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photoconductivity

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photoconductivity , the increase in the electrical conductivity of certain materials when they are exposed to light of sufficient energy. Photoconductivity serves as a tool to understand the internal processes in these materials, and it is also widely used to detect the presence of light and measure its intensity in light-sensitive devices.

Certain crystalline semiconductors , such as silicon , germanium , lead sulfide, and cadmium sulfide, and the related semimetal selenium , are strongly photoconductive. Normally, semiconductors are relatively poor electrical conductors because they have only a small number of electrons that are free to move under a voltage. Most of the electrons are bound to their atomic lattice in the set of energy states called the valence band. But if external energy is provided, some electrons are raised to the conduction band, where they can move and carry current. Photoconductivity ensues when the material is bombarded with photons of sufficient energy to raise electrons across the band gap , a forbidden region between the valence and conduction bands. In cadmium sulfide this energy is 2.42 electron volts (eV), corresponding to a photon of wavelength 512 nanometres (1 nm = 10 −9  metre), which is visible green light. In lead sulfide the gap energy is 0.41 eV, making this material sensitive to infrared light.

Because the current ceases when the light is removed, photoconductive materials form the basis of light-controlled electrical switches. These materials are also used to detect infrared radiation in military applications such as guiding missiles to heat-producing targets. Photoconductivity has broad commercial application in the process of photocopying , or xerography , which originally used selenium but now relies on photoconductive polymers . See also photoelectric effect .

Photoconductivity in Materials Research

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Photoconductivity is the incremental change in the electrical conductivity of a substance upon illumination. Photoconductivity is especially apparent for semiconductors and insulators, which have low conductivity in the dark. Significant information can be derived on the distribution of electronic states in the material and on carrier generation and recombination processes from the dependence of the photoconductivity on factors such as the exciting photon energy, the intensity of the illumination or the ambient temperature. These results can in turn be used to investigate optical absorption coefficients or concentrations and distributions of defects in the material. Methods involving either steady state currents under constant illumination or transient methods involving pulsed excitation can be used to study the electronic density of states as well as the recombination. The transient time-of-flight technique also allows carrier drift mobilities to be determined.

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Photocurrent as a multiphysics diagnostic of quantum materials

An Introduction to Steady-State and Time-Resolved Photoluminescence

Abbreviations.

alternating current

conduction band

constant-photocurrent method

chemical vapor deposition

dual-beam photoconductivity

density of states

interrupted field time-of-flight

modulated photoconductivity

steady-state photoconductivity

time of flight

transient photoconductivity

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Brinza, M., Willekens, J., Benkhedir, M., Adriaenssens, G. (2006). Photoconductivity in Materials Research. In: Kasap, S., Capper, P. (eds) Springer Handbook of Electronic and Photonic Materials. Springer Handbooks. Springer, Boston, MA. https://doi.org/10.1007/978-0-387-29185-7_7

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• Published: 21 September 2023

Ultra-broadband photoconductivity in twisted graphene heterostructures with large responsivity

• H. Agarwal   ORCID: orcid.org/0000-0002-9418-7966 1   na1 ,
• K. Nowakowski   ORCID: orcid.org/0000-0002-4598-9831 1   na1 ,
• A. Forrer 2 ,
• A. Principi 3 ,
• R. Bertini   ORCID: orcid.org/0000-0001-7225-0277 1 ,
• S. Batlle-Porro   ORCID: orcid.org/0000-0001-9663-8268 1 ,
• A. Reserbat-Plantey   ORCID: orcid.org/0000-0002-9106-8750 1 , 4 ,
• P. Prasad 1 ,
• L. Vistoli 1 ,
• K. Watanabe   ORCID: orcid.org/0000-0003-3701-8119 5 ,
• T. Taniguchi   ORCID: orcid.org/0000-0002-1467-3105 6 ,
• A. Bachtold 1 , 7 ,
• G. Scalari   ORCID: orcid.org/0000-0003-4028-803X 2 ,
• R. Krishna Kumar   ORCID: orcid.org/0000-0003-0857-4466 1 &
• F. H. L. Koppens   ORCID: orcid.org/0000-0001-9764-6120 1 , 7  

Nature Photonics volume  17 ,  pages 1047–1053 ( 2023 ) Cite this article

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• Photonic devices

The requirements for broadband photodetection are becoming exceedingly demanding in hyperspectral imaging. While intrinsic photoconductor arrays based on mercury cadmium telluride represent the most sensitive and suitable technology, their optical spectrum imposes a narrow spectral range with a sharp absorption edge that cuts their operation to <25 μm. Here we demonstrate a large ultra-broadband photoconductivity in twisted double bilayer graphene heterostructures spanning the spectral range of 2–100 μm with internal quantum efficiencies of approximately 40% at speeds of 100 kHz. The large response originates from unique properties of twist-decoupled heterostructures including pristine, crystal field-induced terahertz band gaps, parallel photoactive channels and strong photoconductivity enhancements caused by interlayer screening of electronic interactions by respective layers acting as sub-atomic spaced proximity screening gates. Our work demonstrates a rare instance of an intrinsic infrared–terahertz photoconductor that is complementary metal-oxide-semiconductor compatible and array integratable, and introduces twist-decoupled graphene heterostructures as a viable route for engineering gapped graphene photodetectors with three-dimensional scalability.

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Plasmonic antenna coupling to hyperbolic phonon-polaritons for sensitive and fast mid-infrared photodetection with graphene

Intelligent infrared sensing enabled by tunable moiré quantum geometry

Synergistic-potential engineering enables high-efficiency graphene photodetectors for near- to mid-infrared light

Data availability.

The data that support the plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request.

Code availability

All codes used to produce the findings of this study are available from the corresponding authors on reasonable request.

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Acknowledgements

We thank D. B. Ruiz, S. Castilla, D. De Fazio, M. Amir Ali, G. Li, A. Berdyugin, M. Polini, V. Mkhitaryan, G. Kumar, and I. Torre for technical discussions. We further thank M. Ceccanti for making the illustration presented in Fig. 1a . H.A., K.N. and R.B. acknowledge funding from the European Union’s Horizon 2020 research and innovation programme under Marie Skłodowska-Curie grant agreement no. 665884, 713729 and 847517, respectively. S.B.-P. acknowledges funding from the Presidencia de la Agencia Estatal de Investigación within the PRE2020-094404 predoctoral fellowship. G.S. and A.F. gratefully acknowledge funding from the ERC grant CHIC (no. 724344), and J. Faist for discussions. A.P. acknowledges support from the European Union’s Horizon 2020 research and innovation programme under Marie Sklodowska-Curie grant agreement no. 873028 and from the Leverhulme Trust under grant agreement RPG-2019-363. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by MEXT Japan with grant no. JPMXP0112101001, JSPS KAKENHI (JP19H05790, JP20H00354 and JP21H05233) and CREST (JPMJCR15F3), JST. R.K.K. acknowledges the EU Horizon 2020 programme under MarieSkłodowska-Curie grants 754510 and 893030 and the FLAG-ERA grant (PhotoTBG, PCI2021-122020-2A), by ICFO, RWTH Aachen and ETHZ/Department of Physics. A.B. acknowledges support from ERC advanced grant no. 692876, MICINN grant no. RTI2018-097953-B-I00 and PID2021-122813OB-I00, AGAUR (grant no. 2017SGR1664), the Fondo Europeo de Desarrollo, the Spanish Ministry of Economy and Competitiveness through Quantum CCAA, EUR2022-134050, and CEX2019-000910-S [MCIN/AEI/10.13039/501100011033], MCIN with funding from European Union NextGenerationEU (PRTR-C17.I1), Fundacio Cellex, Fundacio Mir-Puig, Generalitat de Catalunya through CERCA. F.H.L.K. acknowledges support from the ERC TOPONANOP (726001), Fundació Cellex, Fundació Mir-Puig, Generalitat de Catalunya (CERCA, AGAUR, SGR 1656, program TWIST), the Government of Spain [PID2019-106875GB-I00; PCI2021-122020-2A; PDC2022-133844-I00 (Teracomm); Severo Ochoa CEX2019-000910-S] funded by MCIN/AEI/10.13039/501100011033 and by the European Union NextGenerationEU/PRTR. Furthermore, the research leading to these results has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 881603 (Graphene flagship Core3), 820378 (Quantum flagship) and 101034929 (Fastera). This material is based upon work supported by the Air Force Office of Scientific Research under award number FA8655-23-1-7047. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the United States Air Force.

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These authors contributed equally: H. Agarwal, K. Nowakowski.

Authors and Affiliations

Institut de Ciències Fotòniques, The Barcelona Institute of Science and Technology, Castelldefels, Spain

H. Agarwal, K. Nowakowski, R. Bertini, S. Batlle-Porro, A. Reserbat-Plantey, P. Prasad, L. Vistoli, A. Bachtold, R. Krishna Kumar & F. H. L. Koppens

Quantum Optoelectronics Group, Institute of Quantum Electronics, ETH Zürich, Zürich, Switzerland

A. Forrer & G. Scalari

School of Physics and Astronomy, University of Manchester, Manchester, UK

A. Principi

Université Côte d’Azur, CNRS, CRHEA, Valbonne, France

A. Reserbat-Plantey

Research Center for Functional Materials, National Institute for Materials Science, Tsukuba, Japan

K. Watanabe

International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Japan

T. Taniguchi

Institució Catalana de Recerca i Estudis Avançats, Barcelona, Spain

A. Bachtold & F. H. L. Koppens

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Contributions

F.H.L.K., H.A. and R.K.K. conceived the presented idea. H.A. fabricated the devices. H.A. and K.N. performed the mid-infrared measurements with inputs from R.K.K. and F.H.L.K. K.N. built the mid-infrared measurement setup with inputs from A.R.-P., R.K.K. and F.H.L.K. R.K.K, and H.A. performed terahertz far-field photocurrent measurements A.R.-P, R.K.K. and H.A. designed and built the measurement setup for terahertz photocurrent experiments. A.F., H.A. and G.S. performed low-temperature FTIR spectroscopy measurements. A.P., R.K.K. and F.H.L.K. developed the theoretical formalism and performed the analytical calculations. R.B. performed absorption calculations. K.W. and T.T. provided hBN crystals. H.A, P.P., L.V. and A.B. performed magnetotransport experiments. H.A., K.N. and R.K.K. analysed the results. R.K.K., H.A. and F.H.L.K. wrote the manuscript. R.K.K. and F.H.L.K. supervised the project. All authors provided critical feedback and helped shape the research, analysis and manuscript.

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Correspondence to R. Krishna Kumar or F. H. L. Koppens .

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Agarwal, H., Nowakowski, K., Forrer, A. et al. Ultra-broadband photoconductivity in twisted graphene heterostructures with large responsivity. Nat. Photon. 17 , 1047–1053 (2023). https://doi.org/10.1038/s41566-023-01291-0

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DOI : https://doi.org/10.1038/s41566-023-01291-0

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Photoconductivity – Definition, Working and its Applications:

Photoconductivity – When excess electrons and holes are produced in a semiconductor, there is a corresponding increase in conductivity of a sample as indicated by the following equation

Conductivity of semiconductor,

The above equation is obtained from Eqs. (6.48) and (6.59).

When the excess carriers in a semiconductor are due to opti­cal luminescence, the resulting conductivity is called photoconductivity . This is an important effect, with useful applications in the analysis of semiconductor materials and in the operation of different types of devices.

The photoconductive effect is explained as follows:

The conductivity of a material is proportional to the concentration of charge carriers present, as indicated by Eq. (25.10). Radiant energy supplied to the semiconductor causes covalent bonds to the broken, and EHPs in excess of those generated thermally are produced. These increased current carriers reduce the resistance of the material and, therefore, such a device is called a photoconductor or photoresistor .

Energy diagram of a semiconductor having both acceptor and donor impurities is given in Fig. 25.10. When this specimen is illuminated by photons of sufficient energies, the possible transitions are as follows :

An EHP can be generated by a high-energy photon, in what is called intrinsic excitation ; a photon may excite a donor electron into the conduction band ; or a valence electron may move into an acceptor state. The last two transitions are called the impurity excitations . Since the density of states in the conduction and valence bands greatly exceeds the density of impurity states, photoconductivity is due principally to intrinsic excitation.

Spectral Response: The minimum energy of a photon required for intrinsic excitation is the forbidden-gap energy E G , in electron volts, of the semiconductor material. The long-wavelength threshold of the material is defined as the wavelength corresponding to the energy gap E G and is given by Eq. (25.11)

For silicon, E G = 1.1 eV and λ C = 1.13 microns

whereas for germanium, E G  = 0.72 eV and λ C = 1.73 microns at room temperature.

The spectral-sensitivity curves for silicon and germanium are given in Fig. 25.11. The worth noting point is that the long-wavelength limit is slightly greater than the calculated values of λ C . This is due to impurity excitations. With the decrease of the wavelength, the response increases and attains a maximum value.

Photoconductive Current: The carriers generated by photoexcitation will move under the influence of an applied field, If they survive recombination, they will reach ohmic contacts at the ends of the semiconductor bar, and thus the device current will be constituted. The current may be given by

where G L is the generation rate of excess carriers, in cm -3 s -1 , produced by optical luminescence, τ is the average life time of the newly generated carriers and t t is the average transit time for carriers to reach the ohmic contacts.

Photoconductor:

Figure 25.12 depicts a semiconductor bar with ohmic contacts at each end and a voltage applied between the terminals. The initial thermal-equilibrium conductivity is given by

When the excess carriers are created in the semiconductor, the conductivity becomes

where δn and δp are the excess electron and hole concentrations, respectively. Considering an N-type semiconductor, it can be assumed that δn = δp. Using δp as the concentration of excess carriers, in steady-state, the carrier concentration is given by

where G L is the generation rate of excess carriers and τ h  is the lifetime of excess minority carriers.

The conductivity from Eq. (25.14) can be written as

The change in conductivity due to the optical excitation, called a photoconductivity, is then

Applied voltage induces an electric field in the semiconductor and a current is caused by this electric field. The current density is given by

where J 0 is the current density in the semiconductor prior to optical excitation and J L is the photocurrent density. The photocurrent density, J L  = ΔσE. In case of uniform generation of excess electrons and holes throughout the semiconductor, photocurrent is given as

where A is the x-sectional area of the semiconductor device. The photocurrent varies directly as the excess carrier generation rate, which in turn is proportional to the incident photo flux .

If excess carriers (electrons and holes) are not generated uniformly throughout the semiconductor material, then the total photocurrent is determined by integrating the photoconductivity over the cross-sectional area.

Since μ e E is the electron drift velocity, the electron transit time (time required for an electron to flow through the photoconductor) is given as

where L is the length of the semiconductor device.

The photocurrent is given, by Eq. (25.18),

Photoconductor gain can be defined as the ratio of the rate at which charge is collected by the contacts to the rate at which charge is created within the photoconductor i.e.

and substituting the value of I L from Eq. (25.20) in Eq. (25.21) we have

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Condensed Matter > Materials Science

Title: photoluminescence and high temperature persistent photoconductivity experiments in sno2 nanobelts.

Abstract: The Persistent Photoconductivity (PPC) effect was studied in individual tin oxide (SnO2) nanobelts as a function of temperature, in air, helium, and vacuum atmospheres, and low temperature Photoluminescence measurements were carried out to study the optical transitions and to determine of the acceptor/donors levels and their best representation inside the band gap. Under ultraviolet (UV) illumination and at temperatures in the range of 200 to 400K we observed a fast and strong enhancement of the photoconductivity, and the maximum value of the photocurrent induced increases as the temperature or the oxygen concentration decreases. By turning off the UV illumination the induced photocurrent decays with lifetimes up to several hours. The photoconductivity and the PPC results were explained by adsorption and desorption of molecular oxygen at the surface of the SnO2 nanobelts. Based on the temperature dependence of the PPC decay an activation energy of 230 meV was found, which corresponds to the energy necessary for thermal ionization of free holes from acceptor levels to the valence band, in agreement with the photoluminescence results presented. The molecular-oxygen recombination with holes is the origin of the PPC effect in metal oxide semiconductors, so that, the PPC effect is not related to the oxygen vacancies, as commonly presented in the literature.

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