Step-flow Mechanism Research Articles

Domain structure and surface morphology of epitaxial PbTiO 3/SrRuO 3 heterostructures on vicinal (001) SrTiO 3 substrates

Epitaxial ferroelectric PbTiO 3/SrRuO 3 heterostructures have been grown on (001) SrTiO 3 substrates with 1.7 and 5° miscut toward [010] by rf magnetron sputtering. X-ray diffraction indicates that heterostructures with thin PbTiO 3 layer (120, 400, and 1250 Å) consist of purely c-axis PbTiO 3 domain, while the thicker PbTiO 3 layer (2500 Å) contains both a-axis and c-axis PbTiO 3 domains. The bottom SrRuO 3 layer is single domain with [110] normal orientation. Furthermore, both PbTiO 3 and SrRuO 3 layers are of very good crystalline quality from the measurements of the full width at half maximum of rocking curve and in-plane φ scans. Atomic force microscopy studies show that both SrRuO 3 and PbTiO 3 layers are grown by step flow mechanism, which results in very smooth surfaces (the root mean square roughness ≈12 Å for a 400 Å PbTiO 3/SrRuO 3 heterostructure on a 1.7° miscut substrate over a 2×2 μm area). Our results demonstrate that the high quality epitaxial ferroelectric PbTiO 3 heterostructures can be prepared by using SrRuO 3 bottom electrodes on miscut (001) SrTiO 3 substrates for ferroelectric device applications.

Epitaxial growth of Fe on Mo(110) studied by scanning tunneling microscopy

Scanning tunneling microscopy has been used to investigate the growth of Fe on Mo(110). Growth was carried out at room temperature with growth rates between 0.03 and 0.5 ML min −1. Initially, the first atomic layer is completed independently of the rate of evaporation. At low deposition rates the further growth proceeds in a multi-layer fashion whereas at higher rates the layer growth is partially preserved. Preferred growth along the [001] direction is observed in some cases. One-dimensional dislocation lines are observed in the second-layer islands. At higher coverage a two-dimensional dislocation network occurs which vanishes for thicker films. At a temperature of 600 K Fe on Mo(110) grows by a step flow mechanism in the first layer followed by the formation of wedge-shaped three-dimensional islands. A comparison with Fe grown on W(110) at a similar deposition rate is provided.

Screw-like structures in ex situ Tl2Ba2CuO6 films

Ba2CuO6 films on single-crystal LaAlO3 was investigated using atomic force microscopy and X-ray diffraction. Most of the film surface consisted of flat areas separated by single or several unit-cell steps. This morphology suggests that the films grow predominantly by a step-flow mechanism. A Tl-2201 film grown using a precursor film deposited at 700 °C exhibited stacks of planes arranged in a screw-like manner on a flat background. Islands of 1–3 μm size and a height of 2–20 unit cells were dominant structures on the surface. Possible causes for the formation of the screw-like structures are discussed.

Transport, Growth Mechanisms, and Material Quality in GaN Epitaxial Lateral Overgrowth

AbstractGrowth kinetics, mechanisms, and material quality in GaN epitaxial lateral over-growth (ELO) were examined using a single mask of systematically varied patterns. A 2-D gas phase reaction/diffusion model describes how transport of the Ga precursor to the growth surface enhances the lateral rate in the early stages of growth. In agreement with SEM studies of truncated growth runs, the model also predicts the dramatic decrease in the lateral rate that occurs as GaN over-growth reduces the exposed area of the mask. At the point of convergence, a step-flow coalescence mechanism is observed to fill in the area between lateral growth-fronts. This alternative growth mode in which a secondary growth of GaN is nucleated along a single convergence line, may be responsible for producing smooth films observed to have uniform cathodoluminescence (CL) when using 1 μm nucleation zones. Although emission is comprised of both UV (∼365nm) and yellow (∼550nm) components, the spectra suggest these films have reduced concentrations of threading dislocations normally associated with non-radiative recombination centers and defects known to accompany growth-front convergence lines.

Study for fabricating large area diamond single-crystal layers

A study on the feasibility of a process for making large area diamond, single-crystalline films using a microwave plasma assisted chemical vapour deposition reactor is presented. After choosing the (100) orientation for the seed crystals' top faces, the operating conditions of the plasma reactor were optimised in such a way that step flow mechanism or two-dimensional nucleation mechanism can be favoured. To prevent the development of facets at the crystal edges, (100) orientation was chosen for side faces and step flow growth was favoured. Penetration twins formation has been limited, using low substrate temperature and adding a very low amount of nitrogen in the feed gas. Finally, growth on top of 0.08-0.8 cm 2 mosaics is presented, and the layer grown at the interface analysed.

A novel bi-directional step-flow growth mode: C 60 on Ge(100) and GaAs(110)

A novel epitaxial step-flow growth mechanism is revealed in the growth of C 60 overlayers on Ge(100) and GaAs(110) surfaces using scanning tunneling microscopy. In contrast to the conventional step-flow growth mode, molecules attached to a substrate atomic step protrude well above its top edge and hence create an inverted step. This new step in turn acts as a nucleation site for subsequent growth over the upper terrace, and flows in the direction of ascending steps, in conjunction with the step flow on the lower terrace in the direction of descending steps. This bi-directional step-flow growth mode is a direct consequence of a large out-of-plane lattice mismatch, and thus should be common in the growth of large molecules or clusters on monoatomically stepped substrates.

Power-law behavior of step roughening with surface diffusion

We study the scaling behavior of the step width w while the crystal is growing from vapor via the step-flow mechanism. The growth exponent β ≡ d ln w d ln t varies in several stages when the microscopic mechanism of step fluctuation changes. Eventually, after the density of kinks becomes sufficiently high so that the step can be regarded as rough, the exponent β takes the value ∼ 1 4 or ∼ 1 3 depending on the impingement flux f, as is predicted by the theory.

Ultra-smooth, highly ordered, thin films of La0.67Ca0.33MnO3±d

We report the growth and characterization of ultra-smooth, highly ordered, strained La2/3Ca1/3MnO3±d thin films (approximately 600 Å in thickness). The thin films were deposited by ozone-assisted, block-by-block, molecular beam epitaxy on SrTiO3 (001) substrates without any post-deposition annealing. Scanning tunneling microscopy images showed oriented, unit-cell-high terraces characteristic of a step-flow growth mechanism. The root-mean-square roughness of the surface of an imaged film was determined to be 2 Å. This same film showed negative magnetoresistance values, [R(H=0)−R(Happlied)]/R(H=0), at 150 K of 93% and 5% in applied magnetic fields of 5.12 and 4×10−2 T, respectively.

Fabrication of YBa2Cu3Ox thin-film flux transformers using a novel microshadow mask technique for in situ patterning

A novel microshadow mask technique for in situ patterning of multilayers is presented. It is ideally suited to fabricate YBa2Cu3Ox(YBCO) and insulator lines with gently sloping edges, needed for high quality insulated superconducting crossovers. The critical current density jc (T=77 K) of a YBCO/SrTiO3/YBCO crossover exceeds 2×106 A/cm2 in both the bottom and the top YBCO stripline. The insulating SrTiO3 layer of 200 nm thickness displays a high resistivity of ρ≳108 Ω cm (T=77 K). The extremely smooth morphology of the edges has been revealed by cross sectional transmission electron microscopy, indicating a stepflow mechanism of YBCO growth. Multiturn flux transformers with a 15 μm linewidth input coil spiral have been fabricated by this microshadow mask technique. A transformer with a pickup loop area of 7 mm2 has been coupled to a 1 mm2 washer dc SQUID in flip chip geometry. In comparison to the bare SQUID a magnetic flux gain factor of 9 has been obtained. The white noise level of this setup was determined to be 8×10−5 Φ0/Hz1/2 at 77 K. It was entirely due to the intrinsic noise of the employed dc SQUID itself. The 1/f noise level increased a factor of 2.

Microstructural Study of CMR Films as a Function of Growth Temperature, As-Deposited and Annealed

AbstractThe properties encompassed by the family of complex metal oxides span the spectrum from superconductors to insulating ferroelectrics. Included in this family are the new colossal magnetoresistive perovskites with potential applications in advanced high density magnetic data storage devices based on single or multilayer thin films units of these materials fabricated by vapor phase deposition (PVD) methods. The realization of this potential requires solving basic thin film materials problems requiring understanding and controlling the growth of these materials. Toward this end, we have grown La0.7Ca0.3MnO3 and La0.7Sr0.3MnO3 on LaAlO3 single crystal substrates by pulsed laser and RF sputter deposition at temperatures from 500° C to 900° C and annealed at over 900° C for about 10 hours. The evolution of the microstructure of these films was studied by scanning probe microscopies and transmission electron microscopy (TEM).The results of SPM characterization showed that at the lower end of the growth temperature range, the as-grown films were polygranular with grain size increasing with temperature. The 500° C as-grown films appeared to be amorphous while the 750° C film grains were layered with terrace steps often one unit cell high. In contrast, films grown at 900° C consisted of coalesced islands with some 3-D surface crystals. After annealing, all films had coalesced into very large extended layered islands. The change in microstructure was reflected in a decreased resistivity of coalesced films over their unannealed granular precursors. Previous reported work on the growth of La0.84 Sr0.16MnO3 and La0.8Sr0 2CoO3 grown demonstrated the sensitivity of the microstructure to substrate and deposition conditions. Films grown on an “accidental” vicinal surface grew by a step flow mechanism.

Molecular beam homoepitaxial growth of MgO(001)

We describe homoepitaxial growth and detailed in situ characterization of MgO(001). We have used, for the first time, high-speed Auger electron spectroscopy as a real-time probe of film composition during growth. Excellent short-range and long-range crystallographic order are achieved in films grown to a thickness of several hundred angstroms in the substrate temperature range of 450 °C to 750 °C. Moreover, the films become more laminar as the growth temperature increases, suggesting that MgO grows homoepitaxially by the step-flow growth mechanism at elevated temperature. The surfaces of films grown at 650°and 750 °C are smoother than those obtained by cleaving MgO(001).

Surface morphology of Pb overlayers grown on Si(100)-(2 x 1).

The behavior of Pb overlayers deposited on Si(100)-(2\ifmmode\times\else\texttimes\fi{}1) at room temperature was investigated by scanning-tunneling microscopy and low-energy electron microscopy. Surface reconstructions of (2\ifmmode\times\else\texttimes\fi{}2), c(4\ifmmode\times\else\texttimes\fi{}8), (2\ifmmode\times\else\texttimes\fi{}1), and c(4\ifmmode\times\else\texttimes\fi{}4) were observed as Pb coverage increased from 0.5 to 1.5 monolayers. After the appearance of the c(4\ifmmode\times\else\texttimes\fi{}4) phase, Pb islands with threefold-symmetric (111) orientation were observed on the twofold- or fourfold-symmetric Si(100) surface. The transformation of the island shape during growth can be broadly categorized as hexagonal\ensuremath{\rightarrow}triangular\ensuremath{\rightarrow}hexagonal. An internal structure of stepped-down depression was formed on all islands. As deposition continued, the depression was gradually filled by a step-flow growth mechanism. Our observations suggest that the initial growth of the islands is dominated by diffusion of Pb atoms on the substrate to the island edges; later, when the islands become sufficiently large, growth is determined mainly by atom diffusion on the Pb(111) islands.

Gas phase etching of Si(111)-(7×7) surfaces by oxygen observed by scanning tunneling microscopy

Structural and mechanistic aspects of the initial stages of gas phase etching of Si(111)-(7×7) surfaces, by reaction with O2 at temperatures between 910 and 1070 K, were investigated by scanning tunneling microscopy. On smaller terraces, the reaction is found to proceed via a step flow mechanism, where Si is effectively removed at the edges of retracting terraces. On very large terraces, nucleation and growth of vacancy islands contribute in addition. The data lead to a reaction mechanism involving the formation of volatile SiO on the terraces, by reaction of O adatoms and Si surface atoms, subsequent vacancy diffusion, and finally the annihilation of vacancies at step edges or in vacancy islands. The temperature dependence of the resulting step structures, ranging from irregular and nearly dendritic forms at ∼910 K via faceted steps at ∼970 K to rounded shapes above 1000 K, results from step pinning by oxide nuclei and annealing effects at higher temperatures.

Surface topography changes induced by chemical vapor deposition of aluminum on Al(111)

Aluminum was deposited by chemical vapor deposition (CVD) on an Al(111) surface, using trimethylamine alane (TMAA). The precursor dissociates on the surface and the products (trimethylamine and hydrogen) desorb at temperatures in excess of 350 K. Surface topographical changes are observed during the CVD process by thermal energy atomic helium scattering. Growth at 400 K proceeds by a step-flow mechanism.

RHEED studies of steps, islanding and faceting on singular, vicinal and high-index surfaces

The atomic-scale morphology of singular, vicinal and high-index surfaces and their growth mechanisms during molecular beam epitaxy (MBE) have been studied by reflection high-energy electron diffraction (RHEED). The growth mechanism of the singular GaAs (001) surface is governed by a competition between layer-by-layer growth and kinetic roughening. On vicinal GaAs (001) surfaces tilted toward the (1 1 1) As plane the terrace width fluctuations increase with the decrease in misorientation, although a step flow mechanism is maintained. The GaAs (311) Ga surface shows a unique behaviour. After breaking up into {331} facets, the well-ordered corrugated surface reproduces during growth, since neither nucleation nor diffusion processes over long distances are essential. On single-layer stepped Si (001) surfaces misoriented toward [110], a preferential incorporation of Si atoms in one of the inequivalent step types is possible even for low tilt angles. It is, however, hindered by arsenic adsorption.

Trends in Solute Segregation Behavior During Silicon Solidification

ABSTRACTAt the high growth rates accessible during pulsed-laser induced melting and solidification and explosive crystallization, crystal growth kinetics are dominated not by equilibrium thermodynamics, but by the atomistic mechanisms by which crystallization proceeds. These Mechanisms can be probed by testing the predictions of solute trapping models based on various crystal/Melt interface structures against Measurements. We have measured the dependence of solute trapping of several group III, IV, and V elements in silicon on both interface orientation and crystallization speed. The Aperiodic Stepwise Growth Model of Goldman and Aziz accurately fits both the velocity and orientation dependence of the solute trapping observed in these systems. The success of the model implies a ledge structure for the crystal/Melt interface and a step-flow mechanism for crystal growth. In addition, we have observed an empirical inverse correlation between the two free parameters (“diffusive speeds”) in this model and the equilibrium solute partition coefficient of a system. This correlation may be used to estimate values of the diffusive speeds for other systems in which solute trapping has not been or cannot be Measured.

Stranski-Krastanov growth of AgBr on NaCl(001)

The Stranski-Krastanov growth of AgBr on NaCl cleavage faces has been studied by means of the surface decoration method. The existence of an intermediate monolayer, formed by nucleation and coalescence of two-dimensional (2D) islands and by step-flow mechanism, has been revealed by the gold decoration technique. The transition from 2D growth to 3D growth of AgBr is observed at a coverage above one monolayer of AgBr. The 3D islands are epitaxially grown on top of the intermediate layer and are aligned with respect to the NaCl substrate with the epitaxial relationship [100](001)AgBr‖[100](001)NaCl.

Kinetic mechanism for the transformation of single-layer steps into double-layer steps by Si deposition on a vicinal Si(100) surface.

We perform energy calculations with the Stillinger-Weber potential to determine how the Si atoms deposited on a Si(100) surface with single-layer steps migrate to produce a surface with double-layer steps, and why the system does not evolve by the usual step flow mechanism.

In situ STM imaging of high temperature oxygen etching of Si(111)(7 × 7) surfaces

Gas phase etching of Si(111)(7 × 7) surfaces, by reaction with O 2 at ≈950 K, was directly followed by in situ STM imaging. The reaction mechanism was found to strongly depend on the density of structural defects in the respective (7 × 7) surface layer, which can act as nucleation centers for pit formation, and on the terrace size. Under present conditions we observed a step-flow mechanism for removal of the topmost, defect-free Si layer and formation of monolayer deep pits in the defective (7 × 7) layers in the further reaction process. The resulting surface is rough on an atomic scale, with typically three layers exposed.

Homo-epitaxial growth of CdTe by sublimation under low pressure

A new method to obtain a twin-free single crystal of CdTe on a CdTe substrate by sublimation is described. When CdTe(111)A substrates were employed for the homo-epitaxial growth of CdTe, twin crystals were frequently obtained. The substrate of CdTe(211)A and (211)B, however, gave no twins resulting in single crystals of high quality. The difference may come from the existence of many steps, sufficient to suppress two-dimensional nucleation and to promote step flow mechanism. To obtain twin-free films, therefore, a fairly large tilt angle of the substrate from a singular plane and a fairly low supersaturation are essential.