Improved Inversion Channel Mobility in Si-face 4H-SiC MOSFETs by Phosphorus Incorporation Technique
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1246-B06-06
Improved Inversion Channel Mobility in Si-face 4H-SiC MOSFETs by Phosphorus Incorporation Technique Dai Okamoto, Hiroshi Yano, Shinya Kotake, Kenji Hirata, Tomoaki Hatayama, and Takashi Fuyuki Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama, Ikoma, Nara 630-0192, Japan. ABSTRACT We propose a new technique to fabricate 4H-SiC metal–oxide–semiconductor field-effect transistors (MOSFETs) with high inversion channel mobility. P atoms were incorporated into the SiO2/4H-SiC(0001) interface by post-oxidation annealing using phosphoryl chloride (POCl3). The interface state density at 0.2 eV from the conduction band edge was reduced to less than 1 × 1011 cm−2eV−1 by the POCl3 annealing at 1000 °C. The peak field-effect mobility of 4H-SiC MOSFETs on (0001) Si-face processed with POCl3 annealing at 1000 oC was approximately 90 cm2/Vs. The high channel mobility is attributed to the reduced interface state density near the conduction band edge. INTRODUCTION The inversion channel mobility of 4H-SiC MOSFETs is deteriorated by a high density of interface states at SiO2/4H-SiC [1], which is a major obstacle to the commercialization of SiC MOS devices. It is well known that the performance of 4H-SiC MOSFETs strongly depends on the crystal face and oxidation process. One of the best methods to improve MOS interface properties on 4H-SiC(0001) has been nitridation of the gate oxide using NO, N2O, or NH3 [2][10]. Relatively high channel mobilities of up to about 50 cm2/Vs [6], [10] and improved reliability [3], [9] can be obtained via nitridation. However, it seems to be difficult to further improve the channel mobility by nitridation even after optimizing the nitridation conditions. Besides N atoms, the incorporation of Na atoms into the SiO2/4H-SiC(0001) interface improves the peak channel mobility to above 150 cm2/Vs [11]. However, this method is not suitable for practical use because Na atoms act as mobile ions in the SiO2 layer. Recently, we have reported that the interface state density can be decreased by overoxidation of P-implanted Si-face 4H-SiC substrates [12]. However, this method has drawbacks such as large implantation damage due to heavy P ions and a negative flat-band voltage due to the implanted P donors beneath the gate oxide [12]. In this letter, we propose a new technique to incorporate P atoms into the SiO2 side of the interface (not the 4H-SiC side) by thermal annealing using phosphoryl chloride (POCl3). The interface properties of MOS capacitors and the inversion channel mobility of MOSFETs on (0001) Si-face were drastically improved by post-oxidation annealing with POCl3. EXPERIMENT N-type, 4°-off 4H-SiC (0001) Si-face epilayers with a net donor concentration of 8 × 1015 cm were used to fabricate n-type MOS capacitors. After the standard Radio Corporation of –3
America (RCA) cleaning, approximately 55-nm-thick oxides were formed by dry oxidation at 1200 °C for 160 min. Then, the samples were annealed at 900, 950, and 1000 oC for 10 min in a gas
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