Annealing study of electrodeposited CuInSe<Subscript>2</Subscript> and CuInS<Subscript>2</Subscript> thin films

In this study CuInSe₂ and CuInS₂ thin films were prepared onto ITO glass substrate using the electrodeposition technique in aqueous solution. The electrodeposited films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis. The annealing effects on electrodeposited precursors were investigated. The chalcopyrite structure of CuInSe₂/CuInS₂ showed an enhancement of crystallinity after subsequent selenization/sulfurization treatment in Se/S atmosphere, respectively. XRD and SEM studies revealed a dramatic improvement of the crystalline quality of CIS films after annealing treatments. Mott–Schottky measurements were used to assess the conductivity type of the films and their carrier concentration. The prepared samples underwent an etching process to remove the binary accumulated Cu_2?x(Se,S) phases shown in FESEM pictures. This etching process has shown a noticeable decrease in both, the flat band potential, V_fb (V), and the number of acceptors, N_A (cm^?3) in selenized CuInSe₂ and sulfurized CuInS₂ samples.     

Influence of nanocrystalline structure and composition on hardness of thin films based on TiO<Subscript>2</Subscript>

In this work, the influence of Tb-doping on structure, and especially hardness of nanocrystalline TiO₂ thin films, has been described. Thin films were formed by a high-energy reactive magnetron sputtering process in a pure oxygen atmosphere. Undoped TiO₂-matrix and TiO₂:Tb (2 at. % and 2.6 at. %) thin films, had rutile structure with crystallite sizes below 10 nm. The high-energy process produces nanocrystalline, homogenous films with a dense and close packed structure, that were confirmed by X-ray diffraction patterns and micrographs from a scanning electron microscope. Investigation of thin film hardness was performed with the aid of a nanoindentation technique. Results of measurements have shown that the hardness of all manufactured nanocrystalline films is above 10 GPa. In the case of undoped TiO₂ matrix, the highest hardness value was obtained (14.3 GPa), while doping with terbium results in hardness decreasing down to 12.7 GPa and 10.8 GPa for TiO₂:(2 at. % Tb) and TiO₂:(2.6 at. % Tb) thin films, respectively. Incorporation of terbium into TiO₂-matrix also allows modification of the elastic properties of the films.     

Photoelectrochemical characteristics of pulse electrodeposited CuInS<Subscript>2</Subscript> films

CuInS₂ films were deposited by the single-step pulse electrodeposition technique at different duty cycles in the range of 6–50% at 80 °C using conducting glass and titanium substrates. The films exhibited a single-phase tetragonal structure. Optical transmission spectra exhibited interference fringes. A direct band gap value of 1.42 eV was obtained. The refractive index value calculated by the envelope method was 1.2. Electrical resistivity of the films was in the range of 15–33 Ω cm. Photoelectrochemical cells made with the as deposited films yielded low photo output. After post-heat treatment in argon atmosphere at different temperatures, the films yielded higher photo output compared to earlier reports.     

Single-oscillator model and determination of optical constants of spray pyrolyzed amorphous SnO<Subscript>2</Subscript> thin films

It has recently been shown that growth of a multilayer structure with one or more delta-layers at high temperature leads to spreading and asymmetrization of the dopant distribution [see, for example, E.F.J. Schubert, Vac. Sci. Technol. A. 8, 2980 (1990), A.M. Nazmul, S. Sugahara, M. Tanaka, J. Crystal Growth 251, 303 (2003); R.C. Newman, M.J. Ashwin, M.R. Fahy, L. Hart, S.N. Holmes, C. Roberts, X. Zhang, Phys. Rev. B 54, 8769 (1996); E.F. Schubert, J.M. Kuo, R.F. Kopf, H.S. Luftman, L.C. Hopkins, N.J. Sauer, J. Appl. Phys. 67, 1969 (1990); P.M. Zagwijn, J.F. van der Veen, E. Vlieg, A.H. Reader, D.J. Gravesteijn, J. Appl. Phys. 78, 4933 (1995); W.S. Hobson, S.J. Pearton, E.F. Schubert, G. Cabaniss, Appl. Phys. Lett. 55, 1546 (1989); Delta Doping of Semiconductors, edited by E.F. Schubert (Cambridge University Press, Cambridge, 1996); Yu.N. Drozdov, N.B. Baidus', B.N. Zvonkov, M.N. Drozdov, O.I. Khrykin, V.I. Shashkin, Semiconductors 37, 194 (2003); E. Skuras, A.R. Long, B. Vogele, M.C. Holland, C.R. Stanley, E.A. Johnson, M. van der Burgt, H. Yaguchi, J. Singleton, Phys. Rev. B 59, 10712 (1999); G. Li, C. Jagadish, Solid-State Electronics 41, 1207 (1997)]. In this work analytical and numerical analysis of dopant dynamics in a delta-doped area of a multilayer structure has been accomplished using Fick's second law. Some reasons for asymmetrization of a delta-dopant distribution are illustrated. The spreading of a delta-layer has been estimated using example materials of a multilayer structure, a delta-layer and an overlayer.     

Methane sensor based on SnO<Subscript>2</Subscript>/In<Subscript>2</Subscript>O<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> nanostructure

Two sets of samples of SnO₂/In₂O₃/TiO₂ system have been fabricated with different concentrations of component materials. In the first set TiO₂ with rutile structure was used, while in the second set it has the structure of anatase. Thin films (up to 50 nm) of obtained mixtures were deposited. Their sensitivity and selectivity with respect to methane (CH4) were studied. Nanostructure on the basis of 70%SnO₂ — 10%In₂O₃ — 20%TiO₂(anatase) exhibits sufficient sensitivity to methane.     

The study of optical properties of amorphous thin films of mixed oxides In<Subscript>2</Subscript>O<Subscript>3</Subscript>—SnO<Subscript>2</Subscript> system,deposited by co-evaporation

A discussion of optical properties of mixed oxides In₂O₃—SnO₂ system is presented. Film thickness, substrate temperature, composition (in molar %) and annealing have a profound effect on the structure and optical properties of these films. Initially the increase in band gap with the increase of SnO₂ content in In₂O₃ is due to the increase in carrier density as a result of donor electrons from tin. The decrease in band gap above the critical Sn content is caused by the defects formed by Sn atoms, which act as carrier traps rather than electron donors. The increase in band gap with film thickness is caused by the increase in free carrier density which is generated by (i) Sn atom substitution of In atom, giving out one extra electron and (ii) oxygen vacancy acting as two electrons donor. The decrease in band gap with substrate temperature and annealing is due either to the severe deficiency of oxygen, which deteriorate the film properties and reduce the mobility of the carriers, or to the formation of indium species of lower oxidation state (In²⁺).     

Volume of precursor solution effect on the properties of SnO<Subscript>2</Subscript> thin films prepared by nebulized spray pyrolysis technique

Undoped SnO₂ thin films have been deposited on amorphous glass substrates with different precursor solution volume (10, 15, 20 and 25 ml) using simple and cost-effective nebulized spray pyrolysis technique. The influence of precursor solution on structural, optical, photoluminescence and electrical properties had been studied. The X-ray diffraction spectra prove the polycrystalline nature of SnO₂ with tetragonal structure. All the films show a preferred growth orientation along (110) diffraction plane. The average transmittance of SnO₂ thin films varied between 82 and 75% in the visible as well as IR region. The band gap energy decreases from 3.74 to 3.64 eV corresponding to direct transitions with the precursor solution volume had increased from 10 to 20 ml and then increased as 3.72 eV for 25 ml. SEM pictures demonstrated polyhedrons like grains. EDX confirmed the existence of Sn and O elements in all the prepared SnO₂ thin films. Photoluminescence spectra at room temperature revealed that the four emission bands in all the samples such as sharp dominant peak at 361 nm with shoulder peak at 377 nm (UV region), a broad and low intensity peak at 492 nm (blue region) and 519 nm (green region). The electrical parameters were examined by Hall effect measurements, which demonstrated that the film prepared at 20 ml precursor solution volume possess minimum resistivity 2.76?×?10^?3 Ω-cm with activation energy 0.10 eV and maximum figure of merit 1.54?×?10^?2 (Ω/sq)^?1.     

Absorption spectrum of thin K<Subscript>2</Subscript>CdI<Subscript>4</Subscript> films

The absorption spectrum of thin films of a new compound, K₂CdI₄, was studied. It was established that this compound belongs to direct-bandgap dielectrics and that its low-frequency electronic and excitonic excitations are localized in CdI ₄ ^2? structural blocks of the crystal lattice. It was found that, in M₂CdI₄ compounds (M = K, Rb, Cs), the bandgap width E _g grows and the spin-orbit splitting of the valence band top decreases with increasing ionic radius of the alkali metal.     

Impedance spectroscopy analysis of Li<Subscript>2</Subscript>SnO<Subscript>3</Subscript>

In this work, Li₂SnO₃ has been synthesized by the sol–gel method using acetates of lithium and tin. Thermogravimetric analysis (TGA) has been applied to the precursor of Li₂SnO₃ to determine the suitable calcination temperature. The formation of the compound calcined at 800 °C for 9 h has been confirmed by X-ray diffraction (XRD) analysis. The Li₂SnO₃ is then pelletized and electrically characterized by using electrochemical impedance spectroscopy (EIS) in the frequency range from 50 Hz to 1 MHz. The complex impedance spectra clearly show the dominating presence of the grain boundary effect on electrical properties whereas the complex modulus plots reveal two semicircles which are due to the grain (bulk) and grain boundary. The spectra of imaginary parts of both impedance and modulus versus frequency show the existence of peaks with the modulus plots exhibiting two peaks that are ascribed to the grain and grain boundary of the material. The peak maximum shifts to higher frequency with an increase in temperature and the broad nature of the peaks indicates the non-Debye nature of Li₂SnO₃. The activation energy associated with the dielectric relaxation obtained from the electrical impedance spectra is 0.67 eV. From the electric modulus spectra, the activation energies related to conductivity relaxation in the grain and grain boundary of Li₂SnO₃ are 0.59 and 0.69 eV, respectively. The conductivity–temperature relationship is thermally assisted and obeys the Arrhenius rule with the activation energy of 0.66 eV. The conduction mechanism of Li₂SnO₃ is via hopping.     

Semiconducting SnO<Subscript>2</Subscript>-TiO<Subscript>2</Subscript> (S-T) composites for detection of SO<Subscript>2</Subscript> gas

SnO₂-TiO₂ (S-T) composites with different molar ratios were prepared by mechanical mixing followed by sintering at 700 °C for 4 h in air. The structural and microstructural properties of the composites were investigated using powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM). S-T composites were investigated by introducing SO₂ to test their chemical stability using PXRD and SEM coupled with energy dispersive X-ray (EDX) analysis. The sensing performance was measured at different temperatures using various SO₂ concentrations (10–100 ppm). A composite comprising 25 mol% of SnO₂ and 75 mol% TiO₂ (S25-T75) exhibited the highest sensitivity comparing to other S-T composites studied under the presently investigated conditions. t ₉₀ (90 % of response time) was found to be ～5 min for thick pellet (~2 mm in thickness). SO₂ sensing mechanism has been explained through the band structure model.     

Magnetotransport in thin epitaxial Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> films

The magnetoconductivity of thin Bi₂Se₃ films covered by a protective Se layer and grown at (111) BaF₂ substrates is studied. It is shown that the negative magnetoconductivity observed at low magnetic fields and caused by the effect of weak antilocalization, as well as the Shubnikov?de Haas oscillations at higher fields, is determined only by the magnetic field component perpendicular to the film plane. The obtained experimental results can be reasonably interpreted under the assumption that the studied films exhibit two-dimensional topologically protected electron states. Moreover, the contribution of these states to the total conductivity turns out to be the dominant one.     

Low-macroscopic field emission from nanocrystalline Al doped SnO<Subscript>2</Subscript> thin films synthesized by sol–gel technique

We have observed low-macroscopic field electron emission from wide bandgap nanocrystalline Al doped SnO₂ thin films deposited on glass substrates. The emission properties have been studied for different anode-sample spacings and for different Al concentrations in the films. The turn-on field and approximate work function were calculated and we have tried to explain the emission mechanism from this. The turn-on field was found to vary in the range 5.6–7.5 V/μm for a variation of anode sample spacing from 80–120 μm. The turn-on field was also found to vary from 4.6–5.68 V/μm for a fixed anode-sample separation of 80 μm with a variation of Al concentration in the films 8.16–2.31%. The Al concentrations in the films have been measured by energy dispersive X-ray analysis. Optical transmittance measurement of the films showed a high transparency with a direct bandgap ∼3.98 eV. Due to the wide bandgap, the electron affinity of the film decreased. This, along with the nanocrystalline nature of the films, enhanced the field emission properties. PACS 81.20.Fw; 61.10.-i; 79.70.+q     

Lattice structure and magnetization of LaCoO<Subscript>3</Subscript> thin films

We investigate the structure and magnetic properties of thin films of the LaCoO₃ compound. Thin films are deposited by pulsed laser deposition on various substrates in order to tune the strain from compressive to tensile. Single-phase (001) oriented LaCoO₃ layers were grown on all substrates despite large misfits. The tetragonal distortion of the films covers a wide range from -2% to 2.8%. Our LaCoO₃ films are ferromagnetic with Curie temperature around 85 K, contrary to the bulk. The total magnetic moment is below 1μ _B /Co³⁺, a value relatively small for an exited spin-state of the Co³⁺ ions, but comparable to values reported in literature. A correlation of strain states and magnetic moment of Co³⁺ ions in LaCoO₃ thin films is observed.     

Electronic absorption spectrum of thin K<Subscript>2</Subscript>ZnI<Subscript>4</Subscript> films

The absorption spectrum of thin K₂ZnI₄ films has been investigated in the range of 3–6 eV at temperatures from 90 to 340 K. It is established that this compound belongs to direct-gap insulators; low-frequency exciton excitations are localized in the ZnI₄ lattice layers and have a quasi-two-dimensional character. The spectral analysis revealed the existence of two K₂ZnI₄ modifications, which presumably belong to the monoclinic (I) and orthorhombic (II) phases. The temperature dependences of the spectral position and halfwidth of the low-frequency exciton band in K₂ZnI₄ revealed the existence of first-order phase transitions at 215 and 225 K in the monoclinic and orthorhombic phases, respectively.     

Fabrication and optical study of Ag@SnO<Subscript>2</Subscript> core-shell structure nanoparticle thin films

Ag@SnO₂ core-shell nanoparticles dispersed in poly-(vinyl) alcohol films were fabricated on glass substrate by employing a dip-coating technique. Synthesis of Ag@SnO₂ nanoparticles with core-shell morphology is carried out by a soft-chemical technique in aqueous phase at 60°C. Formation of core-shell structure is monitored by the red-shift of the surface plasmon band of Ag nanoparticles (from 390 to 410 nm) in the UV-visible spectrum. These nanoparticles are deposited on the glass substrate. The structure and morphology of these films were investigated by X-ray diffraction technique and field-emission transmission electron microscopy, respectively. Optical properties of these pseudo-solids were studied by UV-visible spectroscopy. Surface plasmon spectrum of the core-shell nanoparticles film remained unaltered with increase in the number of layers. However, silver nanoparticles films have shown peak broadening and development of additional peaks with increase in the number of layers. Our investigations showed that the surface plasmon band of the silver nanoparticles could be preserved by controlled deposition of the tin dioxide shell.     

Electronic absorption spectrum of Cs<Subscript>2</Subscript>ZnI<Subscript>4</Subscript> thin films

The absorption spectrum of Cs₂ZnI₄ thin films in the energy range 3–6 eV at temperatures from 90 to 340 K has been investigated. It is established that this compound belongs to direct-gap insulators. Low-frequency exciton excitations are localized in ZnI₄ structural elements of the lattice. Phase transitions at 280 K (paraelectric phase ? incommensurate phase), 135 K (incommensurate phase ? monoclinic ferroelastic phase), and 96 K (monoclinic phase ? triclinic ferroelastic phase) have been found from the temperature dependences of the spectral position and halfwidth of the low-frequency exciton band. Additional broadening of the exciton band is observed for ferroelastic phases; it is likely to be due to exciton scattering from strain fluctuations near domain walls.     

Effects of LaNiO<Subscript>3</Subscript> on the structures and properties of SrBi<Subscript>2</Subscript>Ta<Subscript>2</Subscript>O<Subscript>9</Subscript> thin films

SrBi₂Ta₂O₉(SBT)/LaNiO₃(LNO)/Si and SBT/Pt/TiO₂/SiO₂/Si multilayers were fabricated by pulsed laser deposition. With Pt top electrodes, the measured remanent polarization (2P_r) of Pt/SBT/LNO/Si and Pt/SBT/Pt/TiO₂/SiO₂/Si capacitors was 6.5 C/cm² and 5.2 C/cm², respectively. Using LNO as both bottom electrodes and buffer layers, enhanced non-c-axis crystalline SBT films were induced, which resulted in a 2P_r greater than that of the Pt/SBT/Pt/TiO₂/SiO₂/Si capacitor. The hysteresis loop of the Pt/SBT/LNO/Si capacitor showed a great external-field-dependent horizontal shift. Using an electron-injection model, this dependence was addressed. The fatigue-free property of the Pt/SBT/LNO/Si capacitor was experimentally established, in that the non-volatile polarization decreased by less than 5% of the initial value after 1.44×10⁹ switching cycles . PACS 77.84.Dy; 68.65.+g     

A study of Gaussianity and stationarity of noise in gas-sensitive SnO<Subscript>2</Subscript> films

Chemisorption processes significantly influence the surface states of semiconductor films. It is assumed that these processes should also influence electric-current noise generated by the semiconductor structure. We study stationarity and Gaussianity of the resistance fluctuations of semiconductor gas sensors. It is shown that Gaussianity and stationarity of noise are shown as functions of the gas-phase composition and temperature. On this basis, we conclude that it is in principle possible to increase the selectivity of gas sensors by jointly measuring their sensor and noise characteristics.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 47, No. 9, pp. 784–788, September, 2004.     

Structural and sensing properties of nanocrystalline SnO<Subscript>2</Subscript> films deposited by spray pyrolysis from a SnCl<Subscript>2</Subscript> precursor

We report the fabrication and characterization of tin dioxide gas sensing layers. The tin dioxide layers were synthesized using a convenient, simple and low-cost technique of spray pyrolysis. The formation of stoichiometric SnO₂ layers with fine-grain structure is revealed by Rutherford backscattering spectroscopy. The microstructure, phase, nanoparticle size distribution and surface morphology were studied by transmission electron microscopy, electron diffraction and atomic force microscopy. Most of the grains were of 10–20 nm size; however, some particles were up to 100 nm in size and had a microtwin lamellae structure of SnO₂ phase (cassiterite) with lattice parameters a= 0.474 nm and c= 0.319 nm. The sensitivity of the layers with respect to 1000–10000 ppm CH₄ in air was obtained from both resistivity (S_R) and capacity (S_C) measurements at 330 °C and values of S_R=5–7 and S_C=22–31 were extracted. PACS 68.43.-h; 68.55.-a; 81.05.Hd; 81.07.-b; 81.15.Rs     

Red photoluminescence mechanism in SnO<Subscript>2</Subscript> nanostructures

Tin dioxide nanoribbons were fabricated for clarifying the origin of the red photoluminescence band. It is found that the red band abruptly decreases its intensity after annealing the nanoribbons in O₂. The time-resolved photoluminescence decay curve shows that the red band has a luminescence lifetime of ns. The electron spin resonance spectrum discloses that the red band is related to a kind of combined oxygen-vacancy (V _o⁺ and V _o⁺⁺) centers. Spectral analysis and theoretical calculation confirm that the red band arises from a transition between the combined oxygen-vacancy defect states in the band gap.