Satoshi Shigemori

Zn1−xCdxO systems with visible band gaps

Zn1−xCdxO films in the range of the content x from x=0 to x=1 were grown by remote-plasma-enhanced metal organic chemical vapor deposition. The crystal structure of Zn1−xCdxO film changed with increase of the content x from wurtzite structure to rocksalt structure around x=0.7. The relationship between the cadmium content and axis length in the Zn1−xCdxO films was studied. Photoluminescence spectra were observed from the wurtzite Zn1−xCdxO films in the range of 3.3–1.8eV at room temperature. Stokes’s shift in the restricted composition range was compared with the previous results.

Zn1-xCdxO Film Growth Using Remote Plasma-Enhanced Metalorganic Chemical Vapor Deposition

Zn1-xCdxO films were successfully grown by remote plasma-enhanced metalorganic chemical vapor deposition (RPE-MOCVD). The content ratio of Zn1-xCdxO films was controlled by changing the molar ratio of diethyl zinc (DEZn) to dimethyl cadmium (DMCd). The wurtzite structure of Zn1-xCdxO films was obtained by increasing the Cd content up to x=0.697. The optical-band-gap energy of Zn1-xCdxO films was tuned between 1.85 eV and 3.28 eV at room temperature. The photoluminescence emission of hexagonal Zn1-xCdxO films up to x = 0.697 was observed at room temperature.

Characterization of Wurtzite Zn1-xCdxO Films Using Remote Plasma-Enhanced Metalorganic Chemical Vapor Deposition

The optical properties of wurtzite Zn1-xCdxO films were investigated. The films were grown on (1120) a-plane sapphire substrates by remote plasma enhanced metalorganic chemical vapor deposition (RPE-MOCVD) and a characterized by X-ray diffraction, photoluminescence (PL), ultraviolet/visible/near-infrared (UV/VIS/NIR) optical transmission spectroscopy and Hall measurement. The content ratio of the films was controlled by changing the molar ratio of diethyl zinc (DEZn) to dimethyl cadmium (DMCd). The optical band gap of the films at nearly 3.28 eV was shifted by alloying with Cd down to 1.85 eV depending on the alloy composition. The evolution of the PL lineshape was studied as a function of temperature. The activation energy of the films calculated from the variation of the PL peak intensity was as large as the binding energy of ZnO. In addition, the films showed a large Stokes shift at room temperature, which is discussed in terms of the localization of excitons in a ternary alloy.

Zn1-xCdxO/ZnO Heterostructures for Visible Light Emitting Devices

Wurtzite Zn1-xCdxO/ZnO heterostructures were successfully grown by remote plasma enhanced metalorganic chemical vapor deposition (RPE-MOCVD) and were investigated by photoluminescence (PL) spectroscopy. The flatness of Zn1-xCdxO films was investigated by an atomic force microscope (AFM), indicating the typical RMS value of 0.5 nm. The optical properties of the Zn0.96Cd0.04O film were characterized by micro-PL at 4 K, exhibiting micro-structural and positional uniformities in the films. In the double heterostructure consisting of ZnO/Zn0.92Cd0.08O/ZnO, temperature and excitation intensity dependencies of PL spectra were examined. The PL emission is characterized as localized and free exciton emission. A dependence with a slope near unity is obtained from the excitation dependence of the PL intensity. Blue-green emission (2.78 eV) was demonstrated from the double-heterostructure at room temperature.

Hydroxyl-radical-assisted growth of ZnO films by remote plasma metal-organic chemical vapor deposition

The hydroxyl-radical-assisted growth of ZnO films by remote plasma-enhanced metal-organic chemical vapor deposition (RPE-MOCVD) was investigated. Plasma was generated by an RF discharge in O2. The change of the carrier gas from N2 to H2 resulted in a significant increase in deposition rate. It was found from the spectroscopic characterization of light emitted by the reactor gas in the vicinity of a substrate that the effect of the carrier gas on deposition rate is related to the occurrence of OH radicals. Oxygen radicals and OH radicals, which were observed by spectroscopic measurement, promoted film growth and suppressed deep-level emissions in photoluminescence spectra. To elucidate the effects of O and OH radicals, hydrogen and helium remote plasma techniques were also used in film growth for comparison. Finally, the ZnO films fabricated by the OH radical-assisted growth were investigated by photoluminescence and X-ray diffraction (XRD) analyses. In addition, the Zn-termination effects of the sapphire substrate surface on heteroepitaxial ZnO film growth weres investigated.

OH Radical Activation of ZnO Growth in Remote Plasma Metalorganic Chemical Vapor Deposition

A possible role of OH radicals in the formation mechanism of ZnO thin films by remote plasmaenhanced metalorganic chemical vapor deposition (RPE-MOCVD) was investigated. The plasma was generated by a radio frequency discharge in O2. The change of the carrier gas from H2 to N2 resulted in a significant decrease in deposition rate. Moreover, growth rate changed also with the content of H2 of an oxygen-hydrogen mixture gas in a reactor. Optical spectroscopic investigation of the light emitted by the reactor gas in the vicinity of a substrate showed that the effect of the carrier gas on deposition rate is related to the occurrence of OH radicals.

Spatially Ordered Self-Assembled Quantum Dots with Uniform Shapes Fabricated by Patterning Nanoscale SiN Islands

We report a method of obtaining position-controlled quantum dots with the uniform spatial shapes. The self-assembled InGaAs quantum dots are grown on a GaAs (311) B substrate with patterned SiN islands. The SiN patterns determine the position of the quantum dots as well as their optical properties. The uniformity of the positions and photoluminescence properties strongly depend on the pitch of the hexagonally patterned SiN islands. With an optimum pattern, the quantum dots have a uniform spatial arrangement. These uniform quantum dots exhibit strong photoluminescence spectra with sharp peaks and spatially isotopic polarization resolved photoluminescence signals. These results show that quantum dots have efficient photoemission processes and that their shapes have the same spatial symmetry.

Positional control of self-assembled quantum dots by patterning nanoscale SiN islands

We propose a method for obtaining position-controlled self-assembled quantum dots. The self-assembled InGaAs quantum dots are grown on a GaAs (311) B substrate on which SiN islands have been patterned using a nanolithographic technique. The SiN pattern determines the position of the quantum dots as well as their optical properties. The positional uniformity and photoluminescence spectrum strongly depend on the pitch of the SiN pattern. At an optimum pitch, uniformly arranged quantum dots and intense photoluminescence spectra with sharp peaks are obtained.