Characterization of ZnSiP2 Films Grown on Si Substrate by Liquid

Article Cite This: Cryst. Growth Des. 2019, 19, 3681−3687

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Characterization of ZnSiP2 Films Grown on Si Substrate by Liquid Phase Epitaxy: Morphology, Composition, and Interface Microstructure Longzhen Zhang, Guodong Zhang,* Kui Cheng, Peng Zhang, Lin Liu, Rongzhen Li, Xiang Li, and Xutang Tao* State Key Laboratory of Crystal Materials, Institute of Crystal Materials, Shandong University, Jinan 250100, P. R. China Downloaded via NOTTINGHAM TRENT UNIV on August 11, 2019 at 12:11:27 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: The exploration and growth of novel cost-effective top cell materials is one of the great hot topics for the development of Si-based tandem solar cells. ZnSiP2 was thought to be a promising candidate of the top cell material for the Si tandem solar cell. In this work, ZnSiP2 single crystal film with the stoichiometric ratio was successfully grown on a Si substrate by the liquid phase epitaxy method for the first time. The surface morphology, thickness, composition, and the orientation of the film were characterized by SEM, EDS, EPMA, and the high-resolution XRD. The results show that a continuous epitaxial ZnSiP2 film with the thickness of 3 μm was grown on the Si substrate. In addition to this continuous film, many island single crystals were also observed on the surface of the film. The high-resolution XRD φ scan results indicated that the film as well as these isolated islands crystallized with the same orientation, c-orientation with respect to the (100) Si substrate, without any misoriented domains. The microstructure of the ZnSiP2/Si interface characterized by the high-resolution transmission electron microscopy image, as well as the selected area electron diffraction patterns of the ZnSiP2 thin films and Si substrates, also confirmed the epitaxial growth of ZnSiP2 on the Si substrate. The electrical conductivity σ, Hall mobility μH, and carrier concentrations n of the ZnSiP2 film were 1.38 × 10−5 Ω−1 cm−1, 94.5 cm−2 V−1 s−1, and 9.11 × 109 cm−3, respectively. Our work paves the way for the development of ZnSiP2 silicon-based tandem solar cell devices.

1. INTRODUCTION To date, the photoconversion efficiency of a Si single-junction solar cell has been 26.6% under AM1.5D, 100 mW/cm2, very close to the theoretical limit of 29.4%.1,2 One of the most efficient ways to further enhance the efficiency of the Si cell is to develop the tandem cells, in which the top cell materials are the key factor for its real application.3 In the past few years, some semiconductors including GaInP, InGaN, GaAsP, AlGaAs, Cu2ZnSnS4, and GaP have been adopted as the top layer of the Si tandem cell, mainly because of their proper band gap between 1.5 and 2.0 eV for harvesting visible photons efficiently. GaInP is a suitable layer of the Si tandem solar cell top material with 38.9% theoretical efficiency, while GaInP is difficult to epitaxially grow directly on the Si substrate due to the differences in lattice constants and thermal expansion coefficients with Si.4 GaAs is one of the most representative III−V compound semiconductors based on which the efficiency of the Si tandem solar cell has been as high as 32.8%.5 However, the as-grown GaAs film faces the problem of cracking due to the high lattice mismatch and the difference in thermal expansion coefficient between GaAs and Si.6 In addition, the prices of the elements In and Ga are expensive in the compounds of GaInP, InGaN, and AlGaAs. Though some compounds such as Cu2ZnSnS4, GaP, and AlPSi3 with good quality could be grown on Si as they have low lattice mismatch and a similar structure with that of single crystalline Si,7−11 the © 2019 American Chemical Society

exploration of novel efficiency top cell materials with low cost and environmentally friendly elements is still a hot topic for the Si tandem solar cell. ZnSiP2 crystallizes in a chalcopyrite structure with I4̅2d space group, which can be thought of as ternary analogues of the binary GaP zinc-blende semiconductors. In GaP, each anion P is coordinated by four identical Ga cations, whereas, in ZnSiP2, each anion P is coordinated by two Zn cations and two Si cations (Figure 1a,b).12 As a result, ZnSiP2 possesses several potential merits compared with GaP as the top layer of Sibased tandem solar cells: (1) The band gap of ZnSiP2 is about 2.0 eV that can be tuned to be a favorable gap of 1.7−1.8 eV by doping.13 (2) Its lattice parameters matched well with those of Si (for ZnSiP2, a = 5.400 Å, c = 10.440 Å; for Si, a = 5.431 Å). The mismatch rate between ZnSiP2 and Si is only 0.5%. (3) ZnSiP2 has a similar refractive index with Si which is beneficial for absorption of light in a heterojunction. (4) Finally, yet importantly, a ZnSiP2 crystal is physically and chemically stable and the three elements of a ZnSiP2 crystal are abundant, nontoxic, environmentally friendly, and cheap, which is very suitable for low cost industrialization.14,15 In 2016, Martinez et al. characterized the photovoltaic properties of ZnSiP2 with a Received: December 19, 2018 Revised: May 21, 2019 Published: June 4, 2019 3681

DOI: 10.1021/acs.cgd.8b01877 Cryst. Growth Des. 2019, 19, 3681−3687

Crystal Growth & Design

Article

Figure 1. (a) Crystal structure of GaP in a unit cell. (b) Crystal structure of ZnSiP2 in a unit cell. (c) Crystal structure of Si in a unit cell.

diffraction (HR-XRD), scanning electron microscopy (SEM), electron probe micro analyzer (EPMA), transmission electron microscope (TEM), and selective area electron diffraction (SAED). Furthermore, many island single crystals with the uniform orientation were observed on the film. The growth mechanism and the influence of growth parameters on the size and density of island crystals were also studied.

regenerative photo-electrochemistry method, yielding a high open circuit voltage (Eoc) of 1.30 V.16 At the same time, Kudryashov et al. calculated the efficiency of ZnSiP2/Si double-junction solar cells, showing that ZnSiP2/Si heterostructure solar cells can provide efficiencies of 28.8% at AM1.5D, 100 mW/cm2, and 33.3% at AM1.5D, 200 W/cm2.17 First-principles investigation of optoelectronic properties of ZnSiP2 also indicated that ZnSiP2 is an attractive material in optoelectronic devices especially as a lattice matched material with silicon for tandem solar cells applications.18−21 However, there is still no experimental report about the efficiency of a ZnSiP2/Si solar cell because of the lack of ZnSiP2 epitaxial film on Si. In the 1970s, some researchers have reported the growth of ZnSiP2 crystals by iodine assisted chemical vapor transport or by Sn or Zn metal flux methods, while only millimeter-sized needle-like crystals were obtained.22−25 In 2015, the dark-red needle ZnSiP2 crystals up to 2 cm in length were successfully grown in Zn flux.26 However, unlike GaP or Cu2ZnSnS4 single crystal films which have been grown on a Si substrate by various vapor growth methods including molecular beam epitaxy (MBE), chemical vapor deposition, pulsed laser deposition, and sputtering, there are few reports about the epitaxial growth of ZnSiP2 film on Si.27−29 In the 1970s, Si layers were deposited epitaxially on natural facets of solutiongrown ZnSiP2 crystals by hydrogen reduction of SiHCI3 and the orientation relationships of ZnSiP2(112)/Si(111) and ZnSiP2(101)/Si(201) were determined.27 Soon after, the Si/ ZnSiP2 alloy on Si substrates was grown by the vapor−liquid− solid (VLS) growth technique.30 Very recently, the amorphous nanoscale thin films of ZnSiP2−Si alloys with a tunable Si content were grown on Si substrates with a carbon-free chemical vapor deposition method by Martinez et al, and the amorphous structure will crystallize by postgrowth annealing at 850 °C.28 Despite the widely tunable Si content in the amorphous films, tuning the stoichiometry to reach nearstoichiometric ZnSiP2 is likely to be challenging using this method. In contrast to the vapor growth methods mentioned above, the main advantage of the LPE technique lies in the nearequilibrium growth conditions, which could achieve the high quality film with extremely low dislocation density. In this work, we demonstrate the growth of stoichiometric ZnSiP2 single crystalline film on a Si substrate by the LPE method for the first time, providing a near equilibrium, low-cost, and largesized ZnSiP2 single crystal film growth technique. The surface morphology, composition of the film, and the interface microstructure of the ZnSiP2/Si heterojunction were characterized by X-ray diffraction (XRD), high-resolution X-ray

2. EXPERIMENTAL SECTION 2.1. Film Growth. The LPE technology was adopted to grow ZnSiP2 single crystal films on Si substrates. Figure 2 illustrates the

Figure 2. Schematic diagram of LPE furnace for ZnSiP2 epitaxial growth on a Si wafer. schematic graph for ZnSiP2 film epitaxial growth. High purity elemental zinc powder (99.999% purity), silicon powder (99.999% purity), and red phosphorus blocks (99.999% purity) were used as the solute, and tin pellet (99.999% purity) was used as the solvent. These raw materials were placed in an evacuated fused silica tube (2.0 × 10−4 Pa) with a ratio of Zn:Si:P:Sn = 1:1:2:20; i.e., the concentration of the solution is 5 mol % with respect to the amount of Sn. According to the reported ZnSiP2-Sn phase diagram,31 the supersaturation temperature for the solution of 5 mol % concentration is about 1035 °C. Therefore, the furnace was slowly heated to 1050 °C at a rate of ∼30 °C/h for completely dissolving the ZnSiP2. Note that the heating rate must be slow enough to make sure P, Zn, and Si powder could dissolve into the Sn solvent and react to form ZnSiP2 completely. After holding for 30 h at 1050 °C, the furnace was slowly cooled to 600 °C at 1−10 °C/h.26 In the cooling process, ZnSiP2 will deposit on the Si substrate from the supersaturated ZnSiP2-Sn solution. The as-grown sample was centrifuged with Sn solvent at 600 °C. The remanent Sn flux on the surface of the as-grown sample was removed by the concentrated hydrochloric acid. 3682



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Figure 3. (a) SEM image showing morphology of ZnSiP2 film and island crystal on the Si surface. (b) SEM image showing the cross section of ZnSiP2 film grown on a Si substrate, and an enlarged image of the rectangular region marked with a white line was inserted in the top right of the image. (c) EPMA image and (d) composition distribution of Zn, Si, P in the as-grown ZnSiP2/Si sample as a function of distance from the substrate (Si substrate), as measured by EPMA. The substrates used in this study were mirror polished Si(100) wafers, typically with 1 cm2 in area and 500 μm in thickness. Before placing the substrate inside the quartz crucible, the wafer was cleaned organically and inorganically by Standard Radio Corporation of America (RCA) cleaning techniques, then etched in a solution of 2% HF for 3 min, and then washed with deionized water in a N2 atmosphere for 3 times. 2.2. Characterization. XRD patterns were measured using a Bruker D8ADVANCE instrument with a rotating Cu Kα anode X-ray source operating at 40 kV/40 mA. The HR-XRD φ scan was performed using a Bruker D8 Discover instrument with a Cu Kα anode X-ray source and four-crystal monochromator operating at 40 kV/30 mA. The surface morphology and the composition of the film were determined by a Hitachi S-4800 SEM and the accessory Horiba EMAX Energy EX-350 instrument operated at 15 kV, respectively. In addition, element distribution in the depth direction of the film was analyzed by a Shimadzu EPMA-1720H instrument. The microstructure and lattice mismatch in the interface of ZnSiP2/Si was observed by TEM using an FEI Tecnai G2 F20 S-Twin instrument operated at 200 kV. In order to fabricate TEM crosssectional samples, the sample was tacked with M-bond 610 and double-sided milled to less than 20 μm. While gradually reducing from 4.8 kV to 3.2 kV with the Gatan 691 Ion Thinner instrument, the tilt angle was gradually reduced from 10° to 4°. A thin area was prepared in 30 min and then was observed under an electron microscope. Electrical properties such as electrical conductivity σ, Hall mobility μH, and carrier concentrations n of the ZnSiP2 film were measured by a Lake Shore 8400 Hall measurement instrument at room temperature.

3. RESULTS AND DISCUSSION 3.1. Surface Morphology and Composition. The optical microscope image of the surface morphology of the as-grown ZnSiP2 film clearly shows many crystal islands (Figure S1). To examine whether a continuous epitaxial film formed or the island crystals directly grew on the Si(001) surface, the SEM, EDS, and EPMA measurements were performed on one typical sample holding for 30 h at 1050 °C with the cooling rate of 5 °C/h to 600 °C. Figure 3a shows the SEM image of the surface morphology of the ZnSiP2 film which comprises a relative flat surface and many island crystals on the surface. The average size of an island crystal is about 40 × 50 × 30 μm3. We noted that most of these island crystals are in the same orientation as marked with a white line in Figure 3a. Therefore, further X-ray diffraction detection was carried out to determine the misoriented domains that would be elucidated in section 3.2. Figure 3b is the cross-sectional SEM image of the film, and an enlarged image of the rectangular region marked with a white line was inserted in Figure 3b to show the thin film clearly. The thickness of the film is about 3 μm which is enough to fabricate a Si tandem solar cell,17 and these island crystals are confirmed grown on the film. The composition for both the flat film and the island crystals was measured by EDS. The result shows that the composition of the flat film is Zn 24.4, Si 25.1, and P 50.5 at. % and the composition of the island crystals is Zn 24.8, Si 23.5, and P 51.7 at. %, both of which are in good accordance with the stoichiometric ratio of ZnSiP2 (Table 1); the detailed data are shown in Figure S2. 3683



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Table 1. Compositions of Film and Island Crystal Measured by EDS and EPMA element Zn Si P

film (EDS at. %) island crystal (EDS at. %) 24.4 25.1 50.5

24.8 23.5 51.7

film (EPMA at. %) 24.49 25.90 49.61

The composition distribution along the depth direction of the as-grown ZnSiP2/Si sample was measured by EPMA (X−Y line in Figure 3c). As shown in Figure 3d, the changes of the content of Zn, P, and Si depended on the E-probe position. With moving the probe from the Si substrate to the epitaxial films, the Si content decreased while Zn and P contents increased in the 0−2 μm region. All three elements contents declined to 0 with the probe moving out of the ZnSiP2/Si sample area in the 6−8.5 μm region. The intensity of the three elements contents remains substantially unchanged between 2 and 6 μm region, indicating that the distribution of Zn, Si, and P in the film is relatively uniform (Figure 3b). Therefore, the thickness of the ZnSiP2 film calculated from EPMA measurements is about 4 μm which is in accordance with the crosssectional SEM result. But the height of the curve in Figure 3d does not represent the content of each element, the quantitative analysis of film composition in Figure 3c marked with Z was measured by EPMA. As shown in Table 1, the composition of the three elements Zn, Si, and P is 24.49, 25.90, and 49.61 at. %, respectively, which satisfies the stoichiometric ratio of ZnSiP2. It is worth noting that the accuracy of EPMA is 0.1%, which is higher than the measurement accuracy of EDS. The EDS elemental mapping results shown in Figure S3 further demonstrate the uniform distribution of these elements as well. In conclusion, our SEM, EDS, and EPMA composition measurements confirm that a continuous stoichiometric ZnSiP2 film was successfully grown on the Si(001) surface by the LPE technique despite the formation of many isolated ZnSiP2 islands on this continuous film. 3.2. Orientation of ZnSiP2 Film on Si. XRD measurement was performed to visualize the phase of the epitaxial film on the Si substrate. The result is shown in Figure 4a, plotted on a linear scale. By comparing XRD patterns of the as-grown ZnSiP2/Si sample with the Si substrate, two additional diffraction peaks at 2θ ≈ 34° and 72° can be obviously observed apart from the (400) diffraction peaks of the Si substrate which were (004) and (008) peaks of ZnSiP2, respectively. This result indicates that the as-grown film as well as the island crystals is highly oriented ZnSiP2 with the direction of ⟨001⟩ perpendicular to the Si(100) face. But there indeed existed some other peaks whose intensities are very low besides (004) and (008) peaks of ZnSiP2 (Figure S4). These little peaks may arise from some island crystals. Highresolution TEM was performed to observe the interface of ZnSiP2 film and the Si substrate in order to prove the film’s crystallinity that would be elucidated in the next section. The XRD patterns of six samples with different growth conditions were measured, and the results have good repeatability (Figure S5). To examine whether ZnSiP2 film was epitaxially grown on the Si substrate, an HR-XRD φ scan was performed to check the in-plane orientations of ZnSiP2 film, as shown in Figure 4b. Only four diffraction peaks from the allowed diffraction of ZnSiP2(101) faces were clearly observed, agreeing with 4-fold symmetry of ZnSiP2(101) reflections. Considering the 90°

Figure 4. (a) XRD pattern of ZnSiP2/Si sample. (b) High-resolution X-ray diffraction φ scan pattern of film on ZnSiP2(101), indicating four diffraction peaks apart 90°. (c) The rocking curve taken around the (004) diffraction peak of ZnSiP2 films grown on the Si(100) plane with the fwhm of approximately 1°.

space of the adjacent peaks, as well as the almost equal relative intensity of these four peaks, we can conclude that there is probably only a c-axis growth mode of ZnSiP2 on the Si(100) substrate. High-resolution TEM was performed to conform this conclusion that would be elucidated in section 3.3. Therefore, our diffraction measurements confirm that twin-free ZnSiP2 was heteroepitaxially grown on the Si(100) face for the first time. Figure 4c shows the rocking curve taken around the (004) diffraction peak of ZnSiP2 films grown on the Si(100) plane. The full width at half-maximum (fwhm) of the film is approximately 1°. The high value of fwhm is attributed to the presence of the island crystals on the ZnSiP2 film, which need 3684



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spots of the substrate Si and ZnSiP2 film are very clear, and their arrangements are very regular, confirming the nature of high crystallinity. Therefore, only one set of periodic diffraction spots is evident in Figure 6b, indicating that no other epitaxial relationships exist. The SAED pattern of the ZnSiP2 film follows that of the Si substrate, suggesting that a transfer of crystallographic information has taken place from the Si substrate into the ZnSiP2 epilayer. As the high-resolution TEM image and SAED patterns of the ZnSiP2/Si interface show, the film’s growth is only along the c axis perpendicular to the substrate. 3.4. Effect of Growth Parameters on the Morphology of the Film. As mentioned above, one of the hallmarks of the as-grown film is the formation of the oriented island crystals on the continuous flat ZnSiP2 film. As for the growth mechanism of these islands, we found it followed the Stranski−Krastanov growth mode which is an intermediary process characterized by both 2D layer and 3D island growth. Different with the other two epitaxial growth mechanisms, Volmer−Weber mode and Frank−van der Merwe mode, the transition from the layerby-layer to island-based growth occurs at a critical layer thickness in Stranski−Krastanov growth, which is highly dependent on the chemical and physical properties such as surface energies and lattice parameters of the substrate and film.32 During the exploration process, we found that the holding time and cooling rate are the two key factors, which affect the size and distribution of these islands. As shown in Figure 7a−c, the size of the island crystals grown on ZnSiP2 film increased from about 10 to 60 μm with the holding time increasing from 0 to 15 h. The reason for this phenomenon is attributed to the time of the dissolution and reaction of Zn, Si, and P three elements in Sn solvent. With the holding time increasing from 0 to 15 h, the concentration of the ZnSiP2 in the Sn solvent gradually increases. Therefore, the size of the island crystals increases after the cooling process ends. While further increasing the holding time from 15 to 30 h (Figure 7c,d), the raw materials have completely dissolved and reacted to be ZnSiP2 in the solvent Sn. That is, the concentration of ZnSiP2 does not change any more when the holding time exceeds 15 h, so the size of the island crystals does not change. As shown in Figure 7d−f, the islands’ density increased from about 34% to 90% with the cooling rate increased from 1 °C/h to air cooling rate (about 35 °C/h). The relationship between

to be eliminated by optimizing the growth condition in our near future work. 3.3. Cross-Sectional TEM Analysis. High-resolution TEM was performed to observe the interface lattice structure between ZnSiP2 and Si, as shown in Figure 5. The ZnSiP2 layer

Figure 5. High-resolution TEM image of the ZnSiP2/Si interface.

well aligned with the Si substrate, illustrating the successful heteroepitaxial growth of ZnSiP2 on Si. The interplanar distance of 0.19 and 0.31 nm was measured from the presented clear lattice fringes which can be indexed as the (111) plane of Si and (220) plane of ZnSiP2, respectively. These lattice fringes in the ZnSiP2 film area are orientated, flat and continuous, indicating a single crystal film with no obvious defects present. Figure 6 shows the SAED patterns of the Si substrate and the ZnSiP2 film in the TEM image of Figure 5. The diffraction

Figure 6. Selective area electron diffraction patterns of (a) Si substrate and (b) ZnSiP2 film.

Figure 7. SEM images showing the size and distribution of island crystals at different holding times and different cooling rates. (a) Hold 0 h at 1050 °C, air cooling. (b) Hold 10 h at 1050 °C, air cooling. (c) Hold 15 h at 1050 °C, air cooling. (d) Hold 30 h at 1050 °C, air cooling. (e) Hold 30 h at 1050 °C, 5 °C/h cooling rate. (f) Hold 30 h at 1050 °C, 1 °C/h cooling rate. 3685



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large excess of Zn or Sn is higher than that in crystals grown by the vapor phase transport method, indicating the crystals grown from the metal flux have lower impurities.36 The Hall mobility μH of ZnSiP2 film in Figure 7e (hold 30 h at 1050 °C, 5 °C/h cooling rate) is 94.5 cm2 V−1 s−1, which is consistent with the literature mentioned above, demonstrating the quality of the ZnSiP2 film in our work is comparable to ZnSiP2 single crystals. The electrical conductivity σ and the carrier concentrations n of ZnSiP2 film were 1.38 × 10−5 Ω−1 cm−1 and 9.11 × 109 cm−3 that are smaller than those reported in the literature. That is because the ZnSiP2 film we grew is an intrinsic nondoped semiconductor. The ideal conductivity and carrier concentration can be easily obtained by a doping method.

growth rate, nucleation rate, and supersaturation can be expressed as G = K g ·ΔC g B = Kb·ΔC b

with G the growth rate, Kg the growth constant, ΔC the supersaturation, g the growth order, B the nucleation rate, Kb the nucleation constant, and b the nucleation order.33 When the cooling rate is increased, the supersaturation ΔC will increase, which means both growth rate G and the nucleation rate B increase. However, if the cooling rate (supersaturation) is too large, crystal nucleation takes precedence over crystal growth, inducing the spontaneous nucleation of some crystal nuclei of the precipitate from the solvent instead of heterogeneous nucleation on the Si substrate. At the same time, those nuclei of the precipitate on the Si substrate will lack sufficient time for growth. From Figure 7f, we can see all of the island crystals connected together with a cooling rate of 1 °C/ h, which means it is possible to grow a continuous thick (or bulk) single crystal with millimeter thick by improving growth conditions. Actually, we have successfully obtained millimetersized mosaic crystals connected together with the same orientation by the LPE method (see Figure S6). The work on the growth of a bulk single crystal with a flat surface is in progress. We repeated the EDS measurements on all of the samples grown in different conditions mentioned, and the elements Zn, Si, P with the stoichiometric ratio could be found when the holding time is more than 10 h (see Table S1). The XRD patterns and cross-sectional SEM images of ZnSiP2 films under different growth conditions in Figure 7 are shown in Figures S5 and S7, respectively. The thicknesses of the ZnSiP2 film show an upward trend with the increase of the holding time and the decrease of the cooling rate. The thickness of the film has already increased up to 4 μm when the holding time is 30 h and cooling rate is 1 °C/h. The films we discussed above are all grown on a Si(100) substrate. We also have tried the growth of ZnSiP2 thin film on a Si(111) substrate. The SEM image shows the morphology of ZnSiP2 film and island crystals on the Si(111) plane (Figure S8a). Both ZnSiP2 film and island crystals can be observed on the Si surface, which is the same as the result of Si(100). Figure S8b is EDS data of the ZnSiP2 film area in Figure S8a (marked with a symbol, O) and the stoichiometric ratio between Zn, Si, and P satisfies 1:1:2 relationships. Figure S9 shows XRD patterns of ZnSiP2 films grown on the Si(111) plane. Two diffraction peaks were clearly observed at 2θ ≈ 25.9°, 25.5°, corresponding to the reflections from the ZnSiP2(112) plane and Si(111) plane, respectively. This result is consistent with the ZnSiP2(112)/Si(111) orientation relationship reported in ref 27. In addition, Zn cannot be used as flux in epitaxial growth of ZnSiP2 on the Si substrate because of the large solubility of Si in Zn. According to the phase diagram of Si-Sn and Si-Zn, the solubility of Si in Sn and Zn is about 4.54% and 24% at 1050 °C, respectively.34,35 In other words, the Si substrate can be almost preserved due to that the silicon powder in the 5% raw material has almost dissolved into the Sn and saturated at 1050 °C. Unfortunately, because of the larger solubility (24 mol %) of Si in Zn,35 the Si substrate will easily dissolve in solvent Zn for the 5% concentration solution in our experiment. 3.5. Electrical Properties of ZnSiP2/Si. The Hall mobility μH in ZnSiP2 single crystals grown from the melt containing a

4. CONCLUSIONS In conclusion, to the best of our knowledge, we have demonstrated the first growth of ZnSiP2 single crystalline films on Si substrates with the liquid phase epitaxial method. The stoichiometric composition and the distribution of Zn, Si, P elements in the film were examined by EPMA and EDS elemental mapping measurements. In addition to the continuous ZnSiP2 epitaxial film, many island single crystals were also observed on the surface of the film. The highresolution XRD φ scan results indicated that the film as well as the isolated islands crystallizes with the c-axis orientation with respect to the (100) face of the Si substrate, without any misoriented domains. The interface microstructure of ZnSiP2/ Si was determined by high-resolution TEM images and the SAED patterns, which further confirmed the successful heteroepitaxial growth of ZnSiP2 single crystalline film on the Si substrate. The undoped as-grown ZnSiP2 film shows very high Hall mobility μH which is comparable to that of ZnSiP2 single crystals.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.8b01877. Optical micrograph of ZnSiP2 film on Si(001), EDS data of the film and island crystals, EDS mapping, magnifying XRD data of Figure 4a, XRD data of ZnSiP2 films under different growth conditions, optical micrograph of millimeter-sized ZnSiP2 island crystals grown on Si(001), cross-sectional SEM images of ZnSiP2 films under different growth conditions, SEM image of ZnSiP2 film and island crystal on Si(111), XRD data of ZnSiP2 films grown on Si(111), and EDS data of film composition under different growth conditions (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: 86-531-88364963 (X.T.). *E-mail: [email protected]. Phone: 86-531-88369099 (G.Z.). ORCID

Guodong Zhang: 0000-0002-9048-9270 Xutang Tao: 0000-0001-5957-2271 Notes

The authors declare no competing financial interest. 3686



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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 51602178, 51321091), the Program of Introducing Talents of Disciplines to Universities in China (111 program Grant No. BP2018013), the Fundamental Research Funds of Shandong University, and the Shandong Provincial Key Research and Development Program (Grant No. 2018GGX102003).

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