Enhanced Diffusion of High-Temperature Implanted Aluminum in Silicon Carbide
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ABSTRACT The diffusion of aluminum in silicon carbide during high-temperature Al+ ion implantation was studied using secondary ion mass spectrometry (SIMS). Transmission electron microscopy (TEM) has been used to determine the microstructure of the implanted sample. A 6H-SiC wafer was implanted at a temperature of 1800 'C with 40 keV Al ions to a dose of 2 x 1016 cm-2. It was established that an Al step-like profile starts at the interface between the crystal region and the damaged layer. The radiation enhanced diffusion coefficient of Al at the interface was determined to be Di = 2.8 x 10-12 cm 2 /s, about two orders of magnitude higher than the thermally activated diffusion coefficient. The Si vacancy-rich near-surface layer formed by this implantation condition is believed to play a significant role in enhanced Al diffusion. INTRODUCTION Selective doping is an essential technique for most device technologies. Ion implantation is the prevailing method for achieving p-type regions in SiC. Previously it was established [1,2], that to reduce crystal lattice damage and to increase the electrical activation of Al implants in n-type SiC, high-temperature (HT) implantation is required. However, HT implantation could result in an enhanced impurity diffusion coefficient [3]. In this work the concentration profile of Al implanted into n-type 6H-SiC at 1800 'C was studied using secondary ion mass spectrometry (SIMS), and the crystal structure was studied using transmission electron microscopy (TEM). EXPERIMENT A3
min 6H-SiC n-type epitaxial film (Nd - Na = 8 x 1016 cm-3) on a 6H-SiC (Si-face) substrate
2 was implanted with 40 keV AP- ions and an ion current density of 20 pA/cm to a dose of 2 x 1016 cm-2 (implantation time 160 s) at 1800 'C. During implantation, the sample was tilted 7' off the normal to minimize channeling. Part of the sample was masked by graphite clips to retain the sample against the heater and to preserve unirradiated reference regions by preventing evaporation. The thickness of the evaporated layer was determined by step height measurements with a surface profilometer. The SIMS depth profiles of Al, Si and C were performed using a CAMECA IMS-4F spectrometer. Cross-sectional analysis was performed using a PHILIPS EM 420 TEM at 100 keV. The 6H-SiC wafers used in this study were obtained from Cree Research Inc.[4].
RESULTS Figure 1 shows a plot of concentration profiles of Al, Si and C obtained by SIMS and a theoretical model curve drawn by a solid line. The step-like Al concentration profile from 0.15 [tm to 1.2 ttm indicates that diffusion had taken place during HT implantation. The Al content of the specimen measured by SIMS is about 10% of the total dose. This indicates that Al evaporation from the film surface occurs in conjunction with this diffusion. C and Si concentration profiles are consistent with stoichiometry and are seen to be constant except for the near surface region where concentrations are lower than in the bulk (Fig. 1). This near surface instability commonly observed in SIMS depth
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