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.     

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.     

Effect of the coherence of free electron-hole pairs on excitonic absorption in GaAs/AlGaAs superlattices

The effect of photoexcited free carriers on the absorption spectra dynamics of GaAs/Al_xGa_1−x As superlattices is investigated experimentally by the pump-probe method. A sharp change in the shift of the excitonic resonance energy from the low-to the high-energy direction is found to occur at the moment that the electromagnetic radiation of the pump and probe beams overlap in the case of band-band excitation. This phenomenon is explained in a model of scattering of high-energy electron-hole pairs. The dephasing time of free high-energy particles is experimentally estimated to be several tens of femtoseconds. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 1, 62–67 (10 January 1998)     

Effect of free electron-hole pairs on the saturation of excitonic absorption in GaAs/AlGaAs quantum wells

The pump-probe experimental method is used to investigate the effect of photoexcited carriers on the dynamics of the exciton absorption spectra of GaAs / Al_xGa_1–x As-multilayer quantum wells. Use of the method of moment analysis for processing the results makes it possible to identify the simultaneous contribution of changes in oscillator strength and width of the exciton lines in the saturation of exciton absorption. It was found that the oscillator strength recovers its initial value in the course of the first 100–130 ps, whereas broadening and energy-shift of the exciton lines is observed for 700–800 ps. These are the first experimental measurements of the excitation densities at which the oscillator strength of the excitonic state saturates when the latter is perturbed only by free-electron-hole pairs, and when it is perturbed only by other excitons. Fiz. Tverd. Tela (St. Petersburg) 40, 1130–1133 (June 1998)     

Radius of electron-hole drops in silicon

The radius of electron-hole drops at nucleation threshold is investigated as a function of temperature in Si using a small shift of their main luminescence line due to their surface energy. For T > 10 K, experiment and theory are consistent if we take into account a sticking coefficient (～ 4%) at the drop surface. For T ? 10 K our data indicate that the drop size is certainly small, but the measured radii are not likely. This suggests that other effects should be considered, such as the drop curvature energy for example.     

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.     

Effect of an electric field on the dissociation of electron-hole pairs interacting with triplet excitons in amorphous molecular semiconductors

The photoconductivity of amorphous molecular semiconductor films increases with the simultaneous photogeneration of singlet electron-hole pairs (EHPs) and triplet excitons but decreases when singlet EHPs are replaced by triplet EHPs. As the electric field increases, the influence of the triplet excitons on the photoconductivity of the films due to the dissociation of EHPs becomes less. It is concluded that as the electric field increases, the current-carrier mobility increases and the dissociation rate of EHPs becomes comparable to the spinconversion rate of EHPs interacting with triplet excitons. Fiz. Tverd. Tela (St. Petersburg) 39, 1020–1023 (June 1997)     

Influence of the electron-hole density profile on the reflectivity of laser irradiated silicon

We study the influence of the spatial extension of the electron-hole plasma created by a pump pulse on the reflectivity of a probe pulse. We show that the density deduced from reflectivity measurements is the surface density value with a very good accuracy, except very close to the plasma resonance. We also show that the resonance broadening due to the spatial inhomogeneity can be larger than the one due to free carriers absorption and has to be included in the usual experimental determination of the plasma relaxation time.     

Role of electron-hole pairs in the mechanisms of desorption from semiconductor surfaces

In the presence of a dense electron-hole plasma (due to high power laser irradiation or resulting from a plasma driven phase transition or melting), it is shown that desorption is drastically enhanced. This permits us to propose a test for the validity of the hypothesis of the plasma driven melting in Si. Also the creation of surface excitons is shown to lead to enhanced desorption.     

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.     

Stress dependence of electron-hole liquid parameters in silicon

The electron-hole liquid recombination luminescence in silicon is studied as a function of stress up to corresponding valence band splittings of 30 meV for <100>- and <111>-stress directions. EHL densities and exchange-correlation energies as obtained from lineshape fits are in general agreement with recent theoretical data. Systematic deviations in detail, however, indicate that - contrary to simplifying theoretical assumptions - a band structure dependence of the exchange and correlation energy E_xc has to be taken into account.     

Iron-aluminum pairs in silicon

Iron-aluminum pairs in silicon are investigated with conventional and optically detected electron paramagnetic resonance (EPR). For the trigonal and orthorhombic pairs known from previous EPR measurements we found for the first time optical absorption bands by measuring their magnetic circular dichroism of the absorption (MCDA). Direct experimental evidence is presented for the configurational bistability of both pairs by showing that the MCDA of the trigonal configuration can be transformed into that of the orthorhombic configuration by the combined effect of light and temperature. A new trigonal pair was discovered by conventional EPR having the same EPR intensity as the known one. Total energy calculations of various (Fe_i-Al_s) pair configurations show that two trigonal (Fe_i-Al_s)⁰ pairs with different Fe_i-Al_s separations have almost the same binding energy and should occur with the same probability. Fe _i ⁺ is always on a tetrahedral interstitial site, while Al _s ^– is nearest neighbor along 111 in one pair, second nearest neighbor in the other one with one silicon lattice site in between.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

Phonon-assisted radiative electron-hole recombination in silicon quantum dots

The temperature dependence of the photoluminescence (PL) spectrum of silicon quantum dots (QDs) is studied both theoretically and experimentally, and the time of the corresponding electron-hole radiative recombination is calculated. The dependence of the recombination time on the QD size is discussed. The experiment shows that the PL intensity decreases by approximately 60% as the temperature increases from 77 to 293 K. The calculated characteristic recombination time has only a weak temperature dependence; therefore, the decrease in the PL intensity is associated primarily with nonradiative transitions. It is also shown that the phonon-assisted radiation is much more efficient than the zero-phonon emission. Moreover, the zero-phonon recombination time depends on the QD radius R as R⁸, whereas the phonon-assisted recombination time depends on this radius as R³.     

Cascade avalanche production of electron-hole pairs in type II quantum wells

It is shown that the photon avalanche mechanism can be used for producing nonequilibrium electron-hole pairs by low-intensity IR light with a photon energy smaller than the energy gap of a semiconductor by a factor of 3–5. A type II heterostructure with deep quantum wells is proposed to be employed for this purpose. In the model under investigation, the photon avalanche effect is due to a combination of a cascade of one-and two-photon transitions and Auger-type transitions.