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The optical properties of InGaN/GaN violet light-emitting multiple quantum wells with different thicknesses of GaN quantum barriers are investigated experimentally. When the barrier thickness decreases from 20 to 10 nm, the photoluminescence intensity at room temperature increases, which can be attributed to the reduced polarization field in the thin-barrier sample. However, with a further reduction in the thickness to 5 nm, the sample's luminescence intensity decreases significantly. It is found that the strong nonradiative loss process induced by the deteriorated crystal quality and the quantum-tunneling-assisted leakage of carriers may jointly contribute to the enhanced nonradiative loss of photogenerated electrons and holes, leading to a significant reduction in photoluminescence intensity of the sample with nanoscale ultrathin GaN quantum barriers. [ABSTRACT FROM AUTHOR]

We have demonstrated the role of thermal annealing treatment after InGaN quantum well layer growth to improve the interface morphology and luminescence properties of InGaN/GaN multiple quantum wells (MQWs). X-ray diffraction and transmission electron microscope analyses reveal that better structural properties are achieved by appropriately increasing the ramp-up time as well as relatively decreasing the annealing temperature as the interface quality of MQWs is improved. Moreover, the photoluminescence (PL) and electroluminescence (EL) measurements confirm the higher crystal quality and optical properties of InGaN/GaN MQWs. The reason may be the redistribution of indium atoms improves the homogeneity of localized states in MQWs, and the In-rich clustering behavior is obviously alleviated. This annealing method is feasible and can lead to obtaining high-performance semiconductor optoelectronic devices.

Abstract The InGaN/GaN multi-quantum wells (MQWs) are prepared at the same condition by metal-organic chemical vapor deposition (MOCVD) except the thickness of cap layers additionally grown on each InGaN well layer. The photoluminescence (PL) intensity of the thin cap layer sample is much stronger than that of thicker cap layer sample. Interestingly, the thick cap layer sample has two photoluminescence peaks under high excitation power, and the PL peak energy-temperature curves show an anomalous transition from reversed V-shaped to regular S-shaped with increasing excitation power. Meanwhile, it exhibits a poorer thermal stability of thick cap layer sample under higher excitation power than that under lower excitation power. Such an untypical phenomenon is attributed to carrier redistribution between the two kinds of localized states which is induced by the inhomogeneous distribution of indium composition in thick cap layer sample. Furthermore, the luminescence of deep localized states has a better thermal stability, and the luminescence of shallow localized states has a poor thermal stability. In fact, such a severer inhomogeneous indium distribution may be caused by the degradation of subsequent epitaxial growth of InGaN/GaN MQWs region due to longer low-temperature GaN cap layer growth time.

The optical properties of InGaN/GaN violet light-emitting multiple quantum wells with different thicknesses of GaN quantum barriers are investigated experimentally. When the barrier thickness decreases from 20 to 10 nm, the photoluminescence intensity at room temperature increases, which can be attributed to the reduced polarization field in the thin-barrier sample. However, with a further reduction in the thickness to 5 nm, the sample’s luminescence intensity decreases significantly. It is found that the strong nonradiative loss process induced by the deteriorated crystal quality and the quantum-tunneling-assisted leakage of carriers may jointly contribute to the enhanced nonradiative loss of photogenerated electrons and holes, leading to a significant reduction in photoluminescence intensity of the sample with nanoscale ultrathin GaN quantum barriers.

Optical properties of wurtzite violet InGaN/GaN quantum well (QW) structures, with the same well-plus-barrier thickness, grown by metal-organic chemical vapor deposition (MOCVD) on c-plane sapphire substrates, were investigated using temperature-dependent photoluminescence (TDPL) and excitation-power-dependent photoluminescence (PDPL). Two samples were compared: one had a thicker well (InGaN/GaN 3/5 nm); the other had a thicker barrier (InGaN/GaN 2/6 nm). It was found that the GaN barrier thickness in the InGaN/GaN MQWs plays an important role in determining the optical characteristics of the MQWs. The peak energy of the two samples varied with temperature in an S-shape. The thicker-barrier sample had a higher turning point from blueshift to redshift, indicating a stronger localization effect. From the Arrhenius plot of the normalized integrated PL intensity, it was found that the activation energy of the nonradiative process also increased with a thicker barrier thickness. The radiation recombination process was dominated in the sample of the thicker barrier, while the non-radiation process cannot be negligible in the sample of the thicker well.

Abstract We report and analyze luminescence spectra for rectangular and trapezoidal InGaN/GaN multiple quantum wells (MQWs) under various bias voltages using well-calibrated numerical simulations. The effect of carrier localization due to clustering of In atoms in In-rich regions is taken into account by considering quantum dot-like structure in the QW for In contents exceeding 30%. The shape of the luminescence spectra obtained from different MQWs is explained physically on the basis of energy band diagram, electric field in the well and shift of the peak of electron and hole concentrations. Our findings demonstrate that under various bias conditions the trapezoidal MQWs produce luminescence peaks at the same wavelength whereas the rectangular MQWs do not. Such a striking feature associated with the trapezoidal MQWs may be exploited for color-stable photonic applications in which varying bias conditions modulate the light intensity while maintaining the peak emission at a constant wavelength. Highlights • Luminescence spectra are studied for rectangular and trapezoidal InGaN/GaN multiple QWs with biases. • Analysis is based on quantum dot-like structure in the In-rich QWs, and other quantum effects. • The luminescence peak increases near zero bias, showing more bias sensitivity for rectangular QWs. • For trapezoidal QWs, the color-stable peak spectrum is obtained with varying bias voltages. [ABSTRACT FROM AUTHOR]

In InGaN/GaN multi-quantum wells (MQWs), a low temperature cap (LT-cap) layer is grown between the InGaN well layer and low temperature GaN barrier layer. During the growth, a temperature ramp-up and ramp-down process is added between LT-cap and barrier layer growth. The effect of temperature ramp-up time duration on structural and optical properties of quantum wells is studied. It is found that as the ramp-up time increases, the Indium floating layer on the top of the well layer can be diminished effectively, leading to a better interface quality between well and barrier layers, and the carrier localization effect is enhanced, thereby the internal quantum efficiency (IQE) of QWs increases surprisingly. However, if the ramp-up time is too long, the carrier localization effect is weaker, which may increase the probabilities of carriers to meet with nonradiative recombination centers. Meanwhile, more nonradiative recombination centers will be introduced into well layers due to the indium evaporation. Both of them will lead to a reduction of internal quantum efficiency (IQE) of MQWs. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) properties of four blue-violet light emitting InGaN/GaN multiple quantum well (MQW) structures with varying well thickness were studied by means of room temperature PL (RTPL) spectra and temperature-dependent PL (TDPL). From the TDPL, two different kinds of dependencies of PL peak energy with increasing temperature are observed, i.e. an S-shape dependence and an inverted V-shape one. Since the In content in MQW structures is nearly identical for four samples, the difference in luminescence properties is mainly attributed to an increase in localization effects with increasing well thickness. [ABSTRACT FROM AUTHOR]

We present a study of polar and semi-polar InGaN/GaN multiple quantum well (MQW) samples. Power density dependent photoluminescence (PL) was conducted on the samples. The polar sample exhibited a large blue shift of 65 nm in the peak PL from low to high power density compared to 10 nm for the semi polar sample. The semipolar sample displayed "chevron" shaped surface features. These chevrons were mapped using micro-PL spectroscopy and showed a red shift of 10 nm at the "join" of the chevron. Raman mapping of the chevron also showed an increase in amplitude of the Raman shift in this area. Angle dependent polarised Raman measurements were conducted on the semi-polar sample to potentially isolate the scattering from the InGaN layers. However, polarised mapping exhibited similar behaviour to the unpolarised maps at the chevron join, indicating that the GaN bulk layer is responsible for the observed scattering. [ABSTRACT FROM AUTHOR]

Highlights: [•] A nanoporous InGaN/GaN multiple quantum wells has been fabricated. [•] The overall external quantum efficiency for nanoporous InGaN/GaN MQWs is enhanced. [•] The nanoporous structure led to more light scattering and lower defects density. [Copyright &y& Elsevier]

Highlights: [•] Correlations between carrier dynamics and structural defects of InGaN MQWs exist. [•] PL intensity is much weaker in the V-shaped pits than in the regular c-plane. [•] Blue-shifted photoluminescence spectrum is not observed at the V-pits. [•] A kinetic model including carriers recombination and diffusion is presented. [•] The model estimates the nonradiative recombination time as 10ps at the V-pits. [ABSTRACT FROM AUTHOR]

We investigated the comparative structural and optical properties of semipolar InGaN/GaN multiple quantum wells (MQWs) grown on the (1\bar101) facet GaN/sapphire substrate by metal–organic chemical vapor deposition using lateral epitaxial overgrowth. The scanning electron microscopy (SEM), photoluminescence (PL), and temperature-varying time-resolved photoluminescence measurement were performed to investigate the structure and optical properties. The cross-sectional SEM image shows that the stripe triangular structure of the QW with semipolar (1\bar101) planes is obtained as sidewall facets with the mask stripes aligned along the GaN a-axis. The structural and optical advantages of semipolar orientations were confirmed by a modurate shift of the PL peak energy, higher internal quantum efficiency, and lower radiative recombination lifetime than the MQWs on (0001) GaN grown by conventional methods. The results were obtained because of the reduced polarization fields in semipolar InGaN/GaN MQWs comparing with that in polar (0001) MQWs. [ABSTRACT FROM PUBLISHER]

A-plane InGaN/GaN multiple-quantum wells (MQWs) were grown on a series of nanorod epitaxial lateral overgrowth (NRELOG) templates with varied nanorod depth. Optical properties of these samples were investigated by excitation power and temperature-dependent photoluminescence (PL). Due to the absence of quantum-confined Stark effect, the negligible PL emission peak shift and nearly identical power index for all samples were observed. In contrast to the as-grown MQWs, the thermal activation energy and internal quantum efficiency of NRELOG MQWs exhibit 1.6-fold and 4-fold increases, respectively, which are attributed to the improvement of crystal quality by NRELOG. Furthermore, the Shockley-Read-Hall nonradiative coefficient, determined from the fits of power-dependent PL quantum efficiency, is also apparently reduced while MQWs are grown on NRELOG GaN template. The results show the feasibility to fabricate high radiative efficiency a-plane devices via NRELOG. [ABSTRACT FROM AUTHOR]

Abstract: We report a study on the nanostructure properties of InGaN/GaN multiple quantum well (MQW) heterostructures. The temperature dependent photoluminescence (PL) spectra from the InGaN/GaN MQWs with different indium contents were carried out. Based on the band-tail states model, the degree of the localized states dispersion is estimated. The broadening of localized states in the samples is enhanced as the indium content is increasing. To further investigate the results, X-ray diffraction spectra of the samples were measured and analyzed. The nanostructure size distribution of the MQW heterostructure is determined via the calculation of the diffracted intensity of X-ray beam in Fourier space. It is found that the sample with higher indium content exhibits a wider nanostructure size distribution, and is consistent with the experimental results from PL spectra. [Copyright &y& Elsevier]

Abstract: Sapphire substrates patterned by a selective chemical wet and an inductively coupled plasma (ICP) etching technique was proposed to improve the performance of GaN-based light-emitting diodes (LEDs). GaN-based LEDs were fabricated on sapphire substrates through metal organic chemical vapor deposition (MOCVD). The LEDs fabricated on the patterned substrates exhibit improved device performance compared with the conventional LED fabricated on planar substrates when growth and device fabricating conditions were the same. The light output powers of the LEDs fabricated on wet-patterned and ICP-patterned substrates were about 37% and 17% higher than that of LEDs on planar substrates at an injection current of 20mA, respectively. The enhancement is attributable to the combination of the improvement of GaN-based epilayers quality and the improvement of the light extraction efficiency. [Copyright &y& Elsevier]

Optical properties of wurtzite violet InGaN/GaN quantum well (QW) structures, with the same well-plus-barrier thickness, grown by metal-organic chemical vapor deposition (MOCVD) on c-plane sapphire substrates, were investigated using temperature-dependent photoluminescence (TDPL) and excitation-power-dependent photoluminescence (PDPL). Two samples were compared: one had a thicker well (InGaN/GaN 3/5 nm); the other had a thicker barrier (InGaN/GaN 2/6 nm). It was found that the GaN barrier thickness in the InGaN/GaN MQWs plays an important role in determining the optical characteristics of the MQWs. The peak energy of the two samples varied with temperature in an S-shape. The thicker-barrier sample had a higher turning point from blueshift to redshift, indicating a stronger localization effect. From the Arrhenius plot of the normalized integrated PL intensity, it was found that the activation energy of the nonradiative process also increased with a thicker barrier thickness. The radiation recombination process was dominated in the sample of the thicker barrier, while the non-radiation process cannot be negligible in the sample of the thicker well. [ABSTRACT FROM AUTHOR]

• Thermal annealing treatment is employed to improve the performance of MQWs. • The luminescence uniformity and interface quality of MQWs is improved obviously. • The In-rich clustering behavior is effectively alleviated. • The excellent structure and optical properties of InGaN/GaN MQWs are achieved. We have demonstrated the role of thermal annealing treatment after InGaN quantum well layer growth to improve the interface morphology and luminescence properties of InGaN/GaN multiple quantum wells (MQWs). X-ray diffraction and transmission electron microscope analyses reveal that better structural properties are achieved by appropriately increasing the ramp-up time as well as relatively decreasing the annealing temperature as the interface quality of MQWs is improved. Moreover, the photoluminescence (PL) and electroluminescence (EL) measurements confirm the higher crystal quality and optical properties of InGaN/GaN MQWs. The reason may be the redistribution of indium atoms improves the homogeneity of localized states in MQWs, and the In-rich clustering behavior is obviously alleviated. This annealing method is feasible and can lead to obtaining high-performance semiconductor optoelectronic devices. [ABSTRACT FROM AUTHOR]

The optical properties of GaN-based green light-emitting diodes (LED) are investigated by transmission electron microscopy (TEM) and temperature-dependent micro-photoluminescence (μ-TDPL) in this study. Compared with the emission of normal c-plane multiple quantum wells (C-MQWs), the emission of C-MQWs close to V-shaped pits (V-pits) exhibits a shoulder peak phenomenon at cryogenic temperature, and the shoulder peak gradually disappears as the temperature increases. At room temperature, the peak of C-MQWs close to V-pits (C 2) exhibits a red-shift. The formation of V-pits would induce strain relaxation. Strain relaxation can result in Indium (In) incorporation and localization, and reduced quantum-confined stark effects (QCSE). In incorporation and localization can lead to the red-shift of C 2 , reduced QCSE can cause the blue-shift of C 2. Therefore, the red-shift of C 2 arises from further In incorporation and localization of C-MQWs. It is worth noting that In incorporation and localization is dominant in the formation of V-pits for green InGaN/GaN C-MQWs. Additionally, the carriers limited in sidewall MQWs (S-MQWs) of V-pits escape into C-MQWs close to V-pits can mitigate the common phenomenon in which the emission intensity of C-MQWs decreases as the temperature increases, which helps improve the emission intensity of GaN-based green LEDs. • The double peak phenomenon of the peak C 2 disappears as the temperature exceeds 140 K. • When the temperature exceeds 160 K, the integrated intensity ratio of C S /C 2 exhibit a rapid decrease. • The decline of emission efficiency for the peak C 2 can be relieved by the formation of V-pits. [ABSTRACT FROM AUTHOR]

• The height of potential barrier and band offset were discussed by band theory. • IQE was improved by decreasing band offset of valence band. • The relationship among these photo-electronic parameters was analyzed. Band alignment of InGaN/GaN multiple quantum wells (MQW) light-emitting diode was investigated by regulating the widths of well and barrier. Novel structure of InGaN/GaN MQW was designed with wider well and narrower barrier with respect to conventional InGaN/GaN MQW with the layer thickness of 2.5/12.5 nm, respectively. The analyses of band alignment, and hole and electron concentrations within the MQW zone show that the novel MQW structure could improve hole transport and block electron leakage owing to the increased potential barrier for electron and the decreased potential barrier for hole in MQW zone, as can be confirmed by decreased electron density and increased hole density. Both the intensity of electroluminescence and internal quantum efficiency for novel MQW are enhanced by increasing radiative recombination. Moreover, it was found that non-radiative recombination current density mainly stems from Auger recombination current density that mainly depends on excess electron. The novel MQW structure reduces built-in field at well/barrier interface owing to increased hole injection. This work provides an effective approach for achieving high performance for InGaN/GaN LED by regulating band offset. [ABSTRACT FROM AUTHOR]

The piezo-phototronic effect is the tuning of piezoelectric polarization charges at the interface to largely enhance the efficiency of optoelectronic processes related to carrier separation or recombination. Here, we demonstrated the enhanced short-circuit current density and the conversion efficiency of InGaN/GaN multiple quantum well solar cells with an external stress applied on the device. The external-stress-induced piezoelectric charges generated at the interfaces of InGaN and GaN compensate the piezoelectric charges induced by lattice mismatch stress in the InGaN wells. The energy band realignment is calculated with a self-consistent numerical model to clarify the enhancement mechanism of optical-generated carriers. This research not only theoretically and experimentally proves the piezo-phototronic effect modulated the quantum photovoltaic device but also provides a great promise to maximize the use of solar energy in the current energy revolution.