Abstract
Zinc oxide is a promising multifunctional material. The practical use of nano- and polycrystalline ZnO devices faces a serious problem of instability of electrical and luminescent characteristics, due to the adsorption of oxygen by the surface during aging. In this paper, the aging effect in ZnO films and nanorod arrays was studied. It was found that ZnO samples demonstrate different behavior of the degradation process, which corresponds to at least two different types of adsorbing surface sites for O2, where O2 adsorption is of a different nature. The first type of surface sites is rapidly depassivated after hydrogen passivation and the aging effect takes place due to these centers. The second type of surface sites has a stable structure after hydrogen passivation and corresponds to HO–ZnO sites. The XPS components of these sites include the Zn2p3/2 peak at 1022.2 ± 0.2 eV and Zn2p1/2 peak at 1045.2 ± 0.2 eV, with a part of the XPS O1s peak at 531.5 ± 0.3 eV. The annealing transforms the first type of site into the second one, and the subsequent short-term plasma treatment in hydrogen results in steady passivation, where the degradation of characteristics is practically reduced to zero.
Highlights
Zinc oxide is an n-type conductivity semiconductor with a wide band gap of 3.37 eV, strong luminescence at room temperature, high conductivity and optical transparency [1,2]
It is known that the luminescence, optical, and electrical properties of ZnO device structures are largely determined by the surface states and effects that take place on the surface of nano- and microcrystallites of ZnO films, nanorod arrays and nanoparticles [2,3,4,5,7]
The stability of this plasma treatment effect is quite high only in ZnO films synthesized at high temperatures (i.e., T > 400 ◦C) [18,21] and special treatment methods are required to ensure the high conductivity of ZnO films obtained by low temperature methods
Summary
Zinc oxide is an n-type conductivity semiconductor with a wide band gap of 3.37 eV, strong luminescence at room temperature, high conductivity and optical transparency [1,2]. The practical application of nanostructured and polycrystalline ZnO is complicated by the fact that the performance of non-encapsulated ZnO layers does not have long-term stability since the surface is modified during storage under ambient conditions or under cyclic exposure to an oxidizing atmosphere and humidity as a result of which the bulk properties are subject to strong variations with shelf aging in atmospheric oxygen [22,23,24,25]. The presence of two different types of sites with active O2 adsorbing centers in the near-surface region, showing significantly different stability after hydrogen passivation during subsequent aging, is confirmed by the combined data of XPS, PL and the electrical characteristics. A simple method for obtaining a stable passivated HO–ZnO surface by means of preliminary heat treatment is demonstrated
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