Optical gain of 1.3 /spl mu/m GaAsSbN/GaAs quantum well lasers

Optical gain of 1.3 mum GaAsSbN/GaAs quantum well (QW) structure is investigated using the multiband effective mass theory. The results are compared with those of 1.3 mum InGaNAs/GaAs and GaAsSb/GaAs QW structures. The optical gain of the GaAsSbN/GaAs QW structure is found to be similar to that of the InGaAsN/GaAs QW structure. In contrast, GaAsSbN/GaAs and InGaNAs/GaAs QW structures show significantly larger optical gain than the GaAsSb/GaAs QW structure. This is mainly attributed to the fact that the former has a larger optical matrix element than the latter. In addition, GaAsSbN/GaAs and InGaNAs/GaAs QW structures have much smaller threshold current density than the GaAsSb/GaAs QW structure. This is because the Auger recombination current density gives dominant contribution to the threshold current density and the former has smaller threshold carrier density than the latter. On the contrary, the threshold current density of the GaAsSbN/GaAs QW structure is shown to be similar to that of the InGaAsN/GaAs QW structure     

Photoreflectance study of strained GaAsN/GaAs T-junction quantum wires grown by metal-organic vapor phase epitaxy

Strained GaAsN T-junction quantum wires (T-QWRs) with different N contents grown on GaAs by two steps metal-organic vapor phase epitaxy in [001] and [110] directions, namely QW1 and QW2 respectively, have been investigated by photoreflectance (PR) spectroscopy. Two GaAsN T-QWRs with different N contents were formed by T-intersection of (i) a 6.4-nm-thick GaAs0.89N0.011 QW1 and a 5.2-nm-thick GaAs0.968N0.032 QW2 and (ii) a 5.0-nm-thick GaAs0.985N0.015 QW1 and a 5.2-nm-thick GaAs0.968N0.032 QW2. An evidence of a one-dimensional structure at T-intersection of the two QWs on the (001) and (110) surfaces was established by PR resonances associated with extended states in all the QW and T-QWR samples. It is found that larger lateral confinement energy than 100 meV in both of [001] and [110] directions were achieved for GaAsN T-QWRs. With increasing temperature, the transition energy of GaAsN T-QWRs decreases with a faster shrinking rate compared to that of bulk GaAs. Optical quality of GaAsN T-QWRs is found to be affected by the N-induced band edge fluctuation, which is the unique characteristic of dilute III-V-nitrides.     

An active lasing region with a quantum well and a quantum dot array

A combined active lasing region of the new type, containing an In_0.2Ga_0.8As quantum well (QW) and a single-layer array of InAs quantum dots (QDs) located outside the QW, was studied. In this system, the QW accumulates the injected charge carriers and the QD array serves as a radiator. The energy levels of electrons and holes in a QD were calculated. It is shown that the QDs can be filled by the resonance tunneling of holes from the QW to an unoccupied QD. The electron energy level in an unoccupied QD is markedly higher than that in the QW, but occupation of the QD by a hole leads to a resonance of the electron levels. Theoretical conclusions agree with the results of observations on a prototype laser with a combined active region.     

Dynamic Transport of Interacting Electrons in a Mesoscopic Quantum Wire

Electron density distribution in a quantum wire (QW) coupledto reservoirs is investigated, treating this structure as aunified quantum system and taking into account the Coulombinteraction of electrons. We show that electrons aretransferred between the QW and the reservoirs when theequilibrium state is established. As a result the QWacquires a positive or negative charge or remains neutral as awhole, depending on the QW size and the background charge.Electron transport in a QW is investigated for the lattercase using the model which allows one to treat exactly theCoulomb interaction. We study the Coulomb interaction effecton the dynamic conductance. The real part of the impedanceshows a resonant behavior versus the frequency which is causedby the reflection of the charge waves from the contacts andtheir interference in the QW. The Coulomb interaction effectconsists in a nonlinear dependence of the resonant frequencieson the wave number. No effective interaction parameter of theLuttinger liquid can simulate the frequency dependence of theimpedance calculated with the real Coulomb interaction.     

(In)GaSb/AlGaSb quantum wells grown on Si substrates

We have successfully grown GaSb and InGaSb quantum wells (QW) on a Si(001) substrate, and evaluated their optical properties using photoluminescence (PL). The PL emissions from the QWs at room temperature were observed at around 1.55 μm, which is suitable for fiber optic communications systems. The measured ground state energy of each QW matched well with the theoretical value calculated by solving the Schrödinger equation for a finite potential QW. The temperature dependence of the PL intensity showed large activation energy (∼ 77.6 meV) from QW. The results indicated that the fabricated QW structure had a high crystalline quality, and the GaSb QW on Si for optical devices operating at temperatures higher than room temperature will be expected.     

Hole transport due to shallow acceptors along boron doped SiGe quantum wells

Lateral conductivity and magnetotransport measurements were performed with SiGe single quantum well (QW) structures doped with boron in the QW. The conductivity at low temperatures (T) is shown to be due to hopping over B centers while at higher T, it is due to two-stage excitation: thermal activation of holes from the ground to strain-split B states are followed by hole tunneling into the valence band. The tunneling is due to a potential drop across the QW which is due to hole capture at surface states of the Si cap layer making the surface charged. The external potential applied across the QW essentially changes the lateral conductivity as well as the activation energy. The calculations of band profile, free carrier concentration in the QW and acceptor population, as well as an effect on the transverse electric field were carried out taking into account the charging of surface states.     

Optical properties of a wrinkled nanomembrane with embedded quantum well

A wrinkled nanomembrane with embedded quantum well (QW), fabricated by the partial release and bond back of epitaxial layers upon underetching, is investigated by spatially resolved micro-photoluminescence spectroscopy. From the observed QW transition energies and calculations based on the linear deformation potential theory, we find that the bonded back regions are fully relaxed and act on the strain state of the wrinkled QW. Light emission enhancement observed in the wrinkled QW is explained by interference contrast theory.     

Strained InGaAs/GaAs Quantum-Well Laser Emitting at 1054 nm

In this paper, we present an InGaAs/GaAs strain QW laser with tilted waveguide emitting at 1054 nm that has been fabricated by Metal-Organic Chemical-Vapor Deposition (MOCVD) on a GaAs substrate. The active region consists of 6 nm QW separated by a 60-nm barrier within a two-step graded-index separated-confinement heterostructure (GRINSCH) region. The use of a Strained Buffer Layer (SBL) is shown to significantly improve the laser performance. The composition and micro-structure quality of the QW was measured by high-resolution X-ray diffraction. The strain QW laser exhibits a threshold current of 9 mA and a slope efficiency of 0.4 W/A (unused antireflection coating on the facets). Stable fundamental transverse mode operation was obtained up to 33 mW emitted power at an injection current of 100 mA. The spectral width of the strain QW laser is 1.6 nm at an injection current of 50 mA.     

A piezoelectric photothermal study of InGaAs/GaAs quantum well heterostructures

The optical absorption spectrum of InGaAs quantum well (QW) heterostructure samples were measured by using the piezoelectric photothermal (PPT) spectroscopy. From the room-temperature-PPT measurements and curve fitting analysis, the exciton contribution was clearly distinguished from the two-dimensional step-like band-to-band transition. Two samples of different QW structures, a molecular beam epitaxy grown single-QW (MBE-SQW) and an atomic layer epitaxy grown multiple-QW (ALE-MQW), were prepared in order to examine the availability of the PPT technique to the QW structure samples. The binding energies and FWHMs of the PPT exciton peaks were found to be 8 and 20 meV for MBE-SQW, and to be 13 and 43 meV for ALE-MQW samples, respectively. The present results show that the PPT methodology is a powerful tool for investigating the optical properties of QW structures only at room-temperature measurements.     

Reliability and degradation of 980 NM InGaAs/GaAs strained quantum well lasers

Degradation behaviours of 980 nm InGaAs/GaAs strained quantum well (QW) lasers are clarified and compared with normal AlGaAdGaAs, InGaAsP/InP and GaAs/GaAs QW lasers. Through various ageing tests, it is confirmed that 980 nm InGaAs/GaAs strained QW lasers are applicable to optical fibre transmission systems where the components are required to be highly reliable.     

Optical evidence of a quantum well channel in low temperature molecular beam epitaxy grown Ga(AsBi)/GaAs nanostructure

A Ga(AsBi) quantum well (QW) with Bi content reaching 6% and well width of 11 nm embedded in GaAs is grown by molecular beam epitaxy at low temperature and studied by means of high-resolution x-ray diffraction, photoluminescence (PL), and time-resolved PL. It is shown that for this growth regime, the QW is coherently strained to the substrate with a low dislocation density. The low temperature PL demonstrates a comparatively narrow excitonic linewidth of ～ 40 meV. For high excitation density distinct QW excited states evolve in the emission spectra. The origins of peculiar PL dependences on temperature and excitation density are interpreted in terms of intra-well optical transitions.     

Increasing laser excitation power induces large blue shift of the photoluminescence peak of quantum wells in gallium nitride

It is experimentally demonstrated that, as the output power density of a nitrogen laser is increased from 10 to 1000 kW/cm², a peak in the photoluminescence spectrum of quantum wells (QWs) in GaN shifts by approximately 150 meV. This behavior cannot be interpreted within the framework of the ideal QW model. The observed phenomenon is theoretically explained by the presence of a “tail” in the localized density of states in the QW bandgap and by the filling of bands in the QW by nonequilibrium photogenerated charge carriers. A phenomenological expression for the density of states is proposed, which takes into account the tail in the localized density of states and provides qualitative agreement between theoretical and experimental spectra.     

Time-resolved photoluminescence of type-II Ga(As)Sb/GaAs quantum dots embedded in an InGaAs quantum well

Optical properties and carrier dynamics in type-II Ga(As)Sb/GaAs quantum dots (QDs) embedded in an InGaAs quantum well (QW) are reported. A large blueshift of the photoluminescence (PL) peak is observed with increased excitation densities. This blueshift is due to the Coulomb interaction between physically separated electrons and holes characteristic of the type-II band alignment, along with a band-filling effect of electrons in the QW. Low-temperature (4?K) time-resolved PL measurements show a decay time of [Formula: see text]?ns from the transition between Ga(As)Sb QDs and InGaAs QW which is longer than that of the transition between Ga(As)Sb QDs and GaAs two-dimensional electron gas ([Formula: see text]?ns).     

Localized surface plasmon-induced emission enhancement of a green light-emitting diode

The output enhancement of a green InGaN/GaN quantum-well (QW) light-emitting diode (LED) through the coupling of a QW with localized surface plasmons (LSPs), which are generated on Ag nanostructures on the top of the device, is demonstrated. The suitable Ag nanostructures for generating LSPs of resonance energies around the LED wavelength are formed by controlling the Ag deposition thickness and the post-thermal-annealing condition. With a 20?mA current injected onto the LED, enhancements of up to 150% in electroluminescence peak intensity and of 120% in integrated intensity are observed. By comparing this with a similar result for a blue LED previously published, it is confirmed that surface plasmon coupling for emission enhancement can be more effective for an InGaN/GaN QW of lower crystal quality, which normally corresponds to the emission of a longer wavelength.     

Quasi-wavelet formulations of turbulence and other random fields with correlated properties

Quasi-wavelets (QWs) are similar to customary wavelets in that they are based on translations and dilations of a parent function; however, their positions and orientations are random. QWs are convenient for representing random fields with a self-similar structure. In this paper, a general, multi-dimensional treatment of QW fields is presented that includes scale-dependence in the number density and amplitude of the QWs. Previous QW formulations are extended to include anisotropy and correlations among several properties of the random fields. These extensions would be difficult (if not impossible) to achieve systematically by Fourier methods. As an example application, it is shown how QW models can be constructed to produce constant turbulent flux layers. Heat flux in buoyantly driven turbulence is modeled as a collection of QWs with predominantly horizontal rotation coupled with dipole scalar perturbations. Predictions for spectra in the presence of fluxes are obtained.     

Giant Zeeman Splitting of Excitons in a III–V Nonmagnetic Quantum Well Electronically Coupled With a II–VI Semimagnetic Quantum Well

We report on design, fabrication by molecular beam epitaxy, and photoluminescence (PL) studies of GaAs/AlGaAs/ZnSe/ZnCdMnSe double quantum wells (QWs), where resonant electronic coupling occurs through a heterovalent interface. The resonant conditions achieved in the properly designed sample facilitate penetration of the electron wave function from the nonmagnetic GaAs QW into the diluted magnetic semiconductor ZnCdMnSe QW. It results in the sign reversal and drastic increase of a GaAs QW excitonic g factor. The exciton spin splitting observed in the magneto-PL spectra is in general agreement with the calculation performed within the envelope function approximation, taking into account both the inter-well electron coupling and Brillouin-like paramagnetic behavior of the Mn²⁺ ions.     

Formation of various metal nanostructures with thermal annealing to control the effective coupling energy between a surface plasmon and an InGaN/GaN quantum well

We demonstrate the variations of the photoluminescence (PL) spectral peak position and intensity through the surface plasmon (SP) coupling with an InGaN/GaN quantum well (QW) by forming Ag nanostructures of different scale sizes on the QW structure with thermal annealing. By transferring an Ag thin film into a nanoisland structure, we can not only enhance the PL intensity, but also adjust the SP dispersion relation and hence red-shift the effective QW emission wavelength. Such an emission spectrum control can be realized by initially coating Ag films of different thicknesses. Although the screening process of the quantum-confined Stark effect, which can result in PL spectrum blue-shift and intensity enhancement, also contributes to the variations of the emission behaviour, it is found that the SP-QW coupling process dominates in the observed phenomena.     

Optical nutation in GaAs/GaxA11−xAs quantum well structures

Abstract

The possibility of occurrence of the coherent optical transient effect known as optical nutation has been analytically established in the semiconductor quantum well (QW) structure, namely GaAs/Ga_xA1_1?xAs most extensively used in optical electronics. Ultra-short-pulse low-intensity band-to-band excitation of electrons to the 1s Wannier-Mott exciton state of the crystal has been considered to play an important role in the coherent radiation—QW interaction. Numerical estimations of the complex optical susceptibility and the transmitted intensity under the transient regime reveal ringing behaviour confirming the occurrence of optical nutation in III-V semiconducting QW structures.     

Piezoelectric gate for a two-dimensional electron system transport in LiNbO3-GaAs/AlGaAs sandwich structures

Standing-wave piezoelectric fields in the LiNbO(3) driving plate are used to form depleted and accumulated electron densities in GaAs/AlGaAs quantum wells (QWs). The photoluminescence spectrum of the two-dimensional electron system varies both spatially and temporally, exhibiting an electron-hole plasma recombination and exciton and trion emissions at large and small electron densities, respectively. Controlling the piezoelectric field component perpendicular to the QW layers offers a versatile tool to achieve the spatially indirect exciton luminescence in double QW structures.