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Optical Studies of Spin Polarized 2Deg in Modulation-Doped (Zn,Mn)Se/(Zn,Be)Se Quantum Wells in High Magnetic Fields
Abstract
Optical properties of (Zn,Mn)Se/(Zn,Be)Se quantum wells containing a two-dimensional electron gas with densities varying from
109 to 5.5 × 1011 cm-2 have been studied in magnetic fields up to 45 T. A strong s — d exchange interaction of free electrons with magnetic Mn ions results in a giant Zeeman splitting of conduction band states,
that has been exploited for manipulation with a spin polarization of the electron gas. Critical behavior of the photoluminescence
and reflectivity linewidths and of the energy shift of the lines in magnetic fields in vicinity of integer filling factors
2 and 1 has been found and analyzed.
Key wordsquantum well–diluted magnetic semiconductor–excitons–trions
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A combined exciton-cyclotron resonance is found in photoluminescence excitation and reflectivity spectra of semiconductor quantum wells containing an electron gas of low density. In external magnetic fields, an incident photon creates an exciton in the ground state and simultaneously excites one of the resident electrons from the lowest to one of the upper Landau levels. A theoretical model is developed, which gives a good quantitative description of the energy position and the intensity of the combined exciton-cyclotron resonance.
Modulation-doped CdxMn1-xTe/CdyMg1-yTe quantum wells are studied at 1.7 K by magnetoabsorption experiments performed near the fundamental heavy-hole–electron (HH1-E1) transition. In the investigated range of Mn content (x≈0.02) and electron concentration [ne≈(2–3)×1011 cm-2], the two-dimensional electron gas is fully spin-polarized at magnetic fields as low as H=1.5 T. The interband optical spectra exhibit unusual magnetic field dependence resulting from electron-hole interactions and phase-space-filling effects. An additional enhancement of the spin splitting of Landau levels found in this study is attributed to the electron-exchange interaction. A simplified theoretical model including the exchange energy and the electron-hole interaction provides a coherent quantitative description of the overall experimental features.
Excitons and charged excitons (trions) are investigated in ZnSe-based quantum well structures with (Zn,Be,Mg)Se and (Zn,Mg)(S,Se) barriers by means of magneto-optical spectroscopy. Binding energies of negatively () and positively (X+) charged excitons are measured as functions of quantum well width, free carrier density and in external magnetic fields up to 47 T. The binding energy of shows a strong increase from 1.4 to 8.9 meV with decreasing quantum well width from 190 to 29 A. The binding energies of X+ are about 25% smaller than the binding energy in the same structures. The magnetic field behavior of and X+ binding energies differ qualitatively. With growing magnetic field strength, increases its binding energy by 35-150%, while for X+ it decreases by 25%. Zeeman spin splittings and oscillator strengths of excitons and trions are measured and discussed.
An optical method is suggested to determine the concentration of two-dimensional electrons in modulation-doped quantum wells at low and moderate electron densities between 10^{9} and 2x10^{11} cm^{-2}. The method is based on an analysis of magneto-reflectivity spectra of charged excitons (trions). The circular polarization degree and the oscillator strength of the charged excitons contain information about the density and spin polarization of two-dimensional electron gas. The method is applied to CdTe/(Cd,Mg)Te and ZnSe/Zn,Mg)(S,Se) heterostructures.
Low-temperature magneto-photoluminescence studies of negatively charged excitons (X- trions) are reported for n-type modulation-doped ZnSe/Zn(Cd,Mn)Se quantum wells over a wide range of Fermi energy and spin-splitting. The magnetic composition is chosen such that these magnetic two-dimensional electron gases (2DEGs) are highly spin-polarized even at low magnetic fields, throughout the entire range of electron densities studied (5e10 to 6.5e11 cm^-2). This spin polarization has a pronounced effect on the formation and energy of X-, with the striking result that the trion ionization energy (the energy separating X- from the neutral exciton) follows the temperature- and magnetic field-tunable Fermi energy. The large Zeeman energy destabilizes X- at the nu=1 quantum limit, beyond which a new PL peak appears and persists to 60 Tesla, suggesting the formation of spin-triplet charged excitons. Comment: 5 pages (RevTex), 4 embedded EPS figs. Submitted to PRB-RC
Heating of the spin system of magnetic Mn ions by means of photoexcited carriers has been studied in undoped (Zn,Mn)Se/(Zn,Be)Se multiple quantum well structures. Elevated spin temperature of the magnetic ions has been documented by a suppression of the giant Zeeman splitting of excitonic states measured in photoluminescence and reflectivity spectra. Low densities of photoexcitation (about 1 W/cm2) induce strong heating of the Mn spin system. The heating shows a strong dependence on the Mn content varying from 0.004 to 0.06. It decreases with increasing Mn content due to the shortening of the spin-lattice relaxation time.
We have performed magnetoluminescence and magnetoreflectance studies on
novel semimagnetic semiconductor heterostructures based on
(ZnMn)Se/(ZnBeMg)Se. The successful fabrication and various optical
properties of that system are demonstrated by
Zn0.95Mn0.05Se/''
''Zn0.76Be0.08Mg0.16Se single quantum
wells. The exciton spin splitting was used to determine the valence-band
offset for (ZnMn)Se/(ZnBeMg)Se to 0.22 of the total band-gap
discontinuity. Weakly confining multiple quantum wells made of
Zn0.91Mn0.09Se/Zn0.972Be0.028Se
demonstrate a change of the band alignment from type I to type II for
one of the exciton spin components in external magnetic fields which
results in the formation of a spin superlattice. This spin superlattice
formation manifests itself in an asymmetric Zeeman splitting of a
spatially direct exciton caused by a spin-dependent change of the
exciton binding energy. A pronounced broadening of the exciton in
reflectance and photoluminescence excitation spectra was observed when
scattering into the (ZnBe)Se barriers becomes possible.
Making use of a unique spin engineering possible in diluted magnetic semiconductors (DMS), we study magneto-optical properties of quantum wells with electrons as a function of their g factor (tuned widely from g*eff=-1.46 to +55). The study is made on samples containing different densities of electrons in various spatial regions of a single structure. An unusual nonmonotonic behavior of the intensity of the trion-related transition is detected. We show that it reflects a reversal of the ordering of the spin-split electron levels with the magnetic field. We provide an experimental evidence that polarization properties of exciton-cyclotron resonances are governed by the spin splitting of the conduction band states only. A very strong band gap renormalization due to the presence of electrons is also determined in our quasi-two-dimensional DMS system.
Magnetization of Zn1−xMnxSe () was measured for magnetic field up to in the temperature range 2.2 – 30 K. The obtained data were described by Brillouin-like function. The direct comparison of magnetization data with the previous magnetooptical data enabled us to determine the s-d exchange constants in , .
Magnetic semiconductors offer a unique possibility for strongly tuning the intrinsic alloy disorder potential with applied magnetic field. We report the direct observation of a series of steplike reductions in the magnetic alloy disorder potential in single ZnSe/Zn(Cd, Mn)Se quantum wells between 0 and 60 T. This disorder, measured through the linewidth of low-temperature photoluminescence spectra, drops abruptly at ~19, 36, and 53 T, in concert with observed magnetization steps. Conventional models of alloy disorder (developed for nonmagnetic semiconductors) reproduce the general shape of the data, but markedly underestimate the size of the linewidth reduction.
Summary The influence of the electron spin splitting on the stability of quasi-two-dimensional donor bound excitons (D
0
X) has been investigated in the Cd0.97Mn0.03Te/Cd0.7Mg0.3Te semi-magnetic semiconductor quantum well (QW) structure. TheD
0
X binding energy has been measured directly from emission spectra recorded at magnetic field parallel to the QW plane. Complexes
have been shown to dissociate into a neutral donor and a free exciton with a spin flip of the electron at high magnetic fields
when the electron spin splitting exceeds the binding energy.
In modulation-doped ZnSe/(Zn,Mg)(S,Se) quantum well structures with electron concentration of about 1011 cm—2 a new resonant optical transition involving four particles (one hole and three electrons) was found in external magnetic fields in photoluminescence excitation and reflectivity spectra. This transition reveals as a narrow line between exciton and trion lines. It is identified with a combined process in which a photocreated exciton binds with an electron forming a trion and promotes for the second electron a transition to higher Landau levels.
We have carried out a photoluminescence study of an n-type modulation-doped ZnSe/Zn0.825Cd0.14Mn0.035Se/ZnSe single quantum well structure (electron areal density nA=1.6×1012cm-2) in magnetic fields up to 30 T. In the presence of a magnetic field, the broad emission band in the vicinity of the gap evolves into a series of discrete features. These are attributed to interband transitions of electrons occupying Landau levels to photogenerated holes. The Landau transitions associated with both electron spin states (mj=±1/2) are clearly resolved due to the large electron and hole g factors exhibited by the magnetic wells. An analysis of the Landau-level occupation as a function of magnetic field yields the electron concentration.
The oscillator strength of negatively charged excitons (trions) in ZnSe/(Zn,Mg)(S,Se) quantum-well structures with n-type modulation doping is studied by means of reflectivity as a function of electron concentration, temperature, and external magnetic fields. The trion oscillator strength is found to increase linearly with increasing electron concentration up to 6×1010cm-2. The trion nonradiative damping shows no dependence on the electron concentration or external magnetic field. An optical method, based on the analysis of the polarization of the trion line, is proposed to investigate two-dimensional electron gases of low density from 109 up to 1011cm-2.
A model for the high-field magnetization of II-VI semimagnetic compound semiconductors at low temperatures is presented. It applies for low Mn concentrations. The model explains quantitatively and without adjustable parameters the magnetization of Cd1-xMnxTe, Cd1-xMnxSe, Cd1-xMnxS, Zn1-xMnxSe, and Zn1-xMnxS near 100 kOe for T<2 K. At higher magnetic fields, magnetization steps are predicted. The first step is observed in Cd1-xMnxSe and Zn1-xMnxSe, and is used to determine the nearest-neighbor exchange constant. The results support a random distribution of Mn ions.
Heating of the spin system of magnetic Mn ions by means of photoexcited carriers has been studied in undoped (Zn, Mn)Se/(Zn, Be)Se multiple quantum well structures. Elevated spin temperature of the magnetic ions has been documented by a suppression of the giant Zeeman splitting of excitonic states measured in photoluminescence and reflectivity spectra. Low densities of photoexcitation (about 1 W/cm2) induce strong heating of the Mn spin system. The heating shows a strong dependence on the Mn content varying from 0.004 to 0.06. It decreases with increasing Mn content due to the shortening of the spin-lattice relaxation time.
Effective g factors of electrons and heavy holes in CdTe/CdMgTe quantum wells have been measured with high accuracy as a function of the angle between the magnetic field and the structure growth axis. As a result, we have observed the electron g factor anisotropy in A2B6-based heterostructures. The dependences of the g factor tensor components on the quantum-well width and the solid solution composition have been calculated within the k · p method and compared with the available experimental data.
Properties of a two-dimensional electron gas with a density of in a ZnSe/(Zn,Be,Mg)Se quantum well structure have been investigated by means of polarized magneto-luminescence in fields up to and by an optically detected resonance (ODR) technique. With increasing magnetic field, the optical spectra of the electronic system changes its appearance from Landau-level-like with a linear shift of lines, to excitonic-like with a diamagnetic shift. The transition occurs at filling factor 2. Pronounced oscillatory features at even filling factors are found for the luminescence intensity and polarization degree, the energy of optical transitions, and in ODR signal.
Photoluminescence measurements are made on the semimagnetic semiconductors {\mathrm{Cd}}_{1\mathrm{-{}}\mathrm{x}}Te and {\mathrm{Cd}}_{1\mathrm{-{}}\mathrm{x}}Se for 0<x{}0.15. Large energy shifts ({}50 meV) of the exciton recombination peaks and large optical polarizations ({}100%) are produced by applying a magnetic field. This study examines changes in luminescence using magnetic fields up to B=15 T and at temperatures between T=1.5 and 60 K. When viewed along the direction of the field (Faraday geometry), the luminescence due to both exciton recombination and impurity-related transitions becomes circularly polarized. At T=2 K the polarization shows saturation at only 0.5 T.
Magnetoluminescence studies of highly excited undoped CdTe/Cd0.88Mn0.12Te and Cd0.97Mn0.03Te/Cd0.75Mg0.25Te quantum wells (QW's) have been carried out in magnetic fields up to 14 T at 4.2 K. Qualitatively different excitonic systems, spin aligned in (Cd,Mn)Te QW and spin depolarized in CdTe QW, have been generated. The interaction of spin-aligned excitons are found to be weakly repulsive. In contrast, an additional well pronounced emission line M has been found to appear at approximately 5 meV below the excitonic line in the spectra of the spin-degenerate system. This line is assigned to the emission of excitonic molecules consisting of excitons with different spins. Finally, we have shown that the renormalization of Landau-level transitions in dense electron-hole magnetoplasmas with several occupied Landau levels depends on their spin state, it is markedly weaker in the spin-polarized plasmas.