Studies On The Effect Of The Dielectric-constant-controlled Hole Injection Layer For ?-nitride Near-UV Light-emitting Diodes

As a promising light source,?-nitride-based light-emitting diodes(LEDs)have been found in tremendous applications.However,the internal quantum efficiency for GaN-based LED is partly affected by the hole injection efficiency.The hole injection efficiency is strongly subject to the p-type electron blocking layer(p-EBL),the valence band barrier height of which hinders the hole injection.Besides that,the hole concentration is lower than the electron concentration in the multiple quantum wells(MQWs)which is due to the low hole mobility and the low Mg ionization rate in the p-type ?-nitride layer at the room temperature.Moreover,the valence band offset between the quantum well and the quantum barrier also leads to the uneven hole distribution in the active region.Researchers have proposed some measures to optimize the LED structure for improving the hole injection efficiency,and the structural optimization lies on the p-EBL,the MQWs,the hole injection layer.This thesis seeks for novel design for LED structures to improve the hole injection and increase the IQE.The contents and the obtained results are described as follows.Firstly,we have proposed a dielectric-constant-controlled hole injection layer,i.e.,an n+-GaN/AlGaN/p+-GaN heterojunction,in which the dielectric constant for the AlGaN layer is smaller than that for the GaN layer.When compared to the standard LED device and the LED with the n+-GaN/p+-GaN tunnel junction,the adopted LED with the dielectric-constant-controlled tunnel junction has the improved hole injection and the enhanced IQE if the n+-GaN/AlGaN/p+-GaN heterojunction is properly designed.We have found that the improved hole injection is ascribed to the stronger electric field intensity within the tunnel junction.Furthermore,the proposed device can better spread the current when compared to the LED without a tunnel junction.As a result,the internal quantum efficiency can be improved for the LED possessing the dielectric-constant-controlled tunnel junction.Secondly,we also investigate the LEDs that have different n+-GaN/AlGaN/p+-GaN tunnel junctions with various relative dielectric constants for the AlGaN layer within the n+-GaN/AlGaN/p+-GaN tunnel junctions.We find that a smaller dielectric constant enables a stronger electric field intensity within the n+-GaN/AlGaN/p+-GaN tunnel region and a higher hole concentration level in the MQW region provided that the polarization induced charge density at the p+-GaN/AlGaN and the AlGaN/n+-GaN interfaces are unchangedBased on the fact that the relative dielectric constant is closely related to the Al composition,the relative dielectric constant for the AlGaN layer can be reduced by increasing the Al composition.We find that,although the relative dielectric constant is smaller,the polarization induced charges may reduce the net charge density and becomes the dominating factor leading to a decrease of the electric field intensity in the tunnel region Fortunately,the properly smaller electric field intensity in the tunnel region can properly increase the vertical resistance and better spread the current which in turn improves IQE.Lastly,we also point out that the thickness for the AlGaN layer is of essential importance in deciding the depletion region width for the n+-GaN/AlGaN/p+-GaN tunnel junction while a properly thicker AlGaN layer enables a better current spreading and an improved IQE.Therefore,although the LEDs that have the n+-GaN/AlGaN/p+-GaN tunnel junctions can improve the IQE and the optical power when compared with the reference LED device.The function of the n+-GaN/AlGaN/p+-GaN tunnel junction can be maximized only if it is fully optimized.We strongly believe that the findings in this work introduce the additional physical understanding to the n+-GaN/AlGaN/p+-GaN tunnel junction and are very useful for the ?-nitride community.