High Quality a-Ge:H Films and Devices Through Enhanced Plasma Chemistry
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0989-A04-04
High Quality a-Ge:H Films and Devices Through Enhanced Plasma Chemistry Erik V. Johnson, and Pere Roca i Cabarrocas LPICM (CNRS, UMR 7647), Ecole Polytechnique, Palaiseau Cedex, F-91128, France
ABSTRACT We present a material study on RF PECVD-grown a-Ge:H showing that thin films of such material can be produced without using the conventional techniques of high power density or powered-electrode substrate placement. We demonstrate the production of material with PDS signatures superior to material produced at ten times higher power density. This is achieved through the use of Ar and H2 dilution and by growing the films at high pressures under conditions where nanoparticles and nanocrystals formed in the gas phase contribute significantly to the growth as confirmed by HRTEM. The conditions described result in material which demonstrates activated conduction down to room temperature. Additionally, the quality of the material has been demonstrated through its application in n-i-p diodes. A spectral response at 0.9um of 0.38 and an AM1.5 efficiency of 2.1% have been demonstrated utilizing an absorber layer thickness of only 60nm. INTRODUCTION The goal of producing PECVD-grown, device-quality a-Ge:H for use as a low-bandgap semiconductor has lead to important discoveries about the ideal growth conditions of the material. It is generally accepted that the best quality a-Ge:H is grown under conditions that are in stark contrast with those ideal for a-Si:H, a similar tetrahedrally-bonded amorphous semiconductor. For example, the quality of a-Ge:H is greatly improved when it is grown under conditions of high H2 dilution of germane (GeH4) and under high power conditions, whereas the growth of a-Si:H under these conditions (without an accompanying increase in pressure) leads to low quality, porous material. High quality a-Ge:H has been produced through deposition on the powered cathode of asymmetric RF-PECVD systems [1,2] and in ECR systems [3-6]. Recently, hot-wire CVD using undiluted GeH4 has been employed to grow high-quality films [7], demonstrating that high H2-dilution is not a strict requirement but that the conditions for good quality films are more complicated. The causes behind the higher quality of films grown under the above-listed conditions are still under dispute. Authors variously ascribe the high quality of the material to be due to 1) copious amounts of atomic hydrogen arriving at the growth surface, 2) ion bombardment during growth, 3) growth in the gas depletion regime for germane, and/or (4) heated precursors arriving at the surface. In this work, we promote this growth environment through the use of simultaneous Ar+H2 dilution of GeH4 and an elevated growth pressure. The metastable argon ions (Ar*) created through electron collision will ionize GeH4 molecules, increase the density of atomic H, and release energy to the surface through relaxation of the metastable state. As well, under the high-pressure conditions employed, plasma-formed and heated nanoparticles will contribute to the growth,
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