InGaN QWs Research Articles

Internal quantum efficiency improvement of InGaN-based red LED by using double V-pits layers as strain relief layer

Micro/mini light emitting diodes (LEDs) based on AlInGaN material system have vast potential in display applications. Nevertheless, the low internal quantum efficiency (IQE) of InGaN-based red LED limits its development and application. In the epitaxial structure of our designed red LED, double V-pits layers were used as strain relief layers to reduce compressive strain and improve the IQE of the active layer. First, InGaN/GaN superlattices (SLs) were grown below the active layer to form low-density large V-pits layer. Subsequently, multi-period green and red composite quantum wells were adopted as the active layer. A high-density small V-pits layer was introduced into the active region to release the compressive strain by adjusting the growth parameters of green multiple quantum wells (MQWs). The V-shaped pits divide the continuous large-area of active layer into mutually isolated small pieces, which prevents the transmission of strain and converts the long-range strain into separated local strain. The peak IQEs of LED A2 with single V-pits layer and LED B4 with double V-pits layers were measured to be 10.5% at 613 nm and 21.5% at 612.1 nm, respectively. The IQE is greatly improved by 204.7%. The research results indicate that the double V-pits layers structure can alleviate the compressive strain of InGaN QWs more effectively, reduce the influence of piezoelectric polarization field, and improve the IQE.

Strain visualization enabled in dual-wavelength InGaN/GaN multiple quantum wells Micro-LEDs by piezo-phototronic effect

Optical strain sensor for stress visualization is a significant topic in biomechanical imaging or electronic skin. In this study, the dual-wavelength InGaN/GaN multiple-quantum-wells (MQWs) micro light emitting diode (Micro-LED) is demonstrated with color response under mechanical stimulation by piezo-phototronic effect. The peak intensity of green light increases by about 37%, while the blue decreases by 10% at the external strain of 0.20%. Simultaneously, the intensity integral ratio of green and blue light changes from 43:57–53:47, leading to the display color shift from light blue to turquoise significantly. It is noted that electrical injection shows a greater color response than photoluminescence, indicating that the piezo-phototronic effect changes both the recombination in MQWs and the carrier injection. The different piezoelectric modulation between green QW and blue QW lies on the different strain-induced piezoelectric polarized charges in the interface of InGaN QWs, revealed by experiments and APSYS simulation in details. This study is of high significance for the development of optical-based stress sensor with strain visualization and high spatial resolution for smart sensing and micro-opto-electromechanical systems.

Redistribution of carrier localization in InGaN-based light-emitting diodes for alleviating efficiency droop

We propose an efficient method for reducing efficiency droop in InGaN QWs by redistributing carrier localization via thermal annealing, which results in increased internal quantum efficiency (IQE) in this study. Entire structures of InGaN-based light-emitting diode (LED) were grown using metal-organic chemical vapor deposition. After thermal annealing, the photoluminescence (PL) intensity of the InGaN LED was gradually increased up to 900 °C, as confirmed by room-temperature macro-PL spectroscopy. The temperature-dependent PL spectra indicated that the In-rich InGaN cluster's local energy band was flattened by the thermal annealing treatment. Moreover, excitation power-dependent PL spectroscopy confirmed that redistribution of carrier localization in InGaN QWs alleviated the efficiency droop phenomenon. Furthermore, the flattened local energy band of the In-rich InGaN cluster and the increased carrier localization site implied improved IQE.

Structural and emission improvement of cyan-emitting InGaN quantum wells by introducing a large substrate misorientation angle

Within this work, we studied InGaN QWs with nominally 17% InN mole fraction grown within an 80 × 80 μm area with local misorientation angle change from 0.3° to 3.2°. We observed a significant improvement of the photoluminescence intensity for the area with misorientation above 1.5°, which we attribute to the quenching of nonradiative recombination processes. From the structural point of view, the increase of the misorientation angle above 1.5° is accompanied by the improvement of the morphology of the sample and quality of the quantum wells observed through atomic force microscopy and transmission electron microscopy. We show that the structural and emission qualities in high-InN- mole fraction layers can be improved just by increasing the misorientation angle of the substrate and that the improved qualities are preserved even for large misorientation angles.

Understanding the Luminescence Characteristics of Ultraviolet InGaN/AlGaN Multiple Quantum Wells with Different In Gradients

The electroluminescence (EL) properties of InGaN/AlGaN ultraviolet light-emitting multiple quantum wells (MQWs) with identical average In content but different In gradients (In content increases linearly, along the growth direction) are investigated numerically. It is found that the luminescence efficiency is improved, and the EL spectral peak wavelength becomes longer for the MQW sample with a larger In gradient. Since the influence of In gradient is different for the conduction and valence bands in InGaN layers, the distribution of electrons and holes in QWs may be changed, leading to a redshift of EL spectra. In particular, when the In gradient increases, the overlap integral of electron-hole wavefunction in InGaN QWs increases, resulting in a higher radiative recombination rate and an enhanced EL intensity.

Improving the homogeneity and quality of InGaN/GaN quantum well exhibiting high In content under low TMIn flow and high pressure growth

Two series InGaN/GaN MQWs samples were prepared by metal-organic chemical vapor deposition (MOCVD) to study the effect of TMIn flow and pressure on structural parameters, emission propertied and surface morphology of InGaN well layer. This study revealed that both high In flow rate and high pressure will improve the incorporation of atoms undergoing material deterioration. However, an elevated pressure will expedite the incorporation of indium atoms while maintaining the surface morphology of InGaN well layer under low TMIn flow. For those InGaN QWs exhibiting fixed In composition and thickness, this study proposed to decrease the TMIn flow to improve the homogeneity and quality of InGaN layer, while increasing pressure to keep the In content unchanged.

Synthesis and Properties of InGaN/GaN Multiple Quantum Well Nanowires on Si (111) by Molecular Beam Epitaxy

Herein, InGaN/GaN multiple quantum well (MQW) nanowire ensembles grown on Si (111) substrates by plasma‐assisted molecular beam epitaxy (PA‐MBE) are investigated. Well‐oriented InGaN/GaN nanowires with perfect wurtzite crystal structure can be achieved by optimizing the growth parameter of bottom GaN nanowires. The morphology of the nanowires is strongly influenced by the AlN seeding layer growth time as well as the Ga/N flux ratio and by introducing excessive In flux and relatively higher quantum barrier (QB) growth temperatures. Strong emission from InGaN/GaN nanowires can be achieved by varying InGaN growth temperatures to change the incorporation of indium, therefore tuning the emission spectra from violet to amber. The observations from transmission electron microscopy (TEM) and energy dispersive spectrometer (EDS) mapping prove the high quality of MQWs with sharp interfaces and the uniform indium distribution in InGaN QWs.

Effects of photogenerated carriers in GaN layers on the photoluminescence characteristics of violet light-emitting InGaN/GaN multiple quantum wells

Two violet light-emitting InGaN/GaN multiple quantum well (MQW) structures with different GaN barrier thicknesses are investigated by the temperature-dependent photoluminescence measurements with 325 nm He–Cd laser. At low temperatures the luminescence intensity of thick-barrier MQWs is enhanced by the increased photogenerated carriers from the thicker GaN barriers, even though the polarization field in QWs is promoted. As the temperature rises, an anomalous increase of luminescence intensity is observed from 100 to 150 K for both samples. This may be ascribed to the increased nonequilibrium carriers in the InGaN QWs due to the migration of thermal carriers from p-GaN and n-GaN layers into the depletion region. Therefore, the influence of photogenerated carriers in GaN layers on the luminescence properties of InGaN/GaN MQWs should be considered carefully for the 325-nm photoluminescence system.

Influence of GaN buffer layer under InGaN/GaN MQWs on luminescent properties

Although InGaN layers or InGaN/GaN superlattices are commonly used as efficiency improving buffers for LED structure production, there is still a controversy and active discussion about the mechanisms improving the luminescence properties of InGaN QWs grown above such buffers. In this manuscript it is shown that presence of In in the buffer layer is not the primary reason for photoluminescence improvement which can be also achieved by introduction of GaN buffer layer grown at lower temperature under nitrogen atmosphere. SIMS analysis suggests that low temperature buffer layer does not influence the impurity incorporation and hence the PL improvement is caused by suppressed contamination of MQW region grown above the low temperature buffer. AFM images for two samples that differ mostly in morphology however supports another explanation in which formation of larger V-pits is the main reason for the luminescence improvement.

Effect of strain modification on crystallinity and luminescence of InGaN/GaN multiple quantum wells grown by MOCVD

The effects of compressive strain modification on green-emitting InGaN/GaN multiple quantum wells (MQWs) are comprehensively investigated by inserting a bulk InGaN prestrain underlying layer with different indium doping contents. Systematic measurements of the structural, morphological and optical properties are conducted. Better crystallinities are obtained for both modified samples, in which the density of V-pits is reduced from 4.0 × 108 /cm2 in the original sample to 1.9 × 108 /cm2 and 3.3 × 108 /cm2. Compared with the conventional InGaN/GaN MQWs, the photoluminescence (PL) emission wavelength of the strain-modified MQW structure with an InGaN (In = 0.03) prestrain layer is blueshifted by ~ 6 nm as a result of the reduced built-in piezoelectric field, while the other MQW structure with an InGaN (In = 0.1) layer has an ~ 15 nm redshift, which is attributed to not only the ~ 1% increase in the In content in the QWs and the ~ 0.6 nm increase in the well width due to the composition pulling effect, but also the ~ 0.19 degree of strain relaxation in the InGaN QWs. The low-temperature PL (LTPL) intensities of both samples are approximately twice as large as that of the conventional one, whereas the internal quantum efficiency is three times larger at the maximum. These tremendous improvements are mainly due to the weakened quantum confined stark effect and stronger carrier localization states, as confirmed by LTPL and temperature-dependent PL (TDPL) measurements.

Site-Controlled Growth of GaN Nanorods with Inserted InGaN Quantum Wells on μ-Cone Patterned Sapphire Substrates by Plasma-Assisted MBE

We report on a new approach to fabricate regular arrays of GaN nanorods (NRs) with InGaN QWs by plasma-assisted molecular-beam epitaxy (PA MBE) on micro-cone patterned sapphire substrates (μ-CPSSs). A two-stage PA MBE fabrication process of GaN NRs has been developed, starting with a high temperature nucleation layer growth at metal-rich conditions to aggregate selectively GaN nucleus on c-oriented areas of the μ-CPSSs and followed by growth of 1-μm-thick GaN NRs at strongly nitrogen-rich conditions exactly on the cone tips. These results are explained by energetically favorable GaN growth on the (000-) oriented sapphire surface. Both micro-photoluminescence and micro-cathodoluminescence confirm the formation of regular array of optically and spectrally isolated NRs without usage of any nanolithography.

Influence of in doping in GaN barriers on luminescence properties of InGaN/GaN multiple quantum well LEDs

Room-temperature photoluminescence (RT PL) spectra of InGaN/GaN multiple quantum well (MQW) structures grown by metalorganic chemical vapor deposition (MOCVD) was investigated. It is found that with increasing In content in GaN barriers, the FWHM and emission intensity decreases, and the emission wavelength is first red shift and then blue shift. The shrinkage of FWHM and emission wavelength blue shift can be attributed to the reduction of piezoelectric field, and the lower height of potential barrier will make carrier confinement weaker and ground state level lower, which resulting in emission intensity decreasing and wavelength red shift. In addition, doping the barrier with In will induce more inhomogeneous and deeper localized states in InGaN QWs, which also contribute to a red shift of PL emission wavelength.

Quantum Barrier Growth Temperature Affects Deep Traps Spectra of InGaN Blue Light Emitting Diodes

Electroluminescence (EL) efficiency, deep electron and hole traps spectra, microcathodoluminescence (MCL), electron beam induced current (EBIC) imaging, and MCL spectra were studied for blue GaN/InGaN multi-quantum-well (MQW) light emitting diodes differing by the temperature at which the GaN barriers of the MQW active region were grown. It was found that increasing the growth temperature from 850 to 920°C very strongly suppressed the formation of deep electron traps with level at Ec-1 eV in the GaN barriers and of the hole traps with levels at Ev+0.7 eV in InGaN QWs. The suppression of the formation of the Ec-1 eV electron trap, a known prominent nonradiative recombination center in n-GaN, improved the carrier injection efficiency into the InGaN QWs and increased the external quantum efficiency by about 9%. EBIC and MCL imaging showed that the density of threading dislocations and terminating them V-pits was relatively low and similar for both studied growth temperatures, close to 108 cm−2. The cross-sectional dimensions of the V-pits were measurably higher for increased growth temperature. However, the rather low dislocation density and rather high dimensions of the V-pits were believed to result in minor contribution of these defects to the observed EL efficiency changes.

Site-controlled growth of GaN nanorods with inserted InGaN quantum wells on mu-cone patterned sapphire substrates by plasma-assisted MBE

AbstractWe report on a new approach to fabricate regular arrays of GaN nanorods (NRs) with InGaN QWs by plasma-assisted molecular-beam epitaxy (PA MBE) on micro-cone patterned sapphire substrates (μ-CPSSs). A two-stage PA MBE fabrication process of GaN NRs has been developed, starting with a high temperature nucleation layer growth at metal-rich conditions to aggregate selectively GaN nucleus on c-oriented areas of the μ-CPSSs and followed by growth of 1-μm-thick GaN NRs at strongly nitrogen-rich conditions exactly on the cone tips. These results are explained by energetically favorable GaN growth on the (000-) oriented sapphire surface. Both micro-photoluminescence and micro-cathodoluminescence confirm the formation of regular array of optically and spectrally isolated NRs without usage of any nanolithography.

Comparative study on the InGaN multiple-quantum-well solar cells assisted by capacitance-voltage measurement with additional laser illumination

The investigation on the photovoltaic characteristics of two InGaN/GaN multiple-quantum-well (MQW) solar cells with different QW parameters, but having nearly the same spectral response range, is assisted by the capacitance-voltage (C-V) measurements. Based on the laser-assisted C-V technique as well as the X-ray reciprocal space mapping measurements, it is found the enhanced external quantum efficiency of solar cell consisting of the InGaN QWs having thicker well layer and lower In content can be ascribed to the lower effective potential barrier height, the larger active volume and the better material quality. In addition, it suggests that for InGaN MQW solar cells, a proper increase of the InGaN well layer thickness may be a good choice to extend the spectral response into longer wavelength region.

Influence of Indium Content on the Unintentional Background Doping and Device Performance of InGaN/GaN Multiple-Quantum-Well Solar Cells

We experimentally investigate the influence of indium composition on the photovoltaic performance of InGaN/GaN multiple-quantum-well (MQW) solar cells. Three MQW structures with different In content are grown via metal-organic chemical vapor deposition, and then fabricated into solar cell devices. The solar cells with lower In content show stronger photovoltaic response, which may be ascribed to both smaller piezoelectric fields in InGaN well layers and better material quality of MQW active region. In addition, based on the capacitance–voltage measurements by which the profiles of apparent carrier concentration can be determined, we find that in the higher-In-content solar cells the background electron concentration in unintentionally doped InGaN QWs is higher. Consequently, the effective volume of active region, or the width of depletion region, may become smaller at zero bias and correspondingly the light absorption by InGaN active layers is reduced. This may be attributed to the increased nitrogen vacancy-related defects which are easier induced into In-rich InGaN alloys as a source of shallow donors.

Performance improvement of InGaN/GaN multiple quantum well visible-light photodiodes by optimizing TEGa flow**Project supported by the Science and Technology Major Project of Guangdong Province, China (Grant Nos. 2014B010119003 and 2015B010112001).

The performance of an InGaN/GaN multiple quantum well (MQW) based visible-light Schottky photodiode (PD) is improved by optimizing the source flow of TEGa during InGaN QW growth. The samples with five-pair InGaN/GaN MQWs are grown on sapphire substrates by metal organic chemical vapor deposition. From the fabricated Schottky-barrier PDs, it is found that the smaller the TEGa flow, the lower the reverse-bias leakage is. The photocurrent can also be enhanced by depositing the InGaN QWs with using lower TEGa flow. A high responsivity of 1.94 A/W is obtained at 470 nm and −3-V bias in the PD grown with optimized TEGa flow. Analysis results show that the lower TEGa flow used for depositing InGaN may lead to superior crystalline quality with improved InGaN/GaN interface, and less structural defects related non-radiative recombination centers formed in the MQWs.

Optical and microstructural properties of InGaN/GaN multiple quantum wells with embedded graphene coating

We investigate the effects of embedded graphene coating on the optical and microstructural properties of ultrathin InGaN/GaN multiple quantum wells (MQWs). The InGaN/GaN MQWs grown on graphene-buffered GaN templates displayed enhanced internal quantum efficiency compared to conventional ones and showed the internal electric field effect-free characteristic, desirable for general lighting applications. These phenomena were attributed to the enhancement of potential fluctuation with increased indium content and negligible piezoelectric polarization in ultrathin InGaN QWs, respectively. It was found that the atomically rough surface of GaN induced by embedded graphene coating efficiently relieved the biaxial compressive strain in the ultrathin InGaN/GaN QWs and enhanced the In incorporation efficiency during the InGaN growth, suggesting the potential use of atomic-thick carbon layer in niche optoelectronic applications.

Increase of photogenerated carriers in thick quantum wells in InGaN solar cells verified by laser-assisted capacitance-voltage measurement

For the InGaN/GaN multiple quantum well (MQW) solar cells, it is known that in the thicker InGaN QWs the collection of photogenerated carriers suffer from the impact of both stronger piezoelectric field and enhanced non-radiative recombination process, which may lead to an inferior performance for thick-well solar cells. However, we find that the photovoltaic intensities of our MQW solar cells with different well thickness are almost the same. Combined with the profiles of apparent carrier concentration obtained from the capacitance-voltage measurement with additional laser illumination, we conclude that the increase of photogenerated carriers in the thick QWs may partially counteract the adverse effects of both polarization effect and deteriorated material quality on the photocurrent for the MQW solar cells with thick InGaN well layers, compared with the thin-well ones.

Effect of nanopillar sublayer embedded with SiO2 on deep traps in green GaN/InGaN light emitting diodes

The effect of a layer of GaN nanopillars with SiO2 nanoparticles inserted into the n+-GaN contact Layer on the electrical properties, electroluminescence (EL) and photoluminescence (PL), admittance spectra, and deep trap spectra of green multi-quantum-well GaN/InGaN light emitting diodes (LEDs) grown by metalorganic chemical vapor deposition (MOCVD) on patterned sapphire substrates is reported. The PL and EL intensities for these SiO2 LEDs are measurably enhanced compared with reference to LEDs without the nanopillar sublayer. This correlates with the decrease in the SiO2 LEDs of the concentration of 0.25 eV electron traps and 0.45 eV hole traps, both located in the InGaN QWs.