MXene/GaN Van Der Waals Heterostructures For Photodetectors And Light-emitting Diodes

Two-dimensional transition-metal carbides and nitrides(MXenes),as an emerging family of 2D materials,have gained extensive attention since the discovery of Ti3C2 in2011 by Yury Gogotsi and colleagues.Due to their excellent electrical conductivity,large surface area,and hydrophilicity,MXenes have been widely used in supercapacitors,piezoresistive sensors,lithium-ion batteries,and electromagnetic interference shielding.However,van der Waals heterostructures fabricated by MXenes and traditional semiconductors have rarely been investigated so far and still need further study.In order to study the properties of MXene/GaN van der Waals heterostructure and explore their potential applications,this thesis designed and fabricated MXene/(n/p)-GaN van der Waals heterostructure devices and studied their applications in photodetectors and LEDs,respectively.The main contents of this thesis are as follows:(1)Preparation and characterization of MXene.We synthesized the colloidal solution of single-layer MXene nanosheet by etching MAX phase using a mixture of hydrochloric acid(HCl)and lithium fluoride(LiF).We further characterized the prepared MXene and proved that it is of good quality.(2)A self-driven photodetector was fabricated based on the MXene/n-GaN van der Waals heterostructure.We dropcast the MXene colloidal solution onto the n-GaN substrate and let it dry naturally at room temperature to form the MXene/n-GaN van der Waals heterostructure,which was then fabricated into a photodetector device.We further studied the mechanism by which concentration of MXene solution affected device performance and found that devices fabricated by 0.05 mg/mL MXene solution have better photodetection performance.Under the illumination of a 365 nm light source with an intensity of 96.9?W/cm~2,the device was tested with a fast response speed(60 ms)and recovery speed(20 ms),high responsivity(44.3 mA/W)and on/off ratio(~11300).(3)An LED device was fabricated based on MXene/p-GaN van der Waals heterostructure.Similarly,we used a drop-cast method to fabricate LED devices.The LED device maintains stable orange light emission under a wide range of bias voltages(from 4 V to 22 V),indicating that the device is of good electrical stability and thermal stability.We further performed a Gaussian fitting on the electroluminescence(EL)spectrum at a 22 V bias and divided it into four wave bands:ultraviolet light(376 nm),blue light(436 nm),orange light(602 nm),and red light(706 nm).We believe that ultraviolet light(376 nm)is derived from the near-band emission of p-GaN,blue light(436 nm)is Mg-acceptor-related emission of p-GaN,the orange(602 nm)and red(706nm)emission are related to the deep level of p-GaN.Based on the CIE 1931 XYZ standard,we further calculated the chromaticity coordinate(0.4541,0.4432)of the LED device under a 22 V bias.