InAs avalanche photodiodes

The ability to efficiently detect low-level light in the infrared above wavelengths of 1.7 μm is becoming increasingly important for many applications such as gas sensing, defence/geoscience ranging and clinical thermography. The III-V narrow gap semiconductor InAs, with a bandgap of 0.36 eV, is well known for its use as a conventional photodiode. The aim of this thesis was to design, build and test InAs devices for use as reverse biased avalanche photodiodes. In order to fabricate a lownoise detector, a passivation study was conducted. For the first time we report the achievement of high quality single crystal II-VI passivation layers on InAs mesa structures. Pre-growth surface oxide removal processes were developed to improve surface morphology of II-VI layers grown on InAs samples. ZnSe and ZnTe successfully terminate the InAs mesa devices preventing atmospheric oxidation. Low surface leakage currents are observed at low reverse bias and at room temperature for both materials. LIDAR at wavelengths greater than 2 μm was studied using these InAs mesa photodiodes, showing potential to take advantage of the low solar background at these wavelengths. For the first time, laboratory based LIDAR experiments, with ranges of around 0.5 metre stand-off distance, were performed with InAs n-i-p edge illuminated mesa photodiodes, used in linear multiplication mode. Time-of-flight measurements were demonstrated at wavelengths from 1.3 μm to 2.365 μm. A 6 mm ranging error was observed in these short range measurements

Photodetectors

In this book some recent advances in development of photodetectors and photodetection systems for specific applications are included. In the first section of the book nine different types of photodetectors and their characteristics are presented. Next, some theoretical aspects and simulations are discussed. The last eight chapters are devoted to the development of photodetection systems for imaging, particle size analysis, transfers of time, measurement of vibrations, magnetic field, polarization of light, and particle energy. The book is addressed to students, engineers, and researchers working in the field of photonics and advanced technologies

Quantitative electroluminescence measurements of PV devices

Electroluminescence (EL) imaging is a fast and comparatively low-cost method for spatially resolved analysis of photovoltaic (PV) devices. A Silicon CCD or InGaAs camera is used to capture the near infrared radiation, emitted from a forward biased PV device. EL images can be used to identify defects, like cracks and shunts but also to map physical parameters, like series resistance. The lack of suitable image processing routines often prevents automated and setup-independent quantitative analysis. This thesis provides a tool-set, rather than a specific solution to address this problem. Comprehensive and novel procedures to calibrate imaging systems, to evaluate image quality, to normalize images and to extract features are presented. For image quality measurement the signal-to-noise ratio (SNR) is obtained from a set of EL images. Its spatial average depends on the size of the background area within the EL image. In this work the SNR will be calculated spatially resolved and as (background independent) averaged parameter using only one EL image and no additional information of the imaging system. This thesis presents additional methods to measure image sharpness spatially resolved and introduces a new parameter to describe resolvable object size. This allows equalising images of different resolutions and of different sharpness allowing artefact-free comparison. The flat field image scales the emitted EL signal to the detected image intensity. It is often measured through imaging a homogeneous light source such as a red LCD screen in close distance to the camera lens. This measurement however only partially removes vignetting the main contributor to the flat field. This work quantifies the vignetting correction quality and introduces more sophisticated vignetting measurement methods. Especially outdoor EL imaging often includes perspective distortion of the measured PV device. This thesis presents methods to automatically detect and correct for this distortion. This also includes intensity correction due to different irradiance angles. Single-time-effects and hot pixels are image artefacts that can impair the EL image quality. They can conceivably be confused with cell defects. Their detection and removal is described in this thesis. The methods presented enable direct pixel-by-pixel comparison for EL images of the same device taken at different measurement and exposure times, even if imaged by different contractors. EL statistics correlating cell intensity to crack length and PV performance parameters are extracted from EL and dark I-V curves. This allows for spatially resolved performance measurement without the need for laborious flash tests to measure the light I-V- curve. This work aims to convince the EL community of certain calibration- and imaging routines, which will allow setup independent, automatable, standardised and therefore comparable results. Recognizing the benefits of EL imaging for quality control and failure detection, this work paves the way towards cheaper and more reliable PV generation. The code used in this work is made available to public as library and interactive graphical application for scientific image processing

An InGaAlAs-InGaAs two-colour detector, InAs photodiode and Si SPAD for radiation thermometry

This work aims to develop infrared detectors and to introduce a new measurement technique for infrared radiation thermometry. It consists of two-colour detectors for ratio thermometry, InAs photodiode for 3.43 m narrow band thermometer and photon counting thermometer using a Si single photon avalanche photodiode (SPAD). In addition to research in these detectors, a Monte Carlo model for modelling impact ionisation in Si was also developed. InGaAlAs is attractive material for multi-colour detection at wavelengths up to 1.7 m, as it is lattice matched to InP substrate. InGaAlAs-InGaAs two-colour detector was evaluated as a ratio thermometer. When compared to a commercial Si-InGaAs detector, the InGaAlAs diode produces slightly higher (lower) output than Si at temperature below (above) 500 °C, while the InGaAs diode in this work also produces slightly higher output than that in the commercial Si-InGaAs detector. The InGaAlAs and InGaAs diodes detect blackbody temperatures as low as 275 and 125 oC, respectively, with signal to noise ratios (SNRs) above 10. As a ratio thermometer, the two-colour InGaAlAs-InGaAs photodetector achieves a temperature error of 12.8 °C at 275 °C, but this improves with temperature to 0.1 °C at 450 °C. If the maximum temperature error of 2 °C is defined, the InGaAlAs-InGaAs is capable of detecting an object temperature down to 325 °C. These results demonstrate the potential of InGaAlAs-InGaAs two-colour photodetector for development of high performance two-colour array detectors for radiation thermometry and thermal imaging of hot objects. The InAs photodiode offers huge potential for infrared sensing applications at wavelengths above 1.7 m. The performance of InAs photodiode was evaluated for use in radiation thermometry at wavelengths beyond InGaAs photodiode. For uncooled InAs, it successfully measured a blackbody temperature of 50 oC with an acceptable error of 0.17 oC. In order to evaluate its performance as a 3.43 m narrow band thermometer, measurements were repeated with a narrow band filter. InAs was demonstrated to have lower temperature error than a commercial PbSe detector. The temperature error was 1.88 oC for InAs at 50 oC compared to 3.78 oC for PbSe. This suggests that InAs is ideally X. ZHOU III suited for applications requiring 3.43 m operating wavelength. Further improvement was achieved by cooling InAs to 200 K. It was found that a temperature as low as 37 oC, with an error of less than 0.5 oC, can be measured indicating its potential for human body temperature sensing. An alternative to using a photodetector with longer wavelength response is to increase the sensitivity of the photodetector via internal gain mechanisms such as impact ionisation. By employing a very high internal gain in SPAD, the photon counting technique was evaluated for radiation thermometry. Photon induced avalanche pulses were successfully measured at temperature as low as 225 oC with an error less than 2 oC using Si SPAD. This is significantly lower than the lower temperature limit of 400 oC in conventional Si photodiode based radiation thermometer. The photon counting technique is therefore demonstrated to be a feasible technique to achieve lower temperature sensing

Characterization of silicon photovoltaic wafers using polarized infrared imaging

Photovoltaic (PV) solar industry uses low-cost material processing methods to produce silicon wafer-based solar cells. Thermal process can introduce crystal defects and residual stress in a PV wafer, which can impact the electrical performance and mechanical reliability of a finished solar cell. This research presents characterization methods for mono-crystal and multi-crystal silicon PV wafers, using an integrated polarized infrared imaging tool capable of both photoelastic (PE) and polarized photoluminescence (PL) imaging. Infrared PE imaging is used to investigate the thermal process-induced residual stress and defect-related stress in mono-crystal silicon PV wafers. The measured stress pattern shows that dislocation structures interact with the thermal residual stress, forming slip band structures oriented at 45 degrees to the wafer edges. The measured PE images are then interpreted using a discrete dislocation-based numerical modeling approach that accounts for stress relaxation in the wafer due to the dislocation structures. The model leads to simulated PE images and is used to analyze the preferred dislocation slip band orientations for wafer strain energy reduction. The analysis is consistent with experimental observations, forming the basis for a more quantitative infrared PE-based inspection method. Crystal growth process for multi-crystal silicon PV wafers results in grain boundaries and dislocation structures that impact solar cell performance. These defects are investigated using the polarized PL imaging setup, which can spatially resolve the defect structures from both the band-to-band and defect-related PL emission. The polarization resolving ability allows the identification and the correlation among different defect types. The technology described here creates a pathway to rapid full-field wafer quality inspection in a manufacturing setting, and will help to improve PV wafer material processing

Low-power CMOS digital-pixel Imagers for high-speed uncooled PbSe IR applications

This PhD dissertation describes the research and development of a new low-cost medium wavelength infrared MWIR monolithic imager technology for high-speed uncooled industrial applications. It takes the baton on the latest technological advances in the field of vapour phase deposition (VPD) PbSe-based medium wavelength IR (MWIR) detection accomplished by the industrial partner NIT S.L., adding fundamental knowledge on the investigation of novel VLSI analog and mixed-signal design techniques at circuit and system levels for the development of the readout integrated device attached to the detector. The work supports on the hypothesis that, by the use of the preceding design techniques, current standard inexpensive CMOS technologies fulfill all operational requirements of the VPD PbSe detector in terms of connectivity, reliability, functionality and scalability to integrate the device. The resulting monolithic PbSe-CMOS camera must consume very low power, operate at kHz frequencies, exhibit good uniformity and fit the CMOS read-out active pixels in the compact pitch of the focal plane, all while addressing the particular characteristics of the MWIR detector: high dark-to-signal ratios, large input parasitic capacitance values and remarkable mismatching in PbSe integration. In order to achieve these demands, this thesis proposes null inter-pixel crosstalk vision sensor architectures based on a digital-only focal plane array (FPA) of configurable pixel sensors. Each digital pixel sensor (DPS) cell is equipped with fast communication modules, self-biasing, offset cancellation, analog-to-digital converter (ADC) and fixed pattern noise (FPN) correction. In-pixel power consumption is minimized by the use of comprehensive MOSFET subthreshold operation. The main aim is to potentiate the integration of PbSe-based infra-red (IR)-image sensing technologies so as to widen its use, not only in distinct scenarios, but also at different stages of PbSe-CMOS integration maturity. For this purpose, we posit to investigate a comprehensive set of functional blocks distributed in two parallel approaches: • Frame-based “Smart” MWIR imaging based on new DPS circuit topologies with gain and offset FPN correction capabilities. This research line exploits the detector pitch to offer fully-digital programmability at pixel level and complete functionality with input parasitic capacitance compensation and internal frame memory. • Frame-free “Compact”-pitch MWIR vision based on a novel DPS lossless analog integrator and configurable temporal difference, combined with asynchronous communication protocols inside the focal plane. This strategy is conceived to allow extensive pitch compaction and readout speed increase by the suppression of in-pixel digital filtering, and the use of dynamic bandwidth allocation in each pixel of the FPA. In order make the electrical validation of first prototypes independent of the expensive PbSe deposition processes at wafer level, investigation is extended as well to the development of affordable sensor emulation strategies and integrated test platforms specifically oriented to image read-out integrated circuits. DPS cells, imagers and test chips have been fabricated and characterized in standard 0.15μm 1P6M, 0.35μm 2P4M and 2.5μm 2P1M CMOS technologies, all as part of research projects with industrial partnership. The research has led to the first high-speed uncooled frame-based IR quantum imager monolithically fabricated in a standard VLSI CMOS technology, and has given rise to the Tachyon series [1], a new line of commercial IR cameras used in real-time industrial, environmental and transportation control systems. The frame-free architectures investigated in this work represent a firm step forward to push further pixel pitch and system bandwidth up to the limits imposed by the evolving PbSe detector in future generations of the device.La present tesi doctoral descriu la recerca i el desenvolupament d'una nova tecnologia monolítica d'imatgeria infraroja de longitud d'ona mitja (MWIR), no refrigerada i de baix cost, per a usos industrials d'alta velocitat. El treball pren el relleu dels últims avenços assolits pel soci industrial NIT S.L. en el camp dels detectors MWIR de PbSe depositats en fase vapor (VPD), afegint-hi coneixement fonamental en la investigació de noves tècniques de disseny de circuits VLSI analògics i mixtes pel desenvolupament del dispositiu integrat de lectura unit al detector pixelat. Es parteix de la hipòtesi que, mitjançant l'ús de les esmentades tècniques de disseny, les tecnologies CMOS estàndard satisfan tots els requeriments operacionals del detector VPD PbSe respecte a connectivitat, fiabilitat, funcionalitat i escalabilitat per integrar de forma econòmica el dispositiu. La càmera PbSe-CMOS resultant ha de consumir molt baixa potència, operar a freqüències de kHz, exhibir bona uniformitat, i encabir els píxels actius CMOS de lectura en el pitch compacte del pla focal de la imatge, tot atenent a les particulars característiques del detector: altes relacions de corrent d'obscuritat a senyal, elevats valors de capacitat paràsita a l'entrada i dispersions importants en el procés de fabricació. Amb la finalitat de complir amb els requisits previs, es proposen arquitectures de sensors de visió de molt baix acoblament interpíxel basades en l'ús d'una matriu de pla focal (FPA) de píxels actius exclusivament digitals. Cada píxel sensor digital (DPS) està equipat amb mòduls de comunicació d'alta velocitat, autopolarització, cancel·lació de l'offset, conversió analògica-digital (ADC) i correcció del soroll de patró fixe (FPN). El consum en cada cel·la es minimitza fent un ús exhaustiu del MOSFET operant en subllindar. L'objectiu últim és potenciar la integració de les tecnologies de sensat d'imatge infraroja (IR) basades en PbSe per expandir-ne el seu ús, no només a diferents escenaris, sinó també en diferents estadis de maduresa de la integració PbSe-CMOS. En aquest sentit, es proposa investigar un conjunt complet de blocs funcionals distribuïts en dos enfocs paral·lels: - Dispositius d'imatgeria MWIR "Smart" basats en frames utilitzant noves topologies de circuit DPS amb correcció de l'FPN en guany i offset. Aquesta línia de recerca exprimeix el pitch del detector per oferir una programabilitat completament digital a nivell de píxel i plena funcionalitat amb compensació de la capacitat paràsita d'entrada i memòria interna de fotograma. - Dispositius de visió MWIR "Compact"-pitch "frame-free" en base a un novedós esquema d'integració analògica en el DPS i diferenciació temporal configurable, combinats amb protocols de comunicació asíncrons dins del pla focal. Aquesta estratègia es concep per permetre una alta compactació del pitch i un increment de la velocitat de lectura, mitjançant la supressió del filtrat digital intern i l'assignació dinàmica de l'ample de banda a cada píxel de l'FPA. Per tal d'independitzar la validació elèctrica dels primers prototips respecte a costosos processos de deposició del PbSe sensor a nivell d'oblia, la recerca s'amplia també al desenvolupament de noves estratègies d'emulació del detector d'IR i plataformes de test integrades especialment orientades a circuits integrats de lectura d'imatge. Cel·les DPS, dispositius d'imatge i xips de test s'han fabricat i caracteritzat, respectivament, en tecnologies CMOS estàndard 0.15 micres 1P6M, 0.35 micres 2P4M i 2.5 micres 2P1M, tots dins el marc de projectes de recerca amb socis industrials. Aquest treball ha conduït a la fabricació del primer dispositiu quàntic d'imatgeria IR d'alta velocitat, no refrigerat, basat en frames, i monolíticament fabricat en tecnologia VLSI CMOS estàndard, i ha donat lloc a Tachyon, una nova línia de càmeres IR comercials emprades en sistemes de control industrial, mediambiental i de transport en temps real.Postprint (published version

Luminescence-based characterization of crystalline silicon solar cells

[no abstract

Advanced Luminescence-based characterisation of silicon wafer solar cells

Ph.DDOCTOR OF PHILOSOPH

Comparing properties of different infrared cameras

Infrared (IR) camera enables to measure the temperature of an object without contact. As a result, a thermal image is created, and it is usually visualized by colours/contrast representing different temperature values. Originally, IR cameras were developed for military and surveillance purposes. As the features of IR cameras evolved and cost decreased, the application field expanded also to other areas such as industry and medicine. This thesis work is conducted on requirements and desires of medical technology company Imaqen Ltd. The aim was to acquire a better understanding of different IR camera technologies, compare those to each other on paper and carry out measurements, where the ability of the IR cameras to detect fast effects is tested and the noise levels of the cameras are determined. On this ground, one could find the most suitable IR camera to a future application of the company. Among other things, spectral range, sensitivity and response time of IR cameras depend on the material and technology of the detector. These properties influence e.g. in which temperature range the camera is working, how small temperature differences can be distinguished and how fast effects can be detected. The real functionality of an IR camera is revealed only in practical testing, so in addition to theoretical study, five different IR cameras were tested. Three cameras were cooled photovoltaic (PV) detectors, which measure the temperature of an object by counting incident photons. Two cameras were microbolometers (MB), which operation is based on variation in temperature of sensing elements of the detector. The aim was to measure how well the cameras are detecting heat effects lasting only a couple of milliseconds. A tech-nical phantom was built for the measurement, where a rotating plate with a small hole was adjusting the time a warm object behind it was visible. By increasing the angular velocity of the plate, the time of the heat effect was reduced to 2.7 ms. Noise level of the camera affects how small temperature differences the camera can distinguish. Therefore, the characteristic noise level of every camera was calculated, and this was compared to the NETD (noise-equivalent temperature difference) value provided by the manufacturer. MBs are not suitable for detecting heat effects that last only a couple of milliseconds due to their long response time i.e. the thermal time constant of the sensing element. The PV detectors on the other hand are very suitable for fast recognition tasks as their response time is short. However, an aliasing effect was discovered because of a too low frame rate. This can be a limiting factor when the effect time decreases and if the frame rate of the camera cannot be increased. All the noise levels were higher than values provided by the manufacturers, especially with MBs. This difference of noise levels between MB and PV detectors makes the latter more suitable for detecting small temperature differences on a scene. Based on the results and theory, one can conclude that PV detectors have better fast recognition and noise features than MBs. However, cost and size of MBs are considerably reduced compared to PV detectors. This can turn out to be a selection criterion if the minimum requirements are fulfilled. There are still other detectors used in IR cameras e.g. quantum well infrared photodetector, which were not tested in this work. Thus it would be beneficial to reproduce these tests with a larger variety of different IR detector technologies; only then more precise conclusions about the best detector could be made. Sometimes a trade-off between some features must be made, for example, it has to be decided if a fast response time is more important than high sensitivity with a large resolution. The application will set the requirements for optimal features of the IR camera. Therefore, different cameras cannot be put in order without knowing what the target application is.Lämpökamera mahdollistaa kohteen lämpötilan mittaamisen ilman kontaktia. Sillä saadaan kohteesta lämpökuva, jossa värit/sävyt edustavat eri lämpötiloja. Alun perin lämpökameroita käytettiin ja kehitettiin sotilas- ja valvontakäyttöön. Kameroiden hintojen laskiessa ja ilmaisimien ominaisuuksien parantuessa sovellusalue on laajentunut myös muun muassa teollisuuteen ja lääketieteeseen. Tämä diplomityö on toteutettu terveysteknologia-alan yrityksen, Imaqen Oy:n tarpeita ja toiveita varten. Tavoitteena oli parantaa ymmärrystä eri lämpökamerateknologioiden ominaisuuksista sekä vertailla näitä keskenään niin teoriassa kuin käytännössä. Lähtökohtana oli toteuttaa mittaus, jossa vertaillaan eri kameroiden kykyä havaita nopeita ilmiöitä sekä määrittää kameroiden kohinatasot. Työn tavoitteena oli edesauttaa yrityksen päätöksentekoa tulevaisuuden laitehankintoja ajatellen. Käytetystä ilmaisinmateriaalista ja –teknologiasta riippuen muun muassa lämpökameroiden spektrivaste, herkkyys sekä vasteaika vaihtelevat. Nämä ominaisuudet vaikuttavat esimerkiksi siihen, millä lämpötila-alueella kameralla pystytään kuvaamaan, kuinka pieniä lämpötilaeroja pystytään erottamaan ja kuinka nopeita ilmiöitä havaitsemaan. Todellinen toiminta paljastuu kuitenkin vasta käytännön testeissä. Teoreettisen tarkastelun lisäksi työssä suoritettiin testit viidelle eri lämpökameralle. Kolme kameroista oli jäähdytettyjä fotojännitekameroita, joiden kohteen lämpötilan mittaus perustuu kohteesta saapuvien fotonien laskemiseen. Lisäksi testattiin kaksi jäähdyttämätöntä mikrobolometria, joiden toiminta perustuu ilmaisinmateriaalin lämpötilan vaihteluun. Tavoitteena oli testata, kuinka hyvin eri kamerat havaitsevat lämpöilmiöitä, jotka ovat nopeimmillaan muutaman millisekunnin mittaisia. Tätä varten rakennettiin testijärjestelmä, jossa lämpölähteen näkymisaikaa kameralle säädettiin pyörivän reikälevyn avulla. Pyörimisnopeutta kasvattamalla saatiin lämpöilmiötä lyhennettyä aina 2,7 millisekuntiin asti. Kameran kohinataso vaikuttaa siihen, kuinka pieniä lämpötilaeroja kameralla pystytään havaitsemaan. Tämän vuoksi jokaiselle kameralle määritettiin sen ominainen kohinataso, jota verrattiin kameravalmistajan ilmoittamaan NETD-arvoon (noise-equivalent temperature difference). Mikrobolometrit eivät sovellu millisekunnin luokkaa olevien lämpöilmiöiden havaitsemiseen niiden pitkän vasteajan vuoksi, joka riippuu materiaalin termisestä aikavakiosta. Fotojännitekameroissa vasteaika on lyhyt, joten ne soveltuvat nopeiden ilmiöiden havaitsemiseen. Tuloksissa havaittiin kuitenkin taajuuksien laskostumista liian matalan kuvanottotaajuuden vuoksi. Tämä voi muodostua rajoittavaksi tekijäksi ilmiönopeuden kasvaessa, jos kameran kuvanottotaajuutta ei pystytä nostamaan. Kohinatasot olivat kaikilla kameroilla valmistajien ilmoittamia arvoja korkeammalla, erityisesti mikrobolometreilla. Ero mikrobolometri- ja fotojännitekameroiden kohinatasojen välillä tekee fotojännitekameroista soveltuvampia pienten lämpötilaerojen havaitsemiseen. Tulosten ja teorian perusteella voidaan todeta, että fotojännitekamerat ovat sekä nopeus- että kohinaominaisuuksiltaan mikrobolometreja parempia. Mikrobolometrien koko sekä kustannukset ovat kuitenkin fotojännitekameroita huomattavasti pienemmät. Tämä voi osoittautua valintakriteeriksi, jos kuvattavan kohteen asettamat vaatimukset täyttyvät. On kuitenkin olemassa myös muita lämpökameroissa käytettyjä ilmaisimia, kuten kvanttikaivoilmaisin, joita ei tämän työn aikana päästy testaamaan. Vaaditaan siis testien toistamista laajemmalla otannalla eri ilmaisinteknologioita, jotta tarkempia johtopäätöksiä parhaasta lämpökamerasta pystytään tekemään. Osa kameran ominaisuuksista vaatii myös kompromissien tekemistä. Täytyy esimerkiksi päättää, onko tärkeämpää kuvausnopeus vai pienten lämpötilaerojen havaitseminen suurella resoluutiolla. Käyttökohde asettaa vaatimukset optimaaliselle kameralle, eikä lämpökameroita voida siis asettaa paremmuusjärjestykseen tietämättä ensin, mitä ollaan kuvaamassa