Investigation On The Fabrication Of Tunable N-ZnCdO/p-GaN Heterojunction Light Emitting Diodes

ZnO is an Ⅱ-Ⅵ group direct transition semiconductor with a wide band gap of 3.37 eV and a high exciton binding energy of 60 meV at room temperature,which is suitable for the preparation of ultraviolet（UV）lighting emitting devices and laser diodes.At present,about the researches on ZnO light emitting devices,it is hoped that the ZnO alloy thin films can be designed by using energy band engineering to achieve the purpose of adjusting and improving the performance of Zn O based light-emitting devices.For example,the band gap of the prepared ZnCdO alloy thin films can be effectively reduced by doping Cd substitute for Zn2+ ion in ZnO,and a continuously tunable luminescence from the UV to the green light region can be realized.On the other hand,because of the preparation difficulty of low resistance p-type ZnO,it has attracted much attention to fabricate the heterojunction by using n-ZnO and p-GaN,and UV light emitting devices and lasers have been successfully realized in this structure.Therefore,if ZnCdO alloy thin films as the active layers can be used to prepare ZnO based light-emitting devices,the luminescence from ultraviolet to visible light region in n-ZnCdO/p-GaN heterojunction can be relized by controlling the alloy composition of the n-ZnCdO layer.This is of great significance to make it apply to the fields of the illumination and display.In this paper,a series of ZnCdO alloy films were grown at 300℃ on c-Al₂O₃ substrates by pulsed laser deposition（PLD）technique.The influences of the oxygen pressure,the laser frequency and target components on the crystal strucrures,morphology and optical properties of ZnCdO thin films were studied.Thus,a controllable ZnCdO film on the composition can be obtained by optimizing fabrication technology.On this basis,tunable n-ZnCdO/p-GaN heterojunction light-emitting diode was fabricated.The main contents are as follows:（1）The charcterization of the ZnCdO alloy thin films,which were prepared under different oxygen pressures and laser frequencies by using Zn_0.9Cd_0.1O as the target material,were studied.The results show that there is the phase separation structure of ZnO and CdO in thin films prepared at low oxygen pressure.With the increase of the oxygen pressure,the ZnCdO films with a single hexagonal wurtzite structure can be obtained and the content of Cd in the films increases gradually.So the band gap of ZnCdO alloy films can be controlled by varying the oxygen pressure.The laser frequency can seriously affect the growth rate,morphology and crystal quality of the thin films,but it is not sensitive to the influence of Cd content in the film.A proper laser frequency is helpful to improve the crystal quality and luminescent properties of ZnCdO thin films.The Zn0.8Cd0.2O target with higher content of Cd was selected as souce materials.Under the condition of the laser frequency of 2Hz,it is realized that the band gap of ZnCdO films can be adjusted continuously in the range of 3.15-2.63 eV by changing the oxygen pressure.The highest content of Cd in ZnCdO alloy films is up to 0.15,and the corresponding emission peak is located near 500 nm.The resistivity of the films is as low as 0.16 Ω·cm.（2）The luminescence mechanism of ZnCdO films with different Cd contents was studied.The emission of the sample in low Cd content was mainly attributed to the recombination of free exciton and bound exciton localized at the isolated Cd atom.With the increase of Cd content in the films,the emission originating from the excitons bound to a few isolated Cd atoms and exciton localized at the clusters of Cd become the main in the PL spectra.ZnCdO thin films with different Cd contents were used to construct n-ZnCdO/p-GaN heterojunction devices.The devices exhibit the characteristics of the diode rectification.Under the forward bias,the emission peak observed at 440 nm from the electroluminescence of the device is considered to be related to the Mg acceptor level in p-GaN.Anderson model is used to analyze the band structure of n-ZnCdO/p-GaN heterojunction devices,and the reason why the electroluminescence of the devices is mainly from the GaN layer is explained.（3）By the analysis using the energy band engineering,the MgZnO and MgO was inserted between the n-ZnCdO and p-GaN heterojunction as the electron blocker layer.We designed and fabricated n-ZnCdO/i-layer/p-GaN heterojunction light emitting devices,and observed the emission mainly from the n-ZnCdO layer in the heterojunction devices.The luminescence mechanism was analyzed by using Anderson model.n-ZnCdO/MgO/p-GaN heterojunction devices were prepared by using ZnCdO films with different Cd contents as the active layer of n region.The electroluminescence spectra at room temperature show that the main emission peak gradually shiftes from 390 nm to 500 nm with the increase of Cd content.This indicates that a tunable the light emitting device from near ultraviolet to green light region is realized.