David G. Cooke

Optical and transient photoconductive properties of pentacene and functionalized pentacene thin films: Dependence on film morphology

We present a comprehensive study of the optical and transient photoconductive properties of pentacene and functionalized pentacene thin films grown by evaporation or from solution onto a variety of substrates. The transient photoconductivity was studied over picosecond time scales using time-resolved terahertz pulse spectroscopy. The structure and morphology of the films were assessed using x-ray diffraction, atomic force microscopy, and scanning electron microscopy. Regular pentacene films grown by evaporation under similar conditions but on different substrates yielded polycrystalline films with similar morphology and similar optical and transient photoconductive properties. Single exponential or biexponential decay dynamics was observed in all of the regular pentacene films studied. Functionalized pentacene films grown by evaporation at two different substrate temperatures (as well as from solution) yielded significant variations in morphology, resulting in different optical-absorption spectra and tran...

Electron mobility in dilute GaAs bismide and nitride alloys measured by time-resolved terahertz spectroscopy

We report time-resolved terahertz spectroscopy measurements of the electronic transport properties of dilute GaAs bismide and nitride alloys. The electron mobility for GaAs1−yBiy (y=0.84%) extracted from Drude fits to the transient complex conductivity was ∼2800cm2∕Vs at a carrier density of 2.7×1018cm−3, close to the mobility of 3300cm2∕Vs measured for GaAs at a similar carrier density. The electron mobility did not decrease significantly for Bi concentrations up to 1.4%. In contrast, the GaNxAs1−x (x=0.84%) and GaNxAs1−x−yBiy (x=0.85%, y=1.4%) films exhibited non-Drude behavior with a highly reduced electron mobility and suppressed conductivity at low frequencies indicative of carrier localization.

Ultrafast carrier dynamics in pentacene, functionalized pentacene, tetracene, and rubrene single crystals

We measure the transient photoconductivity in pentacene, functionalized pentacene, tetracene, and rubrene single crystals using optical pump-terahertz probe techniques. In all of the samples studied, we observe subpicosecond charge photogeneration and a peak photoconductive response that increases as the temperature decreases from 297 down to 20K, indicative of bandlike transport. Similar decay dynamics are observed at room temperature, but at low temperatures the decay dynamics measured in pentacene, rubrene, and tetracene crystals are much faster than those observed in functionalized pentacene crystals, revealing different charge trapping properties.

Graphene Conductance Uniformity Mapping

We demonstrate a combination of micro four-point probe (M4PP) and non-contact terahertz time-domain spectroscopy (THz-TDS) measurements for centimeter scale quantitative mapping of the sheet conductance of large area chemical vapor deposited graphene films. Dual configuration M4PP measurements, demonstrated on graphene for the first time, provide valuable statistical insight into the influence of microscale defects on the conductance, while THz-TDS has potential as a fast, non-contact metrology method for mapping of the spatially averaged nanoscopic conductance on wafer-scale graphene with scan times of less than a minute for a 4-in. wafer. The combination of M4PP and THz-TDS conductance measurements, supported by micro Raman spectroscopy and optical imaging, reveals that the film is electrically continuous on the nanoscopic scale with microscopic defects likely originating from the transfer process, dominating the microscale conductance of the investigated graphene film.

Anisotropy of transient photoconductivity in functionalized pentacene single crystals

We report on the anisotropy of transient photoconductivity in functionalized pentacene single crystals using ultrafast optical pump–terahertz probe techniques. Functionalized pentacene crystals with tri-isopropylsilylethynyl (TIPS) and tri-ethylsilylethynyl (TES) side groups were studied, characterized by crystal structures favoring two-dimensional and one-dimensional charge transports, respectively. Charge carrier mobility anisotropies in the a-b plane of 3.5±0.6 and 12±6 were obtained in the TIPS and TES crystals, respectively, consistent with the degree of π overlap along different directions in the crystals. A photogeneration efficiency anisotropy was also observed in both types of crystals.

Optical modulation of terahertz pulses in a parallel plate waveguide

In this work we present a technique for optically modulating a terahertz pulse inside a parallel plate waveguide. A novel semiconductor filled waveguide is formed by coating both sides of a thin, high resistivity silicon slab with a transparent conducting oxide. While the waveguide is intrinsically lossy due to the low conductivity of the oxides, it permits photoexcitation through the plates, generating free carriers within the silicon that modulates the terahertz pulse transmission. We demonstrate this modulation by observing the Drude response of photoexcited carriers within the silicon in a narrow strip inside the waveguide.

Intrinsic femtosecond charge generation dynamics in single crystal CH3NH3PbI3

Hybrid metal–organic perovskite solar cells have astounded the solar cell community with their rapid rise in efficiency while maintaining low-cost fabrication. The intrinsic material photophysics related to the generation of free charges, their dynamics and efficiency, however, remains to be understood. As fabrication techniques improve, larger crystal grain sizes have been shown to be a critical factor for improving both the optical and transport properties of the hybrid metal halide perovskites. In this work, we use pulses of multi-THz frequency light in the ultra-broadband 1–30 THz (4–125 meV) range to observe the ac conductivity in large single crystal CH3NH3PbI3. Our spectra reveal the ultrafast dynamics and efficiencies of free charge creation and extremely high charge carrier mobility as high as 500–800 cm2 V−1 s−1. While quasi-equilibrium analysis of efficiencies through the Saha equation suggests a binding energy on the order of 49 meV, an observed reflectance feature appearing at high pump fluence occurs at 12 meV and is consistent with an orbital transition of the exciton, indicating a much lower Rydberg energy of 17 meV at room temperature. The signature of the exciton is found to vanish on a 1 ps time scale commensurate with the appearance of mobile carriers, consistent with thermal dissociation of the exciton to the continuum in the room temperature tetragonal phase.

Anisotropic photoconductivity of InGaAs quantum dot chains measured by terahertz pulse spectroscopy

We report results of time-resolved terahertz (THz) pulse spectroscopy experiments on laterally ordered chains of self-assembled InGaAs quantum dots photoexcited with 400nm, 100fs laser pulses. A large anisotropy in the transient photoconductive response is observed depending on the polarization of the THz probe pulse with respect to the orientation of the dot chains. Fast (3.5–5ps) and efficient carrier capture into the dots and one-dimensional wetting layers underneath the dot chains is observed below 90K. At higher temperatures, thermionic emission into the two-dimensional wetting layers and barriers becomes significant and the anisotropy in the photoconductive signal is reduced.

Ultrabroadband terahertz conductivity of Si nanocrystal films

The terahertz conductivity of silicon nanoparticles embedded in glass with varying density is studied with ultra-broadband terahertz spectroscopy on picosecond time scales following fs optical excitation. The transition from relatively isolated charge carriers to densities which allow inter-particle transport is clearly observed. For the times immediately following carrier injection, we observe Drude-like long range transport that is rapidly replaced with a localized response on picosecond time scales. The localized response can be very well described by a phenomenological Drude-Smith model, verifying the applicability of this simple model to the conductivity of nanoparticle ensembles over the entire THz spectral window.

Carrier density dependence of the nonlinear absorption of intense THz radiation in GaAs

We study the carrier density dependence of nonlinear terahertz (THz) absorption due to field-induced intervalley scattering in photoexcited GaAs using the optical-pump/THz-probe technique. The intervalley scattering in GaAs is strongly dependent on the photo-carrier density. As the carrier density is increased from 1 × 10(17) to 4.7 × 10(17) cm(-3), the nonlinear absorption bleaching increases. However, if the carrier density is increased further above 4.7 × 10(17) cm(-3), the trend reverses and the bleaching is reduced. The initial increase in absorption bleaching is because, unlike low THz field, high THz field experiences intervalley scattering and nonparabolicity of the conduction band. On the other hand, a simple electron transport model shows that the reduction in intervalley scattering is mainly due to the increase in the electron-hole scattering rate with the increase in the carrier density. This increase in the electron-hole scattering rate limits the maximum kinetic energy attainable by the electrons and thus reduces the observed nonlinear absorption.