Tomoaki Terasako

High electron mobility and low carrier concentration of hydrothermally grown ZnO thin films on seeded a-plane sapphire at low temperature

Hydrothermal zinc oxide (ZnO) thick films were successfully grown on the chemical vapor deposition (CVD)-grown thick ZnO seed layers on a-plane sapphire substrates using the aqueous solution of zinc nitrate dehydrate (Zn(NO3)2). The use of the CVD ZnO seed layers with the flat surfaces seems to be a key technique for obtaining thick films instead of vertically aligned nanostructures as reported in many literatures. All the hydrothermal ZnO layers showed the large grains with hexagonal end facets and were highly oriented towards the c-axis direction. Photoluminescence (PL) spectra of the hydrothermal layers were composed of the ultraviolet (UV) emission (370 to 380 nm) and the visible emission (481 to 491 nm), and the intensity ratio of the former emission (IUV) to the latter emission (IVIS) changed, depending on both the molarity of the solution and temperature. It is surprising that all the Hall mobilities for the hydrothermal ZnO layers were significantly larger than those for their corresponding CVD seed films. It was also found that, for the hydrothermal films grown at 70°C to 90°C, the molarity dependences of IUV/IVIS resembled those of mobilities, implying that the mobility in the film is affected by the structural defects. The highest mobility of 166 cm2/Vs was achieved on the hydrothermal film with the carrier concentration of 1.65 × 1017 cm−3 grown from the aqueous solution of 40 mM at 70°C.

ZnO Nanowires Grown by Atmospheric Pressure Chemical Vapor Deposition Using ZnCl2 and H2O as Source Materials and Their Growth Mechanisms

Single crystalline ZnO nanowires (NWs) with diameters ranging from 80 to 700 nm were successfully grown on Ni-coated SiO2/Si(100) substrates by atmospheric pressure chemical vapor deposition (CVD) using ZnCl2 and H2O as source materials. Scanning electron microscope (SEM) and transmission electron microscope (TEM) observations revealed that the ZnO NWs grown by two types of CVD systems with different source feeding configurations exhibited different morphologies and growth directions, reflecting the difference in growth mechanism, i.e., vapor–liquid–solid (VLS) growth and vapor–solid (VS) growth. Photoluminescence spectra of the ZnO NWs exhibited a dominant near-band-edge (NBE) emission, indicating their high crystalline quality.

Photoluminescence from highly oriented MgxZn1-xO films grown by chemical spray pyrolysis

Highly c-axis oriented polycrystalline MgxZn1−xO (x<0.20) thin films were prepared by chemical spray pyrolysis. It is found that the optical gap energy of the zinc oxide (ZnO) films is affected not only by the Burstein–Moss band-filling effect, but also by the bandgap narrowing effects such as electron–electron and electron–impurity interactions. The influence of the highly introduced donors and free-electrons is also confirmed on photoluminescence spectra of ZnO films via the broadening of the bound exciton line and the appearance of the non-k-conserving band-to-band emission. The MgxZn1−xO (x<0.20) films exhibited a broad unstructured near-band-edge (NBE) emission shifting continuously towards higher energy side with increasing composition x. The NBE emission is ascribed to radiative transitions from tail states of the conduction band to those of the valence band.

Decay Characteristics of Photoluminescence Lines in CuGaS 2 Crystals

Time-resolved photoluminescence (PL) measurements have been carried out for CuGaS2 single crystals with a slightly Cu-rich composition grown by an iodine transport method. PL lines at 2.460 eV, 2.457 eV, 2.417 eV, 2.404 eV, 2.397 eV and 2.295 eV exhibited biexponential decay. The relationship between the decay time of the fast component (τf) and the localization energy (Eloc) can be given by τf∝Eloc1.25. On the other hand, the decay times of the slow components (τs) have been classified into two groups having different trends τs∝Eloc2.7 and Eloc0.5. The difference between the two groups is discussed in terms of native defects and the iodine impurity included in the luminescence centers.

Vapor-Phase Atomic Layer Epitaxy of CuGaS2 at Atmospheric Pressure Using Metal Chlorides and H2S

In the alternate feeding of metal chlorides (CuCl, GaCl3) and H2S sources, the growth rate saturation of CuGaS2 on GaP was observed for substrate temperatures between 530° C and 570° C, and also for some range of H2S flow rate. Together with the growth rate saturation behavior for metal chlorides reported in our previous paper, these results can be considered to indicate achievement of atomic layer epitaxy (ALE) of CuGaS2. The growth orientation change leading to c-axis growth was observed for a large H2S flow rate. Some possibility of stoichiometry control by ALE was suggested from photoluminescence spectra.

Growth of ZnO films on R-plane sapphire substrates by atmospheric-pressure chemical vapor deposition using Zn powder and H2O as source materials

Films of ZnO have been grown on R-plane sapphire substrates by atmospheric-pressure chemical vapor deposition (AP-CVD) using Zn powder and H2O as source materials. Variation in growth rate as a function of substrate temperature can be divided into three regions: re-evaporation region, mass transport controlled region, and surface controlled region. Scanning electron microscope observations revealed that the surface morphology of the film is dependent not only on substrate temperature but also on Zn source and H2O source temperatures. X-ray diffraction patterns of most of the films showed a dominant (110) peak, indicating highly A-axis oriented growth. A single crystalline A-plane ZnO film without mixed domain was successfully grown by optimizing the growth condition. Visible region photoluminescence changed from orange-red emission to blue-green emission systematically by adjusting Zn source and H2O source temperatures. This result suggests the possibility of defect control in our AP-CVD system. For the f...

PHOTOACOUSTIC SPECTRA OF CUINSE2 THIN FILMS PREPARED BY CHEMICAL SPRAY PYROLYSIS

Photoacoustic (PA) spectra of CuInSe2 thin films prepared by chemical spray pyrolysis have been measured and compared with the Raman and X-ray diffraction results. Stoichiometric films having a sphalerite structure exhibit a PA band at ~0.95 eV, which is considered to be due to the electronic transition from copper atom substituting indium site (CuIn) acceptor to indium atom substituting copper site (InCu) donor. In-rich films having the chalcopyrite structure with the In2Se3 second phase exhibit PA bands at ~0.90 eV and ~0.83 eV. The former PA band is considered to be due to the electronic transition from copper vacancy (VCu) acceptor to indium atom substituting selenium site (InSe) donor. The latter PA band is considered to be related to the defect complex involving InCu. For Cu-rich films, a PA signal below the absorption edge increases with increasing Cu/In ratio. This behavior is attributed to the increase in the content of the Cu-Se compound.

Raman Scattering and Two-Phonon Infrared Transmission Spectra of Cu(AlxGa1-x)S2 Crystals.

First- and second-order Raman scattering and two-phonon infrared (IR) transmission measurement results for Cu(Alx Ga1-x )S2 single crystals have been examined as a function of x. It is apparent that the highest frequency B2 and E modes at the zone center exhibit two-mode(persistence)-type behavior. The appearance of the two-mode-type behavior in a ternary alloy system is shown to be explainable by theoretical consideration based on a modified model of the type judging criterion. In addition, some of the two-phonon IR absorption bands and second-order Raman peaks are found to be due to phonons located at zone-boundary points. Compositional dependence of the A1(W1) mode Raman line shape and the appearance of a cross-coupled phonon between the two modes indicate that the clustering effect is essentially suppressed in this alloy system.

Lattice dynamics of CuAlS2 and CuAlSe2

Abstract We have measured the first-order Raman spectra of CuAlSe 2 and the second-order Raman spectra of CuAlS 2 and CuAlSe 2 , and reexamined two-phonon absorption spectra of CuAlS 2 . Lattice dynamical calculations have been performed using a rigid ion model. Force constants and charges as fitting parameters have been determined so as to reproduce zone-center phonon frequencies and the two-phonon spectra observed. Calculated two-phonon density of states shows reasonable agreements with those spectra.

Various Shapes of ZnO and CdO Nanostructures Grown by Atmospheric-Pressure Chemical Vapor Deposition

Various shapes of ZnO and CdO nanostructures were successfully grown on a- and c-plane sapphire substrates coated with Au nanocolloidal solution by atmospheric-pressure chemical vapor deposition methods under a simultaneous source supply of metal powder (Zn or Cd) and H2O. The ZnO and CdO nanorods (NRs) grown at higher substrate temperatures (TSs) exhibited tapered shapes, resulting from the competition between the axial growth due to the vapor–liquid–solid (VLS) mechanism and the radial growth due to the vapor–solid (VS) mechanism. The alternate source supply of Zn and H2O was found to be effective for suppressing the tapering of ZnO NRs. The appearance of the Y- and T-shaped nanotrees of CdO may be due to the splitting and migration of catalytic particles during the growth process. These results suggest that both the source supply sequence and the substrate temperature are important factors for the shape design of oxide nanostructures.