Photoluminescence Of Quantum Wells Research Articles

Low-threshold AlGaN-based UVB VCSELs enabled by post-growth cavity detuning

The performance of vertical-cavity surface-emitting lasers (VCSELs) is strongly dependent on the spectral detuning between the gain peak and the resonance wavelength. Here, we use angle-resolved photoluminescence spectroscopy to investigate the emission properties of AlGaN-based VCSELs emitting in the ultraviolet-B spectral range with different detuning between the photoluminescence peak of the quantum-wells and the resonance wavelength. Accurate setting of the cavity length, and thereby the resonance wavelength, is accomplished by using doping-selective electrochemical etching of AlGaN sacrificial layers for substrate removal combined with deposition of dielectric spacer layers. By matching the resonance wavelength to the quantum-wells photoluminescence peak, a threshold power density of 0.4 MW/cm2 was achieved, and this was possible only for smooth etched surfaces with a root mean square roughness below 2 nm. These results demonstrate the importance of accurate cavity length control and surface smoothness to achieve low-threshold AlGaN-based ultraviolet VCSELs.

GHz guided optomechanics in planar semiconductor microcavities

Hybrid opto, electro, and mechanical systems operating at several GHz offer extraordinary opportunities for the coherent control of opto-electronic excitations down to the quantum limit. We introduce here a monolithic platform for GHz semiconductor optomechanics based on electrically excited phonons guided along the spacer of a planar microcavity (MC) embedding quantum well (QW) emitters. The MC spacer bound by cleaved lateral facets acts as an embedded acoustic waveguide cavity with a high quality factor ( Q ∼ 10 5 ) at frequencies well beyond 6 GHz, along which the acoustic modes live over tens of µs. The strong acoustic fields and the enhanced optomechanical coupling mediated by electronic resonances induce a huge modulation of the energy (in the meV range) and strength (over 80%) of the QW photoluminescence, which, in turn, becomes a sensitive local phonon probe. Furthermore, we show the coherent coupling of acoustic modes at different sample depths, thus opening the way for phonon-mediated coherent control and interconnection of three-dimensional epitaxial nanostructures.

Combined effects of surface plasmon coupling and Förster resonance energy transfer on the light color conversion behaviors of colloidal quantum dots on an InGaN/GaN quantum-well nanodisk structure

By forming nanodisk (ND) structures on a blue-emitting InGaN/GaN quantum-well (QW) template, the QWs become close to the red-emitting quantum dots (QDs) and Ag nanoparticles (NPs) attached onto the sidewalls of the NDs such that Förster resonance energy transfer (FRET) and surface plasmon (SP) coupling can occur to enhance the efficiency of blue-to-red color conversion. With a larger ND height, more QWs are exposed to open air on the sidewall for more QD/Ag NP attachment through QD self-assembly and Ag NP drop casting such that the FRET and SP coupling effects, and hence the color conversion efficiency can be enhanced. A stronger FRET process leads to a longer QD photoluminescence (PL) decay time and a shorter QW PL decay time. It is shown that SP coupling can enhance the FRET efficiency.

GaAs layer on c-plane sapphire for light emitting sources

High-quality cubic GaAs (111)A buffer layers have been grown on an atomically flat c-plane trigonal sapphire substrate having well-defined steps and terraces. A two-step growth method has been used where, at an early stage, a GaAs layer has been grown at low temperature (LT), followed by second high-temperature GaAs growth layer. In addition to the two-step growth process, an AlAs nucleation layer and multiple annealing steps have been employed. The effectiveness of the LT GaAs layer in this highly dissimilar epitaxy was then investigated. An LT GaAs layer resulted in a relaxed GaAs buffer with smooth surface morphology and high crystalline quality. An InGaAs quantum well (QW) was epitaxially grown on the 70 nm GaAs buffer and compared with a reference InGaAs QW grown on a GaAs (111)A substrate. Along with x-ray and high-resolution cross-section transmission electron microscopy, comparable QW photoluminescence intensity and linewidth with respect to reference InGaAs QW confirmed the effectiveness of our growth strategies to produce high-quality GaAs on sapphire. This demonstrates the opportunity for GaAs photonics on sapphire and the potential to realise an integrated microwave photonic chip on a sapphire platform.

Photoluminescence of extended defects in heterostructures with InAlAs metamorphic buffer at different excitation wavelengths

We report the observation of photoluminescence (PL) of energy levels presumably associated with extended defects in metamorphic buffer InxAl1–xAs with x = 0.05 → 0.65 at photon energy range of 0.8–1.2 eV, while the quantum well PL was observed at 0.6–0.8 eV. The metamorphic HEMT heterostructures with In0.65Al0.35As/In0.70Ga0.30As/In0.65Al0.35As quantum well (QW) and with different defectiveness degree were studied. Interband excitation by lasers with a wavelength of 409, 450, 532 nm, and subband excitation by infrared (IR) laser with 1064 nm wavelength (with photons energy less than the bandgap of metamorphic buffer layer) both result in the appearance of “defect” PL. Compared to the visible excitation, the IR illumination with the same power density increases “defect” PL intensity in samples with high twins density by an order of magnitude, whereas in more perfect samples with stacking faults and threading dislocations it causes an increase in “defect-to-QW” PL intensities ratio. The study of PL intensity as a function of the pump power in the range of power density 10–3600 W/cm2 reveals that the non-radiative recombination plays an important role during the recombination of photoexcited charge carriers from “defect” levels because the “defect” PL intensity dependence on the pump power density is mainly sublinear while for the QW PL the power law with exponent 1.2–1.4 is observed.

Band structure engineering in strain-free GaAs mesoscopic systems

We investigate the optical properties of strain-free mesoscopic GaAs/AlxGa1 − xAs structures (MGS) coupled to thin GaAs/AlxGa1 − xAs quantum wells (QWs) with varying Al content (x). We demonstrate that quenching the QW emission by controlling the band crossover between AlGaAs (X-point) and GaAs (Γ-point) gives rise to long carrier lifetimes and enhanced optical emission from the MGS. For x = 0.33, QW and MGS show typical type-I band alignment with strong QW photoluminescence emission and much weaker sharp recombination lines from the MGS localized exciton states. For x ≥ 0.50, the QW emission is considerably quenched due to the change from type-I to type-II structure while the MGS emission is enhanced due to carrier injection from the QW. For x ≥ 0.70, we observe PL quenching from the MGS higher energy states also due to the crossover of X and Γ bands, demonstrating spectral filtering of the MGS emission. Time-resolved measurements reveal two recombination processes in the MGS emission dynamics. The fast component depends mainly on the X − Γ mixing of the MGS states and can be increased from 0.3 to 2.5 ns by changing the Al content. The slower component, however, depends on the X − Γ mixing of the QW states and is associated to the carrier injection rate from the QW reservoir into the MGS structure. In this way, the independent tuning of X − Γ mixing in QW and MGS states allows us to manipulate recombination rates in the MGS as well as to make carrier injection and light extraction more efficient.

Photoluminescence of CdTe quantum wells doped with cobalt and iron

Cobalt and iron dopants in semiconductors are killers of excitonic photoluminescence. In particular, photoluminescence of quantum wells doped with Co and Fe poses a challenge and has not been reported so far. Here, we show molecular beam epitaxy growth and photoluminescence studies of (Cd,Co)Te and (Cd,Fe)Te quantum wells in (Cd,Mg)Te. We compare the results to well-known structures: CdTe and (Cd,Mn)Te quantum wells, and we conclude that 0.2% of Fe reduces photoluminescence intensity by 3 orders of magnitude, and similar concentration of Co reduces PL intensity by about 5 orders of magnitude. We also present the influence of the dopant's concentration on emission efficiency in external magnetic field. Structures with low doping levels require weaker fields to reduce the quenching effect.

Magnetically controlled exciton transfer in hybrid quantum-dot–quantum-well nanostructures

A magnetophotoluminescence study of the carrier transfer with hybrid InAs/GaAs quantum dot(QD)-InGaAs quantum well (QW) structures is carried out where we observe an unsual dependence of the photoluminescence (PL) on the GaAs barrier thickness at strong magnetic field and excitation density. For the case of a thin barrier the QW PL intensity is observed to increase at the expense of a decrease in the QD PL intensity. This is attributed to changes in the interplane carrier dynamics in the QW and the wetting layer (WL) resulting from increasing the magnetic field along with changes in the coupling between QD excited states and exciton states in the QW and the WL.

Revealing Temperature-Dependent Absorption and Emission Enhancement Factors in Plasmon Coupled Semiconductor Heterostructures

Localized surface plasmon resonances can increase the quantum efficiency of photon emitters through both absorption and spontaneous emission enhancement effects. Despite extensive studies, experimental results that clearly distinguish the two plasmonic enhancement effects are rarely available. Here, we present clear spectral signatures of the plasmonic enhancement effects on the absorption (excitation) and spontaneous emission (Purcell factor) by analyzing the temperature-dependent photoluminescence (PL) properties of an InGaAs/GaAs single quantum well (QW) coupled to colloidal gold nanorods (AuNRs) at different GaAs capping layer thicknesses (d). We find that when the emitting InGaAs layer is close to the AuNRs (d = 5 nm), the plasmonic enhancement effect on the QW PL is dominated by the Purcell factor, which significantly increases the external quantum efficiency of the QW that otherwise barely emits. When d is increased to 10 nm, the temperature dependence of the PL enhancement factor (F) reflects absorption enhancement in the capping layer followed by carrier diffusion and capture by the well. First, F increases with temperature and then decreases following the temperature dependence of the carrier diffusion coefficient in GaAs. By factoring out the contribution of the captured carriers to F, it is shown that carrier transfer to the well reaches saturation with increasing incident laser power. In addition to providing insight into the plasmonic enhancement mechanism, the results presented in this work suggest that colloidal plasmonic nanoparticles can be used as simple probes for understanding carrier transport phenomena in arbitrary semiconductor heterostructures.

Functionalized GaN/GaInN heterostructures for hydrogen sulfide sensing

Near-surface GaN/GaInN quantum wells (QWs) were investigated as optical transducers for the detection of hydrogen sulfide. The heterostructure sensors were grown by metal organic vapor phase epitaxy and later covered by a thin layer of Au by electron beam evaporation. The QW photoluminescence (PL) is sensitive to changes in the sensor surface potential. By the adsorption of hydrogen sulfide (H2S) on the Au cover layer, downward near-surface band bending results in an increase of the quantum confined Stark effect in the GaInN QW producing a red shift in its luminescence. Unexpectedly, an increase in PL intensity is also observed. A concentration of 0.01 parts per million of H2S in nitrogen has been successfully detected. This phenomenon may be helpful to detect trace amounts of H2S present in the human breath for early detection of diseases.

Strong suppression of In desorption from InGaN QW by improved technology of upper InGaN/GaN QW interface

The aim of this work is to elucidate how different growth mode and composition of barriers can influence the QW properties and their PL and to find optimal QW capping process, to suppress the In desorption from QWs and to maintain the QW PL efficiency. It concentrates on the technology procedure for growth of upper quantum well (QW) interfaces in InGaN/GaN QW structure when different temperature for QW and barrier epitaxy is used. We have found that optimal photoluminescence (PL) results were achieved, when the growth after QW formation was not interrupted, but immediately continued during the temperature ramp by the growth of (In)GaN capping layer with small introduction of In precursor into the reactor. Optimal barrier between QW with respect to PL results was found to be pure GaN. We have shown according to SIMS and HRTEM results that by this technological procedure the InGaN desorption was considerably suppressed and three times higher In concentration and two times thicker QWs were achieved for the same QW growth parameters without deterioration of PL intensity in comparison to sample with usually used thin GaN low temperature capping protection. Additionally, for samples covered by the QW capping layer during the temperature ramp the defect band is almost completely missing, thus we can conclude that this defect band is connected with quality of the upper QW interface.

Impurity-assisted terahertz photoluminescence in quantum wells under conditions of interband stimulated emission

Terahertz and near-infrared photoluminescence under conditions of interband stimulated emission are studied in n-GaAs/AlGaAs quantum well laser structure. The observed terahertz emission is related to the optical transitions of nonequilibrium electrons from the first electron subband and excited donor states to donor ground states in quantum wells. The opportunity to increase the intensity of impurity-assisted terahertz emission due to interband stimulated emission with the participation of impurity centres is demonstrated.

III/V nano ridge structures for optical applications on patterned 300 mm silicon substrate

We report on an integration approach of III/V nano ridges on patterned silicon (Si) wafers by metal organic vapor phase epitaxy (MOVPE). Trenches of different widths (≤500 nm) were processed in a silicon oxide (SiO2) layer on top of a 300 mm (001) Si substrate. The MOVPE growth conditions were chosen in a way to guarantee an efficient defect trapping within narrow trenches and to form a box shaped ridge with increased III/V volume when growing out of the trench. Compressively strained InGaAs/GaAs multi-quantum wells with 19% indium were deposited on top of the fully relaxed GaAs ridges as an active material for optical applications. Transmission electron microcopy investigation shows that very flat quantum well (QW) interfaces were realized. A clear defect trapping inside the trenches is observed whereas the ridge material is free of threading dislocations with only a very low density of planar defects. Pronounced QW photoluminescence (PL) is detected from different ridge sizes at room temperature. The potential of these III/V nano ridges for laser integration on Si substrates is emphasized by the achieved ridge volume which could enable wave guidance and by the high crystal quality in line with the distinct PL.

Influence of near-field coupling from Ag surface plasmons on InGaN/GaN quantum-well photoluminescence

We have investigated the borderline between photoluminescence quenching and enhancement of InGaN/GaN quantum-wells due to Ag nanoparticles and their surface plasmon modes. By embedding Ag nanoparticles inside nanohole structures on the p-type layer GaN, luminescence quenching is observed. Increasing the distance between the nanoparticles and quantum-wells has shown to enhance the emission. We have found that the nano-structure geometry of the metal-semiconductor interface in the near-field of the quantum-wells plays a crucial role in determining whether the emitter performance is enhanced or degraded.

Photoluminescence spectra of CdTe multi-quantum wells sandwiched by ultrathin MnTe layers and their temperature dependence

To study the energy transfer (ET) between the Mn2+ intra-center luminescence (IL) state and the band state in CdTe quantum wells (QWs), we studied the temperature dependences of the photoluminescence (PL) spectra of the CdTe multi-QWs sandwiched by ultrathin MnTe layers and found an anomalous thermal variation of their PL intensities. With elevating temperature, the PL intensities of the thinner QWs having 4 and 8 monolayers (MLs) remain at rather high temperatures than 100K despite the PL of the thicker QWs disappears rapidly. In the thinner QWs, the PL bands appear in an overlap region with the Mn2+ IL-band and energy transfer is expected between the IL-state and the band states in the thinner QWs. The anomalous thermal variations of the PL intensities can be explained by considering the ET and the role of Mn2+ IL-state as reservoir states against the thermal quenching processes.

Optical excitation dependent emission properties of InGaN quantum wells

Fully microscopic many-body calculations are used to study the carrier and polarization dynamics in optically excited InGaN quantum wells. For long-pulse or stationary (CW) optical excitation conditions, it is shown that the strong excitation-induced dephasing due to the excited charge carriers leads to an effective spectral width of the created polarization in the range of several hundred meV. The subsequent polarization to population conversion results in broad carrier distributions spreading over energy regions well above and below the central excitation energy. Without invoking Auger transitions, it is shown that this model can explain experiments observing ultraviolet quantum-well photoluminescence after resonant CW excitation of a neighboring green emitting quantum well Binder et al. (Appl. Phys. Lett. 103:071108, 2013).

The quantum‐confined Stark effect and localization of charge carriers in AlxGa1‐xN/AlyGa1‐yN quantum wells with different morphologies

AbstractWe present a comparative study of three samples with a single Al0.3Ga0.7N/Al0.4Ga0.6N quantum well (QW) grown by plasma‐assisted molecular beam epitaxy on c ‐sapphire substrates with an AlN buffer deposited under different Al/N flux ratios. We show by means of transmission electron microscopy that strong N‐rich growth conditions result in a three‐dimensional morphology that is inherited by the QW. As a result, the QW is corrugated on the size scale of the QW photoluminescence (PL) wavelength (∼300 nm).The Al‐rich conditions lead to a two‐dimensional growth and formation of a plane QW with flat interfaces. The PL line in the corrugated QW is 5 times more intensive and shifted toward larger energies by ∼200 meV, as compared with the plane QW. The superior emission properties are explained by simultaneous action of several factors such as suppression of the quantum‐confined Stark effect, enhanced carrier localization, and improved efficiency of light extraction. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of tunneling transfer on thermal redistribution of carriers in hybrid dot-well nanostructures

The thermally induced redistribution of carriers between quantum well (QW) and quantum dot (QD) layers in a hybrid dot-well system composed of InAs QDs and an InGaAs QW is studied by means of photoluminescence (PL) spectroscopy. This redistribution significantly affects the QD and QW PL intensities depending both on the dot-well barrier thickness and height. For comparatively thin barriers, the interplay between tunnel and thermal carrier fluxes becomes crucial, governing the exciton dynamics in a tunnel injection dot-well structure at elevated temperatures. For a sufficiently thick spacer, it is shown that exciton localization within the QW, apparently induced by QD strain fields, has a profound influence on the transfer dynamics at low temperatures.

High quality beaming and efficient free-space coupling in L3 photonic crystal active nanocavities

We report on far-field measurements of L3 photonic crystal (PhC) cavities with high quality beaming. This is achieved by means of the so-called "band folding" technique, in which a modulation of the radius of specific holes surrounding the cavity is introduced. Far-field patterns are measured from photoluminescence of quantum wells embedded in the PhC. A very good agreement between experimental results and simulated radiation patterns has been found. Laser effect is demonstrated in the beaming cavity with a threshold comparable to the regular one. In addition, free-space input coupling to this cavity has been achieved. In order to fully analyze the coupling efficiency, we generalize the approach developed in S. Fan, et al., [J. Opt. Soc. Am. A 20, 569 (2003)], relaxing the hypothesis of mirror symmetry. The obtained coupling efficiencies are about 15% with quality factors (Q) exceeding 10(4). These results further validate the "folding" technique on L3 cavities for nanocavity realization with efficient free-space coupling and high Q factors.

Magneto-optics of heterostructures with an InGaAs/GaAs quantum well and a ferromagnetic delta-layer of Mn

Magneto-optics of heterostructures with an InGaAs/GaAs quantum well (QW) and a 3–5 nm spaced ultra-thin ferromagnetic δ-layer of Mn in the GaAs barrier is investigated in detail. While nonmagnetic structures demonstrate very low degree of circular polarization PC of QW photoluminescence, the existence of the close Mn δ-layer gives rise to high values of PC even above its Curie temperature TC ∼ 35 K. Fast growth of PC and Zeeman splitting of the QW emission band, observed at low B < 0.2 T, is followed by a slow linear increase at B > 0.5 T in heterostructures with strictly oriented GaAs (001) substrates. The characteristic fast increase slows down with temperature and disappears above TC. The effect depends non-monotonically on QW depth and is explained by the model of strong QW fluctuation potential caused by the highly charged Mn δ-layer. The spin polarization of carriers is believed to be related to the effective (p–d)-exchange interaction of holes in the QW with the ferromagnetic δ-layer and to appear due to a normal-to-plane magnetization emerging from the plane of the δ-layer with saturation of the normal component of magnetization at high magnetic fields.