Monolayer Of Sr Research Articles

Atomic structure of Sr/Si(0 0 1)(1 × 2) surfaces prepared by Pulsed laser deposition

A buffer layer formed by depositing a ½ monolayer of Sr on Si(0 0 1) is known to passivate the Si surface, while its surface structure constitutes a suitable template for the integration of various functional oxides with the existing Si platform. We used Pulsed Laser Deposition (PLD) to prepare a Sr/Si(0 0 1)(1 × 2) surface and analysed it using in-situ Reflection High-Energy Diffraction (RHEED) in combination with low-temperature Scanning Tunneling Microscopy (STM). The STM images reveal an atomically ordered surface with terraces composed of one-dimensional (1D) chains running along perpendicular directions on neighbouring terraces. The 1D chains are separated by 0.78 nm and exhibit a low-amplitude corrugation with a period of 0.39 nm. The measured values agree well with the size of the (1 × 2) unit cell observed for similar MBE-grown surfaces, while the density of the surface defects is somewhat higher in the presented case. According to simulated STM images based on DFT calculations, two types of surface defects were identified and explored: arrays of Sr vacancies and Sr adatoms. These results show that PLD can offer precise control for the preparation of high-quality Sr-buffered Si(0 0 1) surfaces.

Mechanism of oxidation protection of the Si(001) surface by sub-monolayer Sr template

We investigate theoretically the oxidation stability of the Si(001) (2 × 1) reconstructed surface passivated by Sr. Using density functional theory, we find that the Sr surface with ½ monolayer of Sr is protected against oxidation. The presence of Sr delays the oxidation of the surface dimer, and even when the dimer is oxidized, O does not react with the back-bond, preventing the unwanted vertical growth of SiO2. We also show that ¼ monolayer of Sr protects the Si surface in a different way. In the presence of ¼ monolayer of Sr, O atoms are attracted to the Sr-Si dimer complexes, thus preventing the formation of SiO2.

Dependence of silicon oxidation channel on distribution of surface electrons at initial stage of oxide growth on Si(001)

Our first principles simulations for a single oxygen atom barrierless adsorption on Si(001) show that only a few channels exist for silicon oxidation. The reason is that surface electrons reduce O's electronegativity. According to the dependence of the silicon oxidation channels on the distribution of the surface electrons, we explain at atomic level that if the narrow regions of the oxidation channels can be occupied by pre-growth, such as depositing one monolayer of Sr on the Si(001), the silicon oxidation channels can be closed and the high-quality interface between oxides and silicon can be realized.

Charge transfer in Sr Zintl template on Si(001)

The formation of the half monolayer (ML) Sr Zintl template layer on Si(001) is investigated in a combined experimental and theoretical work consisting of in situ reflection high energy electron diffraction, in situ x-ray photoelectron spectroscopy (XPS), and density functional theory. Starting with clean 2 × 1 reconstructed Si(001), we demonstrate that Sr deposition leads to a charge transfer from the metal to the Si substrate resulting in the disappearance of the asymmetry of Si dimers—an essential structural change that enables direct perovskite epitaxy on Si, and likely, other semiconductors. XPS reveals an unexpected shift to higher binding energy of the Si 2p core-level components, including the bulk. This unusual behavior is attributed to final state effects using first principles calculations. As measured by ultraviolet photoelectron spectroscopy, the deposition of 0.5 ML of Sr lowers the work function of the system by 1.35 eV, and is in good agreement with our theoretical calculations.

First-principles study of Sr adsorption on InN (0001)

Structures of Sr adsorbed on InN (0001) surfaces are theoretically investigated by first-principles calculations. The adsorption energies of Sr on InN (0001) decrease with decreasing Sr coverage. An InN (0001)-(2 ×2) surface structure covered by a 1/4 monolayer of Sr at the T4 sites may be the most energetically favourable. Sr atoms may substitute indium atoms, or accumulate at the voids inside InN films. The interstitial Sr defects may act as a potential source of compensation for the p-type behaviour of Sr-doped InN at the surface.

Theory of the Sr-induced reconstruction of the Si (001) surface

By using first-principles calculations, we theoretically investigate the surface reconstruction caused by the adsorption of Sr on the Si (001) surface. For the Sr coverage below 12 monolayer (ML), the system behavior is dominated by the charge transfer between Sr and silicon that results in the “unbuckling” of Si dimers. At a higher coverage, the surface begins to “undimerize.” At the 1 ML coverage, a geometric size constraint induces a series of reconstructions such as 3×, 5×, 7×, etc., characterized by 1×1 reconstructed patches separated by a dimer row. Calculations of the surface energy as function of coverage suggest that for 1 ML of Sr 3×, 5×, and 7× reconstructions of the surface are preferred under the Sr rich conditions, which is in good agreement with the experiment.

Impact of O 2 exposure on surface crystallinity of clean and Ba terminated Ge(1 0 0) surfaces

In analogy with the case of Sr on Si [Y. Liang, S. Gan, M. Engelhard, Appl. Phys. Lett. 79 (2001) 3591], we studied surface crystallinity and oxidation behaviour of clean and Ba terminated Ge(1 0 0) surfaces as a function of oxygen pressure and temperature. The structural and chemical changes in the Ge surface layer were monitored by LEED, XPS and real-time RHEED. In contrast to the oxidation retarding effect, observed for 1/2 monolayer of Sr on Si, the presence of a Ba termination layer leads to a pronounced increase in Ge oxidation rate with respect to clean Ge. In fact, while the Ge(1 0 0) surface terminated with 1/2 ML Ba amorphizes for a pO 2 of 10 −2 Torr, LEED indicates that clean Ge forms a thin (4.5 Å), 1 × 1 ordered oxide upon aggressive O 2 exposure (150 Torr, 200 °C, 30 min). We briefly discuss the origins for the difference in behaviour between Ba on Ge and Sr on Si.

Inclusion compounds of l, d-dipeptide with small sulfoxides: flexible sheet structure of ( S)-phenylglycyl-( R)-phenylglycine

A heterochiral l, d-dipeptide, ( S)-phenylglycyl-( R)-phenylglycine ( SR- 1), formed inclusion compounds with some small dialkyl sulfoxides. By their single-crystal X-ray analyses, we observed monolayer structures, where SR- 1 molecules are arranged in parallel to construct a wavy sheet. The sulfoxides were accommodated in a channel cavity between the monolayers of SR- 1 by hydrogen bonding with +NH 3 of SR- 1. Notably, the sheet of SR- 1 is so flexible to change its wavy degree in response to the volume of the included sulfoxides. Furthermore, we could analyze the structure of crystalline SR- 1 without any sulfoxide, which has a bilayer structure.

Mechanism of cleaning Si(100) surface using Sr or SrO for the growth of crystalline SrTiO3 films

A method for removing SiO2 and producing an ordered Si(100) surface using Sr or SrO has been developed. In this technique, a few monolayers of Sr or SrO are deposited onto the as-received Si(100) wafer in an ultrahigh vacuum molecular-beam epitaxy system. The substrate is then heated to ∼800 °C for about 5 min, the SiO2 is removed to leave behind a Sr- or SrO-terminated ordered Si(100) surface. This Sr- or SrO-terminated Si(100) surface is well suited for the growth of crystalline high-k dielectric SrTiO3 films. Temperature programmed desorption measurements were carried out to understand the mechanism of removing SiO2 from Si(100) using Sr or SrO. The species we observed coming off the surface during the temperature cycle were mainly SiO and O, no significant amount of Sr containing species was observed. We conclude that the SiO2 removal is due to the catalytic reaction SiO2+Sr(or SrO)→SiO(g)+O+Sr(or SrO). The reaction SiO2+Si→2SiO(g) at the SiO2/Si interface is limited and the pit formation is suppressed. The main roles that Sr or SrO play during the oxide removal process are catalysts promoting SiO formation and passivating the newly exposed Si surface, preventing further etching and the formation of pits in the substrate.

Si(100) Surface Cleaning Using Sr and SrO

AbstractA method for removing SiO2 and producing an ordered Si(100) surface using Sr or SrO has been developed. In this technique, a few monolayers of Sr or SrO are deposited onto the as received Si(100) wafer in a ultrahigh vacuum molecular beam epitaxy system. The substrate is then heated to ∼800°C for about 5 minutes, the SiO2 is removed to leave behind a Sr terminated Si(100) surface. This Sr terminated Si(100) surface is well suited for the growth of crystalline high k dielectric SrTiO3 films. Temperature programmed desorption measurements were carried out to understand the mechanism of removing SiO2 from Si(100) using Sr or SrO. The species we observed coming off the surface during the temperature cycle was mainly SiO and O, no significant amount of Sr containing species was observed. We conclude that the SiO2 removal is due to the catalyst reaction SiO2 + Sr (or SrO) → SiO (g) + O + Sr (or SrO). The reaction happened through several intermediate steps. The reaction SiO2 + Si → 2SiO (g) at the SiO2/Si interface is limited and the pit formation is suppressed. The main roles that Sr or SrO play during the oxide removal process are catalysts promoting SiO formation and preventing further etching and the formation of pits in the substrate.