High-mobility ZrInO thin-film transistor prepared by an all-DC-sputtering method at room temperature

Thin-film transistors (TFTs) with zirconium-doped indium oxide (ZrInO) semiconductor were successfully fabricated by an all-DC-sputtering method at room temperature. The ZrInO TFT without any intentionally annealing steps exhibited a high saturation mobility of 25.1 cm2V−1s−1. The threshold voltage shift was only 0.35 V for the ZrInO TFT under positive gate bias stress for 1 hour. Detailed studies showed that the room-temperature ZrInO thin film was in the amorphous state with low carrier density because of the strong bonding strength of Zr-O. The room-temperature process is attractive for its compatibility with almost all kinds of the flexible substrates, and the DC sputtering process is good for the production efficiency improvement and the fabrication cost reduction.

There have been growing interests in transparent oxide semiconductors (TOSs) for their potential application in thin-film transistors (TFTs) backplanes of flat-panel displays (FPDs) such as active matrix organic light-emitting diodes (AMOLEDs) 1-3 and liquid-crystal displays (LCDs) 4,5 . InGaZnO (IGZO) is one of the most common TOSs owing to its advantages of relatively high mobility compared to amorphous silicon, good uniformity, visible-light transparency, and low cost [6][7][8][9][10] . However, the processing temperature of IGZO TFTs is still too high for most of the transparent plastic substrates, such as polyethersulphone (PES), polyethylene naphthalate (PEN), polyethylene terephthalate (PET), and polycarbonate (PC), which require processing temperature below 180 °C [11][12][13] . On the other hand, the mobility of IGZO (usually around 10 cm 2 V −1 s −1 ) is still not enough for the future high-resolution, high-frame rate, or 3D displays which require TFT backplanes with mobility of higher than 20 cm 2 V −1 s −1 .
To address the above issues, many groups have attempted to attain high-mobility and low-temperature TFTs using InO x -based TOSs, such as In 2 O 3 [14][15][16] , InZnO 17,18 , Sn-InZnO 19 , Hf-InZnO 20,21 , Si-InZnO 22,23 , Zr-InZnO 24,25 , InWO 26 and InSiO 27,28 , as the channel materials. However, the stability of most of the low-temperature TOS-TFTs are not good enough for applications in FPDs. In another aspect, most of the TOS films are fabricated with RF sputtering which is not favorite in the mass-production lines because of the low deposition rate and the RF-radiation dangers.
In this paper, TFTs with zirconium-doped indium oxide (ZrInO) channel layer were prepared by an all-DC-sputtering method at room temperature. Instead of RF sputtering, DC sputtering was chosen to deposit ZrInO films for its advantages of high deposition rate, good reproducibility, good uniformity for multi-component film, no need of power matching, and no RF-radiation danger [29][30][31] . The gate dielectric layer of the ZrInO TFTs was prepared by room-temperature anodization, rather than by plasma-enhanced chemical vapor deposition (PECVD) which requires another expensive vacuum instrument and would cause environment pollution due to the requirement of expensive, toxic, flammable, and explosive gases.
The Zr element of ZrInO is considered to be a superior oxygen binder to suppress the formation of oxygen vacancies for its low electronegativity (1.4) 24 and strong bonding strength with oxygen (the bonding energy of Zr-O is as high as 776 KJ/mol) 32 . In comparison, the electronegativity of In is 1.78 33 , and the bonding strength of In-O is only 348 KJ/mol 28 . In addition, the radius of Zr 4+ is 0.72 Å which is close to that of In 3+ (0.79 Å), so incorporation of Zr into In 2 O 3 will not cause serious lattice distortion.  Figure 2 shows the X-ray diffraction (XRD) patterns of the ZrInO thin films (140 nm) with different annealing temperature. The as-deposited ZrInO thin film was almost in amorphous phase. The ZrInO thin film annealed at 150 °C exhibited a clear crystalline peak at 30.71 o , which coincided well with the (222) peaks of In 2 O 3 bixbyite structure. It implied that almost no lattice distortion took place after incorporation of Zr into In 2 O 3 , which was ascribed to the similar ionic radii of Zr 4+ (0.72 Å) and In 3+ (0.79 Å). The 250 °C-annealed thin film exhibited more peaks at 35.47 o , 51.04 o , and 60.68 o , corresponding to the (400), (440), and (622) peaks of In 2 O 3 bixbyite structure, respectively. As the annealing temperature increased to 350 °C, another peak for In 2 O 3 (543) was found. The results suggest that the film structure of ZrInO is almost the same as that of In 2 O 3 , and increasing the annealing temperature will cause crystallization which is not good for the uniformity of the electrical performances.

Results and Discussion
Field-emission scanning electron microscopy (FESEM) was used to study the surface morphology of ZrInO thin films with different annealing temperature, as shown in Figure S1. All the ZrInO thin films showed uniform and dense surfaces, which is critical to obtain high performance TFTs. In addition, the changes of the grain size with the annealing temperature were consistent with the result of XRD and TEM experiments above. Figure 3(a) shows the transmittance spectra of the 140 nm-thick ZrInO thin films coated on a quartz substrate with different annealing temperature. All ZrInO thin films were optically transparent and showed an average transmittance of exceeding 85% at wavelengths ranging between 400 and 1000 nm, indicating that ZrInO thin film could be used as an active channel layer for fully transparent displays. The optical band gap (E g ) of the ZrInO thin films were estimated from the Tauc Plot (Equation 1) 34 , where A is the constant of material; α is the absorption coefficient; h is Planck's constant and ν is the frequency; and n is the coefficient value (herein, n = 2). Figure 3(b) shows the curves of (αhν) 2 versus hν. The as-deposited and 150 °C-annealed ZrInO thin films showed the same E g of 3.42 eV. As the annealing temperature increased to 250 °C, the values of E g increased sharply to 3.67 eV which was the same as that of the 150 °C-annealed one. The variation of E g was ascribed to the higher degree of crystallinity when the annealing temperature reached 250 °C, as discussed above. X-ray photoelectron spectroscopy (XPS) analysis was also conducted to observe the relationship between the oxygen bonding states and the annealing temperature. Figure 4(a-c) show In 3d 5/2 , Zr 3d 5/2 , and O 1 s spectra of ZrInO thin films with different annealing temperature, respectively. Compared to the as-deposited and 150 °C annealed ZrInO thin films, the 250 °C and 350 °C annealed thin films showed a smaller hump at 531 ± 0.1 eV which is corresponding to the oxygen vacancy (V o ). And the content of V o of ZrInO thin films with different annealing temperature were obtained from Figure S2 and listed in Table 1. Compared to the as-deposited ZrInO thin film with V o content of 38.3%, the V o content of 150 °C-annealed one slightly increased up to 38.7%. And the V o -content sharply decreased down to 29.1% and 19% for 250 °C and 350 °C annealed ones, respectively. The same phenomena were observed in the spin-orbit split XPS data of In 3d 5/2 and Zr 3d 5/2 , as shown in Fig. 4(a,b), respectively. Both of the peaks of In 3d 5/2 and Zr 3d 5/2 in ZrInO thin film shifted toward the high binding energy direction as the annealing temperature increased, indicating an increase of the coordination number of the Zr or In with the increasing annealing temperature. It should be noted that the binding energy shift (Δ E b ) of In 3d 5/2 was only 0.11 eV, much smaller than that of Zr 3d 5/2 (0.46 eV), implying that the Zr ions would gain more oxygen ions compared to indium ions.     Table 2. The as-deposited ZrInO TFT exhibited good performance with an average saturation mobility (μ sat ) of 25.1 cm 2 V −1 s −1 (from 20 devices, as shown in Fig. 5(f)), a threshold voltage (V th ) of − 0.94 V, a sub threshold swing (SS) of 0.42 V/decade, and an on/off current ratio (I on /I off ) of 3.3 × 10 7 . The small hysteresis in transfer characteristic between forward and reverse sweeps was ascribed to few adsorption state 35 (such as O 2− and O − etc.) on the ZrInO back channel because of its insensitivity to air. As the annealing temperature increased to 150 °C, the ZrInO TFT became conductive, which indicated that the carrier concentration was too high to be depleted. Further increased the annealing temperature to 250 or 350 °C, the turn-on voltage shifted towards the positive direction, but hysteresis and the off current (I off ) increased largely. These results suggest that the room-temperature processed ZrInO TFTs have the best performance which is attributed to the lowest carrier density. It is worth noting that the undoped In 2 O 3 TFTs exhibited high conductivity (unable to be turn off) even without any annealing steps. Therefore, the Zr dopant has an effect of suppressing free carrier generation. Figure 6 shows the electrical stability of the unannealing ZrInO TFT under positive bias stress (PBS). During the test, a positive bias (V GS = 10 V, V DS = 10.1 V) was applied as an electrical stress for 60 min, and the transfer curves were recorded every 10 min. It is clearly to see that ZrInO TFT without passivation layer exhibited excellent electrical stability with a threshold voltage shift of only 0.35 V, indicating that the ZrInO material was insensitive to air (H 2 O or O 2 ). It suggests that the room-temperature, all DC-sputtered ZrInO TFTs is stable enough for the backplanes of AMOLEDs.
In conclusion, ZrInO thin film prepared by DC magnetron sputtering were investigated as an active channel layer for TFTs. The structural properties of ZrInO thin films were analyzed using XRD, SEM and TEM, which showed an increase in the crystallinity as the annealing temperature increased. The average transmittance of the ZrInO thin films were over 85% in the wavelength ranging between 400 and 1000 nm. The as-deposited ZrInO TFTs exhibited an average μ sat of 25.1 cm 2 V −1 s −1 ; a V th of − 0.94 V; a SS of 0.42 V per decade and an I on /I off of 3.3 × 10 7 with excellent positive bias stress stability (Δ V th = 0.35 V/h). The room-temperature processes without any intentionally annealing steps show a great potential for the applications in the flexible displays, and the DC sputtering method is good for the production efficiency improvement and cost reduction.

Methods
ZrO 2 and In 2 O 3 powders were weighed in a stoichiometric ratio (ZrO 2 :In 2 O 3 = 1:99 wt.%) and were thoroughly mixed after griding. Then the powders were pressed into a pellet and were sintered at ~1450 °C for 48 h to form a ZrInO target. Then the ZrInO target was fixed in a sputter. The ZrInO semiconducting thin film was prepared by      TFTs with ZrInO channel layers were fabricated with bottom-gate structure, as shown in Fig. 1(a). A layer of 300-nm-thick Al-Nd alloy was deposited on a glass substrate by DC magnetron sputtering and patterned by wet etch. Then, a layer of Nd:Al 2 O 3 was prepared by an anodization process as the gate dieletric layer 36,37 . After that, a 20-nm-thick ZrInO active layer was deposited onto Nd:Al 2 O 3 by DC magnetron sputtering and patterned by shadow mask. For the source/drain electrodes, a 380-nm-thick ITO thin film was sputtered through a shadow mask defining a channel width/length (W/L) of 300/300 μ m. The whole preparation process was performed at room temperature without intentionally annealing.
The surface morphology and structure properties of the oxide thin films were characterized by scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, FEI Titan Themis 200) equipped with an energy dispersive X-ray spectrometer (EDS), respectively. The crystal structures were confirmed via X-ray diffraction (XRD, Philips X pert pro M). The optical properties of the ZrInO thin films were analyzed using ultraviolet fluorescence spectrometer (Shimadzu, UV-3600). X-ray photoelectron spectroscopy (XPS, Thermo Scientific, Escalab 250 XI) using monochromatic Al Ka radiation (~1486.6 eV) was used to examine chemical composition of the ZrInO thin films. The XPS data were calibrated with C 1 s peak at ~284.6 eV. The electrical measurements were performed using a semiconductor parameter analyzer (Agilent 4155 C) in air.