The influence of fabrication process on top-gate thin-film transistors

Amorphous indium zinc oxide (a-IZO) thin-film transistors (TFTs) with bottom- and top-gate structures were fabricated at room temperature by direct current (DC) magnetron sputter in this research. High dielectric constant (κ) hafnium oxide (HfO₂) films and a-IZO were deposited for the gate insulator and the semiconducting channel under a mixture of ambient argon and oxygen gas, respectively. The bottom-gate TFTs showed good TFT characteristics, but the top-gate TFTs did not display the same characteristics as the bottom-gate TFTs despite undergoing the same process of sputtering with identical conditions. The electrical characteristics of the top-gate a-IZO TFTs exhibited strong relationships with sputtering power as gate dielectric layer deposition in this study. The ion bombardment and incorporation of sputtering ions damaged the interface between the active layer and the gate insulator in top-gate TFTs. Hence, the sputtering power was reduced to decrease damage while depositing HfO₂ films. When using 50 W DC magnetron sputtering, the top-gate a-IZO TFTs showed the following results: a saturation mobility of 5.62 cm²/V-s; an on/off current ratio of 1 × 10⁵; a sub-threshold swing (SS) of 0.64 V/decade; and a threshold voltage (V_th) of 2.86 V.     

Characteristics of gravure printed InGaZnO thin films as an active channel layer in thin film transistors

Characteristics of oxide semiconductor thin film transistor prepared by gravure printing technique were studied. This device had inverted staggered structure of glass substrate/MoW/SiNx/ printed active layer. The active layer was printed with precursor of indium gallium zinc oxide solution and then annealed at 550 °C for 2 h. Influences of printing parameters (i.e. speed and force) were studied. As the gravure printing force was increased, the thickness of printed film was decreased and the refractive index of printed active layer was increased. The best printed result in our study was obtained with printing speed of 0.4 m/s, printing force of 400 N and the thickness of printed active layer was 45 nm. According to AFM image, surface of printed active layer was quite smooth and the root-mean square roughness was approximately 0.5 nm. Gravure printed active layer had a field-effect mobility of 0.81 cm²/Vs and an on-off current ratio was 1.36 × 10⁶.     

C60 thin-film transistors with high field-effect mobility,fabricated by molecular beam deposition

We report performance of C₆₀ thin-film field-effect transistors and characterizations of C₆₀ thin-films on SiO₂ substrates fabricated by molecular beam deposition. Devices, fabricated and characterized under high vacuum without exposing to air, routinely showed current on/off ratios >10⁸ and field-effect mobility in the range of 0.5–0.3 cm²/V s. The obtained mobility is comparable to the highest value among n-type organic thin-film transistors and close to that derived from the photocurrent measurements on C₆₀ thin-films.The grain size of C₆₀ thin-film, condensed in an fcc solid, increases with the substrate temperature, while themobility did not exhibit a clear relation with substrate temperature.

© 2003 Elsevier Ltd. All rights reserved.     

Electrical properties of AlNx/Al/Mo composite film prepared for use in thin-film transistors

In the present study, AlN_x/Al/Mo composite films with various thicknesses of AlN_x and Al layers were prepared to replace commercial AlNd/Mo composite film as the gate metal of the two metal layers (namely the gate metal and the source-drain metal) in thin-film transistor (TFT) specimens. The prerequisite for the TFT device is that no hillock is formed. The electrical properties of the AlN_x/Al/Mo TFT device rival those of the AlNd/Mo TFT device. One of eight kinds of AlN_x/Al/Mo composite films (0.05 µm/0.2 µm/0.07 µm) without hillocks was compared with the AlNd/Mo (0.25 µm/0.07 µm) composite film. The line width after development and strip inspections, the I_g (gate leakage current)-V_g (gate voltage) curve, the coating film resistance to electricity, the contact resistance between the indium tin oxide (ITO) film and the metal film, the I_d-V_g curve, and the critical dimension loss (CD loss) were compared. The experimental results indicate that the metal line widths for these two composite films are similar. The coating film resistance, the contact resistance between the ITO film and the metal film, and the I_d-V_g curve for the AlN_x/Al/Mo TFT device were similar to those for the AlNd/Mo TFT device. The CD loss shown in the AlN_x/Al/Mo TFT device was lower than that for the AlNd/Mo TFT device.     

Analysis of effective channel length variation for thin-film transistors with edge waviness in source/drain electrodes

We analyzed the effective channel length variation of hydrogenated amorphous silicon thin-film transistors (TFTs) that have wavy edge source/drain (S/D) electrodes. Edge waviness is frequently observed when narrow electrodes are fabricated by using printing methods. We used hydrogenated amorphous silicon (a-Si:H) TFTs and photolithographically patterned wavy edge S/D electrodes for accurate analysis. From a transmission line method (TLM), we successfully related the channel current variation to the variation of current transfer length (L_{T_wavy}) of the wavy edge S/D electrodes originated from current spreading and geometrical edge waviness effects which can be separately extracted.     

Mn:SnO2 ceramics as p-type oxide semiconductor

SnO₂-based ceramics substituted with manganese as a new p-type oxide semiconductor were prepared by conventional solid state reaction. The Mn was ranged from 5 to 20 mol%, and the microstructure as well as the physical and chemical properties was characterized. Single-phase rutile of Mn:SnO₂ solid solution was obtained in all compositions. Lattice parameter was decreased with the increase of amount of Mn. The compositional change and electrical properties of the Mn:SnO₂ ceramics were confirmed by X-ray photoelectron spectroscopy and Hall effect measurement. The 5-10 mol% Mn:SnO₂ solid solutions exhibited electrically p-type behavior. The simultaneous presence of Mn²⁺, Mn³⁺ and Mn⁴⁺ states was approved and Sn⁴⁺ in Mn:SnO₂ ceramics was partially substituted with Mn³⁺ which contributes p-type behavior. SnO₂ substituted with higher contents of Mn³⁺ of 50% exhibits p-type semiconductor.     

Pentacene thin-film transistors and inverters with plasma-enhanced atomic-layer-deposited Al2O3 gate dielectric

The performances of pentacene thin-film transistor with plasma-enhanced atomic-layer-deposited (PEALD) 150 nm thick Al₂O₃ dielectric are reported. Saturation mobility of 0.38 cm²/V s, threshold voltage of 1 V, subthreshold swing of 0.6 V/decade, and on/off current ratio of about 10⁸ have been obtained. Both depletion and enhancement mode inverter have been realized with the change of treatment method of hexamethyldisilazane on PEALD Al₂O₃ gate dielectric. Full swing depletion mode inverter has been demonstrated at input voltages ranging from 5 V to − 5 V at supply voltage of − 5 V.     

Evaluation of Y2O3 gate insulators for a-IGZO thin film transistors

In this work, Y₂O₃ was evaluated as a gate insulator for thin film transistors fabricated using an amorphous InGaZnO₄ (a-IGZO) active layer. The properties of Y₂O₃ were examined as a function of various processing parameters including plasma power, chamber gas conditions, and working pressure. The leakage current density for the Y₂O₃ film prepared under the optimum conditions was observed to be ~ 3.5 × 10^− 9 A/cm² at an electric field of 1 MV/cm. The RMS roughness of the Y₂O₃ film was improved from 1.6 nm to 0.8 nm by employing an ALD (Atomic Layer Deposition) HfO₂ underlayer. Using the optimized Y₂O₃ deposition conditions, thin film transistors (TFTs) were fabricated on a glass substrate. The important TFT device parameters of the on/off current ratio, sub-threshold swing, threshold voltage, and electric field mobility were measured to be 7.0 × 10⁷, 0.18 V/dec, 1.1 V, and 3.3 cm²/Vs, respectively. The stacked insulator consisting of Y₂O₃/HfO₂ was highly effective in enhancing the device properties.     

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.     

Fabrication and texture evolution of hexagonal YMnO3 nanofibers by electrospinning

Hexagonal YMnO₃ nanofibers were successfully fabricated by sol-gel preparation based on electrospinning. The as-spun fibers dried at 125 °C were round and had a rather uniform diameter around 0.7 μm-2 μm over its length. In order to get pure hexagonal YMnO₃ nanofibers, crystalline structures and microstructures of fibers at various temperatures for 6 h were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The reasonable evaluations for the change of morphology with the increasing temperature were proposed. After being heated at 1100 °C for 6 h, the pure hexagonal YMnO₃ nanofibers were obtained with a reduced diameter ranging from 200 nm to 800 nm and the fibers were homogenous in chemical constitution over its length.     

Electron doping into the surface of SrTiO3 single crystal by using a field effect transistor structure having a polyvinyl alcohol gate insulator layer

We fabricated an n-channel accumulation-type field-effect transistor (FET) on a SrTiO₃ single crystal having a polyvinyl alcohol (PVA) layer and a Na⁺-doped PVA layer as a gate insulator. Both devices show an n-type FET performance having a response time of 10 - 100 second order. This slow response is due to the gradual formation of an electric double layer by solvated Na⁺ ions moving through the water absorbed in the PVA layer towards the surface of SrTiO₃ by applying a gate voltage (V_G), which is manifested by the fact that Na⁺-doping into the PVA layer dramatically reduces the driving voltages and the response time. In these devices, metallic or semiconducting behaviors are observed if the applied V_G is large enough to form a conducting channel during cooling of the devices. These results indicate that electrons are certainly doped into the surface of SrTiO₃ by applying V_G in both devices.     

PL and EL properties of Tm-activated vanadium oxide-based phosphor thin films

The preparation of high-luminance blue emitting Tm-activated multicomponent oxide phosphor thin films is described. The phosphor thin films were deposited on thick BaTiO₃ ceramic sheets by r.f. magnetron sputtering using powder targets. A very high photoluminescence (PL) intensity in blue emission could be observed in postannealed Tm-activated gadolinium oxide-vanadium oxide Gd_0.5-V_0.5-O:Tm thin films prepared using Tm₂O₃-doped (Gd₂O₃)_0.5-(V₂O₅)_0.5 targets (V₂O₅ content of 50 mol.%). It should be noted that thin films of this phosphor postannealed at 1100 °C in air exhibited higher PL intensity than those from commercially available blue BaMgAl₁₄O₂₃:Eu (BAM) phosphor powder and Gd_0.5-V_0.5-O:Tm powders. In addition, a Gd_0.5-V_0.5-O:Tm thin-film electroluminescent device exhibited a blue emission that was the same as the PL emission.     

First-principles calculations of vacancy formation in In-free photovoltaic semiconductor Cu2ZnSnSe4

To quantitatively evaluate the formation energies of Cu, Zn, Sn, and Se vacancies in kesterite-type Cu₂ZnSnSe₄ (CZTSe), first-principles pseudopotential calculations using plane-wave basis functions were performed. The formation energies of neutral Cu, Zn, Sn and Se vacancies were calculated as a function of the atomic chemical potentials of constituent elements. The obtained results were as follows: (1) the formation energy of Cu vacancy was generally smaller than those of the other Zn, Sn and Se vacancies, (2) under the Cu-poor and Zn-rich condition, the formation energy of Cu vacancy was particularly low, (3) the formation energy of Zn vacancy greatly depended on the chemical potentials of the constituent elements and under the Zn-poor and Se-rich condition, the formation energy of Zn vacancy was smaller than that of Cu vacancy, and (4) the formation energy of Sn vacancy did not greatly depend on the chemical potentials of the constituent elements and was much larger than those of Cu, Zn, and Se vacancies. These results indicate that Cu vacancy is easily formed under Cu-poor and Zn-rich conditions, but Zn vacancy is easily formed under the Zn-poor and Se-rich conditions.     

Electron transport in InGaO3(ZnO)m (m=integer) studied using single-crystalline thin films and transparent MISFETs

We have investigated the characteristics of transparent metal-insulator-semiconductor field-effect transistors (MISFETs) fabricated using InGaO₃(ZnO)_m (m=integer) single-crystalline thin films as n-channel layers and amorphous alumina as gate insulator films. The MISFETs exhibit good characteristics such as insensitivity to visible light illumination, off-current as low as ∼1 nA with a positive threshold voltage of ∼3 V and on/off current ratio of 10⁵. The field-effect mobility increased from ∼1 to ∼10 cm² (V s)⁻¹ as the m-value increased. Room temperature Hall mobility also increased. However, unexpectedly these values were lower than the field-effect mobility. It is explained by existence of shallow localized state in the homologous compounds.     

Impurity dependent semiconductor type of epitaxial CuFeO2 (111) thin films deposited by using a pulsed laser deposition

We have prepared CuFeO₂ thin films successfully oriented to the (111) direction on amorphous glass substrates by PLD. The average grain size analyzed by SEM images is about 80-90 nm, and CuFeO₂ grains are formed to the hexagonal flat shape which means CuFeO₂ with the rhombohedral structure was hexagonally grown on the amorphous glass substrate. P-type conductivities are commonly governed by impurities of the amount of metallic Cu phase. However, it was found that the highly (111) oriented CuFeO₂ film shows insulation properties and CuFe₂O₄ phase affects the change of the type of semiconductor from p-type to n-type.     

Fabrication of transparent CuCrO2:Mg/ZnO p-n junctions prepared by pulsed laser deposition on glass substrate

Transparent p-n heterojunctions composed of zinc oxide, copper-chromium, and indium tin oxide films were fabricated by the pulsed laser deposition technique on a glass substrate. The effect of the deposition temperature of the p-CuCrO₂:Mg layer in the junction on photovoltaic properties was investigated. Post-annealing was performed to improve the crystallinity of the semiconductor layers deposited at a relatively lower temperature. The rectifying characteristics were observed in the current-voltage curves of the prepared junctions for both p- and n-layers as thin as 100 nm. A sample in which the copper-chromium oxide layer was deposited at 250 °C and annealed at 500 °C for 10 min exhibited the highest photovoltage—as large as 184 mV—under irradiation at λ ≈ 375 nm. The optical transmission of the p-n junction sample was 70% in the visible region.     

Optical and electrical properties of multiferroic bismuth ferrite thin films fabricated by sol-gel technique

Single-phase multiferroic BiFeO₃ thin films have been prepared on LaNiO₃/Si(100) and Si(100) wafer via sol-gel technique. The films are polycrystalline with preferring orientation of (101). The film has a conspicuous absorption in the blue and green light region, and band gap of 2.74 eV. The refractive index and the extinction coefficient of the film is about 2.36 and 0.06 at 600 nm, 2.26 and close to zero in the range of 800-1200 nm, respectively. The films also exhibit favorable ferroelectric and dielectric properties. A large photo induced open-circuit voltage was observed, indicating that the film exhibits photovoltaic behaviours.     

Crystal structure and ferroelectric properties of Ca(Cu3−xMx)Ti4O12 (M = Fe and Ni) ceramics

M-substituted Ca(Cu_3−xM_x)Ti₄O₁₂ (CCMTO) ceramics, where M = Fe and Ni, were synthesized and the influence of M substitutions for Cu on the crystal structure and ferroelectric properties of CCMTO ceramics were investigated in this study. From the variations in the lattice parameters of CCMTO ceramics, the solubility limit of Ni substitution for Cu in CaCu_3−xNi_xTi₄O₁₂ (CCNTO) ceramics was x = 0.2, whereas that of CaCu_3−xFe_xTi₄O₁₂ (CCFTO) ceramics was x = 0.05. The crystal structural analysis of CCMTO ceramics revealed that the single phase of CCMTO ceramics belongs to the I23 non-centrosymmetric space group of I23; as a result, the P_r and E_c values of CCFTO ceramics at x = 0.05 were 1.8 μC/cm² and 40 kV/cm, respectively. The ferroelectric behavior of CCMTO ceramics by the M substitutions for Cu may be related to the displacement of a Ti⁴⁺ cation in the TiO₆ octahedra and tilting of the Ti–O–Ti angle because of the non-centrosymmetric space group.     

The investigation of (Bi,La)(Ga,Fe)O3-PbTiO3 thin film prepared by PLD technique

In this report, (Bi,La)(Ga,Fe)O₃-PbTiO₃ (BLGF-PT) thin film was prepared on platinum coated Si wafer by pulsed laser deposition (PLD) method. BLGF-PT ceramic with morphotropic phase boundary (MPB) composition was used as the PLD target. The spot of Bi₂Fe₄O₉ impurity phase in X-ray diffraction profile implies a composition deviation in the BLGF-PT thin film from that of target. However, the MPB feature of co-existence of rhombohedral and tetragonal phases remains remarkable in the film. The characterization results of the leakage current and ferroelectric hysteresis loop indicate a desirable insulation in this BLGF-PT thin film. Polarization response by positive-up-negative-down pulse measurement and retention property are further carried out at room temperature to check the intrinsic ferroelectric performance. Moreover, high Curie temperature of 465 °C has been discovered in this BLGF-PT thin film, which may promote the application scope of the thin film.