Transient Photoresponse from Co Schottky Barriers on AlGaN
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TRANSIENT PHOTORESPONSE FROM Co SCHOTTKY BARRIERS ON AlGaN R. Schwarz1, M. Niehus1, L. Melo1, P. Brogueira1, S. Koynov1, M. Heuken 2, D. Meister3, B.K. Meyer3 1 Physics Department, Instituto Superior Técnico, IST, P-1096 Lisboa, Portugal Tel: +351-21-841 7775, e-mail: [email protected] 2 Aixtron GmbH, D-52072 Aachen, Germany 3 I. Physics Department, University of Giessen, D-35392 Giessen, Germany ABSTRACT Co on AlGaN is expected to form a large barrier Schottky contact due to its high work function. We have used this material combination with 18 % of Al in AlxGaN for the study of transient photoresponse in the photovoltaic mode and in secondary photocurrent measurements after pulsed laser excitation. In reverse bias and in shortcircuit mode a fast decay with a characteristic time of a few microseconds is dominant at room temperature. This mode is appropriate for UV detector operation. At elevated temperature, a much slower tail extending to several milliseconds is also observed. In forward bias operation the slow tail is dominating at any temperature. We discuss this asymmetry with respect to fast minority carrier collection within the space charge region for primary photocurrents and the slower majority carrier transport in forward bias. INTRODUCTION When GaN and its alloy with Al is used in UV detector applications the so-called persistent photoconductivity effect might limit the detector speed [1]. For example, a slow response with a characteristic time of 8.2 ms was reported for a p-i-n GaN detector [2]. Faster devices were realized with simple Schottky barrier devices having a response time of only 118 ns [3]. An even faster device with a 12 ns decay time was realized with a heterojunction of AlGaN/GaN that showed a responsivity of 0.15 A/W [4]. Usually high work function metals are expected to yield high values for the Schottky barrier height qΦb on n-type AlGaN. In a recent study a value of qΦb = 0.9 eV was obtained from capacitance-voltage (C-V) and current-voltage (I-V) analysis in an AlGaN sample with 11 % Al content grown by MOCVD [5]. As an alternative method, Polyakov et al. [6] performed internal photoemission spectroscopy on a Schottky barrier detector formed by a Ni contact on top of a 15 % Al content AlGaN layer with a large barrier height of qΦb = 1.31 eV. In this work we want to focus on another large work function metal, Co, to study the time dependence of transient photoconductivity (TPC) after pulsed laser excitation. We observe a strong asymmetry in the magnitude and the decay time of the photocurrent transients in forward and reverse bias operation. The dependence on bias voltage, on temperature, and, to a certain extent, on light intensity and wavelength, together with a schematic band diagram of the Schottky barrier device should give a consistent explanation for the measured photocurrent transients. T6.15.1
SAMPLE PREPARATION The samples were prepared by metal-organic vapor phase epitaxy (MOVPE) on sapphire substrates with Al contents between 2.5 and 18 %. Ohmic contacts, about
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