Istituto dei materiali per l'elettronica ed il magnetismo     
Dubecky F., Hubik P., Gombia E., Zatko B., Dubecky M., Bohacek P. Anomalous charge current transport in semi-insulating GaAs with a new contact metallization: Influence of 2DEG formed at the M-S interface. In: Progress in Applied Surface, Interface and Thin Film Science 2009 (Firenze, 15-20 November 2009).
Semi-insulating (SI) GaAs became one of the most important candidates for the fabrication of monolithic detector arrays (mainly in X-ray [1] or neutron imaging) due to its high resistance to radiation and neutron damage, good physical characteristics including attenuation factor, quality of the base material and well developed material technology. Applications in the field of UV detection have been also investigated ( [2]). However, the electrical charge transport related to the particular electrodes technologies of the SI GaAs detectors has to be more deeply understood [3-5]. In the present work, recent results of development of novel electrodes applied to SI GaAs detectors are presented. The idea of the novel electrodes is based on the replacement of the standard blocking Ti/Pt/Au metallization by another contact using metals (In,Gd, Mg) with low work function. In a previous study [4] such metals were d deposited on SI GaAs with the aim to form a new ohmic contact. The basic idea was to form an "anti-blocking" band-bending contact i.e. a blocking barrier for minority carriers (holes in undoped SI GaAs) on the cathode - ohmic side of the detector structure. We reported that the observed I-V characteristics of such structures show, in a low bias region, lower reverse currents in comparison with the „standard“ AuGeNi metallization. In this study detector structures having small In/Au, Mg/Au and Gd/Au anode „blocking“ contacts combined with the reference full area „quasi-ohmic“ AuGeNi eutectic alloy were fabricated. The structures were based on “detector-grade” [2] undoped VGF bulk SI GaAs and characterized by current-voltage and low frequency admittance-voltage measurements performed at various temperatures. An anomalous lowering of the reverse current (1-3 orders of magnitude in comparison with the standard Ti/Pt/Au blocking electrode) at low bias voltages (<10 V) was observed for electrodes formed by metals with low work function (Gd, Mg). This effect is explained by the creation of a 2DEG at the Gd-GaAs or Mg-GaAs interface. Finally, additional physical methods (PR, OBIC scanning, molecular dynamic simulation) have been applied to confirm the 2 DEG existence at the Gd-GaAs interface. The results obtained, the interpretation of the observed phenomena and the development of new possible applications will be presented. This work was supported by the Slovak Research and Development Agency under the contract No. APVV-0459-06 and Grant Agency for Science through grant No. 2/7170/27.
Subject GaAs
Semi-insulating (SI)

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