SiGe Nanoislands Research Articles

Structural and photoluminescence study of TiO2 layer with self-assembled Si1−xGex nanoislands

We study the surface morphology and growth process of SiGe nanoislands on TiO2 films, deposited over Si (001) substrates by high power impulse magnetron sputtering, followed by varying annealing parameters (i.e., 500–750 °C for 30 min to 20 h). Structural analysis was performed by atomic force microscopy, scanning electron microscopy, and grazing incidence x-ray diffraction. Two structural schemes were taken into consideration, i.e., SiGe grown over pre-annealed TiO2 (scheme I) and as-grown TiO2 (scheme II). Photoluminescence (PL) study of the structures revealed spectral features comprised of multiple peak features related to localized and surface states within the oxide layer, along with a peak due to the SiGe nano-islands. It was observed that the spectral feature and intensity depend on the surface morphology and the crystallinity of the underlying TiO2 layer. The structures were subjected to low temperature PL measurements, and the spectra were de-convoluted in order to validate the origin of the obtained spectra. Structural analysis revealed that pre-annealing the underlying polycrystalline TiO2 film, prior to deposition of SiGe layers (scheme I), facilitates the formation of SiGe nanoislands, preferably along the grain boundaries (due to their higher interfacial energy). In comparison, for the case of SiGe deposited over as-grown TiO2 (scheme II), annealing of the structure resulted in random distribution of nanoislands across the entire film. The size of the nanoislands grew with increased annealing time up until the point they started to coalesce, forming discontinuous SiGe layers and eventually leading to melting of the layer.

Preferential MBE Growth and Characterization of SiGe Nanoislands on Depth‐Selective Si Pits Etched by Ar+ Plasma

In this study, the size selective deposition of SiGe nanoislands is demonstrated to be possible only in Si nanopits using a molecular beam epitaxy (MBE) system. The depth of the etched Si substrate prepared by Ar+ plasma etching just before the deposition seems to be playing a role in the selectivity of keeping the SiGe nanoislands only inside the nanopits. We observed that, when the thickness of the deposited SiGe layer is around the mean pit depth, which is 4 nm in this case, Ge nucleation takes place selectively on the pre‐etched pits. Relatively larger deposition thickness (e.g., 40 nm) is demonstrated to suppress the preferential growth of the Ge nanocrystals (NCs)/Si NCs which are in return observed all along the surface of the Si substrate. On the other hand, surface migration is considered to play a role in very small depth (relatively more shallow pits) and yielding the unfilled Si nanopits (ca. 1.5 nm) whereas Ge NCs selectively nucleate only within those having larger depths (ca. 3 nm). Such site‐specific 3D controlled growth of nanoislands is shown for the deposition of different semiconductor nanocrystals on top of another for formation of nanodevices fabricated in a single nanopit.

Structural and electrical investigations of MBE-grown SiGe nanoislands

SiGe nanoislands were grown by Molecular Beam Epitaxy (MBE) method on Si (100) substrates with comparative growth parameters such as annealing temperature, top Ge content and layer-by-layer annealing (LBLA). XRD and Raman data suggest that annealing temperature, top Ge content and layer-by-layer annealing (LBLA) can overall give a control not only over the amorphous content but also over yielding the strained Ge layer formation in addition to mostly Ge crystallites. Depending on the layer design and growth conditions, size of the crystallites was observed to be changed. Four Point Probe (FPP) Method via Semiconductor Analyzer shows that 100 °C rise in annealing temperature of the samples with Si0.25Ge0.75 top layers caused rougher islands with vacancies which further resulted in the formation of laterally higher resistive thin film sheets. However, vertically performed I-AFM analysis produced higher I–V values which suggest that the vertical and horizantal conductance mechanisms appear to be different. Ge top-layered samples gained greater crystalline structure and better surface conductivity where LBLA resulted in the formation of Ge nucleation and tight 2D stacking resulting in enhanced current values.

Photofield emission from SiGe nanoislands under green light illumination

Photofield emission from SiGe nanoislands formed by molecular beam epitaxy (MBE) have been investigated. Two types of nanoislands, namely the domes and pyramids with different heights, have been addressed. It was found that the arrays of SiGe nanoislands exhibited a low onset voltage for field emission. The increase of emission current and the decrease of the curve slope in Fowler-Nordheim coordinates under green light illumination have been revealed. Electron field emission and photoemission from SiGe nanoislands have been explained based on the energy band diagram of Si-Ge heterostructure and some energy barriers have been determined.

Unique surface structure formations on a Ge-covered Si(110)-16×2 surface

Si–Ge structures forming new shapes on a Si(110)-16×2 reconstructed surface were investigated via scanning tunneling microscopy. Pyramidal-shaped Si–Ge nanoislands lying along the <111> directions were formed on the striped structure at Ge coverage between 3 and 6 monolayers. However, when a single monolayer of Ge was deposited on the Si(110)-16×2 surface, single-domain of 16×2 striped structure disappeared, and a new double-domain striped structure was formed over the surface along directions that differed from the <112> directions. This structure represents a new Si–Ge striped structure that forms by the mixing of Ge and Si due to high temperature annealing. These results indicate that the surface structure changes specifically with trace amounts of Ge.

Effects of the lateral ordering of self-assembled SiGe nanoislands grown on strained Si1 − x Ge x buffer layers

Atomic-force microscopy, micro-Raman spectroscopy, and high resolution X-ray diffraction are applied to study the spatial ordering in single layers of SiGe nanoislands grown on a strained Si1 − xGex buffer sublayer. It is shown that, apart from stimulating the spatial ordering of nanoislands, the introduction of a Si1 − xGex sublayer leads to an enhanced role for interdiffusion processes. An unusually high increase in the volume of nanoislands in the process of the epitaxy is related to the anomalously strong diffusion from the buffer sublayer into the islands that is induced by nonuniform fields of elastic strains. The anisotropy of the islands shape and spatial ordering is discussed in terms of the anisotropy of the diffusion processes in spatially nonuniform fields of elastic strains.

Ordering Mechanisms in Epitaxial SiGe Nanoislands

We investigate and elucidate the surprising observation of atomically ordered domains in dome-shaped SiGe nanoislands. We show, through atomistic Monte Carlo simulations, that this ordering is a surface-related phenomenon, and that is driven by surface equilibrium rather than by surface kinetics. The ordering depends on facet orientation. The main source of ordering is the {15 3 23} facet, while the {105} and {113} facets contribute less. Subsurface ordered configurations self-organize under this facet and are frozen-in and buried during island growth, giving rise to the ordered domains. Ordering mechanisms based on constrained surface kinetics, requiring step-mediated segregation at the island facets, are shown to be much less likely.

Photovoltaic properties and photoconductivity in multilayer Ge/Si heterostructures with Ge nanoislands

Interband optical transitions in multilayer heterostructures with SiGe nanoislands were investigated using photocurrent spectroscopy and photo-emf. The n-p heterostructures containing Ge nanoislands in the area of the potential barrier were prepared by molecular-beam epitaxy at the temperature about 500 °C. It was shown that electron transitions from the ground state of the valence band in a nanoislands to the conduction band of Si surrounding made the main contribution into the vertical photo-emf in the range 0.75–1.05 eV, which is below the interband absorption edge of Si. The lateral photoconductivity observed in the range 0.63–0.8 eV at 77 K can be attributed to indirect interband transitions from the ground state of a nanoisland to L-state of the conduction band of a nanoisland. Analysis of Raman scattering spectra revealed that the Ge composition x in a nanoisland is about 0.87, while elastic deformation value amounts to exx = −0.016. The calculated energies of interband transitions from the ground state of a nanoisland to the conduction band of Si surrounding (0.63 eV) and to L-state of the conduction band of a nanoisland (0.81 eV) fit the experimental data with a rather good accuracy.

Photovoltage Performance of Ge/Si Nanostructures Grown on Intermediate Ultrathin SiO&lt;sub&gt;X&lt;/sub&gt; Layers

An enhanced photovoltage is reported to occur in Ge/Si structures with a SiOx layer having a thickness of 0.5-2 nm and placed between a Si substrate and Ge nanoislands. The effect is interpreted in terms of an increased separation distance for photoexcited electrons and holes occurring in the stress fields generated in the oxidized Ge/SiOx/Si structure. The electron-hole separation is modeled utilizing finite-element method techniques, and a good agreement between the experimentally observed enhancement and the computationally increased inter-charge distance is obtained. It is also found that insertion of the oxide layer accelerates the photovoltage decay. This result is interpreted in terms of competing processes, involving the direct recombination of the separated electrons and holes and multi-trapping behavior typical of disordered systems caused by Ge islands.

Silicon Dioxide Embedded Germanium Nanocrystals Grown Using Molecular Beam Epitaxy for Floating Gate Memory Devices

SiO2/Ge nanocrystals/SiO2 trilayer memory structure has been fabricated by oxidizing and subsequent annealing of self assembled SiGe nanoislands grown by molecular beam epitaxy. The optical and charge storage characteristics of trilayer structures have been studied through Raman spectroscopy and capacitance-voltage measurements, respectively. An anti-clockwise hysteresis in the C-V characteristics indicated the net electron trapping in the floating gate containing Ge nanocrystals. Frequency dependent measurements of device characteristics indicate that neither interface defects nor deep traps are dominant for the charging or discharging processes of nanocrystal floating gates.

Imaging of nanoislands in coherent grazing-incidence small-angle x-ray scattering experiments

Coherent x-ray scattering in a grazing-incidence geometry was used to image SiGe nanoislands grown by liquid phase epitaxy on Si(001). Due to their narrow size distribution, identical shape, and orientation, the total scattered intensity obtained in this geometry represents a coherent diffraction pattern from an average island. Iterative phase retrieval techniques were used for the reconstruction of the projected electron density of hundred nanometer size islands. So far other x-ray imaging methods do not achieve the obtained spatial resolution of 10--15 nm for islands of this size.

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.

Band offsets and photocurrent spectroscopy of Si/Ge heterostructures with quantumdots

Raman and lateral photoconductivity spectra of self-assembled SiGe nanoislands were studied with a height of∼2 nm and a base of∼20 nm formed at atemperature of 500 °C. It was estimated that the value of elastic deformation(εxx)was −0.022 (εzz = 0.017), while the germanium content in the islands(x) was 0.66. Theobtained values of x and ε were used to calculate band offsets at the interfaces and the energy of interbandtransitions of structures under study. It was shown that the minimal energy ofphotocurrent observation is 0.52 eV, which is below the bandgap of the QDsunder study. The first photocurrent component which began to contribute at0.52 eV and had a peak at 0.68 eV is explained by optical transitions of electronsfrom the QD HH localized states of the valence band to the conduction bandΔ2 valley of the surrounding silicon matrix in which tensile strains are present.The second component with limiting energy of 0.73 eV can be caused byinterband electron transitions from the HH valence band of the QDs to theΔ4 valley of the QD conduction band.

Optical and photoelectrical properties of GeSi nanoislands

Raman and lateral photoconductivity spectra of self-assembled SiGe nanoislands with heights of ∼1 to 2 nm and bases of 10–20 nm were studied. Investigated heterostructures formed at a temperature 300 °C after deposition of Ge with a nominal layer thickness of dGe ⩽ 8 monolayers have the shape of the so-called hut clusters. It was estimated that the value of elastic deformation (εxx) is −0.028 ± 0.002 (εzz = 0.015 ± 0.002), the germanium content in islands (x) is 0.87 ± 0.05. It was determined that the value of the valence band energy offset in the heterojunction between the strained Si0.13Ge0.87 layer of nanoislands and adjacent stretched Si is ∼0.47 eV.

The correlation between the surface-energy minima and the shape of self-induced SiGe nanoislands

The effects of the composition, size, and thermal expansion coefficient of self-induced Ge and SiGe nanoislands formed on Si on the value of the islands’ total energy are examined. A correlation between the discrete minima in the surface energy of the islands and their shape is considered. The interdiffusion processes that are important at high temperatures of epitaxy are taken into account. The results of calculations are compared with experimental data obtained using atomic-force microscopy.

Effect of surface energy minima on the shape of self‐induced SiGe nanoislands

AbstractThe results of a numerical modelling of the influence of discrete surface energy minima on the island shape transition are reported. The real composition of self‐induced (Ge)SiGe/Si nanoislands grown at high temperatures is taken into account in deriving the expression of the island total energy. The simulated values of the critical island size at which the transition occurs are compared with those observed in experiments. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Experimental and theoretical study of the influence of growth temperature on composition in self-assembled SiGe QD's

A detailed study of self-induced SiGe nanoislands on Si substrate formed at different substrate temperatures is presented. As a result of AFM investigations, the dependencies of the density, volume and shape of the islands on the growth temperature are established. Using Raman spectroscopy and HRXRD, the dependences of average values of strain and composition in the islands on the growth temperature are determined. Based on the experimental results and theoretical calculations, the dependence of SiGe island volume on Si content is established.

Raman study of self-assembled SiGe nanoislands grown at low temperatures

We report on Raman scattering measurements on Si-capped Ge quantum structures grown bymolecular beam epitaxy on Si(001) at low temperatures. We find a double band structure in theGe–Ge frequency range for nanoislands grown at substrate temperatures ranging in the interval300–500 °C. Complementary information has been obtained from performing Raman scatteringexperiments on annealed samples. The results are interpreted in terms of a model thatconsiders quantum structures (hut clusters) composed of a strained Ge core and a morerelaxed SiGe shell.

Raman Spectroscopy and Electroreflectance Studies of Self-Assembled SiGe Nanoislands Grown at Various Temperatures

SiGe nanoislands grown in a silicon matrix at temperatures of 300 to 600°C are studied using Raman spectroscopy and electroreflectance. For islands grown at relatively low temperatures (300–500°C), phonon bands are observed to have a doublet structure. It is shown that changes in the percentage composition, size, and shape of nanoislands and, hence, in the elastic stresses (depending on the growth temperature of the structures) have a significant effect on the energies of optical electronic interband transitions in the islands. As a consequence, the resonance conditions for Raman scattering also change. It is found that interdiffusion from the silicon substrate and the cover layer (determining the mixed composition of SiGe islands) is of importance even at low growth temperatures of nanostructures (300–400°C).

Correlation between the energy of SiGe nanoislands and their shape and size

The total energy of self-assembled SiGe nanoislands on a silicon substrate is investigated theoretically as a function of their geometric and physical parameters. It is demonstrated that the growth temperature and the silicon content in nanoislands affect the minimum of their energy. The results of numerical calculations for nanoislands are compared with experimental data obtained by atomic-force microscopy.