Ultrafast carrier trapping in microcrystalline silicon observed in optical pump–terahertz probe measurements

Jepsen, Peter Uhd; Schairer, W.; Libon, I. H.; Lemmer, Uli; Hecker, N. E.; Birkholz, M.; Lips, K.; Schall, M.

doi:10.1063/1.1394953

Cited by 103 publications

(99 citation statements)

References 13 publications

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“…From the observed dynamics, trapping processes occurring on picosecond time scales can be identified (Jepsen et al, 2001;Messner et al, 2001). Especially in materials that possess a low degree of crystallinity or porous materials, the conduction pathway can be significantly obstructed, resulting in the localization of charge carriers.…”

Section: Ultrafast Dynamicsmentioning

confidence: 99%

Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

et al. 2011

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Time-resolved, pulsed terahertz spectroscopy has developed into a powerful tool to study charge carrier dynamics in semiconductors and semiconductor structures over the past decades. Covering the energy range from a few to about 100 meV, terahertz radiation is sensitive to the response of charge quasiparticles, e.g., free carriers, polarons, and excitons. The distinct spectral signatures of these different quasiparticles in the THz range allow their discrimination and characterization using pulsed THz radiation. This frequency region is also well suited for the study of phonon resonances and intraband transitions in low-dimensional systems. Moreover, using a pump-probe scheme, it is possible to monitor the nonequilibrium time evolution of carriers and low-energy excitations with sub-ps time resolution. Being an all-optical technique, terahertz time-domain spectroscopy is contact-free and noninvasive and hence suited to probe the conductivity of, particularly, nanostructured materials that are difficult or impossible to access with other methods. The latest developments in the application of terahertz time-domain spectroscopy to bulk and nanostructured semiconductors are reviewed.

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Section: Ultrafast Dynamicsmentioning

confidence: 99%

Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

et al. 2011

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“…It can be seen that the carrier lifetime increases with excitation fluence for all nanowire samples, suggesting the existence of trap states that begin to saturate toward higher photoinjected carrier density. 11 In order to assess the effects of both overcoating and defect-free growth, we constructed a model that accounts for both trapping of charges and other, nonsaturable, freecharge annihilation routes. While the process of overcoating will have a significant impact on the density of available surface states, a change in growth conditions for the GaAs core will mainly affect the nonsaturable charge recombination pathways.…”

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Carrier Lifetime and Mobility Enhancement in Nearly Defect-Free Core−Shell Nanowires Measured Using Time-Resolved Terahertz Spectroscopy

et al. 2009

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We have used transient terahertz photoconductivity measurements to assess the efficacy of two-temperature growth and core-shell encapsulation techniques on the electronic properties of GaAs nanowires. We demonstrate that two-temperature growth of the GaAs core leads to an almost doubling in charge-carrier mobility and a tripling of carrier lifetime. In addition, overcoating the GaAs core with a larger-bandgap material is shown to reduce the density of surface traps by 82%, thereby enhancing the charge conductivity.Semiconductor nanowires are promising new materials for implementation in nanoscale electronic and optoelectronic devices. Of particular interest are III-V semiconductor nanowires, which can exhibit a direct bandgap and a high electron mobility.1 However, the large surface-to-volume ratio inherent to nanowires results in the presence of surface traps offering easy access to carrier and exciton recombination pathways.2,3 In addition, one-temperature growth techniques have been shown to cause a significant twin-defect (stacking-defect) density within the nanowires. 4 Refinements in the epitaxial growth of these nanowires are therefore essential in order for their optoelectronic and crystallographic standards to approach those of bulk material.2,6,7 Such efforts are complicated by the fact that electrical measurements conducted on nanowires to determine charge-carrier mobility are often obscured by properties of the electrical contacts. Most contactless spectroscopic probes of nanowires to date have relied upon low-temperature photoluminescence measurements to characterize optoelectronic quality by measuring excitonic dynamics and radiative quantum efficiency. 2,6However, for use of these materials in nanoelectronics and optoelectronics, it is essential to determine charge-carrier mobility and lifetime at room temperature.In this study, we have conducted transient photoconductivity measurements on an ensemble of nanowires in order to assess the effect of nearly defect-free (two-temperature) growth and core-shell encapsulation technologies on charge-carrier trapping and mobility. Optical-pump terahertz-probe spectroscopy was employed as a noncontact ultrafast probe of the room-temperature photoconductivity with subpicosecond resolution. We demonstrate that both two-temperature growth and encapsulation of the GaAs nanowires with a higher band gap material lead to significant increases in the lifetime of free charge carriers. Encapsulation of the nanowires is shown to be highly effective, reducing the areal density of surface traps to one-seventh of that for the untreated wires. Importantly, we find that moving from one-temperature growth to a two-temperature procedure (comprising a brief high-temperature step for nucleation and a longer lower-temperature phase for prolonged growth 4 ) increases the intrinsic carrier mobility of the wires from 1200 cm 2 /(V s) to 2250 cm 2 /(V s).All nanowire samples were initially grown onto a GaAs substrate as shown in a representative scanning electron microscopy ...

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“…THz pulses are essentially single cycle electromagnetic pulses of about 1 ps duration. The transmitted field strength E t is measured directly in the time domain, through electrooptical sampling with 800 nm, 150 fs pulses: This ''gating'' technique provides a time resolution significantly better than the 1 ps THz pulse width [16]. Exciting MEH-PPV above the absorption gap ( 2:5 eV [5]) using 266 nm (4.5 eV) or 400 nm (3 eV), 150 fs pulses, allows investigation of photoexcited species.…”

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confidence: 99%

Efficiency of Exciton and Charge Carrier Photogeneration in a Semiconducting Polymer

et al. 2004

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We determine the efficiencies for the formation of excitons and charge carriers following ultrafast photoexcitation of a semiconducting polymer (MEH-PPV). The simultaneous, quantitative determination of exciton and charge photoyields is achieved through subpicosecond studies of both the real and the imaginary components of the complex conductivity over a wide frequency range. Predominantly excitons, with near-unity quantum efficiency, are generated on excitation, while only a very small fraction ( < 10 ÿ2 ) of free charges are initially excited, consistent with rapid ( 100 fs) hot exciton dissociation. These initial charges are very short lived, decaying on subpicosecond time scales. DOI: 10.1103/PhysRevLett.92.196601 PACS numbers: 72.80.Le, 71.35.Aa, 73.50.Gr, 73.61.Ph Since their discovery in the 1970s, semiconducting conjugated polymers have received considerable interest owing to their potential in technological applications, particularly in electronics [1]. These materials have many advantages over conventional semiconductors: They are low cost, easy to process, lightweight and malleable, and have shown significant promise in lightemitting diodes [2], photovoltaics, and laser optics [3]. Despite their widespread optical applications, the nature of the photoexcitation physics in these materials is currently subject to intense debate [4 -6]. One of the key questions that has remained controversial is whether, initially upon photoexcitation, excitons or charge carriers are primarily formed. From photoconductivity measurements [7], it is evident that free charges are formed, but the mechanism and efficiency of free charge formation remains polemical: Some studies suggest that excitons (Coulombically bound electron-hole pairs) are the primary photoexcitation product, which may dissociate into free charges after migration to electron (or hole) accepting defects, by absorption of a second photon or by bimolecular exciton-exciton annihilation [8,9]. Other studies suggest electron-hole pairs as the direct and predominant initial excitation product [10]. This apparent contradiction may be related to the selective sensitivity of different experimental approaches: For example, photoconductivity (PC) measurements [7] are sensitive only to free charges (responsible for real conductivity), whereas transient absorption (TA) and stimulated emission (SE) measurements [11][12][13] can probe excitons (bound electron-hole pairs responsible for imaginary conductivity). However, the simultaneous detection of the real and imaginary conductivity with sufficient time resolution is necessary to allow a quantitative comparison of both free and bound charges upon photoexcitation. The relatively recent technique of terahertz time domain spectroscopy [14] (THz-TDS) provides this opportunity.In this Letter we present the first investigation of a semiconducting polymer using THz-TDS. This technique allows us to monitor the evolution of free and bound charges on subpicosecond time scales following photoexcitation, through the time-depen...

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Ultrafast carrier trapping in microcrystalline silicon observed in optical pump–terahertz probe measurements

Cited by 103 publications

References 13 publications

Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

Carrier Lifetime and Mobility Enhancement in Nearly Defect-Free Core−Shell Nanowires Measured Using Time-Resolved Terahertz Spectroscopy

Efficiency of Exciton and Charge Carrier Photogeneration in a Semiconducting Polymer

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