Instability of vicinal crystal surfaces with transparent steps: Transient kinetics and non-local electromigration

The steps at the crystal surfaces could be partially transparent in the sense that the migrating adatoms can cross the steps without visiting a kink position. In the case of significant transparency the velocity of a given step in a given moment is affected by atomic processes which took place at rather distant terraces in rather earlier moments. The reason is that an adatom needs time to cross many terraces before attaching to a kink position. That is why the theory of crystallization with non-local kinetics should account for the time dependence of the adatom concentrations on the terraces. Such a transient growth model (going beyond the quasi-static approximation of the BCF theory of crystal growth) is developed here for the limit of slow kinetics at the steps and fast surface diffusion. This model predicts instability of the vicinal surface (in agreement with the earlier studies) but rather different expression for the wavelength of the most unstable mode, which depends only on the ratio between the step transparency and the step kinetic coefficients.     

Orientation and structure of triple step staircase on vicinal Si(1 1 1) surfaces

The vicinal Si(1 1 1) surface, inclined towards the direction, was investigated by scanning tunnelling microscopy and spot profile analysing low energy electron diffraction. It has been established that the surface, consisting of regularly spaced triple steps and (1 1 1) terraces with a width equal to that of a single unit cell of the Si(1 1 1)-7 × 7 surface structure, has the (7 7 10) orientation. An atomic model of the triple step is proposed.     

A two-region diffusion model for current-induced instabilities of step patterns on vicinal Si(1 1 1) surfaces

We study current-induced step bunching and wandering instabilities with subsequent pattern formations on vicinal surfaces. A novel two-region diffusion model is developed, where we assume that there are different diffusion rates on terraces and in a small region around a step, generally arising from local differences in surface reconstruction. We determine the steady state solutions for a uniform train of straight steps, from which step bunching and in-phase wandering instabilities are deduced. The physically suggestive parameters of the two-region model are then mapped to the effective parameters in the usual sharp step models. Interestingly, a negative kinetic coefficient results when the diffusion in the step region is faster than on terraces. A consistent physical picture of current-induced instabilities on Si(1 1 1) is suggested based on the results of linear stability analysis. In this picture the step wandering instability is driven by step edge diffusion and is not of the Mullins–Sekerka type. Step bunching and wandering patterns at longer times are determined numerically by solving a set of coupled equations relating the velocity of a step to local properties of the step and its neighbors. We use a geometric representation of the step to derive a nonlinear evolution equation describing step wandering, which can explain experimental results where the peaks of the wandering steps align with the direction of the driving field.     

Semiconductor–metal–semiconductor transition: valence band photoemission study of Ag/Si(1 1 1) surfaces

The , and 6×6 phases of Ag/Si(1 1 1) have been studied by angle-resolved photoemission and low-energy electron diffraction. The Ag/Si(1 1 1) surface has an intrinsic semiconducting character with two fully occupied, dispersing surface state bands. We find that only one of the additional surface bands on the surface is metallic in contrast to the two metallic bands discussed in the literature. On the 6×6 surface, the partially occupied surface band of the surface seems to be absent, resulting in a gap of about 0.2 eV with respect to the Fermi-level.     

Step bunching to step-meandering transition induced by electromigration on Si(1 1 1) vicinal surface

The step configuration of a vicinal Si surface is studied under electromigration and a gradient of temperature. An abrupt transition (ΔT = 4 °C) from step-meandering to step bunching is found at 1225 °C for a step-down direct-current direction. This transition starts by random fluctuations which then extend on the whole surface. The transition is studied in the framework of a linear stability analysis of the usual Burton-Cabrera-Frank model by comparing the amplification factors of step-meandering and step bunching instabilities. Both compete at a given temperature, but since the amplification factors behave differently with temperature, bunching abruptly supersedes meandering above a critical temperature.     

On the microscopic origin of the kinetic step bunching instability on vicinal Si(0 0 1)

A scanning tunneling microscopy/atomic force microscopy study is presented of a kinetically driven growth instability, which leads to the formation of ripples during Si homoepitaxy on slightly vicinal Si(0 0 1) surfaces miscut in [1 1 0] direction. The instability is identified as step bunching, that occurs under step-flow growth conditions and vanishes both during low-temperature island growth and at high temperatures. We demonstrate, that the growth instability with the same characteristics is observed in two dimensional kinetic Monte Carlo simulation with included Si(0 0 1)-like diffusion anisotropy. The instability is mainly caused by the interplay between diffusion anisotropy and the attachment/detachment kinetics at the different step types on Si(0 0 1) surface. This new instability mechanism does not require any additional step edge barriers to diffusion of adatoms. In addition, the evolution of ripple height and periodicity was analyzed experimentally as a function of layer thickness. A lateral “ripple-zipper” mechanism is proposed for the coarsening of the ripples.     

Stable surface termination on vicinal 6H-SiC(0 0 0 1) surfaces

Ordered nanofacet structures on vicinal 6H-SiC(0 0 0 1) surfaces, consisting of pairs of a (0 0 0 1) basal plane and a facet, are investigated in terms of stable surface stacking of the (0 0 0 1) basal planes. The surface termination of S3 (or S3*), i.e., ABC (or A*C*B*), was suggested by a structural model based on quantized step-bunching, which typically gives a one-unit-cell bunched step configuration at the facet. Here, we evaluate the surface termination at basal planes covered with a layer of silicon oxynitride by means of quantitative low-energy electron diffraction (LEED) analysis combined with scanning tunneling microscopy (STM), and show the validity of the structural model proposed.     

Surface structure evolution during Sb adsorption on Si(1 1 1)–In(4×1) reconstruction

The Sb adsorption process on the Si(1 1 1)–In(4×1) surface phase was studied in the temperature range 200–400 °C. The formation of a Si(1 1 1)–InSb (2×2) structure was observed between 0.5 and 0.7 ML of Sb. This reconstruction decomposes when the Sb coverage approaches 1 ML and Sb atoms rearrange to and (2×1) reconstructions; released In atoms agglomerate into islands of irregular shapes. During the phase transition process from InSb(2×2) to Sb (θ_Sb>0.7 ML), we observed the formation of a metastable (4×2) structure. Possible atomic arrangements of the InSb(2×2) and metastable (4×2) phases were discussed.     

Growth of single-domain monatomic In chain arrays on the vicinal Si(0 0 1) surface

Using the annealed vicinal Si(0 0 1) surface with 4° miscut toward the [1 1 0] direction as a substrate, single-domain monatomic In chain arrays have been fabricated. High-resolution STM images reveal that deposited In atoms preferentially form In dimers between two neighboring Si dimer rows along the step edges on the lower terrace. Formation of In dimers removes the surface dangling bonds and saturates the In valency. With increasing coverage, the In dimers develop into straight monatomic In chains along the step running direction. It is found that the ordered narrow terrace and rebonded double-layer (D_B) step edge are the keys for the formation of monatomic In chains.     

Si 2p and O 1s photoemission from oxidized Si(0 0 1) surfaces depending on translational kinetic energy of incident O2 molecules

The influence of translational kinetic energy of incident O₂ molecules for the passive oxidation of the partially oxidized Si(0 0 1) surface has been studied by photoemission spectroscopy. The incident energy of O₂ molecules was controlled up to 3 eV by a supersonic molecular beam technique. Two incident energy thresholds (1.0 and 2.6 eV) were found out in accordance with the first-principle calculations. Si 2p and O 1s photoemission spectra measured at representative incident energies showed the incident energy induced oxidation at the backbonds of the dimer and the second layer (subsurface) Si atoms. Moreover, the difference of oxygen chemical bonds was found out to be as the low and the high binding energy components in the O 1s photoemission spectra. They were assigned to bridge sites oxygen and dangling bond sites oxygen, respectively.     

Evidence for diffusion-limited kinetics during electromigration-induced step bunching on Si(1 1 1)

We have measured how the initial terrace width l₀ on vicinal Si(1 1 1) surfaces influences the rate of step bunching and the minimum terrace width within a bunch when direct-current heated at 940-1290 °C. A comparison of this data with analytic solutions and numerical simulations of the conventional “sharp-step” model give strong evidence that the kinetic length d is relatively small (d < ∼20 nm) in both temperature regime I (∼850-950 °C) and regime III (∼1200-1300 °C), in which step-down current is required for step bunching. This indicates that surface mass transport is diffusion-limited in both regimes I and III when l₀ > 20 nm, and hence that the adatom attachment- and terrace diffusion-hopping rates are of comparable magnitude. We also observe similar scaling with initial terrace width in temperature regime II (∼1040-1190 °C), in which step-up current is required for bunching, suggesting a similar step bunching mechanism in all three temperature regimes.     

RAS: An efficient probe to characterize Si(0 0 1)-(2 × 1) surfaces

A complete inspection of the capabilities of reflectance anisotropy spectroscopy (RAS) in studying the adsorption of molecules or atoms on the Si(0 0 1)-(2 × 1) surface is presented. First, a direct comparison between RA spectra recorded on the clean Si(0 0 1)-(2 × 1) and the corresponding topography of the surface obtained using scanning tunneling microscopy (STM) allows us to quantify the mixing of the two domains that are present on the surface. Characteristic RA spectra recorded for oxygen, hydrogen, water, ethylene, benzene are compared to try to elucidate the origin of the optical structures. Quantitative and qualitative information can be obtained with RAS on the kinetics of adsorption, by monitoring the RA signal at a given energy versus the dose of adsorbate; two examples are presented: H₂/Si(0 0 1) and C₆H₆/Si(0 0 1). Very different behaviours in the adsorption processes are observed, making of this technique a versatile tool for further investigations of kinetics.     

Anisotropic profile decay on perturbed Au(111) vicinal surfaces

The anisotropy of the surface free energy has a large influence on the decay rate of periodic surface profiles at temperatures below the roughening temperature. This has been demonstrated by a study of the periodic surface profiles that were etched into vicinal Au(111) crystals in two orientations: with their wavevector parallel (ψ = 0) and perpendicular (ψ = π/2) to the intrinsic step direction of the vicinal surface. Due to this modulation, steps become either oscillatory in shape or remain straight, with their average separation modulated. Accordingly, profile decay is driven primarily by step self-energy (ψ = 0) or step interaction energy (ψ = π/2). The rate of decay was measured under ultra-high vacuum conditions at 750°C using a scanning tunneling microscope for imaging. A large anisotropy of decay was observed on two vicinal crystals of 1.5 and 5° miscut, with the ψ = 0 decay being much faster than with ψ = π/2). The results are evaluated and discussed in the framework of recent theory.     

Water adsorption on hydrogenated Si(1 1 1) surfaces

The adsorption of water on the hydrogen terminated Si(1 1 1) surface is studied by means of first-principles calculations as well as contact angle measurements. Possible initial adsorption configurations for single water molecules and the potential energy surface are calculated. Only small adsorption energies of the order of meV are predicted. Calculations for higher coverage show that the water-water interactions are stronger than the water-surface bonding. The contact angle formed between a water droplet on the surface approximated from the total-energy calculations amounts to 88°, while our measured value is 91°.     

Mobility enhancement of 2DEG in MOVPE-grown AlGaN/AlN/GaN HEMT structure using vicinal (0 0 0 1) sapphire

Al_0.25Ga_0.75N/AlN/GaN high electron mobility transistor (HEMT) structures were grown on (0 0 0 1) sapphire substrates with vicinal angles of 0.0^°,0.25^°,0.5^°$0.0^{°},0.25^{°},0.5^{°}$ and 1.0^°$1.0^{°}$ by metalorganic vapor phase epitaxy (MOVPE). Vicinal sapphire was demonstrated to enhance the step-flow growth to improve morphology, crystal and optical qualities, which eventually suppressed interface scattering and dislocation scattering to enhance the mobility of 2-dimension-electron-gas (2DEG). The optimum vicinal degree was determined to be 0.5^°$0.5^{°}$, and the corresponding 300 K Hall mobility and sheet resistance were 1720 cm²/V s and 301 Ω/sq, respectively. Furthermore, the temperature dependence of Hall measurements proved good high-temperature performance of the 0.5-off sample.     

Si(111)衬底上GaN的MOCVD生长

利用LP-MOCVD在Si（111）衬底上,以高温AIN为缓冲层,分别用低温GaN（LT-GaN）和偏离化学计量比富Ga高温GaN（HT-GaN）为过渡层外延生长六方相GaN薄膜。采用高分辨率双晶X射线衍射（DCXRD）,扫描电子显微镜（SEM）,原子力显微镜（AFM）和室温光致荧光光谱（RT-PL）进行分析。结果表明,有偏离化学计量比富Ga HT-GaN为过渡层生长的GaN薄膜质量和光致荧光特性均明显优于以LT-GaN为过渡层生长的GaN薄膜,得到GaN（0002）和（1012）的DCXRD峰,其半峰全宽（FWHM）分别为698s和842s,室温下的光致荧光光谱在361nm处有一个很强的发光峰,其半峰全宽为44．3meV。     

Chemical interactions in noncontact AFM on semiconductor surfaces: Si(111), Si(100) and GaAs(110)

Total-energy pseudopotential calculations are used to study the imaging process in noncontact atomic force microscopy (AFM) on Si(111), Si(100) and GaAs(110) surfaces. The chemical bonding interaction between a localised dangling bond on the atom at the apex of the tip and the dangling bonds on the adatoms in the surface is shown to dominate the forces and the force gradients and, hence, to provide atomic resolution. The lateral resolution capabilities are tested in both the Si(100) and the GaAs(110) surfaces. In the first case, the two atoms in a dimer can be resolved due to the dimer flip induced by the interaction with the tip during the scan, while in the GaAs(110), we identify the anion sublattice as the one observed in the experimental images.     

Observation of triangular terraces and triangular craters of CaF2 film on Si(1 1 1) substrate

We have investigated the growth mode and surface morphology of CaF₂ film on Si(1 1 1)7×7 substrate by reflection high-energy electron diffraction (RHEED) using very weak electron beam and atomic force microscopy (AFM). It was found by RHEED intensity oscillation measurements and AFM observations that three-dimensional (3D) islands grow at RT; however, rather flat surface appears with two-dimensional (2D) islands around 300 °C. Especially, at high temperature of 700 °C, characteristic equilateral triangular terraces (or islands) with flat and wide shape grow with the tops directed toward [1 1 −2] of substrate Si(1 1 1). On the other hand, the desorption process of the CaF₂ film due to electron stimulated desorption (ESD) was also examined. It was found that the ESD process at 300 °C forms characteristic equilateral triangular craters on the film surface with the tops (or corners) directed toward [−1 −1 2] of substrate Si(1 1 1), provided that the film was grown at 700 °C.     

Si（100）（2×1）表面光学二次谐波强度随温度的变化关系

通过测量基频光波长为１．０６４μｍ时几个不同掺杂类型和掺杂浓度的Ｓｉ（１００）（２×１）样品表面反射二次谐波强度随温度的变化关系，说明在此波长上二次谐波不是来源于表面耗尽场的影响，而是来源于表面态电子。Ｓｉ（１００）（２×１）表面反射二次谐波强度反比于温度的平方。本文提出了一个简单模型，给出了初步解释。