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Orientation of Zn3P2 films via phosphidation of Zn precursors
Journal of Crystal Growth 459 (2017) 95–99
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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Orientation of Zn3P2 films via phosphidation of Zn precursors Ryoji Katsube, Yoshitaro Nose
⁎
MARK
Department of Materials Science and Engineering, Kyoto University, Yoshida Honmachi, Sakyo-ku, Kyoto 606-8501, Japan
A R T I C L E I N F O
A BS T RAC T
Keywords: A3. Polycrystalline deposition B1. Phosphides B2. Semiconducting materials B3. Solar cells
Orientation of solar absorber is an important factor to achieve high efficiency of thin film solar cells. In the case of Zn3P2 which is a promising absorber of low-cost and high-efficiency solar cells, (110)/(001) orientation was only reported in previous studies. We have successfully prepared (101)-oriented Zn3P2 films by phosphidation of (0001)-oriented Zn films at 350 °C. The phosphidation mechanism of Zn is discussed through STEM observations on the partially-reacted sample and the consideration of the relationship between the crystal structures of Zn and Zn3P2 . We revealed that (0001)-oriented Zn led to nucleation of (101)-oriented Zn3P2 due to the similarity in atomic arrangement between Zn and Zn3P2 . The electrical resistivity of the (101)-oriented Zn3P2 film was lower than those of (110)/(001)-oriented films, which is an advantage of the phosphidation technique to the growth processes in previous works. The results in this study demonstrated that well-conductive Zn3P2 films could be obtained by controlling orientations of crystal grains, and provide a guiding principle for microstructure control in absorber materials.
1. Introduction Zinc phosphide (Zn3P2 ) is a p-type semiconductor with a direct bandgap of 1.55–1.6 eV [1], which is suitable for energy conversion of sunlight into electricity. In addition, the constituent elements of Zn3P2 , zinc and phosphorus, are earth-abundant. Zn3P2 is therefore a candidate material for inexpensive and high-efficiency solar cells. The devices using Zn3P2 have been studied for over four decades, and the highest conversion efficiency for cells based on polycrystalline p-type Zn3P2 bulk crystals was reported to be 6.08% under 100 mW cm−2 and air mass 1 (AM 1) illumination by Catalano et al. in 1979 [2]. In addition, the light absorption coefficient of Zn3P2 is enough to realize thin film devices [3]. Many researchers have thus studied about fabrication of Zn3P2 thin films because solar cells based on thin films have some advantages in series resistance and in amount of required materials. However, the highest efficiency of thin film Zn3P2 solar cells is much lower than that of bulk crystals [4]. One of the reasons is a trap site of carriers at grain boundaries. The grain boundary density of the champion thin-film cell was larger than that of the bulk-crystalline cell [4], because the average grain size of Zn3P2 in the film was 100 times smaller than that in the bulk crystals [5]. For improvement of efficiency of thin-film based devices, increase of grain size, passivation of grain boundaries, and/or orientation control are effective approaches from the viewpoint of grain boundary engineering. In the case of Cu(In, Ga)Se 2 (CIGS)-based solar cells with high conversion efficiency, there is a well-developed strategy, alkaline metal incorporation, for ⁎
passivation of grain boundaries [6–8]. CIGS-based solar cells therefore show high efficiency around 20% even though the grain size is small. Furthermore, the absorber layers in the high-efficiency cells have columnar grain structure with orientations of (112) or (110)/(102) [9–11]. The orientations of (110) and (102) are equivalent. This is an advantage for carrier transport due to fewer grain boundaries in the transport direction. It was also reported that the quantum efficiency of solar cells was affected by the orientations, which dependent on the conditions such as [Se]/[In + Ga] flux ratio in molecular-beam epitaxial growth. In the case of polycrystalline Zn3P2 thin films, there are some reports on surface passivation, alkaline metal doping, and orientation of thin films. Kimball et al. reported surface passivation of bulk crystalline Zn3P2 by chemical etching and oxidation [12], and Paquin et al. revealed that Zn3P2 films on mica (compound including alkaline metals) have larger carrier density than those on quartz (without alkaline metals) [13]. However, there are no reports on passivation of the grain boundaries. This is still a topic under investigation in Zn3P2 -based solar cells. Zn3P2 films with (110)/(001) orientation were successfully grown by gas phase growth techniques such as metalorganic chemical vapor deposition (MOCVD) [14], ionized cluster beam deposition (ICBD) [15], radio frequency sputtering (RF sputtering) [16], evaporation [17], and close space sublimation (CSS) [18]. The (110) and (001) orientations are equivalent to each other in Zn3P2 with the ideal axial ratio (c / a = 20.5). From the reported results, it seems that Zn3P2 is self-organized in the deposition of Zn3P2 films. On the other
Corresponding author. E-mail address: [email protected] (Y. Nose).
http://dx.doi.org/10.1016/j.jcrysgro.2016.11.040 Received 9 September 2016; Received in revised form 25 October 2016; Accepted 10 November 2016 Available online 12 November 2016 0022-0248/ © 2016 Elsevier B.V. All rights reserved.
Journal of Crystal Growth 459 (2017) 95–99
R. Katsube, Y. Nose
hand, Cimaroli et al. reported that the resistivity of films with high (110)/(001) orientation was higher than that of films with random orientation [18]. Therefore, it is necessary to prepare films with various orientations and investigate their properties in order to clarify the relationship between orientations and electric properties in Zn3P2 films, and thereby optimize the microstructure of Zn3P2 films. In the present study, we thus tried to fabricate such films by phosphidation of oriented Zn precursors. In the case of Zn-chalcogenides such as ZnS and ZnO, it was reported that (0001)-oriented chalcogenides films were obtained by the reaction of (0001)-oriented Zn films with sulfur or oxygen [19,20]. In this case, the structure of ZnS and ZnO was wurtzite, which is a similar arrangement of zinc atoms to pure Zn with hcp structure. It is known that the orientation of metals deposited on glass substrates tends to be the closest-packed plane of metals [21], which is (0001) plane in the case of hcp metals such as Zn. We also consider the above tendency. 2. Experimental procedure Zn film as precursor was fabricated by magnetron sputtering. We prepared soda lime glasses (SLG) with and without coating Mo as substrates. Mo was also coated by magnetron sputtering. The sputtering was carried out under an Ar atmosphere with the pressure of 7 × 10−1 Pa. The power density for the target and the duration of sputtering were 1.4 W cm−2 and 30 min for Zn, and 6 W cm−2 and 80 min for Mo, respectively. The thicknesses of Zn were approximately 100 – 150 nm, which was estimated from the theoretical density of Zn and the deposited amount evaluated by inductively-coupled plasma atomic emission spectroscopy (ICP-AES, SPS3520UV, SII Nano Technology). The phosphidation experiments were carried out in the tube furnace with two heating zones, which was similar to the formation of ZnSnP2 . [22] The temperatures of a Zn film and a phosphorus source were individually controlled during phosphidation. As a source of phosphorus gas, we used a composite with dual phase of Sn and Sn 4 P3, which was prepared from Sn (5N, Kojundo Chemical Lab. Co., Ltd.) and (6N, Kojundo Chemical Lab. Co., Ltd.). The total amounts of the Sn/Sn 4 P3 composites were 1.7–1.9 g. Ar gas deoxygenated by passing through the furnace containing Ti sponge heated at 850 °C was used as a carrier gas with the flow rate of 20 sccm. The partial pressure of oxygen was monitored using a zirconia oxygen sensor (SD/LD-450, Toray Engineering Co., Ltd.), and controlled below 10−20 atm, which is the minimum limit of detection of the sensor. Before the experiments, the substrate covered by Zn and the Sn/Sn 4 P3 composite were placed out of the heating zones in the furnace. First, the zone for the composite was heated up to 450 °C in 15 min and held for 1 h for stabilization. After that, the composite was inserted into the heating zone and phosphorus was evaporated. The partial pressure of P4 on the composite is approximately 3 × 10−3 atm at 450 °C, which was calculated from the thermodynamic data of the previous report [23]. The substrate was then inserted into the heating zone after the stabilization for 15 min and heated up to 350 °C with the rate of 18 °C min−1. The substrate was kept at 350 °C for 30 min and then cooled down in the furnace. In addition, we fabricated a sample quenched at the early stage of phosphidation in order to consider the reaction behavior. In this experiment, Zn films as precursors were prepared on SLG substrates. The sample was annealed at 350 °C for 2.5 min under P4 atmosphere ( p P4 = 3 × 10−3 atm ), and then moved out from the heating zone. The Zn films before and after phosphidation were analyzed by X-ray diffraction (XRD, SmartLab, Rigaku) for identification of products and evaluation of crystal orientation. The sample was observed by scanning transmission electron microscopy (STEM, JEM-2100F, JEOL) and elemental mapping using energy dispersive X-ray spectroscopy (EDX). The resistivity of the film on the SLG substrate was measured by van der Pauw method [24] at room temperature (ResiTest 8310,
Fig. 1. XRD profiles of the films on SLG substrates (a) before (153 nm-thick) and (b) after phosphidation (266 nm-thick).
TOYO Corporation). 3. Results and discussion Fig. 1 shows the XRD profiles of Zn films on SLG substrates before and after phosphidation. A single-phase Zn was observed in the film before phosphidation, while all peaks were assigned to Zn3P2 and no secondary phase was confirmed in the profile after phosphidation. It is also understood that the Zn film shows (0001) orientation, as described in the literature [21]. In the Zn3P2 film, we observed (101) orientation, which is different with the plane orientation in the previous reports. For the films on Mo-coated substrates, the orientation of Mo film was observed in the [110] direction, while a strong orientation was not confirmed in the Zn films deposited on Mo films as shown in Fig. 2(a). Fig. 2(b) shows that the films after phosphidation do not have a strong
Fig. 2. XRD profiles of the films on Mo-coated SLG substrates (a) before (96 nm-thick) and (b) after phosphidation.
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Fig. 3. XRD profiles of the Zn film on the SLG obtained by phosphidation for 2.5 min.
Fig. 5. Cross-sectional STEM-EDX images of the Zn film on the SLG substrate obtained by phosphidation 30 min (a) STEM-DF image, elemental mappings of (b) Zn and (c) P, and (d) overlapped image.
orientation, although Zn3P2 films without secondary phases were obtained. It is thus clarified that the oriented Zn3P2 films can be obtained from the oriented Zn precursors, while the Zn films with random orientations lead to randomly-oriented Zn3P2 films. In addition, the grain sizes of the Zn3P2 films calculated using the Scherrer equation were 9.8 ± 2.1nm for the film on SLG and 15.6 ± 2.4nm for that on Mo. The types of substrate appear to be affect the grain size, and therefore the interface between Zn and substrates should play important role in the initial stage of phosphidation. For further consideration of the relationship between films before and after phosphidation, we investigated initial stage of phosphidation by cross-sectional STEM observation of a sample on the SLG which was obtained by interrupting the phosphidation experiment in 2.5 min Figs. 3 and 4 show the XRD profile and the cross-sectional STEM-EDX images of the sample, respectively. The phases of Zn and Zn3P2 are identified in the XRD profile and the STEM-EDX images indicate that the film was partially phosphidated in comparison to those of the completely phosphidated film shown in Fig. 5. Consequently, it is understood that the sample was en route of phosphidation. As seen in Fig. 4, Zn3P2 formed at the interface between Zn precursor and substrate in the initial stage of phosphidation, although the chemical potential of phosphorus on the surface of Zn film was controlled by the partial pressure of phosphorus gas and it was enough high to the
formation of Zn3P2 . Fig. 4 indicates that phosphorus penetrates into Zn film before the reaction with Zn. This is a similar phenomenon to the phosphidation of indium by the thin-film vapor-liquid-solid process [25], where nucleation of InP occurs at the interface between liquid indium and molybdenum substrate. We here consider on impurity diffusion in Zn, however, the diffusion coefficient of phosphorus in Zn has not been reported. The diffusion coefficient of Ni in Zn, which is the smallest among reported impurity diffusion coefficients, is 2.8 × 10−11 m2 s−1 at 350 °C in the [0001] direction [26,27], and the average diffusion length is 5.3 × 10−6 m s−1. Considering that the diffusion coefficient of phosphorus in Zn is at least more than that of Ni, phosphorus atoms can penetrate from the surface to the substrate through the Zn film with the thickness of 150 nm in just 7.9 × 10−4 s. Consequently, it is likely that phosphorus penetrates to the bottom of the Zn film before the formation of Zn3P2 on the surface. In addition, it is reasonably considered that the concentration of phosphorus increases at the interface between Zn and SLG and the nucleation of Zn3P2 occurs because the diffusion coefficient of phosphorus in SLG might be much smaller than that in Zn. In order to discuss the orientation relationship between Zn3P2 and Zn, we here take account of the crystal structures of Zn and Zn3P2 . The crystal structure of Zn is hcp with the lattice constants of a = 2.66 Å and c = 4.95 Å , and the atomic arrangement of (0001) plane is a two dimensional triangular lattice. On the other hand, the crystal structure of Zn3P2 is a defect antifluorite type structure [28]. The phosphorus sublattice is fcc, and zinc atoms occupy 3/4 of tetrahedral sites of the phosphorus sublattice. The zinc sublattice can be regarded as a simple cubic lattice containing vacancies (unoccupied tetrahedral sites). The vacancies are ordered in the structure, and therefore the symmetry of Zn3P2 belongs to the tetragonal system with the lattice constants of a = 8.09 Å and c = 11.45 Å . In the following discussion on the orientation, the displacement of zinc atoms from the ideal tetrahedral positions is not considered for simplicity. The relationship between the unit lattice vectors of Zn3P2 (a b c) and the fcc-based phosphorus sublattice (a′ b′ c′) can be written as
a = a′ + b′,
b = a′ − b′,
c = 2c′.
(1)
Accordingly, the relationship between the indices of the lattice plane of Zn3P2 (h k l ) and the phosphorus sublattice (h′ k′ l′) is
h = h′ + k′,
k = h′ − k′,
l = 2l′.
(2)
Considering the above relationship, the (101) plane of Zn3P2 corresponds to the (111) plane of the phosphorus sublattice. Similarly, it also corresponds to the (111) plane of the zinc sublattice.
Fig. 4. Cross-sectional STEM-EDX images of the Zn film on the SLG substrate obtained by phosphidation for 2.5 min (a) STEM-DF image, elemental mappings of (b) Zn and (c) P, and (d) overlapped image.
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Fig. 8. Schematic illustrations of phosphidation process. (a) Precursor Zn film with (0001) orientation, (b) nucleation of (101)-oriented Zn3P2 at the interface of Zn and substrate, and (c) grain growth of Zn3P2 by the reaction of Zn and phosphorus while keeping the orientation. Fig. 6. Atomic arrangements of Zn atoms in (a) vacancy-rich (101) plane of Zn3P2 (type I), (b) vacancy-deficient (101) plane of Zn3P2 (type II), (c) (0001) plane of Zn, and (d) a lattice consisting of 1/3 points of Zn (0001) plane.
Table 1 Electrical resistivity of oriented polycrystalline Zn3P2 thin films.
Consequently, the atomic arrangement of the zinc sublattice in the (101) planes is a two-dimensional triangular lattice, including vacancies. Figs. 6(a) and (b) show the atomic arrangements of zinc atoms in (101) plane of zinc sublattice in Zn3P2 . There are two types of arrangements depending on the ordering of vacancies, hereafter they are called type I and type II arrangements for Figs. 6 (a) and (b), respectively. In any cases, the arrangement of lattice points is a triangular lattice like the (0001) plane in Zn with hcp structure. However, the distances between the nearest neighbor lattice points in Figs. 6(a) and (c) are quite different with each other: 4.04 and 2.66 Å. Here, we consider a two dimensional lattice shown in Fig. 6(d), which consists of 1/3 lattice points of the (0001) plane in the hcp structure shown in Fig. 6(c). The lattice constant of the triangular lattice shown in Fig. 6(d) is 4.61 Å, which is close to that of the zinc sublattice of Zn3P2 shown in Fig. 6(a). On the other hand, the stacking structure of the zinc sublattice in the direction perpendicular to the (101) plane of Zn3P2 is shown in Fig. 7. The stacking can be understood by considering that the sequence in the [111] direction of the simple cubic lattice is … ABCABC… like fcc, since the zinc sublattice is simple cubic as previously described. Assuming that the stacking of the zinc sublattice consists of the lattice shown in Fig. 6(d), the zinc sublattice in Zn3P2 can be obtained without as much as possible changing the arrangement of lattice points. In other words, the (101) plane in Zn3P2 is based on the atomic arrangement derived from hcp Zn shown in Fig. 6(d). In actual, two types of the atomic arrangement in the (101) plane, type I and II, should be considered as shown in Figs. 6 (a) and (b), and the stacking sequence is 4 layers in a cycle, I-I-II-II, from the viewpoint of the atomic arrangement. It is thus understood that the structure in Fig. 7 consists of a stacking of 12 layers considering from the stacking of lattice points and the atomic arrangements, which are 3 and 4 layers in a cycle, respectively. We here show the phosphidation mechanism of zinc at 350 °C as shown in Fig. 8. First of all, phosphorus atoms penetrate into crystal grains of zinc and nucleation of Zn3P2 occurs at the interface of Zn and substrate. At this stage, the orientation of Zn3P2 is (101) due to the
Growth technique
Resistivity/ Ωcm
Orientation
MOCVD[14]
2.5 × 10 3 106 – 107
(110)/(001)
ICBD[15] RF sputtering[16] CSS[18] Phosphidation (this study)
1.0 × 10 4 – 1.5 × 10 5 4 × 101 – 2 × 10 3 30 ± 2
(110)/(001) (110)/(001) (110)/(001) (101)
similarity in atomic arrangements between Zn and Zn3P2 as discussed above. Then, Zn3P2 grains grow by the reaction of Zn and phosphorus while keeping the (101) orientation as shown in Fig. 8(c). Table 1 shows the resistivity of Zn3P2 in this study together with reported values. Cimaroli et al. reported that highly (110)/(001)oriented Zn3P2 films showed higher resistivity than the random films [18], and therefore (110)/(001) orientation in Zn3P2 films seems to have a negative influence on conductivity. On the other hand, the (101)-oriented Zn3P2 film in this study have the resistivity in the same order of magnitude as those of less (110)/(001)-oriented Zn3P2 films. The resistivity of thin film semiconductors is commonly affected by factors including concentration of electroactive dopants and density of defects such as grain boundaries acting as trapping sites of carriers. The results of this study indicate that (101) orientation of Zn3P2 is more favorable than (110)/(001) orientation from the viewpoint of electrical conduction, and microstructure control including orientation is also important for electrical conductivity of Zn3P2 films. As discussed above, the orientation of Zn3P2 films obtained by phosphidation depends on that of Zn precursors because of the formation of Zn3P2 by the reaction of solid Zn precursors with phosphorus gas. This is an advantage to obtain films with any orientations compared to conventional gas phase growth techniques. 4. Conclusions We successfully fabricated Zn3P2 films with (101) orientation on SLG substrates, which had never reported before. The process includes preparation of (0001) oriented precursor Zn films by sputtering and phosphidation of them at 350 °C which is slightly lower than the melting point of Zn. From the result of STEM observation of the sample partially phosphidated, nucleation of Zn3P2 was revealed to occur at the interface between Zn and the substrate in the phosphidation. The relationship between the orientations of the films before and after phosphidation was discussed from the viewpoint of atomic arrangements of zinc atoms in Zn and Zn3P2 . Taking vacancies in the (101) plane of Zn3P2 into consideration, it is understood that the twodimensional lattice of the (101) plane is similar to that of (0001) plane of Zn with hcp lattice. Moreover, the distance between nearest neighbor points is close to each other in Zn3P2 (101) and Zn (0001), considering the lattice consisting of 1/3 lattice points of Zn. In other words, the zinc sublattice of Zn3P2 is based on the lattice of Zn precursor, which is one
Fig. 7. Stacking structure of (101) plane of Zn in Zn3P2 . Type I and II shown in Figs. 6(a) and (b) are expressed as I and II, respectively. A, B, and C represent stacking sequence of triangular lattice.
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of the origins for the growth of oriented Zn3P2 films. The electrical resistivity of the (101) oriented Zn3P2 film was measured and compared with those in previous studies. The (101) oriented Zn3P2 film in this study was revealed to have lower resistivity than the (110)/(001)-oriented films in the previous articles. As described above, phosphidation of zinc films is a new process for preparation of well-conductive Zn3P2 films with an orientation different from that of the films grown by conventional growth techniques.
[11]
[12]
[13]
Acknowledgements [14]
This work was partly supported by JSPS KAKENHI Grant number 26289279. The authors are grateful to Prof. H. Hayashi (Kyoto Univ.) for giving us the opportunity to use the equipment for XRD measurements and resistivity and Hall effect measurements, and Prof. A. Kitada (Kyoto Univ.) for fruitful discussions about the mechanism of the phase transformation. The authors would also like to express our gratitude to Dr. K. Kazumi, Mr. N. Sasaki, and Ms. Y. Uno (Kyoto Univ.) for their experimental supports for STEM observations.
[15]
[16]
[17]
[18]
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Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Orientation of Zn3P2 films via phosphidation of Zn precursors Ryoji Katsube, Yoshitaro Nose
⁎
MARK
Department of Materials Science and Engineering, Kyoto University, Yoshida Honmachi, Sakyo-ku, Kyoto 606-8501, Japan
A R T I C L E I N F O
A BS T RAC T
Keywords: A3. Polycrystalline deposition B1. Phosphides B2. Semiconducting materials B3. Solar cells
Orientation of solar absorber is an important factor to achieve high efficiency of thin film solar cells. In the case of Zn3P2 which is a promising absorber of low-cost and high-efficiency solar cells, (110)/(001) orientation was only reported in previous studies. We have successfully prepared (101)-oriented Zn3P2 films by phosphidation of (0001)-oriented Zn films at 350 °C. The phosphidation mechanism of Zn is discussed through STEM observations on the partially-reacted sample and the consideration of the relationship between the crystal structures of Zn and Zn3P2 . We revealed that (0001)-oriented Zn led to nucleation of (101)-oriented Zn3P2 due to the similarity in atomic arrangement between Zn and Zn3P2 . The electrical resistivity of the (101)-oriented Zn3P2 film was lower than those of (110)/(001)-oriented films, which is an advantage of the phosphidation technique to the growth processes in previous works. The results in this study demonstrated that well-conductive Zn3P2 films could be obtained by controlling orientations of crystal grains, and provide a guiding principle for microstructure control in absorber materials.
1. Introduction Zinc phosphide (Zn3P2 ) is a p-type semiconductor with a direct bandgap of 1.55–1.6 eV [1], which is suitable for energy conversion of sunlight into electricity. In addition, the constituent elements of Zn3P2 , zinc and phosphorus, are earth-abundant. Zn3P2 is therefore a candidate material for inexpensive and high-efficiency solar cells. The devices using Zn3P2 have been studied for over four decades, and the highest conversion efficiency for cells based on polycrystalline p-type Zn3P2 bulk crystals was reported to be 6.08% under 100 mW cm−2 and air mass 1 (AM 1) illumination by Catalano et al. in 1979 [2]. In addition, the light absorption coefficient of Zn3P2 is enough to realize thin film devices [3]. Many researchers have thus studied about fabrication of Zn3P2 thin films because solar cells based on thin films have some advantages in series resistance and in amount of required materials. However, the highest efficiency of thin film Zn3P2 solar cells is much lower than that of bulk crystals [4]. One of the reasons is a trap site of carriers at grain boundaries. The grain boundary density of the champion thin-film cell was larger than that of the bulk-crystalline cell [4], because the average grain size of Zn3P2 in the film was 100 times smaller than that in the bulk crystals [5]. For improvement of efficiency of thin-film based devices, increase of grain size, passivation of grain boundaries, and/or orientation control are effective approaches from the viewpoint of grain boundary engineering. In the case of Cu(In, Ga)Se 2 (CIGS)-based solar cells with high conversion efficiency, there is a well-developed strategy, alkaline metal incorporation, for ⁎
passivation of grain boundaries [6–8]. CIGS-based solar cells therefore show high efficiency around 20% even though the grain size is small. Furthermore, the absorber layers in the high-efficiency cells have columnar grain structure with orientations of (112) or (110)/(102) [9–11]. The orientations of (110) and (102) are equivalent. This is an advantage for carrier transport due to fewer grain boundaries in the transport direction. It was also reported that the quantum efficiency of solar cells was affected by the orientations, which dependent on the conditions such as [Se]/[In + Ga] flux ratio in molecular-beam epitaxial growth. In the case of polycrystalline Zn3P2 thin films, there are some reports on surface passivation, alkaline metal doping, and orientation of thin films. Kimball et al. reported surface passivation of bulk crystalline Zn3P2 by chemical etching and oxidation [12], and Paquin et al. revealed that Zn3P2 films on mica (compound including alkaline metals) have larger carrier density than those on quartz (without alkaline metals) [13]. However, there are no reports on passivation of the grain boundaries. This is still a topic under investigation in Zn3P2 -based solar cells. Zn3P2 films with (110)/(001) orientation were successfully grown by gas phase growth techniques such as metalorganic chemical vapor deposition (MOCVD) [14], ionized cluster beam deposition (ICBD) [15], radio frequency sputtering (RF sputtering) [16], evaporation [17], and close space sublimation (CSS) [18]. The (110) and (001) orientations are equivalent to each other in Zn3P2 with the ideal axial ratio (c / a = 20.5). From the reported results, it seems that Zn3P2 is self-organized in the deposition of Zn3P2 films. On the other
Corresponding author. E-mail address: [email protected] (Y. Nose).
http://dx.doi.org/10.1016/j.jcrysgro.2016.11.040 Received 9 September 2016; Received in revised form 25 October 2016; Accepted 10 November 2016 Available online 12 November 2016 0022-0248/ © 2016 Elsevier B.V. All rights reserved.
Journal of Crystal Growth 459 (2017) 95–99
R. Katsube, Y. Nose
hand, Cimaroli et al. reported that the resistivity of films with high (110)/(001) orientation was higher than that of films with random orientation [18]. Therefore, it is necessary to prepare films with various orientations and investigate their properties in order to clarify the relationship between orientations and electric properties in Zn3P2 films, and thereby optimize the microstructure of Zn3P2 films. In the present study, we thus tried to fabricate such films by phosphidation of oriented Zn precursors. In the case of Zn-chalcogenides such as ZnS and ZnO, it was reported that (0001)-oriented chalcogenides films were obtained by the reaction of (0001)-oriented Zn films with sulfur or oxygen [19,20]. In this case, the structure of ZnS and ZnO was wurtzite, which is a similar arrangement of zinc atoms to pure Zn with hcp structure. It is known that the orientation of metals deposited on glass substrates tends to be the closest-packed plane of metals [21], which is (0001) plane in the case of hcp metals such as Zn. We also consider the above tendency. 2. Experimental procedure Zn film as precursor was fabricated by magnetron sputtering. We prepared soda lime glasses (SLG) with and without coating Mo as substrates. Mo was also coated by magnetron sputtering. The sputtering was carried out under an Ar atmosphere with the pressure of 7 × 10−1 Pa. The power density for the target and the duration of sputtering were 1.4 W cm−2 and 30 min for Zn, and 6 W cm−2 and 80 min for Mo, respectively. The thicknesses of Zn were approximately 100 – 150 nm, which was estimated from the theoretical density of Zn and the deposited amount evaluated by inductively-coupled plasma atomic emission spectroscopy (ICP-AES, SPS3520UV, SII Nano Technology). The phosphidation experiments were carried out in the tube furnace with two heating zones, which was similar to the formation of ZnSnP2 . [22] The temperatures of a Zn film and a phosphorus source were individually controlled during phosphidation. As a source of phosphorus gas, we used a composite with dual phase of Sn and Sn 4 P3, which was prepared from Sn (5N, Kojundo Chemical Lab. Co., Ltd.) and (6N, Kojundo Chemical Lab. Co., Ltd.). The total amounts of the Sn/Sn 4 P3 composites were 1.7–1.9 g. Ar gas deoxygenated by passing through the furnace containing Ti sponge heated at 850 °C was used as a carrier gas with the flow rate of 20 sccm. The partial pressure of oxygen was monitored using a zirconia oxygen sensor (SD/LD-450, Toray Engineering Co., Ltd.), and controlled below 10−20 atm, which is the minimum limit of detection of the sensor. Before the experiments, the substrate covered by Zn and the Sn/Sn 4 P3 composite were placed out of the heating zones in the furnace. First, the zone for the composite was heated up to 450 °C in 15 min and held for 1 h for stabilization. After that, the composite was inserted into the heating zone and phosphorus was evaporated. The partial pressure of P4 on the composite is approximately 3 × 10−3 atm at 450 °C, which was calculated from the thermodynamic data of the previous report [23]. The substrate was then inserted into the heating zone after the stabilization for 15 min and heated up to 350 °C with the rate of 18 °C min−1. The substrate was kept at 350 °C for 30 min and then cooled down in the furnace. In addition, we fabricated a sample quenched at the early stage of phosphidation in order to consider the reaction behavior. In this experiment, Zn films as precursors were prepared on SLG substrates. The sample was annealed at 350 °C for 2.5 min under P4 atmosphere ( p P4 = 3 × 10−3 atm ), and then moved out from the heating zone. The Zn films before and after phosphidation were analyzed by X-ray diffraction (XRD, SmartLab, Rigaku) for identification of products and evaluation of crystal orientation. The sample was observed by scanning transmission electron microscopy (STEM, JEM-2100F, JEOL) and elemental mapping using energy dispersive X-ray spectroscopy (EDX). The resistivity of the film on the SLG substrate was measured by van der Pauw method [24] at room temperature (ResiTest 8310,
Fig. 1. XRD profiles of the films on SLG substrates (a) before (153 nm-thick) and (b) after phosphidation (266 nm-thick).
TOYO Corporation). 3. Results and discussion Fig. 1 shows the XRD profiles of Zn films on SLG substrates before and after phosphidation. A single-phase Zn was observed in the film before phosphidation, while all peaks were assigned to Zn3P2 and no secondary phase was confirmed in the profile after phosphidation. It is also understood that the Zn film shows (0001) orientation, as described in the literature [21]. In the Zn3P2 film, we observed (101) orientation, which is different with the plane orientation in the previous reports. For the films on Mo-coated substrates, the orientation of Mo film was observed in the [110] direction, while a strong orientation was not confirmed in the Zn films deposited on Mo films as shown in Fig. 2(a). Fig. 2(b) shows that the films after phosphidation do not have a strong
Fig. 2. XRD profiles of the films on Mo-coated SLG substrates (a) before (96 nm-thick) and (b) after phosphidation.
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Fig. 3. XRD profiles of the Zn film on the SLG obtained by phosphidation for 2.5 min.
Fig. 5. Cross-sectional STEM-EDX images of the Zn film on the SLG substrate obtained by phosphidation 30 min (a) STEM-DF image, elemental mappings of (b) Zn and (c) P, and (d) overlapped image.
orientation, although Zn3P2 films without secondary phases were obtained. It is thus clarified that the oriented Zn3P2 films can be obtained from the oriented Zn precursors, while the Zn films with random orientations lead to randomly-oriented Zn3P2 films. In addition, the grain sizes of the Zn3P2 films calculated using the Scherrer equation were 9.8 ± 2.1nm for the film on SLG and 15.6 ± 2.4nm for that on Mo. The types of substrate appear to be affect the grain size, and therefore the interface between Zn and substrates should play important role in the initial stage of phosphidation. For further consideration of the relationship between films before and after phosphidation, we investigated initial stage of phosphidation by cross-sectional STEM observation of a sample on the SLG which was obtained by interrupting the phosphidation experiment in 2.5 min Figs. 3 and 4 show the XRD profile and the cross-sectional STEM-EDX images of the sample, respectively. The phases of Zn and Zn3P2 are identified in the XRD profile and the STEM-EDX images indicate that the film was partially phosphidated in comparison to those of the completely phosphidated film shown in Fig. 5. Consequently, it is understood that the sample was en route of phosphidation. As seen in Fig. 4, Zn3P2 formed at the interface between Zn precursor and substrate in the initial stage of phosphidation, although the chemical potential of phosphorus on the surface of Zn film was controlled by the partial pressure of phosphorus gas and it was enough high to the
formation of Zn3P2 . Fig. 4 indicates that phosphorus penetrates into Zn film before the reaction with Zn. This is a similar phenomenon to the phosphidation of indium by the thin-film vapor-liquid-solid process [25], where nucleation of InP occurs at the interface between liquid indium and molybdenum substrate. We here consider on impurity diffusion in Zn, however, the diffusion coefficient of phosphorus in Zn has not been reported. The diffusion coefficient of Ni in Zn, which is the smallest among reported impurity diffusion coefficients, is 2.8 × 10−11 m2 s−1 at 350 °C in the [0001] direction [26,27], and the average diffusion length is 5.3 × 10−6 m s−1. Considering that the diffusion coefficient of phosphorus in Zn is at least more than that of Ni, phosphorus atoms can penetrate from the surface to the substrate through the Zn film with the thickness of 150 nm in just 7.9 × 10−4 s. Consequently, it is likely that phosphorus penetrates to the bottom of the Zn film before the formation of Zn3P2 on the surface. In addition, it is reasonably considered that the concentration of phosphorus increases at the interface between Zn and SLG and the nucleation of Zn3P2 occurs because the diffusion coefficient of phosphorus in SLG might be much smaller than that in Zn. In order to discuss the orientation relationship between Zn3P2 and Zn, we here take account of the crystal structures of Zn and Zn3P2 . The crystal structure of Zn is hcp with the lattice constants of a = 2.66 Å and c = 4.95 Å , and the atomic arrangement of (0001) plane is a two dimensional triangular lattice. On the other hand, the crystal structure of Zn3P2 is a defect antifluorite type structure [28]. The phosphorus sublattice is fcc, and zinc atoms occupy 3/4 of tetrahedral sites of the phosphorus sublattice. The zinc sublattice can be regarded as a simple cubic lattice containing vacancies (unoccupied tetrahedral sites). The vacancies are ordered in the structure, and therefore the symmetry of Zn3P2 belongs to the tetragonal system with the lattice constants of a = 8.09 Å and c = 11.45 Å . In the following discussion on the orientation, the displacement of zinc atoms from the ideal tetrahedral positions is not considered for simplicity. The relationship between the unit lattice vectors of Zn3P2 (a b c) and the fcc-based phosphorus sublattice (a′ b′ c′) can be written as
a = a′ + b′,
b = a′ − b′,
c = 2c′.
(1)
Accordingly, the relationship between the indices of the lattice plane of Zn3P2 (h k l ) and the phosphorus sublattice (h′ k′ l′) is
h = h′ + k′,
k = h′ − k′,
l = 2l′.
(2)
Considering the above relationship, the (101) plane of Zn3P2 corresponds to the (111) plane of the phosphorus sublattice. Similarly, it also corresponds to the (111) plane of the zinc sublattice.
Fig. 4. Cross-sectional STEM-EDX images of the Zn film on the SLG substrate obtained by phosphidation for 2.5 min (a) STEM-DF image, elemental mappings of (b) Zn and (c) P, and (d) overlapped image.
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Fig. 8. Schematic illustrations of phosphidation process. (a) Precursor Zn film with (0001) orientation, (b) nucleation of (101)-oriented Zn3P2 at the interface of Zn and substrate, and (c) grain growth of Zn3P2 by the reaction of Zn and phosphorus while keeping the orientation. Fig. 6. Atomic arrangements of Zn atoms in (a) vacancy-rich (101) plane of Zn3P2 (type I), (b) vacancy-deficient (101) plane of Zn3P2 (type II), (c) (0001) plane of Zn, and (d) a lattice consisting of 1/3 points of Zn (0001) plane.
Table 1 Electrical resistivity of oriented polycrystalline Zn3P2 thin films.
Consequently, the atomic arrangement of the zinc sublattice in the (101) planes is a two-dimensional triangular lattice, including vacancies. Figs. 6(a) and (b) show the atomic arrangements of zinc atoms in (101) plane of zinc sublattice in Zn3P2 . There are two types of arrangements depending on the ordering of vacancies, hereafter they are called type I and type II arrangements for Figs. 6 (a) and (b), respectively. In any cases, the arrangement of lattice points is a triangular lattice like the (0001) plane in Zn with hcp structure. However, the distances between the nearest neighbor lattice points in Figs. 6(a) and (c) are quite different with each other: 4.04 and 2.66 Å. Here, we consider a two dimensional lattice shown in Fig. 6(d), which consists of 1/3 lattice points of the (0001) plane in the hcp structure shown in Fig. 6(c). The lattice constant of the triangular lattice shown in Fig. 6(d) is 4.61 Å, which is close to that of the zinc sublattice of Zn3P2 shown in Fig. 6(a). On the other hand, the stacking structure of the zinc sublattice in the direction perpendicular to the (101) plane of Zn3P2 is shown in Fig. 7. The stacking can be understood by considering that the sequence in the [111] direction of the simple cubic lattice is … ABCABC… like fcc, since the zinc sublattice is simple cubic as previously described. Assuming that the stacking of the zinc sublattice consists of the lattice shown in Fig. 6(d), the zinc sublattice in Zn3P2 can be obtained without as much as possible changing the arrangement of lattice points. In other words, the (101) plane in Zn3P2 is based on the atomic arrangement derived from hcp Zn shown in Fig. 6(d). In actual, two types of the atomic arrangement in the (101) plane, type I and II, should be considered as shown in Figs. 6 (a) and (b), and the stacking sequence is 4 layers in a cycle, I-I-II-II, from the viewpoint of the atomic arrangement. It is thus understood that the structure in Fig. 7 consists of a stacking of 12 layers considering from the stacking of lattice points and the atomic arrangements, which are 3 and 4 layers in a cycle, respectively. We here show the phosphidation mechanism of zinc at 350 °C as shown in Fig. 8. First of all, phosphorus atoms penetrate into crystal grains of zinc and nucleation of Zn3P2 occurs at the interface of Zn and substrate. At this stage, the orientation of Zn3P2 is (101) due to the
Growth technique
Resistivity/ Ωcm
Orientation
MOCVD[14]
2.5 × 10 3 106 – 107
(110)/(001)
ICBD[15] RF sputtering[16] CSS[18] Phosphidation (this study)
1.0 × 10 4 – 1.5 × 10 5 4 × 101 – 2 × 10 3 30 ± 2
(110)/(001) (110)/(001) (110)/(001) (101)
similarity in atomic arrangements between Zn and Zn3P2 as discussed above. Then, Zn3P2 grains grow by the reaction of Zn and phosphorus while keeping the (101) orientation as shown in Fig. 8(c). Table 1 shows the resistivity of Zn3P2 in this study together with reported values. Cimaroli et al. reported that highly (110)/(001)oriented Zn3P2 films showed higher resistivity than the random films [18], and therefore (110)/(001) orientation in Zn3P2 films seems to have a negative influence on conductivity. On the other hand, the (101)-oriented Zn3P2 film in this study have the resistivity in the same order of magnitude as those of less (110)/(001)-oriented Zn3P2 films. The resistivity of thin film semiconductors is commonly affected by factors including concentration of electroactive dopants and density of defects such as grain boundaries acting as trapping sites of carriers. The results of this study indicate that (101) orientation of Zn3P2 is more favorable than (110)/(001) orientation from the viewpoint of electrical conduction, and microstructure control including orientation is also important for electrical conductivity of Zn3P2 films. As discussed above, the orientation of Zn3P2 films obtained by phosphidation depends on that of Zn precursors because of the formation of Zn3P2 by the reaction of solid Zn precursors with phosphorus gas. This is an advantage to obtain films with any orientations compared to conventional gas phase growth techniques. 4. Conclusions We successfully fabricated Zn3P2 films with (101) orientation on SLG substrates, which had never reported before. The process includes preparation of (0001) oriented precursor Zn films by sputtering and phosphidation of them at 350 °C which is slightly lower than the melting point of Zn. From the result of STEM observation of the sample partially phosphidated, nucleation of Zn3P2 was revealed to occur at the interface between Zn and the substrate in the phosphidation. The relationship between the orientations of the films before and after phosphidation was discussed from the viewpoint of atomic arrangements of zinc atoms in Zn and Zn3P2 . Taking vacancies in the (101) plane of Zn3P2 into consideration, it is understood that the twodimensional lattice of the (101) plane is similar to that of (0001) plane of Zn with hcp lattice. Moreover, the distance between nearest neighbor points is close to each other in Zn3P2 (101) and Zn (0001), considering the lattice consisting of 1/3 lattice points of Zn. In other words, the zinc sublattice of Zn3P2 is based on the lattice of Zn precursor, which is one
Fig. 7. Stacking structure of (101) plane of Zn in Zn3P2 . Type I and II shown in Figs. 6(a) and (b) are expressed as I and II, respectively. A, B, and C represent stacking sequence of triangular lattice.
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of the origins for the growth of oriented Zn3P2 films. The electrical resistivity of the (101) oriented Zn3P2 film was measured and compared with those in previous studies. The (101) oriented Zn3P2 film in this study was revealed to have lower resistivity than the (110)/(001)-oriented films in the previous articles. As described above, phosphidation of zinc films is a new process for preparation of well-conductive Zn3P2 films with an orientation different from that of the films grown by conventional growth techniques.
[11]
[12]
[13]
Acknowledgements [14]
This work was partly supported by JSPS KAKENHI Grant number 26289279. The authors are grateful to Prof. H. Hayashi (Kyoto Univ.) for giving us the opportunity to use the equipment for XRD measurements and resistivity and Hall effect measurements, and Prof. A. Kitada (Kyoto Univ.) for fruitful discussions about the mechanism of the phase transformation. The authors would also like to express our gratitude to Dr. K. Kazumi, Mr. N. Sasaki, and Ms. Y. Uno (Kyoto Univ.) for their experimental supports for STEM observations.
[15]
[16]
[17]
[18]
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