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

This work investigates the characteristic of layered In₆Se₇ with varying phosphorus (P) dopant concentrations (In₆Se₇: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 In₆Se₇:P series compounds remain unchanged, exhibiting a monoclinic structure. Room-temperature micro-Raman (μRaman) result of all the compositions of layered In₆Se₇: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 In₆Se₇. An extra peak at approximately 171 ± 2 cm^-1 is observed and it shows enhancement at the highest P composition of In₆Se₇:P 5%. This mode is attributed to P-Se bonding caused by P doping inside In₆Se₇. All the doped and undoped In₆Se₇: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 In₆Se₇:P samples. Kelvin probe measurement shows a decrease in work function (i.e., an energy increase of Fermi level) of the n-type In₆Se₇ multilayers with the increase of P content. The indirect and direct band gaps for all of the multilayer In₆Se₇: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 In₆Se₇/In₆Se₇: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.