Photoconductive Gain Research Articles

Upconversion under Photon Trapping in ZnO/BN Nanoarray: An Ultrahigh Responsivity Solar-Blind Photodetecting Paper.

Solar-blind photodetectors (PDs) are widely applicable in special, military, medical, environmental, and commercial fields. However, high performance and flexible PD for deep ultraviolet (UV) range is still a challenge. Here, it is demonstrated that an upconversion of photon absorption beyond the energy bandgap is achieved in the ZnO nanoarray/h-BN heterostructure, which enables the ultrahigh responsivity of a solar-blind photodetecting paper. The direct growth of ultralong ZnO nanoarray on polycrystalline copper paper induced by h-BN 2D interlayer is obtained. Meanwhile, strong photon trapping takes place within the ZnO nanoarray forest through the cyclic state transition of surface oxygen ions, resulting in an extremely high absorption efficiency (> 99.5%). A flexible photodetecting paper is fabricated for switchable detections between near UV and deep UV signals by critical external bias. The device shows robust reliability, ultrahigh responsivity up to 700 A W-1 @ 265-276nm, and high photoconductive gain of ≈2 × 103 . A negative differential resistance effect is revealed for driving the rapid transfer of up-converted electrons between adjacent energy valleys (Γ to A) above the critical bias (3.9V). The discovered rationale and device structure are expected to bring high-efficiency deep UV detecting and future wearable applications.

Ultrafast carrier dynamics in terahertz photoconductors and photomixers: beyond short-carrier-lifetime semiconductors

AbstractEfficient terahertz generation and detection are a key prerequisite for high performance terahertz systems. Major advancements in realizing efficient terahertz emitters and detectors were enabled through photonics-driven semiconductor devices, thanks to the extremely wide bandwidth available at optical frequencies. Through the efficient generation and ultrafast transport of charge carriers within a photo-absorbing semiconductor material, terahertz frequency components are created from the mixing products of the optical frequency components that drive the terahertz device – a process usually referred to as photomixing. The created terahertz frequency components, which are in the physical form of oscillating carrier concentrations, can feed a terahertz antenna and get radiated in case of a terahertz emitter, or mix with an incoming terahertz wave to down-convert to DC or to a low frequency photocurrent in case of a terahertz detector. Realizing terahertz photoconductors typically relies on short-carrier-lifetime semiconductors as the photo-absorbing material, where photocarriers are quickly trapped within one picosecond or less after generation, leading to ultrafast carrier dynamics that facilitates high-frequency device operation. However, while enabling broadband operation, a sub-picosecond lifetime of the photocarriers results in a substantial loss of photoconductive gain and optical responsivity. In addition, growth of short-carrier-lifetime semiconductors in many cases relies on the use of rare elements and non-standard processes with limited accessibility. Therefore, there is a strong motivation to explore and develop alternative techniques for realizing terahertz photomixers that do not rely on these defect-introduced short-carrier-lifetime semiconductors. This review will provide an overview of several promising approaches to realize terahertz emitters and detectors without short-carrier-lifetime semiconductors. These novel approaches utilize p-i-n diode junctions, plasmonic nanostructures, ultrafast spintronics, and low-dimensional materials to offer ultrafast carrier response. These innovative directions have great potentials for extending the applicability and accessibility of the terahertz spectrum for a wide range of applications.

Advances in Self-Powered Ultraviolet Photodetectors Based on P-N Heterojunction Low-Dimensional Nanostructures.

Self-powered ultraviolet (UV) photodetectors have attracted considerable attention in recent years because of their vast applications in the military and civil fields. Among them, self-powered UV photodetectors based on p-n heterojunction low-dimensional nanostructures are a very attractive research field due to combining the advantages of low-dimensional semiconductor nanostructures (such as large specific surface area, excellent carrier transmission channel, and larger photoconductive gain) with the feature of working independently without an external power source. In this review, a selection of recent developments focused on improving the performance of self-powered UV photodetectors based on p-n heterojunction low-dimensional nanostructures from different aspects are summarized. It is expected that more novel, dexterous, and intelligent photodetectors will be developed as soon as possible on the basis of these works.

High-sensitivity transparent photoconductors in voltage-controlled silicon waveguides.

On-chip optical power monitors are essential elements to calibrate, stabilize, and reconfigure photonic integrated circuits. Many applications require in-line waveguide detectors, where a trade-off has to be found between large sensitivity and high transparency to the guided light. In this work, we demonstrate a transparent photoconductor integrated on standard low-doped silicon-on-insulator waveguides that reaches a photoconductive gain of more than 106 and an in-line sensitivity as high as -60 dBm. This performance is achieved by compensating the effect of electric charges in the cladding oxide through a bias voltage applied to the chip substrate or locally through a gate electrode on top of the waveguide, allowing one to tune on demand the conductivity of the core to the optimum level.

Sensitive direct x-ray detectors based on the In–Ga–Zn–O/perovskite heterojunction phototransistor

Direct x-ray detectors are essential in many applications including medical tomography, security inspection, nondestructive testing, crystallography and astronomy. Despite the rapid advances in recent years, the currently available direct x-ray detectors are still limited by the insufficient photon-to-charge conversion, compromising the detection sensitivity, ease of fabrication, cost and flexibility. Here we demonstrate a device concept of heterojunction phototransistor with high internal-gain effect to realize the sensitive x-ray direct detection. Specifically, the heterojunction phototransistors are mainly composed of an industrially available In–Ga–Zn–O channel and all-inorganic perovskite nanocrystals used as x-ray photoconductor. In contrast to the conventional diode-based x-ray detectors, phototransistor allows both electrical gating and photodoping effect for efficient carrier density modulation, leading to the low dark-current and high photoconductive gain. The introduction of such high-gain mechanism into x-ray detectors can offer internal signal amplification for photogenerated currents without the increment of noise, thereby leading to the high sensitivity over 106 μC Gyair −1 cm−2 and detection limit down to 3 μGyair s−1. These results suggest that the heterojunction x-ray phototransistor can provide the most promising platform to achieve high-performance direct x-ray detectors with both high sensitivity, light weight, flexibility and low cost.

MoSe2/n‐GaN Heterojunction Induced High Photoconductive Gain for Low‐Noise Broadband Photodetection from Ultraviolet to Near‐Infrared Wavelengths

AbstractHeterojunction photodiodes comprising of layered metal chalcogenides and wide‐bandgap semiconductors are a promising candidate for broadband photodetection. In this work, nanolayered Molybdenum diselenide (MoSe2)/Gallium Nitride (GaN) based photodetector has been demonstrated from ultraviolet to near‐infrared range (300–1000 nm). The performance is investigated by conducting Kelvin probe force microscopy to measure the conduction band offset at the interface of the heterojunction. It is used to analyze the energy‐band diagrams to understand the current transport mechanism. The device exhibits a high photoconductive gain of 1.8 × 104, responsivity of 5580 A W−1, detectivity of 1.9 × 1011 Jones and low noise equivalent power of 10 fW Hz−1/2 at 365 nm. Additionally, finite difference time domain simulations are performed for the MoSe2/n‐GaN heterostructure to authenticate the broadband photodetection spectrum. Therefore, the outcomes of this study validate the suitability of MoSe2/n‐GaN heterojunction devices for high responsivity and detectivity applications.

Innovative all-silicon based a-SiNx:O/c-Si heterostructure solar-blind photodetector with both high responsivity and fast response speed

The photoresponsivity and response speed are two key figures of merit for the photodetector (PD). According to the previous reports, there is an inherent contradiction between high photoresponsivity and fast response speed in normal photoconductive-type PDs. Facing the challenge of coordinating this inherent contradiction, we propose an innovative design idea, which employs a luminescent wide-bandgap (WBG) amorphous oxynitride (a-SiNx:O) film as an absorption layer combining with monocrystalline silicon (c-Si) as a carrier transport layer, to construct an all-silicon based a-SiNx:O/c-Si heterostructure photoconductive-type solar-blind photodetector (SBPD). Benefiting from the built-in electric field in the a-SiNx:O/c-Si heterojunction and good passivation at the SiNx:O/Si interface, the photogenerated carriers in the a-SiNx:O layer can be injected into the c-Si layer, which separates the carrier transport process from the carrier photogeneration/recombination process in the different layers. Since the transport process of injected carriers in the c-Si layer is much faster than their recombination process, the detector yields a large photoconductive gain, thus overcoming the above-mentioned inherent contradiction in normal photoconductive-type PDs, where both the defect-related carrier photogeneration/recombination process and carrier transport process occur in the same active layer. The designed SBPDs exhibit highlighted performance with both the high responsivity (R) of 4 × 103 A/W at 225 nm and the fast response speed of 4.3 µs. Compared to most other WBG semiconductor SBPDs, e.g., AlxGa1−xN, MgxZn1−xO, Ga2O3, and diamond, the advantages of the a-SiNx:O/c-Si heterostructure SBPD lie not only in adopting economic Si-based materials but also in manufacturing processes compatible with mature CMOS technology, thereby rendering it preferable for the development of cost-effective large-area SBPD arrays.

Enhanced UV photosensing properties by field-induced polarization in ZnO-modified (Bi0.93Gd0.07)FeO3 ceramics

Self-powered photodetection was studied using x wt% ZnO-modified (Bi0.93Gd0.07)FeO3 (abbreviated as B7GFO-xZn) ferroelectric ceramics. The ITO/B7GFO-xZn ceramic/Au photovoltaic (PV) cell was constructed for photosensing study at ultraviolet-A (λ = 360 nm) and near-ultraviolet (λ = 405 nm). The +2kV/cm poled PV cell using B7GFO-1 wt%Zn ceramic under 360-nm irradiation displays maximal photocurrent density of ~364 μA/cm2 at 102 mW/cm2. Furthermore, a remarkable photosensing performance was observed with photoresponsivity (R) of ~3.02×10−2A/W, specific detectivity (D*) of ~2.27×1012 Jones, and photoconductive gain (G) of ~10.4%. A sensitive photosensing response time (rise time, τr) of ~9 ms was acquired under illumination from the 360-nm laser at 102 mW/cm2. The enhanced photodetection performance originated from the collective influence of the narrower bandgap, enhanced p-n junction effect, E-field-modulated energy band tilt, and improved photocurrent generation due to the local conductive pathways formed by the interconnected domain walls and grain boundaries. This work provides an extensive analysis to elucidate the photovoltaic mechanisms in BiFeO3-based ceramics and explores their potential in self-powered UV photosensing.

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.

Solar-blind UV detection by ultra-wide-bandgap 4HCB organic single crystal semiconductor

In this work, the solar-blind ultraviolet (UV) detection performance of organic single crystals 4-hydroxycyanobenzene (4HCB) is demonstrated. The ultra-wide bandgap and low dark current make 4HCB an important candidate for this application. Detectors with two electrode configurations, i.e., sandwiched electrode (SWE) and interdigital electrode (IDE), are fabricated based on 4HCB single crystals and measured under the illumination of 254 nm-UV light. Apparently, the IDE detector exhibits a responsivity R of 14 000 μA W−1 at a bias voltage of 1000 V, which is 2000 times higher than that of the SWE detector, due to its enhanced photoconductive gain by the surface layer edge states. To explore the possibility for the space UV detection applications in the radiation environment, the effect of neutron radiation on 4HCB detector performance is revealed. The point defects introduced by fast neutrons, mainly H vacancies, dominate the variation of the Fermi energy level and electric properties; however, this effect on photodetection is limited when the neutron flux is below 1013 n cm−2.

Surface-dominant transport properties in MoSe2 nanosheets

The electronic transport and photoconduction properties of the molybdenum diselenide (MoSe2) nanosheets were investigated. The transfer length method analysis suggests the electric current following an anomalously two-dimensional (2D) flow rather than the general 3D mode. The 2D current implies a higher electron concentration and downward band bending at the surface. A broad spectral photoresponse from ultraviolet to near-infrared wavelengths was also observed for the MoSe2 nanosheets. The optimal responsivity and photoconductive gain respectively at 58 A/W and 179 under the wavelengths at 405 nm were achieved. A surface-controlled photoconduction mechanism based on the surface band bending and electron accumulation is proposed to explain the long carrier lifetime and its dependence of light intensity and environment in MoSe2 nanostructures.

Gallium oxide thin film-based deep ultraviolet photodetector array with large photoconductive gain

Gallium oxide (Ga<sub>2</sub>O<sub>3</sub>) has the natural advantages in deep ultraviolet absorbance for performing deep ultraviolet photodetection. Owing to the vital application of photodetector array in optical imaging, in this work, we introduce a 4×4 Ga<sub>2</sub>O<sub>3</sub>-based photodetector array with five-finger interdigital electrodes, in which the high-quality and uniform Ga<sub>2</sub>O<sub>3</sub> thin film is grown by using metal-organic chemical vapor deposition technique, and the device is fabricated by using the following methods: ultraviolet photolithography, lift-off, and ion beam sputtering . The photodetector cell possesses a responsivity of 2.65×10<sup>3</sup> A/W, a detectivity of 2.76×10<sup>16</sup> Jones, an external quantum efficiency of (1.29×10<sup>6</sup>)%, and a photoconductive gain as high as 12900. The 16-cells in this array show good uniformity. In this work the great application potential of gallium oxide deep ultraviolet detector array is illustrated from the perspective of optoelectronic performance and application prospect.

A Micro Broadband Photodetector Based on Single Wall Carbon Nanotubes-Graphene Heterojunction

A room-temperature broadband (visible to near infrared spectrum) photodetector based on single wall carbon nanotubes (SWCNTs) and graphene heterojunction was fabricated in this work, by drop-casting the SWCNTs suspension onto the graphene field-effect transistor with the buried-gate electrode. A high photoresponsivity of 109 A/W and a fast photoresponse speed of 2.9 ms were obtained within the visible region based on the photogating effect. This high gain mechanism leads to a high photoconductive gain (9.1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> ). In addition, the photodetector exhibits a photoresponsivity of 51.5 A/W at 940 nm, indicating a wide spectrum detection capability. Moreover, a higher photoresponsivity can be obtained by tuning the source-drain and gate bias voltages. This approach provides an easy and feasible way to achieve broadband heterojunction photodetectors with high-performance.

Broad Range (254–302 nm) and High Performance Ga2O3:SnO2 Based Deep UV Photodetector

The harmful UV radiation leaking out of the ozone hole can have a detrimental effect on mother nature. To monitor any UV rays leaking out of the ozone hole requires an electronic device such as deep UV photodetectors. In this context, Sn-doped Ga₂O₃ incorporated with SnO₂ nanostructures has been grown on a c-plane sapphire substrate using low-pressure chemical vapor deposition (LPCVD) followed by the fabrication of metal-semiconductor-metal (MSM) based deep ultraviolet (UV) photodetector (PD) using Pt as electrodes with interdigitated geometry. The PD possesses a low dark current of 21 nA even at 50 V bias with a very high photo-to-dark current ratio of 9 &#x00D7; 10⁴ and exceptionally large responsivity of 1532 and 262 A&#x002F;W under 254 nm and 302 nm UV-illumination respectively. Consequently, an extremely high detectivity of 1.7 &#x00D7; 10¹⁵ Jones and external quantum efficiency of 7.4 &#x00D7; 10⁵&#x0025; has been recorded under 254 nm illumination with a fast fall time of 0.2 sec. The PD works well in UV-B range with high responsivity and is attributed to the long wavelength absorption by the SnO₂ nanostructures accompanied by a charge transfer from SnO₂ to the Ga₂O₃ layer. The high gain has been attributed to the photoconductive gain due to interface trapped charges and self-trapped holes, along with light trapping on the textured Ga₂O₃ surface.

Ultrasensitive and Broad‐Spectrum Photodetectors Based on InSe/ReS2 Heterostructure

AbstractPhotogating effect based on vertical structure of 2D materials allows for the realization of a highly sensitive photodetector. A highly sensitive and broad‐spectrum (365–965 nm) photodetector is reported based on the indium selenide (InSe)/rhenium disulfide (ReS2) vertical heterostructure, where the top layer InSe serves as the photogate to regulate the channel current, enabling a large photoconductive gain of 106. The detectivity of the photodetector can reach 6.51 × 1013 Jones, making this one of the highest values among reported transition metal dichalcogenide photodetectors. The photodetector represents a high responsivity of 1921 A W−1, an ultrahigh external quantum efficiency (EQE) of 6.53 × 105%, and a fast response time of 21.6 ms. The outstanding properties of the InSe/ReS2 heterojunction reveal the promising potential in high‐efficient, ultrasensitive, broadband photodetectors.

FIB-Assisted Fabrication of Single Tellurium Nanotube Based High Performance Photodetector.

Nanoscale tellurium (Te) materials are promising for advanced optoelectronics owing to their outstanding photoelectrical properties. In this work, high-performance optoelectronic nanodevice based on a single tellurium nanotube (NT) was prepared by focused ion beam (FIB)-assisted technique. The individual Te NT photodetector demonstrates a high photoresponsivity of 1.65 × 104 AW−1 and a high photoconductivity gain of 5.0 × 106%, which shows great promise for further optoelectronic device applications.

Hybrid 1D/2D heterostructure with electronic structure engineering toward high-sensitivity and polarization-dependent photodetector

The widespread application of photodetectors has triggered an urgent need for high-sensitivity and polarization-dependent photodetection. In this field, the two-dimensional (2D) tungsten disulfide (WS2) exhibits intriguing optical and electronic properties, making it an attractive photosensitive material for optoelectronic applications. However, the lack of an effective built-in electric field and photoconductive gain mechanism in 2D WS2 impedes its application in high-performance photodetectors. Herein, we propose a hybrid heterostructure photodetector that contains 1D Te and 2D WS2. In this device, 1D Te induces in-plane strain in 2D WS2, which regulates the electronic structures of local WS2 and gives rise to type-II band alignment in the horizontal direction. Moreover, the vertical heterojunction built of 2D WS2 and 1D Te introduces a high photoconductive gain. Benefiting from these two effects, the transfer of photogenerated carriers is optimized, and the proposed photodetector exhibits high sensitivity (photoresponsivity of ~27.7 A W−1, detectivity of 9.5 × 1012 Jones, and short rise/decay time of 19.3/17.6 ms). In addition, anisotropic photodetection characteristics with a dichroic ratio up to 2.1 are achieved. This hybrid 1D/2D heterostructure overcomes the inherent limitations of each material and realizes novel properties, opening up a new avenue towards constructing multifunctional optoelectronic devices.

Gate tunable photoresponse of a two-dimensional p-n junction for high performance broadband photodetector

Optoelectronic devices based on two-dimensional (2D) van der Waals (vdWs) materials and their heterostructures are promising owing to their intriguing properties. However, the achievement of high-performance photodetectors over a wide detection range is still limited. In particular, detection in the near-infrared (NIR) region has not been attained successfully because of the relatively low responsivity to date. Herein, we report an efficient photodetection from visible to NIR ranges using a ReS2/Te vdWs heterostructure. The dependence of the photocurrent on the incident power reveals that the dominant operating mechanism can be switched between the photogating effect and photoconductive effect by applying the back-gate voltage. The ReS2/Te photodetector exhibits remarkable responsivities of 3 × 106 A W‒1 at 458 nm and 1 × 106 A W‒1 at 1062 nm laser irradiations, and an ultrahigh photoconductive gain (up to ∼108) resulting from the strong photogating effect. These outstanding performances demonstrate that the ReS2/Te vdWs heterostructure is suitable for high-performance broadband photodetectors in practical applications.

Robust Visible-Blind Wearable Infrared Sensor Based on IrP2 Nanoparticle-Embedded Few-Layer Graphene and the Effect of Photogating.

It is challenging to realize a visible-blind infrared photodetector as the materials that absorb infrared light also absorb visible light. Here, we report the synthesis of IrP2 nanoparticle-embedded few-layer graphene by one-step solid-state pyrolysis and its application in visible-blind infrared sensing. A linear photodetector device was fabricated by drop casting IrP2 nanoparticle-embedded few-layer graphene onto a flexible PET substrate with two gold electrodes separated by ∼16 μm. The photoconductive gain was found to be as high as ∼145% with response and decay times of ∼0.4 and ∼2.8 s, respectively, under 1550 nm irradiation of 800 mW cm-2. The room-temperature responsivity was ∼1.81 A W-1 at 80 mW cm-2 and ∼0.54 A W-1 at a high incident power of ∼2200 mW cm-2 under a bias of 1 V. Interestingly, the device showed response even in the long-wavelength infrared region, but no response was found under visible light. The embedded IrP2 nanoparticles act as trap centers inducing photogating in the device, and the average trap state energy was estimated to be ∼16.5 ± 1.5 meV from the temperature-dependent photocurrent studies. The device was found to be immune to air exposure and bending, suggestive of use a a wearable sensor.

High-detectivity tin disulfide nanowire photodetectors with manipulation of localized ferroelectric polarization field

Abstract Tin sulfide semiconductor nanowires (NWs) have been widely investigated for photodetection applications because of their good optical and electrical properties. Herein, we synthesized n-type SnS2 NWs and then fabricated SnS2 NW photodetectors with a ferroelectric polymer side-gate. The strong electric field induced by ferroelectric polymer can effectively suppress the dark current and improve the detectivity in SnS2 NW photodetectors. The photodetectors after polarization depletion exhibit a high photoconductive gain of 4.0 × 105 and a high responsivity of 2.1 × 105 A W−1. Compared with devices without polarization depletion, the detectivity of polarization-depleted photodetectors is improved by at least two orders of magnitude, and the highest detectivity is 1.3 × 1016 Jones. Further, the rise and fall time are 56 and 91 ms respectively, which are about tens of times faster than those without polarization depletion. The device also shows a good spectral response from ultraviolet to near-infrared. This study demonstrates that ferroelectric materials can enhance optoelectronic properties of low-dimensional semiconductors for high-performance photodetectors.