Fast Degradation with Pulsed Light of a-Si:H P-I-N Photodiodes
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FAST DEGRADA TION WITH PULSED LIGHT OF a-Si:H P-I-N PHOTODIODES
J. PUIGDOLLERS, J. BERTOMEU, J.M. ASENSI, J. ANDREU AND J.C. DELGADO Universitat de Barcelona. Departament de Ffsica Aplicada i Electr6nica. Avda Diagonal, 647. 08028 - Barcelona (SPAIN) ABSTRACT
Amorphous Silicon p-i-n photodiodes were obtained by PECVD in a reactor with a rotating substrate holder. Reverse currents as low as 5 x 1011 A/crM2 at a bias of - 2 V were measured using a guard ring electrode to minimize lateral edge currents. The devices were degraded by a Xenon flash lamp in open circuit conditions. The kinetics of the degradation process was evaluated by studying the long time dark current transient under reverse bias. INTRODUCTION
Amorphous Silicon p-i-n devices are used as solar cells for photovoltaic applications, and as sensors for light and radiation detection. In their use as solar cells, the most important parameter is, of course, the efficiency. To be used as detector, one of the main parameters to optimize is the dark reverse current (IR). The lower the 1R the higher the dynamic range of the sensor. The three main origins of 'R are bulk thermal generation, contact injection, and edge leakage. The study of the reverse dark current when it is controlled by bulk thermal generation yields useful information not only about the properties of the device but also about the quality of the i-layer. The long term transient evolution of 'R after the voltage is applied allows us to determine the depletion charge and from that, an estimation of the defect density of the i layer [1]. As in the case of solar cells the performance of the detector deteriorates when they are exposed to light. Although the influence of light on the p-i-n solar cells has been extensively studied, this is not the case for the p-i-n detectors, especially for the reverse dark current. One drawback in studying degradation by light is that it is usually slow when continuous light is used. Recently, degradation of solar cells with pulsed light to accelerate the saturation has been reported [2]. This paper describes measurements of the influence of this kind of light on the dark reverse current in p-i-n detectors. EXPERIMENTAL
Samples used in this study were obtained in a capacitively-coupled diode type rf plasma reactor, operating at 13.56 MHz, as describe elsewhere [3]. The reactor chamber has a rotating substrate holder, which enables us to remove the samples from the plasma during the gas exchange period and plasma stabilization. This is important to produce abrupt interfaces. Deposition parameters were: substrate temperature 3001C, rf power 5 W (27 mW/cm 2). Film thicknesses were 1 pm for the intrinsic layer, 500 A for the p-layer and 100 A for the n-layer. The p-layer was deposited using a flow of 5 sccm of B2H. (diluted 2% in H2)/5 sccm SiH 4, the / layer with 10 sccm of SiH 4, and the n-layer with 2 sccm of PH 3 (diluted 5% in SiH 4)/8 sccm SiH 4 . Dark conductivities at room temperature were 3 x 10-5 S cm*1 and 2 x 10-3 S cm 1 for p and n-layers, and 1 x 10-10
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