InGaN-based Multiple Quantum Wells Research Articles

Effects of GaN cap layer thickness on photoexcited carrier density in green luminescent InGaN multiple quantum wells

The effects of GaN cap layers on the optical properties of green luminescent InGaN-based multiple quantum wells were studied by photoluminescence (PL) spectroscopy. The PL peak energy under the selective excitation of the InGaN well layers was lower than that under the band-to-band excitation of the GaN barrier layers. The difference in the PL peak energies between the selective and band-to-band excitations decreased as the cap layer thickness increased, indicating an increase in the nonradiative recombination of photogenerated carriers in the barrier layers. Moreover, the internal quantum efficiency under selective excitation decreased as the cap layer thickness increased because of the increase in the internal electric field strength.

Preparation and enhanced photocatalytic properties of the large-area transferred mesoporous InGaN-based MQWs with mesoporous GaN reflectors

The self-supporting mesoporous (MP) InGaN-based multiple quantum wells (MQWs) with MP GaN reflectors, which were prepared by a combination of doping and electrochemical etching, were transferred to a quartz substrate for the first time. Surface photocurrent (SPC), electrochemical impedance spectroscopy (EIS) and fluorescence lifetime results show that the unique mesoporous structures can effectively inhibit the recombination of photogenerated electron-hole pairs, thus improve the separation and transfer efficiency of charge. Interestingly, the MP InGaN-based MQWs with MP GaN reflectors present good photocatalytic ability at various pHs, even under strong acidity and alkalinity. And the transferred sample exhibited the best photocatalytic activity among the three samples, which are due to the stress relaxation, large surface area, increased light absorption capacity and decreased defect density of the multiple quantum wells (MQWs) layer. In addition, the mechanism of the InGaN-based MQWs for enhancing removal of methyl orange is expounded.

Performance improvement of InGaN-based multiple quantum wells embedded nanoporous n-GaN layers

InGaN-based multiple quantum wells (MQWs) embedded nanoporous (NP) GaN layers were fabricated by metal-organic chemical vapor deposition (MOCVD) and followed by electrochemical (EC) etching in oxalic, HF or HNO3 electrolytes. The oxalic-etching and HF-etching do not lead to enhanced luminescence of MQWs, which should be contributed to the damage of InGaN/GaN stacking layers. For the HNO3-etching, the photoluminescence (PL) strength of MQW layer is increased by a factor of 0.52 compared with the as-grown sample, which should be contributed to stress relaxation, light scattering and reflection of quasi vertical nanopore array, and amplify spontaneous emission by resonant cavity. If the nanopore walls in NP-GaN layer are smoothed by the annealing in NH3 atmosphere, the light extraction efficiency of MQWs can be further improved. The study provides a useful method for improving the light extraction efficiency of GaN-based light emitting devices.

Preparation and photoluminescence of self-standing nanoporous InGaN/GaN MQWs via UV-assisted electrochemical etching

The self-standing InGaN-based multiple quantum wells (MQWs) thin films with the vertically aligned nanoporous InGaN/GaN layer, were electrochemically etched in nitric acid (HNO 3 ) solution under ultraviolet (UV) light, and then transferred to stainless steel cloth and quartz substrates, respectively. Compared to the as-grown sample, the etched and transferred thin films show an enhanced and blue-shift photoluminescence (PL) emission. The PL enhancement is attributed to the improved internal quantum efficiency (IQE) which is ascribed to the improved crystalline quality of MQWs, and the increased light extraction efficiency (LEE) which is ascribed to the high light-extracting surface area and light-guiding effect of the vertically aligned nanoporous InGaN/GaN structure. And the blue-shift can be attributed to stress relaxation of MQWs and variable In content in InGaN layer. PL spectra of As-grown, etched and two transferred InGaN-based MQWs. • The self-standing nanoporous InGaN/GaN MQWs are prepared by a UV-assisted electrochemical etching. • Compared to the as-grown sample, the etched and transferred thin films show an enhanced and blue-shift PL emission. • Among the samples, the transferred thin films show the largest PL intensity.

Recombination rates in green-yellow InGaN-based multiple quantum wells with AlGaN interlayers

The recombination rates in InGaN/AlGaN/GaN multiple quantum wells (MQWs) emitting in the green-yellow and grown with different Al compositions in the AlGaN interlayer (IL) are shown. By transforming measurements on radiative efficiency, absorption, and differential carrier lifetime, the radiative and nonradiative rates are determined. The IL Al composition controls lattice relaxation of the MQWs, as determined by X-ray reciprocal space mapping, and, therefore, defect formation. For the most pseudomorphic MQWs, the Shockley-Read-Hall (SRH) A coefficient is minimized and is similar to reports at shorter (blue and green) wavelengths. It is an order of magnitude smaller than a conventional InGaN/GaN MQW and is the most significant factor behind the improvement in radiative efficiency using the IL. The radiative B coefficient is also reduced and a minimum for the most pseudomorphic MQWs due to a reduction in the electron-hole wavefunction overlap. However, the decrease in A is more significant and leads to an overall improvement in the radiative efficiency. These recombination rate measurements confirm that if the SRH recombination is controlled, then the severe reduction of radiative recombination with an increased emitting wavelength is one of the main challenges in realizing high efficiency, long-wavelength InGaN-based MQW emitters operating at low to moderate current densities.

Analysis of efficiency curves in near-UV, blue, and green-emitting InGaN-based multiple quantum wells using rate equations of exciton recombination

The internal quantum efficiency (IQE) of near-UV, blue, and green-emitting InGaN-based multiple quantum wells was evaluated as functions of excitation power density and temperature using photoluminescence spectroscopy. The IQE curves under weak excitation densities were analyzed using a rate equation model based on the radiative and nonradiative recombination of excitons. The analysis clarified that the initial increase in the estimated IQE could be explained by the filling of nonradiative recombination centers. Moreover, the analysis elucidated that the nonradiative recombination process of excitons was independent of the emission wavelength and that the radiative recombination rate of excitons was strongly affected by the exciton localization and depended on the emission wavelength. Furthermore, the analysis suggested that the maximum IQE reached was 100% in all the samples at a low temperature.

Electric-field driven photoluminescence probe of photoelectric conversion in InGaN-based photovoltaics.

The spatial distribution of electric field in photovoltaic multiple quantum wells (MQWs) is extremely important to dictate the mutual competition of photoelectric conversion and optical transition. Here, electric-field-driven photoluminescence (PL) in both steady-state and transient-state has been utilized to directly investigate the internal photoelectric conversion processes in InGaN-based MQW photovoltaic cell. As applying the reversed external electric field, the compensation of the quantum confined stark effect (QCSE) in InGaN QW is beneficial to help the photoabsorbed minor carriers drift out from the localized states, whereas extremely weakening the PL radiative recombination. A directly driven force by the reversed external electric field decreases the transit time of photocarriers drifting in InGaN QW. And hence, the overall dynamic PL decay including both the slow and fast processes gradually speeds up from 19.2 ns at the open-circuit condition to 3.9 ns at a negative bias of -3 V. In particular, the slow PL decay lifetime declines more quickly than that of the fast one. It is the delocalization of photocarriers by electric-field drift that helps to further enhance the high-efficiency photoelectric conversion except for the tunneling transport in InGaN-based MQW photovoltaics. Therefore, it can be concluded that the electric-field PL probe may provide a direct method for evaluating the photoelectric conversion in multilayer quantum structures and related multijunction photovoltaic cells.

Spatial redistribution of photoexcited carriers in InGaN/GaN structures emitting in a wide spectral range

Capabilities of pulsed mode in metalorganic chemical vapor deposition in growing InGaN-based multiple quantum wells (QWs) emitting in a wide spectral range by varying deposition temperatures are tested. Lowering of the temperatures for growing short-period superlattices, which were used as templates, and the active QWs enabled redshifting the photoluminescence peak position to 580 nm. Spatially-resolved photoluminescence spectroscopy using confocal microscope revealed that the structures emitting at the wavelengths longer than ~530 nm have specific features in the profile of potential fluctuations, which are qualitatively different from those observed in the structures emitting in blue region. The transition from the dominance of the potential fluctuations of a small spatial scale to the prevalence of large-scale potential fluctuations is accompanied by the considerable decrease in emission intensity.

Strain compensation in InGaN-based multiple quantum wells using AlGaN interlayers

Data are presented on strain compensation in InGaN-based multiple quantum wells (MQW) using AlGaN interlayers (ILs). The MQWs consist of five periods of InxGa1-xN/AlyGa1-yN/GaN emitting in the green (λ ∼ 535 nm ± 15 nm), and the AlyGa1-yN IL has an Al composition of y = 0.42. The IL is varied from 0 - 2.1 nm, and the relaxation of the MQW with respect to the GaN template layer varies with IL thickness as determined by reciprocal space mapping about the (202¯5) reflection. The minimum in the relaxation occurs at an interlayer thickness of 1 nm, and the MQW is nearly pseudomorphic to GaN. Both thinner and thicker ILs display increased relaxation. Photoluminescence data shows enhanced spectral intensity and narrower full width at half maximum for the MQW with 1 nm thick ILs, which is a product of pseudomorphic layers with lower defect density and non-radiative recombination.

Polarized white light from hybrid organic/III-nitrides grating structures

Highly polarised white light emission from a hybrid organic/inorganic device has been achieved. The hybrid devices are fabricated by means of combining blue InGaN-based multiple quantum wells (MQWs) with a one-dimensional (1D) grating structure and down-conversion F8BT yellow light emitting polymer. The 1D grating structure converts the blue emission from unpolarised to highly polarised; Highly polarised yellow emission has been achieved from the F8BT polymer filled and aligned along the periodic nano-channels of the grating structure as a result of enhanced nano-confinement. Optical polarization measurements show that our device demonstrates a polarization degree of up to 43% for the smallest nano-channel width. Furthermore, the hybrid device with such a grating structure allows us to achieve an optimum relative orientation between the dipoles in the donor (i.e., InGaN/GaN MQWs) and the diploes in the acceptor (i.e., the F8BT), maximizing the efficiency of non-radiative energy transfer (NRET) between the donor and the acceptor. Time–resolved micro photoluminescence measurements show a 2.5 times enhancement in the NRET efficiency, giving a maximal NRET efficiency of 90%. It is worth highlighting that the approach developed paves the way for the fabrication of highly polarized white light emitters.

Surface morphology and optical properties of InGaN/GaN multiple quantum wells grown on freestanding GaN (0001) substrates

Surface morphology and optical properties of InGaN-based multiple quantum wells grown on c-plane freestanding GaN substrates by metalorganic vapor phase epitaxy was investigated. The surface shows a dependence on the substrate crystallographic orientation, presenting wavy and mirror-like feature with the substrate off-cut towards the [112¯0] direction and the [11¯00] direction, respectively. To get sharp hetero-interfaces, InGaN multiple-quantum-wells-based blue laser diode wafers were grown on substrates with off-cut towards the [11¯00] direction. The photoluminescence peak wavelength of the blue laser wafers shows a linearly blue shift as the substrate off-cut angle increases. A minimum spectra line width of 90 meV@450 nm can be obtained with a substrate off-cut angle of about 0.5°. These surface and optical properties were ascribed to the step growth kinetics of the wurtzite type materials.

Enhanced light output power of InGaN-based amber LEDs by strain-compensating AlN/AlGaN barriers

We investigated the effect of a combined AlN/Al0.03Ga0.97N barrier on InGaN-based amber light-emitting diodes (LEDs) grown by metalorganic vapor-phase epitaxy. InGaN-based multiple quantum wells with a combined AlN/Al0.03Ga0.97N barrier showed intense photoluminescence with a narrow full-width at half-maximum. The amber LEDs with a combined AlN/Al0.03Ga0.97N barrier achieved a light output power enhanced approximately 2.5-fold at 20mA compared to that of the LED with a combined AlN/GaN barrier, owing to the reduction of defects in InGaN active layers. Thus, the efficiency of high-In-content InGaN-based LEDs can be improved in the spectrum range of amber.

Growth of semipolar high-indium-content InGaN quantum wells using InGaN tilting layer on Si(001)

Low-toxity high-In-content InGaN is an attractive option for short-distance communications through plastic optical fibers because its performance is only slightly affected by temperature. However, its fabrication on the c-plane is impaired by In droplets and V pits, which form at low-growth temperature. On the other hand, unlike the c-plane, InGaN relaxes with tilting. Therefore, in this study, we first grew a high-In-content InGaN single layer, and then we fabricated an InGaN tilting layer between InGaN-based multiple quantum wells (MQWs) and GaN stripes/(001)Si. The emission wavelength increased with the InGaN tilting layer’s growth time because the strain was relaxed by misfit dislocations at the heterointerface. This layer also extended the emission peak of InGaN/GaN MQWs and increased the photoluminescence intensity with respect to that of a single-layered InGaN. Therefore, the InGaN tilting layer is effective for growing high-In-content InGaN MQWs.

On the increased efficiency in InGaN-based multiple quantum wells emitting at 530–590 nm with AlGaN interlayers

InGaN/AlGaN/GaN-based multiple quantum wells (MQWs) with AlGaN interlayers (ILs) are investigated, specifically to examine the fundamental mechanisms behind their increased radiative efficiency at wavelengths of 530–590nm. The AlzGa1−zN (z~0.38) IL is ~1–2nm thick, and is grown after and at the same growth temperature as the ~3nm thick InGaN quantum well (QW). This is followed by an increase in temperature for the growth of a ~10nm thick GaN barrier layer. The insertion of the AlGaN IL within the MQW provides various benefits. First, the AlGaN IL allows for growth of the InxGa1−xN QW well below typical growth temperatures to achieve higher x (up to~0.25). Second, annealing the IL capped QW prior to the GaN barrier growth improves the AlGaN IL smoothness as determined by atomic force microscopy, improves the InGaN/AlGaN/GaN interface quality as determined from scanning transmission electron microscope images and x-ray diffraction, and increases the radiative efficiency by reducing non-radiative defects as determined by time-resolved photoluminescence measurements. Finally, the AlGaN IL increases the spontaneous and piezoelectric polarization induced electric fields acting on the InGaN QW, providing an additional red-shift to the emission wavelength as determined by Schrodinger-Poisson modeling and fitting to the experimental data. The relative impact of increased indium concentration and polarization fields on the radiative efficiency of MQWs with AlGaN ILs is explored along with implications to conventional longer wavelength emitters.

High brightness InGaN-based yellow light-emitting diodes with strain modulation layers grown on Si substrate

InGaN-based multiple quantum wells (MQWs) yellow light-emitting diodes (LEDs) were grown on Si substrate by metal organic vapor deposition. Blue MQWs were introduced as strain modulation layers for yellow MQWs. The LED chips emitted 72-mW yellow light with 566-nm dominant wavelength and 9.4 % external quantum efficiency (EQE) at 350 mA under room temperature, and it reached a peak EQE of 22.2 % at 0.7 mA. A comparison sample without strain modulation layers exhibited much weaker performance. The results reveal that long-wavelength emission of InGaN system is reliable if the strain of MQWs has been properly modulated.

Effect of interface grading and lateral thickness variation on x-ray diffraction by InGaN/GaN multiple quantum wells

We develop a method for simulating the effects of interface grading and lateral variation in layer thickness on x-ray diffraction by InGaN/GaN multiple quantum wells (MQWs). Using the resulting simulation scheme, we perform detailed fitting of symmetric (0002) ω/2θ scans measured for a selection of typical InGaN/GaN MQW heterostructures. We find that incorporation of the combined effects of interface grading and thickness variation substantially improves the goodness of fit relative to a conventional model that assumes ideal MQW structure. The improved simulations of experiments reveal that the examined InxGa1−xN/GaN MQWs (0.17≤x≤0.24) grown on the basal plane of GaN have graded heterointerface widths, w, in the range 0.5≤w≤1.1nm concomitant with lateral variations in total MQW thickness of 0.7–6.3nm rms. Atomic force microscopy of 10×10μm2 areas of the as-grown MQWs finds surface roughnesses of 1.0–5.6nm rms in agreement with corresponding rms thickness variations found by simulating the XRD measurements. For samples with smaller thickness variations, higher order MQW satellites are observed in high-dynamic-range diffraction experiments and best-fit simulations find evidence for asymmetric MQW interface widths. The lower interfaces are narrower than the upper interfaces in agreement with recent transmission electron microscopy and atom-probe studies of MQW interfaces by other groups. These under-recognized structural features—heterointerface grading and lateral film-thickness variation—will influence not only x-ray diffraction, but also polarization, bandstructure, and carrier localization within InGaN-based MQW heterostructures.

Characteristics of indium incorporation in InGaN/GaN multiple quantum wells grown on a-plane and c-plane GaN

We investigated the characteristics of InGaN-based multiple quantum wells (MQWs) grown on a-plane and c-plane GaN templates, which were grown by metal-organic chemical vapor deposition onto r-plane and c-plane sapphire, respectively. A shorter photoluminescence peak wavelength and peaks with larger full-width at half-maximum are observed for MQWs grown on an a-plane GaN template compared with a c-plane GaN template, despite the same growth conditions used. A growth model based on the atomic configuration of the growing surfaces is proposed to explain the difference in optical emission properties and indium incorporation between a-plane and c-plane MQWs.

Electron microscopy investigations of V defects in multiple InGaN/GaN quantum wells and InGaN quantum dots

The mechanism of high emission of InGaN-based multiple quantum wells, which exhibit exceptionally high light emission efficiency despite their high defect density, is still not fully understood. Here, we deal with this problem, showing the details of structure and formation of V defects in the multiple quantum wells and reviewing interpretations proposed so far. Then, we show a structural investigation of three-dimensional high-density quantum dots, fabricated instead of quantum wells in the active layer. The shape and size of the InGaN quantum dots and the SiN(x) masks for the growth of the dots have been revealed using high-angle annular dark field scanning transmission electron microscopy, energy dispersive X-ray spectroscopy nanoanalysis and high-resolution transmission electron microscopy.

Influence of threading dislocations on the properties of InGaN-based multiple quantum wells

The relationship of the threading dislocations (TDs) and InGaN-based quantum well (QW) optical characteristics is studied by temperature-dependent photoluminescence, high-resolution X-ray diffraction and atomic force microscopic. It is found that TDs deteriorate the QW interfacial abruptness, and can enhance the localization effect. Temperature-dependent photoluminescence measurements give evidence that the TDs act as nonradiative recombination (NR) centers in InGaN active layer. According to these results, it is inferred that the influence of TD on the optical properties of InGaN-based QW depends on the competition of localization induced by TDs and the NR effect of TDs.