High Density Plasma Damage Induced in n-GaN Schottky Diodes Using Cl 2 /Ar Discharges
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In this work we have examined the effect of Inductively Coupled Plasma (ICP) on the reverse breakdown voltage (VB), Schottky barrier height (φB) and forward turn-on voltage of (VF) of GaN diode rectifiers with the contacts already in place. Several methods for damage removal were investigated, including annealing, UV ozone oxidation/stripping or wet etching with NaOH solutions. EXPERIMENTAL The diodes were fabricated on nominally undoped (n∼1017cm-3) or Si-doped
(5×1017 cm-3) GaN layers ∼3µm thick grown on an n+(1018 cm-3) GaN buffer on c-plane Al2O3 substrate. Ohmic contacts were formed with lift-off Ti/Au subsequently annealed at
600 °C, followed by evaporation of the 250 µm diameter Pt(250Å)/Au(1500Å) Schottky contacts through a stencil mask. A schematic is shown in Figure 1. The samples were
briefly exposed (∼10secs controlled by the system software) to 10Cl2/5Ar (total gas load 15 standard cubic centimeters per minute) or 15Ar ICP discharges in a Plasma Therm 790 reactor. During the ignition stage of discharge, the dc self-bias takes ∼2secs to reach its final value. Longer exposure time resulted in more damage. Damage layer thickness was limited due to Cl2/Ar etching of damaged GaN at a rate of around 1000 Å/min. From limited measurements we found that damage depth saturates in 10secs (~500Å). The gases were injected directly into the source through electronic mass flow controllers, and the 2MHz source power was varied from 100-1000 W. The samples were placed on an rfpowered (13.56MHz, 5-300W), He backside-cooled chuck. Process pressure was held constant at 2 m Torr. Ti/Au Ti/Au Ni/Pt 3µm undoped (n~1017 cm-3) GaN 0.3 µm n+ (1018 cm-3) GaN Al2O3 substrate Figure 1. Schematic of the planar diodes RESULTS AND DISCUSSION The dependence of VB and φB on rf chuck power for 300W source power discharges is shown in the upper part of Figure 2. Both of these parameters, at least initially, decrease with increasing power. The φB values saturate beyond 50W. The main
effect on φB is from damage created around the contact periphery. This would expected to
saturate once a N2-deficient region is created because much of the resultant φB is still determined by the unexposed region under the contact metal. Under these conditions, the dc chuck self-bias increases from –105 V at 50 W to –275 V at 200 W. The average ion energy is roughly the sum of this voltage plus the plasma potential which is 20-25 V in this system under these conditions. After plasma exposure, the diode ideality factor was always
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≥2 (the reference ideality factor wa
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