Investigation of Implant Temperature on Dopant Activation Efficiency for Single Fluence Al+ Implanted 4H-SiC

Abstract

Abstract Body: As a wide bandgap semiconductor, silicon carbide (SiC) possesses unique properties that make it an advantageous replacement for silicon in high power, high frequency, and high temperature electronic devices. Ion implantation followed by high temperature annealing (>1600oC) is often used to introduce and activate dopants in the material. However,the high concentration of point defects created from implantation evolve into extended defects upon annealing, resulting in device failure. In order to address this issue, elevated temperature implants have been used extensively to prevent amorphization but little is known about higher temperature implant effects on defect evolution and how this correlates with dopant activation efficiency. In this study, 60 keV Al+ was implanted into single crystal n-type 4H-SiC wafers to a fluence of either 5x1013 or 1x1015 cm-2. The implant temperature was varied between 300°C and 700°C. High temperature furnace annealing was performed with a carbon cap from 1300°C to 1680°C for 30 minutes. Structural characterization using cross-sectional and plan-view BF-TEM and DF-TEM was performed for extended defect analysis. Ohmic contacts were created from sputter deposition of a Ti/Al/Ni metal stack followed by alloying at 1000oC for 1 minute. Electrical characterization using variable-temperature Hall Effect was performed to study dopant activation. As implanted cross-sectional TEM analysis showed amorphization is avoided for the low fluence (5x1013 cm-2) implants at all implant temperatures. For the high fluence case (1x1015 cm-2), amorphization is avoided above 400°C due to the increasing effects of dynamic annealing. The reduction of lattice damage with increasing implant temperature can be seen in UV/Vis/NIR measurements for the unannealed samples. The absorption peaks corresponding to lattice damage show a consistent reduction as the implant temperature is increased up to 700°C. A carbon cap was introduced to reduce surface decomposition during the high temperature annealing process with a variety of capping methods studied. Graphitized photoresist was shown by AFM measurements to result in the least amount of surface roughening upon annealing at around 1 nm. Extended defects were observed by TEM for the high fluence implants at all implant temperatures after annealing but were absent for the low fluence case. The loops are seen to coarsen with implant temperature as expected but are not eliminated by the anneal. TEM results suggest the concentration of trapped interstitials in the extended defect decreases with increasing implant temperature. Further analysis on the effect of implant temperature on the evolution of these extended defects will be presented. In addition to structural characterization, electrical measurements were carried out. Ohmic contacts were successfully created from the combination of an effective annealing cap and from the optimal metal thickness ratios in the metal stack. However, initial Hall effect measurements at room temperature were not successful in determining the dopant activation values even after 1680°C annealing. This is thought to be due to the relatively deep acceptor level associated with Al in SiC. Subsequently, variable field measurements at 400°C showed a dopant type change from the n-type substrate to the p-type implant enabling dopant activation values to be determined. These electrical results will be compared with structural images to highlight correlations between extended defect evolution and dopant activation efficiency.