CVD Growth Of Transfer-free Graphene With Different Orientation And Its Optoelectronic Device Application

Graphene as a carbon nanomaterial with a unique structure,has excellent advantages in the field of semiconductor optoelectronic devices and microelectronic devices application.According to the growth direction of graphene,it is divided into horizontally oriented two-dimensional structure of graphene(generally directly called graphene)and vertically oriented three-dimensional structure of graphene(generally called vertical graphene).Two kinds of graphenes have excellent electrical conductivity,heat dissipation ability,and mechanical properties,etc.However,they shows opposed characteristics on the optical appearance.The horizontal graphene only has atomic-scale thickness,and the light transmittance of single layer graphene is up to 97.7%.Therefore,it is an ideal material for transparent conductive electrodes on GaN LEDs(Gallium nitride light-emitting diodes).Corresponds to the vertical graphene,the direction of growth is almost perpendicular to the substrate surface,with an unique flake branch structure.So that the incident light can be absorbed in the vertical graphene,making it almost opaque,which provide a great advantage in the photodetectors application.Chemical vapor deposition(CVD)is currently the most widely used method for high-quality graphene films synthesis,which is simple,easy to operate,high repeatability,and low cost.It also can obtained large-scale graphene films with controllable number of layers on substrate.However,this method usually referred transfer process from metal substrate to the target substrate,and often need high-temperature growth environment of graphene,which is complex and inefficient as well as the lowly repeatable.And it is also easy to cause graphene damage,holes,folds,doping,etc.So it becomes the bottlenecks on graphene industrialization and mechanization application.Therefore,growing graphene directly on the target substrate,avoiding the complex transfer process becomes a very effective way to solve this problem.On the one side,the growth parameters of horizontally oriented graphene CVD synthesis are very strict due to its atomic scale thickness,and the growth is highly dependent the high temperatures(generally ≥ 1000 ℃).However,the damage is easy caused in the interior substrate by the high temperature.In addition,the metal layer is often located between the graphene and the substrate,which is difficult to remove.There are two main methods to obtained transfer-free graphene recently.The first one is to grow graphene on target substrate directly without metal catalyzes,however,this method usually sacrifice the quality of graphene and the performance of the device.The second method is to use metal to catalyze the graphene growth and work as a composite graphene/metal electrode on LED,which is deeply influence the luminous of device.Based on this background,we propose three methods to grow transfer-free graphene on sustrate.All of these methods have the advantages of high light transmission,high conductivity,low temperature(600 ℃)growth,and rapid graphene growth.Finally,we successfully obtained a two-inch wafer-scale transfer-free graphene GaN LED device,with high-performance on electrical,optical,and thermal application.On the other side,the growth direction of vertical graphene is perpendicular to the substrate surface,which is more compatibility with the substrate.In fact,it can almost grow on any substrate directly without transfer problem.However,the type morphology of vertical graphene is too complex,there are many factors affecting its growth.So the growth mechanism of vertical graphene has not yet been clearly concluded.It is more recognized that the plasma electric field plays a very important role on it,but how it works and effect,it is controversial because there still haven’t relevant detailed proof.In our thesis,the plasma electric field plays a key role to reducing the growth temperature of horizontal graphene.Therefore,studying the relationship between vertical graphene and plasma electric field can not only to get the point of the growth orientation of graphene accurately,but also supplemente the short of the growth mechanism research of vertical graphene.The main research work and achievements are as follows:1.By depositing ultra-thin layer metal(≤ 2 nm)platinum as a catalyst for graphene growth,the two-dimensional graphene films directly grown GaN as a transparent conductive electrode on GaN blue LED at only 600 ℃,avoiding the complex transfer process of graphene.The thichness of platinum is only 2nm,which is easily absorbed by the substrate and evaporat by plasma and temperature in PECVD(plasma enhanced CVD)which make graphene almost contact with the GaN substrate directly without the negative influence on electrode transparency.Finally,the effect of annealing on platinum and LEDs was studied.It was found that the use of graphene not only compensat the negative impact on the electrical characteristics by annealing temperature,but also further improved the properties of the device on electrical,optical and thermal.2.By using the metal proximity catalytic effect,we catalytic the growth of transfer-free graphene as GaN LED transparent conductive electrode.Metal catalytic high quality graphene without contact,avoiding the metal doping by metal,and achieve the in situ and transfer-free patterned graphene on different substrates by different metals catalytic.3.By using(≥ 200 nm)metal as a sacrificial layer,which means metal act as both the a catalyst for graphene growth and a mask layer for GaN etching.In addition,the transfer process of graphene and the contact between graphene and photoresist are avoided in this technology,where protect the graphene from photoresist doping.Although graphene grows on metal,we remove the metal successfully by using the method of penetrating corrosion,so that the graphene will contact with the GaN substrate directly.This method achieves the transfer-free and lithography-free patterned growth of graphene,which also reduces the doping by photoresist.Finally,the graphene GaN LED is prepared successfully with high-performance.4.Further optimize the conditions of graphene GaN LED by sacrificial metal layer,solve the fall off problem of large-size graphene arrays in the process,and get the main points of manufacture process from small scale towards large scale.A two-inch wafer-scale GaN LED with transfer-free,uniform,and patterned graphene array is prepared successfully.5.To study the effects of nonlocal plasma and local plasma-assisted growth on graphene growth orientation in the same equipment.By analyzing the vertical graphene quality at different growth condition,it is found that the growth of vertical graphene deeply depends on the tip electric field in the local plasma.The growth time and growth temperature can control the edge size of vertical graphene scales,and the power of plasma can also affect the degree of differentiation of vertical graphene.The morphological characteristics of vertical graphene are not observed in the nonlocal plasma electric field at all.Finally,we studied the absorption of light by vertical graphene with different thicknesses and prepared a high-performance vertical graphene thermopile photodetector.This study not only supplemente the short of the growth mechanism research of vertical graphene,but also provides the reference value for graphene growth at different orientations.At the same time,the application of vertical graphene on photodetectors is also rarely reported.