Visualization of 2D subband states formed in Si(111)-$\beta -\surd $3x$\surd $3-Bi surface by STM

We have investigated electronic subbands formed underneath of Si(111)-$\beta -\surd $3x$\surd $3-Bi ($\beta -\surd $3-Bi) surface by using STM. The $\beta -\surd $3-Bi surface is semiconducting, and the band gap is larger than that of bulk Si. In the dI/dV images, standing waves are found around defects, and the obtained energy dispersion is in good agreement with the free-electron-like model. Apparently, formation of a 2D electronic state is suggested. However, this standing wave is observed only on p-type, but not on n-type Si substrates, and consequently the 2D state is attributed to the subband formed in the depletion layer just below the $\beta -\surd $3-Bi surface. The subband formation is also consistent with the calculation of electronic states inside a potential-well caused by interfacial band bending.