The energy relaxation rate for hot electrons in n-type GaN epilayers has been measured over the temperature range 1.5–300 K. Several samples grown by molecular-beam epitaxy and having different electron concentrations have been studied. At low electron temperatures (Te<20 K), the energy relaxation is via acoustic phonon emission. The magnitude and temperature dependence of the energy relaxation are found to be in good agreement with theoretical calculations using appropriate values of the deformation potential and piezoelectric coupling constants and ignoring screening. For Te⩾70 K, the dominant mechanism of energy loss is optic phonon emission. For the several samples studied, consistent values of the optic phonon energy and electron-optic phonon relaxation time, 90±4 meV and 5–10 fs, respectively, are measured. The energy agrees well with values obtained by other methods and the relaxation time is consistent with theoretical calculations of the Fröhlich interaction and indicate that hot phonon effects are absent up to 10−8 W/electron dissipation.

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We have used heat pulse techniques to study the energy relaxation of a hot two-dimensional electron gas (ZOEG) in a GaAsIAIGaAs heterojunction. The 20EG was heated by applying short ( 2 loons) electrical pulses to the drainsource contacts of the device. The electrons lost energy by emitting phonons which were detected by a CdS bolometer on t h e opposite side of the GaAs substrate. A change in the nature of the phonon signal occurring at a n excitation level of about 5 p W per electron indicated a change in the phonon emission process. The corresponding electron temperature, T.. at which optic phonon emission is expected to become the dominant energy relaxation process was estimated to be about 60 K. At powers well below the change-over, we found that the energy loss rate per electron, P., due to acoustic phonon emission is proportional to E . At much higher powers, P, a exp(-hw,,lkT,), where hw,, is the longitudinal optic phonon energy. We obtained a value of 3.3ps for the electron-optic phonon scattering time, which is consistent with the range Of values found in the literature.

We have used bolometric detection to observe directly the phonons emitted by photoexcited carriers in the InAs/GaAs self-organized quantum-dot system. We find that about 74% of the energy lost by carriers in the InAs dots and wetting layer is via emission of low-frequency acoustic phonons and argue that this is facilitated by Auger scattering.

We have used pulse time-of-flight techniques to examine the phonon emission from an optically excited GaAs/AlAs superlattice structure. For laser excitation wavelengths shorter than 767 nm ͑the energy of E1HH1 transition͒, we detect a significant longitudinal acoustic phonon component directed in a narrow beam normal to the structure. Under identical excitation conditions, generation of coherent longitudinal acoustic phonons has previously been observed in this structure. We suggest that the excitation wavelength and angular characteristics of the longitudinal acoustic emission is consistent with those of propagating modes produced as coherent phonons ''leak'' from the superlattice structure. © 2000 American Institute of Physics. ͓S0003-6951͑00͒00246-1͔In recent years a number of experiments have been reported in which coherent phonons have been generated in semiconductor structures using ultrafast laser pulses. In the first experiments 1 coherent optical phonons were generated. However, more recently, coherent tetrahertz acoustic phonons have been generated by ultrafast excitation of electrons and holes in GaAs/AlGaAs quantum wells 2 and superlattices 3,4 and detected by surface deflection 2 and timeresolved reflectivity. 3,4 Specifically, it has been shown that coherent zone-folded longitudinal acoustic ͑LA͒ phonons are excited when a femtosecond laser pulse is shined onto a superlattice.3,4 The excitation process is attributed to impulsively stimulated Raman scattering which is expected to exhibit a strong resonant enhancement where the formation of an electron-hole pair is accompanied by the creation of a LA phonon. The acoustic miniband structure of the superlattice permits the creation of relatively high frequency phonons of low q with the principal feature being a triplet of phonon modes centered around q ϳ0 with sidebands at qϳ2k laser produced by backscattering. The central frequency corresponds to the unfolded zone boundary at qϭ2/d SL , where d SL is the period of the superlattice, leading to Ϸ2c LA /d SL , where c LA is the velocity of the LA phonons and, hence, ϭc LA /d SL which is typically around 500 GHz for the structures used. A triplet of modes centered on twice this frequency corresponding to the unfolded zone boundary at qϭ4/d SL is also observed with an intensity more than ten times weaker.The presence of the phonons has been observed by studying the reflection of a probe pulse from the sample surface following the excitation pulse.3,4 The reflected signal amplitude oscillates at a few hundred gigahertz which is attributed to modulation of the refractive index by the lattice vibrations. The modulation amplitude decays in a few nanoseconds. This is likely to be the result of the confined modes leaking into propagating monochromatic phonons. This has been observed in optical two-color pump-probe experiments but only over small distances ͑500 nm͒ 5 and not in phonon spectroscopy using incoherent detection schemes. In this work our aim has been to observe these propagating phonons directly using...