Tongjun Yu

Effects of symmetry of GaN-based two-dimensional photonic crystal with quasicrystal lattices on enhancement of surface light extraction

We demonstrate enhancement of surface light extraction from two-dimensional photonic crystals (2D-PCs) on the electrical injected GaN-based light emitters. The effects of symmetry of PCs on light extraction were studied. 2.5 times enhancement of surface emission was obtained from the PCs with an octagonal symmetric quasicrystal lattice (8PQC) compared to that from a nonpatterned region. Additionally the surface emission from PCs with dodecagonal symmetric quasicrystal lattice (12PQC) exhibited about 1.7 and 1.4 times higher emission than regular PCs with triangular lattice and 8PQC, respectively. Consequently, the 12PQC provides a favorable consideration of 2D-PC in light extraction from light emitting diode.

High conductive gate leakage current channels induced by In segregation around screw- and mixed-type threading dislocations in lattice-matched InxAl1−xN/GaN heterostructures

A correlation between microstructures and high gate leakage current density of Schottky contacts on lattice-matched InxAl1−xN/GaN heterostructures has been investigated by means of current-voltage measurements, conductive atom force microscopy (C-AFM), and transmission electron microscopy (TEM) investigations. It is shown that the reverse-bias gate leakage current density of Ni/Au Schottky contacts on InxAl1−xN/GaN heterostructures is more than two orders of magnitude larger than that on AlxGa1−xN/GaN ones. C-AFM and TEM observations indicate that screw- and mixed-type threading dislocations (S/M-TDs) are efficient leakage current channels in InxAl1−xN barrier and In segregation is formed around S/M-TDs. It is believed that In segregation around S/M-TDs reduces local Schottky barrier height to form conductive channels and leads to high leakage current density of Schottky contacts on InxAl1−xN/GaN heterostructures.

Performance improvement of GaN-based LEDs with step stage InGaN/GaN strain relief layers in GaN-based blue LEDs

The performance of nitride-based LEDs was improved by inserting dual stage and step stage InGaN/GaN strain relief layer (SRL) between the active layer and n-GaN template. The influences of step stage InGaN/GaN SRL on the structure, electrical and optical characteristics of GaN-based LEDs were investigated. The analysis of strain effect on recombination rate based k·p method indicated 12.5% reduction of strain in InGaN/GaN MQWs by inserting SRL with step stage InGaN/GaN structures. The surface morphology was improved and a smaller blue shift in the electroluminescence (EL) spectral with increasing injection current was observed for LEDs with step stage SRL compared with conventional LEDs. The output power of LEDs operating at 20 mA was about 15.3 mW, increased by more than 108% by using step stage InGaN/GaN SRL, which shows great potential of such InGaN/GaN SRL in modulating InGaN/GaN MQWs optical properties based on its strain relief function.

Effect of substrate temperature on the structural and electrical properties of CIGS films based on the one-stage co-evaporation process

Cu(In, Ga)Se2 thin films were deposited on Mo/soda-lime glass substrates by the one-stage co-evaporation process at the substrate temperatures (Tsub) from 350 °C to 550 °C. The structural and electrical properties of CIGS films have been studied by x-ray diffraction (XRD), scanning electron microscopy (SEM) and Hall effect measurement. The experimental results indicate that a temperature of 450 °C is critical for CIGS films grown by the one-stage process. The (In, Ga)2Se3 phase with high resistivity is found below this temperature. The higher Tsub will lead to the formation of single-phase CIGS films with larger grain size and better electrical properties. A higher carrier concentration and lower resistivity of CIGS films are ascribed to sodium incorporation diffused from the glass substrate and the disappearance of the (In, Ga)2Se3 phase in CIGS films. Additionally, the performance of the CIGS solar cells improves significantly with the increase of Tsub. It can be attributed to the reduction of the grain-boundary recombination and the sufficient reaction between the additional (In, Ga)2Se3 phase and the CuxSey binary phase at Tsub above 500 °C.

Study on the formation of dodecagonal pyramid on nitrogen polar GaN surface etched by hot H3PO4

Hot phosphor acid (H3PO4) etching is presented to form a roughened surface with dodecagonal pyramids on laser lift-off N face GaN grown by metalorganic chemical vapor deposition. A detailed analysis of time evolution of surface morphology is described as a function of etching temperature. The activation energy of the H3PO4 etching process is 1.25 eV, indicating the process is reaction-limited scheme. And it is found that the oblique angle between the facets and the base plane increases as the temperature increases. Thermodynamics and kinetics related factors of the formation mechanism of the dodecagonal pyramid are also discussed. The light output power of a vertical injection light-emitting-diode (LED) with proper roughened surface shows about 2.5 fold increase compared with that of LED without roughened surface.

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.

The origin and evolution of V-defects in InxAl1−xN epilayers grown by metalorganic chemical vapor deposition

Near-lattice-matched and highly compressive-strained InxAl1−xN epilayers were grown on GaN templates by metalorganic chemical vapor deposition. The V-defects associated with screw-component threading dislocations (TDs) were found in all the InxAl1−xN layers. Their origin and evolution were investigated through near-lattice-matched In0.173Al0.827N layers with different thicknesses. Furthermore, small V-defects not associated with TDs were also found in InxAl1−xN layers with high In composition (x=0.231). Stacking mismatch boundaries induced by lattice relaxation in InxAl1−xN epilayers under large strain is believed to be another mechanism forming V-defects.

Analysis of mass transport mechanism in InGaN epitaxy on ridge shaped selective area growth GaN by metal organic chemical vapor deposition

In this work, the evolution of the InGaN layer growth on the ridge shaped GaN was studied. A mass transport model was presented to simulate the epitaxy process of the InGaN layer. The model consisted of two consecutive components, gas-phase diffusion process and surface diffusion process. The mean lifetime of adatoms on epitaxial surface was associated with their reaction rate in this model. An InGaN layer on ridge shaped GaN, including (0002) and {112¯2} facets, was grown by metal organic chemical vapor deposition to confirm the mass transport model. Gradient indium content distribution and inhomogeneous thickness of the InGaN layer were observed. Simulation of the InGaN layer growth process was performed by finite difference method with the mass transport model. By analyzing the results from calculations and experiments, the origins of the InGaN layer characteristics were attributed to the two diffusion components in the growth process. Surface diffusion resulted in the inhomogeneous thickness and gas-p...

Study of the structural damage in the "0001… GaN epilayer processed by laser lift-off techniques

The structural influences of the laser lift-off (LLO) techniques on the created (0001) GaN surface region are characterized by cross-sectional high-resolution transmission electron microscopy and fitted using the model of stress waves caused by a longitudinal impact at the end of a cylindrical bar extending to infinity. The authors study reveals that, in addition to the superficial damage caused by laser absorption, the stress saltation in GaN crystal where the shock waves come into being induces deformation of the lattices and generates a cluster of half loops above the LLO interface. After that, the lattice deformation will be induced every time the partial dissipation of the steady-state shock waves takes place until the shock wave is dissipated to elastic mode.

Resonant absorption and scattering suppression of localized surface plasmons in Ag particles on green LED

The metallic-structure dependent localized surface plasmons (LSPs) coupling behaviors with InGaN QWs in a green LED epitaxial wafer are investigated by optical transmission, scanning electron microscopy (SEM) and photoluminescence (PL) measurements. Ag nanoparticles (NPs) are formed by thermal annealing Ag layer on the green LED wafer. SEM images show that for higher annealing temperature and/or thicker deposited Ag layer, larger Ag NPs can be produced, leading to the redshift of absorption peaks in the transmission spectra. Time resolved PL (TRPL) measurements indicate when LSP-MQW coupling occurs, PL decay rate is greatly enhanced especially at the resonant wavelength 560 nm. However, the PL intensity is suppressed by 3.5 folds compared to the bare LED. The resonant absorption and PL suppression are simulated by three dimension finite-difference-time-domain (FDTD), which suggests that Ag particle with smaller size and lower height lead to the larger dissipation of LSP.