Layer Islands Research Articles

Lateral alloy segregation in thin heteroepitaxial films

Alloy segregation is believed to be an important factor in the growth of the wetting layer and subsequent formation of islands during heteroepitaxy. We report simulated annealing results that determine the lowest-energy configuration of a strained system. Our results indicate that in addition to vertical segregation, there is also an energetic driving force for lateral segregation and formation of subsurface features before island formation. We believe that these subsurface features play a crucial role in the nucleation of islands.

X-ray diffraction investigation of a three-dimensional Si/SiGe quantum dot crystal

High-resolution x-ray diffraction was employed to study the structural properties of a three-dimensional periodic arrangement of SiGe quantum dots in a Si matrix. Using extreme ultraviolet lithography at a synchrotron source a two-dimensional array of pits (period $90\ifmmode\times\else\texttimes\fi{}100\text{ }{\text{nm}}^{2}$) was defined and transferred into a (001) Si wafer by reactive ion etching. By molecular-beam epitaxy SiGe islands of about 30 nm diameter and 3 nm height were grown into the pits. Subsequent deposition of Si spacer layers of 10 nm thickness and SiGe island layers results in a three-dimensionally periodic arrangement of quantum dots, mediated by the strain fields of the buried dots. Their so far unmatched structural perfection is assessed by coplanar x-ray diffractometry using synchrotron radiation. Reciprocal-space maps around the (004) and (224) reciprocal-lattice maps were recorded and analyzed to get quantitative information on the disorder of the dot positions and to obtain the mean Ge content of the dots. In addition, information on the strain fields was deduced from the analysis of the diffraction data. Together with atomic force microscopy data on the island shape and size distribution, a complete structural characterization is achieved.

Optimization of Inkjet-Based Process Modules for Printed Transistor Circuits

We report on the development of inkjet-based process modules for printed transistor-based circuits. The requirement for realization of printed transistor-based circuits is the capability to print consistent fine lines for transistor electrodes and uniform/conformal layers of insulators and semiconductor islands on top of the printed electrodes by optimizing the printing parameters and surface energy of the underlying layers. The baseline processes accomplished the above-mention requirement were developed. In addition, we demonstrate novel process modules such as self-alignment of transistor electrodes, printed self-aligned interconnect and printed local interconnect by simultaneous dissolving and connecting the underlying metal layers. Lastly, a printed inverter circuit utilizing the complete printing modules was demonstrated to illustrate the simplicity and the feasibility of the inkjet-based process modules towards the realization of low-cost printed electronics.

Surface- and Air-Temperature Studies in Relation to Land Use/Land Cover of Nagpur Urban Area Using Landsat 5TM Data

Continuously changing temperature trends within the urban regions are worth noticing. This paper investigates the influence of land use/ land cover (LU/LC) of an urban area on the air temperatures. Landsat 5 Thematic Mapper image of December 2000 of Nagpur city is used for the analysis. The calculated correlated coefficients depict a high degree of correlation between the digital number values of thermal band and the air temperatures, which is indicative of the surface temperatures (Ts) as the controlling parameter for the air temperature (Ta). The surface temperature and air temperature were correlated to different LU/LC class in the city. Ts are calculated from the Markham–Barker equation and Ta from the actual field records in each class. The analysis shows that the presence of a specific LU/LC class in the city may be related to the formation of surface heat island and canopy layer heat island within the urban area in Nagpur.

Promoting strain relaxation of Si 0.72Ge 0.28 film on Si (1 0 0) substrate by inserting a low-temperature Ge islands layer in UHVCVD

A flat, fully strain-relaxed Si 0.72Ge 0.28 thin film was grown on Si (1 0 0) substrate with a combination of thin low-temperature (LT) Ge and LT-Si 0.72Ge 0.28 buffer layers by ultrahigh vacuum chemical vapor deposition. The strain relaxation ratio in the Si 0.72Ge 0.28 film was enhanced up to 99% with the assistance of three-dimensional Ge islands and point defects introduced in the layers, which furthermore facilitated an ultra-low threading dislocation density of 5 × 10 4 cm −2 for the top SiGe film. More interestingly, no cross-hatch pattern was observed on the SiGe surface and the surface root-mean-square roughness was less than 2 nm. The temperature for the growth of LT-Ge layer was optimized to be 300 °C.

Electrical conductivity and luminescence of metal-organic nanocomposites

We presented the results of experimental study of electrical and luminescence properties of planar nanocomposites based on gold island films and layers of efficient organic luminophores — aluminum (III) 8-hydroxyquinolinate (Alq3) and europium β-diketonates [Eu(DBM)3 bath, Eu(DBM)3 phen, and Eu(DBM)3]. The mechanisms of electroluminescence of such hybrid systems are discussed and the processes of luminescence excitation in the organic component of nanocomposites are considered in detail. It is shown that the spectral characteristics of europium β-diketonates are controlled by the Eu3+ ion fluorescence. The proposed mechanisms of nanocomposite electroluminescence are confirmed by the results of measurements of cathodoluminescence spectra of organic luminophores.

Study of acoustical phonon modes in superlattices with SiGe QDs

Multilayers with SiGe nanoislands (QD's) grown in a broad temperature range are studied using Raman spectroscopy, HRXRD and compared with similar multilayers without islands. As the growth temperature increases, Si content in the islands increases, partially relieving strain. These structural transformations manifest themselves in both the intensity and frequency of the low‐frequency Raman peaks. Due to composition‐ and strain‐induced changes in the island band structure, excitation conditions come out of resonance, reducing Raman peak intensity. We have shown that at the interpretation of the Raman scattering by folded acoustic phonons for structures with nanoislands the real morphology of the island layer should be considered. The observed series of the low‐frequency Raman peaks, for the multilayered structures with the number of QD layers below ten, is due to the acoustic phonon modes within the islands. The enhancement of the scattering intensity due to resonance of the excitation light with the electronic transitions within the islands plays a significant role. Theoretical analysis was fulfilled in the framework of microscopic approach with using Green function method and taking into account the real structure of QD's and their interaction with surrounding matrix.

Electrochemical deposition of polypyrrole nanolayers on discontinuous ultrathin goldfilms

Ultrathin layers of polypyrrole (PPy) were electrochemically grown between microelectrodes on aSi/SiO2 substrate. Conducting nanolayers of PPy are directly grown onto ultrathin discontinuousgold (Au) film between the microelectrodes, with thicknesses in the range 10–100 nm. Thesystem therefore forms a novel (PPy/Au) nanocomposite conductor. Atomic forcemicroscopy (AFM) imaging and conductivity measurements indicate that at allthicknesses a relatively uniform film is formed but with significant roughness thatreflects the roughness of the metallic island layer. In PPy/Au films with thickness∼10 nm, the small barriers around the gold islands dominate the conduction, and as the filmthickness increases to 100 nm the intrinsic conductivity of highly doped PPy dominates thecharge transport.

Effect of interlayer strain interaction on the island composition and ordering in Ge/Si(001) island superlattices

In this article we present a quantitative study of the influence of the number and the thickness of the silicon spacer layer on the optical and structural properties of single and multilayers of self-assembled Ge/Si(001) islands. By means of cathodoluminescence spectroscopy, high resolution x-ray diffraction, and transmission electron microscopy, we will show that the island composition and strain status of single-layer samples do not depend on the silicon cap-layer thickness. On the contrary, we found that the strain interaction existing between island layers separated by a silicon spacer layer 33 nm thick (i.e., ∼3 times the mean island height), enhances the SiGe intermixing during the deposition of subsequent layers bringing to a composition inhomogeneity of the intermixed islands of about 18% along the stack. This inhomogeneity is not present in samples having a thicker spacer layer (60 nm) although the strain interaction between different layers is strong enough to drive the island stacking along the vertical direction. We conclude that the latter spacer layer thickness allows us to obtain a highly ordered three-dimensional superlattice of an island having a homogeneous size and composition along the stack itself.

Mechanism of formation and electronic structure of semiconducting ZnSb nanoclusters electrodeposited from an ionic liquid

Electrocrystallization of Sb and the compound semiconductor ZnSb has been investigated by in situ SPM methods at the electrified ionic liquid/Au(1 1 1) interface at an elevated temperature of 50 °C for the first time employing the ionic liquid ZnCl 2–[C 4mim] +Cl − (45:55). Prior to the underpotential deposition (UPD) process of Sb, ZnCl 3 − anions adsorb on the gold surface at the open-circuit potential (OCP). An ordered region – showing the characteristic of a Moiré-like pattern – coexists with a disordered region indicative of an interfacial phase transition. When the potential is reduced to −0.40 V versus Pt/Pt(II), 2D electrocrystallization of Sb starts showing a typical 3 × 3 structure of the first monolayer. Further decreasing the potential to −0.5 V a second layer of Sb islands occurs. Stepping the potential from the UPD region to −0.60 V, the OPD of Sb sets in showing randomly dispersed clusters of homogeneous size. Near the ZnSb deposition potential, at ∼−0.95 V, a nearly homogeneous distribution of clusters of spherical shape with diameters up to 15 nm is found. Their corresponding STS curves exhibit an obvious semiconducting behaviour with a gap-energy of ∼0.6 ± 0.2 eV. Experiments at deposition conditions on the Sb-rich or Zn-rich side relative to the ZnSb deposition potential show an obvious doping effect – in the case of Zn excess – which is revealed by the corresponding normalized conductance (NC) spectra.

VARIATION OF IN-PLANE LATTICE CONSTANT OF Si/Ge/Si HETEROSTRUCTURES WITH Ge QUANTUM DOTS

Experimental data are presented on variations of the in-plane lattice constant of Ge and Si films in the course of the MBE film growth on the silicon (100) surface. The in-plane lattice constant of the silicon film is shown to alter as the film grows; the changes reflect the process of relaxation of elastic strains that result from the misfit of the germanium and silicon lattice constants. Due to the presence of germanium islands, a considerably thicker silicon film is required to provide the strain relaxation. The dependence of distortion penetration depth to the silicon film on the effective germanium film thickness is obtained. TEM studies indicate the vertical ordering of the germanium island layers when the thickness of the Si layer in between Ge layers is not sufficient to provide the full strain relaxation.

Vertical and lateral ordering of Ge islands grown on Si(001): theory and experiments

A set of recent results concerning lateral and vertical ordering of Ge islands grown onSi(001) is reviewed. Experimental data generated by chemical vapour deposition andanalysed by atomic force microscopy and photoelectron spectroscopy are compared withcomputer simulations and modelling based on atomistic approaches and continuum theory.In particular, we show that it is possible to probe experimentally the detailedstrain field generated by buried Ge islands at the surface of the Si capping layer.The observed arrangement of small Ge islands grown over the capping layer isdemonstrated to be very close to the one predicted by a simple model where the localchemical potential is inferred from the strain field at the atomic scale, as given byTersoff-potential molecular dynamics simulations. Moreover, we review recentexperimental evidence for lateral ordering, triggered by partial Si capping, in thefirst layer of Ge islands on Si(001). Theoretical support is given by showing thatwhen two islands lie in close proximity the elastic field is likely to generate aflow of atoms leading to an effective gliding motion along opposite directions ofboth islands, eventually stopped by the presence of further neighbouring islands.

Effect of inter-layer strain interaction on the optical properties of Ge/Si(001) island multi-layers

In this paper we present a study on the influence of the number and the thickness of silicon spacer layer on the optical properties of single-and multi-layers of self assembled Ge/Si (001) islands performed by means of cathodoluminescence spectroscopy, high resolution X-ray diffraction and transmission electron microscopy. In single-layer sample, we do not evidence dependence of the island no-phonon emission peak position on the silicon cap-layer thickness. In multi-layer samples having a thin (33 nm) silicon spacer layer the no-phonon emission energy value progressively blue-shifts for an increasing number of island layers. This is interpreted as an enhanced intermixing driven by the strain interaction existing between island layers. On the contrary, island emission energy position is independent on the number of layers in the sample series having a thicker spacer layer (60 nm). These findings are consistent with the X-ray diffraction observation that islands belonging to different layers have the same composition. As a consequence we can conclude that multilayers with 60-nm spaced islands layer are more homogeneous and ordered.

Influence of growth and annealing conditions on photoluminescence of Ge/Si layers grownon oxidized Si surfaces

Ge/Si structures with a layer of Ge islands grown on oxidized Si surfaces and covered withSi were recently found to exhibit intense photoluminescence (PL) in the D1 region(∼0.8 eV) after annealing at high temperatures. We show that this PL is a propertyof the Ge/Si structures grown at low temperatures from about 400 to500 °C, at which crystal defects are introduced through the coalescence of strained three-dimensionalislands. PL features are found to be independent of Ge thickness over a wide range from 0.7to 3 nm. A monotonic increase in PL with annealing temperature is observed up to1000 °C, but PL completely vanishes after annealing at higher temperatures. The transmissionelectron microscopy data show the presence of dense arrays of crystal defects in the Si layercapping the layer of Ge islands.

Localization of electrons in type‐II Ge/Si quantum dots stacked in a multilayer structure

AbstractSpace‐charge spectroscopy was employed to study electronic structure of a single layer of Ge quantum dots and of a stack of four layers of Ge islands embedded in an n ‐type Si(001) matrix. For a multilayer sample, the evidence for an electron localization in strained Si in the vicinity of Ge dots was found. From the temperature‐ and frequency‐dependent measurements the electron binding energy was determined to be 40–70 meV for different quantum states. The electron accumulation was not observed in a sample with a single layer of Ge dots. Existence of localized electronic states is explained by a modification of the conduction band alignment induced by inhomogeneous tensile strain in Si around the buried Ge dots. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

MBE growth of vertically ordered Ge quantum dots on Si

AbstractThe in plane lattice constant of the silicon film on germanium is shown to alter as the film grows; the changes reflect the process of strains relaxation of that result from the misfit of the Ge and Si lattice constants. The thickness allowing detection of changes in the in plane lattice constant of the Si film during the growth on the Ge surface depends on the germanium layer thickness. The thickness of the silicon film where elastic relaxation is determined as dependent on the germanium layer thickness. TEM studies indicate the vertical ordering of the germanium island layers when the thickness of the Si layer in between Ge layers is not sufficient to provide the full strain relaxation. The vertical ordering of Ge islands in the multilayer structure results in a decrease in the island density at reduced dispersion of their shape and size. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

In-situ X-ray diffraction study of the initial dealloying of Cu 3Au(001) and Cu 0.83Pd 0.17(001)

Dealloying and selective dissolution are serious corrosion processes, but also employed in technology. We studied the surface structure of Cu 0.83Pd 0.17(001) and Cu 3Au(001) during electrochemical selective Cu dissolution below the critical potential in-situ in 0.1 M H 2SO 4 (pH = 1) electrolyte. In both cases we observe the formation of an epitaxial layer of nano-scale islands of the noble component of the original alloy (Au or Pd). These islands form a metallic passivation layer suppressing massive dissolution of Cu. The Au islands developed {111} facets. By re-deposition of Cu ions from the electrolyte solution onto Pd islands, an epitaxial Cu layer is formed.

Desmoplastic ameloblastoma in Indians: Report of five cases and review of literature

In the recent World Health Organization classification of odontogenic tumours, desmoplastic ameloblastoma has been characterized as a variant of ameloblastoma, with specific clinical, radiographical, and histological features. Till date, 145 cases have been reported in Japanese, Chinese, Malaysian, Western, and African populations, with very few cases described in Indians. Here, we report five cases in the Indian population. The male to female ratio was 3:2. The mean age at diagnosis was 33.2 years. Four of the tumours were located in the maxilla, in the anterior premolar region. The lone mandibular tumour was located anteriorly, crossing the midline. Three of the tumours had a mixed radiologic appearance with poorly defined borders. Unilocular change was seen in one of them. Two tumours presented as unilocular radiolucencies with specks of radiopacities and well-circumscribed borders. Histologically, irregular odontogenic islands, with a stretched-out 'kite-tail' appearance, were seen in a dense desmoplastic stroma. The peripheral layer of the epithelial islands was made up of flattened cells and the inner core was made up of spindle-shaped and, in some instances, squamous-shaped cells. In two cases, odontogenic epithelium in the form of follicles, typical of solid/multicystic ameloblastoma, was seen and these were typed as 'hybrid' variants. All the cases were treated by resection.

Photoluminescence Properties of Two-dimensional Planar Layer and Three-dimensional Island Layer for ZnO Films Grown Using MOCVD

ZnO (002) films with different thicknesses ranging from 7 to 300 nm were grown on sapphire (006) substrates via metal-organic chemical vapor deposition (MOCVD). The two-dimensional (2D) planar layer and the three-dimensional(3D) island layer were studied by using of X-ray diffraction(XRD) rocking curves and atomic force microscopy (AFM). The room temperature photoluminescence(PL) spectra show a blue shift of the peak positions of the ultraviolet(UV) emission with increasing film thickness. The blue shift is remarkably high(393—380 nm) when an increase in film thickness(7–15 nm) is accompanied by the change of structure from a 2D planar layer to a 3D island layer. The PL spectra at 77 K also indicate that there are different transition mechanisms in the film thickness from a 2D planar layer to a 3D island layer near the 2D layer region.

Freezing shape and composition of Ge∕Si(001) self-assembled islands during silicon capping

We use atomic force microscopy, x-ray photoemission spectroscopy, and x-ray absorption spectroscopy to study the effect of the deposition of a Si cap layer by chemical vapor deposition on the morphology and composition of a Ge island layer grown at 600°C. We found that the capping of self-assembled Ge islands under a silicon layer results in high-quality, atomically flat layer only at deposition temperature above 700°C. On the other hand at this temperature Ge–Si intermixing and island coarsening are greatly enhanced, resulting in an increased average island volume. Here we show that the predeposition at low temperature of a thin cap layer preserves island shape, size, and composition when the capped islands undergo a subsequent process at higher temperature up to 750°C. It is shown, therefore, that with a two-step capping process it is possible to combine the benefit of a low temperature capping, which reduces island alloying and coarsening, with that of a high temperature capping which is needed to recover a flat surface.