2,792 research outputs found

    State-of-the-art all-silicon sub-bandgap photodetectors at telecom and datacom wavelengths

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    Silicon-based technologies provide an ideal platform for the monolithic integration of photonics and microelectronics. In this context, a variety of passive and active silicon photonic devices have been developed to operate at telecom and datacom wavelengths, at which silicon has minimal optical absorption - due to its bandgap of 1.12 eV. Although in principle this transparency window limits the use of silicon for optical detection at wavelengths above 1.1 μm, in recent years tremendous advances have been made in the field of all-silicon sub-bandgap photodetectors at telecom and datacom wavelengths. By taking advantage of emerging materials and novel structures, these devices are becoming competitive with the more well-established technologies, and are opening new and intriguing perspectives. In this paper, a review of the state-of-the-art is presented. Devices based on defect-mediated absorption, two-photon absorption and the internal photoemission effect are reported, their working principles are elucidated and their performance discussed and compared

    Accurate strain measurements in highly strained Ge microbridges

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    Ge under high strain is predicted to become a direct bandgap semiconductor. Very large deformations can be introduced using microbridge devices. However, at the microscale, strain values are commonly deduced from Raman spectroscopy using empirical linear models only established up to 1.2% for uniaxial stress. In this work, we calibrate the Raman-strain relation at higher strain using synchrotron based microdiffraction. The Ge microbridges show unprecedented high tensile strain up to 4.9 % corresponding to an unexpected 9.9 cm-1 Raman shift. We demonstrate experimentally and theoretically that the Raman strain relation is not linear and we provide a more accurate expression.Comment: 10 pages, 4 figure

    Giant Gating Tunability of Optical Refractive Index in Transition Metal Dichalcogenide Monolayers

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    We report that the refractive index of transition metal dichacolgenide (TMDC) monolayers, such as MoS2, WS2, and WSe2, can be substantially tuned by > 60% in the imaginary part and > 20% in the real part around exciton resonances using CMOS-compatible electrical gating. This giant tunablility is rooted in the dominance of excitonic effects in the refractive index of the monolayers and the strong susceptibility of the excitons to the influence of injected charge carriers. The tunability mainly results from the effects of injected charge carriers to broaden the spectral width of excitonic interband transitions and to facilitate the interconversion of neutral and charged excitons. The other effects of the injected charge carriers, such as renormalizing bandgap and changing exciton binding energy, only play negligible roles. We also demonstrate that the atomically thin monolayers, when combined with photonic structures, can enable the efficiencies of optical absorption (reflection) tuned from 40% (60%) to 80% (20%) due to the giant tunability of refractive index. This work may pave the way towards the development of field-effect photonics in which the optical functionality can be controlled with CMOS circuits

    Fourier Transform Infrared Spectroscopy for the measurement of GeSn/(Si)GeSn

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    Photoluminescence (PL) and Electroluminescence (EL) characterization techniques are important tools for studying the optical and electrical properties of (Si)GeSn. Light emission from these PL and EL measurements provides relevant information on material quality, bandgap energy, current density, and device efficiency. Prior to this work, the in-house PL set-up of this lab which involves the use of a commercially-obtained dispersive spectrometer was used for characterizing both GeSn thin film and fabricated devices, but these measurements were limited by issues bordering on low spectral resolution, spectral artifacts, and poor signal-to-noise ratio (SNR) thereby resulting in the possible loss of vital information and inaccurately reported parameters. To maintain the progress of the GeSn material development toward replacing the group III-V semiconductors in the optoelectronics industry, high-resolution spectroscopy with high SNR are necessary to accurately determine the unknowns in the GeSn growth conditions, device fabrication, and material development. In this work, PL and EL characterization systems were designed, built, and aligned using a Bruker Fourier transform infrared (FTIR) set up to collect the emission from fabricated GeSn samples using an external source with an external photodetector. This thesis presents an optical setup of a Bruker FTIR spectrometer aligned with external optical components and external light sources (532 nm laser and 1064 nm laser) to achieve high-resolution spectroscopy with high SNR. The setup employs the use of OPUS operational software for controlling the components of the FTIR hardware and a LabVIEW program for controlling all motorized devices on the external optical bench. An indium antimonide (InSb) external photodetector was used with the FTIR for a wider spectral range detection. Finally, a step-scan mode of operation which required both lock-in and a chopper was used to measure various GeSn bulk and laser samples at high resolution. The results obtained from these characterizations demonstrate a high SNR spectrum compared to a dispersive spectrometer

    Photodetectors based on low-dimensional materials and hybrid systems

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    Premi extraordinari doctorat UPC curs 2015-2016, àmbit de CiènciesIn the last decade, two-dimensional (2D) materials have attracted attention both in the nascent field of flexible nanotechnology as well as in more conventional semiconductor technol-ogies. Within the rapidly expanding portfolio of 2D materials, the group of semiconducting transition metal dichalcogenides (TMDCs) has emerged as an intriguing candidate for various optoelectronic applications. The atomically thin profile, favorable bandgap and outstanding electronic properties of TMDCs are unique features that can be explored and applied in novel photodetecting platforms. This thesis presents highly sensitive two-dimensional phototransistors made of sub-nanometre thick TMDC channels. Firstly, an encapsulation route is developed to address the detrimental and, to date, uncontrollable impact of atmospheric adsorbates, which severely deteriorate detector performance. The passivation scheme improves the transport properties of TMDCs, leading to high photoconductive gain with gate dependent responsivity of 10 -10^4 A/W throughout the visible, and temporal response down to 10 ms, which is suitable for imaging applications. The atomic device thickness yields ultra-low dark current operation and record detectivity of 10^11 - 10^12 Jones for TMDC-based detectors is achieved. The use of monolayer TMDCs, however, has disadvantages like limited spectral absorption due to the bandgap and limited absorption efficiency. In order to increase the absorption and to extend the spectral coverage, TMDC channels are covered with colloidal quantum dots to make hybrid phototransistors. This compelling synergy combines strong and size-tunable light absorption within the QD film, efficient charge separation at the TMDC-QD interface and fast carrier transport through the 2D channel. This results in large gain of 10^6 electrons per absorbed photon and creates the basis for extremely sensitive light sensing. Colloidal quan-tum dots are an ideal sensitizer, because their solution-processing and facile implementation on arbitrary substrates allows for low-cost fabrication of hybrid TMDC-QD devices. Moreover, the custom tailored bandgap of quantum dots provides the photodetector with wide spectral tunability. For photodetection in the spectral window of NIR/SWIR, which is still dominated by expensive and complex epitaxy-based technologies, these hybrid detectors have the potential to favorably compete with commercially available systems. The interface of the TMDC-QD hybrid is of paramount importance for sensitive detector operation. A high density of trap states at the interface is shown to be responsible for inefficient gate-control over channel conductivity, which leads to high dark currents. To maintain the unique electrical field-effect modulation in TMDCs upon deposition of colloidal quantum dots, a passivation route of the interface with semiconducting metal-oxide films is developed. The buffer-layer material is selected such that charge transfer from QDs into the channel is favored. The retained field-effect modulation with a large on/off ratio allows operation of the phototransistor at significantly lower dark currents than non-passivated hybrids. A TMDC-QD phototransistor with an engineered interface that exhibits detectivity of 10^12 - 10^13 Jones and response times of 12 ms and less is reported. In summary, this work showcases prototype photodetectors made of encapsulated 2D TMDCs and TMDC-QD hybrids. Plain TMDC-detectors have potential for application as flexible and semi-transparent detector platforms with high sensitivity in the visible. The hybrid TMDC-QD device increases its spectral selectivity to the NIR/SWIR due to the variable absorption of the sensitizing quantum dots and reaches compelling performance thanks to im-proved light-matter interaction and optimized photocarrier generation.En la última década ha surgido un gran interés por los materiales bidimensionales (2D) tanto para las tecnologías emergentes de dispositivos flexibles, como para las tecnologías de semiconductores tradicionales. Dentro del creciente catálogo de materiales 2D, los semiconductores basados en dicalcogenuros de metales de transición (DCMTs) han surgido como candidatos para aplicaciones optoelectrónicas. Sus características únicas, tales como grosor atómico, banda prohibida y propiedades electrónicas pueden ser examinadas y aplicadas en nuevas plataformas de fotodetección. En esta tesis se presentan nuevos fototransistores bidimensionales ultrasensibles basados en canales de DCMTs subnanométricos. Se presenta una ruta de encapsulación para intentar solucionar el impacto negativo, e incontrolable hasta la fecha, producido por la adsorción de sustancias atmosféricas que degradan el funcionamiento de los detectores. Este proceso mejora el transporte en los DCMTs dando lugar a una gran ganancia fotoconductora, una respuesta, dependiente de la tensión aplicada en el gate, de 10-10^4 A/W en el visible y una respuesta temporal de tan solo 10 ms, todo ello adecuado para aplicaciones de imagen. El grosor atómico de los dispositivos da lugar a corrientes de oscuridad muy bajas y una detectividad de 10^11-10^12 Jones. Sin embargo, el uso de monocapas de DCMTs presenta ciertas desventajas como por ejem-plo una eficiencia en la absorción baja. Con el fin de mejorar la absorción, los canales de DCMTs se han recubierto con puntos cuánticos (QDs) para fabricar fototransistores híbridos. Esta sinergia combina la alta absorción de los QDs, una eficiente separación de cargas en la interfaz DCMT-QD y un rápido transporte de cargas a través del canal 2D. Todo esto resulta en una ganancia de 10^6 electrones por fotón absorbido y crea la base para sensores de luz extremadamente sensibles. Los puntos cuánticos coloidales son sensibizadores ideales ya que su procesado en disolución y su fácil incorporación sobre cualquier sustrato permiten la fabricación de sistemas híbridos DCMT-QD a bajo coste. Además, la posibilidad de modifi-car la banda prohibida, ofrecida por los QDs, proporciona al fotodetector una amplia respuesta espectral. Para fotodetección en la ventana espectral del infrarrojo cercano (NIR/SWIR), estos detectores híbridos presentan el potencial de competir favorablemente con los sistemas comerciales disponibles. La interfaz entre el híbrido DCMT-QD es de la mayor importancia para la sensibilidad del detector. Se ha demostrado que una alta densidad de trampas en la interfaz es la responsable del ineficiente control mediante el gate de la conductividad del canal, dando lugar a corrientes de oscuridad muy altas. Para mantener la excepcional modulación de efecto campo aún después de la deposición de los QDs, se ha desarrollado una ruta de pasivación de la interfaz con óxidos metálicos semiconductores. El material de esta capa amortiguadora (buffer) es seleccionado de tal manera que permita la transferencia de cargas desde los puntos cuánticos hasta el canal DCMT. Esto retiene la modulación de efecto campo con una relación encendido/apagado muy alta, permitiendo el funcionamiento del fototransistor con corrientes de oscuridad significativamente menores que las de los híbridos sin pasivar. Así, se presenta un fototransistor híbrido DCMT-QD, con una interfaz cuidadosamente diseñada, que exhibe una detectividad de 10^12-10^13 Jones. En resumen, este trabajo presenta unos prototipos de fotodetectores basados en DCMT 2D encapsulados y en híbridos DCMT-QD. Los fotodetectores basados en DCMT simples presentan potencial para su aplicación en detectores flexibles y semitransparentes, con gran sensibilidad en el visible. Los híbridos DCMT-QD amplían la selectividad espectral al infrarrojo cercano gracias a la absorción variable ofrecida por los puntos cuánticos y alcanzan un muy interesante rendimiento gracias a una mejor interacción luz-materia.Award-winningPostprint (published version

    Compact on-chip optical interconnects on silicon by heterogeneous integration of III-V microsources and detectors

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