Excitation of electron-hole pairs in low-energy electron scattering from surfaces

We calculate the Distorted Wave Born Aprroximation differential cross section for the inelastic scattering of low-energy electrons reflected from metallic surfaces via the dynamically screened Coulomb potential, and in particular the excitation of low-energy electron-hole pairs. We discuss the angular and energy dependence of the loss spectra, and the application of this method to study the spectrum of the electron-hole excitations at surfaces.     

Thermal emission spectrum from a semiconductor electron-hole plasma

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 55, No. 4, pp. 624–629, October, 1991.     

Photoexcitation of electron-hole pairs during SIMS

During analysis with SIMS (secondary ion mass spectroscopy) a HeNe laser beam was focussed on the sample surface. While sputtering Si with oxygen ions, the laser irradiation produced a strong increase of the target current and the SIMS intensities as well. This was found for lightly p-doped Si only, whereas no effect was observed for highly p-doped or n-doped Si. To explain this we assume that a depletion layer exists under the surface oxide layer and free charged carriers are created therein by laser excitation. The laser induced effects observed in the SIMS intensity or the target current can be used for measuring the profile of an ion beam or for measuring the alignment of an ion beam at a laser marked target. In addition, laser irradiation combined with SIMS allows one to measure qualitatively both the profile of the doping impurity and its electrically active part.     

Ultrafast Fano resonance between optical phonons and electron-hole pairs at the onset of quasiparticle generation in a semiconductor

Based on the many-body time-dependent approach applied to the ultrafast time region, we investigate the dynamics of creation of an optical phonon incorporating with the electron-hole continuum in a semiconductor. In the transient Fano resonance, due to an interference between those sharp (optical phonon) and continuum (electron-hole pair) quasiparticles, we find the robust destructive interference at birth of them, i.e., tau approximately 0 if the created phonon is coherent under the irradiation of ultrashort optical pulses. The origin is found to be the potential scattering of the electron-hole pair by the q=0 coherent phonon. This finding agrees well with the recent experiment.     

Effect of electron-hole pairs on the melting of silicon

We compute the variation of the melting temperature T_m of silicon, with the density np of electron-hole pairs created by laser illumination: T_m0(1?αnp)2, where α^?11 = 8 × 10²¹ cm^?3. A similar formula is given for amorphous silicon and the application to laser annealing of damaged silicon is discussed.     

Role of patterning in islands nucleation on semiconductor surfaces

Quantum dots (QDs) grown on semiconductors surfaces are actually the main researchers' interest for applications in the forthcoming nanotechnology era. New frontiers in nanodevice technology rely on the precise positioning of the nucleation site and on controlling the shape and size of the dots. In this article we will review some recent studies regarding the control of the nucleation process on semiconductor surfaces. A few approaches to form ordered patterns on surfaces are described: natural patterning induced by surface instabilities (as step bunching or step meandering), in situ substrate patterning by Scanning Tunneling Microscopy (STM), high resolution patterning by Focused Ion Beam (FIB). Growth of epitaxial layers of semiconductors (Ge/Si(100) and InAs/GaAs(100)) on patterned surfaces has been studied by STM or Atomic Force Microscopy (AFM) unveiling the way in which the first atoms start to aggregate and identifying their exact nucleation site. Control of the dot size to match the patterning typical wavelength has been achieved by using surfactants on misoriented substrates. STM images acquired in real time allows one to identify the mechanism of Ge cluster formation on patterned Si(100), and to follow the island transition from pre-pyramid to pyramid. Nucleation of ordered Ge dots on SiO₂ substrates has been obtained thanks to FIB tight patterning, achieving island densities of 3.5×10¹⁰/cm². To cite this article: N. Motta et al., C. R. Physique 7 (2006).     

Role of electron-hole pair excitations in the dissociative adsorption of diatomic molecules on metal surfaces

We quantitatively evaluate the contribution of electron-hole pair excitations to the reactive dynamics of H2 on Cu(110) and N2 on W(110), including the six dimensionality of the process in the entire calculation. The interaction energy between molecule and surface is represented by an ab initio six-dimensional potential energy surface. Electron friction coefficients are calculated with density functional theory in a local density approximation. Contrary to previous claims, only minor differences between the adiabatic and nonadiabatic results for dissociative adsorption are found. Our calculations demonstrate the validity of the adiabatic approximation to analyze adsorption dynamics in these two representative systems.     

Quantum partition noise of photon-created electron-hole pairs

We show experimentally that even when no bias voltage is applied to a quantum conductor, the electronic quantum partition noise can be investigated with GHz radio frequency excitation. Using a quantum point contact configuration as the ballistic conductor we are able to make an accurate determination of the partition noise Fano factor resulting from the photon-assisted shot noise. Applying both voltage bias and rf irradiation we are able to make a definitive quantitative test of the scattering theory of photon-assisted shot noise.     

Interaction-assisted propagation of Coulomb-correlated electron-hole pairs in disordered semiconductors

A two-band model of a disordered semiconductor is used to analyze dynamical interaction induced weakening of localization in a system that is accessible to experimental verification. The results show a dependence on the sign of the two-particle interaction and on the optical excitation energy of the Coulomb-correlated electron-hole pair. Received 4 September and Received in final form 26 November 1998     

Singlet-triplet splitting of geminate electron-hole pairs in conjugated polymers

The singlet-triplet splitting of geminate polaron pairs in a ladder-type conjugated polymer has been studied by the thermally stimulated luminescence technique. The energy gap separating the singlet and triplet states of the geminate pairs is measured to be in the range of 3-6 meV, depending on the polymer morphology. The results of correlated quantum-chemical calculations on a long ladder-type oligomer are fully consistent with the observed values of the geminate polaron pair singlet-triplet gap. Such low splitting values have important implications for the spin-dependent exciton formation in conjugated polymers.     

Theoretical analysis of desorption from surfaces

The general theory of the desorption of atoms from surfaces is discussed and a method given for the calculation of the surface residence time. Potential parameters are deduced for a number of experimental systems using a variety of adsorption sites to model the interactions. By using this technique valuable information about the surface-atom potential may be obtained.     

THz emission from semiconductor surfaces

We provide a review of experimental and theoretical work on electromagnetic terahertz pulse emission from semiconductor surfaces excited by femtosecond laser radiation. The main terahertz emission mechanisms are analysed. The terahertz emission from InAs and Ge is explained by the photo-Dember effect and electric field induced optical rectification. Electronic band structure and carrier scattering mechanisms are investigated by means of terahertz emission and absorption spectroscopy in InAs, InSb and Ge. To cite this article: V.L. Malevich et al., C. R. Physique 9 (2008).     

Entangled photon pairs from semiconductor quantum dots

Tomographic analysis demonstrates that the polarization state of pairs of photons emitted from a biexciton decay cascade becomes entangled when spectral filtering is applied. The measured density matrix of the photon pair satisfies the Peres criterion for entanglement by more than 3 standard deviations of the experimental uncertainty and violates Bell's inequality. We show that the spectral filtering erases the "which path" information contained in the photons' color and that the remanent information in the quantum dot degrees of freedom is negligible.     

Instability of photoexcited electron-hole plasma in short semiconductor structures

The instability of the electron-hole plasma produced by continuous photoexcitation in short semiconductor structures is investigated theoretically. The applied electric field is considerably disturbed by photogenerated charge carriers. At a sufficiently intensive photogeneration plasma instability occurs. The frequency of current oscillations due to the instability, as shown by numerical simulation for a GaAs structure, is in the range of 10¹¹–10¹²s^–1.     

Excitation-induced atomic desorption and structural instability of III–V compound semiconductor surfaces

We discuss the laser-induced structural instability of III–V compound semiconductor surfaces. The electronic instability is characterized by local bond rupture of both metallic and nonmetallic atoms at intrinsic surface sites, with the bond-rupture rate super linearly dependent on the excitation density. Spectroscopic studies show that bulk-valence excitation triggers surface bond rupture, and that valence holes are the responsible species. From our results, we propose a mechanistic model based on the two-hole localization induced by a high density of non-equilibrated valence holes, and demonstrate that the model describes all the important features quantitatively and consistently. Additional evidence that further supports the validity of the two-hole mechanism is presented.     

Formation of excitons in semiconductor nanostructures in the presence of electron-hole plasma

We propose a mechanism of increase in the binding energy of an exciton in wide band-gap semiconductors in the presence of optically pumped electron-hole plasma. These excitons with relatively high binding energy (>150 meV) can exist at room temperature when the dielectric constant of semiconductor in the infrared region of spectrum approaches zero. Calculations for CdS show that the density of electron-hole plasma should be higher than 10¹⁹ cm^?3 for formation of such excitons. We show that there exist a considerable number of close-lying energy levels of excitons with high binding energy in the forbidden band of the semiconductor. We guess that these excitons participate in the process of laser generation in optically pumped semiconductor nanocrystals.