Blueshift Of Emission Energy Research Articles

Optical characterization of type-I to type-II band alignment transition in GaAs/AlxGa1−xAs quantum rings grown by droplet epitaxy

Optical properties of GaAs/AlxGa1−xAs quantum rings (QRs) grown on GaAs (1 0 0) by droplet epitaxy have been investigated as a function of the Al-composition in the AlxGa1−xAs barrier. A transition from type-I to type-II band alignment is observed for the QRs via photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements. While x ⩽ 0.45, the QR PL spectra show a blue-shift and an increasing intensity with increasing Al-composition, revealing the enhancement of quantum confinement in the QRs with type-I band alignment. While x ⩾ 0.60, the characteristic large blue-shift with excitation intensity and the much longer lifetime indicate the realization of a type-II band alignment. Due to the height fluctuation of QR structures grown by droplet epitaxy mode, it is not the large blue-shift of emission energy, but the long lifetime that becomes the more important feature to identify the type-II band alignment.

Effects of matrix layer composition on the structural and optical properties of self-organized InGaN quantum dots

Self-organized InGaN quantum dots (QDs) with emission wavelength from green to red range have been grown on GaN templated c-plane sapphire substrates by metal-organic chemical vapor deposition (MOCVD). The effects of matrix layer composition on the structural and optical properties of InGaN QDs have been investigated. A continued growth of QDs is observed during the growth of In0.1Ga0.9N matrix layer, which results in an increase of the QDs' size. By using In0.1Ga0.9N matrix layer instead of GaN one, the annealing induced blue-shift in emission energy of the InGaN QDs can be suppressed. After the growth of top GaN cap layer, a larger red-shift caused by the quantum confined Stark effect is observed in the sample with In0.1Ga0.9N matrix layer. Employing this method, InGaN QD sample emitting at 615 nm with an internal quantum efficiency of 24.3% has been grown. The significance of this method is that it allows a higher growth temperature of InGaN QDs with emission wavelength in the green range to improve the crystalline quality, which is beneficial to enhance the efficiency of green InGaN QD light-emitting-diodes and laser diodes.

CdS/CdSe lateral heterostructure nanobelts by a two-step physical vapor transportmethod

The two-dimensional heterostructure nanobelts with a central CdSe region andlateral CdS structures are synthesized by a two-step physical vapor transportmethod. The large growth rate difference between lateral CdS structures on both ± (0001) sidesof the CdSe region is found. The growth anisotropy is discussed in terms of the polar nature of theside ± (0001) surfaces of CdSe. High-resolution transmission electron microscopy reveals the CdSe centralregion covered with non-uniform CdS layer/islands. From micro-photoluminescencemeasurements, a systematic blueshift of emission energy from the central CdSe region inaccordance with the increase of lateral CdS growth temperature is observed. This resultindicates that the intermixing rate in the CdSe region with CdS increases with the increaseof lateral CdS growth temperature. In conventional CdSSe ternary nanostructures,morphology and emission wavelength were correlated parameters. However, the morphologyand emission wavelength are independently controllable in the CdS/CdSe lateralheterostructure nanobelts. This structure is attractive for applications in visibleoptoelectronic devices.

Theoretical studies on square-planar pincer platinum(II) complex and its SO 2 adducts: SO 2 detector and potential luminescent probe

We explored the electronic structures and spectroscopic properties of [Pt(Me 4NCN)]Cl ( 1) (NCN − = [2,6-(CH 2NMe 2) 2-C 6H 3] −) and its SO 2 adducts, [Pt(Me 4NCN)(SO 2)]Cl, which show three penta-coordinated modes ( trans- η 1-pyramidal ( 2), cis- η 1-pyramidal ( 3) and η 1-coplanar ( 4)), using the theoretical methods. For 2, a comparative study of ab initio and density functional theory (DFT) shows that the DFT method cannot describe molecular structures appropriately, especially the Pt–S bonding. Therefore, the structures of 1– 4 in the ground state were optimized by the ab initio MP2 method and those of the first two complexes in the lowest-lying triplet excited state were studied by the unrestricted MP2 method. The present studies reveal that trans- η 1-pyramidal SO 2 coordination mode is the most stable in the three conformations of [Pt(Me 4NCN)(SO 2)]Cl because of its lower total energy as well as the most overlapping between the HOMO and LUMO of SO 2 and the filled Pt d orbital, and least repulsion between the lone pair orbitals of SO 2 and the Lewis base (PtNCN). The spectroscopic calculations at the TD-DFT associated with the Polarizable Continuum Model (PCM) level indicate that the adsorption of SO 2 changes the nature of low-lying absorptions of 1, agreeing well with the color change observed in experiments. We predicted the phosphorescent emissions in the CH 2Cl 2 solution at 778 and 552 nm for 1 and 2, respectively. The coordination of SO 2 into Pt atom results in a significant blue shift in emission energy.

The influence of a capping layer on optical properties of self-assembled InGaN quantum dots

Optical properties of InGaN quantum dots (QDs) with and without a GaN capping layer have been investigated, showing a major difference between each other. Compared with the InGaN QDs with a GaN capping layer, those grown under identical conditions but without the GaN capping layer showed much stronger photoluminescence (PL) emission and a ∼350 meV blueshift in emission energy. The excitation power-dependent PL measurements indicated that the emission energy of the QDs with the capping layer showed a large blueshift with increasing excitation power, while there was a negligible shift in the QDs without the capping layer. The major difference between them is attributed to existence of a strong quantum-confined Stark effect (QCSE) in the QDs with the capping layer, while there is no clear QCSE observed in the uncapped QDs. The transition energy has been calculated for both QDs within the framework of effective-mass approximation and variational approach, showing a good agreement with the experimental data. The results obtained should be highly taken into account in investigating the optical properties of InGaN QDs on a GaN surface.

Optical and morphological properties of InGaAs/AlGaAs self-assembled quantum dot nanostructures for 980 nm room temperature emission

This work deals with the study of optical and morphological properties of InGaAs/AlGaAs quantum dot (QD) structures grown by molecular beam epitaxy (MBE). Photoluminescence (PL) emission energies, activation energies of PL quenching and QD sizes are studied as functions of the Al content in the AlyGa1-yAs confining layers (CL). We show that the PL emission energy of In(Ga)As/AlyGa1-yAs QD structures increases with increasing y and that the sizes of InAs/AlyGa1-yAs QDs decrease with increasing y. By the comparison of the experimental results with those of an effective-mass model developed to calculate the QD fundamental transition energies, we show that the blueshift of emission energy has to be ascribed not only to the increase in barrier discontinuities that confine the carriers into QDs but even to effects related to changes of the QD morphology dependent on CL composition. Moreover, we show that the Al content in the barriers determines also the activation energy of thermal quenching of PL, which depends on the thermal escape of carriers from QD levels. These studies resulted in the preparation of structures with efficient light-emission in the 980 nm spectral window of interest for lightwave communications.

Blueshifts of emission energy from InAs quantum dots in GaAs matrix due to narrowed interdot spacing: a token of the integrity of a nanostructure

The shifts of emission energy from InAs quantum dots (QDs) in the GaAs matrix due to change in the QD stacking period have been numerically analyzed by investigating the strain modified shifts of band edges. Narrower stacking results in a blueshift of emission energy from direct band gap transition provided the QD spacing is wide enough to avoid electronic coupling. Through a comparison with existing experimental results in the literature, processing methods and variables that have benefits in achieving a blueshift have been inferred. A blueshift through decreased QD spacing has been proposed as a token of the integrity of a processed QD nanostructure .

MBE growth and properties of GaAsSbN/GaAs single quantum wells

The growth and properties of GaAsSbN single quantum wells (SQWs) are investigated in this work. The heterostructures were grown on GaAs substrates in an elemental solid source molecular beam epitaxy (MBE) system assisted with a RF plasma nitrogen source. A systematic study has been carried out to determine the influence of various growth conditions, such as the growth temperature and the source shutter-opening sequence, on the quality of the grown layers and the incorporation of N and Sb. The effects of ex situ and in situ annealing under As overpressure on the optical properties of the layers have also been investigated. Substrate temperature in the range of 450–470 °C was found to be optimum. Simultaneous opening of the source shutters was found to yield sharper QW interfaces. N and Sb incorporations were found to depend strongly upon substrate temperatures and source shutter-opening sequences. A significant increase in PL intensity with a narrowing of PL line shape and blue shift in emission energy were observed on annealing the GaAsSbN/GaAs SQW, with in situ annealing under As overpressure providing better results, compared to ex situ annealing.

1.28μm lasing from stacked InAs∕GaAs quantum dots with low-temperature-grown AlGaAs cladding layer by metalorganic chemical vapor deposition

We report the device characteristics of stacked InAs∕GaAs quantum-dot lasers cladded by Al0.4Ga0.6As layer grown at a low temperature by metalorganic chemical vapor deposition. A blueshift in emission energy by the effect of postgrowth annealing can be suppressed when the annealing temperature is below 570°C. We achieved the 1.28μm continuous-wave lasing at room temperature of five layer stacked InAs∕GaAs quantum dots embedded in In0.13Ga0.87As strain-reducing layer whose p-cladding layer is grown at 560°C. From the experiments and calculations of the gain spectra of fabricated quantum-dot lasers, the observed lasing originates from the first excited state of stacked InAs quantum dots.

Comparison of the Strain-modified Band Gap Energies of Truncated and Untruncated InAs Quantum Dots in GaAs Matrix at Varying Inter-dot Spacings

The change in strain-modified band gap energies in truncated and untruncated InAs quantum dots (QDs) in the GaAs matrix has been comparatively investigated as a function of vertical inter-dot spacing, based on the strain-modified band lineups. The direct band gap energy is larger for truncated QDs at a given inter-dot spacing and shows more uniform band lineups in the truncated QD than in the untruncated counterpart, regardless of stacking distance. For both QD shapes, a narrower stacking increases the direct band gap energy. However, excessively close inter-dot spacing yields an indirect transition in which band gap decreases as dot spacing is narrowed further. From the viewpoint of band lineups, for both QD shapes, a blueshift of emission energy with a closer inter-dot spacing is anticipated in a large dot spacing regime and a redshift in a small spacing regime. The experimental results in the literature, reporting either blueshift or redshift as the QDs are stacked closer, have been compared with the current results and discussed.

In As ∕ Ga As self-assembled quantum-dot lasers grown by metalorganic chemical vapor deposition—Effects of postgrowth annealing on stacked InAs quantum dots

We investigated the effects of postgrowth annealing on stacked InAs∕GaAs quantum dots. The blueshift in emission energy by postgrowth annealing depends on the temperature of postgrowth annealing and the growth conditions of stacked InAs quantum dots, such as a spacer thickness or a stacking number. We can control the peak wavelength of stacked InAs quantum dots by changing the temperature of postgrowth annealing and the growth conditions of stacked InAs quantum dots. We achieved continuous-wave lasing with a threshold current of 16.4mA at the wavelength of 1.245μm from five layer vertically aligned InAs quantum dots whose upper cladding layer was grown at 600°C.

MBE Growth Study of GaAsSbN/GaAs Single Quantum Wells

ABSTRACTIn this work, the growth and properties of GaAsSbN single quantum wells are investigated. The heterostructures were grown on GaAs substrates in an elemental solid source molecular beam epitaxy system with a RF plasma nitrogen source. A systematic study has been carried out to determine the influence of growth temperature on the optical properties of the layers. For reference low temperature photoluminescence (PL) characteristics of the GaAsSb/GaAs QWs as a function of Sb is also presented. A significant increase in PL intensity with a corresponding blue shift in emission energy and a decrease in full width at half maximum (FWHM) has been observed on annealing the GaAsSbN/GaAs sample in a nitrogen ambient at 700°C. PL emission wavelength as long as 1.52 μm at room temperature has been obtained on annealed samples.

Optical properties of InGaN/GaN double quantum wells with varying well thickness

The optical properties and recombination kinetics of the InGaN/GaN double quantum well (DQW) structures with different well thickness (L w ) have been studied by means of photoluminescence (PL), time-resolved PL, and cathodoluminescence (CL) measurements. With increasing quantum well thickness up to 4 nm, the PL emission energy decreases and the blueshift of the PL emission energy increases with increasing excitation density. On the other hand, the PL emission energy of the DQWs with L w =16 nm is higher than that of the DQWs with L w =4 nm, and is independent of the excitation density. With increasing L w from 1 to 4 nm, the PL decay times increase. In contrast, the decay times of 16 nm DQWs are faster than those of 4 nm DQWs. These different results for 16 nm DQWs such as the blueshift of the emission energy, the decrease of the excitation density dependence, and the increase of recombination rate can be ascribed to the relaxation of the piezoelectric field. We also observed the inhomegeneity in the CL spectra of the DQWs with L w =1 nm on 1 μm scale.

Syntheses, Structures, and Electroluminescence of New Blue/Green Luminescent Chelate Compounds: Zn(2-py-in)2(THF), BPh2(2-py-in), Be(2-py-in)2, and BPh2(2-py-aza) [2-py-in = 2-(2-pyridyl)indole; 2-py-aza = 2-(2-pyridyl)-7-azaindole

Four novel blue/green luminescent compounds, Zn(2-py-in)2(THF) (1), BPh2(2-py-in) (2), Be(2-py-in)2 (3), and BPh2(2-py-aza) (4), where 2-py-in = 2-(2-pyridyl)indole and 2-py-aza = 2-(2-pyridyl)-7-azaindole, have been synthesized and fully characterized. The 2-py-in ligand and 2-py-aza ligand in the new compounds are chelated to the central atom. Compounds 2−4 are air stable and readily sublimable, with a melting point above 250 °C. In the solid state, compounds 1−4 have an emission maximum at λ 488, 516, 490, and 476 nm, respectively. The structures of compounds 2 and 4 are similar. The blue shift of emission energy displayed by compound 4, in comparison to that of 2, is attributed to the presence of an extra nitrogen atom in the 2-py-aza ligand as confirmed by ab initio calculations on compounds 2 and 4. Electroluminescent devices of compounds 3 and 4 were fabricated by using N,N‘-di-1-naphthyl-N,N‘-diphenylbenzidine (NPB) as the hole transporting layer, Alq3 (q = 8-hydroxyquinolato) as the electron tran...