Sb Surface Segregation Research Articles

On determining the surface segregation kinetics of Sb in binary and ternary Fe alloys during continuous annealing

In the present research, Sb surface segregation kinetics for Fe-(0.01/0.03)Sb and Fe-2Mn-(0.01/0.03)Sb at.% alloys were determined based on the modified Darken model and linear heating followed by isothermal annealing. The modified Darken model was used to evaluate the measured Sb segregation profile during continuous galvanizing heat treatments. Based on the kinetic analysis, increasing temperatures and isothermal holding times led to an increase in Sb surface segregation. Sb segregation was observed at the surface for both the Fe-xSb and Fe-2Mn-xSb alloys after annealing. It was found that Sb surface diffusion in the Fe-Sb alloys was faster than in the Fe-2Mn-Sb alloys, which was attributed to the crystal structure and the defect density in the matrix. The segregation rate was determined from the Darken curves and a higher segregation rate was observed in Fe-xSb alloys. The activation energy of Sb diffusion for the Fe-Sb and Fe-2Mn-xSb alloys was determined to be ∼ 193±18 kJ/mol, which is close to the activation energy of bulk Sb diffusion in α-Fe.

On the study of antimony incorporation in InAs/InAsSb superlattices for infrared sensing

Advanced infrared detector materials utilizing InAs/InAsSb superlattices (SLs) are emerging due to the long minority carrier lifetimes observed in this material system. However, compositional and dimensional changes through Sb segregation alter the detector properties from the original design, and precise compositional control of the Sb in the SL is crucial to advance the state-of-the-art of this novel material system. In this study, epitaxial conditions that can mitigate Sb segregation during growth are explored in order to achieve high-quality SL materials. A nominal SL structure of 77 Å InAs/35 Å InAs0.7Sb0.3 tailored for a midinfrared gap was used to optimize our epitaxial parameters. Since the growth of mixed anion alloys is complicated by the potential reaction of Asx with Sb surfaces, the substrate temperature (Ts), and arsenic cracker temperature (TAs) was varied in order to control the Asx surface kinetics on a Sb surface. Experimental results indicate that the SL sample grown at the lowest investigated Ts of 400 °C produces the highest Sb mole fraction x of ∼0.3 in InAs1-xSbx layers, which is then decreased by 14% as the Ts increases from 400 to 440 °C. This reduction originates from Sb surface segregation during InAsSb growth through the As-Sb exchange process. Although this incorporation was increased with a lower TAs, the crystalline quality of SL layers quickly degraded with the TAs below 850 °C due to the poor adsorption coefficient of As4 at the growth front. Since a change in the designed compositions and effective layer widths related to Sb segregation disrupts strain balance and also significantly impacts the detector performance, further studies to prevent Sb segregation are needed.

Quantitative study of the effect of deposition temperature on antimony incorporation in InAs/InAsSb superlattices

InAs/InAsSb superlattices (SLs) are being actively explored for infrared detector applications owing to their superior carrier lifetimes. However, antimony (Sb) segregation during growth can alter the properties of the grown material. In this study, using X-ray energy dispersive spectrometry, authors quantify the compositional profile of individual layers and establish epitaxial parameters for high-quality InAs/InAsSb SL materials. Epitaxial conditions are determined for a nominal 7.7nm InAs/3.5nm InAs0.7Sb0.3 SL structure tailored for an approximately 6μm response at 150K. Since the growth of mixed anion alloys is complicated by the potential reaction of As2 with Sb surfaces, authors varied the deposition temperature (Tg) in order to control As2 surface reactions on Sb surfaces. Authors find that Sb incorporation is suppressed by 21%, with the increase of Tg from 395 to 440°C. This incorporation likely stems from Sb surface segregation during InAsSb layer growth that is driven by the As–Sb exchange mechanism, which can lead to significant compositional and dimensional deviations from the intended design.

Effects of Ga- and Sb-precursor chemistry on the alloy composition in pseudomorphically strained GaAs 1−ySb y films grown via metalorganic vapor phase epitaxy

GaAs-based multiple quantum well (MQW) heterostructures comprised of metastable alloys such as GaAs 1− y Sb y -GaAs 1− z N z have potential for realizing high-performance and low temperature-sensitivity lasers in the 1.55 μm wavelength region. However, strain-induced ‘lattice-latching’ and Sb-surface segregation effects limit the Sb-mole fractions in the pseudomorphically strained GaAs 1− y Sb y layers to y⩽0.2. The effect of Ga- and Sb-precursor chemistry and the growth temperature on the Sb-incorporation efficiency in strain-relaxed and strained, pseudomorphic GaAs 1− y Sb y films was studied using metalorganic vapor phase epitaxy. Both trimethyl- and triethyl-gallium (TMGa and TEGa) and trimethyl- and triethyl-antimony (TMSb and TESb) were used as Ga- and Sb-precursors, in four different source combinations, with arsine. The Sb-mole fraction in the strained GaAs 1− y Sb y films was found to be lower than that in the relaxed films for all of the precursor chemistries. The highest Sb-incorporation rates were found with the TEGa-based growth with strained-layer compositions up to y∼0.48 being obtained for the growth conditions employed. The results were discussed in terms of the strain-related thermodynamic effects and the chemical kinetics of precursor surface decomposition for the different precursor chemistries.

Surface segregation of Sb atoms during molecular-beam epitaxy of InSb quantum dots in an InAs(Sb) matrix

Surface segregation of Sb atoms during molecular-beam epitaxy (MBE) of the InSb/InAs quantum dot nanostructures has been investigated using the reflection high-energy electron diffraction (RHEED). It has been shown that an increase of the average specular spot intensity (SSI) during growth of a cap InAs layer immediately after the surface exposure to the Sb flux is due to the presence of residual population of Sb atoms at the growth surface. The Sb surface segregation is responsible for formation of a wetting layer in the InSb QD structures obtained without intentionally deposition of InSb, when the Stranski–Krastanow growth mode is impossible. Temperature dependences of the length of Sb segregation and the Sb segregation coefficient are deduced from the RHEED SSI. The RHEED data are in a good agreement with both X-ray diffraction and potoluminescence data which confirm the presence of the InAsSb wetting layer surrounding InSb QDs.

Surfactant-mediated epitaxy of high-quality low-doped relaxed germanium films on silicon (001)

Direct growth of relaxed Ge layers on Si(001) substrates was achieved using Sb as a surfactant. Deposition of Ge at substrate temperatures around 670 °C under large Sb flux resulted in complete compensation of lattice mismatch via a regular array of 90° dislocations at the interface. A residual 0.20% tensile strain is found caused by thermal mismatch between Ge and Si. The density of defects threading through the Ge films is as low as 5 × 10 7 cm − 2 . This is ascribed to an abrupt strain release during the initial micro-rough growth phase, which occurs only under the selected growth conditions. We also observed n-type Sb background doping levels in the Ge layers well below 10 17 cm − 3 presumably related to an enhanced Sb surface segregation due to the high growth temperature. Such relaxed Ge films grown by surfactant-mediated epitaxy on Si(001) open attractive perspectives for integration of novel Ge devices into mainstream Si technology.

Ultra-low-temperature homoepitaxial growth of Sb-doped silicon

An ultra-low-temperature process for homoepitaxial growth of high-quality, surface-confined, Sb-doped silicon layers is presented. Non-equilibrium growth by molecular beam epitaxy (MBE) is used to achieve dopant incorporation in excess of 2×10 14 cm −2 in a thin, surface-confined layer. Sb surface segregation larger than expected from theoretical models was observed, in agreement with other experimental works. Furthermore, this work details an entirely low-temperature process (<450 °C) that can be applied to fully processed and aluminum-metallized silicon devices. One application of this process is the formation of a back-surface electrode for back-illuminated high-purity silicon imaging arrays.

Fundamental reactions controlling anion exchange during the synthesis of Sb/As mixed-anion heterojunctions

As/Sb and Sb/As anion exchange reactions are characterized and modeled in order to better understand and control mixed-anion heterojunction synthesis by molecular-beam epitaxy. The importance of substrate temperature, anion flux exposure time, and incident anion molecular species is examined. Sb/As structures exhibit Sb surface segregation, and the subsequent incorporation of the segregating surface population into epitaxial overlayers. As/Sb exchange is significant in extent of the reaction, and enhanced with incident dimer fluxes. Competitive reactions between exchange and isoelectronic compound formation yield complex structures.

Sb surface segregation during epitaxial growth of SiGe heterostructures: The effects of Ge composition and biaxial stress

Antimony is the most widely used n-type dopant for Si molecular-beam epitaxy (MBE). However, because of surface segregation during growth, the control of doping profiles remains difficult. The case of ${\mathrm{S}\mathrm{i}/\mathrm{S}\mathrm{i}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ heterostructures is complicated by the existence of stresses, which may affect both the thermodynamics and kinetics of segregation. In this study, we analyze the segregation of Sb resulting from the MBE growth of ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}/\mathrm{Si}(100)$ heterostructures using secondary ion mass spectrometry as a function of (i) growth temperature $(200\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}l~T\ifmmode^\circ\else\textdegree\fi{}l~550\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}),$ (ii) germanium content $(0l~xl~0.2),$ and (iii) stresses (compressively strained and relaxed layers). We show that Sb segregation: (i) increases with temperature, (ii) increases with Ge content in biaxially compressed layers, (iii) decreases with Ge content in relaxed layers. The temperature variation indicates that Sb surface segregation during growth is kinetically controlled. The contrasting behaviors observed as a function of Ge content in stressed and relaxed layers can thus be explained by a decrease of the segregation enthalpy induced by Ge addition and an increase of near-surface diffusion in stressed layers.

Compositional Nonuniformity in Molecular Beam Epitaxy Grown InAsSb on GaAs(111)A Substrates

The compositional nonuniformity of InAsSb alloys on GaAs(111)A substrates grown by molecular beam epitaxy (MBE) has been investigated. In contrast to the three-dimensional (3D) island growth on GaAs(001) substrates, the alloy growth occurs in the two-dimensional (2D) growth mode from the initial stage when the GaAs(111)A substrate is used. The lattice constant fluctuation estimated from X-ray diffraction linewidths is much smaller in the alloys grown on (111)A substrates than in the alloys grown on (001) substrates, indicating that the InAsSb alloys grown on (111)A substrates are more homogeneous. We have also found that homogeneous InAsSb alloys are grown at lower temperatures when the (111)A substrates are used instead of the (001) substrates. Because the adhesion force of Sb atoms to the (111)A surface is much smaller than to the (001) surface, the desorption rate of Sb atoms from the (111)A surface should be much larger than that from the (001) surface. Even in the InAsSb alloys grown on (111)A substrates, however, a new inhomogeneity, which includes As-enriched stripes and As precipitates appears at low growth temperatures. This phenomenon is not associated with Sb surface segregation, but is induced by the excess As incorporation in the epilayer. The As-enriched stripes as well as the As precipitates can be avoided by optimizing the growth conditions. Thus, homogeneous InAsSb alloys are successfully grown even at temperatures as low as 370°C less than the critical temperature of 415°C which is obtained on GaAs(001) substrates.

Sb surface segregation effect on the phase separation of MBE grown InAsSb

Mechanism of the phase separation observed in molecular beam epitaxial (MBE) grown InAsSb is investigated. We have reported that the alloys grown at or below ∼400°C show phase separation whereas the expected critical temperature ( T c) from the DLP model is 303°C. The phase-separated alloys consist of two alternating plates with different alloy compositions lying approximately parallel to the layer surface. The structural studies by transmission electron microscopy (TEM) showed that the alternating plates are ∼50 nm thick relatively uniform InAs-rich plates and ∼20 nm thick InSb-rich plates containing dense misfit dislocations. We believe that the spontaneous formation of such platelet structures at such temperatures is not associated with the spinodal decomposition but is induced by the Sb surface segregation. To confirm this model, InAsSb are grown below the critical temperatures with a periodic annealing during the growth to enhance desorption of the excess Sb accumulated on the growth surface due to the Sb surface segregation. With sufficient annealing time, no platelet structures are observed, i.e., homogeneous InAsSb form inside the empirical miscibility gaps.

Sb segregation in Si and SiGe: effect on the growth of self-organised Ge dots

The segregation and incorporation coefficients of antimony (Sb) in Si 1− x Ge x buried doped layers were investigated simultaneously using specific temperature sequences. We first showed an exponential kinetic evolution of Sb surface segregation in Si. In contrast such an evolution could not be observed in Si 1− x Ge x because of the Sb thermal desorption, at growth temperatures of 550°C. We also showed an increased surface segregation increasing with the partial Ge concentration in Si 1− x Ge x alloys, which was explained by a decrease of the kinetic barrier for Sb atoms mobility. It was, therefore, possible to determine the growth conditions to obtain a Si 1− x Ge x doped layer with a controlled incorporation level and a negligible surface segregation obtained by the thermal desorption of the Sb surface coverage. Finally, using Sb surfactant mediated growth, we found Ge dots with lateral sizes reduced by a factor of 2.8 and density multiplied by a factor of four as compared to dots directly deposited on Si(001).

Ge concentration dependence of Sb surface segregation during SiGe MBE

Surface segregation of antimony atoms during Si 1− x Ge x molecular beam epitaxy (MBE) was studied by using the delta-doping technique. The segregation of Sb increases with increasing growth temperature and increasing Ge concentration of Si 1− x Ge x . This result is well explained by a two-state model in which the energy barrier to Sb surface segregation decreases with increasing Ge concentration of Si 1− x Ge x .

RHEED-AES observation of Sb surface segregation during Sb-mediated Si MBE on Si(0 0 1)

The Sb surface segregation during Si molecular beam epitaxy (MBE) on a Sb-saturated Si(0 0 1) surface has been investigated by the simultaneous usage of reflection high-energy electron diffraction and Auger electron spectroscopy (RHEED-AES). From a quantitative analysis of Sb MNN Auger electron spectra as excited by a grazing-incidence electron beam for RHEED, the Sb bulk incorporation rate, k, and the Sb surface coverage, Θ Sb, during the initial Si MBE growth were evaluated as a function of the substrate temperature T s. The temperature dependence of k, and Θ Sb suggests that the Sb surface segregation kinetics are divided into three temperature regions: (I) T s<∼500°C, (II) ∼500°C< T s<∼700°C and (III) ∼700°C< T s. k decreases with increase in T s in region I whereas k increases with T s in region II. k also shows long-time-scale decrease with growth time for the regions I and II. These findings are explained in terms of Sb diffusion through MBE Si layers and the crystallinity of the MBE Si layers.

Effect of Surface Segregation on the Temperature Dependence of Ion-Bombardment Induced Surface Morphology

ABSTRACTWe present a scanning tunneling microscopy (STM) study, performed at different elevated temperatures, on the influence of Sb surface segregation on the morphology of the (111) surface of a Cu-0.45 at.% Sb solid-solution single-crystal; the surface was initially cleaned at room temperature by Ar+ ion sputtering. Unexpectedly, when the temperature is increased from room temperature to 380°C, the typical (111) surface morphology, obtained after sputtering, evolves in two very different manners that depend on the heating rate. If the heating rate is rapid (approximately a few minutes), it evolves to a structure with large terraces, whereas if it is slow (about 10 hours) the morphology does not evolve -- i.e., it is frozen. These unexpected results are interpreted in terms of excess subsurface point defects (vacancies and self-interstitials) created during ion bombardment, which are mobile and can mediate Sb diffusion at low temperatures. This is mainly on step edges, but the point defects can precipitate out of solution, for a rapid heating rate, thereby forming small secondary clusters in the near-surface region, in which Sb atoms are trapped.

Interfacial segregation of competitive impurities in embrittled and de-embrittled Fe-3%Ni

The kinetics of surface segregation in embrittled and de-embrittled Fe-3 at.-%Ni-0.0ll at.-%Sb containing 20 at.-ppm S was studied by Auger electron spectroscopy at temperatures ranging from 550 to 630°C. In addition, information on the grain boundary segregation in the embrittled material has been obtained from analysing intercrystalline fracture surfaces. Grain boundary segregation is characterised by cosegregation of Sb and Ni, which is increasingly promoted as the grain boundary concentration of C decreases. On the surface of the embrittled material, segregation competition of S and Sb is observed. This is stimulated by Sb enrichment via grain boundaries. In the de-embrittled material, Sb surface segregation is inhibited so that S remains the dominant segregant. These differences in the kinetic behaviour are consistent with the demixing model of temper embrittlement. Further, the S-Sb segregation competition leads to an inhomogeneous surface chemistry; some of the individual facets are enriched with Sb, others with S. Thus, the S-Sb surface segregation competition is not only characterised by the presence of fast diffusion paths but also by anisotropy. The conditions for the segregation dominance are analysed using Guttmann's equilibrium segregation modelfor multicomponent systems.

Interfacial segregation of competitive impurities in embrittled and de-embrittled Fe-3%Ni

The kinetics of surface segregation in embrittled and de-embrittled Fe-3 at.-%Ni-0.0ll at.-%Sb containing 20 at.-ppm S was studied by Auger electron spectroscopy at temperatures ranging from 550 to 630°C. In addition, information on the grain boundary segregation in the embrittled material has been obtained from analysing intercrystalline fracture surfaces. Grain boundary segregation is characterised by cosegregation of Sb and Ni, which is increasingly promoted as the grain boundary concentration of C decreases. On the surface of the embrittled material, segregation competition of S and Sb is observed. This is stimulated by Sb enrichment via grain boundaries. In the de-embrittled material, Sb surface segregation is inhibited so that S remains the dominant segregant. These differences in the kinetic behaviour are consistent with the demixing model of temper embrittlement. Further, the S-Sb segregation competition leads to an inhomogeneous surface chemistry; some of the individual facets are enriched with Sb, others with S. Thus, the S-Sb surface segregation competition is not only characterised by the presence of fast diffusion paths but also by anisotropy. The conditions for the segregation dominance are analysed using Guttmann's equilibrium segregation modelfor multicomponent systems.

Sb-surface segregation and the control of compositional abruptness at the interface

Abstract Quantitative measurements of Sb surface segregation in pseudomorphic GaAs 0.8 Sb 0.2 layers grown on GaAs (001) using molecular beam epitaxy (MBE) were performed in situ using the line-of-sight mass spectrometry (MS) technique. It was observed that substantial surface accumulation of Sb occurs under normal growth conditions, giving rise to compositional broadening of the GaAsSb on GaAs interface. This compositional grading is found to strongly depend on the growth temperature as well as the presence of Sb at the GaAs film surface prior to the initiation of GaAsSb layer growth. We report the effect to substrate temperature and shutter sequence on the evolution of Sb surface accumulation near the interface and demonstrate that a compositionally abrupt interface can be formed by populating the GaAs surface with an amount of Sb equivalent to that expected on the GaAsSb surface during growth at steady state. Additionally, compositional abruptness at the GaAs on GaAsSb interface can be achieved by a “chemical flashoff” of the surface Sb population during exposure to an incident As 2 flux.

Surface segregation and effect of mechanical stress on Sb-stabilised tetragonal zirconia

Compact pressure effects on Sb-doped tetragonal zirconia have been monitored using XRD, Raman and FTIR spectroscopies. A martensitic tetragonal→monoclinic transformation, driven by compact pressure, has been observed upon pelletisation of stabilised samples. By contrast, no monoclinic phase has been observed on amorphous powders upon pelletisation and firing. Sb surface segregation has been observed upon firing of pellets. The phenomenon has been quantitised by angle resolved XPS using the recent Gries algorithms to evaluate the Zr 3d photoelectron inelastic mean free path in ZrO 2 which, in turn, compares well with previous experimental data. Surface Sb segregation occurs over a depth range and involves many ionic planes. An experimental estimate of the Sb segregation enthalpy has been evaluated.

Surface segregation and structure of Sb-doped Si(100) films grown at low temperature by molecular beam epitaxy

Sb surface segregation and doping during Si(100) molecular beam epitaxy were studied for growth temperatures of 320–500°C. Surface segregation was analyzed by depth profiling with secondary ion mass spectrometry and the results indicate the existence of several distinct dopant concentration- and temperature-dependent surface segregation regimes: (1) For dilute Sb surface concentrations the measurements reveal a region where bulk and surface concentrations are linearly related, and the surface segregation is described by a constant. However, the experimentally determined temperature dependence of the segregation does not follow simple kinetics theory, and appreciable surface segregation is observed at temperatures ≤ 400°C. (2) At temperatures ≥ 350°C, the surface segregation reaches a maximum for Sb surface concentrations of 0.5 monolayers. (3) For surface concentrations near 1 monolayer, the surface segregation decreases with increasing surface Sb coverage due to dopant interaction within surface and subsurface layers. In cases where films were grown under very high dopant fluxes, we have identified cone-like defects and stacking faults that are the result of the apparent surface concentration exceeding 1 monolayer.