On the growth morphology and crystallography of the epitaxial Cu7Te4/CdTe interface

Fang Wang a, Yimin Lei b, Dapeng Wang a, Zhibin Lei a, Jie Sun *a and Zonghuai Liu *a
aKey Laboratory of Applied Surface and Colloid Chemistry (MOE), Shaanxi Key Laboratory for Advanced Energy Devices, Shaanxi Engineering Laboratory for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, 620 West Chang'an Street, Xi'an, Shaanxi 710119, China. E-mail: jiesun@snnu.edu.cn; zhliu@snnu.edu.cn
bSchool of Advanced Materials and Nanotechnology, Xidian University, 266# Xifeng Road, Xi'an, Shaanxi 710126, China

Received 4th December 2017 , Accepted 3rd January 2018

First published on 3rd January 2018


Controlling interface structures in the epitaxial growth of thin films is quite essential as it is closely related to some special physical and chemical properties. Using crystallographic calculation methods, growth behavior of the thin film as well as the interface geometry can be satisfactorily interpreted. In this study, the growth morphology and crystallography of the epitaxial Cu7Te4/CdTe interface were investigated in detail through comprehensive TEM analysis and crystallographic calculations. The results indicated that the growth interface was not smooth at the atomic level, which may be caused by chemical etching of the CdTe surface. The crystallographic orientation relationship between CdTe and Cu7Te4 was confirmed to be [101]CdTe//[11[2 with combining macron]0]Cu7Te4, (1[1 with combining macron][1 with combining macron])CdTe//(0001)Cu7Te4 and ([1 with combining macron][2 with combining macron]1)CdTe//([1 with combining macron]100)Cu7Te4, which is a typical relationship in FCC/HCP system. Two variants of the Cu7Te4 phase were found to be randomly distributed across the interface region, indicating the equivalent crystallographic relationship between the 2 variants. Both the invariant line and the near coincident site lattice calculations confirmed that the observed orientation relationship between CdTe and Cu7Te4 had the lowest lattice mismatch and interface energy. The mutual rotation angles of ±54.74° and ±35.26° could be applied to obtain 2 types of coherent interfaces. Based on the TEM observation and crystallographic calculations, the possible growth process of the Cu7Te4 thin film on the CdTe substrate was discussed.


I Introduction

Copper chalcogenides (Cu2−xY, 0 ≤ x ≤ 1, Y = S, Se, Te) have attracted great attention in recent years due to their diverse structures and excellent properties.1–3 The application of such materials involves near infrared detection,4,5 thermoelectric devices,6,7 solar cells,8 lithium ion batteries,9 and gas sensing.10 Among all copper chalcogenides, copper tellurides (CuxTe), which include various stoichiometric (CuTe, Cu2Te) and non-stoichiometric (Cu3Te4, Cu7Te4, Cu7Te5) compounds with different crystal structures, have been widely used as back contact materials in CdS/CdTe solar cells according to their variable band-gap (1.0 to 1.5 eV) and excellent electrical properties.11 In the synthesis and application of CuxTe, both thin films and nanocrystals are fabricated via different methods, including the sonochemical approach,12 hydrothermal synthesis,5,13 template-free electrochemical deposition14 and the cation exchange reaction.15–17 In particular, the solid-phase interface generated after chemical reaction, crystallization or phase transformation during the synthesis of CuxTe has an important influence on the morphology and structure of the synthesized products, but crystallography research in this field is quite limited.

In case of a thin film, the lattice correspondence is of great importance to the growth morphology and crystallographic orientation relationship (COR) between the substrate and the film. For example, Mn3O4 exhibits a triangular prism, a quadrangular frustum pyramid and a mixture of cylindrical and triangular prism-morphology when grown on the substrate of SrTiO3 along the (111), (001) and (110) planes, respectively.18 In addition, different interface structures with diverse CORs can also be obtained accordingly. Through the manipulation of the epitaxial surface structures, it is not difficult to obtain thin films with certain growth morphology and different defect distributions at the interface region. It is easy to acquire different interfaces with very little lattice mismatch when the parent phase has a similar crystal structure to the new phase. The cation exchange reaction is a very good example because the replacement of the new cations will not significantly change the crystal symmetry. This may lead to the generation of coherent interfaces with low defect density.16,19,20 For CuxTe, although the preparation methods of nanocrystals and thin films with various growth morphologies have been reported, the growth behaviours of CuxTe as well as the interface geometry such as lattice correspondence and COR between CuxTe and the parent phase still remain a mystery.

It has been a long time since the crystallographic calculation model was used to investigate the geometry of the artificial synthesized or phase transformation-induced interfaces.21,22 In general, the orientation of the substrate will be confirmed first in the case of epitaxial growth. After the growth process, interfaces with certain COR between the substrate and the thin film can be obtained when the cell parameter differences are small; otherwise, an amorphous layer may appear between the substrate and the thin film.23 This situation applies to most growth processes without the participation of chemical reactions. Once chemical reaction is involved, the atoms are considered more active during the growth process and this may lead to a different lattice correspondence at the initial stage of interface formation.23 The final COR between the substrate and the thin film will also be determined by the crystal structures and symmetry.

From a crystallography perspective, epitaxial growth can be regarded as a case of phase transformation from the substrate to the film with one set of crystallographic parallel relations at all times. In other words, the epitaxial surface is considered to be the “habit plane” of this transformation. In this case, the classic crystallographic calculation method including the 2-D invariant line (IL) strain model,24 the edge-to-edge matching (E2EM) model,25 and the invariant deformation element (IDE) model26 is easy to apply as one set of parallel relations (the parallel plane pairs between the parent and new phase at the epitaxial surface) has been fixed. More recently, Liu et al. have developed a universal method for the prediction of the interface structure of heterogeneous epitaxial nanocrystals based on the IDE model.27 This is quite helpful for the analysis of interface geometry between the substrate and the epitaxial thin film.

To investigate the CdTe/Cu7Te4 interface, this study concentrated on the formation and interface crystallography between the Cu7Te4 thin film and the CdTe substrate. Through comprehensive transmission electron microscopy (TEM) investigation, the growth morphology and interface structure of the CdTe/Cu7Te4 contact was analysed in detail. Furthermore, the IDE model and near coincident site lattice (NCSL) calculation was carried out to interpret the particular COR between Cu7Te4 and CdTe. The possible formation mechanism of the Cu7Te4 thin film was finally proposed. The results of this study may provide theoretical and experimental support towards deeper understanding of the interface formation mechanism and further guide the precise regulation on the interface structure between different metal chalcogenides.

II Experimental

2.1 Preparation of the CdTe/Cu7Te4 interface

CdTe single crystal was grown by using the vertical Bridgman method. The wafer was cut along the [001] direction first and then mechanical polished by using MgO suspension with different concentrations. Chemical etching, which has been widely used in Te group compound semiconductors, was then carried out by immersing the wafers into a 2% Br–MeOH solution for 5 min to remove the mechanically damaged surface layer. This process will also introduce a Te-rich layer with a thickness of approximately 5 nm at the surface of the wafer, which is helpful to the following formation of Cu7Te4 thin films. A pure Cu film was then prepared on a CdTe wafer with a chemically etched surface by using the thermal evaporation method under a vacuum of 2 × 10−4 Pa. After removing the sample from the thermal evaporation chamber, the wafer was placed inside a tube furnace to anneal at 500 °C under an argon flow of 0.5 L min−1 for 1 h to promote the formation of Cu7Te4.

2.2 TEM sample preparation and characterization

In TEM sample preparation, the as-prepared wafer was cut into 2 small fragments with the dimension of 5 mm × 2 mm × 2 mm. After pasting the two fragments to each other with the thin film layers face to face, the cross-sectional sample was clamped by tweezers and then placed in an oven to anneal at 200 °C for 1 hour. Tripod polishing was then employed to reduce the thickness of the samples to 10–20 μm by using diamond lapping films. A Gatan 691 precision ion polishing system (PIPS) with a cold stage was used for the final thinning. The beam energy and angle were set to be 3 keV and 7° in the first stage and 2.5 keV and 6° for final thinning. The TEM experiments were carried out on a JEM-3000F field emission TEM. All the images were processed using the Gatan Digital Micrograph software.

III Results

3.1 The morphology of the CdTe/Cu7Te4 interface

Fig. 1 shows the TEM bright field (BF) and dark field (DF) images with the corresponding selected area electron diffraction (SAED) patterns of the CdTe/Cu7Te4 interface. As shown in Fig. 1(a), the contrast across the interface between the CdTe substrate and the Cu7Te4 thin film is very uniform. The interface is marked with a red line in Fig. 1(a). Taking the SAED patterns at the entire region, two sets of diffraction patterns are clearly visible which shown in Fig. 1(c). After indexing the patterns, the zone axis is found to be [110]CdTe//[2[1 with combining macron][1 with combining macron]0]Cu7Te4 as shown in Fig. 1(d). Here, the lattice parameters of CdTe and Cu7Te4 used for indexing are a = 0.6280 nm (F[4 with combining macron]3m) (PDF#65-1046) and a = 0.8328 nm, c = 0.7219 nm (P3m1) (PDF#37-1024), respectively. A 3.5° deviation of (1[1 with combining macron][1 with combining macron])CdTe from (000[1 with combining macron])Cu7Te4 can be clearly observed from the figure. This may be attributed to the interface-stress induced after the reaction. In order to illustrate the detailed morphology of the interface, a TEM DF image was taken by using the (0[1 with combining macron]10) spot of Cu7Te4 as shown in Fig. 1(b). It is can be clearly observed that the film has a mean thickness of approximately 55 nm. The interface shown in the figure is not flat at the microscopic level. This is because the chemically etched surface cannot exhibit a flat status, which may lead to the formation of a zigzag shaped final interface.
image file: c7ce02091k-f1.tif
Fig. 1 TEM analysis of the CdTe/Cu7Te4 interface: (a) BF image; (b) DF image using the (0[1 with combining macron]10) spot of Cu7Te4; (c) corresponding SAED patterns; (d) the indexing of the patterns.

3.2 The structure and lattice correspondence at CdTe/Cu7Te4 interface

In order to illustrate the detailed structure of the CdTe/Cu7Te4 interface at the atomic level, high resolution transmission electron microscopy (HRTEM) images were taken at the interface region as shown in Fig. 2. Two images taken from the same place under different zone axes can be obtained by carefully tilting the samples as shown in Fig. 2(a) and (b). Both Cu7Te4 and CdTe show clear lattice fringes, indicating excellent crystalline status. Similar to the BF images in Fig. 1, the CdTe/Cu7Te4 interface displays a curved shape in the figure. As previously mentioned, this is very common in the chemically etched Te group compounds.23 In addition, some stacking faults marked with blue arrows can be found located at the CdTe side near the interface. It is difficult to observe these faults under another viewing direction (VD) because the stacking fault plane is found to be (1[1 with combining macron]1), which is not a visible diffraction plane under the zone axis [[1 with combining macron][2 with combining macron]1]. The existence of these planar defects (stacking faults or twins) is very common in heteroepitaxial growth systems, particularly for some low symmetry structures.27–29 After taking the FFT images from different areas marked with I, II, III and IV, the 4 patterns were indexed accordingly. It was found that (11[1 with combining macron])CdTe and (000[2 with combining macron])Cu7Te4 completely overlap in the FFT images, which is different from that in Fig. 1(c). This indicates that the interface region, captured by HRTEM images, has lower interface strain. The 3.5° orientation difference is unavailable in this area. In this case, COR under 2 different VDs can be summarized as:
VD1: [110]CdTe//[2[1 with combining macron][1 with combining macron]0]Cu7Te4, (11[1 with combining macron])CdTe//(000[1 with combining macron])Cu7Te4

VD2: [[1 with combining macron][2 with combining macron]1]CdTe//[1[1 with combining macron]00]Cu7Te4, (11[1 with combining macron])CdTe//(000[1 with combining macron])Cu7Te4

image file: c7ce02091k-f2.tif
Fig. 2 The HRTEM images with two viewing directions [110]CdTe//[2[1 with combining macron][1 with combining macron]0]Cu7Te4 (a) and [[1 with combining macron][2 with combining macron]1]CdTe//[1[1 with combining macron]00]Cu7Te4 (b) illustrate the detailed structures of CdTe/Cu7Te4 interfaces. The estimated tilting angle (28.42°) calculated from the goniometer system of the TEM is very close to the real orientation difference (30°). The FFT images taken from the different areas and the corresponding indexing of the patterns are also illustrated accordingly. By using the stereographic projections 2 (SP2) software,30 the stereographic projections of vectors for the relative position of two VDs are shown in panel (c). Part of the crystallographic parallel relationships of the two VDs were also drawn in the stereographic projection maps shown in panels (d) and (e), respectively. The black, red and blue spots refer to the overlapping, CdTe and Cu7Te4, respectively.

Here, it is worth mentioning that VD2 can be reached by tilting the sample starting from VD1 along the normal direction of (11[1 with combining macron])CdTe//(000[1 with combining macron])Cu7Te4 by a certain angle, which can be estimated through the following equation:

 
cos[thin space (1/6-em)]θ ≈ cos[thin space (1/6-em)]ΔTX[thin space (1/6-em)]cos[thin space (1/6-em)]ΔTY(1)
where θ represents the actual orientation differences between the two VDs. ΔTX and ΔTY are the tilting angle differences along X and Y directions between the two VDs. These two values can be read and calculated directly from the TEM goniometer system. In this study, the TX and TY values of VD1 and VD2 are TX1 = −10.6°, TY1 = 1.4° and TX2 = 14.8°, TY2 = 14.6°, respectively. Therefore, ΔTX and ΔTY can be calculated to be 25.4° and 13.2°. Then, the value of θ can be obtained to be 28.42°. By projecting the 2 VDs onto the same plane, the stereographic projection figure can be drawn to illustrate the relative position of the 2 VDs as shown in Fig. 2(c). There are 12 parallel relationships with the included angle of 30° distributed over the circumference when the pole center is set to be [[1 with combining macron]11]CdTe//[0001]Cu7Te4. Our estimated angle (28.42°) between the 2 VDs is very close to the theoretically calculated value. Moreover, the detailed COR between CdTe and Cu7Te4 for the 2 VDs are plotted onto the stereographic projection figure shown in Fig. 2(d) and (e). These two figures illustrate that several parallel relations are located along the trace of ([1 with combining macron]11)CdTe//(0001)Cu7Te4. In addition, similar symmetry can be observed for both VDs.

3.3 The variant structures of Cu7Te4 thin film

We also observed that the Cu7Te4 thin film exhibits two variant growth habits at the surface of CdTe, the results are summarized in Fig. 3. Compared with the images shown in Fig. 2, numerous stacking defects at (0001)Cu7Te4 can be confirmed from the FFT images at the corresponding region. This indicates that the planar defects not only appear in the substrate, but also exist inside the film during the epitaxial growth process. In addition, the Cu7Te4 thin film in this region is found to have 2 variants with a mutual rotation of 70.53°. This is also the angle between (1[1 with combining macron]1) and (1[1 with combining macron][1 with combining macron]) of CdTe, which indicates that (0001)Cu7Te4 has equal probabilities parallel to the two equivalent {111}CdTe. The white circles marked in Fig. 3(a) indicate that the double lattice plane spacing of CdTe appeared near the interface. This phenomenon will be discussed in section 4.2. The interface models of CdTe/V1 and CdTe/V2 are also drawn based on the TEM results as shown in Fig. 3(b) and (c). Here, the mutual rotation of 70.53° is carried out on the CdTe substrate in order to display the 2 figures under the same layout. Some important directions are also illustrated accordingly. Furthermore, Fig. 3(d) shows the enlarged image of the rectangular region marked in Fig. 3(a), demonstrating the detailed interface structure between the 2 variants of Cu7Te4. Clearly, a fully incoherent interface structure can be observed in the figure, indicating a very large orientation difference between the two variants. The kinks and steps present a regular distribution along the interface with the parallel relationships of (0001)V1//(0[3 with combining macron]31)V2 and (0001)V2//(0[3 with combining macron]31)V1, respectively.
image file: c7ce02091k-f3.tif
Fig. 3 The HRTEM image (a) of the CdTe/Cu7Te4 interface illustrating different variants of the Cu7Te4 thin film. Numerous planar defects are observed, which can be confirmed inside the film through the FFT images at the corresponding region. The reciprocal lattices of the 2 variants of Cu7Te4 can be completely coincident after a 70.53° rotation, which can be confirmed through the FFT panel from different regions. The white circles marked in panel (a) indicate the double expansion of the lattice spacing for CdTe near the interface. The interface models between CdTe and the 2 variants are drawn in panel (b) and (c). Panel (d) is the enlarged figure taken from the white rectangular area in panel (a), illustrating the detailed interface structure between the 2 variants. The zigzag shaped interface has the kinks and steps parallel to (0001)V1//(0[3 with combining macron]31)V2 and (0001)V2//(0[3 with combining macron]31)V1, respectively.

IV Discussions

4.1 The crystallographic calculations on the CdTe/Cu7Te4 interface geometry

Crystallographic calculations were employed to interpret the formation of specific COR between CdTe and Cu7Te4. As mentioned previously, the epitaxial growth can be regarded as the phase transformation process from substrate to the film with the epitaxial surface being the habit plane. According to the IDE model,26 the habit plane in a diffusion controlled phase transformation must contain the burgers vector (b) of the matrix. A virtual unit vector P1 can be defined to be b/|b|. This vector should be invariant both in length and direction during the transformation. In addition, the invariant normal Q1 (the invariant line in the reciprocal lattice) and the corresponding 2 vectors after the transformation, P2 and Q2 can also be defined accordingly. By establishing the reference coordinate system, a rotation axis can be found to allow COR changing from the initial lattice correspondence to the new COR after the transformation. The rotation axis u and angle θ can be determined by resolving the Euler equation as follows:
 
image file: c7ce02091k-t1.tif(2)

According to the definition of the 4 vectors P1, P2, Q1, and Q2, the following equations can be obtained:

 
image file: c7ce02091k-t2.tif(3)
and
 
P1·Q1 = 0, |P1| = |P2|, |Q1| = |Q2|(4)

Here, B is the initial lattice strain matrix, which can be determined by the ratio of the corresponding parallel vectors' length between substrate and the thin film in the reference coordination system. Fig. 4(a) illustrates the initial lattice correspondence between CdTe and Cu7Te4 established according to the TEM observations. The COR can be summarized as follows:

 
image file: c7ce02091k-t3.tif(5)


image file: c7ce02091k-f4.tif
Fig. 4 The initial lattice correspondence between CdTe and Cu7Te4 (a), which is established based on the TEM observations. The stereographic projection figure (b) is also illustrated accordingly.

This is a very common COR in the FCC/HCP phase transformation system as both the close packed planes and the close packed directions are parallel for the two phases.31 In this case, the Bain strain matrix can also be calculated accordingly. Fig. 4(b) also draws the initial lattice correspondence on the pole figure.

As the epitaxial surface in this study is very close to (001)CdTe, the habit plane can be regarded as (001)CdTe according to the approach described by Liu et al.27 In other words, the following expressions can be obtained:

 
image file: c7ce02091k-t4.tif(6)

This indicates that the direction is not changed when the system is transformed from Q1 to Q2. Moreover

 
P1 = [uv0], P2 = B·P1 = [η1u,η2v,0](7)
where η1, η2, and η3 represent the main strain along the x, y and z axis, respectively. Clearly, P1 and P2 also have the same direction, but with different modulus according to eqn (7). In other words, the direction of the inclined vector has not changed after the transformation. Therefore, no rotation is needed to minimize the strain of the system. This indicates that the observed COR simultaneously exhibits an excellent lattice mismatch and the lowest interface energy. In brief, the growth direction and habit plane of the Cu7Te4/CdTe system are [001]CdTe and (001)CdTe, respectively. The final COR between the two materials is illustrated in eqn (5).

The NCSL calculation method was also performed to interpret the COR between CdTe and Cu7Te4 as well as the existence of the variants. To start the calculation, a 2D reference coordination system was first established as shown in Fig. 5. For CdTe, 2 vectors, RS1 and RS2, which are perpendicular to each other, were set as basic vectors. On expanding the 2 vectors m and n times, the basic vector of the NCSL V1 can be obtained by adding the vectors mRS1 and nRS2, as shown in Fig. 5(a). Similarly, the basic vectors of Cu7Te4, RF1 and RF2, can also be established. According to the definition of NCSL, a mutual rotation must be applied to one of the two basic vectors. Therefore, two vectors, RRF1 and RRF2 after the mutual rotation can be obtained. After enlarging RRF1 and RRF2p and q times, another basic vector of the NCSL V2 can be obtained by adding the vectors pRRF1 and qRRF2 as shown in Fig. 5(b). In fact, both V1 and V2 are the basic NCSL vectors established using the substrate and the film, respectively. By applying V1 equals to V2, the four integers, m, n, p and q, the rotation angle θ as well as the reciprocal density ∑ can be calculated through basic mathematical operations. The detailed calculation process can be found in the ESI of this paper.


image file: c7ce02091k-f5.tif
Fig. 5 The process of establishing the NCSL unit vectors from the substrate (a) and the film (b), respectively. The schematic figures of the predicted COR are also drawn in panels (c) and (d). The red and blue rectangles refer to the projected unit cell of CdTe and Cu7Te4, respectively. The 54.74° and 35.26° rotation is applied to obtain the COR between variant 1/2 of Cu7Te4 and CdTe, respectively.

Based on the principle of taking the smallest values of the four integers, m, n, p and q, 4 groups of values can be obtained as shown in Table S1 in the ESI. Moreover, the rotation angle θ can be calculated to be ±54.74° and ±35.26°, respectively, just corresponding to the 2 variants. Under this condition, the reciprocal density ∑ is calculated to be 9, which is also the smallest odd number in the CdTe/Cu7Te4 system when projected along [110]CdTe//[10[1 with combining macron]0]Cu7Te4. Fig. 5(c) and (d) are the schematic figures of the predicted COR between variant 1 and 2 of Cu7Te4 and CdTe. A 54.74° and 35.26° rotation, which corresponds to variant 1 and 2, is applied respectively.

4.2 Possible growth mechanism of epitaxial Cu7Te4 thin film

According to the crystallographic calculation results, the growth of the Cu7Te4 thin film in this study strictly follows a certain COR, which indicates that the initial lattice correspondence still constrains the growth process. Therefore, the epitaxial growth of the Cu7Te4 thin film on the CdTe substrate can be considered as the phase transformation process from CdTe to Cu7Te4 when considering the growth habit and crystallography. This indicates that the chemical reaction does not affect the COR between the reactant and product.

The possible growth mechanism of the epitaxial Cu7Te4 thin film can be summarized in Fig. 6. At the first stage, Cu was deposited onto CdTe, which contains a thin Te-rich layer induced by chemical etching at the top surface, as shown in Fig. 6(a). Both the as-prepared Cu and the Te-rich layer exhibit the amorphous status at this time. After starting the heat treatment process, these Cu and Te atoms began to mix with each other due to the inter-diffusion process, leading to the formation of a mixed layer between the Cu film and the CdTe substrate, as shown in Fig. 6(b). The formation of this mixed layer will combine with the cation exchange reaction to form the new Cu7Te4 layer. In general, the final reaction products are the stoichiometric copper chalcogenide phases in the case of a perfect cation exchange reaction.17,19,20 But in our case, only the Cu7Te4 phase can be found across the interface, which may be attributed to the combination effect of cation exchange between CdTe and Cu as well as the chemical reaction between Cu and Te at the surface. It is speculated that the first CuxTe phase is generated by the reaction of Cu and the Te-rich layer. As the reaction proceeds, Cu, CuxTe and CdTe will react with each other, leading to the formation of the Cu7Te4 layer. This process is caused by the simultaneous reconstruction of the mixed layer and the cation exchange when the temperature reached 500 °C, as shown in Fig. 6(c). This may also be the reason why no stoichiometric CuxTe phase, such as Cu2Te, can be obtained in our study as the reaction is not a perfect cation exchange reaction. The as-deposited Cu film will be fully consumed as the reaction proceeds. With this process, the 2 variants of Cu7Te4 can occur with the same probability. After the final adjustment, the epitaxial Cu7Te4 thin film with 2 variants can be obtained. The Cu7Te4 thin film with excellent crystallinity prepared by this method has specific crystallographic relationships with the substrate. Further investigations may be needed to understand the structure–property relationships of such CuxTe thin films.


image file: c7ce02091k-f6.tif
Fig. 6 The schematic diagram of the formation process of the Cu7Te4 thin film at the surface of CdTe: (a) after Cu deposition, (b) the mix of Cu and Te atoms at the surface, (c) the reaction during heat treatment and (d) the final adjustment after the reaction.

Conclusions

Through comprehensive TEM analysis, the growth morphology and crystallography of the CdTe/Cu7Te4 interface was investigated in detail. It was found that the interface generated after the reaction exhibits a zigzag shape in the atomic scale, which may be associated with the Te-rich layer formed after chemical etching on the CdTe surface. In addition, the COR between CdTe and Cu7Te4 was confirmed to be [101]CdTe//[11[2 with combining macron]0]Cu7Te4, (1[1 with combining macron][1 with combining macron])CdTe//(0001)Cu7Te4 and ([1 with combining macron][2 with combining macron]1)CdTe//([1 with combining macron]100)Cu7Te4, which is a typical COR in the FCC/HCP system. Two variants of Cu7Te4 were also found to be distributed randomly across the interface. By carrying out the invariant line and NCSL calculation, it was found that the observed COR had the lowest lattice mismatch. The reciprocal density (∑) was calculated to be the smallest value (9) for the CdTe/Cu7Te4 interface. Moreover, (0001)Cu7Te4 had the same probability to parallel with (1[1 with combining macron][1 with combining macron])CdTe and (1[1 with combining macron]1)CdTe, which led to the formation of 2 variants. The mutual rotation angles were calculated to be ±54.74° and ±35.26° for the 2 variants by the NCSL calculation. The results of this study confirmed that the COR between the film and the substrate still follows the basic lattice correspondence in the chemical reaction involving the epitaxial growth process.

Conflicts of interest

There are no conflicts to declare for this manuscript.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (Grant No. 21601117, 21701129), Natural Science Basic Research Plan in Shaanxi Province of China (Program No. 2017JQ2038, 2017KW-023) and the Fundamental Research Funds for the Central Universities (GK201603054, GK201603053).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ce02091k

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