Masahito Kurouchi

Radio frequency-molecular beam epitaxial growth of InN epitaxial films on (0001) sapphire and their properties

Growth of InN by radio frequency plasma-excited molecular beam epitaxy on (0001) sapphire was systematically studied. To improve the crystalline quality of InN, the following growth conditions were found to be essential: (1) nitridation of sapphire, (2) two-step growth, (3) precise control of V/III ratio, and (4) selection of optimum growth temperature. Results of structural characterization using x-ray diffraction, transmission electron microscopy, and extended x-ray absorption fine structure have clearly demonstrated that InN grown in this study had single crystalline with ideal hexagonal wurtzite structure. It is confirmed, however, that the InN had a threading dislocation density on the order of 1010/cm2 and large twist distribution. Photoluminescence studies on these well-characterized InN clearly demonstrated that band-gap energy of InN should be less than 0.67 eV at room temperature.

Kelvin probe force microscopy study of surface potential transients in cleaved AlGaN∕GaN high electron mobility transistors

The surface potential of cleaved cross sections of AlGaN∕GaN high electron mobility transistors was measured by Kelvin probe force microscopy. For the bias conditions of Vgs=−5V and Vds=20V, the electric field was concentrated near the GaN∕SiC interface under the gate and between the gate and drain electrodes. A negative potential that decreased over time was observed in the GaN layer beginning 10min after the bias stress was removed. The transient surface potential was found to be well described by an exponential dependence with two time constants: 11 and 55s.

Fabrication and characterization of InN-based quantum well structures grown by radio-frequency plasma-assisted molecular-beam epitaxy

InN/InxGa1-xN quantum well structures have been successfully fabricated on InN templates grown on (0 0 0 1) sapphire substrates by RF plasma assisted molecular beam epitaxy for the first time. The structure was confirmed by observing the 1st and 2nd satellite peaks of X-ray diffraction. From InN/In0.8Ga0.2N single quantum well structures with the different well widths of 2.3, 3.4 and 5.7 nm, photoluminescence (PL) emission from the well layers was observed at 77 K, and the PL peak energy slightly lowered with increasing well width. This dependence can be explained by combined effects of the quantum size effect, quantum confinement Stark effect, and band filling effect.

Band-to-band character of photoluminescence from InN and In-rich InGaN revealed by hydrostatic pressure studies

The authors studied the hydrostatic pressure dependence of photoluminescence (PL) from In-rich InxGa1−xN with In contents x between 0.58 and 1.00. The observed PL pressure coefficients of 20–25meV∕GPa agree well with previously reported experimental and theoretical values of the band gap pressure coefficient, from which they conclude that band-to-band recombination is responsible for PL emission. This contrasts with earlier reports, where relatively low PL pressure coefficients were interpreted as evidence of the involvement of strongly localized states in the PL emission. The reported observation of band-to-band recombination in In-rich InGaN is encouraging from the point of view of the construction of light emitters, since band-to-band recombination is more efficient than recombination via localized states. Furthermore, significant bowing of the band gap pressure coefficient in In-rich InxGa1−xN, as predicted by theory, is confirmed.

The influence of alloy disorder and hydrostatic pressure on electrical and optical properties of In-rich InGaN compounds

We discuss the influence of indium segregation-induced disorder effects in InxGa1-xN alloys. Changes of the transport mechanism between InN and InxGa1-xN with x=0.58 were demonstrated by means of temperature-dependent conductivity measurements. Furthermore, an increase of (i) full width at half maximum of photoluminescence (PL) and (ii) the Stokes shift between PL and absorption was seen for samples approaching an In content of 0.5, which can also be attributed to growing disorder. Hydrostatic pressure dependent PL measurements of In-rich InGaN alloys are diacussed. Due to the fact that PL in InGaN originates from regions with higher-than-average In content, the luminescence pressure coefficient dEE/dp should not be associated with the average In content, but with the In content which is in accordance with the energy of the photon emission. This correction leads to a reduction of the large bowing of dEE/dp (associated with the band gap) which was reported earlier. Furthermore, it is shown that the electron concentration in InN has a significant influence on the measured value of dEE/dp.

Effects of post exposure bake temperature and exposure time on SU-8 nanopattern obtained by electron beam lithography

SU-8 is a photoresist imaged using UV rays. However, we investigated the characteristics of an SU-8 nanopattern obtained by electron beam lithography (EBL). In particular, we studied the relationship between post-exposure bake (PEB) temperature and exposure time on an SU-8 nanopattern with a focus on phase transition temperature. SU-8 residue was formed by increasing both PEB temperature and exposure time. To prevent the formation of this, Monte Carlo simulation was performed; the results of such simulation showed that decreasing the thickness of SU-8 can reduce the amount of residue from the SU-8 nanopattern. We confirmed that decreasing the thickness of SU-8 can also prevent the formation of residue from the SU-8 nanopattern with EBL.

The composition dependence of the optical properties of InN-rich InGaN grown by MBE

Study of the relationship between the composition and optical energies of InxGa1-xN has generated much interest and intrigue over the last decade and beyond. In this paper we describe data from InxGa1-xN epilayers covering the full range of composition (0 < x < 1), grown by both Metal-Organic Vapour Phase Epitaxy (MOVPE) and Molecular Beam Epitaxy (MBE). In particular we concentrate on a set of state-of-the-art InN rich MBE layers (0.6 < x < 1.0). Wavelength dispersive X-ray microanalysis is employed for accurate measurement of the InN fraction and of the group III : group V ratio. The InN rich layers are shown to be highly stoichiometric. The composition results are correlated with luminescence spectra, which show peaks covering the range 1.3 to 0.7 eV. Inclusion of our data from sets of MOVPE and MBE epilayers with InN fractions up to 0.4, measured using identical techniques, allows the composition dependence of the luminescence peak energy to be plotted across the entire composition range. A quadratic fit gives good agreement with both the low-InN MOVPE and high-InN MBE samples but not for the intermediate region. Possible reasons for this are discussed.

Formation of highly planarized Ni–W electrodeposits for glass imprinting mold

We confirmed that increasing the total metal concentration is effective for the planarization of Ni–W films and Ni–W nanopatterns formed with a uniform height and a 480 nm pitch. At the same time, the W content in Ni–W films decreased. We investigated the relationship between the planarization of Ni–W films and the W content in Ni–W films, and confirmed that increasing the total metal concentration is effective for the inhibition of hydrogen generation. We pointed to the inhibition of hydrogen gas generation as a cause of the planarization of Ni–W films, and the reduction in the hydrogen generation amount necessary for the deposition of W as a cause of the reduction in the W content in Ni–W films. In order to obtain a flat plating film with a high W content, it is necessary to generate an adequate amount of hydrogen on the surface of the cathode and to remove hydrogen gas from the cathode surface immediately.

Effect of metal ion concentration in Ni–W plating solution on surface roughness of Ni–W film

Since nanopatterns are used for various purposes including solar cells, super-hydrophilicity, and biosensors, it is necessary to miniaturize the patterns on glass devices from micro- to nano-order. We have studied glass imprinting as an excellent microfabrication technology for glass devices. Uniformity of the nanopattern height is required for a mold, since a nodular structure on the Ni–W surface is recognized as a problem in Ni–W nanopattern formation. We confirmed that the Ni–W plating bath increasing metal ion concentration is effective for inhibition of the nodular structure on the Ni–W film, and succeeded in Ni–W nano pattern formation with uniform height. However, the W content rate of plated Ni–W film was reduced in exchange for enhancing the flatness of the Ni–W film. It is necessary to examine the Ni–W plating condition for obtaining planarization of the Ni–W surface and a high content rate of W in the Ni–W film.

Expansion of lattice constants of aluminum nitride thin film prepared on sapphire substrate by ECR plasma sputtering method

Wurzite aluminum nitride is prepared on a c-plane sapphire substrate by electron cyclotron resonance plasma-enhanced sputtering deposition (ECR sputtering). Atomic force microscopy (AFM) showed flat AlN thin-film surfaces, and X-ray diffraction (XRD) analysis verified the epitaxial growth of AlN films with the full-width at half-maximum (FWHM) of the rocking curve of 0.025 deg on the film with the thickness of 100 nm. XRD analysis also verified the change in the peak position for the AlN film along both out-of-plane and in-plane directions. The effect of lattice constants on the energy gap was theoretically estimated by the first principles method.