Femtosecond hot carrier energy redistribution in GaAs and AlGaAs

Solid-State ElectronicsVol. 31, No. 3/4, pp. 443-446, 1988 Printed in Great Britain

0038-1101/88 $3.00+0.00 Pergamon Journals Ltd

F E M T O S E C O N D HOT C A R R I E R E N E R G Y R E D I S T R I B U T I O N IN GaAs AND AIGaAs

R. W. Schoenlein, W. Z. Lin, S. D. Brorson, E. P. Ippen, and J. G. Fujimoto Department of Electrical Engineering and Computer Science and Research Laboratory of Electronics Massachusetts Institute of Technology Cambridge, MA 02139

ABSTRACT Excited carrier dynamics in GaAs and A1.2Ga.sAs are investigated using femtosecond pump and continuum probe techniques. Absorption saturation measurements provide evidence for transient spectral hole burning due to split-off as well as heavy and light hole valence to conduction band transitions. The initial nonthermal carrier distribution thermalizes and assumes a broad energy distribution on a femtosecond time scale. KEYWORDS GaAs, semiconductor, hot-carrier, nonthermal distribution, intervalley scattering, femtosecond. Studies of hot carrier dynamics in GaAs are of scientific interest because they involve the fundamental processes of intercarrier scattering and electron-phonon coupling. Furthermore, an understanding of these processes has important applications for high speed electronic and electro-optic devices. Previous studies of excited carrier dynamics in GaAs have focussed on band edge renormalization, intercarrier scattering, and electron-phonon interaction. Femtosecond pump and continuum probe techniques have been used to demonstrate band edge renormalization on a femtosecond time scale (Shank, 1979; Leheny, 1979). Single frequency femtosecond pump probe (Lin, 1987a, 1987b) and two pulse correlation (Taylor, 1985; Erskine, 1984) measurements have been applied to investigate scattering times from nonthermal electronic distributions. P u m p and continuum probe techniques have been used to study transient spectral hole burning (Cruz, 1986; Oudar, 1985; Knox, 1987). R a m a n scattering (Kash, 1985) and photoluminescense ( Tsang, 1986) have been applied to study electron-phonon interaction and intervalley scattering, and degenerate four wave mixing (Oudar, 1984a, Oudar, 1984b) has been used to study orientational relaxation and absorption anisotropy. In this paper we present studies of hot carrier thermalization and energy relaxation using femtosecond pump and continuum probe techniques. We observe spectral hole burning due to a transient nonthermal carrier distribution, and a rapid energy redistribution or thermalization on a time scale of several tens of femtoseconds. Subsequent cooling of the thermalized electronic distribution occurs on a picosecond time scale. Time resolved studies of hot carrier dynamics in semiconductors have been made possible by recent advances in femtosecond optical pulse generation and amplification. Our studies utilize a pump probe technique in which an amplified femtosecond pulse is used to excited carriers in the sample while a second pulse, delayed by translation stage, probes the change in transmission, A T / T , as a measure of the transient electronic distribution. Ultrashort pulses, ~ 45 fs duration F W H M at 625 nm (1.99 eV), are generated by a dispersion-balanced colliding pulse modelocked ring dye laser (Valdmanis, 1985). These pulses are amplified at 8 kHz to energies in excess of 2 ~J by a copper vapor laser pumped dye amplifier (Knox, 1984). Excited carrier relaxation is probed by a broadband femtosecond continuum generated through the nonlinear process of self-phase modulation. We combine lock-in amplification with real-time differential detection by chopping the pump beam, and simultaneously analyzing the transmitted probe and a reference from the continuum with a tunable monochromator. Time resolved measurements of absorption saturation were performed in 0.5 ~ m thick samples of GaAs and A1.2Ga.sAs with thin Al.ssGa.15As cladding layers grown by liquid phase epitaxy. All measurements were performed at room t e m p e r a t u r e at carrier densities of ~ 1018 cm -s. Amplified femtosecond pulses excite hot carriers in GaAs and A1.2Ga sAs via optical transitions from the valence bands to the conduction b a n d near k = 0. The initial nonthermal distributions thermalize, and cool to the lattice temperature through intercarrier scattering and electron-phonon interaction.

443

444 Figure 1 shows the band structure of GaAs near k = 0 and indicates the allowed optical transitions for the 1.99 eV pump energy. The dashed lines are a schematic representation of the initial nonthermal carrier distributions generated at N 500 and ~ 150 meV above the conduction band minimum via optical transitions from the split-off band as well as the heavy and light hole bands. The dotted line represents the excited carrier distribution after reaching thermal equilibrium with the lattice. Both intercarrier scattering and electron-phonon interaction contribute to the thermalization of the initial distribution. Thermal equilibrium with the lattice proceeds through electron-phonon scattering. Furthermore, carriers excited from the heavy and light hole bands have sufficient excess energy to scatter to the X and L valleys through phonon interaction. Figure 2 shows time resolved measurements of the absorption saturation dynamics for various probe photon energies. An excited carrier density of ~ 10 is cm -s is generated by a pump pulse of ~ 60 fs duration at 625 nm (1.99 eV). The magnitude of the absorption saturation, AT~T, at each photon energy is indicated; however, the curves have been normalized for display purposes. At 2.0 eV probe energy, we observe a two component response of the absorption saturation. Initial saturation corresponds to the presence of excited carriers generated by optical transitions from the split-off, heavy hole and light hole bands. The fast recovery component is comparable to the incident pulse duration and indicates the rapid scattering of carriers out of their initial excited states. The picosecond response results from a cooling of the hot electron distribution to the lattice temperature. This result is in agreement with our previous single frequency transient absorption saturation measurements (Lin, 1987a, 1987b).

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Absorption measurements for photon energies ranging from 1.88 to 2.07 eV display an initial saturation and partial recovery within the first 100 fs. This behavior indicates a spectral absorption hole resulting from an initial nonthermal carrier distribution. The N 200 meV width of the hole is an indication of the initial scattering time. However, the initial nonthermal distribution consists of carriers excited from both the heavy and light hole bands. Furthermore, the carrier distribution excited from the split-off band is also probed. Thus, a more quantitative analysis of the hole width must take these effects into account. Absorption saturation traces for photon energies between 1.68 and 1.73 eV display a similar ultrafast partial recovery corresponding to the nonthermal electronic distribution excited from the split-off band. Experimental measurements for all photon energies show a rapid onset of absorption saturation. This indicates that excited carriers scatter out of their initial states and assume a broad energy distribution within the first several tens of femtoseconds. The ultrafast time scale of this process suggests intercarrier scattering, however, electron-phonon interaction may also contribute to the initial energy redistribution. Other investigators have used hot carrier luminescence (Tsang, 1986) to demonstrate the importance of

445

intercarrier scattering at high carrier densities. The rising absorption saturation observed at 1.55 eV, near the band edge, may be explained by an increasing electronic occupancy below the Fermi energy as the thermalized hot electron distribution cools to the lattice temperature. The rising signature appears again for photon energies above 1.78 eV because optical transitions from the split-off band begin to probe the electronic distribution below the Fermi energy. Additionally, carriers that scatter to the X and L sattelite valleys may return to the r valley on a 2-3 ps time scale and will contribute to the observed response (Osman, 1987). The decaying response for photon energies above 2.0 eV is consistent with thermal equilibration with the lattice via electron-phonon interaction. In order to eliminate the effects of the split-off band, similar measurements have been performed in A12Ga.sAs which has a band gap of 1.68 eV. Figure 3 shows the band structure of Al.~Ga.sAs near k --0, and indicates the allowed optical transitions. The dashed line represents the initial nonthermal carrier distribution generated N 300 meV above the conduction band minima via optical transitions from the heavy and light hole bands. These excited carriers have sufficient excess energy to scatter to the L valley, but X valley scattering is not allowed. Figure 4 shows time resolved measurements of absorption saturation dynamics in AI.2Ga.sAs for various probe photon energies with excited carrier densities of ~ 1018 cm -s. Measurements near the pump energy, 1.88 to 2.0 eV, display an initial absorption saturation and ultrafast partial recovery characteristic of a transient spectral hole. This results from an initial nonthermal carrier distribution due to optical excitation from the heavy and light hole bands. Furthermore, the rapid onset of absorption saturation is evident at all probe energies. This indicates that the carriers scatter from the initial nonthermal distribution and assume a broad energy distribution within the first ~ 100 fs. The picosecond response behavior is consistent with a cooling of the thermalized distribution to the lattice temperature via electron-phonon interaction as well as intervalley scattering effects. The rising signal observed at 2.07 eV results because of probe transitions from the split-off band to the conduction band minimum. Since L valley scattering is allowed at these pump energies~ some contribution to the observed response from intervalley scattering is expected.

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-0,5 0 0.51.0 1.5 2.0 2.5 DELAY (PS) Fig. 4. Transient absorption saturation measurements in A10.2Ga0.sAs at probe photon energies, Epro, ranging from 1.78 to 2.07 eV at a carrier density of 101 8 cm- 3 . AT/T indicates the transient transmission change for each probe energy.

446 In summary, femtosecond pump and continuum probe techniques have been used to study hot carrier energy relaxation in GaAs and AI.2Ga.sAs. Transient absorption saturation holes are observed, and correspond to initial nonthermal carrier distributions generated by optical transitions from the heavy and light hole bands and also from the split-off band in GaAs. The ultrafast thermalization of these holes indicates that electrons scatter from the initial nonthermal distribution within tens of femtoseconds. Furthermore, the rapid onset of absorption saturation observed at all probe energies demonstrates that the excited carriers assume a broad energy distribution within the first 100 fs. Intercarrier scattering is believed to play an important role in this initial scattering process because of the high density of excited carriers. The response of the absorption saturation signal after the initial excitation is consistent with thermal equilibration of the hot carrier distribution with the lattice, as well as intervalley scattering effects. The increase in transmission on a picosecond time scale may be attributed to decreased absorption for probe photons with energies below the Fermi energy as the distribution cools and carriers scatter from satellite valleys. Further studies at different carrier densities may provide additional information about the role of intercarrier scattering in the initial thermalization process. Additional pump probe measurements using a tunable femtosecond source should provide a better understanding of intervalley scattering effects. ACKNOWLEDGMENTS We wish to thank R. A. Logan from AT&T Bell Laboratories for providing the samples used in this investigation. This work was supported in part by the Air Force Office of Scientific Research Grant 85-0213. JGF acknowledges support from the National Science Foundation Presidential Young Investigator Program Grant ECS-8552701. WZL is visiting from Zhongshan University, Guangshou, People's Republic of China. REFERENCES Cruz, C. H. Brito, R. L. Fork, and C. V. Shank (1986). Annual Meeting of the Optical Society of America, Paper I-ThB4. Erskine, D. J., A. J. Taylor, and C. L. Tang (1984). Appl. Phys. Lett., 45, 54. Kash, J. A., J. C. Tsang, and J. M. Hvam (1985). Phys. Rev. Lett., 54, 2151. Knox, W. H., and D. S. Chemla (1987). International Quantum Electronics Conference. Paper PDI-1 XV I. Knox, W. H., M. C. Downer, and C. V. Shank (1984). Opt. Lett., 9, 552. Leheny, R. F., J. Shah, R. L. Fork, C. V. Shank, and A. Migus (1979). Solid State Commun. 31,809. Lin, W. Z., J. G. Fujimoto, E. P. Ippen, and R. A. Logan (1987a). Appl. Phys. Lett., 50, 124. Lin, W. Z., J. G. Fujimoto, E. P. Ippen, and R. A. Logan (1987b). Appl. Phys. Lett., to be published July 1987. Osman, M. A., M. J. Kann, D. K. Ferry, and P. Lugli (1987). Picosecond Electronics and Optoelectronics Conference, Paper ThB3. Oudar, J. L., I. Abram, and C. Minot (1984a). Appl. Phys. Lett., 44, 689. Oudar, J. L., A. Migus, D. Hulin, G. Grillon, J. Etchepare, and A. Antonetti (1984b). Phys. Rev. Lett., 53, 384. Oudar, J. L., D. Hulin, A. Migus, A. Antonetti, and F. Alexandre (1985). Phys. Rev. Lett., 55, 2074. Shank, C. V., R. L. Fork, R. F. Leheny, and J. Shah (1979). Phys. Rev. Lett., 42, 112. Taylor, A. J., D.J. Erskine, and C.L. Tang (1985). J. Opt. Soc. Am. B, 2, 663. Tsang, J. C., and J.A. Kash (1986). Phys. Rev. B, 34, 6003.