The relative importance of atomic defects and electron transfer in explaining conductivity at the crystalline LaAlO3/SrTiO3 interface has been a topic of debate. Metallic interfaces with similar electronic properties produced by amorphous oxide overlayers on SrTiO3 [Y. Chen et al., Nano Lett. 11, 3774 (2011); S. W. Lee et al., Nano Lett. 12, 4775 (2012)] have called in question the original polarization catastrophe model [N. Nakagawa et al., Nature Mater. 5, 204 (2006)]. We resolve the issue by a comprehensive comparison of (100)-oriented SrTiO3 substrates with crystalline and amorphous overlayers of LaAlO3 of different thicknesses prepared under different oxygen pressures. For both types of overlayers, there is a critical thickness for the appearance of conductivity, but its value is always 4 unit cells (∼1.6 nm) for the oxygen-annealed crystalline case, whereas in the amorphous case the critical thickness could be varied in the range 0.5 to 6 nm according to the deposition conditions. Subsequent ion milling of the overlayer restored the insulating state for the oxygen-annealed crystalline heterostructures but not for the amorphous ones. Oxygen post-annealing removes the oxygen vacancies, and the interfaces become insulating in the amorphous case, but the interfaces with a crystalline overlayer remain conducting with reduced carrier density. These results demonstrate that oxygen vacancies are the dominant source of mobile carriers when the LaAlO3 overlayer is amorphous, while both oxygen vacancies and polarization catastrophe contribute to the interface conductivity in unannealed crystalline LaAlO3/SrTiO3 heterostructures, and the polarization catastrophe alone accounts for the conductivity in oxygen-annealed crystalline LaAlO3/SrTiO3 heterostructures. Furthermore, it was found that the crystallinity of the LaAlO3 layer is crucial for the polarization catastrophe mechanism in the case of crystalline LaAlO3 overlayers. PACS numbers: 73.20.-r 73.21.Ac 73.40.-c 71.23.CqThe two-dimensional electron gas (2DEG) appearing at the interface between the band insulators LaAlO 3 (LAO) and SrTiO 3 (STO) has attracted much attention since its discovery by Ohtomo and Hwang [1]. It has stimulated a substantial body of experimental and theoretical work [2-25], but, its origin is still controversial [26]. Three different mechanisms have been proposed. First is interface electronic reconstruction to avoid the polarization catastrophe induced by the discontinuity at the interface between polar LAO and nonpolar STO [2][3][4]. Second is doping by thermal interdiffusion of Ti/Al or La/Sr atoms at the interface [13]. A third possible mechanism is creation of oxygen vacancies in STO substrates during the deposition process [9][10][11]27,28]. Oxygen vacancies are known to introduce a shallow intragap donor level close to the conduction band of STO [29], and their action may be specific to this one substrate. The thermal interdiffusion mechanism was discounted in recent work [25], which studied the effect of a mixed interface layer. It i...
Semiconductor ZnO nanotube arrays have been synthesized by direct electrochemical deposition from aqueous solutions into porous anodic alumina membranes. Scanning electron microscopy and transmission electron microscopy indicate that large-area and highly ordered nanotube arrays have been obtained. X-ray diffraction and selected-area electron diffraction analyses show that the as-synthesized nanotubes are polycrystalline. Photoluminescence spectra of the ZnO nanotube arrays show that a violet peak and a blue peak are centered at 414 and 464 nm, respectively. The ordered polycrystalline ZnO nanotube arrays may find potential applications in optoelectronic and sensor devices. The growth mechanism and the electrochemical deposition process are discussed.
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