Fucheng Lin

Structural and optical properties of self-assembled InAs/GaAs quantum dots covered by InxGa1−xAs (0⩽x⩽0.3)

Optical and structural investigations of InAs quantum dots (QDs) covered by InxGa1-xAs (0 less than or equal to x less than or equal to 0.3) overgrowth layer have been systematically reported. The decrease of strain in the growth direction of InAs quantum dots covered by InGaAs layer instead of GaAs is demonstrated by transmission electron microscopy experiments. In addition, the atomic force microscopy measurement shows that the surface of InAs islands with 3-nm-thick In0.2Ga0.8As becomes flatter. However, the InGaAs islands nucleate on the top of quantum dots during the process of InAs islands covered with In0.3Ga0.7As. The significant redshift of the photoluminescence peak energy and reduction of photoluminescence linewidth of InAs quantum dots covered by InGaAs are observed. The energy gap change of InAs QDs covered by InGaAs could be explained in terms of reducing strain, suppressing compositional mixing, and increasing island height

Influence of growth conditions on self-assembled InAs nanostructures grown on (001)InP substrate by molecular beam epitaxy

Self-assembled InAs nanostructures on (0 0 1) InP substrate have been grown by molecular beam epitaxy (MBE) and evaluated by transmission electron microscopy (TEM) and photoluminescence (PL). It is found that the morphologies and PL properties of InAs nanostructures depend strongly on the growth condition. For the same buffer layer, elongated InAs quantum wires (QWRs) and no isotropic InAs quantum dots (QDs) can be obtained using different growth conditions. At the same time, for InAs quantum dots, PL spectra also show several emission peaks related to different islands size. Theoretical calculation indicated that there are size quantization effects in InAs islands

Nonlinear photoluminescence of fullerene-doped optical glasses

Strong broadband white photoluminescence was observed in fullerene-doped phosphate and fluorophosphate optical glasses irradiated by an ultraviolet laser. Microphotoluminescence measurements demonstrated the existence of microislands in those amorphous glasses, where fullerene dopants provided high photosensitivity and optical nonlinearity. Nonlinear photoluminescence was observed under ultralow continuous-wave laser excitations. The photoluminescence peak wavelengths were demonstrated to depend nonlinearly on the laser excitation power.

Conductivity of fullerene-doped optical glasses

Appropriate amount doping of fullerenes caused insulating phosphate and fluorophosphate optical glasses to be conductive at room temperature. Microphotoluminescence measurements indicated the existence of ordered and disordered microstructures. A large number of fullerene-related medium-range microdomains aggregated as self-organized islands along the glass networks, and brought about connective tissues in the vitreous matrices for percolation migration of metal cations. The variation of microphotoluminescence induced by electric field presented us an indirect way to characterize the random activation, percolation migration, and retrap of metal cations near fullerene-related amorphous islands.

Optical properties of microstructures in fullerene-doped optical glasses

Microphotoluminescence and second-harmonic microscopy were used to study islands formed inside phosphate and fluorophosphate optical glasses with fullerene dopants. Scanning microphotoluminescence and second-harmonic microscopy indicated that fullerene dopants are subjected to strong influence from the glass matrix, which causes distortion of molecular symmetry, enhancement of fluorescence, and occurrence of noncentrosymmetry required by the macroscopic second-order optical nonlinearity.

Photoluminescence and its temperature dependence of phosphate and fluorophosphate optical glasses doped with fullerenes

Microphotoluminescence is studied for fullerene-doped phosphate and fluorophosphate optical glasses, where microstructures are formed by connecting nearby -[PO4]- tetrahedra with chemical bonds between fullerenes and non-bridging oxygen anions. Fullerene-related tetrahedral chains result in significant changes of photoluminescence. Scanning photoluminescence measurements indicate that a large number of fullerene-related microdomains aggregate as self-organized islands. The temperature dependence of the photoluminescence demonstrates that the non-radiative decay and ionic environments of fullerene complexes are different at the various temperatures. The changes of the ionic environments and anionic potentials of the fullerene complexes are caused by laser excitation induced activation of cations followed by their percolation migration and retrapping in the vitreous matrix.

Conductive phosphate and fluorophosphate glasses with fullerene doping

The appropriate amount of fullerene doping in phosphate and fluorophosphate optical glasses modifies the structures of glass matrices by bonding the nearby -[PO4]- tetrahedra with nonbridging oxygen anions, and consequently building up observable conductivity at room temperature. Non-Arrhenius ionic conductivity is observed, which is interpreted as a result of the temperature dependence of the activation energies of the mobile cations. The variation of microphotoluminescence induced by the electric field presents us with an indirect way to characterize the random activation, percolation migration, and retrap of metal cations near fullerene-related amorphous islands.

InAs self-assembled nanostructures grown on InP(001)

The 6-period stacked layers of self-assembled InAs quasi-quantum wires(qQWRs) and quantum dots(QDs) embedded into InAlAs on InP(001) substrates have been prepared by solid molecular beam epitaxy. The structures are characterized by atomic force microscopy(AFM) and transmission electron microscopy(TEM). From AFM we have observed for the first time that InAs qQWRs and QDs coexist, and we explained this phenomenon from the view of the energy related to the islands. Cross-sectional TEM shows that InAs qQWRs are vertically aligned every other layer along the growth direction [001], which disagrees with conventional vertical self-alignment of InAs QDs on GaAs substrate.

Arsenic clusters on the surface of vertically aligned InAs islands on GaAs substrate by annealing

The formation of arsenic clusters in a system of vertically aligned InAs quantum islands on GaAs during thermal annealing under As overpressure has been investigated by transmission electron microscopy (TEM) and Raman scattering. Semicoherent arsenic clusters, identified by TEM examination, have been formed on the surface of the GaAs capping layer. The existence of arsenic precipitates is also confirmed by Raman spectra, showing new peaks from the annealed specimen at 256 and 199 cm−1. These peaks have been ascribed to A1g and Eg Raman active phonons of crystalline arsenic. The phenomenon can be understood by a model of strain-induced selected growth under As overpressure.