Study Of Improving Light Emitting Efficiency Of GaN-based LED By Micro- And Nanostructures

The world’s energy crisis and environmental pollution become serious and worsening. As a "green" light source, light emitting diodes (LEDs) are attractive in solid state lighting, backlights and display due to the advantages of high reliability, long lifetime and energy saving. LED will lead the future trend of development in the lighting field. It is expected to become the fourth generation artificial light source after incandescent lamp, fluorescent lamp, high intensity discharge lamp. Solid-state lighting can potentially reduce the electricity consumption by 25%. Many governments have begun to attach importance to the research and development of energy-saving solid state lighting based on LEDs. There are two approaches for white light sources based on LED and LED-plus-phosphor. High efficiency blue, green and red LEDs across the visible spectrum are needed in two approaches for white light sources.The development of material growth technology, especially the MOCVD epitaxy, makes it possilbe to fabiraicate III-V compound semiconductor with high quality. As a representative of III-V compound semiconductor materials, GaN has been the preferred materials for semiconductor optoelectronic devices, due to its unique chemical and physical properties such as high melting point, chemical inertness, high thermal conductivity, very high hardness and high breakdown field strength. GaN and related materials (AlGaN, InGaN) have direct bandgap across the entire visible spectrum, covering a wavelength range from the infrared (InN,-0.7 eV) to the ultraviolet (A1N,-6.2 eV) by changing the chemical composition percentage of In and Al, and was used in energy-saving solid-state lighting LEDs. InGaN-based quantum well structures have been widely used for active layers in LEDs due to the tenability of the emitted light from UV through visible spectral range.The photoluminescence (PL) efficiency of LEDs is generally determined as the product of the light extraction efficiency (LEE) and the internal quantum efficiency (IQE). Major challenges exist however which need to be overcome to further increase the adoption of III-V nitride-based LEDs with an active layer of InGaN MQW. However, due to most commercial LEDs being fabricated based on a conventional structure with a planar light-extracting surface, further development of efficient InGaN-based LEDs has being obstructed by two main drawbacks. First, the LEE of a conventional planar GaN-based LED is limited by an internal reflection effect due to the large difference in the refractive index between the GaN (n= 2.5) and air (n= 1). Second, it is well-known that there is a strong strain-induced piezoelectric polarization field in the conventional c-plane InGaN/GaN multi-quantum-well (MQW) active region due to the lattice mismatch between GaN and InN. The piezoelectric polarization field results in a quantum-confined Stark effect (QCSE), which can cause a red-shift in emission peak and a decrease in wave function overlap between the electron and the hole in a QW. Total internal reflection, materials defects, polarization charges, and have thus far restricted the development of high-efficiency InGaN-based LED, in particular InGaN LEDs in the green/yellow wavelength range, which are critical in achieving highly efficient LED luminaires with an excellent color-rendering index. Further improvement in the GaN-based LED luminous efficiency are important practical significance for energy conservation and the development of lighting industry.In this paper, we focus on how to improve the LED luminous efficiency. InGaN/GaN MQW LEDs were grown on a c-plane sapphire substrate using metal organic chemical-vapor deposition (MOCVD). The emission mechanism of green and blue light-emitting InGaN/GaN multiple quantum well structure with a strong phase separation in InGaN epilayer are investigated. In order to overcome the drawbacks of luminous efficiency, we study the optical behavior of nanoporous GaN, nanopillar LED fabricated using FIB, surface-textured LED fabricated using ICP by measuring the photoluminescence spectrum of the sample. The main conclusions of the dissertation are listed below.(1) Nanoporous GaN fabricated by electrochemical wet etching.An nanoporous GaN film was prepared by electrochemical etching on an initial GaN epilayer grown on a sapphire substrate using MOCVD. The formation of nanopores in the NP GaN film has been confirmed by SEM. PL measurements showed that the NP GaN had a stronger UV intensity compared to the as-grown GaN. This improved UV intensity was mainly attributed to the enhancement of the photon extraction efficiency due to more photons scattering from the sidewalls of the NP GaN. At the same time, it was believed that fewer impurities or defects in the NP GaN should also result in an enhancement of the UV intensity, since the electrochemical etching began preferentially around the dislocations and surface-related traps. The presence of fewer impurities or defects in the NP GaN could be confirmed further by its weaker YL intensity than that of the as-grown GaN. Furthermore, the almost identical line-shape and line-width of the PL peaks for these two samples indicated that the etching process did not result in the incorporation of as many impurities or defects. The red-shift of the UV peak energy of the NP GaN relative to the as-grown GaN was ascribed to the relaxation of compressive stress in the NP GaN. Such strain-relaxed NP GaN could be used as a buffer, or intermediate layer, for the overgrowth of GaN layers with low stress and defect densities.(2) The light-emitting mechanism of the phase-separated InGaN/GaN MQW.The PL spectra of an MOCVD-grown InGaN/GaN MQW with clear phase separation are investigated over an excitation power range from 0.001 mW to 50 mW and a temperature range from 6 K to 300 K. Two InGaN-related PL peaks located at around 2.4 eV and 2.7 eV in the full PL spectrum are assigned to the high-In-content QDs and the InGaN matrix, respectively, due to the strong phase separation supported by HRTEM.With an increase in the excitation power in a low excitation power range at 6 K, there is an increased peak energy and an unchanged FWHM for both emission peaks PD and PM, which are attributed to the combined action of the Coulomb screening effect of QCSE and the state-filling effect of localized states. On the other hand, at 300 K and in the low excitation power range, the PM peak energy shows a decrease, which is accompanied with the broadening of FWHM, while the PD peak energy shows a decrease accompanied with the narrowing of FWHM. The former is attributed to the non-radiative recombination process of the carriers in the InGaN matrix, and the latter is due to the fact that increasing numbers of carriers are scattered away from the shallower localized QDs and preferentially transferred into deeper localized QDs by tunnelling, resulting in the weight of carrier distribution among the QDs tending towards lower QD energies. In addition, in the low excitation power range, the unchanged PL efficiencies for both emission peaks at 6 K, are attributed to the fact that the radiative recombination dominates the emission process of MQWs, including the QD regions and the InGaN matrix region. In contrast, the increased PL efficiencies for emission peaks PM and PD at 300 K are due to the recombination mechanism conversion from non-radiative recombination to radiative recombination and to the transfer of carriers among the QDs, respectively. This is in good consistence with the excitation power dependences of the PL peak energy and line-width.(3) The light-emitting mechanism of the nanopillar LED.Planar InGaN/GaN MQWs LED, with phase separation in the MQWs confirmed by HRTEM, was grown by MOCVD. Two high-density, top-down, nanopillar InGaN/GaN MQW LEDs with different etching depths (100 and 700 nm) were fabricated by subsequent focused ion beam milling. In comparison with the planar sample, the shorter (100 nm) nanopillar array structure (fabricated only on the p-GaN layer) shows no obvious PL peak shift, but undergoes a two-fold enhancement of PL intensity due to the increased light extraction efficiency arising as a result of its surface nanotexture. By contrast, the longer (700 nm) nanopillar array structure (penetrating through the InGaN/GaN MQW active region) show an obvious PL peak blue-shift, and an approximately four-fold enhancement in PL intensity compared with the planar sample. The former is attributed to strain relaxation of the MQWs embedded in the nanopillars due to the high surface-to-volume ratio of the MQW region; the latter is ascribed to the increase of IQE caused by the aforementioned strain relaxation, and to the improvement of LEE due to the larger vertical sidewall surface area and the stronger vertical light-guiding effect. Furthermore, the green emission from In-rich QDs shows a smaller peak blue-shift than the blue emission from the InGaN matrix after fabrication of the nanopillar array structure. This is attributed to the QCSE in the strong localized QDs of small size being smaller than in the InGaN matrix for the planar sample. Additionally, the longer nanopillar array structure shows a smaller peak blue-shift relative to the planar one at 300 K than that at 4 K. This is attributed to the fact that, with increasing temperature, the thermal-mismatch strain in the MQWs gradually decreases. The present work illustrates that FIB provides a technique for fabricating high optical quality InGaN-based nanopillar LED arrays. The nanopillar profiles, such as etching depth, can be further optimized to obtain a higher PL efficiency.(4) The influence of surface roughness on the luminescence of LED.InGaN/GaN MQW LED were grown on a c-plane sapphire substrate using MOCVD. A top nanoroughened p-GaN surface are fabricated using inductively coupled plasma (ICP) dry etching. Au nanoparticles obtained by thermal annealing were used as as etch masks for ICP etching. The etching depth in the p-GaN was about 50 nm confirmed by AFM measurement results. Compared with the as-grown sample, PL spectra of roughened LEDs display a slight blueshift due to partial strain release in quantum wells by surface roughness. PL measurements at different excitation power showed an S-shaped (decrease-increase-decrease) temperature dependence of the peak energy for InGaN-related emisson with increasing temperature. The parameter σ indicating the degree of localization effect was obtained by fitting the "S-shape". The decreasing trend of the parameter σ as a function of the excitation power implies the reduced localized effect. The parameter σ of the surface roughened sample were larger than those of the as-grown sample. The results provided experimental basis for the utilization of surface roughness on how to increase the luminous efficiency.