Nanoscale electro-structural characterisation of ohmic contacts formed on p-type implanted 4H-SiC

Alessia Frazzetto,Filippo Giannazzo,Vito Raineri,Corrado Bongiorno,Edoardo Zanetti,Raffaella Lo Nigro,Mario Saggio,Salvatore Di Franco,Fabrizio Roccaforte

doi:10.1186/1556-276x-6-158

Abstract

This work reports a nanoscale electro-structural characterisation of Ti/Al ohmic contacts formed on p-type Al-implanted silicon carbide (4H-SiC). The morphological and the electrical properties of the Al-implanted layer, annealed at 1700°C with or without a protective capping layer, and of the ohmic contacts were studied using atomic force microscopy [AFM], transmission line model measurements and local current measurements performed with conductive AFM.The characteristics of the contacts were significantly affected by the roughness of the underlying SiC. In particular, the surface roughness of the Al-implanted SiC regions annealed at 1700°C could be strongly reduced using a protective carbon capping layer during annealing. This latter resulted in an improved surface morphology and specific contact resistance of the Ti/Al ohmic contacts formed on these regions. The microstructure of the contacts was monitored by X-ray diffraction analysis and a cross-sectional transmission electron microscopy, and correlated with the electrical results.

Highlights

Silicon carbide (SiC) is surely the most attractive among the wide band gap semiconductors for the fabrication of high-power and high-temperature electronic devices [1,2].Ion implantation is the most commonly used technique for selective doping during the fabrication of electronic devices in SiC [3]
While after high-temperature thermal treatment a significant increase of the surface roughness occurs in the sample annealed without capping layer, determined by the appearance of the typical step bunching on the surface (Figure 2a), the morphology of the sample annealed with a capping layer (Figure 2b) does not exhibit such a surface degradation, and only a slight increase of the roughness with respect to the asimplanted sample is observed
Whilst in the sample without a capping layer (Figure 5a) large Ti3SiC2 grains are located close to the interface with SiC and interrupted by small Al-rich regions, in the sample with a capping layer (Figure 5b), larger Al-rich regions are found, in some parts forming an almost continous interfacial layer. These results suggest that the reaction mechanism is affected by the original SiC surface roughness

Summary

Introduction

Silicon carbide (SiC) is surely the most attractive among the wide band gap semiconductors for the fabrication of high-power and high-temperature electronic devices [1,2]. Ion implantation is the most commonly used technique for selective doping during the fabrication of electronic devices in SiC [3]. Phosphorous implantation is typically used for n-type doping, and an almost complete electrical activation of the dopants can be achieved already at 1500°C [4]. Al implantation is used for p-type doping of SiC [5], and it is typically followed by annealing at higher temperatures (T = 1600°C to 1800°C) to promote the electrical activation of the dopant in substitutional lattice sites [6]. Efficient p-type doping by Al implantation is difficult due both to the high ionisation energies of acceptors and to the high thermal budget required to achieve the

Discussion

Conclusion