Device characteristics improvement of a-In-Ga-Zn-O TFTs by low-temperature annealing

A low-temperature process to improve performances of a-In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) fabricated at room temperature was examined. Two deposition methods, pulsed laser deposition (PLD) and RF magnetron sputtering were employed to deposit the a-IGZO channels. For the PLD case, the TFT characteristics were improved significantly by wet annealing at dew point (d.p.) of 50 °C at the annealing temperature of 200 °C. For the sputtered TFTs, a wider range of annealing temperature from 100 to 200 °C was examined. It was found that annealing at ≥ 150 °C improved the TFT characteristics when dry annealing was employed. On the other hand, wet annealing also improved μ_sat and S values, but very large negative threshold voltage (V_th) shift was observed. These results indicate that the annealing at 150 °C is enough to obtain mobility (μ_sat) as large as 8 cm² Vs^− 1, but annealing temperature as high as 200 °C provides larger μ_sat comparable to those obtained by 400 °C annealing. It is speculated that the large negative V_th shift originates from compensated donors in as-deposited sputtered films.     

Operation model with carrier-density dependent mobility for amorphous In-Ga-Zn-O thin-film transistors

An operation model for an amorphous In-Ga-Zn-O (a-IGZO) based thin film transistor (TFT) is studied. The model is not based on the exponential tail states employed in hydrogenated amorphous Si (a-Si:H) TFT, but on a power function of the carrier density which is observed in the TFT and Hall mobilities of a-IGZO. A 2D numerical simulator employing this model reproduced current-voltage characteristics under on operation of coplanar homojunction a-IGZO TFTs. Although the mathematical expression of the mobility is similar to the field effect mobility of a-Si:H TFT, the present model explains the temperature dependence of the on characteristics of a-IGZO TFT.     

Steady-state photoconductivity of amorphous In-Ga-Zn-O

Photoresponse was investigated for an amorphous oxide semiconductor, In-Ga-Zn-O, by the steady-state photoconductivity (SSPC) method. All the films exhibited extremely slow reversible photoresponses. Analysis of the transient photocurrent at varied temperatures provided similar activation energies of ~ 0.5 eV for both the time constants and the photoconductivity. Mobility-lifetime (μτ) products were estimated from the photoconductivity spectra measured at the sweep rate of 2 nm/s, which monotonically increased with increasing dark conductivity σ_D (i.e. the Fermi level E_F becomes shallower). The obtained μτ values are larger than those of hydrogenated amorphous silicon even if the E_F dependence is considered.     

Stability and high-frequency operation of amorphous In-Ga-Zn-O thin-film transistors with various passivation layers

The stability of amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) was investigated focusing on the effects of passivation layer materials (Y₂O₃, Al₂O₃, HfO₂, and SiO₂) and thermal annealing. Positive bias constant current stress (CCS), negative bias stress without light illumination (NBS), and negative bias light illumination stress (NBLS) were examined. It was found that Y₂O₃ was the best passivation layer material in this study in terms of all the stability tests if the channel was annealed prior to the passivation formation (post-deposition annealing) and the passivation layer was annealed at ≥ 250 °C (post-fabrication annealing). Post-fabrication thermal annealing of the Y₂O₃ passivation layer produced very stable TFTs against the CCS and NBS stresses and eliminated subgap photoresponse up to the photon energy of 2.9 eV. Even for NBLS with 2.7 eV photons, the threshold voltage shift is suppressed well to − 4.4 V after 3 h of test. These results provide the following information; (i) passivation removes the surface deep subgap defects in a-IGZO and eliminates the subgap photoresponse, but (ii) the bulk defects in a-IGZO should be removed prior to the passivation process. The Y₂O₃-passivated TFT is not only stable for these stress conditions, but is also compatible with high-frequency operation with the current gain cut-off frequency of 91 kHz, which is consistent with the static characteristics.     

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.     

Transparent amorphous In-Ga-Zn-O thin film as function of various gas flows for TFT applications

The electrical and optical properties of amorphous indium gallium zinc oxide (a-IGZO) films, which can be used as a channel layer, deposited by radio frequency (rf) magnetron sputtering system at room temperature (RT), were investigated as function of various gas flows. The optical transmittance of films deposited under Ar, O₂ / Ar + O₂ and O₂ / Ar-4% H₂ + O₂ atmospheres in the visible wavelength was consistently above 90% at a wavelength of 550 nm at all gas flows, although the film deposited under Ar-4% H₂ atmosphere exhibited a transmittance of below 50%. The carrier concentration and mobility of the a-IGZO films fabricated under Ar and Ar-4% H₂ were observed slight decrease as a function of the flow, respectively. The thin film transistors (TFTs) with an a-IGZO channel deposited under Ar and Ar-4% H₂ atmosphere exhibited the following good characteristics: V_th of 0.34 V, µ_FE of 3.6 cm² V^− 1 s^− 1, on/off ratio of 10⁶, and S value of 0.04 V decade^− 1.     

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.     

Effect of oxygen partial pressure on electrical characteristics of amorphous indium gallium zinc oxide thin-film transistors fabricated by thermal annealing

We report the fabrication and electrical characteristics of high-performance amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) with a polymer gate dielectric prepared by spin coating on a glass substrate at different oxygen partial pressure values. The transmittance of the deposited polymer film was greater than 90% at 600 nm a-IGZO thin films were deposited on glass substrates using RF magnetron sputtering at different oxygen partial pressure values. The a-IGZO TFTs were prepared by rapid thermal annealing at 350 °C for 10 min at a 0.2% oxygen partial pressure. It was observed that a-IGZO TFTs with an active channel layer exhibited enhanced mode operation, a threshold voltage of 1 V, an on-off current ratio of 10³, and a field-effect mobility of 18 cm²/Vs.     

Degradation characteristics and light-induced effects of polymer thin-film transistors

Polymer thin-film transistors based on poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene) have been fabricated by spin-coating process and characterized. The electrical characteristics of the devices stored in dry air show obvious degradation with a smaller mobility due to oxygen effect, and lower threshold voltage. The devices present good optical response in low-light condition and optically induced memory effects, demonstrating their use as promising smart light-detection devices. Moreover, solution preparation, deposition and device measurements have been all performed in the air for the purpose of large-area applications.     

An investigation of contact resistance between metal electrodes and amorphous gallium-indium-zinc oxide (a-GIZO) thin-film transistors

This paper reports our investigation of different source/drain (S/D) electrode materials in thin-film transistors (TFTs) based on an indium-gallium-zinc oxide (IGZO) semiconductor. Transfer length, contact resistance, channel conductance, and effective resistances between S/D electrodes and amorphous IGZO thin-film transistors were examined. Intrinsic TFT parameters were extracted by the transmission line method (TLM) using a series of TFTs with different channel lengths measured at a low drain voltage. The TFTs fabricated with Cu S/D electrodes showed the lowest contact resistance and transfer length indicating good ohmic characteristics, and good transfer characteristics with intrinsic field-effect mobility (μ_FE-i) of 10.0 cm²/Vs.     

Photovoltaic properties of n-type amorphous In-Ga-Zn-O and p-type single crystal Si heterojunction solar cells: Effects of Ga content

Photovoltaic properties and electronic structures of n-type amorphous In-Ga_x-Zn-O/p-type Si heterojunction solar cells (x = 1, 2, and 3) were investigated focusing on the effects of Ga content based on expectation that Ga-rich films have larger band gaps and improve open circuit voltages (V_OC) of solar cells. To know the electronic structures such as the conduction band minimums (CBMs) and the valence band maximums (VBMs) of these materials, hard x-ray photoemission spectroscopy (HX-PES) was performed. Contrary to the above expectation, the best result was obtained for x = 1 with an energy conversion efficiency of 5.3%. Although the Ga-rich films had larger optical band gaps and higher CBMs, V_OC were remained low and poorer fill factors were obtained due to larger densities of defects. The low V_OCwere partly resulted from the deep VBM levels of the Ga-rich films. The defect densities are discussed in relation also to near-VBM states and near-CBM states observed in HX-PES and subgap optical absorptions.     

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.     

Surface-induced time-dependent instability of ZnO based thin-film transistors

We report on the surface-induced time-dependent instability of ZnO based thin-film transistors (ZnO-TFTs) with interdigitated source/drain (S/D) electrodes. As time elapsed, a considerable shift of threshold voltage (V_T) was observed (by ~ − 16 V) from our TFT. Contact angle of de-ionized water on ZnO surface also changed from 30° to 110°, revealing time-dependent surface state change. According to X-ray photoemission spectroscopy (XPS) measurements, the Zn 2p_3/2 core-level peak and the valence band maximum (VBM) of aged ZnO surface shifted to the higher binding energy by 0.3 eV, which implies a downward energy band bending of the ZnO back channel-surface. We conclude that without passivation layer any bottom gate ZnO-TFT meets the surface-induced electrical instabilities due to the time-dependent conductance of ZnO surface.     

Fabrication of oxide-gate thin-film transistors using PECVD/PLD multichamber system

We developed a new multichamber system which combines a pulsed-laser deposition (PLD) and a plasma-enhanced chemical vapor deposition (PECVD) with shadow masks installed to define the film deposition area on a substrate. In order to verify thecapability of this PLD/PECVD multichamber system, hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFTs) using MgO and Al2O₃ gate dielectrics have been fabricated on glass/indium-tin-oxide (ITO) substrates. The MgO and Al2O₃ films fabricated on fused silica substrates by PLD exhibited transparency higher than 90% and a low leakage current (1 nA/cm² at 1 MV/cm). After depositions of the MgO or Al2O₃ film on the glass/ITO, thesample was transferred to the PECVD chamber for a-Si:H deposition without exposing them to the air. TFTs thus fabricated exhibited such high characteristics as the threshold voltage (VTH) as low as 0.35 V and gate bias dependence of source±drain current exceeding five orders of magnitude. Theresults indicate a high quality a-Si:H/oxide interface and that heterojunction devices can be produced by using the PLD/PECVD multichamber system.     

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.     

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.     

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.     

Wet etching rates of InGaZnO for the fabrication of transparent thin-film transistors on plastic substrates

The wet etch process for amorphous indium gallium zinc oxide (a-IGZO or a-InGaZnO) by using various etchants is reported. The etch rates of a-IGZO, compared to another indium-based oxides including indium gallium oxide (IGO), indium zinc oxide (IZO), and indium tin oxide (ITO), are measured by using acetic acid, citric acid, hydrochloric acid, perchloric acid, and aqua ammonia as etchants, respectively. In our experimental results, the etch rate of the transparent oxide semiconductor (TOS) films by using acid solutions ranked accordingly from high to low are IZO, IGZO, IGO and ITO. Comparatively, the etch rate of the TOS films by using alkaline ammonia solution ranked from high to low are IGZO, IZO, IGO and ITO, in that order.Using the proposed wet etching process with high etch selectivity, bottom-gate-type thin-film transistors (TFTs) based on a-IGZO channels and Y₂O₃ gate-insulators were fabricated by radio-frequency sputtering on plastic substrates. The wet etch processed TFT with 30 µm gate length and 120 µm gate width exhibits a saturation mobility of 46.25 cm² V^− 1 s^− 1, a threshold voltage of 1.3 V, a drain current on-off ratio > 10⁶ , and subthreshold gate voltage swing of 0.29 V decade^− 1. The performance of the TFTs ensures the applicability of the wet etching process for IGZO to electronic devices on organic polymer substrates.     

Formation of polycrystalline thin-film transistors with stacked poly-SiGe/poly-Si channel layer for low-voltage applications

A thin-film transistor (TFT) with polycrystalline SiGe/Si stacked channel layer has been proposed for low-voltage applications. For the stacked poly-SiGe/poly-Si channel layer, the resultant 1-μm TFT device can achieve an on/off current ratio above 7 orders and a relatively large on-state current at a low operating voltage, and also cause better transfer characteristics than both the conventional poly-Si and poly-SiGe channel layers. As compared to the poly-Si channel layer, the poly-SiGe channel layer may cause a larger on-state current at a small gate bias of 3 V, due to smaller difference between conduction band and intrinsic level. However, even at a small drain bias of 3 V, the poly-SiGe channel layer leads to an off-state leakage current of about 2 order larger than the poly-Si channel layer, since a smaller energy bandgap may cause more carrier field emission via trap states. As a result, when a poly-SiGe/poly-Si stacked channel layer is employed, the leakage current may be suppressed to a low level as that for the poly-Si channel layer, and the resultant on-state current at a low gate bias voltage can be close to a relatively high level as that for the poly-SiGe channel layer.     

Hot-wire thin-film transistors on PET at 100 °C

Bottom-gate amorphous silicon thin-film transistors were fabricated using active layers deposited by r.f. and hot-wire (HW) chemical vapor deposition on polyethylene terephthalate (PET) and polyimide (PI) substrates. The maximum processing temperature was 100 °C for PET and 250 °C for PI. For transistors deposited at 100 °C by r.f. on PET and at 175 °C by HW on PI the transistor characteristics are comparable, although still inferior, to those of standard amorphous silicon transistors fabricated on glass substrates at 250 °C. HW transistors fabricated at 100 °C showed poor device characteristics. For devices fabricated at 100 °C, an extended anneal at this temperature was required to improve the transistor characteristics, independently of the film deposition technique used.