Effect of titanium oxide-polystyrene nanocomposite dielectrics on morphology and thin film transistor performance for organic and polymeric semiconductors

Previous studies have shown that organic thin film transistors with pentacene deposited on gate dielectrics composed of a blend of high K titanium oxide-polystyrene core-shell nanocomposite (TiO₂-PS) with polystyrene (PS) perform with an order of magnitude increase in saturation mobility for TiO₂-PS (K = 8) as compared to PS devices (K = 2.5). The current study finds that this performance enhancement can be translated to alternative small single crystal organics such as α-sexithiophene (α-6T) (enhancement factor for field effect mobility ranging from 30-100× higher on TiO₂-PS/PS blended dielectrics as compared to homogenous PS dielectrics). Interestingly however, in the case of semicrystalline polymers such as (poly-3-hexylthiophene) P3HT, this dramatic enhancement is not observed, possibly due to the difference in processing conditions used to fabricate these devices (film transfer as opposed to thermal evaporation). The morphology for α-sexithiophene (α-6T) grown by thermal evaporation on TiO₂-PS/PS blended dielectrics parallels that observed in pentacene devices. Smaller grain size is observed for films grown on dielectrics with higher TiO₂-PS content. In the case of poly(3-hexylthiophene) (P3HT) devices, constructed via film transfer, morphological differences exist for the P3HT on different substrates, as discerned by atomic force microscopy studies. However, these devices only exhibit a modest (2×) increase in mobility with increasing TiO₂-PS content in the films. After annealing of the transferred P3HT thin film transistor (TFT) devices, no appreciable enhancement in mobility is observed across the different blended dielectrics. Overall the results support the hypothesis that nucleation rate is responsible for changes in film morphology and device performance in thermally evaporated small molecule crystalline organic semiconductor TFTs. The increased nucleation rate produces organic polycrystalline films with small grain size which are better connected and exhibit lower barriers for charge transport and as such higher field effect mobilities are measured in these devices.     

Improved performance of fluorinated copper phthalocyanine thin film transistors using an organic pn junction: Effect of copper phthalocyanine film thickness

We have investigated the effect of film thickness of copper phthalocyanine (CuPc) on improving fluorinated copper phthalocyanine (F₁₆CuPc) thin film transistor (TFT) performance with an organic pn junction. Electron field-effect mobility is exponentially enhanced up to 2.0 × 10^− 2 cm² V^− 1 s^− 1 with increasing of CuPc film thickness, and then unchanged when the CuPc thickness is over the saturation thickness (3 monolayers). The charge carrier density at the interface of F₁₆CuPc/CuPc decreases the total TFT resistance, which leads to the increase of mobility. Threshold voltage is suppressed with increasing CuPc films. On the other hand, larger current on/off ratio is obtained when islanded CuPc films are formed on the surface of F₁₆CuPc films. Therefore, employing an organic pn junction is an effective and simple method to fabricate high performance of n-channel transistors for practical applications.     

Electrical and morphological properties of polystyrene thin films for organic electronic applications

This study investigates the electrical and morphological properties of polystyrene layers in the nanometre thickness range for organic and polymer based electronic applications. The paper aims to providing conduction data and information on trapped charges present in the polystyrene layer, as well as investigating how polystyrene properties change under differing annealing conditions. The maximum dielectric strength was found to be 4.0 MV cm⁻¹, while fixed trapped charge and mobile trapped charge average densities were calculated to be 9.9 × 10¹¹ cm⁻² and 2.6 × 10¹² cm⁻² respectively. Optimum electrical characteristics were obtained at an anneal temperature of 90 °C, which is just below the glass transition temperature for polystyrene.     

Experiments and theory on pentacene in the thin film phase: structural, electronic, transport properties, and gas response to oxygen, nitrogen, and ambient air

We investigated the morphological, structural, electronic, and transport properties of pentacene thin films grown by vacuum thermal evaporation on different inert substrates at room temperature. The results of our atomic force microscopy (AFM), X-ray diffraction and scanning tunnelling microscopy (STM) analysis show a structure in the so called “thin film phase” with 1-2 μm sized grains. Atomic terraces are clearly evidenced with AFM and give an inter-planar spacing of 1.54 nm corresponding to the (001) distance. The Scanning Tunneling Spectroscopy measurements show an HOMO-LUMO gap of 2.2 eV. After vacuum thermal evaporation on patterned substrates with different inter-electrodes distances, we have performed in situ measurements of the electrical response of such thin films. We found for these films a resistivity of ρ = 4.7 ± 0.2 · 10⁴ Ω m, that is an order of magnitude lower than the value reported to date in literature for single crystals of pentacene. This value is not affected by the presence of grain boundaries. The resistivity is further reduced by a factor 8.9 ± 0.7, 14 ± 1, 2.3 ± 0.3 upon exposure to oxygen, nitrogen and ambient air, respectively. In addition density functional theory calculations have been performed to investigate the electronic structure of pentacene in this specific phase, focusing on the effects on the relevant electronic properties of the relative orientation of the molecules within the crystalline unit cell, so far experimentally unknown. Our results show that the energy bandwidth and band-gap are crucially affected by the molecular stacking. Furthermore, by comparing our theoretical spectra with the scanning tunneling spectroscopy (STS) measurements, we propose a molecular arrangement that gives a good agreement with experiments as far as the relevant orbitals are concerned. For this polymorph, we find a HOMO and LUMO bandwidth of ≈ 0.7 eV and ≈ 0.8 eV, respectively, which are significantly larger than those obtained for the pentacene bulk-phase and are consistent with the larger conductivity experimentally observed in pentacene thin films.     

Thermoelectric properties of boron-carbide thin film and thin film based thermoelectric device fabricated by intense-pulsed ion beam evaporation

Crystallized B₁₃C₂ thin films were fabricated by intense pulsed-ion beam evaporation (IBE) method. Electrical conductivity and Seebeck coefficients of the obtained films were 1×10^—4 l/Ωm and 200 μV/K at 1000 K, respectively. These values were comparable to those of bulks. For the application of the thin films, since reasonable thermoelectric (TE) properties were confirmed for the B₁₃C₂ films fabricated, we attempted to develop ’in-plane’ type TE device using B₁₃C₂ and SrB₆ as p-type and n-type elements, respectively. With applying temperature difference to the fabricated device, thermo-electromotive force and electrical power were generated from the device we made, indicating that the device worked as a TE device. To the best of our knowledge, this is the first demonstration of the TE device composed of only boron-rich solids.     

CdTe thin film solar cells: Interrelation of nucleation, structure, and performance

The performance of CdTe solar cells as prototype of thin film solar cells strongly depends on film morphology. The needs for high solar cell performance using thin film materials will be addressed covering nucleation and growth control of thin film materials. In order to understand the basic growth mechanisms and their impact on cell performance, we have systematically investigated the growth of CdTe thin films by Close Spaced Sublimation (CSS) using the integrated ultra-high vacuum system DAISY-SOL. CdTe thin films were deposited on TCO/CdS substrates (transparent conductive oxide) held at 270 °C to 560 °C. The properties of the films were determined before and after CdCl₂ treatment using X-ray diffraction and electron microscopy. In addition, solar cells were prepared to find correlations between material properties and cell efficiency. At low sample temperature the films tend to form compact layers with preferred (111) orientation which is lost at elevated temperatures above 450 °C. For CdS layers without (0001) texture there is in addition a low temperature regime (350 °C) with (111) texture loss. After activation treatment the (111) texture is lost for all deposited layers leading to strong recrystallisation of the grains. But the texture still depends on the previous growth history. The loss of (111) texture is evidently needed for higher performance. A clear correlation between cell efficiency and the texture of the CdTe film is observed.     

Studies on the electronic transport properties of some organic semiconductors in thin films

The investigated compounds are some derivatives of orthotolidin-N,N-bis (4-aminobenzene-2-sulphonic) acid. The temperature dependences of the electrical conductivity and Seebeck coefficient are studied on the thin films deposited from dimethylformamide solution onto glass substrates. It is known that the investigated compounds have typical semiconducting properties. The values of some important parameters of the films (thermal activation energy of electrical conduction, concentrations and mobilities of charge carriers) have been determined. The correlations between some of these parameters and molecular structure of the respective compounds are discussed.     

Morphology and electronic structure of thin 3,4,9,10-perylenetetracarboxylic dianhydride layers on Si(001)

The growth of 3,4,9,10-perylenetetracarboxylic dianhydride on Si(001) was examined in the light of varying flux of impinging molecules. Using atomic force microscopy and synchrotron radiation photoelectron spectroscopy Vollmer-Weber growth mode was observed on a wide range of growth rates. The island size initially decreases rapidly with growth rate, for the low growth rate reaches a minimum, and then gradually increases. Polarization dependent photoemission indicates that the orientation of the molecules within the islands remains flat on the substrate.     

Effect of the electrode structure on the electrical properties of alkoxide derived ferroelectric thin film

Effect of the thermal expansion coefficient of electrode on the electrical properties in lead zirconate titanate (PZT) with morphotropic phase boundary (Pb(Zr_0.53,Ti_0.47)O₃: MPB) composition film was demonstrated in this paper. The lanthanum nickel oxide (LaNiO₃: LNO) and lanthanum strontium cobalt oxide ((La_0.5,Sr_0.5)CoO₃: LSCO) was deposited by chemical solution deposition (CSD) as bottom electrode on Si wafer. Highly (100)-oriented LSCO layers were successfully prepared by CSD on Si wafer using (100)-oriented LNO layers as seeding layer for the crystal orientation control. As a result, (100) and (001) oriented PZT film was also successfully prepared on LSCO/LNO/Si stacking structure. The obtained dielectric and ferroelectric properties changed according to the thermal stress which was influenced by the bottom electrode thickness.     

射频溅射工艺参数对AZO薄膜结构和性能的影响

采用射频磁控溅射法制备了掺铝氧化锌(AZO)薄膜,研究了衬底温度及溅射工作压强对沉积薄膜的晶体结构、表面形貌及电学性能的影响。结果显示,随衬底温度增加,薄膜的结晶结构发生显著变化,而溅射工作气压增加主要影响沉积薄膜(103)面与(002)面的相对强度。薄膜的表面形貌受温度影响严重,而气压对形貌的影响相对较小。衬底温度增加,薄膜的电阻率急剧降低,迁移率和载流子浓度都显著增加,而工作气压增加则导致电阻率先减小后增大。     

Revealing structure and electronic properties at organic interfaces using TEM

Molecules and atoms at material interfaces have properties that are distinct from those found in the bulk. Distinguishing the interfacial species from the bulk species is the inherent difficulty of interfacial analysis. For organic photovoltaic devices, the interface between the donor and acceptor materials is the location for exciton dissociation. Dissociation is thought to occur via a complex route effected by microstructure and the electronic energy levels. The scale of these devices and the soft nature of these materials create an additional level of difficulty for identification and analysis at these interfaces. The transmission electron microscope (TEM) and the spectroscopic techniques it incorporates can allow the properties of the donor-acceptor interfaces to be revealed. Cross-sectional sample preparation, using modern focused ion beam instruments, enables these buried interfaces to be uncovered with minimal damage for high resolution analysis. This powerful instrument combination has the ability to draw conclusions about interface morphology, structure and electronic properties of organic donor-acceptor interfaces at the molecular scale. Recent publications have demonstrated these abilities, and this article aims to summarise some of that work and provide scope for the future.     

Effect of NH3 plasma treatment on the device performance of ZnO based thin film transistors

We fabricated an enhancement-mode thin film transistor (TFT) using ZnO as an active channel layer deposited by radio frequency (rf) magnetron sputtering. The NH₃ plasma passivation was performed in order to improve the electrical properties of the ZnO TFTs. We observed that the NH₃ plasma treated ZnO TFTs revealed improved device performances, which include the field effect mobility of 34 cm²/Vs, threshold voltage of 14 V, subthreshold swing of 0.44 V/dec, off-current of 10⁻¹¹ A and on to off ratio higher than 10⁵. These results demonstrate that NH₃ plasma treatment could effectively enhance the performance of the ZnO based TFT device.     

Electrical properties of iron phthalocyanine thin film device using gold and aluminium electrodes

The electrical conductivity of FePc thin film sandwich structures using gold and aluminium electrodes has been investigated for the freshly prepared devices and device after exposure to oxygen for 30 days. Current density-voltage characteristics of the devices in the forward bias showed an ohmic conduction in lower voltages and a space charge limited conduction (SCLC) controlled by a single and an exponential trapping levels at two different ranges of applied voltages. The hole concentrations are obtained as P _o = 3.92 × 10¹⁶ m⁻³ with a hole mobility μ = 5.81 × 10⁻⁶ m⁻¹ V⁻¹ s⁻¹. In the SCLC region a discrete trap level of 1.88 × 10²¹ m⁻³ is found at 0.66 eV followed by an exponential trap distribution of P _e = 4.63 × 10⁴⁶ J ⁻¹ m⁻³ at N _t(e) = 2.23 × 10²⁶ m⁻³. From the current limitations in the reverse bias, the conduction is identified as an electrode limited Schottky type of conduction. In the oxygen-doped samples, both in the forward and reverse bias the order of currents are much enhanced and a transition from the ohmic conduction to a space charged conduction is observed.     

Effect of carbon nanotube network morphology on thin film transistor performance

The properties of electronic devices based on carbon nanotube networks (CNTNs) depend on the carbon nanotube (CNT) deposition method used, which can yield a range of network morphologies. Here, we synthesize single-walled CNTs using an aerosol (floating catalyst) chemical vapor deposition process and deposit CNTs at room temperature onto substrates as random networks with various morphologies. We use four CNT deposition techniques: electrostatic or thermal precipitation, and filtration through a filter followed by press transfer or dissolving the filter. We study the mobility using pulsed measurements to avoid hysteresis, the on/off ratio, and the electrical noise properties of the CNTNs, and correlate them to the network morphology through careful imaging. Among the four deposition methods thermal precipitation is found to be a novel approach to prepare high-performance, partially aligned CNTNs that are dry-deposited directly after their synthesis. Our results provide new insight into the role of the network morphologies and offer paths towards tunable transport properties in CNT thin film transistors.      

沉积气压对相变域硅薄膜性能的影响

采用甚高频等离子体增强化学气相沉积（VHF-PECVD）法,成功制备出一系列从非晶到微晶过渡区域的硅薄膜。研究了气体压强对样品的微结构、光电特性、输运性质以及沉积速率的调控作用。结果表明,增大沉积气压可以提高材料的光敏性及沉积速率,但材料的结构有序度以及输运特性变差。     

Effect of Ag doping on the performance of ZnO thin film transistor

Ag-doped zinc oxide (SZO) thin film transistors (TFTs) have been fabricated using a back-gate structure on thermally oxidized and heavily doped p-Si (100) substrate. The SZO thin films were deposited via pulsed laser deposition (PLD) from a 1, 3, and 5 wt.% Ag-doped ZnO (1SZO, 3SZO, and 5SZO, respectively) target using a KrF excimer laser (λ, 248 nm) at oxygen pressure of 350 mTorr. The deposition carried out at both room-temperature (RT) and 200 °C. The SZO thin films had polycrystalline phase with preferred growth direction of (002) as well as a wurtzite hexagonal structure. Compare with ZnO thin films, the SZO thin films were characterized by confirming the shift of (002) peak to investigate the substitution of Ag dopants for Zn sites. The as-grown SZO TFTs deposited at RT and 200 °C showed insulator characteristics. However the SZO TFTs annealed at 500 °C showed good n-type TFT performance because Ag was diffused from Zn lattice site and bound themselves at the high temperature, and it caused generation of electron carriers. The post-annealed 5SZO TFT deposited at 500 °C exhibited a threshold voltage (V_th) of 11.5 V, a subthreshold swing (SS) of 2.59 V/decade, an acceptable mobility (μ_SAT) of 0.874 cm²/V s, and on-to-off current ratios (I_on/off) of 1.44 × 10⁸.     

Investigations on the structure, composition and performance of nanocrystalline thin film thermocouples deposited using anodic vacuum arc

This paper deals with the performance study of nanocrystalline thin film thermocouples (TFTCs) fabricated using anodic vacuum arc plasma aided deposition technique. Various single junction single elemental metal-metal pairs, elemental metal-metal alloy pairs, and metal alloy-metal alloy pairs were developed on glass substrates Elemental metal films were annealed at 10^− 4 Pa for 4 h while metal alloy films were annealed for 5 h. Their thermoelectric response has been studied in ambient air up to a maximum temperature difference of 300 °C between hot junction and cold junction. The phase purity, microstructure and composition of individual layer films were extensively studied. Elemental metal pairs agree well with their wire thermocouple equivalents. Thermoelectric power (TEP) of Cu-Ni and Fe-Ni TFTCs were found to be 17.81 μV/°C and 27.94 μV/°C at 300 °C, respectively. Among metal alloy-metal alloy TFTCs, a TEP of 32.87 μV/°C at 300 °C was obtained for Chromel-Alumel TFTCs which agree fairly well with its wire counterpart. However, Constantan based TFTCs deviated considerably from their wire counterparts. Cu-Constantan, Fe-Constantan and Chromel-Constantan showed a TEP of 26.48 μV/°C, 35.76 μV/°C and 37.41 μV/°C at 300 °C respectively. This deviation in thermoelectric power of Constantan based TFTCs with their wire counterparts were due to the fractionation of the Constantan arm. This fractionation leads to decrease of Ni content in the film which in turn reduces their TEP.     

Effect of composition,film thickness and annealing on the optical properties of Bi-Sb-Se thin films

Thin films of Bi₁₀Sb _x Se_90–x (x35, 40, 45) of different composition and thickness, were deposited on glass substrates by vacuum evaporation. Optical absorption measurements show that the fundamental absorption edge is a function of glass composition, film thickness and annealing temperature. The optical absorption is due to indirect electronic transitions. The value of the optical band gap was found to increase with thickness and decreasing the antimony content and with increasing temperature of heat treatment. The validity of the Urbach rule was investigated and the respective parameters estimated. X-ray diffraction was used to obtain an insight into the structural information.     

Influence of ambient air exposure on surface chemistry and electronic properties of thin copper phthalocyanine sensing layers

In this paper we present photoemission studies of the influence of 12-hour exposure to the ambient air on the chemical and electronic properties of thin 16-nm copper phthalocyanine (CuPc) sensing layers deposited on n- and p-type silicon Si(111) substrates covered with the native oxide. The surface chemistry and electronic parameters of organic thin film including surface band bending, work function, electron affinity and their variations upon the exposure have been monitored with X-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy techniques. We found that after the exposure, the surface chemistry of CuPc remained unaffected, however the work function and surface band bending increased by 0.55 eV and 0.45 eV for the layers on n-Si and by 0.25 eV and 0.30 eV for those on p-Si. Additionally, we detected a slight surface dipole at CuPc on n-Si manifested by a small shift in electron affinity of 0.10 eV. In order to explain these changes we developed a model basing on the interaction of ionic species with the phthalocyanine surface.