Mechanistic Study On Photoelectrochemical Etching Of Gallium Nitride

Gallium nitride?GaN?with a wide bandgap?3.4 eV?is one of the third generation semiconductor materials,and has broad application prospects in the field of electronics and optoelectronics.Nevertheless,the chemical properties of GaN are extremely inert;employing traditional wet etching method is difficult to achieve its high-efficiency and high-quality material removal.Recently,it has been found that photoelectrochemical?PEC?etching based on photogenerated holes can be as a feasible approach,but it is impossible to produce perfect GaN wafer due to the limitation of preparation techniques,and the surface quality after etching can be seriously affected on the heterogeneous distribution of holes in the crystal surface which is caused by various dislocation defects.Therefore,it is of great theoretical significance to understand the influence of surface/interface of GaN especially various dislocation defects on the PEC etching,and to study the method of wet etching.In general,aqueous solutions are used to PEC etching,theoretically both H₂O and OH^-can form oxide layer of GaN,but none further studies on the possible difference of them up until now.In this paper,Deep Eutectic Solvents?DESs?and aqueous etching solutions were taken to study the effects of electrode potential,OH^-ions concentration,water and ultraviolet?UV?light intensity on the influence and regularity of PEC etching process of n-type GaN with a?0001?plane.The study targeted at the understanding of theoretical foundation and the developing of the etching GaN methods,including the chemical-mechanical planarization?CMP?of GaN wafer and the electrodeless PEC?ELPEC?fabricating of GaN devices.In the chapter 3rd,the subjects included:?1?under the darkness,either the electrochemical or the photoelectrochemical etching of GaN in reline?choline chloride + urea?enabled to reveal the surface/subsurface damages and the various kinds of dislocations;?2?surface/interfacial reaction of GaN in different DESs and aqueous solutions,including the influence of H₂O,UV intensity,electrode potential,OH^-concentration and other factors;?3?the effects of solution components,etching potential and electrochemical methods on the PEC etching rate and surface morphology by using GaN electrode with Ti mask.The experimental results are as follows:?1?under the darkness,either the electrochemical or the photoelectrochemical etching of GaN in reline?choline chloride + urea?enabled to reveal the surface/subsurface damages and the various kinds of dislocations;?2?the etching potential played a key role to regulate the PEC processes and the morphologies of the finishing surface;at the GaN/ethaline?choline chloride + glycol?or GaN/H₂O interface,the reaction of hole with OH' anions occurred at lower potential than that with H₂O,hydroxyl groups or Cl' anions;the reaction of hole with OH^-anions not only led to the etching of GaN but also the formation of OH·radicals;?3?the etching reaction mainly involved OH^-anions at lower etching potential;the low OH^-concentration would lead the diffusion to dominate the etching and result in a smooth surface;the high OH^-concentration would lead the kinetics to dominate the etching and cause a whisker structure at a slow rate,because the preferential recombination of electron-hole at the dislocations and the thick oxide layer would cause an uneven distribution of holes at the GaN/solution interface;?4?the etching reaction involved both H₂O and OH" anions at high etching potential;the oxide layer,generated from H₂O in a self-limiting way,could result in a even etching,while high electronic filed established within the space charge layer could increase the etching rate through the decreasing of the recombination,thereby leading to a smooth surface with the atomic step structure.In the 4th chapter,Pt mask was fabricated for ELPEC etching by lithography and magnetron sputtering,and a thin-film electrolytic cell was designed.Differences of etching rate and surface morphology were investigated under the conditions of standiig and thin-film flowing when O₂?S₂O₈^2-and H₂O₂ were used as oxidizing agent of ELPEC etching respectively.The results shows that?1?thin-film flowing improved material balance and promoted the homogenous of etching in the direction of planar.?2?H₂O₂ had a more positive reduction potential than S₂O₈^2-,but it consumed surface holes resulting in a slow etching rate;employing thin-film flowing could reduce the edge effect.?3?S₂O₈^2-could produce SO₄^-by electrochemical and photochemical methods and gain a high etching rate as it made hole injection valence band and reacted with GaN;in a alkaline solution,SO₄^-formed OH· with H₂O or OH^-and decreased the pH value rapidly,leading to a thick oxide layer.?4?under the condition of alkalinity,the surface roughness Ra was smaller than that in an acid solution and was not selective for the defect areas.Given the above results,a distance-sensitive ELPEC machining method based on soft template was developed.