Y. Merle d'Aubigné

Ordered magnetic phase in Cd1-xMnxTe/Cd1-y-zMgyZnzTe : N heterostructures: magnetooptical studies

Photoluminescence magnetospectroscopy is employed to examine the low-temperature magnetic phase that is induced by the carrier-mediated ferromagnetic exchange interaction in modulation-doped Cd1-xMnxTe quantum well. Unusual properties of the domain structure are linked to tendency towards spin-density wave formation in this low-dimensional magnetic system

Optical properties of CdTe/Cd1-xZnxTe quantum wells and superlattices

Abstract MBE growth of high-quality CdTe / Cd 1- x Zn x Te single quantum wells and superlattices with x =0.08−0.15, on Cd 0.96 Zn 0.04 Te substrates, is described. Due to lattice-parameter mismatch, the different layers are strained, and these strains predominate in determining the valence-band discontinuity at the CdTe / Cd 1- x Zn x Te interfaces. Deformation potentials for CdTe are determined from optical spectra of strained CdTe layers. Optical properties of excitons in single quantum wells are described, emphasizing methods for identifying the different transitions (optical pumping experiments) and the nature of the quantized exciton states observed (centre-of-mass quantization versus separate-carrier quantization). Oscillator strengths for excitons in CdTe layers and quantum wells are determined as a function of layer thickness from transmission measurements. A high quality superlattice shows both direct (Type I) heavy-hole and indirect (Type II) light-hole exciton transitions; for this sample the 2s excited state of the heavy exciton is identified by its diamagnetic shift and the band offsets are deduced directly from the energies of direct and indirect transitions.

Magnetic tuning of the exciton binding energy and band-offsets determination in a CdTe/Cd1-xMnxTe superlattice

Abstract Magnetic tuning of the electronic properties of heterostructures incorporating diluted magnetic semiconductors offers various opportunities which are well illustrated in this optical study of a Cd1-xMnxTe superlattice (x = 6.6%). Due to the exceptional quality of this structure grown on a InSb substrate both the 1S and 2S states of the heavy hole exciton are observed, which allows the determination of the valence band discontinuity at the interface and offers the unique opportunity to study the variation of the exciton binding energy with the quantum well depth. The partition ratio of the band gap discontinuity at the interface between the conduction and valence band is found to be close to 60/40 ie smaller than earlier values obtained from quantum wells with larger values of x but in good agreement with a theoretical estimation.

Nitrogen doping of Te-based II–VI compounds

Abstract We report on the doping properties of the II–VI Te-related compounds, ZnTe, CdTe and CdZnTe, CdMgTe and ZnMgTe, using two different nitrogen plasma sources. High doping levels are measured for ZnTe, slightly lower for CdTe (8 × 10 17 cm −3 ). Following the comparative study of nitrogen doping in these materials, we invoke the possible formation of nitride compounds as responsible for the large nitrogen amount incorporated in Mg-containing alloys. The formation of such compounds might be reduced by using appropriate growth conditions and doping levels in the 4 × 10 17 cm −3 range are obtained in ternary and quaternary alloys containing Mg. Such doping levels allow the growth and the study of high-quality modulation-doped heterostructures.

ENDOR of the self-trapped exciton in KCl

ENDOR of the self-trapped exciton in KCl is studied using the optical detection technique. The contact interaction with the neighbouring nuclei is very similar to that of the F centre in its ground state which indicates that the radial extent of electronic wavefunctions are similar in the two cases. The quadrupole resonance of the two central halogen nuclei was also detected, observation of two different quadrupole splittings indicates that the self-trapped exciton is slightly displaced from the centrosymmetrical position between two lattice sites.

Ferromagnetism induced by free carriers in p-type structures of diluted magnetic semiconductors

Abstract An overview is given of studies aiming at elucidation of the dominant effects that control the character of magnetic ordering and the magnitude of carrier-mediated ferromagnetic interactions in various dimensionality structures of diluted magnetic semiconductors. The modulation-doped p-type quantum wells of Cd1−xMnxTe/Cd1−y−zMgyZnzTe:N as well as epilayers of Zn1−xMnxTe:N and of Ga1−xMnxAs grown at low temperatures are discussed as examples of systems, in which ferromagnetic correlation induced by free carriers is observed. The effect of kp and spin–orbit couplings, notably in heavily doped, strained or reduced-dimensionality systems, is addressed. It is pointed out that disorder and carrier–carrier interactions have a strong influence on the spin–spin interaction, particularly in the vicinity of the metal-to-insulator transition. Outstanding properties of low-dimensional spin systems, resulting from their tendency towards the formation of spin-density waves, are described in the context of the observed peculiar domain structure.

Indication of ferromagnetic ordering in p-Zn1−xMnxTe

Abstract P-type doped Zn 1− x Mn x Te has been grown by molecular beam epitaxy and by the Bridgman method. Our SQUID measurements indicate ferromagnetic ordering above 1 K, for p⩾10 19 cm −3 . It is argued that weakly localized holes are responsible for the ferromagnetic correlation in the insulating phase.

A microwave cavity for EPR studies under light illumination

A microwave cavity for e.p.r. work under light illumination is described. The structure is very open so that light scattering is minimized. In addition this cavity is very easy to make.

Spectroscopy of polaritons in CdTe-based microcavities

Abstract We report on cavity polaritons arising from the strong coupling between quantum well excitons and the Fabry—Perot mode of a CdTe-based microcavity at 4 K. The sample, fully grown by molecular beam epitaxy, is a 3λ/2 cavity containing six CdTe quantum wells, embedded in CdMnTe/CdMgTe Bragg reflectors. The wedge-shaped structure of the sample allows the study of a multiple anticrossing between the photon mode and three excitonic levels: E1H1(1s), E1H1(2s) and E1L1(1s). This provides an accurate determination of the oscillator strength for each exciton. Finally, the Rabi splitting for the E1H1(1s) exciton is measured versus temperature up to 270 K.

Optical pumping study of light and heavy hole states in CdTe/Cd1−xZnxTe strained quantum wells

Abstract The optical pumping study of CdTe/Cd1−xZnxTe strained quantum wells allows the identification of the light and heavy hole states of both the well and the barriers. The electron spin polarization optically induced in the barriers is well conserved after the carrier capture in the quantum well. The polarized excitation spectra are well resolved which allows the identification of the e1 h1 (2S) transitions and the determination of the excitation binding energy.