Surface Protection during Plasma Hydrogenation for Acceptor Passivation in InP
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SURFACE PROTECTION DURING PLASMA HYDROGENATION FOR ACCEPTOR PASSIVATION IN InP
J. LOPATA, W. C. DAUTREMONT-SMITH, S. J. PEARTON, J. W. LEE, N. T. HA, AND H. S. LUFTMAN AT&T Bell Laboratories, Murray Hill, NJ 07974
ABSTRACT Various dielectric and metallic films were examined as H-permeable surface protection layers on InP during H2 or D2 plasma exposure for passivation of acceptors in the InP. Plasma deposited SiNx, Si0 2, and a-Si(H) films ranging in thickness from 85 to 225 A were used to protect p-InP during D2 plasma exposure at 2500C. Optimum protective layer thicknesses were determined by a trade-off between the effectiveness of the layer to prevent P loss from the wafer surface and the ability to diffuse atomic H or D at a rate greater than or equal to that in the underlying InP. SIMS and capacitance-voltage depth profiling were used to determine the extent of D in-diffusion and acceptor passivation respectively. Sputter deposited W and e-beam evaporated Ti films -100 A thick were also evaluated. The W coated sample yielded similar results to those with dielectric films in that acceptors in p-InP were passivated to a similar depth for the same plasma exposure. The 100 A Ti film, however, did not allow the D to diffuse into the InP substrate. It is surmised that the Ti film trapped the D, thus preventing diffusion into the substrate. INTRODUCTION Hydrogen passivation of shallow dopants and deep levels in semiconductors has been widely reported and reviewed1 ,2 1 but until recently little has been reported on the effects in InP. The use of a hydrogen plasma is the most commonly used technique to introduce hydrogen into crystalline semiconductors. Attempts to hydrogenate InP by this method however result in preferential P loss from the surface as PH 3 and therefore In droplet formation.1 '34 1 In order to successfully diffuse hydrogen into single crystal InP without degradation a surface protection layer which is H permeable but PH3 impermeable must be employed. In recent reports on the passivation of acceptors in p-InP various hydrogen permeable capping layers have been used to protect the InP surface from degradation during the hydrogen in-diffusion process. Chevallier et al.1 51 used 0.55 lgm InGaAs to protect InP during plasma hydrogenation for acceptor passivation and Omeljanovsky et al. 6 1 used a Schottky Au layer to act as a surface protectant. In an earlier paper we reported the use of SiN, cap layers to protect InP during plasma hydrogenation.1 71 A thin layer of -85 A PECVD SiNX was found to be adequate to protect a p-InP surface during a 0.5h, 250 0C hydrogen or deuterium plasma exposure to achieve a depth of acceptor passivation of - 1.0 gim. In this paper we investigate the useful range of SiNx cap layer thickness to provide surface protection and yet allow acceptor passivation to occur during a typical 0.5h plasma exposure. In addition to SiNx, thin PECVD films of SiO 2 and a-Si(H) and metallic films of e-beam evaporated Ti and RF sputtered W were also evaluated as H-permeable protective layers.
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