Large-area epitaxial growth of MoSe2 via an incandescent molybdenum source.

We have developed an incandescent Mo source to fabricate large-area single-crystalline MoSe2 thin films. The as-grown MoSe2 thin films were characterized using transmission electron microscopy, energy dispersive x-ray spectroscopy, atomic force microscopy, Raman spectroscopy, photoluminescence (PL), reflection high energy electron diffraction (RHEED) and angular resolved photoemission spectroscopy (ARPES). A new Raman characteristic peak at 1591 cm-1 was identified. Results from Raman spectroscopy, PL, RHEED and ARPES studies consistently reveal that large-area single crystalline mono-layer of MoSe2 could be achieved by this technique. This technique enjoys several advantages over conventional approaches and could be extended to the growth of other two-dimensional layered materials containing a low-vapor-pressure element.

Self-Assembled Formation of Well-Aligned Cu-Te Nano-Rods on Heavily Cu-Doped ZnTe Thin Films.

Cu doping of ZnTe, which is an important semiconductor for various optoelectronic applications, has been successfully achieved previously by several techniques. However, besides its electrical transport characteristics, other physical and chemical properties of heavily Cu-doped ZnTe have not been reported. We found an interesting self-assembled formation of crystalline well-aligned Cu-Te nano-rods near the surface of heavily Cu-doped ZnTe thin films grown via the molecular beam epitaxy technique. A phenomenological growth model is presented based on the observed crystallographic morphology and measured chemical composition of the nano-rods using various imaging and chemical analysis techniques. When substitutional doping reaches its limit, the extra Cu atoms favor an up-migration toward the surface, leading to a one-dimensional surface modulation and formation of Cu-Te nano-rods, which explain unusual observations on the reflection high energy electron diffraction patterns and apparent resistivity of these thin films. This study provides an insight into some unexpected chemical reactions involved in the heavily Cu-doped ZnTe thin films, which may be applied to other material systems that contain a dopant having strong reactivity with the host matrix.