III-V interfaces and thin layers are largely applied to gas-phase detection, e.g. H₂, NO₂ and O₃. The key role in the sensing mechanism of these structures plays the near-surface region of sub-micrometer size, which is strongly affected by the electronic surface states. The charge captured at these states largely modifies the height of interface potential barriers and often pins the surface Fermi level [1]. However, conventional theoretical approaches of the electronic properties of semiconductor gas sensors assume usually an unrealistically low density of the surface states in the band gap and thus lack of their contribution to the mechanism of thin layer conductivity.
In this work we performed a rigorous theoretical analysis of the conductivity of thin n- and p-type GaAs and InP layers as a function of the surface state density N_SS and the surface fixed charge Q_FC taking both positive and negative values. Such a charge represents both physically adsorbed ionic species and, on the other hand, the surface doping applied in order to control the gas sensitivity of the layer. The band bending induced by Q_FC changed from accumulation to inversion. In the calculations, a U-shaped continuous surface state distribution in the energy gap was assumed in accordance with the Disorder Induced Gap State model [2]. The layer thickness was changed in the sub-micrometer range down to the values approaching the depletion region width. In addition, the in-depth profiles of the potential barrier and carrier density were calculated and the Debye length for different doping was determined. For the numerical analysis we used a computer program for self-consistent simulations of the electronic phenomena in semiconductors.
As a result, the influence of surface states and doping on ion-sensitivity of III-V thin layers in terms of their conductivity was determined.
1. B.Adamowicz, et al. Thin Solid Films 436 (2003) 101
2. H.Hasegawa, H.Ohno, J.Vac.Sci.Technol.B4 (1986) 1130
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