Showing papers by "Jun Suda published in 2009"

Temperature and doping dependencies of electrical properties in Al-doped 4H-SiC epitaxial layers

Abstract: The free hole concentration and the low-field transport properties of Al-doped 4H-SiC epilayers with several acceptor concentrations grown on semi-insulating substrates have been investigated in the temperature range from 100to500K by Hall-effect measurements. Samples have been grown by cold-wall chemical vapor deposition (CVD) in the Al acceptor concentration range from 3×1015to5.5×1019cm−3. The dependencies of the acceptor ionization ratio at 300K and the ionization energy on the acceptor concentration were estimated. Numerical calculations of the hole Hall mobility and the Hall scattering factor have been performed based on the low-field transport model using relaxation-time approximation. At the low acceptor concentrations, the acoustic phonon scattering dominates the hole mobility at 300K. At the high acceptor concentrations, on the other hand, the neutral impurity scattering dominates the mobility. A Caughey–Thomas mobility model with temperature dependent parameters is used to describe the dependen...

4H-SiC MISFETs with nitrogen-containing insulators

Abstract: 4H-SiC MISFETs with nitrogen-containing insulators have been fabricated and characterized. Several techniques have been explored to incorporate nitrogen in the gate insulator in order to improve the density of interface states and thereby the channel mobility. The techniques are N2O-grown oxides, the oxidation of a surface layer co-implanted with N+ and Al+, deposited SiO2 annealed in N2O and NO, and deposited SiNx /SiO2 annealed in N2O. By optimizing the formation process of the gate insulators, MIS capacitors with N-containing insulators have demonstrated an interface state density close to the conduction band edge below 2 × 1011 cm–2 eV–1 which is one or two orders-of-magni- tude lower than that of MOS capacitors with oxides grown in dry O2. The channel mobility of the n-channel 4H-SiC(0001) MISFETs with N-containing insulators is increased to about 30 cm2/Vs. In addition, an even higher channel mobility of 50 cm2/Vs has been realized by utiliz- ing N-containing insulators adequately processed on the 4H-SiC (000) face. From the experimental results, the dominant scattering mechanisms in SiC MISFETs have been identified; Coulomb scattering and electron trapping at interface states dominate the channel mobility in SiC MOSFETs with thermally-grown and deposited SiO2. The application of N-containing insulators to p-channel 4H-SiC MIS devices is also discussed. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Triple Shockley type stacking faults in 4H-SiC epilayers

Abstract: 4H-SiC epilayers have been characterized by microphotoluminescence (micro-PL) spectroscopy and micro-PL intensity mapping at room temperature. A type of stacking fault (SF) with a peak emission wavelength at 480 nm (2.58 eV) has been identified. The shape of this SF is triangular revealed by the micro-PL intensity mapping. Conventional and high-resolution transmission electron microscopies have been carried out to investigate the structure of this SF. Its stacking sequence is determined as (3,5) in Zhdanov’s notation, which is consistent with that of the triple Shockley SF. The formation mechanism of this SF is also discussed.

Characterization of major in-grown stacking faults in 4H-SiC epilayers

Abstract: The optical properties of major in-grown stacking faults (IGSFs) in 4H-SiC epilayers have been characterized by micro-photoluminescence (micro-PL) spectroscopy and its intensity mapping. Strong PL emissions from the IGSFs are observed even at room temperature. Three kinds of IGSFs have been identified in the samples based on the micro-PL spectra. Each kind of IGSF shows the distinct PL emission peak located at 460, 480, and 500 nm, respectively. The micro-PL intensity mapping at the emission band of each IGSF has been performed to spatially profile the IGSF. The shapes, distributions, and densities of IGSFs in the epilayers are then presented. The microstructure of each IGSF has been revealed by high-resolution transmission electron microscopy observations. The stacking sequences of three IGSFs are determined as (4,4), (3,5), and (6,0) in the Zhdanov's notation, respectively, which apparently differ from the perfect 4H-SiC, (2,2). Three identified IGSFs are then classified as quadruple Shockley SFs, triple Shockley SFs, and double Shockley SFs, respectively, based on the shear formation model.

Accurate measurements of second-order nonlinear optical coefficients of 6H and 4H silicon carbide

Abstract: The second-order nonlinear optical coefficients of 4H-SiC and 6H-SiC have been measured by use of two second-harmonic generation methods, the rotational Maker-fringe and wedge techniques, at the fundamental wavelength of 1.064 μm. Measurements on high-quality (0001) and (112¯0) plane samples as well as rigorous analyses taking into account the multiple-reflection effects allowed us to accurately determine the magnitudes of the nonlinear optical coefficients. The obtained values are d31=6.7 pm/V, d15=6.5 pm/V, and d33=−12.5 pm/V for 6H-SiC; and d31=6.5 pm/V, d15=6.7 pm/V, and d33=−11.7 pm/V for 4H-SiC.

P-Channel MOSFETs on 4H-SiC {0001} and Nonbasal Faces Fabricated by Oxide Deposition and $\hbox{N}_{2}\hbox{O}$ Annealing

Abstract: In this paper, we have investigated 4H-SiC p-channel metal-oxide-semiconductor field-effect transistors (MOSFETs) with deposited SiO2 followed by N2O annealing. In addition to deposited oxides, dry-O2-grown oxides and N2O-grown oxides were also adopted as the gate oxides of SiC p-channel MOSFETs. The MOSFETs have been fabricated on the 4H-SiC (0001), (0001macr), (033macr8), and (112macr0) faces. The (0001) MOSFETs with deposited oxides exhibited a relatively high channel mobility of 10 cm2/V ldr s, although a mobility of 7 cm2/V ldr s was obtained in the (0001) MOSFETs with N2O-grown oxides. The channel mobility was also increased by utilizing the deposited SiO2 in the MOSFETs fabricated on nonbasal faces, although the MOSFETs on (0001macr) were not operational. Compared with the thermally grown oxides, the deposited oxides annealed in N2O are effective in improving the performance of 4H-SiC p-channel MOSFETs.

1580-V–40- $\hbox{m}\Omega\cdot \hbox{cm}^{2}$ Double-RESURF MOSFETs on 4H- SiC $(\hbox{000}\bar{\hbox{1}})$

Abstract: Double-reduced-surface-field (RESURF) MOSFETs with N2O-grown oxides have been fabricated on the 4H-SiC (0001macr) face. The double-RESURF structure is effective in reducing the drift resistance, as well as in increasing the breakdown voltage. In addition, by utilizing the 4H-SiC (0001macr) face, the channel mobility can be increased to over 30 cm2 / V ldr s, and hence, the channel resistance is decreased. As a result, the fabricated MOSFETs on 4H-SiC (0001macr) have demonstrated a high breakdown voltage (VB) of 1580 V and a low on-resistance (RON) of 40 mOmega ldr cm2. The figure-of-merit (VB 2/RON) of the fabricated device has reached 62 MW/cm2, which is the highest value among any lateral MOSFETs and is more than ten times higher than the "Si limit".

Mobility oscillation by one-dimensional quantum confinement in Si-nanowire metal-oxide-semiconductor field effect transistors

Abstract: Si-nanowire p-channel metal-oxide-semiconductor field effect transistors (MOSFETs), in which the typical cross section of the nanowire is a rectangular shape with 3 nm height and 18 nm width, have been fabricated and the current-voltage characteristics have been measured from 101 to 396 K. The transconductance has shown oscillation up to 309 K. The carrier transport has been theoretically analyzed, assuming that the acoustic phonon scattering is dominant. The electronic states have been determined from the effective mass approximation and the mobility from the relaxation time approximation as a function of the Fermi level. Relation between the gate voltage and the Fermi level has been estimated from the MOSFET structure. The calculated mobility has shown the oscillation with change in the Fermi level (the gate voltage), resulting in the transconductance oscillation. The oscillation originates from one-dimensional density of states (∝E−0.5).

Enhanced Drain Current of 4 H-SiC MOSFETs by Adopting a Three-Dimensional Gate Structure

Abstract: 4H-SiC (0001) metal-oxide-semiconductor field-effect transistors (MOSFETs) with a 3-D gate structure, which has a top channel on the (0001) face and side-wall channels on the {112macr0} face, have been fabricated. The 3-D gate structures with a 1-5-mum width and a 0.8- mum height have been formed by reactive ion etching, and the gate oxide has been deposited by plasma-enhanced chemical vapor deposition and then annealed in N2O ambient at 1300degC. The fabricated MOSFETs have exhibited good characteristics: The I ON/I OFF ratio, the subthreshold swing, and V TH are 109, 210 mV/decade, and 3.5 V, respectively. The drain current normalized by the gate width is increasing with decreasing the gate width. The normalized drain current of a 1-mum-wide MOSFET is 16 times higher than that of a conventional planar MOSFET.

Systematic Investigation of c-Axis Tilt in GaN and AlGaN Grown on Vicinal SiC(0001) Substrates

Abstract: High-resolution X-ray diffraction measurements of GaN and AlGaN grown on 4H- and 6H-SiC(0001) vicinal substrates with misorientation angles of up to 9° are presented. Growth of (Al)GaN was carried out by plasma-assisted molecular beam epitaxy. The c-axis tilt, i.e., inclination of the (Al)GaN c-axis relative to that of SiC, was systematically investigated. The inclination angle clearly depended on the SiC substrate misorientation angle, while it was independent of the (Al)GaN growth temperature, SiC polytype, and substrate misorientation direction. The behavior observed for both GaN and AlGaN is in excellent agreement with the model proposed previously by Nagai for the InGaAs/GaAs system.

Polytype Replication in Heteroepitaxial Growth of Nonpolar AlN on SiC

Abstract: Zinc-blende and wurtzite are the most common structures for binary compound semiconductors. Aluminum nitrides (AIN), one of the most promising materials for deep ultraviolet light-emitting diodes, have a wurtzite structure as an equilibrium phase due to its strong ionicity. Silicon carbide (SiC) is widely used as a substrate for heteroepitaxial growth of AlN, since SiC has a hexagonal structure whose lattice constant is close to that of AIN. Different from other compound semiconductors, SiC can have many different crystalline structures, called polytypism. Among various polytypes of SiC, large-size high-quality wafers are available for 4H and 6H structures. When AlN is grown on a 4H- or 6H-SiC basal plane (0001), normal, wurtzite-structured AIN is obtained. On the other hand, when AlN is grown on a nonbasal SiC plane, such as nonpolar (1100) or (1120), what is expected? If ideal growth is realized, AIN will follow the crystalline structure of SiC (i.e., the polytype of the SiC substrate will be replicated to the AIN epitaxial layer). Nonpolar nitride growth has attracted much attention to eliminate undesirable internal electric fields due to the polarization in nitride heterostructures. In addition, nonpolar nitride growth on SiC also allows an opportunity to obtain nitrides with new crystalline structures. In this article, the polytype replication growth of AIN on nonpolar SiC substrates is reviewed.

Observation of novel defect structure in 2H‐AlN grown on 6H‐SiC(0001) substrates with 3‐bilayer‐height step‐and‐terrace structures

Abstract: 250 nm thick AlN layers without a nucleation layer were grown directly on 6H-SiC(0001) with 3-bilayer-height steps by rf-plasma-assisted molecular-beam epitaxy The structure and morphology of the AlN layers have been studied using atomic force microscopy, X-ray diffraction and transmission electron microscopy Two different types of unique defect structures were observed Rows of pure-edge-type threading dislocations were observed along the pre-existing step-edges of the SiC substrate for AlN grown on as-gas-etched SiC sub- strates, while the planar defects threading through the AlN layer were observed at the step-edges of the substrate for AlN on SiC with sacrificial oxidation We concluded that these planar defects were the stacking mismatch boundaries due to the difference in stacking sequence of AlN layers on different SiC terraces

In situ Gravimetric Monitoring of Thermal Decomposition and Hydrogen Etching Rates of 6H-SiC(0001) Si Face

Abstract: The thermal decomposition and hydrogen etching of a 6H-SiC(0001) Si-face were directly monitored using an in situ gravimetric monitoring system. The monitoring of the weight change of the 6H-SiC Si-face using this system clarified the dependences of the thermal decomposition and hydrogen etching rates on the substrate temperature. Although the thermal decomposition of the 6H-SiC Si-face above 1400 °C generated a graphite layer since only the Si atom directly desorbs from the surface, the etching of the 6H-SiC Si-face by hydrogen did not form this layer, and both Si and C atoms react with hydrogen. Moreover, the surface reaction of the 6H-SiC Si face with H2 and the resultant surface morphology were found to change at approximately 1250 °C.

Improved Current Gain in GaN/SiC Heterojunction Bipolar Transistors by Insertion of Ultra-Thin AlN Layer at Emitter-Junction

Abstract: GaN/SiC Heterojunction Bipolar Transistors (HBTs) with ultra-thin AlN insertion layers at the n-GaN/p-SiC emitter junction are proposed to improve carrier injection efficiency. The current-voltage characteristics of n-GaN/AlN/p-SiC heterojunctions have exhibited very small reverse leakage and good rectification. The capacitance-voltage measurement have revealed that the conduction band offset between n-GaN and p-SiC has been reduced from -0.74 eV to -0.54 eV by insertion of AlN, indicating that the GaN/AlN/SiC heterojunction may show better electron-injection efficiency. A significantly improved common-base current gain (α~0.2) is obtained for GaN/AlN/SiC HBTs with initial N* pre-irradiation, while it was very low (α~0.001) for GaN/SiC HBTs without AlN layers.

A New Class of Step-and-Terrace Structure Observed on 4H-SiC(0001) after High-Temperature Gas Etching

Abstract: Various interesting phenomena related to the step-and-terrace structures of crystal surfaces have been previously observed. In this paper, we report a unique phenomenon not observed previously: the formation of a new class of step-and-terrace structure on the 4H-SiC(0001) surface after high-temperature gas etching. The structure consists of a periodic array of pairs of 1-bilayer-height "down" steps and 5-bilayer-height "up" steps, in contrast to conventional step-and-terrace structures which consist of only "up" (or only "down") steps. We show that the newly observed (5–1)-bilayer-height step-and-terrace structure originates from spiral etching at a screw dislocation. A mechanism for step-flow etching which conserves the "up"-and-"down" step pairs is also proposed.

Anomalously Large Difference in Ga Incorporation for AlGaN Grown on the (1120) and (1100) Planes under Group-III-Rich Conditions

Abstract: The Ga incorporation behavior of AlGaN layers grown on nonpolar planes has been investigated comparing the (1120) and (1100) planes. AlGaN growth was performed on 4H-SiC(1120) and (1100) substrates by molecular-beam epitaxy under group-III-rich conditions. The Ga composition of the AlGaN layers was evaluated by energy-dispersive X-ray spectroscopy analysis as well as X-ray diffraction and cathodoluminescence measurements. The GaN mole fraction x of Al1-xGaxN layers grown on the (1100) plane is 0.12 for the Ga flux ratio [JGa/(JAl+JGa)] of 0.32 at 750 °C, while it is negligibly small (x<0.01) for growth on the (1120) plane.

Accurate Measurement of Nonlinear Optical Coefficients of Gallium Nitride

Abstract: 1Department of Materials Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Phone:+81-3-5841-7093 E-mail: m-abe@castle.t.u-tokyo.ac.jp 2Department of Electrical, Electronic, and Communication Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan 3Department of Electronic Science and Engineering, Kyoto University, Kyoto Daigaku Katsura, Nishikyo-ku, Kyoto 615-8101, Japan 4Division of Electrical, Electronic and Information Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan 5Frontier Reserch Center, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan

Spatial Profiling of Planar Defects in 4H-SiC Epilayers Using Micro-Photoluminescence Mapping

Abstract: The micro-photoluminescence (micro-PL) spectroscopy and its intensity mapping have been utilized to investigate the planar defects, stacking faults (SFs), in 4H-SiC epilayers. Strong PL emissions from the SFs are observed even at room temperature. It is found that each kind of SF shows the distinct PL emission behaviours. Three kinds of SFs: intrinsic Frank SFs, double Shockley SFs, and in-grown SFs, have been identified in the samples based on the micro-PL spectra. At the same time, the micro-PL intensity mapping at the emission band of each SF has been performed to spatially profile the SFs. The shapes, distributions, and densities of SFs in the epilayers are then presented. The PL emission behaviours of each SF at low temperature are also studied.

Strong anharmonicity and lattice dynamics in LiTaO3 by Raman spectroscopy

Abstract: The phonon-dispersion relations of LiTaO3 crystal are calculated using the first-principle calculations approach described earlier [1]. We found that the TO1 (206 cm-1) and the TO2 (256 cm-1) are the z-vibration of Li ions and other ions, respectively, while the z-displacements are negligible in the TO3 (357 cm-1) and the TO4 (600 cm-1) modes using this calculation. The first-order Raman spectra in the LiTaO3 were measured in the temperature range 23–896K for rhombohedral phase. In the experiment the TO2 mode softens more rapidly than the TO1 mode and then pushes it down to lower frequencies. The temperature dependence of linewidth of four TO modes was analyzed using TDOS due to cubic term based on the first-principle calculations. The calculated results reproduce the observed ones in this temperature range. We found that the ionic motions associated with the TO1 and the TO2 modes are transferred to each other, and in the TO4 mode, torsion of individual octahedral take place, giving a strong anharmonic contribution.

Electrostatic-Actuated Suspended Ribbon Structure Fabricated in Single-Crystalline SiC by Selective Photoelectrochemical Etching

Abstract: An electrostatic-actuated suspended ribbon structure composed of single-crystalline SiC is presented. All the main parts of the structure, namely, a suspended ribbon, posts, base plate, and electrical connections, are made of homoepitaxially grown single-crystalline 4H-SiC with selective ion implantations. Electrical isolation between the ribbon and the base plate is established with a pnp junction. The structure is fabricated by a combination of reactive ion etching (RIE) and doping-selective photoelectrochemical (PEC) etching. The suspended ribbon is actuated by applying a voltage between the ribbon and the base plate.

Demonstration of SiC heterojunction bipolar transistors with AlN/GaN short-period superlattice widegap emitter

Abstract: SiC bipolar-junction transistors (BJTs) are attractive candidates for high-power switching devices due to their high breakdown voltage and low on-resistance. However, SiC BJT has so far suffered from the limited current gain. An alternative device structure would be heterojunction bipolar transistors (HBTs). Because it is impossible to grow Si x C 1−x solid solutions with x near 0.5, HBTs cannot be fabricated within group-IV semiconductors. Heteroepitaxial growth of wider bandgap group-III nitride (Al)GaN on SiC is one possible way to realize bandgap engineering in SiC devices. The fabrication of (Al)GaN/SiC HBTs was first reported by Pankove et al. [1] followed by several groups [2]. But any HBTs did not show common-emitter mode operation due to the large leakage at the emitterjunction.

1.5 kV Lateral Double RESURF MOSFETs on 4H-SiC (000-1)C Face

Abstract: SiC lateral double RESURF MOSFETs have been fabricated on the 4H-SiC (000-1)C face. By utilizing the C face, the channel resistance can be reduced because the C-face MOSFETs show higher channel mobility than the Si-face MOSFETs. In addition, by employing the double RESURF structure, the drift resistance is decreased and the breakdown voltage is increased with increasing the RESURF doses. The fabricated RESURF MOSFETs on the 4H-SiC (000-1)C face have demonstrated a low on-resistance of 40 mΩcm2 at an oxide field of 3 MV/cm and a breakdown voltage of 1580 V at zero gate bias. The figure-of-merit of the MOSFET is 62 MW/cm2, which is more than 10 times better than the conventional “Si limit” and the highest value among any lateral MOSFETs to date.

Determination of the thermo‐optic coefficients of GaN and AlN up to 515 °C

Abstract: The refractive index dispersions of hexagonal GaN and AlN have been investigated to determine the thermo-optic coefficients. Measurements were conducted at temperatures ranging from room temperature up to 515 °C in the wavelength range from the near band-edge to 1000 nm. Optical interference measurements with vertical incident configuration were employed to precisely evaluate the ordinary refractive indices. Optical simulation of AlGaN/GaN distributed Bragg reflectors was also carried out by using the obtained thermo-optic coefficients. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Accurate Measurements of Second-Order Nonlinear-Optical Coefficients of Silicon Carbide

Abstract: Second-order nonlinear-optical coefficients of 4H and 6H-SiC have been measured with the wedge technique. Using high-quality (11-20) samples as well as performing rigorous measurements and analyses, the three independent components, d31 (= d32), d15 (= d24), and d33, have been accurately determined. We have found that the nonlinear-optical coefficients are nearly the same between the measured 4H and 6H-SiC samples within the experimental accuracy; d31 = 5.4 pm/V, d15 = 6.2 pm/V, and d33 = 9.7pm/V.

Improved On-Current of 4H-SiC MOSFETs with a Three-Dimensional Gate Structure

Abstract: 4H-SiC (0001) MOSFETs with a three-dimensional gate structure, which has a top channel on the (0001) face and side-wall channels on the {11-20} face have been fabricated. The three-dimensional gate structures with a 1-5 m width and 0.8 m height have been formed by reactive ion etching, and the gate oxide has been deposited by plasma-enhanced chemical vapor deposition and then annealed in N2O ambient at 1300°C. The fabricated MOSFETs have exhibited superior characteristics: ION / IOFF, the subthreshold swing and VTH are 1010, 250 mV/decade and 3.5 V, respectively. The drain current normalized by the gate width is increasing with decreasing the gate width. The normalized drain current of a 1 m-wide MOSFET is ten times higher than that of a conventional planar MOSFET.

High Channel Mobility in P-Channel MOSFETs Fabricated on 4H-SiC (0001) and Non-Basal Faces

Abstract: P-channel MOSFETs have been fabricated on 4H-SiC (0001) face as well as on 4H-SiC (03-38) and (11-20) faces. The gate oxides were formed by thermal oxidation in dry N2O ambient, which is widely accepted to improve the performance of n-channel SiC MOSFETs. The p-channel SiC MOSFETs with N2O-grown oxides on 4H-SiC (0001), (03-38), and (11-20) faces show a channel mobility of 7 cm2/Vs, 11 cm2/Vs, and 17 cm2/Vs, respectively. From the quasi-static C-V curves measured by using gate-controlled diodes, the interface state density was calculated by an original method. The interface state density was the lowest at the SiO2/4H-SiC (03-38) interface (about 1x1012 cm-2eV-1 at EV + 0.2 eV). The authors have applied deposited oxides to the 4H-SiC p-channel MOSFETs. The (0001), (03-38), and (11-20) MOSFETs with deposited oxides exhibit a channel mobility of 10 cm2/Vs, 13 cm2/Vs, and 17 cm2/Vs, respectively. The deposited oxides are one of effective approaches to improve both n-channel and p-channel 4H-SiC MOS devices.