Photoconductive Gain Research Articles

Investigation of RF sputtered, n-Bi2Se3 heterojunction on p-Si for enhanced NIR optoelectronic applications

A significant deal of interest has been generated in heterojunctions made of 2D Bi-Chalcogenides and semiconducting materials due to the wide range of unique properties and functionalities that they exhibit. In this work RF magnetron sputtering technique was used to fabricate the high-quality n-Bi 2 Se 3 /p-Si heterojunction device. The structural properties of the grown Bi 2 Se 3 thin films were examined by XRD, SEM, Raman and Energy dispersive spectroscopy. The current-voltage (I–V) characteristic revealed the development of effective p-n junction with a greatest rectification ratio (RR) of 724 and significant figure of merit (FOM) of 1.11. A visible to near-infrared photodetector was successfully constructed using n-Bi 2 Se 3 /p-Si heterostructure exhibiting a strong response to 1100 nm irradiance and demonstrated outstanding photo-responsivity of 43.3 A/W, an excellent detectivity of 2.08 × 10 12 Jones and convincing photoconductive gain of 48.71. The fabricated n-Bi 2 Se 3 /p-Si heterojunction offers inexpensive optical detection, high performance broadband detector and tremendous potential for superior electrical and optoelectronic applications due to its convenient device fabrication and compatibility with silicon technology. • Using RF sputtering technique with high-quality epitaxial film offering low-cost optical detection. • n-Bi 2 Se 3 /p-Si heterojunction device for ultrafast optoelectronic devices for the future prospectus. • Excellent Responsivity (43A/W) and high Detectivity (2.08 × 10 12 ) from visible to near infrared region making it efficient for broadband detector.

Highly responsive photodetection based on bismuth ferrite ceramics: The roles of depolarization field and domain network

This study presents the highly-sensitive, millisecond-response, and self-powered photosensors based on the indium-tin-oxide (ITO)/Bi0.93R0.07FeO3 ceramic/Au heterostructures (R = Nd, Gd) at 360-nm irradiation. Photocurrent density (Jph), photoconductive gain (G), photoresponsivity (R) and specific detectivity (D*) were significantly boosted by an electric-field poling, and can reach 1.4 mA/cm2, 25%, 71 mA/W and 2.4 × 1011 Jones in the ITO/Bi0.93Gd0.07FeO3/Au; 0.17 mA/cm2, 14.5%, 42 mA/W and 2.6 × 1012 Jones in the ITO/Bi0.93Nd0.07FeO3/Au. Fast response times of 0.6 ms and 1.0 ms are respectively detected in the ITO/Bi0.93Gd0.07FeO3/Au and ITO/Bi0.93Nd0.07FeO3/Au. The enhanced photodetection is driven collectively by the p-n heterojunction, the depolarization field, and the electric-field-induced domain-wall nucleation. The complex network consisting of grain boundaries and nanoscale domain walls provides electrically conductive passages in the grain matrix. This work delivers an energy self-sufficient route using the polycrystalline rare-earth-doped BiFeO3 ceramics for the high-performance photodetection in the ultraviolet-A spectrum.

Trap States Ruling Photoconductive Gain in Tissue‐Equivalent, Printed Organic X‐Ray Detectors

AbstractOrganic semiconductors are excellent candidates for X‐ray detectors that can adapt to new applications, with unique properties including mechanical flexibility and the ability to cover large surfaces. Their chemical composition, primarily carbon and hydrogen, makes them human tissue equivalent in terms of radiation absorption. This is a highly desirable property for a radiation dosimeter to be employed in medical diagnostics and therapy, however a low‐Z composition limits the absorption of ionizing radiation. The detection efficiency can be enhanced by considering the photoconductive gain (PG) effect, a significant contributor to the ionizing radiation detection mechanism in this class of materials. In this work, a process of controlled solution deposition by nozzle printing and crystallization of an organic semiconductor thin film is demonstrated whereby a flexible, arrayed thin‐film X‐ray detector with record X‐ray sensitivities among flexible radiation detectors (S = (9.0 ± 0.4) × 107 µC Gy−1 cm−3) is developed. The excitonic peaks responsible for the activation of the PG effect are investigated and identified using a novel technique called photocurrent spectroscopy optical quenching, and the analysis of the changes in trap states is further demonstrated.

Plasmonic Near-Infrared Photoconductor Based on Hot Hole Collection in the Metal-Semiconductor-Metal Junction.

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.

Al nanoparticles decorated Er:TiO2 thin film based plasmonic photodetector

Aluminum (Al) nanoparticles (NPs) patterned Erbium-doped Titanium dioxide (Er:TiO2) thin films (TFs) were synthesized by the combination of sol-gel and glancing angle deposition (GLAD) technique inside the thermal evaporation system. Effects of Al NPs on electrical and optical properties of the Er:TiO2 TF were experimented and systematically analyzed. Size of NPs was estimated from the field emission scanning electron microscopy (FESEM) image. Diameter of the maximum number of Al NPs was determined to be ∼13 nm. The presence of Al was confirmed from the energy dispersive X-ray (EDX) spectra. The optical absorption at around 315 nm revealed the plasmonic resonance of Al NPs. The Au/Er:TiO2/Al NPs/Er:TiO2/p-Si Schottky contact-based plasmonic photodetector (PD) was fabricated, which showed excellent photosensitivity under both forward and reverse bias conditions. The responsivity was calculated at an applied voltage of 1 V, which exhibited a high response in the UV region with peak responsivity at 330 nm and 380 nm. The device showed fast switching behaviors under the illumination of 350 nm with a rise time of ∼0.55 s and fall time of ∼0.13 s. A simple theoretical approach was adopted to evaluate some important parameters of the plasmonic device like photoconductive gain, transit time, and mobility of electrons. Detectivity (1.13 × 1012 Jones) and noise equivalent power (NEP) (2.46 × 10−14 W) at 330 nm indicate the suitability of the device as a sensitive UV PD. Hence, the embedded plasmonic Al NPs on Er:TiO2 TF-based devices can commercially emerge as an efficient and cost-effective UV photodetector.

Heterojunction structure for suppressing dark current toward high-performance ZnO microrod metal-semiconductor-metal ultraviolet photodetectors

ZnO microrod metal-semiconductor-metal (MSM) photoconductive detectors have generally shown a high photoconductive gain but with a low detectivity owing to the high dark current. Fortunately, the dark current can be suppressed in reverse-biased heterojunctions. In this work, we have reported the fabrication of high-performance MSM ultraviolet photodetectors based on multilayer graphene oxide/ZnO microrod (MLGO/ZnO) heterojunctions. Structural, optical, and electrical properties studies have suggested that the ZnO microrods possessed a high crystalline quality but with a high concentration of defects, which led to a high dark current and low detectivity in the reference ZnO MSM photodetectors. Notably, the dark current has been substantially suppressed by more than 5 orders of magnitude in the MLGO/ZnO heterojunction MSM photodetectors owing to a high potential barrier formed at the MLGO/ZnO heterojunction interface. Finally, the responsivity and detectivity as high as 210 A/W and 3.0 × 1014 Jones, respectively, have been achieved, which are higher than most of the state-of-the-art devices. The results reported in this paper provide a simple and cost-effective approach for the fabrication of high-performance MSM photoconductive detectors.

25.2: High Detectivity Perovskite‐IGZO Phototransistor with Tunable Photoconductive Gain

Organic‐inorganic hybrid perovskite is a highly light sensitive photodetector material(1), which has competitive advantages of low cost, wide and adjustable light absorption range. However, due to its solubility in polar solvent, its fabrication is not compatible with conventional photolithography process, which limits its application as pixel element in high‐resolution image sensors. Therefore, the patterning of perovskite turns out to be a key issue for practical application of perovskite photodetectors in the field of image sensor(2). In this study, we report a Ruddlesden‐Popper (RP) quasi‐two‐dimensional (quasi‐2D) perovskite ‐ indium gallium zinc oxide (IGZO) phototransistor, and their patterning techniques. A patterned quasi‐2D perovskite layer is deposited on the channel region of IGZO thin film transistor (TFT), which enables ultralow off‐state dark currents of 10−11 A and a high responsivity of 103.5 A/W under 400 nm light illumination with tunable photoconductive gain.

Electrically tunable spectral response in vertical nanowire arrays

The semiconductor nanowire (NW) array promises a high photoconductive-gain as well as an enhanced light-absorption in optoelectronic applications. However, to date, the two kinds of advantages are always consuming each other, leading to a low global income. In this work, we show a feasible route to balance the electric gain and the light absorption efficiency. It is accomplished by an inverse injection of photocarriers into NW (from the tip to the bottom of NW or in the opposite direction), which will activate the photoconductive gain in maximal degree. Experimentally, the responsivity reaches up to ∼200 A/W. The spectral response of the GaAs nanowire-array detector is proven to be bias-voltage controlled, allowing it to work at visible or shortwave-infrared enhanced modes. Also, the photoresponse carries on the wavelength information of the incident light, thus, can be used to discriminate monochromatic lights from each other. Together, these findings depict a full image of the photoresponse process in the vertical nanowire array. It might pave a way for the design and fabrication of subwavelength optoelectronic devices.

Role of defects in ultra-high gain in fast planar tin gallium oxide UV-C photodetector by MBE

We report ultra-high responsivity of epitaxial (SnxGa1−x)2O3 (TGO) Schottky UV-C photodetectors and experimentally identified the source of gain as deep-level defects, supported by first principles calculations. Epitaxial TGO films were grown by plasma-assisted molecular beam epitaxy on (−201) oriented n-type β-Ga2O3 substrates. Fabricated vertical Schottky devices exhibited peak responsivities as high as 3.5 ×104 A/W at −5 V applied bias under 250 nm illumination with sharp cutoff shorter than 280 nm and fast rise/fall time in milliseconds order. Hyperspectral imaging cathodoluminescence (CL) spectra were examined to find the mid-bandgap defects, the source of this high gain. Irrespective of different tin mole fractions, the TGO epilayer exhibited extra CL peaks at the green band (∼2.20 eV) not seen in β-Ga2O3 along with enhancement of the blue emission-band (∼2.64 eV) and suppression of the UV emission-band. Based on hybrid functional calculations of the optical emission expected for defects involving Sn in β-Ga2O3, VGa–Sn complexes are proposed as potential defect origins of the observed green and blue emission-bands. Such complexes behave as acceptors that can efficiently trap photogenerated holes and are predicted to be predominantly responsible for the ultra-high photoconductive gain in the Sn-alloyed Ga2O3 devices by means of thermionic emission and electron tunneling. Regenerating the VGa–Sn defect complexes by optimizing the growth techniques, we have demonstrated a planar Schottky UV-C photodetector of the highest peak responsivity.

Electronic global-shutter one-thin-film-transistor active pixel sensor array with a pixel pitch of 50 μm and photoconductive gain greater than 100 for large-area dynamic imaging

In large-area dynamic imaging, an active pixel sensor (APS) is proposed. However, there is a trade-off between signal-to-noise ratio (SNR) and spatial resolution. To resolve this, a 256 × 256 active pixel image sensor array based on a 3-D dual-gate photosensitive thin-film transistor (TFT) is presented in this work, with a pixel pitch of 50 μm, pixel fill factor of 63%, photoconductive gain of 102–104 and spatial resolution of 505 ppi. An electronic global shutter is enabled by dual-gate biasing without additional a shutter TFT. Such an array is capable of dynamic imaging at a frame rate of 34 Hz.

Hetero-radial MgO capped TiO2 nanowire arrays as a deep UV and self-powered photodetector

Nanowires acts as a potential component in photodetectors (PDs) owing to its superior capacity to generate morphology-dependent broad absorption spectrum. In this study, we have demonstrated an efficient self-powered PD based on the radial heterostructure of the MgO capping layer on top of TiO2 nanowires (RHNWs). RHNWs are highly sensitive under deep ultraviolet (UV) illumination when compared to TiO2-MgO axial heterostructure nanowires (AHNWs) PD. The finite-difference time-domain (FDTD) method is used to simulate electric field distribution, absorption spectra and generation rate profile of the structures. The RHNWs device possesses two times lower leakage current (∼ 1.78 × 10−9 A) than AHNWs (∼ 3.84 ×10−9 A) at – 1.5 V. Theoretical analysis shows a higher photoconductive gain for RHNWs (∼ 12) than the corresponding AHNWs device (∼ 9.9). Maximum photoresponsivity of ∼ 18 A/W has been observed for RHNWs with a sharp rise time (Tr) ∼ 0.12 s and fall time (Tf) ∼ 0.13 s at 200 nm (– 9 V) and the device also behaves like a self-powered PD under white light illumination.

Improved photoconductive gain and high responsivity in LT-GaAs on UHV annealing without arsenic overpressure

The study reports peculiar light absorption in LT-GaAs due to the prominence of arsenic vacancies (VAs), interstitials (Asi), and neutral arsenic anti-sites (AsGa0) after UHV annealing under no Arsenic vapour pressure. We compared unannealed and annealed samples’ transient absorption behaviour and vibrational modes through ultrafast transient absorption spectroscopy (UFTS) (over 2.75 eV–0.775 eV spectral range) and RAMAN spectroscopy, respectively. We found that the UHV heating of LT-GaAs has augmented the tensile strain in the film by incorporating split Asi. However, there is overall defect mitigation, as evident from the reduced TO phonon peak (RAMAN) and ∼59 meV reduction in Fermi level (UFTS). Even with reduced defects, the UFTS data for the annealed sample shows a higher switching speed and responsivity (1.75 ps faster) in the NIR range and a higher photoconductive gain in the visible region.

Fabrication of nanocrystalline Ga2O3-NiO heterojunctions for large-area low-dose X-ray imaging

The Ga2O3 photoconductor possesses a wide bandgap, a relatively large density and a high absorption coefficient for low-energy X-ray. Therefore, it is expected to realize a highly sensitive low-energy X-ray detector. Realizing p-n heterojunctions using Ga2O3 thin film is essential for large-area low-dose X-ray imaging. Here, a p-n diode consisting of nanocrystalline Ga2O3 thin film and NiO thin film is successfully prepared by electron beam evaporation, exhibiting a large current rectification ratio of 6.0 × 104. The p-n heterojunction shows a large valence band offset of 1.7 eV, a large conduction band offset of 0.84 eV and a built-in potential of 0.2 eV near the interfacial region, benefiting for separation and migration of photogenerated carriers. In addition, the photoconductive gain mechanism of Ga2O3/NiO heterojunction based X-ray detectors with optimized NiO thickness is investigated, achieving a high internal gain (3.3 × 103), a high sensitivity (3.4 × 104 μCGyair−1 cm−2), a low detection limit (42 μGyair s−1) and a short response time (2.2 ms). The optimized detectors with 10 × 10 pixels show a uniform X-ray response, demonstrating their promising use in large-area X-ray imaging applications.

Unraveling the Mechanism of the Persistent Photoconductivity in InSe and its Doped Counterparts

AbstractDopant levels in layered compound InSe have considerable potential in optoelectronic devices. Dopant‐induced trap states are essential in determining the optoelectrical properties of semiconductors. However, detailed studies of the persistent photoconductivity (PPC) and related mechanism in doped InSe are still not available. Here, the dependence of excitation energy on the shallow donor level caused by the dopants (Ge, Sn) in InSe is systematically investigated. Notably, prolonged decay time originates from extrinsic Ge, Sn dopants and these doping‐assisted states improve the optoelectrical performance of pristine InSe. Those photogenerated carriers are trapped in the Ge, Sn shallow impurities states, which are long‐lived enough to be extracted into Au contacts before annihilation. This renders Ge‐, Sn‐doped InSe photoconductive gain and maximized photocurrent. Sn‐doped InSe single crystal device can achieve a maximum responsivity of around 1.7 × 106 A W−1 under red light and detectivity of 6.18 × 1013 Jones. In addition, Hall measurements identify the carrier concentration and the Hall mobility of pristine InSe is significantly changed by Ge and Sn dopants. It is demonstrated that doping Ge, Sn atoms is responsible for the obvious photoconductivity and beneficial for the high‐performance photodetector, offering intriguing opportunities for novel holographic memory applications.

Printed GaAs Microstructures‐Based Flexible High‐Performance Broadband Photodetectors

AbstractNano/microstructures of compound semiconductors such as gallium arsenide (GaAs) demonstrate enormous potential for advanced photonic technologies as they provide realistic means for miniaturization of optoelectronic devices that feature better performance and low power consumption. However, intimately integrating them onto flexible substrates is challenging and restricts their use in the next generation of applications such as wearables and soft robotics. Herein, printed arrays of well‐defined and laterally aligned semi‐insulating (undoped) and doped GaAs microstructures are presented to develop high‐performance flexible broadband photodetectors. The direct roll transfer printed GaAs microstructures‐based photodetectors exhibit excellent performance under ultraviolet and near‐infrared illumination, including ultrafast response (2.5 ms) and recovery (8 ms) times, high responsivity (>104 AW–1), detectivity (>1014 Jones), external quantum efficiency (>106), and photoconductive gain (>104) at low operating voltage of 1 V. The achieved performance is among the best reported for broadband photodetectors but with an added benefit of the developed devices having a flexible form factor. Further, the photodetectors show stable performance under mechanical bending (500 cycles) and twisting loading. The developed materials and manufacturing route can enable high‐speed communications and computation via high‐performance flexible electronics and optoelectronics and transform numerous emerging applications such as wearable systems and internet of things.

Robust Deposition of Sub‐Millimeter WSe2 Drive Ultrasensitive Gate‐Tunable 2D Material Photodetectors

Abstract2D semiconductors are regarded as the potential channel materials for high‐performance integrated optoelectronics. However, the absence of an effective photoconductive gain mechanism and the adverse effects from traditional SiO2 substrate prevent the further breakthrough of the photosensitivity of 2D semiconductors‐based photodetectors. Here, an ingenious out‐of‐plane heterostructure is proposed for boosting the photosensitivity. Sub‐millimeter scale (>700 µm) tri‐layer WSe2 with high quality and uniformity is robustly deposited and integrated with 2D photosensitive channels for gate‐tunable photodetection. The WSe2 with naturally passivated surface can not only serve as a substrate passivation layer to mitigate the detrimental substrate effects, but also introduces vertical built‐in fields, which efficiently separates the photogenerated carriers and produces high photoconductive gain. Under gate voltage modulation, the proposed InSe photodetector exhibits a series of excellent performances, including responsivity of 112 A W−1, detectivity of 8.6 × 1012 Jones, light on/off ratio of 1.3 × 104 and rise/decay time of 29.1/15.3 ms. More inspiringly, this heterostructure scheme is universally applicable to 2D WS2 photodetector and also significantly improves its photosensitivity, demonstrating broad applicability. This research will provide helpful guidance for the design and optimization of customizable optoelectronic devices based on 2D semiconductors.

Non-Ultrawide Bandgap Semiconductor GaSe Nanobelts for Sensitive Deep Ultraviolet Light Photodetector Application.

In this paper, the authors report the fabrication of a sensitive deep ultraviolet (DUV) photodetector by using an individual GaSe nanobelt with a thickness of 52.1nm, which presents the highest photoresponse at 265nm illumination with a responsivity and photoconductive gain of about 663AW-1 and 3103 at a 3V bias, respectively, comparable to or even better than other reported devices based on conventional wide bandgap semiconductors. According to the simulation, this photoelectric property is associated with the wavelength-dependent absorption coefficient of the GaSe crystal, for which incident light with shorter wavelengths will be absorbed near the surface, while light with longer wavelengths will have a larger penetration depth, leading to a blueshift of the absorption edge with decreasing thickness. Further finite element method (FEM) simulation reveals that the relatively thin GaSe nanobelt exhibits an enhanced transversal standing wave pattern compared to its thicker counterpart at a wavelength of 265nm, leading to an enhanced light-matter interaction and thereby more efficient photocurrent generation. The device can also function as an effective image sensor with acceptable spatial resolution. This work will shed light on the facile fabrication of a high-performance DUV photodetector from non-ultrawide bandgap semiconductors.

Ge–Ge0.92Sn0.08 core–shell single nanowire infrared photodetector with superior characteristics for on-chip optical communication

Recent development on Ge1−xSnx nanowires with high Sn content, beyond its solid solubility limit, makes them attractive for all group-IV Si-integrated infrared photonics at the nanoscale. Herein, we report a chemical vapor deposition-grown high Sn-content Ge–Ge0.92Sn0.08 core–shell based single nanowire photodetector operating at the optical communication wavelength of 1.55 μm. The atomic concentration of Sn in nanowires has been studied using x-ray photoelectron and Raman spectroscopy data. A metal–semiconductor–metal based single nanowire photodetector, fabricated via an electron beam lithography process, exhibits significant room-temperature photoresponse even at zero bias. In addition to the high-crystalline quality and identical shell composition of the nanowire, the efficient collection of photogenerated carriers under an external electric field results in the superior responsivity and photoconductive gain as high as ∼70.8 A/W and ∼57, respectively, at an applied bias of −1.0 V. The extra-ordinary performance of the fabricated photodetector demonstrates the potential of GeSn nanowires for future Si CMOS compatible on-chip optical communication device applications.

Transient analysis of photomultiplication-type organic photodiodes

Photomultiplication-type organic photodetectors have emerged as a class of next generation solution-processed photodetectors with high gain. Despite this promising feature, the reported photodectors still suffer from relatively large dark currents at high bias voltages. To overcome this drawback, a mechanistic understanding of the photomultiplication effect in organic photodiodes is required. In this work, we advanced the performance of photomultiplication-type organic photodetectors by tuning the active layer composition and interfacial layers. The optimized devices exhibit small dark currents and flat dark current–voltage curves under the reverse bias condition up to −10 V. The optimized photodetectors also reached an ultra-high responsivity of 23.6 A/W and the specific detectivity of 1.04 × 1012 Jones at −10 V. More importantly, we investigated the photomultiplication process with multiple transient techniques and revealed that the photoconductive gain effect is a slow process, which relies on the photo-Schottky effect enabled by charge carrier tunneling and the accumulation of holes. Furthermore, we also demonstrated prototypical pulsed-light detection based on the optimized devices, which showed great potential for real applications.

High-performance dual-mode ultra-thin broadband CdS/CIGS heterojunction photodetector on steel.

An ultra-thin CdS/CIGS heterojunction photodiode fabricated on steel firstly exhibits dual-mode broadband photodetection from ultraviolet to near infrared spectrum. In the photovoltaic mode, the CIGS photodiode, working as a self-driven photodetector, shows an outstanding photodetection capability (under a light power density of 20 µW cm-2 at 680 nm), reaching a record detectivity of ∼4.4×1012 Jones, a low noise equivalent power (NEP) of 0.16 pW Hz-1/2 and a high Ilight/Idark ratio of ∼103, but a relatively low responsivity of ∼0.39 A W-1 and an external quantum efficiency (EQE) of ∼71%. Working under the same illumination but in the photoconductive mode (1 V reverse bias), the responsivity and EQE are significantly enhanced to 1.24 A W-1 and 226%, respectively, but with a relatively low detectivity of 7×1010 Jones and a higher NEP of 10.1 pW Hz-1/2. To explain these results, a corrected photoconductive gain (G) model indicates that minority electrons could be localized in the defects, surface states and depletion region of the CIGS photodiode, causing excess hole accumulation in the ultra-thin CIGS photodiode and thus high EQE over 100% (G over 1).