1,721,030 research outputs found
Carbon-acceptor-induced cascade scattering by acoustic phonons above the (e,A(0)) threshold in GaAs
No full textBelow the E-0 band gap of GaAs the transitions of free electrons to bound impurity states play an important role. Our study of Raman scattering above the (e,A(0)) threshold at 2 K shows that the electrons are scattered by acoustic phonons before successively being scattered by one LO phonon and finally recombining the holes bound to the acceptors. This finding indicates one of important mechanisms of relaxation of the hot electrons excited above the minimum of conduction band
Impurity-induced resonance Raman scattering at the (e, A(0)) threshold in lightly carbon-doped p-type GaAs at 2 K
No full textIn lightly carbon-doped p-type GaAs the impurity-induced resonance Raman scattering (RRS) by one longitudinaloptic (LO) phonon has been studied at low temperature by using a tunable Ti-sapphire laser. In spite of involvement of strong luminescence in the spectral region of the Raman scattering, a pronounced scattering resonance at the (e, A(0)) threshold has been well identified. Both the extrinsic LO-Raman scattering and the luminescence of (e, A(0)) LO phonon replicas have been analyzed in a quantitative way. We find that carbon-acceptor-induced extrinsic scattering is fairly intense compared to the intrinsic Raman scattering. It is interpreted in terms of the large oscillator strength of the (e, A(0)) transition. Compared to the intrinsic scattering due to free electron-hole pairs, the large (e, A(0)) oscillator strength leads to about two orders of magnitude enhancement in the Raman intensity. (C) 2002 Elsevier Science B.V. All rights reserved
Carbon-acceptor-induced cascade scattering by acoustic phonons above the (e,A(0)) threshold in GaAs
No full textBelow the E-0 band gap of GaAs the transitions of free electrons to bound impurity states play an important role. Our study of Raman scattering above the (e,A(0)) threshold at 2 K shows that the electrons are scattered by acoustic phonons before successively being scattered by one LO phonon and finally recombining the holes bound to the acceptors. This finding indicates one of important mechanisms of relaxation of the hot electrons excited above the minimum of conduction band
Electron-hole plasma expansion in GaAs: Submicron optical time of flight investigations
No full textPump and probe differential reflection (Delta R) and transmission (Delta T) measurement of subpicosecond light pulses for either co- and counterpropagating pump and probe geometries are performed on thin GaAs samples. With this time of flight (TOF) method we study density dependent electron-hole plasma (EHP) expansion in GaAs perpendicular to the sample surface in the temperature range of 300 K greater than or equal to T-L greater than or equal to 4 K. At a fluence of F = 800 mu Jcm(-2) the expansion velocities increase with decreasing lattice temperature T-L. The expansion velocities v at T-L = 300 K and 4 K show a dependence on the delay time tau like v proportional to tau(-2/3). At low lattice temperatures we find a much stronger dependence of the expansion velocities on the fluence of the pump pulses
Diffusive atom transport along step edges on Ag(111) at 295 K
No full textThe entropy-driven relaxation of a unique, non-equilibrium step edge configuration on the Ag(111) surface was observed using time-resolved STM imaging at room temperature. Using the Gibbs-Thomson relation, the relaxation process is quantitatively described as diffusive mass transport in terms of a gradient in the chemical potential along the monoatomic step edge. The STM data directly show that mass transport on Ag(111) is dominated by step edge diffusion at 295 K, and allow an estimate of the corresponding effective energy barrier. We obtain E-eff = 0.49 +/- 0.05 eV and compare this value with recent results on island diffusion studies. (C) 2003 Elsevier B.V. All rights reserved
Asymmetry of acceptor wave functions caused by surface-related strain and electric field in InAs
No full textThe spatial distribution of the local density of states at Mn acceptors near the (110) surface of p-doped InAs is investigated by scanning tunneling microscopy. The shapes of the acceptor contrasts for different dopant depths under the surface are analyzed. Acceptors located within the first ten subsurface layers of the semiconductor show a lower symmetry than expected from theoretical predictions for the bulk acceptor wave function. They exhibit a (001) mirror asymmetry. The degree of asymmetry depends on the acceptor atoms' depths. The measured contrasts for acceptors buried below the tenth subsurface layer closely match the theoretically derived shape. Two effects are able to cause the observed symmetry reduction, i.e., the strain field of the surface relaxation and the tip-induced electric field. While both effects induce similar asymmetries, a comparison of their relative strengths indicates that surface-related strain is the dominant effect for Mn in InAs
Band structure related wave-function symmetry of amphoteric Si dopants in GaAs
No full textAutocompensated Si-doped GaAs is studied with cross-sectional scanning tunnelling spectroscopy (X-STS). The local electronic contrasts of substitutional Si(Ga) donors and Si(As) acceptors under the (110) cleavage plane are imaged with high resolution. Si(Ga) donor atoms exhibit radially symmetrical contrasts. Si(As) acceptors have anisotropic features. The anisotropic acceptor contrasts are traced back to a tunnel process at the valence band edge. They reflect the probability density distribution of the localized acceptor hole state. (C) 2008 Elsevier Ltd. All rights reserved
Mapping Itinerant Electrons around Kondo Impurities
No full textWe investigate single Fe and Co atoms buried below a Cu(100) surface using low temperature scanning tunneling spectroscopy. By mapping the local density of states of the itinerant electrons at the surface, the Kondo resonance near the Fermi energy is analyzed. Probing bulk impurities in this well-defined scattering geometry allows separating the physics of the Kondo system and the measuring process. The line shape of the Kondo signature shows an oscillatory behavior as a function of depth of the impurity as well as a function of lateral distance. The oscillation period along the different directions reveals that the spectral function of the itinerant electrons is anisotropic.Deutsche Forschungsgemeinschaft [SFB 602
Differential interferometric particle tracking on the subnanometer- and submillisecond-scale
No full textWe describe an interferometric method to measure the movement of a subwavelength probe particle relative to an immobilized reference particle with high spatial (Delta x = 0.9nm) and temporal (Delta t = 200 mu s) resolution. The differential method eliminates microscope stage drift. An upright microscope is equipped with laser dark field illumination (lambda(0) = 532nm, P-0 = 30mW) and a compact modified Mach-Zehnder interferometer is mounted on the camera exit of the microscope, where the beams of scattered light of both particles are combined. The resulting interferograms provide in two channels subnanometer information about the motion of the probe particle relative to the reference particle. The interferograms are probed with two avalanche photodiodes. We applied this method to measuring the movement of kinesin along microtubules and were able to resolve the generic 8-nm steps at high ATP concentrations without external forces. (c) 2013 Optical Society of Americ
Direct observation of Fermi-pressure-driven electron-hole plasma expansion in GaAs on a picosecond time scale
No full textApplying pump and probe differential reflection (Delta R) and transmission (Delta T) of femtosecond light pulses inr either co- or counterpropagating pump and probe geometries, a direct time of flight method with submicrometer resolution is presented. With this technique we study the density-dependent transport of photogenerated carrier plasmas perpendicular to the surface of GaAs samples for delay times 20 ps less than or equal to tau less than or equal to 1 ns. At a pump fluence of 800 mu J cm(-2) a relatively sharp charge-carrier front was observed, with high velocities of 14 x 10(5) cm/s at a delay time tau approximate to 20 ps, decreasing as v proportional to tau(-2/3) to 2 x 10(5) cm/s at tau approximate to 350 ps. The arrival times tau of the carriers at a fixed sample thickness depend on the fluence of the pump pulses F such as tau proportional to 1/F-0.45. The results are discussed in the framework of diffusive transport with a strongly density-dependent diffusivity D. The data can be described consistently with the assumption of Fermi pressure as the dominating driving force fur plasma expansion
- …
