Initial growth behavior of Cu(In,Ga)Se2 on molybdenum substrates

We study the initial growth stage of Cu(In,Ga)Se-2 on Mo films on glass, as the typical back contact for Cu(In,Ga)Se2 solar cells. A thermal evaporation process deposits Cu(In,Ga)Se-2 of nominal 3 nm thickness at different rates R and substrate temperatures T-Sub. An ultrahigh resolution scanning electron microscope serves to visualize the nucleation behavior. The deposited Cu(In,Ga)Se-2 forms three-dimensional isolated nuclei, known as Volmer-Weber growth mode. Deposition rate R and substrate temperature T-Sub control the areal density n of the Cu(In,Ga)Se, nuclei. For the growth on polycrystalline Mo substrates fabricated by sputter process, we observe a power law dependence between the island density n and the deposition rate R, and an exponential dependence of the island density n on substrate temperature T-Sub. The theory of homogeneous nucleation explains the Cu(In,Ga)Se-2 cluster formation on sputtered Mo substrates and the dependence of the island density on the growth conditions. On electron gun evaporated molybdenum, the cluster growth shows a similar behavior, but the nucleation mechanism deviates from the model of homogeneous nucleation and strong island density fluctuations occur. (C) 2003 Elsevier B.V. All rights reserved.