Ultra-Fast Visible Light Photodection with α-Fe₂O₃ Grown on p-Silicon

Ultrafast photo-responsiveness was observed for α-Fe₂O₃thin films grown on p-type Si (100) (r=10-35 W-cm). The samples were prepared by spin coating a solution of Fe(III) acetylacetonate in acetone on Si, followed by annealing at 500°C for 1 hour in air. The process produces a microcrystalline film of α-Fe₂O₃ with average domain size of 5 µm. A pn-junction is formed due to the n-type character of α-Fe₂O₃. The photocurrent and photoresponse of the device were measured using a white light source. The photocurrent of the α-Fe₂O₃/Si device has a very fast rise response of <10 µs and a decay time of <4 µs, which is attributed to the fast carrier dynamics of Si layer. Using lock-in amplifier detection and illumination with a white light source with a power density of 270 µW/cm², a photocurrent at zero bias of 16.3 nA is achieved. A comparative study of zero bias photocurrent between samples prepared using 30 and 140 mM solutions shows that the lower concentrated solution produces higher zero bias photocurrent, which is due to the formation of smaller domains of α-Fe₂O_3,thereby reducing the probability of carrier recombination in the bulk. The high mobility of carriers in p-Si and bandgap of α-Fe₂O₃ (~2.2eV) corresponding to the center of the visible spectrum makes this pn-junction ideal for photodetection in the visible spectrum.