On the Role of the Staebler-Wronski Susceptibility in Hydrogenated Amorphous Silicon
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ON THE ROLE OF THE STAEBLER-WRONSKI SUSCEPTIBILITY IN HYDROGENATED AMORPHOUS SILICON D.Caputo, G. De Cesare, F. Irrera, G. Masini, F. Palma, M. C. Rossi Universitk "La Sapienza", Dip. Ingegneria Elettronica, via Eudossiana 18, 00185 Roma, ITALY G. Conte, G. Nobile, E. Terzini ENEA, Centro Ricerche Fotovoltaiche, P.O. Box 32, 80055 Portici (Napoli), ITALY ABSTRACT Photoconductivity decay during monochromatic illumination has been measured on an ensemble of a-Si:H films deposited at different substrate temperatures. Degradation behaviour has been modelled within the framework of the bond-breaking model (dN/dt = Csw np). Simmons and Taylor recombination kinetics has been assumed, taking into account the divalent nature of dangling bonds and their three possible conditions of occupancy. The Staebler-Wronski susceptibility (Csw) has been extracted through a fitting procedure. As a result, a correlation between the obtained Csw and the measured electronic, optical and structural properties of as deposited a-Si:H films can be inferred. INTRODUCTION In order to understand the origin of the light-induced metastable defects in hydrogenated amorphous silicon (a-Si:H), a lot of efforts has been devoted to investigate the creation and annealing kinetics of metastable dangling bonds related to the as deposited material parameters [1]. Many different models [2,3,4] have been proposed to explain electronic instabilities in aSi:H, and it is widely accepted that the increased defect density after illumination is due to the weak-bond/dangling-bond conversion induced by photogenerated carriers. It is also well established that hydrogen plays a fundamental role [3,5] in the formation, stabilization and annealing of dangling bonds, due to its abundance in the a-Si:H network and to its diffusion at relatively low temperatures. In this work we study the influence of hydrogen microstructure and electronic properties of the as deposited material on metastability, with particular emphasys on the role of the StaeblerWronski susceptibility in the photoconductivity decay during the illumination. EXPERIMENTAL RESULTS Samples were deposited by RF glow discharge with a power density of 25 mW/cm 2 , silane flow of 40 sccm, gas pressure of 700 mTorr and deposition temperature (Td) ranging between 130 and 270 *C. Td was varied in order to modify the optoelectronic and structural properties of the material. The deposition rate was about 2.5 A/sec and the samples thickness was 1I Pm. Structural characterization was performed by infrared spectroscopy: the absorption spectra of the stretching mode were deconvolved to determine the total hydrogen content and the amount of microstructure (R). The former was obtained from the 640 cm- 1 band, whereas the latter is defined as the ratio of the integrated band intensity centered at 2070 cm- 1 (which is ascribed to the clustered phase of hydrogen) to the sum of the integrated band intensities of the modes at 2070 and 2000 cm-1 . The results are shown in Figure 1.In the investigated temperature range, the total h
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