Diborane impurities

It is often necessary to iatroduce dopant atoms iato the epitaxial (epi) layers. Typically, the dopant sources are hydrides (qv) of the impurity atoms. Common dopants are boron hydride, ie, diborane(6) [19287-45-7] 2 6 p-ty e dopiag, and arsiae [7784-42-17, AsH, and phosphoms hydrides for n-ty e dopiag (11). For example ... 

For adding dopiag impurities duriag vapor-phase growth, a gaseous or easily vaporizable Hquid compound is metered, added to the siUcon source gas stream, and reduced along with the siUcon compound. Typical examples are diborane, 2 phosphine, and boron tribromide, BBr. ... 

Boranes oxidize in air to form boron oxides. Diborane spontaneously ignites in air around 50°C. The presence of impurities can lower the ignition temperature. Tetraborane decomposes slowly at ordinary temperatures but rapidly on heating. Decaborane is very stable at ordinary temperatures. When heated at 300°C, it slowly decomposes to boron and hydrogen. All boranes decompose to their elements at elevated temperatures. 

Impure diborane is a hazardous material and may combust explosively on contact with air. Therefore, precautions must be taken to prevent escape from the reaction. 

Ethane, usually present as an impurity, is removed by passing the products into dimethyl ether at —80°, to form a solid complex, BgHg.McgO, from which the ethane may be pumped. The complex is decomposed by warming and the diborane purified by fractional distillation. 

There is, however, a major disadvantage in using boron trifluoride or tetrafluoroborates in the preparation of diborane, for they result in a product that is contaminated with BF3, which is not readily removed (29). Particularly when the diborane is required for selective reductions, this is a most undesirable impurity traces of BF3 enhance the reducing powers of B2Hg and catalyze hydrogenolysis at centers that would not otherwise be attacked (24). 

Pure diborane is insensitive to mechanical shock however, shock and thermally sensitive mixtures may be formed in the presence of impurities such as oxygen, water, halogenated hydrocarbons, and so on [1]. Thermal decomposition of diborane to hydrogen can result in excessive pressure buildup. Vessels for containment of diborane should be designed to contain such resultant decomposition pressure. 

Zorin et al applied gas chromatography to the determination of organic impurities in diborane. The bulk of the sample was removed on a precolumn and the impurities were then determined on a squalane or liquid paraffin column. Nitrogen was used as carrier gas and a flame ionization detector was used. 

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