Indium Gallium Zinc Oxide Thin-film Transistors Research Articles

Beneficial effect of hydrogen in aluminum oxide deposited through the atomic layer deposition method on the electrical properties of an indium–gallium–zinc oxide thin-film transistor

ABSTRACTDescribed herein is the role of hydrogen in aluminum oxide (Al2O3) gate dielectrics in amorphous indium–gallium–zinc oxide (a-InGaZnO or a-IGZO) thin-film transistors (TFTs). Compared to a-IGZO TFTs with a low-temperature (150°C) Al2O3 gate dielectric, a-IGZO devices with a high-temperature (250–300°C) Al2O3 gate dielectric exhibit poor transistor characteristics, such as low mobility, a high subthreshold slope, and huge hysteresis. Through DC and short-pulsed current–voltage (I–V) measurements, it was revealed that the degradation of the transistor performance stems from the charging and discharging phenomenon at the interface traps located in the interface between the a-IGZO semiconductor and the Al2O3 gate insulator. It was found that the low-temperature Al2O3 atomic layer deposition processed film contains a higher density of hydrogen atoms compared to high-deposition-temperature films. The study results show that a high concentration of hydrogen atoms can passivate the defect sites in the interface and bulk, which produces excellent transistor characteristics. This study demonstrated that hydrogen has a beneficial effect on the defect passivation for oxide TFTs.

The Current Behaviors of the Amorphous In-Ga-Zn-O Thin-Film Transistor Under Varying Illumination Conditions

In this study, the time response behavior of the amorphous indium-gallium zinc-oxide (a-IGZO) thin-film transistors (TFTs) to the illumination pulse is analyzed. We modified the previously proposed fitting formula by changing the fitting parameters from constant to time dependent. The mechanism for the response behaviors is proposed based on the analysis of the fitting parameters with respect to measurement time and under different light intensities. The single-pulse measurement results and their corresponding fitting parameters is used as the database to predict the current behaviors of the TFTs under multi-pulse illumination. The predicted and measured results fit fairly well. The method to formulize the time response for the a-IGZO TFT under varying situations of illumination is thus developed, which can be important in the design and simulation of transparent electronic circuits.

Mechanism of Hysteresis for a-IGZO TFT Studied by Changing the Gate Voltage Waveform in Measurement

In this paper, we investigate the response time of oxygen vacancies on the hysteresis for the amorphous indium–gallium–zinc oxide thin-film transistors. The fast pulse measurement method is used to observe the drain current under the different sweeping speeds of gate voltage to clarify the mechanism of the hysteresis. Effects of light intensity and temperature are also discussed. Based on the results and the analysis, the model of oxygen vacancies is proposed to explain the effect of sweep speed on the hysteresis, which would be important for the devices to be driven in their circuit application.

Investigation on Alumina Passivation for Improved IGZO TFT Performance

A study on the influence of the back-channel interface on the operation of bottom-gate indium gallium zinc oxide (IGZO) thin-film transistors (TFTs) is presented. TFTs without any passivation material deposited typically exhibit best-case initial results. However a passivation layer is required for device stability and process integration. The surface of IGZO is very sensitive to deposition conditions of passivation materials; in general additional oxygen vacancies are created which renders the IGZO conductive. Annealing in an oxidizing ambient may reverse this effect and result in devices which exhibit appropriate semiconducting properties. The impact of passivation using alumina deposited via electron beam evaporation and atomic layer deposition (ALD) has been investigated. A decrease in TFT channel mobility is attributed to the IGZO/alumina back-channel interface degradation associated with the alumina passivation process. Low temperature ALD (e.g. T ≤ 150°C) has been explored to minimize this degradation effect. Pre-passivation and post-passivation anneal treatments have also been explored to better understand the mechanisms involved. Improvements in the back-channel interface were shown to depend on the thickness and deposition method of the alumina passivation layer. Temperature-dependent measurements reveal differences in electrical characteristics including bias-stress stability. Modifications in the passivation and annealing procedures and process integration details have resulted in a marked improvement in the performance of passivated devices.

Photoluminescence and Reliability Study of ZnO Cosputtered IGZO Thin-Film Transistors Under Various Ambient Conditions

Zinc oxide (ZnO) cosputtered indium–gallium–ZnO (IGZO) and pure IGZO thin films have been deposited at room temperature by dual RF magnetron sputtering and annealed in argon (Ar), nitrogen (N2), and oxygen (O2) ambients. The beneficial effect of various annealing environments appears because of a reorganization of the amorphous network with controlling the defect chemistry in the films as systematically investigated by photoluminescence analysis. The various defects, such as zinc vacancy ( $V_{\mathrm{ Zn}})$ , zinc interstitial (Zn i ), and oxygen vacancy ( $V_{\mathrm{ O}})$ , were remarkably enhanced in argon, while suppressed in N2 and O2 ambients—leading more reliable cosputtered structure than the IGZO to be utilized in thin-film transistors (TFTs) fabrication for transparent electronics. Furthermore, the ZnO cosputtered IGZO TFTs were also fabricated and systematically investigated the impact of various annealing environments on their performance characteristics. The characteristics were improved in N2 ambient—displaying the field-effect mobility ( $\mu _{\mathrm{ FE}}$ ) of 16.10 cm $^{\textrm {2}}$ /Vs, the threshold voltage ( $V_{\mathrm{ th}}$ ) of 1.50 V, the subthreshold swing of 0.21 V/decade, and the negative bias illumination stress shifting of −2.75 V.

Contact resistance asymmetry of amorphous indium–gallium–zinc–oxide thin-film transistors by scanning Kelvin probe microscopy**Project supported by the Key Industrial R&D Program of Jiangsu Province, China (Grant No. BE2015155), the Priority Academic Program Development of Higher Education Institutions of Jiangsu Province, China, and the Fundamental Research Funds for the Central Universities, China (Grant No. 021014380033).

In this work, a method based on scanning Kelvin probe microscopy is proposed to separately extract source/drain (S/D) series resistance in operating amorphous indium–gallium–zinc–oxide (a-IGZO) thin-film transistors. The asymmetry behavior of S/D contact resistance is deduced and the underlying physics is discussed. The present results suggest that the asymmetry of S/D contact resistance is caused by the difference in bias conditions of the Schottky-like junction at the contact interface induced by the parasitic reaction between contact metal and a-IGZO. The overall contact resistance should be determined by both the bulk channel resistance of the contact region and the interface properties of the metal-semiconductor junction.

Performance of Indium Gallium Zinc Oxide Thin-Film Transistors in Saline Solution

Transistors are often envisioned as alternative transducing devices to microelectrodes to communicate with the nervous system. Independently of the selected technology, the transistors should have reliable performance when exposed to physiological conditions (37°C, 5% CO2). Here, we report on the reliable performance of parylene encapsulated indium gallium zinc oxide (IGZO) based thin-film transistors (TFTs) after prolonged exposure to phosphate buffer saline solution in an incubator. The encapsulated IGZO TFTs (W/L = 500 μm/20 μm) have an ON/OFF current ratio of 107 and field effect mobility of 8.05 ± 0.78 cm2/Vs. The transistors operate within 4 V; their threshold voltages and subthreshold slope are ~1.9 V and 200 mV/decade, respectively. After weeks immersed in saline solution and at 37°C, we did not observe any significant deterioration in the transistors’ performance. The long-term stability of IGZO transistors at physiological conditions is a promising result in the direction of metal oxide bioelectronics.

Solution-processed hybrid organic–inorganic complementary thin-film transistor inverter

We investigated hybrid organic–inorganic complementary inverters with a solution-processed indium–gallium–zinc-oxide (IGZO) n-channel thin-film transistor (TFT) and p-channel TFTs using the high-uniformity polymer poly[2,5-bis(alkyl)pyrrolo[3,4-c]pyrrolo-1,4(2H,5H)-dione-alt-5,5-di(thiophene-2-yl)-2,2-(E)-2-(2-(thiophen-2-yl)vinyl)thiophene] (PDVT-10). The IGZO TFT was fabricated at 150 °C for 1 min. It showed a high field-effect mobility of 0.9 cm2·V−1·s−1 and a high on/off current ratio of 107. A hybrid complementary inverter was fabricated by combining IGZO with a PDVT-10 thin-film transistor and its operation was confirmed.

Electromechanical properties of amorphous indium–gallium–zinc-oxide transistors structured with an island configuration on plastic

A comparative study of the electromechanical properties was carried out on a low-temperature-processed amorphous indium–gallium–zinc-oxide thin-film transistor, particularly with regard to the structural design of the device under the stress accumulation of an outward bending surface. Shown herein is the reliable electromechanical integrity of island-structured devices against the mechanical strain at bending radii of mm order. The onset of crack strain also closely corresponded to the electrical failure of the stressed device. These results revealed that the island configuration on the bending surface effectively suppresses the stress accumulation on sheets composed of inorganic stacked layers in a uniaxial direction.

Interface location-controlled indium gallium zinc oxide thin-film transistors using a solution process

The role of an interface as an electron-trapping layer in double-stacked indium gallium zinc oxide (IGZO) thin-film transistors (TFTs) was investigated and interface location-controlled (ILC) IGZO TFTs were introduced. In the ILC TFTs, the thickness of the top and bottom IGZO layers is controlled to change the location of the interface layer. The system exhibited improved electrical characteristics as the location of the interface layer moved further from the gate insulator: field-effect mobility increased from 0.36 to 2.17 cm2 V−1 s−1, and the on-current increased from 2.43 × 10−5 to 1.33 × 10−4 A. The enhanced electrical characteristics are attributed to the absence of an electron-trapping interface layer in the effective channel layer where electrons are accumulated under positive gate bias voltage.

Improvement in reliability of amorphous indium–gallium–zinc oxide thin-film transistors with Teflon/SiO2 bilayer passivation under gate bias stress

The reliability of amorphous indium–gallium–zinc oxide (a-IGZO) thin-film transistors (TFTs) with Teflon/SiO2 bilayer passivation prepared under positive and negative gate bias stresses (PGBS and NGBS, respectively) was investigated. Heavier electrical degradation was observed under PGBS than under NGBS, indicating that the environmental effects under PGBS are more evident than those under NGBS. The device with bilayer passivation under PGBS shows two-step degradation. The positive threshold voltage shifts during the initial stressing period (before 500 s), owing to the charges trapped in the gate insulator or at the gate insulator/a-IGZO active layer interface. The negative threshold voltage shift accompanies the increase in subthreshold swing (SS) for the continuous stressing period (after 500 s) owing to H2O molecules from ambience diffused within the a-IGZO TFTs. It is believed that Teflon/SiO2 bilayer passivation can effectively improve the reliability of the a-IGZO TFTs without passivation even though the devices are stressed under gate bias.

Self-Aligned Coplanar Top Gate In-Ga-ZnO Thin-Film Transistors Exposed to Various DUV Irradiation Energies

Self-aligned coplanar top gate indium–gallium–zinc-oxide thin-film transistors (TFTs), with source/drain (S/D) regions that were heavily doped by various deep-ultraviolet (DUV) irradiation energies at dual wavelengths of 185 and 254 nm, were investigated. The electrical performances of TFTs improved as the DUV irradiation energy increased. The proposed TFTs, with DUV irradiation of 20 J/cm2, have a field-effect mobility of 22.0 cm2/ $\text {V}\cdot \text {s}$ , a threshold voltage of 2.2 V, a subthreshold swing value of 0.24 V/decade, and an on/off current ratio of $1.34 \times 10^{6}$ . In addition, the channel length modulation ( $\Delta L$ ) and the width-normalized contact resistance ( $R_{\mathrm {SD}}W$ ) were 1.04 $\mu \text{m}$ and 60.5 $\Omega \cdot \text {cm}$ , respectively. Such short $\Delta L$ and low $R_{\mathrm {\mathbf {SD}}}W$ values indicate that the S/D regions were properly formed by the DUV irradiation. This simple and cost-effective doping technique can be useful for the fabrication of self-aligned TFTs.

Investigation of Low-Frequency Noise Properties in High Mobility ZnON Thin-Film Transistors

We investigate the low-frequency noise (LFN) properties of amorphous zinc oxynitride (a-ZnON) thin-film transistors (TFTs) exhibiting high field-effect mobilities ranging from 48.5 to 118.9 cm2/ $\text {V}\cdot \text {s}$ , depending on the gas flow rates during the deposition process. The measured noise power spectral density of the drain current shows that the LFN in a-ZnON TFTs obeys the classical 1/ $f$ noise theory, i.e., it is proportional to 1/ $f^{\mathrm {\gamma }}$ with $\gamma \sim 1$ in the frequency range from 10 Hz to 1 kHz. The LFN from the a-ZnON TFT is successfully interpreted by the correlated number fluctuation-mobility fluctuation model. The near-interface dielectric trap density ( $N_{T})$ and the Coulomb scattering coefficient ( $\alpha _{S})$ extracted from the measured LFN in a-ZnON TFTs are similar to those from the previously reported values for amorphous indium–gallium–zinc oxide TFTs. The relatively large values of $N_{T}$ and $\alpha _{S}$ from the a-ZnON TFTs formed under O2-rich environment are mainly attributed to the high degree of disorder of the a-ZnON channel layer caused by the energetically broad and high density of subgap tail states.

High-density carrier-accumulated and electrically stable oxide thin-film transistors from ion-gel gate dielectric.

The use of indium–gallium–zinc oxide (IGZO) has paved the way for high-resolution uniform displays or integrated circuits with transparent and flexible devices. However, achieving highly reliable devices that use IGZO for low-temperature processes remains a technological challenge. We propose the use of IGZO thin-film transistors (TFTs) with an ionic-liquid gate dielectric in order to achieve high-density carrier-accumulated IGZO TFTs with high reliability, and we discuss a distinctive mechanism for the degradation of this organic–inorganic hybrid device under long-term electrical stress. Our results demonstrated that an ionic liquid or gel gate dielectric provides highly reliable and low-voltage operation with IGZO TFTs. Furthermore, high-density carrier accumulation helps improve the TFT characteristics and reliability, and it is highly relevant to the electronic phase control of oxide materials and the degradation mechanism for organic–inorganic hybrid devices.

Visible-light phototransistors based on InGaZnO and silver nanoparticles

Visible-light phototransistors were fabricated using wide-bandgap indium gallium zinc oxide (IGZO) and silver (Ag) nanoparticles (NPs). A bottom gate made of thin film transistors (TFTs) was fabricated with an amorphous IGZO film as the active channel. Ag NPs were then formed on the surface of the active channel region to be used as a visible-light absorption layer. The prepared Ag NPs were highly absorptive in the wavelength range of 450–600 nm, due to the plasmon effect. A detailed study of photoresponsivity and external quantum efficiency indicated that the TFTs with Ag NPs induced a large photocurrent upon illumination by visible light on the active region, while the TFTs without Ag NPs did not. This result indicates the presence of coupling between the localized plasmons and electrical carriers on the IGZO TFTs with Ag NPs.

Thermally Induced Variation of the Turn-ON Voltage of an Indium–Gallium–Zinc Oxide Thin-Film Transistor

The thermal processing of an indium–gallium–zinc oxide (IGZO) thin-film transistor (TFT) with annealing-induced source/drain (S/D) regions was found to induce a negative shift in the turn-ON voltage ( $V_{\mathrm{\scriptscriptstyle ON}})$ of the TFT. Such shifts are consistent with the presence of positive ions, and correlated with the detection of indium, gallium, and zinc, in the dielectric layer adjacent to the channel region. The origin of these species is attributed to the thermally induced dissociation of the IGZO in the S/D regions of the TFT, and the subsequent migration of some of the dissociated species to the dielectric layer above the channel region. It was found that the process-induced shifts in $V_{\mathrm{\scriptscriptstyle ON}}$ depended on not only the heat-treatment condition but also the structural configuration of the TFT. Since the thermal processes are practically unavoidable during the fabrication of an IGZO TFT and it is unlikely the dissociation of IGZO is unique to the present device architecture, attention must be paid to the effects of the potential migration of such ionic species on the device characteristics, regardless of the transistor structure.

Scaling characteristics of depletion type, fully transparent amorphous indium–gallium–zinc-oxide thin-film transistors and inverters following Ar plasma treatment

We fabricated depletion type, transparent amorphous indium–gallium–zinc-oxide (a-IGZO) thin-film transistors (TFTs) and inverters with an Ar plasma treatment and analyzed their scaling characteristics with channel lengths ranging from 2 to 100 µm. The improvement of the field-effect mobility of a-IGZO TFTs is apparent only for short channel lengths. There is also an unexpected side effect of the Ar plasma treatment, which introduces back-channel interfacial states and induces a positive shift in the threshold voltage of a-IGZO TFTs. The resulting increase in the field-effect mobility and the positive shift in the threshold voltage of each TFT increase the differential gain up to 3 times and the positive shift in the transient point of the transparent inverters.

Analysis of the dependence of indium–gallium–zinc oxide thin-film transistor properties on the gate interface material using a two-stack gate-insulator structure

To study the interface effects on the device performance, we fabricated indium–gallium–zinc oxide (IGZO) thin-film transistors (TFTs) with a two-stack gate-insulator structure. The two-stack gate insulator was composed of a thick main insulator and a thin interfacial insulator; the main insulator determines the effective permittivity of the gate insulator, and the interfacial insulator regulates the gate/active interface properties. The a-IGZO TFTs had about 10cm2V−1s−1 field effect mobility (μFE) values and 107–108 switching ratios. The dependences of μFE and threshold voltage, VTH, on the channel width to length ratio were different according to the electron affinity, χ, of the interfacial insulator. The contact resistance between the source/drain electrode and the active layer, and the electron-injection barrier height from the active layer to the interfacial gate insulator layer could explain this finding. In this work, we successfully demonstrated the method to distinguish the interface-related phenomena from the insulator permittivity-related phenomena.

Performance and Stability Improvements of Back-Channel-Etched Amorphous Indium–Gallium–Zinc Thin-Film-Transistors by CF4+O2 Plasma Treatment

The performance and stability improvement of back-channel-etched amorphous indium–gallium–zinc oxide thin-film transistors (a-IGZO TFTs) by post CF4+O2 plasma treatment is investigated. It is revealed that the metal residue of the wet-etching of source/drain electrodes degrades TFT performance and aggravates the positive threshold voltage ( $V_{\mathrm {th}})$ shift under positive gate bias stress. It is demonstrated that the CF4+O2 plasma treatment effectively removes the metal residue and remarkably improves device performance and the $V_{\mathrm {th}}$ stability. This improvement is attributed to oxygen vacancy repairing at the back channel interface of the a-IGZO TFT by the O free radicals generated by the O2 plasma.

Highly Transparent, Visible-Light Photodetector Based on Oxide Semiconductors and Quantum Dots.

Highly transparent phototransistors that can detect visible light have been fabricated by combining indium-gallium-zinc oxide (IGZO) and quantum dots (QDs). A wide-band-gap IGZO film was used as a transparent semiconducting channel, while small-band-gap QDs were adopted to absorb and convert visible light to an electrical signal. Typical IGZO thin-film transistors (TFTs) did not show a photocurrent with illumination of visible light. However, IGZO TFTs decorated with QDs showed enhanced photocurrent upon exposure to visible light. The device showed a responsivity of 1.35×10(4) A/W and an external quantum efficiency of 2.59×10(4) under illumination by a 635 nm laser. The origin of the increased photocurrent in the visible light was the small band gap of the QDs combined with the transparent IGZO films. Therefore, transparent phototransistors based on IGZO and QDs were fabricated and characterized in detail. The result is relevant for the development of highly transparent photodetectors that can detect visible light.