We report on the optical spectroscopy of a single InAs/GaAs quantum dot (QD) doped with a single Mn atom in a longitudinal magnetic field of a few Tesla. Our findings show that the Mn impurity is a neutral acceptor state A 0 whose effective spin J = 1 is significantly perturbed by the QD potential and its associated strain field. The spin interaction with photo-carriers injected in the quantum dot is shown to be ferromagnetic for holes, with an effective coupling constant of a few hundreds of µeV, but vanishingly small for electrons.PACS numbers: 71.35. Pq, 78.67.Hc,75.75.+a,78.55.Cr The spin state of a single magnetic impurity could be envisaged as a primary building block of a nanoscopic spin-based device [1,2] in particular for the realization of quantum bits [3]. However probing and manipulating such a system require extremely high sensitivity. Several techniques have been successfully developed over the last few years to address a single or few coupled spins: electrical detection [4,5], scanning tunneling microscopy (STM) [6,7,8,9], magnetic resonance force microscopic [10], optical spectroscopy [11]. Recently, Besombes et al. [12,13,14,15] have investigated the spin state of a single Mn +2 ion embedded in a single II-VI self-assembled quantum dot (QD). In this system the magnetic impurity is an isoelectronic center in a 3d 5 configuration with spin S = 5/2. The large exchange interaction between the spin of the photocreated carriers confined inside the dot and the Mn magnetic moment induces strong modifications of the QD photoluminescence (PL) spectrum: 2S + 1 = 6 discrete lines are observed, reflecting the Mn spin state at the instant when the exciton recombines.The case of the Mn ion is different in GaAs, since the impurity is an acceptor in this matrix with a rather large activation energy (113 meV). Two types of Mn centers exist in GaAs, the A 0 and the A − states. In low doped GaAs (below 10 19 cm −2 ), the former is dominant. It corresponds to the 3d 5 + h configuration, where h is a hole bound to the Mn ion with a Bohr radius around 1 nm [16]. When considering a single Mn impurity in InAs QD several issues arise: the impurity configuration, its possible change when photo-carriers are captured, the influence on the binding energy of excitonic complexes, the strength and sign of the effective exchange interaction with each of the carriers (electron or hole) in the QD S-shell. In this Letter, we report the first evidences of a single Mn impurity in an individual InAs QD which enable us to answer most of the above questions. In particular, we find that the formation of excitons, biexciton and trions is weakly perturbed by the impurity center, whereas the effective exchange coupling with the Mn impurity (found in the A 0 configuration) is ferromagnetic for holes (a few 100 µeV's) and almost zero for the electrons.The sample was grown by molecular beam epitaxy on a semi-insulating GaAs [001] substrate. The Mn-doped quantum layer was embedded inbetween an electron reservoir and a Schottky gate. This des...
Epitaxial liftoff is a post-growth process by which the active part of a semiconductor heterostructure, the epitaxial layer, is removed from its original substrate and deposited onto a new substrate. This is a well established technique in GaAs-based heterostructures where epitaxial liftoff can be achieved by exploiting the contrast in the etch rates of GaAs and AlAs in hydrofluoric acid. We report here successful epitaxial liftoff of a ZnSe-based heterostructure. We find that a metastable layer of MgS acts as a perfect release layer based on the huge contrast in the etch rates of ZnSe and MgS in hydrochloric acid. Epitaxial liftoff of millimeter-sized ZnSe samples takes a fraction of the time required for GaAs liftoff. Photoluminescence experiments confirm that the liftoff layer has the same optical characteristics as the original wafer material.
We have determined the direct and exchange electron -electron and electron -hole Coulomb energies in CdSe/ZnSe quantum dots. The experiments are based on single dot photoluminescence at low temperature. By controlling the charging with a vertical transistor structure and by applying a symmetry-breaking magnetic field, we show how we can determine all the Coulomb energies. The direct Coulomb energies are responsible for large, ~20 meV, red-shifts of the emission on charging. The exchange Coulomb energies lead to a very pronounced fine structure splitting, up to 2.6 meV, for the neutral exciton.The physics of self-assembled quantum dots is dominated by quantization. The largest energy gap in the system is clearly the fundamental energy gap. The strong confinement potentials induced by the quantum dot lead to quantized energy levels for both electrons and holes with typical quantization energies of tens of meV. However, additional energy scales play an important role in the photonics of self-assembled quantum dots. Electrons and holes interact through the Coulomb interaction. The typical Coulomb energies are particularly large in CdSe/ZnSe quantum dots on account of the small dot size, relatively large effective masses and relatively small relative permittivity. Exchange energies are also important. While the electron-electron exchange plays a role only for exciton complexes containing two or more electrons, the electron-hole exchange plays a crucial role in determining the energies and spins of the neutral exciton. A hierarchy of energy scales therefore exists, from ~eV for the fundamental band gap to sub-meV for some components of the electron-hole exchange interaction. It is clearly a challenge to determine all these energies experimentally.Spectroscopy of quantum dot ensembles is of limited use in this quest as inhomogeneous broadening obscures all but the coarsest energies in the system. Spectroscopy of single dots removes the inhomogeneous broadening and, particularly at low temperature where the homogeneous broadening can be as small as a few µeV [1], in principle allows access to a complete spectroscopic picture. We report here spectroscopy on single CdSe/ZnSe quantum dots in a variety of experimental configurations. We control the charge through the realization of a vertical field effect transistor to determine the direct electronelectron and electron-hole Coulomb energies. We determine the electron-hole exchange energies from detailed spectroscopy of the neutral excitons in a symmetry-breaking magnetic field. As we show, this enables us to determine all the important energies in this system.The samples for these experiments were grown by molecular beam epitaxy. After n + -GaAs substrate preparation, we grow a ZnSe buffer at 390 °C. CdSe quantum dots are grown at the same temperature using 8 cycles in atomic layer epitaxy. The growth is then completed with 50 nm ZnSe overgrowth. The quantum dots emit at 2.38 eV at low temperature with an ensemble width of 52 meV. The quantum dot
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