Guoyi Zhang

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.

Valence band offset of β-Ga2O3/wurtzite GaN heterostructure measured by X-ray photoelectron spectroscopy

A sample of the β-Ga2O3/wurtzite GaN heterostructure has been grown by dry thermal oxidation of GaN on a sapphire substrate. X-ray diffraction measurements show that the β-Ga2O3 layer was formed epitaxially on GaN. The valence band offset of the β-Ga2O3/wurtzite GaN heterostructure is measured by X-ray photoelectron spectroscopy. It is demonstrated that the valence band of the β-Ga2O3/GaN structure is 1.40 ± 0.08 eV.

Size-dependent efficiency and efficiency droop of blue InGaN micro-light emitting diodes

The mechanisms of size-dependent efficiency and efficiency droop of blue InGaN micro-pixel light emitting diodes (μLEDs) have been investigated experimentally and by simulation. Electrical characterisation confirms the improvement of current spreading for smaller μLEDs, which enables the achievement of the higher efficiency at high injection current densities. Owing to the higher ratio of sidewall perimeter to mesa area of smaller μLEDs, a lower efficiency was observed at a low injection current density, resulting from defect-related Shockley-Read-Hall non-radiative recombination. We demonstrate that such sidewall etch defects can be partially recovered by increased thermal annealing time, consequently improving the efficiency at low current densities.

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.

Polarization of edge emission from III-nitride light emitting diodes of emission wavelength from 395to455nm

Polarization-resolved edge-emitting electroluminescence of InGaN∕GaN multiple quantum well (MQW) light emitting diodes (LEDs) from 395to455nm was measured. Polarization ratio decreased from 3.2 of near-ultraviolet LEDs (395nm) to 1.9 of blue LEDs (455nm). Based on TE mode dominant emissions in InGaN∕GaN MQWs, compressive strain in well region favors TE mode, indium induced quantum-dot-like behavior leads to an increased TM component. As wavelength increased, indium enhanced quantum-dot-like behavior became obvious and E‖C electroluminescence signal increased thus lower polarization ratio. Electroluminescence spectrum shifts confirmed that quantum dotlike behaviors rather than strain might be dominant in modifying luminescence mode of InGaN∕GaN MQWs from near ultraviolet to blue.

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.

Enhancement of surface emission in deep ultraviolet AlGaN-based light emitting diodes with staggered quantum wells.

The optical polarization properties of staggered AlGaN-AlGaN/AlN quantum wells (QWs) are investigated using the theoretical model based on the k·p method. The numerical results show that the energy level order and coupling relation of the valence subband structure change in the staggered QWs and the trend is beneficial to TE polarized transition compared to that of conventional AlGaN/AlN QWs. As a result, the staggered QWs have much stronger TE-polarized emission than conventional AlGaN-based QWs, which can enhance the surface emission of deep ultraviolet (DUV) light-emitting diodes (LEDs). The polarization control by using staggered QWs can be applied in high efficiency DUV AlGaN-based LEDs.

Effects of buffer layer on formation of domain boundaries in epilayer during film growth of GaN by low-pressure metal–organic vapor phase epitaxy on sapphire substrates

Twinning was observed in a GaN buffer layer. The twin boundaries in the buffer layer can extend into the epitaxial layer to form domain boundaries during growth of the epilayer. The domain boundaries, which initiated from the twin boundaries in the buffer layer, are determined to be inversion domain boundaries.

Zero-field spin splitting in AlxGa1−xN/GaN heterostructures with various Al compositions

The zero-field spin splitting in AlxGa1−xN/GaN heterostructures with various Al compositions has been investigated at low temperatures and high magnetic fields. The zero-field spin-splitting energy and the spin-orbit coupling parameter are obtained by means of beating pattern Shubnikov–de Haas measurements. It is found that the spin-orbit coupling parameter can be tuned by the polarization-induced electric field. The AlxGa1−xN/GaN heterostructure is one of the promising materials for the spin-polarized field effect transistor.