Papers by Author: Tomoaki Hatayama

Abstract: In this article, we show some new results regarding the electrical properties of 4H-SiC MOSFETs fabricated by thermal annealing using POCl₃. The temperature dependence of MOSFET properties is described and the effect of POCl₃ annealing followed by forming gas annealing is shown. POCl₃-annealed MOSFETs indicates negative temperature dependence of channel mobility and smaller change in threshold voltage. Forming gas anneal after the POCl₃ treatment further improves channel mobility up to about 101 cm²/Vs. Features and problems of P-doped oxide are summarized and the future challenges are described.

Abstract: Relationship between the chemical reactivity and the orientation of SiC substrates was investigated. Thermal etching of 4H-SiC in the mixed gas of oxygen and chlorine was carried out as the chemical reaction. The etching rate did not change monotonously with the increase of the off angle in 4H-SiC (000-1) C substrate. By the use of such tendency in the thermal etching, the three dimensional structure with the specific pyramidal plane was able to be obtained.

Abstract: The sub-trenches in 4H-SiC Si- and C-faces could be disappeared by the thermal treatment in chlorine ambience at 900-1000oC. The surface morphologies of the thermally treated trench-sidewalls were unchanged. It is considered that the sub-trench is selectively removed because thermally Cl2 etching rate of the (0001) Si- and (000-1) C-face are different to the (11-20) and (1-100).

Abstract: We investigated electrical properties of 4H-SiC trench metal-oxide-semiconductor field-effect transistors (MOSFETs) fabricated on (000_,1) C-face substrates with various off-axis angles. Off-axis angles and directions are 4o, 8o, and 15o towards [__,1120] and 8o towards [1_,100] directions. Most trench MOSFETs showed good on-state performance. Peculiar characteristics that field-effect mobility was 103 cm2/Vs in spite of a relatively high acceptor concentration of 1 × 1017 cm−3 in the channel region were observed for trench MOSFET on 15o-off substrates. From crystallographic analysis, this face is (11_,20) with 15o off towards [000_,1] direction. We can expect that this face has quite good MOS interface properties.

Abstract: Instability of metal-oxide-semiconductor field-effect transistor (MOSFET) characteristics was evaluated by DC and pulse current-voltage (I-V) measurements. MOSFETs with nirided gate oxides were fabricated on C-face 4H-SiC. Their interfaces have near interface traps (NITs) with long time constants, depending on the cooling down process after nitridation. Such devices exhibited a large hysteresis in DC I-V and a large transient current in pulse I-V measurements. These phenomena can be explained by the charge state of NITs due to capture/emission of electrons in the channel.

Abstract: The sloped sidewall angle in 4H-SiC mesa structure could be controlled by a thermal etching at 900oC in chlorine (Cl2) based ambience. 4H-SiC C face with 8o off substrate was used. The SiO2 layers for the etching mask were formed by a deposited SiO2 layer or a thermally oxidized layer. Thermal etching was carried out at Cl2 ambience at 900oC for 15 minutes. The surface morphologies of the mesa structures were observed with the scanning electron microscope (SEM) and atomic force microscope (AFM). The sloped angles at the mesa sidewalls using deposited SiO2 mask and thermal SiO2 mask were about 23o and 60o, respectively. These results mean that the angle of sloped sidewall can change by mask fabrication method.

Abstract: Metal-oxide-semiconductor (MOS) capacitors and MOS field-effect transistors (MOSFETs) were fabricated on C-face 4H-SiC with post-oxidation annealing in phosphorus- containing atmosphere. POCl3/N2 annealing at 1000 °C, which is an effective condition for Si-face, did not bring any improvement in the interface state density (Dit) for C-face due to additional oxide growth. We have developed a new process sequence suitable for C-face MOS structures. As a result, the Dit near the conduction band edge was drastically decreased by the developed process to less than 3x1011 cm−2eV−1. The field-effect mobility of C-face 4H-SiC MOSFETs was effectively increased to 37 cm2/Vs. We found that the incorporation of phosphorus atoms into the SiO2/SiC interface can improve MOSFET performance not only for the Si-face but also for the C-face.

Abstract: Surface properties of the 4H-SiC (0001) Si faces could be evaluated by the contact angle measurements with water droplet method, X-ray photoelectron spectroscopy and an atomic force microscope. The contact angles do not depend on the surface roughness under 3nm. The substrate surfaces with the contact angles over 30o will be terminated by hydrogen related species. The contact angles around 20o on 4H-SiC is caused by the removal of oxide layer with fluoride acid and terminated subsequently by the -CF species on the surface. The hydrophile surface of 4H-SiC is caused by the formation of chemical oxide layer as well as the case of the silicon wafers.

Abstract: We report on electrical and physical investigations aimed to clarify the mechanisms behind the high channel mobility of 4H-SiC metal–oxide–semiconductor field-effect transistors processed with POCl3 annealing. By low-temperature capacitance–voltage analysis, we found that the shallow interface traps are effectively removed by P incorporation. Using x-ray photoelectron spectroscopy, we found that the three-fold coordinated P atoms exist at the oxide/4H-SiC interface. The overall results suggest that P atoms directly remove the Si–Si bonds and thus eliminate the near-interface traps.

Abstract: . 　 Crystallographically specific pyramidal planes in 4H-SiC could be naturally formed at mesa sidewalls by a thermal etching at 900oC in a chlorine (Cl2) based ambience. The mesa structures have been fabricated on the 4H-SiC C-face with 0-45o off-angle toward [11-20]. The etched surface was rather smooth, and bunched step structures on the specific pyramidal planes were not observed with atomic force and scanning electron microscopes. The formation mechanisms of the specific pyramidal planes at mesa sidewalls are discussed on the basis of these experimental results.