Mechanism Study And Method Improvement Of Photoelectrochemical Etching Of Gallium Nitride In Acidic Aqueous Solution

Gallium nitrogen(GaN)as one of the third generation semiconductor materials,has been widely used in many fields,such as electronics and optoelectronics.Nevertheless,the he chemical properties of GaN are extremely inert which makes the traditional wet etching method unable to be applied to the processing of GaN-based devices.At present the preparation of GaN-based devices mainly relies on physical dry etching technology,but the resulting surface inevitably contains damage,which affects the ultimate performance of devices.In recent years,it has been found that photoelectric chemical(PEC)method can effectively and nondestructively etch GaN.However,many theoretical and technical problems still remain unsaved in the preparation of high quality surface,especially super-smooth surface.Therefore,understanding the PEC interfacial reactions and processes of GaN in principle has important theoretical guiding significance for the development of practicable PEC etching method.Previous studies mainly focused on the PEC etching of GaN in alkaline solution,but the results showed that when etching the most common groove structure,it is hard to eliminate the boundary effect caused by OH-ion diffusion.That is,the amount of OH-ions diffuse from solution to both sides of the trench is higher than that diffuse to the middle of the trench,so that the trench would have a curved profile.In order to solve this problem,this thesis intends to develop the PEC method in acidic aqueous solution.By systematically investigating the influence of H+ion concentration(pH),etching electrode potential,and solution flow and other key factors on the PEC etching process of the n-type GaN monocrystalline(0001)plane,this thesis provides a theoretical basis for the development of PEC processing technology.In chapter 3,the relationship between the flat band potential of GaN and the onset potential of photoanode current in acidic solution was firstly studied.It was found that the difference(AE)between the flat band potential and onset potential of photocurrent decreased with the decrease of pH,revealing that AE is generated from the PEC reaction kinetics at the interface of GaN.The higher the concentration of H+ions,the more likely the photogenerated holes(h+)will conduct at the interface,thus triggering the reaction.Subsequently,the effects of etching potential and pH on the flatness and roughness(Ra)of etching surface were investigated systematically using a patterned GaN electrode with Ti gate as a mask,which also works as a barrier layer.The experimental results showed that:(1)in 2 mM H2SO4 solution(pH=2.3),an ultrasmooth surface without boundary effect can be obtained in a wide potential range(+0.4+1.0v vs SCE),while Ra attains 4.17nm in 5x5?m2 and the etching rate at+ 0.4 V is of 16 nm/min higher than 5 nm/min at+2.0 V;(2)in 2 mM H2SO4 solution(pH=2.3),the use of the flow solution to improve the mass balance leads the etching rate at+1.0 V increase to 17.87 nm/min.whereas Ra increase to 20.71 nm;The etching surface shows an ordered single crystal step morphology,indicating that etching can be carried out in step-edge mode under acidic conditions;nm/min,(3)in 0.1 M H2SO4 solution,the etching rate at+1.0 V increases to35.14 nm/min and Ra increases to 15.41nm as well.These results indicate that the high etching potential,stationary solution,and low H+ion concentration are favorable for etching the super-smooth surface,although the etching speed is slow.The reason is that these conditions are conducive to the formation of uniform Ga2O3oxide film on the GaN surface during etching,so that the initial solid(GaN)/solution interface reaction becomes solid(GaN)/solid(oxide film layer)/solution interface reaction.GaN etching is controlled by the diffusion behavior of oxygen species in oxide film,which is similar to the process of metal corrosion.The experiment also found that when the solution(pH<5.6)contained an appropriate amount of phosphoric acid,it could promote the formation of oxide film and reduce Ra.Finally,fluorescence detection method was adopted to prove that OH free radicals were generated in the etching process.In chapter 4,the electrodeless PEC etching system used in industry is explored to fabricate the groove structure.This system is similar to the solar cell system.Pt gate mask deposited on the GaN surface is used as the cathode,and the PEC etching of GaN surface is dynamically controlled by the photogenerated electrons consumed at Pt nano-layer with oxidant.A new oxidant,K2SO4·KHSO4·2KHSO5,was developed and compared with K2S2O8oxidant commonly used in industry.The experimental results showed that:(1)in 2 mM H2SO4+5 mM K2SO4·KHSO4·2KHSO5solution,the PEC etching rate is 3 to 5 times(10.33,25.16 nm/min)higher than that conducted in2 mM H2SO4+5 mM K2S2O8solution,whereas Ra is of 37.88 and 88.44 nm when the solution was flow or not.The reason is that KHSO5 has a higher oxidation potential of oxidable metal Pt which is conducive to the rapid consumption of photogenerated electrons and the promotion of GaN oxidation by photogenerated holes.(2)In 2 mM H2SO4containing20 mM or 40 mM K2SO4·KHSO4·2KHSO5,the etching rate was of 16.52 nm/min,24.56 nm/min,and the etching surface had no boundary effect.Ra decreased to 12.77 nm and 15.33 nm.(3)In 0.1 M H2SO4 containing20 mM or mM K2SO4·KHSO4·2KHSO5,the etching rate was further increased to 27.67 nm/min and 56.50 nm/min,while Ra attained 31.63 and 54.74 nm,indicating that the decrease in pH would accelerate etching but be not beneficial to Ra.(4)In 2mM H2SO4+20 mM K2SO4·KHSO4·2KHSO5 containing the hole scavenge(furfural alcohol and n-propanol),the etching rate decreased from 16.52 nm/min to 6.03 and 10.27 nm/min,while Ra decreased slightly to 12.17 nm and 10.20 nm/min.These preliminary results indicate that the high quality grooves without boundary effect can be obtained by further developing the electrodeless PEC acid etching system.The key is to use high concentration(>20 mM)K2SO4·KHSO4·2KHSO5 as oxidant,without the use of flow solution method and any organic additives,while the pH of etching solution must be high than 2.4 to form a high quality oxidation film.