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.     

Localization effect of a current-path in amorphous In-Ga-Zn-O thin film transistors with a highly doped buried-layer

The highly-doped buried layer (carrier concentration of ~ 10¹⁹ cm^− 3) in an amorphous indium-gallium-zinc oxide (a-IGZO) channel layer of thin film transistor (TFT) led to dramatic improvements in the performance and prolonged bias-stability without any high temperature treatment. These improvements are associated with the enhancement in density-of-states and carrier transport. The channel layer is composed of Ga-doped ZnO (GZO) and a-IGZO layers. Measurements performed on GZO-buried a-IGZO (GB-IGZO) TFTs indicate enhanced n-channel active layer characteristics, such as V_th, μ_FE, I_off, I_on/off ratio and S.S, which were enhanced to 1.2 V, 10.04 cm²/V·s, ~ 10⁻¹³A, ~ 10⁷ and 0.93 V/decade, respectively. From the result of simulation, a current path was well defined through the surface of oxide active layer especially in GB-IGZO TFT case because the highly-doped buried layer plays the critical role of supplying sufficient negative charge density to compensate the amount of positive charge induced by the increasing gate voltage. The mechanism underlying the high performance and good stability is found to be the localization effect of a current path due to a highly-doped buried layer, which also effectively screens the oxide bulk and/or back interface trap-induced bias temperature instability.     

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.     

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.     

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.     

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.     

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.     

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.     

Zinc cadmium oxide thin film transistors fabricated at room temperature

Zinc cadmium oxide (ZnCdO) transparent thin film transistors (TFTs) have been fabricated with a back-gate structure using highly p-type Si (001) substrate. For the active channel, 30 nm, 50 nm, and 100 nm thick ZnCdO thin films were grown by pulsed laser deposition. The ZnCdO thin films were wurtzite hexagonal structure with preferred growth along the (002) direction. All the samples were found to be highly transparent with an average transmission of about 80%~ in the visible range. We have investigated the change of the performance of ZnCdO TFTs as the thickness of the active layer is increased. The carrier concentration of ZnCdO thin films has been confirmed to be increased from 10¹⁶ to 10¹⁹ cm⁻³ as the film thickness increased from 30 to 100 nm. Base on this result, the ZnCdO TFTs show a thickness-dependent performance which is ascribed to the carrier concentration in the active layer. The ZnCdO TFT with 30 nm active layer showed good off-current characteristic of below ~ 10¹¹, threshold voltage of 4.69 V, a subthreshold swing of 4.2 V/decade, mobility of 0.17 cm²/V s, and on-to-off current ratios of 3.37 × 10⁴.     

Threshold voltage shift by controlling Ga in solution processed Si-In-Zn-O thin film transistors

The threshold voltage change of solution processed gallium-silicon-indium-zinc oxide (GSIZO) thin film transistors (TFTs) annealed at 200 °C has been investigated depending on gallium ratio. GSIZO thin films were formed with various gallium ratios from 0.01 to 1 M ratio. The 30 nm-thick GSIZO film exhibited optimized electrical characteristics, such as field effect mobility (μ_FE) of 2.2 × 10^− 2 cm²/V·s, subthreshold swing (S.S) of 0.11 V/dec, and on/off current ratio (I_on/off) of above 10⁵. The variation of gallium metal cation has an effect on the threshold voltage (V_th) and the field effect mobility (μ_FE). The V_th was shifted toward positive direction from − 5.2 to − 0.4 V as increasing gallium ratio, and μ_FE was decreased from 2.2 × 10^− 2 to 5 × 10^− 3 cm²/V s. These results indicated that gallium was acted as carrier suppressor by degenerating oxygen vacancy. The electrical property of GSIZO TFTs has been analyzed as a function of the gallium ratio in SIZO system, and it clearly showed that variation of gallium contents could change on the performance of TFTs.     

Effect of magnesium oxide passivation on the performance of amorphous indium-gallium-zinc-oxide thin film transistors

Effect of hygroscopic magnesium oxide (MgO) passivation layer on the stability of amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) under positive bias stress and positive bias temperature stress has been investigated. The effect of MgO passivation has been observed by comparing the shift of the positive threshold voltage (V_th) after constant bias temperature stress, which were 8.2 V for the unpassivated TFTs and 1.88 V for the passivated TFTs.In addition, MgO passivated a-IGZO TFTs show also excellent stability under a humidity test since MgO passivation layer can prevent the penetration of water into back channel. In order to investigate the origin of humidity test result, we have measured X-ray photoelectron spectroscopy depth profile of both unpassivated and MgO passivated TFTs with a-IGZO back channel layers after N₂ wet annealing.     

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.     

Influence of process steps on the performance of microcrystalline silicon thin film transistors

Bottom gate microcrystalline silicon thin film transistors (μc-Si TFT) have been realized with two types of films: μc-Si(1) and μc-Si(2) with crystalline fraction of 80% and close to 100% respectively. On these TFTs we applied two types of passivation (SiN_x and resist). μc-Si TFTs with resist as a passivation layer present a low leakage current of about 2.10^− 12 A for V_G = − 10 and V_D = 0.1V an ON to OFF current ratio of 10⁶, a threshold voltage of 7 V, a linear mobility of 0.1 cm²/V s, and a sub-threshold voltage of 0.9 V/dec. Microcrystalline silicon TFTs with SiN_x as a passivation present a new phenomenon: a parasitic current for negative gate voltage (− 15 V) causes a bump and changes the shape of the sub-threshold region. This excess current can be explained by and oxygen contamination at the back interface.     

High performance self-aligned top-gate ZnO thin film transistors using sputtered Al2O3 gate dielectric

High performance self-aligned top-gate zinc oxide (ZnO) thin film transistors (TFTs) utilizing high-k Al₂O₃ thin film as gate dielectric are developed in this paper. Good quality Al₂O₃ thin film was deposited by reactive DC magnetron sputtering technique using aluminum target in a mixed argon and oxygen ambient at room temperature. The resulting transistor exhibits a field effect mobility of 27 cm²/V s, a threshold voltage of − 0.5 V, a subthreshold swing of 0.12 V/decade and an on/off current ratio of 9 × 10⁶. The proposed top-gate ZnO TFTs in this paper can act as driving devices in the next generation flat panel displays.     

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.     

Zinc oxide thin film transistors using MgO-Bi1.5Zn1.0Nb1.5O7 composite gate insulator on glass substrate

We report on high mobility ZnO thin film transistors (TFTs) (< 5 V), utilizing a room temperature grown MgO-Bi_1.5Zn_1.0Nb_1.5O₇ (BZN) composite gate insulator on a glass substrate. 30 mol% MgO added BZN composite gate insulators exhibited greatly enhanced leakage current characteristics (~< 2 × 10^− 8 A/cm² at 0.3 MV/cm) due to the high breakdown strength of MgO, while retaining an appropriate high-k dielectric constant of 32. The ZnO-TFTs with MgO-BZN composite gate insulators showed a high field-effect mobility of 37.2 cm²/Vs, a reasonable on-off ratio of 1.54 × 10⁵, a subthreshold swing of 460 mV/dec, and a low threshold voltage of 1.7 V.     

The effect of thermal annealing sequence on amorphous InGaZnO thin film transistor with a plasma-treated source–drain structure

In this paper, the effects of thermal annealing and the plasma treatment sequence on the performance of amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) without conventional source/drain (S/D) layer deposition were investigated. We fabricated TFTs using two different processes, one where S/Ds were plasma-treated after thermal annealing, the second where the S/Ds were plasma-treated before annealing. The performance of the former exhibited a linear mobility of 4.97 cm²/V s, an on/off ratio of 4.6 × 10⁶, a V_th of 2.56 V, and a subthreshold slope of 0.65 V/decade. However, the TFT parameters of the latter sample were reduced to a linear mobility of 0.07 cm²/V s, an on/off ratio of 1.5 × 10⁵, a V_th of 2.33 V, and a subthreshold slope of 3.54 V/decade. It was shown that the sheet resistance of plasma-treated S/D areas increased after thermal annealing by about three orders of magnitude. As a result, the increase of the sheet resistance caused a decrease of TFT performance.     

Interfacial characteristics and dielectric properties of Ba0.65Sr0.35TiO3 thin films

Ba_0.65Sr_0.35TiO₃ (BST) thin films were deposited on Pt/Ti/SiO₂/Si substrates by radio frequency magnetron sputtering technique. X-ray photoelectron spectroscopy (XPS) depth profiling data show that each element component of the BST film possesses a uniform distribution from the outermost surface to subsurface, but obvious Ti-rich is present to BST/Pt interface because Ti⁴⁺ cations are partially reduced to form amorphous oxides such as TiO_x (x < 2). Based on the measurement of XPS valence band spectrum, an energy band diagram in the vicinity of BST/Pt interface is proposed. Dielectric property measurements at 1 MHz reveal that dielectric constant and loss tangent are 323 and 0.0095 with no bias, while 260 and 0.0284 with direct current bias of 25 V; furthermore, tunability and figure of merit are calculated to be 19.51% and 20.54, respectively. The leakage current density through the BST film is about 8.96 × 10^− 7 A/cm² at 1.23 V and lower than 5.66 × 10^− 6 A/cm² at 2.05 V as well as breakdown strength is above 3.01 × 10⁵ V/cm.     

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.