Surface Conversion Effects in Plasma-Damaged p-GaN
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subcollector layer. Energetic ion damage may result in increased surface and bulk leakage currents and changes in the electrical properties of the near-surface region through a change in Ga/N stoichiometry [1]. In other compound semiconductor systems it is often possible to remove plasma-damaged regions using slow wet chemical etching [17]. In the GaN system, much less is known about the electrical effects of dry etch damage, and its subsequent removal by wet etching or annealing. We describe the results of experiments in which p-GaN was exposed to Inductively Coupled Plasmas (ICP) of Cl2/Ar, H2 or Ar. The changes in electrical properties were measured by diode breakdown voltage. EXPERIMENTAL A schematic of the final Schottky diode structures is shown in Figure 1. Plasma exposures were performed with the contacts in place. The layer structure consisted of 1µm of undoped GaN (n-5x1016 cm-3) grown on a c-plane Al2O3 substrate, followed by 0.3µm of Mg doped (p-1017 cm-3) GaN. The samples were grown by rf plasma-assisted Molecular Beam Epitaxy [18]. Ohmic contacts were formed with Ni/Au deposited by ebeam evaporation, followed by lift-off and annealing at 750oC. The GaN surface was then exposed for 1 min to ICP Cl2/Ar, H2 or Ar plasmas in a Plasma-Therm 790 System. The 2MHz ICP source power was varied from 300-1400 W, while the 13.56 MHz rf chuck power was varied from 20-250 W. The former parameter controls ion flux incident on the sample, while the latter controls the average ion energy. Prior to deposition of 250µm diameter Ti/Pt/Au contacts through a stencil mask, the plasma exposed surfaces were either annealed under N2 in a rapid thermal annealing system, or immersed in boiling NaOH solutions to remove part of the surface. As reported previously it is possible to etch damaged GaN in a self-limiting fashion in hot alkali or acid solutions [19-21]. The current-voltage (I-V) characteristics of the diodes were recorded on an HP 4145A parameter analyzer.
Ni/Au
Pt/Au
Ni/Au
1 µm Mg doped (p~1017 cm-3) GaN 0.5 µm undoped Al2O3 substrate
Figure 1. Schematic of completed p-GaN Schottky diode.
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RESULTS AND DISCUSSION We observed an increase in reverse breakdown voltage (VB) on the p-GaN, whose magnitude was dependent on both ion energy and ion flux. The increase in breakdown voltage on the p-GaN is due to a decrease in hole concentration in the near-surface region through the creation of shallow donor states. The key question is whether there is actually conversion to an n-type surface under any of the plasma conditions. Figure 2 shows the forward turn-on characteristics of the p-GaN diodes exposed to different source power Ar discharges at low source power (300 W). The turn-on voltage remains close to that of the unexposed control sample. However there is a clear increase in the turn-on voltage at higher source powers, and in fact at ≥750 W the characteristics are those of an n-p junction [22]. Under these conditions the concentration of plasmainduced shallow donors exceeds the hole concentration and t
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