P-type β-MoO₂ nanostructures on n-Si by hydrogenation process: Synthesis and application towards self-biased UV-visible photodetection

We report on the synthesis and UV-vis photodetection application of p-type MoO₂ nanostructures (NSs) on Si substrate. β-MoO₂ NSs have been synthesized from previously grown α-MoO₃ structures/n-type Si via a hydrogenation process at 450 °C. After hydrogenation, the α-MoO₃ structures were completely converted into β-MoO₂ NSs without the presence of sub-oxidized phases of molybdenum oxide. The as-grown NSs exhibited very good p-type electrical conductivity of ≈2.02 ×10³ S-cm^-1 with hole mobility of ≈7.8 ± 1.3 cm²-V^-1-Sec^-1. To explore optoelectronic properties of p-type β-MoO₂ NSs, we have fabricated a p-MoO₂/n-Si heterojunction photodetector device with Au as the top and Al as the bottom contacts. The device exhibits peak photoresponsivity of ≈0.155 A W^-1 with maximum detectivity ≈1.28 ×10¹¹ cm-Hz^1/2-W^-1 and 44% external quantum efficiency around ≈436 nm, following the highest photoresponse (I _ph/I _d ≈ 6.4 ×10²) and good response speed (rise time ∼29 ms and decay time ∼38 ms) at -1.5 V. Importantly, this device also shows good self-powered high-speed (rise time ∼47 ms and decay time ∼70 ms) photodetection performance with peak responsivity and detectivity of ≈45 mA W^-1 and ≈4.05 × 10¹⁰ cm-Hz^1/2-W^-1, respectively. This broadband UV-visible light detection feature can be attributed to the coordinated effects of MoO₂ band-edge absorption, interfacial defects and self absorption in Si. The photodetection behavior of the device has been understood by proposed energy-band diagrams with the help of an experimentally derived work function, band gap and valence band maximum position of MoO₂ NSs.