The Underlying Micro-mechanism Of Performance Enhancement Of Nonpolar A-plane N-ZnO/p-AlGaN Ultraviolet Light Emitting Diode With I-ZnO Inserted Layer

Recently,light-emitting diode(LED)has become an indispensable optoelectronic device in our daily life,leading to a highly efficient and intelligent direction for our society.Due to its potential applications in the field of scientific detections,military affairs and communications,ultraviolet LED has been extensively studied,especially compound semiconductors that consist of III-V and II-VI elements.ZnO and GaN are direct wide band gap materials with excellent piezoelectric and pyroelectric performance.Besides,ZnO and GaN,with hexagonal wurtzite structures,have close forbidden band widths and lattice constants.Therefore,the heterojunctional material of ZnO/GaN has been widely studied.AlGaN,a direct bandgap semiconductor material,has tunable wavelengths and superior chemical stability.Compared with n-ZnO/p-GaN,n-ZnO/p-AlGaN heterojunction possesses higher valence band offset,preventing holes injection from the p-type layer to n-ZnO layer.Nowadays,most nitrogenous devices in the market are grown along the [0001] c direction with strong internal electric fields,leading to spatial separation of electron and holes,which is commonly known as the quantum confined stark effect(QCSE).In order to eliminate these polarization effects and suppress QCSE,one alternative way is to develop non-polar n-ZnO/p-AlGaN(p-n)and n-ZnO/i-ZnO/p-AlGaN(p-i-n)based LEDs on the non-polar a-plane(11-20)nitrides.After an insertion of i-ZnO layer into the p-n heterojunction,the luminous performance can obviously be enhanced.In this paper,we emphatically analyzed the performance enhancement and the related microscopic mechanism.Here,the study of microscopic mechanism of performance enhancement was carried out using focused ion beam,aberration-corrected transmission electron microscopy and electronic holography.The elemental distributions across heterojunction interfaces were obtained using energy dispersive spectroscopy in scanning transmission electron microscopy mode.The high resolution transmission electron microscopy image(HRTEM),electron diffraction(SAED),and geometric phase analysis(GPA)at the interfaces reveal low lattice distortion,high crystalline quality and low lattice mismatch ratio.By using electronic holography,the distribution of electrostatic potentials across the heterostructures were quantitatively characterized,and the results are listed below: 1.The corresponding piezoelectric polarization related electrostatic fields are much smaller than those reported for polar devices.2.Compared with p-n heterojunction,the electrostatic fields of i-ZnO layer is close to zero.These two structural features contribute to faster movements of the injected electrons and holes,making the i-ZnO layer more conductive to the radiative recombination with enhanced exciton recombination possibilities,and at last the LED performance enhancement.