Band-energy estimation on silicon cap annealed 4H-SiC surface using hard X-ray photoelectron spectroscopy
Introduction
Silicon carbide (SiC) devices have received considerable attention because of their high breakdown-field strength and high-thermal conductivity. However, substantial differences in the work functions of metals and 4H-SiC result in the formation of highly resistive contacts with the deposition of metals on the SiC surface [1]. To form low resistivity contacts on SiC, silicide metal layer insertions are typically used, which are formed by silicidation annealing using Ni, Ti, Al, W, and among others [2]. To induce silicidation, high-temperature annealing around 1000 °C is required that causes carbon segregation at the metal/SiC interface and inside the silicide metal layer [3], [4], [5], which leads to graphite precipitation and highly resistive contacts formation [6]. Owing to the above reasons, low-temperature silicidation or non-silicidation processes are urgently required for ohmic contact formation to improve reliability and reduce the thermal budget. Ohmic contacts where Ni2Si is formed by low-temperature silicidation annealing at 550 °C on 4H-SiC of high phosphorus concentration of 3 × 1020 cm−1 was reported in [7]. Laser annealing using TiSi and Ni/Ti was also proposed with the limitation of inducing C segregation [8], [9]. To solve this limitation, adding a carbophilic material such as Nb into the Ni layer before laser annealing was reported in [10]. Conversely, a silicidation-less ohmic contact formation methods using amorphasization or hydration of the SiC surface by ion implantation or plasma treatment were proposed in [11], [12]. However, relatively thick damage layers owing to the implantation and hydrogen desorption by high-temperature annealing (~400 °C) are problems remaining to be addressed.
Recently, we proposed a silicon-cap annealing (SiCA) method that avoids high-temperature silicidation annealing of the contact metals. The method was proposed for ohmic contact formation on an n-type Si-face 4H-SiC [13]. Moreover, the SiCA method is a simple process that includes amorphous-Si (a-Si) layer deposition and crystallization of the layer. Following SiCA method, ohmic contact on the n-type 4H-SiC is formed only using the metal layer deposition. In the present study, to elucidate the conditions of ohmic contact formation when using SiCA, electrical characteristic measurements and hard X-ray photoelectron spectroscopy (HAXPES) are conducted to directly perform a band-structure under Al and SiCA-processed SiC junctions.
Section snippets
Materials and methods
In this study, commercially available Si-face (0001) 4° off n-type low resistivity 4H–SiC wafers [nitrogen concentration = 1 × 1018 cm−1, 370-μm-thick] was used. After conventional cleaning and dilute hydrofluoric (HF) acid cleaning, a 25-nm-thick a-Si layer was deposited on the Si-face of the SiC wafer at a substrate temperature of 300 °C by the RF-sputtering method with 1000 Ωcm up n-type Si (FZ) target. Subsequently, the a-Si layer was crystallized using rapid thermal annealing from 50 °C to
Results and discussion
Fig. 1 shows the Tmax dependence of I–V characteristics obtained from Al/c-Si/SiCA-SiC structure. The samples at Tmax = 800 and 900 °C showed Schottky characteristics, whereas the slope of I–V curves increased at Tmax = 1000 °C and a clear ohmic contact characteristics were obtained for samples with temperatures above 1100 °C. Here, a minimum ρc of 1.01 × 10−3 Ωcm2 was obtained with the SiCA sample at 1280 °C. Moreover, the SiC sample with the Si layer removed after 1280 °C SiCA showed almost
Conclusions
In summary, HAXPES was conducted to estimate the band-diagram state of silicidation-less ohmic contacts of the SiCA-SiC layer. The study revealed that SiCA could modulate surface potential energy of SiC and lower the potential of barrier lowering, leading to the formation of a thin-depletion layer. Thus, ohmic contact formation without silicidation annealing was achieved by SiCA. The flexibility of SiCA could help to improve the reliability of contact. The specific physical surface structure of
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The synchrotron radiation experiments were performed at BL46XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute(JASRI) (Proposal no. 2017A1762 and 2017B1825). The authors would like to thank Dr. S. Yasuno for technical assistance with the HAXPES experiments. We would also like to thank Prof. N. Iwamuro for HAXPES measurements. Part of this research was conducted with support of The Research Institute for Nanodevice and Bio Systems, Hiroshima University.
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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