A Study Of UV-LEDs With P-type Superlattices Structure

Ultraviolet light emitting diodes (UV-LEDs) have been widely used in the fields such as solid-state white lighting, medical sterilization, UV curing, biological agent detection, large capacity information storage and non-line of sight communications. Short wavelength UV-LEDs should use a high p-doped AlGaN or GaN material as the p-type layer, which makes the production of AlGaN-based UV-LEDs difficult. Because the impurities of high Al Component Mg-doped AlGaN materials is difficult to ionized, AlGaN films have a high doping concentration, which directly affect the quality of the materials. Currently, there still exist many problems in the growth of AlGaN material and its p-type doping, making the light emitting efficiency of wavelength less than 365nm GaN-based Ultraviolet LED still lower a magnitude than that of an InGaN-based Ultraviolet LED. Because of the specific band gap and polarization effect of the AlGaN/GaN short period superlattice (SPSLs), the holes injection efficiency of UV-LEDs with Mg-doped p-SPSLs as the p-layer can be greatly improved. Short period superlattice also can stop the dislocations and cracks of the epitaxial introduced from the growth extending to the surface, to improve the quality of the epitaxial layer. Because of the characteristics that the horizontal resistivity is not equal to the vertical resistivity in the AlGaN/GaN superlattice, the current distribution of the LEDs can be improved, so do the optical properties of the LEDs.On the basis of this theory, we proposed that utilizing the Mg-doped p-AlGaN/GaN SPSLs as the p-type layer of the UV-LEDs. We have demonstrated ultraviolet light emitting diodes with a wavelength of 353 nm in this thesis. High resolution XRD and AFM surface morphology were used to analysis the crystalline quality and morphology characteristics of the denotation chip. After the chip die machining, C-V profiling measurement is preformed to analysis the distribution characteristics of apparent carrier and doping concentration in the device. Then Ⅰ-Ⅴ profiling measurement is carried out to further analyze the current transport mechanism of the device. Finally Electroluminescent (EL) test is preformed to study the optical properties of the sample LED devices. The main contents are as follows:1. Enhanced p-type doping mechanism of AlGaN/GaN Short Period Superlattices is studied in this thesis. The specific band gap and polarization effects of AlGaN/GaN Short Period Superlattices make superlattice band bending occurs at the interface, which can greatly reduce the activation of acceptor Mg. So that the interface of superlattice produces a high density of surface charges, eventually improve the hole concentration in the material. Silvaco ATLAS simulation software is used to calculate the energy band diagram and hole concentrations of p-AlGaN/GaN SPSLs. The theory that the hole concentration can be improved by the superlattice structure is verified.2. Then the UV-LEDs devices with p-AlGaN/GaN Short-Period superlattices as p-type layer are epitaxial grown by MOCVD. The material crystalline quality of the chip is good enough to be used in the device. After die processing, C-V profiling measurements is test to analysis the distribution characteristics of apparent carrier in the device. From the C-V profiling, we know that the doping concentration of Mg impurity is 5.6 X 1019cm-3. The Ⅰ-Ⅴ characteristic curve shows that the working voltage is only 3.55V at 350 mA. Further research of the Ⅰ-Ⅴ characteristic shows that the ideal factor increasing at high current mainly due to the deep level impurity tunneling current. Then we test the optoelectric property of the devices, to deeply analysis the device performance of the device with the p-AlGaN/GaN SPSLs as p-type layer. The wavelength of the sample LED is 353 nm. Luminous power of the sample we obtained can reach 27.67mW at 350 mA, corresponding to an external quantum efficiency of 6.20% at 50 mA.