Reduced photoconductivity observed by time-resolved terahertz spectroscopy in metal nanofilms with and without adhesion layers

Alberding, Brian G.; Kushto, Gary P.; Lane, Paul A.; Heilweil, Edwin J.

doi:10.1063/1.4953208

Cited by 19 publications

(29 citation statements)

References 31 publications

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“…We model the complex conductivity with a phenomenological Drude–Smith model, a modification of the free carrier Drude conductivity that accounts for localization of the mobile carriers on length scales commensurate with their mean free path, such as localization within individual nanosheets . ,− Complex frequency-resolved conductivity is given as where τ DC is a carrier relaxation time, , N is the intrinsic charge carrier density and m* is the carrier effective mass. In this formalism, the DC conductivity is given by σ DC = σ 0 (1 + c ), where c is a phenomenological parameter that is a measure of carrier localization over the probed length scales.…”

Section: Resultsmentioning

confidence: 99%

Two-Dimensional MXenes Mo₂Ti₂C₃T_z and Mo₂TiC₂T_z: Microscopic Conductivity and Dynamics of Photoexcited Carriers

Natu

Shi

et al. 2020

ACS Appl. Energy Mater.

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MXenes are a recently discovered family of two-dimensional transition metal carbides, nitrides, and carbonitrides with electronic properties that can be tuned by their chemistry and structure. Herein THz spectroscopy was used to investigate the microscopic conductivity and photoexcited charge carrier dynamics in two Mo-based MXenes: Mo2Ti2C3T z and Mo2TiC2T z . We find that both have high intrinsic carrier densities (∼1020 cm–3 in Mo2Ti2C3T z and ∼1019 cm–3 in Mo2TiC2T z ) and mobilities and exhibit high conductivities within individual nanosheets. Optical excitations result in a transient conductivity increase in both compositions, in stark contrast with the most studied member of the MXene family, Ti3C2T z , where photoexcitation suppresses the conductivity for nanoseconds. Deintercalation of water, and other species, from between the nanosheets by mild vacuum annealing at 200 °C further improves the long-range, internanosheet transport of the photoexcited carriers and increases their lifetime. High, and long-lived, photoinduced conductivity that can be engineered by substituting Mo for Ti renders these Mo-based MXenes attractive for a variety of optoelectronic, sensing, and photoelectrochemical applications.

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Section: Resultsmentioning

confidence: 99%

Two-Dimensional MXenes Mo₂Ti₂C₃T_z and Mo₂TiC₂T_z: Microscopic Conductivity and Dynamics of Photoexcited Carriers

Natu

Shi

et al. 2020

ACS Appl. Energy Mater.

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“…Terahertz (THz) spectroscopy is a suitable tool for determining these properties and has been used to study an array of metallic and semiconductor materials [1–10]. In particular, it is a non-contact, all optical method which offers high throughput evaluation of materials without the need for complete device fabrication and can follow carrier dynamics over a wide range of time scales (picoseconds to nanoseconds or longer).…”

Section: Introductionmentioning

confidence: 99%

Direct comparison of time-resolved terahertz spectroscopy and Hall Van der Pauw methods for measurement of carrier conductivity and mobility in bulk semiconductors

2017

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Charge carrier conductivity and mobility for various semiconductor wafers and crystals were measured by ultrafast above bandgap, optically excited Time-Resolved Terahertz Spectroscopy (TRTS) and Hall Van der Pauw contact methods to directly compare these approaches and validate the use of the non-contact optical approach for future materials and in-situ device analyses. Undoped and doped silicon (Si) wafers with resistances varying over six orders of magnitude were selected as model systems since contact Hall measurements are reliably made on this material. Conductivity and mobility obtained at room temperature by terahertz transmission and TRTS methods yields the sum of electron and hole mobility which agree very well with either directly measured or literature values for corresponding atomic and photo-doping densities. Careful evaluation of the optically-generated TRTS frequency-dependent conductivity also shows it is dominated by induced free-carrier absorption rather than small probe pulse phase shifts, which is commonly ascribed to changes in the complex conductivity from sample morphology and evaluation of carrier mobility by applying Drude scattering models. Thus, in this work, the real-valued, frequency-averaged conductivity was used to extract sample mobility without application of models. Examinations of germanium (Ge), gallium arsenide (GaAs), gallium phosphide (GaP) and zinc telluride (ZnTe) samples were also made to demonstrate the general applicability of the TRTS method, even for materials that do not reliably make good contacts (e.g., GaAs, GaP, ZnTe). For these cases, values for the sum of the electron and hole mobility also compare very favorably to measured or available published data.

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“…Previous measurements of the current response time in metals were limited to ca. 10 fs accuracy, due to uncertainties in the thickness of the reference substrate 20,21 . Our thickness correction technique allows determining the current response time with an error of ca.…”

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confidence: 99%

Thickness-dependent electron momentum relaxation times in iron films

Krewer

Zhang

Arabski

et al. 2020

Applied Physics Letters

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Terahertz time-domain conductivity measurements in 2 to 100 nm thick iron films resolve the femtosecond time delay between applied electric fields and resulting currents. This response time decreases for thinner metal films. The macroscopic response time depends on the mean and the variance of the distribution of microscopic momentum relaxation times of the conducting electrons.Comparing the recorded response times with DC-conductivities demonstrates increasing variance of the microscopic relaxation times with increasing film thickness. At least two electron species contribute to conduction in bulk with substantially differing relaxation times. The different electron species are affected differently by the confinement because they have different mean free paths.Conductivity in metals is typically described using a highly simplified model: a gas of identical electrons characterized by one relaxation time [1][2][3][4], one mean free path [1][2][3][4][5][6], and hence one velocity. This is in stark contrast to the complexity of the underlying process where all electronic states on the Fermi surface contribute to conduction[7], and velocity and relaxation time often vary strongly across that large and often complex Fermi surface [8,9]. Therefore, rather than a single value for the relaxation time, one should consider a distribution of relaxation times to describe the system, as has been shown for metal oxides[10] and semiconductors [11].Scattering of charge carriers is usually discussed in the similarly simplified framework of Mathiessens rule, where each scattering process can be assigned a scattering rate independent of any other scattering mechanism present. The scaling of the resistivity is then given by the sum of all rates. For thickness scaling however, the probability of an electron scattering on the surface depends on its mean free path in bulk [1,12]. Therefore Mathiessens rule should break down and consequently the shape of the relaxation time distribution should change by surface scattering. By time resolving the conduction in iron films we demonstrate this change of the relaxation time distribution, illustrating the breakdown of the simple picture of one relaxation time and Mathiessens rule.We start by deriving the connection between the response time of the macroscopic current τ C to the distribution of microscopic momentum relaxation times τ . The exponential decay [7,8] of the conductivity of a state j with a relaxation time of τ j is equivalent to a

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Reduced photoconductivity observed by time-resolved terahertz spectroscopy in metal nanofilms with and without adhesion layers

Cited by 19 publications

References 31 publications

Two-Dimensional MXenes Mo₂Ti₂C₃T_z and Mo₂TiC₂T_z: Microscopic Conductivity and Dynamics of Photoexcited Carriers

Two-Dimensional MXenes Mo₂Ti₂C₃T_z and Mo₂TiC₂T_z: Microscopic Conductivity and Dynamics of Photoexcited Carriers

Direct comparison of time-resolved terahertz spectroscopy and Hall Van der Pauw methods for measurement of carrier conductivity and mobility in bulk semiconductors

Thickness-dependent electron momentum relaxation times in iron films

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Reduced photoconductivity observed by time-resolved terahertz spectroscopy in metal nanofilms with and without adhesion layers

Cited by 19 publications

References 31 publications

Two-Dimensional MXenes Mo2Ti2C3Tz and Mo2TiC2Tz: Microscopic Conductivity and Dynamics of Photoexcited Carriers

Two-Dimensional MXenes Mo2Ti2C3Tz and Mo2TiC2Tz: Microscopic Conductivity and Dynamics of Photoexcited Carriers

Direct comparison of time-resolved terahertz spectroscopy and Hall Van der Pauw methods for measurement of carrier conductivity and mobility in bulk semiconductors

Thickness-dependent electron momentum relaxation times in iron films

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Two-Dimensional MXenes Mo₂Ti₂C₃T_z and Mo₂TiC₂T_z: Microscopic Conductivity and Dynamics of Photoexcited Carriers

Two-Dimensional MXenes Mo₂Ti₂C₃T_z and Mo₂TiC₂T_z: Microscopic Conductivity and Dynamics of Photoexcited Carriers