Low energy hydrogen-ion implantation on thermally treated ZnO surfaces

Studies are reported of ZnO surfaces prepared by a three-stage implantation procedure consisting of: (1) bombardment with a 100-eV hydrogen-ion beam of an insulating sample held at 200°C, a process that introduces a very high surface density of free electrons; (2) annealing at 300°C, which restores the initial insulating state of the surface, suggesting loss of all memory of the preceding implantation; and, finally, (3) re-bombardment with 100-eV ions, but now while the sample is kept at room temperature. The surfaces so produced are totally different from those reported previously in which only the last implantation stage had been applied. In addition to the narrow, electron-rich layer resembling very closely the accumulation-like layer observed on the latter surfaces, a second, much wider electron-rich layer also forms. The first layer, extending to about 20 Å below the surface proper, contains hydrogen donors which are fully ionized down to at least 10 K. The second layer is several thousand angstroms wide and the hydrogen donors in it are only partially ionized at room temperature, becoming completely unionized at 100 K. The enormous penetration range of the implanted hydrogen, as evidenced by the thickness of the wide layer, is very difficult to account for. Equally difficult to explain is the marked difference in the characteristics of the hydrogen donors in the narrow and wide layers. Some suggestions concerning these rather puzzling observations are put forward but at this stage they are largely of a speculative nature.