than the open circuit voltage are analysed under different external voltage bias and illumination conditions. A two phase model to explain the transport properties is proposed using as input parameters the measured experimental data. The results suggest that the transport is preferentially concentrated inside the crystalline grains. The conduction within the amorphous regions is poor. The percolation path is different for electrons and holes and is determined by the local fields at the boundaries. These local fields are independent of the externally applied condition, and they can be related to the persistence of the small photocurrent observed when a bias voltage higher than the open circuit voltage is applied. INTRODUCTION The first successful deposition of microcrystalline silicon (gtc-Si) films has been realized in the late sixties [1]. Since then, gc-Si has been a subject of interest due to its high electrical conductivity and absorption coefficient, making this material a good candidate for future photovoltaic applications such as solar cells, thin film transistors, and large-area optoelectronic sensors [2, 3]. The understanding of jac-Si:H based devices is still in an early stage and some questions about its properties are actually a subject of investigation and are under discussion [4, 5]. The gc-Si:H is a two-phase material. Its composition can be considered as grains of crystalline silicon imbedded in an a-Si:H tissue. The structural properties of ktc-Si:H are strongly dependent on the deposition process used. Evidence of different film morphology has been found in samples deposited with different processes [6, 7]. In this study we have used the closed-chamber chemical vapor deposition method [7] to produce thin entirely ac-Si:H p-i-n structures resulting in wide spectral range device with an enhanced sensitivity to the visible and near infrared regions and a positive spectra! response under forward bias. In order to gain insight into the transport mechanism we have correlated the spectral responsivity with a detailed simulation analysis. An anisotropic model, based on the band discontinuities near the grain boundaries, is presented and supported by numerical simulation. EXPERIMENTAL DETAILS AND DEVICE CHARACTERIZATION All layers of the analyzed p-i-n structures have been prepared by Closed-Chamber Chemical Vapor Deposition. The [tc-Si films have a high crystalline fraction (>80%) and can be efficiently doped both and
type. A conic-like growth for the crystallites is assumed with an amorphous-to-crystalline transient sublayer in the initial stages of ftc-Si film growth [8]. For the characterization
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