I describe mid/far-infrared photodetectors based on frequency upconversion in a near-resonant cascade of interband and intersubband transitions in high optical non-linearity asymmetric quantum well structures. Such structures can yield high detectivity and responsivity in the mid/far-infrared range. Resonant uponversion detectors can be designed for both collinear and perpendicular pump and signal beams. They can be integrated with semiconductor pump lasers to yield compact devices. Single photon counting is also achieved by these detectors. I present specific device designs based on GaAs/AlGaAs and InGaAs/AlInAs heterostructures and calculations of their expected figures of merit. This includes a study of the intersubband nonlinear absorption of asymmetric double quantum wells designed for mid/far-IR range. The dependance of second order nonlinear susceptibility on various parameters of the structure is studied. In particular, different values for barrier and well widths are considered. The nonlinear absorption can be obtained by using perturbative calculation of the linear susceptibility up to second order with density matrix approach. The intersubband linear and nonlinear asymmetric double quantum well can be tuned using two design parameters. One is the width of the barrier between the wells that controls the coupling and the second is the width of the narrow well that controls the asymmetry of the structure. As the barrier width narrows the energy gap at the anticrossing increases. The asymmetry of the two well potentials is essential for sum frequency and difference frequency generation since in a symmetric well hZ31i = 0 due to the same parity of the ground and second excited states so that Â(2) = 0. In our detection scheme using frequency up-conversion we demonstrate that these devices can achieve high detectivity, very low noise and high value for Â(2) hence good efficiency. This can be an important advantage for low signal detection and single photon counting.

Optical sensing for imaging applications has traditionally been enabled by single-crystalline photodetectors. This approach has dramatically curtailed monolithic integration of a variety of optically-sensitive materials onto silicon read-out circuits. The advent of solution-processed optoelectronic materials such as colloidal quantum dots offers the potential of a revolution in optoelectronics. Their solution-processibility enables low-cost monolithic integration with an arbitrary substrate. This dissertation presents the first high-sensitivity solution-processed photodetectors. It does so by leveraging the high degree of control offered by nanoscale materials engineering. Material processing routes are developed to achieve sufficient carrier mobility and sensitization that lead to high photoconductive gain up to 10^3 A/W, observed for the first time in soft materials. A method to remove charge-transport-inhibiting moieties from the nanocrystal surface is developed. Surface treatment procedures are then advanced to prolong the carrier lifetime and thus sensitize the material. The sequence of these processing stages is crucial for the noise performance of the device. Processing conditions that lead to high photoconductive gain and low noise current are then reported to achieve highly sensitive photodetectors with reported D* on the order 10^13 Jones. The spectral tunability offered by colloidal quantum dots enables monolithic multispectral photodetectors. The material challenges, imposed by the behaviour of matter in the nanoscale, are addressed to report sensitive photodetectors in the visible and infrared parts of spectrum. Carrier lifetime determines the temporal response of a photoconductor. The abundance of trap states on the nanocrystal surface and their associated carrier lifetimes mandate careful attention in order to preserve the trap states that yield temporal response acceptable for imaging applications. It is shown for the first time that the temporal response of a quantum dot photoconductor can be tailored by careful control over surface chemistry. Materials species were identified as responsible for particular photocurrent temporal components. These findings are then exploited to isolate and remove surface species responsible for undesirably long time constants. A solution-processed photoconductive detector is reported that exhibits high sensitivity (D* ~10^12 Jones) and temporal response of 25 ms, suitable for imaging applications.

Optical sensing for imaging applications has traditionally been enabled by single-crystalline photodetectors. This approach has dramatically curtailed monolithic integration of a variety of optically-sensitive materials onto silicon read-out circuits. The advent of solution-processed optoelectronic materials such as colloidal quantum dots offers the potential of a revolution in optoelectronics. Their solution-processibility enables low-cost monolithic integration with an arbitrary substrate. This dissertation presents the first high-sensitivity solution-processed photodetectors. It does so by leveraging the high degree of control offered by nanoscale materials engineering. Material processing routes are developed to achieve sufficient carrier mobility and sensitization that lead to high photoconductive gain up to 10^3 A/W, observed for the first time in soft materials. A method to remove charge-transport-inhibiting moieties from the nanocrystal surface is developed. Surface treatment procedures are then advanced to prolong the carrier lifetime and thus sensitize the material. The sequence of these processing stages is crucial for the noise performance of the device. Processing conditions that lead to high photoconductive gain and low noise current are then reported to achieve highly sensitive photodetectors with reported D* on the order 10^13 Jones. The spectral tunability offered by colloidal quantum dots enables monolithic multispectral photodetectors. The material challenges, imposed by the behaviour of matter in the nanoscale, are addressed to report sensitive photodetectors in the visible and infrared parts of spectrum. Carrier lifetime determines the temporal response of a photoconductor. The abundance of trap states on the nanocrystal surface and their associated carrier lifetimes mandate careful attention in order to preserve the trap states that yield temporal response acceptable for imaging applications. It is shown for the first time that the temporal response of a quantum dot photoconductor can be tailored by careful control over surface chemistry. Materials species were identified as responsible for particular photocurrent temporal components. These findings are then exploited to isolate and remove surface species responsible for undesirably long time constants. A solution-processed photoconductive detector is reported that exhibits high sensitivity (D* ~10^12 Jones) and temporal response of 25 ms, suitable for imaging applications.

Photodetectors are essential in optoelectronics as they allow the conversion of optical signals into electrical outputs. Silicon, germanium and III-V semiconductors currently dominate the photodetector market. In this dissertation I exploit the potential of layered materials to demonstrate a class of photodetectors able to challenge existing technological issues. I first demonstrate a fabrication method for high-mobility, chemical-vapour-deposited graphene devices which could help to increase the responsivity in graphene-based photodetectors. I then show three examples of graphene-based Schottky photodetectors working at the telecommunication wavelength $\lambda$=1550nm, two for free-space illumination and one for on-chip applications. These are able to achieve responsivities up to 1A/W with relatively-low operation voltage (-3V), similar to those achieved with germanium. I then target the mid-infrared range ($\lambda\sim$10$\mu$m), where emission from objects at room temperature has a peak. I show graphene-based pyroelectric bolometers with temperature coefficient of resistance up to 900\%/K, two orders of magnitude higher compared to current solutions based on thin oxide membranes. I present flexible photodetectors working in the visible range ($\lambda$=642nm) with gate-tunable graphene/MoS$_2$ heterostructures and show responsivity up to 45A/W, 82\% transparency, and low voltage operation (-1V). The responsivity is two orders of magnitude higher compared to semiconducting flexible membranes. Graphene/MoS$_2$ photodetectors can be bent without loss in performance down to a bending radius of 1.4cm. I finally report on the investigation of superconducting properties of layered materials with the target of realizing ultra-sensitive superconducting photodetectors. Unconventional superconductivity is induced in graphene by proximity with a cuprate superconductor. I used gating to turn semiconducting, few-layer MoS$_2$ into a superconductor, which allowed us to unveil the presence of a multi-valley transport in the superconducting state. Electrical properties of the layered superconductor NbSe$_2$ are then studied. I then used NbSe$_2$ ultrathin flakes to realize superconducting photodetectors at $\lambda$=1550nm, reaching a sensitivity down to few thousand photons.

Bulk heterojunction (BHJ) devices represent a very versatile family of organic cells for both the fields of solar energy conversion and photodetection. Organic photovoltaics (OPV) are an attractive alternative to their silicon-based counterparts because of their potential for low-cost roll-to-roll printing, and their intended application in light-weight mechanically conformable devices and in window-type semi-transparent PV modules. Of all proposed OPV candidates, polymer donor with different absorption range are especially promising when used in conjunction with complementary absorbing acceptor materials, like fullerene derivatives (PCBM), conjugated molecules or polymers, achieving nowadays power conversion efficiencies (PCEs) in the range of 10-13% and being a step closer to practical applications. Among the photodetectors (PD), low band gap polymer blended with PCBM decked out the attention, given their extraordinary range of detection from UV to IR and high detectivity values reached so far, compared to the inorganic devices. Since the research has been focused on the enhancement of those numbers for an effective commercialization of organic cells, the topic of the following thesis has been centered on the synthesis of different polymer structures with diverse absorption ranges, used as donor or acceptor, with emphasis on performance in various BHJ devices either for solar cells and photodetectors. In the first part, two new wide band gap polymers, used as donor material in BHJ devices blended with fullerene and small molecule acceptors, are presented. The PBDT_2FT and PBDTT_2FT have shown nice efficiencies from 7% to 9.8%. The device results are implemented with a morphology study and a specific application in a semi-transparent tandem device, reaching a record PCE of 5.4% for average level of transparency of 48%. In another section two new low band gap polymers (Eopt~ 1.26 eV) named DTP_2FBT and (Eopt~ 1.1 eV) named BDTT_BTQ are presented. While the DTP based one resulted to be an optimal candidate for future tandem solar cell application, the other one has been applied for a competitive PD. At last, a comparative study displaying two new acceptor polymers based on modified Isoindigo motifs named PIID(CO)_2FT and PIID(CO)_BTIA brought some prospective for future investigations on fullerene free OSC.

No description available.

<p>In this project we have used Fourier transform infrared spectroscopy to study the photoresponse of two different types of quantum dot-in-a-well infrared photodetectors (DWELL QDIPs). The basic task was to compare the photoresponse of these two detectors, and to study the influence of external resonant laser pumping on the photoresponse. Series of measurements were done at 77K. In the first measurements we investigated the photoresponse for different applied voltages at 77K. </p><p>In a second run of experiments, we used a 1064 nm infrared semiconductor laser to resonantly </p><p>pump the fundamental transition of the quantum dots. The results show that by using this </p><p>additional illumination the photoresponse was dramatically increased by creating additional </p><p>charge carriers in the quantum dots. This could be used to increase the sensitivity of infrared </p><p>detectors based on QDs.</p>

In this project we have used Fourier transform infrared spectroscopy to study the photoresponse of two different types of quantum dot-in-a-well infrared photodetectors (DWELL QDIPs). The basic task was to compare the photoresponse of these two detectors, and to study the influence of external resonant laser pumping on the photoresponse. Series of measurements were done at 77K. In the first measurements we investigated the photoresponse for different applied voltages at 77K. In a second run of experiments, we used a 1064 nm infrared semiconductor laser to resonantly pump the fundamental transition of the quantum dots. The results show that by using this additional illumination the photoresponse was dramatically increased by creating additional charge carriers in the quantum dots. This could be used to increase the sensitivity of infrared detectors based on QDs.

abstract: The long wavelength infrared region (LWIR) and mid wavelength infrared region (MWIR) are of great interest as detection in this region offers a wide range of real time applications. Optoelectronic devices operating in the LWIR and MWIR region offer potential applications such as; optical gas sensing, free-space optical communications, infrared counter-measures, biomedical and thermal imaging etc. HgCdTe is a prominent narrow bandgap material that operates in the LWIR region. The focus of this research work is to simulate and analyze the characteristics of a Hg1-xCdxTe photodetector. To achieve this, the tool `OPTODET' has been developed, where various device parameters can be varied and the resultant output can be analyzed. By the study of output characteristics in response to various changes in device parameters will allow users to understand the considerations that must be made in order to reach the optimum working point of an infrared detector. The tool which has been developed is a 1-D drift diffusion based simulator which solves the 1-D Poisson equation to determine potentials and utilizes the results of the 1-D electron and hole continuity equations to determine current. Parameters such as absorption co-efficient, quantum efficiency, dark current, noise, Transit time and detectivity can be simulated. All major recombination mechanisms such as SRH, Radiative and Auger recombination have been considered. Effects of band to band tunnelling have also been considered to correctly model the dark current characteristics. / Dissertation/Thesis / M.S. Electrical Engineering 2011

The terahertz (THz) and mid-infrared (MIR) spectral regions have many potential applications in the industrial, biomedical, and military sectors. Yet, a wide portion of this region of the electromagnetic spectrum (particularly the THz range) is still relatively unexplored, due mainly to the absence of suitable sources and photodetectors, related to the lack of practical semiconductor materials with adequately small band gap energies. Intersubband transitions (ISBTs) between quantized energy states in quantum heterostructures provide tunable wavelengths over a broad spectral range including the THz region, by choosing appropriate layer thicknesses and compositions. This work focuses on the development of THz and MIR Quantum Well Infrared Photodetectors (QWIPs) based on ISBTs in GaN/AlGaN and Si/SiGe heterostructures. Due to their large optical phonon energies, GaN materials allow extending the spectral reach of existing far-infrared photodetectors based on GaAs, and may enable higher-temperature operation. In the area of MIR optoelectronic devices, I have focused on developing QWIPs based on ISBTs in Si/SiGe heterostructures in the form of on strain-engineered nanomembranes. Due to their non-polar nature, these materials are free from reststrahlen absorption and ultrafast resonant electron/phonon scattering, unlike traditional III-V semiconductors. Therefore, Si/SiGe quantum wells (QWs) are also promising candidates for high-temperature high-performance ISB device operation (particularly in the THz region), with the additional advantage of direct integration with CMOS technology. In this thesis work, numerical modeling is used to design the active region of the proposed devices, followed by sample fabrication and characterization based on lock-in step-scan Fourier transform infrared spectroscopy. Three specific QWIP devices have been developed. The first is a III-nitride THz QWIP based on a novel double-step QW design in order to alleviate the material limitations provided by the intrinsic electric fields of GaN/AlGaN heterostructures. Next, I have developed a THz GaN/AlGaN QWIP grown on semi-polar (202 ̅1 ̅) GaN, where the detrimental effects of the internal fields are almost completely eliminated. Finally, I have demonstrated a Si/SiGe MIR QWIP based on a novel fabrication approach, where nanomembrane strain engineering is used to address the materials quality issues normally found in SiGe QWs. Promising photodetector performance is obtained in all cases. / 2017-06-21T00:00:00Z