H-passivated Surface Research Articles

The interaction of carbon with Si(1 1 1):As and Si(1 1 1):H surfaces, a theoretical study

Density functional theory calculations have been performed to determine the adsorption site of carbon at the Si(1 1 1):As and Si(1 1 1):H surfaces at different coverages. The As- and H-passivated surfaces were simulated by replacing the topmost Si layer by As or by saturating the Si dangling bonds with hydrogen atoms, respectively. Different high symmetry sites were considered. Carbon was placed successively in the fourfold (T 4) or threefold coordinated (H 3), the ontop (T 1) sites or substituted for a Si atom in the S 5 position located underneath the Si adatom in the T 4 site. We found that the preferred carbon adsorption site depends on the coverage of the passivated surfaces. At low coverages i.e. at 1/16 ML and 1/3 ML, it prefers a distorted T 4 position whereas at 1 ML, it occupies an H 3 site. This contrasts with the clean surface where the most energetically favored site is the S 5 at all coverages. Carbon adsorption induces a significant change in the structural geometry of the surface atoms, leading to a charge re-arrangement in the surface layers.

Ab Initio Study of Clean and Hydrogen-Saturated Unreconstructed SiC{0001} Surfaces

Using density functional theory, we investigate the 6H-SiC{0001} surfaces in the unreconstructed 1 × 1 and the H-passivated configuration. The strong correlation effects of the dangling bonds at the surface are treated by spin-polarised calculations including the Hubbard-U parameter. We find that the clean surfaces are semiconducting with surface states in good agreement with experimental data. The impact of the Hubbard-U is stronger on the C-terminated face. For the H-passivated surfaces we find resonances in the valence band. The antibonding C−H state is located in the upper part of the bandgap around the ¯-point.

Controlled Modulation of Conductance in Silicon Devices by Molecular Monolayers

We have controllably modulated the drain current (I(D)) and threshold voltage (V(T)) in pseudo metal-oxide-semiconductor field-effect transistors (MOSFETs) by grafting a monolayer of molecules atop oxide-free H-passivated silicon surfaces. An electronically controlled series of molecules, from strong pi-electron donors to strong pi-electron acceptors, was covalently attached onto the channel region of the transistors. The device conductance was thus systematically tuned in accordance with the electron-donating ability of the grafted molecules, which is attributed to the charge transfer between the device channel and the molecules. This surface grafting protocol might serve as a useful method for controlling electronic characteristics in small silicon devices at future technology nodes.

First-principles study of Ag adsorption on the H-passivatedSi(001)surface with Bi nanolines

This first-principles study examines the adsorption and migration of Ag atoms on the H-passivated $\mathrm{Si}(001)$ surface with Bi nanolines, and finds that Ag nucleates preferentially at the nanolines in the following manner. An Ag atom deposited on the H-passivated surface rapidly migrates through a groove between dimer rows (the barrier is $0.14\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$), to reach a Bi nanoline. Then, the adatom enters a backbond of the Bi nanoline, bonding directly to Si and releasing the energy of $\ensuremath{\sim}0.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. After that, the adatom migrates along the nanoline (the barrier is $0.33\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$), and encounters another adatom to form a dimer, without leaving the backbonds. The dimer is sufficiently stable at room temperature (dissociation energy is $1.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$) to serve as a growth nucleus. Thus, Ag nucleates preferentially at the Bi nanolines, directly on Si.

Wet Chemical Cleaning of Germanium Surfaces for Growth of High-k Dielectrics

AbstractOne of the major difficulties preventing the wide use of germanium (epi or bulk) as a gate material is the poor stability of its oxide, leading to reproducibility and reliability issues. In contrast to silicon, the nature and thickness of Ge “native” oxides are history dependent, and most phases of germanium oxide are water-soluble. As a result, the procedures for passivating Ge surfaces with hydrogen (HF last) are more complex and less forgiving.We have used infrared absorption spectroscopy and x-ray photoelectron spectroscopy to investigate the nature of oxidized and H-terminated Ge surfaces. The GeO2, GeO and GeC phases have been identified and quantified as a function of processing conditions. The stability of the H-terminated surfaces has been examined in air and in controlled environments. The H-passivated Ge surfaces are found to be much less stable in air than H-terminated Si surfaces.

Atomic-scale mechanisms of selective adsorption and dimerization of pentacene on Si surfaces

We report results of first-principles calculations in terms of which we elucidate the mechanisms for nucleation and initial growth of pentacene films on Si. Pentacene molecules bond in flat, distorted configurations on bare surfaces. On H-passivated surfaces, direct bonding or H replacement are not energetically favored. However, molecules bond in an upright configuration at isolated depassivated Si dangling bonds and film growth continues over the passivated area. The results elucidate generic adsorption issues on inert surfaces and suggest procedures for controlling film growth.

Effects in synergistic blistering of silicon by coimplantation of H, D, and He ions

Silicon blistering was achieved at unprecedently low ion fluences of 2×1015He∕cm2 (8keV) followed by 6×1015H∕cm2 (5keV), but no blistering occurs for reversed order (H+He), or (He+D) coimplantation up to a high fluence. Raman scattering data suggest that: (i) the He synergy is due to He assistance in the appearance of H-passivated internal surfaces and their pressurization at high temperature; (ii) the order effect is due to the destruction by the room-temperature He postbombardment of favorable Si–H structures; and (iii) the isotope effect is due to the deuterated multivacancies evolving into surprisingly stable interstitial and bond-centered configurations.

Density functional theory study of initial stage of ZrO 2 atomic layer deposition on Ge/Si(100)-(2×1) surface

The reaction mechanisms of ZrCl 4 adsorption and dissociation on Ge/Si(100)-(2×1) surface as the initial stage of ZrO 2 atomic layer deposition process are investigated with density functional theory (DFT). The Si–Si, Si–Ge and Ge–Ge one-dimer cluster models are employed to represent Ge/Si(100)-(2×1) surface with different Ge composition. The reaction of ZrCl 4 with hydroxylated Ge/Si(100)-(2×1) surface forms a bridged ZrCl 2 site, while on H-passivated surface, ZrCl 3 site is formed. The reaction energy barrier of ZrCl 4 with H-passivated SiGe surface is much higher than ZrCl 4 with hydroxylated SiGe surface, which indicates that reaction proceeds more slowly on H-passivated surface than on OH-terminated surface. In addition, adsorption of ZrCl 4 is favorable on Ge surface atoms than on Si surface atoms.

Atomic-level robustness of the Si(100)-2×1:H surface following liquid phase chemical treatments in atmospheric pressure environments

The UHV-prepared Si(100)-2×1:H surface is studied at atomic resolution following liquid phase chemical processing under atmospheric pressure conditions. A custom experimental setup, consisting of an UHV scanning tunneling microscope (STM) chamber that is directly interfaced to an inert atmosphere glovebox, facilitates liquid phase chemical processing without exposing the pristine H-passivated surface to ambient air. While in the inert atmosphere, the Si(100)-2×1:H surface is treated with a variety of organic and aqueous solvents. Atomic resolution STM images reveal that the hydrogen passivation remains largely intact after treatments in toluene and dichloromethane. In addition, by minimizing oxygen levels during processing, perturbation to the Si(100)-2×1:H surface can be significantly reduced following exposure to water. These results are potentially useful in the fields of microelectronics and molecular-beam epitaxy, where liquid phase chemical processing is often avoided in an effort to preserve atomically pristine Si(100) surfaces. Furthermore, this study delineates the conditions under which various organic and biological molecules can be delivered to nanopatterned Si(100)-2×1:H surfaces via liquid phase solvents.

Surface reconstruction effects on atomic properties of semiconducting nanoparticles

We have performed first-principles and empirical molecular dynamics calculations of the effect of size and surface reconstruction on the atomic structure properties of small Ge nanoparticles (1–2.5 nm). Systems with ideal H-passivated surfaces, or bare reconstructed surfaces have been considered. We show that the surface reconstruction leads to a very strong volume contraction of the nanoparticles, equivalent to an applied pressure of 4 GPa for a 2 nm nanoparticle. We also found that the surface reconstruction shifts significantly the optic mode peak of the vibrational spectrum, thus preventing a straightforward size determination from Raman spectroscopy.

Stability of HF-etched Si(100) surfaces in oxygen ambient

In situ multiple internal reflection infrared absorption spectroscopy of H-passivated silicon surfaces in controlled oxygen environments reveals that direct oxygen incorporation into the surface Si–Si bonds occurs without surface hydrogen removal, in the temperature range of 550–590 K for 1–20 mTorr O2 pressures. The kinetics of the O2 insertion process display overall effective activation energies of 1.6 to 1.7 eV and prefactors controlled primarily by Si–H steric hindrance for O2 to access Si–Si backbonds.

Scanning tunnelling microscopy imaging and modification of hydrogen-passivated Ge(100) surfaces

Scanning tunnelling microscopy (STM) study and modification of hydrogen (H)-passivated Ge(100) surfaces have been investigated. Thermal oxidation procedures were used to minimise surface roughness. Ge samples were passivated in HF solution after thermal oxidation. STM and atomic force microscope (AFM) imaging showed that, using HF etching after thermal oxidation, we can obtain a natural H-passivatedtopographically and chemically flat Ge(100) surface. The root-mean-square (rms) roughness ofa H-passivatedGe(100) surface measured both by STM and AFM is less than 2 A. Electric properties of H-passivatedGe(100) surfaces were studied by scanning tunnelling spectroscopy (STS) in nitrogen ambient. STS showed that the H-passivated Ge surfaces were not pinned. Modification on H-passivated Ge(100) surfaces was carried out using STM by applying an electric voltage between the sample and tip in air. Modified features were characterised by STM and AFM imaging. On the H-passivated Ge(100) surfaces, stable, low-voltage, nanometer-scale modified features can be produced.

Nanostructure fabrication using pulsed lasers in combination with a scanning tunneling microscope: Mechanism investigation

Nanostructure fabrication using lasers in combination with a scanning tunneling microscope has been reported in the past several years. Different mechanisms have been discussed for the formation of these nanostructures. However, they are controversial. In this study, we investigated the mechanism of nanostructure fabrication on both gold films and hydrogen-passivated Ge surfaces. Current-distance curves for a gold film and for an H-passivated Ge surface under an electrochemically etched tungsten tip were measured to determine the tip-sample distance. An analytical model was proposed to explain different mechanisms for nanostructure fabrication on gold films and on H-passivated Ge surfaces. Thermal expansion of the tip under laser irradiation was calculated. With comparison between the tip-sample distance and the thermal expansion of the tip, we can determine whether the mechanism is based on optical enhancement or on thermal mechanical indentation.

Investigation On Laser-Induced Effects In Nanostructure Fabrication With Laser-Irradiated Scanning Tunneling Microscope Tips in Air Ambient

AbstractIn this paper, we report our investigation on the kinetics of nanostructure fabrication on gold films and on H-passivated Ge surfaces. The relationship between the current and the tip-sample distance of the STM junction was measured for both gold films and H-passivated Ge surfaces. The tip-sample distance for gold films under a electrochemically etched W tip is approximately 2 nm, while that for H-passivated Ge sufaces is more than 27 nm. The thermal expansion length of the tip under laser irradiation was calculated. From the comparison of the thermal expansion length and the tip-sample distance, we can reach the conclusion that for gold films, thermal mechanical indention is the primary reason of nanostructure formation, while for H-passivated Ge surfaces, optical enhancement is the only reason.

A monohydride high-index silicon surface: Si(114):H-(2×1)

We describe the adsorption of H on Si(114)-(2×1) as characterized by scanning tunneling microscopy and first-principles calculations. Like Si(001)—and despite the relative complexity of the (114) structure—a well-ordered, low-defect-density monohydride surface forms at ∼400 °C. Surprisingly, the clean surface reconstruction is essentially maintained on the (2×1) monohydride surface, composed of dimers, rebonded double-layer steps, and nonrebonded double-layer steps, with each surface atom terminated by a single H. This H-passivated surface can also be easily and uniformly patterned by selectively desorbing the H with low-voltage electrons.

Models for Hydrogen Extraction from the Passivated Si(100) Surface Induced by the Scanning Tunneling Microscope

This article discusses theoretical models developed to explain spatially controlled hydrogen extraction from H-passivated silicon surfaces by an STM tip. Observed STM images of a hydrogen-free spot are compared to simulated images in an attempt to clarify the nature of the surface modification. A modified rate law for hydrogen desorption in the presence of the STM tip is suggested, which accounts for the vibrational pumping of the Si--H bond by the STM current as well as for a destabilization of the adsorbate due to the electric field. The second effect is discussed in further detail on the basis of a macroscopic dielectric theory of the semiconductor surface. It is demonstrated how the derived model can be applied to experimental data.

Nanoscale patterning and selective chemistry of silicon surfaces by ultrahigh-vacuum scanning tunneling microscopy

Nanometer scale patterning of the monohydride surface has been achieved by using an ultrahigh-vacuum (UHV) scanning tunneling microscope (STM) to selectively desorb the hydrogen. After preparing high-quality H-passivated surfaces in the UHV chamber, patterning is achieved by operating the STM in field emission. The field-emitted electrons stimulate the desorption of molecular hydrogen, restoring clean in the patterned area. This depassivation mechanism seems to be related to the electron kinetic energy for patterning at higher voltages and electron current for low-voltage patterning. The patterned linewidth varies linearly with tip bias, achieving a minimum of less than 10 Å at -4.5 V. The linewidth dependence on electron dose is also studied. For positive tip biases up to 10 V no patterning occurs. The selective chemical reactivity of the patterned surface has been explored by oxygen and ammonia dosing. For the oxygen case, initial oxidation of the patterned area is observed. Ammonia dosing, on the other hand, repassivates the surface in a manner different from that of atomic hydrogen. In both cases the pattern resolution is retained and the surrounding H-passivated areas remain unaffected by the dosing.

Minority carriers contribution and hot-electron injection process in tunnel spectroscopy of H-passivated silicon surfaces

Detailed studies of the current-voltage characteristics of the STM tunnel junction with H-passivated Si(111) surfaces have been performed as a function of type and level of doping, temperature and light illumination of the surface. For slightly doped p-Si crystals we have observed a main contribution of minority carriers in the reverse tunnel current. For n-Si surfaces an abrupt increase of the reverse current at some bias voltage has been found. This effect is explained by a hole accumulation near the surface which changes band bending and initiates the hot-electron injection from the STM tip with the energy sufficient for the impact ionization of the carriers in the silicon. The hole accumulation also causes a vast hysteresis of the tunnel current. It creates a new look at the point tunnel contact as a low-dimensional bistable element.

Photoluminescence of porous silicon exposed to ambient air

The room-temperature photoluminescence and structure were studied concerning porous silicon which was exposed to ambient air. Water vapor in ambient air gradually oxidized the surface of the porous silicon with developing Si—O—Si bonds. This room-temperature oxidation progressively replaced an unstable H-passivated surface with a more stable O-passivated surface, dramatically increasing the intensity of the photoluminescence.

Nanometer scale patterning and oxidation of silicon surfaces with an ultrahigh vacuum scanning tunneling microscope

Nanoscale patterning of the Si(100)-2×1 monohydride surface has been achieved by using an ultrahigh vacuum (UHV) scanning tunneling microscope (STM) to selectively desorb the hydrogen passivation. Hydrogen passivation on silicon represents one of the simplest possible resist systems for nanolithography experiments. After preparing high quality H-passivated surfaces in the UHV chamber, patterning is achieved by operating the STM in field emission. The field emitted electrons stimulate the desorption of molecular hydrogen, restoring clean Si(100)-2×1 in the patterned area. This depassivation mechanism seems to be related to the electron kinetic energy for patterning at higher voltages and the electron current for low voltage patterning. The patterned linewidth varies linearly with the applied tip bias achieving a minimum of <10 Å at −4.5 V. The dependence of linewidth on electron dose is also studied. For positive tip biases up to 10 V no patterning occurs. The restoration of clean Si(100)-2×1 is suggestive of selective area chemical modifications. This possibility has been explored by exposing the patterned surface to oxygen and ammonia. For the oxygen case, initial oxidation of the patterned area is observed. Ammonia dosing, on the other hand, repassivates the surface in a manner different from that of atomic hydrogen. In both cases the pattern resolution is retained and the surrounding H-passivated areas remain unaffected by the dosing.