Highly Ordered Nitride Semiconductor Nanostructures And Hybrid White LED Devices

Solid-state lighting is a new technology to utilize semiconductor materials to directly transfer electricity to light in an efficient way.?-nitride semiconductor materials,which own the advantages of wide and extensive direct-band gap from 0.7eV to 6.2eV and excellent physical and chemical stability,have been the most vital luminescent semiconductor materials in the solid-state lighting.In recent years,IIInitride based optoelectronic devices have a fantastic development and the research about material and device characteristics of nitride semiconductors starts to turn to the low-dimensional systems and functional applications.Especially as the nanofabrication techniques have made a breakthrough recently,the traditional nitride film based optoelectronic technologies are beginning to expand to the nitride low-dimensional nanostructures,which will bring lots of new physical phenomenon and laws and may be potential to solve the problems of traditional devices.As a result,the research about the optoelectronic characteristics of nitride nanostructures,regarded as the basic for achieving the nitride nanostructure based optoelectronic devices,have been one of hot topics on nitride optoelectronics.Besides,GaN based white light-emitting diodes(LEDs)always dominate the advanced position in the field of nitride optoelectronic devices,especially for nitride nanostructure based white emitters,which exhibit interesting physical properties and great application potentials.This dissertation is directly guided by the needs of practical applications and takes the innovation in the field of both technology and physical theory as the main purpose.It mainly discusses the issue of optical properties of highly ordered nitride nanostructures and its white LEDs in a systematic way.The complicated luminescent process of highly ordered nitride nanostructure arrays have been investigated in details.The main conclusion are listed as follows:1.Comparing different kinds of nanoimprint lithography(NIL),we have developed a soft UV-curing NIL and a post-growth pattern transferring technique,achieving GaN nanorod and nano-grating arrays in a large scale with low defect densities.Besides,utilizing novel dry and wet etching techniques,we can control the lateral size of nanostructures and the morphologies of sidewalls,overcoming the limitation of the single-size structure fabricated by NIL.The luminescent properties of both GaN NR and NG arrays have been carefully analyzed.It is observed that the photoluminescence(PL)integral intensity of GaN NR arrays exhibits a great enhancement with a factor of 2.5 compared to GaN films.It also figures out the physical mechanisms of this enhancement.Besides,the strain conditions of both GaN NR and NG arrays are studied,finding out the proportionality factor for the strain-induced PL shift is-27meV·GPa-1,in good accordance with the results reported by other literatures,which demonstrates the PL peak shifts of GaN nanostructures are mainly due to the difference of strain conditions.2.The fabrication processes of both soft UV-curing nanoimprint lithography and top-down etching techniques have been further improved and highly ordered InGaN/GaN multiple quantum well(MQW)NR arrays have been successfully fabricated on a wafer scale.The optical characteristics of a series of InGaN/GaN MQW NR arrays with In content of 0.17 to 0.28 have been extensively studied,demonstrating a clear blue shift of peak positions for all InGaN/GaN MQW NR samples compared to their corresponding as-grown samples,which is mainly due to the reduction of quantum confined Stark effect(QCSE).Besides,the PL integral intensity of InGaN/GaN MQW NR arrays show a remarkable enhancement with a factor up to two orders of magnitude compared with their corresponding as-grown samples at room temperature.The strain conditions and excitonic recombination dynamics of NR arrays have been analyzed carefully,together with their electromagnetic distribution and light extraction process.It can be concluded that the luminescent enhancement is mainly due to internal excitonic recombination and external optical modulation.The uniform electroluminescence(EL)of InGaN/GaN MQW NR arrays has been successfully achieved as well,with a slight enhancement of light output power per unit area compared to that of the planar devices at the low injection current density.3.The polarized properties of c-plane InGaN/GaN MQW elliptic NR and NG arrays have been discovered,with the polarization ratio of as high as 71%.Based on the k-p perturbation theory,the band structure and transition energies under different in-plane asymmetric strain have been simulated,together with the corresponding optical momentum matrix elements.It is demonstrated that the polarized properties of these nanostructure arrays are mainly attributed to the in-plane strain asymmetry,which results from the broken nanostructure symmetry.4.Violet and blue InGaN/GaN MQW nanohole(NH)arrays based LEDs have been successfully fabricated by our developed soft UV-curing nanoimprint lithography and device processes.High color-conversion efficiency(CCE,-69%)and effective quantum yield(QY,-94%)have been achieved by utilization of ?-? nanocrystals as down-conversion materials.The new mechanism,named non-radiative resonant energy transfer(NRET)between InGaN/GaN MQWs and nanocrystals,with efficiency as high as 80%,has been demonstrated carefully.Based on the modified formulation,the modified Forster's radiuses and coupling distances of NRET for all hybrid structures have been obtained,further illustrating the physical schematics of NRET.Besides,a model on the excitonic recombination process has been proposed,explaining the advantage of NRET that avoiding energy loss associated with the intermediate light emission and conversion steps to improve the CCE and effective QY.It is also discovered that the efficiency droop of NH-LEDs in the hybrid structures was validly suppressed,resulting from the reduction of carrier concentrations in InGaN/GaN multiple quantum wells(MQWs)via NRET.Finally,we have set up the optimized model of these hybrid LED devices,and a series of white LEDs with optimized ternary complementary color scheme have been realized with a high color rendering index up to 82,covering different correlated color temperature corresponding to warm white,natural white,and cold white.