Symmetric Double Quantum Wells Research Articles

Enhancement of electron transport mobility in GaAs/InGaAs asymmetrically doped narrow quantum well pHEMT structure

We study the effect of asymmetric doping concentrations on the electron mobility μ in GaAs/InGaAs-based single quantum well (SQW) as well as double quantum well (DQW) pseudomorphic high electron mobility transistor (pHEMT) structures. Unequal doping in the substrate and surface barriers (n d1 and n d2 ) causes asymmetric distributions of subband wave functions, ψ 0 and ψ 1, which influence the subband scattering rate matrix elements (SSRME), thereby affecting the subband mobility μ n . For narrow well widths (w w ), in SQW structures, mostly a single subband is occupied. We show that an increase in n d2 , keeping n d1 fixed, enhances μ nonlinearly. The interface roughness (ir-) scattering mostly dominates μ in thin wells (w w < 70 Å), while generally, μ is determined by ionized impurity (ii-) scattering and to some extent by alloy disorder (ad-) scattering. The influence of ir-scattering enhances, while ad-scattering diminishes, by reducing n d2 . For DQW, a double subband is occupied. In a symmetric DQW structure at resonance, n d1 = n d2 , ψ 0 and ψ 1 equally extend into both the wells. For a minor variation, say n d1 > n d2 , ψ 0 mostly lies in one well while ψ 1 is in the other well. In the case of n d1 < n d2 , the distribution reverts. The substantial changes in ψ 0 and ψ 1 influence the intra- and inter-SSRME differently through intersubband effects, leading to nonlinear μ n as a function of n d2 . Taking n d1 + n d2 = 3 × 1018 cm−3, we show that for w w1 = w w2 = 80 Å, a shallow dip in μ occurs at n d1 = n d2 = 1.5 × 1018 cm−3. Whereas, for w w1 = 60 Å and w w2 = 100 Å, the dip in μ occurs near the corresponding resonance, n d1 = 2.3 × 1018 cm−3 and n d2 = 0.7 × 1018 cm−3. Our results of nonlinear μ can be utilized for performance analysis of pHEMT.

Optical anisotropies of asymmetric double GaAs (001) quantum wells

In the present work, we were able to identify and characterize a source of in-plane optical anisotropies (IOAs) occurring in asymmetric double quantum wells (DQWs), namely a reduction of the symmetry from ${D}_{2d}$ to ${C}_{2v}$ as imposed by asymmetry along the growth direction. We report on reflectance anisotropy spectroscopy (RAS) of double GaAs quantum well structures coupled by a thin ($&lt;2$ nm) tunneling barrier. Two groups of DQW systems were studied: one where both QWs have the same thickness (symmetric DQW) and another where they have different thicknesses (asymmetric DQW). RAS measures the IOAs arising from the intermixing of the heavy and light holes in the valence band when the symmetry of the DQW system is lowered from ${D}_{2d}$ to ${C}_{2v}$. If the DQW is symmetric, residual IOAs stem from the asymmetry of the QW interfaces, e.g., that associated with Ga segregation into the AlGaAs layer during the epitaxial growth process. In the case of an asymmetric DQW with QWs with different thicknesses, the AlGaAs layers (that are sources of anisotropies) are not distributed symmetrically at both sides of the tunneling barrier. Thus the system loses its inversion symmetry, yielding an increase in the RAS strength. The RAS line shapes were compared with reflectance spectra in order to assess the heavy- and light-hole mixing induced by the symmetry breakdown. The energies of the optical transitions were calculated by numerically solving the one-dimensional Schr\"odinger equation using a finite-difference method. Our results are useful for interpretation of the transitions occurring in both symmetric and asymmetric DQWs.

Magnetic control of Rashba splittings in symmetric InAs quantum wells

We propose a mechanism to control the Rashba-induced subband splitting by a magnetic field using a symmetric double quantum well (QW) system, where the lowest two subbands are coupled by a position-dependent Rashba parameter α ( z ) . In such a system, all subbands are spin degenerate due to the time reversal symmetry and the spatial inversion symmetry at zero magnetic field, despite the presence of the Rashba spin–orbit interaction. Applying an external magnetic field parallel to the QW plane ( B ∥ y ^ ) lifts this spin degeneracy breaking the time reversal symmetry, where the spin splitting energies are controllable in the range between zero and 2.9 meV, the latter being on the same order of magnitude as a typical Rashba splitting in a narrow asymmetric QW. We find that the first and second subband energy levels for a selected spin state with k ∥ = ( k F , 0 , 0 ) anticross each other, and that the energy of the subband splitting Δ 0 , equivalent to the Rashba splitting for the case of single QWs, can be determined from the value of the anticrossing magnetic field B ac. These results suggest that the investigation in the symmetric double QWs would provide useful approaches for quantitative understanding of the Rashba spin–orbit interaction.

Transition behaviors from coupled-to-uncoupled [formula omitted] symmetric versus asymmetric double quantum wells

The coupling behaviors between symmetric versus asymmetric CdTe–ZnTe double quantum wells (DQWs) are theoretically investigated based on the application of the sp3s* tight-binding method including the spin–orbit interaction. This method is used to calculate the band structures of the (CdTe)Nw1(ZnTe)Nb(CdTe)Nw2–ZnTe DQWs versus the barrier thickness (Nb), and well widths (Nw1 and Nw2). Special attention was given to the decoupling behaviors between the wells in both symmetric and asymmetric cases. The decoupling of the asymmetric DQWs is found to occur at a critical barrier thickness (of about Lbcrit≃18A˚) shorter than the case of symmetric DQWs (of about Lbcrit≃38A˚) because, in the former case, the bound state wavefunctions have more tendency to localize in one of the two wells. Our modelling of asymmetric DQWs (with Nw1=6 and Nw2=3) yields results in excellent agreement with the available experimental photoluminescence data.

Charge confinements in CdSe–ZnSe symmetric double quantum wells

The bound states in the (CdSe)Nw(ZnSe)Nb(CdSe)Nw–ZnSe(001) symmetric double quantum wells are investigated versus the well width(Nw) and the barrierthickness (Nb). Acalculation based on the sp3s* tight-binding method which includes the spin–orbit interactions is employedto calculate the bandgap energy, quantum-confinement energy, and bandstructures. The studied systems possess a vanishing valence-band offset(VBO = 0) in consistency with the well known common-anion rule, and a large conduction-bandoffset ( eV), which plays an essential role in the confinement of electrons withinthe CdSe wells. The biaxial strain, on the other hand, plays another rolein confining the holes at the interfaces (within the well regions) and thusenhancing the radiative efficiency. The induced-strain energy is estimated to be∼35 meV. More importantly, the results show that, for a fixed barrier thickness, the double wells areable to confine a pair of bound states when they are very thin. By increasing the wells’ width(Nw), further, a new pair of states from the conduction-band continuum falls into the wells every timeNw hits a multiple of four monolayers (more specifically, for4n<Nw≤4(n+1), the number ofbound states is 2(n+1), where n is an integer). On the other hand, the barrier thickness(Nb) is shown to have no effect on the number of bound states, but it solely controlstheir well-to-well interactions. A critical barrier thickness to switch off these latterinteractions is estimated to occur at about (). Rules governing the variation of the quantum-confinement energy versus both barrier thickness(Nb) and wellwidth (Nw) have been derived. Our theoretical results are also shown to have excellent agreementwith the available experimental photoluminescence data.

Magnetoexcitons in near-surface symmetric and asymmetric double quantum wells: Optical properties

We present a theoretical study of the optical properties of excitons in symmetric and asymmetric bulk and near-surface double quantum wells (DQWs) under the action of an external magnetic field oriented along the growth direction. The coupling of magnetoexcitons with light in the semiconductor heterostructure is described with Stahl’s real-space density-matrix approach. After solving the system of equations for the coherent wave amplitude and the electromagnetic fields, we calculate optical spectra (reflectivity and absorption) for specific GaAs/AlGaAs DQWs. Our results for the absorption of bulk symmetric DQWs are compared with those obtained by Vera and Barticevic [F. Vera and Z. Barticevic, J. Appl. Phys. 83, 7720 (1998)]. The effect of the surface-magnetoexciton interaction upon the reflectivity and absorption for near-surface DQWs is analyzed. It is established that such an interaction alters the symmetry or asymmetry of the DQWs, and therefore, the resonance structure of their optical spectra is noticeably modified as the thickness of the cap layer is reduced.

Intersubband optical absorption in double quantum well under intense laser field

The intersubband absorption in symmetric double quantum wells under the laser field is theoretically calculated within the framework of the effective mass approximation. Results obtained show that intersubbband optical transition and energy levels in double quantum well can significantly be modified and controlled by intense laser field. This statement gives a new degree of freedom in various device applications based on the intersubband transition of electrons.

Confined acoustic and optical plasmons in double-layered quantum-wire arrays with strong tunneling

We investigate electronic excitations in $\mathrm{Ga}\mathrm{As}\text{\ensuremath{-}}{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ double-layered quantum wire arrays with strong tunneling coupling by resonant inelastic light scattering. By applying an external electric field, we can change the one-dimensional (1D) electron density and the symmetry of the double quantum-well (DQW) structure at the same time. We identify confined optical 1D intersubband plasmons (COP) and confined acoustic 1D intersubband plasmons (CAP). Due to the tunneling coupling, the energies of the CAP exhibit a minimum for a symmetric DQW potential, whereas the energies of the COP are dominated by the total carrier density, and are nearly insensitive to the symmetry of the potential.

Photoluminescence of n-doped double quantum well—electron subbands under influence of in-plane magnetic fields

We report on photoluminescence (PL) measurements of a GaAs/AlGaAs double quantum well (DQW) in high magnetic fields. Measurements were carried out on a selectively contacted symmetric p- δ n-DQW- δ n-p structure, which allows a variation of the electron density in DQW by a p–n bias and simultaneously a tilting of DQW, when a p–p bias is applied. Attention was paid to phenomena in in-plane magnetic fields, theoretically studied by Huang and Lyo (HL), [Phys. Rev. B 59, (1999) 7600]. In this paper, we compare our results for both symmetric and asymmetric DQWs with the theoretical model made by HL. Whereas the spectra from a symmetric DQW fully confirmed the theoretical predictions, the results gained from DQW with an electric-field-induced asymmetry did not allow a proper study of anticipated effects. The reasons for that are discussed.

Subband structure and band bending in symmetric modulation-doped double quantum wells

We have calculated the subband structure and confinement potential of modulation-doped Ga Al x As-GaAs symmetric double quantum wells a function of the doping concentration. Electronic properties of this structure are determined by solving the Schrodinger and Poisson equations self-consistently. To understand the exchange correlation potential effects on the band bending and subband populations, this potential has been included in the calculation at 0 K and, at room temperature. We find that at low doping concentrations the effect of the exchange correlation potential is more pronounced on the subband populations.

Collective excitations in double quantum wells with strong tunnel coupling

Spectra of collective and magnetic excitations in symmetric double quantum wells with strong tunnel coupling were studied by inelastic scattering. Gaps in the spectrum of collective and single-particle excitations associated with tunnel interwell splitting were measured. New excitation modes, namely, tunnel Bernstein modes, were detected in a magnetic field. Based on the dispersion and magnetic-field dependences of the energies of inelastic light scattering lines, a classification of excitations and magnetic excitations was given.

Plasmons in asymmetric double quantum well structures

We investigate the effects of spatial asymmetry, tunneling coupling, and exchange-correlation correction on the plasmon modes in asymmetric double quantum well (DQW) structures in a time-dependent local-density approximation. Special attention is paid to the properties of the ω - mode which is always damped in symmetric DQW systems. In addition, the results on the spectral weight of the excitations are also presented. In general, all the modes carry finite spectral weights and should be observable in resonant inelastic light scattering experiments for the specified values of the parameters.

Effects of tunneling coupling on plasmon modes in asymmetric double-quantum-well structures

The collective charge density excitations in asymmetric double-quantum-well (DQW) structures with different tunneling strengths are systematically studied. In particular, the damping properties of the plasmon modes in various tunneling strengths are investigated in detail. It is shown that plasmon modes in asymmetric DQW structures are quite different from those in symmetric DQW systems. In weak tunneling regime, an intra-subband mode ω- with an acoustic-like dispersion relation which is damped in symmetric DQW structures arises and coexists with the optical-like mode ω+ while the inter-subband mode ω10 is highly damped. With the tunneling strength being increased, the ω10 branch gradually becomes undamped and emerges out of the (1-0) single-particle continuum, whereas the ω- branch gradually approaches the (0-0) single-particle continuum. In intermediate coupling regime, these three branches of modes coexist undamped. In strong tunneling regime, ω- enters the (0-0) single-particle continuum and becomes damped. Consequently, only the ω+ and ω10 modes exist in this regime.

Intersubband absorption in degenerately doped GaN/AlxGa1−xN coupled double quantum wells

Intersubband absorption in coupled GaN/AlGaN double quantum wells (DQWs) has been measured. The samples were grown by molecular-beam epitaxy on a sapphire substrate and with large (0.65 or 0.9) AlN-mole fraction in the barriers. Peak absorption wavelengths as short as 1.35 and 1.52 μm were measured for a symmetric DQW of 12 Å wide wells coupled by a 10 Å wide barrier, which also showed evidence of excited-state anticrossing. As expected, asymmetric DQWs displayed no such anticrossing, and the ground-state anticrossing energies were found to be much smaller, as a result of the comparatively large effective electron mass, than the energy broadening of individual transitions. Degenerate doping of the DQWs was used to establish a common reference energy at the Fermi level, which allows overcoming uncertainties related to intrinsic internal electric fields. The asymmetric DQWs displayed peak absorption wavelengths between 1.5 and 2.9 μm.

Intersubband optical absorption and electron relaxation rates in GaN/AlGaN coupled double quantum wells

AbstractRecent interest in intersubband (IS) transitions in semiconductor heterostructures with large band offset has been fueled by attempts to extend the wavelength range of IS-based optical devices to the fiber-optics wavelength range around ~ 1.55 μm. GaN/AlGaN-based heterostructures are of particular interest due to their large effective electron mass and large longitudinal optical phonon energy. Both are essential to achieve ultrafast electron relaxation at large transition energies. IS absorption in GaN/AlGaN single and coupled double quantum wells (DQWs) has been measured. The samples were grown by molecular beam epitaxy on sapphire substrate and with a large (0.65 or 0.9) AlN-mole fraction in the barriers. Peak absorption wavelengths as short as 1.35 μm and 1.52 μm were measured for a symmetric DQW of 12 Å wide wells coupled by a 10 Å wide barrier, which also showed evidence of excited-state anti-crossing. As expected, asymmetric DQWs displayed no such anti-crossing, and the ground-state anti-crossing energies were found to be much smaller – as a result of the comparatively large effective electron mass – than the energy broadening of individual transitions. The asymmetric DQWs displayed peak absorption wavelengths between 1.5 and 2.9 μm. The electron relaxation time, attributed to longitudinal optical phonon scattering has been measured by pump-probe technique as 240 fs for a coupled DQW sample.

Optical properties of InGaAs/InAIAs diffused double quantum wells

The effects of interdiffusion on the subbands and optical properties of InGaAs/InAIAs double quantum well (QW) have been theoretically studied. The results show that since a double QW (DQW) can be diffused to become an effectively single QW structure, the characteristic features in the subbands structure and optical properties of a strongly coupled DQW structure and that of a single QW structure can be obtained by a suitable annealing time. Moreover, the increase in separation between the first symmetric and antisymmetric heavy hole subbands and the increase in the spin splitting of the valence subbands of the diffused DQW, due to an applied electric field, diminish when annealing time increases. In optimizing the In0.53(AlaGa1−a)0.47As/In0.52Al0.48As DQW structure, the results show that symmetric DQW with no Al content in wells can provide large material gain and radiative spontaneous recombination rate. With interdiffusion, the material gain and recombination rate reduce but the reduction saturates when the DQW structure is diffused to effectively become a single graded QW. By subjecting the DQW and an as-grown single QW to the same annealing conditions (where the summation of the width of the two wells and the separation barrier of the DQW equals the well width of the single QW), the diffused DQW can provide a larger material gain and radiative recombination rate than the diffused single QW when the annealing time is short. Therefore, the short-time diffused DQW is more useful for laser applications. Besides, since Al diffuses into the wells, the transition energy of the QW structure increases so that the operating wavelength of the optical devices can be adjusted.

The Exciton Transition Energies in Symmetric Double GaAs/AlxGa1-xAs Quantum Wells

The exciton transition energies between valence and conduction subbands of symmetric double quantum wells are studied. To study the effect of geometry coupled wells of rectangular, graded and parabolic shapes are considered. A special emphasis is given to the study of crossover between the symmetric light-hole (e1-11) and the antisymmetric heavy-hole (e2-h2) transitions.

Effect of the anticrossing of excitonic states on their phase relaxation time in a GaAs/AlGaAs symmetric double quantum well

The change in the properties of excitonic states near anticrossing is investigated experimentally. It is shown that the phase relaxation time of light excitons in a GaAs/AlGaAs symmetric double quantum well increases by a factor of five as a result of mixing of the energy states of light and heavy excitons.

Far-infrared s→p ± interexciton transitions in InGaAs/GaAs coupled double quantum wells

We consider s→p± interexciton far-infrared (FIR) magnetooptical transitions in coupled double quantum wells (DQWs). Spatially direct (intrawell) and indirect (interwell) excitons in strained InxGa1−x As/GaAs symmetric DQWs with a simple valence band are considered. The evolution of the transition energies and oscillator strengths as functions of the transverse magnetic field B in different regimes is studied: we consider the direct regime (zero and low transverse electric field ℰ), the indirect regime (high ℰ), and the indirect-direct crossover (induced by increasing B at intermediate ℰ).

Quantum interference effect in [formula omitted] double quantum wells

Quantum interference properties of GaAs AlGaAs symmetric double quantum wells were investigated in a magnetic field parallel to heterointerfaces at 1.9 K. For two types of samples used in our experiments, two GaAs quantum wells with the same width of 60 Å are separated by an AlGaAs barrier layer of 120 Å and 20 Å thick, respectively. The channels with the length of 2 μm are defined by alloyed ohmic contacts. The conductance oscillation as a function of the magnetic flux Φ(= B s) was observed and oscillation period is approximately equal to h e . The results are in agreement with the theoretical expectation of the Aharonov-Bohm effect. Conductance oscillations are apparent slightly in the samples with a thinner AlGaAs barrier.