**1. Introduction**

A fundamental aim of material research and surface science is the development of deposition techniques of compound semiconductors with low impact from either the energetic or environmental points of view. These techniques should ensure a high structural control for the

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

engineering of nanostructures such as quantum dots, quantum well, superlattices, and thin films still preserving the crystalline properties of the grown material. The bottom-up approach is renowned as very favorable for the synthesis of such materials in the form of dispersed nanoparticles from molecular precursor, usually involving several steps and the addition of surfactants to the reaction environment. Since most of these products, obtained following these pathways, are in form of powders, the production of solid-state devices requires several steps for the deposition of the materials. In this context, electrodeposition has the advantage of the direct production of the films from the molecular precursors. In any case, electrodeposition hardly results in highly ordered materials as requested by recent technologies based on semiconductor. However, in specific conditions, electrodeposition enables the assemblage of atomic layers by means of surface limited reactions (SLRs). SLRs give the opportunity of exploiting layer-by-layer deposition of different atomic layers, leading to one of the most clean and energy saving approaches, electrochemical atomic layer epitaxy (ECALE) [1], for the growth of heterostructures. ECALE could be also referred in general as electrochemical atomic layer deposition (E-ALD) since in some cases the growth, though based on under potential deposition (UPD) processes or on any SLR, cannot be rigorously considered epitaxial. Hence, E-ALD joins highly ordered products with the direct access to the final material in the context of the bottom-up approach in a very clean reaction environment. E-ALD has been proven to be very effective for the electrodeposition of ultra-thin films of semiconducting materials. In recent years, thin films of binary [2–5] and ternary semiconductors [6–9] were successfully obtained. E-ALD requires very low energy consumption, diluted solutions, room temperature, and atmospheric pressures. Thus, it can be employed for the sustainable large-scale production of these materials. This is particularly interesting for photovoltaics application, where the improvements of the full life cycle assessment (FLCA) are considered a crucial aspect for the possible large-scale production of new materials [10]. In this context, it is very crucial to study and understand the growth mechanism together with the detailed analysis of the structural features of the resulting thin films. For this purpose, surface analysis techniques play an important role. Among them, surface X-ray diffraction (SXRD), with high brilliance synchrotron sources, enables the operando structural analysis in electrochemical environment during the E-ALD growth and discloses the structural features of such systems during the deposition process. In recent years, several works on the operando characterization of ultra-thin films have been reported thanks to the development of specifically designed flow cells and automated apparatus to perform 100 or more growth cycles in few hours.

### **2. E-ALD for semiconducting materials**

Materials overcoming the properties of silicon-based semiconducting materials are very widely researched. In particular, the interest is devoted toward materials with optoelectronic and electronic properties, in a broad range, able also to work in severe conditions. In this context, compound semiconductors are promising candidates. They result from the combination of two or more elements. The ones formed by elements of the groups symmetrical to the IV group, namely the III–V compound semiconductors are among the most studied. Epitaxy is generally expected to enhance their semiconducting proprieties, such as mean free path and charge carrier mobility. Moreover, epitaxial growth is necessary to obtain superlattices, i.e., materials with a periodic modulation of structure or composition and crystallographic coherence with respect to the atomic planes [11]. For example, without the crystallographic coherence, a bilayer ZnSe/CdSe film or a sequence of multiple layers of ZnSe, CdSe is not called a superlattice, but simply a heterojunction or a multilayer system. In this field, superlattices are of increasing interest, since it is possible to tailor their properties even at the nanoscale, where they are exhibiting quantization effects. The growth of perfectly flat, ultrathin, and even 2D sheets of material is a stringent requirement for the functionality of multilayers or superlattices. E-ALD provides a sustainable solution got the growth of high-quality 2D materials with well-controlled periodicities.

engineering of nanostructures such as quantum dots, quantum well, superlattices, and thin films still preserving the crystalline properties of the grown material. The bottom-up approach is renowned as very favorable for the synthesis of such materials in the form of dispersed nanoparticles from molecular precursor, usually involving several steps and the addition of surfactants to the reaction environment. Since most of these products, obtained following these pathways, are in form of powders, the production of solid-state devices requires several steps for the deposition of the materials. In this context, electrodeposition has the advantage of the direct production of the films from the molecular precursors. In any case, electrodeposition hardly results in highly ordered materials as requested by recent technologies based on semiconductor. However, in specific conditions, electrodeposition enables the assemblage of atomic layers by means of surface limited reactions (SLRs). SLRs give the opportunity of exploiting layer-by-layer deposition of different atomic layers, leading to one of the most clean and energy saving approaches, electrochemical atomic layer epitaxy (ECALE) [1], for the growth of heterostructures. ECALE could be also referred in general as electrochemical atomic layer deposition (E-ALD) since in some cases the growth, though based on under potential deposition (UPD) processes or on any SLR, cannot be rigorously considered epitaxial. Hence, E-ALD joins highly ordered products with the direct access to the final material in the context of the bottom-up approach in a very clean reaction environment. E-ALD has been proven to be very effective for the electrodeposition of ultra-thin films of semiconducting materials. In recent years, thin films of binary [2–5] and ternary semiconductors [6–9] were successfully obtained. E-ALD requires very low energy consumption, diluted solutions, room temperature, and atmospheric pressures. Thus, it can be employed for the sustainable large-scale production of these materials. This is particularly interesting for photovoltaics application, where the improvements of the full life cycle assessment (FLCA) are considered a crucial aspect for the possible large-scale production of new materials [10]. In this context, it is very crucial to study and understand the growth mechanism together with the detailed analysis of the structural features of the resulting thin films. For this purpose, surface analysis techniques play an important role. Among them, surface X-ray diffraction (SXRD), with high brilliance synchrotron sources, enables the operando structural analysis in electrochemical environment during the E-ALD growth and discloses the structural features of such systems during the deposition process. In recent years, several works on the operando characterization of ultra-thin films have been reported thanks to the development of specifically designed flow cells and automated apparatus to perform 100 or more growth cycles in few hours.

32 X-ray Characterization of Nanostructured Energy Materials by Synchrotron Radiation

Materials overcoming the properties of silicon-based semiconducting materials are very widely researched. In particular, the interest is devoted toward materials with optoelectronic and electronic properties, in a broad range, able also to work in severe conditions. In this context, compound semiconductors are promising candidates. They result from the combination of two or more elements. The ones formed by elements of the groups symmetrical to the IV group, namely the III–V compound semiconductors are among the most studied. Epitaxy is

**2. E-ALD for semiconducting materials**

E-ALD constitutes an easy way to deposit suitable ultrathin films and 2D materials alternating atomic layer of different elements in a very straightforward manner. For these reasons, since its infancy, the E-ALD study has been carried on compound semiconductors, with a specific focus on compound semiconductors based on elements of the II–VI groups. **Figure 1** depicts a general scheme for the E-ALD of a ternary chalcogenide, this can be obtained by alternating SLR steps of metals (M1, M2) and chalcogenides (C).

**Figure 1.** General scheme for an E-ALD process aimed to grow a ternary chalcogenide. C, M1 and M2, respectively, stand for an atomic layer of chalcogenide, first metal in the scheme and second metal in the scheme.

In order to tailor the stoichiometry of the compounds, it is possible to define schemes with different number of steps for the two metals, as will be discussed in Section 2.2.2 for Cux Zn<sup>y</sup> S. The E-ALD procedure requires a thorough knowledge of the separated SLRs (often UPD under potential deposition) involved in the deposition of the metals over the nonmetals and vice versa. An open issue is the exact definition of the driving force leading these processes. It is very widely accepted that the main contribution to the driving force is the negative free energies change involved in the interaction with the electrodic surface and in the formation of a thermodynamically stable compound. However, the surface can change during the growth due to formation of the crystal, and it is possible to think of a change also in the driving force when a relatively thick crystal is grown. As reported by almost every paper on this topic, it is worth to notice that, usually, the first step of the E-ALD process is the deposition of an atomic layer of chalcogen (or of the nonmetal in general) over the bare metal substrate. Reasonably, the experimental conditions for the deposition change while increasing the thickness of the deposit. For instance, in the growth of crystals, the accumulation of the elastic energy into the lattice is a nonfavorable contribution to the driving force. This is a well-known issue leading to different growth mechanisms and related morphology. Usually, in the case of the E-ALD process, these effects do not prevent the compound formation. Generally speaking, the changes are in the sense of decreasing the amount of deposition and then the total time of the process increases. This can prevent the deposition of some materials, as CdTe and InAs, with a thickness of practical relevance. In other cases (such as the deposition of Cu2 S), the deposition of the copper layer, implying the reduction of copper ions from the solution, may result in more complex reactions implying the formation of some intermediate compounds and the deposition cannot be accounted strictly by the UPD process. Several experimental evidences for these controversial aspects have been reported and they will be discussed in the following pages. It is worth to notice that they reveal a complex mechanism for the formation of chalcogenides by means of E-ALD. The complexity of these processes occurring during the growth is the reason why we prefer to refer to the E-ALD steps with the acronym SLR rather than using UPD.

In this work, we focus on the deposition of chalcogenides on a Ag single crystal, usually on the Ag(111) surface if not specifically indicated. All the potentials repoeted here are referred to the Ag/AgCl(KCl sat.) reference electrode.

### **2.1. Cadmium-based compound semiconductors**

Cadmium chalcogenides have been among the first compound semiconductors to be deposited by means of E-ALD due to their interesting electronic properties and favorable electrochemical characteristics.

#### *2.1.1. CdS*

One of the first chalcogenides electrodeposited by E-ALD is the CdS, extensively studied on several different single crystals. The E-ALD process for CdS on Ag(111) has been verified to be epitaxial and does not require the formation of any intermediate compounds [12, 13]. Its deposition can be considered a genuine ECALE process. It starts with the oxidative UPD of the sulfur ions on the metal surface. It is worth mentioning that the structure of the resulting sulfur layer has been deeply studied by means of scanning tunneling microscopy (STM) measurements. The second step of the ECALE process is the UPD deposition of cadmium on top of the Ag(111)/S. The charge associated with each layer of either Cd or S corresponds to 0.165 monolayers (referred to as the ideal covering of a layer of the Ag(111) substrate). STM measurements confirm that the fractional coverage of the Cd and S cycles on top of the Ag(111)/S from the 3/7 of the first S layer to the 1/7 (consistent with the coulomb-metric measurement). The epitaxial growth is actually confirmed by the STM images of the first four atomic layers, whereas the unit value of the S/Cd ratio for the successive layers strongly suggests that the epitaxial growth is maintained in these further layers. Moreover, the charge deposited was verified to be linear with the number of ECALE cycles [2] for ECALE scheme as long as 50 cycles. Thus, the results are consistent with the layer-by-layer mechanism proposed for the ECALE process. CdS shows a discrepancy between the deposition on Au(111) and Ag(111). Shannon and Demir reported a (333) structure with a Cd–Cd distance of 4.3 Å for the Cd layer on top of the S layer deposited on Au(111) [14]. This structure is much more compact than the one obtained by our group (0.76 nm for both Cd–Cd and S–S distances) and that difference cannot be ascribed to a difference in lattice constants of Ag and Au since they are practically identical. We proposed that the difference could be ascribed to the different structure of the S layer in contact with the metallic substrate. In fact, the S layer on Au(111) forms a structure with a coverage of one-third, while in the experimental condition we defined on Ag(111) forms a site occupied by a triplet of sulfur atom (coverage of 3/7) and is therefore much more compressed. Thus, Ag denotes a higher affinity with S, resulting in a CdS structure, as determined by STM, corresponding to the basal planes of both wurtzite and zinc blende; these two structures are very similar on the basal plane, and it is not possible to distinguish them by means of STM studies of the first E-ALD cycles. SXRD experiment clarified the structure of CdS and showed that high crystalline quality of the CdS films grown on different surfaces [12, 13]. On the Ag(111), the growth films present only the hexagonal wurtzite structure, while on the other low index planes, a mixture of the hexagonal wurtzite and cubic zinc-blende phases is present.

### *2.1.2. Other Cd-based chalcogenides*

In order to tailor the stoichiometry of the compounds, it is possible to define schemes with different number of steps for the two metals, as will be discussed in Section 2.2.2 for Cux

34 X-ray Characterization of Nanostructured Energy Materials by Synchrotron Radiation

The E-ALD procedure requires a thorough knowledge of the separated SLRs (often UPD under potential deposition) involved in the deposition of the metals over the nonmetals and vice versa. An open issue is the exact definition of the driving force leading these processes. It is very widely accepted that the main contribution to the driving force is the negative free energies change involved in the interaction with the electrodic surface and in the formation of a thermodynamically stable compound. However, the surface can change during the growth due to formation of the crystal, and it is possible to think of a change also in the driving force when a relatively thick crystal is grown. As reported by almost every paper on this topic, it is worth to notice that, usually, the first step of the E-ALD process is the deposition of an atomic layer of chalcogen (or of the nonmetal in general) over the bare metal substrate. Reasonably, the experimental conditions for the deposition change while increasing the thickness of the deposit. For instance, in the growth of crystals, the accumulation of the elastic energy into the lattice is a nonfavorable contribution to the driving force. This is a well-known issue leading to different growth mechanisms and related morphology. Usually, in the case of the E-ALD process, these effects do not prevent the compound formation. Generally speaking, the changes are in the sense of decreasing the amount of deposition and then the total time of the process increases. This can prevent the deposition of some materials, as CdTe and InAs, with a thick-

ness of practical relevance. In other cases (such as the deposition of Cu2

to the Ag/AgCl(KCl sat.) reference electrode.

chemical characteristics.

*2.1.1. CdS*

**2.1. Cadmium-based compound semiconductors**

the copper layer, implying the reduction of copper ions from the solution, may result in more complex reactions implying the formation of some intermediate compounds and the deposition cannot be accounted strictly by the UPD process. Several experimental evidences for these controversial aspects have been reported and they will be discussed in the following pages. It is worth to notice that they reveal a complex mechanism for the formation of chalcogenides by means of E-ALD. The complexity of these processes occurring during the growth is the reason why we prefer to refer to the E-ALD steps with the acronym SLR rather than using UPD.

In this work, we focus on the deposition of chalcogenides on a Ag single crystal, usually on the Ag(111) surface if not specifically indicated. All the potentials repoeted here are referred

Cadmium chalcogenides have been among the first compound semiconductors to be deposited by means of E-ALD due to their interesting electronic properties and favorable electro-

One of the first chalcogenides electrodeposited by E-ALD is the CdS, extensively studied on several different single crystals. The E-ALD process for CdS on Ag(111) has been verified to be epitaxial and does not require the formation of any intermediate compounds [12, 13]. Its deposition can be considered a genuine ECALE process. It starts with the oxidative UPD of the sulfur ions on the metal surface. It is worth mentioning that the structure of the resulting

Zn<sup>y</sup> S.

S), the deposition of

It has been shown that a one-step oxidative SLR is not possible for selenides and tellurides. The E-ALD process for CdSe involves the same steps of the CdS, but the experimental conditions are very different [15]. Hence, the deposition of the Se layer is usually obtained by means of a two-step process [15]. The first implying a massive deposition of a Se film on the Ag(111) followed by reduction of all but the adlayer of Se on the Ag(111) surface. A 1:1.3 (Cd:Se) growth on Ag(111) has been achieved, a tentative explanation is that the transition to a less compact Se structure can occur during the stripping of the bulk Se. The peak related to the transition is probably overlapping with the bulk Se reduction peak. Hence, a lower than expected covering of the substrate is achieved, and consequently, a Cd:Se ratio higher than 1.. Regarding CdTe, cyclic voltammetry showed two peaks related to the reductive UPD of the HTeO2 + on Au(111), and they occur at potential too positive to be easily observed on Ag(111) due to its narrower electrochemical stability window. They are related to a complex chemistry well explained in the literature [16]; hence, it is necessary to use a scheme similar to the CdSe growth. The charge deposited for the first Te layer on Ag(111) is equal to that found on the first UPD of Te on Au(111), for which a (12 × 12) structure was revealed by STM images as well as by low-energy electron diffraction (LEED) patterns. LEED measurements after emersion at a potential corresponding to the second UPD of Te also showed that the (12 × 12) structure originated from a (3 × 3) structure [17]. The structure formed in correspondence with the sole UPD of Te observed on Ag(111) is expected to have the same structure [18]. However, no morphological or structural analysis has been performed on such substrate in order to confirm the expectation. Eventually, E-ALD also gives the possibility of growing ternary and quaternary compounds, and here we describe the case of ternary chalcogenides Cdx Zn1−xS and Cd<sup>x</sup> Zn1−xSe grown on Ag(111) [19–21]. The stoichiometry of the ternary chalcogenides grown by E-ALD can be controlled by the ZnX/CdX (X = S, Se) ratio. Still, CdX deposition seems to be favored with respect to ZnX deposition, hence the ratio between Cd and Zn cycles in the E-ALD sequence does not correspond to the stoichiometric ratio of Cd and Zn in the ternary compounds. Zn-deficiency is a general trend in ternary compounds deposited by means of E-ALD. The authors reported that the most likely explanation for the experiments is a lower deposition rate for Zn. However, electrochemical and XPS characterization confirmed the layer-by-layer growth and the 1:1 ratio between metals and sulfide ions. Eventually, the thickness seems to decrease while decreasing the Cd content. Analogous behavior, although referred to films as thick as 1–5 μm, was found for the compounds grown by the dip technique [22].

#### *2.1.3. An overview on thermodynamics and lattice mismatches*

The potential shifts exploited by the SLR involved in the E-ALD steps are related to the driving force of the process. As discussed in the first part of this text, the definition of the driving force is an open question for the E-ALD, and we can divide it in three terms:


The first term probably dominates the first stages of the growth, while for films with a thickness of several E-ALD cycles the second and third terms become predominant. Experimental results on similar electrodeposition process reported by Golan showed that the growth of such thin films can be either stopped [23, 24] or develop overgrowing quantum dots due to their cumulative strain. Hence, for a qualitative analysis on thin films of practical interest, we consider the potential shift related to the following energy term:

$$
\Delta E = \Delta E\_f \star \Delta E\_{\text{mis}} \tag{1}
$$

where Δ*Ef* is the chemical energy related to the formation of the CdX compound, *ΔE*misis the cumulative elastic energy of the overgrowth due to the mismatch with the substrate. Considering m:n supercells structure, the mismatch is the following:

$$M = \frac{m \, a\_s \, \text{ma}\_s}{\, ^{\text{H.R}\_s}} \tag{2}$$

where *n* is the index of the supercell for the overgrowth, while *m* is for the substrate. As a rule of thumb, we consider larger supercells energetically less stable, even though their mismatch can be smaller. Eventually, an assessment of the cumulative elastic energy is impossible due to the lack of force constants for the hexagonal phases of the system CdX (*x* = S, Se and Te). Still, **Table 1** reports an overview of the ΔE<sup>f</sup> and the mismatch for Ag and Au along selected orientations. Some of these structures have been experimentally verified in literature. Shannon and Demir [14] and Golan et al. [25] reported for CdS over Au(111) a 3:2 supercell with higher mismatch than the 10:7. Despite the lower mismatch of last supercell, we think this is probably related to the lower cumulative elastic energy involved in the latter due to its lower supercell index.

In principle, E-ALD of CdTe should result in a very stable compound. However, the elastic strain induced by the lattice mismatch is higher than the other two chalcogenides, unless we take into considerations very big supercell. On the other hand, CdSe seems to have very favorable chemical and elastic terms for the growth over both the substrates. Still, the experimental condition for a 1:1 growth is not easy to achieve due to the electrochemical behavior of Se. Eventually, Cd and S have a very straightforward electrochemical behavior over Ag(111). The growth of CdS on Ag(111) and Au(111) substrates is quite strained (≈4–5%) unless the substrate and compound are rotated by 30° with respect to the other. Moreover, two structures for the CdS compounds with a very similar stability are known, wurtizte-like (hexagonal grenockite) and zincblende-like (cubic—hawleyite). Hence, the growth and structure for CdS are suitable for the characterization by SXRD. In conclusion, SXRD is a natural follow-up for the characterization of these films due to its coupled surface and bulk selectivities. In particular, CdS resulted to be an ideal system for performing operando studies of E-ALD growth.
