Hydrogen-terminated Si Substrates Research Articles

Initial growth analysis of ALD Al2O3 film on hydrogen-terminated Si substrate via in situ XPS

The initial stage of Al2O3 films deposited by atomic layer deposition (ALD) on a H-terminated Si(001) wafer was investigated with in situ X-ray photoelectron spectroscopy. At deposition temperatures of 250 °C and 300 °C, no Al peak was detected for the first 10 ALD cycles. An Al peak was first observed at ALD cycle 12, and its intensity increased with increasing ALD cycle number. The 10 initial ALD cycles were defined as an incubation period. The Si 2p and Al 2p narrow spectra indicated that Si sub-oxide formed during the incubation period and trimethylaluminum (TMA) adsorbed on it after the incubation period. The incubation period at a growth temperature of 350 °C decreased to five ALD cycles because the H2O exposure at a higher temperature promoted the oxidation of the H-terminated Si surface and formed Si sub-oxide.

Estimation of Molecular Orientation Using Water Drop Angle

In self-assembled monolayer, SAM, molecules are attracted each other to be closely packed. Such a structural feature causes the stable mechanical, chemical, and thermo-dynamical properties. In addition, recent preparation techniques allowed the direct electronic coupling between organic functionality and Si substrate. As the results, SAMs possess great potential in the fields of electrochemical sensors and molecular electronics. From the viewpoint of application, it is desirable to precisely estimate the molecular orientation in SAMs. In the present study, we propose the structural analysis using water drop angle. To form hexadecyl-SAMs, hydrogen-terminated Si substrate was soaked in 1-hexadecene under UV light for 2 hours. UV light is capable of breaking Si-H bond homolytically, which yields the Si dangling bonds that can act as the starting point for radical chain propagation. When a droplet is attached to SAM and the SAM surface is tilted little by little, the droplet will lunge forward and finally slide downward. The tilting angle when the droplet starts sliding is called ‘water drop angle’, WDA. We measured WDA by an original system, in which sample surface could be simultaneously tilted and rotated from 0 to 90 and from 0 to 360 degrees, respectively. Figure 1(a) shows the typical measurement results for the hexadecyl-SAMs. WDAs were found to considerably change with the rotation angles. In addition, the period in the WDA change was nearly equal to 180 degrees. Here in-plane rotation angle was defined the orientation flat of the wafer, Si(111)<110>, as 0 degree. We believe that the periodic change is in relation to the molecular orientation. Namely, when a sample is tilted in the direction as shown in Fig. 1(b), a droplet is hooked by the SAM molecules, resulting in the large WDAs. On the other hand, in the case of Fig. 1(c), the friction between a droplet and the SAM surface is expected to be weak, corresponding to the small WDAs. The maximum WDAs for the hexadecyl-SAMs were observed at the rotation angles about 210 degrees. To justify these experimental results, it is extremely important to notice that the sterific hindrance between the (Si-C-)C-H bonds of the SAM molecules and the surrounding unreacted Si-H bonds should be minimum in six molecular orientation. As a result, the WDA reasonably exhibited the maximum value at the rotation angles of 210 degrees. In the ECS meeting, we will indicate that our measurement method is useful to estimate the orientation direction of the molecules covalently-bonded to the solid surface. Figure 1

Study on deposition of Al2O3 films by plasma-assisted atomic layer with different plasma sources

In this paper, Al2O3 thin films are deposited on a hydrogen-terminated Si substrate by using two home-built electron cyclotron resonance (ECR) and magnetic field enhanced radio frequency plasma-assisted atomic layer deposition (PA-ALD) devices with Al(CH3)3 (trimethylaluminum, TMA) and oxygen plasma used as precursor and oxidant, respectively. The thickness, chemical composition, surface morphology and group reactions are characterized by in situ spectroscopic ellipsometer, x-ray photoelectric spectroscopy, atomic force microscopy, scanning electron microscopy, a high-resolution transmission electron microscope and in situ mass spectrometry (MS), respectively. We obtain that both ECR PA-ALD and the magnetic field enhanced PA-ALD can deposit thin films with high density, high purity, and uniformity at a high deposition rate. MS analysis reveals that the Al2O3 deposition reactions are not simple reactions between TMA and oxygen plasma to produce alumina, water and carbon dioxide. In fact, acetylene, carbon monoxide and some other by-products also appear in the exhaustion gas. In addition, the presence of bias voltage has a certain effect on the deposition rate and surface morphology of films, which may be attributed to the presence of bias voltage controlling the plasma energy and density. We conclude that both plasma sources have a different deposition mechanism, which is much more complicated than expected.

Electron Cyclotron Resonance Plasma-Assisted Atomic Layer Deposition of Amorphous Al2O3 Thin Films

Without extra heating, Al2O3 thin films were deposited on a hydrogen-terminated Si substrate etched in hydrofluoric acid by using a self-built electron cyclotron resonance (ECR) plasma-assisted atomic layer deposition (ALD) device with Al(CH3)3 (trimethylaluminum; TMA) and O2 used as precursor and oxidant, respectively. During the deposition process, Ar was introduced as a carrier and purging gas. The chemical composition and microstructure of the as-deposited Al2O3 films were characterized by using X-ray diffraction (XRD), an X-ray photoelectric spectroscope (XPS), a scanning electron microscope (SEM), an atomic force microscope (AFM) and a high-resolution transmission electron microscope (HRTEM). It achieved a growth rate of 0.24 nm/cycle, which is much higher than that deposited by thermal ALD. It was found that the smooth surface thin film was amorphous alumina, and an interfacial layer formed with a thickness of ca. 2 nm was observed between the Al2O3 film and substrate Si by HRTEM. We conclude that ECR plasma-assisted ALD can grow Al2O3 films with an excellent quality at a high growth rate at ambient temperature.

Highly reproducible technique for three-dimensional nanostructure fabrication via anodization scanning probe lithography

The effects of coated materials for Si probes on the sizes of line structure by using anodization scanning probe lithography (SPL) are investigated by using atomic force microscopy (AFM). Anodization SPL was carried out on the 1-decane monolayer directly attached to hydrogen-terminated silicon. Gold-coated, diamond-coated, and uncoated Si probes were used as the SPL probes in order to fabricate line structures of silicon oxide. In the cases of Au-coated and uncoated Si probes, the widths of line structures are widely influenced by the changes of scanning rate and applied bias voltage. However, the line width fabricated by use of the diamond-coated probe maintained 15 nm under any condition of scanning rate and applied bias voltage. One of the most narrow and three-dimensional nanostructures is found to be successfully fabricated on the hydrogen-terminated Si substrates when the diamond-coated probe was used as a probe of anodization SPL. In addition, this result indicated that high reproducibility of oxide nanostructures is attainable by anodization SPL used the diamond-coated probe.

Scanning probe anodization patterning of Si substrates covered with a self-assembled monolayer dependent on surface hydrophilicity

Contact-mode atomic force microscopy (AFM)-based anodization patterning was performed on silicon (Si) substrates covered with a self-assembled monolayer (SAM) in order to investigate effects of relative humidity (RH), surface wetability of the SAM, and probe-tip material on widths and heights of drawn lines. Three types of SAMs, that is, methyl-terminated, ester-terminated, and carboxyl-terminated SAMs, were prepared on hydrogen-terminated Si substrates by a thermal activation method. These SAMs were covalently fixed on the Si substrates through Si–C bonds without an interfacial oxide layer between the SAM and Si. Rh-coated and boron-doped conductive diamond-coated AFM probe tips were used for patterning with a positive sample bias of 10V. Consequently, the region scanned with the AFM probe became protruded due to degradation of the SAM and anodization of Si. When the Rh-coated Si tip was used, the width of the protruded line increased with increasing RH on each SAM sample. The linewidth on the most hydrophobic methyl-terminated SAM was narrowest, while that on the most hydrophilic carboxylic SAM was widest. However there was no distinct difference in the pattern width at the patterning under low RH of 10%. In the case of patterning with the boron-doped conductive diamond-coated tip on the ester SAM, the width of the patterned line showed no clear increase with increasing RH. These pattern width changes were discussed in terms of the size of adsorbed water meniscus at the AFM-tip/sample junction as confirmed by force curve measurements; in the case of the boron-doped conductive diamond-coated tip which is rather more hydrophobic than the Rh-coated Si tip, the size of water meniscus hardly affected RH.

Comparative study of the molecular aggregation state of alkyl organic monolayers preparedon Si and hydrogen-terminated Si substrates

A comparative study of the molecular aggregation states ofn-octadecyltrichlorosilane (OTS) and octadecene monolayers was carried out based onwater contact angle, x-ray reflectivity (XR), grazing incidence x-ray diffraction(GIXD), and Fourier transform infrared spectroscopy (FT-IR) measurements.Water contact angle, XR, and FT-IR measurements revealed that the packingdensity and orientational order of the OTS were higher than those of octadecene.The OTS monolayer was in a hexagonal crystalline state with (10) spacing of0.417 nm, whereas the octadecene monolayer was in an amorphous state. The growthmechanisms of the OTS and octadecene monolayers are also proposed. The alkylmolecular aggregation states strongly depended on the chemical bonding stateat the interface between the alkyl organic molecule and the substrate surface.

Synthesis of silicon dioxide film using high-concentration ozone and evaluation of the film quality

We investigated the ozone oxidation characteristics on a hydrogen-terminated Si substrate. A high-concentration ozone gas generator with an ozone condensation unit was specially designed and assembled for this study. During the oxidation by ozone with the concentration of 25 vol.% in the temperature range from 340°C to 625°C at 8 Torr (1.1 kPa), the formed oxide film thickness increased with oxidation time in accordance with the parabolic law, which suggests a diffusion-controlled step, while the oxidation by pure oxygen attained saturated states within 3 min of initiating oxidation. The activation energy for parabolic constants in the ozone oxidation was determined to be 0.52 eV. This value is much smaller than the activation energy for dry oxidation with oxygen, while it is almost the same as that in the plasma oxidation with the mixture of rare gas and oxygen. Moreover, the quality of the ozone oxidation film was evaluated by estimating the amount of suboxides (Si3++Si2++Si+) using x-ray photoelectron spectroscopy (XPS) analysis and the compressive stress using Fourier transform infrared (FT-IR) spectroscopic analysis. Both results showed that the quality of film subjected to ozone oxidation at 500°C is equal or superior to that of the film subjected to pyrogenic oxidation at 750°C in spite of the faster oxidation rate, and thus, the significant advantages of ozone oxidation at low oxidation temperatures could be confirmed.

Influence of oxidant source on the property of atomic layer deposited Al 2O 3 on hydrogen-terminated Si substrate

Al 2O 3 thin films were deposited on hydrogen-terminated Si substrate using atomic layer deposition (ALD) technique with tri-methylaluminum (TMA) and an oxidant source of H 2O vapor, O 2 plasma, or O 3. Substrate temperature was maintained at 350 °C when the Al 2O 3 films were grown with the oxidant sources of H 2O vapor and O 3, and with the oxidant source of O 2 plasma, Al 2O 3 films were deposited at the substrate temperature of 200 °C. Growth rates of Al 2O 3 films on HF-cleaned Si surface were saturated at 0.08, 0.14, and 0.06 nm/cycle for H 2O vapor, O 2 plasma, and O 3, respectively. Equivalent oxide thickness (EOT) and leakage current vs. physical thickness of atomic layer deposited Al 2O 3 films grown with various oxidant sources were also measured in this study. To investigate the main cause of different EOT with oxidant sources, interfacial properties were examined by using transmission electron microscopy (TEM) and X-ray photoelectron microscopy (XPS). In the TEM analysis, interfacial layers with the thickness of about 1.7 and 1.3 nm were observed in as-deposited Al 2O 3 films grown using O 2 plasma and O 3. We confirmed that the interfacial layers were mainly composed of SiO x in the XPS depth analysis. Using angle resolved X-ray photoelectron spectroscopy, effect of annealing on the interfacial structure of Al 2O 3 films grown with O 3 and O 2 plasma was also studied, and we found that after annealing, the peak corresponding to silicon suboxide and Al-silicate disappeared and fully oxidized Si 4+ increased.

Direct growth of CdTe(1 0 0) epilayers on Si(1 0 0) substrate by hot wall epitaxy

Strong preferential (1 0 0) orientation is observed for the first time in the CdTe thin films directly grown on Si(1 0 0) substrates without any buffer layers. This result is attributed to the fact that the epilayer is grown directly on the hydrogen-terminated Si substrate without any preheating treatment. The crystal qualities of CdTe(1 0 0)/Si(1 0 0) and CdTe(1 1 1)/Si(1 1 1) epilayers obtained at the same growth conditions were compared. Atomic force microscopy observations reveal different surface morphology at the early stages of the crystal growth for CdTe(1 1 1)/Si(1 1 1) and CdTe(1 0 0)/Si(1 0 0) epilayers, implying that they are governed by different growth mechanisms. The nucleation of CdTe(1 0 0)/Si(1 0 0) starts with 3D islands having a dome shape. It is demonstrated that the height and diameter distributions narrow and the aspect ratio decreases with decreasing the growth time. The crystallinity of CdTe(1 0 0)/Si(1 0 0) epilayers is inferior to that of CdTe(1 1 1)/Si(1 1 1) due to a double-domain structure.

Real-time stress/strain measurement during growth of Sr and SrO epilayer on H-terminated Si

We use real-time measurement of substrate curvature and reflection high energy electron diffraction (RHEED) methods to observe the film stress/strain behavior during epitaxial growth of Sr metal films and their oxidation process. The film stress and the thickness of the strained layer are minimized by using hydrogen-terminated Si substrates. The use of hydrogen-terminated Si(111) enables the growth of stress-free Sr layers from a thickness of 1.05 nm, whereas the films directly deposited on Si(111) 7×7 shows the presence of a strained layer with a thickness of 3.16 nm. We also observed that the intrinsic stress in the films changes drastically from compressive to tensile during the oxidation process, resulting from the variation in lattice mismatch.

STM study of structural changes on Si(100)2×1-Sb surface induced by atomic hydrogen

Using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED), we have studied the structural changes of the Si(100)2×1-Sb surface caused by hydrogen adsorption at both room temperature (RT) and 300°C. We have found that the ordering of a 2×1-Sb surface is more stable against atomic hydrogen exposure at 300°C than at RT, and that some Sb atoms desorb during atomic hydrogen exposure at 300°C. However, upon hydrogen exposure at both temperatures, we have observed neither three-dimensional islands nor the hydrogen terminated Si substrate which were reported for hydrogen interaction with the other metal/Si systems. On the 2×1-Sb surface exposed to atomic hydrogen of 1000 L at RT followed by 550°C annealing, long bright lines similar to those reported for the Bi/Si(100) system have also been found.

Elastic scattering of Si 2p photoelectrons in silicon oxide

The cross-sections for elastic and inelastic scattering of Si 2p photoelectrons in silicon oxide were uniquely determined from the analysis of oxidation-induced changes in Si 2p photoelectron diffraction patterns arising from single crystalline Si substrate under the assumption that the structure of hydrogen-terminated Si substrate does not change by the oxidation. It was found from the Monte Carlo calculation of path of elastically and inelastically scattered Si 2p photoelectrons for the simulation of oxidation-induced changes in photoelectron diffraction patterns that the total elastic scattering cross-section and the inelastic scattering cross-section are 1.54×10 −20 and 1.26×10 −20 m 2, respectively.

Effects of accelerated ion beam deposition to form thin metal films on silicon

Cu or Ag thin film was fabricated on Si and hydrogen-terminated Si substrates at room temperature, by the ion-beam deposition method. A difference based on the substrates between Si and hydrogen terminated Si was found. With the former substrate, the Schottky-barrier height decreased with an increase in film crystallization, while in the case of the latter substrate, there was no decrease. The reason is believed to be because the latter interface was not affected by the ion's kinetic energy on the hydrogen terminated surface of the substrate.

Pulsed Laser Deposition and Characterization of Novel Cu/TiN/Si(100) Heterostructures Grown VIA Domain Epitaxy

Laser physical vapor deposition (LPVD) has been used to study the growth morphology of copper thin films on siliconi 100) substrates with epitaxially grown titanium nitride buffer layers. The epitaxial TiN film was grown on hydrogen-terminated Si substrate at 600°C. The Cu films were grown on TiN/Si at different substrate temperatures ranging from room temperature to 600°C in vacuum (∼10-7 torr) with a pulsed KrF excimer laser (τ = 248 nm, ι = 25 ns). The pulse repetition rates were 15 and 30 Hz. The X-ray diffraction (XRD) results indicate that below 200°C, the Cu film has a mosaic spread of (001) textures of about 2.3° and that a small fraction (0.05 ) is of (111) textures. But above 200°C there is only (001) textures Cu of about 0.42° to 0.40° depending on the substrate temperature (200° - 600°C). The Rutherford backscattering (RBS) spectra show that the TiN has a yield (xmin) of 13.5 % whereas that for Cu is 70 %. A detail high resolution transmission electron microscopy (HRTEM) was performed to reveal the growth morphology of Cu films with substrate temperature. It was observed that at 15 Hz, Cu grows three-dimensionally on TiN with the island sizes ranging from 0.3–1.5 μm depending upon the substrate temperature. All of these islands grow epitaxially on TiN giving rise to Cu ║ TiN ║ Si. At 30 Hz, there is a uniform growth of epitaxial cube-on-cube Cu film on TiN/Si. The Cu/TiN and TiN/Si interfaces are quite abrupt with absolutely no indication of interfacial reactions between them. Moire fringe spacings suggest that both TiN and Cu unit cells are completely relaxed. Although there exist a large lattice misfit between Cu and TiN (15.8%) and as well as between TiN and Si (24.6%), it has been possible to grow epitaxial Cu/TiN/Si(100) heterostructures by means of domain epitaxial growth. In this case, there are 4-to-3 match in unit cells for TiN/Si structure and 7-to-6 match for Cu/TiN structure giving rise to remaining lattice misfits of only 4.0% and 0.6% respectively.

Epitaxial growth of vanadyl-phthalocyanine ultrathin films on hydrogen-terminated Si(111) surfaces

Ultrathin films of vanadyl phthalocyanine (VOPc) have been grown on hydrogen-terminated Si(111) surfaces by molecular beam epitaxy. Epitaxial growth was examined on the two types of substrates. Reflection high energy electron diffraction studies have revealed that VOPc molecules form commensurate lattices [−3[3 1]4], [−1[4 3]3], and [1[4 4]1] on the surface terminated homogeneously with monohydride. On the other hand, epitaxial growth did not occur on the surface terminated with the mixture of polyhydride. Homogeneity and microscopic flatness of the substrate surface seem to be important factors for the epitaxial growth of VOPc on the hydrogen-terminated Si substrate.