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Some studies on gold-doped germanium Syed, Abdus Sattar

Abstract

Tyler used Hall coefficient measurements to evaluate the gold acceptor concentration in germanium crystallized from a germanium-gold melt at a temperature between the melting point of pure germanium and 17°K below this temperature. The present study reports the results of similar measurements in the temperature range of growth from 12° to 80°K below the melting point of germanium. A crystal grower was designed and constructed for growing the monocrystals of gold-doped germanium. It was found that the direction was the most favorable direction for eliminating dendritic growth on gold-rich phase crystallized as inclusions. The phenomenon of retrograde solubility was observed. The maximum solubility (~2.8 x 10¹⁶ gold acceptor atoms per c.c.) occurred at about 30°K below the melting point of pure germanium. Infra-red absorption in gold-doped germanium was studied between 4.2° and 298°K over the energy range from 0.08 to 0.6 e.v. The magnitudes of the absorption cross-sections for the specimens containing about 3.5 x l0¹⁵ and 1.2 x 10¹⁵ gold acceptor atoms per c.c. were in close agreement with the data of Johnson and Levinstein. The cross-section of absorption near the band gap of germanium for the heavily gold-doped germanium ( ≻ 10¹⁶ gold acceptor atoms per c.c.) is about the same as in the case of lightly doped specimens. Whereas in the case of lightly doped specimens, however, the cross-section of absorption drops sharply to negligible values as the energy of the incident photon decreases, the cross-section of absorption for the heavily doped specimen remains very high. The excess absorption is found to remain substantially the same when measurements are extended to the energy range between 0.032 and 0.044 e.v. At 298°K, the absorption of infra-red radiation in gold-doped germanium shows spectral structure associated with valence band intra-band transitions. For the lightly doped specimens, with increasing concentration, the structure in the spectrum becomes less pronounced. Newman and Tyler have reported studies of the same effect in gallium-doped germanium. Our studies indicate that the effect is more pronounced in gold-doped germanium than in gallium-doped germanium. When intra-band absorption is peeled off from the absorption spectra at 195° and 298°K, the threshold of absorption is at a higher energy for the higher temperatures. The transmission of infra-red light through both pure and lightly gold-doped germanium is found to be enhanced by about 16 per cent at 4.2°K compared to that at higher temperatures (≻60° K). The determination of surface reflectivity at 4.2°K gives 0.29 and correspondingly a refractive index (μ) of 3.3 This shows an anomalous behaviour of {equation omitted} at low temperatures. It is known that materials with the diamond structure reveal similar extraordinary behaviour in other lattice properties such as the temperature dependence of their coefficient of thermal expansion and Debye temperatures.

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