T.C.M. Graham

Growth and characterization of CdSe:Mn quantum dots

In this paper we report the growth by atomic layer epitaxy (ALE) and optical properties of Znse/Cdse:Mn magnetic quantum dots. For samples grown without a ZnSe capping layer, dot densities of the order of 10 9 cm 2 were measured by atomic force microscopy (AFM). In capped samples, the ensemble dot photoluminescence (PL) was observed over a range of energies between 2.1 and 2.5 eV and a spectrally broad emission at 2.15 eV from the internal Mn 2 transition was observed at high Mn concentrations. Single dot spectroscopy was carried out by confocal microscopy and the PL linewidth was measured as a function of Mn concentration. At high Mn concentrations the temporal change in magnetization causes a broadening of the FWHM of lines from single dots of up to 4meV. However, for low concentrations single dot PL linewidths were resolution limited at <0.2 mev.

Growth and Spectroscopy of CdSe: Mn Quantum Dots

In this paper we report the growth and optical properties of ZnSe/CdSe:Mn magnetic quantum dots by Atomic Layer Epitaxy. For the uncapped samples, dot densities of the order of 109 cm−2 were measured by Atomic Force Microscopy. The ensemble dot photoluminescence (PL) was observed over a range of energies between 2.1 and 2.5 eV, and a spectrally broad emission at 2.15 eV from the internal Mn2+ transition was observed at high Mn concentrations. Single dot spectroscopy was carried out by confocal microscopy and the PL line width was measured as a function of Mn concentration. For large Mn contents the temporal change in magnetization causes a broadening of the single dot PL line of up to 4 meV FWHM. However, for low concentrations the single dot PL line widths were resolution limited at <0.2 meV.

Growth and spectroscopy of II-VI CdSe quantum dots

In this paper we review the recent progress in the growth and spectroscopy of CdSe quantum dots. In particular, atomic layer epitaxy (ALE) has been used to grow ZnSe/CdSe and ZnSe/CdSe:Mn magnetic quantum dots. For samples grown without a ZnSe capping layer, dot densities of the order of 10/sup 9/ cm/sup -2/ were measured by atomic force microscopy (AFM). In the capped samples, the ensemble dot photoluminescence (PL) was observed over a range of energies between 2.1 and 2.5 eV and in the high Mn concentration samples a spectrally broad emission at 2.15 eV from the internal Mn/sup 2+/ transition. Single dot spectroscopy was carried out by confocal microscopy and the PL linewidth was measured as a function of Mn concentration. Also, CdSe/MgS quantum dots have been grown successfully by molecular beam epitaxy using a thermally activated reorganization process that occurs during growth interruption. Unlike the ZnSe/CdSe dots the PL measurements show emission from both QDs and the wetting layer, with emission energies ranging between (2.3 and 3.8 eV). AFM topography and /spl mu/m-PL measurements also show evidence of quantum dot structures and power dependent PL measurements carried out on the dots give a value of 30 meV for the bi-exciton binding energy at 77 K.

Growth and characterisation of MgS/CdSe self-assembled quantum dots

MgS has a very large bandgap of /spl sim/ 5eV and can form an excellent barrier material for wide-gap II-VI quantum structures. Although its stable crystal structure is rocksalt, our group has recently established a novel molecular beam epitaxy (MBE) technique that allows us to grow zinc-blende MgS lattice matched to GaAs substrates to thicknesses greater than 130 nm [1]. The lattice parameter of zinc-blende MgS is almost the same as that of ZnSe. Accordingly, strain between MgS and CdSe is almost identical to that between ZnSe and CdSe and a transition from 2D to 3D growth is expected with increasing CdSe coverage. Using the barrier material MgS with CdSe dots instead of ZnSe has two advantages. Firstly, large band discontinuities, which are estimated to be 2.1 and 0.9eV for the conduction and valence bands, respectively, will provide strong carrier confinement. Secondly, interdiffusion of MgS and CdSe is inhibited due to the immiscibility of these two materials. The clear material boundary between the dots and the barrier will enhance the confinement even further.

Growth of zinc blende MnS and MnS heterostructures by MBE using ZnS as a sulphur source

Many potentially interesting semiconductor compounds do not have the usual zinc blende (ZB) or wurtzite crystal structures as their lowest energy configuration. In some, the atomic coordination number is six, rather than four, and the stable crystal structures are NaCl or NiAs, e.g. the first row transition metal sulphides and other ionic sulphides such as MgS.

Growth and characterisation of CdSe:Mn quantum dots

In this paper, we report the optical properties of CdSe:Mn dots grown by Atomic Layer Epitaxy (ALE). The samples were grown on GaAs substrates with a 150nm buffer of ZnSe deposited by normal MBE. CdSe layers of different thicknesses were deposited by ALE and then capped with ZnSe. Quantum dot emission was observed for CdSe layers more than 3 ML thick. Two sets of layers containing dots have been grown which were doped with Mn to give concentrations ranging from one to approximately ten Mn ions per dot. One set was capped for optical studies and the other left uncapped. For the uncapped samples, dot densities of the order of /spl sim/10/sup 9/ cm/sup -2/ were measured by Atomic Force Microscopy (AFM) measurements obtained immediately after growth (figure 1).