Preparation And Investigation Of Gallium Nitride Based Near-ultraviolet LED

GaN based materials including aluminum nitride（AlN）,gallium nitride（GaN）,indium nitride（InN）and its alloys,the energy band gap can be turned from 6.2 eV（AlN）to 0.7 eV（InN）.The wavelength of it can be range range from 200 nm to 1770 nm.At present,the GaN base material is very mature in the wavelength of visible light.It has been widely used in the backlight of display,outdoor display,lighting and so on.Recent years,the application of ultraviolet light is constantly expanding.It can be applied to purify water,uv curing,sterilization,disinfection and air purification,security testing,ultraviolet phototherapy treatment,medical instrument disinfection,photoetching and medical diagnostics.At present,the market is using mercury lamp as ultraviolet light source.The luminous efficiency of mercury lamp is very high,but its luminous spectrum is very wide.In our actual application,we only use light at a specific wavelength.A lot of light will be wasted.Gallium nitride based LEDs have a narrower luminous band than the mercury lamps,there will be no waste of light.In addition,gallium nitride based ultraviolet LEDs have the advantages of non-toxic and long life.Although gallium nitride based uv LEDs have many advantages,but the luminescence efficiency of gan-based devices is relatively low and high power devices are difficult to prepare.So at present ultraviolet light source application still use mercury lamp.In order to improve the performance of UV LED,we must overcome the difficulties in the preparation of ultraviolet LED:（1）The AlGaN material with low dislocation density is difficult to prepare.It is because Al atoms have very high bond energies.It is very difficult for Al atoms to migrate on the surface of epitaxial layer.This results in poor crystal quality of the film.Normal AlGaN has a dislocation density of 10¹00 cm^-2 to 10¹11 cm^-2.In InGaN active layer,indium-rich clusters can physically separate carriers from threading dislocations.Absence of this alloy clustering in AlGaN active layer may lead to the reduced of internal quantum efficiency.When the dislocation density is over 10¹⁰cm^-2,the internal quantum efficiency will be less than2%.（2）AlGaN has a large polarizing electric field,which can produce quantum confinement of the stark effect and reduce the internal quantum efficiency of the device.In addition,the polarization electric field can cause the potential barrier for carriers to enter into active layer,which is not conducive to the quantum efficiency of the device.（3）Because of the special band structure of AlGaN,when its Al component is higher than 25%,the light emittied in active layer is mainly TM polarized.It is difficult for light to shoot out of device.It leads to a low external photoelectric effect.（4）Doping is very difficult in AlGaN.It makes it difficult for carriers to inject into active layer.Meanwhile,it may cause current crowding effect in horizontal structure device.In solving the problem of low luminous efficiency of the ultraviolet LED device,the RCLED structure is proposed to enhance the vertical direction of light emission for the problem that UV LEDs have strong TM luminescence.For the problem of poor AlGaN crystal quality,the use of SiC as a substrate is proposed who has similar lattice constant to GaN-based materials.In order to avoid the current crowding effect,a vertical structure LED is prepared on SiC substrate.In order to suppress the absorption of ultraviolet light by the SiC substrate,LED structure with a conductive DBR between active layer and substrate is used here.In order to eliminate the potential barrier that prevents carriers from entering the active region which is caused by the polarization effect,a pn inversion structure is realized by a tunnel junction.To solve the above problems,research have been made and the results achieved are as follows:1.The structure of DBR is designed.The relationship between the peak reflectivity of DBR and the refractive index of each layer and the pair of DBR using the transmission matrix method.The optimization method of DBR reflectivity is studied.The relationship between the stopband and the refractive index is calculated by the formula of the stopband,and the optimization method of the stopband width is proposed.The angle characteristics of DBR are studied through the transmission matrix method,and the optimal scheme of the angle characteristics of DBR is proposed.2.The crack-free near-ultraviolet DBR was prepared by MOCVD method.DBRs with different Bragg wavelengths were prepared,and the reflectivity of the DBR was measured.The DBR structures with different number of pairs were prepared.The peak reflectance of the DBR was verified by testing the peak reflectivity of the DBR and the DBR structure with high reflectivity was obtained.In order to improve the stopband of DBR,a GaN-based double DBR stack was fabricated.The experiment proves that the double DBR stack can effectively increase the stopband of DBR,and it is inferred from theory that the compound DBR can increase the reflection angle of DBR.In order to prepare the crack-free AlGaN/GaN DBR structure with high Al content,by studying the influence of the insert layer on the DBR stress,the low temperature AlN insert layer can effectively relieve the tensile stress in the DBR and suppress the generation of surface cracks.3.Conductive DBR structures were studied on n-type SiC substrates.Ten pairs of conductive DBR structures without cracks were prepared and the peak reflectivity was65%.It was investigated that 20 pairs of conductive DBRs were prepared and the cracks are relieved by the InGaN interlayer and the SiN interlayer.The InGaN interlayer can relieve the cracks,but it takes too long to grow and does not completely eliminate cracks.The SiN insertion layer can suppress the generation of cracks,but it will increases the roughness of the conductive DBR surface.The effect of AlN buffer layer in conductive DBR was studied.It was found that growth of AlN buffer layer under high temperature and high V/III was more conducive to the suppression of cracks in conductive DBRs,and a conductive DBR with no cracks was prepared.Its peak reflectivity could reach80%.4.A horizontal structure near-ultraviolet LED structure was fabricated on sapphire.The LED’s emission wavelength was at 389nm,and the half-peak width of its emission peak was about 13nm.The method of suppressing the light emitted from the side of the substrate by the DBR mirror increases the external quantum efficiency of the device and increases the luminous intensity of the device by about 1.5 times.The RCLED structure was simulated and prepared.The EL test found that the RCLED have multiple diffraction peaks and the device emitted light concentrated mainly in the direction parallel to the c-axis.The contrast shows that the RCLED luminous intensity is 1.9times that of a conventional LED and 1.3 times that of a DBR device.5.Based on the previously studied v ertical conductive structure,a vertical structure near-ultraviolet LED was fabricated on a SiC substrate and has a light emission wavelength of 389 nm.It was tested by changing the angle to find that its angle characteristic appeared circular.The angle characteristics were analyzed and it was considered that this angle characteristic was due to the change in transmittance with the angle.By analyzing the polarization characteristics of the LED light emission and calculating the relationship between the transmittance of different polarized light and the angle,the calculated results are in agreement with the experimental results.A vertical structure near-ultraviolet LED with 10 pairs and 20 pairs of conductive DBRs was fabricated on a conductive DBR and characterized by testing.It was found that near-ultraviolet LEDs with DBR have stronger luminous intensity and better directivity.Simulation work has found that LEDs with a tunnel junction structure can effectively increase the injection of carriers into the active region and effectively suppress the flow of carriers out of the active region.Preparation of pn inverted structure using tunnel junction structure with conductive DBR,and a near-ultraviolet LED structure with an emission wavelength of 395 nm was obtained.