Scanning X-ray strain microscopy of inhomogeneously strained Ge micro-bridges

ISSN:0909-0495ISSN:1600-577

X-ray Nanodiffraction on a Single SiGe Quantum Dot inside a Functioning Field-Effect Transistor

For advanced electronic, optoelectronic, or mechanical nanoscale devices a detailed understanding of their structural properties and in particular the strain state within their active region is of utmost importance. We demonstrate that X-ray nanodiffraction represents an excellent tool to investigate the internal structure of such devices in a nondestructive way by using a focused synchotron X-ray beam with a diameter of 400 nm. We show results on the strain fields in and around a single SiGe island, which serves as stressor for the Si-channel in a fully functioning Si-metal-oxide semiconductor field-effect transistor

X-ray based investigations of semiconductor multilayer and microbridges

Die vorliegende Arbeit beschäftigt sich mit der Strukturanalyse verschiedenster Halbleiternanostrukturen mittels Röntgenstreuverfahren. Die untersuchten strukturellen Merkmale bestimmen maßgeblich elektrische, thermische und optische Eigenschaften der untersuchten Materialien und sind daher zwingend zu bestimmen, um das Wachstum zu optimieren, bzw. gültige Eingabeparameter für Simulationen und Konstruktionen verbesserter Strukturen zu erhalten. Es handelt sich dabei um Strukturen, die zukünftig im Bereich erneuerbarer Energie eingesetzt werden können oder in integrierten optischen Schaltungen Anwendung finden. Einerseits werden konventionelle Laborexperimente mit Röntgenstrahlgrößen im mm Bereich durchgeführt, andererseits aber neuartige Nanofokus-Aufbauten mit Strahlgrößen von einigen hundert nm verwendet, mit denen es möglich ist, lokal aufgelöste Untersuchungen im sub-m Bereich zerstörungsfrei durchzuführen. Der erste Teil der Arbeit beschäftigt sich mit der Untersuchung von SiGe Übergittern. Einerseits werden Übergitter bestehend aus SiGe und Ge Schichten mit Dicken im nm Bereich für thermoelektrische Anwendungen untersucht, andererseits Übergitter, bestehend aus ultradünnen reinen Si und Ge Schichten, konzipiert für optische Anwendungen. Mittels Röntgenbeugungsmethoden werden Perioden, Verspannungen sowie die Zusammensetzung und Dicke einzelner Schichten untersucht. Um Einblick in die Grenzflächenmorphologie und etwaige Interdiffusionsmechanismen zu bekommen, werden Röntgenreflektometrie Untersuchungen durchgeführt. Im Vordergrund steht dabei die Änderung all dieser Parameter in Abhängigkeit der Wachstumsbedingungen. Der zweite Teil der Arbeit befasst sich mit der Untersuchung von tensil verspannten Ge Brückenstrukturen. Durch den Transfer eines Elektronenstrahl-Schemas wurde vorverspanntes Ge weiter verspannt, was große Auswirkungen auf die Bandstruktur hat und eine Möglichkeit darstellen könnte, zukünftig optische Bauelemente zur drahtlosen Kontaktierung herzustellen. Einerseits dienen die Brücken als Modellstrukturen für ein neues Rasterverfahren zur Verspannungsmessung, andererseits werden durch die Untersuchung von verschieden ausgerichteten Brücken Phononen-Deformationspotentiale angepasst und so Raman Verspannungsverschiebungen für den tensil verspannten Bereich von Ge kalibriert. Der dritte Teil der Arbeit befasst sich mit Si Nanodrähten, die erstmals in hexagonaler Gitterstruktur hergestellt werden konnten. Dazu wurde ein hexagonaler GaP Nanodraht als Vorlage genutzt und Si epitaktisch aufgewachsen. Die so entstandene Hülle aus hexagonalem Si wird mittels Röntgenbeugung eindeutig identifiziert und ebnet so den Weg, um vorallem optische, elektrische und mechanische Eigenschaften dieses völlig neuen Materials zu untersuchen.In the herewith presented thesis structural properties of various semiconductor nanostructures are investigated by means of X-ray scattering techniques. The studied structural parameters highly influence electrical, thermal and optical characteristics of the materials. Hence, precise knowledge is mandatory to optimize growth, fabrication and design of optimized structures which are intended for renewable energy applications or for integrated optical circuits, respectively. Besides conventional laboratory scattering methods, where X-ray beams with dimensions of a few mm are used, nanodiffraction experiments with focused X-ray beams are carried out in order to conduct spatially resolved investigations in the sub-m beamsize region. The first part of the work deals with the investigation of SiGe superlattices. One section is devoted to superlattices composed of SiGe and Ge layers with thicknesses in the nm-range intended for thermoelectric applications. A further section is devoted to superlattices composed of ultra-thin pure Si and Ge monolayers intended for optical applications. The periodicity, thicknesses and composition of the layers as well as the strain state is obtained by X-ray diffraction experiments. In order to gain insight on the interface quality and interdiffusion effects also X-ray reflectivity studies are performed. The influence of the growth parameters on all these structural characteristics is studied in great detail. The second part of the thesis focuses on the characterization of tensile strained Ge microbridges. An electron beam pattern was transferred onto a pre-strained Ge layer which enhances the strain at certain regions. Strain has dramatic influence on the bandstructure and could hence pave the way for future light emitters integrable in Si-technology. The studied bridge-structures serve on the one hand as model structures for a new scanning strain microscopy technique developed at the synchrotron, on the other hand for studies on differently oriented bridges to obtain the directional dependent Raman strain shift coefficient for the tensile strain region by fitting phonon deformation potentials. The third part of the work is devoted to Si nanowires where the hexagonal crystal phase is confirmed unambiguously for the very first time. Therefore, hexagonal GaP nanowires served as template for the epitaxial and thus also hexagonal overgrowth of Si shells. The so formed hexagonal crystal growth is confirmed with X-ray diffraction and clears the way for further optical, electrical and mechanical studies on this completely new material.eingereicht von: Tanja EtzelstorferZusammenfassungen in deutscher und englischer SpracheUniversität Linz, Dissertation, 2015OeBB(VLID)81890

Anodic oxide formation on aluminium-terbium alloys

Aluminium terbium alloys were prepared by simultaneous thermal evaporation resulting in a thin film library covering a 5 to 25 at.% Tb compositional spread. Synchrotron x-ray diffraction (XRD) proves all of the alloys to be amorphous. Scanning electron microscopy (SEM) measurements reveal the structural changes upon increase in Tb content with the formation of small, Tb-rich segregations right before a drastic change in morphology around 25 at.% Tb. Anodic oxides were formed systematically in cyclic voltammograms using scanning droplet cell microscopy. Coulometric analysis revealed a linear thickness over formation potential behaviour with film formation factors ranging from 1.2 nm V1 (5 at.% Tb) to 1.6 nm V1 (25 % Tb). Electrochemical impedance spectroscopy was performed for each incremental oxidation step resulting in a linear relation between inverse capacity and formation potential with dielectric constants ranging from 8 (5 at.% Tb) to 16 (25 at.% Tb).(VLID)342323

Determining the directional strain shift coefficients for tensile Ge: A combined x-ray diffraction and Raman spectroscopy study

In this work the calibration of the directional Raman strain shift coefficient for tensile strained Ge microstructures is reported. The strain shift coefficient is retrieved from micro-Raman spectroscopy measurements in combination with absolute strain measurements from x-ray diffraction using focused synchrotron radiation. The results are used to fit the phonon deformation potentials. A linear dependence of the phonon deformation potentials p and q is revealed. The method can be extended to provide strain calibration of Raman experiments also in other material system

The thermoelectric properties of Ge/SiGe based superlattices: from materials to energy harvesting modules

We report recent progresses in the characterization of the vertical electrical and thermal properties of multilayered silicon germanium (SiGe) materials for thermoelectric coolers and generators. Superlattice structures p- and n-type doped with different barriers and quantum well thicknesses were grown by Low Energy Plasma Enhanced Chemical Vapor Deposition. The electrical and thermal characterizations were performed using fully integrated metrology structures, microfabricated on the surface of the sample. Heaters, thermometers and electrical contacts were integrated onto the device to allow a simultaneous measurement of the electrical, thermal and Seebeck coefficient for the extraction of the figure of merit ZT. Enhancements in the Seebeck coefficient up to 450 µV/K and the reduction of the thermal conductivity down to 2.2 W/mK are mainly attributed to the low dimensionality of the system. Preliminary tests on microfabricated modules are performed on a non-optimized material and device dimensions as proof of concept for next generation devices

Scanning X-ray strain microscopy of inhomogeneously strained Ge micro-bridges

ISSN:0909-0495ISSN:1600-577

Structural investigations of the alpha(12) Si-Ge superstructure

This article reports the X-ray diffraction-based structural characterization of the [alpha]12 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