Above-band-gap pulsed optical excitation of electron-hole pairs within the surface-space-charge region of semiconductors alters the surface-space-charge field via free-carrier transport. We report on the direct observation of this ultrafast transient screening and the associated charge-carrier transport by applying reflective electro-optic sampling (REOS) with subpicosecond time resolution to (100)-oriented GaAs surfaces. The REOS measurements performed under different initial surface field conditions and various optical excitation densities are compared to numerical simulations of hotcarrier transport, including the calculation of the optical response. The simulations, which are based on a simple drift-diffusion model for optically excited electron-hole pairs, are in quantitative agreement with the experiment. The strength and sign of the static built-in field can be determined and the carrier drift velocities can be derived on a subpicosecond time scale.
Abstract.We report on details of coherent LO phonon generation in surface-space-charge regions of III-V-compounds by optical injection of free carriers with laser pulses of 50 fs duration at 2 eV. Both the dynamics of the transient surface field, as well as the coherent lattice vibration, are measured via electro-optic sampling techniques under different experimental conditions. The driving force for the coherent phonon vibration is the sudden depolarization of the crystal lattice due to ultrafast screening of the intrinsic electrical surface field by photoexcited free carriers. 78.47.+p, 42.65.Re, Since the invention of the CPM ring dye laser, pump-probe techniques have widely been used to study relaxation of nonequilibria in condensed matter down to the subpicosecond time regime [1,2]. In many of these experiments periodic modulations of transmission or reflectivity changes are observed [3][4][5][6][7][8]. They are related to coherent lattice vibrations, modifying the optical dielectric functions. First theoretical explanations of this phenomenon invoked impulsive stimulated Raman scattering (ISRS) [9]. There, the coherent phonons are driven by the difference-frequency of two laser fields contained within an ultrashort laser pulse of sufficient spectral bandwidth. More recently, a displacive excitation mechanism of coherent phonons (DECP) has been discussed. In this case the phonon amplitude is driven by sudden perturbations of the electronic system [10]. Anisotropic hole distributions constitute the driving force for coherent phonons in Ge [11]. PACS:A completely different excitation mechanism is found in polar III-V-materials. Optical injection of free carriers by ultrashort laser pulses leads to an ultrafast depolarization of the crystal lattice by screening the electrical field in the surface-space-charge region [7]. This process can be so fast, that coherent longitudinal lattice vibrations are launched.In this paper, a detailed investigation of the generation mechanism for coherent LO phonons in surface-spacecharge regions of III-V-compounds will be presented. In Sect. 1, the classical equation of motion for the harmonic oscillator is presented, including specific driving force terms for coherent longitudinal optical phonons. In sect. 2 the experimental apparatus for performing time-resolved pumpprobe experiments is outlined. We use a reflective electrooptic detection geometry (time resolved REOS technique [12,13]), which is sensitive to longitudinal electrical field changes. Detailed studies of the generation process for coherent LO phonons in surface-space-charge fields are presented in Sect. 3. These experiments confirm that coherent LO phonons in surface-space-charge regions are launched by an ultrafast longitudinal depolarization of the crystal lattice induced by the screening of the surface field via the photoexcited free carriers. Driving Forces for Coherent LO PhononsThe most distinct feature of a coherent vibration is the fixed phase relationship among individual modes. Under this condition the a...
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