The Effects of Doping and Temperature on the Fermi Level and its Relationship to the Recrystallization Growth Velocity i
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THE EFFECTS OF DOPING AND TEMPERATURE ON THE FERMI LEVEL AND ITS RELATIONSHIP TO THE RECRYSTALLIZATION GROWTH VELOCITY IN ION-IMPLANTED SILICON L.E. Mosley, M.A. Paesler, and P.D. Richard North Carolina State Univ., Dep't. of Physics,
Raleigh,
NC 27695
ABSTRACT It has been observed that doping produces an enhancement in the recrystallization growth rate of silicon made amorphous by ionimplantation. This enhancement has been attributed to a shift of the Fermi level with doping. Evidence supporting this is based on the compensating effect of implantation of n- and p-type dopants together. We have previously proposed a model of the recrystallization growth process based on the diffusion of dangling bonds. We suggested that the rate enhancement is due to band bending at the amorphous-crystalline interface produced by doping. We have calculated the change in activation energy for the recrystallization growth velocity for a number of doping concentrations as a function of temperature. The major contribution to the apparent lowering of the activation energy with doping in an Arrhenius plot of the growth velocity versus I/kT is due to the temperature dependence of the Fermi level. Experimental data are compared with the calculated results. In addition differences in the measured growth rates in thermal and laser annealed samples are discussed, with primary emphasis on the lack of a change in the activation energy with doping in the laser annealed case. Introduction In the recrystallization of silicon made amorphous by ion implantation, the growth rate can be described by an Arrhenius equation which depends on the position of the Fermi level'. A compensating effect 2 , 3 , found when equal concentrations of phosphorous and boron are implanted, demonstrates the Fermi level dependence of the growth rate. Comparison of growth velocity measurements made by thermal 3 and laser annealing 4 , suggest that the origin of this Fermi level dependence is different in the two processes. With increased doping, thermal annealing results show a decrease in the activation energy and prefactor, whereas, laser annealing results show an increase in the prefactor with little change in the activation energy. To better understand the Fermi level dependence, we have measured the growth rate 'bn a family of thermally annealed n-type samples. In the following we present our results and compare them with a model of the growth process which we had previously developed 5 . Although we suggest that the dangling bond is responsible for the growth process in our model, the Fermi level dependence requires only that the defect is free to diffuse and can exist in various charge states. Finally, we offer an explanation for differences found between thermal and laser annealed growth rates in terms of quasi-Fermi level shifts. Experimental Since the recrystallization rate depends on the doping concentration, multiple phosphorous implants were used in order to obtain a uniform doping profile. Table 1 shows the implant schedules and peak concentrations for eac
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