Recently, ZnO has garnered widespread attention in the semiconductor community for its large set of useful properties, which include a wide bandgap and its resulting optical transparency, a large exciton binding energy, a significant piezoelectric response, and good electrical conductivity. In many ways, it shares many properties with a widely used and technologically important semiconductor GaN, which is widely used for blue LEDs and lasers. However, ZnO cannot substitute for GaN in most optoelectronic applications, because it cannot be doped p-type. On the other hand, unlike many traditional, covalently bonded semiconductors like GaN, ZnO can be easily formed aqueous solutions at close to room temperature and pressure in the form of large crystals or a variety of nanostructures, making possible applications that are normally very difficult with traditional semiconductors. In this light, we aimed to take advantage of aqueous solution-based, ZnO growth techniques and incorporated ZnO structures novel optoelectronic and photonic structures. By controlling the morphology of ZnO, we studied the effects of nanowire-based \(ZnO/Cu_2O\) solar cells. Carrier collection was increased using a nanowire-based device architecture. The main result, however, was the time evolution of the performance of these devices due to the movement of ionized defects in the material. The effects of geometry on the ageing characteristics were studied, which showed that the carrier collection could be increased further with ageing in a nanowire \(Cu_2O\) solar cell. The aging behavior was substantially different between nanowire and planar solar cells, which implies that future design of nanostructured solar cells must long term aging effects. In addition to solar cells, we explored the possibilities of using aqueous solution growth of ZnO to fabricated whispering gallery mode optical cavities and waveguides for enhancing extraction from a single photon source. In both applications, we used templated growth of ZnO to fabricate geometrically (near) perfect rods and disks for these photonics applications. Finally, since epitaxy is important in the process of optimizing device performance and fabrication, we showed the ability to grow ZnO epitaxially on single crystalline plates of Au, expanding the options of epitaxial substrates to include a metal. / Engineering and Applied Sciences

Information anytime and anywhere has ushered in a new technological age where massive amounts of ‘big data’ combined with self-aware and ubiquitous interactive computing systems is shaping our daily lives. As society gravitates towards a smart living environment and a sustainable future, the demand for faster and more computationally efficient electronics will continue to rise. Keeping up with this demand requires extensive innovation at the transistor level, which is at the core of all electronics. Up until recently, classical silicon transistor technology has traditionally been weary of disruptive innovation. But with the aggressive scaling trend, there has been two dramatic changes to the transistor landscape. The first was the re-introduction of metal/high-K gate stacks with strain engineering in the 45 nm technology node, which enabled further scaling on silicon to smaller nodes by alleviating the problem of gate leakage and improving the channel mobility. The second innovation was the use of non-planar 3D silicon fins as opposed to classical planar architectures for stronger electrostatic control leading to significantly lower off-state leakage and other short-channel effects. Both these innovations have prolonged the life of silicon based electronics by at least another 1-2 decades. The next generation 14 nm technology node will utilize silicon fin channels that have gate lengths of 14 nm and fin thicknesses of 7 nm. These dimensions are almost at the extreme end of current lithographic capabilities. Moreover, as fins become smaller, the parasitic capacitances and resistances increase significantly resulting in degraded performance. It is of popular consensus that the next evolutionary step in transistor technology is in the form of gate-all-around silicon nanowires (GAA NWFETs), which offer the tightest electrostatic configuration leading to the lowest possible leakage and short channel characteristics in over-the-barrier type devices. However, to keep scaling on silicon, the amount of current generated per device has to be increased while keeping short channel effects and off-state leakage at bay. The objective of this doctoral thesis is the investigation of an innovative vertical silicon based architecture called the silicon nanotube field effect transistor (Si NTFET). This topology incorporates a dual inner/outer core/shell gate stack strategy to control the volume inversion properties in a hollow silicon 1D quasi-nanotube under a tight electrostatic configuration. Together with vertically aligned source and drain, the Si NTFET is capable of very high on-state performance (drive current) in an area-efficient configuration as opposed to arrays of gate-all-around nanowires, while maintaining leakage characteristics similar to a single nanowire. Such a device architecture offsets the need of device arraying that is needed with fin and nanowire architectures. Extensive simulations are used to validate the potential benefits of Si NTFETs over GAA NWFETs on a variety of platforms such as conventional MOSFETs, tunnel FETs, junction-less FETs. This thesis demonstrates a novel CMOS compatible process flow to fabricate vertical nanotube transistors that offer a variety of advantages such as lithography-independent gate length definition, integration of epitaxially grown silicon nanotubes with spacer based gate dielectrics and abrupt in-situ doped source/drain junctions. Experimental measurement data will showcase the various materials and processing challenges in fabricating these devices. Finally, an extension of this work to topologically transformed wavy channel FinFETs is also demonstrated keeping in line with the theme of area efficient high-performance electronics.

Το φαινόμενο της μαγνητικής πόλωσης ανταλλαγής (exchange bias, EB) έχει προσελκύσει το επιστημονικό ενδιαφέρον εξαιτίας των σημαντικών εφαρμογών σε διατάξεις μαγνητικής αποθήκευσης πληροφορίας. Ιδιαίτερο ενδιαφέρον εμφανίζουν οι ΕΒ ιδιότητες των πολυστρωματικών φιλμ με σύσταση [La2/3Ca1/3MnO3(FM)/La1/3Ca2/3MnO3(AF)]15, που αποτελούνται από αντισιδηρομαγνητικά, La1/3Ca2/3MnO3 (AF), και σιδηρομαγνητικά, La2/3Ca1/3MnO3 (FM), στρώματα επειδή αυτή η κατηγορία ενώσεων ανήκει στα ισχυρώς συσχετιζόμενα ηλεκτρονικά συστήματα (strongly correlated electronic systems), όπου οι μαγνητικές-, ηλεκτρονικές-, και κρυσταλλικές-δομές αλληλεπιδρούν ισχυρά μεταξύ τους. Στην παρούσα διατριβή χρησιμοποιήθηκαν μετρήσεις περίθλασης και σκέδασης συντονισμού ακτίνων-Χ στην Κ-ακμή του Mn για την μελέτη της επίδρασης των ελαστικών τάσεων επιταξίας στο μηχανισμό της μαγνητικής πόλωσης ανταλλαγής της πολυστρωματικής δομής [La2/3Ca1/3MnO3/La1/3Ca2/3MnO3]15 που εμφανίζεται κάτω από την θερμοκρασία εμπλοκής ΤΒ≈80 Κ. Οι μετρήσεις περίθλασης ακτίνων-Χ έδειξαν ότι ο λόγος c/a, που αποτελεί το μέτρο της τετραγωνικής πλεγματικής παραμόρφωσης της ψευδοκυβικής δομής, φθάνει στην μέγιστη τιμή της, και ότι το μήκος συσχετίσεως εντός των (101) και (102) κρυσταλλογραφικών επιπέδων μεταβάλλεται σημαντικά κοντά στην ΤΒ. Αυτό το φαινόμενο οφείλεται στο γεγονός ότι στην ΤΒ η ψευδοκυβική πλεγματική σταθερά aML του φιλμ προσεγγίζει την τιμή της ψευδοκυβικής πλεγματικής σταθερά της ένωσης La1/3Ca2/3MnO3 από την οποία αποτελούνται τόσο το ενδιάμεσο στρώμα (buffer layer) όσο και τα AF στρώματα, αποδεικνύοντας έτσι ότι το EB φαινόμενο σχετίζεται με την εξισορρόπηση των επιταξιακών τάσεων εντός των AF και FM στρωμάτων. Οι μετρήσεις σκέδασης συντονισμού ακτίνων-Χ (RXS) αποκαλύπτουν ότι οι επιταξιακές τάσεις εξωθούν τα πολύεδρα MnO6 σε σιδηροπαραμορφωτική (ferrodistortive, FD) ευθυγράμμιση, όπου η FD υπερκυψελίδα συμπίπτει με την μοναδιαία κυψελίδα της κρυσταλλικής δομής. Εφαρμογή της τεχνικής συμβολής στα RXS φάσματα αποκαλύπτει μια κορυφή συντονισμού από την κύρια ακμή απορρόφησης των ιόντων Mn στα ~6.555 keV και μια, μικρότερης έντασης, δευτερεύουσα κορυφή συντονισμού στα ~6.55 keV, του οποίου η ενεργειακή μετατόπιση ελαττώνεται γραμμικά με την θερμοκρασία και το λόγο c/a μέχρι τους 80 Κ. Η εξαφάνιση της δευτερεύουσας κορυφής πάνω από τους 80 Κ(≈ΤΒ) μπορεί να σχετίζεται με κάποια αναδιάταξη των γωνιών που σχηματίζουν οι δεσμοί Mn-O-Mn εξαιτίας της εξισορρόπησης των επιταξιακών τάσεων στις FM/AF διεπιφάνειες κάτω από 80 Κ. / The exchange bias (EB) phenomenon has received considerable attention because of its important applications in magnetic storage devices. Of particular interest are the EB properties of colossal magnetoresistance (CMR) compositionally modulated structures consisting of antiferromagnetic (AF) and ferromagnetic (FM) (La,Ca)MnO3 layers because manganites are strongly correlated electron systems, in which the magnetic, electronic and crystal structures interact strongly with each other. Complementary x-ray synchrotron radiation diffraction (XRD) and resonant scattering (RXS) measurements were performed at the Mn K-edge between 10 and 300 K in order to analyze the effect of epitaxial strain and tetragonal lattice distortions on the exchange bias (EB) mechanism observed in [La2/3Ca1/3MnO3(FM)/La1/3Ca2/3MnO3(AF)]15 multilayers below a blocking temperature, TB, of 80 K. XRD measurements showed that the c/a axial ratio, an indication of the tetragonal lattice distortion in pseudocubic lattice settings, reaches its maximum at the onset of the EB effect and the corresponding structural correlation length varies substantially at the onset of TB. The in-plane lattice parameter a at TB is close to the bulk lattice parameters of the AF layers, thus indicating that the EB effect is related with the accommodation of strain inside the FM and AF layers. RXS measurements revealed that such anisotropic lattice strains force the MnO6 octahedral sites into a ferrodistortive (FD) alignment, where the FD supercell coincides with the unit cell of the crystalline lattice. The RXS intensity difference signal exhibits a main-edge feature and a post-edge feature at 6.57 keV that scales linearly with temperature and the c/a ratio up to 80 K. The disappearance of the post-edge feature above 80 K(=TB) may signify a rearrangement of Mn-O-Mn bonding angles due to strain-driven effects at the FM/AF interfaces, inducing disorder in FD octahedral tilt ordering which may pin the local distortions below the TB.

A close relation between structural and optical properties of any semiconductor material does exist. An adequate knowledge and understanding of this relationship is necessary for fabrication of devices with desired optical properties. The structural quality and hence the optical properties can be influenced by the growth method and the substrate used. The aim of this work was to investigate the change in optical properties caused by growth techniques and substrate modification. To study the influence of growth technique on optical properties, ZnO nanostructures were grown using atmospheric pressure metal organic chemical vapor deposition (APMOCVD) and chemical bath deposition (CBD) technique. The structural and optical investigations were performed using scanning electron microscopy (SEM) and micro photoluminescence (μ-PL), respectively. The results revealed that the grown structures were in the shape of nano-rods with slightly different shapes. Optical investigation revealed that low temperature PL spectrum for both the samples was dominated by neutral donor bound excitons emission and it tends to be replaced by free exciton (FX) emission in the temperature range of 60-140K. Both excitonic emissions show a typical red-shift with increase in temperature but with a different temperature dynamics for both the sample and this is due to difference in exciton-phonon interaction because of the different sizes of nano-rods. Defect level emission (DLE) is negligible in both the sample at low temperature but it increased linearly in intensity after 130 K up to the room temperature.Modification in substrate can also play a significant role on structural and optical properties of the material. Specially variation in the miscut angle of substrate can help to control the lateral sizes of the Nanostructures and thus can help to obtain better structural andoptical quality. Also optical quality is a key requirement for making blue and ultraviolet LEDs. Therefore, ZnO Nanostructures were grown on SiC on-axis and off-axis substrates having different off-cut angles. Morphological investigation revealed thatgrown structures are epitaxial for the case when substrate off-cut angle is higher and deposition rate is low. Low temperature PL spectrum of all the samples was dominated by neutral donor bound excitons and free exciton emission become dominant at 100 K for all the samples which completely eliminate the neutral donor bound excitonic emission at 160K. Two electron satellite of the neutral donor bound excitons and LO phonons of excitonic features are also present. A typical red-shift in excitonic features was evident in temperature dependence measurement. Red-shift behavior of free exciton for all the samples was treated by applying Varshni empirical expression and several important parameter, such as, the Debye temperature and the band gap energy value was extracted. Thermal quenching behavior was also observed and treated by thermal quenching expression and value of the activation energy for non-radiative channel was extracted. The results that are obtained demonstrate a significant contribution in the fields of ZnO based nano-optoelectronics and nano-electronics.

Graphene is widely considered to be a promising candidate for a new generation of electronics, but there are many outstanding fundamental issues that need to be addressed before this promise can be realized. This thesis focuses on the production and properties of graphene grown epitaxially on the carbon terminated face (C-face) of hexagonal silicon carbide leading to the construction of a novel graphene transistor structure. C-face epitaxial graphene multilayers are unique due to their rotational stacking that causes the individual layers to be electronically decoupled from each other. Well-formed C-face epitaxial graphene single layers have exceptionally high mobilities (exceeding 10,000 cm^2/Vs), which are significantly greater than those of Si-face graphene monolayers. This thesis investigates the growth and properties of C-face single layer graphene. A field effect transistor based on single layer graphene was fabricated and characterized for the first time. Aluminum oxide or boron nitride was used for the gate dielectric. Additionally, an all graphene/SiC Schottky barrier transistor on the C-face of SiC composed of 2DEG in SiC/Si2O3 interface and multilayer graphene contacts was demonstrated. A multiple growth scheme was adopted to achieve this unique structure.

No description available.

This thesis deals with the synthesis and characterization of Ge-Te-Sb (GST) thin films. The films were deposited using a Pulsed Laser Deposition (PLD) method and mainly characterized with XRD, SEM, AFM and TEM. For amorphous and polycrystalline films, un-etched Si(100) was used. The amorphous films showed a similar crystallization behavior as films deposited with sputtering and evaporation techniques. When depositing GST on un-etched Si(100) substrates at elevated substrate temperatures (130-240°C), polycrystalline but highly textured films were obtained. The preferred growth orientation was either GST(111) or GST(0001) depending on if the films were cubic or hexagonal. Epitaxial films were prepared on crystalline substrates. On KCl(100), a mixed growth of hexagonal GST(0001) and cubic GST(100) was observed. The hexagonal phase dominates at low temperatures whereas the cubic phase dominates at high temperatures. The cubic phase is accompanied with a presumed GST(221) orientation when the film thickness exceeds ~70 nm. Epitaxial films were obtained with deposition rates as high as 250 nm/min. On BaF2(111), only (0001) oriented epitaxial hexagonal GST films are found, independent of substrate temperature, frequency or deposition background pressure. At high substrate temperatures there is a loss of Ge and Te which shifts the crystalline phase from Ge2Sb2Te5 towards GeSb2Te4. GST films deposited at room temperature on BaF2(111) were in an amorphous state, but after exposure to an annealing treatment they crystallize in an epitaxial cubic structure. Film deposition on pre-cleaned and buffered ammonium fluoride etched Si(111) show growth of epitaxial hexagonal GST, similar to that of the deposition on BaF2(111). When the Si-substrates were heated directly to the deposition temperature films of high crystal-line quality were obtained. An additional heat treatment of the Si-substrates prior to deposition deteriorated the crystal quality severely. The gained results show that PLD can be used as a method in order to obtain high quality epitaxial Ge-Sb-Te films from a compound target and using high deposition rates.

In this work we present the results of the analysis of the surface structures and absorption properties with respect to the CO and O2 molecules of the Sn/Rh and Rh/SnO2 model systems. In the part dedicated to the Sn structures on Rh surfaces with two different orientations - Rh(110) and Rh(111) - we have investigated the development of the core electron levels and valence band during the development of surface reconstructions and absorption of CO molecules. The surface reconstructions of the Sn/Rh(110) systems were studied for the first time. Difference in behaviour w.r.t. Sn/Rh(111) was observed and explanation offered. Finally, on in-situ prepared epitaxial SnO2 layers, the surface reconstruction (4×1) was observed. The CO adsorp- tion properties of Rh on polycrystalline and epitaxial SnO2 layers were also studied and difference in behaviour explained.

abstract: Carrier lifetime is one of the few parameters which can give information about the low defect densities in today's semiconductors. In principle there is no lower limit to the defect density determined by lifetime measurements. No other technique can easily detect defect densities as low as 10-9 - 10-10 cm-3 in a simple, contactless room temperature measurement. However in practice, recombination lifetime τr measurements such as photoconductance decay (PCD) and surface photovoltage (SPV) that are widely used for characterization of bulk wafers face serious limitations when applied to thin epitaxial layers, where the layer thickness is smaller than the minority carrier diffusion length Ln. Other methods such as microwave photoconductance decay (µ-PCD), photoluminescence (PL), and frequency-dependent SPV, where the generated excess carriers are confined to the epitaxial layer width by using short excitation wavelengths, require complicated configuration and extensive surface passivation processes that make them time-consuming and not suitable for process screening purposes. Generation lifetime τg, typically measured with pulsed MOS capacitors (MOS-C) as test structures, has been shown to be an eminently suitable technique for characterization of thin epitaxial layers. It is for these reasons that the IC community, largely concerned with unipolar MOS devices, uses lifetime measurements as a "process cleanliness monitor." However when dealing with ultraclean epitaxial wafers, the classic MOS-C technique measures an effective generation lifetime τg eff which is dominated by the surface generation and hence cannot be used for screening impurity densities. I have developed a modified pulsed MOS technique for measuring generation lifetime in ultraclean thin p/p+ epitaxial layers which can be used to detect metallic impurities with densities as low as 10-10 cm-3. The widely used classic version has been shown to be unable to effectively detect such low impurity densities due to the domination of surface generation; whereas, the modified version can be used suitably as a metallic impurity density monitoring tool for such cases. / Dissertation/Thesis / M.S. Materials Science and Engineering 2013

With recent trends in miniaturization in the electronics sector, ferroelectrics have gained popularity due to their applications in non-volatile RAM. Taking one step further researchers are now exploring multiferroic devices that overcome the drawbacks of ferroelectric (FE) and ferromagnetic (FM) RAM’s while retaining the advantages of both. The work presented in this dissertation focuses on the growth of FE and FM thin film structures. The primary goals of this work include, (1) optimization of the parameters in the pulsed laser deposition (PLD) of FE and FM films and their heterostructures, (2) development of a structure-property relation that leads to enhancements in electric and magnetic polarizations of these structures, (3) investigation of doping on further enhancement of polarizations and coupling between the FE and FM layers. The materials of choice are La0.7Sr0.3MnO3 (LSMO) as the ferromagnetic and PbZr0.52Ti0.48O3 (PZT) as the ferroelectric component. Epitaxial thin film capacitors were grown using PLD. The work starts with the establishment of the optimum deposition conditions for PZT and goes on to describe results of attempts at performance enhancement and tuning using two methods. It is demonstrated that ferroelectric and ferromagnetic properties can be tuned by inserting a ferromagnetic buffer layer of CoFe2O4 (CFO) between PZT and LSMO. One of the key findings of this work was the anomalously high ferroelectric polarizations produced by lanthanum (La) doped PZT films. This work attempts to shine light on a possible mechanism that leads to such high enhancements in polarization.