Dopant ion size and electronic structure effects on transparent conducting oxides. Sc-doped CdO thin films grown by MOCVD

A series of Sc-doped CdO (CSO) thin films have been grown on both amorphous glass and single-crystal MgO(100) substrates at 400 degrees C by MOCVD. Both the experimental data and theoretical calculations indicate that Sc3+ doping shrinks the CdO lattice parameters due to its relatively small six-coordinate ionic radius, 0.89 angstroms, vs 1.09 angstroms for Cd2+. Conductivities as high as 18100 S/cm are achieved for CSO films grown on MgO(100) at a Sc doping level of 1.8 atom %. The CSO thin films exhibit an average transmittance >80% in the visible range. Sc3+ doping widens the optical band gap from 2.7 to 3.4 eV via a Burstein-Moss energy level shift, in agreement with the results of band structure calculations within the sX-LDA (screened-exchange local density approximation) formalism. Epitaxial CSO films on single-crystal MgO(100) exhibit significantly higher mobilities (up to 217 cm2/(V x s)) and carrier concentrations than films on glass, arguing that the epitaxial CSO films possess fewer scattering centers and higher doping efficiencies due to the highly textured microstructure. Finally, the band structure calculations provide a microscopic explanation for the observed dopant size effects on the structural, electronic, and optical properties of CSO.     

CdO as the archetypical transparent conducting oxide. Systematics of dopant ionic radius and electronic structure effects on charge transport and band structure

A series of yttrium-doped CdO (CYO) thin films have been grown on both amorphous glass and single-crystal MgO(100) substrates at 410 degrees C by metal-organic chemical vapor deposition (MOCVD), and their phase structure, microstructure, electrical, and optical properties have been investigated. XRD data reveal that all as-deposited CYO thin films are phase-pure and polycrystalline, with features assignable to a cubic CdO-type crystal structure. Epitaxial films grown on single-crystal MgO(100) exhibit biaxial, highly textured microstructures. These as-deposited CYO thin films exhibit excellent optical transparency, with an average transmittance of >80% in the visible range. Y doping widens the optical band gap from 2.86 to 3.27 eV via a Burstein-Moss shift. Room temperature thin film conductivities of 8,540 and 17,800 S/cm on glass and MgO(100), respectively, are obtained at an optimum Y doping level of 1.2-1.3%. Finally, electronic band structure calculations are carried out to systematically compare the structural, electronic, and optical properties of the In-, Sc-, and Y-doped CdO systems. Both experimental and theoretical results reveal that dopant ionic radius and electronic structure have a significant influence on the CdO-based TCO crystal and band structure: (1) lattice parameters contract as a function of dopant ionic radii in the order Y (1.09 A) < In (0.94 A) < Sc (0.89 A); (2) the carrier mobilities and doping efficiencies decrease in the order In > Y > Sc; (3) the dopant d state has substantial influence on the position and width of the s-based conduction band, which ultimately determines the intrinsic charge transport characteristics.     

Effect of Crystallization Kinetics on Microstructure and Charge Transport of Polythiophenes

We have examined the effects of crystallization kinetics of poly(3‐hexylthiophene) and poly[2,5‐bis(3‐hexadecylthiophen‐2‐yl)thieno(3,2‐b)thiophene] on microstructure and charge transport. Rapid crystallization increases the density of tie molecules in polythiophenes. As a consequence, ordered regions are better connected resulting in higher charge carrier mobilities. Our results suggest that controlling the crystallization kinetics might be an important factor for maximizing the charge mobility in semicrystalline polythiophene thin films.     

Effect of dielectric roughness on performance of pentacene TFTs and restoration of performance with a polymeric smoothing layer

The morphology, structure, and transport properties of pentacene thin film transistors (TFTs) are reported showing the influence of the gate dielectric surface roughness. Upon roughening of the amorphous SiO2 gate dielectric prior to pentacene deposition, dramatic reductions in pentacene grain size and crystallinity were observed. The TFT performance of pentacene films deposited on roughened substrates showed reduced free carrier mobility, larger transport activation energies, and larger trap distribution widths. Spin coating roughened dielectrics with polystyrene produced surfaces with 2 A root-mean-square (rms) roughness. The pentacene films deposited on these coated surfaces had grain sizes, crystallinities, mobilities, and trap distributions that were comparable to the range of values observed for pentacene films deposited on thermally grown SiO2 (roughness also approximately 2 A rms).     

MOCVD-derived highly transparent, conductive zinc- and tin-doped indium oxide thin films: precursor synthesis, metastable phase film growth and characterization, and application as anodes in polymer light-emitting diodes

Four diamine adducts of bis(hexafluoroacetylacetonato)zinc [Zn(hfa)(2).(diamine)] can be synthesized in a single-step reaction. Single crystal X-ray diffraction studies reveal monomeric, six-coordinate structures. The thermal stabilities and vapor phase transport properties of these new complexes are considerably greater than those of conventional solid zinc metal-organic chemical vapor deposition (MOCVD) precursors. One of the complexes in the series, bis(1,1,1,5,5,5-hexafluoro-2,4-pentadionato)(N,N'-diethylethylenediamine)zinc, is particularly effective in the growth of thin films of the transparent conducting oxide Zn-In-Sn-O (ZITO) because of its superior volatility and low melting point of 64 degrees C. ZITO thin films with In contents ranging from 40 to 70 cation % (a metastable phase) were grown by low-pressure MOCVD. These films exhibit conductivity as high as 2900 S/cm and optical transparency comparable to or greater than that of commercial Sn-doped indium oxide (ITO) films. ZITO films with the nominal composition of ZnIn(2.0)Sn(1.5)O(z)() were used in fabrication of polymer light-emitting diodes. These devices exhibit light outputs and current efficiencies almost 70% greater than those of ITO-based control devices.     

Organocadmium aminoalcoholates: synthesis, structure, and materials chemistry

The novel methylcadmium aminoalkoxides MeCd(dmae) (Hdmae = dimethylaminoethanol), MeCd(bdmap) [Hbdmap = 1,3- bis-(dimethylamino)-propan-2-ol], and MeCd(tdmap) [tdmap = 1,3- bis(dimethylamino)-2-(dimethylaminomethyl)-propan-2-ol] have been synthesized and structurally characterized. MeCd(dmae) (1) forms a tetrameric heterocubane with a Cd4O4 core, while MeCd(bdmap) (2) is trimeric and MeCd(tdmap) (3) is a dimer. Only in the case of MeCd(dmae) are all the ligand donors fully utilized. In solution, MeCd(tdmap) undergoes a Schlenk equilibrium, with Me2Cd and Cd(tdmap)2 evident at 218 K. The structure and solution-state chemistry of Cd(tdmap)2 (5) have been independently studied and, in the solid-state, found to exist as a dimer whose coordination number at cadmium (CN = 6) is greater than in the organocadmium complexes (CN = 4, 5). MeCd(tdmap) has been used as a single-source precursor for CdO films by LPCVD with a glass substrate temperature of only 140 degrees C. Evidence is also presented for the formation of a heterometallic precursor, [(MeZn)(MeCd)(tdmap)2] (6), which has been used to deposit films of CdO mixed with ZnO by LPCVD at 140 degrees C. The structure of Me4Cd4(tdmap)2Cl2 (4), obtained serendipitously, is also included. Crystal data: 1, C20H52Cd4N4O4, FW 862.26, triclinic, P1, a = 11.47560(10), b = 13.55400(10), c = 21.5966(2) A, alpha = 99.7869(4), beta = 90.7476(4), gamma = 98.7823(4) degrees, V = 3268.82(5) A(3), Z = 4; 2, C27H67Cd3N6O3, FW 861.07, triclinic, P1, a = 11.4148(2), b =13.1886(2), c = 14.3139(3) A, alpha = 102.1962(10), beta = 108.3064(10), gamma = 100.8446(10) degrees, V = 1923.09(6) A(3), Z = 4; 3, C22H54Cd2N6O2, FW 659.51, monoclinic, P2(1)/n, a = 10.2912(1), b = 13.46930(1), c = 11.79130(1) A, beta = 112.8051(1) degrees, V = 1506.59(2) A(3), Z = 2; 4, C24H60Cd4Cl2N6O2, FW 985.28, monoclinic, P2(1)/c, a = 10.89780(10), b = 20.3529(2), c = 16.5317(2) A, beta = 94.8550(10) degrees, V = 3653.61(7) A(3), Z = 4; 5, C40H96Cd2N12O4, FW 1034.09, orthorhombic, P2(1)cn, a = 12.33290(10), b = 14.25060(10), c = 29.9003(2) A, V = 5255.01(7) A(3), Z = 4.     

Recent advances in characterization of CaCu3Ti4O12 thin films by spectroscopic ellipsometric metrology

CaCu3Ti4O12 (CCTO) thin films were successfully grown on LaAlO3(100) and Pt/TiO2/SiO2/Si(100) substrates by a novel MOCVD approach. Epitaxial CCTO(001) thin films have been obtained on LaAlO3(100) substrates, while polycrystalline CCTO films have been grown on Pt/TiO2/SiO2/Si(100) substrates. Surface morphology and grain size of the different nanostructured deposited films were examined by AFM, and spectroscopic ellipsometry has been used to investigate the electronic part of the dielectric constant (epsilon2). Looking at the epsilon2 curves, it can be seen that by increasing the film structural order, a greater dielectric response has been obtained. The measured dielectric properties accounted for the ratio between grain volumes and grain boundary areas, which is very different in the different structured films.     

Synthesis and characterization of volatile,fluorine-free beta-ketoiminate lanthanide MOCVD precursors and their implementation in low-temperature growth of epitaxial CeO(2) buffer layers for superconducting electronics

A new class of volatile, low-melting, fluorine-free lanthanide metal-organic chemical vapor deposition (MOCVD) precursors has been developed. The neutral, monomeric Ce, Nd, Gd, and Er complexes are coordinatively saturated by a versatile, multidentate ether-functionalized beta-ketoiminato ligand series, the melting point and volatility characteristics of which can be tuned by altering the alkyl substituents on the keto, imino, and ether sites of the ligand. Direct comparison with conventional lanthanide beta-diketonate complexes reveals that the present precursor class is a superior choice for lanthanide oxide MOCVD. Epitaxial CeO(2) buffer layer films can be grown on (001) YSZ substrates by MOCVD at significantly lower temperatures (450-650 degrees C) than previously possible by using one of the newly developed cerium beta-ketoiminate precursors. Films deposited at 540 degrees C have good out-of-plane (Deltaomega = 0.85 degrees ) and in-plane (Deltaphi = 1.65 degrees ) alignment and smooth surfaces (rms roughness approximately 4.3 A). The film growth rate decreases and the films tend to be smoother as the deposition temperature is increased. High-quality yttrium barium copper oxide (YBCO) films grown on these CeO(2) buffer layers by pulsed organometallic molecular beam epitaxy exhibit very good electrical transport properties (T(c) = 86.5 K, J(c) = 1.08 x 10(6) A/cm(2) at 77.4 K).     

Charge transfers at metal/oxide interfaces: a DFT study of formation of K delta+ and Au delta- species on MgO/Ag(100) ultra-thin films from deposition of neutral atoms

Ultra-thin oxide films grown on a metal substrate and of thickness smaller than 1 nm may exhibit unusual properties with respect to thicker films or single crystal oxide surfaces. In a previous study [G. Pacchioni, L. Giordano and M. Baistrocchi, Phys. Rev. Lett., 2005, 94, 226104] we have suggested that a Au atom adsorbed on a MgO/Mo(100) thin film becomes negatively charged by direct electron tunneling from the Mo metal and that this is related to the low MgO/Mo(100) work function. Here we show, based on periodic DFT supercell calculations, that charge transfer can occur also in the opposite direction by adsorption of electropositive K atoms on MgO/Ag(100) films. We predict the occurrence of a charge transfer also for Au on MgO/Ag(100) films despite the fact that here the work function is 1 eV larger than in MgO/Mo(100). The formation of a layer of adsorbed negative (Au delta-/MgO/Ag) or positive (K delta+/MgO/Ag) adsorbates results in an increase or decrease, respectively, of the MgO/Ag(100) work function as predicted by the classical Gurney model for ionic adsorbates on metal surfaces.     

Effect of Fluorine Doping on Semiconductor to Metal-Like Transition and Optical Properties of Cadmium Oxide Thin Films Deposited by Sol–Gel Process

Highly conducting fluorine doped n-type cadmium oxide thin films have been synthesized by sol–gel dip coating process on glass and Si substrates for various fluorine concentrations in the films. X-ray diffraction pattern confirmed the cubic CdO phase formation and SEM micrograph showed fine particles of CdO with size ~0.3 m. F concentration in the films was varied from 1.8% to 18.7% as determined from energy dispersive X-ray analysis (EDX). The resistivity of the CdO films decreased with increase of F doping and increase of temperature below 14.6% of F, as usual for semiconductors. Above this F concentration the resistivity increased with increase of temperature like metals. Hall measurement showed very high carrier concentrations in the films lying in the range of ~2.93 × 10²⁰ cm^–3 to 4.56 × 10²¹ cm^–3. UV-VIS-NIR spectrum of the films showed the optical bandgap energy increased with increase of F doping and corresponding carrier concentrations obtained from Burstein-Moss shift also support the Hall measurement results.     

Mobility enhanced photoactivity in sol-gel grown epitaxial anatase TiO2 films

Epitaxial anatase thin films were grown on single-crystal LaAlO3 substrates by a sol-gel process. The epitaxial relationship between TiO2 and LaAlO3 was found to be [100]TiO2||[100]LaAlO3 and (001)TiO2||(001)LaAlO3 based on X-ray diffraction and a high-resolution transmission electron microscopy. The epitaxial anatase films show significantly improved photocatalytic properties, compared with polycrystalline anatase film on fused silica substrate. The increase in the photocatalytic activity of epitaxial anatase films is explained by enhanced charge carrier mobility, which is traced to the decreased grain boundary density in the epitaxial anatase film.     

Formation of intra-island grain boundaries in pentacene monolayers

To assess the formation of intra-island grain boundaries during the early stages of pentacene film growth, we studied sub-monolayers of pentacene on pristine silicon oxide and silicon oxide with high pinning centre density (induced by UV/O(3) treatment). We investigated the influence of the kinetic energy of the impinging molecules on the sub-monolayer growth by comparing organic molecular beam deposition (OMBD) and supersonic molecular beam deposition (SuMBD). For pentacene films fabricated by OMBD, higher pentacene island-density and higher polycrystalline island density were observed on UV/O(3)-treated silicon oxide as compared to pristine silicon oxide. Pentacene films deposited by SuMBD exhibited about one order of magnitude lower island- and polycrystalline island densities compared to OMBD, on both types of substrates. Our results suggest that polycrystalline growth of single islands on amorphous silicon oxide is facilitated by structural/chemical surface pinning centres, which act as nucleation centres for multiple grain formation in a single island. Furthermore, the overall lower intra-island grain boundary density in pentacene films fabricated by SuMBD reduces the number of charge carrier trapping sites specific to grain boundaries and should thus help achieving higher charge carrier mobilities, which are advantageous for their use in organic thin-film transistors.     

Substrate dependent structure and in-plane dielectric properties of Ba(Sn0.15Ti0.85)O3 thin films

The structure, microstructure and in-plane dielectric properties of Barium tin titanate Ba(Sn_0.15Ti_0.85)O₃ (BTS) thin films grown on (100) LaAlO₃ (LAO) and (100) MgO single crystal substrates through sol–gel process were investigated. The films deposited on (100) LAO substrate exhibited a strong (100) preferred orientation while the film deposited on (100) MgO substrate showed polycrystalline structure. The in-plane ɛ–T measurements reveal that the films grown on (100) LAO substrate exhibited an obvious room-temperature ferroelectric state, while the film grown on MgO substrate showed paraelectric state in the temperature range of 10–130 °C. A high tunability of 52.11% was observed for the BTS films deposited on (100) LAO substrate at the frequency of 1 MHz with an applied electric field of 80 kV/cm, which is about two times larger than that of the BTS films deposited on (100) MgO substrate. The obvious differences in the dielectric properties could be attributed to the stress in the films, which come from lattice mismatch and difference in the thermal expansion coefficients between the film and substrates. This work clearly reveals the highly promising potential of BTS films for application in tunable devices.     

Effect of side-chain length on charge mobilities in neutral poly(3-alkylthiophene)s: Determination from dielectric measurement

For neutral poly(3-alkylthiophene)s (P3ATs), the electric modulus formalism of dielectric relaxation measurement together with the use of nonexponential decay function can be applied to describe the behavior of conductivity relaxation and carrier transport. The charge mobilities of neutral P3ATs calculated from conductivity relaxation with the use of the defect-diffusion model are in agreement with the data from field-effect transistor measurement. The temperature dependence of charge mobility exhibits a local maximum right after the end of glass transition region, which can be attributed to the transition of soft conformons in the disordered phase to localized conformons. The charge mobilities and activation energies of mobilities for P3ATs are dependent on conjugation length and volume fraction of conducting units. © 1994 John Wiley & Sons, Inc.     

Correlation of the number of thiophene units with structural order and carrier mobility in unsubstituted even- and odd-numbered alpha-oligothiophene films

We investigate the correlation of the number of thiophene units with the structural order and carrier mobility of the films through studies on thin-film transistors (TFTs) based on alpha-quinquethiophene (5T), alpha-sexithiophene (6T), and alpha-septithiophene (7T). The X-ray diffraction (XRD) data of the nT films deposited at low substrate temperatures present obviously different structural orders depending on the parity of the number of thiophene units. Although even-numbered nT films present well-ordered structures and large carrier mobilities, odd-numbered nT films present two different crystalline polymorphs and vastly low carrier mobilities reflecting the coexistence of two crystalline polymorphs. However, the XRD data of both even- and odd-numbered nT films deposited at high substrate temperatures indicate that the nT molecules form single well-ordered structures. Those ordered TFTs exhibit large carrier mobilities accompanying an increase in the number of thiophene units, 0.05, 0.08, and 0.13 cm2 V(-1) s(-1) for 5T, 6T, and 7T, respectively. The parity of the number of thiophene units affects the structural order intrinsically in grown thin films, and affects carrier mobilities extrinsically in their TFTs.     

CdO auf Cadmiumanoden in Alkalihydroxidlösungen??

Anodic films formed potentiostatically in 1M NaOH on Cadmium electrodes were examined by means of electron diffraction under carefully controlled conditions. Glancing incidence electron diffraction indicates the presence of Cd(OH)₂ on the electrolyte side of the film and of CdO on the metal side. Transmission electron diffraction of thin isolated films indicates only Cd(OH)₂ in films formed below the Flade-potential of the CdO electrode, but CdO along with Cd(OH)₂ above the Flade-potential. In these films selected area diffraction reveals spots consisting exclusively of a very thin oxid layer.     

Molecular weight dependent structure and charge transport in MAPLE‐deposited poly(3‐hexylthiophene) thin films

In this work, poly(3‐hexylthiophene) (P3HT) films prepared using the matrix‐assisted pulsed laser evaporation (MAPLE) technique are shown to possess morphological structures that are dependent on molecular weight (MW). Specifically, the structures of low MW samples of MAPLE‐deposited film are composed of crystallites/aggregates embedded within highly disordered environments, whereas those of high MW samples are composed of aggregated domains connected by long polymer chains. Additionally, the crystallite size along the side‐chain (100) direction decreases, whereas the conjugation length increases with increasing molecular weight. This is qualitatively similar to the structure of spin‐cast films, though the MAPLE‐deposited films are more disordered. In‐plane carrier mobilities in the MAPLE‐deposited samples increase with MW, consistent with the notion that longer chains bridge adjacent aggregated domains thereby facilitating more effective charge transport. The carrier mobilities in the MAPLE‐deposited simples are consistently lower than those in the solvent‐cast samples for all molecular weights, consistent with the shorter conjugation length in samples prepared by this deposition technique. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 652–662     

Structural and optoelectrical properties of nanocrystalline Gd-doped CdO films prepared by sol gel method

Gd-doped CdO thin films with various Gd concentrations have been prepared on glass and Si wafer substrates using sol gel technique. The films were characterised by X-ray fluorescence (XRF), X-ray diffraction (XRD), optical absorption spectroscopy, and dc-electrical measurements. XRF method was used to determine the %Gd content in the films while XRD was used to study the influence of Gd doping on the detailed crystalline structure. Experimental data indicate that Gd³⁺ doping with level of less than 2.4% slightly enlarge the CdO crystalline unit cell. The bandgap (E _g) of Gd-doped CdO suffers narrowing by about 13% due to a small (0.2%) doping level but with %Gd doping level larger than 2.4%, E _g becomes wider than that of undoped CdO. The electrical behaviours of the Gd-doped CdO films show that they are degenerate semiconductors. The 2% Gd-doped CdO film shows increase in its mobility by about 92%, conductivity by 320%, and carrier concentration by 127%, relative to undoped CdO film. From transparent-conducting-oxide point of view, the Gd doping of CdO by sol gel method is not effective. Finally, the absorption in the NIR spectral region was investigated to be due to the free electrons.     

Rational Design of Ambipolar Organic Semiconductors: Is Core Planarity Central to Ambipolarity in Thiophene–Naphthalene Semiconductors?

Herein, we report a new family of naphthaleneamidinemonoimide-fused oligothiophene semiconductors designed for facile charge transport in organic field-effect transistors (OFETs). These molecules have planar skeletons that induce high degrees of crystallinity and hence good charge-transport properties. By modulating the length of the oligothiophene fragment, the majority carrier charge transport can be switched from n-type to ambipolar behavior. The highest FET performance is achieved for solution-processed films of 10-[(2,2'-bithiophen)-5-yl]-2-octylbenzo[lmn]thieno[3',4':4,5]imidazo[2,1-b][3,8]phenanthroline-1,3,6(2H)-trione (NDI-3 Tp), with optimized film mobilities of 2×10(-2) and 0.7×10(-2) cm(2) V(-1) s(-1) for electrons and holes, respectively. Finally, these planar semiconductors are compared with their twisted-skeleton counterparts, which exhibit only n-type mobility, in order to understand the origin of the ambipolarity in this new series of molecular semiconductors.