Diffusion of Hydrogen and Deuterium in Stack Systems of Si x N y H z /Si x N y D z and Crystalline Si
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Diffusion of Hydrogen and Deuterium in Stack Systems of SixNyHz/ SixNyDz and Crystalline Si Christoph Boehme and Gerald Lucovsky Department of Physics, North Carolina State University, Raleigh, North Carolina, 27695-8202, U.S.A. ABSTRACT H/D-, N-H/D- and Si-H/D-bond density changes were investigated in stacks consisting of a Cz-Si substrate, a thin layer of SiO2, amorphous deuterated silicon nitride as well as amorphous hydrogenated silicon nitride in order to see if the post deposition anneal of a-SixNyHz layers on crystalline silicon wafers can actually lead to a migration of H atoms into the Si-bulk, which is an important question in regard to emitter passivation of Si-solar cells. The stacks were grown with remote plasma enhanced chemical vapor deposition (RPECVD). A low temperature (≈200°C) process of down stream injected ammonia (NH3) and silane (SiH4) activated by an upstream injected He-plasma, produced through RF-radiation (13.65MHz) was used. Thermal treatment was executed by ex situ rapid thermal anneal in Ar ambient. For the measurements of H and D bond densities, FTIR was employed while SIMS determined atomic densities of H, D and O in the c-Si/nitride interface region. The experiments showed that H transport in silicon nitride is determined by several mechanisms including diffusion and dissociation processes and that silicon nitride deposited with high ammonia to silane ratios can produce molecular species like ammonia and H2. The study of the reaction dynamics showed that the production of molecular hydrogen is the most dominant process as long as Si-H-bonds are present in the system. After their exhaustion, an ammonia producing reaction prevails that leads with increasing temperatures to lower densities in the nitride films. INTRODUCTION Even though the release of hydrogen out of amorphous hydrogenated silicon nitride during post deposition anneal has been studied extensively since the end of the 1970s, little attention has been paid to the actual microscopic processes responsible for this effect. In the past ten years improvement through anneal of c-Si solar cells underneath silicon nitride anti reflection layers has repeatedly been observed [1,2]. This effect was assumed to be due to hydrogen passivation by diffusion of hydrogen from the AR-coating into the underlying c-Si bulk. Since hydrogen permeation through the silicon nitride/silicon interface depends mostly on the dynamics of the hydrogen concentration in the silicon nitride, the nature of the microscopic H-transport is of great importance for such a permeation process. A dominance of atomic H diffusion in the SixNyHz as described by Bik [3] can lead to a very inhomogeneous H density depth profile and therefore high H densities at the nitride/c-Si interface. However, a dissociative mechanism as proposed by Stein et. al. [7] which consists of the rapid diffusion of molecules with H atoms after a local dissociation out of the network, produces very homogeneous H depth profiles and therefore low H-densities at the interface.
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