Study Of Growth Of GaN Films On Polyimide Substrateson Under Low Temperature

Gallium nitride(GaN)is a type III nitride semiconductor with direct band gap structure.It has a wide band gap(Eg=3.39 eV),good thermal stability and high electron mobility.It is ideal for the preparation of optoelectronic devices(semiconductor light emitting diodes and semiconductor lasers)and high power and high frequency microelectronic devices.In addition,high efficiency solar cell devices can be made by using GaN and InN InGaN alloy.The choice of substrates which is used for depositing GaN thin films has always been the primary concern of researchers.At present,?-Al2O3,silicon(Si),silicon carbide(SiC)and other materials are generally used as substrates.But the ?-Al2O3 substrate is not conductive,and its heat dissipation is not good.It is difficult to directly prepare the vertical structure of high-power devices.Although the SiC substrate is conductive,it is expensive.Moreover,these substrates are hard substrates,which can not meet the demand for flexible electronic devices.Therefore,we consider the growth of GaN thin films on flexible substrates such as Poly imide(PI)and research the growth rule of GaN at low temperature.Polyimide(PI)refers to a class of polymers containing imine rings on the main chain.They are usually prepared by a polycondensation reaction with a monomer containing two amines and two anhydrides,and then prepared by dehydration.Because the aromatic heterocycle structure of PI molecule is very stable,it has high glass transition temperature(Tg)and excellent thermal stability.If used as a substrate for GaN thin film,it has a large temperature adjustable range.The PI used in this experiment is condensed from pyromellitic dianhudride(PMDA)and 4,4 one or two amino two benzylether(ODA).Its Tg is about 300 ~ 400?,the thermal decomposition temperature can reach 500~600?.In addition,this type of polyimide(PMDA-ODA)has excellent mechanical properties and relatively low thermal expansion coefficient.The growth of GaN on PI substrate has the advantage of low cost.In addition,due to the flexibility of polyimide,GaN films grown on PI can be used in folding solar cells,flexural screens,and other industrial fields.In addition,sliver nanowires(AgNWs)can be coated on the PI substrate by liquid phase chemical reduction to make flexible electrodes for GaN.Silver is a relatively inexpensive precious metal material with high conductivity and thermal conductivity.Previous researchers have discussed the growth of GaN films on Ag,and the growth results are good.The use of AgNWs coated PI substrate(hereinafter referred to as AgNWs/PI)is conducive to the convenient fabrication of vertical structure flexible GaN devices.On the other hand,the AgNWs structure can guide GaN to grow in nanowire shape,thus preparing GaN nanowires.In this paper,on the flexible substrate PI and AgNWs/PI,the electron cyclotron resonance plasma enhanced metal organic chemical vapor deposition(ECR-PEMOCVD)was used to prepare a GaN film with high c-axis preferred orientation at low temperature.The effects of different TMGa flow rate and different growth temperature on the crystal structure,surface morphology and optical properties of GaN thin films were studied.The results of RHEED,XRD and AFM show that the suitable growth conditions of GaN growth layer are: TMGa flow rate is 0.9sccm,and temperature is 250?on flexible PI substrate.The average grain size of GaN films grown under this condition is about 42.1nm,and the RMS roughness of the surface is RMS 2.31 nm,and on the flexible AgNWs/PI substrate,the GaN of the structure of the rice line is grown.The test results of RHEED,XRD and AFM show that the crystal quality of GaN films is the best at 300?.the average grain size is 25.5nm,and RMS is 4.64 nm.