Flexible Electronic Devices Based On High-performance Oxide Semiconductors

Oxide semiconductors have broad application prospects in the fields of flexible display and flexible radio frequency(RF)circuits due to their optical transparency,flexibility,and high mobility.Due to the low working temperature of flexible substrates,the research of this paper focuses on fabricating high-mobility thin-film transistors(TFTs)at a low process temperature.On the one hand,although flexible zinc oxide(Zn O)TFTs have been fabricated as early as 2010 by G.F.Boesen,and their mobility can reach about15 cm²/V·s.It is generally required to achieve higher mobility by annealing,and the mobility rapidly reduces as the thickness decreases.Zn O could be applied to flexible TFTs,but not suitable for flexible RF devices with higher performance and higher technology node.On the other hand,although flexible RF devices based on indium gallium zinc oxide(IGZO)have been fabricated as early as 2012 by N.Münzenrieder,the cut-off frequency(f _T)of all flexible oxide RF devices is still in the megahertz magnitude,which is far from meeting the high-speed communication needs of the Io T.To improve the performance of RF devices,it is necessary to use ITO with higher mobility at low process temperature as channel material.To date,there has been a limited study on the flexible mixer based on oxide semiconductors.In this work,first,oxide TFTs have been fabricated on Si substrate to explore the best material and electrical properties.Then,flexible electronic devices and circuits have been fabricated.The following detailed studies have been carried out in terms of material properties,flexible TFTs,flexible RF devices,and flexible mixer:1.Si Zn O TFT.Zn O TFT has been fabricated on Si substrate using atomic layer deposition(ALD).By adjusting the growth temperature and film thickness,the device achieved great electrical performance.With 10 nm Zn O deposited at 190?,the?_FEreached 18.5 cm²/V·s.Zn O TFT has been annealed in vacuum by rapid thermal annealing(RTA)to improve the material quality and electrical property.When annealed in vacuum at 350°C for 1 minute,the device achieved a?_FEof 43.2 cm²/V·s and showed good working stability at a temperature of 20?to 180?.The mechanism of annealing to improve electrical performance has been investigated through XRD,XPS,and 1/f noise measurements.2.Flexible Zn O TFT.In this work,flexible Zn O TFT has been fabricated based on the work of Si Zn O TFT.However,the mature technology for fabricating devices on Si is not completely suitable for polyimide(PI),appropriate process improvements are needed to fabricate flexible devices.For example,given the problem that PI is easy to bend under external force or high temperature,it is necessary to use polydimethylsiloxane(PDMS)to adhere PI on Si;a lower temperature of 140?was used during the device fabrication;high-quality alumina(Al₂O₃)was used instead of silicon dioxide(Si O₂)as the gate dielectric.Flexible Zn O TFT achieved a?_FE of 13 cm²/V·s and an I_on/I_off of 1.5×10⁸.Due to the high-quality materials and interfaces deposited by ALD,the device exhibited excellent bending stability.The flexible device still exhibited a?_FE of 8.9 cm²/V·s or 14.1cm²/V·s even under a maximum bending cycle of 200,000 with a tensile strain of 0.63%or a static tensile strain of up to 2.08%.Finally,the bias-stress stability and high-temperature stability have been studied in detail.3.Flexible ITO RF device and mixer.To achieve excellent RF and mixer performance,TFT is required to have higher transconductance and smaller parasitic effect,which puts higher demands on the material and the device structure.However,Zn O must be annealed to further improve performance and the mobility decreases rapidly when the thickness is reduced,so it cannot meet the requirements of flexible RF devices.Next,the channel material is changed from Zn O to ITO,and the flexible ITO RF device is researched based on the thinner channel,shorter channel length,and lower process temperature.By adjusting the oxygen partial pressure during magnetron sputtering at200°C,ITO can be transformed from a metallic state to a semiconductor state.To characterize the uniformity of ITO deposited by magnetron sputtering,a four-inch wafer-scale ITO TFT array has been fabricated and characterized.The flexible ITO RF device with a channel length of 160 nm showed a mobility of 26cm²/V·s,and exhibited excellent RF characteristics with a record-high extrinsic f _T of 2.1GHz and an extrinsic maximum oscillation frequency(f_max)of 3.7 GHz.Then,the bending stability of DC and RF performances was characterized in detail.Finally,the flexible ITO mixer has been fabricated for the first time,and the mixing characteristics have been studied in detail.The above research is an application extension of oxide semiconductors in the field of flexible devices,demonstrating the application potential of high-performance oxide semiconductors in the field of flexible display and high-frequency communication.