An Examination of H Effusion in a-Si:H Using Infrared Spectroscopy
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AN EXAMINATION OF H EFFUSION IN a-Si:H USING INFRARED SPECTROSCOPY A. H. Mahan, E. J. Johnson, and J. D. Webb, National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401, USA ABSTRACT We present the results of H diffusion studies on glow discharge (GD) and hot wire (HW) deposited a-Si:H films studied by infrared (IR) spectroscopy. In this technique, a-Si:H films deposited on crystalline silicon are annealed isochronally for fixed times, and after each anneal the decrease in the amount of SiH infrared absorption is carefully measured. We then incorporate these data into the rate equation for loss of Sill bonds due to annealing, enabling the determination of an activation energy to break these bonds and place the H in a site that cannot be measured by IR. The activation energies for device quality GD films are similar to those found in the a-Si:H diffusion literature, suggesting that diffusion parameters can be measured by this method. On the other hand, a similar analysis of data for HW films containing similar H contents is not clear cut. GD and HW samples with lower H content, deposited at higher Ts, are also examined. We discuss these results in the context of film microstructure. INTRODUCTION The influence of H on the structural and electronic properties of a-Si:H has been extensively investigated. According to the a-Si:H literature, the incorporated H in device quality material is needed not only to passivate dangling bonds, but is also apparently needed to relieve lattice strain [1], since more H than is needed just to passivate the -1019 cm-3 dangling bonds is routinely incorporated. It is also clear that a large majority of this H exhibits an activated behavior upon sample annealing, with values in the range of 1.35-1.50 eV routinely obtained [2-6]. Various interpretations of the meaning of this value have been advanced, and companion measurements of the electron spin resonance signal with anneal, which does not increase significantly with H evolution [2], suggest that this H is clustered in some fashion. Typical H contents needed for device quality glow discharge (GD) a-Si:H range on the order of 7-12 at.%. Attempts to reduce the H content in GD material have met with only limited success. Recently, a-Si:H grown by the hot wire (HW) technique was found to incorporate much less H with a preservation of device quality electronic properties [7-8]. In particular, these films exhibited their best electronic properties (Eo, Ld) for H contents in the range 1-4 at.%. Other measurements have also suggested that the HW films are different from GD films over a wider range of H contents [8,9). Thus, the question arises as to whether the H is bonded differently in the HW films compared to the GD films. The technique chosen to explore this question is that of H effusion. Because of a suspension of deposition laboratory activities due to safety issues involving the use of concentrated silane, we have been unable to deposit the specific series of films needed to pursue H effusion (evolution) measurements in th
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