Intergrain Potential Barriers Research Articles

Synthesis, Characterization and Gas Sensing Study of ZnO-SnO2 Nanocomposite Thin Films

Thin nanocomposite films composed of ZnO and SnO2 at 0.5–5 mol.% concentrations were synthesized by a new solid-phase low-temperature pyrolysis under the developed protocols. This hetero-oxide material was thoroughly studied by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) techniques to be compared with electrical and gas-sensing properties. We have found that the films have a poly-nanocrystal structure of ZnO and SnO2 crystals with characteristic grain sizes at 10–15 nm range. When comparing the chemiresistive response of the films with varied tin dioxide content, the sample of Sn:Zn optimum ratio taken as 1:99 yields 1.5-fold improvement upon to 5–50 ppm NO2 exposure at 200 °C. We argue that these remarkable changes have matured from both a reducing the intergrain potential barrier down to 0.58 eV and increasing the concentration of anionic vacancies at this rational composite. The results demonstrate that solid-phase low-temperature pyrolysis is a powerful technique for adjusting the functional gas-sensing properties of hetero-oxide film via modifying the ratio of the oxide components.

Mechanism of band gap persistent photoconductivity (PPC) in SnO2 nanoscrystalline films: Nature of local states, simulation of PPC and comparison with experiment

A phenomenological model of room temperature photoconductivity in nanocrystalline SnO2 under photon excitation below the fundamental bandgap based on electronic states located at the bottom part of the band gap was proposed. Nature of these states is related to the surface oxygen vacancies and Sn-derived electronic states. Appropriate distribution of these states was considered. Numerical simulation of the photoconductivity response and decay on the basis of balance rate equation for excited electrons and immobile holes was done. Analysis revealed that response time is determined by the photoionization cross section of these states and intensity of illumination. Stationary photoresponse is saturated due to the limited number of these states. Intergrain potential barrier that originated due to the ionosorbed oxygen is the main factor limiting the reverse annihilation process and determining the photoconductivity decay time. Stretched exponential behavior of the photoconductivity decay was interpreted in terms of structural and electronic film disordering that results in asymmetric probability distribution of intergrain barrier heights and corresponding distribution of time constants.

Influence of the sulphurization time on the morphological, chemical, structural and electrical properties of Cu2ZnSnS4 polycrystalline thin films

The effects of the sulphurization annealing time on the morphological, chemical, structural and electrical properties of CZTS thin films were investigated by scanning electron microscopy, X-ray energy dispersive spectroscopy, Hall effect and electrical conductivity measurements in samples annealed during different time intervals. The increase of the annealing time was found to improve the chemical composition of the samples and to, slightly, increase the crystallite size. Small amounts of Na were measured in the samples. However, the concentration of Na does not increase significantly with the annealing time and should not modify the characteristics of the CZTS thin films. It was also found that at high temperature the electrical conductivity is dominated by thermal emission of carriers over the inter-grain potential barriers. As the temperature decreases different hopping conduction mechanisms start to dominate. At first with nearest-neighbour hopping and successively changing to variable range hopping conduction with a crossover from Mott and Efros–Shklovskii behavior. The electrical conductivity, the concentration of free holes, acceptors and donors, traps' density at the grain boundaries and the grain potential barriers height were found to increase with the annealing time. However, a significant drop in the compensation ratio from 0.8 to 0.5 was also detected.

Resistividade do filme depositado via sol-gel e estado de oxidação do dopante Ce na matriz SnO2

Incorporação de Ce3+ ou Ce4+ em filmes finos de SnO2 depositados via sol-gel-dip-coating aumenta drasticamente a resistividade elétrica. No primeiro caso, temos comportamento aceitador do dopante, levando a matriz à alta compensação de carga. Por outro lado, para Ce4+, verifica-se aumento na largura da região de depleção do contorno de grão, resultando em maior espalhamento de elétrons. Medidas de caracterização elétrica sob pressão ambiente levam à barreiras de potencial mais altas do que as medidas sob vácuo, devido a adsorção de oxigênio na superfície das partículas. A presença de Ce3+ aumenta a transmitância no infravermelho, o que significa menor quantidade de elétrons livres. Dados de XANES confirmam que o tratamento térmico a 550 ºC dos filmes, ainda que promova oxidação parcial para Ce4+, preserva uma quantidade significativa (em torno de 60%) no estado Ce3+. Espectroscopia Raman mostra a evolução dos modos de vibração intra-grãos de SnO2 com o aumento da temperatura de tratamento térmico.

Electron mobility in CdO films

Electron mobility in degenerate CdO thin films has been studied as a function of carrier concentration. The “optical” mobility has been determined from infrared reflectance measurements of the conduction band plasmon lifetime. The acquired values vary from ∼209 to ∼1116 cm2 V-1 s-1 for carrier concentrations between 2.5×1020 and 2.6×1019 cm-3. Ionized impurity scattering is shown to be the dominant effect reducing the intra-grain mobility of the electrons at room temperature. The transport mobilities from Hall effect measurements range between ̃20 and ̃124 cm2 V-1 s-1 which are much lower than the optical mobilities. Simulation of grain boundary scattering-limited mobility is commonly based on models that assume a depletion layer at the boundaries which causes an inter-grain potential barrier. These models are found not to be applicable to CdO as it has been previously shown to have surface electron accumulation. Therefore, simulation of the transport mobility has been performed using the Fuchs-Sondheimer and Mayadas-Shatzkes models to take into account the grain boundary and surface scattering mechanisms, in addition to intra-grain scattering. The results indicate that electron scattering at grain boundaries with ̃95 % reflection is the dominant mechanism in reducing the mobility across the layer. The effect of surface scattering plays only a minor role in electron transport.

The size dependence of tin oxide atomic cluster nanowire field effect transistors

Tin oxide (SnO2) cluster nanowires have been fabricated using atomic clusters as building blocks.Nanowires with widths of less than 100 nm were defined using electron beam lithographyfollowed by deposition of Sn clusters which were subsequently thermally oxidized. Thecluster nanowires were used to fabricate field effect transistors. The transistors weren-type and demonstrated a clear size-dependent behaviour. With zero gate bias thenarrowest wires were depleted, both the carrier concentration and the conductionquickly increasing with wire width. This behaviour is attributed to the formationof a surface depletion region caused by Fermi level pinning. The width of thedepletion region was estimated using the carrier concentration calculated fromthe transistor threshold voltages. The change in the wire conductance with UVillumination was also investigated. Illumination with 365 nm light increases theconduction by up to 40 times. This is attributed to a combination of an increase incarriers due to photo-desorption of oxygen from the surface of the wires and anincrease in the mobility due to a reduction of inter-grain potential barrier height.

Rare earth centers properties and electron trapping in SnO 2 thin films produced by sol–gel route

Some very relevant optical, electrical, and structural properties of SnO 2 doped with rare-earth ions Er 3+ and Eu 3+ are presented. Films are produced by the sol–gel-dip coating process, and may be described as a combination of nanoscopic dimension crystallites (about 3–10 nm) with their respective intergrain potential barriers. The Er 3+ and Eu 3+ ions are expected to act as acceptors in SnO 2, which is a natural n-type conductor, inducing a high degree of charge compensation. Electron trapping and emission spectra data are presented and are rather distinct, depending on the location of the rare-earth impurity. This behavior allows the identification of two distinct centers: located either in the SnO 2 lattice or segregated at the particles surface. Based on a model for thermally activated cross-section defects, the difference between the capture energy of the photo-excited electron and the intergrain potential barrier is evaluated, leading to distinct values for high and low symmetry sites. A higher distortion in the lattice of undoped SnO 2 and SnO 2:Eu (1 at.%) was evaluated from Rietveld refinements of X-ray diffraction data. This was confirmed by Raman spectra, which are associated with the particles size and disorder. By comparing the samples with the same doping concentration, it was found that this disorder is higher in Eu-doped SnO 2 than in Er-doped SnO 2, which is in agreement with a higher energy for the lattice relaxation in the trapping process by Eu 3+ centers.

Unified description of spin-dependent transport in granular ferromagnetic manganites

We have presented direct experimental evidence that the description of spin-dependent transport in granular ferromagnets at low temperature is incomplete unless both intergrain tunneling and nontunneling transport are incorporated in a unified treatment. Depending on the microstructure of the system, charge-carrier localization inside the grain and the lowering of intergrain potential barrier by electric field will play a crucial role behind the magnetoresistive properties in granular ferromagnets.

Majority‐carrier mobilities in undoped and n ‐type doped ZnO epitaxial layers

Transparent and conductive ZnO:Ga thin films are prepared by laser molecular-beam epitaxy. Their electron properties were investigated by the temperature-dependent Hall-effect technique. The 300-K carrier concentration and mobility were about $n_s \sim 10^{16}$ cm$^{-3}$ and 440 cm$^{2}$/Vs, respectively. In the experimental `mobility vs concentration' curve, unusual phenomenon was observed, i.e., mobilities at $n_s \sim 5\times$ 10$^{18}$ cm$^{-3}$ are significantly smaller than those at higher densities above $\sim 10^{20}$ cm$^{-3}$. Several types of scattering centers including ionized donors and oxygen traps are considered to account for the observed dependence of the Hall mobility on carrier concentration. The scattering mechanism is explained in terms of inter-grain potential barriers and charged impurities. A comparison between theoretical results and experimental data is made.

Photoconductivity in the ordered vacancy compound CuIn5Se8

Thin films of the ordered vacancy compound CuIn5Se8 are deposited on glass substrates by multisource co-evaporation method and photoconductivity measurements are done on the films at various temperatures from 10 up to 300K. The two photoactive bands that are identified in the spectral response spectra of CuIn5Se8 thin films at all measured temperatures are attributed to photoactive transitions between acceptor VCu to donor InCu and valence band to conduction band transitions respectively. From the spectra, a shift in band-to-band gap from 1.36 to 1.3eV is observed with a temperature variation from 10 to 300K. The non-exponential long-term decay observed in the transient photoconductivity spectra suggests a deep level trap-emptying process to be associated with the decay process and the decay constants are calculated by the differential life-time concept. From the steady state photocurrent analysis, a reduction in intergrain potential barrier on illumination has been noted as one reason for increase in conductivity on illumination. X-ray photoelectron spectroscopy analysis has been done to determine the binding energies of Cu, In and Se in the compound.

Square and collinear four probe array and Hall measurements on metal oxide thin film gas sensors

Gas detection by means of metal oxide semiconductor sensors is ascribed to the change of resistance due to the modulation of the intergrain potential barrier (grain contribution) and of the metal semiconductor barrier at the contacts (contact contribution). We present a method for measuring the contribution of the contact and the grain resistance separately during exposure to CO and CH 4 by employing the square and collinear four probe array (David S. Perloff, Four-point sheet resistance correction factors fot thin rectangular samples, Solid State Electron. 20 (1977) 681–687). The electronic properties are investigated by means of Hall effect measurements according to grain boundary scattering mechanism.

Scattering of charge carriers in transparent and conducting thin oxide films with a non-parabolic conduction band

A simple model of the conduction mechanism, with the assumption of a non-parabolic conduction band, has been applied to wide band gap, degenerate thin oxide films. The scattering mechanism is explained in terms of intergrain potential barriers and charged point defects. A comparison between theoretical results and experimental data is made for SnO 2 and CdIn 2O 4.

Determination of grain-boundary potential barriers distribution in p-type polycrystalline Si

Metal-oxide-semiconductor structures employing lightly boron doped polycrystalline Si electrode are studied. The frequency dispersion of their admittance is explained by the existence of a broad distribution of intergrain potential barriers. A simple one-dimensional model is presented which yields the form of the potential barrier distribution. The observed results for the temperature and doping concentration dependence of the distribution are described and shown to be consistent with the resistivity activation energies.

Interpretation of Hall mobility in polycrystalline thin films

In this paper the interpretation of Hall effect measurements on polycrystalline semiconductor thin films where current flow is limited (at least in part) by the presence of intergrain potential barriers is considered. By using a one-dimensional band-bending model and assuming that the Hall coefficient R H measures the average carrier density in a grain an expression for the Hall mobility is derived. This applies over the whole range of barrier widths and takes account of both thermionic emission and diffusion currents. Some limiting cases are considered which support the general correctness of the result.

Effect of ion implantation on CdSe thin film transistors

Thin film transistors have been fabricated by implanting 50 keV Cr, 50 keV Al, and 15 keV B ions into high resistivity (≳105Ωcm), polycrystalline CdSe deposited by thermal evaporation. The implants were made at room temperature into inverted transistor structures fabricated by photoengraving. Both the Cr and Al implants cause a maximum decrease of approximately 1.5% in the c-axis lattice spacing of CdSe, and enhance the (00.1) texture of the grains. Similar but smaller changes in c were observed in B implanted samples. For a fixed CdSe film thickness of ∠800 Å, the drain current, ID, increases rapidly with implant dose above the unimplanted value (∠10 nA). At a dose of 5×1015cm−2, ID was approximately 200 μA (VD = 10 V, VG = 10 V) for the Cr and B implants but was much higher for Al; ID was 1.9 mA at VD = 0.5 V for the same VG. The current-dose dependence is interpreted in terms of filling of grain boundary traps by implanted donors, with the intergrain potential barrier height determined by the implant dose.