Influence of Ohmic Contacts on Semi-Insulating GaAs Detector Performances
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DETECTOR FABRICATION AND EXPERIMENTAL METHODS The detectors studied inthe present work were made by ALENIA S.p.A onHitachiaswellasonNippon SILEC undoped oriented substrates, with n-type resistivity p=3x 107 0 cm. Detectors were 100 pjm thick, with circular (diameter 4)=3 mm) Schottky pads metallized with Au/Pt/Ti [2]. Two sets of detectors with different kinds ofohmic contacts were realized. In set A a new non alloyedohmic contact (NAOC) [2], so called for the very short treatment period, has been realized on the whole back surface of the wafer by implanting Si' ions at two different doses and energies, a dose of 7x 1012 cm2 at 300 keV and a dose of I x 10' cm"2 at 40 keV. The wafers, with a reactively sputtered silicon nitride cap, were then fast annealed at 850'C for 30 s.Finally, the ohmic contact was achieved by alloying an e-beam deposited AuGeNi multilayer at 420'C for 30 s. Secondary Ion Mass Spectroscopy (SIMS) after Si' ion implantation showed that alter the annealing 80% of Si atoms were electrically active to 0.80 pm from the surface [2]. In set B the ohmic contact was obtained [3,4] by an Au/Ge/Ni metallization (4250A/500A/500o) followed by a thermal cycle 430 C (20s) in N2+H2 (10%) atmosphere. Current-voltage (I-V) characteristics have been analyzed, and ideality factor n and series resistance Rs of the detectors under test have been obtained. The samples have been investigated by charge collection scanning microscopy in "edge-on" configuration [5] to study the detector active region W. For this purpose the cross-section view has been afforded by crystal cleavage through the metallization and the detector cleaved edge has been probed by the scanning beam. Electron Beam Induced Current (EBIC) imaging [6] has been performed to visualize the actual active region, and optical beam induced current (OBIC) profiles [7] have been obtained at room temperature to evaluate the electric field extent, also as a function of the bias applied. In the OBIC analyses reported here the probe light wavelength was 700 nm, the beam diameter at the sample surface was 2 jim and the photon flux was 1.1 X1016photons/(cm 2s). Finally, photo-induced currenttransient spectroscopy (PICTS) [8] andphotodeep level transient spectroscopy (P-DLTS) [9] have been utilized to identify minority and majority charge carrier traps. RESULTS AND DISCUSSION Typical examples of forward I-V 1A--,= .8 characteristics are reported in Fig. 1, relevant to 5 . B " 'r Rs=13x10 0 samples of both sets A and B, indicating that the E forward currents are severely space charge n--1.4 0 :.SI1 0-6 A Rs=4.2x105 limited. The current trend denotes that ideality factor and series resistance are high, suggesting that the pure thermionic emission over the barrier C Q) is not the only dominant mechanism and 0U) ,.10-7 contributions of other current-transport C mechanisms must be taken into consideration [10]. The experimental data have been fitted by accounting for these additional contributions 10-8 I I I (generation-recombination, leakage and tunneling 0.0 0.5 1.0 1.
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