| In recent years, GaN-based Ⅲ-nitride semiconductors have attracted a lot of attention in research field. Since Ⅲ-nitride semiconductor material is a direct band gap semiconductor, the band gap can be consisitently tuned from0.7eV to6.2eV and cover the extensive spectrum range between near-infrared to ultraviolet, besides, Ⅲ-nitride material having good thermal stability and excellent electrical thermodynamic properties, they are widely used in lighting, displays, high-density storage, detectors and other fields. As the core of the light-emitting device, light-emitting efficiency of the InGaN/GaN quantum well is particularly important. Although in the past period of time, characteristics and application of InGaN/GaN quantum well structure have made great progress, but there are still many problems to be solved.This dissertation mainly focuses on improving the light-emitting efficiency of InGaN/GaN multiple quantum well (MQW), through the preparation of a variety of quantum well nano-structure. We have used photoluminescence, XRD, SEM and other techniques to analysis the different nanostructures for InGaN/GaN multiple quantum wells in the impact of the internal quantum efficiency (ηint) and light extraction efficiency (ηext), and analyzed carriers tunneling and recombination mechanisms in the MQW nanostructure. The main content and results are listed as follow:1. The MQW nanorod arrays were fabricated by inductively coupled plasma (ICP) top-down etching with low-damage etching technique using a self-assembled Ni nanoparticle mask on a plane InGaN/GaN MQW structure. At room temperature, an enhancement of3.1times in total integrated photoluminescence intensity is achieved from the MQW nanorod arrays, in comparison to that of the as-grown MQW structure. Although the light emitting area has decreased, the internal quantum efficiency and light extraction efficiency of the nanorod arrays have a significantly improved. Compared to the as-grown MQW structure, the ηint and next of the MQW nanorod arrays increased to1.75-fold and7-fold. The average distance between the dislocations is much larger than the diameter and spacing of the nanorod arrays in this work. So that the density of dislocations and other defects In Nanorod is significantly reduced. The transition levels are more concentrated at the band edge as compared to the wider transition in as-grown planar MQWs, which is in agreement with the blue-shift and narrower FWTH in the PL spectrum. At the sametime, the non-radiative recombination on the dislocations and other defects is greatly avoided, and the internal quantum efficiency significant improved. The light extraction efficiency of the nanorod array through a top surface rough process which breaks the total internal reflection and increase the emitting area can greatly increase the output emission intensity.2. The nanoporous InGaN/GaN MQWs are fabricated by inductively coupled plasma (ICP) top down etching using self-assembled Ni nanoparticle masks. The nanoporous InGaN/GaN MQWs is composed of coalesced nanoporous structure instead of normal nanocolumns or nanopillars, and the average pore size of the nanoporous structure is between400nm and900nm, while the grain boundaries are in the range of100-300nm. By the photoluminescence (PL) measurements, higher internal quantum efficiency (2.75-fold), higher thermal activation energy (107.44meV), and more stable optical properties with temperature for the nanoporous MQWs are obtained. Detailed X-ray RSM measurement and temperature dependent PL measurements have confirmed that a drastic reduction of dislocations and other defects in the nanoporous structure, which effectively reduces non-radiative recombination and band-tail in the active region. The thermal activation energy of nanoporous structure (107.44meV) is significantly higher than that of the as-grown sample, indicating that carriers are well confined and the nonradiative recombination caused by the dislocations and other defects has been reduced. The nanoporous MQW structure has great potential to improve the devices’performance.3. By adjusting the thick nickel film thermal annealing temperature and time, we fabricated different nano-Ni mask, and obtained nano pits, flower-like MQW nanostructure. By the PL measurements, the emission intensity of nano-pit structures has a great extent reduced. The main reason is the ICP etching brings a lot of surface damage and surface states, so that the non-radiative recombination greatly enhanced. Meanwhile, considering the nano-pit morphology, enhancement of light from the edge of the pit is very limited, and it is difficult to reduce the density of dislocations and other growth defects, so the total luminous intensity will be reduced. The ηint and next of the flower-like nanostructure has a small improvement, but compared to the nanorods and nanoporous structure of the luminescent properties of the samples greatly enhanced, nano-flower-like structure is not a strong improvement nanostructure for MQW.4. We have covered an antireflection layer (SiO2and Si3N4) with different thickness on InGaN/GaN MQW nanorod array. By the light-emitting properties of with/without antireflection layer nanorod array and planar MQW structure measured, the luminescent properties of the nanorod array with antireflection layer has been raised compared with the nanorod array, the maximum rate of the enhancement is40nm Si3N4(1.7-fold). Larger refractive index antireflection layer can be more increased the chance of photons emitting from multiple quantum wells. Meanwhile, the thickness of the antireflection layer is too thick to reduce the number of photons emitted from the quantum well, the luminous efficiency will drop. Further, we analyzed the effect of the etching depth on light-emitting efficiency, the results showed that:with etching depth increases, the ηint, of nanorod will be decreased a little, but the overall luminous intensity gradually improved. Fabrication the planar InGaN/GaN MQW into different nanostructures is a promising approach in order to improve MQW efficiency. Nanostructure morphology was found to huge affect the MQW luminous efficiency, closely related to the characteristic dimensions of nanostructures. Nanorods and nanoporous structure can effectively enhance the luminous efficiency of multiple quantum wells, nano flower-like structure can enhance the luminous efficiency a few, and nano-pit multi-quantum well structure will reduce the luminous efficiency. Meanwhile, use an appropriate thickness antireflection layer can be further improving luminous efficiency by increasing the chance of photons emitting from multiple quantum wells. |
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