Fabrication Of Nanoporous GaN-based Thin Films And Epitaxial Growth Of Beta-Gallium Oxide Thin Films

As a typical representative of the third generation of wide band gap semiconductor materials,GaN has become a hot topic in current research.Due to its excellent photoelectric characteristics,such as wide band gap(3.4 eV),high breakdown electric field,and high electron drift speedy therefore,it has been widely used in UV-visible light emitting diodes(LEDs),lasers(LDs),photodetectors,and high-frequency and high-power electronic devices.High-quality GaN film is a key factor in the preparation of high-performance optoelectronic devices.At present,sapphire as a heteroepitaxial substrate has been widely used for the epitaxial growth of GaN films due to its low price.Because of the large lattice mismatch and thermal mismatch between sapphire and GaN,the GaN film has a large residual stress and a high defect density(101-109 cm-2),which makes GaN-based optoelectronic devices have the disadvantages of low photoelectric performance and poor reliability.The preparation and re-growth of high-quality nanoporous(NP)GaN thin films have become one of the most popular research fields,since the wet etching method can overcome the above-mentioned shortcomings of GaN-based thin films.Herein,the photoelectric characteristics of InGaN-based films with NP-GaN distributed Bragg reflectors(DBRs)were systematically studied on the basis of preparing nanoporous GaN-based thin films by electrochemical etching(EC)method,and then the organic chemical vapor deposition(MOCVD)was used to grow GaN LEDs and P-Ga2O3 single crystal films on NP-GaN-based films.The specific research contents are as follows:(1)Using nitric acid(HNO3)or sodium nitrate(NaNO3)as an electrolyte,the n-GaN/u-GaN periodic structure with InGaN/GaN multi-quantum well layer was etched by the EC etching method,the structure and photoluminescence properties of the samples before and after etching were systematically studied.We found that HNO3 solution is more suitable than NaNO3 solution for the preparation of NP-GaNDBR mirrors with large area,high reflectivity and MQW layer.Compared with the unetched sample,the enhancement factor of the photoluminescence(PL)intensity of the etched sample in HNO3 solution at 16 V was about 3.2 times.In order to verify whether the etched film can be applied,Mg-doped p-GaN layer was grown on the etched sample by MOCVD at 950?.Compared to the sample before regrowth,the reflectance and PL intensity showed a slight decrease and a significant decrease(a decrease of about 5 times).The decrease in reflectivity was due to the absorption of incident light by the p-GaN layer,whereas the significant decrease in PL intensity was attributable to mass transport caused by regrowth,resulting in the MQW structure was destroyed.(2)HNO3 electrolyte has the shortcomings of strong corrosivity,difficult to recycle and environmentally unfriendly,so it is particularly important to seek a safe and environmentally friendly electrolyte.In this chapter,the above-mentioned thin film structure was etched by oxalic acid(Oxalate)solution,NaNO3 solution,and their mixed solution.Compared with the samples etched by a single oxalic acid or sodium nitrate solution,the nanoporous layer formed by the samples etched by the mixed solution had a higher porosity(?50%)and a more uniform pore distribution.DBR structure had higher reflectivity.The PL intensity of the etched sample was increased by 4-6 times in comparison with the unetched sample.It should be attributed to the improvement of the crystal quality in the MQW layer and the enhancement of the light extraction efficiency(LEE).In order to prevent the MQW layer from being damaged during the re-growth process,the 2-cycle InGaN/GaN quantum well layer and p-GaN layer were regrown on the etched samples by MOCVD technology.Compared with the regrown film in the unetched area,(i)the roughness of the regrown film in the etched area was higher,which should be attributed to the etch damage of the InGaN/GaN MQW surface;(ii)the luminous intensity of GaN-based LED with DBR mirrors had increased about 1.8 times,which should be contributed to the light coupling effect induced by the DBR mirror.(3)In contrast to GaN,?-Ga2O3,which has the advantages of wide band gap,high breakdown electric field,good chemical and thermal stability,has potential applications such as high-power power electronics,solar blind detectors,and light-emitting devices.Because of the main form of semiconductor devices is thin film,epitaxial growth of high-quality ?-Ga2O3 single crystal thin films has become one of the central research issues.In this section,the MOCVD technology was used to grow ?-Ga2O3 single crystal films on NP-GaN substrates obtained by EC etching technology.The effects of growth temperature and etching voltage on the structure and morphology of ?-Ga2O3 thin films were systematically studied,which indicated the ?-Ga2O3 film grown on as-grown GaN at 950? is a single crystal film with a growth direction along[201].Based on the above research,?-Ga2O3 single crystal was heteroepitaxially grown on NP-GaN substrate at 950?.Through structural analysis,we found that the ?-Ga2O3 single crystal thin film grown on the 9 V-etched sample had the highest quality.The out-of-plane epitaxial relationship between ?-Ga2O3 film and GaN is ?-Ga2O3(201)?GaN(0001),and the in-plane epitaxial relationship is?-Ga2O3<010>IIGaN<1210>and ?-Ga2O3<102>?GaN<1010>.