Formation and study of $p$–$i$–$n$ structures based on two-phase hydrogenated silicon with a germanium layer in the $i$-type region

Four pairs of $p$–$i$–$n$ structures based on polymorphous Si:H ($pm$-Si:H) are fabricated by the method of plasma-enhanced chemical vapor deposition. The structures in each pair are grown on the same substrate so that one of them does not contain Ge in the $i$-type layer while the other structure contains Ge deposited by molecular-beam epitaxy as a layer with a thickness of 10 nm. The pair differ from one another in terms of the substrate temperature during Ge deposition; these temperatures are 300, 350, 400, and 450$^\circ$C. The data of electron microscopy show that the structures formed at 300$^\circ$C contain Ge nanocrystals ($nc$-Ge) nucleated at nanocrystalline inclusions at the $pm$-Si:H surface. The $nc$-Ge concentration increases as the temperature is raised. The study of the current–voltage characteristics show that the presence of Ge in the $i$-type layer decreases the density of the short-circuit current in $p$–$i$–$n$ structures when they are used as solar cells, whereas these layers give rise to an increase in current at a reverse bias under illumination. The obtained results are consistent with known data for structures with Ge clusters in Si; according to these data, Ge clusters increase the coefficient of light absorption but they also increase the rate of charge-carrier recombination.