Persistent photoenhancement of hydrogen-implanted quantized accumulation layers on ZnO surfaces

Strong hydrogen-implanted accumulation layers on ZnO surfaces, produced and maintained at 80 K, can be further enhanced by illumination with visible light. The attainable enhancement is surprisingly large, resulting in an enormous surface electron density of up to 6 × 10¹⁴cm^-2, and persists indefinitely after the light is switched off. The photoenhanced layer is practically identical in width (10-20 Å) and transport characteristics to the implanted layer upon which it is based. Thus, it constitutes, just as the initial hydrogen-implanted layer, a two-dimensional electron accumulation layer, by far the strongest ever attained on any semiconductor surface. The photoenhancement effect is not fully understood. Various aspects of this effect are studied in an attempt to gain some insight into the processes involved. On the basis of the results presented we tentatively suggest that latent centers consisting of H⁺₂ species are introduced by the hydrogen implantation, in addition to the fully ionized proton donors responsible for the implanted accumulation layer. Illumination dissociates these species and shifts the protons so produced into sites in which they become fully ionized, thus augmenting the surface electron density of the implanted surface.