GaAs Wells Research Articles

Properties of spectral characteristics of quasi-stationary electron states in an open multi-cascade nanostructure

The exact expressions for scattering matrix and transmission coefficient in open multi-cascade nanostructure are obtained. The theory of spectral characteristics of electron quasi-stationary states is developed in two approaches. For the nanostructure with GaAs wells and AlGaAs barriers, it is established that resonance bands appear in the N-cascade structure. Each one is formed by the energies of N states. Only the scattering matrix allows to unambiguously determine the resonance energies and resonance widths of all electron states in the multi-cascade structure. The approach of transmission coefficient, due to the collapse of resonances, gives the incomplete information about the spectral characteristics.

Deep X-Ray Reflectometry of Supermultiperiod A3B5 Structures with Quantum Wells Grown by Molecular-Beam Epitaxy

Elastically strained supermultiperiod (100–1000 periods) AlGaAs/GaAs superlattices with different doping levels and slightly differing period thicknesses have been investigated. The proposed technique of characterization consisting of combined application of deep X-ray reflectometry based on a rigorous calculation method, as well as the well-known method of high-resolution X-ray reflectometry, has made it possible to investigate 100-period structures with 2-nm-wide Al0.3Ga0.7As barriers and 10-nm-wide GaAs wells and determine with a high accuracy the layer thicknesses and spread of interfaces. This achievement can be considered as a first step in further analysis of thick structures using bright synchrotron radiation source. The difference between the expected and obtained by the proposed method layer thicknesses is several percent, including that for samples with a high doping level (up to 1018 cm–3). All supermultiperiod structures are characterized by sharp interfaces with a standard deviation of about 0.1 nm. Based on the obtained data on the thicknesses, one can accurately determine the layer compositions using high-resolution X-ray diffraction.

Глубокая рентгеновская рефлектометрия сверхмногопериодных A3B5-структур с квантовыми ямами, выращиваемых методом молекулярно-пучковой эпитаксии

We studied the elastically strained AlGaAs/GaAs super-multiperiod (SMP – 100–1000 periods) superlattices with different degrees of doping and a small difference in the period thickness. The proposed characterization technique, consisting of the matched application of the deep X-ray reflectometry method, based on a rigorous calculation method, as well as the well-known high-resolution X-ray reflectometry method, made it possible to study 100-period structures with 2-nm wide Al0.3Ga0.7As barriers and 10-nm wide GaAs wells and to determine the layer thicknesses and the blurring of interfaces with high accuracy. It can be considered as the first step in the further analysis of thick structures using bright sources of synchrotron radiation. The difference between the expected and obtained, as a result of restoration by the proposed method, values of the layer thicknesses was several percent, including for samples with a high degree of doping (up to 10^18 cm^−3). All SMP structures are characterized by sharp interfaces with a standard deviation of the order of 0.1 nm. Based on the thickness data obtained, it is possible to accurately determine the composition of the layers using high-resolution X-ray diffractometry.

Strong terahertz response in quantum well photodetector based on intradonor transition by magnetic field

We report on spectral investigation of photocurrent of a nonconventional GaAs/AlGaAs quantum well photodetector (QWP) which realizes a response to terahertz (THz) radiation by intradonor transitions (IDTs) in AlGaAs barriers rather than typical intersubband transitions (ISBTs) in GaAs wells. The photodetector shows dramatically enhanced photocurrent intensity and THz response when under a perpendicular magnetic field. This magnetic field helps to improve the absorption quantum efficiency, remove the restriction of the polarization selection rule, and extract high density electrons from two-dimensional electron gas in GaAs wells after electrons transfer into AlGaAs barriers. The effect of the magnetic field on the peak intensity and the linewidth of the photocurrent peak responses is exploited to distinguish IDT and ISBT and to identify the crucial role of electron transfer in THz detection in QWP. This work is useful for exploring detection strategy and technology for high responsivity THz photodetector.

Wide tunability and electron transfer in GaAs/AlGaAs quantum well photodetector by magnetic field

One strategy for terahertz (THz) detection in GaAs/AlGaAs quantum well photodetectors under the magnetic field is reported. The THz detection begins to operate after the normally empty hydrogenic donor ground states in the AlGaAs barriers become populated by electrons transferred from the GaAs wells. Through the Landau quantization arising from a perpendicular magnetic field, we achieved the electron transfer from subband Landau levels in the GaAs wells at liquid helium temperature when the magnetic field reaches a certain threshold. One detector based on this strategy exhibited a dramatic range of frequency tunability of 3.20–6.13 THz. Our photothermal ionization spectroscopy measurements show quantitative agreement with the theoretical calculation of intradonor transition energies, verifying the origin of the strongly enhanced frequency tunability from the Zeeman behavior of transferred electrons in the AlGaAs barriers. This finding is useful for exploring magneto-optical effects and realization of wide tunability in THz photodetectors.

Landau levels and shallow donor states in GaAs/AlGaAs multiple quantum wells at megagauss magnetic fields

Landau levels and shallow donor states in multiple GaAs/AlGaAs quantum wells (MQWs) are investigated by means of the cyclotron resonance at mega-gauss magnetic fields. Measurements of magneto-optical transitions were performed in pulsed fields up to 140 T and temperatures from 6 to 300 K. The $14\times14$ \textbf{P}$\cdot$\textbf{p} band model for GaAs is used to interpret free-electron transitions in a magnetic field. Temperature behavior of the observed resonant structure indicates, in addition to the free-electron Landau states, contributions of magneto-donor states in the GaAs wells and possibly in the AlGaAs barriers. The magneto-donor energies are calculated using a variational procedure suitable for high magnetic fields and accounting for conduction band nonparabolicity in GaAs. It is shown that the above states, including their spin splitting, allow one to interpret the observed magneto-optical transitions in MQWs in the middle infrared region. Our experimental and theoretical results at very high magnetic fields are consistent with the picture used previously for GaAs/AlGaAs MQWs at lower magnetic fields.

Persistent Skyrmion Lattice of Noninteracting Electrons with Spin-Orbit Coupling.

A persistent spin helix (PSH) is a robust helical spin-density pattern arising in disordered 2D electron gases with Rashba α and Dresselhaus β spin-orbit (SO) tuned couplings, i.e., α=±β. Here, we investigate the emergence of a persistent Skyrmion lattice (PSL) resulting from the coherent superposition of PSHs along orthogonal directions-crossed PSHs-in wells with two occupied subbands ν=1, 2. For realistic GaAs wells, we show that the Rashba α_{ν} and Dresselhaus β_{ν} couplings can be simultaneously tuned to equal strengths but opposite signs, e.g., α_{1}=β_{1} and α_{2}=-β_{2}. In this regime, and away from band anticrossings, our noninteracting electron gas sustains a topologically nontrivial Skyrmion-lattice spin-density excitation, which inherits the robustness against spin-independent disorder and interactions from its underlying crossed PSHs. We find that the spin relaxation rate due to the interband SO coupling is comparable to that of the cubic Dresselhaus term as a mechanism of the PSL decay. Near anticrossings, the interband-induced spin mixing leads to unusual spin textures along the energy contours beyond those of the Rahsba-Dresselhaus bands. Our PSL opens up the unique possibility of observing topological phenomena, e.g., topological and Skyrmion Hall effects, in ordinary GaAs wells with noninteracting electrons.

Optical bistability and multistability in a defect slab doped by GaAs/AlGaAs multiple quantum wells

We proposed a new model for controlling the optical bistability (OB) and optical multistability (OM) in a defect slab doped with four-level GaAs/AlGaAs multiple quantum wells with 15 periods of 17.5 nm GaAs wells and 15-nm Al0.3 Ga0.7As barriers. The effects of biexciton energy renormalization, exciton spin relaxation, and thickness of the slab on the OB and OM properties of the defect slab were theoretically investigated. We found that the transition from OB to OM or vice versa is possible by adjusting the controllable parameters in a lab. Moreover, the transmission, reflection, and absorption properties of the weak probe light through the slab were also discussed in detail.

Two distinct regimes for the electrical control of the spin–orbit interaction in GaAs wells

We investigate the electrical control of the spin–orbit (SO) interaction in GaAs wells, involving both one- and two-subband electron occupations altered by a gate potential Vg, over a wide range of well widths w's. Through the self-consistent Schrödinger–Poisson calculation, we determine all the intrasubband Rashba αν(ν=1,2) and Dresselhaus βν, and also the intersubband Rashba η and Dresselhaus Γ couplings. We observe two distinct regimes marked off around w=wc=30−35nm. In the first regime with w<wc, we find the usual behavior of SO couplings, i.e., the intrasubband Rashba couplings α1 and α2 have the same sign and both change almost linearly with Vg, while the Dresselhaus couplings β1 and β2 obeying the relation β1<β2 are almost constant. In contrast, in the second regime with w>wc, we observe that α1 and α2 can either have the same sign or opposite signs depending on Vg. In addition, even though α1 sensitively depends on Vg, we find that α2 remains essentially constant within a certain gate voltage range. As for the Dresselhaus couplings in this second regime, the inequality β1<β2 (valid in the first regime) only holds near the symmetric configuration of our wells, but otherwise the inequality tends to be reversed. On the other hand, we find that both the intersubband Rashba η and Dresselhaus Γ couplings change almost linearly with Vg in the first regime while in the second one a maximum of |η| occurs near the symmetric configuration. We also determined the persistent-spin-helix symmetry points of the two subbands, where the Rashba and the renormalized (due to cubic corrections) Dressehaus couplings are matched. We find that the helix symmetry for the second subband retains over a broad range of Vg's, thus possibly facilitating its locking in practice. Our results should be essential for experiments pursuing an universal electrical control of the SO interaction in semiconductor nanostructures.

All-optical switching between optical bistability and multistability in a defect dielectric medium doped with a multiple quantum well nanostructure.

In this paper, optical bistability (OB) and multistability (OM) properties of a defect dielectric slab are studied. A GsAs multiple quantum well nanostructure (MQW) with 17.5 nm GaAs wells and 15 nm Al(0.3) Ga(0.7) barriers is used in the dielectric medium as a defect layer. Therefore, properties of the refractive index of the slab can be changed in the presence of MQWs. It is observed that switching from OB to OM can be obtained by controlling some external parameters. We find that the exciton spin relaxation and thickness of the slab have essential roles in adjusting the OB and OM properties of the probe light through the slab. Moreover, transmission, reflection, and absorption properties of the propagating pulse through the slab are also discussed. We show that the subluminal and superluminal light transmission or reflection can be obtained via amplification of the probe pulse through the medium. We hope that our proposed model may be suitable for the development of nanoscale devices in all-optical quantum information technology.

Long wavelength superluminal pulse propagation in a defect slab doped with GaAs/AlGaAs multiple quantum well nanostructure

In this paper, long wavelength superluminal and subluminal properties of pulse propagation in a defect slab medium doped with four-level GaAs/AlGaAs multiple quantum wells (MQWs) with 15 periods of 17.5 nm GaAs wells and 15 nm [Formula: see text] barriers is theoretically discussed. It is shown that exciton spin relaxation (ESR) between excitonic states in MQWs can be used for controlling the superluminal and subluminal light transmissions and reflections at different wavelengths. We also show that reflection and transmission coefficients depend on the thickness of the slab for the resonance and nonresonance conditions. Moreover, we found that the ESR for nonresonance condition lead to superluminal light transmission and subluminal light reflection.

Temperature dependence of the Rashba and Dresselhaus spin–orbit interactions in GaAs wells

We have recently shown [Fu and Egues, Phys. Rev. B 91 (2015) 075408] unusual properties of the spin–orbit (SO) interaction in relatively wide quantum wells, e.g., the second subband Rashba term can vanish even in asymmetric configurations. Here we report our theoretical investigation on the temperature dependence of Rashba and Dresselhaus SO interactions in GaAs both relatively narrow and wide wells, having the electron occupancy of one and two subbands, respectively. We consider all relevant intra- and intersubband SO terms. We find that the variation of intrasubband couplings as temperatures range from 0.3 to 300K could attain, ∼meVÅ, the order of usual magnitudes for SO terms in GaAs wells. Moreover, we observe distinct behaviors of the SO interaction of the two subbands, as functions of temperature. On the other band, we find that the intersubband SO terms have a relatively weak temperature dependence.

Phase control of Goos–Hänchen shift via biexciton coherence in a multiple quantum well

The behavior of the Goos–Hänchen (GH) shifts of the reflected and transmitted probe and signal pulses through a cavity containing four-level GaAs/AlGaAs multiple quantum wells with 15 periods of 17.5nm GaAs wells and 15-nm Al0.3Ga0.7As barriers is theoretically discussed. The biexciton coherence set up by two coupling fields can induce the destructive interference to control the absorption and gain properties of probe field under appropriate conditions. It is realized that for the specific values of the intensities and the relative phase of applied fields, the simultaneous negative or positive GH shift in the transmitted and reflected light beam can be obtained via amplification in a probe light. It is found that by adjusting the controllable parameters, the GH shifts can be switched between the large positive and negative values in the medium. Moreover, the effect of exciton spin relaxation on the GH shift has also been discussed. We find that the exciton spin relaxation can manipulate the behavior of GH shift in the reflected and transmitted probe beam through the cavity. We show that by controlling the incident angles of probe beam and under certain conditions, the GH shifts in the reflected and transmitted probe beams can become either negative or positive corresponding to the superluminal or subluminal light propagation. Our proposed model may supply a new prospect in technological applications for the light amplification in optical sensors working on quantum coherence impacts in solid-state systems.

Controlling of Goos-Hänchen shift via biexciton coherence in a quantum dot

Controlling of the Goos-Hanchen (GH) shifts of the reflected and transmitted probe pulses through a cavity containing four-level GaAs/AlGaAs quantum dot with 15 periods of 17.5 nm GaAs wells and 25-nm Al0.3Ga0.7As barriers is investigated. Under appropriate conditions, the probe absorption can be converted to the probe gain, therefore, the controlling of negative and positive GH shift in the both reflected and transmitted probe beams can be occurred simultaneously. Our obtained results show that the group index of the probe beams could be negative or positive in both reflected and transmitted pulses. Therefore, simultaneous subluminal or superluminal light propagation in reflected and transmitted pulses can be achieved.

Spin-orbit interaction in GaAs wells: From one to two subbands

We investigate the Rashba and Dresselhaus spin-orbit (SO) couplings in GaAs quantum wells in the range of well widths $w$ allowing for a transition of the electron occupancy from one to two subbands. By performing a detailed Poisson-Schr\"odinger self-consistent calculation, we determine all the intra- and intersubband Rashba (${\ensuremath{\alpha}}_{1}$, ${\ensuremath{\alpha}}_{2}$, $\ensuremath{\eta}$) and Dresselhaus (${\ensuremath{\beta}}_{1}$, ${\ensuremath{\beta}}_{2}$, $\ensuremath{\Gamma}$) coupling strengths. For relatively narrow wells with only one subband occupied, our results are consistent with the data of Koralek et al. [Nature (London) 458, 610 (2009)], i.e., the Rashba coupling ${\ensuremath{\alpha}}_{1}$ is essentially independent of $w$ in contrast to the decreasing linear Dresselhaus coefficient ${\ensuremath{\beta}}_{1}$. When we widen the well so that the second subband can also be populated, we observe that ${\ensuremath{\alpha}}_{2}$ decreases and ${\ensuremath{\alpha}}_{1}$ increases, both almost linearly with $w$. Interestingly, we find that in the parameter range studied (i.e., very asymmetric wells), ${\ensuremath{\alpha}}_{2}$ can attain zero and change its sign, while ${\ensuremath{\alpha}}_{1}$ is always positive. In this double-occupancy regime of $w$'s, ${\ensuremath{\beta}}_{1}$ is mostly constant and ${\ensuremath{\beta}}_{2}$ decreases with $w$ (similarly to ${\ensuremath{\beta}}_{1}$ for the single-occupancy regime). On the other hand, the intersubband Rashba coupling strength $\ensuremath{\eta}$ decreases with $w$, while the intersubband Dresselhaus $\ensuremath{\Gamma}$ remains almost constant. We also determine the persistent-spin-helix symmetry points, at which the Rashba and the renormalized (due to cubic corrections) linear Dresselhaus couplings in each subband are equal, as a function of the well width and doping asymmetry. Our results should stimulate experiments probing SO couplings in multisubband wells.

Enhanced Kerr nonlinearity with nonlinear amplification based biexciton coherence

In this paper, the theoretical investigation of giant Kerr nonlinearity based on biexciton coherence in GsAs multiple quantum wells with 17.5 nm GaAs wells and 15nm Al0.3Ga0.7 barriers is presented. The destructive quantum interference in this configuration is set up by two continuous wave control fields that couple to a biexciton state. It is shown that nonlinear susceptibilities of two weak probe and signal fields can be controlled by exciton spin relaxation and biexciton coherence which resulted from many-particle Coulomb interactions in semiconductors. It is found that the exciton spin relaxation can enhance the Kerr coefficient in the absence of probe absorption. We hope that our proposed method could be helpful for the next generation of all-optical systems based on quantum coherence and interference in semiconductor nanostructures.

Simultaneous slow and fast light transmission-reflection with amplification in a defect slab via exciton spin relaxation

The absorption, transmission and reflection properties of two incident probe and signal pulses through a defect slab doped with four-level GaAs/AlGaAs multiple quantum wells with 15 periods of 17.5 nm GaAs wells and 15 nm Al0.3 Ga0.7 as barriers is theoretically discussed. It is shown that the biexciton coherence leads to the phase control of the absorption, transmission and reflection properties of two weak probe and signal beams. We will also show that the exciton spin relaxation between excitonic states can affect the slow and fast light propagation of transmitted and reflected pulses with amplification in a defect slab with dispersive material. Our results show that many particle interactions due to Coulomb correlations in semiconductors can lead to the controlling of light propagation through the defect dielectric medium.

Phase control of light transmission and reflection based biexciton coherence in a defect dielectric medium.

Phase control of two weak probe lights' transmission and reflection based biexciton coherence in a defect dielectric medium doped by four-level GaAs/AlGaAs multiple quantum wells with 15 periods of 17.5nm GaAs wells and 15nm Al0.3Ga0.7As barriers is theoretically investigated. The biexciton coherence in this scheme is set up by two continuous wave control fields that couple to a resonance of biexcitons. It is shown that the transmission and reflection properties versus relative phase between applied fields can be controlled by the intensity of control fields and exciton spin relaxation between exciton states. Our studies show that many-particle interactions due to Coulomb correlations in semiconductors can be harnessed by quantum coherence in an interacting many-particle system.

Effect of uniaxial stress on the interference of polaritonic waves in wide quantum wells

A theory of polaritonic states is developed for a nanostructure with a wide quantum well stressed perpendicular to the growth axis of the heterostructure. The role of the $K$-linear terms appearing in the exciton Hamiltonian under the stress is discussed. Exciton reflectance spectra are theoretically modeled for the nanostructure. It is predicted that the spectral oscillations caused by interference of the exciton-like and photon-like polariton modes disappear with the increase of applied pressure and then appear again with opposite phase relative to that observed at low pressure. Effects of gyrotropy and convergence of masses of excitons with heavy and light holes due to their mixing by the deformation is also considered. Numerical estimates performed for the GaAs wells show that these effects can be experimentally observed at pressure $P < 1$~GPa for the well widths of a fraction of micron.

Transmission and reflection properties of propagated pulse through defect slab based biexciton coherence

In this paper, we theoretically investigate transmission and reflection properties of incident light through dielectric medium doped by GaAs/AlGaAs multiple quantum wells with 15 periods of 17.5nm GaAs wells and 15-nm Al0.3 Ga0.7As barriers, grown by metal organic chemical vapor deposition. The destructive quantum interference is set up by a control pulse that couples to a resonance of biexcitons. We found that many-particle interactions such as biexciton binding energy and biexciton decoherence which are inherent in semiconductors can affect the transmission and reflection properties of incident light on the slab. We have also shown that simultaneous subluminal or superluminal transmission, reflection can be achievable at different frequencies of probe field.