Improvements in Pt-based Schottky contacts to 3C–SiC

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Abstract

Despite its structural shortcomings (stacking faults, twins and threading dislocations), 3C–SiC heteroepitaxially grown on Si still has potential for high temperature sensor applications for which stable electrical contacts are of extreme importance. Pt/Si multilayered metallisations were compared to conventional Pt ones in order to investigate the effect of excess Si to the metal/SiC interface and the possible improvements in the thermal stability. These contacts were annealed up to 750°C. Their electrical behaviour was analysed by IV measurements while the interface between the metal system and the 3C–SiC surface was examined by transmission electron microscopy (TEM) and the formed phases were determined by X-ray diffraction (XRD).

Introduction

Silicon carbide has been attracting considerable attention [1], [2], [3], [4], [5] because of its high electron saturation velocity (2.5×107cm s−1) and its thermal and chemical stability. These unique physical and electrical properties make SiC a viable material for high power, high frequency and high temperature device applications.

The thermal stability of ohmic and rectifying contacts of SiC electronic devices is of extreme importance. In the case of 3C–SiC/Si, the fabrication of good quality Schottky contacts is hindered by the large concentration of stacking faults, associated with the low stacking energy of this polytype, as well as with twins and threading dislocations occurring in heteroepitaxial films. Thus, the mediocre quality of 3C–SiC heteroepitaxial material and the availability of good quality 4H and 6H SiC substrates has turned the mainstream SiC research towards these SiC polytypes. However, there are still active research groups in 3C–SiC with the emphasis on high temperature sensors since the fabrication of sensing elements (e.g. membranes) can be achieved easier due to the etching selectivity between Si and SiC. Our present contribution adheres to that framework.

Several metals have been used as Schottky contacts to 3C–SiC [6] and platinum is one of them.

In the present study, platinum (Pt) contacts and platinum/silicon (Pt/Si) multilayer contacts have been compared. Pt/Si multilayer contacts (MLS) were employed to allow the formation of a platinum silicide layer at the top of the SiC with negligible consumption of the top SiC layer. Thus the generation of a detrimental to the contact, carbon layer [7], even at high annealing temperatures, can be avoided thanks to the total reaction between Pt and Si prior to the reaction between SiC and Pt. The carbon segregation can be avoided by supplying excess Si in the multilayer structure.

Section snippets

Experimental

The material used for the fabrication of the Schottky contacts was a 4 μm thick n-type CVD grown 3C–SiC on Silicon, unintentionally doped, with a donor concentration of 3×1017 cm−3 (SiC1) [8]. The sample was degreased by sequentially immersion in trichloroethylene, acetone and propanol followed by thorough rinsing in DI-water and blow-drying with high purity nitrogen. Then the surface was mechanically polished with diamond pastes of various diminishing sizes (smaller paste size 0.1 μm), and the

Electrical

For the Pt contacts the best results in relation to ideality factor and leakage current were obtained after annealing at 300°C (Fig. 1). The diodes remained rectifying up to annealing at 500°C but exhibited ohmic like behaviour following annealing at 600°C.

For the MLS contacts the best results in relation to ideality factor and leakage current were obtained after annealing at 400°C (Fig. 1). The diodes remained rectifying up to annealing at 750°C (Fig. 1).

The results are summarised in Table 1.

SEM

Conclusions

It is thought that the Pt contacts could fail after a specific temperature due to the presence of a carbon layer following the reaction of Pt and SiC [5], [7], [11], [12]. From our study of the Pt contacts it is obvious that at high temperatures large grains are created into the SiC due to Pt diffusion resulting in the degredation of the contact. The formation of PtxSi is generally accepted but there are different interpretations for the interfacial reaction and for the final structure [5], [7]

Acknowledgements

The seventh protocol of scientific and technological Cooperation between Greece and Hungary supported this work.

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