A Fully Transparent Resistive Memory for Harsh Environments

A fully transparent resistive memory (TRRAM) based on Hafnium oxide (HfO2) with excellent transparency, resistive switching capability, and environmental stability is demonstrated. The retention time measured at 85 °C is over 3 × 104 sec, and no significant degradation is observed in 130 cycling test. Compared with ZnO TRRAM, HfO2 TRRAM shows reliable performance under harsh conditions, such as high oxygen partial pressure, high moisture (relative humidity = 90% at 85 °C), corrosive agent exposure, and proton irradiation. Moreover, HfO2 TRRAM fabricated in cross-bar array structures manifests the feasibility of future high density memory applications. These findings not only pave the way for future TRRAM design, but also demonstrate the promising applicability of HfO2 TRRAM for harsh environments.

device reliability, a more efficient way is to find/fabricate/modify the metal oxide material as inert as possible to suppress the surface effects.
HfO 2 , recognized as the most stable and reliable candidate in the field of RRAM has been widely investigated in several aspects, such as high density memory architecture 19 , nanosecond switching capability 20 , high temperature stability 21 , and neuromorphic computation system 22 . Compared with ZnO, HfO 2 exhibits not only relative inertness to the ambient oxygen adsorption, but also comparable transparent nature, which can be beneficial for the development of future TRRAM to operate under harsh conditions 23 . However, toward practical applications of utilizing TRRAMs for future harsh environments, a critical issue is to understand their device durability and switching uniformity under various kinds of harsh conditions in addition to the high temperature, and fewer reports can be found currently to reach relevant results.
In this study, a sandwiched structure of indium-tin oxide/hafnium oxide/indium-tin oxide (ITO/HfO 2 /ITO) fabricated at room temperature for TRRAM is demonstrated, which exhibits average transmittance of 77.64% within the visible wavelength region from 400 to 800 nm. The ON/OFF ratio, defined as the high resistance state (R H ) over the low resistance state (R L ), is approximately 15 can be obtained for HfO 2 TRRAM, and no significant degradation can be observed for more than 100 cycles within cycling endurance test. The retention time measured at 85 °C is 3 × 10 4 sec. The statistical analysis including cell-to-cell and device-to-device tests for over 100 cells are conducted, verifying the excellent switching uniformity of HfO 2 TRRAM. Moreover, little fluctuations in switching parameters of HfO 2 TRRAM can be perceived under various oxygen partial pressure, moisture, radiation and corrosive agent exposure, validating its outstanding durability in contrast to ZnO TRRAM. Furthermore, the HfO 2 TRRAM is fabricated into the cross-bar array configuration, confirming its feasibility for future high-density memory applications. This work demonstrates a comprehensive investigation of utilizing a highly potential HfO 2 TRRAM for harsh environment applications, offering not only excellent environmental stability against various ambiences, but also high density compatibility toward future transparent electronics.

Results
Optical property and binding energy characterization. To quantitatively examine transparency, the transmittance spectrum of the as-fabricated structure ITO/HfO 2 /ITO/glass was investigated, as shown in Fig. 1(a). The average transmittance of the ITO/HfO 2 /ITO/glass is 77.64% within the visible wavelength region from 400 to 800 nm. The photograph of the ITO/HfO 2 /ITO/glass is marked in a dashed-line rectangle in the inset of Fig. 1(a). The "King Abdullah University of Science and Technology-Nano Energy Lab" logo beneath the device can be perceived clearly due to the optical transparency of the device.
The chemical composition of the HfO 2 thin film was characterized by an X-ray photoelectron spectroscopy (XPS) at room temperature. As shown in Fig. 1(b,c), the peaks located at 18.4 and 16.7 eV can be referred to the binding energies of Hf 4 f 5/2 and 4 f 7/2 orbitals, respectively. In addition, the peak located at 530.2 eV is related to the standard O 1 s orbital. These results confirm the formation of HfO 2 by sputtering technique 21 . Resistive switching characteristics. Figure 2 shows the typical resistive switching characteristics of HfO 2 TRRAM, including current-voltage (I-V) characteristics, endurance, and retention test at 85 °C. For comparison, a control sample with the structure of ITO/ZnO/ITO (ZnO TRRAM) was also prepared. During the measurements, a DC voltage was applied on the top electrode while the bottom electrode was grounded. Current compliance, imposed for the forming processes to prevent permanent destruction of dielectric thin films, was set to 1 μ A. The forming voltage is approximately 9 V. After the forming process, a bipolar switching characteristic can be obtained. As shown in Fig. 2(a), the device is initially situated in R H . By sweeping the voltage above a positive threshold value, a sudden increase in current is observed (as denoted by the arrow of Set) indicating that the device is switched to the low resistance state. Then, an abrupt drop of current occurs when the voltage decreases below a negative threshold value (as denoted by the arrow of Reset), which indicates that the device switches back to the high resistance state. These results demonstrate the reversible and steady bipolar switching characteristics of HfO 2 TRRAM. (Description of the schematic) To evaluate the reliability of HfO 2 TRRAM, endurance, and retention properties were measured. Figure 2(b) shows the endurance property for 130 successive resistive switching cycles. The resistance values were read at − 0.1 V in each DC sweep. It is clear that the ON/OFF ratio is larger than 10 within 130 switching cycles and no conspicuous decay can be observed in both resistance states. The two well-resolved distributions of resistance in the two states ensure a sufficient and clear window for read operation. These results indicate that the switching characteristics of HfO 2 TRRAM are reproducible and stable. Figure 2(c) shows the retention property of HfO 2 TRRAM at 85 °C. It is clear that, for both states, the resistance can be maintained over 3 × 10 4 sec, demonstrating the excellent non-volatility of HfO 2 TRRAM. Effect of oxygen adsorption. Next, the durability of HfO 2 TRRAM for harsh environments is explored. It is well-known that oxygen adsorption at surface of metal oxides act as electron traps for charge carriers, which results in the increase of surface potential and deterioration of device performance [16][17][18][24][25][26][27][28] . Hence, the effect of oxygen partial pressure on performance of HfO 2 TRRAM is first examined. We performed endurance test of 100 cycles for each cell under four different ambient conditions (vacuum, N 2 , air, and O 2 ), simulating environments with low, medium and high oxygen concentrations. A statistical analysis for over 100 cells is conducted for evaluating switching yields, resistance distributions (R H and R L ) and switching voltage distributions (V Set and V Reset ), as shown in Fig. 3(a-c), respectively. As it can be seen, the percentage of switching yield, defined as the ratio of the amount of cells exhibiting resistive switching characteristics over 100 cycles without any Set or Reset failure to the amount of total cells, is quite high (> 90%) and uniform for all the ambient conditions. Moreover, the resistance states and switching voltages remain fairly stable with negligible deviation under all the ambiences. The results show the insensitive properties of HfO 2 TRRAM toward oxygen adsorption, indicating that the detrimental surface effects on resistive switching characteristics can be suppressed by using HfO 2 as a replacement of ZnO. In fact, HfO 2 conventionally serves as a surface passivation layer on ZnO-based transistors and diodes due to its chemical stability and inertness 23,29 . Effect of moisture adsorption. To get further insight into the environmental influence on resistive switching characteristics of metal oxides, a damp-heat (DH) treatment conducted at 85 °C and 90% relative humidity (RH) was implemented to study the effect of moisture adsorption 30   characteristics during the DH treatment. For ZnO TRRAM, the resistance in both states was greatly degraded, and the two states were indistinguishable after 12-hour treatment (i.e., device failure). In contrast, the two distinct memory states of HfO 2 TRRAM remain stable and uniform after 100-hour DH treatment. The DH test results validate that HfO 2 TRRAM is more sustainable than ZnO TRRAM in both humidified and high-temperature environments due to its superior chemical stability.
Effect of atmospheric corrosion. Moreover, the corrosion robustness of ZnO and HfO 2 TRRAMs were investigated under formic acid exposure, as shown in Fig. S1(c),(d) in the Supplementary Information. Though the ZnO-based devices have exhibit− ed excellent performances in the field of electronics and optoelectronics, the atmospheric corrosion due to Zn 2+ dissociated from the surface remains a significant issue 15,31,32 . When exposed to an acidic environment, the ON/OFF ratio of ZnO TRRAM decreases significantly with exposure time, and fails after 2100-min, as shown in Fig. S1(c) in the Supplementary Information. The resistance degradation of R H can be attributed to the adsorption of formic acid molecule which causes the dissociation of Zn 2+ near the surface and the decrease in thickness shown in Fig. S2(a) I-IV in the Supplementary Information. In contrast, the effect of atmospheric corrosion on HfO 2 TRRAM is remarkably eliminated, and the resistance exhibits little dependence on acid exposure, as shown in Fig. S1(d) in the Supplementary Information. After 6000-min acid exposure, the window between R H and R L remains clear, demonstrating the superior corrosion robustness of HfO 2 TRRAM to acid solutions. These results are supported by the negligible decrease in thickness and roughness of the HfO 2 thin films after formic acid exposure, as shown in Fig. S2(a) V-VIII in the Supplementary Information. Figure S2(b),(c) in the Supplementary Information also present that the transmittance spectra of HfO 2 remain constant with exposure time, while the transmittance of ZnO increases from 71.56% to 91.38% at 400 nm. The increase of transmittance can be attributed to the deterioration of ZnO film thickness and roughness during formic acid exposure, which can correspond to the device failure of ZnO TRRAM after 300 min in Fig. S1(c) in the Supplementary Information. Meanwhile, it has been reported that the etching rates of HfO 2 thin fim are extremely low in formic acid, sulfuric acid, and oxalic acid solutions 33 . With the excellent corrosion robustness, HfO 2 thin films have been widely used as a surface passivation layer in microelectromechanical systems 34 . Effect of proton irradiation. The other important environmental factor that might cause device damage is proton irradiation 35,36 . Long-term exposure under proton irradiation can cause shifts of I-V characteristics, larger leakage current, high power consumption, and malfunction of electronic devices. In general, these degradations are related to the interaction between proton-induced charges and bulk defects, where oxide and interface traps are usuallly created 37 . To investigate the radiation tolerance, the as-fabricated TRRAMs were irradiated with 2 MeV protons, where proton fluences range from 10 11 to 10 16 cm −2 . Note that the protons with the energy less than 2 MeV and the fluences ranging from 10 1 cm −2 to 10 8 cm -2 occupy a region about 1L-2L above earth's surface, where L is approximately equal to the geocentric distance of a field line in the geomagnetic equator 38   Conversely, the resistance distributions of HfO 2 TRRAM are congruent, showing reliable switching characteristics under proton irradiation. These results suggest that HfO 2 TRRAM is more reliable than ZnO TRRAM in highly-radiative environments.
Specifically, variations in switching parameters of HfO 2 TRRAM are relatively lower than ZnO TRRAM after proton irradiation, which may correlate to the difference in radiation-hardness of ZnO and HfO 2 . Previously, it has been reported that radiation hardenss of metal oxides are closely related to ionic binding strength 39 . In addition, proton irradiation damge which primarliy comes from fomation of radiation-induced defects has also been reported to associate with binding energies in metal oxides 40 . It is likely that, for metal oxides, the higher the binding strength, the better the radiation hardeness. Meanwhile, it is well understood that HfO 2 possess higher bonding strength than ZnO, which may further imply better radiation hardness of HfO 2 . More experiments are currently conducted for clarifying the mechanism and will published elsewhere. While the precise mechanism cannot be determined, it is reasonable to state that HfO 2 TRRAM is a promising candidate for future transparent memory devices to operate under extremely radiative environments.
ON/OFF ratio. Toward practical RRAM application, maintaining stable ON/OFF ratio of memory devices under various environmnets is one of the key issues that needs to be addressed. Herein, we evaluate the variation of ON/OFF ratio of ZnO and HfO 2 TRRAMs under a variety of environments (including different oxygen partial pressure, moisture, acid exposure, and proton irradiation) by a simple equation below. where μ treatment and μ bare represent the average value of ON/OFF ratio obtained from treated (treatment) and untreated (bare) devices, respectively, after 100 cycling tests. Bare condition means that the ON/ OFF ratio of TRRAMs are measured under the ambience of air without moisture treatment, acid exposure, or proton irradiation. In Fig. 4(a-d), it can be found that the ON/OFF ratio of ZnO TRRAM strongly depends on the environmental conditions. Deterioration and fluctuation of ZnO TRRAM significantly increase with the oxygen concentration, humidity, formic acid, and proton irradiation fluences. Conversely, little variation in the ON/OFF ratio of HfO 2 TRRAM (less than 15%) is observed for diverse environmental conditions. In short, the as-fabricated HfO 2 TRRAM exhibits not only high resistance to oxygen chemisorption, but also excellent durability under moisture, acid exposure, and proton irradiation.
To access the applicability of HfO 2 TRRAM for future memory technology, a cross-bar array configuration of HfO 2 TRRAM is illustrated in Fig. 5(a). The optical image of the as-fabricated HfO 2 TRRAM in the cross-bar array in the (upper-right inset) is the enlargement from the central area of the as-fabricated sample (upper left inset) in Fig. 5(a). Note that the packing density of RRAM in cross-bar array can be 1000-times higher than that of current static random-access memory cells 41 . The endurance property of HfO 2 TRRAM cell in cross-bar array was measured and presented in Fig. 5(b). Little fluctuations can be observed for more than 100 cycles, indicating the feasibility of HfO 2 TRRAM for future high-density memory applications.

Discussion
Stacked layers of ITO/HfO 2 /ITO deposited at room-temperature are demonstrated as a TRRAM for harsh environment applications. The HfO 2 TRRAM exhibits an average transmittance of 77.64% in the visible range (from 400 nm to 800 nm), and reliable resistive switching characteristics. To investigate effects of the harsh conditions on resistive switching characteristics of TRRAMs, various ambient conditions, including high oxygen partial pressure, high moisture (relative humidity = 90% at 85 °C), and corrosive agent exposure were implemented. In comparison with ZnO TRRAM, HfO 2 TRRAM shows outstanding tolerance and consistent switching characteristics against diverse ambiences. Moreover, HfO 2 TRRAM exhibits superior immunity from proton irradiation (2 MeV with fluences up to 10 16 cm −2 ), showing great potential to operate under extremely radiative environments. Furthermore, the cross-bar array fabricated with HfO 2 TRRAM demonstrates the feasibility for future high-density memory applications in see-through electronics. These explorations give insights not only in realizing an environmentally stable and high-density compatible TRRAM, but also in developing practical applications of TRRAM for transparent electronic systems with high reliability requirements.

TRRAM Fabrication.
A commercial glass substrate was pre-cleaned by alcohol and deionized water to avoid the contamination from the ambience. ITO thin film of 100 nm thickness as the bottom electrode was deposited on glass substrate by rf-sputtering technique. A HfO 2 thin film with a thickness of 50 nm was deposited by rf-sputtering technique afterwards. Finally, as the top electrode of the device, a 100 nm thick ITO thin film with a diameter of 200 μ m was deposited by sequential sputtering process with a metal shadow mask. Note that all the processes mentioned above were carried out at room temperature.
Characterization. The transmission spectrum of the whole device was measured by (UV/visible V670).
For radiation tolerance testing, the HfO 2 TRRAM was irradiated at room temperature using a 2 MeV proton beam from a 3 MV tandem accelerator (NEC 9SDH-2, National Electrostatics Corporation). The typical current of the proton beam was 2-50 nA (the current increases with increasing fluences). with the beam fluences ranged from 10 11 cm −2 to 10 16 cm −2 at the sample target. Keithley 4200-SCS semiconductor characterization system was used to measure resistive switching characteristics of the as-fabricated HfO 2 TRRAM. Field-emission transmission electron microscopy (JEOL JEM-7100F) was used to investigate the microstructures of ZnOand HfO 2 thin films.