Theoretical explanation of scanning tunneling spectrum of cleaved (110) surface of InGaAs

Abstract

The cross-sectional (110) surface of In<sub>0.53</sub>Ga<sub>0.47</sub>As/InP hetero-structure grown by molecular beam epitaxy on an InP (001) substrate is characterized by the cross-sectional scanning tunneling microscopy (XSTM). The cleaved (110) surface across the interface between the In<sub>0.53</sub>Ga<sub>0.47</sub>As layer and InP layer is atomically flat but displays slight different image contrast between the two neighbor regions. The scanning tunneling spectroscopy (STS) is used to measure the current/voltage (<i>I-V</i>) spectra. The <i>I-V</i> data of the InGaAs surface and InP (110) surface show the different characteristics. The voltage range of zero-current plateau (apparent band gap) in the <i>I-V</i> spectrum of InP displays the values close to its energy band gaps whereas the plateau ranges in the spectra of In<sub>0.53</sub>Ga<sub>0.47</sub>As are by contrast generally 50% larger than the energy band gap of In<sub>0.53</sub>Ga<sub>0.47</sub>As. The above phenomenon implies the different physical pictures on the tunneling of two surfaces. In the case of InP, the flat band model is feasible since the band edge states existing in the InP (110) surface can prevent the surface from being affected by the tip –induced band bending (TIBB) effect. In contrast, the TIBB effect must be taken into account to explain the <i>I-V</i> spectra of the In<sub>0.53</sub>Ga<sub>0.47</sub>As (110) surface. A statistical analysis of the <i>I-V</i> data of In<sub>0.53</sub>Ga<sub>0.47</sub>As reveals that the width of current plateau in the <i>I-V</i> spectrum is generally between 1.05 eV and 1.20 eV and the current onset points (turn-points) with the plateau for the different spectra are slightly different from each other. We are able to explain quantitatively the above features based on the three-dimensional TIBB model given by Feenstra (<ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://doi.org/10.1116/1.1606466">2003 <i>J.</i> <i>Vac. Sci. Technol. B</i> <b>21</b> 2080</ext-link>). Our calculation reveals that the parameter of density of surface states (DOSS) is a sensitive parameter responsible for the <i>I-V</i> features mentioned above. According to an appropriate assignment of the value of DOSS, which is generally taken in the scope of (0.8–3.0) × 10<sup>12</sup> (cm<sup>2</sup>·eV)<sup>–1</sup>, we well predict both the width and the onset points of the current-plateau. Moreover, the model also reproduces the line-shapes of the <i>I-V</i> spectra measured on In<sub>0.53</sub>Ga<sub>0.47</sub>As.