Ultrafast Carrier Thermalization in Hydrogenated Amorphous Silicon

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Ultrafast Carrier Thermalization in Hydrogenated Amorphous Silicon S. L. Dexheimer,1 C. P. Zhang,1 J. Liu,1 J. E. Young,1 and B. P. Nelson2 1 2

Department of Physics, Washington State University, Pullman, WA National Renewable Energy Laboratory, Golden, CO

ABSTRACT We present detailed studies of the initial relaxation processes of photoexcited carriers in hydrogenated amorphous silicon. We have carried out time-resolved measurements of the photoexcited carrier response in HWCVD a-Si:H thin films using a wavelength-resolved femtosecond pump-probe technique, in which an intense 35-fs pump pulse excites carriers in the sample and a time-delayed probe pulse measures the resulting change in optical properties as a function of time delay following the pump pulse. Measurements of the transient optical absorbance were carried out as a function of the density of excited carriers, sample temperature, and probe wavelength. These studies indicate fast carrier thermalization via phonon emission on a ~ 150 fs time scale and rapid phonon equilibration on a ~ 230 fs time scale.

INTRODUCTION An important unresolved issue in the dynamics of photoexcitations in amorphous silicon has been the determination of the time scale for the initial carrier thermalization processes. The carrier thermalization rate is a key parameter for understanding the transport properties of the material, and also provides important insight into the role of disorder in the physics of the carrier processes. In this work, we present femtosecond time-resolved optical measurements that reveal the carrier thermalization dynamics, in which carriers that are initially photoexcited into extended states in the conduction and valence bands relax in energy toward the band tails via phonon emission. In addition, we detect spectral signatures associated with the subsequent lattice equilibration, providing evidence for significantly enhanced phonon redistribution rates in the amorphous material relative to those characteristic of crystalline material. EXPERIMENT Thin films of a-Si:H were grown on glass substrates by the hot-wire chemical vapor deposition (HWCVD) technique. Time-resolved differential transmittance measurements were carried out as described previously [1]. Briefly, the optical pulses were generated using an amplified Ti:sapphire laser system operating at a repetition rate of 1 kHz. Pulses 35 fs in duration centered at 800 nm (1.55 eV) were used to excite the samples, and broadband probe pulses were generated from a compressed femtosecond white light continuum, giving an overall time resolution of ~ 40 fs in the pump-probe measurements. Wavelength resolution was achieved by spectrally filtering the probe beam with a ~ 10-nm bandwidth interference filter after it had been transmitted through the sample. All measurements were carried out with mutually A2.1.1 Downloaded from https://www.cambridge.org/core. Access paid by the UCSB Libraries, on 19 Jul 2018 at 10:33:42, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/term

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