C. H. Li

Electrical detection of charge-current-induced spin polarization due to spin-momentum locking in Bi2Se3

Topological insulators exhibit metallic surface states populated by massless Dirac fermions with spin-momentum locking, where the carrier spin lies in-plane, locked at right angles to the carrier momentum. Here, we show that a charge current produces a net spin polarization via spin-momentum locking in Bi2Se3 films, and this polarization is directly manifested as a voltage on a ferromagnetic contact. This voltage is proportional to the projection of the spin polarization onto the contact magnetization, is determined by the direction and magnitude of the charge current, scales inversely with Bi2Se3 film thickness, and its sign is that expected from spin-momentum locking rather than Rashba effects. Similar data are obtained for two different ferromagnetic contacts, demonstrating that these behaviours are independent of the details of the ferromagnetic contact. These results demonstrate direct electrical access to the topological insulators surface-state spin system and enable utilization of its remarkable properties for future technological applications.

A phosphorous-rich structure of InP (001) produced by metalorganic vapor-phase epitaxy

A phosphorous-rich structure is generated on the InP (001) surface during metalorganic vapor-phase epitaxy. It consists of phosphorous dimers, alkyl groups, and hydrogen atoms adsorbed onto a layer of phosphorous atoms. The adsorbed dimers produce c(2×2) and p(2×2) domains, with total phosphorous coverages of 2.0 and 1.5 ML. The alkyl groups and hydrogen atoms adsorb onto half of the exposed phosphorous atoms in the first layer. These atoms dimerize producing a (2×1) structure. It is proposed that the first layer of phosphorous atoms is the active site for the deposition reaction, and that the organometallic precursors compete with phosphorous dimers, alkyl radicals, and hydrogen for these sites during growth.

Reflectance difference spectroscopy of mixed phases of indium phosphide (001)

Reflectance difference spectra of mixed (2×1) and (2×4) phases of indium phosphide (001) have been recorded and benchmarked against scanning tunneling micrographs of the surface. The line shapes are found to be linear combinations of the spectra of the pure (2×1) and (2×4) structures, Δr/rmixed=xΔr/r(2×4)+(1−x)Δr/r(2×1), where x is the weighting factor. Thus, in the absence of adsorbates, the reflectance difference spectra can be used to estimate the surface composition, i.e., the fractional coverage of phosphorous is ΘP=1−0.81x±0.06x.

Carrier transport in reverse-biased graphene/semiconductor Schottky junctions

Reverse-biased graphene (Gr)/semiconductor Schottky diodes exhibit much enhanced sensitivity for gas sensing. However, carrier transport across these junctions is not fully understood yet. Here, Gr/SiC, Gr/GaAs, and Gr/Si Schottky junctions under reverse bias are investigated by temperature-dependent current-voltage measurements. A reduction in barrier height with increasing bias is observed for all junctions, suggesting electric-field enhanced thermionic emission. Further analysis of the field dependence of the reverse current reveals that while carrier transport in Gr/SiC Schottky junctions follows the Poole-Frenkel mechanism, it deviates from both the Poole-Frankel and Schottky mechanisms in Gr/Si and Gr/GaAs junctions, particularly for low temperatures and fields.

Electrical Detection of the Helical Spin Texture in a p-type Topological Insulator Sb2Te3.

The surface states of 3D topological insulators (TIs) exhibit a helical spin texture with spin locked at right angles with momentum. The chirality of this spin texture is expected to invert crossing the Dirac point, a property that has been experimentally observed by optical probes. Here, we directly determine the chirality below the Dirac point by electrically detecting spin-momentum locking in surface states of a p-type TI, Sb2Te3. A current flowing in the Sb2Te3 surface states generates a net spin polarization due to spin-momentum locking, which is electrically detected as a voltage on an Fe/Al2O3 tunnel barrier detector. Measurements of this voltage as a function of current direction and detector magnetization indicate that hole spin-momentum locking follows the right-hand rule, opposite that of electron, providing direct confirmation that the chirality is indeed inverted below Dirac point. The spin signal is linear with current, and exhibits a temperature dependence consistent with the semiconducting nature of the TI film and freeze-out of bulk conduction below 100u2009K. Our results demonstrate that the chirality of the helical spin texture of TI surface states can be determined electrically, an enabling step in the electrical manipulation of spins in next generation TI-based quantum devices.

Hydrogen atoms as a probe of the optical anisotropy of indium phosphide (001)

The reflectance difference spectra of the InP(0 0 1) (2×1) and δ(2×4) reconstructions have been characterized using hydrogen as a probe of the surface bonds. Bands observed at 1.9, 3.1, 4.1, and 4.6 eV on the (2×1) and at 2.8, 3.7, and 4.6 eV on the δ(2×4) decrease in direct proportion to the hydrogen coverage. By comparing the changes in the reflectance difference spectra to the changes in the atomic structure of the surfaces, it is possible to relate the peaks to transitions involving specific valence bond states.

Reflectance difference spectroscopy of an ultrathin indium arsenide layer on indium phosphide (001)

A model system has been created which allows the surface and bulk contributions to the reflectance difference spectrum to be distinguished. In particular, an indium arsenide film, less than 10 A thick, has been grown on indium phosphide (001). Reflectance difference spectra of the InAs/InP surfaces were collected and compared to those of InP and InAs. It was found that the InAs/InP heterostructures exhibited electronic transitions between surface states characteristic of InAs (001), while retaining the surface-perturbed bulk transitions characteristic of InP (001). Furthermore, the optical anisotropy arising from the arsenic dimer bonds was shifted 0.2 eV higher for InAs/InP compared to that for InAs. This shift is proportional to 1/a2, where a is the bulk lattice constant.

Stress-induced anisotropy of phosphorous islands on gallium arsenide

The initial growth of (2×4) phosphorous islands on (4×2) terraces of gallium arsenide (001) has been studied. The islands grow anisotropically in the [110] direction with an aspect ratio of approximately 8 to 1 at moderate coverages. The distribution of island widths in the [110] direction follows a Gaussian function. The mean width increases from 24±6 to 47±11 A as the phosphorous coverage increases from 0.10 to 0.85 monolayers. Evidently, the island anisotropy is caused by stress imposed on the underlying gallium layer by the smaller, more tightly bound phosphorous dimers.

Formation of etch pits during carbon doping of gallium arsenide with carbon tetrachloride by metalorganic vapor-phase epitaxy

Scanning tunneling microscopy was used to examine the effects of carbon tetrachloride concentration and temperature on the morphology of carbon-doped gallium arsenide films grown by metalorganic vapor-phase epitaxy. Deposition was carried out at 505–545u200a°C, a V/III ratio of 75, and IV/III ratios between 0.5 and 5.0. The growth rate declined monotonically with increasing carbon tetrachloride concentration. Step bunching and pinning was observed at a IV/III ratio of approximately 2.5. Increasing this ratio further resulted in the formation of pits ranging from 20 to 50 nm in diameter. These results can be explained by two competing processes that occur at the step edges: (1) the reaction of chlorine with adsorbed gallium from the group III precursor, and (2) the reaction of chlorine with gallium arsenide. Both reactions desorb gallium chlorides and reduce the growth rate, but only the latter reaction produces pits.

Atomically resolved investigation of InGaAs/InP heterojunction formation during metalorganic vapor-phase epitaxy

We have studied the formation of indium gallium arsenide/indium phosphide heterojunctions during metalorganic vapor-phase epitaxy (MOVPE). The films are characterized using scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and x-ray photoelectron spectroscopy (XPS). Exposing an InP [001] film to 10 mTorr of tertiarybutylarsine below 500/spl deg/C results in the deposition of a thin InAs layer from 1.5 to 5.0 atomic layers thick (2.3 to 7.5 /spl Aring/). The surface of this epilayer remains atomically smooth independent of arsenic exposure time. However, in an overpressure of tertiarybutylarsine at or above 500/spl deg/C, the arsenic atoms diffuse into the bulk, creating strained InAsP films. These films form three-dimensional island structures to relieve the built-up strain.