Enhancing transparent thin-film transistor device performances by using a Ti-doped GaZnO channel layer

Abstract

This study improved the performances of a thin-film transistor (TFT) device with n-type Ti-doped GaZnO (GTZO) as the channel layer. Various O₂/Ar ratios were used during radio-frequency magnetron sputtering deposition to modify the carrier concentration and the thin-film surface flatness. Atomic force microscopy results indicate that the lowest surface roughness (0.38 nm) was observed in the GTZO films fabricated at an O₂/Ar ratio of 12/30 sccm. In addition, a room-temperature X-ray diffraction and photoluminescence study verified the improved crystal quality and decreased oxygen vacancies as the O₂/Ar ratio increased. The GTZO films fabricated at an O₂/Ar gas flow of 6/30 sccm were adopted as the transistor channel layer of a TFT, which exhibited an improved carrier mobility of 16.1 cm² V⁻¹ s⁻¹, a subthreshold swing of 0.43 V dec⁻¹, an off current of 5.6 × 10⁻¹³ A, and an on–off current ratio of 2.2 × 10⁸. From the comparison of transfer characteristic curves for the TFTs with channels of ZnO, indium gallium zinc oxide, and GTZO in this study, the GTZO TFTs exhibit superior characteristics which demonstrated the potential for high-performance optoelectronic device applications.

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Introduction

Thin-film transistors (TFTs) have been extensively applied in active-matrix liquid crystal displays. Various channel layer materials for fabricating TFTs have been considered, such as amorphous silicon (a-Si),¹ polycrystalline silicon (poly-Si),² indium tin oxide (ITO),³ indium zinc oxide,⁴ zinc oxide (ZnO),⁵ hafnium indium zinc oxide,⁶ and indium gallium zinc oxide (IGZO).⁷ Using these materials in the channel layer of TFTs yields TFTs that feature excellent performance, high field-effect mobility, a high on–off current ratio, and operational stability. However, a-Si TFTs have problems such as low field-effect mobility, light sensitivity, and opaqueness. To overcome these disadvantages, substituting conventional Si-based TFTs with transparent oxide-based semiconductors that have high carrier mobility is essential. The transparent oxide-based transistor exhibits a large energy bandgap (>3 eV); thus, the backlight can be transparent to the transistors, improving the aperture ratio of display devices to address the light sensitivity of an a-Si TFT. Therefore, transparent oxide-based transistors are suitable for use in various optoelectronic devices. Although a-IGZO TFTs have been extensively studied, exhibiting high mobility and a reasonable on–off current ratio, concerns have recently been raised regarding reduced access to indium supplies, which increases the fabrication cost of indium oxide thin-film-based device applications. A low-cost high-performance substituent channel layer is thus essential as a replacement in novel optoelectronic devices.

Recently, ZnO has attracted increasing attention as a potential candidate for use in the active channel layer because this facilitates the development of TFTs with a wide energy bandgap, optical transparency, and a low fabrication cost. In particular, the conductivity of ZnO films were improved by performing extrinsic impurity doping.⁸ However, the ZnO films with a rough surface that served as channel layers in the TFTs were sputter-deposited using a high-energy ion bombardment process. This process can cause surface carrier scattering as well as charged states between the interface of the channel and insulator layers, deteriorating the device performance levels of ZnO-based TFTs. Previous studies have indicated that the smooth channel surface improved device operation characteristics such as carrier mobility, reliability, and the subthreshold swing (S.S.) of bottom-gate TFTs.^2–5 To improve the level of device performance, the increase of ZnO surface flatness was essential for the channel layer, thereby reducing the interface trap density and improving the carrier mobility.

Improved carrier mobility and reduced carrier scattering were demonstrated by reducing the surface roughness of the interface between the channel and insulator layer.⁹ Kim et al. reported that laser-annealing a ZnO thin film produces a smooth thin-film surface and, therefore, TFTs that exhibit a low threshold voltage of 0.6 V and high field-effect mobility of 5.08 cm² V⁻¹ s⁻¹.¹⁰ The smoothness of the ZnO thin-film surface was verified to be beneficial for the TFT device characteristics. However, the fabrication cost was too high and the fabrication yield was low across the large panel area when adopting the laser-annealing process.

Recently, Ga-doped ZnO (GZO) films have attracted a considerable amount of attention for improving carrier concentration due to the substitutional incorporation of Ga³⁺ ions into Zn sites and carrier mobility.¹¹ In addition, GZO films show the advantageous features such as the low reactivity of the gallium dopant with oxygen atoms, resulting in the high reliability of GZO-based optoelectronic devices. Regarding the lattice strain, GZO thin films with closed Ga–O and Zn–O bond lengths of 1.92 Å and 1.97 Å, respectively, exhibit reduced crystal defects due to a reduced lattice strain and high electrochemical stability.

GZO films also feature a smooth surface morphology, which is beneficial for developing various optoelectronic devices. This smooth surface could be due to the surfactant effect of Ga doping.¹² The GZO TFT was studied to exhibit the enhancement in the device performances of field-effect mobility and saturation current compared with the ZnO TFTs because of the smooth channel surface and an increased carrier concentration of the GZO thin films.¹³ Moreover, the ZnO surface flatness can be further improved by performing Ti doping, which can be used to reduce the structural defects in ZnO-based films.¹⁴ Chang et al. reported that ZnO-based thin films can be lightly doped with Ti to reduce the structural defects and root-mean-square (RMS) roughness, which was determined to be advantageous to high-performance TFT channel layer applications.¹⁵

In this study, we combined the benefits of Ga and Ti doping to form Ti-doped GaZnO (GTZO), which is an attractive material for TFT channel application. The performances of GTZO channel TFTs are reported in this paper. Various O₂/Ar ratios (ratios of oxygen/argon process gas flow) were applied during radio-frequency (RF) magnetron sputtering deposition to modify the carrier concentration and the thin-film surface flatness.

Experimental section

The GTZO films were grown using a 3 inch GTZO target composed of ZnO/TiO₂/Ga₂O₃ (96/1/3 wt%). All of the films were deposited at room temperature by using RF magnetron sputtering on glass substrates (Eagle 2000). Prior to deposition, the targets were presputtered for 10 min before growth. To deposit the GTZO films, the chamber was evacuated to the base pressure of approximately 4 × 10⁻⁶ Torr and the working pressure was maintained at 5.0 × 10⁻³ Torr with an O₂/Ar gas flow of 0/30, 3/30, 6/30, and 12/30 standard cubic centimeters per minute (sccm). Although the O₂/Ar gas flow was increased to 15/30 sccm, the high oxygen ratio caused the instability of sputtering plasma generation. The ZnO and GZO films used for performance comparison were grown using 3 inch ZnO (purity: 99.995%) and ZnO/Ga₂O₃ (97/3 wt%) targets, respectively.

The sputtering power was maintained at 150 W for all oxide-based sputtering targets. The chamber was cooled using a water-cooled chiller system during deposition, and the thickness of all deposited GTZO, ZnO, and GZO films was 50 nm. The surface morphology of the GTZO films was observed using atomic force microscopy (AFM: Seilo, SPA400). The 325 nm line of a He–Cd laser was used as the excitation source for room-temperature photoluminescence (PL) measurements. To fabricate transparent TFTs, 100 nm-thick ITO layers were deposited as gate electrodes on the glass substrate. Consequently, a 150 nm-thick SiO₂ layer was deposited using plasma-enhanced chemical vapor deposition (PECVD) as the gate insulator. The 100 nm-thick ITO layers were deposited as source and drain electrodes on the GTZO channel layer by using RF magnetron sputtering at room temperature. Finally, a 50 nm-thick GTZO layer was deposited at various O₂/Ar ratios as the channel layer of TFTs. In the present study, the channel length and width of the TFT device were 50 and 500 μm, respectively. The TFT characteristics were measured using a semiconductor parameter analyzer (HP 4156) at room temperature. All of the samples were measured under the same condition to ensure the reliability of the results.

Results and discussion

The X-ray photoelectron spectroscopy (XPS) measurement was employed to identify the elemental composition incorporation in the GTZO thin-films. Fig. 1(a) shows the XPS spectra survey of the as-grown GTZO thin film, which indicates the chemical states of each constituent element, as follows: 1147 eV for Ga 2p_1/2, 1120.3 eV for Ga 2p_3/2, 1046.8 eV for Zn 2p_1/2, 1023.6 eV for Zn 2p_3/2, 458.8 eV for Ti 2p_3/2, and 531.3 eV for O 1s. The high-resolution XPS spectral scan of Ti 2p_3/2 in the GTZO film was represented in Fig. 1(b), and a reference element of C 1s (285.3 eV) in the inset was used to calibrate all the XPS signals. The titanium compositions of the GTZO thin film is determined as 1.29 at% by using XPS at an etching depth of 40 nm. The binding energy of Ti 2p_3/2 was found as 458.8 eV, which is higher than that of pure Ti (453.8 eV), and could be ascribed to the replacement of Zn²⁺ with Ti⁴⁺ in the ZnO lattice.^16,17

Fig. 1 (a) The X-ray photoelectron spectrum survey scan shows the elemental doping compositions of the as-grown Ti-doped GZO thin film. (b) High-resolution X-ray photoelectron spectrum of Ti-doped GZO thin films at Ti 2p_3/2 and Ti 2p_1/2. The XPS signals were calibrated using 285.3 eV for C 1s as a reference element shown as an inset.

The electrical properties including the dependence of the thin-film resistivity, mobility, and carrier concentration of the GTZO films on the sputtering power of the GTZO thin films were analyzed using the Hall measurement in Fig. 2. For clear comparison, the carrier concentrations of GZO and ZnO thin films are also shown in the figure. The GZO and ZnO thin films were deposited using RF power of 150 W at the identical sputtering environment with GTZO thin-film deposition. All the thin films studied for resistivity study in Fig. 2 were deposited without oxygen incorporation. The high carrier mobility of 3.1 cm² V⁻¹ s⁻¹ was observed for the GTZO thin film at an RF power of 150 W. However, a decrease in the carrier mobility was observed resulting from the increased sputtering power, which may have correlated with the ion bombardment effect at a high level of sputtering power.¹⁸ The superior electron concentration of GTZO thin films (>10²¹ cm⁻³) compared with those of typical GZO (approximately 10²⁰ cm⁻³) and ZnO (approximately 10¹⁹ cm⁻³) was contributed by Ti doping, indicating the advantageous current-driven ability of a GTZO thin film as a channel layer in the TFT application.^19,20 The increased electron concentration of GTZO films could be ascribed to the increased oxygen vacancies (V_O) in the Ti-doped ZnO films.²¹ Additionally, the GTZO films with Ti-doping exhibit Ti⁴⁺ (ionic radius: 0.68 Å) substitution for Zn²⁺ (ionic radius: 0.74 Å) ions, and thus, behaves as a donor providing two additional free electrons; therefore, they can increase thin-film conductivity considerably.^22,23 The detailed material studies of GTZO thin films with advanced optoelectronic performances can be referenced from previous reports.^24–26

Fig. 2 Hall measurements of resistivity ρ, carrier concentration n, and carrier mobility μ of the Ti-doped GaZnO (GTZO) thin films at various sputtering powers. The carrier concentrations of GZO and ZnO thin films are also shown for comparison.

For understanding the conductive mechanism of GTZO films and the doping role of Ti, the temperature-dependent electrical properties of GZO and GTZO thin films were studied using the Hall measurement. The temperature-dependent Hall effect of both samples was measured from 10 to 320 K by using a Lakeshore 325 temperature controller in a van-der-Pauw geometry with a magnetic field of 0.48 Tesla (Accent HL5500 Hall System). The properties of thin-film resistivity, carrier concentration and mobility as a function of temperature for the GZO and GTZO films are shown in Fig. 3. For GZO and GTZO films, there doesn't show significant variation in carrier concentration and mobility over the entire temperature range. The dominant carrier scattering mechanisms for carrier mobility could be attributed to intra-grain scatterings with ionized impurity and phonon scattering in these two films with such high carrier concentration.²⁷ In contrast, the behavior of thin-film resistivity as a function of temperature shows a dependence on the Ti doping in the ZnO matrix. In the temperature-dependent Hall measurement, the resistivity of GTZO thin film was lower than that of GZO thin film. The decrease of thin-film resistivity could be predominantly accounted by the increase in carrier concentration because of Ti doping. In addition, the metal–semiconductor transition behavior of the temperature-dependent resistivity at the temperature of 170 K for GTZO thin films was found because of weak localization of degenerate electrons and the increase in disorder induced by Ti atoms. By contrast, the resistivity of GZO thin film decreases with increasing temperature, which exhibits a negative temperature coefficient behavior and is a critical feature of a nondegenerate semiconductor.²⁸ The temperature-dependent Hall measurements in this study help to clarify the effects of Ti-doping on electrical properties of GTZO films and are important for the optoelectronic device application.

Fig. 3 Temperature dependence of the thin-film resistivity, free carrier concentration, and Hall mobility obtained from temperature-dependent Hall effect measurements.

The surface morphologies of the 50 nm-thick ZnO, GZO, and GTZO films were studied using the AFM technique, as shown in Fig. 4. The insets of Fig. 4 show 3D-AFM images of the ZnO, GZO, and GTZO thin films deposited using RF magnetron sputtering on glass substrates. According to AFM analysis of the thin films, the surface roughness values were 1.21, 0.67, and 0.60 nm for the ZnO, GZO, and GTZO, respectively. The improved surface flatness of GZO can be ascribed to the surfactant effect caused by Ga doping; this is consistent with a previous study that reported a reduced surface roughness from 20.8 to 3.2 nm as the Ga concentration was increased from undoped to 8%.²⁹

Fig. 4 The summary of the average surface roughness of the ZnO, GZO, and GTZO thin films. The 3D-AFM images of 50 nm-thick ZnO, GZO, and GTZO thin films deposited through RF sputtering at 150 W with a O₂/Ar ratio of 6/30 sccm are shown in the inset for clear comparison.

For further improvement, GTZO films exhibited a dense and smooth surface with a reduced surface roughness to 0.60 nm; this concurred with the reduced surface roughness from 15.2 to 3.9 nm through Ti incorporation.^30,31 In addition, Ti-doped ZnO films prepared using sputtering and sol–gel techniques also exhibited improved surface roughness, verifying that incorporating ZnO with Ti doping can yield smooth thin-film surfaces and dense microstructures.^30,31

By incorporating titanium doping into the ZnO material, the roughness of a GTZO layer was reduced, achieving films substantially denser and smoother compared with undoped ZnO and GZO films. The smooth GTZO layer was useful to improve the carrier mobility as well as the driven current density.

For further improvement in the surface flatness and reduced oxygen vacancy defects, GTZO films deposited with various O₂ flows while the total Ar flow was maintained at 30 sccm were studied in this work. The AFM topographic images and roughness of the GTZO films deposited with various O₂ flows are shown in Fig. 5(a). The RMS roughness of GTZO films decreased from 1.89 to 0.38 nm as the oxygen flow increased from 0 to 12 sccm. The GTZO thin film deposited with O₂ gas flattened the surface structure and suppressed the oxygen vacancies.

Fig. 5 (a) The summarized root-mean-square roughness and (b) deposition rate of GTZO thin films deposited with 0, 3, 6, and 12 sccm O₂ flow gas. The 3D-AFM images of 50 nm-thick GTZO thin films with various O₂ process flows are shown in the insets of Fig. 5(a) for clear comparison.

The dependence of the surface roughness of ZnO thin films as a function of oxygen flow was reported, and reduced roughness was observed as the oxygen flow increased because of the decreased sputtering deposition rate.³² Since the thin film deposition rate (and, thus, surface roughness and crystalline quality) varies with oxygen flow, a series of GTZO thin films with different deposition rates was characterized as a function of oxygen flow. Fig. 5(b) depicts the correlation between oxygen flow and deposition rate. The decreased deposition rate from 1.6 to 1.0 Å s⁻¹ was observed as increasing the oxygen flow from 0 to 12 sccm while the total Ar flow was maintained at 30 sccm. Therefore, given uniform thin-film thickness, the effect of oxygen flow on thin-film surface roughness and crystal quality can be investigated.

In addition, the generation of high-energy neutral oxygen resputtering atoms could also contribute to the formation of grain agglomerations in the ZnO film deposited with the introduction of oxygen gas flow.³³ Because the energy was transferred from the impinging particles to the film atoms through a sequence of collisions, the nucleation of crystallite and grain growth will be affected by the sputtering bombardment with high-energy neutral atoms.³⁴ Therefore, the GTZO thin films grown with oxygen incorporation exhibited reduced surface roughness and improved grain growth in this study.

Fig. 6 shows the optical transmission spectra of GTZO films deposited with various O₂ flow. The films exhibit high transparency levels and average transmittance values above 88% in the visible wavelength range (400–800 nm). The GTZO films prepared without oxygen exhibited the lowest transmittance values, and increased transmittance was observed as the oxygen flow increased from 0 to 3 sccm; this was because of the reduced oxygen vacancy defect scattering and improved ZnO crystal quality.⁸ However, a high oxygen flow reduced the thin-film transmittance from 92.6% to 89.7% as the oxygen flow increased from 3 to 12 sccm; this resulted from an increased surface flatness as verified in the AFM studies.

Fig. 6 Optical transmittance of GTZO thin films prepared under various O₂ flow gases from 0 to 12 sccm. The inset shows the optical bandgap of GTZO films prepared under various O₂ flow gases.

The optical bandgap (E_g) of the GTZO film in Fig. 6 was calculated by considering an electron transition from the valance to conduction band when the thin film absorbed a photon of energy (hν). In semiconductors that involve a direct bandgap, the absorption coefficient (α) obeys the following relation for E_g:³⁵

(αhν)² = (hν − E_g) (1)

where h is Plank's constant and ν is the photon frequency. The bandgap (E_g) is therefore deduced by extrapolating the linear portion of a plot (αhν)² against the (hν) axis. The inset of Fig. 6 shows a summary of the optical bandgap values for the GTZO films, exhibiting a decrease from 3.35 to 3.08 eV as the oxygen flow increased from 0 to 12 sccm. The red-shift behavior of the absorption edges as the oxygen flow increased was primarily attributed to the Burstein–Moss effect; in other words, Fermi level shifts occurred because of the reduced conduction electrons provided by the oxygen vacancies.^36,37

Fig. 7 shows the X-ray diffraction (XRD) patterns of GTZO thin films sputter deposited under various O₂ flows. As the oxygen partial flow in the sputtering gas mixture increased from 0 to 12 sccm, the growth of the (002) crystal plane along with an evident increase of (002) orientation XRD intensity was observed. The GTZO film deposited in 12 sccm-oxygen flow ambient exhibited a strong (002) XRD peak intensity with a narrow spectral linewidth, which indicates that the crystallite quality of the ZnO thin films improved with the increased grain size calculated from Scherrer's formula.²² The improvement in the (002) XRD peak intensity as well as the crystal quality could be ascribed to the reduction of oxygen vacancy defects, deposition rate and surface roughness in the ZnO matrix in the increased oxygen-flow sputtering ambient.

Fig. 7 The dependence of XRD intensity, full width at half maximum (FWHM), and the grain size calculated through Scherrer's theorem of GTZO films with various O₂ gas flows from 0 to 12 sccm while the total Ar flow was maintained at 30 sccm during sputtering deposition.

PL measurement of the GTZO fabricated using various oxygen process flows was performed to facilitate a detailed material characteristics study. Typically, the PL spectrum of sputter-deposited ZnO-based transparent conductive oxide (TCO) thin films exhibits a poly-crystallized structure and noticeable deep-level (DL) emission. A possible reason for this DL emission to energy transition is related to defects such as antisite oxygen (O_Zn), zinc vacancies (V_Zn), oxygen vacancies (V_O), oxygen interstitials (O_i), and zinc interstitial (Zn_i). In this study, the oxygen process flow was adopted to reduce the oxygen vacancies to improve the crystallinity and, hence, improve the optoelectronic properties of GTZO thin films. Fig. 8 shows the room-temperature PL spectra from 330 to 800 nm of the GTZO films processed at various O₂/Ar gas flows. The characteristics and crystal quality of GTZO thin films produced at different O₂/Ar gas flows can be analyzed and studied by measuring the PL. The PL results exhibited a broad band in the green emission region, which can be resolved into two branches situated at 540 nm and 600 nm; this was caused by the DL emissions of V_O and O_i.³⁸ Both oxygen deficiencies decreased in intensity as the O₂/Ar ratio increased. In the reported studies, a lower oxygen gas flow can lead to the inhomogenous and structural fluctuation of ZnO thin films.

Fig. 8 The room-temperature photoluminescence spectrum of GTZO films with various O₂ gas flows from 0 to 12 sccm.

This increased oxygen incorporation can reduce the oxygen deficiencies and improve the structural uniformity of ZnO thin films.³⁹ The samples fabricated at a high oxygen flow ratio exhibited improved surface roughness and reduced oxygen vacancies. Therefore, the material characteristics of GTZO films deposited with oxygen gas are expected to yield further improvement in TFT device characteristics.

Bottom-gate TFTs with a GTZO channel layer were fabricated and their current–voltage (I–V) characteristics were detail analyzed. The drain current vs. gate voltage (I_DS–V_GS) output characteristics and the corresponding square root of the drain current–gate voltage ((I_DS)^1/2–V_GS) curve of the TFTs with a GTZO channel layer fabricated at various O₂/Ar gas flows were thoroughly compared. The output characteristics of all of the GTZO TFTs exhibited n-type transistor behavior at a drain-to-source voltage (V_DS) of 2.5 V. Gate voltages from −5 V to 10 V were applied to enable a graph of the transfer characteristics to be made. The I–V curve of the TFT with a GTZO channel layer deposited without O₂ gas did not achieve the saturation condition because the carrier concentration of the GTZO thin film was too high (10²⁰ cm⁻³). When the O₂/Ar ratio was increased to 3/30 sccm and the off current was 9.1 × 10⁻¹¹ A, the on–off current ratio increased to 3.4 × 10⁵ and the threshold voltage was 3.05 V, according to the graph of (I_DS)^1/2–V_GS shown in Fig. 9(a). When the O₂/Ar ratio was further increased to 6/30 sccm (Fig. 9(b)) and the off current was reduced to 5.6 × 10⁻¹³ A, the on–off current ratio further improved to a maximum value of 2.2 × 10⁸ and the threshold voltage was 1.76 V. The mobility (μ_sat) of field-effect transistors was calculated from the drain current in the saturated region, and was determined using the following equation:⁴⁰

where μ_sat is saturation mobility, V_th is threshold voltage, C_ox is the unit capacitance of the gate dielectric, and W and L are the channel width and length, respectively. The μ_sat of the GTZO TFTs was enhanced from 1.2 to 16.1 cm² V⁻¹ s⁻¹, and the S.S. can be determined by calculating the maximum slope in the transfer characteristics from 0.84 to 0.43 V per decade as the O₂ gas flow increased from 3 to 6 sccm, compared with the transfer characteristic results shown in Fig. 9. The reduced S.S. indicates the reduced interfacial trap density (N_it) and the improved interface quality between the dielectric and channel. The N_it can be obtained using the following equation:⁴¹where q is the electron charge, T is the absolute temperature, and k is the Boltzmann constant. The N_it decreased from 2.76 × 10¹² to 2.31 × 10¹² as the O₂ gas flow increased from 3 to 6 sccm, which suggests that N_it decreased because of the high oxygen flow. However, when the O₂ gas flow was increased to 12 sccm, the carrier concentration of the GTZO films decreased substantially and showed insulated characteristics; therefore, the TFT device cannot be operated at a high gate voltage. The properties of the GTZO TFTs fabricated using various O₂ gas flows, such as mobility, threshold voltage, on–off current ratio, S.S., interfacial trap density, and off current, are summarized in Table 1. The electrical properties, including the dependence of the thin-film resistivity, mobility, and carrier concentration, on the O₂/Ar ratios of the GTZO films, were also presented using the Hall measurement (Lakeshore model 75013) in Table 1. The increased resistivity from 1.345 × 10⁻³ to 3.696 × 10⁸ with obvious reduction of carrier concentration was observed for GTZO thin films as increasing the O₂/Ar ratio from 0/30 to 3/30 sccm during the deposition process. Further increasing the O₂/Ar ratio to 6/30 sccm, the GTZO film shows the resistivity of 7.344 × 10¹⁰ ohm cm. Since the increased oxygen process flow was verified to decrease the oxygen vacancies as well as the conductive electrons in the ZnO based materials, the GTZO thin films deposited with increased O₂/Ar ratios to 12/30 show the significantly increased thin-film resistivity with the insulator-like characteristics which cannot be measured by the Hall system.

Fig. 9 The transfer characteristics of drain current (I_DS) vs. gate voltage (V_GS) and (I_DS)^1/2 vs. gate voltage (V_GS) of the GTZO channel TFTs fabricated with a O₂ gas flows of (a) 3 sccm, and (b) 6 sccm. The corresponding output drain current–drain voltage (I_DS–V_DS) curves were shown for GTZO TFTs with varied O₂ gas flows of (c) 3 sccm, and (d) 6 sccm.

Table 1 The properties of the GTZO TFTs fabricated using various O₂ gas flows, such as S.S., threshold voltage, mobility, interfacial trap density (N_it), off current, and on–off current ratio. The electrical properties, including the dependence of the thin-film resistivity, mobility, and carrier concentration on the O₂/Ar ratios of the GTZO films, were presented using the Hall measurement

O2/Ar ratio	Properties of the GTZO TFTs						Hall
	S.S. (V per decade)	Vth (V)	Mobility (cm^2 V^−1 s^−1)	Nit (cm^−2)	Off current (A)	On/off ratio	Resistivity (ohm cm)	Mobility (cm^2 V^−1 s^−1)	Concentration (cm^3)
0/30 sccm	∞	×	×	×	2.5 × 10^−5	1	1.345 × 10^−3	6.236	7.443 × 10^20
3/30 sccm	0.84	3.05	1.2	2.76 × 10^12	9.1 × 10^−11	3.4 × 10^5	3.696 × 10^8	1.125	1.518 × 10^10
6/30 sccm	0.43	1.76	16.1	2.31 × 10^12	5.6 × 10^−13	2.2 × 10^8	7.344 × 10^10	0.364	2.353 × 10^8
12/30 sccm	×	×	×	×	×	×	×	×	×

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Fig. 9(c) and (d) show the drain current–drain voltage (I_DS–V_DS) curves obtained from GTZO TFTs with varied O₂/Ar gas flows of 3/30 and 6/30 sccm. The saturation drain currents I_DS were observed as 8.0 × 10⁻⁴ A and 2.4 × 10⁻⁴ A for Fig. 9(c) and (d) when the applied gate V_GS and drain V_DS bias were 6 V and 20 V, respectively. Both samples exhibited clear current saturation behavior, and good ohmic properties between the channels and the source/drain electrodes.

Reducing the S.S. from 0.84 to 0.43 V per decade and increasing the carrier mobility from 1.2 to 16.1 cm² V⁻¹ s⁻¹ improved the electrical characteristics, as shown in Fig. 9. The reduced surface roughness and surface scattering of the GTZO films fabricated at high oxygen gas flows were observed in the AFM studies, as shown in Fig. 5, and the reduced surface scattering and interface trapped charged states also corresponded well with the improved device performances of GTZO TFTs, shown in Fig. 9.

Fig. 10 shows the comparison of transfer characteristic curves for the GTZO, IGZO, and ZnO TFTs at a V_DS of 2.5 V, and the statistic device characteristics of field-effect mobility, S.S., off current, and on/off current ratio for the aforementioned devices and a reference GZO TFT, were summarized with error bar in Fig. 11. The ZnO and IGZO TFT exhibited a carrier mobility of 0.35, 5.8 cm² V⁻¹ s⁻¹, off current of 3.3 × 10⁻¹⁰, 1.9 × 10⁻¹⁰ A, on/off current ratio of 3.5 × 10⁵, 4.9 × 10⁶, and S.S. of 0.77, 0.76 V per decade, which were similar with the reported results of 0.1–2.5 cm² V⁻¹ s⁻¹ for carrier mobility and 10³ to 10⁶ for the TFT on/off current ratio, and indicated the applicability of this work.^42–44 The rough surface and the reduced carrier concentration of ZnO thin films were correlated with the reduced carrier mobility and saturation current. However, the GTZO TFT with an improved surface flatness and structural quality exhibited enhanced carrier mobility of 16.1 cm² V⁻¹ s⁻¹, off current of 5.6 × 10⁻¹³ A, on/off current ratio of 2.2 × 10⁸, and S.S. of 0.43 V per decade.

Fig. 10 The transfer characteristics of drain current (I_DS) vs. gate voltage (V_GS) of GTZO, ZnO and IGZO TFTs. The picture of GTZO TFT through an optical microscope and schematic illustration are shown as the insets (a) and (b), respectively.

Fig. 11 The statistic device characteristics of field-effect mobility, S.S., off current, and on/off current ratio shown with error bar for GTZO, IGZO, GZO and ZnO TFTs fabricated in this study.

Conclusions

In summary, this study demonstrated that improving the surface flatness of GTZO films enhances the operation characteristics of TFTs. The AFM results indicate that the lowest surface roughness (0.38 nm) was observed in the GTZO films fabricated at an O₂/Ar ratio of 12/30 sccm. The PL results indicate an improvement in the DL emissions of oxygen vacancies and interstitial oxygen; both oxygen deficiencies obviously decreased in intensity as the O₂/Ar ratio increased. According to the results of AFM and PL measurements, the samples fabricated at high oxygen flows of 6 sccm exhibited improved material quality and surface roughness. The GTZO films fabricated at an O₂/Ar gas flow of 6/30 sccm were adopted as the TFT channel layer, such that the TFT device can exhibit an improved carrier mobility of 16.1 cm² V⁻¹ s⁻¹, an S.S. of 0.43 V per decade, an off current of 5.6 × 10⁻¹³ A, and an on–off current ratio of 2.2 × 10⁸. From the comparison of transfer characteristic curves for the TFTs with channels of ZnO, IGZO, and GTZO in this study, the GTZO TFTs exhibit advanced device characteristics which indicating that the GTZO TFT demonstrated the potential for high-performance optoelectronic device application.

Acknowledgements

The authors are grateful to Prof. Chyi at the National Central University for instrument support, and the National Science Council, Taiwan, R.O.C., for its financial support under contracts MOST 104-2623-E-155-001-ET and MOST 104-2221-E-155-028-. The provision of research equipment by the Optical Sciences Center and Center for Nano Science and Technology at National Central University is greatly appreciated.

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