Preparation And Characterization Of Atomic Layer Deposited Nano-structured Transparent And Flexible Capacitors

In recent years,there has been substantial attention focused on fully transparent electronic devices,such as transparent mobile phones,computers,large-sized active matrix liquid crystal displays(AMLCDs)or organic light-emitting diodes(AMOLEDs).The fabrication technology of fully transparent screens is of vital importance in fabricating these transparent devices.While the driven circuitry of pixels is the main factor that inhibits the transmittance of screens.Conventionally,a single pixel driven circuit is composed of two basic electrical components:thin film transistor(TFT)and capacitor.TFTs have been extensively investigated during the past few decades owing to their indispensability in the driven circuits of LCD and OLED.However,it should be noted that the capacitor plays the same important role as the TFT.A high capacitance is needed to ensure the color brightness of the pixel while a low leakage current means a prolonged charge storage time when the pixel is in the off state.However,there are few system results reported so far surrounding high-performance transparent capacitors.In this paper,we focus on the following six research topics:1.The effects of dielectric layers on the performances of transparent planar capacitors.We fabricated the entire capacitor structures on quartz glass substrates by the in-situ atomic layer deposition(ALD)method.Both the top and bottom electrodes were aluminum doped zinc oxide(AZO)films.And nanolaminate Al2O3/TiO2/Al2O3(ATA)films were used as the dielectric layer.Here,H2O and O3 were adopted to serve as the oxidizers in the TiO2 growth.We found that capacitors with H2O as the oxidizer showed higher capacitance and leakage current densities than that with O3 as the oxidizer.We analyzed the detailed leakage mechanisms to understand the roles of different oxidizers.To further balance the capacitance and leakage current properties,we explored the growth techniques of ZrO2 dielectrics.2.The effects of transparent electrodes with different sheet resistances on the dielectric loss of capacitors.To reduce the sheet resistance of AZO films,silver nano wires(AgNWs)were sandwiched between two AZO layers to form the composite AZO/AgNW/AZO electrode which had a much lower sheet resistance of about 10 Ω and a bigger surface area than the ordinary AZO electrode.Then the composite films were used as both the top and bottom electrodes of transparent capacitors with ZrO2 as the dielectric layer.A large capacitance density of 20 fF/μm2 and an ultra-low leakage current density of 2.5×10-9 A/cm2 at 1 V were obtained.More importantly,the quality factor at 1 MHz was improved from the initial 3.9 to 17.0,which can be comparable to some ordinary metal electrodes.3.The fabrication of transparent capacitors based on nano-structured substrates.To further increase the capacitance density without losing the transparency of capacitors,we transferred the anodic aluminum oxide(AAO)templates to AZO/glass substrates.Then the AZO/Al2O3/AZO capacitor structures were deposited on AAO by ALD.This device structure design enhanced the current diffusion ability across the bottom AZO electrode.And thus,the capacitor can work normally in a broad frequency range from 1 to 200 kHz.The capacitance density reached 37 fF/μm2,which was 5.8 times bigger than that of planar capacitors.In addition,a low leakage current density of 1.7×10-7 A/cm2 at 1 V was achieved,demonstrating good insulating properties.4.Transparent and flexible capacitors based on indium doped tin oxide(ITO)bottom electrodes.To simplify the fabricating process,we fabricated the capacitor structures on the purchased ITO/polyethylene naphthalate(PEN)substrates.The capacitor device can only work normally under an outward bending radius which is above 40 mm due to the fragile properties of ITO films.To improve the flexibility,the composite AZO/AgNW/ITO films were adopted as the bottom electrode,the capacitors based on which can work steadily under an outward bending radius of 17 mm,showing a significantly improved bendability.5.Transparent and flexible capacitors with AZO as the bottom electrode.The commercially available ITO/PEN substrates are expensive,fragile,and thickness uncontrollable.To intrinsically improve the flexibility,it is necessary to abandon ITO and prepare the capacitors directly on low-cost PEN substrates.Firstly,we deposited an AZO layer with a thickness of about 100 nm on the PEN surface by magnetron sputtering to serve as the buffer layer which can provide enough hydroxyl groups.Then the AZO/ZrO2/AZO capacitor structures were prepared by ALD.The fabricated capacitors can work normally under an outward bending radius of 7 mm,but the device failure happened after the capacitors were bended 100 times.To improve the anti-fatigue properties,the composite AZO/AgNW/AZO films were used as the bottom electrode,the capacitors based on which can be bended up to 1000 times without any deterioration in electrical performances.Both the top and bottom electrodes of such capacitors are based on ZnO materials,which can effectively reduce the manufacturing cost and promote the mass production process.6.Thin film transistors based on ZnO channels.To fabricate LCD and OLED driven circuits with an ultra-high aperture ratio,it is necessary to fabricate thin film transistors and integrate them with transparent capacitors.Here,we fabricated a kind of ZnO-based TFT by using the in-situ ALD method under an ultra-low growth temperature(80℃).The TFT device demonstrated a field effect mobility of 46.4 cm2V-1s-1,an on-to-off current ratio of about 108 and a threshold voltage of 0.88 V without any post-annealing treatment.This device structure can be directly transferred on flexible plastic substrates which can not endure high temperatures.In addition,the TFTs fabricated at 100 °C exhibited a mobility of 49.2 cm2V-1s-1,an on-to-off current ratio of about 108 and a threshold voltage of 1.55 V after annealing at 200 °C for 15 minutes in air.We found that thermal annealing in air can effectively reduce the subthreshold swing of TFTs and the electrical performances of TFTs after long-time annealing at low temperatures are better than that after short-time annealing at high temperatures.