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Abstract
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GaAs/AlGaAs 2DEG structures are extensively used to fabricate electronic and optoelectronic devices. AuGe/Ni/Au based alloyed contacts are widely used to obtain low resistance contacts to these structures. This recipe gives low contact resistance with moderate surface roughness which can be further improved by increasing Ni layer thickness. The 2DEG structures also make excellent materials for Hall.effect based magnetic field sensors in applications such as non destructive testing. However, in these applications, any residual magnetism of Ni containing contact layer can distort the field at the active areas. Researchers in this area use alternatives to Ni, such as Cr, Ti etc. These alternatives however result in higher contact resistance and increased roughness, a trade off which is undesirable in other electronic devices such as HEMTs which have potentialon.chip fabrication along with the sensors.
For implementing sensors and devices on the same substrate as well as for improved yield, magnetism of Ni containing layer needs to be investigated. Explicit studies of the magnetic properties of the processed Ohmic contact metallization structure are rare in literature. We have investigated the magnetism of the annealed AuGe/Ni/Au structures and confirmed that the structures are rendered non.magnetic when processing conditions(temperature, time, Ni.layer thickness) optimized for minimum contact resistances (but not roughness), are used, implying that the conventional process is suitable for room temperature applications of the magnetic field
sensor.
The magnetization measurements also offered some insights into changes
taking place in the AuGe/Ni/Au metallization structure on annealing and
before alloying takes place. The magnetization data indicate that Ni
undergoes solid.state, solubility.limited, dissolution into the AuGe layer beginning at temperatures as low as 100oC. The dissolved Ni increases the melting temperature of the AuGe layer accounting for increase of alloying temperatures and decrease of roughness with increase of Ni.layer thickness. Use of non.eutectic alloy compositions with lower Ge content for AuGe, also reduces roughness, increases contact resistance, and have reduced solubility for Ni; these appears to be a better option in the trade off between contact resistance and roughness than increased Ni layer thickness. Low temperature
contact resistance measurements indicate that the contacts have both
tunneling and thermally.activated conduction characteristics, with the
barrier height increasing with increasing Ni layer thickness. Current
mapping of the contacts using Current AFM indicate that the integrity of the top Au layer is compromised as far as conductivity is concerned.
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