Transfer-free Growth Of Patterned Graphene And Its Optoelectronic Device Application

Graphene,as the first discovered two-dimensional material,has many excellent properties,such as an ultrafast carrier mobility,a high mechanical strength and a high transmittance,which have attracted extensive attention of researchers.As the upstream research in graphene,the growth of graphene has always been a hot research field.Currently,there are four main methods to synthesize graphene:mechanical or liquid exfoliation,silicon carbide epitaxy,reduction of graphene oxide and chemical vapor deposition(CVD).Among them,CVD is the most commonly used method to prepare large area graphene films because of the high quality of CVD graphene and suitability for mass production.Metal substrates such as Cu and Ni are usually used as catalysts in the CVD process,and the graphene is grown on the metal surface.Metal as a good conductor is not suitable as substrate for the fabrication of graphene devices on its surface,otherwise it will lead to short circuit.The use of graphene requires the transfer of graphene from the metal surface to the target substrate(semiconductor substrate,insulating substrate,etc.).The transfer process is not compatible with the current semiconductor technology,and it is very complex and inefficient.Meanwhile,graphene has only a single atomic layer thickness,which is very fragile and sensitive to the environment.It is inevitable that graphene will be damaged and doped during the transfer.At present,there is no effective solution to this problem yet.Therefore,the transfer of graphene has become one of the bottlenecks limiting the large-scale application of graphene.In this context,the research on the direct growth of graphene was born.The direct growth of graphene refers to graphene grown directly on target substrates,which can avoid the transfer process.This can not only improve the fabrication efficiency of graphene devices,but also ensure the integrity of graphene.In this thesis,a commercial vertical chamber cold wall graphene CVD system was used to study the transfer-free growth methods of graphene.Generally,the methods reported in this thesis can be categorized into two types:metal-assisted growth and metal-catalyst-free growth.On this basis,the applications of direct-grown graphene in the field of optoelectronic devices were explored including Ga N light-emitting diode(LED)and graphene photodetector.In addition,in the field of transferred graphene,some research projects were also carried out.We realized the preparation of large-scale metal nanoparticles assisted by graphene and the fabrication of photoconductive detector using liquid-Cu-catalyzed CVD graphene.The main contents of this paper are as follows:1.A new method of in-situ growth of graphene with metal sacrificial layer as catalyst was proposed and studied.Different from the previously reported metal-assisted growth methods,our method uses the penetration etching mechanism to remove the metal sacrificial layer,greatly improving the repeatability of the experiment and the integrity of the graphene.This method realized the rapid growth of transfer-free patterned graphene film with high quality.At 800°C growth,the ratio of D/G peak of the Raman spectrum of the obtained graphene is generally below 0.3 and the transmittance is 97%,which is close to that of Cu-foil-catalyzed graphene grown at1000°C.Selected area electron diffraction results show that the graphene is single layer.The penetration etching mechanism was studied,and the important phenomenon that graphene accelerates the metal etching was found.In addition,new growth techniques of graphene are explored,including low temperature growth,Cu-Ni alloy catalytic growth and Co catalytic growth.Among them,the influence of carbon diffusion rate on the uniformity of graphene in the low temperature growth was first proposed.The parameters that affect the quality of graphene in low temperature growth were studied.At 600°C growth,the D/G peak ratios of the Raman spectra of the obtained graphene are about 0.12-0.69,which are better than most of the previous reports.As an application of low-temperature growth,graphene has been used as transparent conductive film on Ga N LED.The direct-grown graphene has obvious current expansion effect and improves the device performance.Based on the metal-sacrificial-layer growth method,a technique of metal proximity effect growth of graphene was developed.2.Metal-catalyst-free growth methods were systematically studied.First,the technique of patterned graphene growth using cross-linked Parylene as carbon source is proposed.It is a new direct growth method of patterned graphene,which realizes large area and uniform graphene growth.Metal-catalyst-free growth is simple,efficient,no contamination of substrate and compatible with traditional CMOS process,which has potential application prospects.We analyzed the growth mechanism of the method and explored its application in photodetector.Graphene-Si Schottky photodetector was fabricated.Illuminated by 792 nm laser,the responsivity of the detector under 4 V reverse bias at room temperature is 275.9 m A/W,and the specific detection rate is4.93×10⁹ cm Hz^1/2/W.Under the condition that the performance of the direct-grown graphene detector is basically the same as that of the transferred graphene detector,this transfer-free growth method greatly improves the fabrication efficiency and consistency of the devices,which has much commercial value.Second,the direct growth technique of vertical graphene was studied.The quality and morphology of the graphene were characterized.Two kinds of detector structure including Schottky structure and thermopile structure,were proposed.Among them,the performance of the thermopile structure detector was improved about 8 times by using the vertical graphene as an optical collector.3.In addition to the study of direct-grown graphene,some studies of transferred graphene were also carried out.First,a technique of graphene-asisted preparation of Ag nanoparticles was proposed.On the basis of ion implantation technique,graphene is selected as the evaporation barrier layer.By using its high mechanical strength and atomic impermeability combined with the substrate periodic treatment technique,large-area,large-size(?150 nm)and single-crystal Ag nanoparticles with regular arrangement are obtained.The Ag nanoparticles can enhance the G peak of the Raman spectrum of graphene by 20 times.Second,the photoconductive detector using liquid-Cu-catalyzed CVD graphene was studied.The photoconductive response mechanism of graphene was analyzed,and the graphene detector with high response was realized by using the liquid-Cu-catalyzed high-quality graphene.