Ultraviolet detectors in thin sputtered ZnO films.

Sputtering of ZnO is a well-known technique.'9 The application of sputtered thin films of ZnO has mainly been as surface acoustic wave transducers. To our knowledge nobody has yet used these films as detectors for UV radiation. Measurements of the intensity of UV radiation, for example in medical applications in connection with UV irradiation of the skin, are very inaccurate due to the lack of suitable detectors in this spectral range.10 The problem is to sufficiently block out the visible part of the spectrum. To avoid these problems we have in the present work chosen ZnO films which are only sensitive to UV light because the band gap (Eg = 3.2 eV) is in the near-UV spectral range. ZnO is a material with n-type semiconducting properties.11 It is so ionic that it cannot be made p-type due to self-compensation effects.'2 Hence, the usual pn photodiodes cannot be fabricated. ZnO photodetectors instead have to be made as either photoconductors or metal-semiconductor junctions (Schottky diodes). Both types of detector have been investigated in this work. A number of ZnO photoconductors was produced. The photoconducting properties of ZnO are well known."",13'14 There exist a fast photoconductive process related to the bulk properties and a slow component related to desorption and adsorption of oxygen atoms on the surface. The fast process is related to and strongly influenced by the slow one.