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(Color online) The dashed curves show band bending tracking the bottom of the conduction band and the top of the valence band (VB) as a function of depth, x, from the surface of the material. The Fermi energy is indicated by Ef. (a) Cleaved bulk samples exhibit an initial depletion region near the surface. (b) Unavoidable n-type doping occurs by exposure to atmosphere or vacuum, and the depletion region becomes an accumulation region. Thin films that are grown in vacuum and exposed to atmosphere always result in an accumulation region as in (b).
(Color online) (a) A cartoon showing the bulk contamination process. Se atoms diffuse out leaving Se vacancies (V) and O atoms and water molecules (W) naturally diffuse into the structure. (b) The carrier density (left axis open triangles) and the mobility (right axis solid squares); immediately after growth, 1 week in vacuum (Vac), 1week in O2 gas, and 1 week in N2 gas. (c) A similar plot showing the response of the carrier density and mobility to exposure to vacuum for 2 weeks, air for 2days, and lastly liquid water for 1 h. (d) The response of the carrier density to air exposure.
(a) Nonlinear Hall effect shows that two types of carriers contribute to the conduction. Sample A (solid squares) at t = 0 days shows the lowest bulk carrier density; at t = 2 days (solid circles) the bulk carrier density of sample A has increased due to n-type doping, thereby overwhelming the surface signal and thus reducing the nonlinearity. The solid curve shows data from sample B whose bulk carrier density is initially larger. (b) The fitting parameters were extracted using G xy rather than R xy for functional simplicity. The fitted curves are represented by solid curves, while the experimental data are depicted by open triangles (sample A, t = 0), and open squares (sample B, t = 0).
Taken from two 16 QL samples grown on Al2O3, this table shows the carrier density, nSS (surface state) and nBS (bulk state), and the corresponding mobilites obtained by the nonlinear fitting process for G xy.
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