Double-height Steps Research Articles

Surface reconstructions of (0001) AlN during metal‐organic vapor phase epitaxy

AbstractWe observed (0001) AlN wurzite surfaces by atomic force microscopy after 500 nm regrowth in metal‐organic vapor phase epitaxy. The steps changed from double to single height with decreasing V/III ratio. The single height step edges were alternating smooth and rough due to the two different step types on (0001) wurzite surfaces. By reducing the V/III ratio, the widths equalize for terraces with smooth and rough edges, until Al terminated steps start to dominate, and thus promote again double height steps. Using in situ ellipsometry at nm under static conditions, we could directly identify three different surface reconstructions at high , low , and without which correlates with the different step terminations.

Atomic Resolution in Situ Imaging of a Double-Bilayer Multistep Growth Mode in Gallium Nitride Nanowires.

We study the growth of GaN nanowires from liquid Au-Ga catalysts using environmental transmission electron microscopy. GaN wires grow in either ⟨112̅0⟩ or ⟨11̅00⟩ directions, by the addition of {11̅00} double bilayers via step flow with multiple steps. Step-train growth is not typically seen with liquid catalysts, and we suggest that it results from low step mobility related to the unusual double-height step structure. The results here illustrate the surprising dynamics of catalytic GaN wire growth at the nanoscale and highlight striking differences between the growth of GaN and other III-V semiconductor nanowires.

Formation of alkaline-earth template layers on Ge(100) for oxide heteroepitaxy: Self-organization of ordered islands and trenches

The first step required for oxide heteroepitaxy with atomically abrupt interfaces on Si and Ge(100) is the formation of an alkaline-earth template layer. The atomic structure of this template layer on Ge(100) was characterized using scanning tunneling microscopy and electron diffraction. At elevated temperatures, Sr immediately roughens the surface; a transition that can be associated with Sr displacing Ge from the surface. With increasing Sr coverage a series of ordered (3 \ifmmode\times\else\texttimes\fi{} 4), (3 \ifmmode\times\else\texttimes\fi{} 2), (9 \ifmmode\times\else\texttimes\fi{} 1), and (6 \ifmmode\times\else\texttimes\fi{} 1) phases were observed. Transitions between these phases were accompanied by morphological changes: formation of the (3 \ifmmode\times\else\texttimes\fi{} 4) phase smoothed surface; transition to a local (3 \ifmmode\times\else\texttimes\fi{} 2) ordering was accompanied by trench formation; ordering of the trenches led to the (9 \ifmmode\times\else\texttimes\fi{} 1) structure; and finally, the (6 \ifmmode\times\else\texttimes\fi{} 1) structure was characterized by atomic rows. For both Sr and Ba, highly ordered arrays of one-dimensional islands could be produced with double-height steps preventing orthogonal domain formation. We associate the morphological transitions with strain relief of the surface phases and interactions of step ledges.

Growth and Characterization of InSb films on Si (001)

ABSTRACTThe replacement of native oxides with deposited oxides in CMOS technology opens the door to replacing the Si with semiconductors without high-quality native oxides. For example, the use of InSb in logic applications could allow much lower operating voltages and power dissipation due to the InSb channels reaching saturation at significantly lower electric fields. Epitaxy of InSb onto Si could be done directly or using an intermediate layer such as GaP, GaAs, or InP. In the current work we describe the growth of InSb on Si (001) and discuss the structural and electrical properties of the resulting InSb films. The samples were characterized in terms of background electron concentration, mobility, deep level traps, Hall sensitivity, and x-ray rocking curve width.Samples were grown using molecular-beam epitaxy in a Riber-Compact 21T system. Antimony was supplied with a Veeco valved cracker cell. Vicinal Si(001) substrates offcut by 4º toward [110] were prepared by repeated oxidation and oxide-removal and then loaded into the MBE system. After the substrate temperature had been increased to about 820ºC, the surface shows a clear 24 reconstruction and appears to be free of oxide. This reconstruction remains until the substrate temperature reaches 1015ºC, at which temperature a 21 appears, indicating a dominance of double-height steps. After allowing the substrate to cool to the intended growth temperature for InSb, it is exposed to cracked Sb, resulting in the surface going from 21 to 11. This 11 reconstruction remains throughout the subsequent InSb deposition. InSb was deposited with a Sb/In flux ratio of about 5 and a growth rate of 0.2 nm/s. We have investigated growth temperatures between 300 and 420ºC for growth. To prevent the formation of the defects we introduced in some samples GaSb/AlSb supperlattice buffer layer. The best structural quality has been achieved at a growth temperature of 420ºC using GaSb/AlSb supperlattice buffer layer, resulting in our best electron mobility of 2.6104 cm2/Vs for a 2m film at room temperature. The samples grown at 420°C have the narrowest x-ray rocking curve width (FWHM of about 950 arcsec). Deep level noise spectra indicate the existence of the deep levels. The sample with the best crystal quality and highest mobility has the lowest traps. The deep levels have a temperature dependent behavior.

Early stage of Ge growth on Si(001) vicinal surfaces with an 8° miscut along[110]

The atomistic pathway towards the growth of semiconductor heterostructures on vicinal surfaces is investigated in a special experiment. A step-by-step study of the early stages of Ge deposition at T=600 degrees Celsius on a 8 degrees off Si(001) surface miscut along [110] is performed by scanning tunneling microscopy (STM). The microscopic processes occurring during growth are identified. Highly resolved STM images show how double height steps, which characterize the clean substrate, evolve by a step flow process generated by ad-dimer chains located at specific positions. This process leads to the formation of metastable single domains until the development of {105} faceted ripples extending along the whole surface in the miscut direction.

Scanning tunneling microscopy study of Au growth on Ge(0 0 1): Bulk migration, self-organization, and clustering

Gold growth on Ge(0 0 1) was studied as a function of coverage and growth temperature using scanning tunneling microscopy (STM). Initial Au deposition at 475 K resulted only in two-dimensional Ge islands and numerous vacancies on the surface. The Ge islands were attributed to Ge atoms ejected onto the surface by Au moving sub-surface. At higher temperatures the vacancies ordered into (1 + 2 + 1) dimer vacancy complexes, and step structures not seen on clean Ge surfaces were observed. When more Au was deposited between 475 and 800 K, alternating bright and dim chains appeared on the surface that were attributed to Au–Au and Au–Ge dimer rows, respectively. An increase in the density of double-height steps was observed as the chains formed suggesting that the Ge atoms incorporated into the mixed dimer rows were extracted from step edges. Increasing the Au coverage further caused the chains to organize into well-ordered (4 × 2) domains. After depositing 1.5 ML of Au at 675 K the surface was completely covered by the chains. Similar chains have been observed during Pt growth on Ge but not during Ag growth, supporting the idea that the 5d metals favor the formation of one-dimensional chains. After the surface was covered with chains, additional Au aggregated into three-dimensional clusters. The smaller of these clusters exhibited a rectangular shape consistent with the formation of Au(1 1 0) clusters. The [1 1 0]-orientation was confirmed through atomic resolution imaging of the tops of larger clusters. Larger clusters, however, adopted a different shape—an asymmetric octagon—and had their top surfaces tilted with respect to the Ge(0 0 1) surface. The tilt was attributed to stress relief due to the lattice mismatch, which broke the equivalence between the surface energies of inclined facets on opposite sides of the cluster, accounting for the asymmetric shape. Annealing the Au-covered surfaces to 1000 K caused all of the Au to move into the bulk leaving a defective Ge surface.

Site-specific dissociation of N 2 on the stepped Ru(1 0 9) surface

The dissociative adsorption of molecular nitrogen on Ru(1 0 9) is investigated by temperature-programmed desorption (TPD) and low-energy electron diffraction (LEED). This surface presents double-height steps separated by ∼9-atom wide Ru(0 0 1) terraces. Thermal dissociation of N 2 is shown to be site-specific to the steps leaving atomic N on the steps (step-N), while electron-stimulated dissociation of N 2 adsorbed on Ru(1 0 9) can also provide atomic N on the terraces (terrace-N) as well as on the steps. The corresponding activation energy for the thermally-activated N 2 dissociation on the steps is measured to be 0.26 ± 0.05 eV. We postulate that the steps on Ru(1 0 9) catalyze not only a unique reaction channel for the N 2 dissociation but also provide active sites during subsequent catalytic reactions by providing more stable adsorption sites for dissociated N atoms.

Sb-Mediated Si heteroepitaxial growth on Ge(001)

Abstract Silicon heteroepitaxial growth on Ge(0 0 1) was studied with and without an Sb surfactant using scanning tunneling microscopy. Growth was studied at 570 K where Ge segregation was kinetically inhibited but Sb segregation was not. Without Sb, elongated rectangular 2D islands were initially observed. As the 2D islands grew they left behind long, straight trenches in the first layer that paralleled the island dimer rows. Second layer islands grew until they reached the trenches and formed double height steps. A barrier to descend the double height steps along with nucleation at domain boundaries in the second layer islands then caused 3D pyramids followed by more rounded 3D mounds to form. The 3D features caused the surface roughness to rapidly increase with Si coverage. In contrast, Si growth on Sb-terminated Ge(0 0 1) led to a high density of smaller, more isotropic islands. The growth was still not layer-by-layer, however, the surface roughness increased only very slowly with coverage. The smoother growth was attributed to the change in island shape. Because diffusion along dimer rows is much faster than across them, for the same island area, the downward flux is enhanced on square islands compared to islands elongated along their dimer row direction. In addition, the shape change prevents the long straight trenches from forming in the first layer that subsequently caused the 3D pyramids and mounds to form. The results for Si are compared with previous data for Ge homoepitaxy with and without Sb.

Nitrogen-induced nanometre-scale faceting of Cu(410)

Abstract Scanning tunnelling microscopy has been used to investigate the morphological and atomic-scale structural changes to a Cu(4 1 0) surface induced by adsorbed atomic nitrogen. This (1 0 0) vicinal face is found to restructure to form ordered arrays of (1 0 0) nanofacets, separated by double-height steps of local (2 1 0) or (3 1 0) orientation, with a periodicity of approximately three times that of the step spacing of the original (4 1 0); step bunches to produce (2 1 0) and (3 1 0) nanofacets restore the correct average orientation. The (1 0 0) nanofacets show a c(2 × 2) structural phase similar to that found for N adsorption on extended Cu(1 0 0) surfaces, but do not show the novel c(2 × 2)-N 50 A island ordering seen on Cu(1 0 0), usually attributed to the role of local compressive surface stress. However, single atomic steps occur across the (1 0 0) nanofacets at 100–200 A intervals, and this modified morphology is attributed to the role of the narrow facets in allowing lateral strain to reduce the compressive stress. The absence of (1 1 0) facets associated with full agglomeration of surface steps is discussed in terms of the structure of the pseudo-(1 0 0) outermost layer of the (2 × 3) reconstruction which characterises the adsorption of N on extended Cu(1 1 0) surfaces.

Nitrogen-induced nanometre-scale faceting of Cu(410)

Scanning tunnelling microscopy has been used to investigate the morphological and atomic-scale structural changes to a Cu(4 1 0) surface induced by adsorbed atomic nitrogen. This (1 0 0) vicinal face is found to restructure to form ordered arrays of (1 0 0) nanofacets, separated by double-height steps of local (2 1 0) or (3 1 0) orientation, with a periodicity of approximately three times that of the step spacing of the original (4 1 0); step bunches to produce (2 1 0) and (3 1 0) nanofacets restore the correct average orientation. The (1 0 0) nanofacets show a c(2 × 2) structural phase similar to that found for N adsorption on extended Cu(1 0 0) surfaces, but do not show the novel c(2 × 2)-N 50 Å island ordering seen on Cu(1 0 0), usually attributed to the role of local compressive surface stress. However, single atomic steps occur across the (1 0 0) nanofacets at 100–200 Å intervals, and this modified morphology is attributed to the role of the narrow facets in allowing lateral strain to reduce the compressive stress. The absence of (1 1 0) facets associated with full agglomeration of surface steps is discussed in terms of the structure of the pseudo-(1 0 0) outermost layer of the (2 × 3) reconstruction which characterises the adsorption of N on extended Cu(1 1 0) surfaces.

Structure and stability of the Si(1 0 5) surface

Recent experimental studies have shown that well-annealed, unstrained Si(1 0 5) surfaces appear disordered and atomically rough when imaged using scanning tunnelling microscopy. We construct new models for the Si(1 0 5) surface that are based on single- and double-height steps separated by Si(0 0 1) terraces, and propose that the observed surface disorder of Si(1 0 5) originates from the presence of several structural models with different atomic-scale features but similar energies. This degeneracy can be removed by applying compressive strains, a result that is consistent with recent observations of the structure of the Ge/Si(1 0 5) surface.

Investigation of stable high-index silicon surfacesby means of LEED pattern analysis

In the course of a systematic search for stable silicon surfaces, we have studied four stable high-index silicon surfaces, i.e., Si(1,1,11), (1 0 8), (2 1 2), and (15,1,17), by analyzing their low-energy electron diffraction (LEED) patterns. The results show not only that all these surfaces are stable but also that Si(15,1,17) is a major stable surface,whereas the rest three are minor stable surfaces, meaning that their unit cells consist of a nanometer facet of a major stable surface along with a double height step. In view of the periodic facet-step morphology of these minor stable surfaces, we suggest that these surfaces have the potential to be used as templates for growth of periodic nanowires of some suitable materials.

Theoretical modelling of steps on the CaF2(111)surface

The atomic and electronic structure and relaxation of the perfect(111) surface and several low-index surfaces of CaF2 arecalculated using an ab initio Hartree-Fock method. We findmarked differences between the bulk and surface valence bandstructures; however, the band gap is reduced by only 0.5 eV for thestepped surface. The (111) surface is found to exhibit nosignificant relaxation and we obtain a surface energy of0.47 J m-2. The atomic structure of two types of steppedsurface both having 0.32 nm high steps aligned along the [110]crystallographic direction is determined and the step energies arefound to be 0.25 and 0.35 nJ m-1, respectively. Stepenergies for configurations involving other orientations of stepsare considerably higher. The formation of double-height steps along[110], however, is found to be energetically favourable. Aconsiderable amount of relaxation is found for stepped surfaces andwe explain qualitatively why it results in a roughening of thesurface in some cases. The energetic position of gap electronicstates induced by steps on the (111) surface is found to be verysimilar to that for surface states of the perfect (111) surface;i.e. such states do not yield a significant density of states morethan 0.5 eV above the bulk valence band maximum. We discuss how thestep formation energies found here might explain surfacetopographical features found on cleaved and grown CaF2(111) surfaces.

Step bunching on TaC(910) due to attractive step-step interactions

Using scanning tunneling microscopy, we study the step configuration on TaC(910) which is vicinal to the (100) plane, miscut 6.34{sup o} toward the [010] direction. After annealing at {approx}2100{sup o}C, the surface is dominated by double-height steps which are bunched between relatively long intervening (100) terraces. The step-separation distribution is very skewed and sharply peaked. Monte Carlo simulations show that a strong, medium-range, repulsive step interaction A/x{sup 2} must be combined with a weak, long-range, attractive interaction, -B/x{sup 1.0{+-}0.5} to fit the measured distribution. Possible physical origins for these interactions are discussed.

Atomic structure of cluster-ordered array on the Si(001) surface induced by aluminum

We studied the atomic structure of the Si(00l)-c(4×12)-Al surface, which appears in scanning tunneling microscopy (STM) images as an array of clusters in the four-fold direction. Transmission electron diffraction (TED) patterns showed ( h/4, k/12) fractional-order spots after depositing about four monolayers of aluminum on clean Si(00l) surfaces at a substrate temperature of 600°C. A model, which accords with the TED intensity and the observed STM images, consists of a periodical terrace-and-trench geometry in the 12-fold direction. In the four-fold direction, an aluminum ad-dimer is adsorbed every 4 a distance ( a =0.384 nm) on the terraces terminated by double-height steps, making up the cluster array corresponding to bright protrusions in STM images. In addition to ad-dimers, rebonded atoms at the double-height step edge and zigzag isolated dimers in the trench are found to be the building blocks of the c(4×12) structure.

Roughening and preroughening transitions in crystal surfaces with double-height steps

We investigate phase transitions in a solid-on-solid model where double-height steps as well as single-height steps are allowed. Without the double-height steps, repulsive interactions between up-up or down-down step pairs give rise to a disordered flat phase. When the double-height steps are allowed, two single-height steps can merge into a double-height step (step doubling). We find that the step doubling reduces repulsive interaction strength between single-height steps and that the disordered flat phase is suppressed. As a control parameter a step doubling energy is introduced, which is assigned to each step doubling vertex. From transfer matrix type finite-size-scaling studies of interface free energies, we obtain the phase diagram in the parameter space of the step energy, the interaction energy, and the step doubling energy.

Evolution of steps on vicinal (0001) surfaces of α-alumina

Evolution of the step arrangement on vicinal (0001) surfaces of an α-alumina crystal annealed for 1 h in the range of 1000–1500°C in air was investigated by atomic force microscopy. For T<1200°C, coalescence of monosteps ( h 1=0.2 nm) forms mainly parallel double-height steps. At temperatures >1200°C, steps with larger heights grow either due to direct coalescence or due to the displacement of single steps. During coalescence of steps in the whole temperature range investigated, the direction of the higher steps keeps parallel to that of the initial monosteps over hundreds of microns although it does not correspond to any low index crystalline orientation. Different mechanisms of growth of the steps are discussed. A quantitative estimation of the activation energy of one effective self diffusion process can be performed.

Step Fluctuations on Vicinal Si(113)

The fluctuation properties of steps relevant to step coalescence have been measured using scanning tunneling microscopy on a Si(113) surface miscut along a low symmetry azimuth. In local thermal equilibrium at $710\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, coexistence of single, double, triple, and quadruple steps has been observed. From direct measurement of the step-correlation function, the step stiffness is shown to be proportional to the step height, with an additional stabilization of double-height steps. This result is shown to require a step-step attraction energy close to $\mathrm{kT}$.

The influence of monolayer coverages of Sb on the optical anisotropy of vicinal Si(001)

The influence of monolayer coverages of antimony (Sb) on vicinal silicon (001) is studied by reflectance anisotropy spectroscopy (RAS). Adsorption of Sb at room temperature, which is known to break the Si dimer bond and to cause step reconstruction, changes the shape of the RAS spectrum completely, even before annealing. An interesting feature appears in the region of 3.7 eV, which sharpens when the system is annealed to 300°C to form the Si(001)-(1 × 1)-Sb structure. Our results support the view that Si(001)-(1 × 1)-Sb is a distinct phase and we associate the 3.7 eV peak with optical transitions involving SiSb backbonds. Further annealing to 550°C forms Si(001)-(2 × 1)-Sb, where the Sb atoms are now dimerised. This heat treatment decreases the vicinal surface domain imbalance by splitting up the double-height steps, and the RAS signal is reduced in size. Annealing to 750°C causes Sb desorption and the formation of the 0.25 ML Si(001)-c(4 × 4)-Sb structure, with an increase in optical anisotropy, which we attribute to double-height steps reforming as the Sb dimer-induced strain in the system is alleviated. Complete desorption of Sb restores the Si(001)-(1 × 2) structure, and the original RAS signal.

Formation of Ordered, Anisotropic Organic Monolayers on the Si(001) Surface

It is shown that the SidSi dimers of the reconstructed Si(001) surface can react with the ﷿ bonds of unsaturated organic molecules to produce well-defined organic films with novel physical properties. Scanning tunneling microscopy (STM) studies show that the resulting layers are ordered both translationally and rotationally, with the SidSi dimers acting as a template for extending the translational and rotational order from the silicon substrate to the organic film. STM images and infrared spectroscopy experiments show that by using vicinal Si(001) surface having primarily double-height steps, the rotational order of the molecules can be preserved over macroscopic length scales, leading to measurable anisotropy in optical properties. It is proposed that this chemistry may provide a general method for formation of controlled organic films on Si(001) surfaces.