Investigation of the electrical properties and stability of HfInZnO thin-film transistors

In this study, amorphous HfInZnO (a-HIZO) thin films and related thin-film transistors (TFTs) were fabricated using the RF-sputtering method. The effects of the sputtering power (50–200 W) on the structural, surface, electrical, and optical properties of the a-HIZO films and the performance and NBIS stability of the a-HIZO TFTs were investigated. The films’ N_e increased and resistivity decreased as the sputtering power increased. The 100 W deposited a-HIZO film exhibited good optical and electrical properties compared with other sputtering powers. Optimization of the 100 W deposited a-HIZO TFT demonstrated good device performance, including a desirable μ_FE of 19.5 cm²/Vs, low SS of 0.32 V/decade, low V_th of 0.8 V, and high I_on/I_off of 10⁷, respectively. The 100 W deposited a-HIZO TFT with Al₂O₃ PVL also exhibited the best stability, with small V_th shifts of -2.2 V during NBIS testing. These high-performance a-HIZO thin films and TFTs with Al₂O₃ PVL have practical applications in thin-film electronics.     

Extraction of density-of-states in amorphous InGaZnO thin-film transistors from temperature stress studies

The instability of amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) with different active layer thicknesses under temperature stress has been investigated through using the density-of-states (DOS). Interestingly, the a-IGZO TFT with 22 nm active layer thickness showed a better stability than the others, which was observed from the decrease of interfacial and semiconductor bulk trap densities. The DOS was calculated based on the experimentally-obtained activation energy (E_A), which can explain the experimental observations. We developed the high-performance Al₂O₃ TFT with 22 nm IGZO channel layer (a high mobility of 7.4 cm²/V, a small threshold voltage of 2.8 V, a high I_on/I_off 1.8 × 10⁷, and a small SS of 0.16 V/dec), which can be used as driving devices in the next-generation flat panel displays.     

Electrical behavior of amorphous indium–gallium–zinc oxide thin film transistors by embedding Au nanoparticles in the channel layer

We reported the effects on the electrical behavior of amorphous indium–gallium–zinc oxide (a-IGZO) thin film transistors (TFTs) after introducing various positions and sizes of Au nanoparticles (NPs) in the channel layer. These TFTs showed an off-current increase and threshold voltage (V_th) shift compared to conventional a-IGZO TFTs. The effects of Au NPs are explained to form the carrier conduction path which causes the current leakage in the channel layer, and act as either electron injection sites or trap sites. Therefore, this study demonstrates that the optimized control of size and position of Au NPs in the channel layer is crucial for its application in the electrical stability improvement and V_th control of a-IGZO TFTs.     

The effect of annealing in forming gas on the a-IGZO thin film transistor performance and valence band cut-off of IGZO on SiNx

In this investigation, the carrier concentration gradient between channel and contact region is achieved to improve the Thin film Transistors (TFT) performance by employing annealing at 350 °C in forming gas (N₂ + 5% H₂). The contact region is covered with Mo metal and the channel region is only exposed to forming gas to facilitate the diffusion controlled reaction. The TFT using a-IGZO active layer is fabricated in ambient of Ar:O₂ in ratio 60:40 and the conductivity of the order of 10⁻³ S/cm is measured for as-deposited sample. The electrical conductivity of an annealed sample is of the order of 10² S/cm. The device performance is determined by measuring merit factors of TFT. The saturation mobility of magnitude 18.5 cm²V⁻¹ s⁻¹ has been determined for W/L (20/10) device at 15 V drain bias. The extrapolated field effect mobility for a device with channel width (W) 10 μm is 19.3 cm²V⁻¹ s⁻¹. The on/off current ratio is 10⁹ and threshold voltage is in the range between 2 and 3 V. The role of annealing on the electronic property of a-IGZO is carried out using X-ray photoelectron spectroscopy (XPS). The valance band cut-off has been approximately shifted to higher binding energy by 1 eV relative to as-deposited sample.     

Control of electrical properties and gate bias stress stability in solution-processed a-IZO TFTs by Zr doping

Zr-doped indium zinc oxide (IZO) thin film transistors (TFTs) are fabricated via a solution process with different Zr doping ratios. The addition of Zr suppressed the carrier concentration in the IZO films, which was confirmed by Hall Effect measurements. As the amount of Zr was increased in the oxide active layer of TFTs, the subthreshold swing (S.S) reduced, the ON/OFF ratio improved, and the threshold voltage (V_th) shifted positively. Moreover, the starting points of the ON state for TFTs near the point zero gate voltage could be controlled by the addition of Zr. The 0.3% Zr-doped IZO TFT exhibited a high saturation mobility of 7.0 cm² V⁻¹ s⁻¹, ON/OFF ratio of 2.6 × 10⁶ and S.S of 0.57 V/decade compared the IZO TFT with 10.1 cm² V⁻¹ s⁻¹, 1.7 × 10⁶ and 0.75 V/decade. The Zr effect of the gate bias stability was examined. Zr-doped IZO TFTs were relatively unstable under a positive bias stress (PBS), whereas they showed good stability at a negative bias stress (NBS). The gate bias stability of the oxide TFTs were compared with the extracted parameters through a stretched-exponential equation. The characteristic trapping time under NBS of 0.3% Zr-doped IZO TFTs was improved from 8.3 × 10⁴ s for the IZO TFT to 3.1 × 10⁵ s.     

The effect of post-metal annealing on the electrical performance and stability of two-step-annealed solution-processed In2O3 thin film transistors

In this work, solution-processed indium oxide (In₂O₃) thin film transistors (TFTs) were fabricated by a two-step annealing method. The influence of post-metal annealing (PMA) temperatures on the electrical performance and stability is studied. With the increase of PMA temperatures, the on-state current and off-state current (I_on/I_off) ratio is improved and the sub-threshold swing (SS) decreased. Moreover, the stability of In₂O₃ TFTs is also improved. In all, In₂O₃ TFT with post-metal annealing temperature of 350°С exhibits the best performance (a threshold voltage of 4.75 V, a mobility of 13.8 cm²/V, an I_on/I_off ratio of 1.8 × 10⁶, and a SS of 0.76 V/decade). Meanwhile, the stability under temperature stress (TBS) and positive bias stress (PBS) also show a good improvement. It shows that the PMA treatment can effectively suppress the interface trap and bulk trap and result in an obviously improvement of the In₂O₃ TFTs performance.     

A study of electrical enhancement of polycrystalline MgZnO/ZnO bi-layer thin film transistors dependence on the thickness of ZnO layer

A polycrystalline MgZnO/ZnO bi-layer was deposited by using a RF co-magnetron sputtering method and the MgZnO/ZnO bi-layer TFTs were fabricated on the thermally oxidized silicon substrate. The performances with varying the thickness of ZnO layer were investigated. In this result, the MgZnO/ZnO bi-layer TFTs which the content of Mg is about 2.5 at % have shown the enhancement characteristics of high mobility (6.77–7.56 cm² V⁻¹ s⁻¹) and low sub-threshold swing (0.57–0.69 V decade⁻¹) compare of the ZnO single layer TFT (μ_FE = 5.38 cm² V⁻¹ s⁻¹; S.S. = 0.86 V decade⁻¹). Moreover, in the results of the positive bias stress, the ΔV_on shift (4.8 V) of MgZnO/ZnO bi-layer is the 2 V lower than ZnO single layer TFT (ΔV_on = 6.1 V). It reveals that the stability of the MgZnO/ZnO bi-layer TFT enhanced compared to that of the ZnO single layer TFT.     

High-performance amorphous indium gallium zinc oxide thin-film transistors with sol-gel processed gate dielectric and channel layer fabricated using microwave irradiation

In this study, we fabricated high-performance a-IGZO TFTs by forming Al₂O₃ and a-IGZO thin films for gate insulator and active channel layer, respectively, using a sol-gel process. MWI for low thermal budget process was used to condensate Al₂O₃ and a-IGZO films, which was compared with the CTA. It is found that the MWI is superior process to the conventional method in terms of precursor and solvent decomposition and has proven to be more effective for eliminating residual organic contaminants. In addition, the MWI-treated Al₂O₃ and IGZO films have smoother surfaces, higher visible light transmittance, lower carbon contamination and impurities than the CTA-treated films. We have demonstrated that a-IGZO TFTs with sol-gel solution-processed Al₂O₃ gate insulator and a-IGZO channel layer can achieve a field effect mobility of 69.2 cm²/V·s, a subthreshold swing of 86.2 mV/decade and a large on/off current ratio of 1.48 × 10⁸, by the MWI process even at temperatures below 200 °C. In addition, the MWI-treated a-IGZO TFTs have excellent resistance to electron trapping and good stability to positive and negative gate-bias stress. Therefore, the sol-gel processed a-IGZO TFTs with Al₂O₃ gate oxide and the MWI treatment with a low thermal budget are promising for emerging transparent flat panel displays applications.     

Effect of modifying a methyl siloxane-based dielectric by a polymer thin film for pentacene thin-film transistors

We report on the fabrication of pentacene thin-film transistors (TFTs) utilizing a spun methyl siloxane-based spin-on-glass (SOG) dielectric and show that these devices can give a similar electrical performance as achieved by using pentacene TFTs with a silicon dioxide (SiO₂) dielectric. To improve the electrical performance of pentacene TFTs with the SOG dielectric, we employed a hybrid dielectric of an SOG/cross-linked poly-4-vinylphenol (PVP) polymer. The PVP film was deposited onto the spun SOG dielectric prior to pentacene evaporation, resulting in an improvement of the saturation field effect mobility (μ_sat) from 0.01 cm²/(V s) to 0.76 cm²/(V s). The good surface morphology and the matching surface energy of the SOG dielectric that was modified with the polymer thin film allow the optimized growth of crystalline pentacene domains whose nuclei are embedded in an amorphous phase.     

Low-voltage-drive and high output current ZnO thin-film transistors with sputtering SiO2 as gate insulator

Low-voltage-drive ZnO thin-film transistors (TFTs) with room-temperature radio frequency magnetron sputtering SiO₂ as the gate insulator were fabricated successfully on the glass substrate. The ZnO-TFT operates in the enhancement mode with a threshold voltage of 4.2 V, a field effect mobility of 11.2 cm²/V s, an on/off ratio of 3.1 × 10⁶ and a subthreshold swing of 0.61 V/dec. The drain current can reach to 1 mA while the gate voltage is only of 12 V and drain voltage of 8 V. The C–V characteristics of a MOS capacitor with the structure of ITO/SiO₂/ZnO/Al was investigated. The carrier concentration N_D in the ZnO active layer was determined, the calculated N_D is 1.81 × 10¹⁶ cm⁻³, which is the typical value of undoped ZnO film used as the channel layer for ZnO-TFT devices. The experiment results show that SiO₂ film is a promising insulator for the low voltage and high drive capability oxide TFTs.     

SiOx interlayer to enhance the performance of InGaZnO-TFT with AlOx gate insulator

We have fabricated indium–gallium–zinc (IGZO) thin film transistor (TFT) using SiO_x interlayer modified aluminum oxide (AlO_x) film as the gate insulator and investigated their electrical characteristics and bias voltage stress. Compared with IGZO-TFT with AlO_x insulator, IGZO-TFT with AlO_x/SiO_x insulator shows superior performance and better bias stability. The saturation mobility increases from 5.6 cm²/V s to 7.8 cm²/V s, the threshold voltage downshifts from 9.5 V to 3.3 V, and the contact resistance reduces from 132 Ωcm to 91 Ωcm. The performance improvement is attributed to the following reasons: (1) the introduction of SiO_x interlayer improves the insulator surface properties and leads to the high quality IGZO film and low trap density of IGZO/insulator interface. (2) The better interface between the channel and S/D electrodes is favorable to reduce the contact resistance of IGZO-TFT.     

Investigation of electronic states of infinite-layer SrFeO2 epitaxial thin films by X-ray photoemission and absorption spectroscopies

We investigated the electronic states of a single-crystal SrFeO₂ epitaxial thin film in the valence-band and conduction-band regions using synchrotron-radiation X-ray photoemission and absorption spectroscopies. Fe 2p–3d resonant photoemission measurements revealed that the Fe 3d states have higher densities of states at binding energies of 3–5 eV and 5–8.5 eV in the valence-band region. The O K-edge X-ray absorption spectrum exhibited three peaks in the Fe 3d-derived conduction band hybridized with O 2p states; these can be assigned to Fe 3d_xy, 3d_xz + 3d_yz, and 3d_x²_–y². In addition, the indirect bandgap value of the SrFeO₂ film was determined to be 1.3 eV by transmission and absorption spectroscopies.     

Improved performance of InGaZnO thin-film transistors with Ta2O5/Al2O3 stack deposited using pulsed laser deposition

Ta₂O₅/Al₂O₃ stacked thin film was fabricated as the gate dielectric for low-voltage-driven amorphous indium–gallium–zinc-oxide (IGZO) thin film transistors (TFTs). The Ta₂O₅/Al₂O₃ stacked thin film exhibits a combination of the advantages of Al₂O₃ and Ta₂O₅. The IGZO TFT with Ta₂O₅/Al₂O₃ stack exhibits good performance with large saturation mobility of 26.66 cm² V⁻¹ s⁻¹, high on/off current ratio of 8 × 10⁷, and an ultra-small subthreshold swing (SS) of 78 mV/decade. Such small SS value is even comparable with that of submicrometer single-crystalline Si MOSFET.     

A self-powered β-Ga2O3/CsCu2I3 heterojunction photodiode responding to deep ultraviolet irradiation

In this paper, a lead-free halide perovskite CsCu₂I₃ film with high stability was prepared by the anti-solvent assisted crystallization method. Then, we coupled it with Ga₂O₃ to prepare a corresponding heterojunction deep ultraviolet (UV) photodetector. After testing, we concluded that the photodetector is sensitive to 254 nm UV light. The photodetector has good reproducibility, and has an ultra-high photo-to-dark current ratio (PDCR) of more than 10⁵. In addition, under a bias of 10 V and an illuminated intensity of 200 μW/cm², the responsivity (R) and specific detectivity (D*) reached 20 mA/W and 10⁷ cm Hz^1/2 W⁻¹ (Jones), and the external quantum efficiency (EQE) is 10%. Meanwhile, the prepared photodetector could operate at zero bias, i.e., self-powered operation, along with a photocurrent of about 1 nA under illumination with UV light intensity of 200 μW/cm².     

Improving the electrical performance of HfInZnO-TFTs by introducing a thin ITO interlayer

We have fabricated hafnium–indium–zinc-oxide (HfInZnO) thin film transistors (TFT) with indium–tin-oxide (ITO) interlayer. Compared with conventional HfInZnO-TFT, the electrical performance and bias stability of HfInZnO-TFTs with ITO interlayer are improved. HfInZnO-TFT with 4-nm-thick ITO interlayer shows a high mobility of 7.2 cm²/V s, a low threshold voltage of 0.13 V and a better bias stability. The performance enhancement is attributed to a decrease in interface trap state and an increase in carrier concentration. It suggests that introducing ITO interlayer at the ALD Al₂O₃/HfInZnO interface is an effective way to improve the electrical performance and bias stability.     

Enhancement of the electrical characteristics of indium–zinc tin-oxide thin-film transistors utilizing dual-channel layers

Thin film transistors (TFTs) with indium–zinc tin-oxide (IZTO) dual-channel layers were fabricated on heavily-doped p-type Si substrates by using a tilted dual-target radio-frequency magnetron sputtering system. The number of oxygen vacancies in the IZTO channel layer decreased with increasing oxygen partial pressure, resulting in a decrease in the conductivity. The threshold voltage (V_th) shifted toward positive gate-source voltage with increasing oxygen partial pressure for the growth of the IZTO layer because of a decrease in the carrier concentration. The V_th, the mobility, the on/off-current ratio, and the subthreshold swing of the dual-channel IZTO TFTs were 3.5 V, 7.1 cm²/V s, 1.3 V/decade, and 8.2 × 10⁶, respectively, which was enhanced by utilizing dual-channel layers consisting of a top channel deposited at a high oxygen partial pressure and a bottom channel deposited at a low oxygen partial pressure.     

Oxygen plasma and high pressure H<Subscript>2</Subscript>O vapor heat treatments used to fabricate polycrystalline silicon thin film transistors

Oxygen plasma and high pressure H₂O vapor heat treatment were applied to fabrication of n-channel polycrystalline silicon thin film transistors (poly-Si TFTs). 13.56 MHz-oxygen-plasma treatment at 250 °C, 100 W for 5 min effectively reduced defect states of 25-nm-thick silicon films crystallized by 30 ns-pulsed XeCl excimer laser irradiation. 1.3×10⁶-Pa-H₂O vapor heat treatment at 260 °C for 3 h was carried out in order to improve electrical properties of SiO_x gate insulators and SiO_x/Si interfaces. A carrier mobility of 470 cm²/V s and a low threshold voltage of 1.8 V were achieved for TFTs fabricated with crystallization at 285 mJ/cm². Received: 18 November 2002 / Accepted: 25 November 2002 / Published online: 11 April 2003 RID="*" ID="*"Corresponding author. Fax: +81-42/388-7109, E-mail: tsamesim@cc.tuat.ac.jp     

High mobility organic thin-film transistors on plastic substrate

We have fabricated organic thin-film transistors (OTFTs) based on di-n-decyldinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C₁₀-DNTT) on a polyimide gate dielectric coated on a polycarbonate substrate with a bottom-gate, top-contact configuration. Mobilities of the C₁₀-DNTT-based TFTs were as high as 2.4 cm² V^?1 s^?1, which are much better than those of the parent dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT)-based TFTs (mobility ～ 0.5 cm²/V) fabricated on the same substrate. Compared to the C₁₀-DNTT-TFTs on the conventional Si/SiO₂ substrate, the present mobility of C₁₀-DNTT-TFTs are somewhat reduced, which can be attributed to reduced crystallinity on the polyimide gate dielectric, although the crystalline phase on the polyimide is the same as on the Si/SiO₂ substrate.     

Effects of Ti addition on sol-gel derived InO and InZnO thin film transistors

In this study, we have newly developed titanium-indium oxide (TiInO) and titanium-indium zinc oxide (TiInZnO) thin films as the active channel layer in thin film transistors (TFTs) by the sol-gel process. The effects of adding Ti on TiInO and TiInZnO TFTs were investigated. The addition of Ti elements can suppress formation of oxygen vacancies because of the stronger oxidation tendency of Ti relative to that of Zn or In. TiInO and TiInZnO TFTs showed lower off currents and higher on/off current ratios than pure InO and InZnO TFTs. A TiInO TFT doped with 10.31 mol% Ti showed good performance with an on/off current ratio greater than 10⁷, and a field-effect mobility of 1.91 cm² V^?1 S^?1. A TiInZnO TFT doped with 2.92 mol % Ti showed an on/off current ratio greater than 10⁶, and a field-effect mobility of 0.45 cm² V^?1 S^?1.     

Oxygen nonstoichiometry and transport properties of strontium substituted lanthanum cobaltite

Oxygen nonstoichiometry, structure and transport properties of the two compositions (La_0.6Sr_0.4)_0.99CoO_3−δ (LSC40) and La_0.85Sr_0.15CoO_3−δ (LSC15) were measured. It was found that the oxygen nonstoichiometry as a function of the temperature and oxygen partial pressure could be described using the itinerant electron model. The electrical conductivity, σ, of the materials is high (σ > 500 S cm^− 1) in the measured temperature range (650–1000 °C) and oxygen partial pressure range (0.209–10^− 4 atm). At 900 °C the electrical conductivity is 1365 and 1491 S cm^− 1 in air for LSC40 and LSC15, respectively. A linear correlation between the electrical conductivity and the oxygen vacancy concentration was found for both samples. The mobility of the electron-holes was inversely proportional with the absolute temperature indicating a metallic type conductivity for LSC40. Using electrical conductivity relaxation the chemical diffusion coefficient of oxygen was determined. It was found that accurate values of the chemical diffusion coefficient could only be obtained using a sample with a porous surface coating. The porous surface coating increased the surface exchange reaction thereby unmasking the chemical diffusion coefficient. The ionic conductivity deduced from electrical conductivity relaxation was determined to be 0.45 S cm^− 1 and 0.01 S cm^− 1 at 1000 and 650 °C, respectively. The activation energy for the ionic conductivity at a constant vacancy concentration (δ = 0.125) was found to be 0.90 eV.