Investigation of Pd/MoOx/n-Si diodes for bipolar transistor and light-emitting device applications

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Sub-stoichiometric molybdenum oxide (MoOx) has recently been investigated for application in high efficiency Si solar cells as a “hole selective” contact. In this paper, we investigate the electrical and light-emitting properties of MoOx-based contacts on Si from the viewpoint of realizing functional bipolar devices such as light-emitting diodes (LEDs) and transistors without any impurity doping of the Si surface. We realized diodes on n-type Si substrates using e-beam physical vapor deposition of Pd/MoOx contacts and compared their behavior to implanted p+n-Si diodes as a reference. In contrast to majority-carrier dominated conduction that occurs in conventional Schottky diodes, Pd/MoOx/n-Si diodes show minority-carrier dominated charge transport with I–V, C–V, and light-emitting characteristics comparable to implanted counterparts. Utilizing such MoOx-based contacts, we also demonstrate a lateral bipolar transistor concept without employing any doped junctions. A detailed C–V analysis confirmed the excessive band-bending in Si corresponding to a high potential barrier (>0.90 V) at the MoOx/n-Si interface which, along with the observed amorphous SiOx(Mo) interlayer, plays a role in suppressing the majority-carrier current. An inversion layer at the n-Si surface was also identified comprising a sheet carrier density greater than 8.6×1011 cm−2, and the MoOx layer was found to be conductive though with a very high resistivity in the 104 Ω-cm range. We refer to these diodes as metal/non-insulator/semiconductor diodes and show with our device simulations that they can be mimicked as high-barrier Schottky diodes with an induced inversion layer at the interface.