Modeling Diffusion in Gallium Arsenide: Recent Work
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Device Fabrication
Second to silicon (Si), the most highly developed technology for semiconductor processing exists for gallium arsenide (GaAs).1'2 Unfortunately, GaAs processing is more complex than that of Si, mainly because GaAs is a compound semiconductor. Additionally, the lack of a stable native GaAS oxide and other disadvantages relative to Si have prevented this material from expanding beyond the small niche of applications where its high intrinsic electron mobility, superior radiation hardness, and direct bandgap are essential. Adequate understanding and modeling of the process physics are important for extending the "process window" available to GaAs manufacturers and for increasing the appeal of this material. This article deals with one of the most important process events: dopant diffusion. In the next section we briefly describe device-fabrication technology and show the importance of diffusion modeling in the prediction of device characteristics. We then review some elementary diffusion mechanisms and outline the dopants that are important in GaAsprocessing technology as well as the methods by which these dopants are introduced into the substrate. In subsequent sections we review the research community's current understanding of diffusion mechanisms as well as model parameters for specific dopants. Much work has been done in this field, at Stanford and by other groups, since the publication of a major review of the subject by Tan et al.31 in 1991. In this article, we focus on these recent contributions.
A typical process for fabricating an nchannel MESFET (MEtal-Semiconductor Field Effect Transistor) can be seen in Figure I.3 A low-dose, shallow Si implant is required to form the channel and a deeper, higher dose Si implant is made for the source and drain contact regions. Associated with each of the implants is an activation anneal. In p-channel devices Be is the likely candidate for the channel and the source/drain implants. This is also true of the base implant for a npn heterojunction bipolar transistor (HBT). For both implanted n- and p-type dopants, a relatively high-temperature anneal is necessary to cause the mostly interstitial dopant to become substitutional and electrically active. The annealing process occurs either through furnace annealing at temperatures lower than 850°C for long periods of time (about half an hour), or through rapid thermal annealing performed at much higher temperatures (greater than 900°C) for very short times (a few seconds). Because of the requisite high temperature step, diffusion of the implanted dopant is a major concern. In the case of the MESFET, the junction depth and the abruptness of the transition from n- to p-type GaAs are primary factors in determining both DC characteristics (e.g., the threshold voltage, Vt) and AC characteristics (e.g., switching speed). 4 The generation of point defect damage through ion implantation is an added complication since dopant diffusion mechanisms involve point defects as we will show.
MRS BULLETIN/APRIL 1995
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