QW Emission Research Articles

Influence of pulsed Al deposition on quality of Al-rich Al(Ga)N structures grown by molecular beam epitaxy

Surfaces of AlN epitaxial layers grown by molecular beam epitaxy (MBE) in continuous mode were analyzed by means of X-ray photoelectron spectroscopy, atomic force microscopy and reflection high-energy electron diffraction without breaking the ultra-high vacuum. Our studies show the presence of the Al-Al and N-N bonds on the AlN samples, as well as a rather rough 3D-like structure. Significant improvement of the surface quality was achieved by employing a low flux Al deposition in pulsed mode at the end of the MBE growth (still under a nitrogen background pressure). Subsequently, on the obtained AlN buffer layers, Al0.85Ga0.15N/GaN multiple quantum wells (MQWs) were grown to show the influence of the buffer layers quality on the entire structure properties. The photoluminescence studies indicate that the introduction of pulsed low flux Al deposition improves optical properties of the grown MQWs layers, i.e., the relative intensity between the QW emission and the low energy emission is clearly in favor of pulsed Al deposition.

Simulation study on light color conversion enhancement through surface plasmon coupling.

A theoretical model together with a numerical algorithm of surface plasmon (SP) coupling are built for simulating SP-enhanced light color conversion from a shorter-wavelength radiating dipole (representing a quantum well - QW) into a longer-wavelength one (representing a quantum dot - QD) through QD absorption at the shorter wavelength. An Ag nanoparticle (NP) located between the two dipoles is designed for producing strong SP couplings simultaneously at the two wavelengths. At the QW emission wavelength, SP couplings with the QW and QD dipoles lead to the energy transfer from the QW into the QD and hence the absorption enhancement of the QD. At the QD emission wavelength, SP coupling with the excited QD dipole results in the enhancement of QD emission efficiency. The combination of the SP-induced effects at the two wavelengths leads to the increase of overall color conversion efficiency. The color conversion efficiencies in using Ag NPs of different geometries or SP resonance behaviors for producing different QD absorption and emission enhancement levels are compared.

Enhancing the spontaneous emission rate by modulating carrier distribution in GaN-based surface plasmon light-emitting diodes.

Based on the nanorod structure, we have fabricated GaN-based surface plasmon light-emitting diodes with Ag nanoparticles deposited laterally proximity to the multiple quantum wells (MQWs) region, which allows us to investigate the quantum well - surface plasmon (QW-SP) coupling effect. Our results show that the QW-SP coupling effect increases significantly when the SP resonant wavelength of Ag nanoparticles is close to the QW emission wavelength, especially by using a shorter wavelength light source, which will further enhance the spontaneous emission rate. Combined with the simulations, we find that the enhancement is due to the decreased excitation light penetration depth into the active region, which can modulate the carrier distribution and increase the proportion of SP-coupled carriers in the MQWs of LEDs. To increase the spontaneous emission rate for the electrical QW-SP coupled LEDs, we can use single QW or MQW structure to confine the carriers in the topmost QW, which will effectively increase the proportion of SP-coupled carriers. Our findings pave a way to design the ultrafast LED light source for the application of visible light communication (VLC).

The photoluminescence properties of QWs with asymmetrical step-like InGaN/GaN quantum barriers

The asymmetrical structures were created by inserting a low-indium-content layer between the QW and barrier to form a step-like quantum barrier (QB) at one side of QW. The optical effects of the inserting layer on QW emission were investigated with low-temperature photoluminescence (PL) and time-resolved PL (TRPL). The inserted layer partially relaxed the strain within QW layer and induced about 25nm red-shift in the PL emission compared with conventional QW, while the presence of localization centers around QW affected the emission mechanism and increased the radiative decay time. Furthermore, the position of the inserted layer played different roles in the changed structures, and whilst the n-side step-barrier exhibited strong localization in the energy levels of the inserted layer, the p-side step-barrier showed stronger localization center for the QW levels.

Blue and green electroluminescence from CdSe nanocrystal quantum-dot-quantum-wells

CdS/CdSe/ZnS quantum dot quantum well (QDQW) nanocrystals were synthesized using the successive ion layer adsorption and reaction technique, and their optical properties were tuned by bandgap and strain engineering. 3-monolayer (ML) CdSe QWs emitted blue photoluminescence at 467 nm with a spectral full-width-at-half-maximum of ∼30 nm. With a 3 ML ZnS cladding layer, which also acts as a passivating and strain-compensating layer, the QDQWs acquired a ∼35% quantum yield of the QW emission. Blue and green electroluminescence (EL) was obtained from QDQW light-emitting devices with 3–4.5 ML CdSe QWs. It was found that as the peak blueshifted, the overall EL was increasingly dominated by defect state emission due to poor hole injection into the QDQWs. The weak EL was also attributed to strong field-induced charge separation resulting from the unique QDQW geometry, weakening the oscillator strength of optical transitions.

Enhancement of GaAs/InGaAs quantum well emission by disordered gold nanoparticle arrays

Coupling effect of surface plasmon (SP) with InGaAs/GaAs QW emission is demonstrated experimentally. The SP resonance is generated by disordered arrays of Au nanodisks on the InGaAs/GaAs QW surface. More than twofold enhancement in QW PL is observed. Theoretical simulations also indicated that the disordered arrays of Au structures enlarged the cone angle for which light can be radiated out. The larger angle enhances the PL intensity.

Modeling of III-Nitride Multiple-Quantum-Well Light-Emitting Structures

Spatial inhomogeneity of carrier injection across the multiple-quantum-well (MQW) active region of a semiconductor light emitter can impose severe limitations on the device efficiency. In III-nitride-based devices, the large disparity of electron and hole transport and the excessive depth of active QWs trigger the process of inhomogeneous QW injection which is further aggravated by strong dependence of QW radiative characteristics on the QW injection conditions due to 1) intra-QW screening of polarization fields in polar and semipolar materials, 2) phase-space filling effect in lowest QW subbands at higher levels of carrier injection, and 3) exceedingly nonequilibrium character of the electron and hole populations in deep QWs. All these tendencies become more pronounced in longer wavelength emitters. The residual QW charges provide strong feedback to the QW injection conditions, thus requiring a high level of self-consistency between the active region transport simulation and the QW emission modeling.

Microwire (Mg,Zn)O/ZnO and (Mg,Zn)O/(Cd,Zn)O non-polar quantum well heterostructures for cavity applications

Pulsed-laser deposited, non-polar MgxZn1−xO/ZnO and MgxZn1−xO/Zn1−yCdyO quantum well heterostructures were fabricated in radial direction on ZnO microwires with well-defined hexagonal cross section. Optical resonances modulate room-temperature luminescence spectra for all fabricated heterostructures demonstrating their applicability as microcavities. Quantum confinement was proven by time-integrated and time-resolved luminescence. The ZnO quantum well emission was tuned between 3.76 and 3.35 eV by adjusting the well thickness and barrier composition. In order to further reduce the QW emission energy, active Zn1−yCdyO quantum wells in MgxZn1−xO barriers were grown emitting between 3.07 and 2.70 eV for different well thicknesses but fixed barrier composition.

Spontaneous polarization field in polar and nonpolar GaInN/GaN quantum well structures

AbstractWe present a UHV cathodoluminescence study of the spontaneous polarization field in polar [0001] and nonpolar $[1{\bar {1}}00]$ GaN/GaInN single quantum well structures. Employing electron‐stimulated desorption a descreening of the spontaneous field in the polar c‐plane samples was observed, which counteracts the piezoelectric field in the quantum well. While in c‐plane samples a strong blueshift of the emission energy takes place and a drastic increase of emission intensity occurs, no change of the QW emission energy and intensity from m‐plane samples was detected. The different behaviour under the same experimental conditions can only be explained with the absence of the spontaneous polarization field in the nonpolar $[1{\bar {1}}00]$ m‐direction.

Tuning the spectrometric properties of white light by surface plasmon effect using Ag nanoparticles in a colour converting light-emitting diode

We report on the spectral tunability of white light by localized surface plasmon (LSP) effect in a colour converting hybrid device made of CdSe/ZnS quantum dots (QDs) integrated on InGaN/GaN blue light-emitting diodes (LEDs). Silver (Ag) nanoparticles (NPs) are mixed with QDs for generating LSP effect. When the plasmon absorption of Ag NPs is synchronized to the QW emission at 448 nm, the NPs selectively absorb the blue light and subsequently enhance the QD emission. Using this energy transfer scheme, the ( x, y) chromaticity coordinates of the hybrid white LED was tuned from (0.32, 0.17) to (0.43, 0.26), and thereby generated warm white light emission with correlated colour temperature (CCT) around 1800 K. Moreover, a 47% enhancement in the external quantum efficiency (EQE) was realized.

Band alignment of lattice-matched InGaPN/GaAs and GaAs/InGaPN quantum wells grown by MOVPE

The band alignment of nearly lattice-matched In 0.528Ga 0.472P 1− y N y /GaAs and GaAs/In 0.528Ga 0.472P 1− y N y quantum wells with N content of y=0.027 on GaAs (0 0 1) substrates grown by metalorganic vapor phase epitaxy were studied using low-temperature and temperature dependent photoluminescence (PL). Low temperature (10 K) PL shows the emission ( E PL,QW) in the infrared region which related to sample’s structures. For GaAs/In 0.528Ga 0.472P 0.973N 0.027 QW, the E PL,QW emission was observed at temperature up to 260 K. The results reveal that the In 0.528Ga 0.472P 0.973N 0.027/GaAs and GaAs/In 0.528Ga 0.472P 0.973N 0.027 quantum wells exhibit a type-II quantum structure. Valence band offset Δ E V as large as 450 meV was estimated for GaAs/In 0.528Ga 0.472P 0.973N 0.027 quantum well while the conduction band offset Δ E C was estimated to be 160 meV for In 0.528Ga 0.472P 0.973N 0.027/GaAs quantum well. This type-II quantum structures refer to the natural type-II band alignment between In x Ga 1− x P 1− y N y ( x=0.528, y=0.027) and GaAs, which is useful for separation electrons and holes in the electronic and photovoltaic applications.

Formation of nonuniformity in ZnSe/ZnMgSSe quantum well structures during MOVPE on GaAs(001) misoriented by 10° to (111)A plane

ZnSe/ZnMgSSe quantum well structures grown by metal-organic vapor-phase epitaxy on GaAs substrate misoriented from (0 0 1) to (1 1 1) A by 10° were studied. Low-temperature cathodoluminescence spectra of these structures showed doubling QW emission lines. The doubling was observed also at addition of Cd, Mg and S in the QWs with concentration of several percents. The reason of this effect was investigated by X-ray reflectometry and diffraction methods. Increasing of scattering with increasing penetration depth was found. It evidences changing QW parameters during the growth. Decomposition of ZnMgSSe solid solution was also observed. The main reason of the doubling was supposed to be due to mutual diffusion of Zn and Mg atoms at QW interfaces. The diffusion is stimulated by relaxation of internal stains or nonuniform distribution of point defects due to the ZnMgSSe decomposition.

Effect of the AlGaN electron blocking layer thickness on the performance of AlGaN-based ultraviolet light-emitting diodes

The effect of the AlGaN electron blocking layer (EBL) thickness on the electrical and optical properties of 310 nm AlGaN single quantum well (SQW) light-emitting diodes (LEDs) has been investigated. A large ideality factor extracted from the current–voltage ( I– V) characteristics indicates that a tunneling mechanism dominates the carrier transport process in the LEDs. The ideality factor decreases with increasing EBL thickness suggesting that deep-level state assisted tunneling is reduced. In addition, the QW emission intensity is enhanced with the introduction of an EBL due to the reduction of electron overflow to the p-type layer. The QW emission intensity is sensitive to the EBL thickness. This is attributed to the reduction of electron tunneling to the p-type layer with an EBL.

Dynamical study of the radiative recombination processes in GaN/AlGaN QWs

The effects of the Si doping level on the recombination dynamics and carrier (exciton) localization in modulation-doped GaN/Al0.07Ga0.93N multiple-quantum-well (MQW) structures were studied by means of photoluminescence (PL) and time-resolved PL measurements. All samples with different doping levels show a QW emission which is blue shifted with respect to the 3.48 eV PL peak from the GaN buffer layer. The decay time at the peak position remains nearly constant in the range of 320–420 ps at 2 K for all doping levels. For the undoped and low doped samples (3 × 1018 cm−3), which have less free electrons in the QWs, a non-exponential PL decay behaviour at 2 K is attributed to localized exciton recombination. The more highly doped samples (5 × 1018 cm−3 to 1020 cm−3) show almost exponential decay curves at 2 K, suggesting the recombination of free electrons and localized holes. The internal polarization-induced fields of the medium and highly-doped samples are partly screened by the electrons originating from the doping in the barriers. The emission peaks in time delayed PL spectra of these samples exhibit almost no shift as time evolves. Only the PL peak of the undoped and low-doped samples shows a redshift with time delay, related to the photogenerated carriers [1]. The decay time for the undoped sample shows non-exponential behaviour typical for localized excitons in III-N QWs. The same behaviour of decay time as a function of emission energy has been reported for InGaN QWs [2].

Recombination dynamics and lasing in ZnO∕ZnMgO single quantum well structures

We report a time-resolved study of the recombination dynamics in molecular beam epitaxy grown ZnO∕ZnMgO single quantum wells (SQWs) of 1.0–4.5nm width. The SQWs exhibit different emission properties, depending on both the well width and defect density. Stimulated emission has been achieved at room temperature in a separate confinement double heterostructure having a 3nm wide SQW as an active region. It has been found that a critical parameter for the lasing is the inhomogeneous broadening of both QW and barrier emission bands.

LOW TEMPERATURE ELECTROLUMINESCENCE OF GREEN AND DEEP GREEN GaInN/GaN LIGHT EMITTING DIODES

Electroluminescence of GaInN/GaN multiple-quantum-well (QW) light-emitting diodes emitting in the green spectral region is analyzed at variable low temperature. Spectra are dominated by QW emission at RT throughout 7.7 K. Below 150 K, donor-acceptor pair recombination appears that must be assigned to residual impurities in either the barriers or the p-layers. The current-voltage behavior reveals shunt paths that carry up to several mA at low bias voltages. Below 20 K those paths are frozen out, but the device still emits predominantly from the QW. The peak energy exhibits a blue shift from RT to 158 K followed by a red shift from 158 K to 7.7 K. The deeper binding energy at low temperatures can hardly be affect by the injection current indicating that saturation of low-density states cannot be responsible for the transition between red and blue shifts.

Monolithic white light emitting diodes with a broad emission spectrum

Monolithic white light emitting diode (LEDs) grown by molecular beam epitaxy (MBE) is an original approach to realize white LEDs. Using a Ga0.8In0.2N/GaN multi-quantum well (QW) structure with different QW thicknesses in the active region of a GaN p-n junction leads to white light emission provided that the different QW emission wavelengths and relative intensities are properly adjusted. However, the color rendering index (CRI) and the output power are still a limitation for the development of these LEDs. In this study, we propose to use a broad emission spectrum – from blue to red – with the objective of improving the CRI. Carrier recombination mechanisms in the LEDs are studied by electroluminescence (EL) measurements at room temperature and discussed as a function of the structure design. Results show a strong modification of the EL spectrum as a function of the applied voltage and an enhancement of the red emission component with the insertion of a GaN undoped spacer layer between the active region and the p-type layers. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence study of MOCVD-grown GaN/AlGaN MQW nanostructures: influenceof Al composition and Si doping

A detailed study of low-temperature photoluminescence (PL) in GaN/AlGaN multiplequantum well (MQW) nanostructures has been reported. We have investigated the effect ofSi doping and Al content on PL spectra and PL decay time of these structures. Thetemperature dependence of radiative as well as non-radiative lifetimes have beenevaluated between 2 K and room temperature for different Si doping. We found thatradiative recombination at higher temperatures even up to RT is stronger in thedoped sample, compared to the undoped one. Hole localization in GaN/AlGaNMQWs with different compositions of Al is demonstrated via PL transient decaytimes and LO phonon coupling. It is found that there is an increasing of thedecay time at the PL peak emission with increasing Al composition. For theundoped sample, a non-exponential PL decay behaviour at 2 K is attributed tolocalized exciton recombination. A slight upshift in QWs PL peak with increasingAl composition is observed, which is counteracted by the expected rise of theinternal QW electric field with increasing Al. The localization energies have beenevaluated by studying the variation of the QW emission versus temperature and wefound out that the localization energy increases with increasing Al composition.

Diamagnetic shift of disorder-localized excitons in narrowGaAs∕AlGaAsquantum wells

A correlation between the diamagnetic shift and transition energy of disorder-localized excitons is observed in magnetomicrophotoluminescence $(\ensuremath{\mu}\mathrm{PL})$ on narrow $\mathrm{GaAs}∕{\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ quantum wells (QW's). The QW's were grown by molecular-beam epitaxy without growth interruption at the interfaces. $\ensuremath{\mu}\mathrm{PL}$ spectra were obtained in a confocal setup with the magnetic field applied normal to the QW plane. The lowest-energy exciton states have the smallest diamagnetic coefficients; the exciton diamagnetic shift in the localized exciton tail of the QW emission spectra increases by a factor of 2 as a function of transition energy. The positive correlation between diamagnetic shift and emission energy reveals exciton localization by short-range correlated interface disorder.

Growth and characterization of GaN-based structures on SiCOI-engineered substrates

Silicon carbide (SiC) still shows the best properties as substrate for the growth of GaN and its alloys but suffers from a very high price. An innovative alternative for this substrate are SiC/SiO 2/Si (SiCOI) substrates, combining SiC and Si (silicon) substrate advantages thanks to the Smart Cut™ technology. These substrates consist of thin SiC layers (∼270 nm) bonded on (0 0 1) Si substrates. Using SiCOI substrates, up to 3 μm of GaN could be grown crack-free. The structures were investigated by atomic force microscopy (root mean square=0.86 nm), high-resolution X-ray diffraction and low-temperature (20 K) photoluminescence (PL) (FWHM (full-width at half-maximum)=4.9 meV). Electroluminescence test heterostructures consisting of InGaN/GaN multiple quantum wells (MQWs) were also deposited on SiCOI. Room temperature PL measurements resulted in a QW emission at around 440 nm with a FWHM of 7.8 nm. At electrical excitation, blue light emission was observed.