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Poster

  1. 1. An investigation into the impact of surface passivation techniques using metal- semiconductor interfaces Y. Bonyadi1, P.M. Gammon1, Y.K. Sharma2, G. Baker1*, P.A. Mawby1 1 School of Engineering, University of Warwick, Coventry, CV4 7AL, UK 2 Dynex Semiconductor Ltd, Lincoln, UK Introduction & Objectives Schottky barrier diodes (SBDs) were formed on untreated 4H-SiC surfaces, and after thermal oxidation, direct N2O growth, and direct PSG growth have been performed, and the resulting oxides removed. Then Mo, Ni, and Ti SBDs were fabricated to study the impact of the surface passivation techniques using these metal-semiconductor interfaces. These diodes were then characterised using Current-Voltage (I-V), Capacitance-Voltage (C-V), and Current-Voltage-Temperature (I-V-T) analysis. Background Minimising the interface traps density (Dit) is vital for realising the advantageous properties of 4H-SiC devices. The existence of interface traps/defects, which originate from dangling bonds and carbon clusters is a major obstacle in development of SiC MOS devices [1]. Hence, several passivation treatments have been investigated to improve the surface quality and passivate traps after oxide growth [2]: • Direct Nitrogen growth • Direct Phosphorus growth Table 1. Each result is the average of at least 7 of each diode tested. Results and Discussion Fig. 4 Forward characteristics of (a) Mo/SiC, (b) Ni/SiC and (c) Ti/SiC SBDs using different surface passivation treatments. Fig. 2 (a) Forward and (b) Reverse I-V-T characteristics of PSG treated Mo/SiC at different temperatures. Fig. 3 The C-2-V used to extract the doping value (N2O treated diodes before contact annealing). 1. After depositing a contact metal (Mo, Ni and Ti), but prior to contact annealing (no silicides were formed), IVT and C-V measurements helped assess the effects of the passivation treatments on the SiC surface. 2. After annealing the contacts and the formation of metal silicides at the interface, the same electrical characterisation techniques were used to examine the impact of these treatments on SBDs themselves. Fig. 1 SBD Cross section schematic. In Table 1, the SBH extracted from C-V analysis before contact annealing show just 0.14 eV variation across all the diodes, suggesting that the interface is greatly suffering from Fermi-Level pinning, the result of significant interface traps. Contact annealing process largely reduced the amount of charge at the metal-semiconductor interface. The surface treatments result in SBH lowering and doping profile dropping. No improvement was observed in ideality factor. The doping extracted from C-V data varies from the Ni diodes, at the expected value of 4×1015 cm-3, to the Ti (~3×1015 cm-3) and Mo diode (~2×1015 cm-3). This occurs due to bending in the C-2-V used to extract the value which is most pronounced in the Mo, and the result of charge at the interface that skews the measurement (Fig.3) Experimental Details An unexpected result from this work was the performance of PSG treated Mo diodes. These appear to break the trade-off between turn-on voltage and leakage current by having a low barrier height (consistent with all the Mo and Ti diodes), but also the lowest leakage of any device (4.44×10-5 A/cm2). Even when tested at 300°C, a leakage of 7.26×10-4 A/cm2 is lower than any of the other devices at room temperature (Fig.2) Conclusions • Although, the results of this study did not reveal any consistent patterns between the different treatments, a Mo diode formed on a surface after PSG treatment, displays exceptionally low leakage (4.44×10-4 A/cm2 at 19°C). Even when tested at 300°C, a leakage of 7.26×10-4 A/cm2 is lower than any of the other devices at room temperature. • Barrier heights extracted from C-V analysis before contact annealing show a variation across all the diodes, suggesting that the interface is greatly suffering from Fermi-Level pinning, the result of significant interface traps. After contact annealing the amount of charge at the metal-semiconductor interface was largely reduced and both I-V and C-V data showed the expected work function dependence. *Guy.Baker@Warwick.ac.uk

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