I. INTRODUCTION

Zinc oxide (ZnO) is a II-VI semiconductor with a room temperature direct band gap of ${\sim}$3.4 eV and a wurtzite lattice structure. ZnO with diverse morphologies such as bulk crystals, thin films, nanorods, nanowires, nanobelts, and nanotubes have been widely studied today for possible applications as ultraviolet light emitters, solar cells, gas sensors, and transparent electronics due to their unique properties ^[1-4].

More recently, the demand for electronic devices used in space applications has been increasing with the growth of the space industry. An important consideration in space applications is that the material should be as resistant to radiation as possible to operate reliably for extended periods. Presently, the primary wide-bandgap material with radiation-hard properties for space applications is GaN, SiC, and diamond. However, recent results suggest that the wide-bandgap ZnO shows better radiation resistance than GaN for irradiation experiments by electrons, protons, and heavy ions ^[4-6]. The origin of ZnO radiation resistance is fascinating but still unclear.

The components of space radiation are the high-energy, charged nuclei of elements from hydrogen (protons) to heavy ions. Approximately, it consists of 85% protons, 14% helium, and 1% heavier particles. Since the performance of ZnO devices are affected significantly by the density of oxygen vacancies (V_o) and interstitial zinc atoms (Zn_i) point defects, it is important to develop a more detailed understanding of these basic native defects. Therefore, the different structures of ZnO film and ZnO nanorods (NRs) thin-film transistors (TFTs) were prepared, and we investigated the effects of proton irradiation on the structure of the different active layer focusing on the native defects.

II. EXPERIMENTAL DETAILS

To compare devices with different active layer structures, the ZnO film and ZnO NRs TFTs were fabricated as illustrated in Fig. 1.

Fig. 1. Schematics of (a) the ZnO film; (b) the ZnO NRs TFTs.

First, 120 nm-thick SiO₂ were deposited on n⁺ silicon substrates as a gate oxide. 30 nm-thick ZnO film were deposited by atomic layer deposition (ALD) at 80 ℃. The Diethylzinc (Zn(C₂H₅)₂) was used as a zinc precursor.

For the ZnO NRs TFT, the hydrothermal growth of ZnO NRs on a substrate was prepared following two steps: (i) uniform coating on the substrate with a seeding layer of ZnO NRs which provides the starting points of the ZnO NRs growth; and (ii) directional nucleation of ZnO NRs from the seeding layer. In the first step, we deposited the 30 nm-thick ZnO film by ALD as previously described. After annealing the ZnO film at 500 ℃ for 1 h under air ambient conditions. Before performing the second step of NRs, both film and NRs substrates were patterned with 0.1% diluted HCl to form the channel layer. The titanium source/drain and gate electrodes were formed by RF sputtering and patterned by lift-off process. After then, we prepared the ZnO NRs via the following procedures. 100 mL of 0.02 M zinc nitrate hexahydrate (Zn(NO₃)₂･6H₂O) and 100 mL of 0.02 M hexa-methylenetetramine ((CH₂)₆N₄) were prepared separately in DI water by heating and stirring on a hotplate.

After both solutions were mixed with constant stirring, the ZnO NRs substrate was then dipped into the solution for 20 min at 90~95 ℃. Finally, both ZnO film and NRs devices were subjected to thermal annealing under pure O₂ ambient conditions of 250 ℃ for 1h at 1 atm.

In this study, the TFT dimension was 100 ${\times}$ 10 ${\mathrm{\mu}}$m² (W ${\times}$ L). The electrical characteristics of the TFTs were analyzed using an Agilent 4155B semiconductor parameter analyzer at room temperature under air ambient conditions and in the dark. The fabricated devices were exposed in various proton energies at room temperature, using a 100 MeV proton linear accelerator (Linac) and a 1.7 MV Tandem accelerator at Korea Multi-purpose Accelerator Complex. The proton energy of 50 MeV was provided using the 100 MeV Linac, and the proton energy of 1 MeV was delivered using a Tandem accelerator. The Linac was operated with a repetition rate of 1 Hz and a pulse width of 100 ${\mathrm{\mu}}$sec. A flux of the Linac was about 1 ${\times}$ 10¹⁰ p/cm²/pulse, and a fluence was fixed at 1 ${\times}$ 10¹⁴ p/cm². On the other hand, the Tandem accelerator may deliver the CW beam, instead of any pulse beam. Unlike 50 MeV proton energies that pass through the TFTs, a result of stopping and range of ions in matter (SRIM) simulation ^[7] predicted stopping the 1 MeV proton beam in the ZnO channel layer.

III. RESULTS AND DISCUSSIONS

Fig. 2 shows an SEM image of the ZnO NRs synthesized by hydrothermal method, as mentioned above. Most ZnO NRs have a diameter between 40 nm and 50 nm with a length of 280 nm. NRs are well-formed in the z-axis direction.

Fig. 2. SEM image of the ZnO NRs synthesized by hydrothermal method.

V_GI_D characteristics of the ZnO film and NRs TFTs were measured, as shown in Fig. 3. The measurements were performed by a gate bias voltage from -40 to +30 V at V_D = +20 V. The data indicates that the electrical properties of the NRs TFTs with anomalous hump were inferior to those of the film TFTs. This abnormal hump in the transfer curve was attributed the generation of a parasitic current path ^[8-10]. Although several studies have been introduced the hump characteristics in ZnO-based TFTs ^[11-13], the origin of the hump has not been clarified yet. The cause of the hump will not be further discussed in this study.

Fig. 3. Voltage-current characteristics of the ZnO film and the ZnO NRs TFTs at VD=+20 V.

To investigate the effects of various proton energies irradiation, we compared the ZnO TFTs with different active structures of film and NRs before and after the 1 and 50 MeV proton beam irradiation at fixed fluence of 1${\times}$10¹⁴ p/cm². Fig. 4 shows the V_GI_D characteristics of ZnO film and NRs TFTs before and after proton irradiation. When the 50 MeV protons was irradiated, a negative threshold voltage (V_th) shift was observed in both ZnO TFTs. In general, a negative V_th shift is known to be associated with the increase in electron concentration in the active layer. Irradiated protons can exist two types of donor-like states in the forms of interstitial hydrogen (H_i⁺) or substitutional hydrogen (H_O⁺) in oxide semiconductor. However, the performance of both ZnO TFTs after the 1 MeV proton irradiation was improved with lower subthreshold swing (SS) and positively V_th shift. In particular, the anomalous hump characteristic of the ZnO NRs TFTs was disappeared after the irradiation. Because the 1 MeV proton beam is expected to stop in the ZnO channel layer, we may speculate that the channel resistance of the proton irradiated ZnO NRs was lowered so that the current can flow through the primary current path rather than through the parasitic current path.

Fig. 4. Voltage-current characteristics of (a) the ZnO film; (b) the ZnO NRs TFTs before and after different proton energies irradiation at fluence of 1${\times}$1014protons/cm2.

To investigate the change of the native defects of the ZnO TFTs, an x-ray photoelectron spectroscopy (XPS) was analyzed for the proton beam irradiated ZnO TFTs. Fig. 5 shows O_1s peaks in the XPS spectra of the ZnO film and NRs TFTs before and after the proton irradiation. The original O_1s peaks were deconvoluted by Gaussian fitting into two subpeaks including O_I and O_II. The peak at the lower binding energy of ~530 eV (O_I) is attributed to oxygen bonded with Zn, whereas the peak at higher binding energy of ~532 eV (O_II) is attributed to oxygen vacancy (V_o) related ^[14].

Fig. 5. O1speaks in the XPS spectra of (a) the ZnO film; (b) the ZnO NRs before and after different proton energies irradiation.

The ratio of peak area (O_II / O_tot) of both ZnO TFTs, indicating the relative quantity of V_o defect, are increased after proton irradiation. It is well known that the increase of V_o defects plays a role in the enhancement of conductivity of ZnO-based thin films ^[15,16]. However, in the case of 50 MeV irradiation, the V_GI_D transfer curve showed poor electrical characteristics despite having more V_o defects compared to the 1 MeV case. This may be due to the total ionizing defect (TID) effect caused by the formation of interface traps in the insulator rather than the effect of the increase in V_o in the ZnO film, since the high-energy proton has a lower linear energy transfer (LET) than the low-energy one and thus penetrates through the TFTs. Our irradiated ZnO-TFTs may have high performance in which high dose proton irradiation may enhance the electrical properties of the ZnO active layer.

According to Moon et. al. ^[17,18], the V_o defects of ZnO-based TFTs decreased with increasing proton irradiation dose, but increased at a high proton irradiation dose of over 10¹⁵ p/cm². Thus, if the proton beam irradiation is performed to improve the electrical properties of ZnO-based devices, the relationship between proton dose and V_o defects created should be considered. On the other hand, it should be noted that the proton irradiation effect may depend on the structure of the ZnO active layer. Although a systematic study on varying proton beam energies and doses should be made to understand the affect for the TFTs, it is revealed that the irradiation effects are more sensitive in the ZnO nanostructures than the ZnO film.

IV. CONCLUSION

In summary, electrical and physical properties of ZnO TFTs with different active layers of films and nanorods structures were investigated after various proton energies irradiation. For the 50 MeV proton energies, the performance of both ZnO TFTs were degraded by the TID effect. On the contrary, the 1 MeV irradiated TFTs showed improved characteristics. From the result of XPS analysis, we confirmed that the V_o defects are increased after proton beam irradiation, and. In particular, the performance of the ZnO NRs TFTs was considerably improved than that of the ZnO film TFTs. Therefore, it can be explained that the defects of the ZnO TFTs with nanostructure morphologies are more sensitive to proton irradiation compared to the ZnO film.