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

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Abstract
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 × 1014 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.