Chuanyu Jia

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.

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.

Polarization modification in InGaN/GaN multiple quantum wells by symmetrical thin low temperature-GaN layers

Light emitting diodes (LEDs) using InGaN/GaN quantum wells (QWs) with thin low temperature GaN (LT-GaN) layers bounding each InGaN layer are grown by metal-organic vapor phase epitaxy. The light output power of such LEDs increases by a factor of 2 at a drive current density of 35 A/cm2 compared to that from reference LEDs without the LT-GaN. The blueshift in the emission wavelength is 5.2 nm when the current density increases from 3 to 50 A/cm2, which is much smaller than the shift 8.1 nm from reference LEDs. Moreover, the efficiency droop at high current injection is also reduced by 28%, and current density at which peak efficiency is observed increases from 1 to 2 A/cm2. High resolution transmission electron microscopy of the QWs bounded with LT-GaN shows higher quality and less strain compared to the reference samples. The better performance of LEDs incorporating the LT-GaN layers is attributed to suppressed polarization from piezoelectric fields.

High quality GaN epilayers grown on carbon nanotube patterned sapphire substrate by metal–organic vapor phase epitaxy

Aligned carbon nanotube films were utilized to pattern a c-plane sapphire substrate. High quality GaN was obtained by epitaxy growth on carbon nanotube patterned sapphire substrates (CPSS) by a metal–organic vapor phase epitaxy system. Results of X-ray diffraction and transmission electron microscopy (TEM) confirmed the improvement of crystalline quality of GaN by introduction of the carbon nanotube film. A large number of voids with sizes of tens to hundreds of nanometers were observed on carbon nanotubes by scanning electron microscopy (SEM) and TEM. The dislocation annihilation around voids was believed to be helpful for the reduction of dislocation density. Performance of light-emitting diodes manufactured on CPSS was also clearly improved, as 15% enhancement of luminous intensity at 20 mA, and 50% delay of saturation current from 80 mA to 120 mA.

Modulating lateral strain in GaN-based epitaxial layers by patterning sapphire substrates with aligned carbon nanotube films

AbstractA novel carbon nanotube-patterned sapphire substrate (CPSS) has been utilized for the growth of GaN material and fabrication of a InGaN/GaN light emitting diode (LED) by metal-organic vapor phase epitaxy. Different lateral strain distributions and stress reductions were observed in a GaN thin film on CPSS compared with those on a conventional sapphire substrate. Nanoheteroepitaxy induced by small size nucleation islands of about 50 nm is ascribed to this significant strain modulation. The crystalline quality of the GaN thin film was also improved, as illustrated by X-ray diffraction. Performances of 1 mm × 1 mm LEDs on CPSS were also enhanced, with an operational power increase of 37.5% and higher saturation current.

Valence subband coupling effect on polarization of spontaneous emissions from Al-rich AlGaN/AlN Quantum Wells

The optical polarization properties of Al-rich AlGaN/AlN quantum wells (QWs) were investigated using the theoretical model based on the k·p method. Numerical results show that there is valence subband coupling which can influence the peak emission wavelength and emission intensity for TE and TM polarization components from Al-rich AlGaN/AlN QWs. Especially the valence subband coupling could be strong enough when CH1 is close to HH1 and LH1 subbands to modulate the critical Al content switching dominant emissions from TE to TM polarization. It is believed that the valence subband coupling may give important influence on polarization properties of spontaneous emissions and should be considered in designing high efficiency AlGaN-based ultraviolet (UV) LEDs.

Characteristics of GaN-based light emitting diodes with different thicknesses of buffer layer grown by HVPE and MOCVD

GaN-based light emitting diodes (LEDs) have been fabricated on sapphire substrates with different thicknesses of GaN buffer layer grown by a combination of hydride vapor phase epitaxy and metalorganic chemical vapor deposition. We analyzed the LED efficiency and modulation characteristics with buffer thicknesses of 12 µm and 30 µm. With the buffer thickness increase, cathodoluminescence hyperspectral imaging shows that the dislocation density in the buffer layer decreases from ∼1.3X10 8 cm-2 to∼1.0 X 10 8 cm-2, and Raman spectra suggest that the compressive stress in the quantum wells is partly relaxed, which leads to a large blue shift in the peak emission wavelength of the photoluminescence and electroluminescent spectra. The combined effects of the low dislocation density and stress relaxation lead to improvements in the efficiency of LEDs with the 30 µm GaN buffer, but the electrical-to-optical modulation bandwidth is higher for the LEDs with the 12 µm GaN buffer. A rate equation analysis suggests that defect-related nonradiative recombination can help increase the modulation bandwidth but reduce the LED efficiency at low currents, suggesting that a compromise should be made in the choice of defect density.

Spontaneous luminescence polarizations of wurtzite InGaN∕GaN quantum wells

In this paper, we analyze the polarization selection rules of wurtzite InGaN∕GaN quantum wells using the k⋅p perturbation method. It was found that the symmetry properties of the valence subbands’ wavefunctions at kt≠0 are quite different with those at kt=0. These symmetry properties of valence subbands’ wavefunctions influence the momentum matrix element for TE and TM modes, leading to a different polarization selection rules from the ones at kt=0 and the absence of spectra peak shift between TE and TM modes. It is suggested that the polarization selection rule at kt≠0 should be considered in the main transition process for wurtzite III-V semiconductors.

Performance Improvement of VLC System Using GaN-Based LEDs With Strain Relief Layers

The GaN-based blue light-emitting diodes (LEDs) with superlatticelike strain relief layer (SRL) and without SRL were fabricated by metal organic vapor phase epitaxy (MOVPE). The luminescence characteristics of the different LEDs were measured experimentally and calculated theoretically. It is demonstrated that the efficiency droop of GaN-based LEDs is suppressed using the SRL due to the reduction of polarization electric field in quantum wells. This is beneficial to the mitigation of LED light output power-injection current (L-I) nonlinearity. Furthermore, using the different LEDs as light source, the performance of visible-light communication (VLC) system was also investigated. The results reveal that the VLC system performance is improved when the LEDs with SRL are applied. The reason is that the transmitted signal error decreases as the LED L-I nonlinearity mitigates. As a result, the GaN-based LEDs with SRL can be applied in the VLC to improve the system performance.