Two technologies, namely Laser Technology and Surface Modification Technology, have made rapid strides in the last few decades. The lasers have evolved from a simple laboratory curiosity to a matured industrial tool and its applications are limited only by imagination. Intense, coherent and monochromatic laser sources with power outputs ranging over several orders of magnitude have found innumerable applications in the realm of materials engineering. Reactive Pulsed Laser Deposition (PLD) is a powerful technique that utilises the power of a nanosecond pulsed laser for materials synthesis. Unlike conventional PLD, which require high density targets that are difficult to synthesize at a reasonable cost, the RPLD circumvents the need for one such ceramic target. This thesis presents a detailed and judicious use of this technique for synthesis of hard ceramic multilayer coatings using elemental metal targets. Transition metal nitrides having rock salt structure are known to exhibit superior properties such as hardness and wear resistance and hence formed the basis for the development of first generation coatings. Further improvements through alloying of these binary compounds with metal or metalloid components lead to the development of second generation coatings. As the demand for functional materials increased, surface modification technology alias surface engineering, grew in leaps and bounds. As the large number of coating requirements for optimal performance could not be fulfilled by a single homogeneous material, third generation coatings, comprising multilayer coatings, were developed. It is this aspect of combining the advantages of RPLD process to synthesize ceramic multilayer coatings, provides the main motivation for the present research work. In this thesis, a systematic study presented for synthesis of nanocrystalline and stoichiometric TiN and NbN thin films using RPLD through ablation of high purity titanium and niobium targets, in the presence of low pressure nitrogen gas. A novel Secondary Ion Mass Spectrometry (SIMS) based analysis was developed to effectively deduce the important process parameters in minimum trials to arrive at desired composition. The validity of this SIMS based method, for optimization of process parameters to get stoichiometric nitride films, was proved beyond any speculation by corroborative Proton Elastic Backscattering Spectrometric (PEBS) analysis. SIMS was also used to characterize the [NbN/TiN] multilayers. The feasibility of growing nanocrystalline multilayers with varying thicknesses has been demonstrated. Nanomechanical properties including hardness and adhesion strength of monolithic TiN and NbN films and multilayers were evaluated. The thesis is organised into six chapters. The first chapter gives a brief account on the history and development of ‘surface engineering’. The second chapter provides a comprehensive description of the experimental facility developed in-house to pursue research on PLD grown ceramic thin films and multilayers. Thin film synthesis procedure for ex-situ SIMS and TEM analyses is described. Brief introduction is also presented on the characterization techniques used in this study to investigate the surface, interface and microstructural aspects of PLD grown films with underlying basic principles. The third and fourth chapter describes the synthesis and characterization of titanium nitride and niobium nitride thin films using RPLD technique, respectively. SIMS was used in depth profiling mode, for optimization of three important process parameters, viz., nitrogen gas pressure, substrate temperature and laser pulse energy, to get stoichiometric nitride films. Further, films were characterized using GIXRD, TEM, XPS and PEBS for their structure and composition. AFM measurements were made to elucidate the surface morphological features. PEBS was effectively used to estimate the nitrogen concentration in a quantitative manner and the results corroborate well with the SIMS measurements. Having succeeded in synthesizing stoichiometric TiN and NbN films, further studies on the nanomechanical properties of monolithic TiN and NbN films and their multilayers were carried out and these results form the contents of the fifth chapter. The findings of the work reported in this thesis are concluded in Chapter 6 and few possible suggestions were presented as future directions. Both the monolithic TiN and NbN coatings showed a deposition pressure dependent hardness variation. The hardness of these monolithic films was found to be around 30 GPa, higher than the hardness values obtained by other conventional techniques. Keeping total thickness of the multilayers constant at 1 μm, [NbN/TiN] multilayers having bilayer periods ranging from 50 nm to 1000 nm, were synthesized. A systematic enhancement in hardness upto ~ 40 GPa was observed for [NbN/TiN]10 with the modulus of the multilayer remaining almost constant. The pileup observed around the indentation edge is indicative of toughening in multilayers. The tribological properties of multilayer films showed a better performance in terms of low coefficient of friction and regeneration of coating surfaces as revealed from the nanotribological studies. Overall, the multilayer coatings exhibited better performance in terms of hardness, toughness and adhesion with the substrate material.

White light emitting diodes (LED) have drawn increasing attention due to their low energy consumption, high efficiency and potential to become primary lighting source by replacing conventional light sources. White light emission is usually generated either by coating yellow phosphor on a blue-LED or blending red, green and blue phosphor in an appropriate ratio. Maintaining appropriate proportions of individual components in the blend is difficult and the major demerit of such system is the overall self-absorption, which changes the solution concentration. This results in uncontrolled changes in the whiteness of the emitted light. Zinc Oxide (ZnO), a wide bandgap semiconductor with a large exciton binding energy at room temperature has been recognized as a promising material for ultraviolet LEDs and laser diodes. Tuning of structural, optical and electrical properties of ZnO thin films by different dopants (Lithium, Indium and Gallium) is dealt in this thesis. The achievement of white light emission from a semiconducting material without using phosphors offers an inexpensive fabrication technology, good luminescence, low turn-on voltage and high efficiency. The present work is organized chapter wise, which has 8 chapters including the summary and future work. Chapter 1: Gives a brief discussion on the overview of ZnO as an optoelectronic material, crystal structure of semiconductor ZnO, the effect of doping, optical properties and its possible applications in optoelectronic devices. Chapter 2: Deals with various deposition techniques used in the present study, includes pulsed laser deposition and thermal evaporation. The experimental set up details and the deposition procedures are described in detail. A brief note on the structural characterization equipments, namely X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and the optical characterization techniques namely Raman spectroscopy, transmission spectroscopy and photoluminescence (PL) spectroscopy is presented. The electrical properties of the films were studied by current- voltage, capacitance - voltage and Hall Effect measurements and the experimental details are discussed. Chapter 3: High quality ZnO/Si heterojunctions fabricated by growing ZnO thin films on p-type Si (100) substrate by pulsed laser deposition without using buffer layers are discussed in this chapter. The crystallinity of the heterojunction was analyzed by high resolution X-ray diffraction and atomic force microscopy. The optical quality of the film was analyzed by room temperature (RT) photoluminescence measurements. The high intense band to band emission confirmed the high quality of the ZnO thin films on Si. The electrical properties of the junction were studied by temperature dependent resistivity, current- voltage measurements and RT capacitance-voltage (C-V) analysis. ZnO thin film showed the lowest resistivity of 6.4x10-3 Ω.cm, mobility of 7 cm2/V.sec and charge carrier concentration of 1.58x1019cm-3 at RT. The charge carrier concentration and the barrier height (BH) were calculated to be 9.7x1019cm-3 and 0.6 eV respectively from the C-V plot. The BH and ideality factor, calculated by using the thermionic emission (TE) model were found to be highly temperature dependent. We observed a much lower value in Richardson constant, 5.19x10-7 A/cm2K2 than the theoretical value (32 A/cm2K2) for ZnO. This analysis revealed the existence of a Gaussian distribution (GD) with a standard deviation of σ2=0.035 V. By implementing GD to the TE, the values of BH and Richardson constant were obtained as 1.3 eV and 39.97 A/cm2K2 respectively from the modified Richardson plot. The obtained Richardson constant value is close to the theoretical value for n-ZnO. These high quality heterojunctions can be used for solar cell applications. Chapter 4: This chapter describes the structural and optical properties of Li doped ZnO thin films and the properties of ZnO/Li doped ZnO multilayered thin film structures. Thin films of ZnO, Li doped ZnO (ZLO) and multilayer of ZnO and ZLO (ZnO/ZLO) were grown on silicon and Corning glass substrates by pulsed laser deposition technique. Single phase formation and the crystalline qualities of the films were analyzed by X-ray diffraction and Li composition in the film was investigated to be 15 Wt % by X-ray photoelectron spectroscopy. Raman spectrum reveals the hexagonal wurtzite structure of ZnO, ZLO and ZnO/ZLO multilayer, confirms the single phase formation. Films grown on Corning glass show more than 80 % transmittance in the visible region and the optical band gaps were calculated to be 3.245, 3.26 and 3.22 eV for ZnO, ZLO and ZnO/ZLO respectively. An efficient blue emission was observed in all films that were grown on silicon (100) substrate by photoluminescence (PL). PL measurements at different temperatures reveal that the PL emission intensity of ZnO/ZLO multilayer was weakly dependent on temperature as compared to the single layers of ZnO and ZLO and the wavelength of emission was independent of temperature. Our results indicate that ZnO/ZLO multilayer can be used for the fabrication of blue light emitting diodes. Chapter 5: This chapter is divided in to two parts. The fabrication and characterization of In doped ZnO thin films grown on Corning glass substrate is discussed in the first section. Zinc Oxide (ZnO) and indium doped ZnO (IZO) thin films with different indium compositions were grown by pulsed laser deposition technique. The effect of indium concentration on the structural, morphological, optical and electrical properties of the film was studied. The films were oriented along the c-direction with wurtzite structure and are highly transparent with an average transmittance of more than 80 % in the visible wavelength region. The energy band gap was found to be decreasing with increasing indium concentration. High transparency makes the films useful as optical windows while the high band gap values support the idea that the film could be a good candidate for optoelectronic devices. The value of resistivity observed to be decreasing initially with doping concentration and subsequently increasing. The XPS and Raman spectrum confirm the presence of indium in indium doped ZnO thin films. The photoluminescence spectrum showed a tunable red light emission with different In concentrations. Undoped and In doped ZnO (IZO) thin films were grown on Pt coated silicon substrates (Pt/Si) to fabricate Pt/ZnO:Inx Schottky contacts (SC) is discussed in the second section. The SCs were investigated by conventional two probe current-voltage (I-V) measurement and by the I-V spectroscopy of conductive atomic force microscopy (C-AFM). X-ray diffraction technique was used to examine the thin film quality. Changes in various parameters like Schottky barrier height (SBH) and ideality factor (IF) as a function of temperature were presented. The estimated BH was found to be increasing and the IF was found to be decreasing with increase in temperature. The variation of SBH and IF with temperature has been explained by considering the lateral inhomogeneities in nanometer scale lengths at metal–semiconductor (MS) interface. The inhomogeneities of SBH in nanometer scale length were confirmed by C-AFM. The SBH and IF estimated from I-V spectroscopy of C-AFM showed large deviation from the conventional two probe I-V measurements. IZO thin films showed a decrease in SBH, lower turn on voltage and an enhancement in forward current with increase in In concentration. Chapter 6: In this chapter the properties of Ga doped ZnO thin films with different Ga concentrations along with undoped ZnO as a reference is discussed. Undoped and Ga doped ZnO thin films with different Ga concentrations were grown on Corning glass substrates by PLD. The structural, optical and electrical properties of Ga doped ZnO thin films are discussed. The XRD, XPS and Raman spectrum reveal the phase formation and successful doping of Ga on ZnO. All the films show good transmittance in the visible region and the photoluminescence of Ga doped ZnO showed a stable emission in the blue- green region. The resistivity of Ga doped ZnO thin films was found to be first decreasing and then increasing with increase in Ga concentrations. Chapter 7: The effect of co-doping to ZnO on the structural, optical and electrical properties was described in this chapter. Ga and In co-doped ZnO (GIZO) thin films together with ZnO, In doped ZnO (IZO), Ga doped ZnO (GZO), IZO/GZO multilayer for comparison, were grown on Corning glass and boron doped Si substrates by PLD. GIZO showed better structural, optical and electrical properties compared with other thin films. The Photoluminescence spectra of GIZO showed a strong white light emission and the current-voltage characteristics showed relatively lower turn on voltage and larger forward current. The CIE co-ordinates for GIZO were observed to be (0.31, 0.33) with a CCT of 6650 K, indicating a cool white light and established a possibility of white light emitting diodes. Finally the chapter 8 presents the summary derived out of the work and a few suggestions on future work.

Die vorliegende Arbeit behandelt die Herstellung und optische Untersuchung von Halbleiterheterostrukturen, genauer Mikrokavitäten, in denen ein Bose–Einstein Kondensat (BEK) von sogenannten Exziton-Polaritonen im Festkörper erzeugt und beobachtet werden soll. Diese Strukturen bestehen aus zwei hochreflektierenden Braggspiegeln (BS) und einer ZnO-Kavität als aktivem Material. Zunächst wurde die Abscheidung der BS hinsichtlich genauer Schichtdickenkontrolle und Reproduzierbarkeit verbessert. Um Kavitätsschichten hinreichender Qualität herzustellen, wurden mehrere Ansätze zur Optimierung dieser planaren Schichtabscheidung mittels gepulster Laserdeposition verfolgt. Dabei kamen Techniken, wie das Ausheizen der Proben oder deren Glättung durch Ionenstrahlbeschuß zum Einsatz, um die elektronischen Eigenschaften bzw. die Oberflächen der Kavitätsschichten erheblich zu verbessern. Desweiteren wurde erfolgreich ein Verfahren entwickelt, freistehende, nahezu einkristalline ZnO-Nanodrähte mit Braggspiegeln zu ummanteln. Alle hergestellten Strukturen wurden in ihren strukturellen Eigenschaften, speziell hinsichtlich ihrer Rauhigkeit und Kristallinität, verglichen und mittels orts- und/oder winkelaufgelöster Photolumineszenzspektroskopie sowie Reflexionsmessungen bezüglich ihrer optischen Eigenschaften untersucht. Dabei konnte in fast allen Proben die starke Kopplung, welche die Grundlage für ein BEK darstellt, gezeigt werden. Hinweise für eine höhere Kopplungsstärke in den Nanodraht-basierten Mikrokavitäten wurden gefunden. Der Nachweis von BEK bis nahe Raumtemperatur gelang an der vielversprechendsten planaren Probe, die einen Qualitätsfaktor von ca. 1000 aufweist. Die Eigenschaften des BEK wurden für verschiedene Temperaturen und Detunings untersucht. Es hat sich gezeigt, daß ein negatives Detuning unerläßlich für die Bildung eines BEK in ZnO-basierten Mikrokavitäten ist. Die Impulsraumverteilung der Kondensat-Polaritonen läßt auf ausgeprägte dynamische Eigenschaften dieser Teilchen bei tiefen Temperaturen schließen. / The present work covers the fabrication and optical investigation of semiconductor microcavities for Bose–Einstein condensation (BEC) of exciton-polaritons. These microcavities consist of highly reflective distributed Bragg reflectors (DBR) surrounding a ZnO-cavity as active medium. In the first step, the growth of DBRs was optimised with respect to exact thickness control and high reproducibility. For the active material, several growth strategies have been pursued, in order to optimise the conditions for the growth of planar thin films by pulsed laser deposition. Techniques like annealing or ion beam smoothing were successfuly applied in order to either improve the electronic properties or decrease the roughness of the ZnO-cavity layer. Furthermmore, a successful technology was developed in order to coat highly-crstalline free-standing ZnO nanowires with concentrical DBR shells. All samples have been investigated regarding their roughness and crystallinity as well as their optical properties. For the latter spatially and/or angular-resolved photoluminescence spectroscopy and reflection measurements have been carried out. Thereby, the strong coupling regime – being prerequisite for BEC – could be demonstrated in almost all of the synthesized structures. For the nanowire-based microcavities hints for an enhanced coupling strength have been found. In one of the planar samples, showing the high quality factor of 1000, the formation of BEC almost up to room temperature was observed and was studied as a function of temperature and detuning. Negative detuning was found to be mandatory for the formation of a BEC in ZnO-based microcavities. The distinct momentum- respective in-plane wavevector distribution of the condensate polaritons revealed a strong dynamic character of these particles at low temperatures.