Transparent Memory For Harsh Electronics

As a new class of non-volatile memory, resistive random access memory (RRAM) offers not only superior electronic characteristics, but also advanced functionalities, such as transparency and radiation hardness. However, the environmental tolerance of RRAM is material-dependent, and therefore the materials used must be chosen carefully in order to avoid instabilities and performance degradation caused by the detrimental effects arising from environmental gases and ionizing radiation. In this work, we demonstrate that AlN-based RRAM displays excellent performance and environmental stability, with no significant degradation to the resistance ratio over a 100-cycle endurance test. Moreover, transparent RRAM (TRRAM) based on AlN also performs reliably under four different harsh environmental conditions and 2 MeV proton irradiation fluences, ranging from 1011 to 1015 cm−2. These findings not only provide a guideline for TRRAM design, but also demonstrate the promising applicability of AlN TRRAM for future transparent harsh electronics.

(6.2 eV) 19 , high thermal conductivity (134 W/cm K) and thermal stability 20 , high breakdown field (0.95 MV/cm) 21 , and superior radiation hardness [22][23][24] , making it an ideal material for harsh electronics. Previously, researchers demonstrated an AlN-based photodetector that was stable at high temperature and in radiative environments, verifying the potential of these AlN-based devices in extreme conditions 6 . Moreover, since transparent conducting indium tin oxide (ITO) is highly resistant to proton damage for fluences up to 10 16 ions cm −2 25 , ITO/AlN/ITO TRRAM has also been reported to exhibit nanosecond switching and low-power operation 12 . To build on these developments, it is important to understand and explicitly evaluate the switching characteristics of TRRAM made with such materials under extreme environmental conditions for use in harsh electronics.
In this study, we investigated the reliability of ITO/AlN/ITO TRRAM under various conditions, including different environments (vacuum, air, N 2 , and O 2 ) and proton irradiation fluences. Our results indicate that the device features a high-to-low resistance ratio (R H /R L ) of 30 without significant degradation after 100 endurance cycles, validating its excellent durability and resistive switching capability even during such extreme exposures. Furthermore, statistical analyses, including cycle-to-cycle and device-to-device tests of 50 cells, verify the excellent switching stability and uniformity under high oxygen partial pressure and proton irradiation. These experimental results demonstrate the potential of AlN TRRAM for applications in harsh electronics due to the device's superior reliability in extreme environments.

Results and Discussion
To quantitatively examine transparency, we investigated the transmittance spectrum of the as-fabricated TRRAM device, which was composed of alternating layers of indium tin oxide (ITO) and AlN on a glass substrate (i.e., ITO/AlN/ITO/glass). As shown in Fig. 1b, the average transmittance of the device was 80% within the visible wavelength region (400 nm to 800 nm). In the inset of Fig. 1a, we demonstrate the transparent nature of the device (marked by a red rectangle), beneath which the National Taiwan University logo can be clearly observed. Figure 2a shows the bipolar resistive switching characteristics of AlN-based TRRAM. During the measurements, we applied a DC voltage to the top electrode while the bottom electrode was grounded. The current compliance was set to 1 mA to prevent permanent destruction of the dielectric thin films during the forming process,  in which the resistance of the device transitioned from an initial resistance state to a low resistance state for the first time. As shown in Fig. 2a, the current jump at − 1.5 V indicates the completion of the forming process. By sweeping the voltage above a positive threshold value (V RESET = 0.8 V). a sudden decrease in the current was then observed (as denoted by the Reset arrow in Fig. 2a), indicating that the device has been switched to a high resistance state (R H ). When we subsequently decreased the voltage below a negative threshold value (V SET = − 0.8 V), there was an abrupt jump in the current (as denoted by the Set arrow in Fig. 2a), which indicated that the device had been switched to a low resistance state (R L ). We then performed a 100-cycle endurance test to extract the statistical distributions of the resistance states, R H and R L, and the operation switching voltages (V SET and V RESET ), respectively. Each cycle was composed of the set and reset processes, as indicated in Fig. 2a, interleaved with voltage pulses of 0.1 V/100 ms to read the current after each set and reset process. Figure 2b demonstrates the endurance properties of AlN TRRAM, in which the resistance of both R H and R L showed only small fluctuations over 100 cycles. These results demonstrate the reversible and steady bipolar resistive switching (RS) characteristics of AlN TRRAM.
This bipolar switching characteristics can be explained according to the metal nitride-based RS mechanism, which relies on the formation/rupture of CNFs via nitride-related electron trapping/detrapping processes 12,26 . At the forming and set processes, an electro-reductive/electromechanical process triggered by the forward electric field causes the electrons to be trapped at crystal defects, leading to the creation of nitrogen vacancies through interstitial nitrogen ion release 27 . Nitrogen ions then migrate/drift toward the metal-nitride/electrode interface while the nitrogen vacancies, located at ~0.25 eV below the conduction band of AlN 27 , trap injected electrons, enabling trap-to-trap electron hopping through the CNFs. As a result the device enters the R L . Conversely, at the reset process the local Joule heating effect and reverse field cause electron detrapping events from nitrogen vacancies, leading to the annihilation of nitrogen vacancies through nitrogen fixation, and thus blocking trap-to-trap electron hopping at the ruptured area. As a result the device enters R H . Furthermore, it is well-known that the chemisorption of oxygen acts as an electron trap for charge carriers on the surface of metal oxides, which leads to an increase in the surface potential and deterioration of the device's performance. To examine the surface effects of AlN TRRAM, we performed a 100-cycle endurance test of the device under four ambient conditions to simulate nitrogen-rich (air and N 2 ) and nitrogen-poor (vacuum and O 2 ) environments. We conducted a statistical analysis of 50 cells in order to evaluate the high and low resistance states (R H and R L ) and switching voltage distributions (V SET and V RESET ), as shown in Fig. 3a and b, respectively. Although the resistance in both states and switching voltages remained fairly stable with a high-to-low resistance ratio (R H /R L ) around one order of magnitude under all ambient conditions, we observe that nitrogen-rich and -poor environments had reverse effects on the variability of the operating voltages (ΔV SET and ΔV RESET ) and resistances states (ΔR L and ΔR H ). Under a nitrogen-rich environment, ΔR H and ΔV SET were reduced, while ΔR L and ΔV RESET increased. According to the previously explained RS mechanism of AlN, it is reasonable to assume that the nitrogen-rich environment exerts a high nitrogen partial pressure into the CNFs. As a consequence, nitrogen vacancies are steadily annihilated during the reset process, which results in nearly uniform rupture of the CNFs during subsequent reset cycles. Such stabilization reduces the fluctuations of R H and anchors V SET , leading to the reduced values of ΔR H and ΔV SET , respectively. In contrast, the high nitrogen partial pressure alters the transient currents in the R L due to undesired nitrogen vacancy annihilation events that compromise the stability of the CNFs and randomize ΔV RESET , leading to our observation of the increased ΔR L and ΔV RESET values.
On the other hand, under a nitrogen-poor environment, we observe that ΔR L and ΔV RESET became lower, while ΔR H and ΔV SET increased. To explain this opposite effect, we recall not only the nitride-related RS mechanism of AlN, but also the tunable electrical characteristics of ITO via the modulation of the oxygen concentration 28 . For example, oxygen-deficient polycrystalline ITO can act simultaneously as a reservoir of oxygen vacancy sites and oxygen ions, and hence the CNFs could be extended into the ITO electrode during the forming and set processes [29][30][31] . In fact, the oxygen deficient properties of ITO allow the injection/extraction of oxygen ions at the AlN/ITO interface owing to the high solubility of oxygen in the AlN lattice 32 , which could result in the formation/ dissolution of an AlO x barrier interlayer. Indeed, according to the high chemical affinity of oxygen towards aluminum, it is likely that a thin layer of alumina is present on the surface of the AlN samples 23 . Therefore, the high oxygen content under the nitrogen-poor environment constrains further dissolution of the AlO x barrier interlayer during the set processes, leading to fluctuating transient currents at the high resistance state that increase the V SET mean value and ΔR H 33 . Conversely, the high oxygen content stabilizes the reformation of the AlO x barrier interlayer during the reset process, leading to the reduced ΔR L . Despite the small differences in variability under nitrogen-rich or -poor environments, the environmentally stable results shown in Fig. 3 clearly indicate that the detrimental surface effects on the RS characteristics of metal oxide-based RRAM can be suppressed with the use of AlN films.
To investigate the radiation tolerance of AlN TRRAM, we irradiated the device with 2 MeV proton fluences, ranging from 10 11 to 10 15 cm −2 . It should be noted that protons with an energy less than 2 MeV and fluences ranging from 10 1 to 10 8 cm −2 occupy a significant volume of the Earth's geomagnetic cavity (10-12 earth radii; Re = 6380 km) 4 and thus can potentially affect electronic devices. The I-V characteristics of AlN TRRAM after different proton fluences are shown in Fig. 4a. We also performed a 100-cycle endurance test after each proton radiation exposure. Consequently, the distributions of the resistance states and operation voltages of the AlN TRRAM devices are shown in Fig. 4b and c, respectively. The results demonstrate narrow distributions and overall congruence. We also note that the resistance window (R H /R L ) increased up to 30 regardless of proton irradiation fluences up to 10 15 cm −2 . Furthermore, these distributions were consistently similar to the control sample under air (Fig. 3a and b). It is also noteworthy that the transparency of the device did not degrade after proton irradiation 25 . Therefore, it is clear that the RS characteristics of the device are stable after proton irradiation of different fluences, suggesting a bright future for AlN-based TRRAM in aerospace applications.

Conclusion
In this work, we investigated the reliability of AlN-based TRRAM for applications in harsh electronics. The AlN TRRAM exhibited an average transmittance of 80% in the visible wavelength range, and reliable resistive switching characteristics. Furthermore, statistical analyses of the cycle-to-cycle test in Fig. 2 and the device-to-device tests of 50 cells in Figs 3 and 4, demonstrated that AlN TRRAM is stable and can function properly under high oxygen partial pressure and proton irradiation. These experiments give insight not only into environmentally stable TRRAM design, but also for developing practical applications of TRRAM for future harsh electronics.
Experimental Section TRRAM Fabrication. The structure of the AlN TRRAM device is shown in Fig. 1a. First, a 100 nm thick ITO thin film was deposited as the bottom electrode on a glass substrate by RF-sputtering (Ar flow 50 sccm, working pressure 3 m Torr, power 80 W). Second, a 50 nm thick AlN thin film was then deposited on the ITO layer by RF-sputtering (Ar flow 50 sccm, working pressure 5 mTorr. power 180 W). Finally, a patterned 100 nm thick ITO thin film was deposited as the top electrode by RF-sputtering (Ar flow 50 sccm, working pressure 3 mTorr, power 80 W) assisted with a metal shadow mask featuring an array of 200 μ m diameter circles. These fabrication processes were carried out at room temperature.