Light beam induced current measurements on ZnO Schottky diodes and MESFETs
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Light beam induced current measurements on ZnO Schottky diodes and MESFETs H. von Wenckstern, Z. P. Zhang, M. Lorenz, C. Czekalla, H. Frenzel, A. Lajn and M. Grundmann Universität Leipzig, Fakultät für Physik und Geowissenschaften, Institut für Experimentelle Physik II, Linnéstrasse 5, 04103 Leipzig, Germany ABSTRACT The homogeneity of the Schottky barrier potential of reactively sputtered PdOy/ZnO Schottky contacts has been investigated by light beam-induced current measurements on the micrometer scale. It is found that a metallic capping layer, acting as an equipotential surface, is not necessary for PdOy/ZnO Schottky contacts in contrast to AgxO/ZnO Schottky diodes. Further, we probed the generated photocurrent of a ZnO-based metal-semiconductor field-effect transistor for a closed and open channel, respectively. The photocurrent is, in general, one order of magnitude larger for closed channel conditions. The position of maximum photocurrent generation shifts towards the drain for higher source drain voltages for closed channel conditions, whereas it is nearly independent of the source drain potential for an open channel. INTRODUCTION Properties of Schottky barrier contacts on ZnO have improved considerably in the past years either due to refined pretreatment of the ZnO surface [1] or due to the usage of nonstoichiometric metal oxides as Schottky contact instead of the pure metal [2, 3]. It has been shown that in the case of a non-stoichiometric silver oxide Schottky contact a metallic capping layer is necessary to provide an equipotential surface on top of the metal oxide [3]. This ensures that the electrically active contact area equals the geometric contact area (the contact area is input to most formulas used to calculate the Schottky contact parameters). Further in, e.g. AgxO/ZnO photodiodes, photo-generated carriers are not efficiently collected if a metallic capping is omitted [3]. However, comparisons of the spatially resolved photocurrent between capped and uncapped metal oxide/ZnO Schottky contacts are only published for silver oxide so far. We investigated the homogeneity of non-stoichiometric palladium oxide Schottky diodes on ZnO thin films by light beam-induced current (LBIC) measurements on the micrometer scale. Further we used this method to study the spatial dependence of the generated photocurrent between the source and drain contacts of a ZnO-based metal-semiconductor field-effect transistor (MESFET) having a PdOy Schottky contact as gate. EXPERIMENT The ZnO thin films used for the Schottky diodes were grown by pulsed-laser deposition (PLD) on a-plane sapphire substrates. First a metallic conducting ZnO:Al layer of about 400 nm thickness was deposited serving as Ohmic back contact of the Schottky diodes [4]. On top of the ZnO:Al, a nominally undoped ZnO layer of about 2 µm thickness was grown. The net doping density of this layer is in the mid 1016 cm-3 range. The growth temperature was about 650°C, the oxygen partial pressure was 0.016 mbar. The circular Schottky contacts were def
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