Growth of oriented crystalline Ag nanoislands on air-exposed Si(0 0 1) surfaces

Growth of Ag islands under ultrahigh vacuum condition on air-exposed Si(0 0 1)-(2 × 1) surfaces has been investigated by in-situ reflection high energy electron diffraction (RHEED). A thin oxide is formed on Si via exposure of the clean Si(0 0 1)-(2 × 1) surface to air. Deposition of Ag on this oxidized surface was carried out at different substrate temperatures. Deposition at room temperature leads to the growth of randomly oriented Ag islands while well-oriented Ag islands, with (0 0 1)_Ag||(0 0 1)_Si, [1 1 0]_Ag||[1 1 0]_Si, have been found to grow at substrate temperatures of ≥350 °C in spite of the presence of the oxide layer between Ag islands and Si. The RHEED patterns show similarities with the case of Ag deposition on H-passivated Si(0 0 1) surfaces.     

Electron and lattice structure of ultra thin Ag films on Si(1 1 1) and Si(0 0 1)

We studied the low temperature (T ? 130 K) growth of Ag on Si(0 0 1) and Si(1 1 1) flat surfaces prepared by Si homo epitaxy with the aim to achieve thin metallic films. The band structure and morphology of the Ag overlayers have been investigated by means of XPS, UPS, LEED, STM and STS. Surprisingly a (√3 × √3)R30° LEED structure for Ag films has been observed after deposition of 2-6 ML Ag onto a Si(1 1 1)(√3 × √3)R30°Ag surface at low temperatures. XPS investigations showed that these films are solid, and UPS measurements indicate that they are metallic. However, after closer STM studies we found that these films consists of sharp Ag islands and (√3 × √3)R30°Ag flat terraces in between. On Si(0 0 1) the low-temperature deposition yields an epitaxial growth of Ag on clean Si(0 0 1)-2 × 1 with a twinned Ag(1 1 1) structure at coverage’s as low as 10 ML. Furthermore the conductivity of few monolayer Ag films on Si(1 0 0) surfaces has been studied as a function of temperature (40-300 K).     

Growth of (√3 × √3)-Ag and (1 1 1) oriented Ag islands on Ge/Si(1 1 1) surfaces

We have studied the growth of Ag on Ge/Si(1 1 1) substrates. The Ge/Si(1 1 1) substrates were prepared by depositing one monolayer (ML) of Ge on Si(1 1 1)-(7 × 7) surfaces. Following Ge deposition the reflection high energy electron diffraction (RHEED) pattern changed to a (1 × 1) pattern. Ge as well as Ag deposition was carried out at 550 °C. Ag deposition on Ge/Si(1 1 1) substrates up to 10 ML has shown a prominent (√3 × √3)-R30° RHEED pattern along with a streak structure from Ag(1 1 1) surface. Scanning electron microscopy (SEM) shows the formation of Ag islands along with a large fraction of open area, which presumably has the Ag-induced (√3 × √3)-R30° structure on the Ge/Si(1 1 1) surface. X-ray diffraction (XRD) experiments show the presence of only (1 1 1) peak of Ag indicating epitaxial growth of Ag on Ge/Si(1 1 1) surfaces. The possibility of growing a strain-tuned (tensile to compressive) Ag(1 1 1) layer on Ge/Si(1 1 1) substrates is discussed.     

Effect of Pb adatom flux rate on adlayer coverage for Stranski-Krastanov growth mode on Si(1 1 1)7 × 7 surface

Lead (Pb) has been a prototypical system to study diffusion and reconstruction of silicon surfaces. However, there is a discrepancy in literature regarding the critical coverage at which island formation takes place in the Stranski-Krastanov (S-K) mode. We address this issue by studying the initial stages of evolution of the Pb/Si(1 1 1)7 × 7 system by careful experiments in ultra-high vacuum with in situ characterization by auger electron spectroscopy, electron energy loss spectroscopy and low-energy electron diffraction. We have adsorbed Pb onto clean Si(1 1 1 )7 × 7 surface with sub-monolayer control at different flux rates of 0.05 ML/min, 0.14 ML/min and 0.22 ML/min, at room temperature. The results clearly show that the coverage of the Pb adlayer before the onset of 3D Pb islands in the S-K mode depends on the flux rates. LEED results show the persistence of the (7 × 7) substrate reconstruction until the onset of the island formation, while EELS results do not show any intermixing at the interface. This suggests that the flux rates influence the kinetics of growth and the passivation of dangling bonds to result in the observed rate-dependent adlayer coverages.     

Ga-induced superstructures on the Si(1 1 1) 7 × 7 surface

Monolayer Ga adsorption on Si surfaces has been studied with the aim of forming p-delta doped nanostructures. Ga surface phases on Si can be nitrided by N₂⁺ ion bombardment to form GaN nanostructures with exotic electron confinement properties for novel optoelectronic devices. In this study, we report the adsorption of Ga in the submonolayer regime on 7 × 7 reconstructed Si(1 1 1) surface at room temperature, under controlled ultrahigh vacuum conditions. We use in-situ Auger electron spectroscopy, electron energy loss spectroscopy and low energy electron diffraction to monitor the growth and determine the properties. We observe that Ga grows in the Stranski-Krastanov growth mode, where islands begin to form on two flat monolayers. The variation in the dangling bond density is observed during the interface evolution by monitoring the Si (LVV) line shape. The Ga adsorbed system is subjected to thermal annealing and the residual thermal desorption studied. The difference in the adsorption kinetics and desorption dynamics on the surface morphology is explained in terms of strain relaxation routes and bonding configurations. Due to the presence of an energetic hierarchy of residence sites of adatoms, site we also plot a 2D phase diagram consisting of several surface phases. Our EELS results show that the electronic properties of the surface phases are unique to their respective structural arrangement.     

K-induced surface structural change of Si(1 1 1)-7 × 7 probed by second-harmonic generation

The growth of thin K films on Si(1 1 1)-7 × 7 has been investigated by selecting the input and output polarizations of second-harmonic generation (SHG) at room temperature (RT) and at an elevated temperature of 350 °C. The SH intensity at 350 °C showed a monotonic increase with K coverages up to a saturated level, where low energy electron diffraction (LEED) showed a 3 × 1 reconstructed structure. The additional deposition onto the K-saturated surface at 350 °C showed only a marginal change in the SH intensity. These variations are different from the multi-component variations up to 1 ML and orders of magnitude increase due to excitation of plasmons in the multilayers at RT. The variations of SHG during desorption of K at 350 °C showed a two-step decay with a marked shoulder which most likely corresponds to the saturation K coverage of the Si(1 1 1)-3 × 1-K surface. The dominant tensor elements contributing to SHG are also identified for each surface.     

Interaction of copper with sulfur on the sulfur-terminated Si(1 1 1)-(7 × 7) surface

The adsorption of S₂ on the Si(1 1 1)-(7 × 7) surface and the interaction of copper and sulfur on this sulfur-terminated Si(1 1 1) surface have been studied using synchrotron irradiation photoemission spectroscopy and scanning tunneling microscopy. The adsorption of S₂ at room temperature results in the passivation of silicon dangling bonds of Si(1 1 1)-(7 × 7) surface. Excessive sulfur forms S_n species on the surface. Copper atoms deposited at room temperature directly interact with S-adatoms through the formations of Cu-S bonds. Upon annealing the sample at 300 °C, CuS_x nanocrystals were produced on the sulfur-terminated Si(1 1 1) surface.     

Growth mode and novel structure of ultra-thin KCl layers on the Si(1 0 0)-2 × 1 surface

This study investigates ultra-thin potassium chloride (KCl) films on the Si(1 0 0)-2 × 1 surfaces at near room temperature. The atomic structure and growth mode of this ionic solid film on the covalent bonded semiconductor surface is examined by synchrotron radiation core level photoemission, scanning tunneling microscopy and ab initio calculations. The Si 2p, K 3p and Cl 2p core level spectra together indicate that adsorbed KCl molecules at submonolayer coverage partially dissociate and that KCl overlayers above one monolayer (ML) have similar features in the valance band density of states as those of the bulk KCl crystal. STM results reveal a novel c(4 × 4) structure at 1 ML coverage. Ab initio calculations show that a model that comprises a periodic pyramidal geometry is consistent with experimental results.     

Structures and stability of Ag clusters on Ag(1 1 1) and Ni(1 1 1) surfaces

The structures of the lowest total energy for small Ag_N clusters with N = 2-20, which are grown on Ag(1 1 1) and Ni(1 1 1) surfaces, have been determined using a combination of the embedded-atom method and the basin-hopping algorithm. It is found that the particularly stable Ag clusters with N<18 have similar geometries on both surfaces when comparing clusters of the same size. On the other hand, the geometries of the less stable Ag clusters in the same size range differ for the two surfaces. From N?18, the sizes of the particularly stable structures are different for the two different substrates. Due to the large size mismatch of the two types of atoms it is energetically unfavorable for Ag to form a pseudomorphic monolayer structures on Ni(1 1 1) and there is considerable strain produced at the interface. The effect of this strain and the increased adatom-substrate interactions lead to irregular and elongated structures of the adsorbed Ag clusters.     

Metal organic chemical vapor deposition of ultrathin ZrO2 films on Si(1 0 0) and Si(1 1 1) studied by electron spectroscopy

The growth of ultrathin ZrO₂ films on Si(1 0 0)-(2 × 1) and Si(1 1 1)-(7 × 7) has been studied with core level photoelectron spectroscopy and X-ray absorption spectroscopy. The films were deposited sequentially by chemical vapor deposition in ultra-high vacuum using zirconium tetra-tert-butoxide as precursor. Deposition of a > 50 Å thick film leads in both cases to tetragonal ZrO₂ (t-ZrO₂), whereas significant differences are found for thinner films. On Si(1 1 1)-(7 × 7) the local structure of t-ZrO₂ is not observed until a film thickness of 51 Å is reached. On Si(1 0 0)-(2 × 1) the local geometric structure of t-ZrO₂ is formed already at a film thickness of 11 Å. The higher tendency for the formation of t-ZrO₂ on Si(1 0 0) is discussed in terms of Zr-O valence electron matching to the number of dangling bonds per surface Si atom. The Zr-O hybridization within the ZrO₂ unit depends furthermore on the chemical composition of the surrounding. The precursor t-butoxy ligands undergo efficient C-O scission on Si(1 0 0), leaving carbonaceous fragments embedded in the interfacial layer. In contrast, after small deposits on Si(1 1 1) stable t-butoxy groups are found. These are consumed upon further deposition. Stable methyl and, possibly, also hydroxyl groups are found on both surfaces within a wide film thickness range.     

Photoemission study of cobalt interaction with the oxidized Si(1 0 0)2 × 1 surface

The interaction of cobalt atoms with an oxidized Si(1 0 0)2 × 1 surface was studied by photoelectron spectroscopy with synchrotron radiation at room and elevated temperatures. The SiO_x layer grown in situ on the crystal surface was ∼0.3 nm thick, and the amount of deposited cobalt was varied within eight atomic layers. It was found that Co atoms could penetrate under the SiO_x layer even at room temperature in the initial growth. As the Co amount increased, a ternary Co-O-Si phase was formed at the interface, followed by a Co-Si solid solution. Silicide synthesis associated with the decomposition of these phases started under the SiO_x layer at ∼250 °C, producing cobalt disilicide with a stable CaF₂-type of structure.     

Preferential heights in the growth of Ag islands on Si(1 1 1)-(7 × 7) surfaces

Growth behavior of thin Ag films on Si substrates at room temperature has been investigated by scanning tunneling microscopy and reflection high energy electron diffraction. In the layer-plus-island growth Ag islands show strongly preferred atomic scale heights and flat top. At low coverage (1 ML), islands containing two atomic layers of Ag are overwhelmingly formed. At higher coverages island height distribution shows strong peaks at relative heights corresponding to an even number (2, 4, 6, …) of Ag atomic layers. Beyond some coverage the height preference vanishes due to the appearance of screw dislocations and spiral growth.     

Negative differential resistance of TEMPO molecules on Si(1 1 1)

Negative differential resistance (NDR) has been observed for individual 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) molecules on Si(1 1 1) in ultra high vacuum (UHV) scanning tunneling microscopy (STM) and spectroscopy (STS) measurements at room temperature. NDR effects were observed exclusively at negative bias voltage using an n-type Si(1 1 1) sample. At 77 K no NDR effects were observed, but the I(V) curves were similar in shape to those recorded on bare Si(1 1 1) sites. TEMPO was observed to adsorb preferentially at corner adatom sites of the Si(1 1 1)-7 × 7 structure. Although the Si(1 1 1)-7 × 7 reconstruction was conserved, local defects were frequently observed in the vicinity of the TEMPO adsorbates.     

Co growth on Si(0 0 1) and Si(1 1 1) surfaces: Interfacial interaction and growth dynamics

In situ X-ray photoelectron spectroscopy (XPS) and ex situ atomic force microscopy (AFM) were used to study the growth of thin cobalt films at room temperature (RT) on both clean and H-terminated Si(0 0 1) and Si(1 1 1) surfaces. The growth proceeds by first forming an initial CoSi₂-like phase at the growth front of the Si substrate. With increasing Co coverage the interfacial layer composition becomes richer in Co and eventually a metallic Co film is formed on top. Hydrogen termination of the Si surface did not suppress the reaction of Co and Si. A pseudo-layer-by-layer growth mode is proposed to describe the growth of Co on H-terminated Si surfaces, while closed-packed small island growth occurs on clean Si surfaces. The difference in growth mode can be attributed to the increase in the surface mobility of Co adatoms in the presence of hydrogen.     

Hydrogen desorption kinetics and band bending for 6H-SiC(0 0 0 1) surfaces

The desorption kinetics of hydrogen from polished 6H-SiC(0 0 0 1) surfaces exposed to various sources of hydrogen have been determined using temperature programmed desorption (TPD). For (3 × 3) 6H-SiC(0 0 0 1) surfaces prepared via annealing and cooling in SiH₄, desorption of 0.2 ± 0.05 monolayer of molecular hydrogen was observed to occur at ≈590 °C. This β₁ H₂ desorption peak exhibited second order kinetics with an activation energy of 2.4 ± 0.2 eV. For (3 × 3) 6H-SiC surfaces exposed to atomic hydrogen generated via either a hot rhenium filament or remote hydrogen plasma, low energy electron diffraction patterns showed an eventual conversion back to (1 × 1) symmetry. Spectra acquired using Auger electron and X-ray photoelectron spectroscopies revealed that the atomic hydrogen exposure removed the excess Si. Photoelectron spectroscopy results also showed a 0.5 eV increase in binding energy for the Si2p and C1s core levels after removal of the Si-Si bilayer that is indicative of a decrease in band bending at the SiC surface. TPD from the (3 × 3) 6H-SiC(0 0 0 1) surfaces exposed to atomic hydrogen showed substantially more molecular hydrogen desorption (1-2 ML) through the appearance of a new desorption peak (β_2,3) that started at ≈200 °C. The β_2,3 peak exhibited second order desorption kinetics and a much lower activation energy of 0.6 ± 0.2 eV. A third smaller hydrogen desorption state was also detected in the 650-850 °C range. This last feature could be resolved into two separate desorption peaks (α₁ and α₂) both of which exhibited second order kinetics with activation energies of 4.15 ± 0.15 and 4.3 ± 0.15 eV, respectively. Based on comparisons to hydrogen desorption from Si and diamond surfaces, the β and α desorption peaks were assigned to hydrogen desorption from Si and C sites, respectively.     

Low reactivity of molecular oxygen with Si(1 1 1)-c(2 × 8)

The adsorption of molecular oxygen on the c(2 × 8) reconstruction of quenched Si(1 1 1) surfaces has been studied at the atomic scale using scanning tunneling microscopy (STM) at room temperature (RT). It has been found that clean well reconstructed c(2 × 8) adatoms do not react with O₂ molecules but that a limited oxidation can start where adatom sites arranged in reconstructed structures are present. Comparison between O₂ adsorption on Si(1 1 1)-c(2 × 8) and Si(1 1 1)-7 × 7 reconstructions coexisting on the same quenched silicon surface has been carried out in detail. For each atomic site present on the surface the variation of reacted sites with exposure has been measured. For low O₂ exposures, bright and dark oxygen induced sites appear on the Si(1 1 1)-7 × 7, while Si(1 1 1)-c(2 × 8) does not oxidized at all. At high O₂ exposures, large oxidation areas have spread on the 7 × 7 reconstruction, preferentially on the faulted halves of the unit cell, and smaller oxidation areas induced by topological defects have grown all around clean un-reacted c(2 × 8) regions.     

Bimodal growth of manganese silicide on Si(1 0 0)

Two different growth modes of manganese silicide are observed on Si(1 0 0) with scanning tunneling microscopy. 1.0 and 1.5 monolayer Mn are deposited at room temperature on the Si(1 0 0)-(2 × 1) substrate. The as-grown Mn film is unstructured. Annealing temperatures between room temperature and 450 °C lead to small unstructured clusters of Mn or Mn_xSi_y. Upon annealing at 450 °C and 480 °C, Mn reacts chemically with the Si substrate and forms silicide islands. The dimer rows of the substrate become visible again. Two distinct island shapes are found and identified as MnSi and Mn₅Si₃.     

An ab initio-based approach to adsorption-desorption behavior of Si adatoms on GaAs(1 1 1)A-(2 × 2) surfaces

The adsorption-desorption behavior of Si adatoms on GaAs(1 1 1)A-(2 × 2) surfaces is investigated using our ab initio-based approach, in which adsorption and desorption behavior of Si adatoms is described by comparing the calculated desorption energy obtained by total-energy electronic-structure calculations with the chemical potential estimated by quantum statistical mechanics. We find that the Si adsorption at the Ga-vacancy site on the (2 × 2) surfaces with As adatoms occurs less than 1140-1590 K while the adsorption without As adatom does less than 630-900 K. The change in adsorption temperature of Si adatoms by As adatoms is due to self-surfactant effects of As adatoms: the promotion of the Si adsorption triggered by As adatoms is found to be interpreted in terms of the band-energy stabilization. Furthermore, the stable temperature range for Si adsorbed surfaces with As adatoms agrees with the experimental results. The obtained results provide a firm theoretical framework to clarify n-type doping processes during GaAs epitaxial growth.     

A scanning tunneling microscopy study of PH3 adsorption on Si(1 1 1)-7 × 7 surfaces, P-segregation and thermal desorption

PH₃ adsorption on Si(1 1 1)-7 × 7 was studied after various exposures between 0.3 and 60 L at room temperature by means of scanning tunneling microscopy (STM). PH₃-, PH₂-, H-reacted, and unreacted adatoms can be identified by analyzing empty-state STM images at different sample biases. PH_x-reacted rest-atoms can be observed in empty-state STM images if neighboring adatoms are hydrogen terminated. Most of the PH₃ adsorbs dissociatively on the surface, generating H- and PH₂-adsorbed rest-atom and adatom sites. Dangling-bonds at rest-atom sites are more reactive than adatom sites and the faulted half of the 7 × 7 unit cell is more reactive than the unfaulted half. Center adatoms are overwhelmingly preferred over corner adatoms for PH₂ adsorption. The saturation P coverage is ∼0.18 ML. Annealing of PH₃-reacted 7 × 7 surfaces at 900 K generates disordered, partially P-covered surfaces, but dosing PH₃ at 900 K forms P/Si(1 1 1)- surfaces. Si deposition at 510 K leaves disordered clusters on the surface, which cannot be reordered by annealing up to 800 K. However, annealing above 900 K recreates P/Si(1 1 1)- surfaces. Surface morphologies formed by sequential rapid thermal annealing are also presented.