We report a spin dynamic investigation with the use of the optical polarisation pump-probe technique in bulk and quantum well narrow gap semiconductors (NGSs) of the Ill-V and IV-VI families. By taking advantage of their zinc-blende (Ill-Vs) and rock-salt (IV-VIs) crystal symmetries, their direct energy band-gap, small effective mass, high electron effective g-value, strong relativistic effects, and with the use of the Surrey Ultrafast Laser system and the Free Electron Laser system (FELIX) for interband excitation process we prove that they are attractive for high speed electronic and new proposed spintronic concepts. The electron spin relaxation times have been measured in InSb and InAs epilayers in Faraday and Voigt configurations. A strong and opposite field dependence of the spin lifetime has been observed. We report the existence of a new spin relaxation process, which was theoretically predicted but not observed before. We demonstrate that for these NGSs, and specifically for the Ill-Vs, the electron spin lifetime can be modified by more than one order of magnitude simply by changing the direction of a moderate, externally applied magnetic field. We used circular two-photon absorption (CTPA) and investigated the "allowed- forbidden" and "allowed-allowed" transitions in n-InSb, the theory of which has not been investigated experimentally until now. The detailed CTPA spectrum presented here shows that the sign of photo-excited spin polarisation can in principle be controlled by the excitation wavelength in the two-photon absorption processes. We report the analysis of optical measurement of the spin dynamics at elevated temperatures and in zero magnetic field, for degenerately n-doped InSb/lnAISb QWs, one asymmetric and one symmetric. For the asymmetric QW, by making use of directly determined experimental parameters, we have made a direct measurement of the zero field spin splitting without the influence of the large Zeeman effect. The extracted Rashba parameter is more than an order of magnitude larger than that measured earlier for GaAs QW s. Pump-pump photoconductivity experiments have been performed and we successfully measured the recombination time in InSb-based devices of different geometries, allowing new investigation methods of low dimensional structures and manipulation of samples of weak transmittance signals. We investigated the dependence of spin lifetime on controlled asymmetry in IV- VI Multi-QWs. We show for the first time experimentally that lead-cha1cogenide semiconductor heterostructures can exhibit long spin lifetimes by virtue of their centro-symmetric crystal structure, and a tuning of the lifetime of over one order of magnitude after appropriate structure control corresponding to a large zero field spin splitting. The results imply that this system can be the material' of choice for certain , semiconductor spintronic applications requiring control of spins. We report a significant temperature dependence of the transverse electron g*- factor in symmetric IV-VI Multi-QWs. A second oscillation frequency has been observed, suggesting a possible method for distinguishing the actual excited states and the activation of an electron-hole mechanism. Although temperature tuning of lead salt laser emission wavelengths has been the method of choice in these systems for many years, we demonstrate that temperature can also be used to modulate g*, and hence the spin lifetime in lead salt QW spintronic devices.

This thesis describes the synthesis and characterisation of three related series of lithium-containing garnets. All compounds have been synthesised by conventional solid-state ceramic methods at temperatures up to 960 °C.

No description available.

Degradation processes in AIGaN/GaN high electron mobility transistors were investigated by optical methods. Temperatures within device channels, as well as electric fields are important for on-state degradation. Raman and photoluminescence (PL) thermography were used to investigate these temperatures and the thermal conductivity of GaN channels in both conventional GaN-on-SiC structures and novel AIGaN/GaN/AIGaNon- Si double heterostructure field effect transistors (DHFETs). The thin (150 nm) GaN channel layer in the DHFET had a lower thermal conductivity, at - 60 W m-I K- 1 than typical epilayers, which at - 2 pm thick have more than twice this value. This reduced thermal conductivity has implications for the design of devices employing thin GaN layers, especially when combined with the thick strain relief layers common on Si substrates, as the resulting high temperatures will affect their reliability by on- state thermal degradation processes. The depth resolution of Raman thermography on devices with typical GaN buffers usually limits results to one temperature, averaged through the buffer thickness. A method was developed to improve the depth resolution using a spatial filter and azimuthal polarisation; when combined with offset focal planes it was possible to obtain temperatures of the top and bottom of the GaN epilayer separately. Off-state degradation processes are more closely related to electric fields than self-heating; the generation of leakage current paths from the gate to the channel is particularly important. This leakage-path generation and associated localised electroluminescence (EL) emission was studied using EL imaging and spectroscopy combined with deep UV PL spectroscopy. The PL from the AIGaN barrier was reduced in regions associated with localised EL, indicating the formation of non-radiative recombination centres in the form of defects in the AIGaN. These non-radiative recombination centres were found to be generated over a larger area than the location of the gate leakage currents - these currents only start to flow when sufficient defects form to constitute a path.

Scanning probe microscopy techniques were employed to investigate C60 molecules adsorbed on Si(111)-(7x7) and Ag-Si(111)-(√3x√3)R30o using imaging, spectroscopy, and manipulation methods. First, dynamic scanning tunnelling microscopy revealed the lowest unoccupied molecular orbital features of C60 molecules adsorbed on Si(111)-(7x7) with extremely high resolution at 77 K. Experimental data were compared with Hückel molecular orbital theory simulations to determine the orientation of the molecules on these surfaces. Second, C60 molecules were imaged with a qPlus atomic force microscope, in the attractive force regime and appeared as bright spherical protrusions. The potential energy of interaction between the AFM tip and C60 molecules adsorbed on Si(111)-(7x7) was quantified by force spectroscopy. Furthermore, a C60 molecule was transferred to the scanning probe microscope tip and used as molecular probe to image the Si(111)-(7x7) surface and other C60 molecules. The on-tip C60 molecule was imaged with high precision. Hückel molecular orbital theory calculations accurately predicted the shape and characteristics of molecular orbitals observed with dynamic scanning tunnelling microscopy, which were strongly dependent on molecular symmetry, orientation, and adsorption angle. Using qPlus atomic force microscopy, chemical reactivity was probed close to or at the carbon atom positions in the C60 cage. Density functional theory simulations showed that an (iono)covalent bond formed between a carbon atom and the underlying Si adatom was responsible for contrast formation. The pair potential for two C60 molecules was also determined experimentally and found to be in very good agreement with the Girifalco potential (Girifalco, L.A., J. Phys. Chem., 1992. 96(2): p. 858). Using Hückel molecular orbital theory, the mutual orientation of a C60 molecule adsorbed on the STM/AFM tip and a C60 molecule adsorbed on the Si(111)-(7x7) surface was determined via comparison of simulated images to the experimental data. Individual C60 molecules were also manipulated with qPlus atomic force microscopy. Manipulation of single C60 molecules was performed on the Ag-Si(111)-(√3x√3)R30o surface using scanning tunnelling microscopy at room temperature and at 100 K. The interaction was predominantly attractive. Due to weak molecule-substrate interaction, a short-range chemical force between the C60 molecule and the tip was considered to be responsible for the manipulation process.

La synthèse de nanoparticules de semi-conducteurs (S-C) de TiO2 ou de ZnS dans le volume poreux de zéolithes à canaux de type ferrierite (FER), ZSM-5, mordenite (MOR) et d’une zéolithe constituée de cages de type faujasite (FAU) a été mise en œuvre par échange cationique entre les cations compensateurs de charges des zéolithes et différents précurseurs de TiO2 et de ZnS. La caractérisation de ces nouveaux matériaux a été réalisée par spectroscopie infrarouge en réflexion diffuse, XPS, diffraction des rayons X, absorption UV-visible en réflexion diffuse, microscopie électronique à transmission et isothermes de sorptions. Ces techniques complémentaires mettent en évidence la présence des S-C en surface sous forme d’agrégats de nanoparticules sphériques et dans le volume poreux pour les zéolithes dont le diamètre des pores est supérieur à 0,56 Å (MOR et FAU). L’utilisation potentielle de ces systèmes pour valoriser l’énergie lumineuse est évaluée par adsorption d’une molécule sonde donneuse d’électrons au sein du réseau poreux. Les mécanismes de transferts d’électrons ayant lieu après incorporation et (photo)ionisation de la molécule de trans-stilbène sont ainsi étudiés et comparés à des systèmes ne comportant pas de S-C. Les résultats montrent la formation d’états de charges séparées de très longue durée de vie dont la stabilité est liée à la nature du nouveau cation compensateur de charge, de la morphologie de la zéolithe et de la teneur en aluminium du réseau. La présence du semi-conducteur se traduit par une augmentation de la durée de vie des espèces transitoires qui pourrait être expliquée par un possible transfert d’électrons vers la bande de conduction du S-C. / The synthesis of semiconductors nanoparticles (S-C) of TiO2 or ZnS in the porous volume of channels types zeolites like ferrierite (FER), ZSM-5, mordenite (MOR) and zeolithe constituted of cages like faujasite (FAU) have been implemented by cationic exchange between the charges compensator cations of zeolites and the different precursors of TiO2 (ammonium/potassium titanyl oxalate, TiCl3) and ZnS (Na2S + ZnCl2). The characterization of these new materials have been realized by diffuse reflectance infrared spectroscopy (DRIFT), XPS, XRD, diffuse reflectance UV-visible spectroscopy, high resolution transmission electron microscopy (HRTEM), and sorption isotherms. These complementary techniques underscore the presence of S-C in the surface as aggregate of spherical nanoparticles and inside of porous volume of zeolites with channels diameter higher than 0.56 Å (MOR and FAU). The potential use of these new systems to valorize light energy is evaluated by adsorption of probe molecule donor of electrons within the porous network. The electron transfer mechanisms taking place after the incorporation and (photo)ionization of trans-stilbene molecules are thus studied and compared to zeolites system without S-C. The results show the formation of long-lived charge separated states of whom the stability is linked to the nature of the new charge compensator cation, the zeolites morphology and the aluminum rate of the network. The presence of S-C is leading to an augmentation of the transient species lifetime which could be explained by a possible electrons transfer towards the S-C’s conduction band.

This thesis concerns the study of equilibrium and photo excited carriers in thin-film semiconductors using terahertz time-domain spectroscopy (THz- TDS) and optical- pump terahertz-probe (OPTP) spectroscopy. In particular, it focuses on the develop- ment and application of a MATLAB program to extract the sample complex refractive index (and thereby the complex conductivity) from THz-TDS data. The parameter extraction procedure was developed to be effective in the case of thin-film samples for which the transmission function contains a substantial contribution from multiple internal reflections in the sample. The program's effectiveness was demonstrated in the case of a 30 nm-thick film of gold deposited on silicon; the program successfully extracted the complex refractive index while the original method failed. The program was applied in a study of thin-film nanoporous indium phosphide (InP). The frequency-dependent complex conductivity-obtained via THz- TDS meas- urements-was not Drude-like, as observed for bulk InP, but was qualitatively con- sistent with a plasmonic response. The time-resolved photoconductivity was obtained from OPTP spectroscopy measurements, and revealed that the presence of pores substantially increases the photo carrier lifetime compared with the value measured for bulk InP. This behaviour is attributed to the presence of surface-bound electronic states that pin the Fermi level at the pore surface, bending the electronic bands upwards. This creates a depletion region which contains the carriers away from the recombination sites at the surface. The significance of the role played by surface states in determining carrier be- haviour is further evidenced by a study of carrier dynamics in etched and surface- passivated semi-insulating gallium arsenide (SI-GaAs). Compared with an untreated sample, surface-passivated SI-GaAs exhibited a greater change in photoconductiv- ity upon generation of photocarriers and an increased photo carrier lifetime. These observations are attributed to an increased photo carrier mobility and a decreased recombination rate resulting from the removal of surface states.

The work of this thesis focuses on the interaction of high Rydberg states of hydrogen molecules and atoms with various doped Si semiconductor surfaces with the results compared with those obtained with an atomically flat gold surface. The major part of the thesis was carried out using para-H₂ molecular Rydberg states with principal quantum number n = 17 - 21 and core rotational quantum number N⁺ = 2. Subsequently, this study was continued using H atomic Rydberg states with principal quantum number n = 29 - 34. The high Rydberg states have been produced using two-step laser excitation. For close Rydberg surface separation (< 6 n² a.u.), the Rydberg states may be ionized due to an attractive surface potential experienced by the Rydberg electron, and the remaining ion core may be detected by applying an external electric field. An efficient ion detectability method is introduced to compare the many surface ionization profiles quantitatively. The p-type doped Si surfaces enhance the detected ion-signal more than the n-type doped Si surfaces due to the presence of widely distributed positive dopant charge fields in the p-type doped Si surfaces. As the dopant density increases, the area sampled by the resultant ions becomes effectively more neutral, and the decay rate of the potential from the surface dopant charge with distance from the surface becomes more rapid. Therefore, the minimum ionization distance is also reduced with increasing dopant density. It is found that the detected ion-signal decreases with increasing dopant density of both p- and n- type doped Si surfaces. The higher-n Rydberg states have shown higher ion detectability than that of lower-n Rydberg states and this variation also becomes smaller when increasing the dopant density. Experiments involving H2 Rydberg molecules incident on various doped Si surfaces in the presence of a Stark field at the point of excitation are also presented here. The surface ionization profiles produced via both electron and ion detection schemes are measured by changing the Stark polarization. Positive surface dopant charges oppose production of backscattered electrons and negative surface dopant charges enhance the electron-signal. For the electron detection scheme, lightly doped n-type Si surfaces show higher detectability but in the case of p-type Si surfaces the more heavily doped Si surfaces give a higher detected signal. This different behaviour of the detected ion or electron signal implies a different production mechanism. Theoretical trajectory simulations were also carried out based on a new 2D surface potential model. The results qualitatively agree with the experimental results and explain the changes of the surface ionization profiles between the various dopant types and dopant densities of the Si surfaces.

The design, fabrication and performance of an Extraordinary Magnetoresistance (EMR) and a Vertical Mesa Hall Sensor (VMHS) are studied. EMR devices have been fabricated from a 2DEG InAs/GaSb structures which exhibit a low carrier density and high mobility that achieve the best performance. The general electrical magneto-transport properties are given. The experiments investigate mainly different metallic patterns, which are Rectangular, Triangular and Tip pattern between 4-300 K. Probe configurations and the enhancement of relative size of metallic patterns are described. EMR effect is due to current deflection around the metal-semiconductor interface. The results are metallic pattern dependent. Using finite element analysis, good agreement between experimental and theoretical results was found. The best performance sensor is a symmetrical metallic Tip pattern. It is enhanced by the length of the Tip’s point and the large metallic area. This pattern when combines with an asymmetrical probe configuration, exhibits the highest EMR of 900% at –0.275T measured by inner probes and the best sensitivity of 54Ω/T at room temperature. The second study presents in-plane Hall effect sensors made from InSb. A simple device geometry has been used in which current flows in a plane perpendicular to the device surface. Device sensitivity depends on its geometry and a series of different contacts are used to investigate the geometry of the current flow distribution. The structures produced are only sensitive to the presence of one in-plane field component, and they also demonstrate good angular selectivity. Multi-electrodes were used to investigate biasing current from both mesa and substrate condition. We are able to examine the Hall voltage as a function of contact positions and also to create multiple VMHS. Offset reduction of devices has been achieved by moving the ground contacts to re-balance the current distribution under the mesa surface.

Η κατασκευή συσκευών μικροηλεκτρονικής με οργανικούς ημιαγωγούς όπως τρανζίστορς και φωτοεκπομποί δίοδοι οργανικών (FETs, OLEDs), αναπτύσσεται ταχύτατα τα τελευταία χρόνια. Οι φυσικές και χημικές αλληλεπιδράσεις που συμβαίνουν στις διεπιφάνειες των οργανικών με τα ηλεκτρόδια παίζουν καθοριστικό ρόλο στη λειτουργία τέτοιων συσκευών και επομένως η μελέτη της διεπιφανειακής ηλεκτρονικής δομής είναι σημαντική για την κατανόηση της λειτουργίας αυτών των διατάξεων. Στην παρούσα εργασία η ηλεκτρονική δομή των διεπιφανειών ενός συζυγιακού ολιγομερούς (Ooct-OPV5) με ανόργανα υποστρώματα, συγκεκριμένα το οξείδιο Ινδίου-Κασσιτέρου (ITO), τον πολυκρυσταλλικό Au, την επιφάνεια Si(111) (Si με προσμίξεις τύπου –Ν και -P) και υπέρλεπτα υμένια SiO2(1-5 nm) / Si(111), μελετήθηκε με φασματοσκοπίες φωτοηλεκτρονίων από ακτίνες –Χ και υπεριώδες (XPS, UPS). Το Ooct-OPV5 χρησιμοποιείται ως πρότυπο για το poly(p-phenylenevinylene) (PPV), ένα πολυμερές που έχει ήδη χρησιμοποιηθεί σε συσκευές OLEDs. Το ITO χρησιμοποιείται ως άνοδος στα OLEDs επειδή είναι διαφανές και έχει υψηλή ηλεκτρική αγωγιμότητα. Ο χρυσός είναι αδρανές υπόστρωμα που χρησιμοποιείται ως ηλεκτρόδιο στα FETs. Τέλος, η μελέτη διεπιφανειών του οργανικού με το Si παρουσιάζει ενδιαφέρον, προκειμένου να ενσωματωθούν οι οργανικοί ημιαγωγοί σε μικροηλεκτρονικές συσκευές με βάση το Si. Η μελέτη έγινε σε σύστημα υπερυψηλού κενού (UHV) με τις τεχνικές XPS και UPS. Τα υποστρώματα καθαρίζονταν in-situ με ιοντοβολή με Ar+ και θέρμανση. Ακολούθως γινόταν σταδιακή απόθεση του ολιγομερούς και παρασκευάζονταν υπέρλεπτα υμένια (πάχους ~10 nm) πάνω στα καθαρά υποστρώματα. Σε κάθε στάδιο της απόθεσης λαμβάνονταν τα φάσματα XPS του οργανικού και των υποστρωμάτων. Από την ανάλυση των φασμάτων αυτών προσδιορίζονται οι διεπιφανειακές αλληλεπιδράσεις και η μεταβολή της κάμψης των ζωνών των ημιαγώγιμων υλικών. Με τη φασματοσκοπία UPS μελετάται η ζώνη σθένους της διεπιφάνειας και μετράται η διεπιφανειακή διπολική στοιβάδα. Από το συνδυασμό των πειραματικών αποτελεσμάτων μπορούν να κατασκευάζονται σχηματικά διαγράμματα των ζωνών στις διεπιφάνειες. Με βάση τα πειραματικά αποτελέσματα καταλήγουμε στα εξής συμπεράσματα: Στις διεπιφάνειες του ολιγομερούς με το ITO, τον Au, το Si (τύπου -p) και το SiO2(1-1.8 nm)/Si(111) υπάρχει διεπιφανειακή διπολική στοιβάδα (eD) ενώ στη διεπιφάνεια Ooct-OPV5/ Si (τύπου -n) όχι. Αυτά τα διεπιφανειακά δίπολα σχετίζονται με την ύπαρξη διεπιφανειακών καταστάσεων και εξυπηρετούν τη μεταφορά φορτίου μεταξύ των υλικών που έρχονται σε επαφή στα πρώτα στάδια του σχηματισμού των διεπιφανειών. Κατά το σχηματισμό της διεπιφάνειας Ooct-OPV5/ Si (τύπου - p), το eD σχετίζεται με την αλληλεπίδραση των μορίων του ολιγομερούς με τις επιφανειακές καταστάσεις του Si. Στις διεπιφάνειες Ooct-OPV5/ με Au και Si, η μεταφορά φορτίου ολοκληρώνεται με τη μεταβολή της κάμψης των ζωνών του οργανικού υμενίου κατά ~0.20 eV. Τα φράγματα έγχυσης οπών (eΦbh) ή τα valence band offsets (ΔEV) καθορίστηκαν επίσης σε όλες τις περιπτώσεις. Στη διεπιφάνεια Ooct-OPV5 / Au το eΦbh μετρήθηκε 1.05 eV και επομένως ο Au είναι ακατάλληλο ηλεκτρόδιο για την έγχυση οπών. Το ITO αποδεικνύεται επίσης ακατάλληλο (eΦbh=1.45 eV) και η επιφάνειά του θα πρέπει να υφίσταται κατεργασία προκειμένου να χρησιμοποιείται ως άνοδος σε συσκευές OLEDs. Στην περίπτωση του Si, το valence band offset μεταξύ αυτού και του ολιγομερούς βρέθηκε ~0.4 eV. Η παραπέρα τροποποίηση της επιφάνειας του Si(111) με υπέρλεπτα υμένια SiO2 αυξάνει το ΔEV κατά ~0.2 eV. / The development of organic-based devices, like transistors and light emitting diodes (FETs, OLEDs), is progressing rapidly over the past few years. A great deal of the physics and chemistry that govern the performance of such devices occur at the interfaces between the organic components and the inorganic electrodes, making the study of the interfacial electronic properties essential. In this work, the electronic structure of the interface formed between a conjugated oligomer (Ooct-OPV5) and inorganic substrates, ιn particular indium-tin oxide (ITO), polycrystalline Au, the Si(111) surface (Si n- and p-doped), and ultrathin (1-5 nm) SiO2 films on Si(111), was studied by X-ray and Ultraviolet photoelectron spectroscopies (XPS, UPS). Ooct-OPV5 is a model for poly(p-phenylenevinylene) (PPV), a polymer that has already been used in OLEDs. ITO is the common anode used in OLEDs because of its transparency and high electrical conductivity. Gold was chosen due to its inert nature and because it is used as a source/drain in FETs. Finally, the study of organic/silicon structures is of great importance for the incorporation of organics in Si-based microelectronic systems. All XPS and UPS measurements were carried out in an ultrahigh vacuum (UHV) apparatus. All substrates were cleaned in-situ by Ar+ sputtering and annealing. High purity oligomer films of up to ~10 nm thickness were produced in-situ by stepwise deposition on the clean substrates. The evolution of the oligomer and substrate-related XPS peaks was followed during Ooct-OPV5 deposition on all substrates. Analysis of these spectra clarified the interfacial chemistry and band bending in the semiconducting materials. UPS spectroscopy is used for the determination of the valence band at the interface and the interfacial dipoles. The interfacial energy band diagrams were deduced in all cases from the combination of experimental results. Based on our experimental data we reached the following conclusions: Dipoles are formed at the interfaces of the oligomer with ITO, Au, Si (p-doped) and SiO2(1-1.8 nm)/Si(111), while the Ooct-OPV5/ Si (n-doped) interface is dipole free. These interface dipoles (eD) are related to the existence of interfacial states and serve for the charge transfer between the materials in contact at the initial stages of the interface formation. In the case of the Ooct-OPV5/ Si (p-doped) interface, eD is related to the interaction of the oligomer molecules with Si surface states. At the Ooct-OPV5/Au and Si interfaces, the charge transfer is completed with a band bending of ~0.20 eV in the oligomer film. The hole injection barriers (eΦbh) or valence band offsets (ΔEV) were also determined in all interfaces. This barrier was measured 1.05 eV at the Ooct-OPV5/ Au interface, and thus Au is inappropriate electrode for hole injection. ITO is also proved a poor hole-injecting electrode (eΦbh=1.45 eV), and thus its surface should be modified by treatments when used as an anode in OLEDs. On the other hand, the valence band offset between the Si substrate and the oligomer is measured ~0.4 eV. Modification of the Si(111) surface with ultra-thin SiO2 layers increases ΔEV by ~0.2 eV.