ABSTRACT
Doping of traditional semiconductors has enabled technological applications in modern electronics by tailoring their chemical, optical and electronic properties. However, substitutional doping in two-dimensional semiconductors is at a comparatively early stage, and the resultant effects are less explored. In this work, we report unusual effects of degenerate doping with Nb on structural, electronic and optical characteristics of MoS2 crystals. The doping readily induces a structural transformation from naturally occurring 2H stacking to 3R stacking. Electronically, a strong interaction of the Nb impurity states with the host valence bands drastically and nonlinearly modifies the electronic band structure with the valence band maximum of multilayer MoS2 at the Γ point pushed upward by hybridization with the Nb states. When thinned down to monolayers, in stark contrast, such significant nonlinear effect vanishes, instead resulting in strong and broadband photoluminescence via the formation of exciton complexes tightly bound to neutral acceptors.
ABSTRACT
Simultaneous increases in electrical conductivity (up to 200%) and thermopower (up to 70%) are demonstrated by introducing native defects in Bi2 Te3 films, leading to a high power factor of 3.4 × 10(-3) W m(-1) K(-2). The maximum enhancement of the power factor occurs when the native defects act beneficially both as electron donors and energy filters to mobile electrons. They also act as effective phonon scatterers.
ABSTRACT
Transmission Electron Microscopy (TEM) and X-ray diffraction (XRD) have been used to study compositional modulation in In(x)Ga(1-x) N layers grown with compositions close to miscibility gap. The samples (0.34 < x < 0.8) were deposited by molecular beam epitaxy using either a 200 nm thick AlN or GaN buffer layer grown on a sapphire substrate. Periodic compositional modulation leads to extra electron diffraction spots and satellite reflections in XRD in the theta-2theta coupled geometry. The ordering period delta measured along c-axis was about delta = 45 A for x = 0.5 and delta = 66 A for x = 0.78 for samples grown on AlN buffer layer. TEM and XRD determinations of delta were in good agreement. Compositional modulation was not observed for the sample with x = 0.34 grown on a GaN buffer layer. Larger values of delta were observed for layers with higher In content and for those having larger mismatch with the underlying AlN buffer layer. The possibility that the roughness of the AlN growth surface promotes strong In segregation on particular crystallographic planes leading to compositional modulation is considered.
