- Previous research 2

The two features were cutting-edge. One was its measuring procedure, although it was quite simple.

Firstly, oxygen molecules that strongly adsorb onto an n-type semiconductor surface are desorbed under ultraviolet-light irradiation.

Secondly, when ultraviolet light is turned off, oxygen molecules in the atmosphere initiate adsorption onto the semiconductor surface. After a certain period of time, a certain number of oxygen molecules are collected onto the semiconductor surface. The collected number of oxygen molecules depends on oxygen concentration in the atmosphere. For example, the collision rate of oxygen onto the semiconductor surface is 5.7X10¹⁶ times·s^-1·cm^-2 at an oxygen concentration of 1X10^-9 (1 ppb).

Lastly, the surface electrical resistance of the semiconductor is measured. The electrical resistance is increased with increasing the collected (=adsorbed) number of oxygen molecules at the semiconductor surface, since adsorbed oxygen withdraw conduction electrons from the n-type semiconductor.

When the n-type semiconductor is illuminated with photons with energies close to or greater than the band gap of the semiconductor, electrons can leave the valence band, generating conduction band electrons and valence band holes: the electrons are then swept into the bulk by the built-in field and the holes are swept to the surface. A constant supply of holes to the surface is facilitated by electron flow from the adsorbed oxygen, leading to oxygen desorption.

The second feature is the use of electron-doped SrTiO₃ as the "semiconductor." The thickness of the SrTiO₃ is set at 2.4 nm, 6 TiO₂ units. It is nearly equal to the maximum of polaron diameter of moderately electron-doped SrTiO₃. This slide shows the oxygen concentration dependence of the maximum of polaron diameter which is calculated using Holstein’s polaron theory and experimental data. By the way, polaron is a conceptional quasiparticle that describes an electron moving in a dielectric crystal where the atom moves from their equilibrium positions to effectively screen the charge of an electron, known as a phonon cloud. The polaron diameter decreases with increasing oxygen concentration, indicating that the conduction electron mobility decreases with increasing oxygen concentration, resulting in the increase of electrical resistance of electron-doped SrTiO₃.

This is the schematics of the relation between the thickness of SrTiO₃ thin film and its polaron diameter. Larger polaron diameter results in a high mobility of conduction electrons; in contrast, smaller polaron diameter results in a low mobility.

This is the conceptional schematic that explains the cause of the polaron diameter modulation induced by oxygen adsorption. Although SrTiO₃ is paraelectric, adsorbed oxygen can induce the ferroelectric-like ionic displacement within the near-surface-region. Such a displacement must result in the decrease of polaron diameter.

This is the Ba²⁺ dose dependence of the change in electrical resistance at a high or a low oxygen concentrations. When Ba²⁺ concentration exceeds 75%, the materials become ferroelectric, losing oxygen sensitivity. Only paraelectric materials can be sensitive to the change in oxygen concentrations, probably because oxygen adsorption induces the change in ionic displacement for paraelectric materials. That’s why I chose SrTiO₃ as a material for the oxygen sensor.

This slide shows the oxygen concentration dependence of a prototype oxygen sensor. Inflection point probably shows the change in mechanism: at higher concentrations in the right region electrical resistance is amenable to a conventional semiconductor theory; at lower concentrations in the left to the polaron theory. No conventional semiconductor can detect such trace levels of oxygen concentrations.

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