Tailoring properties of materials by femtosecond laser processing has been proposed in the last decade as a powerful approach for technological applications, ranging from optics to biology. Although most of the research output in this field is related to femtosecond laser processing of single either organic or inorganic materials, more recently a similar approach has been proposed to develop advanced hybrid nanomaterials. Here, we report results on the use of femtosecond lasers to process hybrid nanomaterials, composed of polymeric and glassy matrices containing metal or semiconductor nanostructures. We present results on the use of femtosecond pulses to induce Cu and Ag nanoparticles in the bulk of borate and borosilicate glasses, which can be applied for a new generation of waveguides. We also report on 3D polymeric structures, fabricated by two-photon polymerization, containing Au and ZnO nanostructures, with intense two-photon fluorescent properties. The approach based on femtosecond laser processing to fabricate hybrid materials containing metal or semiconductor nanostructures is promising to be exploited for optical sensors and photonics devices.

The authors report on the fabrication of GaAs nanopillars with different profiles/topologies using colloidal lithography and dry etching. GaAs nanopillars with different shapes and dimensions were successfully fabricated using inductively coupled plasma reactive ion etching. Two different etch chemistries CH4/H-2/Cl-2 and Ar/Cl-2 were investigated. The fabricated nanopillar arrays had a typical period of similar to 500 nm, and the depths could be varied from a few nanometers to 4 mu m. The CH4/H-2/Cl-2 chemistry with optimized gas flows and plasma powers is shown to produce nanopillars with smooth sidewalls compared to those fabricated with the Ar/Cl-2 chemistry. The GaAs nanopillar arrays have appreciably lower reflectivities in the measured wavelength range from 400 to 850 nm and are typically one order of magnitude lower compared to planar GaAs, which shows their potential for photovoltaic applications.

We report the optical-coding operation of monolithic, ultra-compact optical-CDMA encoder and decoder pair, consisting of InP based integrated AWGs and phase modulators. The encoder and decoder successfully demonstrate eight-bit Walsh code based encoding and decoding.

We report on the fabrication of gallium phosphide (GaP) nanowaveguides of controlled dimensions, as small as 0.03 μm and aspect ratio in excess of 20, using focused ion beam (FIB) milling. A known limitation of this fabrication process for photonic applications is the formation of gallium droplets on the surface. We demonstrate a post-fabrication step using a pulsed laser to locally oxidize the excess surface gallium on the FIB milled nanostructures. The process significantly reduces the waveguide losses. The surface optical quality of the fabricated GaP nanowaveguides has been evaluated by second-harmonic generation experiments. Surface and bulk contributions to second-order optical nonlinearities have been identified by polarization measurements. The presented method can potentially be applied to other III-V nanostructures to reduce optical losses.

A novel interfacial energy driven colloidal lithography technique to fabricate periodic patterns from solution-phase is presented and the feasibility and versatility of the technique is demonstrated by fabricating periodically arranged ZnO nanowire ensembles on Si substrates. The pattern fabrication method exploits different interfaces formed by sol-gel derived ZnO seed solution on a hydrophobic Si surface covered by a monolayer of colloidal silica spheres. While the hydrophobic Si surface prevents wetting by the seed solution, the wedge shaped regions surrounding the contact point between the colloidal particles and the Si substrate trap the solution due to interfacial forces. This technique allows fabrication of uniform 2D micropatterns of ZnO seed particles on the Si substrate. A hydrothermal technique is then used to grow well-defined periodic assemblies of ZnO nanowires. Tunability is demonstrated in the dimensions of the patterns by using silica spheres with different diameters. The experimental data show that the periodic ZnO nanowire assembly suppresses the total reflectivity of bare Si by more than a factor of 2 in the wavelength range 400-1300 nm. Finite-difference time-domain simulations of the wavelength-dependent reflectivity show good qualitative agreement with the experiments. The demonstrated method is also applicable for other materials synthesized by solution chemistry.

It is of paramount importance to our society that the environment, life style, science and amusement flourish together in a balanced way. Some trends in this direction are the increased utilization of renewable energy, like solar photovoltaics; better health care products, for example advanced biosensors; high definition TV or high resolution cameras; and novel scientific tools for better understanding of scientific observations. Advancement of micro and nanotechnologies has directly and positively impacted our stance in these application domains; one example is that of vertical periodic or aperiodic nano or micro pillar assemblies which have attracted significant research and industrial interest in recent years. In particular, Si pillars are very attractive due to the versatility of silicon. There are many potential applications of Si nanopillar/nanowire assemblies ranging from light emission, solar cells, antireflection, sensing and nonlinear optical effects. Compared to bulk, Si pillars or their assemblies have several unique properties, such as high surface to volume ratios, light localization, efficient light guiding, better light absorption, selective band of light propagation etc.

     The focus of the thesis is on the fabrication of Si pillar assemblies and hierarchical ZnO nanowires on Si micro structures in top-down and bottom-up approaches and their optical properties and different applications. Here, we have investigated periodic and aperiodic Si nano and micro structure assemblies and their properties, such as light propagation, localization, and selective guiding and light-matter interaction. These properties are exploited in a few important optoelectronic/photonic applications, such as optical biosensors, broad-band anti-reflection, radial-junction solar cells, second harmonic generation and color filters.  

      We achieved a low average reflectivity of ~ 2.5 % with the periodic Si micropyramid-ZnO NWs hierarchical arrays. Tenfold enhancement in Raman intensity is also observed in these structures compared to planar Si. These Si microstructure-ZnO NW hierarchical structures can enhance the performance and versatility of photovoltaic devices and optical sensors. A convenient top-down fabrication of radial junction nanopillar solar cell using spin-on doping and rapid thermal annealing process is presented. Broad band suppressed reflection, on average 5%, in 300- 850 nm wavelength range and an un-optimized cell efficiency of 6.2 % are achieved. Our method can lead to a simple and low cost process for high efficiency radial junction nanopillar solar cell fabrication.     

      Silicon dioxide (SiO₂) coated silicon nanopillar (NP) arrays are demonstrated for surface sensitive optical biosensing. Bovine serum albumin (BSA)/anti-BSA model system is used for biosensing trials by photo-spectrometry in reflection mode. Best sensitivity in terms of limit of detection of 5.2 ng/ml is determined for our nanopillar biosensor. These results are promising for surface sensitive biosensors and the technology allows integration in the CMOS platform.  

      Si pillar arrays used for surface second harmonic generation (SHG) experiments are shown to have a strong dependence of the SHG intensity on the pillar geometry. The surface SHG can be suitable for nonlinear silicon photonics, surface/interface studies and optical sensing.  

      Aperiodic Si nanopillar assemblies in PDMS matrix are demonstrated for efficient color filtering in transmission mode. These assemblies are designed using the ‘‘molecular dynamics-collision between hard sphere’’ algorithm. The designed structure is modeled in a 3D finite difference time domain (FDTD) simulation tool for optimization of color filtering properties. Transverse localization effect of light in our nanopillar color filter structures is investigated theoretically and the results are very promising to achieve image sensors with high pixel densities (~1 µm) and low crosstalk. The developed color filter is applicable as a stand-alone filter for visible color in its present form and can be adapted for displays, imaging, smart windows and aesthetic applications.

We present fabrication and optical characterization of Si microstructure-ZnO nanowire (NWs) hierarchical structures for light management. Random and periodic hierarchical structures constituting Si micro pillar or micro pyramid arrays with overgrown ZnO NWs have been fabricated. Inexpensive colloidal lithography in combination with dry and wet chemical etching is used to fabricate Si microstructures, and ZnO NWs are grown by hydrothermal synthesis. The periodic Si micro pyramid-ZnO NWs hierarchical structure shows broadband antireflection with average reflectance as low as 2.5% in the 300-1000 nm wavelength range. A tenfold enhancement in Raman intensity is observed in this structure compared to planar Si sample. These hierarchical structures with enriched optical properties and high surface to volume ratio are promising for photovoltaic (PV) and sensor applications.

Radial junction nanopillar Si solar cells are interesting for cost effective efficiency improvement. Here, we address a convenient top-down fabrication of Si nanopillar solar cells using spin-on doping and rapid thermal annealing (RTA) for conformal PN junction formation. Broadband suppressed reflection as low as an average of 5% in the 300-1100 nm wavelength range and un-optimized cell efficiency of 7.3% are achieved. The solar cell performance can be improved by optimization of spin-on-doping and suitable surface passivation. Overall, the all RTA processed, spin-on doped nanopillar radial junction solar cell shows a very promising route for low cost and high efficiency thin film solar cell perspectives.

We present the fabrication of silicon dioxide (SiO2) coated silicon nanopillar array structures and demonstrate their application as sensitive optical biosensors. Colloidal lithography, plasma dry etching and deposition processes are used to fabricate SiO2 coated Si nanopillar arrays with two different diameters and periods. Proof of concept bio recognition experiments are carried out with the bovine serum albumin (BSA)/antiBSA model system using Fourier transform visible and IR spectrometry (FT-VIS-IR) in reflection mode. A limit of detection (LoD) value of 5.2 ng/ml is estimated taking in to account the wavenumber uncertainty in the measurements.

We present experimental demonstration and analysis of enhanced surface second harmonic generation (SHG) from hexagonal arrays of silicon pillars. Three sets of Si pillar samples with truncated cone-shaped pillar arrays having periods of 500, 1000, and 2000 nm, and corresponding average diameters of 200, 585 and 1550 nm, respectively, are fabricated by colloidal lithography and plasma dry etching. We have observed strong dependence of SHG intensity on the pillar geometry. Pillar arrays with a 1000 nm period and a 585 nm average diameter give more than a one order of magnitude higher SHG signal compared to the other two samples. We theoretically verified the dependence of SHG intensity on pillar geometry by finite difference time domain simulations in terms of the surface normal E-field component. The enhanced surface SHG light can be useful for nonlinear silicon photonics, surface/interface characterization, and optical biosensing.

We report a new phenomenon related to Al-induced carrier confinement at the interface in core-shell GaAs/AlxGa1-xAs nanowires grown using metal-organic vapor phase epitaxy with Au as catalyst. All AlxGa1-xAs shells strongly passivated the GaAs nanowires, but surprisingly the peak photoluminescence (PL) position and the intensity from the core were found to be a strong function of Al composition in the shell at low temperatures. Large and systematic red shifts of up to similar to 66 nm and broadening in the PL emission from the GaAs core were observed when the Al composition in the shell exceeded 3%. On the contrary, the phenomenon was observed to be considerably weaker at the room temperature. Cross-sectional transmission electron microscopy reveals Al segregation in the shell along six Al-rich radial bands displaying a 3-fold symmetry. Time-resolved PL measurements suggest the presence of indirect electron-hole transitions at the interface at higher Al composition. We discuss all possibilities including a simple shell-core-shell model using simulations where the density of interface traps increases with the Al content, thus creating a strong local electron confinement. The carrier confinement at the interface is most likely related to Al inhomogeneity and/or Al-induced traps. Our results suggest that a low Al composition in the shell is desirable in order to achieve ideal passivation in GaAs nanowires.

Low temperature (similar to 200 degrees C) grown atomic layer deposition (ALD) films of AlN, TiN, Al2O3, GaN, and TiO2 were tested for protective capping and surface passivation of bottom-up grown III-V (GaAs and InP) nanowires (NWs), and top-down fabricated InP nanopillars. For as-grown GaAs NWs, only the AlN material passivated the GaAs surface as measured by photoluminescence (PL) at low temperatures (15K), and the best passivation was achieved with a few monolayer thick (2 angstrom) film. For InP NWs, the best passivation (similar to 2x enhancement in room-temperature PL) was achieved with a capping of 2nm thick Al2O3. All other ALD capping layers resulted in a de-passivation effect and possible damage to the InP surface. Top-down fabricated InP nanopillars show similar passivation effects as InP NWs. In particular, capping with a 2 nm thick Al2O3 layer increased the carrier decay time from 251 ps (as-etched nanopillars) to about 525 ps. Tests after six months ageing reveal that the capped nanostructures retain their optical properties. Overall, capping of GaAs and InP NWs with high-k dielectrics AlN and Al2O3 provides moderate surface passivation as well as long term protection from oxidation and environmental attack.

The thermal conductivity of polycrystalline aluminum nitride (AlN) films with inhomogeneous structures is experimentally and theoretically investigated. The influence of the grain morphology and size evolution along the cross plane direction of the films is studied by thickness-dependent 3m measurements on AlN monolayers. For AlN/AlN multilayer samples, the impact of oxygen-related defects localized at the interface between two AlN layers, is also analyzed. When the total thickness of these multilayers is downsized from 1107 nm to 270 nm, their measured effective thermal conductivity reduces by 47%, which is smaller than the corresponding reduction of 58% for monolayers. In multilayers, this decrease is due to the additive contributions of the thermal resistances arising from the AlN and AlN/AlN interfaces. The experimental data are interpreted through an analytical model developed for nanocrystalline films with inhomogeneous structures. It is shown that the size effects on the phonon mean free paths and the intrinsic thermal resistance resulting from the inhomogeneous microstructure predominate as the film thickness increases, whilst the contribution of the interface thermal resistance strengthens when the thickness is scaled down.

The effective thermal conductivity of nanocrystalline films of AlN with inhomogeneous microstructure is investigated experimentally and theoretically. This is done by measuring the thermal conductivity of the samples with the 3-omega method and characterizing their microstructure by means of electron microscopy. The relative effect of the microstructure and the interface thermal resistance on the thermal conductivity is quantified through an analytical model. Thermal measurements showed that when the thickness of an AlN film is reduced from 1460 to 270 nm, its effective thermal conductivity decreases from 8.21 to 3.12 WYm-1?K-1, which is two orders of magnitude smaller than its bulk counterpart value. It is shown that both the size effects of the phonon mean free paths and the intrinsic thermal resistance resulting from the inhomogeneous microstructure predominate for thicker films, while the contribution of the interface thermal resistance strengthens as the film thickness is scaled down. The obtained results demonstrate that the structural inhomogeneity in polycrystalline AlN films can be efficiently used to tune their cross- plane thermal conductivity. In addition, thermal conductivity measurements of epitaxially grown InP layers on silicon using Raman spectroscopy are reported.

We demonstrate for the first time the phase-error compensation for a 20-GHz-spacing InP AWG with electrooptic phase shifters placed on 42 arrayed waveguides. The experiment results show successful phase control and 6-dB reduction of crosstalk.

As the age of Moore’s law is drawing to a close, continuing increase in computing performance is becoming increasingly hard‐earned, while demand for bandwidth is insatiable. One way of dealing with this challenge is the integration of active photonic material with Si, allowing high‐speed optical inter‐ and intra‐chip connects on one hand, and the economies of scale of the CMOS industry in optical communications on the other. One of the most essential active photonic materials is InP, stemming from its capability in combination with its related materials to produce lasers, emitting at wavelengths of 1300 and 1550 nm, the two most important wavelengths in data‐ and telecom.

However, integrating InP with Si remains a challenging subject. Defects arise due to differences in lattice constants, differences in thermal expansion coefficients, polarity and island‐like growth behavior. Approaches to counter these problems include epitaxial lateral overgrowth (ELOG), which involves growing InP laterally from openings in a mask deposited on a defective InP/Si substrate. This approach solves some of these problems by filtering out the previously mentioned defects. However, filtering may not be complete and the ELOG and mask themselves may introduce new sources for formation of defects such as dislocations and stacking faults.

In this work, the various kinds of defects present in InP ELOG layers grown by hydride vapor phase epitaxy on Si, and the reason for their presence, as well as strategies for counteracting them, are investigated. The findings reveal that whereas dislocations appear in coalesced ELOG layers both on InP and InP/Si, albeit to varying extents, uncoalesced ELOG layers on both substrate types are completely free of threading dislocations. Thus, coalescence is a critical aspect in the formation of dislocations. It is shown that a rough surface of the InP/Si substrate is detrimental to defect‐free coalescence. Chemical‐mechanical polishing of this surface improves the coalescence in subsequent ELOG leading to fewer defects.

Furthermore, ELOG on InP substrate is consistently free of stacking faults. This is not the case for ELOG on InP/Si, where stacking faults are to some extent propagating from the defective substrate, and are possibly also forming during ELOG. A model describing the conditions for their propagation is devised; it shows that under certain conditions, a mask height to opening width aspect ratio of 3.9 should result in their complete blocking. As to the potential formation of new stacking faults, the formation mechanism is not entirely clear, but neither coalescence nor random deposition errors on low energy facets are the main reasons for their formation. It is hypothesized that the stacking faults can be removed by thermal annealing of the seed and ELOG layers.

Furthermore, concepts for integrating an active photonic device with passive Si components are elucidated by combining Si/SiO2 waveguides used as the mask in ELOG and multi‐quantum well (MQW) lasers grown on ELOG InP. Such a device is found to have favorable thermal dissipation, which is an added advantage in an integrated photonic CMOS device.

Epitaxial lateral overgrowth of InP has been grown by hydride vapor phase epitaxy on Si substrates with a thin seed layer of InP masked with SiO2. Openings in the form of multiple parallel lines as well as mesh patterns from which growth occurred were etched in the SiO2 mask and the effect of different growth conditions in terms of V/III ratio and growth temperature on defects such as threading dislocations and stacking faults in the grown layers was investigated. The samples were characterized by cathodoluminescence and by transmission electron microscopy. The results show that the cause for threading dislocations present in the overgrown layers is the formation of new dislocations, attributed to coalescence of merging growth fronts, possibly accompanied by the propagation of pre-existing dislocations through the mask openings. Stacking faults were also pre-existing in the seed layer and propagated to some extent, but the most important reason for stacking faults in the overgrown layers was concluded to be formation of new faults early during growth. The formation mechanism could not be unambiguously determined, but of several mechanisms considered, incorrect deposition due to distorted bonds along overgrowth island edges was found to be in best agreement with observations.

InP thin films have been grown on InP/Si substrate by epitaxial lateral overgrowth (ELOG). The nature, origin and filtering of extended defects in ELOG layers grown from single and double openings in SiO2 mask have been investigated. Whereas ELOG layers grown from double openings occasionally exhibit threading dislocations (TDs) at certain points of coalescence, TDs are completely absent in ELOG from single openings. Furthermore, stacking faults (SFs) observed in ELOG layers grown from both opening types originate not from coalescence, but possibly from formation during early stages of ELOG or simply propagate from the seed layer through the mask openings. A model describing their propagation is devised and applied to the existent conditions, showing that SFs can effectively be filtered under certain conditions. ELOG layers grown from identical patterns on InP substrate contained no defects, indicating that the defect-forming mechanism is in any case not inherent to ELOG itself.

We have described a number of ways to integrate photonic crystals with active optoelectronic devices. PhCs can be used as mirrors in semiconductor lasers where their high reflectivity allow the fabrication of lasers with small cavity length. Lasers with a length down to 100 μm and two PhC mirrors have been demonstrated.

This thesis deals with one of the promising strategies to monolithically integrate III-V semiconductors with silicon via epitaxial lateral overgrowth (ELOG) technology and is supported by extensive experimental results. The aimed applications are light sources on silicon for electronics-photonics integration and cost effective high efficiency multijunction solar cells.

The work focusses on the growth of III-V semiconductors consisting of indium phosphide (InP) and its related alloys on silicon primarily because of the bandgaps that these offer for the aimed applications. For this purpose, we make use of the epitaxial growth technique called hydride vapour phase epitaxy and exploit its near equilibrium operation capability to achieve primarily ELOG of high quality InP as the starting material on patterned InP(seed)/silicon wafer. The InP/InGaAsP layers are grown by metal organic vapour phase epitaxy.

Different pattern designs are investigated to achieve high quality InP over a large area of silicon by ELOG to realise lasers. First, nano patterns designed to take advantage of aspect ratio trapping of defects are investigated. Despite substantial defect filtering insufficient growth area is achieved. To achieve a larger area, coalescence from multiple nano openings is used. Shallowly etched InP/InGaAsP based microdisk resonators fabricated on indium phosphide on silicon achieved by this method have shown whispering gallery modes. However, no lasing action is observed partly due to the formation of new defects at the points of coalescence and partly due to leakage losses due to shallow etching. To overcome these limitations, a new design mimicking the futuristic monolithic evanescently coupled laser design supporting an efficient mode coupling and athermal operation is adopted to yield large areas of ELOG InP/Si having good carrier transport and optical properties. Microdisk resonators fabricated from the uniformly obtained InP/InGaAsP structures on the ELOG InP layers have shown very strong spontaneous luminescence close to lasing action. This is observed for the first time in InP/InGaAsP laser structures grown on ELOG InP on silicon.

A newly modified ELOG approach called Corrugated ELOG is also developed. Transmission electron microscopy analyses show the formation of abrupt interface between InP and silicon. Electrical measurements have supported the linear Ohmic behaviour of the above junction. This proof of concept can be applied to even other III-V compound solar cells on silicon. This allows only thin layers of expensive III-V semiconductors and cheap silicon as separate subcells for fabricating next generation multijunction solar cells with enhanced efficiencies at low cost. A feasible device structure of such a solar cell is presented. The generic nature of this technique also makes it suitable for integration of III-V light sources with silicon and one such design is proposed.

High-quality InGaAsP/InP multi-quantum wells (MQWs) on the isolated areas of indium phosphide on silicon necessary for realizing a monolithically integrated silicon laser is achieved. Indium phosphide layer on silicon, the pre-requisite for the growth of quantum wells is achieved via nano-epitaxial lateral overgrowth (NELOG) technique from a defective seed indium phosphide layer on silicon. This technique makes use of epitaxial lateral overgrowth (ELOG) from closely spaced (1 m) e-beam lithography-patterned nano-sized openings (∼300 nm) by low-pressure hydride vapor phase epitaxy. A silicon dioxide mask with carefully designed opening patterns and thickness with respect to the opening width is used to block the defects propagating from the indium phosphide seed layer by the so-called necking effect. Growth conditions are optimized to obtain smooth surface morphology even after coalescence of laterally grown indium phosphide from adjacent openings. Surface morphology and optical properties of the NELOG indium phosphide layer are studied using atomic force microscopy, cathodoluminescence and room temperature -photoluminescence (-PL) measurements. Metal organic vapor phase epitaxial growth of InGaAsP/InP MQWs on the NELOG indium phosphide is conducted. The mask patterns to avoid loading effect that can cause excessive well/barrier thickness and composition change with respect to the targeted values is optimized. Cross-sectional transmission electron microscope studies show that the coalesced NELOG InP on Si is defect-free. PL measurement results indicate the good material quality of the grown MQWs. Microdisk (MD) cavities are fabricated from the MQWs on ELOG layer. PL spectra reveal the existence of resonant modes arising out of these MD cavities. A mode solver using finite difference method indicates the pertinent steps that should be adopted to realize lasing.

In this paper we propose a strategy to achieve monolithic integration of III-Vs on Si for photonic integration through a simple process. By mimicking the SiO2/Si/SiO2 waveguide necessary to couple light from the gain medium on its top, we adopt a similar to 2 mu m thick silicon dioxide mask for epitaxial lateral overgrowth (ELOG) of InP on Si. The ELOG InP layer as wells as the subsequently grown quantum wells (similar to 1. 55 mu m) have been analyzed by photoluminescence and transmission electron microscopy and found to have high optical quality and very good interface. The studies are strategically important for a monolithic platform that holds great potential in addressing the future need to have an integrated platform consisting of both III-Vs and Si on same chip.

We present the carrier-transport, optical and structural properties of InP deposited on Si by Epitaxial Lateral Overgrowth (ELOG) in a Low Pressure-Hydride Vapor phase epitaxy (LP-HVPE). Hall measurements, micro photoluminescence (μ-PL) and X-ray diffraction (XRD) were used to study the above-mentioned respective properties at room temperature. It is the first time that electrical properties of ELOG InP on Si are studied by Hall measurements. Prior to ELOG, etching of patterned silicon dioxide (SiO2) mask leading to a high aspect ratio, i. e. mask thickness to opening width >2 was optimized to eliminate defect propagation even above the opening. Dense high aspect ratio structures were fabricated in SiO2 to obtain ELOG InP on Si, coalesced over large area, making it feasible to perform Hall measurements. We examine this method and study Hall mobility, strain and optical quality of large area ELOG InP on Si.

A simple method of growing large areas of InP on Si through Epitaxial Lateral Overgrowth (ELOG) is presented. Isolated areas of high quality InP suitable for photonic integration are grown in deeply etched SiO2 mask fabricated using conventional optical lithography and reactive ion etching. This method is particularly attractive for monolithically integrating laser sources grown on InP with Si/SiO2 waveguide structure as the mask. The high optical quality of multi quantum well (MQW) layers grown on the ELOG layer is promisingly supportive of the feasibility of this method for mass production.

Buried heterostructure (BH) lasers are routinely fabricated for telecom applications. Development of quantum cascade lasers (QCL) for sensing applications has largely benefited from the technological achievements established for telecom lasers. However, new demands are to be met with when fabricating BH-QCLs. For example, hetero-cascade and multi-stack QCLs, with several different active regions stacked on top of each other, are used to obtain a broad composite gain or increased peak output power. Such structures have thick etch ridges which puts severe demand in carrying out regrowth of semi-insulating layer around very deeply etched (>10 mu m) ridges in short time to realize BH-QCL. For comparison, telecom laser ridges are normally only <5 mu m deep. We demonstrate here that hydride vapour phase epitaxy (HVPE) is capable of meeting this new demand adequately through the fabrication of BH-QCLs in less than 45 minutes for burying ridges etched down to 10-15 mu m deep. This has to be compared with the normally used regrowth time of several hours, e.g., in a metal organic vapour phase epitaxy (MOVPE) reactor. This includes also micro-stripe lasers resembling grating-like ridges for enhanced thermal dissipation in the lateral direction. In addition, we also demonstrate HVPE capability to realize buried heterostructure photonic crystal QCLs for the first time. These buried lasers offer flexibility in collecting light from the surface and relatively facile device characterization feasibility of QCLs in general; but the more important benefits of such lasers are enhanced light matter interaction leading to ultra-high cavity Q-factors, tight optical confinement, possibility to control the emitted mode pattern and beam shape and substantial reduction in laser threshold.

Radiative efficiency of highly luminescent bulk InP wafers severely degrades upon heat treatment involved in epitaxial growth of quaternary layers and fabrication of photodiodes on the surface. This unfortunate property impedes the use of bulk InP as scintillator material. On the other hand, it is known that thin epitaxial InP layers, grown by molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD), do not exhibit any degradation. These layers, however, are too thin to be useful in scintillators. The capability of hydride vapor phase epitaxy (HVPE) process to grow thick bulk-like layers in reasonable time is well known, but the radiative properties of HVPE InP layers are not known. We have studied radiative properties of 21 mu m thick InP layers grown by HVPE and found them comparable to those of best luminescent bulk InP virgin wafers. In contrast to the bulk wafers, the radiative efficiency of HVPE layers does not degrade upon heat treatment. This opens up the possibility of implementing free-standing epitaxial InP scintillator structures endowed with surface photodiodes for registration of the scintillation. (C) 2014 Elsevier Ltd. All rights reserved.

AIN layers of thickness of about 2 mu m have been grown with AIN nucleation layers (NLs) on (001) sapphire substrates using metal organic chemical vapor deposition. Increasing the AlN-NL deposition temperature from 850 to 1250 degrees C has been found to have significant effect on the surface morphology and the structural quality of the AIN layers. The surface morphology of the AlN-NLs and the AIN layers has been assessed using atomic force microscopy (AFM). The AM images of the AlN-NLs reveal the coalescence pattern of NLs. AM images of the AlN layers and the in-situ reflectance measurement disclose the surface morphology and the growth pattern of the AIN layers, respectively. Smooth surface with macro-steps and terrace features has been achieved for the AIN layer grown on the NL deposited at 950 degrees C. The structural quality of AIN layers has been studied by high resolution X-ray diffraction and Raman spectroscopy. The screw dislocation density from (002) reflection and the average edge dislocation density from (102), (302) and (100) reflections of the AIN layer on NL deposited at 950 degrees C are estimated to be 9 x 10(7) cm(-2) and 4.4 x 10(9) cm(-2), respectively. Lateral correlation length (L) is calculated from the (114) reciprocal space mapping of the AIN layers and correlated with the edge dislocation density of the AIN layers. Raman E-2 (high) phonon mode indicates compressive strain in the AIN layers grown on the NLs deposited at various temperatures. From this work, it has been inferred that the uniform coalescence of the nucleation islands and the complete coverage of AlN-NL determine the surface morphology and the structural quality of the subsequently grown AIN layers.

We describe a simple, aqueous and low thermal budget process for deposition of polycrystalline indium phosphide on silicon substrate. Using stoichiometric indium oxide films prepared from its spin-coated precursor on silicon as an intermediate step, we achieve stoichiometric indium phosphide films through phosphidisation. Both indium oxide and indium phosphide have been characterized for surface morphology, chemical composition, and crystallinity. The morphology and crystalline structure of the films have been explained in terms of the process steps involved in our deposition method. Incomplete phosphidisation of indium oxide to indium phosphide results in the restructuring of the partly unconverted oxide at the phosphidisation temperature. The optical properties of the indium phosphide films have been analyzed using micro photoluminescence and the results compared with those of a homoepitaxial layer and a theoretical model. The results indicate that good optical quality polycrystalline indium phosphide has been achieved. The Hall measurements indicate that the carrier mobilities of our samples are among the best available in the literature. Although this paper presents the results of indium phosphide deposition on silicon substrate, the method that we present is generic and can be used for deposition on any suitable substrate that is flexible and cheap which makes it attractive as a batch process for photovoltaic applications.

Nanosized octagonal pyramidal frusta of indium phosphide were selectively grown at circular hole openings on a silicon dioxide mask deposited on indium phosphide and indium phosphide pre-coated silicon substrates. The eight facets of the frusta were determined to be {111} and {110} truncated by a top (100) facet. The size of the top flat surface can be controlled by the diameter of the openings in the mask and the separation between them. The limited height of the frusta is attributed to kinetically controlled selective growth on the (100) top surface. Independent analyses with photoluminescence, cathodoluminescence and scanning spreading resistance measurements confirm certain doping enrichment in the frustum facets. This is understood to be due to crystallographic orientation dependent dopant incorporation. The blue shift from the respective spectra is the result of this enrichment exhibiting the Burstein-Moss effect. Very bright panchromatic cathodoluminescence images indicate that the top surfaces of the frusta are free from dislocations. The good optical and morphological quality of the nanopyramidal frusta indicates that the fabrication method is very attractive for the growth of site-, shape-, and number-controlled semiconductor quantum dot structures on silicon for nanophotonic applications.

In an attempt to achieve an InP-Si heterointerface, a new and generic method, the corrugated epitaxial lateral overgrowth (CELOG) technique in a hydride vapor phase epitaxy reactor, was studied. An InP seed layer on Si (0 0 1) was patterned into closely spaced etched mesa stripes, revealing the Si surface in between them. The surface with the mesa stripes resembles a corrugated surface. The top and sidewalls of the mesa stripes were then covered by a SiO2 mask after which the line openings on top of the mesa stripes were patterned. Growth of InP was performed on this corrugated surface. It is shown that growth of InP emerges selectively from the openings and not on the exposed silicon surface, but gradually spreads laterally to create a direct interface with the silicon, hence the name CELOG. We study the growth behavior using growth parameters. The lateral growth is bounded by high index boundary planes of {3 3 1} and {2 1 1}. The atomic arrangement of these planes, crystallographic orientation dependent dopant incorporation and gas phase supersaturation are shown to affect the extent of lateral growth. A lateral to vertical growth rate ratio as large as 3.6 is achieved. X-ray diffraction studies confirm substantial crystalline quality improvement of the CELOG InP compared to the InP seed layer. Transmission electron microscopy studies reveal the formation of a direct InP-Si heterointerface by CELOG without threading dislocations. While CELOG is shown to avoid dislocations that could arise due to the large lattice mismatch (8%) between InP and Si, staking faults could be seen in the layer. These are probably created by the surface roughness of the Si surface or SiO2 mask which in turn would have been a consequence of the initial process treatments. The direct InP-Si heterointerface can find applications in high efficiency and cost-effective Si based III-V semiconductor multijunction solar cells and optoelectronics integration.

Together with the optimal basic design, buried heterostructure quantum cascade laser (BH-QCL) with semi-insulating regrowth offers unique possibility to achieve an effective thermal dissipation and lateral single mode. We demonstrate here for the first time realization of BH-QCLs with a single step regrowth of highly resistive (>1x10(8) ohm.cm) semi-insulating InP:Fe in less than 45 minutes in a flexible hydride vapour phase epitaxy process for burying ridges etched down to 10-15 mu m deep both with and without mask overhang. The fabricated BH-QCLs emitting at similar to 4.7 mu m and similar to 5.5 mu m were characterized. 2 mm long 5.5 mu m lasers with ridge width 17-22 mu m, regrown with mask overhang, exhibited no leakage current. Large width and high doping in the structure did not permit high current density for CW operation. 5 mm long 4.7 mu m BH-QCLs of ridge widths varying from 6-14 mu m regrown without mask overhang, besides being spatially monomode, TM00, exhibited WPE of similar to 8-9% with an output power of 1.5 - 2.5 W at room temperature and under CW operation. Thus, we demonstrate a simple, flexible, quick, stable and single-step regrowth process with extremely good planarization for realizing buried QCLs leading to monomode, high power and high WPE.

Together with the optimal basic design, buried heterostructure quantum cascade laser (BH-QCL) with semi-insulating regrowth offers a unique possibility to achieve an effective thermal dissipation and lateral single mode. We demonstrate here the realization of BH-QCLs with a single-step regrowth of highly resistive (>1 x 10(8) ohm . cm) semi-insulating InP: Fe in <45 min for the first time in a flexible hydride vapor phase epitaxy process for burying ridges etched down to 10 to 15 mu m depth, both with and without mask overhang. The fabricated BH-QCLs emitting at similar to 4.7 and similar to 5.5 mu m were characterized. 2-mm-long 5.5-mu m lasers with a ridge width of 17 to 22 mu m, regrown with mask overhang, exhibited no leakage current. Large width and high doping in the structure did not permit high current density for continuous wave (CW) operation. 5-mm-long 4.7-mu m BH-QCLs of ridge widths varying from 6 to 14 mu m regrown without mu mask overhang, besides being spatially monomode, TM00, exhibited wall plug efficiency (WPE) of similar to 8 to 9% with an output power of 1.5 to 2.5 W at room temperature and under CW operation. Thus, we demonstrate a quick, flexible, and single-step regrowth process with good planarization for realizing buried QCLs leading to monomode, high power, and high WPE.

BH-QCLs were fabricated with regrowth of semi-insulating InP:Fe in hydride vapor phase epitaxy reactor. Two types of lateral ridge QCL designs were considered: (i) closely spaced ridges with double trenches and (ii) widely and uniformly spaced ridges. The etched depth varies from 6 to 15 μm in the former and 6 to10 μm in the latter. Double trenches of about 14 μm deep take only &lt; 40 minutes to planarize while the same time is needed to planarize about 8 μm deep trenches with uniform ridges. In any case the achieved growth rate is higher by at least one order of magnitude than that can be achieved in MBE and MOVPE. Some fabricated BH-QCLs are characterized and they exhibit spatially monomode (TMoo) laser with an output power of as high as 2.4 W and wall plug efficiency of ∼8-9% at RT under CW operation.

III-V semiconductors are suitable for high efficiency and radiation resistant solar cells. However, the high cost of these materials limited the application of these solar cells only for specialty application. High quality polycrystalline III-V thin films on low cost substrate are the viable solutions for the problem. In this work we demonstrate two new approaches to grow polycrystalline InP on Si(001) substrate. (i) A simple chemical solution route which makes use of deposition of In2O3 on Si and its subsequent phosphidisation and (ii) In assisted growth that involves deposition of In metal on Si and subsequent growth of InP from its precursors in hydride vapor phase epitaxy. Both techniques are generic and can be applied to other semiconductors on low cost and flexible substrates.

Polycrystalline InP was grown on Si(001) and Si(111) substrates by using indium (In) metal as a starting material in hydride vapor phase epitaxy (HVPE) reactor. In metal was deposited on silicon substrates by thermal evaporation technique. The deposited In resulted in islands of different size and was found to be polycrystalline in nature. Different growth experiments of growing InP were performed, and the growth mechanism was investigated. Atomic force microscopy and scanning electron microscopy for morphological investigation, Scanning Auger microscopy for surface and compositional analyses, powder X-ray diffraction for crystallinity, and micro photoluminescence for optical quality assessment were conducted. It is shown that the growth starts first by phosphidisation of the In islands to InP followed by subsequent selective deposition of InP in HVPE regardless of the Si substrate orientation. Polycrystalline InP of large grain size is achieved and the growth rate as high as 21 mu m/h is obtained on both substrates. Sulfur doping of the polycrystalline InP was investigated by growing alternating layers of sulfur doped and unintentionally doped InP for equal interval of time. These layers could be delineated by stain etching showing that enough amount of sulfur can be incorporated. Grains of large lateral dimension up to 3 mu m polycrystalline InP on Si with good morphological and optical quality is obtained. The process is generic and it can also be applied for the growth of other polycrystalline III-V semiconductor layers on low cost and flexible substrates for solar cell applications.

This thesis addresses new methods in the growth of indium phosphide on silicon for enabling silicon photonics and nano photonics as well as efficient and cost-effective solar cells. It also addresses the renewal of regrowth of semi-insulating indium phosphide for realizing buried heterostructure quantum cascade lasers with high power and wall plug efficiency for sensing applications.

As regards indium phosphide on silicon, both crystalline and polycrystalline growth methods are investigated. The crystalline growth methods are: (i) epitaxial lateral overgrowth to realize large area InP on Si, for silicon photonics (ii) a modified epitaxial lateral overgrowth method, called corrugated epitaxial lateral overgrowth, to obtain indium phosphide/silicon heterointerface for efficient and cost effective solar cells and (iii) selective growth of nanopyramidal frusta on silicon for nanophotonics. The polycrystalline growth method on silicon for low cost solar cell fabrication has been realized via (i) phosphidisation of indium oxide coating synthesized from solution chemistry and (ii) phosphidisation cum growth on indium metal on silicon. All our studies involve growth, growth analysis and characterization of all the above crystalline and polycrystalline layers and structures.

After taking into account the identified defect filtering mechanisms, we have implemented means of obtaining good optical quality crystalline layers and structures in our epitaxial growth methods. We have also identified feasible causes for the persistence of certain defects such as stacking faults. The novel methods of realizing indium phosphide/silicon heterointerface and nanopyramidal frusta of indium phosphide on silicon are particularly attractive for several applications other than the ones mentioned here.

Both the polycrystalline indium phosphide growth methods result in good optical quality material on silicon. The indium assisted phosphidisation cum growth method normally results in larger grain size indium phosphide than the one involving phosphidisation of indium oxide. These two methods are generic and can be optimized for low cost solar cells of InP on any flexible substrate.

The method of regrowth of semi-insulating indium phosphide that is routinely practiced in the fabrication of buried heterostructure telecom laser has been implemented for quantum cascade lasers. The etched ridges of the latter can be 6-15 µm deep, which is more than 2-3 times as those of the former. Although this is a difficult task, through our quick and flexible regrowth method we have demonstrated buried heterostructure quantum cascade lasers with an output power up to 2. 5 W and wall plug efficiency up to 9% under continuous operation.

III-V nanostructures have attracted substantial research effort due to their interesting physical properties and their applications in new generation of ultrafast and high efficiency nanoscale electronic and photonic components. The advances in nanofabrication methods including growth/synthesis have opened up new possibilities of realizing one dimensional (1D) nanostructures as building blocks of future nanoscale devices. For processing of semiconductor nanostructure devices, simplicity, cost effectiveness, and device efficiency are key factors. A number of methods are being pursued to fabricate high quality III-V nanopillar/nanowires, quantum dots and nano disks. Further, high optical quality nanostructures in these materials together with precise control of shapes, sizes and array geometries make them attractive for a wide range of optoelectronic/photonic devices.

This thesis work is focused on top-down approaches for fabrication of high optical quality nanostructures in III-V materials. Dense and uniform arrays of nanopillars are fabricated by dry etching using self-assembly of colloidal SiO₂ particles for masking. The physico-chemistry of etching and the effect of etch-mask parameters are investigated to control the shape, aspect ratios and spatial coverage of the nanopillar arrays. The optimization of etch parameters and the utilization of erosion of etch masks is evaluated to obtain desired pillar shapes from cylindrical to conical. Using this fabrication method, high quality nanopillar arrays were realized in several InP-based and GaAs-based structures, including quantum wells and multilayer heterostructures. Optical properties of these pillars are investigated using different optical spectroscopic techniques. These nanopillars, single and in arrays, show excellent photoluminescence (PL) at room temperature and the measured PL line-widths are comparable to the as-grown wafer, indicating the high quality of the fabricated nanostructures. The substrate-free InP nanopillars have carrier life times similar to reference epitaxial layers, yet an another indicator of high material quality. InGaAs layer, beneath the pillars is shown to provide several useful functions. It effectively blocks the PL from the InP substrate, serves as a sacrificial layer for generation of free pillars, and as a “detector” in cathodoluminescence (CL) measurements. Diffusion lengths independently determined by time resolved photoluminescence (TRPL) and CL measurements are consistent, and carrier feeding to low bandgap InGaAs layer is evidenced by CL data. Total reflectivity measurements show that nanopillar arrays provide broadband antireflection making them good candidates for photovoltaic applications.  A novel post etch, sulfur-oleylamine (S-OA) based chemical process is developed to etch III-V materials with monolayer precision, in an inverse epitaxial manner along with simultaneous surface passivation. The process is applied to push the limits of top-down fabrication and InP-based high optical quality nanowires with aspect ratios more than 50, and nanostructures with new topologies (nanowire meshes and in-plane wires) are demonstrated.  The optimized process technique is used to fabricate nanopillars in InP-based multilayers (InP/InGaAsP/InP and InP/InGaAs/InP). Such multilayer nanopillars are not only attractive for broad-band absorption in solar cells, but are also ideal to generate high optical quality nanodisks of these materials. Finally, the utility of a soft stamping technique to transfer free nanopillars/wires and nanodisks onto Si substrate is demonstrated. These nanostructures transferred onto Si with controlled densities, from low to high, could provide a new route for material integration on Si.

High material quality InP-based multilayer nanopillar (NP) arrays are fabricated using a combination of self-assembly of silica particles for mask generation and dry etching. In particular, the NP arrays are made from user-defined epitaxial multilayer stacks with specific materials and layer thicknesses. An additional degree of flexibility in the structures is obtained by changing the lateral diameters of the NP multilayer stacks. Pre-defined NP arrays made from InGaAsP/InP and InGaAs/InP NPs are then used to generate substrate-free nanodisks of a chosen material from the stack by selective etching. A soft-stamping method is demonstrated to transfer the generated nanodisks with arbitrary densities onto Si. The transferred nanodisks retain their smooth surface morphologies and their designed geometrical dimensions. Both InP and InGaAsP nanodisks display excellent photoluminescence properties, with line-widths comparable to unprocessed reference epitaxial layers of similar composition. The multilayer NP arrays are potentially attractive for broad-band absorption in third-generation solar cells. The high optical quality, substrate-free InP and InGaAsP nanodisks on Si offer a new path to explore alternative ways to integrate III-V on Si by bonding nanodisks to Si. The method also has the advantage of re-usable III-V substrates for subsequent layer growth.

We present a comprehensive characterization of the optical quality of InP nanopillars (NPs) fabricated by a top down approach using micro-photoluminescence (mu-PL), time-resolved PL, and cathodoluminescence (CL). A lattice matched InGaAs layer provided beneath the 1 mu m tall NPs functions as a "detector" in CL for monitoring carrier diffusion in InP NP. Carrier feeding to the InGaAs layer indicated by a double exponential PL decay is confirmed through CL mapping. Carrier lifetimes of over 1 ns and the appreciably long diffusion lengths (400-700 nm) in the InP NPs indicate very low surface damage making them attractive for optoelectronic applications.

We report on the realization of buried single ridge and μ-stripes quantum cascade lasers using HVPE and MOVPE regrowth steps of semi-insulating InP:Fe and Si doped layers. We present here the preliminary results obtained on these devices. The reduction of the thermal resistance achieved using semi-insulating InP:Fe for regrowth planarization and μ-stripe arrays approaches are shown and performance perspectives are addressed.

We report on design, fabrication and investigation of a buried heterostructure photonic crystal quantum cascade laser operating in the mid-IR (8.5μm) at room temperature, leading to single mode emission on a 600μm by 600μm mesa.

Optical properties of semiconductor nanowires (NWs)/nanopillars (NPs), as individual or ensemble, have attracted significant research interest in recent years. Their potential applications range from solid-state lighting, photovoltaics, lasing and nonlinear optics to sensing and life sciences. Many of III-V NWs/NPs are particularly interesting for optoelectronic and photovoltaic applications, because of their direct band gap, high refractive index and superior electrical properties. These properties are beneficial for use in next generation solar cells by reducing the active cell thickness, while maintaining high efficiency. Furthermore, high second order nonlinearity coefficients of many III-V materials, for example GaAs and GaP, enhanced electric fields and tight confinement of optical modes make nanowaveguide geometries ideal for nonlinear effects.

The focus of this thesis is on the fabrication of III-V NPs, their optical properties and applications. Different methods for fabrication of NPs (top-down approach) are proposed. The fabricated NPs show a broadband suppression of reflectance, which is particularly an interesting feature for photovoltaic applications. The effect of the shape and geometry of GaAs NPs on their reflectance spectra is investigated and the experimental data show a very good agreement with the simulations. In order to decrease surface recombination in the fabricated GaAs NPs, a sulfur-based chemical passivation method was used, resulting in the recovery of photoluminescence (PL) linewidth and enhancing the PL intensity for more than an order of magnitude. Moreover, a unique wafer-scale self-organization process for generation of InP NPs is demonstrated. As a proof of concept, the self-organized InP NPs were used to fabricate solar cell devices. For fabrication of InP NP solar cells, epitaxial overgrowth of NPs arrays was used to realize p-n junctions. A significant increase in the open circuit voltage (0.13 V) of the NP solar cell was obtained after surface passivation.

Second-harmonic generation (SHG) was experimentally observed from GaP NP waveguides (single and in arrays) with vertical geometry. The generated second- harmonic light was analyzed with respect to the size of the NP waveguides and the corresponding effects of surface and bulk nonlinearities. In case of individual NPs, SHG was analyzed considering different modal excitations in GaP NPs. It was demonstrated that by varying the NP diameter and changing the pump polarization, it is possible to alter the field distribution of the radiated SHG light. The importance of tight confinement of the pump in the NP waveguides and consequently the longitudinal component of the electric field in this geometry is shown. A method was proposed to distinguish between surface and bulk contributions in SHG, which also addressed how to employ surface SHG to enhance the generated light. The proposed method was used to estimate the nonlinear coefficient and the effective thickness of the nonlinear region at the surface of GaP NP waveguides. Based on these findings, the corresponding nonlinear coefficient at the surface is estimated to be approximately 15 times higher, compared to the bulk. These findings, suggest that NPs/NWs (in this case GaP NPs) are potential alternatives for future nonlinear nanophotonic devices. Additionally, the SHG light from single GaP NPs are promising candidates for ultrafast light sources at nanoscopic scale, with potential applications in sensing, bio and single cell/ molecular imaging.