Impact of surface treatments on high-κ dielectric integration with Ga-polar and N-polar GaN

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Abstract
Gallium- and nitrogen-polar GaN surfaces are subjected to a variety of pretreatments, including oxidation, before the application of high-κ dielectrics by atomic layer deposition (ALD) in order to assess the “best” preparation of smooth, clean, and electrically high-performing dielectric semiconductor interfaces. In terms of topographical and chemical cleanliness, a pretreatment with a wet chemical piranha etch (H2SO4:H2O2) was found to be optimum for both surfaces, and additionally, (NH4)2S is effective for N-polar surfaces. Both thermal and plasma oxidations were employed for controlled growth of native oxides. For Ga-polar surfaces, all native oxides were as smooth as pretreated surfaces, while for N-polar surfaces, all native oxides are much rougher except for very short, high temperature oxidations. ALD Al2O3 films on Ga-polar surfaces are smoother for pretreated surfaces than for as-received surfaces, whereas for N-polar surfaces the opposite is true. In general, ALD HfO2 films on Ga-polar surfaces are rougher (0.8 nm rms) than Al2O3 films (0.1 nm rms), whereas for piranha treated N-polar surfaces, HfO2 films are smoother than Al2O3 films. For Ga-polar surfaces, capacitance–voltage measurements of simple Al2O3 (measured κ = 9) capacitors show the smallest hysteresis for unintentionally oxidized surfaces (0.37 V), whereas simple HfO2 (measured κ = 14) capacitors show the smallest hysteresis for a thermal GaOx at the interface (0.1 V). In both cases, the thicker the GaOx at the interface the larger the negative threshold voltage shift—suggesting an electron trap. Calculated total trapped charges associated with the dielectrics range from 3.2 × 1011 cm−2 (for HfO2 on thermally oxidized GaN) to 1 × 1012 cm−2 for Al2O3 on thermally oxidized GaN and HfO2 on plasma oxidized GaN. Finally, the leakage current density for nearly all capacitors is <10−5 A-cm−2 at +8 V bias.
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