Optimization Of Electron Blocking Layer In Light Emitting Diodes Consisting Of Wurtzite AlGaN Core-shell Nanowire

The electron leakage in the active region in a Light-emitting diode?LED?can always decrease its lifetime and luminous efficiency,thus inserting a barrier as the electron blocking layer?EBL?is one of the solutions.However,since a single barrier blocks both the transportation of the electrons and holes,a substantial multi-barrier is often used to block the leaking electron while facilitating the injection of the holes by the tunneling effect.At present,the core-shell nanowire?CSNW?LED is expected to be the cell of the next generation of LEDs.In consideration of the electronic leakage in the CSNW LED as that in the planar ones,we discuss the feasibility of a radial double-barrier to serve as an EBL.The structure designed in this work is a wurtzite radial Al_yGa_1-yN/Al_xGa_1-xN/Al_yGa_1-yN double-barrier system.The I-U characteristics of the electrons and holes are calculated.If the tunneling current peak of the light holes is at a certain voltage which corresponds to the tunneling current valley of the electrons,this property can be used to block electron while facilitating the injecting of holes by resonant tunneling to improve the recombination rate.First,we calculate the radial transmission coefficients,resonance energy levels and wave functions of the carriers by transfer matrix and Runge-Kutta methods.Then,considering the contribution from all the carriers with Fermi distribution,the radial tunneling current-voltage characteristic are obtained.Finally,the condition at which the tunneling current peak of the holes has corresponded to the valley of the electrons,is searched by modulating the position,the size,and the Al component of the double-barrier.The results are as follows,?1?Similar to the planar LED,a double barrier is also more suitable than a single barrier to serve as the EBL in a CSNW LED.Resonance tunneling occurs when the incident energy is consistent with the Eigen-energies and leads to the negative differential conductance.And several peaks of resonance tunneling current appears due to the multi resonance energies.Besides,continuous increase of the voltage while keeping the double-barrier height,or continuous reduction of the double-barrier height while keeping the voltage,will cause resonance tunneling phenomenon to disappear since the vanish of the intrinsic state confined between the two barriers.?2?In the same structure,tunneling current peaks of the electrons and light holes are at different voltage due to their different effective mass and barrier height.The contribution from heavy holes in the tunneling current is much smaller than that from light holes and can be ignored.Comparing to the electron,the transmission peaks of holes are concentrated in the lower energy,and the energy spacing is smaller.It means less voltage is required for the tunneling current peak of hole.?3?At room temperature,since the electrons at the level nearby the Fermi-energy are easily to be exited,a tunneling peak with a large peak-to-valley radio is likely to occur when the resonant energy is close to the Fermi-energy.The situation of the light hole is the same,with a little smaller peak-to-valley radio than that of the electron.?4?Adjusting the Al component can increase the barrier height and the number of resonant levels,while causing the non-monotonic change of the current density peak as well as peak to valley ratio due to the non-monotonic change of the resonant levels.The tunneling current peak decreases and becomes sharper with a higher barrier.?5?Broadening the distance between the two barriers will increase the number of resonant tunneling levels with smaller spacing to result in more tunneling current peaks,while having little effect on the current peak and peak-valley ratio.Increasing the thickness of one barrier while keeping the total thickness of the two barriers and spacing between them unchanged,the highest transmission coefficient,as well as the largest peak to valley ratio,are found in the case with the same thickness of the barriers.?6?An increase in the thickness of both two barriers obviously reduces the transmission coefficients as well as the tunneling current density.Especially,the tunneling current density of the electrons decreases much more than that of the holes,while there is a situation that the current peak of the hole and current valley of the electron at the same voltage.This phenomenon could be used to block the electrons while making the holes tunnel through the barriers.According to the above rules,we can adjust the parameters of a two-barrier EBL to block the electron leakage while ensuring the holes to be effectively injected.