4.1 Layer Preparation Methods Based Solely on UPD Processes
95
EC-ALD. In turn, the layer-by-layer deposition mode can result in nanostructured
materials whose bulk counterparts with both the same mean composition and atomic
layer structure cannot be synthesized by any other means. This opportunity offers a
great perspective for the EC-ALD method.
In EC-ALD materials corresponding to the layer structure shown in Fig. 4.7a,
one can find materials that mimic the structure of bulk semiconductors in which the
atomic layers are spontaneously ordered. Prominent examples for such materials are
CuInS 2 [84] and CuInSe 2 [45, 85]. While CuInS 2 proved to be nearly stoichiometric,
CuInSe 2 was found to be relatively poor for In. An even more difficult pursuit was
to obtain CuIn x Ga 1−x Se 2 [47]. For this material, the Cu/(In + Ga) = 1 ratio could be
achieved with a 1:5 ratio of the Cu vs. In + Ga deposition periods, and the Ga/(In +
Ga) ratio was only 0.25 for a 1:1 ratio of In and Ga deposition pulses. This composition
discrepancy clearly indicates how the optimization becomes more and more involved
with the increase of the number of elemental components. Cu 2 ZnSnS 4 films were
also reported as quaternary EC-ALD layers [13], although the composition of the
as-deposited material was quite far from the stoichiometric ratio.
Figure 4.7b shows the case when every other atomic layer in the sequence is the
same (atomic layers of even number in Fig. 4.7b), and the ratio of the numbers of the
other two layer types is practically arbitrary (but is often equal to the ratio of small
integers). In such structures, it is possible to deposit two different metallic layers
with one chalcogenide element. Examples for such materials are Cu x Zn 1−x S [86],
Cd x Zn 1−x S [76, 82], Cd x Zn 1−x Se [44, 76], Pb x Sn 1−x Se [87] and Hg x Cd 1−x Te [48].
Ge x Sb y Te z [88] is special in the sense that the Ge and Sb are of different valencies, and
hence, the composition cannot be described simply with a component replacement,
unlike in the previous examples. Also, it is possible to match one metal with two
chalcogenide element like CdS x Se 1−x [89].
Finally, it has to be mentioned that non-identical semiconductor materials can
also be deposited onto each other by the EC-ALD method. Here, it must be stressed
that every component of the system packed onto each other is composed of a large
enough number of atomic layers so that the individual phase properties, especially
the band gap of the particular component, can manifest themselves and contribute
to the overall functionality of the system. The principle of the deposition order
here is to create a structure with gradually varying band gap from the substrate
to the topmost phase where the light absorption is the most intense. By creating
such structures, the light absorption can be widened to nearly the entire wavelength
range of the visible light and also to enter the UV region. The gradual change of
the band gap results in a diminished recombination of the photogenerated electron–hole pair, which leads to enhanced photocurrents. The desired cascade-type
multiple heterogeneous junctions are illustrated in Fig. 4.8. Superlattice-like architectures with composition of PbSe/PbTe [33] and PbTe/CdTe [90] up to 15 atomic
bilayers of each component have been reported, and fairly thick sandwich structures
with a Ag/(S/Cu/S/Zn) 20 /(S/Cd) 40 /S layer sequence [91] have also been described.
However, the favourable influence of the sandwich structure for the absorption properties has been demonstrated only for TiO 2 /CdSe/Cu 2 Se [92] and TiO 2 /CdSe/CdTe
[81] deposits.
95
EC-ALD. In turn, the layer-by-layer deposition mode can result in nanostructured
materials whose bulk counterparts with both the same mean composition and atomic
layer structure cannot be synthesized by any other means. This opportunity offers a
great perspective for the EC-ALD method.
In EC-ALD materials corresponding to the layer structure shown in Fig. 4.7a,
one can find materials that mimic the structure of bulk semiconductors in which the
atomic layers are spontaneously ordered. Prominent examples for such materials are
CuInS 2 [84] and CuInSe 2 [45, 85]. While CuInS 2 proved to be nearly stoichiometric,
CuInSe 2 was found to be relatively poor for In. An even more difficult pursuit was
to obtain CuIn x Ga 1−x Se 2 [47]. For this material, the Cu/(In + Ga) = 1 ratio could be
achieved with a 1:5 ratio of the Cu vs. In + Ga deposition periods, and the Ga/(In +
Ga) ratio was only 0.25 for a 1:1 ratio of In and Ga deposition pulses. This composition
discrepancy clearly indicates how the optimization becomes more and more involved
with the increase of the number of elemental components. Cu 2 ZnSnS 4 films were
also reported as quaternary EC-ALD layers [13], although the composition of the
as-deposited material was quite far from the stoichiometric ratio.
Figure 4.7b shows the case when every other atomic layer in the sequence is the
same (atomic layers of even number in Fig. 4.7b), and the ratio of the numbers of the
other two layer types is practically arbitrary (but is often equal to the ratio of small
integers). In such structures, it is possible to deposit two different metallic layers
with one chalcogenide element. Examples for such materials are Cu x Zn 1−x S [86],
Cd x Zn 1−x S [76, 82], Cd x Zn 1−x Se [44, 76], Pb x Sn 1−x Se [87] and Hg x Cd 1−x Te [48].
Ge x Sb y Te z [88] is special in the sense that the Ge and Sb are of different valencies, and
hence, the composition cannot be described simply with a component replacement,
unlike in the previous examples. Also, it is possible to match one metal with two
chalcogenide element like CdS x Se 1−x [89].
Finally, it has to be mentioned that non-identical semiconductor materials can
also be deposited onto each other by the EC-ALD method. Here, it must be stressed
that every component of the system packed onto each other is composed of a large
enough number of atomic layers so that the individual phase properties, especially
the band gap of the particular component, can manifest themselves and contribute
to the overall functionality of the system. The principle of the deposition order
here is to create a structure with gradually varying band gap from the substrate
to the topmost phase where the light absorption is the most intense. By creating
such structures, the light absorption can be widened to nearly the entire wavelength
range of the visible light and also to enter the UV region. The gradual change of
the band gap results in a diminished recombination of the photogenerated electron–hole pair, which leads to enhanced photocurrents. The desired cascade-type
multiple heterogeneous junctions are illustrated in Fig. 4.8. Superlattice-like architectures with composition of PbSe/PbTe [33] and PbTe/CdTe [90] up to 15 atomic
bilayers of each component have been reported, and fairly thick sandwich structures
with a Ag/(S/Cu/S/Zn) 20 /(S/Cd) 40 /S layer sequence [91] have also been described.
However, the favourable influence of the sandwich structure for the absorption properties has been demonstrated only for TiO 2 /CdSe/Cu 2 Se [92] and TiO 2 /CdSe/CdTe
[81] deposits.