Surface chemical control of the electronic structure of silicon nanowires: Density functional calculations

Silicon nanowires (NWs) have potential use in many electronic and photonic applications. In this study, we have systematically investigated the modification of Si NW electronic structure for different sized NWs and different surface coverage (H, Br, Cl, and I) using ab initio density functional theory calculations. The surface chemistry is shown to have strong effects, comparable to that of quantum confinement, on the band structure of Si NWs. Electronic structure analysis reveals the primary cause of the band gap change is the suppression of surface states as determined by the strength of the surface bonds. The results elucidate surface chemical control of NW electronic structure and illustrate an example of simulation-based design characterization of nanoelectronic components.

Figure 1

(Color online) Band gaps of H passivated NWs in different growth directions. (a) displays the band gaps as a function of diameter, while (b) shows the cross section of the NWs.

Figure 2

(Color online) Band gaps of H passivated NWs at different diameters as a function of surface coverage %. Surface passivation has the strongest effect at smaller diameters and at passivation above 50% coverage. (a) displays the band gaps as a function of surface coverage at different diameters, while (b) displays the structure of the $2\mathrm{nm}$ diameter NWs at 100% H coverage and 50% I coverage.

Figure 3

Band structure of I passivated NWs about $2\mathrm{nm}$ in diameter at 0%, 50%, and 100% I coverage. The band structures and conduction-band minimum and valence-band maximum states show similar trends for different diameters and different surface passivation species at the same coverage %. The charge density multiplied by the volume $\left[\rho \right(r)\times V_{\mathrm{cell}}]$ with units of Coulombs of the valence-band and conduction-band states is shown below and above the band structure plots. $V_{\mathrm{cell}}\approx 7760{\mathrm{\AA }}^3$. $\rho \left(r\right)=0.09\mathrm{C}∕{\mathrm{\AA }}^3$ is shown in light gray.

Figure 4

Partial charge density of the valence-band maximum and conduction-band minimum for an Si slab of about $3\mathrm{nm}$ thickness with H, no passivation, Cl, Br, and I passivation. The partial charge density is in units of Coulombs after being normalized by the volume of the unit cell $\rho \left(r\right)\times V_{\mathrm{cell}}$. $V_{\mathrm{cell}}\approx 544{\mathrm{\AA }}^3$. The valence- and conduction-band states have bulklike character to them when the slabs are passivated with H. With weaker bonding, the valence-band and conduction-band states become more surfacelike. (b) shows the structure of one of the slabs.

Figure 5

Contour plot of the band gaps of I passivated NWs.