Katsuhiro Akimoto

Epitaxial Growth of ZnMgSSe on GaAs Substrate by Molecular Beam Epitaxy

We propose a new material, ZnMgSSe, as the cladding layer of a blue-light laser diode. Band-gap energy can be varied from 2.8 to near 4 eV, maintaining lattice-matching to a (100)GaAs substrate. The band-gap energies of MgS and MgSe (zincblende structure) are estimated to be about 4.5 eV and 3.6 eV, and the lattice constants are 5.62 ? and 5.89 ?, respectively. The refractive index of ZnMgSSe lattice-matched to GaAs is smaller than that of ZnSSe lattice-matched to GaAs. ZnMgSSe meets the requirements of the cladding layer of ZnSSe for fabricating the blue-light laser diode.

Na-induced variations in the structural, optical, and electrical properties of Cu(In,Ga)Se2 thin films

The systematic variations in the structural, optical, and electrical properties of polycrystalline Cu(In,Ga)Se2 (CIGS) thin films with Na doping level were investigated. Precise control of the Na concentration in CIGS films was demonstrated using alkali-silicate glass thin layers of various thicknesses deposited on substrates prior to CIGS growth. The CIGS grain size was observed to decrease with increasing Na concentration, although the surface morphology became smoother and exhibited a stronger (112) texture, which has been demonstrated consequence of larger grain size. The Ga composition gradient in the CIGS films was found to become large due to the presence of Na during growth, which in turn led to a decrease in the nominal band gap energy. Variations in the photoluminescence spectra and electrical properties suggested that the formation of an acceptor energy state, which may originate from OSe point defects, was enhanced in the presence of Na. This result suggests that not only Na, but also the pres...

Nitrogen Doping into Cu2O Thin Films Deposited by Reactive Radio-Frequency Magnetron Sputtering

The effects of nitrogen doping into Cu2O thin films deposited by reactive radio-frequency magnetron sputtering were studied. It was found that nitrogen is an effective p-type dopant for Cu2O and the hole density can be controlled from 1×1015 cm-3 to approximately 1017 cm-3. The acceptor level of nitrogen was estimated to be about 0.14 eV by temperature-dependent Hall effect measurements and this value roughly agrees with that obtained by the effective mass theory. No significant degradation of structural and optical properties induced by nitrogen doping were observed. The resistivity of 15.2 Ωcm was obtained for a relatively high nitrogen flow rate, which is the lowest value reported to date for Cu2O thin films.

Thin-Film Deposition of Cu2O by Reactive Radio-Frequency Magnetron Sputtering

Deposition conditions of cuprous oxide (Cu2O) thin films on glass substrates by reactive radio-frequency (rf) magnetron sputtering method were studied. The substrate temperature was found to be important for obtaining high-quality films, and the optimum substrate temperature was about 500°C. The Cu2O deposited at 500°C shows a band-gap energy of about 2.0 eV and a typical hole concentration of the order of 1015 cm-3 with a Hall mobility of 60 cm2/Vs, which is the highest mobility reported thus far.

EPITAXIAL GROWTH OF P-TYPE ZNMGSSE

N‐doped p‐type ZnMgSSe was grown by molecular beam epitaxy. Nitrogen was induced by electron cyclotron resonance plasma. The maximum net acceptor concentration (NA−ND) and the activation energy of the nitrogen acceptor (EN) depend on the band‐gap energy of ZnMgSSe. With increasing band‐gap energy, the maximum NA‐ND is decreased and EN is increased. The maximum NA‐ND and the EN of ZnMgSSe with a band‐gap energy of 3.05 eV at 77 K are 2.5×1016 cm−3 and 140 meV, respectively.

Electroluminescence in an Oxygen-Doped ZnSe p-n Junction Grown by Molecular Beam Epitaxy

Blue electroluminescence has been obtained from ZnSe p-n junctions. ZnSe films were grown on n-type GaAs(100) substrates by molecular beam epitaxy. The dopants used for n-and p-type ZnSe were Ga and O, respectively. The electron-beam-induced current strongly suggests the formation of a p-n junction. The built-in Potential of the p-n junction and carrier concentration of p-type ZnSe layer estimated from the capacitance-voltage relation were about 2.3 V and 1.2×1016cm-3, respectively. The electroluminescence spectra from the p-n junction were dominated by band-edge emissions of 466 nm at room temperature and 446 nm at 77 K.

Concentration quenching of Eu-related luminescence in Eu-doped GaN

The dependence of Europium (Eu)-related luminescence intensity on the Eu concentration in Eu-doped GaN was studied. This luminescence is observed at 622nm and originates from the intra-4f transition of the Eu3+ ion. The intensity of the luminescence increased with increasing Eu concentration, up to about 2at.%, and then abruptly decreased. It was found that polycrystalline growth began to be induced at Eu concentrations of more than 2at.%. In addition, clear evidence for the formation of EuN compounds was obtained by x-ray diffraction and extended x-ray absorption fine structure analysis. The cause of the concentration quenching is likely to be related to the polycrystalline growth as well as EuN formation.

Defects in Cu2O studied by deep level transient spectroscopy

Hole traps in p-type Cu2O were studied by means of deep level transient spectroscopy in the heterostructure of p-Cu2O∕i-ZnO∕n-ZnO. In addition to the trap level at about 0.45eV from the valance band edge, which is already reported as being due to Cu vacancy, we found a new trap level at about 0.25eV. The new trap is tentatively assigned as Cu-di-vacancy from the trap concentration dependence on oxygen flow rate and substrate temperature.

Control of hole carrier density of polycrystalline Cu2O thin films by Si doping

The effects on the electrical properties of Si doping into Cu2O thin films deposited by reactive sputtering were studied. The hole density increased from 1×1015 to 1×1017 cm−3 with increasing Si content and the minimum resistivity obtained was 12 Ω cm. It was suggested that the electrically active acceptor with an activation energy of 0.19 eV was generated by Si doping. Infrared absorption measurements indicated the formation of silicate in Si-doped Cu2O. The mechanism for Si acting as an acceptor in Cu2O is discussed and modeled based on the silicate formation in Cu2O.

Influence of gap states on electrical properties at interface between bathocuproine and various types of metals

A systematic study on the energy level alignment, chemical interaction, and electron doping at interfaces between bathocuproine (BCP) and various types of metals (Au, Cu, Ag, Mg, and Ca) was carried out by performing ultraviolet photoelectron spectroscopy and electronic conductivity measurements. The energy level alignment at BCP/metal interfaces was found to depend on the metal work function (Φm). For BCP on Au and Cu, whose Φm exceeds 4.3 eV, the energy shift in the highest occupied molecular orbital (HOMO) level with respect to the metal Fermi level (EF) almost accords with the variation in Φm. For BCP on Ag, Mg, and Ca, whose Φm is below 4.3 eV, the HOMO energy level is fixed at 3.7 eV with respect to EF regardless of Φm and new electronic states, called gap states, appeared at BCP/metal interfaces. Since the appearance of gap states is correlated with the energy of the lowest unoccupied molecular orbital (LUMO) level with respect to EF, these states appear to have formed mainly through the interactio...