Ultrafast Acoustic Phonon Ballistics in Semiconductor Heterostructures
Using a two-color ultrafast surface deflection spectroscopy, we demonstrate the time-resolved observation of acoustic phonon wave packets emitted from a single buried GaAs quantum well. A longitudinal acoustic phonon pulse is generated at a preselected depth within a highly confined region ($10 nm thick) through the coherent nonequilibrium deformation potential which was previously unobserved in such structures. Subsequent detection with subpicosecond resolution at the surface resolves propagating high-wave-vector ballistic phonons and quasiballistic or “snake” phonons which subsequently merge into a quasidiffusive phonon pulse. [S0031-9007(97)03074-3] PACS numbers: 73.20.Dx, 42.65.Re, 63.20.Kr, 78.47. + p Much is known about the energy relaxation of electrons in semiconductors, mainly due to the success of optical spectroscopies [1]. These can track both the hot carriers and the emitted optic phonons which absorb the bulk of the excess electron energy. Detailed studies have been driven by new theories which cope with the many-body electron-electron and electron-phonon interactions, and by growing numbers of technological applications. Recently developed ultrafast laser sources with pulse widths shorter than a single oscillation period of the optic phonon in semiconductors now enable direct temporal observation of the high-frequency long-wavelength lattice response [2]. However, in spite of this, only part of the system is ever being probed—the ultimate sink of the excess carrier energy is an acoustic phonon cascade. Initially, the acoustic phonons are highly nonthermal and coherent, undergoing both rapid energy conversion and elastic scattering off isotope and alloy fluctuations in the semiconductor [3]. Capabilities for probing THz acoustic phonons have improved in recent years, predominantly in metallic systems [4]. However, it has proved extremely difficult to access the early acoustic phonon distributions in semiconductors and the mechanisms that act to thermalize them. Here, we introduce a complementary ultrafast optical technique which allows the study of acoustic pulses generated by exciting electrons and holes in a single GaAs quantum well (QW). Using pump and probe pulses of different wavelengths and an appropriately designed wafer cross section, carriers are selectively excited at a specific depth and the emitted phonons probed at the sample surface. In principle, this allows the emission processes to be discriminated and correlated with the carrier cooling dynamics. Using a single QW degrades the signal-to-noise ratio compared with analogous experiments on multiple wells or superlattices [5], but the discrimination of the phonon generation region in space and time, coupled with the detection at a preselected distance away, allows more concrete conclusions to be drawn from the observations. Initial results show ballistic phonon wave packets emerging from the quantum well and a quasiballistic and quasidiffusive phonon cascade from the GaAs substrate. These experiments open the way to the investigation of the generation, propagation, and detection of acoustic phonons in geometries relevant to heat transport and carrier relaxation in optoelectronic devices such as semiconductor lasers. Samples grown by molecular beam epitaxy on (001) semi-insulating GaAs substrates contain single quantum wells of GaAs. These are placed at different depths relative to the sample surface, surrounded by Al0.3Ga0.7As barrier layers, and capped at the surface with GaAs [Fig. 1(a)]. Growth was characterized by electron diffraction, and produced samples of high quality as shown by sub-meV photoluminescence linewidths with negligible Stokes shift. The 70 fs pulsed output of a mode-locked Ti:sapphire laser is tuned to excite electron-hole pairs in the lowest quantum-confined subband of the GaAs QWs FIG. 1. (a) Vertical structure of wafers A, B, and C. ( b) Schematic of the experimental arrangement (F = filter, BS = beam splitter, T › 300 K). (c) Calculated strain pulse dynamics after the QW is permanently stressed.