Investigations Into The Physical Mechanism For Micro-or Nano-nitride Based Light Emitting Diodes

With the increasing development of display technology,in order to achieve higher resolution and lower power consumption,the size of light-emitting diode(LED)for each pixel unit is also decreasing with the increase in resolution requirements,which has attracted intensive research interest.However,for the high-indium-content LEDs,the quantum confinement Stark effect(QCSE)caused by the polarized electric field in the active region severely restricts the improvement of luminous efficiency.With the increasing development of nanostructure fabrication processes,the core-shell nanorod LEDs possessing lots of advantages have become the alternative to overcome this problem.Because the core-shell nanorod structure has a natural three-dimensional morphology,the sidewall of the nanorod can conveniently provide a non-polar surface,thereby eliminating the usage of high-cost nonpolar GaN substrates.Although experiments have confirmed that core-shell nanorod LEDs can significantly improve luminous efficiency when compared with conventional structures.Another important member for display family is Micro-LED,which based on the welldeveloped GaN based epitaxial technogoly.Micro-LED have also obtained significant research effort.However,one of the bottlenecks limiting the further development of Micro LED based displaysis that when the size of the LED device is reduced to less than 100?m,the external quantum efficiency for Micro-LEDs decreases sharply with the device size decrease,and the surface recombination effect becomes more and more obvious.Although tremendous demonstrations have been made for these nano-and micro-LEDs,in-depth research on charge transport and device physics for the these types of LED is particularly not reported yet.Hence,in this thesis,we have revealed very important device physics,which will provide clear design ideas for further improvement of device performance.This paper systematically studies the carrier transport mechanism for core-shell nanorod green LEDs with non-polar active region.The results show that core-shell nanorod LEDs have better electron injection efficiency,which can effectively avoid quantum confined Stark effect,thereby increasing luminous efficiency and suppressing efficiency droop.In addition,further studies show that,in addition to the lateral current spreading effect,vertical current injection is closely related to the external quantum efficiency for core-shell nanorod LEDs.The vertical charge injection is more sensitive to the doping concentration and the thickness of the p-GaN layer than of the n-GaN layer.Moreover,the effect of the nanorod height on the vertical charge injection is most obvious.As the nanorod height increases,the overall hole concentration in the quantum well decreases,thereby effectively suppressing Auger recombination.Therefore,the external quantum efficiency increases first and then stabilizes with the increasing nanorod height.In order to reveal the physical mechanism of the size-dependent effect for blue MicroLEDs,we have established two physical models to compare and analyze the device performance.The size-dependent effect is owing to the higher surface to volume ratio for small Micro-LEDs,which makes the Shockley-Read-Hall non-radiative recombination at the sidewall defects not negligible.When modeling Micro-LEDs,the impact of the sidewall defects cannot be ignored.The modeled results agree well with meausured ones for those fabricated Micro-LEDs.Both theoretically and experimentally,the sidewall defects serve as the current leakage paths.The sidewall defects also severely affect the injection capability for electrons and holes,such that the electrons and holes are captured by sidewall defects for the SRH recombination.In particular,due to the large density and the capture cross-section for the hole traps,holes are more easily captured by the sidewall defects than electrons.Therefore,the hole injection efficiency is one of the key factors affecting the luminous efficiency for Micro-LEDs.