Properties And Anodic Etching Of GaN-based Thin Films

GaN is a direct band gap semiconductor with a wide band gap(Eg=3.4 eV).Due to its high breakdo,wn voltage,high electron mobility,stable chemical properties(at room temperature,insoluble in acid solution),a small dielectric constant,and excellent high thermal conductivity,the GaN material has been widely used in the field of high temperature,high power and high frequency microwave devices,UV detectors,microwave,high-brightness light-emitting diodes,optical storage devices and high-frequency,high-pressure,high-temperature,and anti-radiation devices.GaN-based single crystal thin films were deposited on sapphire substrates by metalorganic chemical vapor deposition(MOCVD)technique.However,n-GaN and sapphire substrate have high lattice mismatch rate,which leads to the high defect density and large internal stress of GaN-based films.Thus,the devices fabricated by GaN-based thin films have disadvantages such as low luminous efficiency,low voltage resistance and low execution.Nanoporous GaN thin films prepared by wet etching can solve the above problems,so more and more attentions have been paid to the preparation and properties of nanoporous GaN-based films.In this paper,the effects of nanopores and nanocavities on the photoelectric properties of GaN-based thin films were systematically studied on the basis of nanoporous GaN films.The main research contents are as follows:(1)Acidic or basic solutions such as HF or NaOH can be used as etching solutions,which is not safe and environmentally friendly,so it is necessary to use neutral solution instead of acidic and basic solutions for GaN.To this end,we etched the GaN thin films in NaNO3 solution.The influencing factors of etching in neutral solution were investigated.The variation of the internal stress of GaN film induced by nanopores was calculated by high resolution transmission electron microscopy(HRTEM)images and verified by Raman spectroscopy.(2)One of the most important applications of porous GaN films is the regeneration of GaN films or InGaN/GaN superlattices and quantum well structures.Due to the fact that the growth temperature is usually between 800? to 1050?,it is of great theoretical and practical significance to systematically study the effect of NH3 atmosphere on the thermal annealing of nanoporous GaN films.Based on such a realization,the porous GaN thin films were annealed at high temperature(800 ?)in the NH3 ambience,and it was found that the nanopores can be transformed into nanocavity after the annealing.To thns end,we studied the effect of annealing time on the nanoporous GaN films and transformation mechanism from pores to cavities.In addition,the effect of annealing on the internal stress of GaN thin films was analyzed by HRTEM and Raman spectroscopy.(3)InGaN/GaN multiple quantum well structures grown on sapphire substrates have high defect density and large internal stress.In order to improve the luminous efficiency,therefore,the decrease of the film defect density is essential.Wet etching has the advantages of reducing defect density and internal stress.Therefore,we etched GaN-based films with InGaN/GaN multiple quantum structures in acid solution,and the etching mechanism is analyzed systematically.The influence factors of the etching of GaN-based LED materials were characterized by various methods,and then the free-standing GaN based LED films were fabricated.