Unconventional metallizations for GaAs radiation detectors


Traditional topic solved in our group, electron transport in high-resistivity and semi-insulating materials, – has evolved in the research into electrical properties of GaAs-based radiation detectors. In cooperation with Institute of Electrical Engineering of Slovak Academy of Sciences Bratislava we deal mainly with the properties of metal-semiconductor interface, i.e. the interface between detectors electrodes (the metallization) and bulk portion presented by semi-insulating GaAs, i.e. by material with very low equilibrium free electron concentration of ca 107 cm-3. We investigate the detector samples chiefly by current-voltage characteristics depending on material and geometry of electrodes, and possibly on temperature. The instruments used allow us to measure and analyse the characteristics in both their linear (very low voltage) parts and in the region of the so-called saturated current, i.e. for tens and hundreds volts. The measurements show significant differences in characteristic depending on the electrode metal. It is observed, that some less used metals (e.g. Mg) induce an order-of-magnitude reduction of the current through detector, feature being favourable for detector efficiency. In the frame of this topic we also endeavour to explain this phenomenon considering work functions of particular metals, and their tendency to form oxides and surface states and applying various metal-semiconductor interface models. For a detailed analysis of the interfaces we use the results of photoelectrical measurements on the interfaces and external measurement of X-ray photoelectron spectroscopy.

I-V characteristics of GaAs radiation detectors
Log-log current-voltage characteristics of GaAs radiation detector structure obtained at room temperature. Red curves correspond to a detector with double-side platinum metallization while in the characteristics where on one side platinum electrode is replaced by magnesium one are shown in blue
Probe station
JANIS ST-500 Probe-station for transport measurements in the temperature range 80 - 475 K


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