The effect of annealing on amorphous indium gallium zinc oxide thin film transistors

This paper presents the post-annealing effects, caused by rapid thermal annealing (RTA), on amorphous indium gallium zinc oxide (a-IGZO) thin film transistor's (TFT) electrical characteristics, and its contact resistance (R_C) with thermally grown SiO₂ gate dielectric on silicon wafer substrates. The electrical characteristics of two types of TFTs, one post-annealed and the other not, are compared, and a simple model of the source and drain contacts is applied to estimate the R_C by a transmission line method (TLM). Consequently, it has been found that the post-annealing does improve the TFT performances; in other words, the saturation mobility (μ_sat), the on/off current ratio (I_ON/OFF), and the drain current (I_D) all increase, and the R_C and the threshold voltage (V_T) both decrease. As-fabricated TFTs have the following electrical characteristics; a saturation mobility (μ_sat) as large as 0.027 cm²/V s, I_ON/OFF of 10³, sub-threshold swing (SS) of 0.49 V/decade, V_T of 32.51 V, and R_C of 969 MΩ, and the annealed TFTs have improved electrical characteristics as follows; a μ_sat of 3.51 cm²/V s, I_ON/OFF of 10⁵, SS of 0.57 V/decade, V_T of 27.2 V, and R_C of 847 kΩ.     

Amorphous structure and electrical performance of low-temperature annealed amorphous indium zinc oxide transparent thin film transistors

The effect of low-temperature (200 °C) annealing on the threshold voltage, carrier density, and interface defect density of amorphous indium zinc oxide (a-IZO) thin film transistors (TFTs) is reported. Transmission electron microscopy and x-ray diffraction analysis show that the amorphous structure is retained after 1 h at 200 °C. The TFTs fabricated from as-deposited IZO operate in the depletion mode with on-off ratio of > 10⁶, sub-threshold slope (S) of ~ 1.5 V/decade, field effect mobility (μ_FE) of 18 ± 1.6 cm²/Vs, and threshold voltage (V_Th) of − 3 ± 0.7 V. Low-temperature annealing at 200 °C in air improves the on-current, decreases the sub-threshold slope (1.56 vs. 1.18 V/decade), and increases the field effect mobility (μ_FE) from 18.2 to 23.3 cm²/Vs but also results in a V_Th shift of − 15 ± 1.1 V. The carrier density in the channel of the as-deposited (4.3 × 10¹⁶ /cm³) and annealed at 200 °C (8.1 × 10¹⁷ /cm³) devices were estimated from test-TFT structures using the transmission line measurement methods to find channel resistivity at zero gate voltage and the TFT structures to estimate carrier mobility.     

The effect of metallization contact resistance on the measurement of the field effect mobility of long-channel unannealed amorphous In-Zn-O thin film transistors

The effect of contact resistance on the measurement of the field effect mobility of compositionally homogeneous channel indium zinc oxide (IZO)/IZO metallization thin film transistors (TFTs) is reported. The TFTs studied in this work operate in depletion mode as n-channel field effect devices with a field effect mobility calculated in the linear regime (μ_FE) of 20 ± 1.9 cm²/Vs and similar of 18 ± 1.3 cm²/Vs when calculated in the saturation regime (μ_FE^sat). These values, however, significantly underestimate the channel mobility since a large part of the applied drain voltage is dropped across the source/drain contact interface. The transmission line method was employed to characterize the contact resistance and it was found that the conducting-IZO/semiconducting-IZO channel contact is highly resistive (specific contact resistance, ρ_C > 100 Ωcm²) and, further, this contact resistance is modulated with applied gate voltage. Accounting for the contact resistance (which is large and modulated by gate voltage), the corrected μ_FE is shown to be 39 ± 2.6 cm²/Vs which is consistent with Hall mobility measurements of high carrier density IZO.     

Contact resistivity measurements of the buried Si-ZnO:Al interface of polycrystalline silicon thin-film solar cells on ZnO:Al

An experimental method is developed for contact resistivity measurements of a buried interface in polycrystalline silicon (poly-Si) thin-film solar cell devices on aluminum doped zinc oxide (ZnO:Al) layers. The solar cell concept comprises a glass substrate covered with a temperature-stable ZnO:Al film as transparent front contact layer, a poly-Si n⁺/p⁻/p⁺ cell, as well as a metal back contact. Glass/ZnO:Al/poly-Si/metal test stripe structures are fabricated by photolithographic techniques with the ZnO:Al stripes locally bared by laser ablation. The high-temperature treatments during poly-Si fabrication, e.g. a several hours lasting high-temperature step at 600 °C, are found to have no detrimental impact on the ZnO:Al/Si interface contact resistivity. All measured ρ_C values range well below 0.4 Ω cm² corresponding to a relative power loss ΔP below 3% for a solar cell with 500 mV open circuit voltage and 30 mA/cm² short circuit current density. By inclusion of a silicon nitride (SiN_x) diffusion barrier between ZnO:Al and poly-Si the electrical material quality of the poly-Si absorber can be significantly enhanced. Even in this case, the contact resistivity remains below 0.4 Ω cm² if the diffusion barrier has a thickness smaller than 10 nm.     

Comparative analysis of temperature thermally induced instability between Si-In-Zn-O and Ga-In-Zn-O thin film transistors

Thermally induced instability of amorphous Si-In-Zn-O (SIZO) with 1 wt.% silicon (Si) concentration and Ga-In-Zn-O (GIZO) with gallium (Ga) of 30 wt.% thin film transistors (TFTs) has been investigated, by comparing the density of states extracted from multi-frequency method. It was observed that the density of state of SIZO-TFT was lower than that of GIZO-TFT, in spite of low processing temperature of SIZO-TFT and thermally induced instability of SIZO- and GIZO-TFT was strongly related with the total trap density. We report that Si of only 1 wt.% in SIZO can improve thermal stability of threshold voltage of In-Zn-O based TFTs more effectively than Ga of 30 wt.% in GIZO.     

Contact resistance variation in top-contact organic thin-film transistors with the deposition rate of Au source/drain electrodes

We investigated the effect of the deposition rate of Au source/drain electrodes on the contact resistance of the top-contact organic thin-film transistors (OTFTs). For the formation of source/drain contacts, Au was thermally deposited at the different rates of 0.5, 1.0, 5.0, and 13.0 Å/s. With increasing the Au deposition rate, the contact resistance extracted at the gate voltage of − 30 V could be reduced from 14 × 10⁶ to 2.4 × 10⁶ Ω, resulting in the characteristic improvements of the top-contact OTFT. It is also found that the contact resistance significantly affects the off-state currents of the device having the short channel length of 10 μm. The control of the deposition rate of source/drain electrodes is suggested to optimize the contact properties of the top-contact OTFTs as well as the device performance.     

Effects of low-temperature ozone annealing on operation characteristics of amorphous In-Ga-Zn-O thin-film transistors

Effects of low-temperature annealing were examined for amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs). In a previous study, we reported that O₂ annealing is effective to improve performances of a-IGZO TFTs when annealed at ≥ 300 °C, but causes large negative threshold voltage shift when annealed at ≤ 200 °C. Here, we examined effects of ozone (O₃) annealing on physical properties and TFT characteristics of a-IGZO in comparison with conventional O₂ annealing. We found little differences in chemical composition, band gap and photoemission spectra between the O₂ and the O₃ annealed films. On the other hand, free electron density was suppressed well by the O₃ annealing even at low temperatures ≤ 200 °C. Moreover, even at 150 °C, the TFTs characteristics were improved to the subthreshold voltage swing of 217 mV/decade, the saturation mobility of ~ 11.4 cm²(Vs)^− 1 and the threshold voltage of 0.1 V by the O₃ annealing. It was also found that the effects of the O₃ annealing is more effective for thicker channel TFTs, which would be due to stronger oxidation power and the larger diffusion constant of oxygen atoms produced from O₃ molecules than those of O₂. These results substantiate that the O₃ annealing is more effective to improve TFT characteristics in particular for low-temperature processes at ≤ 200 °C.     

Nonvolatile memory characteristics of solution-processed oxide thin-film transistors using Ag nanoparticles

We developed a nonvolatile memory device based on a solution-processed oxide thin-film transistor (TFT) with Ag nanoparticles (NPs) as the charge trapping layer. We fabricated the device using a soluble MgInZnO active channel on a SiO₂ gate dielectric, Ag NPs as a charge trapping site at the gate insulator-channel interface, and Al for source and drain electrodes.The transfer characteristics of the device showed a high level of clockwise hysteresis that can be used to demonstrate its memory function, due to electron trapping in the Ag NPs charge trapping layer. A large memory window (?V_th) was observed with a forward and backward gate voltage sweep, and this memory window was increased in size by increasing the gate voltage sweep. These results show the potential application of memory on displays and disposable electronics.     

An amorphous oxide semiconductor thin-film transistor route to oxide electronics

Amorphous oxide semiconductor (AOS) thin-film transistors (TFTs) invented only one decade ago are now being commercialized for active-matrix liquid crystal display (AMLCD) backplane applications. They also appear to be well positioned for other flat-panel display applications such as active-matrix organic light-emitting diode (AMOLED) applications, electrophoretic displays, and transparent displays. The objectives of this contribution are to overview AOS materials design; assess indium gallium zinc oxide (IGZO) TFTs for AMLCD and AMOLED applications; identify several technical topics meriting future scrutiny before they can be confidently relied upon as providing a solid scientific foundation for underpinning AOS TFT technology; and briefly speculate on the future of AOS TFTs for display and non-display applications.     

High mobility top-gated poly(3-hexylthiophene) field-effect transistors with high work-function Pt electrodes

We report high-performance top-gated organic field-effect transistors (OFETs) with regio-regular poly(3-hexylthiophene) (rr-P3HT). The high charge carrier mobility in rr-P3HT FETs (0.4 cm²/Vs) was achieved due to the relatively low contact resistance and high crystallinity of rr-P3HT films. The contact resistance was controlled mainly through the use of high work-function platinum (Pt) (5.6 eV) for the charge injection electrode and a top-gate, bottom-contact geometry that enabled an enhanced current injection via current crowding in the staggered device structure. Moreover, the top-gate configuration provided improved device stability in air ambient conditions via the presence of a gate dielectric and gate electrode on top of the organic semiconductor.     

Surface states related the bias stability of amorphous In–Ga–Zn–O thin film transistors under different ambient gasses

This paper investigates the origin of the bias stability under ambient gas (oxygen, moisture and vacuum) of In–Ga–Zn–O thin film transistors with different annealing temperatures. In Zn-based TFTs, the electrical characteristic of device is a strongly function with the ambient gas, the simultaneous gas ambient and bias stresses are applied on devices annealed in atmosphere ambient to study this issue. The result shows the device which is annealed at temperature up to 330 °C has worst reliability. We suppose that the sensitivity of gas ambient depend the defect state, which is associated to the annealing temperature, of surface in a-IGZO.     

Sputtering deposition of amorphous cadmium stannate as transparent conducting oxide

In this work we studied deposition conditions by RF sputtering of ternary oxides of Cd and Sn, starting from Cd₂SnO₄ target and varying substrate temperature, sputtering power and deposition gas (from inert Ar to oxidizing 50% Ar-50% O₂ atmosphere). The aim of this study was to obtain thin films for use as Transparent Conducting Oxide (TCO). TCOs are oxides that couples low sheet resistance and high transparency that find application in many fields like solar cells, light emitting diodes and transparent thin film transistors.Thin films functional properties were characterised by means of sheet resistance and transmittance measurements in the visible region, and film composition and structure were investigated by total reflection X ray fluorescence and glancing incidence X ray diffraction. Morphology was studied by Atomic Force Microscopy and Scanning Electron Microscope and showed very smooth surface suitable for solar cells application. Composition and phase analysis allowed us to discuss possible correlation of film structure with functional properties. Deposition in inert atmosphere at 400 °C substrate temperature was selected for its low sheet resistance and high transparency that are comparable to the ones of commercial TCOs like indium tin oxide or SnO₂: F. The thin film obtained in these conditions was amorphous, and it crystallized into CdSnO₃ ilmenite phase when annealed at 700 °C; segregation of Sn₃O₄ was also observed. Since sheet resistance of thin films increases after annealing treatments, amorphous thin film was selected for future applications.     

Annealing effects of In2O3 thin films on electrical properties and application in thin film transistors

Considering practical applications in electronic devices, we studied the growth of In₂O₃ thin films on amorphous glasses by magnetron sputtering at room temperature and annealing effect on the structural and electrical properties. The vacuum annealed In₂O₃ thin films display a grain size enlargement and preferential orientation. Electrical characterization shows that the vacuum annealed In₂O₃ thin films exhibit a significant enhancement of both electron density and mobility, while air ambient annealing leads to a remarkable drop. The mechanism of the electrical characteristic changes in In₂O₃ thin films by annealing is explored by using different scattering mechanisms. Finally, a thin film transistor device using vacuum annealed In₂O₃ nano-meter thin films as active channel material is demonstrated.     

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.     

Modeling of amorphous InGaZnO thin film transistors using an empirical mobility function based on the exponential deep and tail states

We demonstrate that the voltage-dependent average mobility of In-Ga-Zn-O (IGZO) thin-film transistors (TFTs) can be excellently fitted by an empirical mobility function based on the exponential density of deep and tail states. The proposed mobility function needs only 5 parameters and does not need the concept of the threshold voltage which is inherently ambiguous in amorphous oxide TFTs. Both the transfer and output curves of the device are well reproduced by integrating the mobility function.     

DC sputter deposition of amorphous indium-gallium-zinc-oxide (a-IGZO) films with H2O introduction

Amorphous indium-gallium-zinc-oxide (a-IGZO) films were deposited by dc magnetron sputtering with H₂O introduction and how the H₂O partial pressure (P_H2O) during the deposition affects the electrical properties of the films was investigated in detail. Resistivity of the a-IGZO films increased dramatically to over 2 × 10⁵ Ωcm with increasing P_H2O to 2.7 × 10^− 2 Pa while the hydrogen concentration in the films increased to 2.0 × 10²¹ cm^− 3. TFTs using a-IGZO channels deposited under P_H2O at 1.6-8.6 × 10^− 2 Pa exhibited a field-effect mobility of 1.4-3.0 cm²/Vs, subthreshold swing of 1.0-1.6 V/decade and on-off current ratio of 3.9 × 10⁷-1.0 × 10⁸.