Shin Ichi Nishizawa

Chloride-based CVD growth of silicon carbide for electronic applications.

Chloride-Based CVD Growth of Silicon Carbide for Electronic Applications Henrik Pedersen,* Stefano Leone, Olof Kordina, Anne Henry, Shin-ichi Nishizawa, Yaroslav Koshka, and Erik Janz en Department of Physics, Chemistry and Biology, Link€oping University, SE-581 83 Link€oping, Sweden National Institute of Advanced Industrial Science and Technology (AIST), Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan Department of Electrical and Computer Engineering, Mississippi State University, Mississippi State, Mississippi 39762, United States

Rapid enlargement of SiC single crystal using a cone-shaped platform

We investigated the enlargement of SiC single crystal during physical vapor transport growth by modifying the shape of graphite lid. The single crystals grown on the cone-shaped platform were larger in diameter than those grown on the conventional one. The enlargement of ingot is discussed in terms of the dual role of polycrystals during crystal growth: (i) it provides a platform for single crystal and (ii) an obstacle to the lateral growth of ingot. The dependence of the broadening angle (β) of single crystal on the taper angle (θ) of the cone-shaped platform was also investigated and an optimum angle at a given growth condition found.

High Throughput SiC Wafer Polishing with Good Surface Morphology

We report SiC wafer polishing study to achieve high throughput with extremely flat, smooth and damageless surface. The polishing consists of three process, wafer grinding, lapping and chemical mechanical polishing (CMP), which are completed in shortest about 200 minutes in total for 2 inch wafer. Specimens of 4H- and 6H-SiC were provided from slicing single crystal as wafers oriented (0001) with 0 or 8 degrees offset angle toward to <112 _ 0>. By the first grinding using a diamond whetstone wheel, we realized flat surface on the wafers with small TTV error of 1 μm in 15 minutes. After second process of lapping, the wafers were finished by CMP using colloidal silica slurry. AFM observation showed not only scratch-free surface but also atomic steps on the wafers after CMP. Rms marks extremely flat value of 0.08 nm in 10 μm square area.

Ultralow-Loss SiC Floating Junction Schottky Barrier Diodes (Super-SBDs)

We have applied the floating junction (FJ) structure, which has been confirmed to be effective in reducing the on-resistance of Si power devices, to SiC FJ Schottky barrier diodes (SiC Super-SBDs). Optimization of the device parameters, which are derived by making improvements in the device simulator, and development of the fabrication process have enabled realization of Super-SBDs with a breakdown voltage of 2700 V and a specific on-resistance of 2.57 mOmega ldr cm2. These values correspond to the world record of 11.3 GW/cm2 for Baligas figure-of-merit (BFOM = 4Vbd 2/Ron-sp).

Generation and transportation mechanisms for two-dimensional hole gases in GaN/AlGaN/GaN double heterostructures

The electrical properties of two-dimensional hole gases (2DHGs) in GaN/AlGaN/GaN double heterostructures were investigated. The layers were grown on sapphire substrates and a high-quality bulk GaN substrate. The coexistence of 2DHG and 2D electron gases on both sides of the AlGaN layer was confirmed by Hall effect measurements at 80–460 K. It was also verified that the 2DHGs were generated by negative polarization at the undoped GaN/AlGaN interface, which did not have a doped Mg acceptor. It was also demonstrated that the 2DHG density could be controlled by varying the AlGaN layer thickness and was inversely related to the 2DHG mobility. The measured relation indicated that the 2DHG mobility is mainly limited by phonon scatterings at around room temperature. As a result, the maximum 2DHG mobility of 16 cm2/Vs at 300 K was achieved with a density of 1 × 1013 cm−2.

Calculation of Lattice Constant of 4H-SiC as a Function of Impurity Concentration

Calculations of lattice constant of 4H-SiC and diamond have been carried out. Lattice constant of 4H-SiC trends to decrease when nitrogen concentration increases. On the other hand, lattice constant of 4H-SiC trends to increase when aluminum concentration increases. Lattice constant of boron and phosphorus doped diamond trends to increase when impurity concentration increases. The effect of phosphorus on diamond lattice constant is about six times larger than that of boron.

High-Quality SiC Bulk Single Crystal Growth Based on Simulation and Experiment

The numerical simulation and in-situ X-ray topography were applied to observe the phenomena inside a crucible. Numerical simulation pointed out that macroscopic grown crystal quality such as grown crystal shape strongly depends on the temperature distribution inside a crucible. In-situ X-ray topography revealed that when the defects were generated, and how the defects were propagated. Most of defects were generated at the initial growth stage. It is important to control the initial stage in order to obtain a high quality SiC single crystal. Numerical simulation also suggested that it is important reduce the residual stress in a grown crystal in order to avoid the dislocation occurrence. From these results based on numerical simulation and experiment, SiC sublimation growth was controlled actively, and the large and high quality SiC single crystal have been grown. Introduction Silicon carbide single crystal is usually grown by sublimation (modified Lely method). Since the first report of modified Lely method [1], more than 20 years has passed. However, there is a lot of remaining issues that should be solved. The main reason of this situation is that sublimation process is a black box process inside a closed carbon crucible above 2000 K. It is so difficult to know what going on inside a crucible, that it is much difficult to control the sublimation process actively. In order to overcome this point, the authors have applied the numerical simulation to see the phenomena inside a furnace [2,3,4,5]. The authors also developed the in-situ X-ray topography system to observe the crystal growth features inside a closed carbon crucible [6,7]. By using these observation tools, the SiC sublimation growth could be understand more detail [7,8,9,10], and could be controlled actively [3,11,12]. In this paper, the observation results and the example active control of SiC sublimation growth are described. Simulation The configuration of numerical modeling was based on the conventional RF induction-heating furnace that we used in experiments. Electromagnetic and thermal fields were analyzed by the commercial software, Flux-Expert [13,14], and CFD-ACE+[15]. Since convective heat transfer could be neglected in our experiments, the equation for momentum transfer was not analyzed. From the thermal fields, the concentration distribution of sublimated species was analyzed according to the LTCE model [13]. The residual stress in a grown crystal was also analyzed. Materials Science Forum Online: 2004-06-15 ISSN: 1662-9752, Vols. 457-460, pp 29-34 doi:10.4028/www.scientific.net/MSF.457-460.29

Single Material Ohmic Contacts Simultaneously Formed on the Source/P-Well/Gate of 4H-SiC Vertical MOSFETs

We fabricated 4H-SiC vertical MOSFETs with contacts to the source, p-well and polycrystalline silicon (polysilicon) gate and these were simult aneously formed from a single material, using one deposition and a single contact annealing process. T ypical specific contact resistances of 4.8×10 -5 cm for the n source region, 1.5×10 -6 cm for the gate polysilicon and 5.2×10 cm for the p-well contact region were obtained using Al/Ni (Al~6%) a s the contact metal. Also, the static characteristics of the vertic al MOSFETs indicated that the MOS interface can withstand an even higher temperature process such as that u ed in ohmic-contact formation. 1.Introduction A widely used technique to form low resistivity ohmic contacts on SiC is to deposit materials such as an Ni [1] for the n-type region and a Ti/Al [2] for the p-t ype region, followed by postdeposition annealing (PDA). It has commonly been thought that it is difficult to form ohmic contacts from a single material on both nand p-type SiC, since a metal having a low-barrier height on an n-type SiC has a high-barrier height on a p-type SiC, and the reverse is also true. However, in the production of SiC power MOSFETs, we urgently need to uti lize he technology to form ohmic contacts on both the n + source and p-well, using a single contact material because it can directly contribute to miniaturizing cell size, resulting in lower specific on-resistance (R on,sp). Nevertheless, a single material contact for practical devices ha s not been discussed to date. This paper describes a simple fabrication process and c ont ct properties that are especially suited to form the ohmic contacts on the n + source, p-well and gate polysilicon in 4H-SiC vertical MOSFETs, using a single contact material. In addition, we discuss the formation of a back side contact that is used as a drain electrode. 2.Device Design and Processing Figure 1 (a) shows a schematic cross section and (b) a plan view of the vertical MOSFET we designed and fabricated. The n + source and p-well contact p + region were formed adjacent to each other in an identical contact window. Also, thin Ni [1] or layered Al/Ni [3] as a c ont t material was formed on the bottom of the contact window to the n + source and p-well contact p region and n + gate polysilicon. An alloyed Ni as a drain electrode was formed on the ground and polished back side of the substrate. The MOS channel length, define d by the p* on leave from Nissan Research Center (NRC), Yokosuka, Japan ** also with NRC Materials Science Forum Online: 2003-09-15 ISSN: 1662-9752, Vols. 433-436, pp 669-672 doi:10.4028/www.scientific.net/MSF.433-436.669

Transition of oscillatory floating half zone convection from Earth's gravity to microgravity

Abstract Oscillatory features of floating half zone convection were experimentally studied by using the drop shaft facility of Japan Microgravity Center which supported microgravity period of 10 s. Coordinated measurements including free surface deformation and oscillation, temperature and flow pattern in both 1g and micro-g environment were obtained. The oscillatory frequency and amplitude in micro-g condition were lower and larger than the ones in 1-g condition, respectively. The results gave, at first time, the oscillatory features such as free surface wave in micro-g, coordinated measurements of more than two physical quantities in the micro-g, and transition of thermocapillary oscillatory convection from 1-g to micro-g.

Numerical Investigation of the Growth Rate Enhancement of SiC Crystal Growth from Silicon Melts

Numerical study has been applied to analyze the high temperature solution growth process for bulk silicon carbide (SiC) crystal growth. A two-dimensional axisymmetric model for 2-in. SiC crystal growth was used for this study. The purpose of this paper is to investigate the possible approaches to enhance the growth rate in this process. In particular, we studied the effect of an AC magnetic field on the carbon transport to the crystal growth interface. The results revealed that the carbon flux to the growing crystal is strongly affected by the coil position and the applied frequency. If these two process parameters are properly chosen, we show that the carbon flux at the growing front, and thus the growth rate of SiC, can be enhanced.