GaAs Photodetector for X-ray Imaging
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GaAs Photodetector for X-ray Imaging G. C. Sun1 , H. Samic1,2, V. Donchev1,3, S. Gautrot4 and J. C. Bourgoin1 1 Laboratoire des Milieux Désordonnés et Hétérogènes, Université Pierre et Marie Curie, CNRS (UMR 7603), Tour 22, Case 86, 4 Place Jussieu, 75252 Paris Cedex 05, France 2 Department of Physics, University of Sarajevo, Saobracajni Fakultet, Zmaja od Bosne 10, 71000 Sarajevo, Bosnia & Hertzegovina 3 Department of Condensed Matter Physics, Sofia University, 5, blvd. James Bourchier, 1164Sofia, Bulgaria 4 Centre de Spectrometrie Nucléiare et Spectrometrie de Masse, Bat. 108, Université Paris-Sud, 91405 Orsay, Paris, France
ABSTRACT We describe briefly a cheap and non polluting technique to grow epitaxial GaAs layers, several hundred microns thick, in a matter of hour. Detectors consisting of a p+/i/n+ structure have been realised with these layers and we present their characteristics obtained from currentvoltage, capacitance-voltage measurements as well as their response versus the energy and flux of X-rays.
INTRODUCTION In the 1960s it has been suggested that GaAs could be a good alternative material for high energy photon detection [1]. Since then, various experiments have demonstrated that GaAs detectors exhibit indeed high performances in terms of charge collection efficiency and energy resolution [2-7]. These detectors were the first to demonstrate high resolution at room temperature (for a review see ref.8). However, no GaAs detector has appeared on the market although the development of a microelectronic technology for this material could have allowed an easy realization of many kinds of structures, ohmic contacts, barriers, junctions and electronics integration. The reason is the following: large thicknesses are required to detect efficiently X and γ photons, thicknesses which are only available from bulk grown materials. Unfortunately, bulk GaAs materials contain a large concentration of defects which, in addition, are not uniformly distributed (for a review see ref.9). For these reasons it has been accepted that these materials are not suitable in practice to develop detectors, in particular imaging detectors. Only good quality materials, i.e. obtained by epitaxy, qualify for detection. All conventional epitaxial growth techniques seem to satisfy this requirement: the data claimed in ref.3 to be due to the very high purity associated with a specific epitaxial growth, are in fact similar to those reported 28 years earlier [4-7], which were obtained with other types of epitaxial growth techniques. But epitaxial layers of large enough thickness do not exist: the data reported in references 2 to 7 have been acquired on unique layers. Indeed, epitaxial growth techniques are slow: they allow to obtain a few microns thick layer in a matter of hour. Hence, to grow several hundred microns thick layers, the ones which would allow efficient absorption for X-ray, days of continuous growth are necessary, which is industrially impracticable. H4.5.1
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