Clean Si Surface Research Articles

An RDS, LEED, and STM Study of MOCVD-Prepared Si(1 0 0) surfaces

Clean, As- and P-terminated Si(1 0 0) surfaces were prepared with H 2 carrier gas and AsH 3 and PH 3 as precursors in an MOCVD chamber. Reflectance difference spectra (RDS) were taken in situ. Samples were then transferred under vacuum to an ultrahigh-vacuum (UHV) chamber for analysis with AES, LEED, and STM. Clean, As-terminated Si(1 0 0) surfaces were achieved by annealing the sample at T<900 °C supplying either AsH 3 or hydrogen (plus background As x ). Various preparation procedures were applied and benchmarked in UHV. Extended annealing under AsH 3 led to strongly faceted surfaces whereas AsH 3 flow of moderate concentration, temperature, and time led to flat, two-domain, (2×1)/(1×2) reconstructed surfaces. RD spectra almost identical to RD spectra of As/Si(1 0 0) surfaces prepared in UHV with MBE were obtained by annealing the samples under AsH 3. Annealing under PH 3 supply bore a new, two-domain (6×3)/(3×6) surface reconstruction at low flows, and a SiP compound at high flows.

Effects of Subsurface Boron and Phosphorus on Surface Reactivity of Si(001): Water and Ammonia Adsorption

Using density functional slab calculations with planewave basis set and pseudopotentials, we have examined the effect of subsurface boron and phosphorus on the surface chemical properties of Si(001). We compare the adsorption behaviors of water and ammonia between B- or P-modified and clean Si(001) surfaces. On the B-modified surface, the charge polarization of two B-connected dimers is altered significantly. Our calculation shows that approximately one electron is transferred from the local surface to the subsurface boron, thereby leading both buckled-up and -down atoms of B-connected dimers to be electron-poor (electrophilic). On the other hand, subsurface P brings about no significant change in surface electronic structure. Only about one-tenth of an electron is transferred from the subsurface P to the local surface. As a result, relative to the clean surface, the adsorption properties of water and ammonia change noticeably on the B-modified surface while varying insignificantly on the P-modified surface.

Ion-bombardment induced light emission from Si(100) surfaces under continuous silane exposures

The intensities of the optical spectral lines emitted from a Si(100) surface under Ar+ ion bombardment are studied as a function of the ion kinetic energy (1–5keV) and the partial pressure of silane. In these experiments, the ion flux is held constant while either the bombardment energy or the partial pressure of silane is varied. The intensities of the spectral lines are normalized with respect to the intensity of the excited neutral silicon (SiI,288nm) optical line emitted from the clean Si surface. The intensities of excited neutral Si (SiI,288nm), molecular SiH (A2Δ–X2Π,414nm), H Balmer beta (486nm) and H Balmer gamma (434nm) optical lines emitted from silane exposed Si surfaces follow a similar increasing trend with respect to increasing partial pressures of silane. During continuous silane exposures, the intensity of these four lines also increases with increasing ion kinetic energy. A qualitative explanation based on the adsorption and decomposition of silane on the Si surfaces can account for this behavior. In contrast to the monotonic increase of the above emission lines with greater incident kinetic energies and higher silane partial pressures, the normalized H Balmer alpha (656nm) optical line exhibits unique behavior. The intensity of the Balmer alpha transition decreases with increasing incident ion kinetic energy at all measured partial pressures of silane.

H 2O on Si(0 0 1): surface optical anisotropy from first-principles calculations

Reflectance anisotropy spectra (RAS) for water covered Si(0 0 1) surfaces have been calculated within the independent-particle approximation. For low coverages we find essentially an attenuation of the features typical for clean Si surfaces. For the saturation coverage of one molecule per dimer, however, dissociative adsorption and oxygen insertion into the Si dimer give rise to clearly different optical anisotropies.

Surface structure of low-coveraged Cs on Si(0 0 1)-( [formula omitted]) system

Alkali metals (AM) on semiconductors have been investigated as a simple model system for the metal-semiconductor interfaces due to their simple electronic structures. Especially, cesium (Cs) on Si(0 0 1) surface has been studied with various experimental techniques. In this study, we investigated the atomic structure of initial Cs adsorption on Si(0 0 1)-( 2 × 1 ) surface using coaxial impact collision ion scattering spectroscopy. When Cs atoms are adsorbed on Si(0 0 1)-( 2 × 1 ) up to 0.2 ML at room temperature, the initial adsorption site is on-top T3 site with poor periodicity and the length of Si dimer is reserved as in the clean Si(0 0 1) surface. It is also found that Cs atoms adsorbed on Si(0 0 1) surface with a height of 2 . 83 ± 0 . 05 Å from the second layer of Si(0 0 1) surface.

Nano-patterned silicon surfaces for the self-organised growth of metallic nanostructures

The aim of this work is to obtain a very regular alignment of metallic and magnetic nanostructures with high density and a narrow size distribution via a self-organised growth process. The method consists of combining microelectronic processes which will provide a periodic surface pattern, with self-assembled growth on the so-obtained surface. The structured surface serves as a template for the controlled positioning of the nanostructures. Concerning the silicon template preparation, the surface periodicity should be of the same order of magnitude as the atomic surface diffusion length used during subsequent growth. These surface patterns were obtained on vicinal silicon surfaces by optimising electron-beam nanolithography and reactive ion etching, to create arrays of nano-holes typically 50 nm in diameter with a 50 nm spacing. A subsequent sample annealing under ultra-high vacuum conditions is studied in order to obtain a corrugated surface mimicking the lithography pattern. As a model system, the growth of Au on vicinal Si(111) surfaces has been chosen. After high temperature annealing, clean Si surfaces present arrays of straight step bunches each separated by ∼50 nm. We discuss the formation of Au islands on these step bunches during growth and/or annealing at a temperature between 300 and 500 ∘C.

Surfactant mediated growth of Sb clusters on Si(1 1 1) surface

Adsorption kinetics of Sb onto the Si(1 1 1) surface has been modified by incorporation of a Si(1 1 1)–In(√3×√3) surface phase (θ In = 1 3 ML) . In contrast to Sb adsorption on the clean Si surface, the formation of Sb nano-size clusters with good crystalline quality was observed below ≈200°C. Hemispherical clusters grow up to the saturation coverage of about 3 ML and stabilize in the average diameter and height of 40 and 5 nm, respectively, occupying approximately 15% of the surface (cluster density is estimated at 1.2×10 10 clusters/cm 2). The structure of clusters is determined to be a double-domain rhombohedral with (0 0 0 1) stacking. Both structure and lattice parameters of the clusters coincide with those of bulk Sb. The inter-cluster surface is covered with a Sb(2×1) surface phase ( θ Sb=1 ML). Using reflection high-energy electron diffraction and scanning tunneling microscopy techniques it has been found that indium acts as a surfactant providing layer-by-layer growth of Sb clusters. By increasing the coverage of initial In layer, the higher cluster density can been achieved.

Beam study of the Si and SiO2 etching processes by energetic fluorocarbon ions

Energy-controlled and mass-selected beam of fluorocarbon ion (CF3+,CF2+) at energies from 50 to 400 eV are directed at Si and SiO2 surfaces, in order to elucidate etch mechanisms and SiO2/Si etch selectivity. During the ion beam injection, the time evolution of neutral radicals desorbed from the etching surface is measured by appearance mass spectrometry. At the same time, in situ surface analysis is also carried out by x-ray photoelectron spectroscopy. The etching rate of a clean Si surface by CF3+ injection is initially high but decreases with time due to the formation of a thin surface layer containing F and C atoms. Finally, a few-nanometers-thick stationary SixCyFz overlayer is formed on the Si surface for ion fluence &amp;gt;3×1016 cm−2, along with saturation of CF2 and SiF2 radical desorption from the surface. Comparison is made with molecular dynamics simulations of CF3+ interactions with Si surfaces. CF2+ injection on the Si surface yields rapid formation of the SixCyFz layer and lower desorption of the CF2 radical compared with the CF3+ injection case. On the other hand, SiO2 etching by CF3+ takes place at an energy above 50 eV. During steady-state etching, a SixOyFz overlayer is formed with C atoms hardly observed. The measured desorption rates of etch products (CO and SiF2) show the same energy dependence of SiO2 etching yield per ion. CF2 desorption during the SiO2 etching is very low compared with Si etching.

Structure of the Sb/Si(112) Surface Studied by Low Energy Electron Diffraction

We have investigated the Sb-induced phase transition of the reconstructed Si(112) surface by low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). A sharp sixfold hexagonal LEED pattern was observed when Sb was deposited on the clean Si(112) surface at 300°C, corresponding to the bulk-terminated 1×1 structure of the Si(111) surface. From the LEED pattern and the XPS results, we suggest a surface structure corresponding to the LEED pattern with (111) - 1×1 + weak 5×1 spots for the Sb/Si(112) surface.

Comparative study of dimer-vacancies and dimer-vacancy lines on Si( [formula omitted]) and Ge( [formula omitted

Although the clean Si(0 0 1) and Ge(0 0 1) surfaces are very similar, experiments to date have shown that dimer-vacancy (DV) defects self-organize into vacancy lines (VLs) on Si(0 0 1), but not on Ge(0 0 1). In this paper, we perform empirical-potential calculations aimed at understanding the differences between the vacancies on Si(0 0 1) and Ge(0 0 1). We identify three energetic parameters which characterize the DVs on the two surfaces: the formation energy of single DVs, the attraction between two DVs in adjacent dimer rows, and the strain sensitivity of the formation energy of DVs and VLs. At the empirical level of treatment of the atomic interactions (Tersoff potentials), all three parameters are favorable for the self-assembly of DVs on the Si(0 0 1) surface, but not on Ge(0 0 1). The most significant difference between the defects on Si(0 0 1) and on Ge(0 0 1) concerns the formation energy of single DVs, which is three times larger in the latter case. By calculating the strain-dependent formation energies of DVs and VLs, we propose that the experimental observation of self-assembly of vacancies on clean Ge(0 0 1) could be achieved by applying compressive strains of the order of 2%.

In situ ESR observation of interface dangling bond formation processes during ultrathin SiO2 growth on Si(111).

We report the formation processes of interface dangling bonds (Pb centers) during initial oxidation of a clean Si(111) surface using an ultrahigh-vacuum electron-spin-resonance technique. At the oxidation of one or two Si layer(s), the Pb center density reached around 2.5-3.0 x 10(12) cm(-2), which is the same density as in the case of thick SiO2. This result shows that the Pb center density does not originate from the long-range accumulation of the structural stress between two materials, but from the chemical reactions of oxidation within a few Si atomic layers.

Calculation of surface optical properties: from qualitative understanding to quantitative predictions

In the last couple of years there has been much methodological and computational progress in the modeling of optical properties from first principles. While the calculation of non-linear optical coefficients is still hampered by numerical limitations—demonstrated here for the case of bulk GaAs—linear optical spectra can now be calculated accurately and with true predictive power, even for large and complex surface structures. This allows on one hand for a much better understanding of the origin of specific features such as surface optical anisotropies. We find that in particular microscopic electric fields at the surface induce slight deformations of bulk-like wavefunctions and thus give rise to optical anisotropies even from sub-surface layers. On the other hand, from the comparison of measured and calculated spectra, one can now confidently reach conclusions on the surface geometry. This short review focuses on the simulation of reflectance anisotropy spectroscopy. The clean, hydrogenated and uracil-covered Si(001) surface is used to illustrate the microscopic origin of surface optical anisotropies and the present state-of-the-art in computational modeling of optical spectra.

Kinetic growth manipulation of Si(0 0 1) homoepitaxy

We have confirmed in a combined diffraction and STM study that the usual kinetic growth manipulation (KGM) applied to Si/Si(0 0 1) (nucleation at relatively low temperatures and completion of monolayer growth at high temperatures) does not lead to a smooth growth front. We have identified the physical reason for this unexpected behaviour: an anti phase boundary (APB) network develops during growth, due to the 2 × 1 reconstruction of the clean Si(0 0 1) surface. The density of this APB network can be substantially reduced by application of a different and optimised KGM procedure. Following a recipe in which 1 ML of Si on Si(0 0 1) is deposited at a relatively low temperature (525 K), followed by a short anneal to 750 K, results in a surface flatness similar to that of the clean Si(0 0 1) 2 × 1 surface. Up to 10 ML of material deposited with a flash anneal after the deposition of each additional layer resulted in a surface with a negligible reduction of the in-phase and out-of-phase intensity of a reflected low energy electron beam, indicative of an almost perfectly smooth growth front. STM images support this observation. The low thermal budget of this method reduces intermixing effects in hetero-epitaxial growth of group IV semiconductor (0 0 1)-faces.

Formation of nanoclusters on silicon from carbon deposition

Changes in the structure of silicon surfaces can be induced by adsorption of carbon-containing molecules followed by thermal treatments. Clean Si(111) surfaces, prepared in vacuum and exposed to different adsorbants such as methanol or carbon monoxide, change their structures with the formation of self-organised nanostructures (15–50nm diameter) after suitable UHV annealing procedures. Evolution of the size and density per unit area over different heating periods indicates that the structures are nucleated by carbon atoms present on the surface while their growth derives from mobile surface silicon atoms during the annealing process. Methanol adsorbs dissociatively on silicon at room temperature thus leading to a high density of nucleation centres, but when the process is applied to partially oxide-masked silicon surfaces using CO as adsorbant the nanostructures form preferentially at the Si/SiO2 interface around the mask border thus offering the possibility to grow more ordered self-organised nanoscale patterns. Monte Carlo simulations of this process correlate well with STM measurements.

Electron beam effects on diethylsilane‐covered Si(100) surfaces investigated by x‐ray photoelectron spectroscopy

AbstractEthylated silanes and germanes are promising candidates for heterojunction atomic layer epitaxy of SiGe devices. However, thermal dissociation of unwanted ligands in the parent molecules usually is not suitable for device fabrication. Hence, alternative means for dissociation employing low thermal budgets are desirable. In this work, an electron beam was used to irradiate diethylsilane (DES)‐covered Si(100) surfaces at 100 K in order to initiate non‐thermal, electron‐driven dissociation and desorption processes in the adsorbed species. Clean Si(100) surfaces were dosed with gas‐phase DES molecules at 100 K followed by irradiation with 600‐eV electrons (current density 5.4 µAcm−2). The effects of electron irradiation were investigated by obtaining x‐ray photoelectron spectra before and after exposing the DES/Si(100) to the electron beam. Shifts in the binding energy of the C 1s photoelectron peak and a decrease in the total surface carbon concentration are indicative of dehydrogenation and removal of carbon species by electron irradiation. Our results suggest the possibility of electron‐beam‐assisted β‐hydride elimination at 100 K as one channel for surface carbon removal. Copyright © 2003 John Wiley &amp; Sons, Ltd.

Characterization of amorphous-Si/1ML-Ge/Si(001) Interface Structure by X-ray Standing Waves

The interface structure of an amorphous-Si/1ML-Ge/Si(001) crystal was investigated by the X-ray standing wave (XSW) method. The sample was prepared by the molocular beam epitaxy (MBE) deposition of 1ML of Ge on a clean Si(001) surface followed by the deposition of an amorphous Si (a-Si) capping layer of 50 Å thickness. The intensity of the reflected X-rays and the yield of the fluorescent X-rays of GeKα were measured around the 111 Bragg point. The fluorescence yield curve, which indicates the interaction between the XSW field and Ge atoms, showed that most of the Ge atoms stay at a position very close to the ideal Si site. This result provides effective feedback for fabricating SixGe1-x/Si devices.

Film-thickness effect of Ag lubricant layer in the nano-region

The tribological properties of Ag films with a wide range of thicknesses on an Si(1 1 1)-7×7 surface have been studied using a pin-on-plate type tribometer under ultrahigh-vacuum (UHV) conditions. Reflection high-energy electron diffraction (RHEED) patterns show that the Ag film deposited on an Si(1 1 1)-7×7 surface at room temperature (RT) forms a two-dimensional fiber crystal structure and gradually grows into a single-crystal Ag film as the film thickness increases. The coefficient of friction of 170 nm as-deposited Ag film was 0.06–0.08, and that coefficient decreased to 0.01–0.03 when the thickness decreased to 8.4–1.5 nm. In the thickness decreased from 1.5 to 0 nm, the coefficient of friction increase as the thickness decreased. The clean Si(1 1 1) surface showed a coefficient of friction of 0.8. The worn track widths on both the 1.5 nm Ag film and 170 nm Ag films were comparable. Auger electron spectroscopy (AES) measurements showed that the Ag MNN intensity ratio of the worn track to non-slide surfaces was 0.9–0.6 after frictional experiments. By annealing the 0.4 and 1.5 nm films at 500 °C, the morphology of the film changed significantly; one atomic layer of Ag formed a 3 × 3 Ag surface ( 3 Ag), and the residual Ag formed fine crystals. The population of the crystals increased as the film thickness increased. Virtually all of the 0.4 nm annealed Ag film area was 3 Ag surface, and the film showed a coefficient of friction of 0.1. The 3 Ag was covered by fine Ag crystals with the 1.5 nm annealed film, which showed a coefficient of friction less than 0.01. The high-lubricity mechanism of the Ag film in the nanometer-thickness region is discussed in terms of its mechanical properties such as the shear strength of Ag depending on the film thickness.

Mechanism of Cu transport along clean Si surfaces

Abstract Cu diffusion along clean Si(111), (110) and (100) surfaces are investigated by Auger electron spectroscopy and low energy electron diffraction. The effective diffusion coefficients of copper are measured in the temperature range from 500 to 650°C. It is shown that the Cu transport along silicon surface occurs by the diffusion of Cu atoms through Si bulk and the segregation of Cu atoms to the surface during the diffusion process. It is found that the segregation coefficients of Cu to silicon surface during the diffusion process depend on surface orientation.

AFM and SNOM characterization of carboxylic acid terminated silicon and silicon nitride surfaces

Silicon and silicon nitride surfaces have been successfully terminated with carboxylic acid monolayers and investigated by atomic force microscopy (AFM) and scanning near-field optical microscopy (SNOM). On clean Si surface, AFM showed topographical variations of 0.3–0.4 nm while for the clean Si 3N 4 surface the corrugation was around 3–4 nm. After material deposition, the corrugation increased in both samples with a value in topography of 1–2 nm for Si and 5–6 nm for Si 3N 4. The space distribution of specific chemical species was obtained by taking SNOM reflectivity at several infrared wavelengths corresponding to stretch absorption bands of the material. The SNOM images showed a constant contribution in the local reflectance, suggesting that the two surfaces were uniformly covered.

Preparation of atomically clean and flat Si(1 0 0) surfaces by low-energy ion sputtering and low-temperature annealing

Si(1 0 0) surfaces were prepared by wet-chemical etching followed by 0.3–1.5 keV Ar ion sputtering, either at elevated or room temperature (RT). After a brief anneal under ultrahigh vacuum (UHV) conditions, the resulting surfaces were examined by scanning tunneling microscopy. We find that wet-chemical etching alone cannot produce a clean and flat Si(1 0 0) surface. However, subsequent 300 eV Ar ion sputtering at room temperature followed by a 700 °C anneal yields atomically clean and flat Si(1 0 0) surfaces suitable for nanoscale device fabrication.