Phosphorus-Doped Multilayer In₆Se₇: The Study of Structural, Electrical, and Optical Properties for Junction Device

Abstract

This work investigates the characteristic of layered In 6 Se 7 with varying phosphorus (P) dopant concentrations (In 6 Se 7 :P) from P = 0, 0.5, 1, to P = 5%. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses indicate that the structure and morphology of the In 6 Se 7 :P series compounds remain unchanged, exhibiting a monoclinic structure. Room-temperature micro-Raman (μRaman) result of all the compositions of layered In 6 Se 7 :P reveals two dominant peaks at 101 ± 3 cm −1 (i.e., In−In bonding mode) and 201 ± 3 cm −1 (i.e., Se−Se bonding mode) for each P composition in In 6 Se 7 . An extra peak at approximately 171 ± 2 cm −1 is observed and it shows enhancement at the highest P composition of In 6 Se 7 :P 5%. This mode is attributed to P−Se bonding caused by P doping inside In 6 Se 7 . All the doped and undoped In 6 Se 7 :P showed n-type conductivity, and their carrier concentrations increased with the P dopant is increased. Temperature-dependent resistivity revealed a reduction in activation energy (for the donor), as the P content is increased in the In 6 Se 7 :P samples. Kelvin probe measurement shows a decrease in work function (i.e., an energy increase of Fermi level) of the n-type In 6 Se 7 multilayers with the increase of P content. The indirect and direct band gaps for all of the multilayer In 6 Se 7 :P of different P composition are identical. They are determined to be 0.732 eV (indirect) and 0.772 eV (direct) obtained by microtransmittance and microthermoreflectance (μTR) measurements. A rectified n−n + homojunction was formed by stacking multilayered In 6 Se 7 /In 6 Se 7 :P 5%. The built-in potential is about V bi ∼ 0.15 V. It agrees well with the work function difference between the two layer compounds.