(Color online) Tauc plot of direct band gap for multilayer alloy CuZnS films in stack configurations 2 and 3. All films have a direct band gap of approximately 2.5 eV, close to the literature direct band gap of Cu2S.
(Color online) Tauc plot of indirect band gap for multilayer alloy CuZnS films in stack configurations 2 and 3. All films have an indirect band gap between 1.3 and 1.5 eV, slightly higher than the indirect band gap of Cu2S in literature.
(Color online) EXAFS r-space plots for films in stack configuration 1a with ZnS deposited first. The result is a thin multilayer film with excess ZnS, where the Zn edge data look very similar to bulk ZnS but the Cu edge plot is disordered.
(Color online) EXAFS results from further attempts at creating ZnS and CuxS multilayers, using configurations 1b and 1c. The deposition is nearly the same as before (stack 1a) but the relative Cu precursor dose time (data 1, 1c) or the number of CuxS cycles within a “super cycle” (data 2, 1b) are doubled. The environment about Zn is highly disordered and looks more like ZnO, while the Cu edge plot closely resembles bulk Cu2S.
(Color online) EXAFS r-space plots for films with a ZnS base from stack configuration 2. The overall reduced amplitude in the Zn data indicates increased disorder, while the Cu resembles a linear combination of CuS and Cu2S. The first peak in the Zn EXAFS looks like Zn-O (see Fig. 10 ).
(Color online) EXAFS r-space plots for films in configuration 2 with a CuxS base. Slightly more Cu2S forms, seen in the increased amplitude of the shoulder (r ∼ 2.3 Å). The Cu2S dominates over ZnS formation, with a distorted Zn environment. Again, the first peak in the Zn EXAFS looks like Zn-O (see Fig. 10 ).
(Color online) EXAFS r-space plots for films in stack configuration 3 (thicker layers of ZnS and CuxS) with ZnS deposited first. The Zn matches bulk results for ZnS, and the Cu contains a combination of CuS and Cu2S. There are equal amounts of Zn and Cu.
(Color online) EXAFS r-space plots for films in stack configuration 3 with CuxS deposited first. This causes a nucleation delay for ZnS while producing a linear combination of CuS/Cu2S.
(Color online) Measured Cu/Zn fluorescence ratio as a function of Cu/Zn cycle ratio for stack configurations 2 and 3. ZnS/CuxS base refers to the 100 cycle base for stack configuration 2, or whichever material is deposited first in stack configuration 3. The measured Cu amount dramatically increases for the thick-layered films (configuration 3, 1:1 cycle ratio) when the Cu is deposited first. All films show more copper than expected based on the cycle ratio.
(Color online) EXAFS theoretical r-space standards of Zn-S in ZnS (top) and Zn-O in ZnO (bottom). The Zn-O peak is shifted to lower r (∼1.55 Å) compared to Zn-S (∼1.95 Å). The phase of the real part of the Fourier transform of Zn-O is also 180° out of phase relative to the envelope, due to a change in backscattering, compared to the heavier S atom.
(Color online) Plot of roughness and % roughness as a function of Cu/Zn cycle ratio for films from stack configurations 2 and 3. There is a clear trend of increasing film roughness with more copper precursor cycles.
(Color online) RMS roughness and % roughness vs measured Cu/Zn fluorescence ratio for films in stack configurations 2 and 3. There is a general trend of increased roughness with increased Cu content.
Percent roughness as a function of percent ZnO content. With one exception, the roughness increases as the % of ZnO increases. The large error bars on the lowest % ZnO value are due to low Zn EXAFS signal.
Summary of the four stack configurations used for deposition in this work. The target Cu/Zn ratios are determined by the ratio of Cu/Zn cycles because the deposition rates for the binary compounds are equal within the thickness measurement uncertainty. The measured Cu/Zn ratios are from the relative fluorescent intensities of Cu and Zn during the EXAFS data collection. A “super cycle” refers to the number of Cu:Zn cycles done before repetition, up to the number of “super cycles,” except for case 4, where the precursors are dosed simultaneously. In configuration 2, the ZnS base and CuxS base are formed from 100 cycles of deposition before any “super cycles” are applied.
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