Transparent Ga and Zn co-doped In2O3 electrode prepared by co-sputtering of Ga:In2O3 and Zn:In2O3 targets at room temperature

This study examined the characteristics of Ga:In₂O₃ (IGO) co-sputtered Zn:In₂O₃ (IZO) films prepared by dual target direct current (DC) magnetron sputtering at room temperature in a pure Ar atmosphere for transparent electrodes in IGZO-based TFTs. Electrical, optical, structural and surface properties of Ga and Zn co-doped In₂O₃ (IGZO) electrodes were investigated as a function of IGO and IZO target DC power during the co-sputtering process. Unlike semiconducting InGaZnO₄ films, which were widely used as a channel layer in the oxide TFTs, the co-sputtered IGZO films showed a high transmittance (91.84%) and low resistivity (4.1 × 10^− 4 Ω cm) at optimized DC power of the IGO and IZO targets, due to low atomic percent of Ga and Zn elements. Furthermore, the IGO co-sputtered IZO films showed a very smooth and featureless surface and an amorphous structure regardless of the IGO and IZO DC power due to the room temperature sputtering process. This indicates that co-sputtered IGZO films are a promising S/D electrode in the IGZO-based TFTs due to their low resistivity, high transmittance and same elements with channel InGaZnO₄ layer.     

Investigation on indium concentration dependence of solution processed indium tin oxide thin film transistors

The effect of the indium content in indium tin oxide (ITO) films fabricated using a solution-based process and ITO channel thin film transistors (TFTs) was examined as a function of the indium mole ratio. The carrier concentration and resistivity of the ITO films could be controlled by the appropriate treatments. The TFTs showed an increase in the off-current due to the enhanced conductivity of the ITO channel layer with increasing indium mole ratios, producing an increase in the field effect mobility. The characteristics of the a-ITO channel TFT showed the best performance (μ_FE of 3.0 cm² V^− 1 s^− 1, V_th of 2.0 V, and S value of 0.4 V/decade) at In:Sn = 5:1.     

Thin film transistors by solution-based indium gallium zinc oxide/carbon nanotubes blend

Solution-based indium gallium zinc oxide (IGZO)/single-walled carbon nanotubes (SWNTs) blend have been used to fabricate the channel of thin film transistors (TFTs). The electrical characteristics of the fabricated devices were examined. We found a low leakage current and a higher on/off currents ratio for TFT with SWNTs compared to solution-based TFTs made without SWNTs. The saturation field effect mobility (μ_sat) of about 0.22 cm²/Vs, the current on/off ratio is ~ 10⁵, the subthreshod swing is ~ 2.58 V/decade and the threshold voltage (V_th) is less than − 2.3 V. We demonstrated that the solution-based blend active layer provides the possibility of producing higher performance TFTs for low-cost large area electronic and flexible devices.     

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.     

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.     

High-performance amorphous indium-gallium-zinc oxide thin-film transistors with polymer gate dielectric

The fabrication of amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs) with a spin-coated polymer gate dielectric on a glass substrate is reported. The interface state density at the poly(4-vinylphenol)/a-IGZO interface is only around 4.05 × 10¹¹ cm^− 2. The TFTs' threshold voltage, subthreshold swing, on-off current ratio, and carrier mobility are 2.6 V, 1.3 V/decade, 1 × 10⁵, and 21.8 cm²/V s, respectively. These characteristics indicate that the TFTs are suitable for use as nonvolatile memory devices and in flexible electronic applications.     

Inkjet-printed InGaZnO thin film transistor

We report inkjet-printed InGaZnO (IGZO) thin film transistors (TFTs). IGZO ink was prepared by dissolving indium nitrate hydrate, gallium nitrate hydrate and zinc acetate dihydrate into 2-methoxyethanol with additional stabilizers. The resulting films were inkjet-printed with a resolution of 300 dots per inch using droplets with a diameter of 40 µm, and a volume of 35 pl. The films exhibited high optical transparency in the visible range and had a polycrystalline phase of InGaO₃(ZnO)₂ after thermal annealing treatment. The chemical composition of this IGZO sample was also determined, and shown to have high stoichiometric characteristics of low oxygen deficiency. The TFTs with a conventional inverted staggered structure using inkjet-printed IGZO as an active channel layer had a field-effect mobility of ~ 0.03 cm²/Vs in saturation region and an on-to-off current ratio greater than ~ 10⁴.     

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.     

Amorphous oxide channel TFTs

Thin film transistors (TFTs) using amorphous oxides of post-transition metals: indium, gallium, and zinc for the channel materials are fabricated with radio-frequency magnetron sputtering methods for the deposition of the channel and the gate insulator layers, at room temperature with no high-temperature post-deposition annealing process. The TFTs operate as n-channel field-effect transistors with various structures of top/bottom gate and top/bottom source-and-drain contact including the inverse-stagger types, and with various materials for the gate insulators, the electrodes, and the substrates. The TFTs having smoother channel interfaces show the better performance at the saturation mobility beyond 10 cm² V^− 1 s^− 1 and the on-to-off current ratio over 10⁸ than the rough channel interfaces. The ring oscillator circuits operate with five-stage inverters of the top-gate TFTs or the inverse-stagger TFTs. Organic light-emission diode cells are driven by a simple circuit of the TFTs. It is also found by a combinatorial approach to the material exploration that the TFT characteristics can be controlled by the composition ratio of the metals in the channel layers. The amorphous oxide channel TFTs fabricated with sputtering deposition at low temperature could be a candidate for key devices of large-area flexible electronics.     

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.     

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.     

Advantages of using amorphous indium zinc oxide films for window layer in Cu(In,Ga)Se2 solar cells

The advantages of using indium zinc oxide (IZO) films instead of conventional Ga-doped zinc oxide (ZnO:Ga) films for Cu(In,Ga)Se₂ (CIGS) solar cells are described. The electrical properties of IZO are independent of film thickness. IZO films have higher mobility (30-40 cm²/Vs) and lower resistivity (4-5 × 10^− 4 Ω cm) compared to ZnO:Ga films deposited without intentional heating, because the number of grain boundaries in amorphous IZO films is small. The properties of a CIGS solar cell using IZO at the window layer were better than those obtained using a conventional ZnO:Ga at the window layer; moreover, the properties tended to be independent of thickness. These results indicate that use of IZO as a transparent conducting oxide layer is expected to increase the efficiency of CIGS solar cells.     

Electrical and optical properties of amorphous indium zinc oxide films

Valence electron control and electron transport mechanisms on the amorphous indium zinc oxide (IZO) films were investigated. The amorphous IZO films were deposited by dc magnetron sputtering using an oxide ceramic IZO target (89.3 wt.% In₂O₃ and 10.7 wt.% ZnO). N-type impurity dopings, such as Sn, Al or F, could not lead to the increase in carrier density in the IZO. Whereas, H₂ introduction into the IZO deposition process was confirmed to be effective to increase carrier density. By 30% H₂ introduction into the deposition process, carrier density increased from 3.08 × 10²⁰ to 7.65 × 10²⁰ cm^− 3, which must be originated in generations of oxygen vacancies or interstitial Zn²⁺ ions. Decrease in the transmittance in the near infrared region and increase in the optical band gap were observed with the H₂ introduction, which corresponded to the increase in carrier density. The lowest resistivity of 3.39 × 10^− 4 Ω cm was obtained by 10% H₂ introduction without substrate heating during the deposition.     

Corrugated Heterojunction Metal‐Oxide Thin‐Film Transistors with High Electron Mobility via Vertical Interface Manipulation

A new strategy is reported to achieve high‐mobility, low‐off‐current, and operationally stable solution‐processable metal‐oxide thin‐film transistors (TFTs) using a corrugated heterojunction channel structure. The corrugated heterojunction channel, having alternating thin‐indium‐tin‐zinc‐oxide (ITZO)/indium‐gallium‐zinc‐oxide (IGZO) and thick‐ITZO/IGZO film regions, enables the accumulated electron concentration to be tuned in the TFT off‐ and on‐states via charge modulation at the vertical regions of the heterojunction. The ITZO/IGZO TFTs with optimized corrugated structure exhibit a maximum field‐effect mobility >50 cm² V^?1 s^?1 with an on/off current ratio of >10⁸ and good operational stability (threshold voltage shift <1 V for a positive‐gate‐bias stress of 10 ks, without passivation). To exploit the underlying conduction mechanism of the corrugated heterojunction TFTs, a physical model is implemented by using a variety of chemical, structural, and electrical characterization tools and Technology Computer‐Aided Design simulations. The physical model reveals that efficient charge manipulation is possible via the corrugated structure, by inducing an extremely high carrier concentration at the nanoscale vertical channel regions, enabling low off‐currents and high on‐currents depending on the applied gate bias.     

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.     

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 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.     

Effects of the composition of sputtering target on the stability of InGaZnO thin film transistor

In this study, we investigated the electrical characteristics and the stability of amorphous indium gallium zinc oxide (a-IGZO) thin film transistors (TFTs) from the viewpoint of active layer composition. Active layers of TFTs were deposited by r.f. sputtering. Two kinds of sputtering targets, which have different compositional ratios of In:Ga:Zn, were used to make variations in the active layer composition. All the fabricated IGZO TFTs showed more excellent characteristics than conventional amorphous silicon TFTs. However, in accordance with the Ga content, IGZO TFTs showed somewhat different electrical characteristics in values such as the threshold voltage and the field effect mobility. The device stability was also dependent on the Ga content, but had trade-off relation with the electrical characteristics.     

Electrical and optical properties of indium tin oxide films prepared on plastic substrates by radio frequency magnetron sputtering

The influence of deposition power, thickness and oxygen gas flow rate on electrical and optical properties of indium tin oxide (ITO) films deposited on flexible, transparent substrates, such as polycarbonate (PC) and metallocene cyclo-olefin copolymers (mCOC), at room temperature was studied. The ITO films were prepared by radio frequency magnetron sputtering with the target made by sintering a mixture of 90 wt.% of indium oxide (In₂O₃) and 10 wt.% of tin oxide (SnO₂). The results show that (1) average transmission in the visible range (400-700 nm) was about 85%-90%, and (2) ITO films deposited on glass, PC and mCOC at 100 W without supplying additional oxygen gas had optimum resistivity of 6.35 × 10⁻⁴ Ω-cm, 5.86 × 10⁻⁴ Ω-cm and 6.72 × 10⁻⁴ Ω-cm, respectively. In terms of both electrical and optical properties of indium tin oxide films, the optimum thickness was observed to be 150-300 nm.