| Wide-gap semiconductors have shown great application potential in artificial intelligence,nanotechnology and aerospace due to excellent properties such as radiation resistance,high temperature resistance and wide band gap.Compared with third-generation semiconductors such as gallium nitride(GaN)and silicon carbide(SiC),gallium oxide(Ga2O3),which is a typical representative of a new generation of wide-gap semiconductors,has excellent physical and chemical properties,such as wider band gap,good stability,high UV transmittance and high breakdown voltage.And it has attracted much attention in the fields of national defense and military,nanomedicine,and green energy.It has five phases such as ?-,?-,?-,?-and ?-Ga2O3.Among the five structures,the ? phase which belongs to the monoclinic system is the most stable,and other crystal forms can be transformed into it at high temperatures.Therefore,the highly stable ?-Ga2O3 can be widely used in radio frequency,high-power electronics and sensors.As a semiconductor matrix material that can emit light in multiple colors,the related research of ?-Ga2O3 thin film in the field of light-emitting devices is still in infancy due to lower luminous efficiency.It is important to improve the light extraction efficiency(LEE)and internal quantum efficiency(IQE)of Ga2O3 for enhancing its luminous efficiency.Generally,LEE could be enhanced by implanting distributed Bragg reflectors(DBR),while IQE could be improved by improving the crystal quality.Ga2O3 homoepitaxial thin film is an ideal material for high-performance devices because of the perfect lattice matching.Due to the high price of Ga2O3 single crystal substrates,it is difficult to achieve large-scale applications of homogeneous epitaxial films.Since the small lattice mismatch between GaN and Ga2O3,GaN could be a suitable heteroepitaxial substrate.However,GaN wafers have the disadvantage of high cost.Although the GaN single crystal film grown on the sapphire substrate has a large residual stress and defect density,the single crystal film etched after the electrochemical etching has the advantages of low residual stress and low defect density.In addition,etched GaN epitaxial films can also be used to fabricate distributable Bragg reflectors(DBRs)which could improve the luminous efficiency.If the rare earth doped Ga2O3 single crystal film is grown on the etched GaN epitaxial wafer,therefore,its application potential will be greatly stimulated.We fabricated the large-area,high reflectance nanoporous(NP-)GaN DBRs and NP-GaN films in oxalic acid or sodium nitrate solution via electrochemical etching,and research the mechanism of etching.On this basis,the high-quality europium(Eu)doped Ga2O3 single crystal films were obtained on the NP-GaN DBR substrate via pulsed laser deposition(PLD)system,and we studied the relationship between annealing temperature and film crystal quality;Finally,NP-GaN films obtained at different etching conditions were used as substrates to grow erbium(Er)doped Ga2O3 single crystal films by PLD method.Otherwise,we studied the influence of porosity and annealing time on the film quality and photoelectric properties.The main researches of this paper are as follows:1.NP-GaN DBRs which were used to deposit Ga2O3 as the substrate were fabricated by undoped GaN/n-GaN films etched via electrochemical etching.(1)The 10-layer u-GaN/n-GaN periodic structures were etched in oxalic acid solution by electrochemical etching,and the etched samples were annealed in an ammonia atmosphere.On this basis,GaN-based LEDs were regrown on the GaN DBRs.The reflectance spectra and the scanning electron microscope(SEM)images of the DBRs show that the NP-GaN DBRs have many advantages such as high reflectivity(>90%),wide stopband(>80 nm),uniform etching process and smooth surface.Compared with the sample before the annealing,the DBR annealed at 800?has almost no change in the morphology of the nanopores in the NP-GaN layer,so its reflectance has not changed significantly.However,the peak reflectance of the DBR annealed at 950? increased by?6%.Such an enhancement should be contributed to improvement of the index contrast between NP-GaN and GaN due to the shape transformation.The reflectance of the DBR sample after the annealing at 1100?shows a significant decrease(?75%),which could be attributed to the dissociation phenomenon of GaN in the NH3 environment after the annealing at 1100?.In order to explore the possible applications of GaN-based DBRs,InGaN light emitted diode(LED)were regrown on the DBRs by organic metal source chemical vapor deposition(MOCVD)method.We found that the photoluminescence(PL)intensity of the LED grown on the DBR was increased by 4 times.In addition to the increase in crystal quality,the increase in PL intensity should also be attributed to the light coupling effect of DBR.(2)Large area(>1cm2),self-standing NP-GaN DBRs were fabricated by electrochemical etching in NaNO3 solution.The DBR mirror was transferred to another substrate(Si substrate as an example),and then the Alq3 film was deposited on the DBR mirror by thermal evaporation,and its optical properties were studied.It is found that the NP-GaN DBR mirrors have the advantages of flat surface,high peak reflectance(>96%),and wide stopband(460-530 nm).For the DBR mirror transferred to the Si substrate,the peak reflectance is significantly reduced(?85%),which is lower than the reflectance of the DBR mirror completely attached to the sapphire substrate(?97%).This result is mainly due to the roughness surface of the transferred DBR mirror.The rough surface increases the light scattering,which causes a significant decrease in the reflectance of the DBR mirror.The regrown Alq3 film is amorphous,and presents a complete chemical structure.Compared with the Alq3 film grown on the reference substrate,NP-GaN DBR/Si can not only improve the luminous efficiency of conjugated organic polymers,but also lead to the blue shift of the PL peak.2.Rare-earth-doped Ga2O3 single crystal films were deposited on NP-GaN substrates via PLD method.(1)Eu-Ga2O3 film was deposited on the NP-GaN DBR substrate by the PLD method and annealed in the air atmosphere.X-ray diffraction(XRD)results show that the annealed films are pure ?-Ga2O3 grown along the normal direction of the(-201)plane.The crystal quality of the film after the annealing at 900? has the best crystal quality,and its crystallinity is 99.31%.The Eu doping concentration is 3.0 at%.According to the XRD ? scanning and transmission electron microscopy(TEM)images of the 900?-annealed film,the epitaxial relationship between the NP-GaN and Eu doped ?-Ga2O3 film was Ga2O3(-201)? GaN(0001)with Ga2O3[010]?GaN[-12-10].For the Eu-Ga2O3 films,the strongest luminescence peak locates at 612 nm,and the luminescence intensity increases with the improvement of the crystal quality.Compared with the reference film,the PL intensity of the 900?-annealed film grown on the DBR is significantly enhanced(?20 times).This enhancement should be mainly attributed to the improvement of the film quality and the enhancement of optical coupling caused by the DBR.The conductivity of the film is an important parameter to measure whether the film could be widely used.It is found that the resistivity of the film will increase with the increase of the annealing temperature,which is mainly related to the decrease of the oxygen vacancies in the doped film.Since the films annealed at 900? have the advantages of high crystal quality,high luminous efficiency,and low resistivity(?8×102 ?·cm),these films have broad application potential in the field of optoelectronic devices.(2)NP-GaN films with different porosity were obtained by electrochemical etching at the different etching voltages(9,12 and 15 V).NP-GaN with different porosity was used to deposited Er-doped Ga2O3 film by PLD method,and its structural characteristics and photoelectric properties were studied.The porosity of etched GaN wafers at 9,12,and 15 V was?20%,?40%,and?60%,respectively.The Er-Ga2O3 films with different doping concentrations(0,1.0 and 2.5 at%)are all pure?-Ga2O3 grown along the normal direction of the(-201)plane.For the films with the different doping concentration,the Er-Ga2O3 films deposited on the substrate with the porosity for 40%(P40)have the best crystalline quality.For the 2.5 at%Er-Ga2O3 film deposited on P40 substrate,the epitaxial relationship between the NP-GaN and Eu doped ?-Ga2O3 film is Ga2O3(-201)? GaN(0001)with Ga2O3[010]? GaN[-12-10],the band gap of the film is about 4.89 eV,the resistivity is 2 ?·cm,and the RMS roughness is 2.680 nm.In addition,it is found that the strongest PL peak of all films is located at 548 nm,and the PL of the films increases with the improvement of crystal quality.Because NP-GaN epitaxial wafers could be used to deposit high-quality?-Ga2O3 single crystal films,this research provides a new idea for the application of NP-GaN epitaxial wafers.(3)Based on the above research,NP-GaN wafers with 40%porosity were fabricated by electrochemical etching at the voltage of 12 V,and then 2.5 at%Er-Ga2O3 films deposited by the PLD method.After the deposition,the films were annealed.Finally,the crystal quality and optical properties of the films were studied.The XRD results show that the films before and after the annealing are all pure?-Ga2O3 grown along the normal direction of the(-201)plane.The 15 min-annealed film has the best crystalline quality,and the full-widths at half-maximum(FWHM)of the rocking curve for(-201)peak is 1.41°;The increase of the annealing time will lead to the XRD diffraction peaks of the ?-Ga2O3 film shift to the large angle direction.The plan-view SEM images of the samples show that the grain size at the films surface would gradually decrease with the increase of annealing time,proving that the XRD diffraction shift.Further research shows that the band gap of the 15 min-annealed film is?4.98 eV,the epitaxial relationship between the film and the substrate is the same as the previous research results,and the PL of the films increases with the improvement of the crystal quality. |
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