The Influence of Electrons From the Filament on the Material Properties of Hydrogenated Amorphous Silicon Grown by the H
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ABSTRACT We observe that under certain conditions, hydrogenated amorphous silicon grown by the hotwire chemical-vapor deposition technique has dark conductivities varying by several orders of magnitude across the same film. Similarly, the ambipolar diffusion lengths fluctuate significantly in these films, yet there is not much evidence of a corresponding structural change. We attribute this electronic nonuniformity to electrons from the filament reaching insulating substrates and charging the substrate negatively in some regions, thus causing films to grow with inferior material properties in those regions. The effect diminishes with increasing substrate temperature, where the film itself may be conductive enough to remove charge reaching the growing surface. Wellgrounded, conducting substrates seem to be immune from this effect. We reduce this effect by putting a conductive grid on insulating substrates, of close enough spacing to remove the charge, and measure the material properties of the film grown between the conductive elements of the grid. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) has a wide variety of commercial applications. The traditional method for growing a-Si:H is by the plasma-enhanced chemical vapor deposition (PECVD) technique. However, there is expanding interest in growing a-Si:H by hot-wire chemical-vapor deposition (HWCVD). This interest is driven by two main factors: a-Si:H grown by the HWCVD process can have stabilized electronic properties surpassing the best films grown by the PECVD process [1], and these HWCVD-films are grown at higher deposition rates than PECVD-grown films [2]. Higher deposition rates reduce the time necessary to grow individual layers in a device and thus reduce manufacturing costs. Better materials give hope to making better devices. In our endeavor to characterize the HWCVD process, we observe that there are deposition conditions that produce films with extremely nonuniform electronic properties. In this paper, we review the HWCVD process we use for depositing a-Si:H, report the results of characterizations on films grown by this process (including the observation of this nonuniformity phenomenon), and propose ways to ensure the growth of films with uniform properties. EXPERIMENT The HWCVD technique by which we grow a-Si:H films consists of a vacuum chamber with a means of heating substrates to a desired growth temperature, a means of supporting and heating a tungsten filament to dissociate silane into reactive species that then deposit onto the substrates, a shutter to shield the substrate from the filament before and after deposition, a gas inlet, and a pumping system to move process gases through the chamber. We employ two different methods for heating our substrates. Our most-common method uses a heater can that extends downward from the top flange on the deposition chamber. Shaped like a can with an open top, the substrates are attached to the bottom of the can using a clamp that fits around the substrates and screws directly to the heater can. The bo
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