RF-radical Source Research Articles

Visible-light-induced dearomative spirocyclization of N -benzylacrylamides toward perfluorinated azaspirocyclic cyclohexadienones

A feasible approach to 2-azaspirocyclic cyclohexadienones via visible-light-induced perfluoroalkylation cyclization of N-benzylacrylamides was reported. Using Rf-X (X = I or Br) as the Rf radical source, the reaction underwent a cascade radical addition/dearomative cyclization process by Ir photocatalyst, leading to various 2-azaspiro[4.5]deca-6,9-diene-3,8-diones bearing perfluorinated groups including CF3, n-C3F7, n-C4F9, n-C6F13, n-C8F17, n-C10F21, CH2CF2 and CF2CO2Et.

Improvement of Oxidation Behavior in Cr(N,O) By Increasing the Oxygen Content

Chromium oxynitride(Cr(N,O)) thin films have been developed by controlling the oxygen partial pressure during the deposition 1). The Cr(N,O) thin films show superior properties not only the hardness but also the oxidation behavior than those of CrN, which are suitable for machining tool applications. However, the reason of the oxidation improvement was not known because of the presence of grain boundaries. Recently, we have succeeded in preparation of epitaxially grown Cr(N,O) on MgO and other single crystal substrates to reduce the density of grain boundaries2). In this work, oxidation behavior of the epitaxially grown Cr(N,O) thin films was investigated from crosssectional transmission electron microscope (TEM) observations. Thin films of Cr(N,O) were epitaxially grown by a pulsed laser deposition method. In the deposition chamber, oxygen gas with a partial pressure of 2x10-5Pa and N radicals from an RF radical source were introduced. A plate of Cr was rotated in the deposition chamber was irradiated by a Nd:YAG pulsed laser with a wavelength of 355 nm. Thin films were deposited on MgO (100) single crystal substrates kept at 973 K in the deposition chamber. After the deposition, oxidation tests were carried out in a furnace at 673-1173 K in air for one hour. After the oxidation tests, powder X-ray diffraction (XRD), crosssectional TEM and electron energy loss spectroscopy (EELS) observations/analyses were conducted for precise phase, microstructure and composition analysis. From the XRD and TEM analyses, the as-deposited thin films were of epitaxially grown Cr(N,O) on the substrates. After the oxidation tests, from XRD patterns, peaks for Cr2O3were seen from samples heated above 873 K. On the other hand, peaks for Cr(N,O) remained up to 1073 K. From TEM and EELS analyses of the sample heated at 1073K, on the existing Cr(N,O) epitaxially grown thin film, a polycrystalline oxide layer was covering. It was found that the oxide layer works as an oxygen diffusion barrier from the surface. References 1) T. Suzuki, J. Inoue, H. Saito, M. Hirai, H. Suematsu, W. Jiang, and K. Yatsui, Thin Solid Films 515, 2161 (2006). 2) K. Suzuki, T. Endo, A. Sato, T. Suzuki, T. Nakayama, H. Suematsu, and K. Niihara, Mater. Trans. 54, 1957 (2013).

X-ray Photoemission Spectroscopy Study of GaAs(111)B Substrate Nitridation using an RF-Radical Source

The mechanism of GaAs(111)B surface nitridation using an RF-radical source was studied while systematically changing the nitridation temperature. Atomic force microscopy and X-ray photoemission spectroscopy measurements indicated that the nitridation of the GaAs(111)B surface showed a similar dependence on nitridation temperature to that of a GaAs(001) surface. Both the degree of nitridation and the Ga adatom density increased with increasing nitridation temperature, and above 510 °C, Ga droplets were formed. Strong photoluminescence signals were obtained from the GaAs(111)B surfaces.

X-ray Photoemission Spectroscopy Study of GaAs(111)B Substrate Nitridation using an RF-Radical Source

The mechanism of GaAs(111)B surface nitridation using an RF-radical source was studied while systematically changing the nitridation temperature. Atomic force microscopy and X-ray photoemission spectroscopy measurements indicated that the nitridation of the GaAs(111)B surface showed a similar dependence on nitridation temperature to that of a GaAs(001) surface. Both the degree of nitridation and the Ga adatom density increased with increasing nitridation temperature, and above 510 °C, Ga droplets were formed. Strong photoluminescence signals were obtained from the GaAs(111)B surfaces.

X-ray Photoemission Spectroscopy Study of Low-Temperature Nitridation of GaAs(001) Surface Using RF Radical Source

The nitridation mechanism of the GaAs(001) surface using an RF-radical source at a low temperature of 350 °C was systematically studied by changing the As supply and nitridation time to obtain a smooth nitrided layer without the formation of Ga droplets. Atomic force microscopy (AFM) measurements indicated that supplying As is useful in suppressing the re-evaporation of As atoms and in maintaining a smooth surface. However, the degree of nitridation was decreased with increasing As pressure in the samples nitrided for 30 min. In contrast, the time dependence of nitridation indicated that the degree of nitridation increases with nitridation time. After optimizing the conditions, a two-monolayer-thick GaN layer was successfully obtained by nitridation for 120 min, regardless of the supply of the As molecular beam. The structure of the nitrided layers was also investigated using angle-resolved X-ray photoemission spectroscopy, and a thin layer that contains As and N atoms was found to cover the nitrided GaN layer.

Influence of substrate temperature on nitridation of (0 0 1) GaAs using RF-radical source

The mechanism of nitridation of (0 0 1) GaAs surface using RF-radical source was systematically studied with changing substrate temperature, nitridation time and supplying As molecular beam. It was found from atomic forth microscopy (AFM) measurements that supplying As is very important to suppress the re-evaporation of As atoms and to keep the surface smooth. Reflection high-energy electron diffraction (RHEED) measurements shows that surface lattice constant (SLC) of GaAs of 0.565 nm decreases with increasing the substrate temperature and that it finally relaxes to the value of c-GaN of 0.452 nm, at 570 °C in both [1 1 0] and [1¯ 1 0] directions without concerning with the supply of As molecular beam. But, in the medium temperature range (between 350 and 520 °C), SLC of [1 1 0] direction was smaller than that of [1¯ 1 0] direction. This suggests a relation between the surface structure and the relaxing mechanism of the lattice. The valence band discontinuity between the nitridated layer and the GaAs layer was estimated by using X-ray photoemission spectroscopy (XPS). It was between 1.7 and 2.0 eV, which coincides well with the reported value of c-GaN of 1.84 eV. This suggests that the fabricated GaN layer was in cubic structure.

Growth of gallium nitride nanorods by metalorganic molecular beam epitaxy

Without a catalyst or template layer, high-quality GaN nanorods have been grown by rf radical source metalorganic molecular beam epitaxy on c-sapphire substrates. Scanning electron microscopy, transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDX), and x-ray diffraction were employed to characterize the GaN nanorods. These nanorods have an average diameter of 50nm, and the rod areal density can reach 1×1010cm−2 depending on the growth parameters. High-resolution TEM results indicate that the well-aligned GaN nanorods have a single-crystal hexagonal wurtzite structure with preferential [0001] orientation, and no droplet was observed at the end of the nanorods. Meanwhile, EDX analysis revealed gallium and nitrogen as the only detectable elements. These high-quality and high-density GaN nanorods might be useful for practical applications in nanoscale optoelectronic and electronic devices.

Growth and Characterization of AlBN Polycrystalline Thin Film by Radio-Frequency Plasma-Assisted Molecular Beam Epitaxy

An AlBN thin film with a boron content (B/(Al+B)) of 0.1 or 0.3 was obtained by radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE) using EB-guns as group-III element sources and an RF radical source for nitrogen supply. We compared the characteristics of the film with those of AlN and BN films. By reflective high-energy electron diffraction (RHEED), we observed ring patterns in the AlBN film. The X-ray photoelectron spectroscopy (XPS) N1s peak of the AlBN film was observed at a binding energy between the peaks of AlN and BN. There was no evidence for phase separation in the film.

Interactions between gallium and nitrogen dopants in ZnO films grown by radical-source molecular-beam epitaxy

It has been recently predicted that the co-doping of an acceptor (nitrogen) and a donor (aluminum, gallium, indium) in a 2:1 ratio will dope ZnO p-type due to a reduction in the Madelung energy making the nitrogen acceptor energy level more shallow. We have been growing gallium and nitrogen co-doped ZnO films by radical-source molecular-beam epitaxy by use of oxygen and nitrogen radicals supplied via rf radical source cells. Diode-like current–voltage characteristics and donor acceptor pair-like photoluminescence emission were observed for a Ga and N doped ZnO film grown on an undoped ZnO buffer layer. However, Hall measurements revealed that the conductivity was n-type. Formation of a non-ZnO phase in the sample was confirmed by secondary ion mass spectroscopy and x-ray diffraction measurements. Zn and Zn+O secondary ion intensities fell sharply by two orders of magnitude in going from the undoped ZnO layer to the highly co-doped ZnO. X-ray diffraction measurements indicated the formation of ZnGa2O4.

Epitaxial growth of LiNbO 3 thin films using pulsed laser deposition

An epitaxial lithium niobate (LiNbO 3) thin film was successfully fabricated on an α-Al 2O 3(0 0 0 1) substrate using a pulsed laser deposition method and the effect of laser fluence on film quality was investigated. The Li concentration in a deposited film was largely influenced by oxygen radicals, which were produced not only by the interaction between the incident excimer laser and the oxygen but also by an rf-radical source. The re-evaporation of Li atoms in the film was suppressed by the oxidation of Li atoms.

Nitrogen-induced defects in ZnO : N grown on sapphire substrate by gas source MBE

Nitrogen-doped ZnO layers were grown on sapphire substrates by radical source molecular beam epitaxy by simultaneously introducing O 2 and N 2 via a RF radical source. Reflection high-energy electron diffraction and X-ray diffraction measurements revealed that high N 2/O 2 flow ratios induced growth twins into the ZnO layer. A nitrogen-doped ZnO fabricated using a N 2/O 2 flow ratio of 10% was found to have a chemical nitrogen concentration of 1×10 19 cm −3. However, type conversion from n-type to p-type did not occur while large nitrogen incorporation was observed to induce extended defects.

Uniaxial locked growth of high-quality epitaxial ZnO films on [formula omitted]-Al 2O 3

ZnO epitaxial films have been grown on sapphire substrates using molecular beam epitaxy (MBE). Elemental sources of Zn and O were used with a RF radical source being employed to increase the reactivity of the oxygen source gas. High-sensitivity pole figure measurements indicated that the films were uniquely (0 0 0 1) oriented with no trace of secondary orientations. The unique orientation is a consequence of the coincidental near zero lattice mismatch of the ZnO a lattice constant of 0.3250 nm and the sapphire c lattice constant over four or 0.3248 nm leading to the term uniaxial locked epitaxy. Atomic force microscopy of as-grown samples indicated that the films were flat with a RMS roughness of less than 0.4 nm. Two dimensional X-ray reciprocal space mapping of the ZnO (1 0 1 ̄ 4) asymmetric reflection using a triple axis configuration indicated that the lateral correlation lengths increased from several hundred nanometers for the case of (0 0 0 1)ZnO grown on sapphire(0 0 0 1) substrates to about 0.5 μm for growth on (1 1 ̄ 2 0) sapphire substrates. This is interpreted as being a consequence of less in-plane twisting of domains due to stress from lattice mismatch. Preliminary photoluminescence measurements indicate a dramatic increase in intensity with bound exciton features less than 0.7 meV.

Growth of high-quality epitaxial ZnO films on α-Al 2O 3

ZnO epitaxial thin films have been grown on sapphire(0 0 0 1) substrates by molecular beam epitaxy using elemental zinc and oxygen supplied by an RF radical source. Despite the large lattice mismatch between ZnO and the underlying sapphire substrate, ZnO layers with (0 0 0 2) rocking curve half-widths of ∼12 arcsec have been grown. X-ray reciprocal lattice scans along the [0 0 0 1] direction show strong Pendellösung fringes indicating the presence of an extremely flat interface and surface as was confirmed by atomic force microscopy experiments. X-ray pole figure measurements indicate that the a -axis of the epilayer was rotated with respect to the a -axis of the substrate by 30°. Preliminary photoluminescence measurements indicate predominant near-bandedge emission.

Self-organization of GaN/Al0.18Ga0.82N multi-layer nano-columns on (0 0 0 1) Al2O3 by RF molecular beam epitaxy for fabricating GaN quantum disks

GaN nanostructures (GaN nano-column) were grown on (0001) Al2O3 by RF-radical source molecular beam epitaxy (RF-MBE) through a self-organization process. An averaged diameter of the GaN nano-column was minimized to 40–45nm by optimizing a Ga cell temperature under a fixed nitrogen supply. The self-organization process of the GaN nano-column was applied for the fabrication of the GaN/Al0.18Ga0.82N multi-quantum disk (MQD). The GaN nano-column with an averaged diameter of 46nm was fabricated, in which the 10 paired GaN(6nm)/Al0.18Ga0.82N(9nm) multi-layer structure was built in to form quantum disks. The blue shift in a photoluminescence (PL) peak wavelength at room temperature was observed for the MQD sample, probably due to the quantum-size effect.

Epitaxial growth of GaN with a high growth rate of 1.4 μm/h by RF-radical source molecular beam epitaxy

GaN epitaxial layers were grown on (0001) sapphire substrates by molecular beam epitaxy (MBE) using RF radical nitrogen sources with relatively high growth rates of 1.2–1.4μm/h. For the growth rate around 1.2μm/h, the N2 flow rate was optimized, so that a low residual carrier density of GaN of 2.8×1016cm−3 and a smooth surface were obtained. The n-type Si-doping of GaN was demonstrated. The carrier densities of Si-doped GaN films were controlled in the range from 1.7×1017 to 1.5×1018cm−3 by changing the Si cell temperature. By increasing the amount of Ga and nitrogen supply, the maximum growth rate of 1.4μm/h was achieved at substrate temperature around 760°C. The surface morphologies were smooth but inferior to that of 1.2μm/h growth, because the growth condition for the growth rate of 1.4μm/h was not optimized yet. The residual carrier density was around 1×1017cm−3. The near band-edge emission was observed in both room temperature and 15K photoluminescence measurements.

High-resolution X-ray diffraction analysis of cubic GaN grown on (0 0 1)GaAs by RF-radical source molecular beam epitaxy

Cubic GaN epilayers were grown on atomic hydrogen treated (001)GaAs substrates by RF-radical source molecular beam epitaxy. The crystalline quality was characterized by a high-resolution X-ray diffractometer. It was found that high-quality c-GaN can be grown at temperatures above 680°C; the FWHM of (002)GaN rocking curve was as small as 80–90arcsec. In order to further precisely investigate how and to what extent the h-GaN phase is included in the c-GaN layer, the X-ray reciprocal space maps were measured for the X-ray beams incident along both 〈110〉 and 〈11̄0〉 azimuths. It was found that the stacking faults exist predominantly on {111}A planes. The inclusion of h-GaN was estimated by comparing the integrated XRD intensities for h-GaN {101̄1} and c-GaN(002) planes. It was found that the inclusion of h-GaN phase drastically decreased with increasing growth temperature above 680°C to reach about 4×10−3 (or below) at temperatures of 710–740°C.

High-speed GaN growth and compositional control of GaN-AlGaN superlattice quasi-ternary compounds by RF-radical source molecular beam epitaxy

Novel growth technologies of III-nitrides for fabricating optical devices by molecular beam epitaxy using RF-plasma excited nitrogen (RF-MBE) were investigated. A relatively high-growth rate, up to 1.4 /spl mu/m/h of GaN with high-electrical and high-optical quality was obtained. The concept of AlGaN quasi-ternary (QT) compounds, consisting of GaN-AlGaN short period superlattice, was demonstrated and the Al composition was controlled with a shutter control method in the range of 0-0.47. Using the QT technology, a GaN-Al/sub 0.07/Ga/sub 0.93/3N-Al/sub 0.3/Ga/sub 0.7/N multiquantum-well heterostructure was fabricated to show the effectiveness of the method.