Abstract:
Photodetectors are among the most widely used optoelectronic devices in numerous commercial and scientific applications. Today, commercially available photodetectors are typically made from gallium phosphide (GaP), silicon (Si) and indium gallium arsenide (InGaAs) for detection in ultraviolet (UV), visible and near-infrared (IR) regimes of the electromagnetic spectrum, respectively. For mid and far infrared lead sulphide (PbS), lead selenide (PbSe), indium antimonide (InSb), and mercury cadmium telluride (HgCdTe) based photodetectors are used. It is therefore highly desirable to have a single, low cost, multi-spectral range photodetector that covers the optical window created by present technologies and does not require cryogenic temperature for efficient operation. In addition, the current photoconductor technologies have high dark currents, poor detectivity, slow response speed, and are not compatible with flexible platforms. The research work in this thesis is aimed to address the major issues related to current photodetector technologies and to develop low cost multispectral photodetector technology. For this, a study on PbS semiconductor nanocrystals and C60 single crystal fullerites is undertaken to develop hybrid photodetector technology. Both these materials have very attractive properties well suited for photodetector device applications such as broadband size-tunable absorption of PbS nanocrystals and high electron mobility of C60 fullerites. A variety of large area (25 mm2) photoconductor devices are demonstrated employing these materials. Photoconductor devices fabricated display a broadband UV-vis-NIR spectral tunability, exhibit a detectivity ~1010 Jones, a responsivity ~0.35 A/W, a linear dynamic range of 80 dB, a rise time 60 μs and signal detection capability up to ~250 kHz. These figures of merit achieved for the photoconductor devices are competitive with the current state of the art technologies. With the additional processing benefits including simple room temperature device fabrication and providing compatibility with large-area flexible platforms, these devices represent significant advances and make PbS nanocrystals and C60 fullerites promising candidates for advanced photodetector technologies. The thesis also includes preliminary studies on photoconductor devices based on single ZnO nanorods.Keywords:
Ultraviolet
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Semiconductors are the backbone of almost every electrical or optical component, one of them being photodetectors. Photodetectors are used in many applications such as digital cameras or solar panels. They can also be designed to detect the omnipresent infrared radiation, discovered in 1800, which is invisible to human eye. Such infrared photodetectors are commercially used in e.g. night-vision, optical communication, environmental monitoring and surveillance. With the advent of nanotechnology, the component size is shrinking rapidly, thus generating a need for new materials compatible with industrial standards. Nanowires possess all the ideal characteristics such as enhanced resonant absorption, tunable spectral response and possible heterogenous integration. This thesis reports on fundamental studies of different types of nanowire-based infrared photodetectors, ultimately designed for industrial applications. The first two studies focused on the influence of doping profile and segment lengths on the performance of p+-i-n+ InP nanowire array photodetectors. An increase in p+-segment length was found to significantly enhance the photocurrent by shifting the depletion region from the substrate far up into the nanowires. Moreover, it was shown that a low doping at the tip of the nanowires made it possible to tune the detector window with an applied bias. A key advantage of nanowires is the possibility to fabricate quantum heterostructures. Broad near-infrared detection was demonstrated in a subsequent study by incorporating multiple InAsP quantum discs in InP n+-i-n+ nanowire array detectors. In low-light conditions, or in applications requiring large bandwidth, an enhanced photocurrent signal is desirable. The last study of the thesis reports on the realization of a spatially separated InAsP absorption region, optimized at 1.55µm for optical communication applications, combined with an InP multiplication region, all integrated in a single nanowire.Summarized, this thesis demonstrates the great promise held by nanowires for future photodetectors. (Less)
Photocurrent
Quantum Efficiency
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The sensitive detection of photons is of central importance in imaging, environmental sensing, and spectroscopy. Thus, the studies of photocarrier dynamics are indeed to understand the operation of photodetectors which typically include photogeneration of electron-hole pairs, their efficient transport, and collection as an electrical signal. In particular, Si-based photodetectors which are routinely used for imaging applications at visible and near-infrared wavelengths (< 1.1 um) are important because the compatibility of Si photodetectors with mature Si electronics is capable of fabricating fast and stable applications with wider bandwidth than electronic devices. However, some severe disadvantages of silicon such as an inefficient light absorption lower the performance of Si-based photodetetor. In order to overcome these limitations, many people have attempted to assemble nanostructures into photodetectors using their unique properties arising from the finite size.
In fact, the representative figure-of-merit to characterize the photodetector is photoconductive gain as the ratio of lifetime to transit time. From the expression, two factors contributing to the strongly enhanced photosensitivity are recognized
Photoconductivity
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Nanocrystalline material
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Small-molecule organic semiconductor crystals (SMOSCs) combine broadband light absorption (ultraviolet–visible–near infrared) with long exciton diffusion length and high charge carrier mobility. Therefore, they are promising candidates for realizing high-performance photodetectors. Here, after a brief resume of photodetector performance parameters and operation mechanisms, we review the recent advancements in application of SMOSCs as photodetectors, including photoconductors, phototransistors, and photodiodes. More importantly, the SMOSC-based photodetectors are further categorized according to their detection regions that cover a wide range from ultraviolet to near infrared. Finally, challenges and outlooks of SMOSC-based photodetectors are provided.
Ultraviolet
Photodiode
Organic semiconductor
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To solve the typical problems of silicon-based invisible photodetectors and image sensors, we focus on the ultraviolet (UV), infrared (IR) photodetection and their imaging integration system. In recent years, we have achieved the following systematic research outcomes: 1) By proposing a new silicon-graphene synergistic absorption theory and using the silicon-on-insulator (SOI) integrated with graphene structure, we broke the limit of traditional silicon-based UV detection, and fabricated high-speed UV photodetectors and imagers; 2) By proposing a cascade structure combined with plasmon resonant absorption, we fabricated high-performance Silicon-based IR photodetectors and imagers; 3) By integrating large photodetector arrays with signal processing circuits, we established high-performance silicon-based broadband imaging system for potential applications.
Photodetection
Ultraviolet
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The paper-based photodetector has recently captivated a great deal of attention in various opto-electronics applications because of facile, cost effective and green synthesis. Two-dimensional transition metal dichalcogenides materials are promising for photodetection under the broad spectral range. In this work, we have fabricated paper-based device by rubbing the tungsten di-selenide (WSe2) crystals on paper substrate. Low-cost, facile and green synthesis technique was employed to make a high-performance paper-based WSe2 photodetector. Paper-based photodetector was fabricated via non-toxic simply rubbing process of WSe2 nanosheets on low-cost bio-degradable paper. The photodetector shows good responsivity of 72.5 μA W−1 and detectivity at around 2.4 × 107 Jones at very low bias (1.0 V) at wavelength of 780 nm, respectively. Due to good photo-absorption strength, photodetector exhibits excellent photo-response over wide wavelength range from visible to near infrared. This device also shows very good flexibility with a stable photo-response. This device shows a general and reliable study for the design of photodetectors that is eco-friendly and cost-effective. Overall studied results of the fabricated device indicate that they have the ability to be used in large-scale preparation of the device.
Photodetection
Specific detectivity
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Emerging applications in sensing, LIDAR, spectroscopy, and SWIR imaging require photodetectors operating at wavelengths beyond the range of silicon technology and that can be produced cost-effectively for very high-volume consumer and commercial markets. In this paper, Array Photonics, Inc. introduces a new generation of photodetectors designed to meet these requirements. We provide an update on PIN photodetectors and detector arrays based on our dilute nitride material (GaInNAsSb) grown epitaxially on and lattice matched to gallium arsenide (GaAs) substrates. We present our latest photodetector results demonstrating broadband operation from 450 nm to 1450 nm with peak responsivity of over 0.85 A/W at 1300 nm and dark current density of 3×10-5 A/cm2 at -1V bias voltage. These dilute nitride-on-GaAs photodetectors are now commercially available as single elements, 1-D arrays and small 2-D arrays and are referred to as eGaAs™ (extended GaAs) photodetectors. We also briefly discuss ongoing work to migrate from 4" (100 mm) to 6" (150 mm) GaAs wafers and the implementation of larger size 2-D arrays for imaging applications in the automotive and industrial markets.
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On-chip light detection is universally regarded as a key functionality that enables myriad of applications, including optical communications, sensing, health monitoring or object recognition, to name a few. Silicon is widely used in the micro-electronics industry. However, its electronics bandgap precludes the fabrication of high-performance photodetectors that operate at wavelengths longer that 1.1 μm, a spectral range harnessed by optical communication windows of low fiber attenuation and dispersion. Conversely, Germanium, a group-IV semiconductor as Silicon, with a cut-off wavelength of ~1.8 μm, yields efficient light detection at near-infrared wavelengths. Germanium-based photodetectors are mature building blocks in the library of silicon nanophotonic devices, with a low dark-current, a fast response, a high responsivity and low power consumption with an established fabrication flow. In this work, we report on the design, fabrication and operation of waveguide pin photodetectors that advantageously exploit lateral Silicon/Germanium/Silicon heterojunctions. Devices were fabricated on 200 mm silicon-on-insulator substrates using standard micro-electronics production tools and processes. This photodetector architecture takes advantage of the compatibility with contact process steps of silicon modulators, thereby offering substantially reduced fabrication complexity for transmitters and receivers, while providing improved optical characteristics. More specifically, at a lowbias reverse voltage of -1 V, we experimentally achieved dark-currents lower that 10 nA, a device photo-responsivity up to 1.1 A/W, and large 3-dB opto-electrical bandwidths over 50 GHz. In addition, high-speed data rate transmission measurements via eye diagram inspection have been conducted, with pin photodetector operation at the conventional 10 Gbps up to the future 40 Gbps link speeds.
Waveguide
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Colloidal quantum dots (CQDs) are emerging solution processed materials combining low cost, easy deposition on large and flexible substrates, and bandgap tunability. The latter feature, which allows spectral tuning of the absorption profile of the semiconductor, makes these materials particularly attractive for light detection applications. Lead sulfide (PbS) CQDs, in particular, have shown astonishing performance as a light sensitive material operating at visible and infrared (IR) wavelengths. Early studies of PbS CQDs used as a photosensitive resistor (photoconductor) showed an impressive responsivity - exceeding 1000 A/W - and a detectivity (D*) higher then 10^13 Jones. This impressive D* was preserved in the successive development of the first PbS CQD photodiode, showing the possibility to realize fast - f_3db > 1Mhz - and sensitive IR detectors. Currently, the field is moving toward the development of hybrid devices and phototransitors. PbS CQDs have been combined in field effect transistors (FETs) with graphene and MoS2 channels, showing ultra-high gain (exceeding 10^8 electrons/photons) and high D*. Recently a photo-junction FET (photo-JFET) has been reported that breaks the inherent dark current/gain/bandwidth compromise affecting photoconductive light detectors. With this presentation we offer a broad overview on CQD photodetection highlighting the past achievements, the benefits, the challenges and the prospects for the future research on this field.
Presentation (obstetrics)
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Colloidal quantum dots (QDs) have been considered in the last years as a promising material for photodetectors due to size-tunable optical absorption spectra through quantum size effect. Another important advantage is solution processability that facilitates integration on a large variety of substrates, including silicon. This paper presents a hybrid PbS/silicon device prepared from solution-processed PbS quantum dot film on top of a Ti-Au/Si device. The complex architecture of the device, combining a PbS photoconductor, a silicon based metal-semiconductor-metal (MSM) detector and a heterojunction p-PbS QDs/n-Si ensures a very good collection of the photogenerated carriers, a long carrier life time, and consequently high responsivity. The hybrid device exhibits a broad spectral range, from ultraviolet to short-wave infrared with responsivities up to 110 A/W, detectivities up to 10 13 Jones and a good stability in time. The broad spectral response of these photodetectors makes them useful for multispectral applications.
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