Suppression Of Surface States Research Articles

Effect of surface passivation on the electronic properties of GaAs nanowire:A first-principle study

Crystal structures of GaAs nanowires prepared by employing molecular beam epitaxy technique are often dominated by the wurtzite (WZ) phase.Recently,Galicka et al.found that the WZ GaAs nanowires grown along the[0001]direction in smaller size are energetically more favorable than other nanowires with the zinc blende phase grown along a specific direction (2008 J.Phys.:Condens.Matter 20 454226).The native nanowire usually has abundant unsaturated surface dangling bonds (SDBs) inducing significant surface states,leading to electrons accumulating at the nanowire surface. Thus the electrical property of the nanowire is very sensitive to the surface condition.However,surface passivation can effectively remove the surface states from the SDBs,and optimize the device performance.In this paper,using the first-principle calculations in combination with density function theory,we investigate the effect of surface passivation on the electronic structure of the GaAs nanowires grown along the[0001]direction.Various passivation species (hydrogen (H),fluorine (F),chlorine (Cl) and bromine (Br)) with different coverage ratios are considered.The GaAs nanowires hydrogenated with different locations and coverage ratios display different electronic properties.It is found that the GaAs native nanowire with a smaller diameter shows a semiconductor characteristic with indirect band gap,which originates from the fact that at smaller diameter,the surface stress becomes more remarkable,and then leads to surface atomic reconstruction.After passivation,the indirect band gap is translated into the direct band gap.For the GaAs nanowire with an As SDB hydrogenated,one deep donor level is located in the gap,and its band structure shows an n-type characteristic.For the GaAs nanowire with a Ga SDB hydrogenated,one shallow acceptor level is located in the gap,and its band structure shows a p-type characteristic.For the GaAs nanowire with a Ga-As dimer hydrogenated, its band structure shows an intrinsic semiconductor characteristic.For the GaAs nanowire with all of the Ga SDBs hydrogenated,the band structure shows a metallic characteristic.The band gap of the GaAs nanowire gradually increases as the hydrogen passivation ratio increases.For 50% hydrogen passivation,the band gap for the symmetrical passivation is slightly bigger than that for the half-side passivation.For the F-,Cl-and Br-passivation,the band gap decreases compared with for H-passivation.This is due to the fact that the ability of passivating atoms to compensate for surface atoms is weak,thereby reducing the band gap.The mechanism for the surface passivation is the suppression of surface states by the ability of the passivating atoms to compensate for surface atoms.These results show that the electronic properties of GaAs nanowires can be modulated by surface passivation,which is helpful for using GaAs nanowires as components and interconnections of nanoscale devices.

Measurement of the topological surface state optical conductance in bulk-insulating Sn-doped Bi1.1Sb0.9Te2S single crystals

Topological surface states have been extensively observed via optics in thin films of topological insulators. However, in typical thick single crystals of these materials, bulk states are dominant and it is difficult for optics to verify the existence of topological surface states definitively. In this work, we studied the charge dynamics of the newly formulated bulk-insulating Sn-doped Bi$_{1.1}$Sb$_{0.9}$Te$_2$S crystal by using time-domain terahertz spectroscopy. This compound shows much better insulating behavior than any other bulk-insulating topological insulators reported previously. The transmission can be enhanced an amount which is 5$\%$ of the zero-field transmission by applying magnetic field to 7 T, an effect which we believe is due to the suppression of topological surface states. This suppression is essentially independent of the thicknesses of the samples, showing the two-dimensional nature of the transport. The suppression of surface states in field allows us to use the crystal slab itself as a reference sample to extract the surface conductance, mobility, charge density and scattering rate. Our measurements set the stage for the investigation of phenomena out of the semi-classical regime, such as the topological magneto-electric effect.

Covalent Fixation of Surface Oxygen Atoms on Hematite Photoanode for Enhanced Water Oxidation

Suppression of surface states is one of the general issues for metal oxide photoanodes in water oxidation. For hematite (α-Fe2O3), the surface states are mainly attributed to Fe3+/Fe2+ redox couples in oxygen deficient regions (surface oxygen vacancies). To date, most of the passivation overlayers against surface states are metal oxides. However, oxygen vacancies are prevalent for most metal oxides. This is because their formation in metal oxides is often thermodynamically favorable. In contrast, the formation of oxygen vacancies is more energy-consuming when oxygen atoms are covalently bonded. On the basis of this understanding, we propose a new strategy to transform the surface of Fe2O3 into amorphous iron phosphate (denoted “Fe-Pi”), where the oxygen atoms are “covalently fixed” in phosphate (PO43–). As a result, the oxygen vacancies are decreased and the surface states are effectively suppressed. The onset potential of corresponding photoanode shifts negatively by 0.15 V and the photocurrent density i...

Structural stability and electronic properties of InSb nanowires: A first-principles study

Using first-principles calculations, we investigate the structural stability and electronic properties of InSb nanowires (NWs). The results show that, in contrast to the bulk InSb phase, wurtzite (WZ) NWs are more stable than zinc-blende (ZB) NWs when the NW diameter is smaller than 10 nm. Nonpassivated ZB and WZ NWs are found to be metallic and semiconducting, respectively. After passivation, both ZB and WZ NWs exhibit direct-gap semiconductor character, and the band gap magnitude of the NWs strongly depends on the suppression of surface states by the charge-compensation ability of foreign atoms to surface atoms. Moreover, the carrier mobility of the NW can be strengthened by halogen passivation.

Tailoring electronic properties of InAs nanowires by surface functionalization

The effect of surface functionalization on the electronic properties of InAs nanowires is investigated by the first-principle calculations. Several surface adsorption species (H, F, Cl, Br, and I) with different coverages are considered. It is found that the electronic structures of InAs nanowires are sensitive to the coverage and adsorption sites of the passivating atoms. The band-gap magnitude of InAs nanowires depends on the suppression of surface states as determined by the charge-compensation ability of passivating atoms to surface atoms. For the halogen passivation, the weak charge-compensation ability induces the band-gap reduction when compared to the hydrogen passivation. The results provide us a feasible way to engineer the bandgap of nanowires by the modification of surface species.

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.