Diffusion of Ion Implanted Mg and Be in GaAs
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DIFFUSION OF ION IMPLANTED Mg AND Be IN GaAs H. G. ROBINSON,* M. D. DEAL,** and D. A. STEVENSON*
*Department of Materials Science and Engineering, Stanford University, Stanford CA 94305 "**Centerfor Integrated Systems, Stanford University, Stanford CA, 94305
ABSTRACT Annealed Mg implants into GaAs show three diffusion regions: 1) rapid uphill diffusion in the peak of the implant; 2) rapid concentration-independent diffusion in the tail; and 3) slow concentration-dependent diffusion in between. Implanted Be, in contrast, exhibits only concentration-dependent diffusion. Constant Fermi level experiments show that this diffusion is actually hole-dependent. Uphill diffusion can be induced in Be implants by co-implanting with a heavier element such as Ar. Paradoxically, this retards the concentration-dependent diffusion. This behavior can be explained with the Substitutional-Interstitial-Diffusion (SID) mechanism and an understanding of the defect chemistry after implantation. In the region of uphill diffusion, the dopants are seen to getter from areas of excess Ga interstitials toward areas of excess Ga vacancies. The magnitude of the Ga interstitial gradient with respect to the dopant concentration is shown to be critical for the uphill diffusion. The reduction in concentration-dependent diffusion with co-implants is thought to be caused by implant damage allowing dopant atoms to shift from interstitial to substitutional sites.
INTRODUCTION Ion implantation has long been a popular technique for introducing dopants into semiconductors. It can be used to implant virtually any dopant into any substrate. Dopants can be implanted precisely over a wide range of concentrations and depths and can often be introduced at concentrations greater than their thermodynamic solid solubility. The most frequently implanted p-type dopants in GaAs are Be and Mg. They are used instead of the traditional GaAs p-type dopant, Zn, due to their lower mass and concomitant deeper penetration into the substrate. They are used in the fabrication of both JFETS and buried p layers in MESFETS. However, like Zn, they both exhibit rapid diffusivity under certain conditions [ 1 - 11 1. This diffusivity is a severe limitation on their use and consequently on the types of devices that can be fabricated with them. Understanding the basic mechanisms that cause this diffusion is a requisite first step in possibly controlling it. The damage caused during the implant complicates the diffusion behavior and damage effects must be separated from non damage effects. It is no longer adequate to speak simply of implant damage. The various types of damage produced during an implant, their spatial distribution, and their annealing properties must all be considered. Mg and Be are nearly ideal candidates for studying implanted dopant diffusion. Their annealed profiles show many similarities but also significant differences that give clues as to the underlying mechanisms of the diffusion. Mg exhibits two annealing phases, one in which rapid uphill diffusion creates "hu
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