Hydrogenated Amorphous Silicon Alloyed with Selenium
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ABSTRACT Hydrogenated amorphous silicon alloyed with selenium has been made by plasma enhanced chemical vapor deposition (PECVD). The activation energy for electrical conduction is essentially unchanged for selenium concentrations < 1 at.%. The photo conductivity changes for selenium concentrations > 0.5 at. %. Photothermal deflection spectroscopy (PDS) and electron spin resonance (ESR), respectively, show that the width of the valence band tail states and the density of neutral silicon dangling bonds also change for selenium concentrations > 0.5 at. %. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) is used in solar cells, thin film transistors and image sensors. But the deleterious properties induced by optical excitation or carrier injection impede the use of a-Si:H in many commercial applications. The most important metastable effect, the StaeblerWronski effect [I ], has been well studied, but the microscopic mechanism is still not understood. One possible route to improve the stability ofthis material is to alloy a-Si:H with other elements, especially those with low nearest-neighbor coordination number. Previously, we reported an improvement in stability for hydrogenated amorphous silicon alloyed with sulfur (a-SiSx:H) [2-4]. In this paper we report parallel experiments on a-Si:H alloyed with selenium (a-SiSex:H). Previous experiments on amorphous silicon alloyed with selenium prepared by thermal evaporation have shown that selenium can terminate the silicon dangling bond and decrease the defect density [5]. In this study, hydrogenated amorphous silicon alloyed with selenium has been made using plasma enhanced chemical vapor deposition (PECVD). Over a wide range of Se concentrations, these amorphous materials exhibit a small dangling band density (< 106 cm-3 ) and a steep slope for the absorption spectrum in the Urbach tail region. Previous results on a-SiSex:H films made by PECVD have been reported for selenium/silicon ratios (Se/Si) >I 0- [6-7]. Here we report on samples with selenium concentrations as low as 2 x 10.' where many optical and electronic properties are similar to those in a-Si:H. EXPERIMENTAL DETAILS The PECVD system, which employed a 13.6 MHz rfpower supply to drive a capacitively coupled reactor, was run at a power of 100 mW/cm2 . The flow rates were 27 sccm for SiH 4 and 3 sccm for the gas mixture of H2Se + He. The gas mixture was prepared in a holding cylinder before flowing through a mass flow controller. Crystal 'silicon, quartz, and coming 7095 glass were used as substrates. The substrate temperature was 230'C. The deposition pressure was 0.1 Torr. All films were over 2 pm thick. The growth rate increases from 1.8 A/s to 3.8 A/s as the doping gas ratio increases from 10-7 to 0.1. Secondary ion mass spectroscopy (SIMS) and electron microprobe analysis were employed to determine the elemental composition of the alloys. Aluminum coplanar electrodes were used for conductivity measurements. Photoconductivity was measured under 100 mW/cm 2 of white light at room temperature. A standard ph
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