Versatile circular test structure for ohmic contact characterisation

Metal to semiconductor contacts can be divided into two groups: rectifying contacts and non-rectifying contacts. For a non-rectifying contact, which is also known as an ohmic contact, there is an electrical junction between the metal and semiconductor that has a linear current–voltage (I-V) characteristic (as with Ohm’s law). A low resistance ohmic contact allows the carriers to flow easily in both directions between the metal and semiconductor. Ohmic contacts are essential for most semiconductor devices e.g. p-n diodes, Schottky diodes, metal–oxide–semiconductor field-effect transistors (MOSFETs), metal–semiconductor field effect transistors (MESFETs) and so on. Lower and lower ohmic contact resistance is desired as the dimensions of devices decrease and nanotechnology demands better means of creating electrical access to devices. Continued improvement in the performance of ohmic contacts requires better characterisation of such contacts. In order to study the properties of ohmic contacts, the parameter specific contact resistivity (SCR) was introduced by [Chang et al., 1971]. Accurate test structures and evaluation techniques needed to be developed to obtain the values of SCR and the transmission line model (TLM), the circular transmission line model (CTLM) and the cross-bridge Kelvin resistor test structure (CBKR) are the most commonly used techniques. Although they have their advantages in either analytical expressions or fabrications, their disadvantages are obvious as well. In this work, new ohmic contact test structure, the two-contact circular test structure, has been developed for determining SCR.

The evaluation techniques are also presented and demonstrated by using finite element modelling (FEM). Experimental results are reported for different metal to semiconductor ohmic contacts to verify the two-contact circular test structure. As examples of contacts to two-dimensional (2-D) semiconductor layers (relatively no vertical voltage drop in the semiconductors), the values of SCR of Ni to heavily doped n-type epitaxial 3C-SiC and Ti to lightly doped n-type epitaxial 3C-SiC ohmic contacts were determined to be (0.8-5.7) E-6 ohm cm2 and (3.0-7.5) E-4 ohm cm2 respectively using the proposed test structure and its corresponding evaluation technique. For contacts to bulk semiconductors (3-D circumstances compared to 2-D circumstances, where semiconductor vertical voltage drop is significant), the values of SCR of Ni to p-type bulk Ge and Ti to n-type 4H-SiC substrate ohmic contacts were determined to be (2.3-27)E-6 ohm cm2 and (1.8-2.5)E-3 ohm cm2 respectively. Furthermore, an investigation of the effect of low energy implantation on the properties of Au/Ni/Ti contacts to heavily doped n-type epitaxial 3C-SiC has been undertaken using the proposed test structure and its corresponding analytical equations.