Electronics Science and Technology - Publications

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Browsing Electronics Science and Technology - Publications by Author "Abhilash, T. S."
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    Adhesion characteristics of Pd/Ge Ohmic contacts on GaAs/AlGaAs multilayer structures
    ( 2011-09-12) Abhilash, T. S. ; Kumar, Ch Ravi ; Rajaram, G.
    The adhesion characteristics of Pd/Ge Ohmic contact metallization on GaAs/AlGaAs multilayer structure are studied using scratch tests in a Nano-indentor. The effect of annealing on the scratch resistance of the metallization was measured and correlated with contact resistance and surface roughness. The scratch depth and qualitative evaluation of adhesion show best adhesion/wear resistance for films annealed at 300°C. It is found that the minimum contact resistance is also observed after annealing at this temperature. Profiles of scratch cross section at a constant force of 200 μUN indicate that, the scratches do not extend into the substrate for anneals at temperature < 400°C. The optimum contact resistance of Pd/Ge contact is ∼(0.75+0.10Ω-mm) which is 15 times larger than those for optimized AuGe/Ni/Au contacts (0.05±0.01Ω-mm). The measured surface roughness is ∼2.0±0.5nm, which is ∼10 times lower than that of AuGe/Ni/Au based contact that gave the lowest contact resistance. © 2011 American Institute of Physics.
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    Dependence of melting, roughness and contact resistances on Ge and Ni content in alloyed AuGe/Ni/Au-type electrical contacts to GaAs/AlGaAs multilayer structures
    ( 2010-02-19) Abhilash, T. S. ; Kumar, Ch Ravi ; Sreedhar, B. ; Rajaram, G.
    Annealed AuGe/Ni/Au film structures on GaAs/AlGaAs multilayers have been examined for contact resistance, roughness, magnetization and melting as functions of anneal temperature, Ni-layer thickness and three AuGe compositions. Magnetization data indicate that a solid state, solubility-limited dissolution of Ni into AuGe takes place even for low-temperature anneals and that this dissolution is complete when alloying occurs at ∼400 °C. An apparent melting temperature, detected in differential scanning calorimetry, increases with increasing Ni-layer thickness and decreasing Ge content in the AuGe alloy. Electrical contact formation and roughening of the surface occur in the range of melting temperatures of the structure. The eutectic alloy with a Ni-layer thickness of ∼25-30 nm gives the optimum contact resistance. The contact resistance can be traded off for the reduction in roughness by either increasing the Ni-layer thickness or reducing the Ge content, with the latter being the better choice of the two. The temperature dependence (4-300 K) of the contact resistance shows indications of both thermionic and tunneling behaviors. The barrier height for the current conduction increases with the increase of the Ni-layer thickness and a decrease of the Ge content in the AuGe layer, relative to that of the structure with optimum contact resistance. © 2010 IOP Publishing Ltd.
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    GaAs/AlGaAs heterostructure based micro-hall sensors and response to A.C. excitation
    ( 2012-08-31) Ravi Kumar, Ch ; Abhilash, T. S. ; Rao, B. P.C. ; Jayakumar, T. ; Rajaram, G.
    Hall sensors are fabricated from GaAs/AlGaAs based heterostructures. The sensors are characterized for sensitivity using A.C and D.C. excitation techniques w.r.to a known magnetic field in the range of 0-100Oe. Sensitivities of the order of 1000V/AT are obtained with a low variation of 0.15% in the temperature range of 30°C-100°C. A.C excitation (30Hz to 1 kHz) for the sensor current and phase-sensitive detection of the Hall Voltage are used to eliminate thermo-emfs and its drift with a consequent improvement in data acquisition rates of > 50% over that of D.C excitation. This improves scan rates in applications such as magnetic flux leakage (MFL) measurements. The sensitivity is independent of the frequency of excitation as expected; however, the zero-field Hall voltage 'offset' value is found to have small frequency dependence. © 2012 The authors and IOS Press. All rights reserved.
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    Influence of nickel layer thickness on the magnetic properties and contact resistance of AuGe/Ni/Au Ohmic contacts to GaAs/AlGaAs heterostructures
    ( 2009-09-18) Abhilash, T. S. ; Kumar, Ch Ravi ; Rajaram, G.
    The magnetization of alloyed Ohmic contact film structures of the form AuGe/Ni/Au deposited on a GaAs/AlGaAs heterostructure substrate are reported as functions of Ni-layer thickness and alloying temperature. The observations are correlated with contact resistance and surface morphology studies. It is found that drops in magnetization, due to conversion of Ni to a non-magnetic phase or alloy, begin at anneal temperatures as low as 100 °C for all Ni-layer thicknesses. The conversion is completed at an anneal temperature, T A, that increases with Ni-layer thickness. TA varies from 100-200 °C to 400-430 °C as Ni-layer thickness is varied from 10 to 100 nm for an AuGe (88 : 12 wt%) layer thickness of 100 nm. The electrical contact formation, however, appears to begin at much higher temperatures than 100 °C. Lowest contact resistance (0.05 0.01 Ω mm) is obtained when Ni thickness is about 25 nm for 100 nm AuGe layer thickness, anneal temperature is 400 °C and anneal duration is 60 s. This contact is non-magnetic. Measurements on samples with other AuGe layer thicknesses suggest that the contact resistances are comparable to this optimum value, if the ratio of AuGe layer thickness to that of Ni is ≥4. Increasing the Ni-layer thickness reduces the roughness of annealed contacts, but also increases contact resistance. The magnetic measurements are suggestive of a transformed Ni-layer thickness proportional to the thickness of the underlying AuGe layer. It is proposed that Ni diffuses into AuGe in a concentration limited diffusive mechanism followed by segregation into Ni3Ge. © 2009 IOP Publishing Ltd.
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    Magnetic evidence for solid-state solubility-limited dissolution of Ni into AuGe in alloyed AuGe/Ni/Au Ohmic contacts to GaAs
    ( 2012-08-15) Abhilash, T. S. ; Ravi Kumar, Ch ; Rajaram, G.
    The unresolved issue of magnetism of the alloyed AuGe/Ni/Au Ohmic contacts in GaAs is investigated in the context of Hall magnetic field sensors. Magnetization measurements provided some interesting insights into changes taking place in the metallization layer before alloying with the substrate takes place. The results show that Ni undergoes solid-state, solubility-limited, dissolution into the AuGe layer during the course of anneal. This results in the metallization structures becoming non-magnetic before alloying with the substrate takes place. Solubility of Ni into AuGe increases with temperature and decreases with decreasing Ge content from the eutectic composition (88:12 wt%) of the AuGe alloy. The melting temperature of the AuGe layer increases as a result of the Ni dissolution into it. Formation of NiGe phase in the structure is observed after cooling from anneal temperature at which the magnetic to non-magnetic transformation is completed, but alloying has not yet occurred. © 2012 American Institute of Physics.
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    Magnetic, electrical and surface morphological characterization of AuGe/Ni/Au Ohmic contact metallization on GaAs/AlGaAs multilayer structures
    ( 2011-12-01) Abhilash, T. S. ; Ravi Kumar, Ch ; Rajaram, G.
    A process issue arising from the use of ferromagnetic Nickel in the AuGe/Ni/Au Ohmic contact metallization is studied in the context of magnetic field sensors and HEMT devices made using GaAs/AlGaAs multilayer structures with the two dimensional electron gas layer. The dependence of magnetization, contact resistance, adhesion, surface roughness and current distribution of alloyed Ohmic contacts on parameters such as Ni layer thickness, anneal temperature and Au-Ge alloy composition are discussed. The magnetization measurements provided some new and interesting insights into changes occurring in the metallization layers prior to alloying. © 2011 SumDU.
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    New insights into formation of Ni-based alloyed Ohmic contacts to GaAs
    ( 2012-12-01) Rajaram, G. ; Abhilash, T. S. ; Ravi Kumar, Ch
    Magnetic properties of AuGe/Ni/Au Ohmic contact for use in fabrication of high sensitivity Hall magnetic field sensors with integrated FET circuits, utilizing GaAs/AlGaAs 2DEG, are investigated. Measurements indicate that Ni undergoes solid-state, solubility limited dissolution into the AuGe layer during the anneal, and prior to alloying of the metallization structure for the formation of Ohmic contact. This results in increase of the melting temperature of the AuGe layer. The ohmic contact structure is rendered non-magnetic upon annealing and prior to the alloying. Therefore, the conventional process Ohmic contact process can be used for integrating Hall sensors with FET and pHEMT in monolithic form.
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    Nickel dissolution into AuGe in alloyed AuGe/Ni/Au Ohmic contacts on GaAs/AlGaAs multilayer structures
    ( 2010-07-30) Abhilash, T. S. ; Ravi Kumar, C. H. ; Rajaram, G.
    Magnetic properties of alloyed Ohmic contacts of the type AuGe/Ni/Au on GaAs/AlGaAs multilayers with n+ cap layer with different AuGe compositions and Ni-layer thicknesses are examined. Magnetization data indicate that the annealed structures are non-magnetic, at room temperature for commonly used anneal temperatures (∼ 400-430 °C) and Ni-layer thicknesses (10-100 nm). The transformation of Ni to non-magnetic phase begins at ∼ 100 °C, well below temperatures at which extensive alloying with the GaAs substrate takes place. The fraction of Ni transformed to non-magnetic phase on annealing appears to scale with AuGe layer thickness, has a quadratic dependence on anneal temperature and is time independent for time scales of minutes. The data indicate that the Ni layer dissolves into the AuGe layer at temperatures well below that at which alloying between AuGe and GaAs substrate takes place. The dissolved Ni concentration is limited by a solubility that increases with anneal temperature and decreases with decreasing Ge content from that of the AuGe eutectic composition. © 2010 Elsevier B.V.