An enhancement of tunnel ionization of deep impurities in semiconductors in an alternating field as compared to static fields has been observed. The transition between the quasistatic and the highfrequency regime is determined by the tunneling time. For the case of deep impurities this is the time of redistribution of the defect vibrational system which depends strongly on temperature and the impurity structure. A theory of tunnel ionization of deep impurities by high-frequency fields has been developed. [S0031-9007(98) PACS numbers: 72.20.Ht, 72.40. + w The effect of high-frequency coherent radiation on tunneling in semiconductor superlattices and nanostructures has attracted considerable attention recently. The superposition of a static electric field and an alternating field causes a wealth of new phenomena as a result of photon assisted tunneling [1][2][3]. In all these cases tunneling is accomplished by a static electric field, and the radiation influences the barrier penetration probability. An intense radiation field, however, can, in fact, both generate the tunneling barrier and initiate tunneling. Such a tunneling process has been observed with the result that the highfrequency field acts like a static field and the tunneling probability does not depend on frequency [4]. The frequency independent tunneling, however, must be limited to frequencies V with Vt , 1, where t is the tunneling time. This has been shown in a number of theoretical works [5][6][7][8][9][10], but has never been explored experimentally. In contrast to static electric fields where the electron tunnels at a fixed energy, in alternating fields the energy of the electron is not conserved during tunneling. In this case the electron can absorb energy from the field, which should lead to a sharp increase of the tunneling probability with increasing frequency for Vt . 1.Here we report on the first experimental demonstration of this effect observed in tunneling ionization of deep impurities in semiconductors. We show that the transition from the quasistatic regime Vt , 1 to the high-frequency regime Vt . 1 occurs at terahertz frequencies. In the quasistatic regime the electron tunnels at the momentary magnitude of the electric field in a time shorter than the period of oscillation, thus the electric field acts like a static field. The ionization probability is independent of frequency and increases with rising field strength E like exp͑E 2 ͞E 2 c ͒ where E c is a characteristic field [4]. In the high-frequency regime the ionization probability, being characterized by the same field dependence, substantially increases with increasing frequency. In contrast to tunneling ionization of atoms, where only electron tunneling takes place [5], ionization of impurities in solids is accomplished by two simultaneous tunneling processes, electron tunneling and the redistribution of the vibrational system by defect tunneling. A theory of the process is developed showing for the first time that in this case the electron tunneling time is controlled by...

The surface modifications induced by treating Cu(In,Ga)(S,Se)2 films in an aqueous ammonia hydroxide-based solution of Cd2+ ions—as used in record Cu(In,Ga)(S,Se)2 solar cells without a CdS buffer layer—have been investigated for different Cd2+ concentrations. Employing a combination of x-ray photoelectron spectroscopy, Auger electron spectroscopy, and x-ray emission spectroscopy, it is possible to distinguish two different surface modifications. For Cd2+ concentrations below 4.5 mM in the solution we observe the formation of a CdS monolayer, while higher Cd2+ concentrations lead to the additional deposition of a cadmium hydroxide film on the CdS/Cu(In,Ga)(S,Se)2 surface.

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For the heterovalent system ZnSe/GaAs(100), we have investigated the influence of a Te pretreatment of the substrate on the electronic structure of the interface by photoelectron spectroscopy. We have paid special attention to correctly determine the valence band maximum in a k-resolved fashion, including the use of photon energies which enable excitation at the Γ point. We find that the Te pretreatment leads to a decrease of the valence band discontinuity as large as 0.3 eV. From photoemission depth profiling we conclude that some Te atoms remain localized at the interface, thus causing the change of the valence band offset while others float on the ZnSe surface, probably acting as surfactants.