Formation Of Step Bunches Research Articles

Phase-field crystal modeling of shape transition of strained islands in heteroepitaxy

The phase-field crystal (PFC) model is employed to study the shape transition of strained islands in heteroepitaxy on vicinal substrates. The influences of both substrate vicinal angles β and the lattice mismatch ξ are discussed. The increase of substrate vicinal angles is found to be capable of significantly changing the surface nanostructures of epitaxial films. The surface morphology of films undergoes a series of transitions that include Stranski-Krastonov (SK) islands, the couple growth of islands and the step flow as well as the formation of step bunching. In addition, the larger ξ indicates an increased strained island density after coarsening, and results in the incoherent growth of strained islands with the creation of misfit dislocations. Coarsening, coalescence and faceting of strained islands are also observed. Some facets in the shape transition of strained islands are found to be stable and can be determined by β and crystal symmetry of the film.

Formation of Step Bunches Induced by Flow in Solution

We study the formation of step bunches induced by flow in solution during growth. In our previous study [M. Inaba and M. Sato: J. Phys. Soc. Jpn. 80 (2011) 074606], we showed that the step-down flow in solution causes bunching. In this research, we study the dependence of step behavior on some parameters. With a slow flow, the separation and coalescence between steps and bunches occur frequently during step bunching. With increasing flow rate, the frequency decreases and tight bunches are formed. The decrease in the frequency also occurs with increasing strength of the repulsion between steps.

Formation and morphology of step bunches during B-segregation on vicinal Si(111)

Degenerately boron-doped vicinal Si(111) substrates were annealed at high temperatures to achieve complete saturation of the ‘dangling bonds’ by surface segregating boron atoms under reconstruction to the Si(111) ( 3 × 3 ) R 30 ∘ phase. Different cooling procedures were applied to obtain well ordered superstructures. The surface morphology of the prepared substrates was studied by spot profile analysis of low energy electron diffraction (SPA-LEED). The splitting of the diffraction peaks into multiple satellites points to the formation of step bunches of different inclination (equivalent to different terrace widths) during the segregation process due to changes in the surface free energy. A model with step bunches consisting of steps with monoatomic or biatomic heights and large (111)-oriented terraces is developed to explain our results. Steep step bunches with short terraces and biatomic step heights are formed independently of the preparation process while flat step bunches with large terraces are dissolved with increasing annealing time.

Formation of epitaxial graphene on SiC(0001) using vacuum or argon environments

The formation of graphene on the (0001) surface of SiC (the Si-face) is studied by atomic force microscopy, low-energy electron microscopy, and scanning tunneling microscopy/spectroscopy. The graphene forms due to preferential sublimation of Si from the surface at high temperature, and the formation has been studied in both high-vacuum and 1 atm argon environments. In vacuum, a few monolayers of graphene forms at temperatures around 1400 °C, whereas in argon a temperature of about 1600 °C is required in order to obtain a single graphene monolayer. In both cases considerable step motion on the surface is observed, with the resulting formation of step bunches separated laterally by ≳10 μm. Between the step bunches, a layer-by-layer growth of the graphene is found. The presence of a disordered, secondary graphitic phase on the surface of the graphene is also identified.

Carbon-Cap for Ohmic Contacts on Ion-Implanted 4H–SiC

A pyrolyzed photoresist film is commonly used as a protective cap of the surface of ion-implanted 4H-SiC wafers during the postimplantation annealing process with the aim to prevent Si sublimation and step bunching formation. Such a film that is called carbon-cap (C-cap) is always removed after postimplantation annealing and before any other processing step of the SiC wafer. Here, we show that this C-cap is a continuous, hard, black, mirrorlike, and planar thin film that can be patterned by a reactive ion etching O 2 -based plasma for the fabrication of ohmic contact pads on both Al + - and P + -implanted 4H-SiC. This C-cap material has an electrical resistivity of 1.5 × 10 -3 Ω cm and a good resistance against scratch. Al (1% Si) wires can be ultrasonically bonded on the C-cap pads. Such a bonding and the C-cap adhesion to the implanted 4H-SiC surface are stable for electrical characterizations in vacuum between room temperature and 450°C. The measured specific contact resistance of the C-cap on a 1 × 10 20 cm -3 p+-implanted 4H-SiC is 9 × 10- 5 Ω cm 2 at room temperature. Micro-Raman characterizations show that this C-cap is formed of a nanocrystalline graphitic phase.

Surface Chemistry and Nanotopography of Step-Bunched Silicon Surfaces: in-system SRPES and SPM Investigations

The formation of step bunching was observed by atomic force microscopy on n-type Si(111) surfaces during the electrodeposition of noble metals under semiconductor depletion conditions. The surface chemical analysis performed by synchrotron radiation photoelectron spectroscopy (SRPES) indicates the formation of an ultra-thin oxide film along with the topological transformation. Step bunching is interpreted in terms of site-specific etching controlled by the reactivity of kink sites and step edges together with the surface accumulation of holes supplied by the reduction of Pt-chloride complexes via the valence band.

Step formation on hydrogen-etched 6H-SiC{0 0 0 1} surfaces

The formation of step bunches and/or facets on hydrogen-etched 6H-SiC(0 0 0 1) and ( 0 0 0 1 ¯ ) surfaces has been studied, using both nominally on-axis and intentionally miscut ( i.e. vicinal) substrates. It is found that small miscuts on the (0 0 0 1) surface produce full unit-cell high steps, while half unit-cell high steps are observed on the ( 0 0 0 1 ¯ ) surface. The observed step normal direction is found to be 〈 1 1 ¯ 0 0 〉 for both surfaces. Hence, for intentionally miscut material, a miscut oriented towards this direction produces much better order in the step array compared to a miscut oriented towards a 〈 1 1 2 ¯ 0 〉 direction. For (0 0 0 1) vicinal surfaces that are miscut towards the 〈 1 1 ¯ 0 0 〉 direction, the formation of surface ripples is observed for 3° miscut and the development of small facets (nanofacets) is found for higher miscut angles. Much less faceting is observed on miscut ( 0 0 0 1 ¯ ) surfaces. Additionally, the (0 0 01) surface is found to have a much larger spatial anisotropy in step energies than the ( 0 0 0 1 ¯ ) surface.

Formation of step bunching in the epitaxial growth of SrRuO 3 thin films

The formation of step bunching in SrRuO 3 epitaxial films has been investigated by exploring the thickness dependence of the morphology. We have observed that pronounced step meandering in the thinner films modifies locally the terrace width. With subsequent growth, areas with double steps form and coexist with other having single steps. The areas with bunching grow and eventually coalesce. We will argue that adatom kinetics is the main driving force for step bunching nucleation.

Step formation, faceting, and bunching in atomically flat SrTiO3 (110) surfaces

We demonstrate that single-terminated atomically flat surfaces of polar SrTiO3 (110) can be obtained by annealing in air at temperatures above 1000°C. We show that a morphological transition occurs from disordered step edges to faceted step edges in the range of 1000–1100°C with appropriate annealing conditions. The influence of miscut angles (polar and azimuthal) on the surface nanostructure, i.e., step bunching and faceting of step edges, is described. We show how the azimuthal angle has a critical role for the step-edge faceting and the step-bunching formation.

Vertical composition fluctuations in (Ga,In)(N,As) quantum wells grown on vicinal (1 1 1)B GaAs

In this work, we present a detailed transmission electron microscopy analysis of the interfacial structure and composition uniformity of (Ga,In)(N,As) quantum wells grown by molecular beam epitaxy on vicinal GaAs(1 1 1)B substrates. Vertical composition fluctuations inside the (Ga,In)(N,As) quantum well are detected depending on the growth conditions, in particular the V/III flux ratio and the growth rate. This vertical composition fluctuation due to the phase separation tendency is in contrast to the (0 0 1) case, where the fluctuations proceed in the lateral direction. The specific character of the phase instabilities is discussed with respect to the spinodal decomposition of the (Ga,In)(N,As) alloy grown by step-flow on the misoriented (1 1 1)B substrates. The vertical composition fluctuations are explained by the formation of step bunches of alternating composition as a consequence of the different propagation velocity of steps with different atom terminations.

Metal organic vapor phase epitaxy of BiSbTe3 films on (001) GaAs vicinal substrates

We have investigated the growth of BiSbTe3 films by metal organic vapor phase epitaxy on (001) GaAs vicinal substrates. A detailed experimental study of the surface morphologies of BiSbTe3 films grown on GaAs (001) vicinal surfaces was carried out and is discussed in this paper with a view to understanding the step-step interaction kinetics that result in step bunching. BiSbTe3 layers grown on nominal (001) GaAs substrates exhibit triangular facet structures consisting of atomically flat plateau areas separated by steps. In contrast, the growth of films on vicinal substrates was found to result in regular arrays of terrace step structures. The formation of step bunches and rectangular terraces on such film surface is attributed to the preferential incorporation of adatoms at step or kink sites and lateral growth rate anisotropy.

Influence of chemical etching on step bunching formation on GaAs (100) during thermal oxide removal

We report on the influence of the chemical etching in the step bunching formation on GaAs (100) 0.2° off and GaAs (100) 2° off toward <010> surfaces after the oxide desorption process of the substrates at high temperatures under an H 2 ambient. The step bunches were observed by in-air atomic force microscopy. Our results reveal that step bunches were formed only on chemically etched samples desorbed under an As 4 overpressure using H 2 as the carrier. When GaAs substrates without chemical etch were deoxidized the step bunches were not observed in despite of the H 2/As 4 presence. From the results we conclude that the chemical etching strongly influence the step bunching formation.

Growth and characterization of InAs epitaxial layer onGaAs(111)B

The behavior of InAs deposition on $\mathrm{Ga}\mathrm{As}(111)B$ substrates and the corresponding routes toward strain relaxation have been investigated. InAs growth was for depositions ranging from 2 monolayers to 30 monolayers. Over this deposition range, different routes for strain relaxation caused by the lattice mismatch were observed. The strain relaxed through ragged step edge formation and Ga-In intermixing for low InAs deposition and through the formation of step bunching and dislocations for thicker depositions.

Coarsening of step bunches in step flow growth: a reaction–diffusion model and its travelling wave solutions

We consider a model of the Frank impurity mechanism in step flow growth based on reaction–diffusion equations. The equations, based on the BCF model of crystal growth, are designed to capture the physics of both the diffusion of adatoms, and the appearance of impurities on the crystal surface following the model proposed by Frank. The model was originally considered by Kandel and Weeks [Physica D 66 (1993) 78; Phys. Rev. B 49 (1994) 5554; Phys. Rev. B 52 (1995) 2154]. The model is a fundamentally two-dimensional one. It exhibits quick formation of step bunches, followed by a much longer period of coarsening, in the direction lateral to the step flow direction. Using tools from the theory of reaction–diffusion equations we are able to reduce the complicated pattern formation of the model to a simple dynamical picture. In most regions, step bunches form quickly, and these step bunches are equilibrium solutions of the equations. As the step bunches form, transitions form between different step bunches. These transitions are travelling wave solutions of the equations. These travelling waves govern the coarsening process. The spatial patterns coarsen as the different travelling waves march across the surface of the crystal and encounter and annihilate each other. By classifying the equilibrium states and travelling waves of the equations we can collapse the number of travelling waves to a small reduced set. This reduction of the dynamics to travelling waves from a small set of equivalent classes, is the basis of a simple reduced model which, despite its simplicity, can capture in entirety the coarsening process arising from the original equations. Using this reduced model, we can investigate large scale nature of the coarsening process, and behaviour of the coarsening process, and reconsider some of the issues raised by Kandel and Weeks in their original analysis.

Step-bunching in SiGe layers and superlattices on Si [formula omitted

The vicinal Si(0 0 1) surfaces exhibit a bunching instability of the atomic height steps. Here we present a study to investigate the effect of germanium on the kinetic instability which occurs in silicon homoepitaxy. For the growth parameters employed we find no evidence for strain-induced step-bunching neither in single Si 1− x Ge x layers nor in Si/Si 1− x Ge x superlattices. The effect of germanium is to kinetically suppress the formation of step-bunches. No significant influence of the Ge-related strain on the surface morphology can be found. The effects of growth temperature and the influence of the amount of deposited pure Si are very pronounced.

Differential phase-shifting interferometry for in situ surface characterization during solution growth of crystals

We have developed a phase-shifting interferometer for high-resolution in situ imaging of the interfacial morphology during the growth of fast-growing crystals from solution. We demonstrate that the evolution of the surface morphology can be captured as the height distribution over surface areas as large as 2×2 mm2 with a depth resolution of 3 nm and a lateral resolution down to 0.5 μm with a frequency of 10–12 surface images per second. We describe the five image phase-shifting algorithm and subsequent processing, which quantify the surface morphology and yield the height differences between surface features. We illustrate the application of the technique to the (101) face of potassium di-hydrogen phosphate crystals. We visualize and monitor the formation of step bunches on the surface of this crystal during growth.

Step-bunching and strain effects in Si 1− xGe x layers and superlattices on vicinal [formula omitted

A comparative study is reported of kinetically and thermodynamically driven growth instabilities in the Si/SiGe heterosystem on vicinal Si(0 0 1) . Investigating single Si 1− x Ge x -layers and Si 1− x Ge x /Si superlattices, a wide range of the relevant growth parameter space is mapped out. In contrast to earlier reports no evidence for strain-induced step-bunching is found. Single Si 1− x Ge x layers replicate the morphology of the underlying buffer layers and tend to form hut clusters near thermal equilibrium. In Si 1− x Ge x /Si superlattices, no significant influence of the Ge-related strain on the step-bunching morphology can be seen. On the other hand, the effect of growth temperature is very pronounced, again indicating the dominant role of kinetics in the formation of step-bunches.

Step-bunching and strain-effects in Si 1− xGe x layers and Superlattices grown on vicinal Si(001)

A comparative study of kinetically and thermodynamically driven growth instabilities in the Si/SiGe heterosystem on vicinal Si(001) is reported. Single Si 1− x Ge x -layers and Si 1− x Ge x /Si superlattices are investigated by mapping out a wide range of the relevant growth parameter space. In contrast to earlier reports no evidence for strain-induced step-bunching is found. Single Si 1− x Ge x layers replicate the morphology of the underlying buffer layers and tend to form hut clusters near thermal equilibrium. In Si 1− x Ge x /Si superlattices no significant influence of the Ge-related strain on the step-bunching morphology can be seen. On the other hand, the effect of growth temperature is very pronounced, again indicating the dominant role of kinetics in the formation of step-bunches.

Investigation of growth modes of cadmium mercury thiocyanate crystal by atomic force microscopy

Growth modes on the {1 1 0} face of cadmium mercury thiocyanate CdHg(SCN) 4 (CMTC) crystal as a function of supersaturation have been investigated using ex situ atomic force microscopy. The results show that in most cases growth occurs in elementary steps generated by either simple or complex screw dislocation sources. Several kinds of spiral hillocks are observed on this surface. In fact, crystal faces formed on a single hillock are very rare for the CMTC crystal. It is notable that almost all spiral hillocks are formed by the dislocation sources with components of the Burgers vectors perpendicular to the {1 1 0} crystal face of b =1 unit step height, which contribute to the absence of hollow cores at the centers of the spiral hillocks. It is also found that the structures of the dominant growth hillocks differ at the different supersaturations. On the {1 1 0} faces of CMTC crystals, 2D nucleation islands are rarely observed except at high supersaturation ( σ=0.68). The 2D nuclei are circular if not deformed by the adsorption of small 3D particles. During multiple scanning on the surface, some 3D nuclei appear at the edges of 2D nuclei and gradually replace them. The new surprising fact is that at low supersaturation ( σ=0.06) both 2D nuclei and spiral hillocks are imaged simultaneously on the surface, which has never been reported previously. Finally, various step morphologies are presented, in which intense roughness of steps is considered as a remarkable feature of complex compound crystals, and the formation of step bunching is described.

Hydrogen as the Cause of Step Bunching Formed on Vicinal GaAs(001)

In order to elucidate the cause of step bunching formed on vicinal GaAs(001) annealed in AsH3/H2 ambient, we investigated the surface of vicinal GaAs(001) annealed in H2, AsH3/N2, AsH3/H2, and N2 by scanning tunneling microscope. Since step bunches always formed on surfaces annealed with hydrogen but not on surfaces annealed in ambients without hydrogen, we conclude that hydrogen is crucial in the formation of step bunching.