Valence band studies of the formation of ultrathin pure silicon nitride films on Si(1 0 0)

The growth of ultrathin films of Si₃N₄ directly on Si surfaces is studied with valence band photoemission. The information from these studies about the growth mechanism and the changes of the electronic structure is enhanced by the use of various photon energies with synchrotron radiation. The silicon nitride films are grown isothermally on the Si(1 0 0) and Si(1 1 1) surfaces by reactions with atomic N. The atomic nitrogen is produced by using a remote, microwave excited nitrogen plasma. The growth under these conditions was earlier shown to be self limiting. The details in the valence band spectra are identified and resolved with numerical methods, and followed systematically during the growth. Thus the identification of Si surface states, Si-nitride interface states and bulk nitride states becomes possible. The previously obtained separation between amorphous and crystalline growth occurring around 500 °C is further supported in the present studies.     

Silicon carbide nanocrystals growth on Si(1 0 0) and Si(1 1 1) from a chemisorbed methanol layer

SiC nanocrystals are grown at high temperature on Si(1 0 0) and Si(1 1 1) surfaces starting from a chemisorbed layer of methanol. The decomposition of this layer allows to have a well defined amount of carbon to feed SiC growth. Nanocrystals ranging from 10 nm to 50 nm with density from 100 μm⁻² to 1500 μm⁻² are obtained, and the total volume of produced SiC corresponds to carbon provided by the chemisorbed organic layer. Large differences in nanocrystal size and density, as well as in surface roughness, are observed depending on substrate orientation. The internal structure, crystallinity and epitaxy of nanocrystals grown on Si(1 0 0) are studied using cross-sectional transmission electron microscopy (XTEM), methanol adsorption and surface evolution using scanning tunnelling microscopy (STM). The joint application of XTEM and STM techniques allows a complete characterization of the geometry and chemical composition of these nanostructures.     

The mechanism of amine formation on Si(1 0 0) activated with chlorine atoms

The dissociation of NH₃ on a Si(1 0 0) surface activated with Cl atoms was investigated using X-ray photoelectron spectroscopy. Gas phase UV-Cl₂ (0.1-10 Torr Cl₂ for 10-600 s under 1000 W Xe lamp illumination) completely replaced the H-termination on aqueous-cleaned Si(1 0 0) with 0.82 ± 0.06 ML of Cl at 298 K. A single spin-orbit split Cl 2p doublet indicated that the Cl atoms were bound to Si dimer atoms, forming silicon monochloride (Cl-Si-Si-Cl). Exposing the Cl-terminated surface at 348 K to NH₃ (1-1000 Torr for 5-60 min) replaced one Cl atom with one N atom up to a coverage of 0.33 ± 0.02 ML. Cl atoms lowered the activation energy barrier for reaction to form a primary amine (Si-NH₂). Oxygen was coadsorbed due to competition by H₂O contamination. The presence of Cl on the surface even after high NH₃ exposures is attributed to site blocking and electrostatic interactions among neighboring Cl-Si-Si-NH₂ moieties. The results demonstrate a low temperature reaction pathway for depositing N-bearing molecules on Si surfaces.     

Polarity inversion of GaN(0 0 0 1) surfaces induced by Si adsorption

We employ density-functional theory within the local-density approximation to study the structural and electronic properties of Si monolayers adsorbed at GaN(0 0 0 1) surfaces. We find that when the N atoms reside in the outermost layer of the surface, the (0 0 0 1) surface converts into a surface, giving rise to a polarity inversion. We explain this inversion as a charge compensation effect between the N dangling bonds states at the outermost surface layer and the Si donor state in the subsurface layer.     

High resolution X-ray diffraction of GaN grown on Si (1 1 1) by MOVPE

High temperature GaN layers have been grown on Si (1 1 1) substrate by metalorganic vapor phase epitaxy (MOVPE). AlN was used as a buffer layer and studied as a function of thickness and growth temperature. The growth was monitored by in situ laser reflectometry. High resolution X-ray diffraction (HRXRD) revealed that optimized monocrystalline GaN was obtained for a 40 nm AlN grown at 1080 °C. This is in good agreement with the results of morphological study by scanning electron microscopy (SEM) and also confirmed by atomic force microscopy (AFM) observations. The best morphology of AlN with columnar structure and lower rms surface roughness is greatly advantageous to the coalescence of the GaN epilayer. Symmetric and asymmetric GaN reflections were combined for twist and stress measurements in monocrystalline GaN. It was found that mosaicity and biaxial tensile stress are still high in 1.7 μm GaN. Curvature radius measurement was also done and correlated to the cracks observations over the GaN surface.     

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).     

DLTS characterization of silicon nitride passivated AlGaN/GaN heterostructures

Passivating the ungated surface of AlGaN/GaN HEMTs with silicon nitride (SiN) is effective in improving the microwave output power performances of these devices. However, very little information is available about surface states in GaN-based HEMTs after SiN passivation. In this work we investigate AlGaN/GaN HEMTs structures having either metal–semiconductor or metal–SiN–semiconductor gate contacts. In short gate devices conductance DLTS measurements point out a hole-like peak that shows an anomalous behaviour and can be ascribed to surface states in the access regions of the device. In insulated gate HEMTs a band of levels is detected and ascribed to surface states, whose energy ranges from 0.14 to 0.43 eV. Capacitance–voltage measurements allow us to point out the existence of a second band of interface states deeper in energy than the former one. This band is responsible for slow transients observed in the characteristics of the insulated gate FAT-HEMT.     

Initial stage of nitridation on Si(1 0 0) surface using low-energy nitrogen ion implantation

The initial stage of the thermal nitridation on Si (1 0 0)-2 × 1 surface with the low-energy nitrogen ion (200 eV) implantation was studied by photoemission spectroscopy (PES). The formation of nitride was shown the different characteristics depending on the annealing temperature. The disordered surface at room temperature was changed to 2 × 1 periodicity with the low-energy electron diffraction (LEED) as increasing the nitridation temperature. By decomposition of Si 2p spectrum, we can identify the three subnitrides (Si¹⁺, Si²⁺, and Si³⁺). By changing the take-off angle of the Si 2p, we can increase surface sensitivity and estimate that Si¹⁺, Si²⁺ and Si³⁺ are the interface states.     

Atomic structure of thin dysprosium-silicide layers on Si(1 1 1)

We report on scanning tunneling microscopy results of thin dysprosium-silicide layers formed on Si(1 1 1). In the submonolayer regime, both a and a 5 × 2 superstructure were found. Based on images taken at different tunneling conditions, a structure model could be developed for the superstructure. For one monolayer, a 1 × 1 superstructure based on hexagonal DySi₂ was observed, while several monolayers thick films are characterized by a superstructure from Dy₃Si₅.     

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.     

Formation of strontium template on Si(1 0 0) by atomic layer deposition

The formation of ordered Sr overlayers on Si(1 0 0) by Atomic Layer Deposition (ALD) from bis(triisopropylcyclopentadienyl) Strontium (Sr(C₅ⁱPr₃H₂)₂) and H₂O has been investigated. SrO overlayers were deposited on a 1-2 nm SiO₂/Si(1 0 0) substrate, followed by a deoxidation process to remove the SiO₂ layer at high temperatures. Auger electron spectroscopy, Rutherford backscattering spectrometry, spectroscopic ellipsometry, and low-energy electron diffraction were used to investigate the progress of both ALD and deoxidation processes. Results show that an ordered Sr/Si(1 0 0) surface with 2 × 1 pattern can be obtained after depositing several monolayers of SrO on Si using ALD followed by an anneal at 800-850 °C. The (2 × 1) ordered Sr/Si(1 0 0) surface is known to be an excellent template for the epitaxial growth of SrTiO₃ (STO) oxide. The present results demonstrate that ALD is a potential alternative to molecular beam epitaxy methods for the fabrication of epitaxial oxides on semiconductor substrates.     

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.     

GaN grown on Si(1 1 1) with step-graded AlGaN intermediate layers

The authors report the growth of crack-free GaN on Si(1 1 1) substrate with step-graded AlGaN intermediate layers all grown at 1120 °C. By preparing all these layers at high-temperature, we can simplify the growth proceduce and minimize the growth time. Using X-ray diffraction and transmission electron microscopy, it was found that the high-temperature step-graded AlGaN intermediate layers can effectively reduce the tensile stress on GaN epitaxial layers. Photoluminescence and Raman measurements also indicate that we can improve the crystal quality of GaN by inserting the step-graded AlGaN intermediate layers.     

Band bending at the Si(1 0 0)-Si3N4 interface studied by photoreflectance spectroscopy

Photoreflectance spectroscopy has been used to measure the band bending at the p-Si(1 0 0)-Si₃N₄ interface subjected to annealing and ion implantation. Upon annealing, unimplanted interfaces exhibit a constant band bending of about 0.77 eV, even though the spectral amplitude changes due to variations in the way minority carriers are annihilated at the interface. Implantation reduces the band banding, although subsequent annealing in stages up to 900 °C progressively restores the bending to its original value through pathways exhibiting a wide range of activation barriers.     

Olefin metathesis reaction on GaN (0 0 0 1) surfaces

Proof-of-concept reactions were performed on GaN (0 0 0 1) surfaces to demonstrate surface termination with desired chemical groups using an olefin cross-metathesis reaction. To prepare the GaN surfaces for olefin metathesis, the surfaces were hydrogen terminated with hydrogen plasma, chlorine terminated with phosphorous pentachloride, and then terminated with an alkene group via a Grignard reaction. The olefin metathesis reaction then bound 7-bromo-1-heptene. The modified surfaces were characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy, and water contact angle measurements following each step in the reaction scheme. The XPS data was used to qualitatively identify surface chemical species and to quantitatively determine molecular surface coverage. The bromine atom in 7-bromo-1-heptene served as a heteroatom for identification with XPS. The reaction scheme resulted in GaN substrates with a surface coverage of 0.10 monolayers and excellent stability towards oxidation when exposed to oxygen plasma.     

SiO adsorption on a p(2 × 2) reconstructed Si(1 0 0) surface

We have investigated the adsorption mechanism of SiO molecule incident on a clean Si(1 0 0) p(2 × 2) reconstructed surface using density functional theory based methods. Stable adsorption geometries of SiO on Si surface, as well as their corresponding activation and adsorption energies are identified. We found that the SiO molecule is adsorbed on the Si(1 0 0) surface with almost no activation energy. An adsorption configuration where the SiO binds on the channel separating the dimer rows, forming a Si-O-Si bridge on the surface, is the energetically most favourable geometry found. A substantial red-shift in the calculated vibrational frequencies of the adsorbed SiO molecule in the bridging configurations is observed. Comparison of adsorption energies shows that SiO adsorption on a Si(1 0 0) surface is energetically less favourable than the comparable O₂ adsorption. However, the role of SiO in the growth of silicon sub-oxides during reactive magnetron plasma deposition is expected to be significant due to the relatively large amount of SiO molecules incident on the deposition surface and its considerable sticking probability. The stable adsorption geometries found here exhibit structural properties similar to the Si/SiO₂ interface and may be used for studying SiO_x growth.     

First-principles study of adsorption and diffusion on Ge/Si(0 0 1)-(2 × 8) and Ge/Si(1 0 5)-(1 × 2) surfaces

Using first-principles total-energy calculations, we have investigated the adsorption and diffusion of Si and Ge adatoms on Ge/Si(0 0 1)-(2 × 8) and Ge/Si(1 0 5)-(1 × 2) surfaces. The dimer vacancy lines on Ge/Si(0 0 1)-(2 × 8) and the alternate S_A and rebonded S_B steps on Ge/Si(1 0 5)-(1 × 2) are found to strongly influence the adatom kinetics. On Ge/Si(0 0 1)-(2 × 8) surface, the fast diffusion path is found to be along the dimer vacancy line (DVL), reversing the diffusion anisotropy on Si(0 0 1). Also, there exists a repulsion between the adatom and the DVL, which is expected to increase the adatom density and hence island nucleation rate in between the DVLs. On Ge/Si(1 0 5)-(1 × 2) surface, the overall diffusion barrier of Si(Ge) along direction is relative fast with a barrier of ∼0.83(0.61) eV, despite of the large surface undulation. This indicates that the adatoms can rapidly diffuse up and down the (1 0 5)-faceted Ge hut island. The diffusion is also almost isotropic along [0 1 0] and directions.     

Si epitaxial growth on LaAlO3(0 0 1)

We report on the growth of Si on c(2 × 2) reconstructed LaAlO₃(0 0 1) surfaces at high substrate temperature (700 °C) by molecular beam epitaxy. An initial Volmer-Weber mode is evidenced using reflection high energy electron diffraction (RHEED), X-ray photoelectron diffraction (XPD) and atomic force microscopy. After the deposition of a few monolayers, the islands coalesce. Using X-ray photoelectron spectroscopy, we demonstrate that Si islands exhibit an abrupt interface with the LaAlO₃ substrate without formation of silicate or silica. Finally, combined RHEED and XPD measurements show the epitaxial growth of Si with a unique Si(0 0 1)//LaAlO₃(0 0 1) and Si<1 0 0>//LAO<1 1 0> relationship.     

Crystal growth of SrTiO3 films on H-terminated Si(1 1 1) with SrO buffer layers

Growth of epitaxial SrTiO₃ (STO) films has been examined on H-terminated Si(1 1 1) with SrO buffer layers. The epitaxial SrO buffer layers have reduced stress on H-terminated Si substrates. On the SrO buffer layers, the STO films grow epitaxially with triple domains at low temperature. Each STO domain has equivalent epitaxial relationship to SrO buffer layers, STO(1 1 0)∥SrO(1 1 1) and .     

The chemisorption of pentacene on Si(0 0 1)-2 × 1

Adsorption structures of the pentacene (C₂₂H₁₄) molecule on the clean Si(0 0 1)-2 × 1 surface were investigated by scanning tunneling microscopy (STM) in conjunction with density functional theory calculations and STM image simulations. The pentacene molecules were found to adsorb on four major sites and four minor sites. The adsorption structures of the pentacene molecules at the four major sites were determined by comparison between the experimental and the simulated STM images. Three out of the four theoretically identified adsorption structures are different from the previously proposed adsorption structures. They involve six to eight Si-C covalent chemical bonds. The adsorption energies of the major four structures are calculated to be in the range 67-128 kcal/mol. It was also found that the pentacene molecule hardly hopped on the surface when applying pulse bias voltages on the molecule, but was mostly decomposed.