Mechanism Of Photoconductivity Research Articles

Highly photoresponsive, ZnO nanorod-based photodetector for operation in the visible spectral range

While significant advances have been made for gold nanoparticle (AuNP)-coupled zinc oxide (ZnO) as visibly blind, ultraviolet photodetection devices, very few ZnO nanomaterial systems have been developed specifically for use in the visible wavelength regime. Further efforts to develop ZnO-based visible photodetectors (PDs) are still highly warranted in order to better understand the precise effect of AuNP load, operation wavelength, and beam position on the device output. In this study, we demonstrate significantly enhanced, photoresponse behaviors of AuNP-coupled ZnO nanorod (NR) network devices in the visible wavelength range with their photoresponse capacity comparable to, if not far exceeding, most commercial PDs as well as recently reported, visible, AuNP-coupled ZnO detectors. In addition, the nature and degree of the photoresponsivity enhancement are systematically elucidated by investigating their light-triggered electrical signals under varying incident wavelengths, AuNP amounts, and illumination positions. We discuss a possible photoconduction mechanism of our AuNP-coupled ZnO NR PDs and the origins of the high photoresponsivity. Specifically related to the AuNP amount-dependent photoresponse behaviors, the nanoparticle density yielding photoresponse maxima is explained as the interplay between localized surface plasmon resonance, plasmonic heating, and scattering in our photothermoelectric effect-driven device. We show that the AuNP-coupled ZnO NR PDs can be constructed via a straightforward method without the need for ultrahigh vacuum, sputtering procedures, or photo/electron-beam lithographic tools. Hence, the approach demonstrated in this study may serve as a convenient and viable means to advance the current state of ZnO-based PDs for operation in the visible spectral range with greatly increased photoresponsivity.

Effect of Nd Impurity on Photoconductivity and Optical Absorption Spectra of GeS Single Crystal

The effect of Nd impurities on the mechanism of photoconductivity and light absorption in GeS single crystals was investigated. The structure of the spectra was studied, and the exciton states and the energy of direct and indirect band–band transitions were investigated.

Mo1−xWxS2-based photodetector fabrication and photoconductive characteristics

We investigated the photoconductive characteristics of tungsten-substituted molybdenum disulfide (Mo1−xWxS2) series materials synthesized by the chemical vapor transport (CVT) method using different x values of W composition (x = 0–1, Δx = 0.1). We applied the Mo1−xWxS2 series materials to photoconductive detectors and then measured photocurrent at different laser source wavelengths: 405, 532, 633, and 808 nm. The band gap bowing effect and absorbance indicated the photoconduction mechanism in the Mo1−xWxS2 series materials. The proportion of x influenced the conductivity and absorbance, and induced the energy-gap bowing effect of Mo1−xWxS2.

Toward Sensitive Room‐Temperature Broadband Detection from Infrared to Terahertz with Antenna‐Integrated Black Phosphorus Photoconductor

Graphene‐like two‐dimensional materials (graphene, transition‐metal dichalcogenides (TMDCs)) have received extraordinary attention owing to their rich physics and potential applications in building nanoelectronic and nanophotonic devices. Recent works have concentrated on increasing the responsivity and extending the operation range to longer wavelengths. However, the weak absorption of gapless graphene, and the large bandgap (>1 eV) and low mobility in TMDCs have limited their spectral usage to only a narrow range in the visible spectrum. In this work, we demonstrate for the first time a high‐performance, antenna‐integrated, black phosphorus (BP)‐based photoconductor with ultra‐broadband detection from the infrared to terahertz frequencies. The good trade‐off between the moderate bandgap and good mobility results in a broad spectral absorption that is superior to that of graphene. Different photoconductive mechanisms, such as photothermoelectric (PTE), bolometric, and electron–hole generation can be distinguished depending on the device geometry, incident wavelength, and power. Especially, the photoconductive response remains highly efficient, even when the photon energy is extended to the terahertz (THz) band at room temperature, which is driven by the thermoelectric‐induced well. The proposed photodetectors have a superior performance with an excellent sensitivity of over 300 V W−1, low noise equivalent power (NEP) (smaller than 1 nW Hz−0.5 (10 pW Hz−0.5) with respect to the incident (absorbed) power), and fast response, all of which play key roles in multispectral biological imaging, remote sensing, and optical communications.

Fabrication and Characterizations of Aluminum-Doped Zinc Oxide (AZO): Cu2O/p-Si Photodiodes

In this paper, aluminum-doped zinc oxide (AZO):Cu2O/p-Si photodiode was fabricated based on simple solution process. The photodiodes were prepared using Zn1-xAlxO:Cu2O thin films grown on p-Si substrates. The photoresponse properties of the p-Si/AZO:Cu2O/Al photodiodes were investigated by current-voltage and capacitance-conductance-voltage characteristics under solar light. The photoconducting mechanism of the diodes were analyzed by the transient photocurrent measurements. It was found that the photoconductivity mechanism is controlled by the continuous distribution of trap levels. The diodes exhibited the photocapacitance and are changed with increasing Cu2O content. The presence of the interface states in the interface of the diodes was confirmed by the series resistancevoltage behavior. The obtained results indicate that the p-Si/Zn1-xAlxO-Cu2O/Al diodes can be used as a photosensor in optoelectronic applications

ITO nanowires-embedding transparent NiO/ZnO photodetector

Ni tipped ITO nanowires (NWs) have been prepared by state-of-the-art method and used as a base layer for NiO/ZnO UV detector. The enhanced opto-electrical properties of the fabricated UV detector were systematically analysed. Ni islands were used as a template to forming vertically grown ITO NWs. The prepared NiO/ZnO/ITO NW UV detector exhibited more than 80% transmittance in the visible and NIR regions. However, as the UV radiation was completely utilized by the device, the transmittance in the UV region was least. The photoconduction mechanism including Schottky contact between NiO and ZnO layers were studied by I⿿V characteristics and Mott-Schottky analysis. The device showed the lowest reverse saturation current of 0.59nA, showing good junction quality. The detector showed excellent UV photoresponse time of 7.05ms. It is suggested that the optical and electrical properties of an UV detector could be enhanced by incorporating the Ni tipped ITO NWs.

Study of photoconduction properties of CVD grown β-Ga2O3 nanowires

We have investigated photoconductive properties and photoconduction mechanism in single crystalline β-Ga2O3 nanowires. To investigate photoconduction, metal-semiconductor-metal (MSM) based photodetectors were fabricated using single crystalline β-Ga2O3 nanowires grown by CVD technique. The current-voltage characteristics of these photodetectors were measured with varying incident laser powers. It was observed that the photocurrent was almost linear with the incident power. Under illumination, the photocurrent increased by three orders of magnitude. The photoconduction mechanism in β-Ga2O3 nanowires has also been discussed. The photoconduction properties of nanowires demonstrate the possibility of future applications of these nanowires in sensors and photodetectors.

Photoconductivities in MoS2 Nanoflake Photoconductors.

Photoconductivities in molybdenum disulfide (MoS2) layered nanostructures with two-hexagonal crystalline structure prepared by mechanical exfoliation were investigated. The photoconductor-type MoS2 nanoflakes exhibit remarkable photoresponse under the above bandgap excitation wavelength of 532 nm at different optical intensity. The photocurrent responsivity and photoconductive gain of nanoflakes can reach, respectively, 30 AW−1 and 103 at the intensity of 50 Wm−2, which are several orders of magnitude higher than those of their bulk counterparts. The vacuum-enhanced photocurrent and power-independent responsivity/gain indicate a surface-controlled photoconduction mechanism in the MoS2 nanomaterial.

Schottky barrier-gated high performance photodetectors using a water-borne polymeric colloid.

Here, we demonstrate the synergetic application of a cationic surfactant (CTAB) for the fabrication of a fast response organic photoconductor via an environmentally benign fabrication process. A water-borne colloid of the semiconducting polymer PBTTT was fabricated via a mini-emulsion process with CTAB as the surfactant, and deposited onto a Au-patterned substrate to complete the photoconductor device geometry. Due to the preferential adsorption of the ammonium cation of the CTAB molecules onto the Au surface, a dipole layer was created and thus the work function of Au was significantly reduced, as confirmed by ultraviolet photoelectron spectroscopic studies. We show that the resulting Schottky barrier between Au-CTAB and PBTTT can be used as an artificial 'gate' for a trap-limited photoconductive mechanism, leading to a fast temporal response of the photoconductor without sacrificing the efficient photoconductive gain-generating mechanism. As a result, a high detectivity of 4.92 × 10(10) Jones, as well as a high gain of 107, can be realized from the PBTTT-based organic photoconductor. This result opens the possibility of fabricating high performance and simple structured organic photodetectors via a nontoxic fabrication process.

On-chip fabrication of lateral growth ZnO nanowire array UV sensor

In this paper, we integrate nano technology into traditional microelectronic processing, and develop an on-chip UV sensor based on lateral growth ZnO nanowire arrays. Traditional procedures are used to fabricate the interdigital electrodes, and ZnO nanowires are self-organized and grown between electrodes laterally by hydrothermal method. Additional inclined nanowires are removed during the post-processing procedures, such as ultrasound cleansing and electrode reinforcement. Two kinds of electrode structures are applied, i.e., Cr and Au. For the Cr electrode device structure, because Cr will restrain nanowires from growing vertically on its top, the laterally grown nanowire is long enough to reach the other side of the electrode. The corresponding photoelectric response mechanism is photoconduction controlled by surface oxide ion adsorption. Although the photocurrent is large, the gain is low, and the response speed is slow. Under the UV radiations of 20 mW/cm2 and of 365 nm in wavelength, the dark current is 2.210-4 A with 1 V bias voltage, the gain is up to 64, the photocurrent cannot reach saturation after 25 s, and the recovery time is 51.9 s. A secondary electrode can be fabricated after growing the nanowire arrays to reinforce the connection between the electrode and the ends of the nanowires. However, the direct contact between metal and semiconductor will form a Schottky contact. The photoelectric response mechanism is then changed to photovoltaic effect, which can greatly improve the gain and response speed. Under UV radiations of 20 mW/cm2 and of 365 nm in wavelength, the dark current is 4.310-8 A with 1 V bias voltage, the gain is up to 1300, the respond time is 3.8 s, and the recovery time is 5.7 s. For the Au electrode device structure, because Au is catalysis for ZnO nanowire growth, nanowires grown in lateral direction will compete with those grown in vertical direction, and hence the laterally grown nanowires are not long enough to reach the other side of the electrode. Nanowires grown from two sides of the electrodes will meet each other and form a bridging junction, however, this will turn the photoconduction mechanism from surface ion controlled into a bridging junction controlled, which yields the best device performance. Before removing the inclined nanowires by ultrasound cleansing, under UV radiations of 20 mW/cm2 and of 365 nm in wavelength, the dark current is 8.310-3 A with 1 V bias voltage, the gain is up to 1350, the respond time is 3.3 s, and the recovery time is 3.4 s. After removing the inclined nanowires, under UV radiations of 20 mW/cm2 and of 365 nm in wavelength, the dark current is 10-9 A with 1 V bias voltage, the gain is up to 8105, the respond time is 1.1 s, and the recovery time is 1.3 s.

Intrinsic photo-conductance triggered by the plasmonic effect in graphene for terahertz detection

Terahertz (THz) technology is becoming more eminent for applications in diverse areas including biomedical imaging, communication, security and astronomy. However, THz detection still has some challenges due to the lack of sources and detectors despite decades of considerable effort. The appearance of graphene and its gapless spectrum enable their applications in sensitive detection of light over a very wide energy spectrum from ultraviolet, infrared to terahertz. Several mechanisms in graphene for THz detection have been proposed, such as photo-thermoelectric, Dyakonov–Shur (DS) and bolometric effects. Here, we propose a photoconductive mechanism assisted by plasma wave in a graphene field-effect transistor (FET). Sensitive response to THz radiation can be realized far below the interband transition at room temperature. The response is due to the contributions of both plasma drag and convection effects. The two effects can both trigger multiple potential wells along the channel, which are different from other quantum-transition mechanisms. The photoconductive effects can be explored in both periodic and non-periodic systems and can be substantially enhanced under the electric field. They could reduce the burden of structural complexity compared to other mechanisms like unilateral thermoelectric and DS detection. This paves the way for more judicious photo-detector design for versatile THz applications.

Resistive switching characteristics of Titanium doping HfO<sub>2</sub> memory cell under different annealing treatment

We fabricated resistive random access memory (RRAM) devices with Titanium doping HfO2 as the switching layer by magnetron sputtering and lithography technology. Resistive switching (RS) characteristics of HfTiO memory devices annealed under N2 and O2 ambient were investigated in this work. It was found that the improved RS properties were obtained in N2 annealing atmosphere, which showed larger RS window as well as improved uniformity for the RS parameters, while the device in O2 annealed atmosphere was the opposite. To clarify the effect of annealing treatment on the devices in various atmospheres, X-ray photo spectra and conduction mechanism, which are related to the RS properties were analyzed. It can be induced the increase of oxygen vacancies in the switching layer by N2 annealing treatment, which was benificial to the connection/ rupture of the conductive filament, was the main reason for the improvement of RS characteristics.

ZnO-core/ZnSe-shell nanowire UV photodetector

ZnO–ZnSe core–shell nanowires were synthesized by the thermal evaporation of a mixture of ZnO and graphite powders and ZnSe powders sequentially. Subsequently, multiple networked ZnO–ZnSe core–shell nanowire photodetectors were fabricated. The diameters and thicknesses of the ZnO-core/ZnSe-shell nanowires ranged from 50 to 150 nm and from 15 to 25 nm, respectively, and the lengths of the nanowires ranged up to a few hundreds of micrometers. Our results showed that the photoconductivity and response time of the ZnO nanowires were enhanced significantly and reduced, respectively, by encapsulating them with a ZnSe thin film. Under a 2 V applied voltage, the dark current and photocurrent of the ZnO NW photodetector were 2.04 × 10−7 A and 9.94 × 10−6 A, respectively. In contrast, the dark current and photocurrent of the ZnO-core/ZnSe-shell nanowire photodetector were 7.50 × 10−8 A and 1.75 × 10−5 A, respectively. The rising and decaying times of the core–shell nanowires were 6.2 and 5.8 s, respectively, whereas both the rising and decaying times of the pristine ZnO nanowires were 7.3 s. The photoconduction mechanism in the core–shell nanowires is discussed.

High-Performance Fully Nanostructured Photodetector with Single-Crystalline CdS Nanotubes as Active Layer and Very Long Ag Nanowires as Transparent Electrodes.

Long and single-crystalline CdS nanotubes (NTs) have been prepared via a physical evaporation process. A metal-semiconductor-metal full-nanostructured photodetector with CdS NTs as active layer and Ag nanowires (NWs) of low resistivity and high transmissivity as electrodes has been fabricated and characterized. The CdS NTs-based photodetectors exhibit high performance, such as lowest dark currents (0.19 nA) and high photoresponse ratio (Ilight/Idark ≈ 4016) (among CdS nanostructure network photodetectors and NTs netwok photodetectors reported so far) and very low operation voltages (0.5 V). The photoconduction mechanism, including the formation of a Schottky barrier at the interface of Ag NW and CdS NTs and the effect of oxygen adsorption process on the Schottky barrier has also been provided in detail based on the studies of CdS NTs photodetector in air and vacuum. Furthermore, CdS NTs photodetector exhibits an enhanced photosensitivity as compared with CdS NWs photodetector. The enhancement in performance is dependent on the larger surface area of NTs adsorbing more oxygen in air and the microcavity structure of NTs with higher light absorption efficiency and external quantum efficiency. It is believed that CdS NTs can potentially be useful in the designs of 1D CdS-based optoelectronic devices and solar cells.

Metal-to-metal charge transfer between dopant and host ions: Photoconductivity of Yb-doped CaF2 and SrF2 crystals.

Dopant-to-host electron transfer is calculated using ab initio wavefunction-based embedded cluster methods for Yb/Ca pairs in CaF2 and Yb/Sr pairs in SrF2 crystals to investigate the mechanism of photoconductivity. The results show that, in these crystals, dopant-to-host electron transfer is a two-photon process mediated by the 4f(N-1)5d excited states of Y b(2+): these are reached by the first photon excitation; then, they absorb the second photon, which provokes the Y b(2+) + Ca(2+) (Sr(2+)) → Y b(3+) + Ca(+) (Sr(+)) electron phototransfer. This mechanism applies to all the observed Y b(2+) 4f-5d absorption bands with the exception of the first one: Electron transfer cannot occur at the first band wavelengths in CaF2:Y b(2+) because the Y b(3+)-Ca(+) states are not reached by the two-photon absorption. In contrast, Yb-to-host electron transfer is possible in SrF2:Y b(2+) at the wavelengths of the first 4f-5d absorption band, but the mechanism is different from that described above: first, the two-photon excitation process occurs within the Y b(2+) active center, then, non-radiative Yb-to-Sr electron transfer can occur. All of these features allow to interpret consistently available photoconductivity experiments in these materials, including the modulation of the photoconductivity by the absorption spectrum, the differences in photoconductivity thresholds observed in both hosts, and the peculiar photosensitivity observed in the SrF2 host, associated with the lowest 4f-5d band.

Cu2ZnSnS4:graphene oxide nanocomposites based photoresponse devices

Graphene oxide:Cu2ZnSnS4 nanocomposites were synthesized by the chemical method to fabricate photodiodes. The structural properties of the nanocomposites were analyzed using a Scanning electron microscopy (SEM) and X-ray Energy Dispersive energy (EDS) technique. Electrical and photoresponse properties of the p-Si/graphene oxide:Cu2ZnSnS4 nanocomposites diodes were investigated by current–voltage and capacitance-conductance-voltages characteristics under dark and various illumination conditions. The photocurrent of the diodes is larger than the dark current suggesting that the prepared diodes exhibited photodiode behavior. The photoconducting mechanism of the diodes was analyzed and it was found that the photoconduction mechanism is controlled by the monomolecular recombination. The diodes exhibited also a photocapacitor behavior. The photoelectrical properties of Au/GO:Cu2ZnSnS4/p-Si/Al devices indicate that the prepared diodes can be used both as a photodiode and a photocapacitor in optoelectronic device applications.

Hot Carrier Trapping Induced Negative Photoconductance in InAs Nanowires toward Novel Nonvolatile Memory.

We report a novel negative photoconductivity (NPC) mechanism in n-type indium arsenide nanowires (NWs). Photoexcitation significantly suppresses the conductivity with a gain up to 10(5). The origin of NPC is attributed to the depletion of conduction channels by light assisted hot electron trapping, supported by gate voltage threshold shift and wavelength-dependent photoconductance measurements. Scanning photocurrent microscopy excludes the possibility that NPC originates from the NW/metal contacts and reveals a competing positive photoconductivity. The conductivity recovery after illumination substantially slows down at low temperature, indicating a thermally activated detrapping mechanism. At 78 K, the spontaneous recovery of the conductance is completely quenched, resulting in a reversible memory device, which can be switched by light and gate voltage pulses. The novel NPC based optoelectronics may find exciting applications in photodetection and nonvolatile memory with low power consumption.

Photoconductivity mechanism in structures with Ge-nanoclusters grown on Si(100) surface

Photoconductivity mechanism in structures with Ge-nanoclusters grown on Si(100) surface

Low-Frequency Noise Spectra of Laterally Bridged ZnO Microrod-Based Photodetectors

The laterally bridged ZnO microrods grown from an Au electrode applied to metal-semiconductor-metal photodetector was fabricated. Interlaced ZnO microrods with approximate single-crystalline structure can be grown from Au electrode fingers. The dark-current was 5.00 × 10 -5 A with an applied voltage of 1 V. Highly dense lateral ZnO microrod-based photodetectors produce remarkable responsivity of 1.93 × 10 5 A/W. Moreover, an extremely high internal photoconductive gain of 6.28 × 10 5 exists in the fabricated photodetectors. For a given bandwidth of 10 kHz and 1 V applied bias, the noise equivalent power of photodetectors were estimated to be 1.86 × 10 -13 W, and correspond to normalized detectivity of 1.12 × 10 12 cm·Hz 0.5 W -1 . This result may be attributed to an internal photoconductive gain mechanism and high-density bridged ZnO microrods. Our approach provides a simple and seed-layer-free method to fabricate high-performance ultraviolet photodetectors.

Mechanisms of photoconductivity in atomically thin MoS2.

Atomically thin transition metal dichalcogenides have emerged as promising candidates for sensitive photodetection. Here, we report a photoconductivity study of biased mono- and bilayer molybdenum disulfide field-effect transistors. We identify photovoltaic and photoconductive effects, which both show strong photogain. The photovoltaic effect is described as a shift in transistor threshold voltage due to charge transfer from the channel to nearby molecules, including SiO2 surface-bound water. The photoconductive effect is attributed to the trapping of carriers in band tail states in the molybdenum disulfide itself. A simple model is presented that reproduces our experimental observations, such as the dependence on incident optical power and gate voltage. Our findings offer design and engineering strategies for atomically thin molybdenum disulfide photodetectors, and we anticipate that the results are generalizable to other transition metal dichalcogenides as well.