Abstract
Hydrogen is known to be present as an impurity in amorphous oxide semiconductors at the 0.1% level. Using amorphous ZnO as a simplified model system, we show that the hydrogens pair up at oxygen vacancies in the amorphous network, where they form metal-H-metal bridge bonds. These bonds are shown to create filled defect gap states lying just above the valence band edge and they are shown to give a consistent mechanism to explain the negative bias illumination stress instability found in oxide semiconductors like In-Ga-Zn-O (IGZO).
Highlights
The water molecule is inserted into the bulk c-ZnO or a-ZnO network at a Zn-O bond
It is proposed that the sizable hydrogen content seen in amorphous oxide semiconductors exist as hydrogen atom pairs trapped at oxygen vacancies
The calculations are carried out using the CASTEP plane wave pseudopotential code[43,44]
Summary
The water molecule is inserted into the bulk c-ZnO or a-ZnO network at a Zn-O bond. The structure is relaxed at constant volum. For example in crystalline Si, H will form the H2* defect, which is a low energy hydrogen configuration caused by inserting two hydrogens into a Si-Si bond[40]. This two hydrogen defect has an even lower energy in the a-Si:H network, and is strongly involved in the network growth process of a-Si:H42. The two-hydrogen defect is more stable in a-ZnO than in c-ZnO and this causes its leading role in the amorphous oxide semiconductors.
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