Preparation And Optical Properties Of GaN One-Dimensional Nanomaterials And Nanofilms

As a direct wide band-gap(3.4 eV)semiconductor material,gallium nitride(GaN)has high chemical and thermal stability,and has great application value in high temperature and high power Radio Frequency(RF)devices and photoelectric components.Compared with bulk GaN,one-dimensional(1-D)GaN nanowire has large aspect ratio,providing a directional electron transport channel,and thus has great carrier transport capacity.Meanwhile,compared with GaN nanowire with disordered growth orientation,vertically aligned 1-D GaN nanoarray with extremely high degree of array and uniformity is more valuable.On the one hand,the vertically aligned GaN nanowires grown on appropriate substrates can effectively filter the dislocations caused by lattice mismatch and thermal mismatch,which greatly improve the crystal quality of the upper nanowires.On the other hand,these vertical GaN nanowires can expose more surface and are thus more sensitive to external stimuli.Therefore,the controllable preparation of 1-D GaN nanowire and nanoarray with low cost and pollution is of great importance.In this paper,a novel chloride vapor phase deposition process is proposed.By changing the structure of the quartz gas path,the effects of different growth processes on the morphology,structure and optical properties of 1-D GaN nanowires or nanoarrays are thoroughly investigated.Firstly,the chloride vapor phase deposition is designed and built independently.Gallium tri-chloride(GaCl3)and ammonia gas(NH3)are used as gallium and nitrogen sources,respectively.The effects of growth parameters on the morphology and optical properties of 1-D GaN nanowires are investigated using gold(Au)as catalyst and[0001]-oriented single-crystal sapphire as substrate.It is found that low Ga source supply and high growth temperature are favorable to obtain GaN nanowires with good linearity.The photoluminescence(PL)results show that the near-band-edge(NBE)emission peak of GaN nano wires gradually blue shifts to the theoretical value of 365 nm and the intensity of defect luminescence gradually decreases with the Ga source flow rate decreasing and the growth temperature increasing,indicating the improvement of crystallization quality.Secondly,on the basis of above work,the experimental gas path is further improved.Highly uniform and vertical GaN nanowire array is successfully prepared at a low GaCl3 flow rate of 10 sccm and a high growth temperature of 1000℃.The results show that low NH3 flow rate is beneficial for the improvement of the array degree of GaN nanowires.Further morphology analyses show that the prepared vertical GaN nanowires are hollow structures.Considering the composition in the reaction atmosphere,the Cl2-based synchronous growth and etching mechanism is proposed.In addition,cathodoluminescence(CL)spectrum shows that a unique blue luminescence peak with the central wavelength at 446 nm appears.The unique luminescence can be attributed to the doping of Cl and O elements and the etching of Cl2 based on the component and element syate analyses.Finally,to further reduce the impurity concentration and the etching of Cl2,lowpressure chloride vapor phase deposition equipment and process are developed.Without using any metal catalyst,self-assembled GaN nanorod array and nano-film array are successfully fabricated under low temperature.The oxygen impurity content of the prepared GaN nanorod array is less than 2%.Then,the effects of growth process on the electrical properties and photocurrent response of GaN nanoarrays under UV excitation are investigated.It shows that the GaN nanoarrays have high intrinsic carrier concentration,which is due to the existence of high-density point defects.At the same time,the larger Ⅴ/Ⅲ growth condition is helpful to reduce the intrinsic carrier concentration in the material,and thus increasing the resistance of the material in the dark state and improving the relative intensity of photocurrent.Transient photocurrent test shows that the prepared GaN nanorods array can maintain the photocurrent for a long time after a short irradiation of 365 nm laser,and the maximum photocurrent attenuation period of 2h can be achieved.It indicates that the as-synthesized material has excellent application prospect in the fields of optical storage and artificial photoelectric synapse.Moreover,the vertical GaN nano-film array is used as a buffer layer to grow GaN epitaxial film at 1000℃.PL spectrum shows that the optical properties of the high temperature GaN epitaxial films are comparable to those prepared by commercial MOC VD,which proves that the chloride vapor phase deposition process is of great application value.