Ultrahigh Vacuum Reflection Electron Microscopy Research Articles

Sn-mediated transformations on Si(111) surface: Reconstructions, Electromigration, Homoepitaxy

We have studied Si mass transport and morphological transformations on the Si(111) surface during (√3 × √3)-Sn reconstruction formation and following Si homoepitaxy in 600–825°C interval by in situ ultrahigh vacuum reflection electron microscopy (UHV REM) and ex situ atomic force microscopy (AFM). We have shown that the formation of a metallic α-(√3 × √3)-Sn phase during Sn deposition at T < 650 °С leads to Si 2D island nucleation on > 1 μm terraces, while the formation of a mosaic γ-(√3 × √3)-Sn phase at T > 650 °С is followed by monatomic step shift in the step-up direction and provides Sn/Si(111) interface with wide atomically flat terraces. The step shift value has been measured as functions of substrate temperature and Sn deposition rate to determine emitted by the steps Si coverage participating in γ-phase formation. The electromigration-induced drift of disordered “1 × 1”-Sn domains has been shown to entail enhanced noncompensated Si mass transport followed by the Si(111)-(√3 × √3)-Sn surface roughening. The kinetics of 2D island nucleation and growth near monatomic steps and on wide (up to 10 μm) terraces on the Si(111)-γ-(√3 × √3)-Sn surface during Si deposition at 700°C is limited by surface diffusion only, and critical nucleus i consists of ∼ 20–100 Si atoms.

Structural transitions on Si(1 1 1) surface during Sn adsorption, electromigration, and desorption studied by in situ UHV REM

Using in situ ultrahigh vacuum reflection electron microscopy, we have studied structural transitions on the Si(111) surface induced by Sn deposition up to 2 ML coverage (1 ML = 7.8∙1014 cm−2) and by Sn desorption at substrate temperatures T = 200–860 °C. We have shown that Si–Sn intermixing in the adsorption layer during Sn deposition onto the Si(111)-(7 × 7) surface at T > 650 °C leads to the shift of the monatomic steps in the step-up direction, and there is formed a mosaic (√3×√3)-Sn phase. The electromigration of Sn adatoms induced by DC resistive heating has been shown to redistribute adsorbed Sn layer and to cause local (√3×√3)-Sn ⇔ “1 × 1”-Sn structural transitions on the surface. We have estimated the lower bound of the Sn adatom positive effective charge on the Si(111) surface qeffSn ≥ 0.001∙e. During AC annealing, the rate of desorption-induced “1 × 1”-Sn domain area shrinkage has been measured as a function of substrate temperature, and the activation energy of Sn adatom desorption has been estimated to be 2.5 ± 0.1 eV.

Drift of two-dimensional vacancy islands on the Si(100) surface under electromigration conditions

The paper presents an experimental study of morphological changes of the Si(100) surface under electromigration conditions that was carried out using the in situ method involving ultrahigh vacuum reflection electron microscopy. The study aims to determine the temperature dependence of the effective electric charge of an adsorbed atom on the Si(100) surface. A system of concentric two-dimensional vacancy islands was formed on the surface of Si(100) samples by low-energy argon ion sputtering and subsequent high-temperature annealing. Quasi-equilibrium conditions were created on the sample surface by compensating the sublimating flow from the surface from an external silicon source. The video images of the drift of vacancy islands were recorded under the conditions of electromigration with the compensation of sublimation. Based on the processing and analysis of video images, the authors described the dependence of the velocity of motion of vacancy islands on the Si(100) surface for various temperatures and the direction of the electric current along and across the dimer rows with the (2 × 1) superstructure inside the island. It is shown that the drift rate of vacancy islands does not depend on their size under quasi-equilibrium conditions. A simplified one-dimensional theoretical model has been constructed. It includes one atomic step moving by a detachment of atoms from the step and their drift under the force of electromigration in the absence of desorption and deposition of atoms on the surface. Based on the proposed model, the effective electric charge is estimated, and the temperature dependence of the effective charge in the temperature range of 1010 to 1120 °C is obtained. The absolute value of the effective charge decreases linearly with increasing temperature. The sign of the effective charge is negative, and its average value is Z = –0.5 ± 0.3 elementary charges. The obtained results can be used for creating structures with a countable number of atomic steps and act as secondary measures of height with reference to the silicon crystal lattice.

Abnormal diffusion profile of adatoms on extremely wide terraces of the Si(111) surface

The authors experimentally studied the distribution of the adatom concentration on extremely wide terraces of the Si(111) surface whose dimensions are comparable with the diffusion length of adatoms. The extremely wide terraces were created during in situ experiments carried out by ultrahigh vacuum reflection electron microscopy by high-temperature annealing of Si(111) samples (more than 1000 °C) followed by rapid cooling to 750 °C to form 7 × 7 superstructure domains. A detailed analysis of the surface morphology of the terraces was carried out by ex situ atomic force microscopy under ambient conditions. Based on high-resolution (1.2 nm/pixel) atomic force microscopy images, panoramic topographic images of the terraces were formed. Digital processing of the panoramic images visualized the distribution of the adatom concentration n. For a terrace cooled from 1070 °C, central terrace regions show minimum n values around 0.13 BL (1 bilayer (BL) = 1.56×1015 cm−2); close to the monatomic step bordering the terrace, n increases to about 0.14 BL. The authors determined that this radial distribution n(r) at 1070 °C corresponds to the adatom diffusion coefficient D = 59 ± 12 μm2/s. It was found that, for a terrace cooled from 1090 °C, the approach assuming the same adatom diffusion length over the entire terrace does not describe the experimental n(r) distribution. For its analysis, the authors used the solution of the stationary diffusion equation under the assumption that D is not constant. Based on a numerical solution, the dependence of D on the experimentally measured n values was obtained. Under the assumption that adatom lifetime does not depend on n at 1090 °C, the adatom diffusion coefficient was found to decrease from 140 μm2/s at n = 0.093 BL (in the central terrace regions) to 5 μm2/s at n = 0.118 BL (near the step). The results of this work experimentally demonstrated that the control over adatom concentration can be used to significantly vary the diffusion properties of the adsorption layer on the crystal surface.

From Self-Organization of Monoatomic Steps on the Silicon Surface to Subnanometer Metrology

The article presents how the understanding of the fundamental processes of self-organization and morphological transformations of the atomically clean Si(111) surface as a result of the study using in situ ultrahigh vacuum reflection electron microscopy can be applied to metrology. The method of high-resolution transmission electron microscopy is used to show that the native oxide formed on the Si(111) surface in atmospheric conditions replicates the atomic step height with high accuracy. The techniques for creating vertical measures in the range 0.31–31 nm with an error of less than 0.05 nm in the entire measurement range are developed on this basis. It is shown that it is possible to create extremely wide atomically smooth surfaces (up to 230 $$\mu$$ m) and use them as reference mirrors in interferometric microscopes. Crystal samples containing a certain number of monoatomic steps and atomically smooth surface areas are included in the State Secondary Reference Standard as a measure of angstrom height and angstrom flatness.

Etching Kinetics of Si(111) Surface by Selenium Molecular Beam

Using in situ ultrahigh vacuum reflection electron microscopy, three modes of the etching kinetics of the Si(111) surface with a selenium molecular beam are revealed. In the low temperature region ( $$\lesssim$$ 650 $${}^{\circ}$$ C depending on the Se deposition rate), the etching kinetics is limited by the energy of formation and desorption of SiSe $${}_{2}$$ molecules and the surface is completely covered by an impurity-induced silicon selenide phase ‘‘1 $$\times$$ 1’’-Se. In the temperature range $${\sim}700{-}1100^{\circ}$$ C the etching rate is limited by the amount of Se deposition flow and does not depend on the temperature, surface structure, and etching mechanism (step-layer or two dimensional-island). At high temperatures ( $${\gtrsim}1150^{\circ}$$ C), the sublimation of Si atoms begins to make the main contribution to the silicon flux from the surface. A theoretical model describing the temperature and kinetics of transitions between etching modes is formulated.

2D island nucleation controlled by nanocluster diffusion during Si and Ge epitaxy on Si(1 1 1)-(7 × 7) surface at elevated temperatures

We have studied two-dimensional (2D) island nucleation at the initial stages of Si and Ge epitaxial growth near monatomic steps and on wide (up to 10 μm) terraces on the Si(1 1 1)-(7 × 7) surface in 500–750 °C interval by in situ ultrahigh vacuum reflection electron microscopy (UHV REM) and ex situ atomic force microscopy (AFM). Concentration of 2D islands nucleated on the terraces and depletion zone width near the steps have been measured as functions of deposition rate and substrate temperature. We have shown that GeSi and Si nanoclusters are dominating diffusion species providing surface mass transport on the Si(1 1 1)-(7 × 7) surface during Ge and Si epitaxial growth at temperatures above 500 °C and 600 °C, respectively; the activation energy of GeSi nanocluster diffusion has been first estimated at 1.3–1.4 eV. The kinetics of 2D island nucleation and growth near the steps (both for Si and Ge epitaxy) and on the wide terraces (for Ge epitaxy) is limited by surface diffusion only, and critical nucleus consists of i ≫ 1 nanoclusters.

Etching of step-bunched Si(1 1 1) surface by Se molecular beam observed by in situ REM

Using in situ ultrahigh vacuum reflection electron microscopy, we have first visualized the evolution of step-bunched Si(1 1 1) surface during Se beam etching depending on substrate temperature T and terrace size required for survival of etching-induced vacancies. Below T ~ 630 °C, the etching proceeds in layer-by-layer mode via periodic nucleation of small 2D vacancy islands without noticeable motion of steps. However, in 630–830 °C interval, the etching of the 7 × 7 reconstructed Si(1 1 1) step-bunched surface is realized by both step flow and continuous nucleation and growth of 2D vacancy islands on wide terraces, which leads to the excavation of initially flat surface between step bunches. At T > 830 °C, the etching proceeds by step flow with preservation of initial surface morphology. We have found that the Si(1 1 1) surface etching by Se beam is characterized by 2.65 eV activation energy that limits etching kinetics in low-temperature range when the whole Si(1 1 1) surface is converted to disordered impurity-induced Si(1 1 1)“1 × 1”-Se phase.

Electromigration Effect on Vacancy Islands Nucleation on Si(100) Surface during Sublimation

By means of in situ ultrahigh vacuum reflection electron microscopy, the nucleation of vacancy islands on wide terraces of the Si(100) surface is investigated. The temperature dependence of the displacement of a vacancy island nucleation center is determined in the process of heating a sample with a dc electric current. On the basis of a theoretical model, the effective electric charge of addimers is estimated in the direction across dimer rows of the surface. The effective charge has a positive sign and does not exceed 15 units of the elementary charge in the temperature range of 1020–1120°C.

Step bunching phenomena on Si(0 0 1) surface induced by DC heating during sublimation and Si deposition

Using in situ ultrahigh vacuum reflection electron microscopy and ex situ atomic force microscopy, we have investigated the influence Si deposition on the step bunching on the Si(0 0 1) surface under direct current (DC) sample heating in 950–1150 °C temperature range. During sublimation time scaling exponents of the average distance between the bunches are about 0.3 for both step-up and step-down DC directions at all temperatures. We have found that the shape of the step bunches becomes straight, and the number of step pairs between the bunches increases at a temperature above 1050 °C for fixed annealing time. We have established that the step bunching rate during step-up DC heating is characterized by 0.24 eV effective activation energy under sublimation conditions. The mean bunch separation changes non-monotonically when net growth rate (R) rises and it decreases as R−1/2 near equilibrium conditions.

Evolution of Micropits on Large Terraces of the Si(111) Surface during High-Temperature Annealing

The transformation of micropits on large terraces of the Si(111) surface containing no vicinal atomic steps has been investigated by in situ ultrahigh-vacuum reflection electron microscopy upon thermal annealing of the substrate in the range of 1200–1400°C. A procedure for the formation of micropits on large terraces of the Si(111) surface with the application of focused-ion-beam (Ga+) technology has been proposed. It has been found that the micropit decay kinetics varies upon reaching the critical radius Rcrit, which is caused by the activation of nucleation of two-dimensional vacancy islands on the micropit bottom. A theoretical model describing variations in the lateral sizes of the micropit both before and after reaching Rcrit has been proposed. Based on analysis of the found temperature dependence of the nucleation frequencies of two-dimensional vacancy pits on the micropit bottom, the effective energy of nucleation of a vacancy island has been determined to be 4.1 ± 0.1 eV.

Эволюция микролунок на широких террасах поверхности Si(111) в процессе высокотемпературного отжига

We have investigated in situ the morphological transformation of the Si(111) surfaces with micro-pits at large terraces during high-temperature annealing at T = 1200–1400°C. Experimental observation of the micro-pits kinetic decay have been performed by means of ultrahigh vacuum reflection electron microscopy. Focused ion beam system have been used for micro-pits creation at Si(111) terraces of large size. We have found that kinetic of micro-pit decay processes is affected by two dimensional vacancy islands nucleation at the micro-pit bottom when the micro-pit reaches the critical lateral size. The simple theoretical model has been proposed for describing the changes of the lateral size of micro-pit. The temperature dependence of two-dimensional vacancy islands nucleation frequency at micro-pit bottom is found to be described by the activation energy of 4.1 ± 0.1 eV.

Влияние электромиграции на зарождение вакансионных островков на поверхности Si(100) при сублимации

AbstractBy means of in situ ultrahigh vacuum reflection electron microscopy, the nucleation of vacancy islands on wide terraces of the Si(100) surface is investigated. The temperature dependence of the displacement of a vacancy island nucleation center is determined in the process of heating a sample with a dc electric current. On the basis of a theoretical model, the effective electric charge of addimers is estimated in the direction across dimer rows of the surface. The effective charge has a positive sign and does not exceed 15 units of the elementary charge in the temperature range of 1020–1120°C.

Nucleation of two-dimensional islands on Si (111) during high-temperature epitaxial growth

The process of two-dimensional island nucleation at the surface of ultra large Si (111) during hightemperature epitaxial growth is studied by in situ ultrahigh-vacuum reflection electron microscopy. The critical terrace size D crit, at which a two-dimensional island is nucleated in the center, is measured in the temperature range 900–1180°C at different silicon fluxes onto the surface. It is found that the parameter D crit 2 is a power function of the frequency of island nucleation, with the exponent χ = 0.9 ± 0.05 in the entire temperature range under study. It is established that the kinetics of nucleus formation is defined by the diffusion of adsorbed silicon atoms at temperatures of up to 1180°C and the minimum critical nucleus size corresponds to 12 silicon atoms.

Advacancy-mediated atomic steps kinetics and two-dimensional negative island nucleation on ultra-flat Si(111) surface

We have investigated Si(111) surface morphology transformations during high-temperature sublimation and oxygen treatments by means of in situ ultrahigh vacuum reflection electron microscopy. By analyzing atomic steps kinetics and two-dimensional negative (vacancy) islands nucleation on ultra-flat Si(111) surface with extremely wide (up to 120μm in size) terraces we have estimated the activation energy associated with the surface-bulk vacancy exchange processes. We show that atomic steps motion and negative islands nucleation kinetics at temperatures above 1180°C can be described by the step-flow model of Burton, Cabrera and Frank taking into account advacancies formation. By comparing experimental results with predictions of model we conclude that the surface mass transport at temperatures above 1180°C is governed by surface vacancies nucleation and interaction with atomic steps rather than via adatoms surface diffusion.

Nucleation of two-dimensional Si islands near a monatomic step on an atomically clean Si(111)-(7×7) surface

The process of nucleation of 2D islands near a monatomic step at the initial stage of growing of a silicon film on the Si(111)-(7 × 7) surface is studied by means of in situ ultrahighvacuum reflection electron microscopy. The dependence of the depletion region width W near the step, where no islands are formed, on the deposition rate R is described by the expression W 2 ∝ R -χ with the exponent χ = 1.18 and χ = 0.63 at temperatures of 650 and 680 °C, respectively. It is demonstrated that the change in χ is associated with the step structure, which provides the transformation from the growth kinetics limited by attachment of adatoms to the step to that limited by diffusion of adatoms. A competition of the processes of nucleation and attachment to the step leads to an increase in the critical size of the island nucleus from i = 1 far from the step to i = 3–5 near the step and to i = 6–8 on the terrace of critical width for 2D nucleation.

Atomic steps on an ultraflat Si(111) surface upon sublimation

The kinetics of atomic steps on an ultraflat Si(111) surface is studied by in situ ultrahigh-vacuum reflection electron microscopy at temperatures of 1050–1350°C. For the first time it is experimentally shown that the rate of displacement of an atomic step during sublimation nonlinearly depends on the width of the adjacent terrace. It is established that the atomic mechanism of mass-transport processes at the surface at temperatures higher than 1200°C is controlled by nucleation and the diffusion of surface vacancies rather than of adsorbed Si atoms. The studies make it possible to estimate the activation energy of the dissolution of vacancies from the surface into the bulk of Si. The estimated activation energy is (4.3 ± 0.05) eV.

Attachment–detachment limited kinetics on ultra-flat Si(111) surface under etching with molecular oxygen at elevated temperatures

Abstract We have used in situ ultrahigh vacuum reflection electron microscopy to investigate the process of two-dimensional negative islands (etching pits) nucleation on extremely wide (up to 120 μm in diameter) terraces at Si(111) surface during thermal etching with molecular oxygen at temperatures above 1000 °C. By analyzing nucleation kinetics at different temperatures and comparing the results with theoretical model, we determined the dominant surface mass transport mechanism. The squared critical terrace size D crit 2 for nucleation of new island is found to follow power law form with scaling exponent χ changing from 0.95 to 1.15 as the temperature increased T crit = 1180 °C. At temperatures above T crit we estimated the energy of the vacancy–step interaction E AD as 1.5 ± 0.15 eV. We conclude that observed changes of the D crit in high temperature region (above T crit ) can occur through detachment–attachment kinetics of vacancy–step exchange process rather than via surface diffusion.

Formation of Two-Dimensional Islands on Si(111) Surface During Homoepitaxial Growth

The nucleation of two-dimensional Si islands has been studied by in situ ultrahigh vacuum reflection electron microscopy on extra-large (~ 10–100 μm) atomically flat terraces of Si(111) surface. The dependence of two-dimensional island concentration N2D on substrate temperature T and silicon deposition rate R is found to obey relation N2D ן Rχ exp(E2D/kT) with χ≈0.58 or 0.82 and E2D ≈ 1.77 eV or 1.02 eV on the Si(111) surface with (7×7) or (1×1) structure, respectively. The critical nucleus during the growth on the extra-large terraces is found to consist of i = 1 particle at T ~ 700°С, and the critical nucleus size increases to i = 7–10 on terraces with smaller width, which is caused by the competition between the 2D island nucleation and the interaction of adatoms with steps bordering the critical terrace

Critical Terrace Width for Two-Dimensional Nucleation during Si Growth on Si(111)-(7×7) Surface

The critical terrace width λ for 2D island nucleation and growth (2DNG) on large-scale atomically flat terraces of a step-bunched Si(111)-(7×7) surface has been studied by in situ ultrahigh vacuum reflection electron microscopy as a function of the substrate temperature T and Si deposition rate R. The dependence of λ(2)(R) is characterized by a power law with scaling exponent χ=1.36-1.46, validating an attachment limited (AL) growth kinetics up to 720 °C. At this temperature, the Arrhenius dependencies lnλ(2)(1/T) change their slope, so that the effective 2DNG activation energy E(2D) drops from 2.4 eV down to 0.5 eV at T>720 °C. We first show that the E(2D) change is caused by a transition between AL and DL (diffusion limited) growth kinetics accompanied by a step shape transformation. The AL growth mode is characterized by kinetic length d(-)~10(5)a and the preferential step-down attachment of atoms to steps limited by an energy barrier E(ES)(-)≈0.9 eV.