Photoconductive Detectors Research Articles

Optimization of Pixel Size and Electrode Structure for Ge:Ga Terahertz Photoconductive Detectors.

To investigate the effects of the pixel sizes and the electrode structures on the performance of Ge-based terahertz (THz) photoconductive detectors, vertical structure Ge:Ga detectors with different structure parameters were fabricated. The characteristics of the detectors were investigated at 4.2 K, including the spectral response, blackbody response (Rbb), dark current density-voltage characters, and noise equivalent power (NEP). The detector with the pixel radius of 400 μm and the top electrode of the ring structure showed the best performance. The spectral response band of this detector was about 20–180 μm. The Rbb of this detector reached as high as 0.92 A/W, and the NEP reached 5.4 × 10−13 W/ at 0.5 V. Compared with the detector with a pixel radius of 1000 μm and the top electrode of the spot structure, the Rbb increased nearly six times, and the NEP decreased nearly 12 times. This is due to the fact that the optimized parameters increased the equivalent electric field of the detector. This work provides a route for future research into large-scale array Ge-based THz detectors.

(Mg)ZnO Photoconductive Detector Development for Direct-Conversion Hard X-Ray Detection

Wide-bandgap semiconductor-based X-ray detectors provide a number of advantages. As presented in this letter, (Mg)ZnO photoconductive direct-conversion X-ray detectors with various Mg contents were fabricated and demonstrated. The signal-to-noise ratio (SNR) and response time of the devices under 6 MV hard X-ray irradiation were significantly improved after doping with Mg. The Ag-Mg <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.19</sub> Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.81</sub> O-Ag photoconductive detector with optimal Mg contents exhibited a sensitivity of 3.03 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{C}$ </tex-math></inline-formula> /(mGy <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\mathrm {air}}\cdot $ </tex-math></inline-formula> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ), a short response time of approximately 0.2 s (rise time) and 0.3 s (decay time), and a large SNR of 278 with a bias of 30 V under hard X-ray irradiation (100 mGy <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">air</sub> /s). The results indicate great application prospects for wide-bandgap MgZnO in direct-conversion X-ray detectors.

Hot-carrier infrared detection in PbS with ultrafast and highly sensitive responses

Traditional infrared semiconductors with direct narrow bandgaps, such as HgCdTe, InGaAs, and lead salts (PbS, PbSe, and PbTe), have been commercialized for decades in various infrared technologies, such as night vision, military communication, and health monitoring. However, traditional infrared (specifically middle- and long-wave infrared) semiconductors suffer from serious noise generation via thermal excitation and external current bias. Although thermal infrared detectors can operate at room temperature, their response speed is very slow, typically on the order of milliseconds or worse, which limits their applications. Herein, we reinvent a classical lead salt semiconductor (PbS) as a room temperature, high speed, and high-detectivity infrared detector. The detection is operated via the self-driven (no bias voltage necessary) photothermoelectric (PTE) effect with a response time reaching 500 ns (limited by the measurement setup)—three orders of magnitude faster than commercial PTE and photoconductive PbS detectors. Furthermore, the physical principle of hot-carrier-dominated heat energy conversion is proposed to understand the unconventional ultrafast response behavior. Combined with high sensitivity at room temperature (noise equivalent power 0.3 pW Hz−1/2) and broadband detection range (0.4–2.3 μm), this hot carrier makes the traditional commercial semiconductor PbS applicable to a class of infrared detection applications.

Effects of Charge Carrier Trapping on Image Resolution of Multilayer Photoconductive Detectors: Application to Amorphous Selenium X-ray Detectors

The effects of charge carrier trapping on image resolution in multilayer photoconductive imaging detectors was analyzed by developing an analytical model for calculating the modulation transfer function (MTF) of the imaging detectors. The MTF model was developed by considering the charge carrier trapping/recombination in the blocking layer and the distributed trapping in the bulk photoconductor layer of multilayer photoconductive imaging detectors. The relative importance of the bulk and blocking layer trapping, as well as the K-fluorescence reabsorption on the resolution, was also analyzed. The model was applied to amorphous selenium- (a-Se) based multilayer detectors for both chest radiographic and mammographic applications. The theoretical model agreed well with experimental results. The charge carrier trapping/recombination in the blocking layer and K-fluorescence reabsorption mainly controlled the resolution of the a-Se mammographic detectors. In addition to these two effects, the bulk trapping also had a significant effect on the resolution of chest radiographic detectors. The blocking layer thickness had more of a profound effect on the resolution than the amount of trap centers in the blocking layer. The resolution deteriorated drastically with increased blocking layer thickness.

Anomalous Absorption in Arrays of Metallic Nanoparticles: A Powerful Tool for Quantum Dot Optoelectronics.

Periodic arrays of noble metal nanoparticles are emblematic nanostructures in photonics. Their ability to sustain localized surface plasmon resonances has been used throughout the years to demonstrate a variety of passive and active functionalities such as enhanced luminescence in dipolar media and LEDs as well as higher responsivities in photoconductive detectors. Here, we show that additional magnetic resonances, associated with inductive current loops between the nanoparticles and accessible with transverse electric waves, emerge in the limit of dense arrays with subwavelength periods. Moreover, their interplay with the plasmons of the system results in spectrally sharp analogues of electromagnetically induced absorption (EIA). We use these metasurfaces to induce changes and enhancements in the emission, absorption, photoconduction, and polarization properties of active layers of PbS nanocrystals, illustrating the potential of EIA beyond the passive functionalities demonstrated so far in literature.

Mechanistic studies of oxidation and iodization of PbSe thin film sensitization for mid-infrared detection

With the resurgence of uncooled PbSe-based photodetectors and the demand for mid-wave infrared (MWIR) imaging systems, a need exists for a better understanding of the sensitization process used for photoconductive detection. A mechanistic study of the oxidation and iodization process is carried out to improve the understanding of the sensitization process with experimental measurement of film composition, morphology, and crystal structure. Comprehensive material characterization is performed for PbSe thin films processed under different sensitization conditions, and results are combined to construct a physical model of the sensitization process. The relative elemental concentration distribution found in cross-sectional energy-dispersive spectroscopy line-scans is correlated with the crystallographic data collected with x-ray diffraction and Raman measurements, respectively, to build a spatially accurate model. The different sensitization conditions are then correlated to the MWIR photoresponse. This study shows that a targeted PbSeO3 layer thickness of 400 nm formed during the oxidation process is necessary to (1) restrict the top PbI2 layer thickness to less than 200 nm, (2) regulate the iodization process, allowing trace amounts of iodine to diffuse along grain boundaries to recrystallize the PbSe base-layer, and (3) preserve the stoichiometric balance of the underlying PbSe layer. An optimum oxidation process window is identified whereby too thick, as well as too thin, of a PbSeO3 layer, both result in a thick PbI2 top layer and a thin PbSe base-layer. This work also shows that PbSeO3 iodizes to form PbI2 at a rate of ∼100 nm/min at 325 °C while pure PbSe and Se-poor PbSe iodizes nearly 5× faster than PbSeO3. Overall, the oxidation step is necessary for oxygen diffusion into PbSe thin film grain boundaries and to control the iodine diffusion rate. The iodization step is necessary to form a PbI2 surface passivation layer, formation of a newly identified compound within sensitized PbSe (Pb3Se2O6I2), and incorporation of iodine into PbSe thin film grain boundaries. We identify and explain the narrow oxidation process window and iodization conditions necessary to achieve high photoconductivity.

Small-diameter p-type SnS nanowire photodetectors and phototransistors with low-noise and high-performance

P-type nanostructured photodetectors and phototransistors have been widely used in the field of photodetection due to their excellent electrical and optoelectronic characteristics. However, the large dark current of p-type photodetectors will limit the detectivity. Herein, we synthesized small-diameter single-crystalline p-type SnS nanowires (NWs) and then fabricated single SnS NW photodetectors and phototransistors. The device exhibits low noise and low dark current, and its noise current power is as low as 2.4 × 10−28 A2. Under 830 nm illumination and low power density of 0.12 mW cm−2, the photoconductive gain, responsivity and detectivity of the photodetector are as high as 3.9 × 102, 2.6 × 102 A W−1 and 1.8 × 1013 Jones, respectively, at zero gate voltage. The rise and fall time of response are about 9.6 and 14 ms. The experimental results show that the small-diameter p-type SnS NWs have broad application prospects in high-performance and low-power photodetectors with high sensitivity, fast response speed and wide spectrum detection in the future.

Vacuum ultraviolet photoconductive detector based on anatase TiO2 thin film deposited on SiO2 substrate

We report on titanium dioxide (TiO2) thin film deposited on a quartz (SiO2) substrate as an improved photoconductive detector for radiation below 200 nm wavelength (vacuum ultraviolet region, VUV). The crystalline film has an anatase phase and an average crystallite size of 10.6 nm. It has a transmission edge at 322 nm and an optical band gap of 3.18 ± 0.02 eV. Under VUV illumination, the TiO2 photoconductive detector exhibited a photocurrent of 5.3 mA at an applied bias voltage of 100 V. The dark current measured without VUV illumination and at the same bias voltage is 8.8 × 10−6 mA, which is 6 orders of magnitude smaller compared to the photocurrent. The photoconductive VUV detector based on TiO2 thin film provides a means of directly measuring VUV radiation compared to detection via the photoluminescence of a scintillator.

Effect of Substrate and Thickness on the Photoconductivity of Nanoparticle Titanium Dioxide Thin Film Vacuum Ultraviolet Photoconductive Detector.

Vacuum ultraviolet radiation (VUV, from 100 nm to 200 nm wavelength) is indispensable in many applications, but its detection is still challenging. We report the development of a VUV photoconductive detector, based on titanium dioxide (TiO2) nanoparticle thin films. The effect of crystallinity, optical quality, and crystallite size due to film thickness (80 nm, 500 nm, 1000 nm) and type of substrate (silicon Si, quartz SiO2, soda lime glass SLG) was investigated to explore ways of enhancing the photoconductivity of the detector. The TiO2 film deposited on SiO2 substrate with a film thickness of 80 nm exhibited the best photoconductivity, with a photocurrent of 5.35 milli-Amperes and a photosensitivity of 99.99% for a bias voltage of 70 V. The wavelength response of the detector can be adjusted by changing the thickness of the film as the cut-off shifts to a longer wavelength, as the film becomes thicker. The response time of the TiO2 detector is about 5.8 μs and is comparable to the 5.4 μs response time of a diamond UV sensor. The development of the TiO2 nanoparticle thin film detector is expected to contribute to the enhancement of the use of VUV radiation in an increasing number of important technological and scientific applications.

Spray‐Stencil Lithography Enabled Large‐Scale Fabrication of Multispectral Colloidal Quantum‐Dot Infrared Detectors

AbstractInfrared HgTe colloidal quantum dots (CQDs) are attracting great interest due to their synthesis scalability, mechanical flexibility, wide spectrum tunability, and excellent photoelectric properties. However, traditional HgTe CQD infrared detector fabrication methods such as drop casting and spin coating cannot achieve spatial control of patterned CQD films, which limits their sensing range to just single waveband and greatly hinders the realization of pixelated or multispectral CQD detectors. In this paper, an efficient spray‐stencil lithography technique is demonstrated for the scalable fabrication of multichannel HgTe CQD infrared detectors that can respond to different spectral ranges. Uniform and smooth CQD films can be deposited on rigid, flexible, and curved substrates up to 4 inch wafer. Both photoconductive and photovoltaic infrared HgTe CQD detectors are fabricated with peak D* above 1011 Jones across short‐wave and mid‐wave regions and show infrared imaging with noise equivalent temperature difference down to 26.7 mK. Flexible multispectral detectors with six channels are fabricated by the proposed spray‐stencil lithography method, and experimentally demonstrate spectral sensing and chemical analysis capability. It is believed that the proposed fabrication method provides a reliable scheme for the production of high‐performance multispectral CQD infrared detector arrays.

Synthesis of Polyaniline–Cerium Oxide Nanocomposite for Photodetector Application

In this paper nanocomposites materials of Polyaniline (PAni) nano-fiber (NFs) and Cerium oxide (CeO2) nanoparticles (NPs) were prepared by two method; hydrothermal and chemical method respectively. The spin coating method was used to prepare PAni and Pani/CeO2 on Si and glass substrates and then screened with XRD, FE-SEM, UV-Vis and as-prepared thin-film photodetectors. The X-ray diffraction pattern of all the prepared films showed the presence of crystalline nature. It was found that the PAni/CeO2 films have a cubic crystal structure.. FESEM results proved that the PAni film prepared have nanofiber like structure, while the PAni/CeO2 films proved that CeO2 NPs fully caped with PAni nanofiber. The UV-Vis spectra showed peaks of PAni 340nm, 651nm and PAni/CeO2 320nm, 620 nm and in the energy gap it is noticed that the band gap value decreases as the CeO2 ratios increases where the maximum values of energy gap of B-band and Q-band (1.65 – 2.74) eV. The maximum sensitivity values of the photoconductive detectors were observed at PAni/CeO2 (2 vol.%) nanoparticles deposited on n-Si substrate which were approximately (2696.5%, 946.15%, 1402.2%, 1613.9%3837.9%, and 2700%) for wavelengths 360, 465, 510, 595,660 and 965 nm respectively.

Depleted Sb2S3 Thin Film Photoconductive Detectors with Fast Response Speed and High Polarization Sensitivity

AbstractAntimony sulfide (Sb2S3) with high absorption coefficient reveals great potential for photodetectors (PDs) in various fields of military and national economy. A new type of depleted Sb2S3 thin film photoconductive detector by using titanium dioxide (TiO2) interlayer with the structure of Au/(glass/TiO2:)Sb2S3/Au is designed. Compared with normal Au/Sb2S3/Au devices, the depleted Sb2S3 PDs obtain a higher switching ratio of 90, greatly enhancing responsivity and specific detectivity. Owing to the electric field of the depletion region, the response time significantly decreases from 23.2 to 0.6 ms. In addition, the anisotropic Sb2S3 film with [120] dominated horizontal orientation is suitable for polarized light detection. The peak‐to‐valley ratio of the depleted Sb2S3 PD reaches 1.3 at polarized light. The results and methods of this study are expected to expand the application of PDs based on Sb2S3 films.

Far Infrared Laser Detector Based on Multi-Walled Carbon Nanotubes and Blend of (Polyaniline - Polymethyl Methacrylate) Polymers with Methyl Blue Dye for Photoconductive Applications

Infrared photoconductive detectors working in the far-infrared region and room temperature were fabricated. The detectors were fabricated using three types of carbon nanotubes (CNTs); MWCNTs, COOH-MWCNTs, and short-MWCNTs. The carbon nontubes suspension is deposited by dip coating and drop–casting techniques to prepare thin films of CNTs. These films were deposited on porous silicon (PSi) substrates of n-type Si. The I-V characteristics and the figures of merit of the fabricated detectors were measured at a forward bias voltage of 3 and 5 volts as well as at dark and under illumination by IR radiation from a CO2 laser of 10.6 μm wavelengths and power of 2.2 W. The responsivity and figures of merit of the photoconductive detector are improved by coating the MWCNTs films with a thin layer of a blend (polyaniline - polymethyl methacrylate) polymer with methylene blue dye. The coated MWCNTs films showed better performances, so this type of coating can be considered as a surface treatment of the detector film, which highly increased the responsivity and specific detectivity of the fabricated IR laser detector-based MWCNTs. The photocurrent response for the coated films was increased about 25 times than that for uncoated films. The results proved the role of the polymer in the enhancement of the performance of the IR photoconductive detectors. Keywords: Carbon nanotubes, Infrared detector, Polyaniline polymer, Polymethyl methacrylate polymer, Methyl Blue dye.

Investigation on method of reducing surface leakage current of the CdZnTe photoconductive detector with MSM structure

The surface leakage current (SLC) of CdZnTe is an important factor affecting detection performance. In this work, we established a two-dimensional model of metal-semiconductor-metal (MSM) structured CdZnTe with a low-resistance layer on the surface. The composition of SLC and the distribution of the internal electric field are analyzed by simulation. The study found that the change of electrode size will change the contribution of SLC from cathode, anode and side surfaces. With the increase of electrode size, the contribution of SLC to total leakage current decreases gradually, and the internal electric field distribution becomes more uniform. According to the conclusions of simulation, we have proposed a simple and accurate method to estimate the bulk resistivity and SLC of CdZnTe. In addition, combined with a series of experiments, we have also summarized several methods which can reduce the contribution of the SLC in MSM structured CdZnTe detector in order to improve energy resolution.

Optimization of infrared detector HgCdTe in photoconductive‏ ‏in the spectral region 2-6 µm‎ ‎

In this paper a new method in design and modeling of infrared detector HgCdTe in photoconductive mode ‎is presented. In this method after scrutiny mechanical, optical, electrical properties of semiconductor ‎HgCdTe and solving the photoconductive equations, software of equations is programmed and with initial ‎value and repeat cycle, their changes to key parameters such as thickness, wavelength and amount of ‎impurities are calculated and finally, performance of detector is optimized.‎ Modeling of photoconductive detector has been done in temperature of 300K and 2-6 µm wavelength range‎‎, according to obtained results, optimal specific detectivity is 3×109 cmHz1/2W-1‎‏ ‏in 5.77 µm wavelength.‎

Enhancement of Photoconductive Detector Based on Carbon Nanotubes Decorated with Silver Nanoparticles by Adding Conductive Polymer

In this work, wide band range photo detector operating in UV, Visible and IR was fabricated using carbon nanotubes (MWCNTs, SWCNTs) decorated with silver nanoparticles (Ag NPs). Silicon was used as a substrate to deposited CNTs/Ag NPs by the drop casting technique. Polyamide nylon polymer was used to coat CNTs/Ag NPs to enhance the photo-response of the detector. The electro-exploding wire technology was used to synthesize Ag NPs. Good dispersion of silver NPs achieved by a simple chemistry process on the surface of CNTs. The optical, structure and electrical characteristic of CNTs decorated with Ag NPs were characterized by X-Ray diffraction and Field Emission Scanning Electron Microscopy. X-ray diffraction patterns of Ag NPs exhibited 2θ values (38.1°,44.3°) corresponding to the Ag nanocrystal, while the XRD pattern of MWCNTs and SWCNTs /Ag NPs peaks appeared at 2θ = 26.2° corresponding to the (002) and at 2theta=44° which corresponds with miller indices (100) for CNTs and (200) for Ag NPs. The optical properties measured by UV-Vis. Spectroscopy. Broad and strong surface plasmon resonance (SPR) peak was detected at 420 nm, for Ag NPs. The absorption of CNTs/Ag NPs increased significantly from UV to near IR region (300-1000 nm). Ag NPs decorated CNTs without any impurities, according to field mission scanning electron microscopy examination, with typical particle sizes of (50-80nm) for Ag-NPs, 44nm for MWCNTs/Ag-NPs, and 30nm for SWCNTs/Ag NPs. ֹThe I-V characteristics at forward bias voltage (0.5-10) volt were studied. The figure of merits (responsivity, photocurrent gain, NEP and detectivity) after coating with polymer of the detector were measured in the dark and after illumination with UV LED (365 nm), Tungsten lamp (500-800 nm) and Laser diode (808 nm).

Efficient photoconductive terahertz detection through photon trapping in plasmonic nanocavities

We present a photoconductive terahertz detector that employs a plasmonic nanocavity to offer high-sensitivity and broadband operation when used in a terahertz time-domain spectroscopy system even at very low optical pump power levels. By employing a plasmonic nanocavity, all of the photocarriers are generated within a 100 nm distance from the photoconductor contact electrodes, enabling a short transport time for almost all of the photogenerated carriers. As a result, the photoconductive detector maintains high quantum efficiency and ultrafast operation simultaneously, enabling high-sensitivity and broadband operation at very low optical pump power levels. We utilize a photoconductive detector based on a plasmonic nanocavity optimized for operation at a 770 nm optical wavelength in a terahertz time-domain spectroscopy system and demonstrate a 100 dB signal-to-noise ratio and a 0.1–6 THz noise-equivalent bandwidth at a record-low average optical pump power of 0.1 mW, compared to the state-of-the-art photoconductive terahertz detectors. The extremely low optical power budget of the demonstrated photoconductive detector makes this detector attractive for multi-pixel terahertz imaging systems.

Photoconductive THz Detector Based on New Functional Layers in Multi-Layer Heterostructures

Photoconductive THz Detector Based on New Functional Layers in Multi-Layer Heterostructures

High responsivity graphene-InGaAs near-infrared photodetector realized by hole trapping and its response saturation mechanism.

Graphene is an ideal material for wide spectrum detector owing to its special band structure, but its low light absorption and fast composite of photogenerated carriers lead to a weak response performance. In this paper, we designed a unique photoconductive graphene-InGaAs photodetector. The built-in electric field was formed between graphene and InGaAs, which can prolong the lifetime of photogenerated carriers and improve the response of devices by confining the holes. Compared with graphene-Si structure, a higher built-in electric field and reach to 0.54 eV is formed. It enables the device to achieve a responsivity of 60 AW-1 and a photoconductive gain of 79.4 at 792 nm. In the 1550 nm communication band, the responsivity of the device is also greater than 10 AW-1 and response speed is less than 2 ms. Meanwhile, the saturation phenomenon of light response was also found in this photoconductive graphene heterojunction detector during the experiment, we have explained the phenomenon by the capacitance theory of the built-in electric field, and the maximum optical responsivity of the detector is calculated theoretically, which is in good agreement with the measurement result.

Highly efficient terahertz photoconductive metasurface detectors operating at microwatt-level gate powers.

Despite their wide use in terahertz (THz) research and technology, the application spectra of photoconductive antenna (PCA) THz detectors are severely limited due to the relatively high optical gating power requirement. This originates from poor conversion efficiency of optical gate beam photons to photocurrent in materials with sub-picosecond carrier lifetimes. Here we show that using an ultra-thin (160 nm), perfectly absorbing low-temperature grown GaAs metasurface as the photoconductive channel drastically improves the efficiency of THz PCA detectors. This is achieved through perfect absorption of the gate beam in a significantly reduced photoconductive volume, enabled by the metasurface. This Letter demonstrates that sensitive THz PCA detection is possible using optical gate powers as low as 5 µW-three orders of magnitude lower than gating powers used for conventional PCA detectors. We show that significantly higher optical gate powers are not necessary for optimal operation, as they do not improve the sensitivity to the THz field. This class of efficient PCA THz detectors opens doors for THz applications with low gate power requirements.