Low-Temperature and Solution-Processable Zinc Oxide Transistors for Transparent Electronics.

Zinc oxide (ZnO) thin-film transistors (TFTs) have many promising applications in the areas of logic circuits, displays, ultraviolet detectors, and biosensors due to their high performances, facile fabrication processing, and low cost. The solution method is an important technique for low-cost and large fabrication of oxide semiconductor TFTs. However, a key challenge of solution-processable ZnO TFTs is the relatively high processing temperature (≥500 °C) for achieving high carrier mobility. Here, facile, low-cost, and solution-processable ZnO TFTs were fabricated under the annealing temperature of ≤300 °C. Dense and polycrystalline ZnO films were deposited by the spin-coating method. The ZnO TFTs showed the maximum electron mobility of 11 cm2/V s and a high on/off ratio of >107 when the ZnO thin films were annealed at 300 °C. The mobility was extremely high among solution-processable undoped ZnO TFTs reported previously, even better than some high-cost indium-doped ZnO TFTs fabricated at low temperature. Furthermore, it is found that the mechanism of oxygen vacancies dominates the electron transport in ZnO thin film and interface behaviors of ZnO thin film and SiO2 gate insulator, and then dominates the performances of devices.

Introduction

In the last decade, metal oxide semiconductors (MOSs) have attracted much attention due to their applications in the field of thin-film transistors (TFTs).[1] The mobility of oxide TFTs can be higher than that of amorphous silicon devices, even reaching the level of polycrystalline silicon, which can make oxide TFTs the promising candidates in the applications of the active matrix organic light-emitting diode displays, photodetectors, and biosensors. Furthermore, because of the high transparency of metal oxide semiconductors,[2] they also can be applied in transparent electronic devices. The key component for transparent electronics is the wide band gap oxide semiconductor.[3] Among metal oxide semiconductors, the wide band gap zinc oxide (ZnO) (3.37 eV) has caught the attention of many researchers due to its high electron mobility, high optical transparency, and low cost. In the past years, the significant improvement has been made in ZnO thin-film transistors prepared by several techniques,[4−7] such as atomic layer deposition and radio frequency magnetron sputtering. However, these techniques require high vacuum system, which limits their application in low-cost and large-area electronics.[8] An alternative method to resolve this issue is to develop low-temperature and solution-processable ZnO thin-film transistors.[2] The TFTs of ZnO have been reported by using a carbon-free aqueous Zn(OH)2(NH3) solution.[9−14] Meyers et al.[10] reported that the ZnO TFTs were fabricated by spin-coating the Zn(OH)2 precursor solution and the high carrier mobility of 4.3 cm2/V s were achieved when the TFT channels were annealed at 300 °C in the N2 glovebox. Very recently, Lin et al.[9] have employed ZnO·xH2O as raw materials to fabricate ZnO TFTs by spin-coating. The TFTs were annealed at 180 °C in the N2 glovebox and the high carrier mobility of 11 cm2/V s was obtained. However, compared to commercial Zn(OH)2 and ZnO raw materials, ZnO·xH2O was difficult to be synthesized and was not stable, which would hamper its application in the future.

To obtain the devices with high carrier mobility, the multicomponent oxide semiconductors-based ZnO, such as indium–zinc oxide (IZO), indium–gallium–zinc oxide (IGZO), and tin–zinc oxide (ZTO), have been synthesized by solution processing and successfully integrated in TFTs.[1,15−17] In 2015, Sirringhaus et al.[1] reported that the IZO TFTs were fabricated by solution processing, and the high carrier mobility of 12 cm2/V s was obtained when the IZO thin films were annealed at 300 °C in the vacuum. However, the vacuum system was required, which would lead to high cost. On the other hand, most of the dopant materials in ZnO were the novel metal, and doping process would add to the complexity of device fabrication. Therefore, it is necessary to explore the simple and low-cost solution processing to fabricate high-performance oxide TFTs.

In this work, we used commercial ZnO as the raw material of precursor solution and successfully fabricated the high-performance undoped ZnO TFTs with the annealing temperature of ≤300 °C. The obtained TFTs showed the carrier mobility of 11 cm2/V s and the on/off ratio of >107 at 300 °C. Even when the annealing temperature of ZnO thin film decreased to 200 °C, the TFTs still indicated a high electron mobility of >3 cm2/V s and a high on/off ratio of >106. We also investigated the influence of oxygen vacancies in ZnO thin film on the electrical properties of TFTs, which would guide to fabricate the high-performance oxide TFTs.

Results and Discussion

Figure a shows the X-ray diffraction (XRD) patterns of the crystallized ZnO thin films annealed at the temperature between 200 and 500 °C. The three strong and narrow diffraction peaks in the XRD patterns were observed clearly, which implied a good crystallization of the ZnO thin films. The three strong peaks corresponded to the (100), (002), and (101) planes of the hexagonal wurtzite structure. Interestingly, the diffraction density ratio of (002) and (100) plane increased when annealing temperature increased from 200 to 300 °C. However, the diffraction density ratio of (002) and (101) plane decreased when the annealing temperature exceeded 300 °C. Therefore, the ZnO thin film annealed at 300 °C had the most preferred orientation of (002) plane. Figure b shows the scanning electron microscopy (SEM) image of the surface morphology of ZnO thin film annealed at 300 °C in the N2 glovebox. The annealed ZnO thin film was dense and uniform. The dense and uniform ZnO grains were beneficial to the transportation of electrons in the ZnO thin film. Further, the surface of ZnO thin films annealed at the temperature between 200 and 500 °C in the glovebox was characterized by atomic force microscope (AFM) (see Figure S1 in the Supporting Information). It was found that all of the surfaces of ZnO thin films were very smooth. Figure c shows the AFM image of the ZnO film annealed at 300 °C in the glovebox. It could be seen that the thin film of ZnO prepared by this simple solution-processable method showed an extremely smooth surface morphology with a root mean square roughness (RRMS) of 0.23 nm. In the polycrystalline oxide semiconductors, low surface roughness was important in establishing effective charge percolation pathways.[18−22] The low roughness of ZnO surface was possibly one of reasons that ZnO TFTs demonstrated a high carrier mobility. The detailed electrical performances of ZnO TFTs would be discussed later. On the other hand, the surface roughness of ZnO thin film annealed at 200 °C in the ambient air was also investigated (see Figure S2 in the Supporting Information). Compared to ZnO thin film annealed in the glovebox, ZnO thin film annealed in the ambient air indicated higher roughness.

Figure 1

(a) XRD patterns of ZnO thin films annealed at the temperatures of 200, 250, 300, 400, and 500 °C. (b) SEM image of ZnO thin film annealed at 300 °C. (c) AFM image (scan size: 2 μm × 2 μm) of ZnO thin film with a root mean square roughness (RRMS) of 0.23 nm. All of the ZnO thin films were prepared by spin coating the precursor solution onto the SiO2/Si2+ substrates for one time with 3000 rpm for 30 s and thermal annealing for 30 min at 300 °C in the glovebox. (d) Transmission spectra of blank glass and ZnO thin films deposited on the glass annealed at 200, 250, and 300 °C.

As shown in Figure d, the transmittances of ZnO thin films on the glass annealed at 200, 250, and 300 °C were investigated. It is clear that all of the samples exhibited an outstanding high transmittance (>90%) for all of the wavelengths in the range of 400–1200 nm. The clear absorption characteristic for all of the thin films appeared at around 380 nm, which implied that ZnO thin films had extremely high transparency in the entire visible region. Therefore, this type of low-cost, solution-processable ZnO TFTs could be promising in the applications of transparent electronics.[23]

Generally, the carrier transport properties of ZnO semiconductor were closely related to the chemical states of ZnO thin films, such as oxygen vacancies. To investigate the chemical states of ZnO thin films, X-ray photoelectron spectroscopy (XPS) spectra were carried out on the ZnO thin films annealed at the temperature between 200 and 500 °C, as shown in Figure . Figure a–e shows the O 1s XPS spectra of ZnO thin films annealed at the temperature of 200, 250, 300, 400, and 500 °C in sequence. The O 1s peak could be fitted by three components representing three different states:[24−26] (1) O2– in a wurtzite structure surrounded by Zn atoms at low binding energy (O–M); (2) oxygen vacancies at medium binding energy (O-deficient); and (3) OH species and/or aqueous adsorbed on the surface of ZnO film at a high binding energy (O–H). As shown in Figure a–f, the O–H groups decreased with the increase in annealing temperature. However, O–M and O-deficient groups demonstrated the different variation tendency. The O–M groups had little variation when the ZnO thin films annealed at the temperature of ≤300 °C, which implied that the O–H groups mainly converted to O-deficient groups during the low temperature phase. When the annealing temperature exceeded 300 °C, the O–M groups began to increase and O-deficient groups began to decrease at the same time. The possible reason was that more oxygen vacancies began to be filled by free oxygen atoms outside the wurtzite structure during the high-temperature phase. The oxygen atoms were easier to enter the atom gaps because the atom gaps of wurtzite structure enlarged or dynamic energy of atoms increased when the temperature was elevated. These oxygen atoms filled the vacancies after cooling down and converted to O–M groups finally. In general, higher concentration of oxygen vacancy usually induced higher carrier concentration, which elevated the Fermi level and shift VT to the negative direction.[27,28] Therefore, ZnO TFTs with the ZnO annealing temperature of ≤300 °C demonstrated excellent performances of TFTs, which would be discussed later.

Figure 3

(a) Transfer characteristics of ZnO TFTs annealed at 200 °C in the glovebox and in the air. (b) Output characteristics of ZnO TFTs annealed at 200 °C in the glovebox. (c) Transfer and (d) output characteristics of ZnO TFT prepared by Zn(OH)2 precursor solution at the annealing temperature of 200 °C in the glovebox.

Figure 2

(a–e) O 1s XPS spectra of ZnO thin film annealed at the temperature of 200, 250, 300, 400, and 500 °C, respectively. (f) Area ratio of the three peaks (O–M groups, O-deficient groups, O–H groups) at the temperature of 200, 250, 300, 400, and 500 °C.

First, the influence of annealing atmosphere on TFT’s performances has been investigated. The ZnO thin films were annealed at 200 °C in the glovebox and ambient air, respectively. Their electrical properties of TFTs are shown in Figure . Figure a shows the transfer characteristics of ZnO TFTs annealed in the glovebox and air. It was obvious that device annealed in the glovebox had a higher field-effect mobility and a higher on/off ratio than those annealed in the air. For the ZnO thin film annealed in the glovebox, the high carrier mobility of 3.26 cm2/V s and high on/off ratio of 106 were obtained, respectively. However, the carrier mobility of device annealed in the air was just 0.32 cm2/V s that was 1 order of magnitude lower than that annealed in the glovebox, which was consistent with the previous results reported by Anthopoulos et al.[9] When the ZnO thin films were annealed in the glovebox, ZnO thin films were easy to form oxygen vacancies and accumulated high concentration of oxygen vacancies due to the lack of O2 atmospheres. The high concentration of oxygen vacancies was beneficial to electron transport in ZnO thin films and resulted in high carrier mobility.[27] On the other hand, the turn-on voltage (Von) of N2-annealed device shifted to negative gate bias. It suggested that N2 annealing would lead to the positive charge doping in the interface of the semiconductor and the insulator. It was attributed to the increase in oxygen vacancies when the device was annealed in the glovebox. Figure b shows the output characteristic of ZnO transistor annealed at 200 °C in the glovebox. It was clear that the output curve of the transistor annealed in the glovebox showed a negligible hysteresis. However, the output curve of the transistor annealed in the air showed an obvious hysteresis (see Figure S5 in the Supporting Information). The large hysteresis of output curve and low on/off ratio in ZnO TFT was because the interface of SiO2 and ZnO thin film got worse when the ZnO thin film was annealed in the air.

To investigate the precursor solution influence on the performances of ZnO TFTs,[10−12] the ZnO TFTs were also fabricated by using the precursor solution of Zn(OH)2 in ammonium hydroxide. The detail preparation processing of the precursor solution and TFTs could be seen in Figure S6 in the Supporting Information. Figure c,d shows the transfer and output characteristics of the devices fabricated by using Zn(OH)2 precursor solution. The extracted saturation mobility of ZnO TFT was about 0.9 cm2/V s, which could be comparable to those of ZnO TFTs prepared by solution processing at the same temperature in previous studies.[29,30] On the other hand, the obvious hysteresis of output curve could be observed, as shown in Figure d. Therefore, the mobility of ZnO TFTs prepared by the precursor of Zn(OH)2 was much lower than those of the ZnO TFTs prepared by the precursor of ZnO in ammonium hydroxide. The as-prepared film by Zn(OH)2 precursor solution needs extra energy during the dissociation procedure in contrast with the ZnO precursor solution. When the films are annealed at the same temperature, the films deposited from Zn(OH)2 precursor solution need extra energy to make Zn(OH)2 to decompose into ZnO, which leads to the deposited films having inferior crystallinity. On the other hand, the precursor solution had a large influence on layer morphology. The surface roughness of film deposited from Zn(OH)2 at 200 °C in the glovebox was characterized (see Figure S8 in the Supporting Information). It could be found that the film deposited from the Zn(OH)2 precursor solution has a rougher surface with the root mean square roughness (RRMS) of 0.74 nm.

The annealing temperature of ZnO thin films was also investigated for optimizing the performances of ZnO TFTs. Figure a shows the transfer curves of ZnO TFTs annealed at the temperature between 200 and 500 °C in the glovebox. The Von shifted to the negative gate bias when the annealing temperature of ZnO thin film increased from 200 to 300 °C, whereas the Von shifted back to the positive gate bias when the annealing temperature exceeded 300 °C. This variable tendency was consistent with oxygen vacancies in the ZnO thin film measured by XPS. The Von was closely relative to the density of traps state of the interface. In other words, the mechanism of oxygen vacancies dominated the Von shift. Figure b shows the dependence of carrier mobilities of ZnO TFTs on the annealing temperatures of ZnO thin film. The average carrier mobilities of ZnO TFTs were 3, 4.1, 8.6, 4.2, and 0.4 cm2/V s for the annealing temperature of 200, 250, 300, 400, and 500 °C, respectively. It was clear that the highest average electron mobility was obtained when the ZnO thin film was annealed at 300 °C. This was most likely attributed to the high concentration of oxygen vacancies and high preferred (002) orientation of ZnO thin film when the ZnO thin film was annealed at 300 °C. These results were confirmed by XPS and XRD measurements. Figure c,d shows the transfer and output characteristics of the ZnO TFT annealed at 300 °C in the glovebox, respectively. It was noteworthy that the highest electron mobility of ∼11 cm2/V s and the on/off ratio of >107 were obtained when the ZnO thin film was annealed at 300 °C. The carrier mobility was extremely high among solution-processable undoped ZnO TFTs,[10,14,31] even better than some solution-processable indium-doped ZnO TFTs fabricated at low temperature. More importantly, the negligible hysteresis of output curves had also been observed, as shown in Figure d. It was very important to employ the ZnO TFTs in the future application. The high performances could be attributed to the low reaction activation energy during the process of zinc-ammine dissociation. The ZnO and ammonium hydroxide could be combined into zinc ammine complex. The zinc ammine in the solution was not stable and easily decomposed into ZnO at low temperature. Therefore, the good crystallinity of films could be obtained when the films were annealed at relatively low temperature, which lead to high performances even when the ZnO TFTs were annealed at low temperature.

Figure 4

(a) Transfer curves of ZnO transistors annealed at the temperatures of 200, 250, 300, 400, and 500 °C in the glovebox. (b) The dependence of carrier mobilities of ZnO TFTs on the annealing temperatures of ZnO thin film. (c) Transfer and (d) output characteristics of ZnO TFT annealed at 300 °C.

Conclusions

In summary, a simple, lost-cost, solution-processable method has been developed to fabricate high-performance ZnO TFTs at low temperature. The performances of ZnO TFTs strongly depend on the annealing temperature of ZnO thin films. The mechanism of oxygen vacancies dominates the electron transport in ZnO thin film and interface behaviors between ZnO thin film and SiO2 gate insulator, and then dominates the performances of the devices. The ZnO TFTs annealed at 300 °C exhibit maximum electron mobility of 11 cm2/V s and a high on/off ratio of >107. Furthermore, when the annealing temperature is reduced to 200 °C, the ZnO TFT still shows excellent performances with the electron mobility of >3 cm2/V s and the on/off ratio of >106. This effort shows extreme attraction in the applications of lost-cost, large-area, flexible devices as well as transparent electronics.

Experimental Section

Precursor Solution

The precursor solution was obtained by directly dissolving pure ZnO powder (99.99%, Aldrich) in ammonium hydroxide (>28%) at the concentration of 8 mg/mL. Then, the complex solution were stirred rigorously at room temperature for 12 h and filtered through a 0.45 μm poly(tetrafluoroethylene) filter to remove the insoluble substance before using. The process of dissolution can be expressed as

Material Characterization

The surface morphology was examined by field emission scanning electron microscope on JEOL7100F and the acceleration voltage was 15 kV. The atomic force microscope (AFM, Solver Nano) was used to measure to the surface roughness. The X-ray diffraction patterns were characterized with Bruker Advanced D8 X-ray diffractometer using Cu Kα (λ = 0.154 nm) radiation. The sweep spacing was 0.02°. The operating voltage and current were 40 kV and 40 mA, respectively. Transmittance spectra were collected by using Shimadzu UV3600. The surface chemical composition and the chemical state were characterized with X-ray photoelectron spectroscope (XPS, Escalab 250Xi) using Al Kα (binding energy 1486.6 eV). The XPS spectra fitting were obtained via Avantage. The high-resolution transmission electron microscopy (HRTEM, FEI Titan F30) was used to measure the particle size distribution of raw ZnO powder (see Figure S11 in the Supporting Information).

Device Fabrication

The detailed flow chart of ZnO TFTs is shown in Figure . The ZnO TFTs with the architecture of bottom gate top contact were fabricated by spin-coating processing. The heavily doped Si with 300 nm thermally grown SiO2 was used as gate electrode and gate dielectric layer. Before depositing the filtered precursor solution, the substrates of SiO2/Si2+ were cleaned via acetone, isopropyl alcohol, and deionized water in sequence in ultrasonic cleaning machine. Next, the clean silicon wafer was treated by O2 plasma for 5 min. Then, the filtered precursor solution was spin-coated on the substrates of SiO2/Si2+ at 3000 rpm for 30 s to form the active layer. This process was repeated for several times to obtain the desired thickness of ZnO films. Afterward, ZnO thin film was annealed on the hot plate for 30 min in the glovebox or in the air. Finally, Al was evaporated on the active layer as the source and drain electrodes through shadow mask. The channel width and length were 2000 and 100 μm, respectively.

Figure 5

Flow chart of fabrication process of ZnO TFTs.

Electrical Characterization

The FETs were characterized with a semiconductor parameter analyzer (Keithley 4200-SCS). The electron saturation mobility (μsat) is extracted by the following equation[32]Here, L and W are the length and width of the channel, respectively. IDS is the channel current. VG and VT are the gate voltage and threshold voltage, respectively. Ci is the capacitance of the gate insulator.