Conventional Quantum Wells Research Articles

Extremely small-chirp electroabsorption-modulator integrated distributed feedback laser diode with a shallow quantum-well absorption layer

We have developed an extremely small-chirp electroabsorption modulator integrated with a distributed feedback laser diode (EAM-DFB-LD) with a novel shallow quantum-well (QW) absorption layer. By using a shallow QW absorption layer, the estimated lifetime of the photogenerated holes and thus the estimated concentration of the holes generated by optical absorption has been reduced to 9% of that for a conventional QW. It was experimentally confirmed that the excess chirp due to the pileup of carriers has been reduced to 10% of that for a conventional QW, and this result is consistent with the estimates. The shallow QW EAM-DFB-LD has shown a chirp parameter |/spl alpha/| less than 0.7 over the entire range of the EAM reverse bias voltage of 0-3 V. Finally, successful 10-Gb/s return to zero transmission has been confirmed through both positive dispersion (+425 ps/nm) and negative dispersion (-425 ps/nm).

Photoluminescence and Lasing Characteristics of GaInNAs Quantum Wells Using GaInAs Intermediate Layers

We have studied photoluminescence (PL) properties of GaInNAs quantum wells (QWs) which have GaInAs intermediate layers (IML) inserted at the GaInNAs/GaAs heterointerfaces. Lasing characteristics of the laser using IML are also investigated. We point out that the total amount of nitrogen (N) in the GaInNAs QW could be reduced by using the GaInAs IML while maintaining the emission wavelength. We also found that the optical quality of the GaInNAs IML QWs depends on both the N composition and the total amount of N incorporated in the QW. Due to the reduced total amount of N, an IML QW evidently exhibited a better PL property than a conventional rectangular potential QW. No deterioration in lasing characteristics was observed for GaInNAs lasers using GaInAs IML. The GaInAs IML structure with an appropriate design is expected to lead to improved lasing characteristics of GaInNAs lasers.

Electrooptic effects in GaAs-AlGaAs narrow coupled quantum wells

The linear and quadratic electrooptic coefficients in narrow single and strongly coupled GaAs-Al/sub x/Ga/sub 1-x/As quantum wells have been measured. The quadratic electrooptic effect is enhanced over that of conventional square quantum wells for both TE and TM polarization in all the structures considered, by up to six times in the case of 2-nm-wide GaAs-Al/sub 0.2/Ga/sub 0.8/As strongly coupled quantum wells. The origin of the enhanced quadratic electrooptic effect was found to correlate with a larger red shift in the absorption edge exciton and strong Coulombic coupling of the bound exciton states with the quasi-continua.

Influence of One Monolayer Thickness Variation in GaAs/AlGaAs Five-Layer Asymmetric Coupled Quantum Well upon Electrorefractive Index Change

The five-layer asymmetric coupled quantum well (FACQW) is a new potential-tailored quantum well (QW) for ultrafast and low-voltage optical modulators and switches. First, the influence of one monolayer (ML) thickness variation of a single layer in the GaAs/AlGaAs FACQW on the electrorefractive index change Δn is theoretically studied. The thickness variation of two thicker GaAs layers has a considerable influence on Δn of the FACQW, while the thickness variation of thin AlAs and AlGaAs barrier layers has a smaller influence on Δn. The ratio of the thicknesses of the two GaAs well layers significantly affects the Δn characteristics of the FACQW. The change Δn does not vary appreciably as long as the ratio is kept constant. Second, the influence of the statistical fluctuation of the layer thickness by 1 ML in all of the layers on the Δn characteristics of the FACQW is also discussed. Even when Δn decreases with the increase of the occurrence probability of a layer being thicker or thinner by 1 ML, the FACQW still has a much larger Δn than conventional rectangular quantum wells do.

Electrorefraction associated with Wannier-Stark localization in strongly coupled three-quantum-well structures

Wannier-Stark localization of heavy holes and the associated refractive index changes in a strongly coupled GaAs-Al/sub 0.75/Ga/sub 0.25/As three-quantum-well structure have been investigated. Electroabsorption has been measured for TE polarization and the results compared with simulations performed by the exciton Green's function method to reveal the dominant contributions to the differential absorption at low applied electric field. The refractive index changes calculated by Kramers-Kronig transformation are large compared with those arising from the quantum-confined Stark effect in conventional square quantum wells and are shown to derive from the emergence of only first-order ladder states due to the strong localization of heavy holes. Preliminary experimental confirmation of strong electrorefraction associated with heavy-hole state localization is obtained at 80 meV detuning. This effect is potentially useful for electrooptic device applications.

Spin Relaxation in GaAs(110) Quantum Wells

We investigated electron spin relaxation time ${\ensuremath{\tau}}_{s}$ in $\mathrm{GaAs}/\mathrm{AlGaAs}$ (110) quantum wells (QWs), in which a predominant spin scattering mechanism [D'yakonov-Perel' (DP) mechanism] for conventional (100) QWs is substantially suppressed; ${\ensuremath{\tau}}_{s}$ in (110) QWs was of nanosecond order at room temperature, more than an order of magnitude longer than that of the (100) counterpart. The mechanism responsible for the spin relaxation was examined by studying the quantized energy, electron mobility, and temperature dependences of ${\ensuremath{\tau}}_{s}$. The results suggest that in the absence of DP interaction, electron-hole exchange interaction limits ${\ensuremath{\tau}}_{\mathrm{s}}$ in a wide temperature range ( $\ensuremath{\sim}0--300\mathrm{K}$).

Ultra-low threshold ZnSe-based lasers with novel design of active region

In this paper, we present the results of recent development of ZnSe-based room temperature blue–green lasers grown by MBE. A novel laser structure design is proposed and realized, involving a combination of alternately-strained ZnSSe/(Zn,Cd)Se layers in a short-period superlattice waveguide with a single 2–3 monolayer thick CdSe fractional monolayer (FM) active region incorporated into a ZnSe quantum well (QW). The CdSe FMs transform under certain MBE conditions into the dense array of 15–30 nm-size CdSe-based self-organized dot-like islands providing effective carrier localization and serving as radiative recombination centers. As a result, significantly improved optical and electronic confinement as well as high quantum efficiency have been obtained, leading to the lowest ever reported threshold power density (4 kW/cm 2, 300 K) and increased degradation stability with respect to conventional ZnCdSe QW lasers.

Effects of thermal annealing on photoluminescence and structural properties of (ZnSe)2(CdSe)n short-period-superlattices multiple quantum wells

This work investigates how thermal annealing affects the optical and structural properties of (ZnSe)2(CdSe)n short-period-superlattices multiple quantum wells (SPS MQW) grown by molecular beam epitaxy using photoluminescence (PL) and high-resolution transmission electron microscopy (HRTEM) techniques. A characteristic that differentiates annealed SPS MQW from annealed conventional quantum wells is that much greater blueshifts can be observed in annealed SPS MQW as the annealing temperature rises above 400 °C. We attribute the larger blueshifts to thermally induced interdiffusion of Zn and Cd atoms between alternate ZnSe and CdSe layers. Furthermore, the PL emission in annealed (ZnSe)2(CdSe)n SPS MQW quenches at higher temperatures and yields a larger value for the activation energy than in as-grown SPS MQW. HRTEM images of samples annealed at 450 °C for 30 min clearly indicate that SPS structures remain in the quantum well regions of as-grown SPS MQW, but intermix and disappear in the well regions of annealed (ZnSe)2(CdSe)n SPS MQW.

Application of self-organized quantum dots to edge emitting and vertical cavity lasers

Self-organised quantum dots (QDs) are used successfully as active media of edge emitting and vertical cavity lasers. Threshold current densities at room temperature (RT) of 60 A/cm 2 for edge emitting lasers are measured in four side cleaved geometry. High internal (>96%) and differential (70%) efficiencies are obtained for InGaAs–AlGaAs lasers based on vertically coupled QDs. 1.5 W continuous wave operation at RT is demonstrated. Vertical cavity surface emitting lasers (VCSELs) based on QDs demonstrate threshold current densities down to 170 A/cm 2, comparable to the best values obtained for quantum well (QW) VCSELs, and total threshold currents as low as 67 μA. QD VCSELs demonstrate a reduction in the total current vs. aperture size down to 1 μm apertures due to reduced nonequilibrium carrier spreading out of the injection area as compared to QW VCSELs. QD lasers exhibit a much larger gain, differential gain and smaller line width enhancement factor as compared to conventional QW devices. These are clear inidcations that QD lasers will significantly overcome the performance of QW lasers in the future.

Excitonic recombination dynamics in shallow quantum wells

We report a comprehensive study of carrier-recombination dynamics in shallow ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ quantum wells. At low crystal temperature (2 K), the excitonic radiative recombination time is shown to be strongly enhanced in shallow quantum wells with $x>0.01,$ consistently with a model that takes into account the thermal equilibrium between the three-dimensional exciton gas of the barrier and the two-dimensional exciton gas, which are closer in energy as x decreases. Furthermore, we demonstrate the existence of a thermally activated escape mechanism due to the low effective barrier height in these structures. The nonradiative recombination is shown to dominate the carrier dynamics for temperatures as low as 10 K for $x\ensuremath{\approx}0.01.$ Our experimental observations are analyzed using three different variational exciton calculations. In particular, we study the crossover from the two-dimensional to the three-dimensional behavior of the exciton, which occurs for x as low as 0.01 and affects mainly the oscillator strength, whereas the transition energies in shallow quantum wells can be calculated, to a large extent, using the same approximations as for conventional quantum wells. The peculiar behavior of the oscillator strength at the crossover to the weak confinement regime is obtained by expansion in a large basis.

Normal-incidence TE intersub-band transitions

Normal-incidence operation, based on TE electron intersub-band transitions in n-type unipolar optoelectronic devices, would have a wide range of applications. However, TE intersub-band transitions are generally accepted to be (nearly) forbidden in direct gap conduction-band quantum wells. The paper describes 14-band k.p effective mass theory calculations of TE electron intersub-band matrix elements in n-type, direct-gap systems, to determine the extent to which TE intersub-band transition strengths can be enhanced by choice of quantum well shape and composition. The possibility of enhanced TE:TM transition strength ratios, owing to remote-band contributions to the matrix elements, is described but no significant enhancement of matrix elements over predictions of the single-band model is calculated for conventional quantum wells. A mechanism is presented for nonsquare wells which is suggested to enhance TE:TM transition strength ratio based on remote-band coupling effects through bulk momentum matrix elements P/sub 1/ and Q.

Semiconductor Quantum Dots

Semiconductor “quantum dots” refer to nanometer-sized, giant (103–105 atoms) molecules made from ordinary inorganic semiconductor materials such as Si, InP, CdSe, etc. They are larger than the traditional “molecular clusters” (~1 nanometer containing ≤100 atoms) common in chemistry yet smaller than the structures of the order of a micron, manufactured by current electronic-industry lithographic techniques. Quantum dots can be made by colloidal chemistry techniques (see the articles by Alivisatos and by Nozik and Mićić in this issue), by controlled coarsening during epitaxial growth (see the article by Bimberg et al. in this issue), by size fluctuations in conventional quantum wells (see the article by Gammon in this issue), or via nano-fabrication (see the article by Tarucha in this issue).

A novel GaInNAs-GaAs quantum-well structure for long-wavelength semiconductor lasers

We propose a novel quantum-well (QW) structure for GaInNAs-GaAs lasers that can emit 1.3 μm or longer wavelength light. The idea is insertion of lattice-matched GaInNAs intermediate layers between well and barrier, which is effective for elongating the emission wavelength and reducing well thickness. It is shown that 1.3-μm emission is achievable by using the proposed GaInNAs-GaAs QW with a well thickness thinner than that of conventional rectangular GaInNAs QWs. This structure will relax the design limitation of strained GaInNAs layers.

0.98-μm multiple-quantum-well tunneling injection laser with 98-GHz intrinsic modulation bandwidth

We demonstrate GaAs-based 0.98-/spl mu/m multiple-quantum-well (MQW) tunneling injection lasers with ultrahigh-modulation bandwidths. Electrons are injected into the active region via tunneling, leading to a cold carrier distribution in the quantum wells (QWs). The tunneling time (2 pS) measured by time resolved differential transmission spectroscopy agrees with the capture time extracted form the electrical impedance measurement. The tunneling barrier prevents electrons from going over the active region into the opposite cladding layer. The carrier escape time in tunneling injection lasers is larger than that in conventional QW lasers. Enhanced differential gain, minimized gain compression and improved high frequency performance have been achieved. The -3-dB modulation bandwidth is 48 GHz and the maximum intrinsic modulation bandwidth is as high as 98 GHz.

Two-dimensional excitonic emission in InAs submonolayers.

Photoluminescence (PL) and time-resolved photoluminescence (TRPL) were used to study optical emissions of ultrathin InAs layers with average layer thickness ranging from 1/12 to 1 ML grown on GaAs substrates. We have found that the inhomogeneous broadening of the PL from InAs layers can be well described by the quantum-well model with InAs islands coupling to each other and being regarded as a quasiwell. From the temperature dependence of the exciton linewidth, the exciton--LO-phonon scattering coefficient was found to be comparable to that in conventional two-dimensional quantum wells. In the TRPL measurements, the PL decay time increases linearly with temperature, which is a typical characteristic of free excitons in quantum wells. All these results indicate that the excitons localized in InAs exhibit two-dimensional properties of quantum wells, despite the topographical islandlike structure. \textcopyright{} 1996 The American Physical Society.

Ground state exciton lasing in CdSe submonolayers inserted in a ZnSe matrix

We study optical properties of ZnMgSSe-ZnCdSe structures with CdSe submonolayers inserted in a ZnSe matrix. Remarkably high exciton oscillator strength is found in ultrashort-period submonolayer CdSe-ZnSe superlattices, as compared to ZnCdSe quantum wells of comparable average width and Cd composition. In conventional ZnCdSe quantum wells the lasing occurs at energies ∼30 meV below the free heavy-hole exciton transition revealed in photoluminescence and in optical reflectance spectra. In the CdSe submonolayer superlattices lasing occurs at energies in the very vicinity of the heavy hole exciton resonance, directly in the region of strongly-enhanced exciton-induced modulation of the reflectance spectrum, and, consequently, refractive index change. We attribute the effects observed to exciton localization by potential fluctuations caused by nanoscale CdSe islands formed during submonolayer deposition.

Carrier capture in quantum well embedded quantum wire structures

We propose a novel quantum wire (QWR) laser structure with improved carrier capture characteristics, where the carriers are injected into a quantum well (QWL) and subsequently recombine within an embedded QWR. The corresponding electron capture rates via polar optical phonon scattering are calculated for this system. An oscillatory behavior of the electron capture rate is observed as a function of the QWR thickness at the temperatures considered (77 K and 300 K). The amplitude of these oscillations also increases as the QWR width decreases. Our calculations show that the electron capture rate in the QWL embedded QWR structure can be improved by more than 30% when compared to single QWLs. Therefore, the proposed QWR laser design provides improved modulation bandwidth and optical gain over conventional QWL and QWR structures.

Tailoring the heavy-hole and light-hole quantum-confined Stark effect using multistrain-stepped quantum wells

We show theoretically how the quantum-confined Stark effect (QCSE) shifts of the E1–HH1 and E1–LH1 energy transitions can be engineered to be closely matched over the entire range of applied fields up to ±100 kV/cm using structures containing more than one strained-layer composition within each well. In contrast, conventional quantum wells that contain a single uniform composition within each well can at best have the energy levels for the HH1 and LH1 states matched at only one field value. Furthermore, we show how the TE and TM absorption spectra are modified using multistrain-stepped structures leading to improved polarization-insensitive QCSE modulators.

Comparison of stepped-well and square-well multiple-quantum-well optical modulators

The responsivity and transmission performance of 60-period p-i-n multiple-quantum-well (MQW) optical modulators having single or double steps in the quantum well (QW) has been experimentally compared to a square-QW control sample. It has been confirmed that significantly increased shift of the exciton with applied electric field [the quantum confined Stark effect (QCSE)] is obtained in the stepped QWs as compared to a conventional square QW. However, no meaningful increase in optical modulation performance is found, due to broadening of the exciton transition in the stepped QWs. All MQWs contained nominal 45 Å Al0.3Ga0.7As barriers. Single-step QWs consisted of a 15 Å GaAs region with an 85 Å Al0.1Ga0.9As step. Double-step QWs consisted of a central Al0.15Ga0.85As plateau, with 20 Å GaAs regions on either side. The square QW consisted of 90 Å of GaAs. Excellent agreement between measured QCSE and tunneling resonance calculations was found. Our results indicate that stepped MQW devices are intrinsically more susceptible to growth induced degradation than square MQW modulators.

The Effect Of Composition Modification on the Optical Polarization Independence In Semiconductor Strain Quantum Wells

AbstractPolarization independent quantum well (QW) materials operating under electroabsorption effect in optical switching and modulation devices are of intense interest recently. This is a theoretical analysis of the optical properties of strained InGaAs/InP QWs. The method of composition modification based on interdiffusion will be introduced to merge the heavy- and light- hole states in order to achieve polarization insensitivity. Results presented here show that the diffused QWs with and without as-growth tensile strain can both serve in polarization independent electro-absorption requirements. With a suitable design in the interdiffused QW materials, the optical polarization independence can operate from 1.465 to 1.540 μm (tunability of 75 nm) with a maximum absorption change of 2000 cm−1. In the case studied here, over 75% reduction in the required as-growth tensile strain is achieved as compared with the conventional rectangular QWs. This provides us with a simpler way to achieve high strain optical polarization independence through interdiffusion.