A postdeposition thermal treatment has been applied to sputtered Al-doped zinc oxide films and shown to strongly decrease the resistivity of the films. While high temperature annealing usually leads to deterioration of electrical transport properties, a silicon capping layer successfully prevented the degradation of carrier concentration during the annealing step. The effect of annealing time and temperature has been studied in detail. A mobility increase from values of around 40 cm2/Vs up to 67 cm2/Vs, resulting in a resistivity of 1.4×10−4 Ω cm has been obtained for annealing at temperatures of 650 °C. The high mobility increase is most likely obtained by reduced grain boundary scattering. Changes in carrier concentration in the films caused by the thermal treatment are the result of two competing processes. For short annealing procedures we observed an increase in carrier concentration that we attribute to hydrogen diffusing into the zinc oxide film from a silicon nitride barrier layer between the zinc oxide and the glass substrate and the silicon capping layer on top of the zinc oxide. Both are hydrogen-rich if deposited by plasma-enhanced chemical vapor deposition. For longer annealing times a decrease in carrier concentration can occur if a thin capping layer is used. This can be explained by the deteriorating effect of oxygen during thermal treatments which is well known from annealing of uncapped zinc oxide films. The reduction in carrier concentration can be prevented by the use of capping layers with thicknesses of 40 nm or more.

Single grain boundaries in CuGaSe 2 have been grown epitaxially. Hall measurements indicate a barrier of 30 -40 meV to majority carrier transport. Nevertheless, local surface potential measurements show the absence of space charge around the grain boundary; i.e., it is neutral. Theoretical calculations [Persson and Zunger, Phys. Rev. Lett. 91, 266401 (2003)] have predicted a neutral barrier for the present 3 grain boundary. Thus, we have experimentally shown the existence of a neutral grain-boundary barrier, however, smaller than theoretically predicted. DOI: 10.1103/PhysRevLett.97.146601 PACS numbers: 72.80.Ey, 61.72.Mm, 71.55.Gs, 72.20.ÿi The electronic structure of Cu chalcopyrites shows numerous peculiarities, including, i.e., their grain-boundary properties. The first model [1,2] for the electronic structure of grain boundaries in chalcopyrites was based on the Seto model [3], where charged defects at grain boundaries cause a barrier for majority carriers (holes in the chalcopyrites discussed here). Barriers and space charge regions have been found in polycrystalline films by Hall measurements and Kelvin probe force microscopy (KPFM) (see, e.g., [1,4 -6] ). Theoretically it has been predicted that grain boundaries in chalcopyrites represent a barrier without charged defects [7,8]. In the Cu chalcopyrites, the top of the valence band is formed from antibonding Cu d states, which lifts it to higher energy compared to the corresponding II-VI compounds [9]. Grain boundaries consisting of f112g tet planes in the tetragonal system [10], corresponding to the f111g cub planes in the cubic system, are proposed to be Cu deficient, which results in a lowering of the valence band maximum. This results in a barrier in the valence band without a space charge at the grain boundary. Recently it has been argued that only a specific structure of a f112g tet grain boundary leads to this barrier and that the experimentally observed structures should not show a neutral barrier [11].Grain boundaries along the f112g tet planes can be described as twins. The coincidence site lattice (CSL) of such grain boundaries is characterized by a value of 3, the lowest value possible. Thus, it is expected that grain boundaries along f112g tet planes show a low defect density. A generating function of the CSL for the cubic system has been derived in Ref. [12]. Since the tetragonal distortion of chalcopyrites is small and since the CSL concerns only the two-dimensional plane of the grain boundary we assume the generating function for the cubic system as an approximation for the chalcopyrite system. Then it becomes clear that a polycrystalline film with f220g tet (i.e., f110g cub ) texture and vertical grain boundaries contains predominantly 3 grain boundaries. In fact, it has been shown that polycrystalline absorber films with f220g tet texture result in higher efficiencies of the corresponding solar cells compared to the usual f112g tet texture [13]. Therefore, 3 grain boundaries are the ones appearing in the most successful solar cells ...

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The chemical structure of the interface between silicon thin films and the transparent conductive oxide ZnO:Al has been investigated by hard x-ray photoelectron spectroscopy. By varying the excitation energy between 2010 and 8040 eV, we were able to probe the Si/ZnO interface buried below 12 nm Si. This allowed for the identification of changes induced by solid phase crystallization (SPC). Based on in-situ SPC annealing experiments, we find clear indications that the formation of Si–O bonds takes place at the expense of Zn–O bonds. Hence, the ZnO:Al acts as the oxygen source for the interfacial Si oxidation.