Xiangning Kang

GaN-based light-emitting diodes with photonic crystals structures fabricated by porous anodic alumina template

In this paper, we propose and demonstrate a convenient and flexible approach for preparation large-area of photonic crystals (PhCs) structures on the GaN-based LED chip. The highly-ordered porous anodic alumina (AAO) with pitch of wavelength scale was adopted as a selective dry etching mask for PhCs-pattern transfer. The PhCs with different pore depths were simultaneously formed on the entire surfaces of GaN-based LED chip including ITO, GaN surrounding contacts and the sidewall of the mesa by one-step reactive ion etching (RIE). The light output power improvement of PhCs-based GaN LED was achieved as high as 94% compared to that of the conventional GaN-based LED.

Light transmission from the large-area highly ordered epoxy conical pillar arrays and application to GaN-based light emitting diodes

To improve the light transmission from the surface, we report a facile and cost-effective approach for the formation of wavelength-scale conical pillar arrays on a large surface area of epoxy resin. The highly ordered epoxy conical pillar arrays with a pitch of about 460 nm and height of about 800 nm have been successfully fabricated by the technique of shape-controlled anodization of Al foil followed by hot embossing. By replicating the tapered pore arrays onto a transparent semi-cylindrical epoxy structure, the incident angular resolved light transmission of the epoxy conical pillar arrays has been obtained. The integrated transmission of conical pillar arrays as high as 62.2% has been achieved which is confirmed to be 223% and 11.3% higher than that of planar epoxy and the cylindrical pillar arrays, respectively. It is reasonable to consider the wavelength-scale conical pillar array as a particular multilayer consisting of a series of two-dimensional photonic crystals with gradually increasing filling factor towards the surface. It can therefore be treated as a multilayer with continuously reducing refractive index towards the air. The conical and cylindrical pillar arrays of epoxy have been directly employed as the encapsulant of a GaN based flip-chip LED. Compared to the LED encapsulated by planar epoxy, the enhancement of light extraction from the LED covered with conical and cylindrical pillar arrays have been demonstrated to be 46.8% and 34.9%, respectively.

Properties of GaN-based light-emitting diode thin film chips fabricated by laser lift-off and transferred to Cu

Conventional GaN-based light-emitting diodes (LEDs) on sapphire substrates and laser lift-off (LLO) lateral current structure GaN LED thin film chips on Cu substrates have been fabricated and their properties are compared. It is found that after the LLO process, the reverse bias leakage current obviously increases and equivalent parallel resistance decreases two orders accordingly. From analyses of I–V curves the fact that tunneling behavior dominates under the reverse bias is confirmed, and the LLO process aids more defects to become tunneling active whereas the similar ideality factors and equivalent series resistances of LLO-LEDs on Cu and conventional LEDs on sapphire suggest that the LLO process does not much damage the electrical characteristics at a forward bias. The analyses of L–I curves reveal that the LLO process induces more nonradiation centers. However, the LLO-LEDs show superior performance under large injection current. The LLO-LEDs have 1.8 times greater maximum output power and 2.5 times higher current operation capabilities than the conventional LEDs within 300 mA for the good thermal conductivity of Cu.

Diffracted transmission effects of GaN and polymer two-dimensional square-lattice photonic crystals

The effects of the lattice pitch of GaN and polymer square-lattice photonic crystal (2PhC) on the diffracted transmission were studied by using rigorous coupled wave analysis (RCWA). Besides the first-order Bragg diffraction, higher-order diffractions from large pitches of PhC are also significant to the light extraction improvement. Three different diffraction mechanisms are illustrated through wave vector analysis. The enhancement factors of integrated transmission are obtained from a wide range of pitches with micro-scale for both GaN and polymer 2PhC. The experimental angular-resolved transmission on a transparent polymer sample of 2 microm pitch 2PhC is comparable to the simulation.

(In,Ga)N∕GaN microcavities with double dielectric mirrors fabricated by selective removal of an (Al,In)N sacrificial layer

Comparable microcavities with 3λ∕2 (∼240nm) active regions containing distributed (In,Ga)N quantum wells, grown on GaN substrates and bounded by two dielectric mirrors, have been fabricated by two different routes: one using laser lift-off to process structures grown on GaN-on-sapphire templates and the second using freestanding GaN substrates, which are initially processed by mechanical thinning. Both exploit the properties of an Al0.83In0.17N layer, lattice matched to the GaN substrate and spacer layers. In both cases cavity quality factors >400 are demonstrated by measurements of the cavity-filtered room-temperature excitonic emission near 410nm.

Light extraction improvement of GaN-based light-emitting diodes using patterned undoped GaN bottom reflection gratings

The Gallium Nitride (GaN) Light-Emitting-Diode (LED) bottom refection grating simulation and results are presented. A microstructure GaN bottom grating, either conical holes or cylindrical holes, was calculated and compared with the non-grating (flat) case. A time monitor was also placed just above the top of the LED to measure both time and power output from the top of the LED. Many different scenarios were simulated by sweeping three parameters that affected the structure of the micro-structure grating: unit cell period (Α) from 1 to 6 microns, unit cell width (w) from 1 to 6 microns, and unit cell grating height (d) from 50 to 200nm. The simulation results show that the cylindrical grating case has a 98% light extraction improvement, and the conical grating case has a 109% light extraction improvement compared to the flat plate case.

Improvement of light extraction from micro-pattern encapsulated GaN-based LED by imprinting

We have demonstrated an improvement of light extraction from GaN based flip-chip LEDs by patterning encapsulant. Two dimensional (2D) micron-scale patterns of encapsulant were realized by using imprint technique of thermosetting polymer. This approach has several advantages such as technical simplification, low cost and freedom of material choice. In this work, we fabricated 2D micron-scale patterns with the triangular or sinusoidal profiles on the polymer encapsulated GaN-based flip-chip LEDs. The enhancement factors of light extraction of GaN LEDs with the patterned encapsulant comparing to the flat encapsulated LEDs are about 32% and 47% corresponding to the triangular and sinusoidal profiles, respectively. To evaluate the concept of a diffraction grating in enhancement of light extraction, we performed a simulation of diffraction based on simplified one-dimensional (1D) rigorous coupled wave analysis (RCWA). The calculation reveals that the grating of sinusoidal profile has greater transmittance than that of triangular profile which is in the same trend with the experimental results. These results provide a guideline for improvement of the LED light extraction.

Angular distribution of polarized light and its effect on light extraction efficiency in AlGaN deep-ultraviolet light-emitting diodes

Angular distribution of polarized light and its effect on light extraction efficiency (LEE) in AlGaN deep-ultraviolet (DUV) light-emitting diodes (LEDs) are investigated in this paper. A united picture is presented to describe polarized lights emission and propagation processes. It is found that the electron-hole recombinations in AlGaN multiple quantum wells produce three kinds of angularly distributed polarized emissions and propagation process can change their intensity distributions. By investigation the change of angular distributions in 277nm and 215nm LEDs, this work reveals that LEE can be significantly enhanced by modulating the angular distributions of polarized light of DUV LEDs.

Nucleation mechanism of GaN growth on wet etched pattern sapphire substrates

Patterned sapphire substrates (PSS) are generally applied and have been proved to be effective in the increasing light extraction efficiency and crystal quality of GaN-based LEDs. The time evolution growth of GaN on triangle platform shaped PSS with an n-plane inclined surface and triangle cone shaped PSS with a part r-plane surface has been performed to research the nucleation and 3D growth mechanisms of GaN grown on the PSS. After the low temperature GaN growth process and high temperature ramping process, the small islands of GaN rotate and gather on the n-plane surface near the ridges of the triangle platform shaped PSS. Then the next 850 seconds high temperature growth process of GaN shows a 3D behavior. From the observations of SEM, it is believed that , and are the three preferred growth directions for GaN and the (1−10k) and (0001) planes are the two preferable planes during the GaN 3D growth.

Top Transmission Grating GaN LED Simulations for Light Extraction Improvement

We study the top transmission gratings improvement on GaN LED light extraction efficiency. We use the finite difference time domain (FDTD) method, a computational electromagnetic solution to Maxwells equations, to measure light extraction efficiency improvements of the various grating structures. Also, since FDTD can freely define materials for any layer or shape, we choose three particular materials to represent our transmission grating: 1) non-lossy p-GaN, 2) lossy indium tin oxide (ITO), and 3) non-lossy ITO (α=0). We define a regular spacing between unit cells in a crystal lattice arrangement by employing the following three parameters: grating cell period (Α), grating cell height (d), and grating cell width (w). The conical grating model and the cylindrical grating model are studied. We also presented in the paper directly comparison with reflection grating results. Both studies show that the top grating has better performance, improving light extraction efficiency by 165%, compared to that of the bottom reflection grating (112%), and top-bottom grating (42%). We also find that when grating cells closely pack together, a transmission grating maximizes light extraction efficiency. This points our research towards a more closely packed structure, such as a 3-fold symmetric photonic crystal structure with triangular symmetry and also smaller feature sizes in the nano-scale, such as the wavelength of light at 460 nm, half-wavelengths, quarter wavelengths, etc.