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We have grown Yb-doped ZnO (ZnO:Yb) on c-plane sapphire substrates by sputtering-assisted metal-organic chemical vapor deposition (SA-MOCVD). X-ray diffraction (XRD) revealed that these ZnO:Yb films exhibit a c-axis orientation and that the lattice constant increases with Yb concentration. After annealing the samples at 600 °C for 0.5 h in O2, they showed a clear 980 nm emission due to the intra-4f shell transitions of 2F5/2–2F7/2in Yb3+ ions. It turns out that Yb lu-minescent centers with different local structures coexist, which exhibit different energy transfer processes from the ZnO host. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Al-doped β-FeSi2 [β-Fe(Si1-xAlx)2] thin films with a thickness of 100 nm were epitaxially grown on Si(111) substrate by reactive deposition epitaxy (RDE). The reflective high-energy electron diffraction (RHEED) images showed epitaxial growth of β-Fe(Si1-xAlx)2 (101)(110)//Si(111) in the composition range of 0 < x < 2.3%. In high-resolution X-ray diffraction (HRXRD) measurements, the β(800) and β(406)(460) peaks shift to lower diffraction angles. The peak shifts linearly increase with Al composition. From an analysis of the peak shifts, Vegard's law of a, b, and c-axis in Al-doped β-FeSi2 was proven for the investigated Al-composition range. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic properties of Er-doped GaAs (GaAs:Er) and Er,O-codoped GaAs (GaAs:Er,O) have been investigated by using superconducting quantum interference devices (SQUID) magnetometer at low temperature. In M-H curves of GaAs:Er,O, a characteristic magnetization due to the codoping with Er and O was observed at 2 K, while GaAs:Er exhibited much smaller magnetization. The saturation magnetization of GaAs:Er,O slightly increased with Er concentration. These results were discussed in relation to an atomic configuration formed selectively around Er3+ ions in GaAs:Er,O. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The electrical transport properties of β-FeSi2 epitaxial films grown on Si(001) or Si(111) substrates were investigated to clarify the growth-orientation dependence of the electrical conduction mechanism in the β-FeSi2 epitaxial films with low electron density of ∼1016 cm-3. As temperature was decreased, the transition from band conduction to hopping conduction was observed in both the epitaxial films. The β-FeSi2 on Si(111) film showed the transition at higher temperature than the β-FeSi2 on Si(001). When the epitaxial films were annealed, the transition temperature shifted to lower temperature. The β-FeSi2 on Si(001) film annealed at 800oC showed the largest electron mobility of 14800 cm2/V·s at 70 K. These results were understood by the conduction through the defect levels in the β-FeSi2 epitaxial films. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence (PL) properties of Eu-doped ZnO (ZnO:Eu) grown by a sputtering-assisted metalorganic chemical vapor deposition technique were investigated. In PL measurements under the high-energy excitation above the band-gap energy of ZnO (indirect-excitation) at room temperature, the annealed sample showed clear red-emission lines due to the intra-4f shell transition of 5D07F2 in Eu3+. Under the low-energy excitation resonant to the energy levels of 7F0–5D3 transitions (direct-excitation), additional PL lines which showed fast thermal quenching were observed. These results revealed that the energy transfer from ZnO host to Eu3+ was accompanied, but there were inactive Eu3+ ions under the indirect-excitation. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We investigated the effect of growth temperature on the luminescence properties of Eu-doped GaN layers grown by organometallic vapor phase epitaxy (OMVPE). The dominant photoluminescence (PL) peak intensity at 621 nm, due to the intra-4f shell transitions of 5D0-7F2 in Eu3+ ions, became the highest when the sample was grown at 1000 °C. We consider three reasons for this temperature dependence. Firstly, Eu concentration became the highest when the sample was grown at 1000 °C. Secondly, Eu-doped GaN layer grown at 1000 °C had the local structure of Eu3+ ions, which emitted at 621 nm preferentially. Thirdly, the density of screw and mixed dislocations decreased with increasing growth temperature. These results indicated that an appropriate growth temperature exists for strong Eu-related luminescence from Eu-doped GaN layer grown by OMVPE. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Transition-metal (TM) doped zinc oxide (ZnO) films were grown by a developed plasma-enhanced chemical vapor deposition (PECVD) technique combined with RF sputtering. In the developed system, TMs such as Fe, Cr and Ni were doped into the ZnO films by RF sputtering with a stainless-steel electrode connected to a RF generator. X-ray diffraction (XRD) measurements revealed that the TM-doped ZnO films were successfully grown with c-axis orientation at 400 °C with RF power of 25 W. The doped TMs were segregated to the surface when the films were annealed at 700 °C in O2 ambient. After higher-temperature annealing at 800 °C, ZnO nanostructures with wire and tube shapes were formed by a catalytic influence of the segregated TMs. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have fabricated GaInP/Er,O-codoped GaAs (GaAs:Er,O)/GaInP double heterostructure (DH) laser diodes (LDs) by organometallic vapor phase epitaxy (OMVPE) and investigated their lasing characteristics at GaAs band-edge. Laser emission at the GaAs band-edge was observed from the DH LDs at room temperature. The threshold current density (Jth) increased by doping of Er. The slope of the Jth against the reciprocal cavity length (1/L) also increased by the Er doping, indicating reduced relaxation time of injected carriers in GaAs:Er,O. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Er-related photoluminescence (PL) properties have been investigated in Er,O-codoped GaAs (GaAs:Er,O) grown by organometallic vapor phase epitaxy. The GaAs:Er,O which was slightly doped with Er exhibited both strong Er-related and band-edge luminescence. In the temperature dependence of the Er-related PL intensity, the intensity decreased with increasing temperature from 4.2 K to 30 K. The temperature region was quite coincident with the region where the Carbon-related PL intensity decreased. This behaviour implies the existence of a Carbon-related nonradiative process in GaAs. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Ultrafast dynamics of photoexcited carriers in Er,O-codoped GaAs (GaAs:Er,O) and Er-doped GaAs (GaAs:Er) have been investigated by means of pump and probe transmission spectroscopy, and discussed in relation to Er-related photoluminescence (PL) intensity. Time-resolved transmission signal in undoped GaAs showed a temporal increase due to saturation of optical absorption, and then decreased reflecting the recovery of the absorption. The decrease consisted of two components corresponding to intra-band relaxation ( 50 ps). In GaAs:Er,O and GaAs:Er, on the other hand, an additional component was observed. The relaxation time was 4 ∼ 11 ps. This relaxation time, assigned to carrier trapping time by an Er-related trapping, became short with increasing Er-related PL intensity. These results suggest the fast carrier capturing plays an important role for the enhancement of the Er-related luminescence. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of Al-doping on the iron silicide growth were investigated in reactive deposition epitaxy (RDE) on Si(111) substrates. e-FeSi(111) layer was grown on the Si(111) substrate when the Fe of 50 A was deposited at 750 °C. In Al-doped samples grown by co-deposition of Fe and Al, both the crystal orientation of e-FeSi(111)//Si(111) and the surface flatness of the layer were improved with increasing Kundsen-cell temperature of Al (TAl). The highly-oriented e-FeSi templates were annealed at 850 °C for a phase transition from e-FeSi to β-FeSi2. X-ray diffraction (XRD) measurements of the annealed samples showed that the crystal orientation of β-FeSi2(202)(220)//Si(111) strongly depended on that of the e-FeSi//Si. In undoped samples, XRD peaks originating from polycrystalline β-FeSi2 were observed in 2θ scan measurements. On the other hand, the peak intensities from the β-FeSi2 polycrystal became much small in the Al-doped β-FeSi2 grown at TAl = 925 °C. These results revealed that Al-doping during the RDE growth promotes the epitaxial growth of e-FeSi(111)//Si(111) and the crystallographic orientation relationship between e-FeSi and Si affects the epitaxial growth of β-FeSi2(101)(110) on Si(111) by the post-annealing. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)