Formation and Transient Photovoltaic Properties of ZnO/Si Isotype Heterojunctions by Magnetron Sputtering

The structural and electro-physical characteristics of ZnO/Si heterostructures deposited by the magnetron sputtering from aluminum-doped ZnO target are addressed. The film recrystallization and microcrystal structure restoration are observed. It is found that the ZnO/Si barrier is about 0.66 eV and depends on the annealing temperature of the implanted structures. Diode structures with a transparent electrode (ITO/ZnO/Si/Al) have photosensitivity in the wide spectral region of 0.4–2.5 µm. The photosensitivity mechanisms in different spectral bands require further refinement. The magnetron sputtering growth and the resulting film consolidation is interesting in which such varied morphologies have an impact on the photovoltaic behavior of ZnO/Si structures. Thus, the magnetron deposited ZnO layers with thicknesses of less than ≈70 nm has roughly 3 times greater photovoltage magnitude compared with that having greater thicknesses, and this coincides with the microstructural evolution of the film. The SPV decays slow down accordingly exhibiting decay times of the order 10 μs at the small film thicknesses and 100 μs at the ones exceeding ≈70 nm. These results can be interpreted in terms of the recombination and carrier trapping centers in the film and at the ZnO/Si boundary, whose concentration varies with the film thickness. In particular, increasing the grain size and grain density affects the composition and concentration of recombination-active defects at grain boundaries, which alter the carrier recombination rates. Carrier trapping at grain surface or interface states can dominate over fast carrier recombination processes, which also increases the photovoltage decay time. It is furthermore shown that forming p-NiO/n-ZnO/Si heterostructure increases the photovoltage magnitude, which is up to several times, and shortens the SPV decays. This can be effectively used in manufacturing rapid photosensitive elements.