Strain relief in Cu-Pd heteroepitaxy

We present experimental and theoretical studies of Pd/Cu(100) and Cu/Pd(100) heterostructures in order to explore their structure and misfit strain relaxation. Ultrathin Pd and Cu films are grown by pulsed laser deposition at room temperature. For Pd/Cu, compressive strain is released by networks of misfit dislocations running in the [100] and [010] directions, which appear after a few monolayers (ML) already. In striking contrast, for Cu/Pd the tensile overlayer remains coherent up to about 9 ML, after which multilayer growth occurs. The strong asymmetry between tensile and compressive cases is in contradiction with continuum elasticity theory and is also evident in the structural parameters of the strained films. Molecular dynamics calculations based on classical many-body potentials confirm the pronounced tensile-compressive asymmetry and are in good agreement with the experimental data.     

应变异质外延生长中的自组织纳米结构

具有纳米尺度的三维岛的阵列能在有应变的外延生长薄膜上自组织生长，包括反量子点和量子分子在内的其他结构在合适的生长或退火条件下也被观测到。讨论了有关这些纳米结构自组织生长的动力学蒙特卡罗模拟计算中的最新进展。     

Virtual-surfactant-induced wetting in strained-layer heteroepitaxy

We show that surface stoichiometry and growth mode are intimately related for heteroepitaxy of InAs on GaO_0.47In_0.53As. Under As-stable conditions during molecular beam epitaxy, the high strain of the InAs film induces a morphological phase-transition from layer-by-layer to island nucleation. In contrast, under In-stable conditions without direct As₄ flux, islanding is inhibited. The In-stabilized surface imposes limitations to the migration of both As and In adatoms and forces layer-by-layer nucleation, thus acting as a virtual surfactant.     

应变异质外延生长中的自组织纳米结构

具有纳米尺度的三维岛的阵列能在有应变的外延生长薄膜上自组织生长,包括反量子点和量子分子在内的其他结构在合适的生长或退火条件下也被观测到.　讨论了有关这些纳米结构自组织生长的动力学蒙特卡罗模拟计算中的最新进展.     

Nucleation of III nitride semiconductors in heteroepitaxy

The nucleation of III nitride semiconductors in heteroepitaxy is theoretically investigated using GaN nucleation on the AlN surface as an example. It is inferred that the mechanism of this process is determined by the temperature at the initial stage of the layer formation (T). At low temperatures (T<500°C), liquid gallium droplets appear and the chemical reaction between the Ga and N atoms results in the formation of GaN nuclei. At substrate temperatures T>650°C, there arise only GaN nuclei. It is revealed that the GaN nucleation is governed by the generalized diffusion coefficient of GaN, which is a combination of the diffusion coefficients for gallium and nitrogen atoms. It is shown that the generalized diffusion coefficient of GaN on the crystal surface increases by seven orders of magnitude as the growth temperature increases from 600 to 800°C. This is accompanied by a change in the growth mechanism of the III nitride semiconductor epitaxial layers.     

Nucleationless three-dimensional island formation in low-misfit heteroepitaxy

The formation of faceted three-dimensional islands during growth of low-misfit Si1-xGex alloys on Si(100) has been investigated by low-energy electron microscopy. The formation of the islands in these alloy systems does not involve three-dimensional nucleation, but rather proceeds via a precursor array of shallow, stepped mounds on the surface that result from the inherent morphological instability of the strained alloy film.     

Deviations from exact epitaxial positions in heteroepitaxy

Epitaxy provides thin films in a perfect periodic structure. In heteroepitaxy the misfit may yield perfect pseudomorphic films or relaxed films, which may show the periodicities of the substrate and the film and their combinations. Then non-periodic defects like mosaics may appear, e.g., due to dislocations. Deviations from exact epitaxial positions in strictly periodic structures can be determined from diffraction intensities. For more complex surfaces, e.g., films with defects like mosaics or large superstructures or on substrates with steps such a strict determination is not possible. From the spot profile analysis of low energy electron diffraction (LEED) or X-ray data some structural information is available for these surfaces with defects. This new type of evaluation is demonstrated with spot profile analysis of LEED (SPA-LEED) for ultrathin Pb films on Si(1 1 1)7 × 7 surface, which grow even at 25 K epitaxially in a layer-by-layer growth mode. The analysis provides the first information on deviation from the exact epitaxial positions of the surface Pb atoms. A vertical shift of domains and an inclination between domains provides an explanation of the experimental results.     

Pulsed laser deposition of complex oxide heteroepitaxy

The modern studies of complex oxides have been mainly driven by the development of advanced growth and characterization techniques, which provide researchers unprecedented access to new insights and functionalities of these materials. Epitaxial growth of thin films and related architectures offers a pathway to the discovery and stabilization of a wide spectrum of new possibilities in conjunction with the availability of high quality materials that produced with larger lateral sizes and being grown constrainedly. Compared with conventional growth techniques, such as sputtering, spin coating, sol–gel processes, metal-organic chemical vapor deposition, molecular beam epitaxy and so on, no other single advance in the creation of oxide materials has had as pronounced an impact as pulsed laser deposition. In pursuit of the fruitful functionalities and exciting physical phenomena among complex oxides, pulsed laser deposition technique has played an important role to fulfill the flurry of complex oxides in recent decades. In this article, we focus on the details of the growth of epitaxial oxide thin films and the related polymorphs, as well as recent advances in control of the oxide heteroepitaxy via pulsed laser deposition.     

Color image encryption by using the rotation of color vector in Hartley transform domains

We propose an algorithm to encrypt color image by using the rotation of color vector based on discrete Hartley transform. The three component images (red, green and blue) of color image are regarded as the axes of Cartesian coordinates. Two random angle shifts are introduced to rotate the color vectors composed by the three color components in discrete Hartley transform domains in image encryption process. The corresponding rotation shifts of the two angles can serve as the key of the scheme. Moreover the encrypted image is encoded with real number. Some numerical simulations have demonstrated the possibility of the proposed scheme.     

Origin of apparent critical thickness for island formation in heteroepitaxy

We find that a continuum model of heteroepitaxy exhibits a sharp crossover with increasing coverage, from planar growth to island formation. The "critical thickness" at which this Stranski-Krastanov transition occurs depends sensitively on misfit strain, with a dependence strikingly similar to that seen experimentally. The initial planar growth occurs because of intermixing of deposited material with the substrate. While the transition is strictly kinetic in nature, it depends only weakly on growth rate. The role of surface segregation is also discussed.     

Local strain-mediated chemical potential control of quantum dot self-organization in heteroepitaxy

From observations of self-assembly of Ge quantum dots directed by substrate morphology, we propose the concept of control of ordering in heteroepitaxy by a local strain-mediated surface chemical potential. Using quite simple lithography, we demonstrate directed quantum dot ordering. The strain part of the chemical potential is caused by the spatially nonuniform relaxation of the strained layer, which in our study is the Ge wetting layer, but, more generally, can be a deposited strained buffer layer. This model provides a consistent picture of prior literature.     

Island, pit, and groove formation in strained heteroepitaxy

We study the morphological evolution of strained heteroepitaxial films using a kinetic Monte Carlo method in three dimensions. The elastic part of the problem uses a Green's function method. Isolated islands are observed under deposition conditions for deposition rates slow compared with intrinsic surface roughening rates. They are hemispherical and truncated conical for high and low temperature cases, respectively. Annealing of films at high temperature leads to the formation of closely packed islands as in instability theory. At low temperature, pits form via a multistep layer-by-layer nucleation mechanism in contrast to the conventional single-step nucleation process. They subsequently develop into grooves, which are energetically more favorable.     

The testing of stress-sensitivity in heteroepitaxy GaN/Si by Raman spectroscopy

In this paper the stress-sensitive features of hexagonal-GaN (H-GaN) and cubic-Si (C-Si) were investigeted. The H-GaN films have been grown on Si (1 1 1) substrates by metal-organic chemical vapor deposition (MOCVD). The Raman peaks of GaN E₂ (high) and Si (TO) have a blueshift when applying displacement-loadings which parallel the (0 0 0 2) plane of H-GaN. According to the relationship between stress changing and Raman peak shifts, the values of compressive stress in both materials were larger with increasing the displacement-loadings. The stress-sensitivity of H-GaN up to 93.5 MPa/μm which higher than C-Si which testing is 467.9 MPa/μm and the nonlinear error σ of GaN films is 0.1639 and Si is 0.0698. The measurement has a great significance to deeply research the piezoelectric polarization of H-GaN in future. This finding is important for the understanding and application of nitride semiconductors.     

GaSb/GaAs heteroepitaxy characterized as a stress-free system

Thin layers of GaSb grown on GaAs by molecular beam epitaxy have been studied by Raman spectroscopy and transmission electron microscopy. In spite of the high value of the lattice mismatch (8%), these systems reveal a good crystalline quality: as a matter of fact, the lattice dynamics analysed through the resonant Raman scattering lead to a large correlation length for the optical modes. Moreover, they evidence the unstrained nature of the epilayers. The absence of the residual strain is connected to the highly regular network of Lomer dislocations observed by TEM. Obtaining such a perfect array of dislocations is a consequence of both the characteristic of the system (high lattice mismatch, same cationic sites) and the peculiar growth conditions (ideal surface preparation due to the buffer layer, adequate temperature allowing the direct relaxation of GaSb by island growth).