Wenlei Yu

Structural, electronic band transition and optoelectronic properties of delafossite CuGa1−xCrxO2 (0 ≤ x ≤ 1) solid solution films grown by the sol–gel method

Pure phase CuGa1−xCrxO2 (0 ≤ x ≤ 1) films were prepared on (001) sapphire substrates by the sol–gel method. The structure, vibration modes, and compositions of the films were analyzed by X-ray diffraction, scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. It was found that the Cr-substituting induced the increase of the film’s roughness, changed the film’s internal structure, and made more crystal defects and grain boundaries. Due to the interatomic potential becoming weaker between Cu and O atoms with increasing the Cu–O bond length, the peak positions of the A1g and Ag phonon modes shifted toward a lower frequency with increasing x. The optical transmittance of the films approached about 60–80% in the visible region and the values of the direct band gap linearly decrease from 3.56 to 3.09 eV with increasing x. The Cr-introduction effects on the electronic band transition have been investigated in detail. The new energy state located at 0.17 eV above the top of the valence band is observed in the CuGa0.8Cr0.2O2 film, which can be derived from the defect energy level. It can induce the increment of the hole in the valence band, contribute to the electrical conductivity, and lower the thermal activation energy. Moreover, the CuGa0.8Cr0.2O2 film is found to be of the larger electrical conductivity of 0.071 S cm−1 at room temperature, which shows the promising application values, as compared to other CuGa1−xCrxO2 films.

Photoluminescence and low-threshold lasing of ZnO nanorod arrays

We report on the photoluminescence (PL) and lasing characteristics of ZnO nanorod arrays (NRAs) fabricated by hydrothermal process on nanocrystalline ZnO seeded Si and post-growth annealing. The morphology of the ZnO NRAs was examined by field emission scanning electron microscopy and the structure was characterized by x-ray diffraction, Fourier-transform infrared and Raman scattering spectroscopy. The properties of light emission were studied by continuous wave (CW) and 30 ps pulsed ultraviolet excitation. The ZnO NRAs consist of aligned nanorods and are nanocrystalline with wurtzite structure and c-axis orientation. At room temperature, the ZnO NRAs are capable of emitting strong CW PL and pulsed stimulated emission, with the latter showing obvious lasing characteristics. The threshold for lasing was observed to be ~16 kW/cm(2).

Diversity of electronic transitions and photoluminescence properties in nanocrystalline Mn/Fe-doped tin dioxide semiconductor films: An effect from oxygen pressure

Transition metal (TM: Mn or Fe) doped tin dioxide (SnO2) films with the compositions of 5% (Sn0.95TM0.05O2) have been deposited on sapphire substrates by pulsed laser deposition under oxygen pressure (Po) varied from 10−4 to 1 Pa. The x-ray diffraction, scanning electron microscopy, and infrared spectra analysis show that different TM dopants can affect the variations of crystallization and lattice distortion. Moreover, x-ray photoelectron spectroscopies indicate that the effective Po during the growth does not change the valence state of Sn4+ in the Sn0.95TM0.05O2 films. The spectral behaviors of the films have been investigated in the photon energy range of 0.47-6.5 eV (2650-190 nm). From transmittance spectra, the shoulder structures become more prominent for the Sn0.95Fe0.05O2 film than those for the Sn0.95Mn0.05O2 film due to the Fe repelling effect of a stronger p-d hybridization. The refractive index values for the Sn0.95Mn0.05O2 film are found to be larger than those for the Sn0.95Fe0.05O2 film at the photon energy of 0.47 eV. The main peaks at about 1.9 and 2.2 eV in photoluminescence (PL) emission spectra for both Sn0.95Mn0.05O2 and Sn0.95Fe0.05O2 films can be observed, and it could be explained by the fact that the electrons in the conduction band of SnO2 relax to defect states and then radiatively recombine with the holes. From direct comparison of PL and transmittance results for the films, the electronic transition energies, the emission peaks’ intensities and positions are shown to present the Po dependent behavior. The distinct trends indicate that the incorporation of Mn and Fe elements can provide a significant difference in the crystalline and electronic band structure. It can be concluded that the oxygen pressure and dopant contributions are responsible for the adjustment of electronic band structures and result in different optical response behaviors for the Sn0.95TM0.05O2 films.

Infrared and raman spectroscopic studies of optically transparent zirconia (ZrO2) films deposited by plasma-assisted reactive pulsed laser deposition.

Plasma-assisted pulsed laser deposited zirconia (ZrO2) films were studied by Fourier transform infrared (FT-IR) and Raman spectroscopy for structural characterization and thermal stability in combination with optical characterization by spectroscopic ellipsometry and optical transmission measurements. Only the monoclinic ZrO2 phase was positively identified from the infrared and Raman spectra of the as-deposited ZrO2 films, which show excellent optical transparency from the ultraviolet to the near infrared as revealed by optical characterization. The as-deposited ZrO2 films are free of any SiOx interfacial layer when deposited on silicon. The prepared ZrO2 films exhibit good thermal stability in their structural, optical, and interfacial properties up to 900 °C. Upon annealing above 1100 °C, a silicon oxide interfacial layer forms due to the oxidation of the silicon substrate surface by the oxygen diffused from the oxide film to the silicon substrate at high temperatures.

Transparent polycrystalline monoclinic HfO2 dielectrics prepared by plasma assisted pulsed laser deposition

The electrical properties of transparent polycrystalline monoclinic HfO2 dielectrics prepared by plasma assisted pulsed laser deposition were studied. The capacitance-voltage and leakage current-voltage characteristics of the capacitors incorporating HfO2 dielectrics were examined in terms of the structural, optical properties of the HfO2 layers. The interfacial properties between the HfO2 layer and the Si substrate were also examined. The HfO2 layers showed excellent thermal stability both in the HfO2 structure and in the HfO2/Si interface. The capacitance-voltage characteristics showed improvements through thermal annealing with a slight increase of leakage current. With an equivalent oxide thickness of 4.7 nm, the 700 °C annealed HfO2 dielectrics had a dielectric constant of 16.5 and leakage current densities of 9.8 × 10−8 and 9.2 × 10−7 A/cm2 at dielectric fields of +0.75 and −0.75 MV/cm, respectively.

Temperature dependence of optical properties in Sn0.925Mn0.075O2 film determined by transmittance spectra

Near-infrared-ultraviolet optical properties of Sn0.925Mn0.075O2 (SMO) film grown on c-plane sapphire substrate have been investigated by the transmittance spectra in the photon energy of 0.45-6.5 eV (190-2650 nm) from 5.3-300 K. The optical constants have been extracted by fitting the experimental data with the Adachis model. The optical band gap of the film can be estimated from the relation (αE)2proportional to(hυ-Eg). It is found that the absorption edge shifts to a lower energy side with increasing the temperature and the band gap Eg decreases from 3.80 to 3.72 eV. The parameters αB and θB of the Bose-Einstein model are 45.4 meV and 221.8 K, respectively, which could be ascribed to the thermal expansion of crystal lattice and the carrier-phonon interaction. The band narrowing coefficient dEg/dT of the SMO film is estimated to be -3.92x10-4 eV/K at room temperature.