Excitons in motion in II–VI semiconductors

Abstract

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 g 3 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.ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction What happens to excitons when they move through semiconductors? To answer this question is not easy, since the usual method of investigating the structure of excitons is to use optical spectroscopy and, unfortunately, the small wavevector of photons means that only excitons with low values of their translational wavevectors can conventionally be studied. To overcome this difficulty, we have in a series of recent investigations [1-6] studied excitons in potential wells which are sufficiently wide for quantum confinement energies to be smaller than the exciton binding energy. In such quantum wells, the exciton can be described in the adiabatic or centre of mass (CoM) approximation [7,8], in which it is considered as a composite particle formed from the electron and the hole mutually orbiting each other. The translational wavevector K z of the exciton as a whole is then quantised approximately according to K z ¼ np=L, where L is the width of the quantum well and N is a non-zero integer, and the recombination transition energies of the exciton then have discrete values. As a result of the loss in translational symmetry, photons can interact with excitons with large

Exchange interactions in Cd1−xMnxTe wide quantum wells

“…There is, however, an unexpected difference between g o for the present specimens and the value of g o = −0.75 found for otherwise identical but nonmagnetic CdTe wells. 11 There is evidence that g o is strain dependent 11,13 but this cannot account for the present difference.…”

“…11. Excellent agreement between the predicted and experimental values of g(K z ) is obtained, not only for CdTe, 11,13 but also for ZnSe 12 and ZnTe. 14 In the present study, the manganese concentrations in the well are very low, so the parameters used to calculate g(K Z ) are taken to be the same as for CdTe.…”

“…The strain in these quantum wells is expected to be similar to that observed in several similar structures that contain no manganese and is expected to cause the heavy-hole (HH) valence band to lie at about 15 meV lower in energy than the light-hole (LH) band, so that the lowest-lying exciton states involve the heavy holes (as for example, in Refs. [10][11][12][13][14].…”

“…In the case of quantum wells that contain no magnetic ions, the approach has already been used to show that the Zeeman splitting of excitons (characterized by the g values) depends strongly on the translational wave vector and the behavior has been shown to be due to mixing between the heavy-hole exciton ground state (of 1S character) and higher-lying P-like light-hole exciton states. [10][11][12][13][14] In wells that contain magnetic ions, this mixing will also be present and its effect on the exchange interaction must also be taken into account, in addition to the mechanisms considered previously.…”

Wave-vector dependence of magnetic properties of excitons in ZnTe

“…10 Recent optical experiments in magnetic fields have shown, however, that the properties of excitons with finite translational wave vectors are more complicated than appears at first sight. [11][12][13][14] In particular, the exciton magnetic moment (characterized by its g value, g exc ) is found to be highly dependent on K z , in some cases changing by an order of magnitude over the range of accessible values of K z . The magnetic moment is sensitive to the electronic structure of the exciton, which itself is thus shown to depend on its state of motion.…”