Growth and relaxation processes in Ge nanocrystals on free-standing Si(001) nanopillars

We study the growth and relaxation processes of Ge crystals selectively grown by chemical vapour deposition on free-standing 90 nm wide Si(001) nanopillars. Epi-Ge with thickness ranging from 4 to 80 nm was characterized by synchrotron based x-ray diffraction and transmission electron microscopy. We found that the strain in Ge nanostructures is plastically released by nucleation of misfit dislocations, leading to degrees of relaxation ranging from 50 to 100%. The growth of Ge nanocrystals follows the equilibrium crystal shape terminated by low surface energy (001) and {113} facets. Although the volumes of Ge nanocrystals are homogeneous, their shape is not uniform and the crystal quality is limited by volume defects on {111} planes. This is not the case for the Ge/Si nanostructures subjected to thermal treatment. Here, improved structure quality together with high levels of uniformity of the size and shape is observed.     

Vertical dislocations in Ge films selectively grown in submicron Si windows of patterned substrates

The structural properties of straight screw dislocations extended in the [001] direction formed in squared- and line shaped- Ge(001) films selectively grown on submicron regions of Si(001) substrates were investigated by transmission electron microscopy. The screw dislocations propagating as a result of spiral surface growth were redirected toward the SiO₂ sidewalls. This redirection is linked to the formation of facets such as {111} facets in the growing Ge films. In the process of strain relaxation upon annealing, the screw dislocations were dissociated into dislocations with Burgers vectors of the a/2<110> type, which glided on the {111} surfaces and disappeared.     

Geometry dependent nucleation mechanism for SiGe islands grown on pit-patterned Si(001) substrates

SiGe islands grown on pit-patterned Si(001) substrates show significant dependence on the surface geometry of the substrate, especially when the period of the patterning is reduced to below 300 nm. With different geometry of pit-patterned substrates, SiGe islands are observed to preferentially nucleate at the bottom of shallow pits after a ripple formation of the SiGe wetting layer, or at the top terrace when the pits are deep and steep.     

Investigation of Si and Ge growth on Si3N4/Si

The growth of Si and Ge on silicon nitride surfaces has been investigated using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and Auger electron spectrometer (AES). In the early stages, Si or Ge nanoclusters appeared irrespective of the different substrates. When annealed, the Si clusters were more stable against coalescence than those of Ge. As these clusters continued to grow, crystalline facets started to form. Both Si and Ge islands grew predominantly with (111)-oriented top facets on the crystalline Si₃N₄(0001)/Si(111). By contrast, they both grew in random orientation on the amorphous Si₃N₄ surface. Low-index facets such as (111) and (001) coexisted with high-index facets such as (113).     

Growth mechanism of ZnO nanowires via direct Zn evaporation

Zinc oxide (ZnO) nanowire synthesized from direct Zinc (Zn) vapor transport in O₂ environment has been studied. The results show that the first step is the formation of ZnO film on the substrate. Then anisotropic abnormal grain growth in the form of ZnO platelets takes place. Subsequently, single-crystalline ZnO platelets grow in [0001] direction to form whiskers. During whisker growth, transformation from layer-by-layer growth to simultaneous multilayer growth occurs when the two-dimensional (2D) Ehrlich–Schwoebel (ES) barrier at the ZnO island edge is sufficiently large and the monolayer island diameter is smaller than the island spacing. As multilayered islands grows far away from the base, isotropic mass diffusion (spherical diffusion) will gradually displace anisotropic diffusion (linear diffusion), which contributes to the formation of pyramid on the top plane of the whisker. When the pyramid contains enough atomic layers, the 2D ES barrier transits to 3-dimensional ES barrier, which contributes to repeated nucleation and growth of multilayered islands or pyramids on the old pyramids. The pyramids play a critical role to taper the whisker to nanorod with a diameter less than 100 nm. The nanorod then grows to nanowire via repeated growth of epitaxial hexagonal-pyramid shape-like islands on the (0001)-plane with facets as the slope planes. During coarsening, the breakage of step motion of facets and the appearance of facets on the base of pyramids may result from the step bunching of {0001} facets, which is consistent with the existence of “2D” Ehrlich–Schwoebel barrier on the edge of (0001) facets. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Si_3N_4/Si表面Ge生长过程的STM研究

利用STM和LEED分析了Ge在Si3 N4 /Si(111)和Si3 N4 /Si(10 0 )表面生长过程的结构演变。在生长早期 ,Ge在两种衬底表面上都形成高密度的三维纳米团簇 ,这些团簇的大小均在几个纳米范围内 ,并在高温退火时体积增大。当生长继续时 ,Ge的晶体小面开始显现。在晶态的Si3 N4 (0 0 0 1) /Si(111)表面 ,Ge的 (111)晶向的小面生长比其他方向优先。最后在大范围内形成以 (111)方向为主的晶面。相反 ,在非晶的Si3 N4 表面 ,即Si3 N4 /Si(10 0 ) ,Ge晶体的高指数侧面生长较顶面快 ,最终形成金字塔形的岛结构。对这样的表面生长过程进行了探讨并给出了合理的物理解释     

Ge island assembly on metal-patterned Si: truncated pyramids, nanorods, and beyond

The organization of semiconductor nanostructures into functional macroassemblies remains a fundamental challenge in nanoscience and nanotechnology. In the context of semiconductor epitaxial growth, efforts have focused on the application of advanced substrate patterning strategies for the directed assembly quantum-dot islands. We present a comprehensive investigation on the use of simple metal patterns to control the nucleation and growth of heteroepitaxial islands. In the Ge on Si model system, a square array of metal dots induces the assembly of Ge islands into an extensive two-dimensional lattice. The islands grow at sites between the metal dots and are characterized by unique shapes including truncated pyramids and nanorods, which are programmed prior to growth by the choices of metal species and substrate orientation. Our results indicate that ordering arises from the metal-induced oxidation of the Si surface; the oxide around each metal dot forms an array of periodic diffusion barriers that induce island ordering. The metals decorate the island surfaces and enhanced the growth of particular facets that are able to grow as a result of significant intermixing between deposited Ge and Si substrate atoms.     

Nucleation and growth of germanium on silicon (111)

The nucleation and growth of Ge on Si(111) substrates were investigated by transmission electron microscopy (TEM). It was found that growth is initiated by the formation of three-dimensional islands. At a very early stage of growth a polycrystalline layer was obtained. The orientation of this Ge film, however, improved as growth proceeded, resulting in an epitaxial film when the islands were fully coalesced. It was found that the major defects present in the Ge epilayer were microtwins (primary and secondary twins) occurring on the inclined {111} planes of Ge.     

Gold-catalyzed oxide nanopatterns for the directed assembly of Ge island arrays on Si

The heteroepitaxial growth of Ge on Au-patterned Si(001) is investigated using in situ spectromicroscopy. Patterning of a hydrogen-terminated Si surface with a square array of Au dots followed by brief exposure to air leads to the spontaneous, local oxidation of Si. The resulting oxide nanopattern limits the surface migration of Au during annealing up to 600 degrees C, resulting in complete preservation of the Au pattern. Subsequent deposition of Ge induces a redistribution of Au across the surface even as the oxide nanopattern persists. As a result, the oxide pattern drives the growth of Ge islands into an ordered assembly, while Au decorates the surfaces of the Ge islands and modifies their shape.     

Growth and characteristics of self-assembly defect-free GaN surface islands by molecular beam epitaxy

GaN surface nano-islands of high crystal quality, without any dislocations or other extended defects, are grown on a c-plane sapphire substrate by plasma-assisted molecular beam epitaxy. Nano-island growth requires special conditions in terms of V/III ratio and substrate temperature, distinct from either film or nanocolumn growth. The insertion of a nitrided Ga layer can effectively improve the uniformity of the nano-islands in both shape and size. The islands are well faced truncated pyramids with island size ranged from 30 to 110 nm, and height ranged from 30 to 55 nm. On, the other hand, the density and facet of the GaN surface islands would be affected by the growth conditions. An increase of the V/III ratio from 30 to 40 led to an increase in density from 1.4 x 10(9) to 4.3 x 10(9) cm(-2) and an evolution from {1-21-1} facets to {1-21-2} facets. The GaN layers containing the surface islands can moderate the compressive strain due to the lattice and thermal mismatch between GaN and c-sapphire. Conductive atomic force microscopy shows that the off-axis sidewall facets are more electrically active than those at the island center. The formation of the GaN surface islands is strongly induced by the Ehrlich-Schwoebel barrier effect of preexisting islands grown in the early growth stage. GaN surface islands are ideal templates for growing nano-devices.     

Real-time studies of Ge growth on nanostructured Si substrates

Nanostructured substrates are an interesting path towards the production of quantum dots devoted to microelectronic applications. In this work we report results on the effects of different nanopatterning methods to obtain lateral ordering of Ge islands grown on Si. By using Scanning Tunneling Microscopy we have studied in real-time the wetting layer growth and islands formation on nanopatterned Si substrates at 500 °C. We compare results obtained on Si substrates nanopatterned by using two different techniques: STM lithography and natural patterning induced by surface instabilities such as step bunching. Different issues on both cases have been addressed: substrate preparation, Ge dots placement and growth mode. We have observed that, on Si(001), the Ge islands nucleate near the holes and on Si(111) step bunching can guide the growth of aligned rows of islands.     

Temperature dependent biaxial texture evolution in Ge films under oblique angle vapor deposition

The morphology and texture of Ge films grown under oblique angle vapor deposition on native oxide covered Si(001) substrates at temperatures ranging from 230 °C to 400 °C were studied using scanning electron microscopy, X-ray diffraction and X-ray pole figure techniques. A transition from polycrystalline to {001}<110> biaxial texture was observed within this temperature range. The Ge films grown at substrate temperatures < 375 °C were polycrystalline. At substrate temperatures of 375 °C and 400 °C, a mixture of polycrystalline and biaxial texture was observed. The 230 °C sample consisted of isolated nanorods, while all other films were continuous. The observed biaxial texture is proposed to be a result of the loss of the interface oxide layer, resulting in epitaxial deposition of Ge on the Si and a texture following that of the Si(001) substrates used. The rate of oxide loss was found to increase under oblique angle vapor deposition.     

Epitaxial growth of CoSi2 on Si(001) by reactive deposition epitaxy: Island growth and coalescence

Epitaxial CoSi₂ layers, which are phase pure but contain {111} twins, are grown on Si(001) at 700 °C by reactive deposition epitaxy. Transmission electron microscopy analyses show that the initial formation of CoSi₂(001) follows the Volmer-Weber mode characterized by the independent nucleation and growth of three-dimensional islands whose evolution we follow as a function of deposited Co thickness t_Co in order to understand the origin of the observed twin density. We find that there are two families of island shapes: inverse pyramids and platelets. The rectangular-based pyramidal islands extend along orthogonal 〈110〉 directions, bounded by four {111} CoSi₂/Si interfaces, and grow with a cube-on-cube orientation with respect to the substrate: (001)_CoSi2||(001)_Si and [100]_CoSi2||[100]_Si. Platelet-shaped CoSi₂ islands are bounded across their long 〈110〉 directions by {111} twin planes (i.e. {111}(001)_CoSi2||{111}_Si) and their narrow 〈110〉 directions by {511}_CoSi2||{111}_Si interfaces. The top and bottom surfaces are {22¯1}, with {22¯1}_CoSi2||(001)_Si, and {1¯1¯1}, with {1¯1¯1}_CoSi2||{11¯1}_Si, respectively. The early stages of film growth (t_Co ≤ 13 Å) are dominated by the twinned platelets due to a combination of higher nucleation rates resulting from a larger number of favorable adsorption sites in the Si(001)2 × 1 surface unit cell and rapid elongation of the platelets along preferred 〈110〉 directions. However, at t_Co ≥ 13 Å island coalescence becomes significant as orthogonal platelets intersect and block elongation along fast growth directions. In this regime, where both twinned and untwinned island number densities have saturated, further island growth becomes dominated by the untwinned islands. A continuous epitaxial CoSi₂(001) layer, with a twin density of 2.8 × 10¹⁰ cm^− 2, is obtained at t_Co = 50 Å.     

Orientation of silicon particles in a binary Al–Si alloy

The orientations Si-crystals take in aluminium, in an alloy with composition Al–1.3at%Si, were investigated by transmission electron microscopy. Hardness was measured for isothermal heat-treatments at 175 °C and 260 °C. Conditions analysed by TEM were 17 h at 175 °C and an additional 3 h at 260 °C, both containing a high density of small Si-crystals, the finest corresponding to 175 °C. Two main orientation relationships were found: The first accounted for approximately 60% of Si precipitates in condition 17 h_175 °C. Despite a high number density and well-aligned interfaces, the Si precipitates have negligible influence on hardness. Findings are consistent with Ge particles in Al–Ge alloys.     

Anatase TiO₂ crystal facet growth: mechanistic role of hydrofluoric acid and photoelectrocatalytic activity

This work reports a facile hydrothermal approach to directly grow anatase TiO(2) crystals with exposed {001} facets on titanium foil substrate by controlling pH of HF solution. The mechanistic role of HF for control growth of the crystal facet of anatase TiO(2) crystals has been investigated. The results demonstrate that controlling solution pH controls the extent of surface fluorination of anatase TiO(2), hence the size, shape, morphology, and {001} faceted surface area of TiO(2) crystals. The theoretical calculations reveal that {001} faceted surface fluorination of anatase TiO(2) can merely occur via dissociative adsorption of HF molecules under acidic conditions while the adsorption of Na(+)F(-) is thermodynamically prohibited. This confirms that the presence of molecular form of HF but not F(-) is essential for preservation of exposed {001} facets of anatase TiO(2). Anatase TiO(2) crystals with exposed {001} facets can be directly fabricated on titanium foil by controlling the solution pH ≤ 5.8. When pH is increased to near neutral and beyond (e.g., pH ≥ 6.6), the insufficient concentration of HF ([HF] ≤ 0.04%) dramatically reduces the extent of surface fluorination, leading to the formation of anatase TiO(2) crystals with {101} facets and titanate nanorods/nanosheets. The anatase TiO(2) nanocrystals with exposed {001} facets exhibits a superior photoelectrocatalytic activity toward water oxidation. The findings of this work clarify the mechanistic role of HF for controlling the crystal facet growth, providing a facile means for massive production of desired nanostructures with high reactive facets on solid substrates for other metal oxides.     

Ge-rich islands grown on patterned Si substrates by low-energy plasma-enhanced chemical vapour deposition

Si(1-x)Ge(x) islands grown on Si patterned substrates have received considerable attention during the last decade for potential applications in microelectronics and optoelectronics. In this work we propose a new methodology to grow Ge-rich islands using a chemical vapour deposition technique. Electron-beam lithography is used to pre-pattern Si substrates, creating material traps. Epitaxial deposition of thin Ge films by low-energy plasma-enhanced chemical vapour deposition then leads to the formation of Ge-rich Si(1-x)Ge(x) islands (x > 0.8) with a homogeneous size distribution, precisely positioned with respect to the substrate pattern. The island morphology was characterized by atomic force microscopy, and the Ge content and strain in the islands was studied by μRaman spectroscopy. This characterization indicates a uniform distribution of islands with high Ge content and low strain: this suggests that the relatively high growth rate (0.1 nm s(-1)) and low temperature (650?°C) used is able to limit Si intermixing, while maintaining a long enough adatom diffusion length to prevent nucleation of islands outside pits. This offers the novel possibility of using these Ge-rich islands to induce strain in a Si cap.     

Faceting mechanisms of Si nanowires and gold spreading

We report detailed structural analysis of 〈111〉 oriented silicon nanowires (NWs) grown by UHV–CVD using the VLS process with a gold catalyst. STEM-HAADF observations have revealed an unexpected inhomogeneous distribution of gold nanoclusters on the NW surface. Gold is mainly distributed on three sides among the six {112}-sidewalls and is anchored on upward {111} facets. This original observation brought us a new comprehension of the faceting mechanisms. The stability of the 〈111〉 growth direction needs the formation of facets on {112}-sidewalls with energetically favorable planes. We demonstrate that the initial formation of covered facets with a three-fold symmetry is driven by the formation of {111} Au/Si interfaces between the nucleated Si NW and the Au droplet.     

Morphology-dependent catalytic properties of nanocupric oxides in the Rochow reaction

Four kinds of CuO catalysts with well-controlled leaf-like (L-CuO), flower-like (F-CuO), sea-urchin-like (U-CuO), and oatmeal-like (O-CuO) morphologies were synthesized by a facile precipitation method assisted by various chelating ligands. High-resolution transmission electron microscopy and fast Fourier transform infrared spectroscopy indicated that the dominant crystal facets of L-CuO, F-CuO, U-CuO, and O-CuO were {001}, {1?10}, {001}, and {110}, as well as {001} and {1?10}, respectively. When tested for the Rochow reaction, it was found that their catalytic performances were dependent on their structures. Among the four CuO catalysts, L-CuO exhibited the best catalytic property, along with the strongest adsorption ability for oxygen and highest reducibility, which are mainly because of its largely exposed {001} facet and large specific surface area. In addition, the amount of the Cu₃Si alloy phase, which is the most important reaction intermediate that generated in the reacted region of the Si surface, was measured for the different catalysts. Based on the findings, a detailed reaction mechanism was proposed. This work demonstrates that shape-controlled synthesis of oxide catalysts could be an effective strategy to design and develop efficient catalysts.

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Self-organized growth of nanometric pyramids in ferrimagnetic epitaxial CoCr2O4 films

Self-organized pyramidal islands of ferrimagnetic spinel CoCr₂O₄ were epitaxially grown on MgAl₂O₄ (0 0 1). The surfaces of the islands are {1 1 1} facets, so they can be considered as true three-dimensional objects. Important characteristics of the structures can be tuned by varying growth parameters. Pyramids form above a critical thickness, and the existing granular-like underlying film grow later at a reduced rate. We discuss on the growth mechanism of pyramids and we end comparing the CoCr₂O₄ pyramids with the widely investigated Si–Ge dots.     

Anisotropic etching behaviour of gallium arsenide junctions

A detailed study of the etching behaviour of junctions between gallium arsenide epitaxial layers and substrates of different carrier concentrations and carrier types has been made. Optical and scanning electron microscope examination of {1 1 0} cleavage faces of (001) surface specimens has shown that high index facets are produced by the etchant used to delineate the junction. The facets are anisotropic on orthogonal {1 1 0} faces: for example on a (1 1 0) face of ann ⁺ layer-n substrate configuration the facet extends from the junction into the substrate, whereas on the orthogonal (110) face the facet extends from the junction into the epitaxial layer. The widths of the facets (in 〈001〉 directions) increase linearly with etching time. A simple model is presented which is able to account qualitatively for all the observed characteristics of the junction etching behaviour, and is based on the occurrence of differential etch rates at the epitaxial layer, junction, and substrate regions. A key assumption of the model is that the junction region is anisotropic in [1 1 0] and [¯1 1 0] directions, and reasons for this anisotropy are considered.