Hydrogen-terminated Silicon Surfaces Research Articles

Evaluation of Gap States in Hydrogen-Terminated Silicon Surfaces and Ultrathin SiO2/Si Interfaces by using Photoelectron Yield Spectroscopy

ABSTRACTThe energy distributions of gap state densities for H-terminated Si surfaces and thermally-grown SiO2/c-Si interfaces have been evaluated by total photoelectron yield spectroscopy (PYS) with a dynamic range of eight orders of magnitude which is sufficient to detect the density of states as low as 1010cm−2eV−1. It is confirmed from the threshold energies for direct and indirect photo-excitations that, for monohydride-terminated Si(111) surfaces prepared by an NH4F treatment, no significant band-bending is observable. For H-terminated n-type sample, the gap state densities of the order of 1011cm−2eV−1 were estimated in the region within 0.4 eV from the valence band edge, which may be attributable to a very little oxidation in the sample preparation. It is also found that, for as-grown 2.5nm-thick SiO2/n+ Si, there exist interface states around midgap with densities as high as ∼1012cm−2eV−1.

Reactions of water and methanol at hydrogen-terminated silicon surfaces studied by transmission FTIR

A simple method for studying the reactivity of hydrogen-terminated silicon surfaces via transmission FTIR spectroscopy is described. Abraded (100) single crystals of silicon, after etching in ammonium fluoride, show features in the IR spectrum that are assigned to Si–H stretching vibrations. Confirmation of the utility of this approach is demonstrated in the reactions of the Si–H surfaces with water and methanol. Native-oxide growth in the former case is evidenced by the appearance of two bands in the Si–O stretch region accompanied by a shift to higher frequency of the longitudinal polar Si–O vibrational mode with increased exposure, consistent with an island-growth model. Similarly, the reaction with methanol generates two bands in the Si–O stretch region. However, the frequency and behaviour of these bands on further exposure to methanol are consistent with a chemisorbed methoxy species with little oxidation taking place, contrary to previous reports.

Combined AFM and laser lithography on hydrogen-passivated amorphous silicon

We report a novel combination of AFM lithography and laser direct writing on hydrogen-passivated amorphous silicon surfaces to fabricate combined silicon milli-, micro- and nanostructures. Selective oxidation is performed by focusing a laser beam (λ=458 nm) on a hydrogen-terminated silicon surface, forming the millimetre-size contact pads for connection of nanometre-scale patterns. The nanostructures are made by electric-field-enhanced oxidation using a contact mode AFM equipped with a metal-coated tip. Both techniques are based on selective oxidation of hydrogen-passivated amorphous silicon, where the oxide is used as an etch mask in a single etch step. The lithographic process has also been demonstrated using a reflection mode scanning near-field optical microscope with an uncoated fiber probe.

Initial stage of oxidation of hydrogen-terminated silicon surfaces

Structural changes produced by the oxidation of hydrogen-terminated Si(111)-1 × 1 and Si(100)-2 × 1 surfaces at 300°C in dry oxygen under a pressure of 1 Torr were investigated by X-ray photoelectron spectroscopy (XPS) and multiple internal reflection infrared absorption spectroscopy (MIR-IRAS). Following results are obtained from the analysis and simulation of the experimental results: (1) the layer-by-layer oxidation reaction occurs locally at SiO 2 Si (111) interface, while that does not occur at SiO 2 Si (100) interface, however, (2) the oxidation on Si(100) surface proceeds more uniformly in atomic scale than that on Si(111) surface.

Contactless Characterization of Thermally Oxidized, Air-Exposed and Hydrogen-Terminated Silicon Surfaces by Capacitance-Voltage and Photoluminescence Methods

Contactless and nondestructive characterization of the electrical properties of the free surfaces of single-crystalline silicon wafers is accomplished for the first time by combined use of the contactless metal-insulator-semiconductor (MIS) capacitance-voltage (C-V) technique and the photoluminescence surface state spectroscopy (PLS3) technique. The thermally oxidized, air-exposed and hydrogen-terminated silicon (111) surfaces were investigated. Thick thermally oxidized surfaces showed well-behaved characteristics with a minimum of a broad U-shaped interface state density (N ss) distributions in a range of 1010 cm-2eV-1. On the other hand, the air-exposed Si surface exhibited a GaAs MIS-like behavior with strong Fermi level pinning near the charge neutrality level E HO caused by a narrow U-shaped state distribution. Surprisingly, hydrogen-terminated surfaces showed even stronger Fermi level pinning at E0 = E c - 0.43 eV due to high density of amphoteric discrete state which may be due to silicon dangling bonds.

Discrete Metal Deposition on Hydrogen Terminated Silicon Surfaces: Kinetics, Morphologies and Sensor Applications

ABSTRACTThe presence of ultra-trace (as low as 1010 – 1013 atoms/cm2) quantities of metal contaminants on silicon substrates is well known to adversely affect the electrical properties of the silicon based microelectronic devices. In this paper, we report our most recent experimental findings on the trace metal deposition mechanisms and its interplays with silicon substrate surface properties at specific chemical environments. The emphases were on the deposition kinetics and morphology, especially at the early stage of metal out-plating process. For instance, our atomic force microscopy results revealed that the Cu deposition on H-Si(100) surface was dominated by nucleation of nanometer-sized Cu nuclei at the initial stage (less than 60 seconds). The growth of the existing Cu nuclei was suppressed for the initial nucleation period until all the nucleation sites were consumed. The number of nucleation sites were found to closely relate to wafer intrinsic properties and solution compositions. A novel sensor which provides direct detection of part-per-trillion level of metal ion impurities in wet cleaning solutions will also be presented.

STM Studies of Electrode/Electrolyte Interfaces and Silicon Surface Reactions in Controlled Atmospheres

ABSTRACTWe have assembled a scanning tunneling microscope with an inverted sample that allows the sample surface to be contacted by fluid electrolytes in a controlled atmosphere. A hanging meniscus is formed between the sample and a small cup surrounding the tunneling tip. In-situ imaging of the electrode/electrolyte interface is conveniently achieved with clean samples under potentiostatic control. The functioning of the microscope is illustrated by the imaging of the electrodeposition of copper on gold. This microscope has been used to image hydrogen-terminated silicon surfaces and to demonstrate that islands, tentatively assigned as silicon oxide, are formed on rinsing in water but can be avoided if the surface is not rinsed on withdrawal from the ammonium fluoride etching solution. Finally, STM shows that the convenient, gas-phase photochlorination of H-Si(111) produces the simple Cl-Si(111)(1×1) structure with little or no etching of the silicon surface.

Scanning probe anodization: Patterning of hydrogen-terminated silicon surfaces for the nanofabrication of gold structures by electroless plating

We have developed a nanofabrication method that uses patterned anodic silicon oxide as a template for pattern transfer to a gold (Au) nanostructure. The oxide patterns were prepared on a hydrogen-terminated silicon (Si–H) surface by scanning probe anodization, that is, electrochemical nanolithography based on scanning probe microscopy. These anodic oxide patterns were transferred to Au patterns through area-selective electroless plating, in which Au deposition proceeds selectively on the unanodized Si–H area while the anodic oxide surface remains free of deposits. Due to low Au nucleation density on the Si–H surfaces, Au lines formed by the electroless plating were discontinuous when their linewidth narrowed to less than 50 nm. The nucleation density was increased by a chemical etching of the Si–H surface prior to plating. We have successfully fabricated a continuous Au line as narrow as ∼30 nm.

STM characterisation of organic molecules on H-terminated Si(111)

We present the first high resolution STM images of organic molecules on the technological important hydrogen terminated silicon surface. Ordered layers of PTCDA and PTCDI were prepared on this surface by organic molecular beam epitaxy. The submolecular contrast of these molecules on Si(111)/H obtained in the high resolution images agrees with the corresponding images on HOPG and MoS2 substrates.

Low energy ion irradiation of H-terminated Si(001): hydrogen sputtering, beam-induced (2 × 1) reconstruction, and Si epitaxy

Low energy (15–50 eV) noble gas ion irradiation at low temperatures (< 300°C) enables formation of a Si(001)-2 × 1 surface reconstruction that has been hydrogen-terminated by a hydrofluoric acid dip prior to irradiation. Using 15 eV He + irradiation, a (2 × 1) reconstruction can be formed at temperatures as low as 50°C, the lowest temperature ever reported for formation of this reconstruction. The (2 × 1) reconstruction occurs as a result of surface hydrogen removal by sputtering and recoil implantation into the silicon substrate. Molecular dynamics simulations of low energy irradiation of a hydrogen-terminated silicon surface indicate that chemically-enhanced physical sputtering of silicon occurs in addition to physical sputtering of hydrogen. The hydrogen sputtering yield is found to be strongly dependent on surface hydrogen coverage. The results suggest that low energy ion irradiation can be used to tailor the hydrogen coverage, and thus also the chemical reactivity of the silicon surface.

Atomic-Scale Desorption Through Electronic and Vibrational Excitation Mechanisms

The scanning tunneling microscope has been used to desorb hydrogen from hydrogen-terminated silicon (100) surfaces. As a result of control of the dose of incident electrons, a countable number of desorption sites can be created and the yield and cross section are thereby obtained. Two distinct desorption mechanisms are observed: (i) direct electronic excitation of the Si-H bond by field-emitted electrons and (ii) an atomic resolution mechanism that involves multiple-vibrational excitation by tunneling electrons at low applied voltages. This vibrational heating effect offers significant potential for controlling surface reactions involving adsorbed individual atoms and molecules.

Chemical Approach to Nanofabrication: Modifications of Silicon Surfaces Patterned by Scanning Probe Anodization

Scanning probe microscope-induced local oxidation of a material surface with adsorbed water is a recent nanolithographic technology. We applied this technique to the nanoscale patterning of hydrogen-terminated silicon (Si-H) surfaces. Using the silicon oxide (SiOx) patterns as masking, examples of two types of pattern transfer method through area-selective chemical modification were demonstrated. Nanostructures of substrate Si or deposited gold were fabricated by wet chemical etching or electroless plating, respectively. These area selectivities arose from the difference in surface chemical reactivities between anodic SiOx and the surrounding Si-H. The oxidation chemistry is discussed in terms of anodization.

Monolayer nitridation of silicon surfaces by a dry chemical process using dimethylhydrazine or ammonia

A hydrogen-terminated silicon surface was successfully converted to a surface covered with a monolayer of nitrogen. Nitridation was carried out in a vacuum chamber using either dimethylhydrazine [H2N–N(CH3)2] or ammonia at a pressure of 1 mTorr and at temperatures ranging from 400 to 600 °C. In situ x-ray photoelectron spectroscopy measurements revealed that the binding energy and the full width at half-maximum in the nitrogen spectra are the same as those in bulk Si3N4. Nitrogen content at the surface increased as the nitridation time increased and, below 500 °C, saturated at a value that approximately corresponds to a monolayer thickness. These results show the effectiveness of dry chemical processes for preparing uniform Si surfaces terminated with specific atoms or molecules other than hydrogen.

Empirical interatomic potential for Si-H interactions.

An empirical many-body interatomic potential has been developed to describe Si-H interactions. The potential was fitted to various gas-phase silicon hydride species and interstitial sites of atomic hydrogen in bulk silicon, and gives a reasonable description of hydrogen-terminated silicon surfaces. The potential is computationally efficient and may be used, with caution, for molecular-dynamics investigations of Si-H interactions on hydrogen-terminated silicon surfaces and hydrogenated amorphous silicon.

Tip-Induced Surface Disorder on Hydrogen-Terminated Silicon(111) Surface Observed by Ultrahigh-Vacuum Atomic Force Microscopy

A sequence of images of a hydrogen-terminated Si(111) surface showing its change to disordered structure observed with an ultrahigh-vacuum atomic force microscope (UHV-AFM) are presented. This phenomenon is in striking contrast with the stability of this surface seen in the previous observations. A change of the chemical nature of the AFM tip is likely to have caused this difference. The silicon oxide layer on the tip is considered to have been removed due to an accidental crash, and consequently the exposed silicon reacted with the sample.

Tunneling-electron-induced adsorption of pyridine on a hydrogen-terminated silicon surface

Pyridine molecules were found to adsorb on a hydrogen-terminated silicon surface when electrons were tunneled through pyridine vapor to the silicon surface. The adsorption was apparently induced by the tunneling electrons, and its desorption also appeared to be induced by tunneling electrons. These findings consist of the first observation of tunneling-electron-induced adsorption–desorption on a solid surface.

Photoinitiation effect during photo-induced deposition of aluminum films using dimethylaluminum hydride

Photo-induced deposition of aluminum films on silicon dioxide and on a hydrogen terminated silicon surface was carried out using dimethylaluminum hydride (DMAH) and a deuterium lamp. After an initial nucleation was induced by irradiation, film growth was sustained without irradiation at a substrate temperature of 200°C where no deposition can occur via a thermal reaction alone. In addition, the growth rate decreased under irradiation during the growth period. An ArF laser also induced a similar photoinitiation effect.

Positron trap sites in the native oxide film grown on a hydrogen-terminated silicon surface

Positron behavior in thin native oxide layers grown on an initially hydrogen-terminated Si(100) surface was investigated and correlated with the chemical structure of the layers determined using Fourier-transform infrared absorption attenuated total reflection spectroscopy, and x-ray photoelectron spectroscopy. Hydrogen termination of the Si surface by 4 vol % HF treatment gave rise to a narrower Doppler-broadened positron-electron annihilation line than that of bulk Si. By a process of oxidation in pure water very thin (up to 7.8 Å) layers were grown on the H-terminated Si. The Doppler broadening of annihilations from the Si surface was seen to increase monotonically with thickness away from the value seen for amorphous bulk SiO2. The positronium fraction was monitored throughout but was found to be independent of the oxidation duration. It was found that the chemical structure of the surface, in particular the oxidized Si-OH bond, was correlated with the positron annihilation mode and to the level of observed Doppler broadening.

Interadsorbate vibrational energy flow on stepped vicinal H/Si(111) surfaces

We report direct measurements of the vibrational energy flow among Si-H stretching modes on hydrogen-terminated stepped vicinal Si(111) surfaces using a two-color, infrared pump/sum frequency generation probe scheme in which one vibrational mode is pumped and another one is probed. The results, which follow the vibrational energy equilibration, reveal that interadsorbate energy transfer from the terrace to the step can be the dominant relaxation channel of the terrace oscillators. Two types of surfaces have been examined. Both have monohydride terminated terraces, but one has monohydride and the other dihydride terminated steps. On the dihydride stepped surface, the terrace Si-H vibrational energy is drained by the short lifetime step modes. The energy flow on the dihydride terminated surface occurs between terrace-localized and step-localized modes and can be resolved into a kinetic model of the vibrational energy equilibration process. Stronger interadsorbate dipole couplings, on the monohydride stepped surface, delocalize the terrace and step modes and make it difficult to separate the energy flow from direct Si-H oscillators excitation. We suggest that on this surface there is a rapid equilibration of all the Si-H stretching modes followed by their collective decay. Estimates of dipole-dipole energy transfer rates are consistent with the kinetic model results and confirm the role of dipolar interactions in vibrational energy flow on the hydrogen-terminated silicon surfaces.

Structural and Electrical Characterization of Ultra-Thin SiO2 Grown on Hydrogen-Terminated Silicon Surfaces

The surface microroughness of Si(100) wafers has been studied by FT-IR-ATR. The final wafer clean in an 0.1% HF + 1% H 2 O 2 aqueous solution significantly improves the hydrogenterminated surface morphology as demonstrated by a sharp SiH 2 stretching vibration peak accompanied with the weak SiH and SiH 3 peaks. The ultra-thin gate oxide grown on such surface exhibits nearly ideal tunneling current transport. The cleaning in 4.5% HF reduces the SiH 2 peak height and enhances SiH 3 , making the surface rough. Nevertheless, the tunneling characteristics are hardly influenced with such spectral change.