Penetration depths of low-energy hydrogen-ion implantation on ZnO surfaces

The penetration depths of protons in the (0001̄) face of ZnO produced by 100- and 400-eV hydrogen-ion bombardment are studied by three methods: calibrated argon-ion sputtering, calibrated etching, and space-charge capacitance measurements in the ZnO/electrolyte system. For the 100-eV implantation, the latter method provides unequivocal support to our previous conclusion that the protons, acting as fully ionized donors, penetrate only to a depth of 10-20 Å below the surface. The narrow space-charge layer so produced, having surface electron densities of up to 2×10^{1 4} cm^-2, constitutes a quantized, two-dimensional electron gas system. In the 400-eV implanted surface, the proton penetration depth is considerably larger. In this case an approximate depth profile could be derived from the combined measurements. It consists of a Gaussian distribution, peaked about 40 Å below the surface with a standard deviation of some 80 Å. However, some 10% of the implanted protons are found to penetrate much deeper, being distributed up to 1000 Å or more below the surface. For 100-eV implanted surfaces, similarly large penetration depths were observed, but the percentage of the deep lying protons is less than 1%. Such huge penetration depths arise most probably from a channeling mechanism.