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Amorphous selenium (a-Se) is a large-area compatible photoconductor that has received significant attention toward the development of UV and X-ray detectors for a wide range of applications in medical imaging, life science, high-energy physics, and nuclear radiation detection. A subset of applications require detection of photons with spectral coverage from UV to infrared wavelengths. In this work, we present a systematic study utilizing density functional theory simulations and experimental studies to investigate optical and electrical properties of a-Se alloyed with tellurium (Te). We report hole and electron mobilities and conversion efficiencies for a-Se1-xTex (x = 0, 0.03, 0.05, 0.08) devices as a function of applied field, along with band gaps and comparisons to previous studies. For the first time, these values are reported at high electric field (>10 V/μm), demonstrating recovery of quantum efficiency in Se-Te alloys. A comparison to the Onsager model for a-Se demonstrates the strong field dependence in the thermalization length and expands on the role of defect states in device performance.

Lead oxide (PbO) photoconductors are proposed as X-ray-to-charge transducers for the next generation of direct conversion digital X-ray detectors. Optimized PbO-based detectors have potential for utilization in high-energy and dynamic applications of medical X-ray imaging. Two polymorphs of PbO have been considered so far for imaging applications: polycrystalline lead oxide (poly-PbO) and amorphous lead oxide (a-PbO). Here, we provide the comparative analysis of two PbO-based single-pixel X-ray detector prototypes: one prototype employs only a layer of a-PbO as the photoconductor while the other has a combination of a-PbO and poly-PbO, forming a photoconductive bilayer structure of the same overall thickness as in the first prototype. We characterize the performance of these prototypes in terms of electron–hole creation energy (W±) and signal lag—major properties that define a material’s suitability for low-dose real-time imaging. The results demonstrate that both X-ray photoconductive structures have an adequate temporal response suitable for real-time X-ray imaging, combined with high intrinsic sensitivity. These results are discussed in the context of structural and morphological properties of PbO to better understand the preparation–fabrication–property relationships of this material.

The demand for integrating infrared (IR) detectors and imaging systems in consumer products and defense applications is rapidly growing. The requirement for IR uncooled, highly responsive, and spectrally tailored devices are driving a resurgence of interest in lead selenide (PbSe) thin films for detectors and focal plane array (FPA). PbSe has been used for MWIR (3 – 5 µm) detectors for over 70 years, however, the photoconductivity mechanism is still not well understood, and higher device performance is desired. The large Bohr exciton radius of PbSe (~46 nm) facilitates strong quantum confinement properties and spectral tailoring as far down as 690 nm. New thin-film deposition methods and processing approaches are needed to better control and expand PbSe thin film properties and to understand the photoconductivity mechanism. The primary goals of this thesis work are (1) to demonstrate a new method for PbSe thin film fabrication using laser sintering deposition (LSD) of micro/nano particles of PbSe (2) to improve the understanding of the photoconductive mechanism in PbSe thin films through optical, electrical, morphological, and structural characterization (3) to fabricate a photodetector sensor and focal plane array device using the LSD method combined with laser patterning, avoiding some of the difficulties of using photolithography liftoff. The new method introduced in this work is a 3-step process called Laser Sinter Deposition (LSD). The first step produces colloidal nanocrystals (NC) by planetary ball milling high-purity PbSe in a methanol solution. The second step is the rapid centrifugal deposition of the PbSe NCs on silicon substrates. Finally, laser sintering is accomplished by rastering a pulsed or continuous-wave laser (UV, visible, or IR) to sinter and form a continuous polycrystalline thin film with superior substrate adhesion and highly tailorable properties. Laser processing has the advantage of allowing spatially defined sintering, ultra-rapid sintering (millisecond to nanosecond) to activate doping, selectively ablation of material with depth control, reorganizing crystalline structures including defect reduction, and the potential for trapping of non-equilibrium structures. Another advantage of LSD is the ability for selective depth sintering (SDS) by exploiting the absorption depth of different laser wavelengths. This work shows that LSD can be tailored to produce a thin film of PbSe with nearly identical properties to films prepared by Chemical Bath Deposition (CBD), in terms of electrical, compositional, and physical properties. In contrast to CBD, LSD produces superior substrate adhesion, unique morphological control (columnar versus equiaxed grains), and provides spatially tailorable grain modification (lateral and transverse)., In contrast to all other deposition methods, the LSD process also allows in-situ doping of the nanocrystals prior to laser sintering which allows precise concentration of dopant potassium iodide, resulting in a more dense film, an inter-grain coating of PbI2, and a 300% increase in photoresponse compared to films without KI. PbSe thin films were oxidized and iodized (sensitization process) to enhance the photoresponse. We report a process agnostic model of the complex interactions between oxidation and iodization to identify the narrow process window that exists in the sensitization process. In doing this, we identify a previously unreported compound (Pb3Se2O6I2) that exists in the transition layer between the base layer of PbSe and the top layer of PbI2. This compound is only observed in highly photoresponsive films. The inter-grain PbSeO3 coating is confirmed through Transmission Electron Microscopy (TEM) and helps in regulating the diffusion of iodine along grain boundaries which assists to densify the film and enhancing the photoresponse. The presence of iodine decreases dark current, while simultaneously assisting in recrystallizing PbSe during sensitization. One interesting discovery is that p-type PbSe film with a carrier concentration of 4.8 x 1018 cm-3 can be converted to n-type with a carrier concentration of 9.2 x 1018 cm-3 with a simple laser treatment using a 355 nm (UV) wavelength laser by selectively reducing the selenium concentration. The UV treatment also improved carrier mobility by nearly 500%, from 6.5 to 37.5 cm2/V·s. Laser processing also simplifies device fabrication by allowing selective in-situ patterning of PbSe, the metal conductor (Au), and substrate, in one system to eliminate the need for photolithography. Unlike other forms of rastered patterning, such as e-beam lithography, laser patterning is accomplished at raster speeds of 10 meters per second in one step without photoresist or chemicals. This work successfully demonstrates the complete laser fabrication of both discrete 3.5 cm2 photodetectors with a Detectivity of 1.01 x 109 cm·Hz-1/2/W and the capability of laser patterning focal plane array (FPA) with vertical contact pixels as small as 60 x 70 µm at a 100 µm pitch to fabricate a 37 x 68 pixel FPA on a 1 cm x 2 cm substrate in < 30 seconds. This work demonstrates the functional operation of a laser patterned 120 pixel FPA prototype of lateral contact pixels (240 µm x 260 µm). This work successfully demonstrates lasers can be used successfully to fabricate both thin films of a few microns in thickness as well as complex integrated devices such as focal plane arrays (FPAs). In summary, we have successfully demonstrated a new method of thin-film deposition using laser sintering of nanoparticles, provided an enhanced understanding of the photoconductive mechanism in PbSe, and IR devices like photodetectors and focal plane arrays can be fabricated by direct laser patterning.

Harvesting energetic carriers from plasmonic resonance has been a hot topic in the field of photodetection in the last decade. By interfacing a plasmonic metal with a semiconductor, the photoelectric conversion mechanism, based on hot carrier emission, is capable of overcoming the band gap limitation imposed by the band-to-band transition of the semiconductor. To date, most of the existing studies focus on plasmonic structural engineering in a single metal-semiconductor (MS) junction system and their responsivities are still quite low in comparison to conventional semiconductor, material-based photodetection platforms. Herein, we propose a new architecture of metal-semiconductor-metal (MSM) junctions on a silicon platform to achieve efficient hot hole collection at infrared wavelengths with a photoconductance gain mechanism. The coplanar interdigitated MSM electrode’s configuration forms a back-to-back Schottky diode and acts simultaneously as the plasmonic absorber/emitter, relying on the hot-spots enriched on the random Au/Si nanoholes structure. The hot hole-mediated photoelectric response was extended far beyond the cut-off wavelength of the silicon. The proposed MSM device with an interdigitated electrode design yields a very high photoconductive gain, leading to a photocurrent responsivity up to several A/W, which is found to be at least 1000 times higher than that of the existing hot carrier based photodetection strategies.

This short review article provides the reader with a summary of the history of organic conductors. To retain a neutral and objective point of view regarding the history, background, novelty, and details of each research subject within this field, a thousand references have been cited with full titles and arranged in chronological order. Among the research conducted over ~70 years, topics from the last two decades are discussed in more detail than the rest. Unlike other papers in this issue, this review will help readers to understand the origin of each topic within the field of organic conductors and how they have evolved. Due to the advancements achieved over these 70 years, the field is nearing new horizons. As history is often a reflection of the future, this review is expected to show the future directions of this research field.

Organic materials are suitable for light sensing devices showing unique features such as low cost, large area, and flexibility. Moreover, transparent photodetectors are interesting for smart interfaces, windows, and display-integrated electronics. The ease of depositing ultrathin organic films with simple techniques enables low light absorbing active layers, resulting in the realization of transparent devices. Here, we demonstrate a strategy to obtain high efficiency organic photodetectors and phototransistors based on transparent active layers with a visible transmittance higher than 90%. The photoactive layer is composed of two phases, each a few nanometers thick. First, an acceptor polymer, which is a good electron-transporting material, on top of which a small molecule donor material is deposited, forming noncontinuous domains. The small molecule phase acts as a trap for holes, thus inducing a high photoconductive gain, resulting in a high photoresponsivity. The organic transparent detectors proposed here can reach very high external quantum efficiency and responsivity values, which in the case of the phototransistors can be as high as ∼74000% and 340 A W-1at 570 nm respectively, despite an absorber total thickness below 10 nm. Moreover, frequency dependent 2D photocurrent mapping allows discrimination between the contribution of a fast but inefficient and highly spatially localized photoinduced injection mechanism at the electrodes, and the onset of a slower and spatially extended photoconductive process, leading to high responsivity

Deformable mirrors are key optical elements in modern astronomical telescopes and instrumentation both for active and adaptive optical systems. Different technological approaches have been exploited for the realization of the deformable mirrors, especially for adaptive optics devices. A new approach is here presented, namely the photo-controlled deformable mirrors, where the size and density of actuators is set by an illumination pattern projected on the back side of a photoconductor. The working principle and an electric model are presented highlighting the features of the material that affect the performances of the mirror in terms of dynamic range and response time. Based on these results, a prototype exploiting ZnSe as photoconductor is reported together with its characterization.

This work reports on the development and characterization of bi-dimensional deep-UV sensor arrays based on synthetic diamond to address the requirements of space-born astrophysical experiments. The material was synthesized at the University of Rome “Tor Vergata” where both heteroepitaxial polycrystalline diamond films and homoepitaxial single-crystal diamonds are grown using a tubular MWCVD reactor. The quality of chemical vapour deposited diamond was characterized by cathodoluminescence, photoluminescence, Raman spectroscopy and thermally stimulated currents. Then, suitable samples were selected and used to fabricate photoconductive single-pixel and 2D array devices by evaporating metal contacts on the growth surface. The electro-optical characterization of the devices was carried out in a wide spectral region, ranging from 120 to 2400 nm. A deuterium lamp and a 0.5 m vacuum monochromator were used to measure the detector responsivity under continuous monochromatic irradiation in the 120–250 nm spectral range, while an optical parametric oscillator tunable laser producing 5 ns pulses was used as light source from 210 up to 2400 nm. Time response, signal-to-noise ratio, responsivity and visible rejection factor were evaluated and the results are hereafter summarized.

New pin-photodiode and metal–semiconductor–metal (MSM) photoconductor devices based on diamond material have been produced showing high responsivity around 200 nm. LYRA, the Large Yield RAdiometer, will use such detectors for the first time for a solar physics space instrument. A set of measurement campaigns was carried out to obtain their XUV-to-VIS characterization (responsivity, linearity, stability and homogeneity). The diamond pin and MSM photodetectors exhibit a high responsivity of 27 mA/W around 200 nm and demonstrate a visible rejection ratio (200 nm versus 500 nm) of six and four orders of magnitude, respectively. We show that these diamond photodetectors are sensitive sensors for the wavelength range of interest (1 nm to 220 nm), stable within a few percent, with a good linearity and moderate homogeneity.

In this work a novel concept based on low power semiconducting, mid infrared components and new, low cost measurement cells for use in harsh environmental conditions, such as permafrost areas, is presented. Due to the small changes of the environmen tal carbon dioxide (CO 2 ) concentration in this area a high resolution of 5 ppm CO 2 is required. The current sensor design achieves a resolution of up to 6 ppm CO 2 and offers further optimization potential at overall low costs.

The phenomenon of image blurring on laser-printed or electro-photocopied paper has been discovered since the 1980s. In the 1990s, the problem was confirmed to be associated with the undesired surface conduction along the unique photoconductive polymer surface during the photoconduction process. Other than this, little progress has been made in investigating this phenomenon, due to the limited experimental techniques. In this thesis, the electrical properties of a commercially available photoconductor as a result of Corona charging were studied. Various techniques including vacuum deposition and step-function impedance spectroscopy were employed, to overcome the nature of the photoconductor that prevented the use of conventional techniques such as AC impedance spectroscopy. Negative dielectric constant (NDC) has been prevalently discovered at a broad range of frequencies (below 1Hz and up to 1 MHz) and it was questioned in the form of a physically-impossible inductor. This precipitous sign switch of dielectric constant is found in various areas ranging from physics, chemistry, biology to electronics. The magnitude of the NDC decreased drastically with the decrease of electric field frequency. The system obeyed the proposed free-carrier plasma model with a resonance frequency at MHz level. Commercially available polymeric photoconducting materials showing NDC at extremely low frequency are expected to provide unusual scattering to electromagnetic waves and therefore demonstrate profound implications with reduced cost. It has paved the way for many applications such as inductors in integrated chips without bulky coils and provides an insight into a possible revolution in electronics and photonics. Doctor of Philosophy (PhD)

Photoconductivity spectra of unstressed and stressed Ge:Ga detectors were measured. The experiments were performed with a polarizing step scan Fourier transform spectrometer using the synchrotron facility BESSY, which was operated in a dedicated mode with a low momentum compaction factor. By this way powerful and coherent synchrotron radiation below 50 cm -1 was generated. We observed a significant response of unstressed and stressed Ge:Ga detectors below 50 cm -1 and 25 cm -1 , respectively. This response can be attributed to transitions between bound excited states or from bound excited states to the valence band. The results indicate that in germanium detectors a fraction of the recombining holes is captured into bound excited states.

A detailed investigation of diamond-based UV planar photoconductors with interdigitated contacts is presented, analyzing in particular the influence of film morphology and texturing on device performance. Photoluminescence and photocurrent spectroscopies reveal a strong correlation between structural defects and detector selectivity, whereas impurities affect photocurrent yield mainly in highly oriented materials. Responsivity measurements show that diamond-based devices give the highest UV-visible discrimination in comparison to UV-enhanced Si- and GaN-based sensors. For λ ≤225 nm, an almost linear photocurrent-light intensity relationship is observed, although, in a few cases, a superlinear dependence on illumination intensity is found. A theoretical model, capable of describing the observed field dependendence of photocurrent gain is presented, giving minority carriers lifetime and mobility in the range 0.1–1 ns and 10–300 cm2V−1s−1, respectively.

We present a method to control individual high-voltage actuators in an array using microfabricated photoconductive switches and a light projection system, thus overcoming the challenges of on-chip control at high voltages. Such switching capabilities go beyond actuators - our proposed approach may open the door to the simple control of other high-voltage processes, such as MEMS, pressure sensors, and RF devices.

We report on colloidal PbS quantum dot near infrared photoconductors operating at low voltage. In order to maximize the device sensitivity, we exploited the advantages of different measurement techniques and methods for dark current cancellation and noise reduction including the DC volt-amperometric measurement with offset cancellation, a Wheatstone-bridge configuration and impedance measurements. We demonstrate that even photodetectors with moderate detectivity (10 9 -10 10 cmHz 1/2 /W), exhibit very large SNR.