Fabrication And Photoelectric Properties Of Anodic Aluminum Oxide-like Nanostructures On GaN Surface

GaN semiconductors have a wide range of applications in LEDs,photocatalysis,sensors,high electron mobility transistors and solar cells.Nanostructures on the surface of GaN help to improve the performance of GaN devices.Nanostructures on the surface of GaN are usually prepared by electron beam lithography,focused ion beam lithography and laser holographic exposure,which are expensive and small in area.Therefore,seeking a large-area and low-cost fabrication method of GaN surface nanostructures is become one of the research hotspots and difficulties in the filed of GaN nanodevices.Anodic aluminum oxide(AAO)can be used as a mask to fabricate nanostructured arrays at low cost.In this paper,the preparation and photoelectric properties of low-cost large-area AAO-like nanostructured GaN are studied based on non-through-hole AAO membranes.A short-time multi-step anodizing method is proposed.Macroporous(pore diameter>400 nm)AAO membranes are prepared.By changing the process parameters,the nanopore diameter can be freely controlled from 10 to 480 nm.In view of the small area and fragility of the through-hole AAO membranes,the barrier layer of AAO is retained to improve the mechanical strength of the AAO membranes.Non-through-hole self-supporting AAO membranes with an area of 7.1 cm~2 are prepared.Using GaN as substrate and non-through-hole AAO as mask,the fabrication of large-area nanostructures on GaN surface is studied.The plasma particle transport mechanism in the plasma etching process is explored,and the regulation of the nanostructure parameters on the GaN surface is studied.During the process of etching the barrier layer,the pore diameter of AAO is enlarged gradually.The transport rate of reactive particles in the AAO channels is accelerated,thereby affecting the etching rate.During the process of expanding the AAO pore diameter from 250 nm to 430 nm,the etching rate v_d increased from 0.141 nm/s to 0.501 nm/s.In the process of etching GaN,the plasma charged particle flow induces charge enrichment on the sidewall surface of the AAO mask.As a result,the trajectory of the incident positive ions in the plasma is bent.Thus,the verticality of the etching profile is affected.The aspect ratio of the mask changes due to ICP etching,which affects the three-dimensional size of the nanostructures on the GaN surface.The correlation between the nanostructure size of GaN surface,the structure size of AAO membrane and the etching process is established.The relationship between the three-dimensional size of nanostructures and the etching time is obtained.When the etching time of AAO satisfies the relationship d_b/v_h<t<(D-d_p)/2v_d,the size of the nanostructures on the GaN surface can be adjusted while removing the barrier layer of AAO.The photoelectric properties of large-area nanostructured GaN are tested and evaluated.AAO-like nanostructures with a period of 487.5 nm are fabricated on the surface of the p-GaN layer of GaN.The photoluminescence efficiency of the GaN is highest when the nanoarray has an aperture of 400 nm and a height of 150 nm.Compared with the planar GaN without nanostructures,the photoluminescence efficiency is increased by 3.5 times.The structure parameters and performance of AAO-like nanostructured GaN are simulated by COMSOL software.The results show that the optimal three-dimensional structure parameters to improve the light extraction efficiency are the height of 150 nm,the period of500 nm,and the duty cycle of 0.865.The simulation results are self-consistent with the experimental results.On this basis,according to the non-strict periodic structure of AAO-like nanoarrays,a certain degree of disorder processing is designed for the pore diameter and position of the ordered nanoarrays in the simulation,and the output optical power is increased by 0.62 times.In addition,the photoelectrochemical water splitting performance of nanostructured GaN is tested.When the incident light wavelength is 365 nm,the incident photon-to-current conversion efficiency of nanoporous structure GaN reaches 46.6%,which is equivalent to 1.88 times that of planar GaN.