Measurements of the conduction band energy surrounding individual InGaAs quantum dots by Cross-Sectional Ballistic Electron Emission Microscopy (XBEEM)

Cross-sectional ballistic electron emission microscopy (XBEEM) at room temperature was used to measure the conduction band (CB) energy in the ``wetting layer'' around and \textit{behind} cleaved InGaAs quantum dots (QDs). Samples with a $\sim $2 nm thick In$_{0.4}$Ga$_{0.6}$As layer embedded in n-doped (5 x 10$^{16}$ cm$^{-3})$ GaAs were grown by organometallic vapor phase epitaxy, then cleaved \textit{ex situ} and 7nm-thick Au Schottky barrier (SB) contacts deposited on the cleaved edge using a shadow mask [1]. With reverse bias $V_{rev}$ = 0V, Schottky barrier heights (SBHs) over different QDs were measured to range from $\sim $0.78 eV - 0.82 eV, compared to $\sim $ 0.84 eV over the wetting layer next to the QDs and $\sim $0.913 eV SBH over the adjacent GaAs. With $V_{rev}$ = 1V, the SBH over the QDs were reduced by $\sim $30 -- 50 meV compared to a much smaller ($<$ 5 meV) measured decrease over the GaAs due to image force lowering, indicating that SBH over the QDs was due to the CB of the wetting layer at an estimated depth of 6 -- 9 nm \textit{behind} the QDs. The XBEEM transmission over the QDs was also strongly enhanced by the applied reverse bias, for reasons that are not yet clear. Work supported by NSF Grant No. DMR-0505165. [1] C. Tivarus \textit{et al}., PRL \textbf{94}, 206803 (2005).