Quantitative measurements of Ge surface segregation during SiGe alloy growth

Ge segregation during the growth of Si_{1 − x}Ge_x alloys (x = 5, 10, 20, and 40%) was studied using X-ray photoelectron spectroscopy. The alloys were grown in thicknesses up to 20.0 nm at 500°C to measure quantitatively the amount of segregated surface Ge. The length of alloy needed to reach steady-state growth edge was found to decrease with increasing alloy concentration (4.8, 2.8, 2.4, and 2.0 nm, respectively). It was found that each alloy had a complete monolayer of Ge on the surface and an increasing amount of segregated Ge in the second layer (20, 55, 80, and 95%, respectively) during steady-state growth. An increase in the temperature of alloy growth (400–750°C) resulted in an increase in the leading edge of alloy growth but did not change the amount of segregated Ge during steady-state growth. We propose that film stress is responsible for the amount of Ge segregation.     

Chlorine chemisorption on Cu(0 0 1) by surface X-ray diffraction: Geometry and substrate relaxation

We report on the precise location of Cl atoms chemisorbed on a Cu(0 0 1) surface and the interlayer relaxations of the metal surface. Previous studies have shown that chlorine dissociates on Cu(0 0 1) to form a c(2 × 2) chemisorbed layer with Cl atoms occupying four-fold hollow sites. A Cu-Cl interlayer spacing of 1.60 Å and a slightly expanded Cu-Cu first interlayer spacing of 1.85 Å (1.807 Å for bulk Cu) was determined by LEED. The resulting Cu-Cl bond length, 2.41 Å, is very similar to the SEXAFS value of 2.37 Å. Contradictory results were obtained by angle-resolved photoemission extended fine structure: while confirming the Cu-Cl interlayer spacing of 1.60 Å, no first Cu-Cu interlayer relaxation has been observed. On the other hand, a small corrugation of the second Cu layer was pointed out. We carried out a detailed structural determination of the Cu(0 0 1)-c(2 × 2)-Cl system using surface X-ray diffraction technique with synchrotron radiation. We find a Cu-Cl interlayer spacing of 1.584(5) Å and confirm the expansion of the first Cu-Cu interlayer, with an average spacing of 1.840(5) Å. In addition, we observe a small corrugation of the second Cu layer, with Cu atoms just below Cl atoms more tightly bound to the surface layer, and even a second Cu-Cu interlayer expansion.     

Surface science investigations of atomic layer deposition half-reactions using TaF5 and Si2H6

Fundamental adsorption and surface chemistry studies of TaF₅ and Si₂H₆ on a polycrystalline Ta surface were performed in ultra-high vacuum (UHV) in the range 303-523 K to understand if these precursors may be used in applications to grow Ta-based films by atomic layer deposition. TaF₅ uptake saturated in UHV conditions for less than 100 L exposure for adsorption on both clean Ta and on 144 L Si₂H₆-treated Ta. TaF₅ dissociatively adsorbed on clean Ta, and F ligands were determined to govern the self-limiting adsorption behavior. The extent of each “half-reaction”, the reaction of TaF₅ with a Si₂H₆-treated surface and the reaction of Si₂H₆ with a TaF₅-treated surface, increased with surface temperature and was dependent on SiH_xF_y byproduct desorption. Neither Si₂H₆ adsorption nor Si₂H₆ half-reaction reached saturation in UHV conditions, as measured by the surface Si concentration. F ligands were removed during the Si₂H₆ half-reaction, and residual F asymtotically approached a constant value.     

Investigation of phosphorus surface segregation by X-ray scattering measurements

The surface segregation of phosphorus in silicon at low temperatures is studied by using δ doping structures grown by molecular beam epitaxy. The samples are characterized by X-ray crystal truncation rod (CTR) scattering using synchrotron radiation as the light source. The 1/e decay length of P segregation and segregation barrier energy are obtained by fitting the CTR curves within kinematical approximation of X-ray diffraction theory. The surface segregation of P is strong at a growth temperature of 450 °C, with a 1/e decay length of 14 nm, while for growth temperatures below 350 °C, P segregation is negligible with a 1/e decay length not larger than 4 nm. The segregation barrier energy is determined to be 0.43 eV.     

Formation of micro-crystals on the (100) surface of SrTiO3 at elevated temperatures

Droplet-like features and regularly shaped micro-crystals appear on the (100) surface of single-crystals of stoichiometric and doped SrTiO₃ as a result of heat treatment around 1000–1100°C under ambient pressure. Secondary ion mass spectrometry, atomic force microscopy and X-ray photoemission are employed to characterize the morphology of the modified surface. The results provide evidence of an accumulation of SrO_x on the surface in a liquid form and subsequent recrystallization as SrO on prolonged annealing. The phenomena are discussed in relation to the restructuring in the near-surface region and the loss of material at the temperatures employed, as evident from thermogravimetrical measurements.     

Raman study of the segregation of GaAs/AlAs heterostructures grown by molecular beam epitaxy

The Raman spectra of the optical confined phonons in the GaAs/AlAs ultra-thin layer superlattices grown with different growth conditions were used to determine the compositional profiles and to study the process of segregation at the heterointerfaces. A modified kinetic model was developed in order to calculate the compositional profiles in the samples under investigation. The comparison between the experimentally obtained compositional profiles and those calculated by the kinetic model allowed us to determine the parameters characterizing the segregation. It was shown that the increase of pressure of As acts equivalently to the decrease of the growth temperature, resulting in a more abrupt compositional profile.     

Optical transitions of InAs/In0.36Ga0.64As/GaAs(311B) surface quantum dots clearly identified by the piezoreflectance technique

The bilayer InAs/In_0.36Ga_0.64As/GaAs(311B) quantum dots (QDs), including one InAs buried quantum dot (BQD) layer and the other InAs surface quantum dot (SQD) layer, have been grown by molecular beam epitaxy (MBE). The optical properties of these three samples have been studied by the piezoreflectance (PzR) spectroscopy. The PzR spectra do not exhibit only the optical transitions originated from the InAs BQDs, but the features originated from the InAs SQDs. After the InAs SQDs have been removed chemically, those optical transitions from InAs SQDs have been demonstrated clearly by investigating the PzR spectra of the residual InAs BQDs in these samples. The great redshift of these interband transitions of InAs SQDs has been well discussed. Due to the suitable InAs SQD sizes and the thickness of In_0.36Ga_0.64As layer, the interband transition of InAs SQDs has been shifted to ∼1.55 μm at 77 K.     

Energy levels in doped SiGe quantum well studied by admittance spectroscopy

SiGe/Si quantum wells (QWs) with different Boron doping concentrations were grown by molecular beam epitaxy (MBE) on p-type Si(1 0 0) substrate. The activation energies of the heavily holes in ground states of QWs, which correspond to the energy differences between the heavy hole ground states and Si valence band, were measured by admittance spectroscopy. It is found that the activation energy in a heavily doped QW increases with doping concentration, which can be understood by the band alignment changes due to the doping in the QWs. Also, it is found that the activation energy in a QW with a doping concentration of 2 × 10²⁰ cm⁻³ becomes larger after annealing at a temperature of 685 °C, which is attributed to more Boron atoms activation in the QW by annealing.     

RHEED observation of BaTiO3 thin films grown by MBE

Ferroelectric BaTiO₃ thin films with a thickness of 10 monolayers (ML) were epitaxially grown on SrTiO₃(0 0 1) substrates by very slow deposition using molecular beam epitaxy (MBE). The investigations were carried out by two growth methods: (i) codeposition and (ii) alternate deposition of the metal elements in an oxygen atmosphere. In situ observation of reflection high-energy electron diffraction confirmed that an epitaxial cube-on-cube structure was prepared. After the deposition, X-ray diffraction measurements were carried out. The 10-ML-thick BaTiO₃ films were highly c-axis oriented single crystals with good film quality.     

Study of nitrogen ion implantation and diffusion phenomena on thin chromium layers followed by the atomic force microscopy and secondary ion mass spectroscopy techniques characterization

This research investigates the effect of ion implantation dosage level and further thermal treatment on the physical characteristics of chromium coatings on Si(1 1 1) substrates. Chromium films had been exposed to nitrogen ion fluencies of 1 × 10¹⁷, 3 × 10¹⁷, 6 × 10¹⁷ and 10 × 10¹⁷ N⁺ cm⁻² with a 15 keV energy level. Obtained samples had been heat treated at 450 °C at a pressure of 2 × 10⁻² Torr in an argon atmosphere for 30 h. Atomic force microscopy (AFM) images showed significant increase in surface roughness as a result of nitrogen ion fluence increase. Secondary ion mass spectroscopy (SIMS) studies revealed a clear increased accumulation of Cr₂N phase near the surface as a result of higher N⁺ fluence. XRD patterns showed preferred growth of [0 0 2] and [1 1 1] planes of Cr₂N phase as a result of higher ion implantation fluence. These results had been explained based on the nucleation-growth of Cr₂N phase and nitrogen atoms diffusion history during the thermal treatment process.     

Homogeneous Si films on CaF2/Si(1 1 1) due to boron enhanced solid phase epitaxy

The structure and morphology of Si/CaF₂/Si(1 1 1) structures have been investigated by X-ray diffraction (XRD, GIXRD) and X-ray photoelectron spectroscopy (XPS). While CaF₂ films were grown via molecular beam epitaxy (MBE), Si films on CaF₂/Si(1 1 1) are fabricated by surfactant enhanced solid phase epitaxy (SE-SPE). Here Boron was used as a surfactant to obtain semiconductor films of homogeneous thickness. The Si films are entirely relaxed while the CaF₂ films have both pseudomorphic and relaxed crystallites. After exposure to ambient conditions, the Si films have a very thin native oxide film. The homogeneous Si film partially prevents the incorporation of impurities at the interface between the Si substrate and CaF₂ via migration along residual defects of the CaF₂ film.     

Assessing Carrier Recombination Processes in Type‐II SiGe/Si(001) Quantum Dots

In this work, it is shown how different carrier recombination paths significantly broaden the photoluminescence (PL) emission bandwidth observed in type‐II self‐assembled SiGe/Si(001) quantum dots (QDs). QDs grown by molecular beam epitaxy with very homogeneous size distribution, onion‐shaped composition profile, and Si capping layer thicknesses varying from 0 to 1100 nm are utilized to assess the optical carrier‐recombination paths. By using high‐energy photons for PL excitation, electron‐hole pairs can be selectively generated either above or below the QD layer and, thus, clearly access two radiative carrier recombination channels. Fitting the charge carrier capture‐, loss‐ and recombination‐dynamics to PL time‐decay curves measured for different experimental configurations allows to obtain quantitative information of carrier capture‐, excitonic‐emission‐, and Auger‐recombination rates in this type‐II nano‐system.     

Thin PTCDA films on Si(0 0 1): 1. Growth mode

We have studied the interface and thin film formation of the organic molecular semiconductor 3,4,9,10 perylene tetracarboxylic dianhydride (PTCDA) on clean and on hydrogen passivated Si(0 0 1) surfaces. The studies were made by means of high resolution X-ray photoelectron spectroscopy (HRXPS), near edge X-ray absorption fine structure (NEXAFS), low energy electron diffraction (LEED), and atomic force microscopy (AFM). On the passivated surface the LEED pattern is somewhat diffuse but reveals that the molecules grow in several ordered domains with equivalent orientations to the substrate. NEXAFS shows that the molecules are lying flat on the substrate. The Si 2p XPS line shape is not affected when the film is deposited so it can be concluded that the interaction at the interface between PTCDA and the substrate is weak. The evolution of the film formation appears to be homogeneous for the first monolayer with a nearly complete coverage of flat lying molecules based on the XPS attenuation. For layer thickness of 0.5-2 monolayers (ML) the molecules start to form islands, attracting the molecules in between, leaving the substrate partly uncovered. For thicker films there is a Stranski-Krastanov growth mode with thick islands and a monolayer thick film in between. For the clean surface the ordering of the film is much lower and angle resolved photoelectron spectroscopy (ARPES) of the molecular orbitals have only a small dependence of the emission angle. NEXAFS shows that the molecules do not lie flat on the surface and also reveal a chemical interaction at the interface.     

Evolution of Si0.8Ge0.2 quantum dots during Si encapsulation

Surface structure, determined by scanning tunneling microscopy (STM), surface morphology, determined by atomic force microscopy (AFM), and surface composition, determined by X-ray photoelectron spectroscopy (XPS) of 20.0 nm Si_0.8Ge_0.2 quantum dots formed at 800 °C and encapsulated with 0-10 nm of Si at 500 °C and 800 °C are presented. It is observed that the quantum dot surface morphology changes during the Si encapsulation at 800 °C by the smoothing of the quantum dots. The height of the quantum dots decreases faster than can be accounted for from the amount of Si deposited, indicating that there is movement of material out of the quantum dots during the encapsulation process. Encapsulation at 500 °C results in a retention of the quantum dot surface morphology with increased Ge segregation compared to Si encapsulation at 800 °C. We conclude that the changing surface morphology at 800 °C is not the result of Ge segregation but due to intermixing resulting from the tensile strain of Si depositing on SiGe.     

Hydroxylated α-Al2O3 (0 0 0 1) surfaces and metal/α-Al2O3 (0 0 0 1) interfaces

X-ray photoelectron spectroscopy was applied to study the hydroxylation of α-Al₂O₃ (0 0 0 1) surfaces and the stability of surface OH groups. The evolution of interfacial chemistry of the α-Al₂O₃ (0 0 0 1) surfaces and metal/α-Al₂O₃ (0 0 0 1) interfaces are well illustrated via modifications of the surface O1s spectra. Clean hydroxylated surfaces are obtained through water- and oxygen plasma treatment at room temperature. The surface OH groups of the hydroxylated surface are very sensitive to electron beam illumination, Ar⁺ sputtering, UHV heating, and adsorption of reactive metals. The transformation of a hydroxylated surface to an Al-terminated surface occurs by high temperature annealing or Al deposition.     

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 .     

Heteroepitaxial praseodymium sesquioxide films on Si(1 1 1): A new model catalyst system for praseodymium oxide based catalysts

The structure, growth and stoichiometry of heteroepitaxial Pr₂O₃ films on Si(1 1 1) were characterized by a combined RHEED, XRD, XPS and UPS study in view of future applications as a surface science model catalyst system. RHEED and XRD confirm the growth of a (0 0 0 1) oriented hexagonal Pr₂O₃ phase on Si(1 1 1), matching the surface symmetry by aligning the oxide in-plane direction along the Si azimuth. After an initial nucleation stage RHEED growth oscillation studies point to a Frank-van der Merwe growth mode up to a thickness of approximately 12 nm. XPS and UPS prove that the initial growth of the Pr₂O₃ layer on Si up to ∼1 nm thickness is characterized by an interface reaction with Si. Nevertheless stoichiometric Pr₂O₃ films of high crystalline quality form on top of these Pr-silicate containing interlayers.     

A spectroscopic study of CNx formation by the keV N2 irradiation of highly oriented pyrolytic graphite surfaces

Amorphous carbon nitride (CN_x) thin layer, formed by the keV N₂⁺ irradiation of highly oriented pyrolytic graphite, has been investigated using X-ray photoelectron and raman spectroscopies, and time-of-flight secondary ion mass spectrometry. C1s X-ray photoelectron spectroscopy (XPS) peak separations indicate that C-N bonds form over and above the graphite fragmentation previously obtained on Ar⁺ irradiation. N1s XPS peak separations indicate three components. Their attributions, and the resultant CN_x structure, are confirmed by angle-resolved XPS and TOF-SIMS analyses.     

A SIMS study on Mg diffusion in Zn0.94Mg0.06O/ZnO heterostructures grown by metal organic chemical vapor deposition

Zn_0.94Mg_0.06O/ZnO heterostructures have been grown on 2 in. sapphire wafer using metal organic chemical vapor deposition (MOCVD). Photoluminescence (PL) mapping demonstrates that Mg distribution on the entire wafer is very uniform (standard deviation of Mg concentration/mean Mg concentration = 1.38%) with average concentration of ∼6%. The effect of annealing on the Mg diffusion in Zn_0.94Mg_0.06O/ZnO heterostructures has been investigated in detail by using secondary ion mass spectrometry (SIMS). All the Mg SIMS depth profiles have been fitted by three Gaussian distribution functions. The Mg diffusion coefficient in the as-grown Zn_0.94Mg_0.06O layer deposited at 700 °C is two orders of magnitude lower than that of annealed samples, which clearly indicates that the deposition temperature of 700 °C is much more beneficial to grow ZnMgO/ZnO heterostructures and quantum wells.     

Silicide formation in Co/Si system investigated by depth-resolved positron annihilation and X-ray diffraction

The transformation of Co/Si to CoSi₂/Si in the temperature range of 300-1170 K has been investigated using depth-resolved positron annihilation and Glancing incidence X-ray diffraction (GIXRD). The different silicide phases formed are identified from the experimental positron annihilation characteristics, which are consistent with the GIXRD results. The present study clearly indicates the absence of vacancy defects in the silicide overlayer.