Epitaxy of SrTiO3 on Silicon: The Knitting Machine Strategy

Saint-Girons, Guillaume et al.SrTiO3 (STO) crystalline layers grown on Si open unique perspectives for the monolithic integration of functional oxides in silicon-based devices, but their fabrication by molecular beam epitaxy (MBE) is challenging due to unwanted interfacial reactions. Here we show that the formation of single-crystal STO layers on Si by MBE at the moderate growth temperature imposed by these interface reactions results from the crystallization of a partially separated amorphous mixture of SrO and TiO2 activated by an excess of Sr. We identify the atomic pathway of this mechanism and show that it leads to an antiphase domain morphology. On the basis of these results, we suggest and test alternative STO growth strategies to avoid antiphase boundary formation and significantly improve the STO structural quality. The understanding provided by these results offers promising prospects to crystallize perovskite oxides on semiconductors at moderate temperature and circumvent the issue of parasitic interface reactions.This work was partly supported by the European projects SITOGA (STREP FP7, grant number 619456) and TIPS (ICT H2020, grant number 107347) as well as by the French ANR programs HIRIS and DIAMWAFEL.Peer reviewe

Poisson ratio and bulk lattice constant of (Sr 0.25 La 0.75 )CrO 3 from strained epitaxial thin films

International audienceAbout 30 nm thick (001)-oriented (Sr0.25La0.75)CrO3 (SLCO) epitaxial thin films were grown by solid-source oxide molecular beam epitaxy on four different single-crystalline cubic or pseudo-cubic (001)-oriented oxide substrates: LaAlO3, (LaAlO3)0.3(Sr2AlTaO6)0.7, SrTiO3 and DyScO3, which result in lattice mismatch ranging from-2% to +1.7%. All the films are of high-quality, flat and strained by the substrates. By assessing the evolution of the out-of-plane lattice parameter as a function of the in-plane lattice parameter of the samples, we determine both the Poisson ratio (ν = 0.32) and the bulk lattice constant (ab = 3.876 Å) of SLCO. The Poisson ratio significantly differs from LaCrO3 (ν = 0.23) and the (SrxLa1-x)CrO3 solid solution appears to obey structural Vegard's law. Since SLCO is the only one p-type transparent conductive oxide of perovskite structure and has promising thermoelectric properties, integrating SLCO in heterostructures and devices is therefore of paramount importance, which confers on our results their strong interest. Besides, the method used here can be straightforwardly applied to other complex oxides

Giant tuning of electronic and thermoelectric properties by epitaxial strain in p-type Sr-doped LaCrO3 transparent thin films

The impact of epitaxial strain on the structural, electronic, and thermoelectric properties of p-type transparent Sr-doped LaCrO3 thin films has been investigated. For this purpose, high-quality fully-strained La0.75Sr0.25CrO3 (LSCO) epitaxial thin films were grown by molecular beam epitaxy on three different (pseudo)cubic (001)-oriented perovskite-oxide substrates: LaAlO3, (LaAlO3)0.3(Sr2AlTaO6)0.7, and DyScO3. The lattice mismatch between the LSCO films and the substrates induces in-plane strain ranging from -2.06% (compressive) to +1.75% (tensile). The electric conductivity can be controlled over two orders of magnitude, σ ranging from ~0.5 S cm-1 (tensile strain) to 35 S cm-1 (compressive strain). Consistently, the Seebeck coefficient S can be finely tuned by a factor of almost two from ~127 μV K-1 (compressive strain) to 208 μV K-1 (tensile strain). Interestingly, we show that the thermoelectric power factor (PF = S2 σ) can consequently be tuned by almost two orders of magnitude. The compressive strain yields a remarkable enhancement by a factor of three for 2% compressive strain with respect to almost relaxed films. These results demonstrate that epitaxial strain is a powerful lever to control the electric properties of LSCO and enhance its thermoelectric properties, which is of high interest for various devices and key applications such as thermal energy harvesters, coolers, transparent conductors, photo-catalyzers and spintronic memories.Financial support from the European Commission through the project TIPS (H2020-ICT-02-2014-1-644453), the French national research agency (ANR) through the projects MITO (ANR-17-CE05-0018), LILIT (ANR-16-CE24-0022), DIAMWAFEL (ANR-15-CE08-0034-02), the CNRS through the MITI interdisciplinary programs (project NOTE), IDEX Lyon-St-Etienne through the project IPPON, the Spanish Ministerio de Ciencia e Innovación, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV2015-0496) and the MAT2017-85232-R (AEI/FEDER, EU), PID2019-107727RB-I00 (AEI/FEDER, EU), and from Generalitat de Catalunya (2017 SGR 1377) is acknowledged. The China Scholarship Council (CSC) is acknowledged for the grant of Dong Han. Ignasi Fina acknowledges Ramón y Cajal contract RYC-2017-22531. Seebeck measurements at ILM were made within the ILMTech transport platform. The authors are also grateful to Jean-Baptiste Goure, Philippe Regreny, Aziz Benamrouche, and Bernat Bozzo for their technical support and the reviewers for their valuable and constructive comments that have improved the quality of the manuscript.Peer reviewe

Instabilities in crystal growth by atomic or molecular beams

The planar front of a growing a crystal is often destroyed by instabilities. In the case of growth from a condensed phase, the most frequent ones are diffusion instabilities, which will be but briefly discussed in simple terms in chapter II. The present review is mainly devoted to instabilities which arise in ballistic growth, especially Molecular Beam Epitaxy (MBE). The reasons of the instabilities can be geometric (shadowing effect), but they are mostly kinetic or thermodynamic. The kinetic instabilities which will be studied in detail in chapters IV and V result from the fact that adatoms diffusing on a surface do not easily cross steps (Ehrlich-Schwoebel or ES effect). When the growth front is a high symmetry surface, the ES effect produces mounds which often coarsen in time according to power laws. When the growth front is a stepped surface, the ES effect initially produces a meandering of the steps, which eventually may also give rise to mounds. Kinetic instabilities can usually be avoided by raising the temperature, but this favours thermodynamic instabilities. Concerning these ones, the attention will be focussed on the instabilities resulting from slightly different lattice constants of the substrate and the adsorbate. They can take the following forms. i) Formation of misfit dislocations (chapter VIII). ii) Formation of isolated epitaxial clusters which, at least in their earliest form, are `coherent' with the substrate, i.e. dislocation-free (chapter X). iii) Wavy deformation of the surface, which is presumably the incipient stage of (ii) (chapter IX). The theories and the experiments are critically reviewed and their comparison is qualitatively satisfactory although some important questions have not yet received a complete answer.Comment: 90 pages in revtex, 45 figures mainly in gif format. Review paper to be published in Physics Reports. Postscript versions for all the figures can be found at http://www.theo-phys.uni-essen.de/tp/u/politi

Développement de nouveaux substrats compliants pour l'hétérroepitaxie de semiconducteurs

Cette thèse montre comment utiliser un matériau visqueux ou élastique pour fabriquer des pseudosubstrats permettant l'épitaxie de matériaux semiconducteurs impossibles à obtenir sans défauts sur les substrats standard. Elle explicite la technique du double report pour réaliser des pseudosubtrats. Une trimembrane InGasAs/InAsP/InGaAs contrainte est obtenue par EJM. Cette tri-membrane est reportée sur un substrat hôte par collage hydrophile - hydrophobe sur une couche épaissede polydimethylsiloxane (PDMS) pour y être relaxée élastiquement. Comme la croissance des semiconducteurs III-V se fait à des températures incompatibles avec la stabilité du PDMS, un second report sur un support hôte est nécéssaire. Les spectres de photoluminescence obtenus pour une couche de InGaAs témoignent de la qualité optoélectronique des reprises d'épitaxie obtenues sur les pseudosubstrats ainsi fabriqués. Deux autres approches sont également explorées. L'une concerne le remplacement du PDMS par une couche de molécule autoassemblée qui sert à la fois d'adhésif et de couche compliante. La seconde approche s'intéresse à un verre, le borophosphosilicate (BPSG) comme couche compliante. Ce verre est compatible avec l'épitaxie des semiconducteurs et à la faculté de fluer à ces températures ce qui permet donc de s'affranchir du second report. Cette étude a clairement démontré l'importance dune part des interfaces de collage qui permettent de répartir la contrainte entre la couche mince et le substrat et d'autre part du rapport d'échelle entre la couche germe fine et la couche compliante épaisse.This work showed how employed a viscous or elastic material in order to make a pseudosubstrate. This allowed to epitaxy new semiconductors that couldn't be obtain without defaults on the standart substrate. We explained the double bonding technic in order to obtain pseudosubstrates. A puedomorphic layer of InGasAs/InAsP/InGaAs strained in compressive was epi-grown onto substrate. this was bonded onto a thick PDMS substrate by hydrophobic-hydrphilic bonding. The PDMS thanks to its great elsticity allowed the relaxation of the epilayer strain. Like the epi-grown of III-V semiconductors was dont to incompatible temperature with the PDMS stability a second bonding on host sunstrate was used. The InGaAs epilayer photolumijnescnce spectrum showed the optoelectronic quality of the re-epitaxy. Others materials were also explored. Once used a self assembled molecular layer in place of PDMS. This layer was used both like adhesif and compliant layer. The second material was a glass, the borophosphosilicate glass (BPSG) that was compatible with III-V semiconductors epi-grown. This allowed to free from double bonding. This study showed the importance of both bonding interface, who permit to divide the strain up between the thin layer and the substrate, and the different thick between the epi-grown layer and the thick compliant layer.LYON-Ecole Centrale (690812301) / SudocSudocFranceF

Elaboration de pseudosubstrats accordables en paramètre de maille à base de silicium mésoporeux pour l'hétéroépitaxie

Ce travail de thèse évalue les potentialités du Silicium Poreux (SiP) nanostructuré comme substrat générateur de contraintes mécaniques pour modifier le paramètre de maille d une fine couche de semiconducteur IV-IV (Si ou SiGe) qui lui est associée. Le but est soit d ajuster les propriétés optoélectroniques de la couche en la contraignant, soit d accorder son paramètre de maille à celui d un autre matériau (Si1-xGex, Ge ou GaAs). Utilisée comme couche germe pour une reprise d épitaxie, cette couche mince peut permettre l obtention d un matériau de bonne qualité cristalline. L hétérostructure (couche mince) / (Si poreux) constitue ainsi un pseudosubstrat accordable en paramètre de maille fabriqué à partir d un substrat peu onéreux, le Si Poreux. La mise au point d une nouvelle technique dite de la double plaque a permis de franchir un verrou technologique majeur et d obtenir des couches de semiconducteur IV-IV (Si ou SiGe), sans défauts structuraux, ultrafines et d épaisseur rigoureusement constante (~50 nm), de grandes dimensions latérales (2"), et présentant une surface peu rugueuse et exempte de toute pollution. Cette couche ultrafine est facilement manipulable et sa dimension latérale n est limitée que par les dimensions de la cellule d anodisation utilisée. L oxydation thermique à basse température du substrat de Si mésoporeux induit son expansion volumique et permet la déformation contrôlée des couches de Si et de SiGe. Le contrôle des paramètres du procédé permet d atteindre des déformations importantes, excédant 1%, tout en évitant la formation des défauts structuraux. Cette technique a permis de rapprocher le paramètre de maille dans le plan du film de celui du Ge et du GaAs et donc de faire un pas en direction de leur intégration sur silicium.This work assesses the potentiality of Porous Silicon (PS) as a mechanical straining substrate for modifying the lattice parameter of IV-IV semiconductor thin films (Si or SiGe). The aim is either to tune the optoelectronic properties of the thin film, or to adapt the film lattice parameter to that of other materials like SiGe, Ge, or GaAs. Such a film can be used as a seed layer for the heteroepitaxial growth of lattice mismatched materials such as SiGe, Ge or GaAs on silicon with a high crystalline quality. In short, the (thin film) / (porous silicon) heterostructure constitutes a lattice tunable virtual substrate obtained from a low cost substrate, viz, the porous silicon. The development of a two wafers technique has been a true breakthrough as it made it possible to obtain a IV-IV semiconductor film as thin as 50 nm, with large lateral dimension (2"), strictly constant thickness, exempt of structural defects and with a perfectly smooth and clean surface. Moreover, this ultra-thin film can be easily handled and its lateral dimension is only dependant on the diameter of the anodization cell. Low temperature thermal oxidation of the bulk porous Si substrate induces its volume expansion, which leads to a straining of the thin film on top. The control of oxidation parameters allows obtaining highly strained (above 1%) films without any structural defects. The technique makes the film lattice in-plane parameter closer to that of Ge or GaAs, which is a step forward toward their integration on silicon.VILLEURBANNE-DOC'INSA LYON (692662301) / SudocSudocFranceF

Sendai virus particle production: basic requirements and role of the SYWST motif present in HN cytoplasmic tail

Sendai virus (SeV) HN protein is dispensable for virus particle production. HN incorporation into virions strictly depends on a cytoplasmic domain SYWST motif. HNAFYKD, with SYWST replaced with the analogous sequence of measles virus (MeV) H (AFYKD), is not incorporated in virus particles produced by LLCMK2 cells, although it is normally expressed at the plasma membrane. Unlike HNSYWST, HNAFYKD is not internalized to late endosomes, raising the possibility that HN internalization is required for uptake into virus particles. Various mosaic MeV-H containing increasing amounts of the SeV-HN all failed to be taken up in SeV virions. However, when co-expressed with HNAFYKD these MeV-H chimera induced HNAFYKD uptake into virions showing that internalization is not a prerequisite for HN uptake into particles. We propose that HN incorporation in virus particles requires first neutralization by HN of a putative inhibitor of infectious particle formation

In situ synthesis of silver benzene-dithiolate hybrid films

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