The Al-Ti based alloys give ohmic contacts on p-type 4H-SiC with reasonably low specific contact resistance (≤ 10^-4­ Ωcm²). Unfortunately during alloying the deposited metals go through a liquid phase [1] and contact areas lose their form. Moreover, during cooling the Al in excess segregates in Al droplets on the top of the contact surface. All that produces ohmic contacts that have not an homogeneous interface with the semiconductor and limits the minimum distance between adjacent contact areas. In previous studies it has been shown that a thin layer of metal with high melting temperature like Ni can be used as a cap on the top of the deposited Al-Ti layer for reducing or avoiding the enlargement of the melted Al-Ti film during alloying, depending on the combination of the Ni, Al and Ti thin film thicknesses [2,3]. In the same studies it was shown that it exists an optimum Ni : Al : Ti thicknesses ratios to preserve the contact form and with optically homogenous and shiny contact surfaces with specific contact resistance values in the low 10^‑4 Ωcm² decade at room temperature for p-type resistivity in the range 0.1-1 Ωcm [3]. This study will focus on the electrical performence of these optimal contacts above room temperature.

4° off-axis <0001> n-type 4H-SiC homo-epitaxial specimens with a 200 nm p-type layer at the top surface have been used for this study. The p-type resistivity of these layers varied in the range 10^-2 – 10¹Ωcm. Ni(50nm)/Al(40nm)/Ti(20nm) were evaporated on the surface of these samples and circular TLM devices (TLM-C) were obtained by the lift-off technology. The intra pad-distances of these devices was in the range 4 – 49 μm, and the internal-pad-radius in the range 51 – 508 μm. After lift-off the specimens were alloyed at 1000°C for 2 min in vacuum. The preservation of the pads form factor and of a mirror like contact surfaces after alloying [3] were verified by optical inspection.

The ohmic behavior of the alloyed contact was tested by current-voltage (I-V) measurements in the temperature range 300-560 K. The specific ohmic resistance r_c and transfer length L_Twere obtained from the I-V resistance values of TLM devices versus intra-pads distance for constant internal-pad radius [4]. As a cross check, the so obtained values were verified to be good for fitting the TLM resistance data obtained from the I-V measurements of devices versus variable internal-pad radius and constant intra-pads distance [4].

The main result of this study is that, for temperatures in the range 300-560 K, the specific contact resistance of the studied contacts is in the range 2 – 4 × 10 ^‑4 Ωcm² over three decades of p-type material resistivity, i.e. 10^-2 – 10¹ Ωcm, that corresponds to a range of 10³- 6×10⁴ Ω_sq sheet resistance values, taking into account the thickness of the p-type layer. Reasons for such a perfomance will be discussed.

[1] Susumu Tsukimoto, Kazuhiro Ito, Zhongchang Wang, Mitsuhiro Saito,Yuichi Ikuhara, Masanori Murakami, Materials Transactions 50(5), 1071-1075 (2009).

[2] P. Fedeli, M. Puzzanghera, F. Moscatelli, R. A. Minamisawa, G. Alfieri, U. Grossner, R. Nipoti, Mater. Sci. Forum 897, 391-394 (2017).

[3] R. Nipoti, M. Puzzanghera, M.C. Canino, P. Fedeli, G. Sozzi, submitted to ICSCRM2017, Washington, DC, Sept. 17-22, 2017

[4] D. K. Schroder, Semiconductor Material and Device Characterization, 3rd Edition, 2006, Wiley-IEEE Press.