We investigate the relaxation dynamics of photogenerated carriers in low-temperature grown GaAs by femtosecond pump-probe measurements. The carrier dynamics in the vicinity of the band edge is disentangled in a two-color technique. The filling of shallow bound states close beneath the band edge is resolved. A temporal delay in the occupation of these states as well as a large optical nonlinearity points towards microscopic potential fluctuations forming these states. © 1997 American Institute of Physics. ͓S0003-6951͑97͒00345-8͔In the last few years the interest in low-temperature grown ͑LT͒ semiconductors rapidly increased because of their unique electrical and optical properties and the resulting wide range of optoelectronic applications. The most promising material is LT GaAs grown at temperatures T g ϭ180°Ϫ300°C by molecular beam epitaxy ͑MBE͒. After annealing at T a ϭ400°Ϫ800°C LT GaAs exhibits an extraordinary high resistance in conjunction with a relatively high photocarrier mobility. 1 Important for THz devices are ultrashort carrier lifetimes in LT GaAs, which can be tailored into the sub-picosecond range. 2-4 All these properties are assigned to the large excess of As ͑1%-2%͒ incorporated during the growth process. In postgrowth annealed LT GaAs, As precipitates of a few nm in diameter nucleate. 5 The density of precipitates and point defects can be tailored with T g and T a and hence the electronic and optic properties can differ largely as a function of these temperatures. Two competing models have been proposed for explaining the properties of LT GaAs. In the defect model, EL2-like point defects stemming from excess As play the most important role. 6 The alternative model assigns the properties of LT GaAs to the formation of As-clusters in a buried Schottkybarrier model. 7 Ultrashort carrier lifetimes are attributed to either trapping in point defects or in As clusters.Presently, LT GaAs is used in photomixing, 8 ultrafast photoconductive switches or substrate material for THz antennas. 9,10 For an optimum performance of these devices the lifetime of photogenerated carriers is crucial. The device bandwidth should be closely related to 1/ , where is the lifetime of mobile carriers. However, a distinct difference is often observed between and the device bandwidth, which can be more than a factor of 2 smaller than 1/ . On the one hand, this difference certainly is influenced by the capacity and the impedance matching of the device, on the other hand carriers trapped in shallow potentials may be reexcited thermally or by the THz electric fields, thus reducing the optimum performance concerning bandwidth and noise.In this letter, we carefully inspect the dynamics of photogenerated carriers close to the band edge in thin LT GaAs films. Two color time-resolved transmission changes allow to resolve the carrier relaxation close to the band edge with high energetic and temporal resolution. We obtain information on shallow bound sub-band-gap states and point out their relevance for the optoelectronic properties...
Bloch oscillations excited in a biased GaAs/Al x Ga 1Ϫx As superlattice are investigated in a time-resolved two-color electro-optic detection scheme. The detection of resonantly excited Bloch oscillations is based on the Pockels effect probed at a wavelength in the center of the band gap. The observed birefringence is induced by the macroscopic polarization of the electronic wave packets relative to the localized holes. The off-resonant detection away from optical transitions directly monitors the spatial dynamics of the electrons in amplitude and phase. The dependence of the amplitudes of the Bloch oscillating electrons on the applied electric fields are in good agreement with the electron-hole dipole lengths calculated by a quantum-mechanical model. Bloch1 and Zener 2 have proposed in their theoretical studies on the dynamics of electronic wave packets in periodic potentials a temporal and spatial oscillation of the carriers in the presence of a static electric field. The frequency and the spatial amplitude L of these Bloch oscillations ͑BO's͒ are ϭeFd/h and Lϭ⌬/eF, ͑1͒respectively, where F is the applied electric field, d the potential period, h Planck's constant, and ⌬ the electronic bandwidth. As proposed by Esaki and Tsu, this intriguing phenomenon can be realized in a biased semiconductor superlattice ͑SL͒.3 Here, BO's are excited by the coherent superposition of several electronic Wannier-Stark ͑WS͒ states 4,5 with a common hole state by short laser pulses. The localization lengths of the electronic wave packets deviate from the semiclassical picture of BO's ͓Eq. ͑1͔͒ if a fully quantum-mechanical calculation of the wave functions in the artificial semiconductor structure is used.6 Additionally, the amplitude of optically excited BO's depends on the exciting laser pulse width and energy that determine the initial conditions of the wave packets. Pathbreaking investigations on BO's have been performed in four-wave-mixing, 8,9 THz emission, 10 and resonant transmittive electro-optic sampling ͑TEOS͒ experiments.11 In recent years, the experimental determination of the spatial amplitude of the oscillating electronic wave packets has become an objective of the studies. The amplitude determination of BO's in a SL has been reported in a THz emission experiment.12 The amplitudes were calculated from the detected THz radiation power by applying a superradiance theory. Subsequently, the BO amplitude has been carefully determined in spectrally and time-resolved four-wave mixing experiments.13 Using this technique, the dependence of the amplitudes on the spectral position of the exciting laser pulses was investigated.14 Resonant TEOS experiments performed with optical detection spectrally integrated over WS transitions can only provide a qualitative analysis of the electronic dynamics due to nonlinearities of the electro-optic effect at the interband resonances.In this paper, we present the off-resonant electro-optic detection of Bloch oscillations in a GaAs/Al x Ga 1Ϫx As SL. BO's are excited resonantly, but detecte...
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