On the Nature of the Effective Surface Charge Transformation on InAs Crystals during Anodic Oxide Layer Growth

The dynamics of fluorine atoms distribution over the thickness of the grown anodic oxide layers and the effective surface charge on InAs crystals under these layers are investigated. Anodic oxidation is performed in an alkaline electrolyte to which a fluorochemical component in the galvanostatic mode is added at the anode current densities of 0.05 and 0.5 mA cm^–2. The layer’s thickness changes by 32–51 nm by setting a final voltage of 15 to 25 V on the electrodes during the growth. The layer’s thickness and refractive index are measured by an ellipsometric technique and the distribution of the thickness of fluorine atoms is measured by photoelectron spectroscopy combined with ion etching. Simultaneously, MIS structures are fabricated from the grown layers and their capacitance–voltage characteristics are calculated to determine the effective surface charge and density of the surface states at different layer thicknesses. The main results of the investigations are that, with regardless of anodic current, density the grown layers are compacted, the fluorine atoms distribution profile shifts toward InAs, and the positive effective surface charge gradually decreases from 3.6 × 10¹¹ to 2.0 × 10¹¹ cm^–2 at densities of the surface states of (6–7) × 10¹¹ eV^–1 cm^–2 in all cases. It is concluded based on comparison of the obtained data with the theoretical concepts on the charge structure of MIS structures that the built-in charge is gradually removed from the interface with InAs during the anodic oxide layer’s growth, which explains the observed decrease in the effective surface charge when the layer’s thickness increases. This result indicates, that the layer’s growth rate is faster than rate of the built-in layer charge shifting toward InAs.