Transparent ZnO Thin Film Transistors Research Articles

Investigation on Transparent, Conductive ZnO:Al Films Deposited by Atomic Layer Deposition Process.

Transparent electrodes are a core component for transparent electron devices, photoelectric devices, and advanced displays. In this work, we fabricate fully-transparent, highly-conductive Al-doped ZnO (AZO) films using an atomic layer deposition (ALD) system method of repeatedly stacking ZnO and Al2O3 layers. The influences of Al cycle ratio (0, 2, 3, and 4%) on optical property, conductivity, crystallinity, surface morphology, and material components of the AZO films are examined, and current conduction mechanisms of the AZO films are analyzed. We found that Al doping increases electron concentration and optical bandgap width, allowing the AZO films to excellently combine low resistivity with high transmittance. Besides, Al doping induces preferred-growth-orientation transition from (002) to (100), which improves surface property and enhances current conduction across the AZO films. Interestingly, the AZO films with an Al cycle ratio of 3% show preferable film properties. Transparent ZnO thin film transistors (TFTs) with AZO electrodes are fabricated, and the ZnO TFTs exhibit superior transparency and high performance. This work accelerates the practical application of the ALD process in fabricating transparent electrodes.

Dual-source device architecture for self-diagnosis and correction of gate bias-stress instability in flexible transparent ZnO thin-film transistors

Flexible transparent ZnO thin-film transistors (TFTs) have gained momentum during the last decade because of the increasingly emerging demands in flexible transparent displays and wearable electronics industry. However, the instability issue regarding a deviation of turn-on voltage under gate bias stress has remained unsettled, greatly impeding the progress towards practical applications. Here we present a solution to this problem by adopting a new dual-source device architecture to self-diagnose and pull back the deviated turn-on voltage. The dual-source TFTs were fabricated on PEN substrate by conventional UV photolithography with ZnO channel prepared by magnetron sputtering technique. By combining the presented device architecture with a rapid correction process, the turn-on voltage shift was reduced to 0.1 V under −15 V gate voltage bias stress of 5000 s, which favors low power consumption circuits by preventing the TFTs drifting from enhancement to depletion mode. To illustrate its unique working mechanism, device physics simulation, square-law model deduction and circuit current analysis were carried out. The methodology we developed enhances the stability of flexible transparent oxide TFTs and can be adapted in other microelectronic and optoelectronic devices.

Low-voltage transparent thin-film transistors with ZnO/ITO double-channel layers

We report an investigation of the effects of thin film transistors (TFTs) using double-channel layers on the electrical properties of transparent ZnO TFTs. TFTs were fabricated at room temperature, which with ZnO channel layers exhibit a performance with an on/off ratio of 105, a subthreshold swing of 0.11 V/decade, and a mobility of 4.6 cm2 V−1 s−1. The field-effect mobility, current on/off ratio, and subthreshold swing of such TFTs with ZnO/ITO double-channel layers were estimated to be 21 cm2 V−1 s−1, 106, and 0.1 V/decade, respectively. The mobility, switching operation, and on/off ratio of the ZnO TFTs were improved considerably after ITO channel deposition.

Understanding the Effect of MgO Interfacial Layer on ZnO/High-K/FTO Transparent Thin Film Transistors for Large-Area Transparent Electronics Applications

We have fabricated transparent ZnO thin film transistors (TFTs) with fluorine doped tin oxide (FTO) as a gate electrode and HfO2 as a gate dielectric. To suppress the high interface state density (Dit), a series of thickness of MgO interfacial layer between ZnO and HfO2 has been investigated. ZnO TFTs with 10 nm MgO interfacial layer displayed improved performance with field effect mobility of 0.3 cm2V-1s-1, threshold voltage of 3.7 V and on/off ratio of 106 compared to 5 nm, 15 nm and 20 nm MgO interfacial layer. Dit and its energy level distribution below 0.25-0.32 eV below the conduction band edge of ZnO were quantified using the conductance method.

Fabrication and comparative study of top-gate and bottom-gate ZnO–TFTs with various insulator layers

Transparent ZnO thin film transistors (ZnO–TFTs) with different structures and dielectric layers were fabricated by rf magnetron sputtering. The PbTiO3, AlOx, SiNx and SiOx films were attempted to serve as the gate dielectric layers in the devices, respectively, and XRD was employed to investigate the crystal structure of ZnO films deposited on these dielectric layers. The optical properties of transparent TFTs were measured and revealed the average transmittance ranged from 60 to 80% in the visible part of the spectrum. Electrical measurement shows the properties of the ZnO–TFTs have great relations with the device structure. The bottom-gate TFTs have better behaviors than top-gate ones with the mobility, threshold voltage and the current on/off ratio of 18.4 cm2 V−1 s−1, −0.7 V and 104, respectively. The electrical difference of the devices may be due to different character of the interface between the channel and dielectric layers.

The improved performance of a transparent ZnO thin-film transistor with AlN/Al2O3 double gate dielectrics

This paper reports on the fabrication of stable and improved performance of transparent ZnO thin-film transistors (TFTs) with AlN/Al2O3 double gate dielectrics. AlN films reduce the surface roughness of the channel/dielectric interface due to an excellent lattice match with the ZnO film. Al2O3 films, which have a large band offset between the channel layers, are helpful to reduce the gate leakage current. Good device characteristics have been obtained from the AlN/Al2O3 TFT with a field effect mobility of 4.3 cm2 V−1 s−1, an on–off current ratio of 2 × 105 and a sub-threshold slope of 0.45 V/decade. By diminishing the charge trap density in the interface, the threshold voltage change from the hysteresis of the transfer characteristics was minimized to 0.3 V. These results should increase the prospects of using transparent TFTs for flat panel display applications.

Light Effects on the Bias Stability of Transparent ZnO Thin Film Transistors

We report on the bias stability characteristics of transparent ZnO thin film transistors (TFTs) under visible light illumination. The transfer curve shows virtually no change under positive gate bias stress with light illumination, while it shows dramatic negative shifts under negative gate bias stress. The major mechanism of the bias stability under visible illumination of our ZnO TFTs is thought to be the charge trapping of photo-generated holes at the gate insulator and/or insulator/channel interface.

Electrical properties of ZnO-based bottom-gate thin film transistors fabricated by using radio frequency magnetron sputtering

We report on enhancement-mode thin film transistors (TFTs) using ZnO as an active channel layer deposited by radio frequency (rf) magnetron sputtering at 300 °C. The TFT structure consisted of ZnO as a channel, SiN x as a gate insulator and indium tin oxide (ITO) as a gate which were deposited onto a Corning glass substrate. X-ray diffraction pattern revealed that dense columnar structure of closely packed ZnO nano grains along the c-axis. The transfer characteristics of a typical ZnO TFT exhibited a field effect mobility of 31 cm 2/V s, a drain current on/off ratio of 10 4, the low off-current value in the order of 10 −10 A, and a threshold voltage of 1.7 V. The transparent ZnO TFT exhibited n-channel enhancement mode behavior.

Transparent ZnO-TFT Arrays Fabricated by Atomic Layer Deposition

Transparent ZnO thin film transistor (TFT) array of 176 X 144 (106 dpi) was fabricated on glass substrate. The V th of the TFT with inverted coplanar structure is about 0.8 V and the mobility is 1.13 cm 2 /V s. The active layer (ZnO), gate insulator (Al 2 O 3 ), and source-drain electrode (ZnO:Al) were deposited by atomic layer deposition. We also compared the performance of TFTs fabricated by lift-off and wet-etching process as the patterning processes of ZnO layer. The carrier density of the ZnO layer was carefully adjusted to reduce off-current of TFT. Good contact with small contact resistance was formed between the active layer and the source-drain electrode.

Transparent ZnO thin film transistor prepared by sol-gel method.

Transparent ZnO thin film transistor prepared by sol-gel method.

Transparent ZnO thin film transistor fabricated by sol-gel and chemical bath deposition combination method

Top-gate thin film transistors with n-type ZnO active channel were performed under 230°C. Especially, ZnO film was deposited by a combined method of sol-gel and chemical bath deposition without any preactivation for film growth. Silicon nitride and indium tin oxide were used as the gate insulator and the conducting electrodes (source, drain, and gate). These transistors were highly transparent in the visible spectrum, with transmittance as high as 75% to approximately 85% at wavelength from 500to700nm. The optimum device has field-effect mobility of 0.67cm2∕Vs and an on-off ratio more than 107.

Fully Transparent ZnO Thin‐Film Transistor Produced at Room Temperature

Fully Transparent ZnO Thin‐Film Transistor Produced at Room Temperature

Transparent ZnO thin-film transistor fabricated by rf magnetron sputtering

We fabricated ZnO thin-film transistors by rf magnetron sputtering on Si substrates held near room temperature. The best devices had field-effect mobility of more than 2 cm2/V s and an on/off ratio>106. These ZnO films had resistivity ∼105 ohm cm, with high optical transparency (>80% for wavelength >400 nm), and compressive stress <0.5 GPa. The combination of transparency in the visible, excellent transistor characteristics, and low-temperature processing makes ZnO thin-film transistors attractive for flexible electronics on temperature sensitive substrates.