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We report preliminary results on InGaP/InGaAs/Ge photovoltaic cells for concentrated terrestrial applications, monolithically integrated on engineered Si(001) substrates. Cells deposited on planar Ge/Si(001) epilayers, grown by plasma-enhanced chemical vapor deposition, provide good efficiency and spectral response, despite the small thickness of the Ge epilayers and a threading dislocation density as large as 107/cm2. The presence of microcracks generated by the thermal misfit is compensated by a dense collection grid that avoids insulated areas. In order to avoid the excessive shadowing introduced by the use of a dense grid, the crack density needs to be lowered. Here, we show that deep patterning of the Si substrate in blocks can be an option, provided that a continuous Ge layer is formed at the top, and it is suitably planarized before the metalorganic chemical vapor deposition. The crack density is effectively decreased, despite that the efficiency is also lowered with respect to unpatterned devices. The reasons of this efficiency reduction are discussed, and a strategy for improvement is proposed and explored. Full morphological analysis of the coalesced Ge blocks is reported, and the final devices are tested under concentrated AM1.5D spectrum. Copyright © 2016 John Wiley & Sons, Ltd.

Display Omitted Patterning by EBL of epitaxial SiGe semiconductor material film.Fabrication method to realize free-standing semiconductor nanostructures.Tuning of the FIB process on epitaxial SiGe material. We target the nanofabrication of free-standing nanostructures made of epitaxial semiconductor material layers of high crystallinity quality and high heterostructure complexity for optical applications at the nanoscale. Here we demonstrate the fabrication method in the case of epitaxial germanium grown on a silicon substrate but the method can be applied to any heterostructure material. The nanostructures are fabricated out of planar epitaxial wafers in the form of pillars with arbitrary section and high aspect ratio by electron-beam lithography and deep reactive-ion etching. The patterned SiGe structures are then released by focused ion-beam milling of the pillar base. In this way, they become free-standing and can be relocated on a suitable substrate by using a nanomanipulator. Microscopic characterizations are ongoing to verify that the high crystal quality typical of epitaxial layers grown on a large-area substrate is preserved throughout the different fabrication steps.

We present an experimental and theoretical analysis of the formation of nanovoids within Si microcrystals epitaxially grown on Si patterned substrates. The growth conditions leading to the nucleation of nanovoids have been highlighted, and the roles played by the deposition rate, substrate temperature, and substrate pattern geometry are identified. By combining various scanning and transmission electron microscopy techniques, it has been possible to link the appearance pits of a few hundred nanometer width at the microcrystal surface with the formation of nanovoids within the crystal volume. A phase-field model, including surface diffusion and the flux of incoming material with shadowing effects, reproduces the qualitative features of the nanovoid formation thereby opening new perspectives for the bottom-up fabrication of 3D semiconductors microstructures., Ordered 3D arrays of nanovoids can be formed during the epitaxial growth of Si microcrystals on patterned substrates. Phase-field simulations provide a clear link between nanovoid formation and the ratio between diffusion coefficient and deposition rate. With the appropriate combination of pattern geometry, deposition rate and deposition temperature control over nanovoids formation could be achieved also with other semiconductors.

The scanning X-ray nanodiffraction technique is used to reconstruct the three-dimensional distribution of lattice strain and Ge concentration in compositionally graded Si1−x Ge x microcrystals grown epitaxially on Si pillars. The reconstructed crystal shape qualitatively agrees with scanning electron micrographs and the calculated three-dimensional distribution of lattice tilt quantitatively matches finite-element method simulations. The grading of the Ge content obtained from reciprocal-space maps corresponds to the nominal grading of the epitaxial growth recipe. The X-ray measurements confirm strain calculations, according to which the lattice curvature of the microcrystals is dominated by the misfit strain, while the thermal strain contributes negligibly. The nanodiffraction experiments also indicate that the strain in narrow microcrystals on 2 × 2 µm Si pillars is relaxed purely elastically, while in wider microcrystals on 5 × 5 µm Si pillars, plastic relaxation by means of dislocations sets in. This confirms previous work on these structures using transmission electron microscopy and defect etching.

In this work we present an innovative approach to realise coherent, highly-mismatched 3-dimensional heterostructures on substrates patterned at the micrometre-scale. The approach is based on the out-of-equilibrium deposition of SiGe alloys graded at an exceptionally shallow grading rate (GR) of 1.5% µm −1 by low energy plasma enhanced chemical vapour deposition (LEPECVD). Fully coherent SiGe/Si crystals up to 6 µm in width were achieved as confirmed by defect etching and transmission electron microscopy (TEM) analyses. The experimental results are supported by calculations of the energy for dislocation formation which indicate that elastic relaxation is energetically favoured over plastic relaxation in the narrower heterostructures. X-ray diffraction measurements show that the SiGe crystals are strain-free irrespective of the stress relieving mechanism which changes from elastic to plastic by increasing their width. The impact of dislocations on the SiGe crystal quality is analysed by comparing the width of X-ray diffraction peaks as a function of the heterostructure size.

Extending the functionality of ubiquitous Si-based microelectronic devices often requires combining materials with different lattice parameters and thermal expansion coefficients. In this paper, scanning X-ray nanodiffraction is used to map the lattice bending produced by thermal strain relaxation in heteroepitaxial Ge microcrystals of various heights grown on high aspect ratio Si pillars. The local crystal lattice tilt and curvature are obtained from experimental three-dimensional reciprocal space maps and compared with diffraction patterns simulated by means of the finite element method. The simulations are in good agreement with the experimental data for various positions of the focused X-ray beam inside a Ge microcrystal. Both experiment and simulations reveal that the crystal lattice bending induced by thermal strain relaxation vanishes with increasing Ge crystal height.

We present an innovative approach to integrate arrays of isolated, strain-free GaN crystals on patterned Si substrates. First, micrometer-sized pillars are patterned onto Si(0 0 1) substrates. Subsequently, 2.5 μm Si substrates are deposited by low-energy plasma-enhanced chemical vapor deposition, forming crystals mostly bounded by {1 1 1}, {1 1 3}, and {15 3 23} facets. Plasma-assisted molecular beam epitaxy is then used for GaN deposition. GaN crystals with slanted {0 0 0 1} facets having a root-mean-square surface roughness of 0.7 nm are obtained for a deposited material thickness of >3 μm. Microphotoluminescence measurements performed at room and cryogenic temperature show no yellow luminescence and a neutral donor-bound A exciton transition at 3.471 eV (10 K) with a full width at half-maximum of 10 meV. Microphotoluminescence and micro-Raman spectra reveal that GaN grown on Si pillars is strain-free. Our results indicate that the shape of GaN crystals can be tuned by the pattern periodicity and that a reduction of threading dislocations is achieved in their top part.

In this report we present a novel strategy in selective epitaxial growth on top of Si pillars, which results in a tessellated Ge film, composed by self-aligned micron-sized crystals in a maskless process. Modelling by rate equations the morphology evolution of fully facetted crystal profiles is extensively outlined, showing an excellent prediction of the peculiar role played by flux shielding among microcrystals, in the case of dense array configuration. Crack formation and substrate bending, caused by the mismatch in thermal expansion coefficients, are eliminated by the mechanical decoupling among individual microcrystals, which are also shown to be dislocation- and strain-free. The method has been also tested for Si 1− x Ge x alloys, with compositions ranging from pure silicon to pure germanium. There are ample reasons to believe that this approach could be extended to other material combinations and substrate orientations, actually providing a technology platform for several device applications.

In this work we show the possibility to increase the optical quality of quantum dots grown by droplet epitaxy on Ge-on-Si substrates in terms of single dot emission linewidth, decay time and efficiency by operating on the As pressure during the crystallization step without increasing the thermal budget. © 2013 Elsevier B.V. All rights reserved.

We report on the growth and optical characterization by macro and micro photoluminescence measurements of high optical quality GaAs quantum nanostructures grown by droplet epitaxy on Ge and Si substrates. The quantum nanostructures show optical performances comparable to those achievable with the most advanced realized on GaAs substrates. The adopted growth procedures show also the possibility to fabricate the active layer maintaining a low thermal budget compatible with back-end integration of the fabricated materials on integrated circuits. We demonstrate the possibility to embed GaAs nanostructured devices such as intersubband detectors and single quantum emitters on Si substrates. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The ordered growth of self-assembled SiGe islands by surface thermal diffusion in ultra high vacuum from a lithographically etched Ge stripe on pit-patterned Si(100) surface has been experimentally investigated. The total surface coverage of Ge strongly depends on the distance from the source stripe, as quantitatively verified by Scanning Auger Microscopy. The size distribution of the islands as a function of the Ge coverage has been studied by coupling atomic force microscopy scans with Auger spectro-microscopy data. Our observations are consistent with a physical scenario where island positioning is essentially driven by energetic factors, which predominate with respect to the local kinetics of diffusion, and the growth evolution mainly depends on the local density of Ge atoms.

We present a detailed experimental and theoretical analysis of the epitaxial stress relaxation process in micro-structured compositionally graded alloys. We focus on the pivotal SiGe/Si(001) system employing patterned Si substrates at the micrometre-size scale to address the distribution of threading and misfit dislocations within the heterostructures. SiGe alloys with linearly increasing Ge content were deposited by low energy plasma enhanced chemical vapour deposition resulting in isolated, tens of micrometre tall 3D crystals. We demonstrate that complete elastic relaxation is achieved by appropriate choice of the Ge compositional grading rate and Si pillar width. We investigate the nature and distribution of dislocations along the [001] growth direction in SiGe crystals by transmission electron microscopy, chemical defect etching and etch pit counting. We show that for 3 μm wide Si pillars and a Ge grading rate of 1.5% μm−1, only misfit dislocations are present while their fraction is reduced for higher Ge grading rates and larger structures due to dislocation interactions. The experimental results are interpreted with the help of theoretical calculations based on linear elasticity theory describing the competition between purely elastic and plastic stress relaxation with increasing crystal width and Ge compositional grading rate.

A method for growing suspended Ge films on micron-sized Si pillars in Si(001) is discussed. In [C.V. Falub et al., Science 335 (2012) 1330] vertically aligned three-dimensional Ge crystals, separated by a few tens of nanometers, were obtained by depositing several micrometers of Ge using Low-Energy Plasma-Enhanced Chemical Vapor Deposition. Here a different regime of high growth temperature is exploited in order to induce the merging of the crystals into a connected structure eventually forming a continuous, two-dimensional film. The mechanisms leading to such a behavior are discussed with the aid of an effective model of crystal growth. Both the effects of deposition and curvature-driven surface diffusion are considered to reproduce the main features of coalescence. The key enabling role of high temperature is identified with the activation of the diffusion process on a time scale competitive with the deposition rate. We demonstrate the versatility of the deposition process, which allows to switch between the formation of individual crystals and a continuous suspended film simply by tuning the growth temperature.

Control of electron spin coherence via external fields is fundamental in spintronics. Its implementation demands a host material that accommodates the desirable but contrasting requirements of spin robustness against relaxation mechanisms and sizeable coupling between spin and orbital motion of the carriers. Here, we focus on Ge, which is a prominent candidate for shuttling spin quantum bits into the mainstream Si electronics. So far, however, the intrinsic spin-dependent phenomena of free electrons in conventional Ge/Si heterojunctions have proved to be elusive because of epitaxy constraints and an unfavourable band alignment. We overcome these fundamental limitations by investigating a two-dimensional electron gas in quantum wells of pure Ge grown on Si. These epitaxial systems demonstrate exceptionally long spin lifetimes. In particular, by fine-tuning quantum confinement we demonstrate that the electron Landé g factor can be engineered in our CMOS-compatible architecture over a range previously inaccessible for Si spintronics., Ge/Si heterojunctions are prominent candidates for spintronics, but the spin-dependent phenomena have been elusive. Here, Giorgioni et al. report long spin relaxation and coherence times in a two dimensional electron gas confined in quantum wells of pure Ge grown on SiGe-buffered Si substrates.

We present a comparison of two different techniques for the evaluation of Threading Dislocation Density (TDD) in Si1−xGex graded buffers: chemical etching and Electron-Beam-Induced-Current (EBIC) imaging. The samples analyzed consist of a series of linearly graded Si1−xGex buffers terminated by a constant composition layer with Ge concentration varying between 0.2 and 0.9. The films were deposited on Si(1 0 0) wafers by means of Low Energy Plasma Enhanced CVD (LE-PECVD). Our results indicates that EBIC proves to be a versatile method, which provides reliable information on the defect concentration up to a Ge concentration of x = 0.4 . For higher values of x ( x = 0.7 ) the information is approximated but yet reliable for the purpose of optimising the growth process. In order to perform EBIC measurements on samples with higher Ge concentrations ( x = 0.9 ) , the Schottky diode fabrication has to be improved.

In this work we address three dimensional heterojunctions demonstrating that photoluminescence from defect free Ge/SiGe multiple quantum well (MQW) micro crystals grown on deeply patterned Si(001) and Si(111) substrates exhibit similar radiative intensity and analogous spectral shape. The finite lateral size and faceted top morphology of the micro crystals guarantee the absence of dislo cations threading through the MQW structure and the dominance of radiative recombination at slanted {113} facets. Our approach yields superior optical quality in comparison to state of the art MQWs grown on SiGe/Si(001) linearly graded buffers.

By transmission electron microscopy with extended Burgers vector analyses, we demonstrate the edge and screw character of vertical dislocations (VDs) in novel SiGe heterostructures. The investigated pillar-shaped Ge epilayers on prepatterned Si(001) substrates are an attempt to avoid the high defect densities of lattice mismatched heteroepitaxy. The Ge pillars are almost completely strain-relaxed and essentially defect-free, except for the rather unexpected VDs. We investigated both pillar-shaped and unstructured Ge epilayers grown either by molecular beam epitaxy or by chemical vapor deposition to derive a general picture of the underlying dislocation mechanisms. For the Burgers vector analysis we used a combination of dark field imaging and large-angle convergent beam electron diffraction (LACBED). With LACBED simulations we identify ideally suited zeroth and second order Laue zone Bragg lines for an unambiguous determination of the three-dimensional Burgers vectors. By analyzing dislocation reactions we confirm the origin of the observed types of VDs, which can be efficiently distinguished by LACBED. The screw type VDs are formed by a reaction of perfect 60° dislocations, whereas the edge types are sessile dislocations that can be formed by cross-slips and climbing processes. The understanding of these origins allows us to suggest strategies to avoid VDs.

Epitaxial growth of Ge films on Si introduces an in-plane biaxial tensile strain, opening up Ge applications in photonic, as recently demonstrated by the room temperature lasing action in Ge-on-Si heterostructure. The optical access to the direct gap transitions remarkably provides the possibility of injecting spin-polarized carriers via absorption of circular-polarized light. Nevertheless the effects of tensile-strain on the spin properties of Ge-on-Si have still to be addressed. In this work we applied polarization-resolved photoluminescence (PL) to spectrally resolve the radiative recombination involving strain-split light and heavy hole bands. We found that at low temperature, the fundamental transition has a record-high circular polarization degree of 85%, despite an off-resonance excitation of 300 meV. We show that this very high value is due to the characteristic energy dependence of the optically induced electron spin population and their concomitant dynamics. Finally, our observation of a direct gap PL doublet clarifies that the light hole contribution to the optical spectrum can dominate the room temperature PL. Our results provide a step forward the investigation of the dynamics of non-equilibrium spin populations and of the evaluation of optical gain in group IV materials. Above all, our findings confirm Ge as a prominent candidate for the development of next-generation CMOS-compatible devices featuring spintronics and photonics functionalities.

This articleThis article will form part of a virtual special issue of the journal, presenting some highlights of the 12th Biennial Conference on High-Resolution X-ray Diffraction and Imaging (XTOP2014). reports the X-ray diffraction-based structural characterization of the α12 multilayer structure SiGe2Si2Ge2SiGe12 [d'Avezac, Luo, Chanier & Zunger (2012). Phys. Rev. Lett. 108, 027401], which is predicted to form a direct bandgap material. In particular, structural parameters of the superlattice such as thickness and composition as well as interface properties, are obtained. Moreover, it is found that Ge subsequently segregates into layers. These findings are used as input parameters for band structure calculations. It is shown that the direct bandgap properties depend very sensitively on deviations from the nominal structure, and only almost perfect structures can actually yield a direct bandgap. Photoluminescence emission possibly stemming from the superlattice structure is observed.

Germanium and silicon-germanium alloys have found entry into Si technology thanks to their compatibility with Si processing and their ability to tailor electronic properties by strain and band-gap engineering. Noticeably, the quasi-direct band structure of Ge allows for optical spin orientation bridging the fields of spintronics and Si photonics where Ge is increasingly applied. Despite recent progress in the investigation of spin dynamics in Ge, very little is known about spin flip scattering by dopants and the role played by impurities in determining spin properties in different temperature regimes. Here we present a combined experimental and theoretical investigation of the spin and energy relaxation of electrons in Ge. By taking advantage of polarization-resolved photoluminescence measurements and Monte Carlo simulations of carrier dynamics we unveil a unique and very rich spin physics and highlight the importance of the multi-valley conduction band of this material. We demonstrate full control over the angular momentum of the direct gap emission with a complete reversal between right- and left-handed circular polarization. In the low temperature regime, the observed state of light polarization is ascribed to the so-far overlooked carrier cooling process that stems from the Coulomb scattering originated within the X-valleys. In the high temperature range, we disclose the role of backscattering from the L-valley in defining the depolarization of direct gap luminescence. This detailed investigation of spin properties of Ge is expected to open new opportunities connected to photonics and spintronics on a CMOS-compatible platform

The cross-plane thermal conductivity κ of multilayers of SiGe nanodots separated either by Si or SiGe can be decreased by reducing the period length or by increasing the nanodot density. It is, however, not clear how far κ can be reduced by using these strategies. In addition, the role of SiGe nanodots on the reduction of κ is still not fully understood. In this work, we addressed these issues by studying experimentally the cross-plane κ of Ge/Si superlattices with period lengths down to 1.5 nm. Although κ tends to preserve the decreasing trend with reducing the period length, for periods shorter than 2 nm we observed a drastic drop of the average thermal resistance per period. This finding indicates a weakening of the effect of the interfaces on phonon scattering and implies a lower limit for κ. To assess the role played by the nanodots in the reduction of κ we studied Ge/Si superlattices with nanodot densities varying from 0 to ∼8×1010cm-2and a fixed Si spacer thickness of 2.7 nm. The experimental results suggest that SiGe nanodots with "pyramid"-shape have an effect comparable to nominally planar wetting layers on the cross-plane thermal transport. Finally, the comparison of superlattices with nanodots separated by Si1-xGex(with x from 0 to 0.2) shows that spacer alloying is beneficial in reducing the κ by ∼20%. The results presented in this work are expected to be relevant to micro/nanoscale energy conversion which requires minimizing the thermal conductivity of superlattice-based thin film thermoelectrics

We show that the Ge concentration in Si1−xGex alloys grown under strong out-of-equilibrium conditions determines the character of the population of threading dislocations (TDs). Above a critical value x ∼ 0.25 vertical TDs dominate over the common slanted ones. This is demonstrated by exploiting a statistically relevant analysis of TD orientation in micrometer-sized Si1−xGex crystals, deposited on deeply patterned Si(001) substrates. Experiments involving an abrupt change of composition in the middle of the crystals clarify the role of misfit-strain versus chemical composition in favoring the vertical orientation of TDs. A scheme invoking vacancy-mediated climb mechanism is proposed to rationalize the observed behavior., APL Materials, 1 (5), ISSN:2166-532X

Spin orientation of photoexcited carriers and their energy relaxation are investigated in bulk Ge by studying spin-polarized recombination across the direct band gap. The control over parameters such as doping and lattice temperature is shown to yield a high polarization degree, namely larger than 40%, as well as a fine tuning of the angular momentum of the emitted light with a complete reversal between right- and left-handed circular polarization. By combining the measurement of the optical polarization state of band-edge luminescence and Monte Carlo simulations of carrier dynamics, we show that these very rich and complex phenomena are the result of the electron thermalization and cooling in the multivalley conduction band of Ge. The circular polarization of the direct-gap radiative recombination is indeed affected by energy relaxation of hot electrons via the X valleys and the Coulomb interaction with extrinsic carriers. Finally, thermal activation of unpolarized L valley electrons accounts for the luminescence depolarization in the high temperature regime. © 2013 American Physical Society.

The structure of self-assembled quantum dots (QDs) grown by Droplet Epitaxy on Ge virtual substrates has been investigated by TEM. The QDs have a pyramidal shape with base and height of 50 nm. By (0 0 2) dark field TEM it was seen that the pyramid top is Ga poor and Al rich most likely because of the higher mobility of Ga along the pyramid sides down to the base. The investigated QDs contain defects identified as As precipitates by Moirè fringes. The smallest ones (3-5 nm) are coherent with the GaAs lattice suggesting that they could be a cubic phase of As precipitation. It seems to be a metastable phase since the hexagonal phase is recovered as the precipitate size increases above ∼5 nm. © 2012 Elsevier B.V. All rights reserved.

The carrier dynamics that occurs in Ge/SiGe QWs when electrons are excited to confined states at Gamma is studied by means of optical spectroscopy at different lattice temperatures. The typical times for the different relaxation and recombination processes are given and discussed

We performed spin-resolved photoluminescence measurements on Ge/SiGe multiple quantum wells with different well thickness and using different exciting power densities. The polarization of the direct emission strongly depends on the relative weight of electrons photoexcited from the light and the heavy hole subbands. The study of the polarization as a function of the exciting power highlights the role of the carrier-carrier interactions in determining spin depolarization. © 2013 AIP Publishing LLC.

We use fs-pump white-light-supercontinuum probe spectroscopy with appropriate polarization configurations to investigate the hole spin and hole cooling dynamics in Germanium quantum wells in the vicinity of the Γ valley. We observe heavy hole spin flip times of up to 1.7 ps at 10, 300 fs after the excitation. Additionally, a strong, late bleaching (Δ ∼10 ps) at the lower-lying energies indicates post-injection heating of the hole system. This effect is virtually independent of the surplus energies of electrons and carrier density as long as no higher bands are involved by direct or two-photon absorption. It is a direct manifestation of energy transfer from thermalizing electrons in the L valleys to the hole system. This mechanism is supported by microscopic simulations based on the semiconductor Bloch equations, clearly identifying the cooling of the hole system. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We describe a method for the direct epitaxial growth of a single photon emitter, based on GaAs quantum dots fabricated by droplet epitaxy, working at liquid nitrogen temperatures on Si substrates. The achievement of quantum photon statistics up to T=80 K is directly proved by antibunching in the second order correlation function as measured with a H anbury Brown and Twiss interferometer. © 2013 AIP Publishing LLC.

The polarization of the direct gap emission from Ge/SiGe multiple quantum wells has been studied in the 4 K to 300 K temperature range, and in samples with different well thicknesses. Our results demonstrate that the polarization type and degree strongly depend on the excitation of electrons from the heavy hole and the light hole subbands, thus providing an effective degree of freedom to control the polarization of the direct interband emission. In addition, the analysis of the temperature dependence of the polarization degree highlights spin depolarization mechanisms.

We present a detailed experimental study of the photoluminescence decay of direct Γ-Γ and indirect L-Γ transitions in compressively strained Ge/Si0.15Ge0.85 type I multiple quantum wells. The lifetime of the fundamental L-Γ indirect-gap related transition is in the 6 to 13 ns range at the lattice temperature of 14 K. These values are just one order of magnitude higher than those typical of type-I direct gap III-V quantum wells and are significantly shorter than those characteristic of type-II indirect gap SiGe/Si quantum wells. The measured decay times show a clear dependence on the quantum well width and lattice temperature. The decay of the Γ-Γ direct-gap related transition is dominated by the ultrafast electron scattering from Γ-type to L-type states of the conduction band.

The emission dynamics in GaAs/AlGaAs coupled ring-disk (CRD) quantum structures fabricated on silicon substrates is presented. The CRD structures are self-assembled via droplet epitaxy, a growth technique which, due to its low thermal budget, is compatible with the monolithic integration of III-V devices on Si based electronic circuits. Continuous wave, time resolved photoluminescence and theoretical calculations in the effective mass approximations are presented for the assessment of the electronic and carrier properties of the CRDs. The CRDs show a fast carrier dynamics which is expected to be suitable for ultrafast optical switching applications integrated on silicon.

We present a dephasing time analysis of the excitonic resonances in Ge/SiGe quantum wells for various lattice temperatures by coherent oscillation spectroscopy (COS). The results are compared to the linewidths of the excitonic resonances determined from linear absorption measurements. Additionally, COS is applied to different samples with varying linewidth, identifying one sample with a dominating homogeneously broadened 1$s$ excitonic resonance down to 7 K.

We present photoluminescence (PL) and spin polarized photo-emission (SPPE) measurements performed on bulk Ge, and Ge/Si(100) epilayers. A set of bi-axially strained Ge epilayers featuring different strain levels has been deposited by low energy plasma enhanced CVD (LEPECVD) and characterized by high resolution X-ray diffraction (HR-XRD). SPPE data indicate that compressive strain effectively lifts the heavy holes - light holes degeneracy raising the polarization of injected electrons above the P=50% limit of bulk material. Circularly polarized light PL measurements performed on p-type bulk crystals confirm the suitability of Ge for spintronics application showing a robust electron spin inizialization at the direct gap of Ge and giving a lower bound value for the spin relaxation time of ? 230 fs. © 2011 SPIE.

We demonstrate optical orientation in Ge/SiGe quantum wells and study their spin properties. The ultrafast electron transfer from the center of the Brillouin zone to its edge allows us to achieve high spin-polarization efficiencies and to resolve the spin dynamics of holes and electrons. The circular polarization degree of the direct-gap photoluminescence exceeds the theoretical bulk limit, yielding ~37% and ~85% for transitions with heavy and light holes states, respectively. The spin lifetime of holes at the top of the valence band is found to be ~0.5 ps and it is governed by transitions between heavy and light hole states. Electrons at the bottom of the conduction band, on the other hand, have a spin lifetime that exceeds 5 ns below 150 K. Theoretical analysis of the electrons spin relaxation indicates that phonon-induced intervalley scattering dictates the spin lifetime., 5 pages, 3 figures

The intricate relaxation dynamics in compressively strained Ge quantum wells with Ge-rich SiGe barriers is investigated systematically for various excitation conditions. Evidence of efficient ultrafast intersubband relaxation in competition with the intrasubband electron cooling is found in continuous wave photoluminescence spectra measured with different excitation wavelengths. These findings are corroborated by time-resolved pumpprobe absorption measurements yielding an intersubband relaxation time in the range of 100 fs.

We investigated the surface diffusion and island nucleation of Ge on Si(100) in presence of a submonolayer coverage of carbon as surfactant by using scanning Auger microscopy and atomic force microscopy. Ge stripes have been deposited and lithographically etched on a Si substrate and used as sources for the surface diffusion of Ge promoted by annealing at 600, 650, and $700\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. The diffusion coefficient has been determined by fitting the postannealing coverage profiles measured by Auger microscopy with a one-dimensional continuous model. The carbon coverage has been spatially modulated on a single sample, allowing the measurement of the diffusion coefficient as a function of the C thickness at $600\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. We show that the reduction in the diffusion coefficient while increasing the surfactant coverage is described by a linear dependence of the diffusion activation energy on the C coverage. This dependence is discussed in terms of the chemical interactions among Si, C, and Ge, of the surface roughness and the local strain field induced by the C surfactant. Spontaneous nucleation of SiGe islands coexists with the continuous surface diffusion of Ge. The transition of the island nucleation as a function of the carbon coverage is observed to be continuous from the Stranski-Krastanov mode to the Volmer-Weber regime. We propose a consistent scenario correlating diffusion and nucleation parameters within a diffusion limited growth regime and show the existence of a threshold for C coverage below which no effect is observed.

p-doped hydrogenated nanocrystalline silicon (p-nc-Si:H) is one of the most critical layers in thin film silicon solar cells. LEPECVD is a new technique for the growth of nc-Si at high growth rate without compromising the layer quality, using a dense but low energy plasma. Thin p-nc-Si:H layers are grown on glass and ZnO:Al coated glass and their structural and electrical properties are investigated as a function of the silane dilution (d) and of the doping ratio (DR). The influence of the substrate on the structural properties is investigated and discussed. The incubation layer is clearly observed on both substrate types and its thickness is estimated. LEPECVD distinguishes itself from other high growth rate methods by a very low impurity distribution coefficient to obtain a comparable conductivity and boron density. The conduction path is shown to be dependent on the density of boron in the layer while a significant decrease of conductivity at high DR is not observed in the studied range (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Polarization resolved absorption spectra of a strain-compensated Ge multiple quantum well (MQW) structure with Ge-rich SiGe barriers have been calculated with an $s{p}^{3}{d}^{5}{s}^{\ensuremath{\ast}}$ tight-binding model and measured for light propagating perpendicular to the growth direction. The MQW was grown by low-energy plasma-enhanced chemical vapor deposition and consists of 50 Ge quantum wells deposited onto a thick graded ${\text{Si}}_{1\ensuremath{-}x}{\text{Ge}}_{x}$ buffer layer. The MQW was structurally characterized by high-resolution x-ray diffraction. The measured absorption spectra show clear quantum confined excitonic transitions related to the $\text{Ge}\text{ }\ensuremath{\Gamma}$ point band gap, and strong dependence on the incident light polarization, as expected from selection rules for type I direct gap quantum confined systems. A good agreement between theoretically predicted spectra and experimental data is found, demonstrating light and heavy hole polarization-dependent selection rules in Ge MQWs.

We report the effect of laser annealing on the structural and electronic properties of nc-Si/a-Si:H samples grown close to the amorphous to nanocrystalline transition. The nc-Si/a-Si:H thin films were produced by low-energy plasma-enhanced chemical vapor deposition through a gas discharge containing SiH4. The samples were subjected to different laser fluencies and were characterized for changes in their structural and electronic properties via Raman spectroscopy and photoluminescence measurements. The laser annealing effects are twofold: i) the nanocrystalline phase grows, during the laser treatment, respect to the amorphous phase; ii) the photoluminescence spectra show the suppression, after laser annealing, of the frequencies above the crystalline Si band–gap.

A combined theoretical and experimental analysis of the crystalline phase fraction in nanocrystalline films grown by low-energy plasma-enhanced chemical vapor deposition is presented. The effect of the key parameters, such as temperature, silane flux, and hydrogen dilution ratio, is analyzed. An atomic-scale Monte Carlo model is developed, where the crystallization probability depends on the local environment of the nanocrystalline film. Good agreement is found between the experiments and theory, despite the use of a single fitting parameter.

An experimental study of the direct-gap related optical transitions in strain-compensated Ge/Si 0.15 Ge 0.85 multiple quantum wells (MQWs) is presented. These structures are of particular interest due to the proximity of Γ -type and L-type conduction band states and due to their type I band alignment. The samples were grown by low-energy plasma-enhanced CVD and consist of Ge MQWs with a large numbers of periods and good morphological quality grown onto thick graded Si 1 - x Ge x buffer layers. The transmission spectra, which shows clear evidence of excitonic transitions, are studied as a function of temperature in the 5–300 K range. Preliminary results of photocurrent measurements performed on the same structures using metal–semiconductor–metal contact are discussed.

Direct-gap and indirect-gap transitions in strain-compensated Ge/SiGe multiple quantum wells with Ge-rich SiGe barriers have been studied by optical transmission spectroscopy and photoluminescence experiments. An s p3 d5 s tight-binding model has been adopted to interpret the experimental results. Photoluminescence spectra and their comparison with theoretical calculations prove the existence of type-I band alignment in compressively strained Ge quantum wells grown on relaxed Ge-rich SiGe buffers. The high quality of the transmission spectra opens up other perspectives for application of these structures in near-infrared optical modulators. © 2008 The American Physical Society.

The knowledge and control of the structural details (texture, crystallite environment and size) of nanocrystalline silicon films is a prerequisite for their proper application in various technological fields. To this purpose, nanocrystalline silicon films grown by low energy plasma enhanced chemical vapour deposition (LEPECVD) on different kinds of substrates were submitted to a systematic characterization using Raman spectroscopy, X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The results showed how the difference in substrate morphology is responsible for a deep difference in the film structural properties, particularly in the case of high silane dilutions.

A detailed spectroscopic and morphological study of GaAs epitaxial layers grown by molecular beam epitaxy on Ge buffer layers deposited by low energy plasma enhanced chemical vapor deposition on Si is presented. The aim is to understand the nature of thermal strain relaxation induced by crack formation in the epilayers. The comparison of the experimental data on the spatial strain relaxation pattern with the theoretical prediction from a purely elastic model indicates that strain relaxation around cracks arises from two contributions. At short distances the main contribution is essentially plastic, due to the presence of extended defects. At large distances, on the contrary, elastic relaxation seems to dominate. It is also shown that GaAs grown on GeSi substrates is in a state of metastable strain as a consequence of the fact that cracks relax the thermal tensile strain only locally. © 2007 American Institute of Physics.

A detailed analysis of the thermal strain relaxation in GaAs/Ge/Si structures by means of crack formation is presented. The study was performed through optical microscopy and photoluminescence measurements. The as grown epilayers were found to be in a metastable state with respect to crack formation. Repeated thermal cycling increased the crack density up to an asymptotic limit. The thermal strain is not fully relaxed even near to the asymptotic crack density. © 2006 Elsevier B.V. All rights reserved.

Nanocrystalline silicon (nc-Si) already attracted a considerable attention as a promising material for photovoltaic applications, while its full optoelectronic potentiality is still under investigation, due to the relatively poor knowledge of the correlations between growth conditions, microstructure and physical properties. This paper aims at the illustration of the preliminary results of a 3 years project, addressed at the demonstration of the multifunctionality of nanocrystalline silicon, via the understanding of the quantitative correlations between growth process parameters and the structural, optical, electrical and physico-chemical properties of nc-silicon. The main topics foreseen and actively pursued were: • The development of a gas phase kinetic model to be used for the simulations of the plasma enhanced chemical deposition (PECVD) process, the development of a kinetic model relative to the interaction of radicals present in the plasma phase at the substrate interface, and last but not least, the development of models relative to the subsequent nc-Si growth. • The growth of undoped and doped nc-Si layers on convenient substrates by the low energy plasma enhanced chemical deposition (LEPECVD) process using silane, phosphine, diborane and hydrogen mixtures. • The quantitative experimental determination of the correlation among the crystallinity fraction, the film microstructure, the grain size/shape/orientation, the hydrogen content, the density of the recombination centres, the optical absorption coefficient the strain/stress state and the carrier mobility and diffusion length in undoped and n-type and p-type films, in view of the optimization of the minority carrier generation and carrier collection. The preliminary results already demonstrate that a proper modelling of the growth process is of great help in the selection of optimised growth procedures for nc-Si films and that a full range experimental characterisation is needed to get the proper inputs to the modelling activity. © 2006 Elsevier B.V. All rights reserved.

We compare four different selective etching solutions commonly used to highlight threading dislocations (TD), in SiGe hetero-epitaxial layers. The aim is to identify, amongst the many reported in the literature, an etching solution effective over the whole Ge concentration range. Etching experiments have been performed on Si1-xGex linearly graded relaxed buffer layers with a final germanium concentration x varying from x = 0.2 to 0.9. All samples have been deposited on Si(1 0 0) by low-energy plasma-enhanced chemical vapour deposition (LEPECVD). The experimental results show that a variant of the so-called Schimmel-etch is successful in revealing TD over the Ge concentration range investigated. (C) 2006 Elsevier Ltd. All rights reserved.

The first systematic investigation of the vibrational properties of epitaxial Si1-xGex alloys in the entire composition range (0 ≤ x ≤ 1) is presented. Reciprocal Space Mapping measurements and a Raman spectroscopy study have been undertaken on alloys grown by Low Energy Plasma Enhanced Chemical Vapour Deposition (LEPECVD) in order to investigate the phonon mode frequency dependence on structural and elastic parameters. It is concluded that the strain relaxation process in the LEPECVD virtual substrates is very effective in the entire composition range and that in these fully relaxed epitaxial SiGe alloys the presence of ordering effects and of local composition fluctuations can be excluded. © 2005 Elsevier B.V. All rights reserved.

Molecular dynamics simulations with the Tersoff potential of the strain distribution around 60degrees misfit dislocation in a heteroepitaxial SiGe film confirm that highly compressed and expanded, cylindrical nanometer-sized regions appear on opposite sides of the (111) glide plane. Such a configuration is suggested to generate opposite chemical potential gradients for Si and Ge diffusion and, as verified by a Monte Carlo simulation, in the formation of Si-rich and Ge-rich nanowires along the dislocation core. This model is supported by photoluminescence measurements as a function of annealing temperature and time. (C) 2004 American Institute of Physics.

Low-energy plasma-enhanced chemical vapor deposition experiments indicate that strain relaxation in thin virtual SiGe layers grown abruptly on a Si substrate may occur more efficiently than in thick graded layers. We provide one possible explanation using molecular dynamics simulations based on Tersoff potentials, showing that the mobility of shuffle dislocation segments is enhanced with increasing misfit in Ge/SiGe(001) alloys. A low strain regime favors the formation of partialized glide segments. The simulated mobility of the perfect shuffle core, in the case of strong compressive stress, appears to be extremely high. Despite being qualitative in nature, these results suggest an intriguing interpretation of the experimental observation. © 2004 Elsevier B.V. All rights reserved.