This paper deals with a novel and rigorous analysis of the electronic properties of n-type InP epitaxial surfaces covered by native oxides, combined with chemical studies of the same material. Such surfaces are known to be extremely sensitive to adsorption of oxidizing gas molecules and are used in sensor structures working in the ppb range of gas concentration. However, the sensing mechanism of InP surfaces is still not well understood. Therefore, we performed a theoretical analysis of the influence of surface states (with a density distribution NSS[E]) and a surface fixed charge (QFC), simulating the effect of adsorbed ionised species or surface δ-doping, on the surface potential barrier, in-depth carrier profile and conductivity of n-InP layers. In our calculation, the surface states density minimum (NSS0) ranged from 10^11 to 10^13 eV^-1cm^-2, corresponding to the surface covered by native oxides. An U-shaped surface states continuum was assumed in accordance with the Disordered Induced Gap State model. For better comprehension of the gas adsorption mechanism, the chemical properties of n-InP epitaxial surfaces before and after oxidising gas exposure, i.e., nitrogen dioxide (NO2) and ozone (O3), were investigated using Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). In addition, Hall effect measurements were carried out to estimate the carrier concentration upon gas action. These results were compared to those of the studies of sensing properties of InP layers in terms of resistance change versus gas adsorption in the ppb concentration range. Additionally, the fabrication process and the structure of ohmic contacts to the sensor device are discussed. The in-depth chemical composition profiles of the contacts were obtained using AES combined with Ar+ ion sputtering. The contact resistance was also determined and Scanning Electron Microscopy (SEM) images of the contact region were analysed.
Keywords: InP, electronic properties, surface states, native oxide, gas sensing.
Acknowledgements: The work was partially supported by the Ministry of Science and Higher Education, Poland (Research Project 3348/T02/2006/31).
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