GaZnO Thin Films Research Articles

The stability analysis of In–Ga–ZnO thin film transistors with polyimide substrates based on Maxwell–Wagner effect

Flexible organic light-emitting diode display devices fabricated on polyimide (PI) substrates have more obvious residual image problems due to the abnormal threshold-voltage (Vth) shifts of a thin film transistor (TFT). In this paper, the Vth shift of TFT fabricated on a PI substrate was analyzed. We explained the worse bias stability and worse recovery of TFT with a PI substrate compared with TFT with a glass substrate, by an interlayer charging effect (Maxwell–Wagner effect) and a technology computer-aided design (Silvaco). When bias stress was applied for a long time, the interface between the PI substrate and the buffer layer will have a charging effect under the action of an electric field, and the charging charge will react on the channel and hinder the formation of the channel. We found that there are differences in the scale of charge under different voltage stress conditions, and this will result in different Vth shifts of driving TFTs for displays units.

Flexible, Transparent, High Mobility Amorphous In–Ga–Zn-O Thin Film Transistors Fabricated on Textile

To implement wearable textile displays (WTDs), it is essential that unique properties such as physical and chemical properties, aesthetics, and flexibility of the textile be maintained. Therefore, thin film transistors (TFTs) for WTDs should exhibit excellent electrical properties to enable 4K and 8K high resolution and high frame-rate (>120 Hz) displays while maintaining the properties of textiles. All layers of TFTs were formed by vacuum deposition and oxide-based transparent materials; amorphous In–Ga–Zn-O (a-IGZO), indium tin oxide (ITO), and Al2O3 were used. A transparent and flexible electrode with an ITO/Ag/ITO (IAI) structure was optimized and applied to the source/drain and gate. All processes were performed below 150 °C, and the wet-step was eliminated to prevent physical and chemical deformation of the substrate. The flexible, transparent, high mobility a-IGZO TFTs (FTH TFTs) for WTDs were fabricated on glass, polyethylene terephthalate (PET) film, and textile substrates, respectively. Their electrical performance, transparency, and flexibility were then compared. The textile-based TFTs showed transmittance of 72.98%, high mobility of 11.5 cm2/(V s), and an on/off ratio of 9.28 × 107 even in a low temperature process (<150 °C). In addition, the electrical properties of the textile-based TFTs were maintained despite testing entailing 1000 cycles of bending with a tensile strain of 0.8%. In particular, the FTH TFTs for WTDs were able to stably drive a red organic light-emitting diode on textile.

A transparent composite electrode composed of AgCr and Mo-doped GaZnO to realize flexible bottom-emitting OLEDs

We propose a new flexible transparent composite electrode that uses an ultra-thin silver-chromium (AgCr) alloy film to increase the conductivity of a Mo-doped GaZnO (MGZO) thin film and to maintain high transmittance.

Optical and electrical properties of GaZnO films deposited by co-sputtering method on two types of substrates

ABSTRACT Gallium zinc oxide GaZnO (GZO) thin films are deposited on silica glass and quartz substrates using DC power and RF power on ZnO and GaO targets, respectively. Structural characteristics are investigated using X-ray diffraction XRD, optical parameters such as refractive index n, optical energy gap Eop and dispersion parameters in addition to the electrical property are investigated for all considered films. The optical properties reveal that GZO films have high transmittance (70–98%) and a low absorption coefficient in the visible range of the spectra. The optical band gap was found to increase from 3.28 to 3.44 eV with increasing the RF power on GaO target. The dielectric constant of GZO was found to be larger than the dielectric loss and has high optical conductivity (≈1015 S−1), The VELF and SELF values of GZO films are calculated and investigated.

A comparison study of ZnO, InZnO, GaZnO and InGaZnO physical properties and optical bandgap

Comparison between ZnO, InZnO, GaZnO and InGaZnO (IGZO) thin films prepared using spin coating method were studied in detail to find out contribution of In and Ga towards changes in the physical properties. From FESEM, ZnO has revealed an uneven and non-uniform distribution of grains on the film. The addition of In has caused the grains to be more separated and inconsistent in sizes. Ga, on the other hand has transformed the grain to be more hexagonal in shape and the surface was more packed with grains. AFM analysis has shown dissimilar topographies and surface roughness values to complement FESEM results. Additionally, optical band gap and elemental atomic percentage of ZnO, InZnO, GaZnO and InGaZnO thin films were also analysed and discussed in this study.

Comparison of characteristic properties of Al, Ga, and In-doped ZnO thin films formed by sol-gel method

Herein, we examine the effect of doping with Indium (In), Gallium (Ga), and Aluminum (Al) (group III elements) on the structural, optical, and vibrational properties of ZnO thin films . The characteristic properties of the ZnO films prepared by the sol-gel dip-coating method are explored by utilizing X-ray diffraction, optical spectroscopy , and Raman scattering measurements. XRD analyzes exhibit that the crystallite size reduces upon doping by Ga and Al, while it increases with In, and all films have hexagonal wurtzite structure. Additionally, Raman measurements indicate that the dominant two peaks at around 104 and 445 cm −1 are related to E 2 low and E 2 high phonon modes of ZnO, respectively. The low-frequency mode ( E 2 low ) is affected by dopant atoms, whereas the high-frequency mode ( E 2 high ) of the wurtzite phase is not influenced by the dopant. Moreover, E dop-atom phonon mode appears at low frequencies and the intensity ratio, I( E dop.atom )/I( E 2 low ), decreases as the ionic radius of dopant atoms increases. UV–Vis spectra reveal that the film transparency, optical band gap, Urbach energy, and refraction index can be effectively tuned by dopant atoms. • Al, Ga and In-doped ZnO films are prepared by sol-gel dip coating method. • XRD and Raman spectra show that films have hexagonal wurtzite structure. • The intensity of E 2 low mode rises as the ionic radius of the dopant atom increases. • The optical band gap of In-doped ZnO reduces due to expansion of crystallite size. • Raman and XRD measurements reveal that the films are under compressive stress.

Improvement of CIGS solar cells with high performance transparent conducting Ti-doped GaZnO thin films

This study reports upon the enhancement of the optoelectronic characteristics and crystallite quality of a typical Ga-doped ZnO ternary alloy achieved by using titanium (Ti) doping to form novel Ti-doped GaZnO (GTZO) transparent conductive oxide (TCO) thin films with characteristics of low-cost and indium-free for employment in Cu(In,Ga)Se2 (CIGS) solar cell applications. These thin films are highly favorable for solar power systems. Photoluminescence measurement and X-ray diffraction were employed to assess the GTZO thin films. A high crystal quality was obtained through radio-frequency magnetron sputtering deposition with a sputtering power of 200 W, a thin-film thickness of 750 nm, a substrate temperature of 250 °C, and a growth pressure of 1 mTorr. The thin-film characteristics were a high transmittance of 86.8%, a low thin-film resistivity of 5.1 × 10−4 Ω-cm (carrier mobility of 10.9 cm2 V−1 s−1, and carrier concentration of 1.1 × 1021 cm−3), and a high figure of merit of 32.6 × 10−3 Ω−1; these characteristics established GTZO thin films as an alternative TCO for CIGS solar cell applications.These devices demonstrated the superior optoelectronic properties of GTZO TCO thin films and the effectiveness of Ti-doping in GaZnO. The CIGS solar cells with high-performance GTZO TCOs exhibited a superior conversion efficiency of 9.1% and on average, a high external quantum efficiency of 87.1% in the wavelength range from 550 to 1100 nm. The performance of these devices demonstrated in this study indicated that GTZO TCO thin films can be used for developing high-performance CIGS solar cells.

Investigation of Dual-Ion Beam Sputter-Instigated Plasmon Generation in TCOs: A Case Study of GZO.

The use of the high free-electron concentration in heavily doped semiconductor enables the realization of plasmons. We report a novel approach to generate plasmons in Ga:ZnO (GZO) thin films in the wide spectral range of ∼1.87-10.04 eV. In the grown GZO thin films, dual-ion beam sputtering (DIBS) instigated plasmon is observed because of the formation of different metallic nanoclusters are reported. Moreover, formation of the nanoclusters and generation of plasmons are verified by field emission scanning electron microscope, electron energy loss spectra obtained by ultraviolet photoelectron spectroscopy, and spectroscopic ellipsometry analysis. Moreover, the calculation of valence bulk, valence surface, and particle plasmon resonance energies are performed, and indexing of each plasmon peaks with corresponding plasmon energy peak of the different nanoclusters is carried out. Further, the use of DIBS-instigated plasmon-enhanced GZO can be a novel mean to improve the performance of photovoltaic, photodetector, and sensing devices.

Effects of Stacked Mo–Ti/Cu Source and Drain Electrodes on the Performance of Amorphous In–Ga–Zn-O Thin-Film Transistors

In this letter, the performance of amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) is investigated using the stacked Mo-Ti/Cu source (S) and drain (D) electrodes. The test samples A and B of the a-IGZO TFT have interlayers with Mo/Ti ratios of 2.29 and 1.15, respectively, for the SD electrodes, and each has a core layer of Cu stacked on the top of the interlayers. They were fabricated and measured to compare their performance in terms of the transfer characteristic and mobility. The measurement results showed that the sample B with a lower Mo/Ti ratio exhibited better performance than the sample A: higher field-effect mobility by 4.4 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V-s, higher saturation mobility by 3.2 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V-s, smaller sub-threshold slope by 0.91 V/decade, and lower threshold voltage by 1.4 V. In addition, the material properties of samples A and B were obtained using the line scan of the energy dispersive spectroscopy and analyzed with technology computer-aided design simulation. As a result, the performance of a-IGZO TFTs can be optimized by tuning the Mo/Ti ratios of the interlayer in the stacked SD electrodes.

Reduction of self-heating effect in (Ga)ZnO thin film transistor

ABSTRACTRF magnetron sputtering technique was employed to fabricate gallium zinc oxide ((Ga)ZnO) semiconductor thin film transistor (TFT) at 100°C. X-ray diffraction and Scanning electron microscopy were used to identify the structure and morphology of (Ga)ZnO layer. Self-heating effect was very much reduced when compared with output characteristics of a TFT fabricated using undoped zinc oxide. The low deposition and processing temperatures make (Ga)ZnO-TFTs very promising for the flexible electronics.

Ga-doped ZnO as an electron transport layer for PffBT4T-2OD: PC70BM organic solar cells

Ga-doped ZnO(GZO) is investigated as an electron transport layer in organic solar cells based on a promising donor: acceptor system of poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3‴-di(2-octyldode-cyl)-2,2′; 5′,2″; -5″,2‴-quaterthio-phen-5,5‴-diyl)] (PffBT4T-2OD):phenyl-C71-butyric acid methyl ester (PC70BM). With the inverted geometry having a configuration of ITO/GZO (40 nm)/PffBT4T-2OD:PC70BM (270 nm)/MoO3 (20 nm)/Al (100 nm), maximum power conversion efficiency (PCE) of 9.74% has been achieved, while it is limited at 8.72% for devices with undoped ZnO. Our study based on the structural, morphological, compositional, and electrical characterizations indicate that suggests enhanced device performance of the GZO-based devices resulted mainly from the improved electrical properties of Ga-ZnO thin films as compared to undoped ZnO.

Drain Current Response to Fast Illumination Pulse for Amorphous In–Ga–Zn-O Thin-Film Transistors

In this paper, the characteristic of the amorphous indium–gallium–zinc-oxide thin-film transistors under steady bias and fast multiple-pulse illumination stress is analyzed. The change in the drain current ( $\Delta I_{D}$ ) with respect to time is measured under different light intensities and pulse frequencies. The mechanism is proposed to explain the results by correlating the oxygen vacancy ( $V_{O}$ ) reacting with the light-induced electron–hole (e-h) pairs. The results can be well analyzed by the fitting formula and parameters for the different light pulse frequencies. The tendency proves that not all the $V_{O}$ react with e-h pairs at the same reaction rate. It is observed for the first time the number of $V_{O}$ with fast reaction decreases with the pulse frequency. A model is proposed to describe the relation between the number and reaction rate of $V_{O}$ .

Optical and electrical characteristics of Ga-Zn-O thin films prepared by RF magnetron co-sputtering system

We investigated the optical, electrical, and structural properties of GaZnO (GZO) thin films which have proven the potential uses in transparent thin film transistor. The doping concentrations of Ga atoms in GZO thin films were continuously controlled via the RF magnetron co-sputtering system in which ZnO target was sputtered at a fixed RF power 200 W, while Ga2O3 target was sputtered with a varied RF power from 30 W to 100 W. The highest electrical conductance and carrier mobility coincides with the dramatic optical band gap widening due to Burstein-Moss effect at the ∼3% atomic concentration of Ga doping. This study clearly indicates that the number of degenerate electrons in the conduction band increases sharply near the 3% doping level, but the band narrowing effect begins to dominate due to the electron-electron and electron-impurity interactions, resulting in the decrease of the carrier mobility beyond the optimized doping level.

Process Optimization and Device Characterization of Nonvolatile Charge Trap Memory Transistors Using In–Ga–ZnO Thin Films as Both Charge Trap and Active Channel Layers

Charge-trap memory thin-film transistors (CTM-TFTs) using In-Ga-ZnO (IGZO) thin films as active channel and charge trap layers (CTLs) were fabricated and characterized. Technical strategies to optimize the device design parameters were categorized into the following three parts. At first, P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O2</sub> conditions during the sputtering deposition of IGZO CTL were varied to 1%, 2%, and 5% to modulate the electronic natures of the IGZO films. The device using the CTL deposited at P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O2</sub> of 1% obtained the largest memory window and exhibited the fastest program speed. Second, to investigate the thickness effects of double-layered tunneling oxide, the configuration was varied to 3/3 nm and 5/5 nm. From the viewpoints of process window, the 5/5 nm configuration was chosen for stable device characteristics. At last, the effects of CTL thickness, which affects the number of trap sites and carrier concentration of the film, was carefully investigated. A 30-nm-thick CTL showed most desirable behaviors, including superior memory operation and uniformity. The CTM-TFTs fabricated with optimum conditions exhibited the memory margin in programmed currents between ON-and OFF-states of 2.9 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> at 1-μs program voltage pulses with ±20 V. Furthermore, the ION/OFF of five-orders-of-magnitude was obtained even after the lapse retention time for 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> s.

Annealing-induced modifications in sol–gel spin-coated Ga:ZnO thin films

Transparent conductive gallium-doped zinc oxide (Ga:ZnO) thin films were prepared by the optimized sol–gel spin-coating technique on glass substrate. After deposition, the films were annealed at temperatures ranging from 200 to 500 °C in the air. The effect of post-annealing temperature on structural, optical and electrical behaviors of Ga:ZnO (GZO) thin films was investigated. Experimental results showed the polycrystalline nature of the films with a hexagonal (wurtzite) crystal structure and a preferred orientation with c-axis, perpendicular to the substrate. The average optical transmittance of all the annealed films is over 89 %. The grain size and optical transmittance of the GZO films increased with increasing annealing temperatures. The photoluminescence spectra of GZO films showed the near-band-edge (NBE) emissions at 390 nm and blue emissions at 456 nm. The NBE emissions might originate from excitonic recombination, and the blue emission peak can be attributed to the zinc interstitial and oxygen vacancies in the ZnO lattice. The lowest resistivity of 2.23 × 10−3 Ω cm and the highest figure of merit are achieved for a GZO film annealed at 500 °C. The new results obtained from this study indicate that the structural, optical and electrical properties of GZO thin films can all be improved through an appropriate post-annealing operation. The obtained results confirm that the thermal annealing has considerable effects on the properties of GZO thin films deposited by sol–gel spin-coating method and can be used as a promising TCO for optoelectronic applications. AFM surface morphology of GZO thin films annealed at (a) 200 °C, (b) 300 °C, (c) 400 °C, (d) 500 °C

Electrical Properties of Solution Processed In-Ga-ZnO Thin Film Transistors with Multi-Stacked Active Layer.

In this study, we prepared solution-based In-Ga-ZnO thin film transistors (IGZO TFTs) having a multistacked active layer. The solution was prepared using an In:Zn = 1:1 mole ratio with variation in Ga content, and the TFTs were fabricated by stacking layers from the prepared solutions. After we measured the mobility of each stacked layer, the saturation mobility showed values of 0.8, 0.6 and 0.4 (cm2/Vs), with an overall decrease in electrical properties. The interface formed between the each layers affected the current path, resulting in reduced electrical performance. However, when the gate bias VG = 10 V was applied for 1500 s, the threshold voltage shift decreased in the stack. The uniformity of the active layer was improved in the stacked active layer by filling the hole formed during pre-baking, resulting in improved device stability. Also, the indium ratio was increased to enhance the mobility from 0.86 to 3.47. These results suggest high mobility and high stability devices can be produced with multistacked active layers.

Investigation of temperature-dependent asymmetric degradation behavior induced by hot carrier effect in oxygen ambiance in In–Ga–Zn-O thin film transistors

The effects of oxygen ambiance on electrical characteristic degradation phenomena in a-InGaZnO thin film transistor with different biases and temperatures are investigated. It can be found that oxygen is substantially adsorbed on the backchannel and results in device instabilities during positive gate bias stress. However, visible light irradiation is found to desorb the adsorbed oxygen ions and this verifies that oxygen dominates the degradation behavior. Moreover, comparing with that in vacuum, hot-carrier stress in oxygen ambiance leads to an extra potential barrier height near the drain side due to oxygen adsorption and causes asymmetric degradation. Furthermore, the asymmetric degradation behavior after hot-carrier stress in oxygen ambiance is suppressed at high temperature due to temperature-induced oxygen desorption or heat-induced holes injecting into the gate insulator.

Quality improvement of high-performance transparent conductive Ti-doped GaZnO thin film

Ti-doped GaZnO transparent conductive oxide films were deposited at various substrate temperatures by radio-frequency magnetron sputtering to examine the effects of crystalline quality on thin-film optoelectronic properties. The results show a low resistivity of 2.97×10−4Ω-cm and a high optical transmittance of 94.15% (wavelength region of 400–800nm) in 300-nm-thick Ti-doped GaZnO thin films grown at the substrate temperature of 300°C, with a post-annealing temperature of 400°C. To evaluate the performance of Ti-doped GaZnO transparent conductive oxide films, this study employed the figure of merit (ΦTC), expressed as ΦTC=T10/RS (T: transmittance RS: sheet resistance), and calculated as 55.2×10−3Ω−1, which is suitable for high-performance optoelectronic device applications.

Sol–gel derived Al and Ga co-doped ZnO thin films: An optoelectronic study

Al and Ga co-doped ZnO (AGZO) thin films with different doping contents of 0.5–4at.% were synthesized via sol–gel route using dip coating method and the results were compared to the single doped specimens Al:ZnO (AZO) and Ga:ZnO (GZO). All samples were highly transparent in visible region (T>85%) with band gap values around 3.3eV. Introduction of Al and Ga to the ZnO crystal structure decreased the crystallinity and reduced the particle size of the films. Electrical resistivity was investigated and engineered in this study as the main parameter. Single doped samples showed reduction of resistivity compared to the un-doped ZnO. In this regard, Ga was more efficient than Al in decreasing the electrical resistivity. Furthermore, samples with 1at.% Al and 1at.% Ga showed the minimum amount of electrical resistivity. Co-doping was performed with two different approaches including variable doping content (Al+Ga≠cte) and constant doping content (Al+Ga=0.5, 1, and 2at.%) for the sake of the comparison with single doped samples. Samples with Al=1at.% and Ga=1at.% showed the lowest electrical resistivity in AGZO samples of former approach. However, in latter approach the lowest resistivity was obtained in Al+Ga=2at.% sample. The results proved the capability of co-doped samples in optoelectronic industry regarding partially substitution of expensive Ga with Al and obtaining co-doped AGZO transparent conductive thin films with lower resistivity compared to conventional AZO thin films and also achieving commercial advantages compared to costly GZO thin films.

Power and gas pressure effects on properties of amorphous In–Ga–ZnO films by magnetron sputtering

Amorphous In–Ga–ZnO thin films were deposited on quartz glass substrate at room temperature utilizing radio frequency magnetron sputtering technique. Sputtering power and oxygen flow rate effects on the physical properties of the In–Ga–ZnO films were systematically investigated. It is shown the film deposition rate and the conductivity of the In–Ga–ZnO films increased with the sputtering power. The as-grown In–Ga–ZnO films deposited at 500 W exhibited the Hall mobility of 17.7 cm2/Vs. Average optical transmittance of the In–Ga–ZnO films is greater than 80% in the visible wavelength. The extracted optical band gap of the In–Ga–ZnO films increased from 3.06 to 3.46 eV with increasing the sputtering power. The electrical properties of the In–Ga–ZnO films are greatly dependent on the O2/Ar gas flow ratio and post-growth annealing process. Increasing oxygen flow rate converted the In–Ga–ZnO films from semiconducting to semi-insulating, but the resistivity of the films was significantly reduced after being annealed in vacuum. Both the as-grown and annealed In–Ga–ZnO films show n-type electrical conductivity.