bluuebunny/crossref_metadata_2025 huggingface dataset:
Electronic structure of the Ge(111)‐5 × 5 wetting layer on Si(111)‐7 × 7
Authors: Psiachos D. , Stott M. J.
Published: June 2007

This work reports an ab initio study of the Ge(111)‐5 × 5 reconstruction, which forms on top of a Si(111)‐7 × 7 substrate. Detailed descriptions of the structural and electronic properties of this surface, obtained from density‐functional calculations, are presented and analyzed. Many similarities with the structural and electronic–electronic properties of related structures such as Si(111)‐7 × 7 and Si(111)‐5 × 5 were found, in particular the similar localisation properties of the eigenstates around the fermi level, the non‐zero density of states at the fermi level, a larger charge density associated with the dangling bonds on the faulted half cell compared with those of the unfaulted half, and the structural relaxations of key surface atoms and bonds, although the magnitudes of these relaxations are somewhat smaller than has been calculated for Si(111)‐7 × 7. The calculated charge density distribution is interpreted in terms of population analysis to obtain information on the bonding mechanisms at the surface. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
First‐principle study of structural stability and electronic properties of Zn1–xBex O semiconductor alloy
Authors: Thangavel R. , Rajagopalan M. , Kumar J.
Published: June 2007

We report the electronic structure calculation of Zn1–x Bex O alloy in the chalcopyrite, wurtzite, zincblende and rocksalt (NaCl) structures. From this study it has been found that Zn1–x Bex O alloy is a direct band gap semiconductor and, from energy considerations, this alloy is found to be more stable in the chalcopyrite‐type structure rather than in the rocksalt‐ or wurtzite‐type structures. The band‐gap value, cohesive energy, bulk modulus, heat of formation and disorder parameter (gap bowing) are deduced and reported for the first time to our knowledge. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A monolithic Cockcroft‐Walton voltage multiplier based on AlGaN/GaN HFET structure
Authors: Ao Jin‐Ping , Matsuda Yoshikazu , Yamaoka Yuya , Ohno Yasuo
Published: June 2007

A five‐stage monolithic Cockcroft‐Walton voltage multiplier is reported which uses metal‐insulator‐metal capacitor and Schottky diode based on AlGaN/GaN heterostructure field‐effect transistor (HFET) structure. The voltage multiplier includes 5 capacitors and 5 Schottky diodes. AlGaN/GaN HFET with L/W = 1 μm/10 μm is used as the Schottky diode, where the gate made of Ni/Au/Ti is used for the anode and the ohmic contact is used for the cathode. The size of the chip is 0.567 mm×0.445 mm. The breakdown voltage of the capacitor is around 40 V and the diode breakdown voltage in reverse direction is more than 100 V. At input AC peak to peak voltage of 20 V under frequency of 10 MHz, the measured output voltage is about 40 V with the output impedance less than 1 MΩ. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Band offset measurements of the pulsed‐laser‐deposition‐grown Sc2O3 (111)/GaN (0001) heterostructure by X‐ray photoelectron spectroscopy
Authors: Liu Chang , Chor Eng Fong , Tan Leng Seow , Dong Yufeng
Published: June 2007

The energy band alignment of the Sc2O3/GaN heterointerface has been studied by means of X‐ray photoelectron spectroscopy. The single‐crystalline Sc2O3 was grown epitaxially on GaN template using pulsed laser deposition. A valence band offset of 0.84 ± 0.05 eV has been obtained across the heterointerface by using Ga 3d and Sc 3p energy core levels as the references. Given the bandgaps of 3.42 and 6.3 eV for GaN and Sc2O3, respectively, the conduction band offset has been estimated to be 2.04 eV in this system. These large band offsets are desirable, especially for GaN‐based electronic devices, which are most suited for high‐temperature operations, where the carrier thermionic emission must be maintained at a sufficient low level. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Time‐resolved four‐wave mixing studies of excitons in GaN
Authors: Yamaguchi K. , Toda Y. , Ishiguro T. , Adachi S. , Hoshino K. , Tadatomo K.
Published: June 2007

Quantum beats in the time‐resolved four‐wave mixing (FWM) signals arising from GaN excitons are observed. Two different types of beats are investigated: beats between the A‐exciton (XA) and B‐exciton (XB) states; beats between the XA and A‐biexciton ((XXA)) states. The former exhibits oscillations along all directions in the delay‐ (τ ) and real‐time (t) plane while the latter exhibits no beat in the direction parallel to the diagonal axis (τ = t). The results are well reproduced by a model calculation, showing good homogeneity of the sample.
Buffer breakdown voltage of AlGaN/GaN HFET on a 4 inch Si(111) substrate grown by MOCVD
Authors: Iwami Masayuki , Kato Sadahiro , Satoh Yoshihiro , Sasaki Hitoshi , Yoshida Seikoh
Published: June 2007

The buffer breakdown voltage and the electron mobility in a two dimensional gas (2DEG) of AlGaN/GaN HFETs on 4 inch Si(111) substrates grown by metalorganic chemical vapor deposition was investigated. The relationship between the electrical properties of HFETs and the crystalline quality of the GaN layer are studied by X‐ray diffraction and photoluminescence measurements. The buffer breakdown voltage decreases with an increase in the FWHM of the X‐ray (0002) diffraction peak. The electron mobility in a 2DEG decreases with an increase in FWHM of the X‐ray (10$ \bar 1 $2) diffraction peak. These results indicate that a screw component of the threading dislocations in the GaN layer is a primary source for buffer leakage, and that edge dislocations in the GaN layer have a scattering effect on the 2DEG transport. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Low on‐resistance of GaN p‐i‐n vertical conducting diodes grown on 4H‐SiC substrates
Authors: Nishikawa Atsushi , Kumakura Kazuhide , Makimoto Toshiki
Published: June 2007

We investigated the resistance of conductive AlGaN buffer layers and the current‐voltage characteristics of GaN p‐i‐n vertical conducting diodes on n‐type 4H‐SiC substrates grown by low‐pressure metalorganic vapor‐phase epitaxy. High Si doping of the AlGaN buffer layer at the AlGaN/SiC interface produces ohmic current‐voltage characteristics in spite of the large band offset between AlGaN and 4H‐SiC. Owing to the optimization of the AlGaN buffer layer, a low on‐resistance (Ron) of 1.12 mΩ cm2 with high breakdown voltage (VB) of 300 V is obtained for a GaN p‐i‐n vertical conducting diode on a 4H‐SiC substrate, leading to the figure of merit (VB2/Ron) of 80 MW/cm2, which is larger than that for the diode with the same structure on a 6H‐SiC substrate (62 MW/cm2). This result indicates that 4H‐SiC is preferable for fabricating GaN‐based electronic devices with a low on‐resistance and high breakdown voltage. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Solution growth of AlN single crystal using Cu solvent under atmospheric pressure nitrogen
Authors: Kamei K. , Shirai Y. , Tanaka T. , Okada N. , Yauchi A. , Amano H.
Published: June 2007

We have grown thick AlN epilayers on SiC substrates by a new solution growth technique using Cu solvents under atmospheric pressure nitrogen. By using growth apparatus based on CZ growth system with inductive heating, we have grown AlN single crystalline layers of which thickness were more than 200 μm on (4H,6H)‐SiC substrates at relatively low growth temperatures such as 1600 °C‐1800 °C. Inch‐size self standing AlN crystals were also prepared by removing the SiC substrate. TEM observation, ω‐scan XRD measurement and cathode luminescence spectroscopy were conducted to characterize the crystallinity of the obtained AlN layers. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Photoluminescence study of isoelectronic traps in dilute GaAsN alloys
Authors: Yaguchi H. , Aoki T. , Morioke T. , Hijikata Y. , Yoshida S. , Yoshita M. , Akiyama H. , Aoki D. , Onabe K.
Published: June 2007

We have studied photoluminescence spectra in detail to clarify the character of the isoelectronic traps in dilute GaAsN alloys. Several sharp lines have been observed at the lower energy side of the GaAs bandgap and are in good agreement with the nitrogen pair‐related emission lines previously reported. In addition to the nitrogen pair‐related lines, some other emission lines have been also observed. Compared with the energies of these emission lines and the nitrogen pair‐related emission lines, it was found that the energy differences agree with the longitudinal optical phonon energy at the Γ point of GaAs, showing that the character of isoelectronic traps due to nitrogen pairs in dilute GaAsN alloys is significantly contributed from the conduction band state at the Γ point. The temperature dependence of the peak energy of luminescence due to nitrogen pairs also indicates that the character of isoelectronic traps in dilute GaAsN alloys is due to the conduction band edge state at the Γ point of GaAs. For a dilute GaAsN alloy with lower nitrogen concentration, we have observed that the intensity of an emission line increased superlinearly with excitation power . (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Initial growth stage in PVT growth of AlN on SiC substrates: Influence of Al2O3
Authors: Heimann Paul , Pfeiffer C. , Bickermann M. , Epelbaum B. M. , Nagata S. , Winnacker A.
Published: June 2007

After the first 2 minutes of growth, the substrate surface demonstrates two areas with very different morphologies: (I) regular arrays of hexagonal hillocks with 3D crystallites of AlN growing well oriented on hillock tops and (II) flat areas with very smooth surface where AlN grows in 2D layered mode. The addition of Al2O3 enhances the second growth mode (II), presumably because of surface melting in a very initial stage of growth. Surface morphologies in these single crystalline layers were studied using scanning electron microscopy (SEM). Chemical composition of selected areas was identified by Auger electron spectroscopy (AES). A chemical model is proposed to explain the obtained layer morphology. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Defect‐selective etching of aluminum nitride single crystals
Authors: Bickermann M. , Schmidt S. , Epelbaum B. M. , Heimann P. , Nagata S. , Winnacker A.
Published: June 2007

We have investigated defect‐selective wet chemical etching of freestanding AlN single crystals in a molten NaOH‐KOH eutectic at temperatures ranging from 240 °C to 400 °C. On nitrogen polar basal planes, hexagonal pyramids/hillocks exceeding 100 µm in diameter may form within seconds of etching at 240 °C. They sometimes are arranged in lines and clusters, thus we attribute them to defects on the surface, presumably originating in the bulk material. On aluminum polar basal planes, the etch pit density after approx. 2…3 min of total etching time at 350 °C corresponds to the screw dislocation density. Additionally, significantly smaller etch pits were found around annealed indentations, in the vicinity of some bigger etch pits after repeated etching, and sometimes also isolated on the surface area. We attribute these etch pits to threading mixed and edge dislocations. On as‐grown rhombohedral and prismatic facets no defect‐related etching features were observed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Aluminum nitride/indium nitride bilayer and multilayer thin film systems as coatings for solar cells and longwave‐pass filters
Authors: Khoshman Jebreel M. , Kordesch Martin E.
Published: June 2007

The optical properties of sputtered AlN/InN/bilayer and multilayer thin film systems onto quartz substrates have been investigated in the visible and near infrared (NIF) regions. The designed multilayer was optimized at 500 nm and 50°–70° incidence for bilayers of alternate high and low index configuration. Analysis of these designs revealed that by increasing number of layers the reflectivity decreased to very low values (0% ‐ 8%) over a broad range of wavelengths (300–1400 nm). The transmissivity tended to increase smoothly from about zero at 750 nm to 88% in the NIR region while the absorptivity showed a tendency to decrease with increasing wavelength in the same region. Also, we found that the absorptivity increases with the total physical thickness of the multilayer system over the visible spectra (20% ‐ 99%). Therefore, the nitride bilayer and multilayer systems could be used as a longwave‐pass filter. However, since these systems provided extremely high absorptivity anywhere within the visible spectra they could be used as coatings for solar cells. X‐ray diffraction (XRD) spectra of AlN/InN/quartz multilayer systems were measured. The XRD patterns of the systems revealed a polycrystalline structure with a strong InN (002) diffraction peak at 31.2°. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
LPE growth of AlN single crystal using cold crucible under atmospheric nitrogen gas pressure
Authors: Tanaka T. , Yashiro N. , Shirai Y. , Kamei K. , Yauchi A.
Published: June 2007

We have grown AlN single crystal on 6H‐SiC substrate using Al‐Cu solution of 1500‐1600 °C. Nitrogen is supplied from gas phase under atmospheric pressure and the cold crucible technique is applied. Thickness of AlN reached 40 μm with the growth rate of 10 μm/h. Rather high growth rate in solution method is due to the increase in the melt surface area, dynamic convection of the melt and high temperature gradient along the seeded axis, which are easily realized in cold crucible technique. TEM observation reveals that the dislocation density beyond 10μm from the interface decreases by a factor of 10 to the power of ‐1 or ‐2. The melt included in between the substrate and AlN epi‐layer eases stress in epi‐layer and reduces cracks. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Nitride film growth morphology using remote plasma enhanced chemical vapor deposition
Authors: Wintrebert‐Fouquet M. , Butcher K. S. A. , Chen P. P.‐T. , Wuhrer R.
Published: June 2007

Gallium nitride and indium nitride films have been grown by remote plasma enhanced chemical vapor deposition (RPECVD) at temperatures between 570 and 650 °C for GaN and between 350 and 570 °C for InN on different substrates. For GaN vast improvements in film morphology and quality have resulted from reductions in background impurities when compared to previous reports. Epitaxial material can now be grown at 650 °C under optimized growth conditions. Columnar growth still occurs for growth on some substrates, however film coalescence is observed when using appropriate buffer layers and epitaxial growth can also be observed. High resolution SEM images show examples of this. The root‐mean‐square surface roughness of epitaxial samples, as measured using atomic force microscopy, shows values of as little as 10 Angstroms. While X‐ray diffraction shows that these surfaces are not amorphous but have a strong (0001) preferred axis with FWHM limited by instrumental effects to (2θ) 0.085 degrees. The improvement in film quality has allowed heavily doped n‐type films to be grown with an electron mobility of 160 cm2/V·s for a carrier concentration of ∼ 1x1019 cm–3 at 650 °C. Moss‐Burstein shifted absorption data confirms the high doping level. For InN film growth by RPECVD, columnar growth is commonly observed in the temperature region of interest for films grown directly on sapphire, however film coalescence and epitaxial films are also observed for this material. X‐ray diffraction indicates very sharp (0002) peaks with FWHM of (2θ) 0.07 degrees. High resolution SEM images show examples of film morphology at different growth temperatures. Electron backscattered diffraction images indicate a wurtzite structure even for InN films with strong deviations from the accepted lattice parameters. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
InGaN/GaN quantum‐well nanocolumn crystals on pillared Si substrate with InN as interlayer
Authors: Hu F. R. , Ochi K. , Zhao Y. , Hane K.
Published: June 2007

Nanocolumn InGaN/GaN crystals were deposited on micropillared Si substrate by molecular beam epitaxy. Low‐temperature InN was used as interlayer. With enough free space, the column crystals grew around all the surface plane of the Si pillars and formed InGaN/GaN quantum‐well flower structure. The QW crystals are about 100 nm in diameter and 1.1‐1.4 μm in length. Raman spectra measurement of the fower structure shows that E2 mode peak line observed at 567.28 cm–1. Photoluminescence measurement indicates a room temperature PL peak position of 620 nm and two peak positions of 404 nm and 519 nm at temperature 15 K. Hg lamp excited photoluminescence demonstrated a clear fluorescence distribution in the flower structure and much stronger emission compared with the quantum‐well crystals on the flat Si substrate. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Significance of vertical carrier capture for electroluminescence efficiency in InGaN multiple‐quantum well diodes
Authors: Inada T. , Satake A. , Fujiwara K.
Published: June 2007

The electroluminescence (EL) spectral intensity has been investigated in the high‐brightness green InGaN multiple‐quantum‐well light emitting diode (LED) in comparison with the single‐quantum‐well LED over a wide temperature range and as a function of injection current. It is found that the EL variation pattern with temperature and current is dramatically improved when the number of active wells increases as a result of enhanced carrier capture. The importance of vertical carrier capture processes is pointed out to explain the anomalous EL intensity variations at low temperatures. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Experimental and theoretical study of substrate modes in (Al,In)GaN laser diodes
Authors: Braun H. , Lauterbach C. , Schwarz U. T. , Laino V. , Witzigmann B. , Rumbolz C. , Schillgalies M. O. , Lell A. , Härle V. , Strauß U.
Published: June 2007

In semiconductor laser diodes, high refractive index epitaxial layers can act as parasitic waveguides and cause severe losses to the mode propagating in the laser waveguide. For (Al,In)GaN laser diodes the parasitic modes are typically caused by the SiC or GaN substrate or buffer layers. Substrate modes have an impact on near‐field and far‐field of the waveguide mode, on laser threshold, and on gain spectra. We present scanning near‐field measurements of the substrate mode and demonstrate that the period of the standing wave in the GaN substrate varies over a wide range and critically depends on the effective refractive index of the waveguide mode.
Suppression of gate leakage current in i‐AlGaN/GaN heterostructures by insertion of anodic Al2O3 layer and influence of thermal annealing on channel electrons
Authors: Sawada T. , Takahashi K. , Ise K. , Suzuki K. , Kitamori K. , Kimura N. , Imai K. , Kim S.‐W. , Suzuki T.
Published: June 2007

We have investigated electrical properties of Ni/anodoc‐Al2O3/i‐AlGaN/GaN gate structures formed by insertion of a thin anodic Al2O3 layer, and the influence of surface preparation or thermal stressing of bare i‐AlGaN/GaN heterostructures on channel electrons. The gate structure effectively suppressed the reverse leakage current down to 10–6‐10–7A/cm2 range depending on the wafer, after optimum annealing in N2. Measured C‐V curves indicated a small density of states at anodic‐Al2O3/AlGaN interface. Hall effect measurements revealed that 2DEG density of bare i‐AlGaN/GaN samples is considerably affected by the surface preparation condition, and that a high temperature thermal stressing in O2 ambient rather degrades the electron mobility of 2DEG, suggesting enhanced incorporation of oxygen in AlGaN layer. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Vibrational modes and strain in GaN/AlN quantum dot stacks: dependence on spacer thickness
Authors: Fresneda J. , Cros A. , Llorens J. M. , García‐Cristóbal A. , Cantarero A. , Amstatt B. , Bellet‐Amalric E. , Daudin B.
Published: June 2007

We have investigated the influence of spacer thickness on the vibrational and strain characteristics of GaN/AlN quantum dot multilayers (QD). The Raman shift corresponding to the E2h vibrational mode related to the QDs has been analyzed for AlN thicknesses ranging from 4.4 nm to 13 nm, while the amount of GaN deposited in each layer remained constant from sample to sample. It is shown that there is a rapid blue shift of the GaN vibrational mode with spacer thickness when its value is smaller than 7 nm while it remains almost constant for thicker spacers. A rapid increase of the Raman line‐width in the thicker samples is also observed. The experimental behavior is discussed in comparison with the results of a theoretical model for the strain in the QDs. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Radiative and non‐radiative transitions in blue quantum wells embedded in AlInGaN‐based laser diodes
Authors: Son J. K. , Lee S. N. , Paek H. S. , Sakong T. , Ha K. H. , Nam O. H. , Park Y.
Published: June 2007

We performed temperature‐dependent time‐resolved optical studies (TRPL) on AlInGaN‐based blue‐light‐emitting quantum well (QW) with different well widths. Radiative and non‐radiative components of PL lifetime were analyzed by measuring intensity and lifetime of PL. We experimentally confirmed that a non‐radiative lifetime was affected by internal field in InGaN QWs. Therefore, non‐radiative lifetimes were reduced as a well width decreasing in the same way that radiative lifetimes do. In addition, the more efficient optical properties of QWs, such as internal quantum efficiency, were achieved in the narrow QWs rather than the wider one. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Synthesis of III‐nitride microcrystals using metal‐EDTA complexes
Authors: Liu Y. H. , Koide S. , Miyake H. , Hiramatsu K. , Nakamura A. , Nambu N.
Published: June 2007

A novel method, using metal ethylenediamine tetraacetic acid (EDTA) complexes as starting materials, is proposed to synthesize III‐nitride microcrystals. Ga (Zn, Ga)‐EDTA·NH4 and In‐EDTA·NH4 complexes were reacted with NH3 in the temperature range of 1020–1150 °C and 300–620 °C, respectively. SEM observation and X‐ray diffraction patterns show that pure phase hexagonal GaN and InN are obtained. The CL peak intensity of GaN increases as the synthesis temperature increases up to 1100 °C. The dependence of CL peak intensity on Zn doping content indicates good doping control. For InN synthesis, the temperature window is very narrow and the reaction evolves from In2O3 to InN. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
XPS study of surface potential in AlGaN/GaN heterostructure with Cat‐CVD SiN passivation
Authors: Onojima N. , Higashiwaki M. , Matsui T. , Mimura T. , Suda J. , Kimoto T.
Published: June 2007

AlGaN surface potentials in AlGaN/GaN heterostructures with and without SiN passivation were investigated using x‐ray photoelectron spectroscopy (XPS). SiN films were formed on AlGaN surfaces by catalytic chemical vapor deposition (Cat‐CVD), which has already been found to increase two‐dimensional electron gas (2DEG) density. Based on a simple electrostatic analysis, the 2DEG density is expected to increase as the AlGaN surface potential decreases. This study experimentally demonstrates that a reduction in the AlGaN surface potential is actually induced by Cat‐CVD SiN passivation. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Stimulated emission from free‐standing GaN/Si micro‐disk structures
Authors: Choi H. W. , Hui K. N. , Lai P. T. , Chen P. , Zhang X. H. , Teng J. H. , Tripathy S. , Chua S. J.
Published: June 2007

Arrays of pivoted GaN microdisks have been fabricated on GaN/Si material by a combination of dry and wet etching. The Si material beneath the GaN microdisks is removed by wet etching, leaving behind a fine pillar to support the disks. The top and botton faces of the microdisks are optically smooth, suitable for forming optical cavities. Resonant modes, corresponding to whispering gallery modes, are observed in the photoluminescence spectra. Stimulated emission is achieved at higher optical pumping intensities. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Gate leakage currents of AlGaN/GaN HEMT structures grown by metalorganic vapour phase epitaxy
Authors: Yamada T. , Tsuchiya T. , Imada K. , Iwami M. , Kikawa J. , Araki T. , Suzuki A. , Nanishi Y.
Published: June 2007

The gate leakage current of AlGaN/GaN HEMTs increased with increasing Al mole fraction x from 0.25 to 0.36. Donor density Nd of AlGaN layer increased to ∼ 1×1018 (cm‐3) with increasing x to 0.36. It is estimated that rather low Nd may not influence the gate leakage. AlGaN surface morphology became worse with increasing x. Reverse‐bias characteristics were well‐fitted by calculation taking into account localized regions with high conductivity. The results suggest that deteriorating imperfections with x of AlGaN layer produce large gate leakage currents. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
AlGaN/GaN MIS‐HEMTs with HfO2 gate insulator
Authors: Kawano A. , Kishimoto S. , Ohno Y. , Maezawa K. , Mizutani Takashi , Ueno H. , Ueda T. , Tanaka T.
Published: June 2007

AlGaN/GaN MIS‐HEMTs using HfO2 gate insulator with a large dielectric constant have been fabricated. HfO2 gate insulator was deposited by pulsed laser deposition at room temperature. It has been demonstrated that the HfO2 gate insulator is effective in suppressing the gate leakage current keeping a large transconductance. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Reduced structural defect densities in a‐plane GaN layers on r‐plane sapphire through buffer layer engineering
Authors: Armitage R. , Hirayama H. , Kondo Y.
Published: June 2007

Nonpolar a‐plane GaN epilayers were grown by metalorganic vapour phase epitaxy on r‐plane sapphire substrates using three different buffer layer structures. The investigated structures consisted of GaN grown on: (A) AlN nucleation layer; (B) AlN plus Al0.25Ga0.75N intermediate layer; (C) AlN, Al0.25Ga0.75N, and GaN/Al0.25Ga0.75N superlattice. The samples were characterized by X‐ray diffraction, transmission electron microscopy, and atomic force microscopy. Structure (B) exhibited much lower overall structural defect densities than structure (A), with narrower X‐ray rocking curves by a factor of 1.5‐2. Compared to structure (B), structure (C) showed a factor of 2 lower density of screw dislocations in TEM although the densities of other types of defects were not significantly reduced. Type (B) and (C) structures also exhibited smoother surface morphologies than Type (A). (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Microstructure of a‐plane AlN grown on r‐plane sapphire and on patterned AlN templates by metalorganic vapor phase epitaxy
Authors: Okada N. , Imura M. , Nagai T. , Balakrishnan K. , Iwaya M. , Kamiyama S. , Amano H. , Akasaki I. , Maruyama H. , Noro T. , Takagi T. , Bandoh A.
Published: June 2007

The growth of a‐plane AlN (a‐AlN) on r‐plane sapphire substrate and the epitaxial lateral overgrowth (ELO) of a‐AlN on patterned a‐AlN has been performed by high‐temperature metalorganic vapor phase epitaxy. The ELO a‐AlN layers coalesced well on the a‐AlN template with trenches formed along 〈10$ \bar 1 $0〉. A detailed study of the microstructure by transmission electron microscopy revealed that the stacking faults due to facet slip are completely annihilated above the trenches of the patterned AlN. Furthermore, a dislocation density as low as 9.1×107 cm–2 was achieved by ELO. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Characterisation of multiple carrier transport in indium nitride grown by molecular beam epitaxy
Authors: Fehlberg Tamara B. , Umana‐Membreno Gilberto A. , Gallinat Chad S. , Koblmüller Gregor , Bernardis Sarah , Nener Brett D. , Parish Giacinta , Speck James S.
Published: June 2007

Quantitative mobility spectrum analysis (QMSA) was performed on multiple magnetic field Hall effect measurements of indium nitride grown by molecular beam epitaxy. This enables two clearly distinct electron species to be identified, which are attributed to the bulk and a surface accumulation layer. In this material, single magnetic field data corresponds to neither electron species, as both contribute significantly to the total conduction. The bulk electron distribution has an extracted average Hall mobility of 3570 cm2/(Vs) at 300 K with a concentration of 1.5 × 1017 cm–3, while the surface electrons have sheet charge density that is an order of magnitude higher than previously reported surface concentrations. The high quality bulk characteristics revealed emphasise the importance of using multi‐carrier analysis when performing transport measurements on InN.
Characterization of free‐standing GaN substrates prepared by self lift‐off
Authors: Lee S. W. , Goto H. , Minegishi T. , Lee W. H. , Ha J. S. , Lee H. J. , Lee Hyo‐jong , Lee S. H. , Goto T. , Hanada T. , Cho M. W. , Yao T.
Published: June 2007

Free‐standing GaN layers were successfully prepared by self lift‐off process. Single crystalline ZnO buffer layer and GaN layer were successively grown on sapphire substrate by plasma assisted molecular beam epitaxy. Thick GaN film was grown on this template substrate for the realization of stress‐free free‐standing substrate by hydride vapor phase epitaxy. The a‐axis and c‐axis lattice constants of free‐standing GaN were 3.189Å and 5.185Å, respectively. Peak positions of photoluminescence spectrum were D0X of 3.4715 eV and FXA of 3.4791 eV. These results suggest that the stress‐free GaN layers were successfully prepared by self lift‐off process. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Molecular dynamics simulation of thermal conductivity of GaN/AlN quantum dot superlattices
Authors: Kawamura Takahiro , Kangawa Yoshihiro , Kakimoto Koichi
Published: June 2007

We calculated thermal conductivity of GaN/AlN quantum dot superlattices by molecular dynamics simulation. The results of investigation of the effect of quantum dots on thermal conductivity as a function of superlattice period are presented in this paper. An empirical potential function of Stillinger‐Weber potential was used for simulations. Thermal conductivity was obtained by Green‐Kubo's equation. The results show that the values of thermal conductivity parallel to the wetting layers decreased due to the effect of quantum dots. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Investigation of local tunneling phenomena in green InGaN‐based LEDs
Authors: Onushkin Grigory , Lee Jinhyun , Yang Jung‐Ja , Park Seong‐Eun , Kim Min‐Ho , Koike Masayoshi
Published: June 2007

The tunneling radiative recombination in green InGaN based Light Emitting Diodes (LEDs) has been studied by micro‐Electroluminescence measurements. It was found that this recombination was very local, which has lateral size less than 1 μm and spectral peak position around 600 nm. Changes of growth conditions led to a reduction in the concentration of these defects, improving the current‐voltage characteristics and slightly enhancing the optical efficiency of green LEDs. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Formation process of sharp‐pointed structures on GaN nanorods during RF‐MBE growth and their field emission characteristics
Authors: Terayama M. , Hasegawa S. , Uchida K. , Ishimaru M. , Hirotsu Y. , Asahi H.
Published: June 2007

We have investigated layer structures of an AlN/GaN superlattice (SL) in nanorods by cross‐sectional transmission electron microscopy to clarify the formation process of sharp‐pointed structures on GaN nanorods during plasma assisted molecular beam epitaxial (PA‐MBE) growth. It is found that during 75‐160 nm MBE growth, flat interfaces of the AlN/GaN SL in nanorods develop into chevron‐shaped interfaces. Beyond a thickness of 165 nm, AlN/GaN SL nanorods continue to grow with nearly the same chevron‐shaped interfaces, and finally these interfaces appear as the sharp‐pointed structures on the top of nanorods. In contrast to GaN and AlN/GaN SL nanorods, no formation of the sharp‐pointed structures is observed for AlN nanorods. This indicates that the formation of the sharp‐pointed structures is ascribed to GaN growth. We will discuss thickness dependence of FE characteristics of GaN nanorods in relation with the formation of the sharp‐pointed structures. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Fabrication of enhancement‐mode AlxGa1–xN/GaN junction heterostructure field‐effect transistors with p‐type GaN gate contact
Authors: Fujii T. , Tsuyukuchi N. , Hirose Y. , Iwaya M. , Kamiyama S. , Amano H. , Akasaki I.
Published: June 2007

We have investigated the device performance, such as the breakdown voltages between gate‐source contacts and drain‐source contacts, threshold voltage (Vth) and on resistance (RON), of enhancement‐mode AlxGa1–xN/GaN junction heterostructure field‐effect transistors (JHFET) with a p‐type GaN gate. Fabricating an Al0.15Ga0.85N/GaN JHFET with a gate length of 2 μm without surface passivation, we have achieved a completely enhancement‐mode JHFET with a threshold voltage of +0.55 V, and breakdown voltages between gate‐source and drain‐source were over 100 V and 250 V, respectively. RON is as low as 4.5 mΩ · cm2 at VG = 3.0 V. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Improvements of quantum efficiency and thermal stability by using Si delta doping in blue InGaN/GaN multiple quantum well light‐emitting diodes
Authors: Wu Gwo‐Mei , Chung Ta‐Jen , Nee Tzer‐En , Kuo Da‐Chuan , Chen Wei‐Jen , Ke Chih‐Chun , Hung Cheng‐Wei , Wang Jen‐Cheng , Chen Nei‐Chuan
Published: June 2007

The improvements of quantum efficiency and thermal stability by using Si delta doping in blue InGaN/GaN multiple quantum well (MQW) structures have been investigated. The high‐resolution X‐ray diffraction (HR‐XRD) indicated better interface properties in the MQWs for Si delta doping structures. The Arrhenius plots of the temperature‐dependent photoluminescence (PL) spectra were used to reveal the enhancements of the radiative recombination. The activation energy was increased from 115 to 140 meV for the InGaN/GaN MQW sample with Si delta doping. The improvements of the thermal stability and activation energy were attributed to the supply of electrons from the Si delta doping barrier region into the InGaN well layer and the increase in hole capturing by the higher energy barrier in the valence band. Si delta doping process plays a crucial role in carrier recombination to significantly suppress carrier leakage, which not only increases the exciton confinement, but also improves the quantum efficiency. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Leak current in high resistive GaN buffer layer and its light responsiveness
Authors: Tanaka T. , Moriya Y. , Sahara M. , Yukimoto T. , Kamogawa H. , Otoki Y. , Mishima T.
Published: June 2007

We observed unique light responsiveness in HR GaN leak current, in which the current was drastically reduced under LED illumination of 2.65 eV or lower photon energy. Free carrier excitation by the illumination and consequential electron capture into the electron traps would rationalize the phenomenon. The threshold energy in our measurement was well associated with the carbon related very deep electron trap reported elsewhere (P. B. Klein et al., Appl. Phys. Lett. 79, 3527 (2001) [3]). Observation of this phenomenon provides us a very simple way to characterize both the HR GaN resistivity and the concentration of electron traps in the GaN buffer simultaneously. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Analysis of electrical properties of insulators (Si3N4, SiO2, AlN, and Al2O3)/0.5 nm Si3N4/AlGaN/GaN heterostructures
Authors: Maeda Narihiko , Hiroki Masanobu , Watanabe Noriyuki , Oda Yasuhiro , Yokoyama Haruki , Yagi Takuma , Makimoto Toshiki , Enoki Takatomo , Kobayashi Takashi
Published: June 2007

The electrical properties in AlGaN/GaN heterostructures with Si‐ and Al‐based insulators (Si3N4, SiO2, AlN, and Al2O3) have been examined and analyzed. By insulators deposition, significant increase in the two‐dimensional electron gas (2DEG) density (Ns) was observed with the order of Ns(Al2O3) > Ns(AlN) ∼ Ns(SiO2) > Ns(Si3N4) > N0 (N0: Ns without insulators). As the result, the decrease in the sheet resistance (R) was observed; the smallest order of R was R(Al2O3) < R(AlN) < R(Si3N4) < R0 ∼ R(SiO2) (R0: R without insulator). The insulators deposition effect has thus been shown to be significant and different among insulators. The increase in Ns was analyzed in terms of the change in the potential profile, and the observed differences in Ns among insulators have been interpreted. The band engineering including insulators is indispensable in understanding and designing AlGaN/GaN HFETs, since insulators are commonly used for the surface passivation as well as for the gate insulators, and the insulators deposition is to alter the essential device parameters such as the source resistance. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Incorporation of self assembled In‐rich InGaN nanostructure to achieve redshift in InGaN/GaN heterostructures
Authors: Soh C. B. , Hartono H. , Chen P. , Chua S. J.
Published: June 2007

The epitaxy growth of InGaN/GaN MQWs posed a great challenge, especially when high In content have to be incorporated for long wavelength applications such as the cyan and green LEDs. To realize devices with superior characteristic, In‐rich InGaN nanostrucutres (QDs) have been fabricated using InGaN wetting layer and trimethylIndium treatment in InGaN well. In InGaN QWs, nanometer scale fluctuation of the indium nanostructures can lead to “QD‐like” states which act as nucleation site to enhance In incorporation. This leads to red‐shift in PL dominant peak emission and a broadening of PL spectrum at its higher wavelength. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
First‐principle study on crystal growth of Ga and N layers on GaN substrate
Authors: Doi K. , Maida N. , Kimura K. , Tachibana A.
Published: June 2007

GaN surfaces show several types of the surface growth depending on the surface structures. In this study, we have simulated some kind of initial states of GaN surface growth from the viewpoint of metal‐organic chemical‐vapor deposition (MOCVD). It is clarified that NH3 molecules and N radicals can not fully terminate the (0001) Ga surface, and that chemical reactions between trimethylgallium and NH3 in gas‐phase is desirable to some extent because the products which contain GaN cores are immediately stabilized on the GaN(0001) surface. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Depth distribution of strain in GaN/AlN/SiC heterostructures by DUV micro‐Raman spectroscopy
Authors: Kitamura T. , Nakashima S. , Mitani T. , Nakamura N. , Furuta K. , Okumura H.
Published: June 2007

We measured the depth distribution of strain in GaN/AlN/SiC heterostructures by using deep ultraviolet (DUV) micro‐Raman and photoluminescence spectroscopies with bevel shaped specimens. Raman signal of AlN were detectable for the layers thicker than 15 nm. AlN and GaN layers had compressive strain in the whole region and the strain relaxation along the growth direction was observed. In this study, the variation of the Raman shift was well explained by a strain relaxation model that the misfit dislocations are distributed not only at the interface, but also throughout the film. The relaxation rate related to the generation rate of misfit dislocations was different for the AlN and GaN layers grown by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD). (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Structural and optical characterization of high In content cubic InGaN on GaAs(001) substrates by RF‐MBE
Authors: Nakamura Teruyuki , Endo Yuta , Katayama Ryuji , Yaguchi Hiroyuki , Onabe Kentaro
Published: June 2007

High In content cubic InGaN films have been successfully grown on GaAs(001) substrates by rf‐plasma assisted molecular beam epitaxy (RF‐MBE). The X‐ray diffraction analysis has confirmed the Ga content of the cubic InGaN films increases with increasing the group‐III flux ratio (Ga/(Ga+In)). The maximum Ga content of the c‐InGaN film is about 29% based on an analysis of the X‐ray diffraction 2θ/ω scan. By 2θ/ω and ω X‐ray reciprocal space mapping measurements, hexagonal‐phase and twin‐phase InGaN are found to be generated from c‐InGaN{111} facets. By low temperature PL measurement, the c‐In1–xGaxN peak shifts from 0.41 eV (x = 0) to 0.72 eV (x = 0.09) with increasing Ga content, showing that the bandgap of c‐InGaN is ∼300meV smaller than that of h‐InGaN with the same alloy composition. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Evaluation of strain in GaN/AlN quantum dots by means of resonant Raman scattering: the effect of capping
Authors: Cros A. , Budagosky J. A. , Garro N. , Cantarero A. , Coraux J. , Renevier H. , Proietti M. G. , Favre‐Nicolin V. , Daudin B.
Published: June 2007

We have studied in detail changes in the strain state of GaN/AlN quantum dots during the capping process. μ‐Raman scattering experiments allowed the detection of a resonant mode which provided information on the evolution of strain with capping. Simultaneously, Multiwavelength Anomalous Diffraction (MAD) and Diffraction Anomalous Fine Structure (DAFS) experiments were performed on the same samples, providing the independent determination of the wurtzite lattice parameters a and c. The remarkable agreement between Raman and X‐ray data stands out the suitability of polar vibrational modes for the determination of strain in nanostructures. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Fabrication of reflective GaN mesa sidewalls for the application to high extraction efficiency LEDs
Authors: Lee Jae‐Soong , Lee Joonhee , Kim Sunghwan , Jeon Heonsu
Published: June 2007

We proposed and demonstrated a modified GaN light‐emitting diode (LED) structure. The structure was made by integrating angled sidewalls to an otherwise conventional LED structure. Various GaN sidewall angles were obtained by adjusting etch conditions and the etched surface roughness was ∼2 nm. Near‐field emission patterns indicated that the angled sidewalls efficiently deflect photons that are initially guided laterally within the GaN epilayer into the off‐surface direction. For sidewall angle of 30°, total surface emission strength was improved by a factor exceeding 3. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
High temperature growth of AlN film by LP‐HVPE
Authors: Tsujisawa K. , Kishino S. , Liu Y. H. , Miyake H. , Hiramatsu K. , Shibata T. , Tanaka M.
Published: June 2007

AlN films were grown on AlN/sapphire templates at 1400–1500 °C using low‐pressure hydride vapor phase epitaxy (LP‐HVPE). Compared to the step‐flow growth of AlN film at 1200 °C with growth rate of 2.1 μm/h, AlN films with atomic steps were obtained at 1400–1500 °C even with high growth rate. For the AlN film grown at 1450 °C with growth rate of 14.3 μm/h, the RMS value is 0.75 nm and the FWHM values of (0002) and (10‐12) X‐ray rocking curve (XRC) are 351 and 781 arcsec, respectively. Since the FWHM value of (10‐12) XRC for the AlN/sapphire template is 1492 arcsec, the crystal quality of HVPE‐grown AlN is greatly improved compared with the AlN/sapphire template, which is also confirmed by TEM observation. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
In situ gravimetric monitoring of decomposition rate on the surface of (0001) c‐plane sapphire for the high temperature growth of AlN
Authors: Akiyama K. , Araki T. , Murakami H. , Kumagai Y. , Koukitu A.
Published: June 2007

The thermal stability of (0001) sapphire was investigated at atmospheric pressure using the in situ gravimetric monitoring (GM) method. The weight change of a sapphire substrate was monitored at various hydrogen partial pressures in carrier gas (P) at temperatures over 1200 °C. Although the sapphire substrate was stable up to 1450 °C in an inert carrier gas (P = 0.0 atm), sapphire decomposition started to occur at 1200 °C in H2 carrier gas (P = 1.0 atm). Moreover the activation energy and order of reaction for sapphire surface decomposition changed at approximately 1300 °C. These results indicate that the rate‐limiting reaction for sapphire decomposition shifts near 1300 °C. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
InAsN quantum dots grown on GaAs(001) substrates by MOVPE
Authors: Kuboya S. , Thieu Q. T. , Nakajima F. , Katayama R. , Onabe K.
Published: June 2007

The self‐assembled InAsN quantum dots (QDs) were grown on the GaAs (001) substrates by MOVPE using 1,1‐dimethlhydrazine (DMHy) and tertiarybutylarsine (TBAs) as the N and As precursors, respectively. The size distribution of the InAs(N) QDs grown at 450 °C and 420 °C is found bimodal. The InAsN QDs are typically 4‐11 nm in height and 30‐41 nm in width. The N incorporation into InAs induces the decrease of the QDs size, due to the increase of the wetting layer thickness. The density of the InAsN QDs with a nominal source supply of 2.6‐3.0 ML, increases from 1.1‐1.3×1010 cm–2 for the 450 °C growth to 2.1‐2.3×1010 cm–2 for the 420 °C growth. The photoluminescence peaks (at 300 K) of the InAs(N) QDs grown at both temperatures clearly show a red‐shift to 1200 nm by N incorporation. This red‐shift is in contrast with the blue‐shift expected from the decrease in the dot sizes (quantum size effect), indicating the huge bandgap bowing induced by N incorporation is dominant over the quantum size effect. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Extraction‐efficiency enhancement of InGaN‐based vertical LEDs on hemispherically patterned sapphire
Authors: Lee Jae‐Hoon , Oh Jeong‐Tak , Choi Seok‐Boem , Woo Jong‐Gun , Lee Su‐Yeol , Lee Myoung‐Bok
Published: June 2007

To improve the external quantum efficiency, a vertical type InGaN LED (VT‐LED) is fabricated on a sapphire substrate with a surface hemispherically‐patterned by laser lift‐off (LLO) technique. As a result, the VT‐LEDs with a concavely patterned surface revealed an enhanced luminous intensity by four times compared to the device with a planar surface. This improvement in the VT‐LED performances is explained to be attributed mainly to the increase in the escaping probability of photons due to the corrugated LED surface. The use of metallic substrate prepared by nickel electroplating seems the other factor for the increase of luminous intensity by considering reduced current crowding and better thermal conductivity. These devices are advantageous for high power operation. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Blue light emitting diode fabricated on a‐plane GaN film over r‐sapphire substrate and on a‐plane bulk GaN substrate
Authors: Naoi Y. , Ikeda K. , Hama T. , Ono K. , Choi R. , Fukumoto T. , Nishino K. , Sakai S. , Lee S. M. , Koike M.
Published: June 2007

We studied the growth technique for the dislocation reduction in a‐plane GaN grown by metal organic chemical vapour deposition (MOCVD) using AlInN buffer layer, high temperature atomic layer epitaxy, and trenched r‐sapphire technique. By using these techniques, the crystal quality was much improved. We also fabricated blue light emitting diodes (LEDs) on a‐plane GaN film over r‐sapphire substrate and on a‐plane bulk GaN substrate. The electroluminescence (EL) characteristics of the LED samples were examined, and we found that the EL near field pattern from homo‐epitaxially grown a‐GaN based LED was spatially uniform, although the pattern from the LED on r‐sapphire substrate was not uniform. The output power at the wavelength of 430nm was 0.72mW at the 20mA injection current for the sample on a‐plane bulk GaN. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Cathodoluminescence characterization of [11$ \bar 2 $0]‐oriented InGaN/GaN thin films grown on r‐plane sapphire substrates by metalorganic vapor‐phase epitaxy
Authors: Kusakabe K. , Furuzuki T. , Ohkawa K.
Published: June 2007

We report the cathodoluminescence (CL) characterization of nonpolar [11$ \bar 2 $0]‐oriented InGaN/GaN thin films grown on r‐plane sapphire substrates by atmospheric metalorganic vapor‐phase epitaxy. The epitaxial GaN films grown on r‐plane sapphire substrates frequently show facet surface morphology. The CL peak wavelength and local indium composition of InGaN grown on them were gradually varied with the probed position from the facet summit to valley. In a‐plane InGaN, the facet summit was In‐rich area while the valley was In‐poor area. This is due to the material transport issue during epitaxial growth. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)