Role of interface states and depletion layer in NO2 sensing mechanism of n-InP epitaxial layers

The electronic properties of n-type InP epitaxial layers covered by their native oxides have been investigated in order to explain the NO2 gas sensing mechanism. The impact of interface states on the interface Fermi level, EF, carrier concentration in-depth profiles, n(x), and resistance, R, of n-InP layers has been studied by means of numerical analysis. The U-shaped interface state continuum, with the density minimum, NSS0, from 1010 to 1013 eV−1 cm−2, has been assumed at the InP interface according to the disordered-induced gap state model. Moreover, the surface fixed charge, QFC, representing adsorbed ions at the semiconductor surface has been introduced. Also the influence of temperature on the electron distribution n(x) has been analysed. The n-InP layer resistance and Hall-effect measurements (using the Van der Pauw method) have been performed for the InP sensors before and upon NO2 action in order to assess the influence of gas adsorption on the electron concentration and mobility as well as on the depletion layer width, W. On this basis the influence of interface states and near-surface region on the InP layer response to NO2 adsorption has been determined. In addition, the conclusions for optimising the InP-based resistive sensor structure (in terms of the layer thickness and doping) have been obtained.