Sputtering Induced Changes in Defect Morphology and Dopant Diffusion for Si Implanted GaAs: Influence of Ion Energy and
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ABSTRACT Experimental observations of dopant diffusion and defect formation are reported as a function of ion energy and implant temperature in Si implanted GaAs. In higher energy implants (>100 keV), little or no diffusion occurs, while at energies less than 100 keV, the amount of dopant redistribution is inversely proportional to energy. The extended defect density shows the opposite trend, increasing with increasing ion energy. Similarly, the diffusion of Si during post implant annealing decreases by a factor of 2.5 as the implant temperature increases from -2 to 40'C. In this same temperature range, the maximum depth and density of extrinsic dislocation loops increases by factors of 3 and 4, respectively. Rutherford Backscattering (RBS) channeling measurements indicate that Si implanted GaAs undergoes an amorphous to crystalline transition at Si implant temperatures between -51 and 40°C. A unified explanation of the effects of ion energy and implant temperature on both diffusion and dislocation formation is proposed based on the known differences in sputter yields between low and high energy ions and crystalline and amorphous semiconductors. The model assumes that the sputter yield is enhanced at low implant energies and by amorphization, thus increasing the excess vacancy concentration. Estimates of excess vacancy concentration are obtained by simulations of the diffusion profiles and are quantitatively consistent with a realistic sputter yield enhancement. Removal of the vacancy rich surface by etching prior to annealing completely suppresses the Si diffusion and increases the dislocation density, lending further experimental support to the model.
INTRODUCTION Increasing the reliability and yields of Si, GaAs and other semiconductor devices requires accurate knowledge and control of doping profiles. As device dimensions shrink, increasingly shallower junction depths and tighter control of channel doping will be required, necessitating precise placement of dopant atoms. Of particular concern in recent years has been transient enhanced diffusion (TED) of annealed shallow implants, leading to anomalously deep junction depths.1, 2 This enhanced diffusion is a observed in many ion/target systems including B implanted Si, which has received the most attention, and Be, Mg, Zn, and Si implanted GaAs. By focusing on understanding fundamental mechanisms, knowledge gained from studying TED in one system should be applicable to modeling other ion/target combinations. The most widely used n-type dopant for fabricating GaAs devices is ion-implanted silicon. Unfortunately, the diffusion of silicon has been found to vary over a large range of values, depending on processing conditions. 3 "7 These process conditions are often not well controlled, leading to large variations in reported diffusivities and anomalous diffusion behavior. 6 Recently, Haynes and Holland reported dramatic variations in implant damage as a function of temperature 337 Mat. Res. Soc. Symp. Proc. Vol. 354 0 1995 Materials Research Society
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