Chiao-Yun Chang

Manipulation of nanoscale V-pits to optimize internal quantum efficiency of InGaN multiple quantum wells

We systematically investigated the influence of nanoscale V-pits on the internal quantum efficiency (IQE) of InGaN multiple quantum wells (MQWs) by adjusting the underlying superlattices (SLS). The analysis indicated that high barrier energy of sidewall MQWs on V-pits and long diffusion distance between the threading dislocation (TD) center and V-pit boundary were crucial to effectively passivate the non-radiative centers of TDs. For a larger V-pit, the thicker sidewall MQW on V-pit would decrease the barrier energy. On the contrary, a shorter distance between the TD center and V-pit boundary would be observed in a smaller V-pit, which could increase the carrier capturing capability of TDs. An optimized V-pit size of approximately 200–250 nm in our experiment could be concluded for MQWs with 15 pairs SLS, which exhibited an IQE value of 70%.

Depth-resolved confocal micro-Raman spectroscopy for characterizing GaN-based light emitting diode structures

In this work, we demonstrate that depth-resolved confocal micro-Raman spectroscopy can be used to characterize the active layer of GaN-based LEDs. By taking the depth compression effect due to refraction index mismatch into account, the axial profiles of Raman peak intensities from the GaN capping layer toward the sapphire substrate can correctly match the LED structural dimension and allow the identification of unique Raman feature originated from the 0.3 μm thick active layer of the studied LED. The strain variation in different sample depths can also be quantified by measuring the Raman shift of GaN A1(LO) and E2(high) phonon peaks. The capability of identifying the phonon structure of buried LED active layer and depth-resolving the strain distribution of LED structure makes this technique a potential optical and remote tool for in operando investigation of the electronic and structural properties of nitride-based LEDs.

Ultraviolet emission efficiency enhancement of a-plane AlGaN/GaN multiple-quantum-wells with increasing quantum well thickness

The defect-induced carrier localization in nonpolar a-plane (Al,Ga)N/GaN multiple quantum wells (MQWs) structures with different well thickness have been investigated. A strong variation of temperature-dependent photoluminescence peak energy was observed and attributed to the existence of the localized states. The degree of carrier localization in these defect-induced states was more prominent in the case of MQWs with the wider well width. In addition, the ultraviolet light emission efficiency revealed a 3-fold enhancement with increasing the well width from 1.6 nm to 7.3 nm, due to the strong carrier localization generated from the quantum-wire-like features formed by the intersection between basal stacking faults and quantum wells.

Inverted Octagonal Surface Defects in a-Plane AlGaN/GaN Multiple Quantum Wells

The structural properties of a-plane AlGaN/GaN multiple quantum wells grown on the r-plane sapphire substrate have been characterized. The pentagonal and inverted octagonal surface pits, consisting of several non-polar and semi-polar crystalline facets, are clearly observed and distinguished. The Al incorporation efficiency of the non-polar and semi-polar facets of these special inverted octagonal surface pits has been verified in the order of (1122) <(1012) <(1100) (1120) <(2021) by cathodoluminescence measurements at room temperature. The evolution of these inverted octagonal surface pits could be due to the results of interaction

Study of efficiency droop in InGaN/GaN light emitting diodes with V-shape pits

We invesstagated the relationship between the emission efficiency of InGaN/GaN multiple quantum wells (MQWs) and the V-shape pits (V-pits) forming along the threading dislocation (TD). The thinner InGaN/GaN MQWs on the side walls around V-pits would create higher local energy barriers, which can resist the carriers trapped into the non-radiative recombination centres within TDs. By inserting different InGaN/GaN superlattice (SLS) layers below the MQWs, sizes of V-pits could be properly controlled. It was found that the V-pit size on InGaN MQWs increased with increasing SLS layers, which could decrease energy barriers. On the contrary, the shorter distance between the TD center and V-pit boundary would increase the carrier capturing capability of TDs in smaller V-pits. By properly controlling the V-shape defect formation, the best internal quantum efficiency of about 70%f was found in the MQWs with underlying 15 periods SLS layers.

Characteristics of Polarized Light Emission in

In this paper, we investigated polarized light emission properties on a series of a-plane GaN/AlGaN multiple quantum wells grown on r-plane sapphire substrates with various well widths by using the polarization-dependent photoluminescence measurement. To clarify reasons of polarization properties in light emission, we applied the 6 × 6 k·p model to simulate the E-K dispersion relationship and wave functions to obtain optical transitions of different polarized emissions. According to our results, the sub-bands of |Y>;-like states are raised toward the top of the valence sub-band level with increasing the well width. And the optical matrix element of y-polarized light emission will dominate the optical transition, leading to the increase of degree of polarization in the thicker well.

a

Non-polar (a-plane) InGaN-GaN multiple quantum wells (MQWs) on the GaN nanorod epitaxially lateral overgrowth templates with different nanorod height have been fabricated. The average in-plane strain in the InGaN MQWs has been determined from 2.73 × 10-2 to 2.58 × 10-2 while the nanorod height in templates increases from 0 to 1.7 μm. The polarization ratio of the emission from InGaN MQWs varies from 85 % to 53 % along with the increase of the GaN nanorod height. The reduction of polarization ratio has been attributed to the partial strain relaxation within the epitaxial structures as a result of growth on the GaN nanorod templates and the micro-size air-voids observed in the nanorod templates.

-Plane GaN-Based Multiple Quantum Wells

We performed depth-resolved PL and Raman spectral mappings of a GaN-based LED structure grown on a patterned sapphire substrate (PSS). Our results showed that the Raman mapping in the PSS-GaN heterointerface and the PL mapping in the InxGa1−xN/GaN MQWs active layer are spatially correlated. Based on the 3D construction of E2(high) Raman peak intensity and frequency shift, V-shaped pits in the MQWs can be traced down to the dislocations originated in the cone tip area of PSS. Detail analysis of the PL peak distribution further revealed that the indium composition in the MQWs is related to the residual strain propagating from the PSS-GaN heterointerface toward the LED surface. Numerical simulation based on the indium composition distribution also led to a radiative recombination rate distribution that shows agreement with the experimental PL intensity distribution in the InxGa1−xN/GaN MQWs active layer.

Investigation of Emission Polarization and Strain in InGaN–GaN Multiple Quantum Wells on Nanorod Epitaxially Lateral Overgrowth Templates

The key issue for light emission strength of GaN-based LEDs is the high defect density and strain in MQWs causing the electric polarization fields. In this work, we construct 3D confocal microspectroscopy to clarify strain distribution and the relationship between photoluminescence (PL) intensity and pattern sapphire substrate (PSS). From 3D construction of E2high Raman and PL mapping, the dislocation in MQW can be traced to the cone tip of PSS and the difference in E2high Raman mapping between substrate and surface is also measured. The ability to measure strain change in 3D structure nondestructively can be applied to explore many structural problems of GaN-based optoelectronic devices.

Three dimensional characterization of GaN-based light emitting diode grown on patterned sapphire substrate by confocal Raman and photoluminescence spectromicroscopy

High quality factor nonpolar GaN two-dimensional photonic crystal (PC) nanocavities have been fabricated and investigated. A dominated resonant mode was observed at 388 nm with quality factor of about 4300 at 77K.