Quantum Well Emission Research Articles

Coherent control and quantum kinetics of LO-phonon emission in CdZnTe quantum wells

We study the quantum kinetics of LO-phonon cascade emission in quantum wells by electron-hole pairs, whose relaxation ends with a trapping in quantum dots. Modulations with a period given by the LO-phonon frequency appear progressively in the time resolved differential transmission spectrum, indicating the transition from the quantum kinetic to the classical regime.

Growth of non-polar ZnO/(Zn,Mg)O quantum well structures on R-sapphire by plasma-assisted molecular beam epitaxy

Zinc oxide (ZnO) has recently attracted considerable attention because of its unique physical properties and its potential applications in the blue and UV spectral range. Up to now, ZnO-based heterostructures have mostly been grown in a c-orientation. The growth of non-polar layers along the a-direction [ 1 1 2 ¯ 0 ] was proposed to avoid any built-in electric fields in the c-direction. A series of A-plane ( 1 1 2 ¯ 0 ) ZnO quantum wells (QWs) and multiple QWs embedded in (Zn,Mg)O barriers were grown on an optimized (Zn,Co)O buffer. After the structural and optical characterization of the buffers and the QWs, we have compared the PL emission of a-oriented and c-oriented QWs. From this comparison, we demonstrate that the QWs exhibit confinement but no indication of quantum-confined Stark effect, contrary to what is observed in c-oriented structures.

Comparison of carrier dynamics in GaN quantum dots and GaN quantum wells embedded in low-Al-content AlGaN waveguides

Comparative analysis of the carrier dynamics of GaN quantum dot (QD) and GaN quantum well (QW) separated confinement heterostructures (SCHs) with low-Al-content AlGaN waveguide layers is reported. A redshift (blueshift) of QD (wetting layer) emission is found with respect to QW emission, as expected from the thickness hierarchy of these objects. The influence of nonradiative processes on QD emission in QD SCH is dramatically reduced compared to the case of QW SCH. It is concluded that GaN QDs in low-Al-content AlGaN matrix are robust localization centers and that the carrier dynamics is seriously affected by the built-in internal field effect.

Optical properties of inGaAs/GaAs quantum wells grown by Sb-assisted molecular beam epitaxy

It is found that both methods using either continuous Sb supply or pre-deposition of a very thin Sb layer are efficient for the Sb-assisted molecular beam epitaxy growth of highly strained InGaAs/GaAs quantum wells (QWs). The emission of QWs is extended to long wavelength close to 1.25 μm with high luminescence efficiency at room temperature. The influence of rapid thermal annealing (RTA) on the photoluminescence intensity critically depends on the annealing temperature and duration for highly strained QWs. A relatively low RTA temperature of 700 °C with a short duration of 10 s is suggested for optimizing the annealing effect.

GaAs-based room-temperature continuous-wave 1.59μm GaInNAsSb single-quantum-well laser diode grown by molecular-beam epitaxy

Starting from the growth of high-quality 1.3μmGaInNAs∕GaAs quantum well (QW), the QW emission wavelength has been extended up to 1.55μm by a combination of lowering growth rate, using GaNAs barriers and incorporating some amount of Sb. The photoluminescence properties of 1.5μm range GaInNAsSb∕GaNAs QWs are quite comparable to the 1.3μm QWs, revealing positive effect of Sb on improving the optical quality of the QWs. A 1.59μm lasing of a GaInNAsSb∕GaNAs single-QW laser diode is obtained under continuous current injection at room temperature. The threshold current density is 2.6kA∕cm2 with as-cleaved facet mirrors.

Effects of In composition on ultraviolet emission efficiency in quaternary InAlGaN light-emitting diodes on freestanding GaN substrates and sapphire substrates

350 - nm -band quaternary InAlGaN quantum-well (QW) structures have been fabricated on low-dislocation-density GaN substrates and sapphire substrates, and the impact of the In composition on ultraviolet emission efficiency has been investigated. Photoluminescence, cathodoluminescence, and atomic force microscopy measurements have revealed that with increasing In concentration, band-gap inhomogeneity in InAlGaN layers is promoted and quantum efficiency becomes less sensitive to dislocation densities. However, this does not necessarily result in the enhancement of QW emission efficiency, because both a decrease in growth temperature and an increase in Al concentration degrade the InAlGaN crystalline quality. It is found that improving the InAlGaN crystalline quality by increasing the growth temperature while maintaining an In fluctuation effect with minimal In composition is essential for highly efficient InAlGaN QWs. The utilization of the optimized InAlGaN growth condition significantly improves the output power of 350-nm-band light-emitting diodes on GaN and sapphire substrates to 7.4mW at 400mA and 2.9mW at 200mA, respectively.

Gallium nitride surface quantum wells

Surface quantum-well emission has been observed from GaN-capped AlGaN∕GaN heterostructures grown by metalorganic chemical vapor deposition. The GaN cap, which forms the surface quantum well, is confined on one side by the vacuum level and on the other side by the AlGaN barrier layer. Photoluminescence at room temperature and cathodoluminescence studies show a strong emission peak corresponding to the lowest bound state of the surface quantum well and a correlation was made to the shift in surface quantum-well emission energy and the thickness of the GaN capping layer, which was varied from ∼15to40Å. The efficient surface quantum-well emission is indicative of low surface recombination velocities even without any surface passivation.

Quantum islands formation and optical properties of CdZnTe/ZnTe quantum wells grown by atomic layer epitaxy

AbstractIn this work we report on optical properties of CdZnTe/ZnTe quantum wells (QWs) grown by atomic layer epitaxy (ALE) and the formation of quantum island during the QWs growth. The photoluminescence (PL) emission of QWs at lower temperature (250 °C) consists of two peaks. From studying the dependence of the PL integrated emission with the excitation intensity, it is found that theses both PL features have different nature. The high‐energy sharp peak is associated to the excitonic recombination in the CdZnTe/ZnTe alloy quantum well while the low‐energy contributions (wide peak) are assigned to the recombination in the quantum islands (compositional fluctuations with high Cd molar fraction embedded inside the active zone of the QW). The temperature dependence of the PL FWHM, energy, and intensity confirm the enhancement of the quantum efficiency of optical recombination in quantum islands. The formation of quantum islands is more likely at lower growth temperature as evidenced by the optical properties of the QWs grown at different temperatures. Since the QWs growth is following an ALE procedure (Cd–Te–Zn–Te cycles), the existence of a Cd desorption induced by Zn exposure (Cd atoms are replaced by Zn atoms) leads to the formation of the compositional fluctuations (islands) on the growing surface at lower temperatures. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Reflection and emission of Brillouin zone edge states for active photonic crystal waveguides

A one-dimensional photonic crystal is patterned into an active planar waveguide containing multiple InGaAs quantum wells. External coupling reflectivity is used to map out the photonic band structure, revealing a clear anti-crossing at ~1.37 eV in the 45° TM polarized spectrum. The band structure is compared to and confirmed by scattering-matrix calculations. Photoluminescence measurements are performed in the same geometry on the patterned and unpatterned regions of the sample. The latter shows conventional quantum-well emission and GaAs emission with increasing pump power. In contrast, the spectra from the patterned sample contain a sharp mode from the dielectric-like band of the anti-crossing, corresponding to the Brillouin zone edge. Power-dependent excitation shows this mode to be strongly super-linear, indicative of band edge lasing.

Local Observation and Spectroscopy of Optical Modes in an Active Photonic-Crystal Microcavity

We report the direct, room-temperature, near-field mapping and spectroscopy of the optical modes of a photonic-crystal microcavity containing quantum wells. We use a near-field optical probe to reveal the imprint of the cavity mode structure on the quantum-well emission. Furthermore, near-field spectroscopy allows us to demonstrate the strong spatial and spectral dependence of the coupling between the sources and the microcavity. This knowledge will be essential in devising future nanophotonic devices.

Kinetics of excitonic complexes on tunneling devices

In this work we have investigated the effects of trion formation on the tunneling current from both experimental and theoretical viewpoints. We have measured the current-voltage characteristics and the quantum-well photoluminescence emission of $\mathrm{Ga}\mathrm{As}\text{\ensuremath{-}}{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ $n\text{\ensuremath{-}}i\text{\ensuremath{-}}n$ double barrier diodes. We have observed a preresonance shoulder in the current-voltage curve under high laser intensities associated with the formation of trions in the quantum well, which increase the number of free states in the resonant and excitonic levels, thus enhancing the tunneling mechanism. These excitonic complexes were detected through the photoluminescence spectra under bias. We have observed that the preresonance shoulder occurs under the same conditions for which a trion peak in the luminescence spectrum is present. This trion-assisted mechanism is terminated when neutral and charged excitons are dissociated either by thermal excitation or by scattering with free carriers in the quantum well. A phenomenological rate equation model has allowed us to confirm our assumptions on the effect of trion formation on the charge transport.

Growth of ZnO∕MgZnO quantum wells on sapphire substrates and observation of the two-dimensional confinement effect

Zn O ∕ Mg Zn O single quantum wells (QWs) in which the well width changes continuously were grown on sapphire (112¯0) substrates by metalorganic chemical vapor deposition. Photoluminescence (PL) measurement revealed two emission peaks: one is position dependent and the other is not. Polarized PL spectra obtained from cleaved facets demonstrated perfect two-dimensional features of the position-dependent emission peak. The position-dependent peak was attributed to emissions due to excitons confined in the ZnO well layer, and the position-independent peak was attributed to emissions due to excitons in MgZnO barrier layers. The width dependence of the emission energy from the ZnO QW was interpreted by a simple theoretical model. Typical PL decay time of the QW emission was 360ps at 77K. It was shorter than that of the MgZnO barrier, 470ps, due to the enhanced confinement effect in the QW.

The Role of Built-in Electric Fields in the InGaN∕GaN Quantum-Well Emission

By analyzing the evolution of time-resolved photoluminescence spectra, it is detected experimentally for the first time that there is a correlated effect of built-in electric fields and of long-lived localized states on the formation of emission in quantum wells based on nitrides of Group III elements. It is shown that light-emitting diode structures can be classified for commercial applications by studying time-resolved photoluminescence spectra.

Transport via excitonic complexes in resonant tunneling structures

In this work we study the formation of neutral and negatively charged excitons in double barrier diodes under bias, and how they contribute to transport. We observe a pre-resonance shoulder in the current–voltage curve, which is associated to trion-assisted tunneling of electrons. We analyze this phenomenon by measuring also the quantum-well photoluminescence emission. This trion-assisted mechanism is terminated when trion complexes are dissociated either by thermal excitation or by scattering with free carriers in the quantum well. A simple phenomenological rate equation model allows us confirming the hypothesis of charge transport via a trion state and the proposed methods of termination.

Studies of InGaN∕GaN multiquantum-well green-light-emitting diodes grown by metalorganic chemical vapor deposition

InGaN ( 3 nm ) ∕ GaN ( 5 nm ) three period multiquantum green-light-emitting diodes (LEDs) grown by the metalorganic chemical vapor deposition technique have been studied using high-resolution transmission electron microscopy (HRTEM), double crystal high resolution x-ray diffraction (HRXRD) and low temperature photoluminescence. HRTEM analysis showed that the defect density gradually decreased in the growth direction with increasing thickness. Self-assembled quantum dot-like structures in the wells and black lumps between the well and barrier due to In segregation and strain contrast were observed, respectively. The HRXRD spectrum of the green LED structure was simulated using the kinematical theory method to obtain the composition and thickness of the well and barrier. The quantum-well (QW) green emission peak 2.557eV at 10K showed “S” shaped shift like a red–blue–red shift with variation of the temperature in the photoluminescence spectra due to potential fluctuations caused by inhomogeneous alloy distribution in the wells. The activation energy of 49meV obtained from the QW green emission line indicated deepening of the localization of the carriers.

High power polarization-insensitive 1.3 µm InGaAsP–InP quantum-well superluminescent emission diodes grown by MOVPE

Polarization-insensitive 1.3 µm InGaAsP–InP multi-quantum-well (MQW) superluminescent diodes (SLDs) were grown by MOVPE. Both compressively-strained wells and tensile-strained wells were employed in the active region. The SLDs were fabricated into ridge waveguide structures with 7° tilted cavity and buried-window end facets. The two facets were coated with four layer anti-reflection TiO2/SiO2 films and residual facet reflectivity was found to be less than 0.02%. The SLDs exhibited up to 16.9 mW optical output power and less than 1 dB polarization dependence of output power with less than 0.2 dB optical spectrum modulation at 200 mA.

Luminescence of stepped quantum wells in GaAs/GaAlAs and InGaAs/GaAs/GaAlAs structures

Luminescence spectra of doped and undoped GaAs/GaAlAs and InGaAs/GaAs/GaAlAs structures containing several tens of stepped quantum wells (QW) are investigated. The emission bands related to free and bound excitons and impurity states are observed in QW spectra. The luminescence excitation spectra indicate that the relaxation of free excitons to the e1hh1 state proceeds via the exciton mechanism, whereas an independent relaxation of electrons and holes is specific to bound excitons and impurity states. The energy levels for electrons and holes in stepped QWs, calculated in terms of Kane’s model, are compared with the data obtained from the luminescence excitation spectra. The analysis of the relative intensities of emission bands related to e1hh1 excitons and exciton states of higher energy shows that, as the optical excitation intensity increases, the e1hh1 transition is more readily saturated at higher temperature, because the lifetime of excitons increases. Under stronger excitation, the emission band of electron-hole plasma arises and increases in intensity superlinearly. At an excitation level of ∼105 W/cm2, excitons are screened and the plasma emission band dominates in the QW emission. Nonequilibrium luminescence spectra obtained in a picosecond excitation and recording mode show that the e1hh1 and e2hh2 radiative transitions are 100% polarized in the plane of QWs.

Characterization of InGaN/GaN multi-quantum-well blue-light-emitting diodes grown by metal organic chemical vapor deposition

The structural, surface morphology, and the temperature dependence photoluminescence of InGaN(3 nm)/GaN(7 nm) 5 period multi-quantum-well blue-light-emitting diode (LED) structures grown by metal organic chemical vapor deposition (MOCVD) have been studied. Quantum dot-like structures and strain contrast evident by black lumps were observed in the quantum wells using high-resolution transmission electron microscopy (HRTEM) analysis. Double-crystal high-resolution x-ray diffraction (HRXRD) spectra of blue LED were simulated using kinematical theory method, to obtain composition, and period thickness of well and barrier. The “S” shape character shift as red–blue–redshift of the quantum-well emission line, i.e., blue emission peak 2.667 eV at 10 K, was observed with variation of temperature in the photoluminescence (PL) spectra. The shift is assigned to the potential fluctuations due to alloy inhomogeneous distribution in the quantum wells. The In composition in the quantum wells obtained by two independent techniques, namely HRXRD and PL, was 8% and 19%, respectively. The reason for this large difference in composition is explained in this letter.

Effects of GaNAsSb intermediate barriers on GaInNAsSb quantum well grown by molecular beam epitaxy

The effects of GaNAsSb intermediate barriers on quinary GaInNAsSb quantum well (QW) grown by molecular beam epitaxy on GaAs substrate were investigated. Series of samples grown with different Sb flux were realized to evaluate the effects. It was demonstrated that introducing GaNAsSb intermediate barriers allowed to increase QW emission. The increase of emission depends on the Sb flux. The presence of GaNAsSb layers degrades slightly the QW optical properties, but it does not degrade the QW structure and crystal quality. By using GaNAsSb barriers, a GaInNAsSb QW with room temperature emission up to 1.61μm was achieved.

High brightness blue and green light emitting quantum wells with graded‐In‐content profile grown by MOCVD

Graded-In-content InGaN/GaN quantum wells (QWs) with high brightness blue and green emission were grown on sapphire by metalorganic chemical vapor deposition. The optical properties were studied by temperature-dependent photoluminescence (PL), PL excitation (PLE), and time-resolved PL. Phase separations are revealed by PL measurement and verified by PLE with different absorption edges. From temperature-dependent PL, the integrated intensity of the main InGaN blue and green emission (∼462 nm for blue and ∼509 nm for green) decreased by about a factor of 13.2 and 7.4, only about one order of magnitude, with increasing temperature from 10 to 300 K, indicating strong carrier localization and high quantum efficiency in graded-In-content multiple QWs. Time-resolved PL shows a clear increase of decay time with temperature from 14 to 90 K for blue emission samples and to 50 K for green emission samples, then decrease with more higher temperature. From the time-resolved PL results, a clear carrier transfer is observed from weakly to strongly localized states, which plays an important role to enhance brightness and quantum efficiency in graded-In-content InGaN QWs. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)