Confining Metal-Halide Perovskites in Nanoporous Thin Films

Controlling size and shape of semiconducting nanocrystals advances nanoelectronics and photonics. Quantum confined, inexpensive, solution derived metal halide perovskites offer narrow band, color-pure emitters as integral parts of next-generation displays and optoelectronic devices. We use nanoporous silicon and alumina thin films as templates for the growth of perovskite nanocrystallites directly within device-relevant architectures without the use of colloidal stabilization. We find significantly blue shifted photoluminescence emission by reducing the pore size; normally infrared-emitting materials become visibly red, green-emitting materials cyan and blue. Confining perovskite nanocrystals within porous oxide thin films drastically increases photoluminescence stability as the templates auspiciously serve as encapsulation. We quantify the template-induced size of the perovskite crystals in nanoporous silicon with microfocus high-energy X-ray depth profiling in transmission geometry, verifying the growth of perovskite nanocrystals throughout the entire thickness of the nanoporous films. Low-voltage electroluminescent diodes with narrow, blue-shifted emission fabricated from nanocrystalline perovskites grown in embedded nanoporous alumina thin films substantiate our general concept for next generation photonic devices

Tip-Enhances Raman Spectroscopy, Enabling Spectroscopy at the Nanoscale

The knowledge on the chemical and the structural properties of substances benefits strongly from characterization methods that can provide access to the sample’s nanoscale building blocks. Not only should the sensitivity of these methods approach a high detection limit up to single molecule, but also the accessible spatial resolution must enable chemical imaging of individual nanoscale features of the substances. High resolution imaging is often provided by electron microscopes through methods such as transmission electron microscopy (TEM) and scanning electron microscopy (STM), nevertheless, these methods lack offering chemical information. Surface-enhanced spectroscopy was developed to improve the sensitivity of the chemical measurements through placing the sample onto rough metallic surfaces. However, in SERS, spatially resolved measurements are not possible since an ensemble of nanoscale features give birth to the SERS effect. The challenge of the simultaneous improvement of the spatial resolution and sensitivity was addressed indeed through combining high resolution optical microscopy with high sensitivity of surface-enhanced spectroscopy and was termed as tip-enhanced Raman spectroscopy. In this thesis, the confined electric field in proximity of the nanoscale apex of the metallic TERS tip is first investigated theoretically through conducting finite-difference time-domain calculations. The results were employed in optimization of the experimental TERS setup which is utilized in this thesis. The power of TERS in high resolution detection of nanoscale substances is then evaluated through TERS study of isolated single walled carbon nanotubes. The accessible high resolution is also used to acquire insight into the impact of structural strains on the molecular vibrations in silicon nanowires. The large surface sensitivity and specificity of TERS is also evaluated through TERS mapping of the adsorption sites of osteopontin phosphoprpteins on the surface of calcium oxalate microcrystal which are responsible for the formation of kidney stones in human body

Defect and yield analysis of semiconductor components and integrated circuits

Semiconductors were studied from the point of material, component, electrical and functional properties. Several methods were used to accomplish this, e.g. X-ray topography, etch pit analysis, statistical methods, and neural nets. The compound semiconductor components, i.e. GaAs varactor diodes, AlGaAs/InGaAs p-HEMTs, and LEDs (GaAs/AlGaAs and GaPN) were studied using the method of synchrotron X-ray topography. First, the silicon wafers studied were selected from fully processed lots with varying, though, low yields. The electrical circuits were fabricated with a CMOS (Complementary Metal-Oxide Semiconductor) process, well suited for mixed-signal applications. Then, synchrotron X-ray topographs and etch pit micrographs of the wafers were analyzed with an image processing software, written entirely for this study, to quantify the strain and defects present in the images. This information was then correlated with electrical parameters previously measured from the wafers, including the yield. Several of the parameters quantified from the synchrotron X-ray images show a strong correlation with certain measured parameters, e.g. PMOS transistor threshold voltage, polysilicon sheet resistance, N- sheet contact chain resistance. Then, some parameters practically do not correlate, e.g. NMOS breakdown voltage. A strong correlation of device yield with near-surface strain measured by synchrotron X-ray topography is found. Finally, the method of self-organizing map (SOM) neural net was applied to analyze a heartbeat rate monitor integrated circuit (IC) yield dependence on CMOS process control monitoring (PCM) data. The SOM efficiently reduces the PCM parameter space dimensions and helps in visualizing the different parameter relations. This makes it possible to identify the most probable PCM parameters affecting the yield. Those were found out to be NMOS transistor drain current and aluminum sheet resistance.reviewe

LASER Tech Briefs, Spring 1994

Topics in this Laser Tech Brief include: Electronic Components and Circuits. Electronic Systems, Physical Sciences, Materials, Mechanics, Fabrication Technology, and books and reports

Ülikiirete relaksatsiooniprotsesside uurimine kolmekomponendilistes heksafluoriidides sünkrotronkiirguse ergastusel

Väitekirja elektrooniline versioon ei sisalda publikatsiooneKross-luminestsents ja tsooni-sisene luminestsents on tahkiste omakiirguste liigid, mis tekivad piisava energiaga footonite või osakeste voogudega ergastamisel. Neid kiirgusi iseloomustab ülilühike luminestsentsi kustumisaeg, mida on võimalik potentsiaalselt ära kasutada väga head ajalist lahutust vajavates rakendustes. Kahjuks on nende kiirguste saagis väga madal ning kross-luminestsentsi kiirgusribad paiknevad lühilainelises spektriosas, kus fotodetektorite tundlikkus on madal. Doktoritöö eesmärk oli kasutada tsoonistruktuuri modifitseerimise lähenemist nende puuduste korvamiseks. Tuginedes materjalide tsoonistruktuuri arvutustele valiti sobivad elemendid, mis koos K, Ba katioonide ja F aniooniga on võimelised moodustama laia keelutsooniga fluoriide. Täiendava elemendi (Ge, Si) lisamisel on võimalik sünteesida heksafluoriide (nt K2GeF6), mille tsoonistruktuuri täiendavad lisaks valentsitsoonile (F seisundid) alamtsoonid (Ge, Si seisundid), luues soodsad võimalused tsooni-sisene luminestsentsi saagise kasvuks. Seisundite vähenenud vahekaugus nihutab kross-luminestsentsi nähtava valguse spektriosa suunas. Sünteesitud heksafluoriidide tsoonistruktuuri, elektron ergastusi ja nende kiirguslikku lagunemist uuriti aeglahutusega luminestsentsspektroskoopia meetoditel nii Tartu Ülikooli füüsika instituudis kui ka sünkrotronkiirguse keskuses MAX IV (Lund, Rootsi). Loodi uus, 32 ps ajalise lahutusega katseseade spetsiaalselt ülikiire luminestsentsi uurimiseks, mille abil näidati, et ülikiirete omakiirguste kustumisaeg uuritud heksafluoriidides jääb alla 500 ps. Kiirguste saagis oli rakendustes kasutamiseks liiga madal, kuid näidati nende esinemist laias spektriosas vaakum-ultravioletist nähtava valguseni. Doktoritöö tulemusena näidati, et tsoonistruktuuri modifitseerimise lähenemist kasutades on võimalik materjalide luminestsentsiomadusi mõjutada rakendamiseks vajalikus suunas.Cross-luminescence and intra-band luminescence are intrinsic emissions in solids excited by photons or particles with sufficient energy. The emissions have very short luminescence decay times and can potentially be used in applications requiring high time resolution. Unfortunately, light yield of the emissions is very low, and cross-luminescence emission bands are located in the short wavelength spectral region, where sensitivity of photodetectors is low. The aim of this doctoral work was to apply band structure engineering approach to overcome these disadvantages. Based on electronic band structure calculations, suitable elements able to form wide band gap ternary fluorides together with K, Ba cations and F anion were selected. Hexafluorides (e.g. K2GeF6) with complex valence band structure formed by F and additional Ge or Si states were synthesised. Valence band in these compounds is split into multiple sub-bands, favouring increased light yield of ultrafast emissions. Reduced energy gap between the valence band and outermost core level allows cross-luminescence emissions to shift towards the visible spectral range. The band structure of synthesized hexafluorides as well as electronic excitations and their radiative recombinations were studied using time-resolved luminescence spectroscopy methods at the Institute of Physics of the University of Tartu and at the MAX IV synchrotron radiation facility (Lund, Sweden). A new experimental setup with 32 ps time resolution was developed for studies of ultrafast luminescence. The experimental results revealed ultrafast emissions in the studied hexafluorides with less than 500 ps decay times. The light yield of ultrafast emissions was too low for use in applications, but they were observed in a broad spectral range from vacuum-ultraviolet to visible. As a result of this work, it was demonstrated that applying the band structure engineering approach, it is possible to achieve luminescence properties desired in materials.https://www.ester.ee/record=b550890

Fabrication of micro-structured surfaces with increased light absorption and their influence on intense laser-plasma experiments

The thesis reports on the influence of customisable and highly light absorbing surfaces on laser-plasma experiments. For the first time, a thin microstructured silicon substrate is interacting with a short laser pulse with peak intensity exceeding 1020 W=cm2. In this process, electrons are accelerated and pushed into the target to relativistic energies. Furthermore, ions are accelerated perpendicular to the target surfaces and electromagnetic radiation is generated. In the framework of this work, a fabrication setup is developed that produces customisable structured silicon surfaces using a laser-assisted ablation and etching process with light pulses of femtosecond pulse length and the effect of laser-induced periodic surface structures (LIPSS). The evolving structure consists of conical silicon spikes with a significant increase in light absorption over a broad spectral range in the visible and infrared region. The experimental setup is demonstrated together with a characterisation of the resulting surface structures. Thereby, a precise prediction of needle height and separation is possible. Following, these structured silicon targets are compared to flat foils and different periodic geometric structures, typically used in laser-plasma experiments, in an experimental campaign using the petawatt class Vulcan laser system of the Central Laser Facility, Oxfordshire, UK. Spectral and spatial investigation of reflected laser light, X-ray generation, electron and ion acceleration in the experiment demonstrate an enhanced performance of the robust microstructured silicon needle structure facing the incident laser pulse. A significant increase in high energetic electrons, ions and brilliant X-ray radiation is observed in comparison to flat foils and targets with geometric structures. Reflection losses from the interaction area are decreased substantially. With the results of the experimental campaign a combination of the microstructured silicon surfaces with different materials is motivated. E.g. proton-rich materials can generate a reliable and auspicious source of laser-accelerated protons. Joining the structured target with a confined piece of material, pointlike sources of brilliant X-ray radiation of selectable photon energy become available. Employing the fabrication setup developed within the framework of this thesis a valuable addition to the scope of the Detektor & Targetlabor is given. The further development of the setup towards high-repetition rate laser facilities, production of customisable and more complex targets and evaluation of applications for highly light absorbing surfaces is promising

Spectrally and temporally resolved single photon counting in advanced biophotonics applications

Biomedicine requires highly sensitive and efficient light sensors to analyse light-tissue or light-sample interactions. Single-photon avalanche diode (SPAD) sensors implemented with complementary metal-oxide-semiconductor (CMOS) technology have a growing range of applications in this field. Single-photon detection coupled with integrated timing circuits enables us to timestamp each detected photon with high temporal resolution (down to picoseconds). Arrays of SPAD based pixels and CMOS technology offer massively parallel time-resolved single-photon counting for spectrally and temporally resolved analysis of various light phenomena.This thesis examines how time-resolved CMOS SPAD based line sensors with per pixel timing circuits can be utilized to advance biophotonic applications. The study focuses on improving the existing techniques of fluorescence and Raman spectroscopy, and demonstrates for the first time CMOS SPAD based detection in optical coherence tomography (OCT). A novel detection scheme is proposed combining low-coherence interferometry and time-resolved photon counting. In this approach the interferometric information is revealed from spectral intensity measurements, which is supplemented by time-stamping of the photons building up the spectra.Two CMOS SPAD line sensors (Ra-I and its improved version, Ra-II) were characterized and the effect of their parameters on the selected techniques was analysed. The thesis demonstrates the deployment of the Ra-I line sensor in time-resolved fluorescence spectroscopy with indications of the applicability in time-resolved Raman spectroscopy. The work includes integration of the sensor with surrounding electrical and optical systems, and the implementation of firmware and software for controlling the optical setup. As a result, a versatile platform is demonstrated capable of micro- and millisecond sampling of spectral fluorescence lifetime changes in a single transient of fast chemical reactions.OCT operating in the spectral domain traditionally uses CMOS photodiode and charge-coupled device (CCD) based detectors. The applicability of CMOS SPAD sensors is investigated for the first time with focus on the main limitations and related challenges. Finally, a new detection method is proposed relying on both the wave and particle nature of light, recording time-resolved interferometric spectra from a Michelson interferometer. This method offers an alternative approach to analyse luminous effects and improves techniques based on the light’s time of flight. As an example, a proof of concept study is presented for the removal of unwanted reflections from along the sample and the optical path in an OCT setup