The buried oxide (BOX) layers of SIMOX structures resemble thermally grown SiO₂ as they are non-crystalline and the density of interface states at both interfaces and the positive oxide charge are of the order of 10¹⁰ cm⁻². However, the oxide network is "strained" in the sense that the proportion of high energy Si-O bonds is higher that in fully relaxed SiO₂. The probable reason for this is that the BOX is formed in a confined manner that hampers the relaxation of the network. The "strained" oxide is more prone to forming defects during its preparation and/or irradiation that a relaxed one, hence, the density of network defects in BOX, such as O vacancies and Si-Si bonds, is usually high. The "strained" bonds and network defects are primarly responsible for trapping of holes generated in or injected into the BOX. Positive charge in the BOX is also generated by interaction with hydrogen at 500 ÷ 700°C. The Box Usually, but necessarily, contains excess silicon ranging from Si-Si bonds associated with the oxide network to separate phases such as amorphous (a) Si clusters and crystalline Si inclusion. The a-Si clusters are photo-active amphoteric defects that can be prositively charged under UV illumination.
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