Nanoscale electro-structural characterisation of ohmic contacts formed on p-type implanted 4H-SiC
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NANO REVIEW
Open Access
Nanoscale electro-structural characterisation of ohmic contacts formed on p-type implanted 4H-SiC Alessia Frazzetto1,3, Filippo Giannazzo1, Raffaella Lo Nigro1, Salvatore Di Franco1, Corrado Bongiorno1, Mario Saggio2, Edoardo Zanetti2, Vito Raineri1, Fabrizio Roccaforte1*
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. 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]. In fact, doping of SiC by conventional diffusion techniques cannot be achieved due to the small diffusion coefficients of impurities in the material. 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]. On the other hand, 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]. However, 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 * Correspondence: [email protected] 1 Consiglio Nazionale delle Ricerche-Istituto per la Microelettronica e Microsistemi-Strada VIII, n. 5, Zona Industriale, 95121, Catania, Italy. Full list of author information is available at the end of the article
electrical activation of the implanted dopants [7]. In particular, the fraction of implanted Al atoms occupying a substitutional position in the SiC lattice has been determined as a function of the post-implantation annealing conditions [8,9]. Furthermore, it has been found that the annealing processes of implanted SiC layers at such high temperatures can induce a significant surface roug
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