Dependence of Steady-State Defect Density in Hydrogenated Amorphous Silicon on Carrier Generation Rate Studied Over a Wi
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DEPENDENCE OF STEADY-STATE DEFECT DENSITY IN HYDROGENATED AMORPHOUS SILICON ON CARRIER GENERATION RATE STUDIED OVER A WIDE RANGE NOBUHIRO HATA, GAUTAM GANGULY, and AKIHISA MATSUDA
Electrotechnical Laboratory 1-1-4 Umezono, Tsukuba-shi, Ibaraki 305, Japan ABSTRACT
Measurements of the steady-state defect density (Nst) in hydrogenated amorphous silicon under illumination of pulse-laser light, as well as of continuous light, were carried out; and the dependence of Nst on the effective rate of carrier generation (G) is presented. The values of G ranged from 8 x 1021 to 2.4 x 1023 cm"3 s-1, while the illumination temperature was kept at 30 'C or at 105 *C. The results showed trends of Nst increasing with G similarly to the trends in the literature, but covered a higher and wider G range, and fitted a defect model which assumes a limited number of possible defect states. INTRODUCTION
Saturation of light-induced defects in hydrogenated amorphous silicon (a-Si:H) under prolonged illumination with intense continuous wave (cw) light has been highlighted either as an exhaustion of possible defect states or as a balance between light-induced creation and annealing of defects. 1-9 Generation-rate (G) and temperature (T) dependences of saturated (steady-state) defect density (Nst) have been studied by several groups, the results being discussed in relation to the saturation mechanisms. 10 -14 The use of light pulses from lasers was recently proposed as an alternative technique for fast creation of light-induced defects, which has an advantage in low average heating of the samples. 15"17 Saturation of light-induced 18 defects under illumination of laser light pulses has also been reported. One of the major limitations in measuring G dependence of Nst has been a narrow G range that can be accessed with cw light sources. At low G, the rate of creation of light-induced defects is so low that light-soaking time necessary to reach Nst is impractically long. On the other hand, experiments at high G are also difficult because of limitation of cw light sources and of sample heating during illumination with intense cw light. In the work reported here, light pulses from a laser, instead of cw light, are employed for the first time to measure G dependences of Nst in a-Si:H over a wide and high G range. The results are then compared with the results from earlier works. Compatibility of the present results with defect models is also discussed. EXPERIMENTS
A device quality a-Si:H film was deposited concurrently on two Corning 7059 glass substrates and a crystalline silicon substrate using a radio-frequency diode-discharge plasma, from silane, at a substrate temperature of 250 'C. 19 Details of the deposition conditions were described elsewhere. 18 The resulting films had a thickness of 1.07 mm, a Tauc optical gap of 1.76 eV, a hydrogen content of 9.7 atomic percent, a dark conductivity (298 K) of 5 x 10-10 S cm" 1, and a photoconductivity (AM1 100 mW cm"2 ) of 1.2 x 10-4 S cm" 1. The second harmonics of light pulses from a Q-switched neodym
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