CVD-SiO₂ and plasma-SiN_x films as Zn diffusion masks for GaAs

Abstract

The effectiveness of plasma-deposited SiN_x and chemical-vapor-deposited SiO₂ as masks for localized diffusion at 600°C of Zn in GaAs has been investigated. Variables included the diffusion time (0.25-16 hrs), and the mask thickness (0.1-0.5 Μm) for both SiO₂ and SiN_x films, and the deposition temperature (300-500°C) and the phosphorous content (0-8 wt.%) for SiO₂ films. Diffusion windows were defined photolithographically and opened by etching in buffered HF or by reactive ion etching of CF₄. Profiles of p-n junctions associated with the Zn diffusion were determined by scanning electron microscopy of an etched cleaved section. Films of SiN_x formed effective diffusion masks, but for masks of SiO₂, enhanced diffusion of Zn took place along the substrate-mask interface, with sideways-to-depth diffusion ratios up to 10. The Inclusion of P in SiO₂ masks reduced, but did not eliminate, this effect. Mechanisms which may contribute to the enhanced sideways diffusion include strain, diffusion through the mask, and degradation of the masksubstrate interface. It is shown that the first of these mechanisms has only a minor effect.