Channeling, exponential tails, and analytical modeling of Si implants into GaAs

The depth distribution N(x) of Si/sup 29/ ion beam implanted into GaAs for a wide range of implant doses, energies, and SiN randomizing-layer thicknesses is discussed. It is found that the variation of implant angle with position on the wafer (typically encountered in scanned-ion-beam implantation systems) results in significant channeling-induced nonuniformity of the doping profiles, even in some cases where a randomizing layer was used. Because of the existence of exponential tails in virtually all implants, an extension of the standard LSS theoretical profile is necessary for accurate characterization of the implants and prediction of the device parameters. The measured profiles were fitted to two different modified-Gaussian analytical forms: the Pearson four-parameter form, which offers excellent flexibility in fitting arbitrary distributions but is relatively devoid of physical content, and a new three-parameter form that is an extension of a Gaussian to profiles hyperbolic in log N. In almost all cases, the hyperbolic-Gaussian form fits the data as well as the Pearson form, and the hyperbolic-Gaussian form is simpler. Parameters characterizing the implants for both types of fit are presented.<
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