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Light-hole quantization in the optical response of ultra-wide GaAs/AlxGa1-xAs quantum wells
Abstract
Temperature-dependent reflectivity and photoluminescence spectra are studied
for undoped ultra-wide 150 nm and 250 nm GaAs quantum wells. It is shown that
spectral features previously attributed to a size quantization of exciton
motion in z-direction coincide well with energies of quantized levels for light
holes. Furthermore, optical spectra reveal very similar properties at
temperatures above exciton dissociation point.
... In the case of direct excitons, the QW-width dependence of the Zeeman splitting in the InGaAs/AlGaAs and GaAs/AlGaAs has been theoretically and experimentally studied in Refs. [11][12][13][14][15]. It was demonstrated that the variation of g-factors characterizing the splittings is caused by the heavy-hole-light-hole mixing. ...
In this work we have investigated the exciton reflectance spectra of a high quality heterostructure with a GaAs/AlGaAs quantum well in a transverse magnetic field (Voigt geometry). It has been shown that application of the magnetic field leads to a decrease of energy distance between spectral features related to the excitonlike polariton modes. This effect has been treated as the magneto-induced increase of the exciton mass. We have shown that the hydrogenlike and diamagnetic exciton models are insufficient to describe the exciton behavior in the intermediate magnetic fields studied. Considering the symmetry of the problem, we have developed a phenomenological model which adequately describes the experimental data.
The magnetic properties of heavy-hole excitons in wide quantum wells of ZnTe with ZnxMg1-xTe barriers have been studied with photoluminescence and reflectivity measurements. The exciton magnetic moments (as characterized by the g values) and the diamagnetic shifts of the exciton transitions are found to depend strongly on the wave-vector component Kz associated with translational motion of the exciton normal to the plane of the quantum well. The case of ZnTe differs from examples of this behavior previously reported for GaS, CdTe, and ZnSe since the ZnTe is under tensile biaxial strain, so that the heavy-hole exciton states lie higher in energy than the corresponding states of the light-hole excitons. The dependence of the magnetic properties on Kz is nevertheless still in excellent agreement with the predictions of a model proposed by Smith et al. Phys. Rev. B 78 085204 (2008), in which mixing of the heavy-hole 1S exciton state with light-hole nP states is found to be responsible for motion-induced changes in the internal structure of the exciton.
Recent studies of excitons in wide quantum wells have shown that the magnetic properties are strongly affected as the excitons acquire kinetic energy. In the center of mass approximation, these motion-induced changes are ascribed to mixing between the 1S exciton ground state and the higher lying nP states. The origin of the mixing is due to the dispersion curves for the valence band not being of simple parabolic form. Detailed previous studies of excitons in CdTe have resulted in excellent agreement between experiment and the predictions of this model. One consequence of the mixing is that the magnetic moment of the exciton is not simply the sum of the magnetic moments of the electron and hole, but contains motion-induced contributions, which can easily dominate the contributions from the individual charge carriers. To confirm the validity of the model, we have carried out detailed investigations of the magnetic properties of center of mass excitons in a second semiconductor, ZnSe, for which the magneto-optical properties of the individual charge carriers are completely different from those of CdTe. Excellent agreement is obtained between theory and experiment with a choice of the Luttinger parameter gamma(3)=0.98, in close agreement with the value determined independently by two-photon magnetoabsorption experiments. The success of the model when applied to both materials provides strong evidence that motion-induced changes in magnetism are a universal feature in zinc-blende semiconductors.
We have shown recently that the magnetic properties of excitons change significantly as the excitons acquire kinetic energy. In particular, the exciton magnetic moments are enhanced considerably, whilst the diamagnetism decreases. The behaviour can be investigated through spectroscopic studies of excitons confined in quantum wells of large width (greater than five times the exciton Bohr radius) and these motion-induced changes in the magnetic properties have now been observed for CdTe, ZnSe, ZnTe and GaAs. The present paper summarises these phenomena, with particular focus on CdTe and ZnSe, and shows that the changes can be accounted for by motion-induced mixing between the exciton ground and higher lying states. The mixing is caused by the gamma3 term in the Luttinger Hamiltonian which describes the dispersion curves for the valence band and, as a result, the form of the exciton wavefunction becomes motion-dependent. For both materials, excellent agreement is obtained between experiment and the results predicted by this mechanism.
Calculations have been made of the temperature dependent linewidths of excitons in semiconductor quantum wells due to coupling to LO phonons. The contributions to the linewidth from exciton scattering to both bound and interacting continuum states have been included. The dependences of the linewidth on the quantum well width and on the choice of heavy hole mass is discussed, and comparison is made with available experimental data.
Photoluminescence spectra, very different from those given by bulk CdTe, are obtained at 1.8 K, for two Cd0.92Zn0.08Te/CdTe/Cd0.92Zn0.08Te quantum-well heterostructures. The thicknesses of the CdTe layers are 500 and 1000 Å, respectively, intermediate between a quasi-two-dimensional quantum well and the bulk. Zero-phonon free-exciton luminescence predominates over bound-exciton recombination; phonon replicas of the free exciton luminescence are not visible. Satellite peaks are observed over a 10-meV range on the high-energy side of the free exciton peak. These are attributed to quantized states of the exciton-polariton confined inside the CdTe layer. A simple theory is given for the relative emission probabilities associated with the quantized states; their energy positions are fitted with a series of indexed, discrete points on a polariton dispersion curve.
Very large changes in the Zeeman splittings and in the diamagnetism of excitons as they acquire kinetic energy in wide quantum wells of CdTe are reported. The changes are found to be functions of the translational wave vector Kz of the exciton in the growth direction of the well, irrespective of the width of the well, and are also found to be strong functions of the direction of the magnetic field. The behavior is accounted for by a model in which mixing occurs between the hydrogenic states which describe the exciton in the center of mass or adiabatic approximation. The mixing is ascribed to terms which arise in the Luttinger Hamiltonian when it is extended to describe excitons. Excellent quantitative agreement with experiment, including the results of changing the strain in the wells, is obtained by using Luttinger parameters close to those previously reported. The model is applicable to wide quantum wells made from any zinc-blende semiconductor and confirms that the huge motion induced changes in magnetic properties, observed here for CdTe and previously also for ZnSe and GaAs, should be universal for such materials.
This paper overviews our recent results on the magneto-optics of excitons in wide quantum wells, when the well width is much larger than the exciton Bohr radius. We have demonstrated that the magnetic moment of the exciton increases by an order of magnitude due to its motion along the magnetic field. The effect was observed for a set of quantum well structures with different well widths and based on various semiconductor compounds. The reason for this effect is mixing of the exciton's center-of-mass motion and the internal motion of electron and hole in the exciton.
We have observed remarkable changes in the magnetic properties of excitons as they acquire kinetic energy. In particular, the Zeeman splittings and diamagnetic shifts of excitonic transitions when magnetic fields are applied along the growth direction of (001) wide quantum wells of CdTe, ZnSe, ZnTe and GaAs are found to to have a strong dependence on the translational wavevector Kz. The behaviour of the Zee-man splittings corresponds to enhancement of the magnetic moments of the excitons. This enhancement is particularly marked when their translational kinetic energy becomes comparable with the exciton Rydberg and can be described by what appears to be a universal function of Kz. A model for the behaviour is outlined which involves motionally-induced mixing between the 1S hydrogenic exciton ground state and excited nP states. The observations imply that there are significant changes in the structure of the exciton as its translational kinetic energy increases. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Reflectance experiments on GaAs/Ga 1-x Al x As single quantum well structures were performed at 4.2 K, with different thicknesses of the front GaAlAs barrier layer (100–1000 Å). The observed exciton reflectance line shapes depend strongly on the thickness of the front barrier layer due to the interferences between the reflected waves from the front surface and the quantum well interfaces. Calculations of the reflectance line shapes show good agreement with the observations. The absorption coefficient for the electron heavy‐hole exciton transition in a single quantum well sample is determined. Our study also provides a new understanding of the line shapes measured in photoreflectance experiments.
We have developed a theory of the longitudinal $g$ factor of light holes in
semiconductor quantum wells. It is shown that the absolute value of the
light-hole $g$-factor can strongly exceed its value in the bulk and, moreover,
the dependence of the Zeeman splitting on magnetic field becomes non-linear in
relatively low fields. These effects are determined by the proximity of the
ground light-hole subband, $lh1$, to the first excited heavy-hole subband,
$hh2$, in GaAs/AlGaAs-type structures. The particular calculations are
performed in the framework of Luttinger Hamiltonian taking into account both
the magnetic field-induced mixing of $lh1$ and $hh2$ states and the mixing of
these states at heterointerfaces, the latter caused by chemical bonds
anisotropy. A theory of magneto-induced reflection and transmission of light
through the quantum wells for the light-hole-to-electron absorption edge is
also presented.
We have observed very sharp lines in magnetoluminescence spectra of a GaAs-AlxGa1-xAs single heterostructure under resonant excitation of 10-70 meV above the GaAs direct energy gap. More than 14 lines were observed in both sigma+ and sigma- polarizations, at magnetic fields as low as 0.6 T. The spectral positions of these lines and their dependence on the magnetic field allow us to attribute the strongest lines to optical transitions between electron and light-hole Landau levels with indices from n=2 to 15. The nonparabolicity of the bands and effects of the resonant electron-phonon interaction were directly observed in the luminescence spectra. The energies of optical transitions obtained for large Landau indices at low magnetic fields allow an accurate determination of band parameters in the classical limit. The effective mass and g factors of the light holes are found to be m*lh=(0.078+/-0.002)m0 and g*lh=30.7+/-0.7 for a magnetic field along the [001] direction.
We report a remarkable enhancement of the magnetic moments of excitons as a result of their motion. This surprising result, which we have observed in magneto-optical studies of three distinct zinc-blende semiconductors, GaAs, CdTe, and ZnSe, becomes significant as the kinetic energy of the exciton becomes comparable with its Rydberg energy and is attributed to motionally induced changes in the internal structure of the exciton. The enhancement of the magnetic moment as a function of the exciton translational wave vector can be represented by a universal equation.
Merle d’Aubigne Y and Million A
- H Tuffigo
- R T Cox
- Magnea
Tuffigo H, Cox R T, Magnea N, Merle d’Aubigne Y and Million A 1988 Phys. Rev. B 37 4310–3
- J Davies
Davies J J et al 2006 Phys. Rev. Lett. 97 187403
- M I Dyakonov
- A Khaetskii
Dyakonov M I and Khaetskii A V 1982 Sov. Phys.—JETP
55 917–20 (Engl. transl.)
- L C Smith
- J J Davies
- D Wolverson
- H Boukari
- H Mariette
- V P Kochereshko
- R Phillips
Smith L C, Davies J J, Wolverson D, Boukari H, Mariette H,
Kochereshko V P and Phillips R T 2011 Phys. Rev. B
83 155206
- V Kochereshko
Kochereshko V P et al 2008 Phys. Status Solidi B 245 1059–63
- M V Durnev
- M M Glazov
- E Ivchenko
Durnev M V, Glazov M M and Ivchenko E L 2012 Physica E
44 797–802 and references therein
- J Davies
Davies J J et al 2010 Phys. Status Solidi B 247 1521–7
- L Smith
Smith L C et al 2008 Phys. Rev. B 78 085204
- Gross Ye
- F Karryev
Gross Ye F and Karryev N A 1952 Dokl. Akad. Nauk SSSR
84 261