H-terminated Si Surfaces Research Articles

Growth and interface of HfO2 films on H-terminated Si from a TDMAH and H2O atomic layer deposition process

Hf O 2 thin films have been deposited by an atomic layer deposition (ALD) process using alternating pulses of tetrakis(dimethyl)amino hafnium and H2O precursors at a substrate temperature of 200–325°C. The initial stage of film growth on OH- and H-terminated Si(100) surfaces is investigated using Rutherford backscattering spectrometry (RBS), x-ray photoelectron spectroscopy (XPS), and spectroscopic ellipsometry (SE). The authors observe an initial growth barrier on the Si–H surface for the first approximately four process cycles, where film growth is more efficient on the OH-terminated surface. Both starting surfaces require about 15cycles to reach a steady growth rate per cycle, with the OH-terminated surface displaying a slightly higher growth rate of 2.7×1014Hf∕cm2 compared to 2.4×1014Hf∕cm2 for Si–H. Combining the RBS and SE data we conclude that the films deposited on the OH-terminated surface are denser than those deposited on the Si–H surface. Angle-resolved XPS measurements reveal the formation of an ∼8Å interfacial layer after four ALD cycles on the H-terminated surface for a deposition temperature of 250°C, and transmission electron microscopy verifies that the thickness of the interfacial layer does not change substantially between the 4th and the 25th process cycles. The interfacial layer appears to depend weakly on the deposition temperature from 200to325°C, ranging from 6.9to8.4Å.

Atomic modeling of surface photovoltage: Application to Si(1 1 1):H

The dependence of surface photovoltages on the wavelength of light are obtained for the first time from an atomic model and ab initio calculations. Photovoltages follow from a time-dependent density matrix treatment using a basis set of Kohn–Sham orbitals and a steady state solution for the time-dependent density matrix. An application to a H-terminated Si(1 1 1) surface gives the main features of calculated photovoltage versus incident photon energies in agreement with experimental trends. Our treatment can be implemented for a wide class of photo-electronic materials relevant to solar energy capture.

Depth distribution of Cs implanted into Si at steady-state during dual beam ToF-SIMS profiling

The steady-state depth distributions of Cs atoms implanted into a H-terminated Si surface at different energies (250 eV to 2 keV, 45° incidence angle) were studied. In situ ToF-SIMS depth profiles of the Si surface preloaded with Cs were obtained immediately after the Cs implantation using low energy Xe + (350 eV, 45°) and O 2 + (500 eV, 45°) sputter beams and a Ga + analysis beam (15 keV, 45°). The crater depth was measured ex situ by a profilometer to calibrate the depth scale. All the Cs profiles exhibited a surface peak attributed to segregation and a maximum under the surface for both Xe + and O 2 + sputtering beams. The relative height of the surface segregation peak, the position of the maximum and the width of the Cs profiles depend on the energy of the Cs primaries. Depth profiles of adsorbed/sputter deposited Cs obtained next to the implantation craters exhibited no surface peak. A dynamic TRIM code was used to obtain the depth distribution of the implanted Cs at steady-state. The results of the simulations were compared to those obtained experimentally.

Extracting maximum information from polarized surface vibrational spectra: Application to etched, H-terminated Si(110) surfaces

A general method to maximize the information extracted from polarized surface absorption spectra is developed and applied to the study of etched Si(110) surfaces. In essence, this technique transforms spectra from the experimental reference frame, which is defined by the direction of the surface electric field during irradiation by s- and p-polarized light, into a more appropriate Cartesian reference frame defined by the surface normal and the plane of incidence. If the Cartesian reference frame is aligned with high symmetry directions of the system, significant spectral simplification can result. This analysis relies on the well-known boundary conditions on interfacial electric fields and is independent of any adsorbate screening or the effective dielectric constant of the adsorbate layer. The validity of this analysis is demonstrated on the spectra of NH4F-etched, H-terminated Si(110). The transition dipole moments of the symmetric and antisymmetric Si[Single Bond]H stretch modes associated with flat terraces are polarized along the [110] and [001] directions, respectively. Two additional modes with transition dipoles polarized along the [001] and [110] directions are assigned to defect species associated with microfaceting and other surface roughness. Data taken in two different experimental geometries are shown to be in excellent quantitative agreement, confirming the validity of the technique. Additionally, the measured adsorbate layer dielectric constant is in good agreement with previously reported values for hydrogen-terminated silicon surfaces.

Theoretical prediction of 1-D molecular lines on the Si(001) surface

We propose a facile method for the self-directed growth of ID molecular lines on the H-terminated Si(001) surface. This method employing a single H-free dimer instead of a single DB greatly enhances the stability of the radical intermediate, thereby facilitating the chain reaction for ID molecular lines. The proposed method will stimulate experiments for fabrication of ID molecular lines which are strongly attached to the Si(001) substrate.

UV-induced immobilization of tethered zirconocenes on H-terminated silicon surfaces

A tethered ethylenebis(indenyl) zirconocene was covalently immobilized on H-terminated Si(111) surfaces using UV-mediated alkene hydrosilylation, thus making possible the development of structured catalytic surfaces with highly controlled properties.

In Situ AFM Studies on Self-Assembled Monolayers of Adsorbed Surfactant Molecules on Well-Defined H-Terminated Si(111) Surfaces in Aqueous Solutions

The formation of self-assembled monolayers (SAMs) of adsorbed cationic or anionic surfactant molecules on atomically flat H-terminated Si(111) surfaces in aqueous solutions was investigated by in situ AFM measurements, using octyl trimethylammonium chloride (C8TAC), dodecyl trimethylammonium chloride (C12TAC), octadecyl trimethylammonium chloride (C18TAC)) sodium dodecyl sulfate (STS), and sodium tetradecyl sulfate (SDS). The adsorbed surfactant layer with well-ordered molecular arrangement was formed when the Si(111) surface was in contact with 1.0x10(-4) M C18TAC, whereas a slightly roughened layer was formed for 1.0x10(-4) M C8TAC and C12TAC. On the other hand, the addition of alcohols to solutions of 1.0x10(-4) M C8TAC, C12TAC, or SDS improved the molecular arrangement in the adsorbed surfactant layer. Similarly, the addition of a salt, KCl, also improved the molecular arrangement for both the cationic and anionic surfactant layers. Moreover, the adsorbed surfactant layer with a well-ordered structure was formed in a solution of mixed cationic (C12TAC) and anionic (SDS) surfactants, though each surfactant alone did not form the well-ordered layer. These results were all explained by taking into account electrostatic repulsion between ionic head groups of adsorbed surfactant molecules as well as hydrophobic interaction between their alkyl chains, which increases with the increasing chain length, together with the increase in the hydrophobic interaction or the decrease in the electrostatic repulsion by incorporating alcohol molecules into the adsorbed surfactant layer, the decrease in the electrostatic repulsion by increasing the concentration of counterions, and the decrease in the electrostatic repulsion by alternate arrangement of cationic and anionic surfactant molecules. The present results have revealed various factors to form the well-ordered adsorbed surfactant layers on the H-Si(111) surface, which have a possibility of realizing the third generation surfaces with flexible structures and functions easily adaptable to circumstances.

The best of both worlds – A novel approach to macromolecular STM studies

The advent of scanning tunnelling microscopy (STM) and its younger sibling atomic force microscopy (AFM) have fundamentally changed the way we investigate phenomena on the molecular scale. Direct visualisation is routine and atomic and molecular level imaging is an expectation of most researchers, if not their supervisors. It might therefore be surprising to learn that whole classes of systems cannot be readily visualised using either technique. AFM is in principle the more versatile technique and ultrahigh vacuum (UHV) studies have demonstrated atomic resolution on surfaces [1] and more recently an ability to distinguish between different surface atoms [2]. However, without significant advances in force sensor technology [3] the prospect for routine molecular (not to mention sub-molecular) imaging is poor. STM, the elder sibling, is itself not without limitations. Apart from the obvious requirement of conducting substrates, it is also important to minimise resolution loss due to unwanted mixing between molecular and substrate electronic levels. In the quest to image large molecular and polymer systems using STM most researchers are faced with deciding between an ambient or UHV approach, including the oftentimes difficult task of developing appropriate sample handling and preparation procedures. Ambient STM studies typically employ gold or graphite substrates and for which samples are typically deposited directly from solution. This approach works well for a broad range of soluble molecular and polymer systems but imaging is often unstable and contamination and artefacts can be problematic [4]. The alternative UHV approach offers more stable tunnelling conditions, together with an atomically clean substrate lattice that provides an in-situ reference for structure analysis. The latter approach, however, is not amenable to systems with ultra-low vapour pressures or which decompose or denature at elevated preparation temperatures. On page XXX of this issue Laracuente and co-workers [5] describe an innovative approach to this problem that combines the best of UHV STM imaging with solution-based sample processing. Starting from a UHV prepared Si(100) surface, the authors formed a passivated H-termination by exposure to atomic hydrogen [6]. H-terminated Si(100) surfaces are stable, exceptionally well ordered and are routinely imaged with atomic resolution [7]. Transferring the H-terminated Si(100) substrate to a nitrogen-purged load-lock, samples were then drop-cast from solution onto the surface, and quickly transferred back to the STM for analysis. CDCl3 was found to be the most suitable solvent for their particular system (a pentiptycene-based polymer) and was chosen following a careful study of the contamination and oxidation levels on the H-terminated Si(100) surface after treatment with different solvents [5]. The beauty of this approach is that it overcomes the vexing problem of sample deposition by eliminating the need for Knudsen cells or even more sophisticated pulsed valve delivery systems, while at the same time provides the full benefits of

UHV characterization of ambient-dosed hydrogen-terminated Si(0 0 1)

We report ultra-high vacuum characterization of a high-molecular weight pentiptycene-based copolymer deposited from solution onto H-terminated Si(0 0 1) surfaces under ambient conditions. We found that NMR-grade deuterated chloroform was the best choice of solvent to preserve the cleanliness of the sample, resulting in the lowest sample C and O contamination as determined by AES and STM. The H-terminated Si(0 0 1) – 2 × 1 surface proved to be an excellent metrology tool for measuring the molecular features of the polymer with nanometer resolution. Atomically resolved STM images of single polymer molecules were obtained. The average periodicity along the polymer chain was 1.26 nm, with the width of the polymer varying between 0.98 nm and 1.5 nm. Kinks observed along the polymer chain are consistent with a reduction of the polymer backbone from alkyne to alkene.

In-situ FTIR Study of Atomic Layer Deposition (ALD) of Copper Metal Films

Growth mechanisms of atomic layer deposition of copper films on various substrates using a novel metal precursor [Cu(sBu-amd)]2 and molecular H2 are investigated by in-situ transmission Fourier transform infrared spectroscopy (FTIR). The Cu-precursor reacts with SiO2 and Al2O3 surfaces by forming chemical bonds with the surface. Upon reduction by H2, Cu atoms agglomerate, yielding additional reactive sites for more Cu-precursors. Cu agglomeration is relatively weaker on nitrided Si surfaces. H-terminated Si surfaces show a minimal reaction with the Cu precursor. The growth rates of the Cu films on all these surfaces are all less than 1 Aå per cycle.

Electrical Properties of Junctions between Hg and Si(111) Surfaces Functionalized with Short-Chain Alkyls

Metal−semiconductor junctions between Hg and chemically modified n- and p-Si(111) surfaces have been prepared and analyzed using current−voltage and differential capacitance−voltage methods. To understand the role of the interfacial dipole on interfacial charge transfer, silicon surfaces were modified with either nonstoichoimetric oxide (SiOx), terminal monohydride, short (CnH2n+1−, n = 1, 2, 3) saturated alkyl chains, or propynyl (CH3−C⋮C−) groups. X-ray photoelectron spectra of the modified Si electrode surfaces taken before and after exposure to Hg contacts showed no evidence of irreversible chemical interactions between the Si and the Hg. Hg/Si contacts made using H-terminated Si(111) surfaces exhibited Schottky junctions having barrier heights (Φb) that were consistent with the known surface electron affinity of Si and the work function of Hg. In contrast, Si coated with a thin, chemically grown oxide formed Hg/Si junctions having barrier heights suggestive of Fermi level pinning. Si(111) surfaces modified with methyl groups yielded Hg junctions having barrier heights in accord with expectations based on the electron affinity (3.67 eV) and surface dipole (0.38 eV) measured on such surfaces by photoemission spectroscopy, attesting to the degree of chemical control that can be exerted over the barrier heights of such systems by surface functionalization methods. Incomplete coverages of functional groups produced by alkylation with ethyl or iso-propyl groups did not greatly impact the observed values of Φb relative to Φb values observed for CH3-terminated Si(111) surfaces. However, the observed variation in Φb between nominally identical samples increased as the number of carbons in the functionalizing alkyl group increased. Junctions between Hg and Si(111) surfaces modified with propynyl groups showed nearly identical behavior to that of CH3−Si(111)/Hg contacts, both in average Φb values and standard deviation between samples. The behavior of Si/Hg interfaces modified with short organic functional groups is consistent with the efficacy and utility of passivated surfaces in modifying the properties of surface-based Si devices.

The effect of controlled ion bombardment on the electronic structure of the Si(0 0 1) surface

We have studied the effects of controlled ion bombardment on the electronic structure of the Si(0 0 1) surface. The surface was exposed to various doses of Ar + ions accelerated towards the surface at 500 eV. X-ray photoelectron spectroscopy (XPS) spectra of the irradiated H-terminated Si(0 0 1) surface reveal the appearance of peaks that are associated with the presence of cleaved Si bonds. Ultraviolet photoelectron spectroscopy (UPS) spectra of the irradiated Si(0 0 1)2 × 1 surface show that the dimer dangling-bond surface state decays monotonically with increasing dose. These results, coupled with previous scanning tunneling microscopy (STM) studies, indicate that the breaking of dimers, and possibly the creation of adatom-like defects, during ion irradiation are responsible for the changes in the electronic structure of the valence band for this surface.

First-principles calculations of Cu adsorption on an H-terminated Si surface

In this study, we use first-principles simulations to study the adsorption of copper onto H-terminated and partially OH-terminated silicon surfaces. We show that, in contrast to previous studies, copper adsorbs strongly to the H-terminated silicon surface and that the adsorption energy is significantly dependent on the local bonding environment. The addition of a hydroxide group increases the average adsorption energy while reducing the range of adsorption energies due to the strong interaction between copper and oxygen. Our results predict that copper will generally prefer to adsorb at dihydride sites on the surface, agreeing with experimental studies of copper nucleation. The adsorption energy hierarchy predicted by the calculations strongly supports the suggestion that copper acts as a micromask in wet chemical etching, blocking reactive sites.

Diffusion and adsorption behavior of Si adatom on defect-induced H-terminated Si(0 0 1) surfaces

Abstracts The diffusion and adsorption behaviors of the Si adatom on the defect-induced H-terminated Si(0 0 1) surfaces are investigated by empirical tight-binding (ETB) method in this paper. The surface adsorption energies are specially mapped out, from which the favorite binding sites and several possible diffusion pathways are achieved. It is observed from this research that the diffusion energy barriers of the Si adatom near the defect on the hydrogenated Si(0 0 1) surface are higher than those on the non-hydrogenated surface. With the shortening of the structural period, it takes more energy for the Si adatom to escape from the single dimer vacancy (SDV). From this paper, we can find that SDV is a good adsorption point.

Simulations of Scanning Tunneling Microscopy for B-/P-doped Si(111) Surfaces

We have investigated scanning tunneling microscopy (STM) for hydrogen-terminated Si(111) surfaces that have substitutional dopants in the subsurface region, using first-principles calculations within the density functional theory. The STM image for the occupied state indicates protrusion around a subsurface B atom and depression around a subsurface P atom. For the STM image of the unoccupied state, protrusions and depressions are interchanged. These dopant features on the STM image can be explained by the change in electrostatic potential around a dopant ion. The most stable depth of the substitutional B or P atom in H-terminated Si(111) surfaces is determined to be the second atomic layer from the topmost Si layer. The effect of a dangling bond created by the desorption of H atoms on the STM image of the subsurface dopant has also been studied.

Nucleation of HfO2 atomic layer deposition films on chemical oxide and H-terminated Si

HfO 2 thin films have been deposited by an atomic layer deposition (ALD) process using alternating pulses of tetrakis-ethylmethylamino hafnium and H2O precursors at 250 °C. The as-deposited films are mainly amorphous and nearly stoichiometric HfO2 (O/Hf ratio ∼1.9) with low bonded carbon content (∼3 at. %). A comparison of the nucleation stage of the films on OH- and H-terminated Si(100) surfaces has been performed using Rutherford backscattering spectrometry, x-ray photoelectron spectroscopy (XPS), and spectroscopic ellipsometry (SE). We find for the initial 5–7 process cycles that the film nucleates more efficiently on the OH-terminated surface. However, after the 7th cycle both surfaces exhibit similar surface coverage, which takes about 40 cycles to reach a steady growth rate per cycle. Angle resolved XPS measurements reveal the formation of a ∼6 Å interfacial layer after four ALD cycles on the H-terminated surface and the thickness of the interfacial layer does not change substantially between the 4th and the 50th process cycles as shown by transmission electron microscopy. Although the surface coverage is comparable for both starting surfaces, film measurements performed by SE suggest that thick films deposited on H-terminated Si are ∼5% thicker than similar films on the chemical oxide surface. Atomic force microscopy (AFM) measurements reveal higher surface roughness for the films deposited in the H-terminated surface. The SE and the AFM data are consistent with higher porosity for the films on H-terminated surfaces.

Enhanced Stability of 1D Molecular Lines on the H-Terminated Si(001) Surface

We present a facile method for the self-directed growth of 1D molecular lines on the H-terminated Si(001) surface. Instead of a previously employed single dangling bond, we here employ a single H-free Si dimer as a reaction site, resulting in an enhanced stability of the radical intermediate for the O-phthalaldehyde (OP) molecule containing two carbonyl groups. This radical intermediate easily abstracts two H atoms from a neighboring Si dimer, thereby allowing the chain reaction for a 1D molecular line. Such a fabricated OP line will be stable at higher temperatures compared to previously reported alkene lines because of its enhanced stability.

Photon-Stimulated H Desorption from Several H/Si(001) Surfaces for Resistless Lithography

Synchrotron radiation photoemission spectroscopy (SRPES) and scanning photoelectron microscopy (SPEM) have been used to investigate photon-stimulated H desorption from several H-terminated Si(001) surfaces for resistless lithography. H was desorbed from a well-defined Si(001)2×1:H surface upon unmonochromatized photon irradiation, but monochromatic photons (100 ∼ 650 eV) did not break H-Si bonds regardless of the incident photon flux per unit area. Photons of ∼7.9 eV or secondary electrons might induce H desorption upon unmonochromatized photon irradiation. On the other hand, contaminated H/Si(001) surfaces were affected by extreme ultraviolet (EUV) (11.8 nm) irradiation. Here, we demonstrate that minute patterns can be formed through the photochemical reaction.

Growth of noble metal nanostructures on the Bi nanoline surface: A first-principles study

Recent experiments suggest that self-assembled Bi nanolines on the passivated Si(0 0 1) surface can be used as templates to grow one-dimensional arrays of noble-metal (Au, Ag) nanoclusters. In order to ascertain this template effect, the gold atom on the H-terminated Si(0 0 1) surface and that on the Bi line are examined by density-functional methods. The calculation indicates that the gold atom enters a Si dimer on the H-terminated surface and the Bi Si backbonds of a Bi dimer on the Bi line. The gold atom is 0.6 eV more stable on the Bi line than on the H-terminated surface. This result suggests the preferential adsorption of gold on the Bi line and confirms its template effect.

Atomic Layer Deposition of Hafnium Oxide from Tetrakis(ethylmethylamino)hafnium and Water Precursors

We have calculated the atomistic mechanism for the HfO2 atomic layer deposition (ALD) using Hf(NEtMe)(4) and H2O precursors using density functional theory. On hydroxylated Si surface, our results show overall Hf(NEtMe)(4) half-reaction is exothermic by 1.65 eV with a small activation barrier of 0.10 eV. The activation barriers for water half-reaction are 0.24 and 0.20 eV. This indicates HfO2 ALD with Hf(NEtMe)(4) and H2O has a faster deposition rate than that with HfCl4 and H2O and can be run at relatively low temperature. However, on H-terminated Si surface, the Hf(NEtMe)(4) half-reaction is endothermic by 2.39 eV with high activation barriers. The H2O half-reaction is similar to that on hydroxylated surface which is kinetically favorable. The results suggest that long Hf(NEtMe)(4) pulse time and high deposition temperature is required during the initial stage of ALD on H-terminated surface. Moreover, our calculations indicate the reaction byproduct HNEtMe is likely to be bound to the surface after each half-reaction because of high desorption energy of 0.7-0.9 eV. Thus, long precursor purge time should be used if HNEtMe is needed to be removed completely.