Formation of Ti ohmic contact on p-SiC by laser annealing

Silicon carbide (4H-SiC) Schottky barrier diodes with a mature technological level and a large variety of products, targeted for different voltage and current values, are now commercially available [1]. Although significant progresses in 4H-SiC devices technology have been recorded in the last decade, there are still some challenging aspects hindering the achievement of devices operating at the theoretical limits, mainly related to surface and interface processing [2,3]. As an example, the formation of good- quality Ohmic contacts to p-type doped SiC still represents a critical issue. Nickel has been widely used as Ohmic contact to p-type 4H-SiC following silicidation at annealing temperatures above 900◦C [4]. Another good candidate to obtain ohmic contact is Titanium (Ti). In fact, reaction between Ti layer and the p-type 4H-SiC emphasizes the role of the ternary phase Ti3SiC2 formed at the interface as responsible for the barrier height lowering and Ohmic behavior [5–7]. For the Ohmic contacts formation, rapid thermal annealing (RTA) was conventionally used. However, during the fabrication process of 4H-SiC power devices, the RTA method causes unnecessarily heating of the whole structure and not only of the contacts. Therefore, a local annealing method for silicidation has become necessary. In this context, a nanosecond non-equilibrium laser annealing (LA) for the metal/4H-SiC silicidation process is a method that could help to overcome the RTA limits [8,9]. In this study, we systematically investigate the influence of the LA conditions, both in terms of energy density in the range 2.0-3.8 J/cm2 and number of shots (n) from 1 to 10, on the properties of Ti-based Ohmic contact on p-type 4H-SiC. In particular, the electrical properties of Ti/4H-SiC Ohmic contacts processed under different LA conditions will be correlated with the structural ones. The starting material was a commercial n-type 4H-SiC wafer. On this sample, transmission line model (TLM) devices were fabricated by using Al implantion for p-type doping and 20 nm Ti deposited by e- beam evaporation for the Ohmic contact. Fig.1 shows the representative I-V characteristics of the devices for two different energy densities. The measurements are acquired between two adjacent pads of TLM structures at different distances from 10 to 50 μm, with 10 μm step. Up to 3.4 J/cm2 (Fig. 1a) the characteristics do not show either a dependence on the TLM pad distance or a linear behavior. Instead, the sample irradiated at the higher fluence from 3.6 J/cm2 (Fig. 1b) shows linear I−V characteristics, with a total resistance increasing with the pad distance. Fig. 2 shows the contact resistance (RC) values as a function of LA energy density. Increasing LA energy density leads to a decrease of the contact resistance values of about one order of magnitude. On the other hand, the increase of number of shots leads to higher RC values. Furthermore, we have studied the behavior of the current, acquired at 1V between two adjacent pads, by fixing the energy density and varying the number of shots. Fig. 3 shows variation of current at different n, spanning from 1 to 10 shots, for 3.4 and 3.6 J/cm2 LA energies. It is possible to observe that the current varies differently with the number of shots in the two cases. For energy density of 3.4 J/cm2 we can observe a first decrease up to 6 shots and then an increment. This behavior is similar to the one observed by Choi et al. [10] and can be related to the different C thickness induced by LA of SiC [11]. On the other hand, the current for the energy density of 3.6 J/cm2 continues to drop down by increasing the number of shots. To confirm this point, additional microstructural analyses are in progress to understand if the evolution of the contact properties is linked with C formation. In the extended paper, a systematic investigation of all the process parameters (LA, C-thickness Ti/4H-SiC interface) will be presented, highlighting their role in controlling the electrical characteristics of the Ohmic contact.