Low-temperature Preparation Of High Quality Graphene And Its Device Application

Graphene preparation is a popular topic of interest for researchers.Among them,Chemical Vapor Deposition（CVD）method has two advantages of large-area preparation and high-quality preparation,and has been applied in the field of graphene materials and electronic devices.Graphene grown based on CVD often needs to be transferred from the catalytic substrate to the target substrate through a transfer process before implementation.Both the complex flow of the transfer process and the inevitable contamination and breakage during the transfer process are extremely detrimental to the devices.Currently,the technologies available for direct growth of graphene on substrates require high growth temperatures（800～1100℃）and are therefore only applicable to high-temperature resistant materials,while there are no corresponding direct growth means for substrates that are in great demand in the market and cannot withstand excessive temperatures（e.g.,integrated circuit chips,flexible sheets,etc.）.In this context,the study of low-temperature graphene growth has started to attract attention,with the aim of lowering the growth temperature of graphene to within the temperature tolerance range of various substrates,and then developing a universal direct graphene growth technology to achieve compatibility between graphene preparation and semiconductor device processes.This thesis addresses the problems of poor quality,discontinuity,and lack of direct growth schemes in the current research related to low-temperature growth of graphene,and systematically investigates the low-temperature growth technology of graphene from three elements that affect the growth temperature of graphene:carbon source,catalyst,and CVD technology,which significantly reduces the growth temperature of high-quality graphene.Based on this,the low-temperature transfer-free growth technology of graphene is proposed to bridge the intermediate link between low-temperature growth and device preparation.A series of studies from preparation to optoelectronic devices have been carried out for the currently popular vertically oriented graphene,which improves the growth quality of vertically oriented graphene and extends the application of vertically oriented graphene in optoelectronic field.In addition,a new graphene photodetector structure was designed,which provides a new idea for the development of graphene-based high-performance optoelectronic devices.The main research contents of this thesis are as follows.1)The use of tetraphenylnaphthalene(C₃₄H₂₄,TPN)as a carbon source for low-temperature growth of graphene was investigated.As a polycyclic aromatic hydrocarbon,TPN has advantages in the low activation energy required for the dehydrogenation and nucleation growth of carbon groups after dehydrogenation,and thus for the low-temperature growth of graphene.A certain component of benzene is added to TPN to repair vacancy defects generated during graphene growth,and the growth temperature of high-quality graphene is lowered to 600℃.The corresponding transfer-free selected growth technique is developed to avoid the problems of wrinkles,breakage and contamination that may be caused by the transfer and lithography patterning processes of graphene.2)A method for low-temperature growth of graphene using multi-temperature thermal CVD is proposed.The front-end temperature zone of the three-temperature thermal CVD is set to high temperature to crack methane,and the back-end temperature zone is set to low temperature and the substrate is placed in this temperature zone for graphene growth.The temperature field of the tube furnace is simulated by computational fluid dynamics,and the results show that this gradient temperature layout ensures that the fraction of cracked carbon monomer at 400℃ in the back-end temperature zone（38.51%）is comparable to that at 1000℃ in the front-end temperature zone（40.02%）.This method provides sufficient carbon radicals for the low-temperature growth of graphene.The temperature field distribution is optimized and experimentally demonstrated that the growth of graphene below400℃ can be achieved using this method.3)To improve the quality of graphene grown by multi-temperature zone thermal CVD at low temperature,the metal catalyst was improved.Single crystal copper was used instead of conventional polycrystalline copper to promote the diffusion of carbon atoms on the surface of the catalytic substrate and to reduce the graphene nucleation density.By optimizing the growth parameters,it was found that appropriately lowering the chamber air pressure could improve the coverage of graphene catalyzed by single crystal copper,and lowering the hydrogen fraction could significantly enhance the growth quality of graphene,thus improving the continuity and monolayer rate of graphene films.The low-temperature growth of high-quality,large-area graphene continuous films at 300℃ was achieved,and Raman measurements showed that the peak intensity ratios of such low-temperature-grown graphene were as low as0.03 for I_D/I_G and 1.67 for I_2D/I_G,which were better than those of graphene grown on polycrystalline copper foil(about 0.05 for I_D/I_G and 1.5 for I_2D/I_G)currently commercially available at 1000℃.A technique for transfer-free growth of graphene with single crystal copper catalytic sacrificial layer,combined with a low-temperature growth process,has been proposed to form a method for low-temperature transfer-free growth of high-quality,large-area continuous graphene films at no more than 350℃.The method has been successfully applied to the preparation of wafer-scale graphene field-effect transistor arrays.The related results were published in ACS Applied Materials&Interfaces,2022,14（47）:53174-53182.4)To address the current problem of excessive defects in the growth of vertically oriented graphene,a method for growing high-quality vertically oriented graphene using a cold-wall plasma enhanced chemical vapor deposition（PECVD）system was investigated.The relevant process parameters were optimized,and vertically oriented graphene with I_D/I_G of 0.42 was prepared with higher quality than most of the literature reports（I_D/I_G>1）.Based on the high quality vertically oriented graphene,a photoconductive detector was prepared with a responsiveness more than 10 times that of monolayer graphene devices of the same structure.The results were published in Optical Materials Express,2020,10（11）:2901-2910.A convenient and efficient method for the growth of vertically oriented graphene selected zones was proposed to avoid the contamination and structural damage of vertically oriented graphene caused by the lithography patterning process.The related results were published in Nanomaterials,2023,13（7）:1242.5)A gate-controlled reverse series graphene-Si Schottky photodetector is proposed and realized.By regulating the Schottky barrier height of two back-to-back graphene-Si heterojunctions through a transparent top gate,the separation and transport efficiency of the photogenerated carriers are enhanced while the dark current of the device is significantly reduced.The responsiveness of the device is 2.6 A/W and 0.42 A/W at 792 nm and 1550 nm,respectively,and the dark current is as low as10-9 A.The related results are published in Optics Express,2022 30（21）:38504-38512.