Zinc Oxynitride Thin-film Transistors Research Articles

Surface potential based modeling of zinc oxynitride thin film transistors

In this work we present analytical models for the drain current and threshold voltage of zinc oxynitride thin film transistors. A surface potential based modeling approach has been adopted and the exact closed form solutions for the potential profile, drain current and threshold voltage have been obtained. In order to account for the effects of both free and localized charges in the semiconductor, an effective charge density method has been used. The models were validated against experimental data from literature. The models show good agreement with the data.

Nonvolatile High-Speed Switching Zn-O-N Thin-Film Transistors with a Bilayer Structure.

Zinc oxynitride (ZnON) has the potential to overcome the performance and stability limitations of current amorphous oxide semiconductors because ZnON-based thin-film transistors (TFTs) have a high field-effect mobility of 50 cm2/Vs and exceptional stability under bias and light illumination. However, due to the weak zinc-nitrogen interaction, ZnON is chemically unstable─N is rapidly volatilized in air. As a result, recent research on ZnON TFTs has focused on improving air stability. We demonstrate through experimental and first-principles studies that the ZnF2/ZnON bilayer structure provides a facile way to achieve air stability with carrier controllability. This increase in air stability (e.g., nitrogen non-volatilization) occurs because the ZnF2 layer effectively protects the atomic mixing between ZnON and air, and the decrease in the ZnON carrier concentration is caused by a shallow-to-deep electronic transition of nitrogen deficiency diffused from ZnON into the interface. Further, the TFT based on the ZnF2/ZnON bilayer structure enables long-term air stability while retaining an optimal switching property of high field-effect mobility (∼100 cm2/Vs) even at a relatively low post-annealing temperature. The ZnF2/ZnON-bilayer TFT device exhibits fast switching behavior between 1 kHz and 0.1 MHz while maintaining a stable and clear switching response, paving the way for next-generation high-speed electronic applications.

Observation of Divacancy Formation for ZnON Thin-Film Transistors With Excessive N Content

This study proposes a model of mobility and negative bias stress stability degradation mechanism of zinc oxynitride (ZnON) thin-film transistors (TFTs) with various anion compositions. The subgap density of states (DOS) for ZnON TFTs were extracted using monochromatic photonic capacitance–voltage measurement. The extracted subgap DOS indicated an additional nitrogen-related subgap peak was observed in ZnON TFTs with an excessive nitrogen condition, which might originate from a nitrogen divacancy formation. Furthermore, the shifted threshold voltage components after the bias stress test were quantitatively divided as subgap DOS and interface trapping.

High Mobility and Air-Stable Zinc Oxynitride Thin-Film Transistors Via ZnF2 Interlayer Insertion

Increasing demands for high-performance thin-film transistors (TFT) in the display application have led to the development of amorphous oxide semiconductors (AOS) such as In-Ga-Zn-O (IGZO), In-Zn-O (IZO), etc. The AOS based TFTs show high field-effect mobility as high as 10 cm2/Vs, and are suitable for the fabrication of large size liquid crystal display (LCD) with high resolution. However, the displays based on organic light-emitting diodes (OLEDs) or micro light-emitting diodes (micro-LED) with very high resolution (super-high vision, 8000 X 4000) require relatively high driving currents compared to the LCD display. Therefore, the AOS based TFTs with higher field-effect mobility (> 50 cm2/Vs) are strongly required into the display backplane array.Recently, zinc oxynitride (ZnON) have researched intensively as an active material for high-performance TFTs due to its high field-effect mobility (> 50 cm2/Vs), excellent stability under bias or light irradiation, and easily tunable bandgap energy. However, some studies reported that the nitrogen-related defects such as nitrogen-deficient site (VN) in ZnON act as carrier trap sites, so that degrade the electrical characteristics of ZnON TFTs. Moreover, the zinc nitride (Zn3N2) is unstable to air due to its low heat of formation, thus the Zn3N2 into ZnON is easily converted to zinc oxide. Therefore, reducing VNs and preventing the degradation of Zn3N2 is necessary for obtaining high-performance ZnON TFTs.In this study, the effects of a thin ZnF2 interlayer between source/drain electrode and ZnON active layer are investigated. The properties of ZnON TFTs were evaluated by the different thicknesses of ZnF2 and the change of deposition conditions. The inserted ZnF2 interlayer acts as i) carrier modulation between source/drain and active layer due to high resistive ZnF2, and ii) passivation layer which prevents Zn3N2 volatilization. Furthermore, in our previous study demonstrated that the incorporated fluorine in ZnON acts as a structural stabilizer, so that reduces VN sites by increasing the effective coordination number of zinc. Moreover, incorporated fluorine which substitutes nitrogen also shown acceptor-like behavior rather than carrier donor. Thus, the VNs near the interface between ZnF2 and ZnON would be reduced, therefore the net density of VNs is decreased.Experimental results showed that an optimized ZnON/ZnF2 TFTs showed high field-effect mobility (> 50 cm2/Vs) and good transfer characteristics (high on/off ratio and low subthreshold swing), whereas ZnON TFTs without ZnF2 interlayer showed completely conductive behavior. In addition, the air stability of ZnON TFTs was improved via the ZnF2 interlayer, the transfer characteristics were almost unchanged even a few months later. Further analysis such as bias and illumination stability and charge transport mechanisms were discussed in detail.

Influence of Nitrogen Content on Persistent Photoconductivity in Zinc Oxynitride Thin Film Transistors

In this work, the influence of anion composition on the photoresponse characteristics of zinc oxynitride thin film transistors is investigated and compared directly to conventional In-Ga-Zn-O TFTs. Increasing the ratio of N to (N+O) led to higher field effect mobility and improved subthreshold swing. The subgap density of states extracted by monochromatic photonic capacitance-voltage measurement indicated that the nitrogen-related defect states, such as nitrogen vacancies, increased with increasing nitrogen gas flow rate. Furthermore, the photoconductivity mechanism of ZnON TFTs is discussed in detail.

Effects of Tantalum Doping on Electrical Characteristics of High-Mobility Zinc Oxynitride Thin-Film Transistors

The effects of Ta doping on the electrical characteristics of high-mobility zinc oxynitride (ZnON) thin-film transistors (TFTs) are investigated. With increasing Ta content ( ${C}_{\text {Ta}}$ ) in ZnON, the threshold voltage ( ${V}_{\text {th}}$ ) of Ta-doped ZnON TFTs increases from negative to positive values, and the subthreshold swing ( SS ) becomes smaller, leading to significantly improved switching characteristics as compared with those for undoped ZnON TFTs. Furthermore, Ta doping effectively suppresses the negative ${V}_{\text {th}}$ shift with time. However, excess Ta doping is found to degrade the field-effect mobility ( $\mu _{\text {fe}}$ ). To elucidate the mechanism behind these results, the bonding states of Ta in ZnON are analyzed using X-ray photoelectron spectroscopy. Through the optimization of ${C}_{\text {Ta}}$ , a high $\mu _{\text {fe}}$ of 49 cm2/Vs is achieved, together with a less negative ${V}_{\text {th}}$ , a smaller SS , and a better long-term ${V}_{\text {th}}$ stability. In addition, the annealing temperature dependence of the properties of Ta-doped ZnON TFTs is investigated. When the annealing temperature is increased to 250 °C, a considerable decrease in the on-current is observed, which is attributed to nitrogen desorption from ZnON, as confirmed by thermal desorption spectroscopy.

Ultra-High-Speed Intense Pulsed-Light Irradiation Technique for High-Performance Zinc Oxynitride Thin-Film Transistors.

In this study, we investigated the effects of intense pulsed light (IPL) on the electrical performance properties of zinc oxynitride (ZnON) thin films and thin-film transistors (TFTs) with different irradiation energies. Using the IPL process on the oxide/oxynitride semiconductors has various advantages, such as an ultrashort process time (∼100 ms) and high electrical performance without any additional thermal processes. As the irradiation energy of IPL increased from 30 to 50 J/cm2, the carrier concentration of ZnON thin films decreased from 5.07 × 1019 to 9.96 × 1016 cm-3 and the electrical performance of TFTs changed significantly, which is optimized at an energy of 40 J/cm2 (field effect mobility of 48.4 cm2 V-1 s-1). The properties of TFTs, such as mobility, subthreshold swing, and hysteresis, and the stability of the device under negative bias degraded as the irradiation energy increased. This degradation contributed to the change in nitrogen-related bonding states, such as nonstoichiometric Zn xN y and N-N bonding, rather than that of oxygen-related bonding states and the atomic composition of ZnON thin films. Optimization of the IPL process in our results makes it possible to produce high-performance ZnON TFTs very fast without any additional thermal treatment, which indicates that highly productive TFT fabrication can be achieved via this process.

Supreme performance of zinc oxynitride thin film transistors via systematic control of the photo-thermal activation process

Zinc oxynitride (ZnON) is a relatively novel class of material, often regarded as a promising alternative to oxide semiconductors, owing to its relatively high electron mobility and low concentration of oxygen-related defects that affect the device reliability.

Effects of Fluorine Doping on the Electrical Performance of ZnON Thin-Film Transistors.

In this work, the effects of fluorine incorporation in high mobility zinc oxynitride (ZnON) semiconductor are studied by both theoretical calculations and experimental evaluation of thin film transistors (TFTs). From density functional theory (DFT) calculations, fluorine acts as a carrier suppressor in the ZnON matrix when it substitutes a nitrogen vacant site (VN). Thin films of ZnON and ZnON:F were grown by reactively cosputtering Zn metal and ZnF2 targets, and their electrical, physical, and chemical characteristics were studied. X-ray photoelectron spectroscopy (XPS) analyses of the nitrogen 1s peaks in ZnON and ZnON:F suggest that as the fluorine incorporation increases, the relative fraction of Zn-N bonds from stoichiometric Zn3N2 increases. On the other hand, the Zn-N bond characteristics arising from nonstoichiometric ZnxNy and N-N bonds decrease, implying that indeed fluorine anions have an effect of passivating the N-related defects. The corresponding TFTs exhibit optimum transfer characteristics and switching ability when approximately 3.5 atomic percent of fluorine is present in the 40 nm thick ZnON:F active layer.

Enhanced Electrical Performance and Stability in Zinc Oxynitride Thin-Film Transistors via Sequential Ultraviolet and Thermal Treatment

In this letter, the sequential combination of ultraviolet (UV) irradiation and thermal annealing is proposed as an effective post-deposition treatment method to improve the electrical performance and stability in high-mobility zinc oxynitride (ZnON) thin-film transistors (TFTs). The results indicated that the sequential combination of UV irradiation and thermal annealing at 270°C more effectively increased the field-effect mobility of the TFT and improved the electrical stability of the device under positive gate-bias stresses when compared with other post-deposition treatment methods of UV irradiation, thermal annealing at 270°C, and thermal annealing at 350°C. A decrease in the number of the defective ZnXNY bonds and an increase in that of the stoichiometric Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> bonds were clearly observed when the ZnON thin-film was post-treated by the sequential combination of UV irradiation and thermal annealing at 270°C. A decrease in the defective ZnXNY bond-related subgap states is considered as the dominant mechanism responsible for the enhanced electrical properties of the UV-after thermally treated ZnON TFTs.

Low temperature carrier transport mechanism in high-mobility zinc oxynitride thin-film transistors

The authors investigate the low-temperature carrier transport mechanism in high-mobility zinc oxynitride (ZnON) thin-film transistors (TFTs) over a wide range of operating temperatures below room temperature (90–293 K) by analyzing the temperature-dependent field-effect characteristics. In the subthreshold and transition regions, the variable range hopping and trap-limited band transport are considered as dominant carrier transport mechanisms in the ZnON TFT at temperatures below ∼207 K and above ∼243 K, respectively. In the above-threshold region, the field-effect mobility almost linearly depends on 1/T (T: absolute temperature) at all temperatures, which represents that the trap-limited band transport is the dominant carrier transport mechanism through the entire temperature range of 90 to 293 K in the fabricated ZnON TFT. Approximately 1 order of magnitude higher subgap density of states is extracted from the fabricated ZnON TFT compared with a conventional amorphous indium-gallium-zinc oxide TFT, which is primarily attributed to the large number of defective ZnXNY bonds inside the ZnON.

Electrical instability of high-mobility zinc oxynitride thin-film transistors upon water exposure

We investigate the effects of water absorption on the electrical performance and stability in high-mobility zinc oxynitride (ZnON) thin-film transistors (TFTs). The ZnON TFT exhibits a smaller field-effect mobility, lower turn-on voltage, and higher subthreshold slope with a deteriorated electrical stability under positive gate bias stresses after being exposed to water. From the Hall measurements, an increase of the electron concentration and a decrease of the Hall mobility are observed in the ZnON thin film after water absorption. The observed phenomena are mainly attributed to the water molecule-induced increase of the defective ZnXNY bond and the oxygen vacancy inside the ZnON thin film based on the x-ray photoelectron spectroscopy analysis.

Effects of oxygen flow rate on the electrical stability of zinc oxynitride thin-film transistors

We investigated the effects of the oxygen flow rate (OFR) during the deposition of a zinc oxynitride (ZnON) channel layer on the electrical performance and stability of high-mobility ZnON thin-film transistors (TFTs). The ZnON TFTs prepared at a lower OFR exhibited higher electrical performance characteristics and a higher electrical stability under positive gate bias stresses than those prepared at a higher OFR, but showed a lower electrical stability under negative gate bias stresses. The lower density of subgap states within the channel layer and the higher hole concentration due to the small bandgap were considered as physical mechanisms responsible for the observed phenomena, respectively.

Effect of Channel Layer Thickness on Electrical and Thermal Stabilities of High-Mobility Zinc Oxynitride Thin-Film Transistors

We investigated the effect of channel layer thickness (tch) on the electrical and thermal stabilities of high-mobility zinc oxynitride (ZnON) thin-film transistors (TFTs). ZnON TFTs with various tch values of 11, 16, 21, and 26 nm were prepared for experiments. The drain current was barely modulated by the gate-to-source voltage in the ZnON TFT with a tch of 26 nm. When tch was less than 21 nm, both the electrical and thermal stabilities of the ZnON TFTs improved with an increase in tch. To explain this phenomenon, the chemical composition and bonding states of the ZnON thin-films with different thicknesses were characterized using X-ray photoelectron spectroscopy (XPS). The XPS results indicated that more oxygen exists in the bulk of the 11-nm-thick ZnON thin-film than in the bulk of the 21-nm-thick ZnON thin-film. In addition, the number of defective ZnXNY bonds decreased in the ZnON with an increase in the distance from the back surface to the characterized layer. Because the excess oxygen and defective ZnXNY bond generate subgap states in ZnON, the observed tch-dependence of the electrical/thermal stability in the ZnON TFT could be mainly attributed to the decrease in the subgap states in ZnON with the increase in tch.

Investigation of Carrier Transport Mechanism in High Mobility ZnON Thin-Film Transistors

In this letter, the carrier transport mechanism in a high-mobility zinc oxynitride (ZnON) thin-film transistor (TFT) is investigated by analyzing the gate bias and temperature dependence of conductance and intrinsic field-effect mobility ( $\mu _{\mathrm {FEi}})$ in the subthreshold and above-threshold regions, respectively. The measured drain currents increase with a temperature and show a thermally activated Arrhenius-like behavior in the subthreshold region. The experimental results are well explained using a Meyer–Neldel rule, which suggests that the trap-limited conduction is the dominant carrier transport mechanism in the ZnON TFT in the subthreshold region. The carrier transport mechanism in the ZnON TFT in the above-threshold region is investigated by examining the gate overdrive voltage ( $V_{\mathrm {OV}})$ and temperature dependence of $\mu _{\mathrm {FEi}}$ . $\mu _{\mathrm {FEi}}$ extracted from the ZnON TFT decreases with an increase in $V_{\mathrm {OV}}$ and temperature, which suggests that the phonon scattering is the most probable mechanism limiting $\mu _{\mathrm {FEi}}$ in the ZnON TFT in the above-threshold region.

Effects of Post-Deposition Thermal Annealing Temperature on Electrical Properties of ZnON Thin-Film Transistors

We investigate the effects of the post-deposition thermal annealing temperature on the physical and chemical structure of zinc oxynitride (ZnON) thin films and on the electrical performance of ZnON thin-film transistors (TFTs). When a ZnON TFT is annealed at 150 °C, it exhibits conductive behavior, which is attributed to the increased electron concentration caused by the increase in the nitrogen vacancies of the defective ZnXNY bonds within the ZnON. The TFT shows the best electrical performance when the annealing temperature is 250 °C, but a degradation in the electrical performance is observed when the annealing temperature is increased to 350 °C. The significantly reduced electron concentration and the relative increase in the oxygen within the ZnON are considered as the possible reasons for the degradation in the electrical performance observed in the ZnON TFTs annealed at 350 °C.

Study on the photoresponse of amorphous In-Ga-Zn-O and zinc oxynitride semiconductor devices by the extraction of sub-gap-state distribution and device simulation.

Persistent photoconduction (PPC) is a phenomenon that limits the application of oxide semiconductor thin-film transistors (TFTs) in optical sensor-embedded displays. In the present work, a study on zinc oxynitride (ZnON) semiconductor TFTs based on the combination of experimental results and device simulation is presented. Devices incorporating ZnON semiconductors exhibit negligible PPC effects compared with amorphous In-Ga-Zn-O (a-IGZO) TFTs, and the difference between the two types of materials are examined by monochromatic photonic C-V spectroscopy (MPCVS). The latter method allows the estimation of the density of subgap states in the semiconductor, which may account for the different behavior of ZnON and IGZO materials with respect to illumination and the associated PPC. In the case of a-IGZO TFTs, the oxygen flow rate during the sputter deposition of a-IGZO is found to influence the amount of PPC. Small oxygen flow rates result in pronounced PPC, and large densities of valence band tail (VBT) states are observed in the corresponding devices. This implies a dependence of PPC on the amount of oxygen vacancies (VO). On the other hand, ZnON has a smaller bandgap than a-IGZO and contains a smaller density of VBT states over the entire range of its bandgap energy. Here, the concept of activation energy window (AEW) is introduced to explain the occurrence of PPC effects by photoinduced electron doping, which is likely to be associated with the formation of peroxides in the semiconductor. The analytical methodology presented in this report accounts well for the reduction of PPC in ZnON TFTs, and provides a quantitative tool for the systematic development of phototransistors for optical sensor-embedded interactive displays.