Near Infrared Response of Amorphous Silicon Detector Grown with Microcompensated Absorber Layer
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ABSTRACT In this work we demonstrate that radiation up to 2 pm induces photocurrent in a single junction amorphous silicon structure at room temperature. The absorber layer is a microcompensated film deposited using very low concentrations of dopant species. Device operation is based on optical excitation of thermal generated carriers from trap states toward valence and conduction band in the high electric field region of the structure. Transient and frequency response under different bias voltages and illuminations conditions are presented. The possibility to use the infrared sensor in low bit rate communication systems has been demostrated by including our detector in a front-end system and measuring its frequency responce. Quantum efficiency measurement have been reproduced with a numerical model, able to take into account sub-band gap absorption into single films. Model results indicate the presence of a large valence band tail and a high number of dangling bonds and shallow defects ascribed to the presence of dopant atoms.
INTRODUCTION Single junction p-i-n amorphous silicon (a-Si:H) structures have been extensively used as solar cells, with quantum efficiency response up to 750 nm or 900 nm when the absorber region is an intrinsic amorphous silicon layer or an intrinsic amorphous silicon-germanium layer, respectively [1, 2, 3]. Constant photocurrent method and photothermal deflection spectroscopy showed that the presence of defects in the forbidden gap allowed to obtain photocurrent response up to 1500 nm in intrinsic a-Si:H films. The first detection of radiation up to 2400 nm by a-Si:H a p-i-n diode, operating under forward bias conditions at low temperature (198 K), has been reported by Wind and Miller [4]. More recently, it has been demonstrated the possibility to achieve near and medium infrared detection at room temperature by using single junction a-Si:H structure with microcompensated absorber zone, whose operation is based on the transitions between the high number of defect density present in this material and the extended states [5, 6]. Here, we present a detailed investigation of the near infrared responce of this device focusing the attention on the detection mechanism and on its transient and frequency responce. Altough near infrared a-Si:H detectors are orders of magnitude slower than detectors based on the Ill-V technology, they could represent a complementary approach, because a-Si:H technology, which requires low deposition temperature, is suitable for almost any kind of substrate and, in particular, it is compatible with silicon IC and integrated optics technology [7].
EXPERIMENTAL RESULTS On a transparent conductive oxide coated glass the deposition of the amorphous layers is performed as the following steps: 1) 300 A thick p-doped amorphous silicon carbide layer; 2) 7000 A thick microcompensated amorphous silicon layer; 3) 500 A thick n-doped amorphous silicon layer. The microcompensated zone has been deposited with 0.25 ppm of PH3 and 18 ppm of B2H6 in the gas mixture. This low dopant
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