Observation of Tunneling Gap in Epitaxial Ultrathin Films of Pyrite-Type Copper Disulfide

We report scanning tunneling microscopy investigation on epitaxial ultrathin films of pyrite-type copper disulfide.Layer-by-layer growth of CuS_（2 ）films with a preferential orientation of（111）on SrTiO_3（001）and Bi_2Sr_2Ca Cu_2O_（8＋）substrates is achieved by molecular beam epitaxy growth.For ultrathin films on both kinds of substrates,we observe symmetric tunneling gap around the Fermi level that persists up to^15 K.The tunneling gap degrades with either increasing temperature or increasing thickness,suggesting new matter states at the extreme twodimensional limit.

Gap States of ZnO Thin Films by New Methods:Optical Spectroscopy,Optical Conductivity and Optical Dispersion Energy

The optical reflectance and transmittance spectra in the wavelength range of 300-2500 nm are used to compute the absorption coefficient of zinc oxide films annealed at different post-annealing temperatures 400, 500 and 600°C.The values of the cross point between the curves of the real and imaginary parts of the optical conductivity ɑ_1 and ɑ_1 with energy axis of films exhibit values that correspond to optical gaps and are about 3.25-3.3 eV. The maxima of peaks in plots dR/dλ and dT/dλ versus wavelength of films exhibit optical gaps at about 3.12-3.25 eV.The values of the fundamental indirect band gap obtained from the Tauc model are at about 3.14-3.2 eV. It can be seen that films annealed at 600°C have the minimum indirect optical band gap at about 3.15 eV. The films annealed at 600°C have Urbach＇s energy minimum of 1.38 eV and hence have minimum disorder. The dispersion energy d of films annealed at 500°C has the minimum value of 43 eV.

Band gap and structure characterization of Tm_2O_3 films

Single crystalline Tm2O3 films were grown on Si （001） substrates by molecular beam epitaxy using metallic Tm source and atomic oxygen source. X-ray photoelectron spectroscopy, atomic force microscopy and high-resolution transmission electron microscopy were employed to investigate the compositions, surface morphology and microstructure of the sample. A very flat surface with a root mean square roughness of 0.3 nm could be reached, and a sharp interface between the film and the Si substrate was achieved. The result of optical spectrum at ultraviolet and visible wavelengths showed that the band gap of the Tm2O3 film was 5.76 eV.

The calculation of band gap energy in zinc oxide films

We investigated the optical properties of undoped zinc oxide thin films as the n-type semiconductor;the thin films were deposited at different precursor molarities by ultrasonic spray and spray pyrolysis techniques.The thin films were deposited at different substrate temperatures ranging between 200 and 500℃. In this paper, we present a new approach to control the optical gap energy of ZnO thin films by concentration of the ZnO solution and substrate temperatures from experimental data, which were published in international journals. The model proposed to calculate the band gap energy with the Urbach energy was investigated. The relation between the experimental data and theoretical calculation suggests that the band gap energies are predominantly estimated by the Urbach energies, film transparency, and concentration of the ZnO solution and substrate temperatures. The measurements by these proposal models are in qualitative agreements with the experimental data; the correlation coefficient values were varied in the range 0.96–0.99999, indicating high quality representation of data based on Equation（2）, so that the relative errors of all calculation are smaller than 4%. Thus, one can suppose that the undoped ZnO thin films are chemically purer and have many fewer defects and less disorder owing to an almost complete chemical decomposition and contained higher optical band gap energy.

Spray pyrolysis of tin selenide thin-film semiconductors:the effect of selenium concentration on the properties of the thin films

Thin films of tin selenide（Sn_xSe_y） with an atomic ratio of r =[y/x]= 0.5,1 and 1.5 were prepared on a glass substrate at T = 470℃using a spray pyrolysis technique.The initial materials for the preparation of the thin films were an alcoholic solution consisting of tin chloride（SnCl_4·5H_2O） and selenide acide（H_2SeO_3）.The prepared thin films were characterized by X-ray diffraction（XRD）,scanning electron microscopy,scanning tunneling microscopy,scanning helium ion microscopy,and UV-vis spectroscopy.The photoconductivity and thermoelectric effects of the Sn_xSe_ythin films were then studied.The Sn_xSe_y thin films had a polycrystalline structure with an almost uniform surface and cluster type growth.The increasing atomic ratio of r in the films,the optical gap,photosensitivity and Seebeck coefficient were changed from 1.6 to 1.37 eV,0.01 to 0.31 and -26.2 to—42.7 mV/K （at T = 350 K）,respectively.In addition,the XRD patterns indicated intensity peaks in r = 1 that corresponded to the increase in the SnSe and SnSe_2 phases.