Epitaxial Growth of Bandgap Tunable ZnSnN2 Films on (0001) Al2O3

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Epitaxial growth of bandgap tunable ZnSnN2 films on (0001) Al2O3 substrates by using a ZnO buffer Duc Duy Le, Trong Si Ngo, Jung-Hoon Song, and Soon-Ku Hong Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01285 • Publication Date (Web): 08 Feb 2018 Downloaded from http://pubs.acs.org on February 8, 2018

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Crystal Growth & Design

Epitaxial growth of bandgap tunable ZnSnN2 films on (0001) Al2O3 substrates by using a ZnO buffer Duc Duy Le,† Trong Si Ngo, † Jung-Hoon Song,‡ and Soon-Ku Hong†,* †

Department of Materials Science and Engineering, Chungnam National University, 99 Daehakro, Youseong-gu, Daejeon 34134, Republic of Korea ‡

Department of Physics, Kongju National University, 56 Gongjudaehak-ro, Shinkwan-dong, Kongju, Chungcheongnam-do 32588, Republic of Korea

*Corresponding author Soon-Ku Hong Department of Materials Science and Engineering, Chungnam National University (CNU), Daejeon 34134, Republic of Korea Phone: +82-42-8216640 Fax: +82-42-8225850 Email: [email protected] Post address: 99 Daehak-ro, Youseong-gu, Daejeon 34134, Republic of Korea

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Growth of ZnSnN2 films on (0001) Al2O3 substrates is performed by plasma-assisted molecularbeam epitaxy by changing the growth temperatures from 350 to 650 °C. Single crystal ZnSnN2 films have been grown by using ZnO buffer while the film grown without the ZnO buffer has shown amorphous-like disordered characteristics addressed by no observation of any diffraction from reflection high-energy electron diffraction. All the grown crystalline ZnSnN2 films with ZnO buffer show a pseudo-wurtzite structure without the formation of an orthorhombic structure. Epitaxial relationships between Al2O3 substrate, ZnO buffer, and ZnSnN2 film are determined to be [1120] ZnSnN2 // [1120] ZnO // [1010] Al2O3 and [0001] ZnSnN2 // [0001] ZnO // [0001] Al2O3. The bandgaps of ZnSnN2 films could be tuned from 1.85 to 2.15 eV, simply by increasing the growth temperatures from 350 to 650 °C. The carrier concentrations and carrier mobilities were investigated and compared. Since the growth of single crystal ZnO films has been reported on various kinds of cheap and large size substrates, our results can expand the method to grow single crystal ZnSnN2 films, which is needed to fabricate new ZnSnN2-based devices.


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INTRODUCTION Group III-nitrides (Al, Ga, In nitrides) have recently shown to exhibit important applications in photovoltaics, photocatalytic energy conversions, and optoelectronic devices.1–5 However, the unpredictability in the supply and price of rare-earth elements has raised the concern to find alternative materials for group III-nitrides. Zn-IV-N2 semiconductors (IV refers to Si, Ge, or Sn) are considered to be potential candidates due to the several merits of these abundant, environmental friendly, non-toxic, and inexpensive elements. Several reports on the synthesis and characterization of both ZnSiN2 and ZnGeN2 exist in the literaure,6–9 but investigations on ZnSnN2 are still in their very early stage since the first paper on its synthesis has been reported five years ago.10 Indeed, ZnSnN2 has attracted strong interest and some groups have already proved its prosperous applications.11–15 As noted by Veal et al., the Zn-IV-N2 family is promising for thin film photovoltaic absorber materials due to its earth abundant components and the possibility to tune the bandgaps for wide spectral coverage.10 In addition, Kou and Chang have recently examined the substantial piezotronic and piezophototronic properties of orthorhombic ZnSnN2.16 Nonetheless, the growth of high quality single crystal ZnSnN2 is highly required to allow the investigation of its fundamental properties and its applications in device fabrication. ZnSnN2 films have been grown on glass, (0001) Al2O3, (0001) GaN, (111) YSZ, and (001) LiGaO2 substrates.10,14,17–19 To the best of our knowledge, only ZnSnN2 films grown on (111) YSZ and (001) LiGaO2 substrates have been reported to be single crystal.17–19 However, the commercial (111) YSZ and (001) LiGaO2 substrates are expensive and available solely in small sizes, which make the industrial applications of single crystal ZnSnN2 films more difficult. Generally, employing a buffer layer is a valuable method to overcome the film-substrate mismatch and to improve the crystallinity of the grown films on various substrates. The lattice



mismatch between wurtzite (0001) ZnSnN2 and c-Al2O3 (JCPDS 88-0826) is -29 %, which is too large to grow high quality ZnSnN2 films. The lattice mismatches of ZnSnN2 films on GaN, YSZ, and LiGaO2 substrates are 6.5 %, -6.8 %, and 6 %, respectively. On the other hand, the lattice mismatch between ZnSnN2 and ZnO (JCPDS 36-1451) is as small as 3.9 %, that ZnO can be a promising substrate material for ZnSnN2 film growth. The ZnO substrate is commercially available, although it is not cheap. However, growth of single crystal ZnO films on a cheaper cAl2O3 substrate has widely been reported.20,21 Therefore, using the single crystal ZnO film as a buffer for growing single crystal ZnSnN2 films on c-Al2O3 is highly encouraging. Succeeding the crystal ZnSnN2 films by using a ZnO buffer layer can also be an innovation for the substrate selection, because single crystal ZnO films were grown on various kinds of substrates.22–28

EXPERIMENTAL SECTION Single-crystal ZnSnN2 films were grown utilizing a plasma-assisted molecular beam epitaxy (PAMBE) system. Each sample was grown on a quarter of 2 inches (0001) Al2O3 substrate, taken from the same batch. The Al2O3 substrates were degreased by ultrasonic agitation in acetone, methanol, and then treated by an etching solution of H2SO4: H3PO4 = 3:1 (vol%) mixture at 160 °C for 15 min followed by rising in deionized water and drying under pure N2 flow. Prior to growth, the substrates were cleaned by exposure to oxygen plasma for 30 min at 800 °C. Metallic Zn and Sn sources (6 N purity) were supplied by standard effusion cells. The fluxes were measured using a quartz crystal microbalance. A radio frequency (RF) plasma source was used for supplying both the oxygen and nitrogen radicals. ZnO buffer layers were grown in 30 min at 700 °C using plasma power of 300 W, oxygen flow rates of 2 sccm, and Zn flux of 2 Å/s.


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After the growth of ZnO buffer layers, the main shutter was closed, and the oxygen plasma was turned off. Nitrogen plasma was turned on 5 min prior to the start of ZnSnN2 growth while the main shutter was closed to remove any residual oxygen in plasma cell. Nitrogen plasma was set at a power of 300 W with a nitrogen flow rate of 0.2 sccm for ZnSnN2 growth. The temperature of the Sn cell was increased to get the designed flux of 0.1 Å/s while the Zn cell was kept at a flux of 2 Å/s. The ZnSnN2 films were grown at 350, 450, 550, and 650 °C for two hours. The film thickness of the samples was measured by observing cross-sectional transmission electron microscope (TEM) images. The ZnSnN2 layers showed a similar thickness of about 40 nm for 2hr growth, and the thickness of the ZnO buffer layer was around 100 nm for 30-min growth. This signifies that ZnSnN2 had a very low growth rate of 3.3 Å/min, while the ZnO had a growth rate of 33 Å/min. The growth evolution at different growth temperatures was real-time monitored by in situ reflection high-energy electron diffraction (RHEED). High-resolution X-ray diffraction (XRD) measurements were performed using a Bruker AXS D8 DISCOVER diffractometer. XRD thetatwo theta, phi-scan, and omega rocking curve measurements were performed to investigate the crystalline properties. Surface morphology and root-mean-square (RMS) roughness of the films were evaluated by atomic force microscope (AFM) measurements in tapping mode (Asylum Research MFP-3D model). In order to investigate the detailed microstructure, we performed cross-sectional transmission electron microscope (TEM) observation by using an atomic resolution TEM (JEOL JEM-ARM200F). TEM specimen was prepared by using focus ion beam process (FEI Nova 200). Energy dispersive spectroscopy (EDS) element mapping was performed using a spherical aberration (Cs) corrected scanning TEM (STEM). The characteristics of crystal structure and epitaxial relationships were determined by RHEED, XRD, and TEM. The optical



bandgaps were calculated from optical absorption spectra measured at room temperature using a Shimadzu UV-2600 spectrophotometer. The electrical mobility and carrier density were measured using HMS-2000 Hall measurement system.

RESULTS AND DISCUSSION Figure 1a-f shows the observed RHEED patterns along [1100] and [1120] azimuths of sapphire substrate and the corresponding RHEED patterns of ZnO buffer layer and ZnSnN2 films grown at different growth temperature. As illustrated in Figure 1a,b, the typical RHEED patterns of single crystal ZnO rotated by 30° compared to the crystallographic directions of sapphire were observed.29 The direction of the [1100] ZnO was aligned with the [1120] direction of the sapphire, which resulted from the ZnO lattice aligning itself with the oxygen sub-lattice in Al2O3.29 The RHEED patterns revealed clear and streaky lines, which are characteristics of very flat surfaces. The ZnSnN2 films were grown on such ZnO buffer layers at different temperatures and were evaluated by RHEED. The effect of the growth temperature on the growth evolution of the ZnSnN2 films is shown in Figures 1c-f. The ZnSnN2 film grown at 350 °C showed blur and diffuse streaky patterns (Figure 1c), which implied a disordered surface. The RHEED patterns of ZnSnN2 film grown at 450 °C and 550 °C showed sharp and clear streaky patterns (Figures 1d,e), which implied a flat surface and high crystallinity of the films grown at these temperatures. However, the film grown at 450 °C seems to be better since its RHEED patterns were much stronger and sharper than those of the film grown at 550 °C. At a growth temperature of 650 °C, the RHEED patterns changed to diffuse spotty patterns with weak rings, suggesting a degradation of the crystal quality.


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Some models of ZnSnN2 crystal structure exists in the literature such as the two orthorhombic octet-rule-preserving phases Pna21, Pmc21, and wurtzite Pm31 phase.13 Quayle et al. have also suggested a model of random stacking of the two octet-rule-preserving structures, which can cause a wurtzite-like diffraction from X-ray diffraction (in which none of (101) peak for Pna21 ordering and the (111) peak for Pmc21 have been observed).30 The complexity of the ordering question is highly relevant to the discussion of bandgaps and their variations with cation ordering.30,31 However, the wurtzite Pm31 phase and Pna21 orthorhombic phase are the prevailing models for the crystal structure of experimentally synthesized ZnSnN2.17–19,31 In the in situ RHEED observations of ZnSnN2 films from the growth start to its end, doubling streaks, which must appear in the case of an orthorhombic ZnSnN2 structure, were not observed as inferred by Figure 1c-f.19 In addition, all the observed RHEED patterns of the films displayed the characteristic features that are identical to the RHEED patterns of wurtzite ZnO buffer layer with 60° rotational symmetry. These patterns are completely different from those with 180° rotational symmetry for the orthorhombic ZnSnN2 case.19 Therefore, we concluded that the ZnSnN2 films grown at 350 ~ 550 °C had a wurtzite structure based on our RHEED results. The epitaxial relationships between ZnO and ZnSnN2 were determined as [1120] and [1100] directions of the ZnSnN2 films are parallel to [1120] and [1100] directions of ZnO, respectively. Such epitaxial relationships are reasonable since (0001) pseudo-wurtzite ZnSnN2 and (0001) wurtzite ZnO have the same symmetry and a small mismatch of about 3.9 %. The schematic drawing for the determined epitaxial relationships is shown in Figure 1g. The calculation of the critical thickness of ZnSnN2 layer is currently difficult due to the lack of necessary information such as the modulus and Poison ratio values of ZnSnN2. Nevertheless, the 3.9 % mismatch to ZnO layer allows to estimate the critical thickness of ZnSnN2 layer to be less than 2 nm based on the



Matthews-Blakeslee framework for some III-V materials.32 In order to examine the growth behavior of the ZnSnN2 film without ZnO buffer layer, ZnSnN2 film was directly grown on cAl2O3 substrate. In this case, the growth temperature was selected to be 450 °C since at this temperature, the ZnSnN2 film with ZnO buffer showed the best crystallinity based on the RHEED evolution. Surprisingly, none of the RHEED patterns were observed in this case of ZnSnN2 film grown without ZnO buffer (Figure 1h). This implied that the ZnSnN2 film was grown as an amorphous or extremely disordered structure due to the large mismatch. Figure 1i shows the cross-sectional TEM images of 4 samples grown at 350°C, 450 °C, 550 °C, and 650 °C; the thicknesses of the ZnSnN2 layers were 51 nm, 39 nm, 39 nm, and 42 nm, respectively. Moreover, the compositional Zn:Sn ratios in these ZnSnN2 films were 2.5 ±0.1, 1.3 ±0.1, 1.2 ±0.1 and 1.1 ±0.1, respectively; based on the TEM-EDS measurements. Even with a fixed Zn:Sn flux ratio, the effective flux and the resultant Zn:Sn ratio can be changed on the growing surface with the variation of the growth temperature. In our set of experiments, the compositional Zn:Sn ratio decreased with the increase of growth temperature. The sample grown at 350 °C had a significantly larger (by around two times) Zn:Sn ratio than the other samples. We also note that the ZnSnN2 layer grown at 350 °C was 20~30 % thicker than the other samples. Hence, it is likely that the sticking coefficient of Zn is much larger than that of Sn at low growth temperature, which might explain the significant increase in the compositional Zn:Sn ratio and thickness of the sample grown at 350 °C. Here, we would like to mention the effect of the ZnO buffer thickness. Single crystal ZnSnN2 film growth was also possible with the thinner buffer. However, there was a minimum buffer thickness to grow single crystal ZnSnN2 films in our experiments. Crystalline quality of ZnO buffer layer is important and it is improved with thickness, in general. Two ZnSnN2 films


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were grown at 450 °C on thinner ZnO buffers to investigate the thickness effect; ZnSnN2 films on 10 nm-thick and on 15 nm-thick ZnO buffer layers. From the in-situ RHEED observations (not shown here), it was confirmed that the ZnSnN2 film on a 10 nm-thick ZnO buffer was grown like an amorphous or a poor crystalline film as addressed by no RHEED patterns. On the other hand, single crystal ZnSnN2 film was grown on 15nm-thick ZnO buffer as addressed by typical characteristic RHEED patterns (not shown here). In order to investigate the crystalline properties of the ZnSnN2 films with ZnO buffer, XRD measurements were carried out. Figure 2a shows the XRD θ–2θ measurement results for the films grown at 450 °C and 550 °C. In addition to the peaks from sapphire substrates (marked by closed squares), reflection peaks from ZnO and ZnSnN2 were observed. Both samples had the same peaks from ZnSnN2 at 2θ positions of 32.65° and 68.35°, which are attributed to pseudowurtzite (0002) and (0004) ZnSnN2 reflections. Furthermore, these measurements revealed the absence of Zn-nitride and Sn-nitride. The film grown at 450 °C showed sharper and narrower peaks than the ones in the sample grown at 550 °C, signifying better crystal quality. In order to determine the c-axis lattice constants from the peak positions of ZnSnN2 and ZnO layers, the obtained peak position from (0006) Al2O3 substrate was calibrated to the known (0006) Al2O3 reference value. Then, the c-axis lattice parameter of ZnSnN2 was determined from the peak position of (0002) reflection as 5.482 ± 0.003 Å, which is compatible with the reported parameters by Feldberg et al. (5.416 ~ 5.567 Å).17 Adjacent to ZnSnN2 peaks, (0002) ZnO peak was observed at 2θ positions of 34.44°. Using this peak, the c-axis lattice parameter of wurtzite ZnO was determined to be 5.197 ± 0.003 Å, which is almost identical to the reference value of 5.207 Å (JCPDS 36-1451). The observations of (0002) and (0004) reflections from ZnSnN2 and



ZnO in the XRD θ–2θ measurements without any other reflection indicated the preferred cdirection growth of films. The XRD phi scan measurement results for (1011) ZnSnN2 and (1011) ZnO diffractions were obtained from the ZnSnN2 film grown at 450 °C (Figure 2b). The phi scans confirmed the in-plane alignment of (1010) ZnSnN2 relative to (1010) ZnO. It should be noted that the XRD phi scan condition for ZnSnN2 was set to the crystallographic information based on the reported structure of the pseudo-wurtzite (monoclinic) ZnSnN2.17 The phi scan diffraction peaks of the (1011) pseudo-wurtzite ZnSnN2 appeared exactly at the expected positions when 2θ position of (1011) ZnSnN2 was 34.68° and the χ, which is equal to the angle between (1011) and (0001) of ZnSnN2, was 62.15°. Peaks from {1011} ZnSnN2 and {1011} ZnO appeared by separation of 60° in phi angle, which confirms the 60° rotational symmetry of the characteristic wurtzite crystal structure. The two facts that only the {000L} peaks of ZnSnN2 and ZnO were observed from the XRD θ–2θ measurements and that 60° rotational symmetry from {1011} ZnSnN2 and {1011} ZnO were observed clearly, present a further evidence that both the ZnO buffer layer and the ZnSnN2 film are single crystal. The crystal qualities of the ZnO buffer and ZnSnN2 films were addressed by measuring the omega rocking curves (Figure 2c). In general, the full width at half maximum (FWHM) values of (0002) reflections in omega rocking represent the tilt mosaics of the epitaxial films.33 The FWHM value of the (0002) reflection for the ZnO buffer layer was 0.23°; smaller than the values for the ZnSnN2 films, which were 0.52° and 0.71° for the films grown at 450 °C and 550 °C, respectively. The smaller FWHM value for the 450 °C sample manifested again the better crystal quality of the ZnSnN2 films grown at 450 °C in comparison to that grown at 550 °C. Qualitatively, similar tendency was observed by Cao et al.34 They reported that high temperature growth led to the decomposition of ZnSnN2.34


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In order to further investigate the microstructure of the grown ZnSnN2 film, crosssectional TEM observation was performed. Figure 3a shows the bright-field TEM micrograph along the [1010] Al2O3 zone axis for the sample grown at 450 °C, and Figure 3b shows the EDS element mapping results from the sample. The position of the boundary between the ZnO buffer layer and the ZnSnN2 layer is indicated by a dashed line in Figure 3a. These figures clearly display a sample structure of ZnSnN2/ZnO/Al2O3. In addition, the selective area diffraction (SAD) patterns from the interface regions of ZnO/ Al2O3 (Figures 3c) and ZnSnN2/ZnO (Figures 3d) were obtained from the corresponding regions circled in Figure 3a. Figure 3c displays the single crystal diffraction patterns for the [1010] zone axis of Al2O3 and the [1120] zone axis of ZnO. Figure 3d parades the single crystal diffraction patterns for the [1120] zone axis of ZnO and the [1120] zone axis of ZnSnN2. The SAD pattern from the ZnSnN2 corresponded to the pseudo-wurtzite structure and did not match with the orthorhombic structure. Therefore, the epitaxial relationships of [1120] ZnSnN2 // [1120] ZnO // [1010] Al2O3 and [0001] ZnSnN2 // [0001] ZnO // [0001] Al2O3 were determined, which were consistent with the ones concluded from the RHEED and the XRD. Inter-planar d-spacings for (1010) and (0001) planes of ZnO layer were determined from the SAD pattern to be 2.799 ± 0.006 Å and 5.192 ± 0.006 Å, respectively. Similarly, the d-spacings for (1010) and (0001) planes of ZnSnN2 were found to be 2.901 ± 0.006 Å and 5.477 ± 0.006 Å, respectively. These lattice parameter values were slightly smaller than those determined from the XRD. The slight variations might be due to the differences in the data acquisition volume and in the calibration of the instruments. These values allowed to establish the lattice constant of ZnSnN2 film as a = 3.350 ± 0.006 Å and c = 5.477 ± 0.006 Å, which are slightly smaller than those reported by Feldberg et al. for pseudo-wurtzite ZnSnN2.17 Since the ZnSnN2 film was grown on the ZnO layer, the contraction of in-plane d-



spacing of the ZnSnN2 film can be understood. In this case, the out-of-plane d-spacing should be expanded considering the biaxial stress. However, the determined d-spacing along the out-ofplane of ZnSnN2 film was also contracted. The disparate vapor pressure of Zn and Sn resulted in difficulties in growing stoichiometric ZnSnN2 films.17 From the quantitative EDS microanalysis, the Zn/Sn atomic percent ratio was 1.3 in the ZnSnN2 film grown at 450 °C. Although the EDS evaluation technique used in our study is less accurate than the Rutherford backscattering spectrometry method used by Feldberg et al.,17 they have mentioned that Zn-rich growth condition is preferred for single crystal ZnSnN2 film growth while Sn-rich growth condition results in polycrystalline film. Although we did not perform the experiments for different Zn:Sn flux ratio but used a fixed flux ratio (Zn:Sn = 20), our growth condition was suitable to get single crystal ZnSnN2 film growth. We believe that the larger Zn content in our ZnSnN2 film, contrary to the result of Feldberg et al,17 causes the contraction of the lattice constant. On the other hand, a recent paper on strain-free ZnSnN2 powder grown by high-pressure metathesis reaction method revealed that a = 0.3376 nm and c = 0.5467 nm for ZnSnN2.35 Base on the lattice parameters of this reference, there were a contraction of in-plane lattice parameter a and an expansion in outof-plane lattice parameter c in our ZnSnN2 film so we would not exclude the possible biaxial strains. HRTEM study was performed for the ZnO/ZnSnN2 interface region (Figure 4a) and the upper region of ZnSnN2 (Figure 4b). The lattice image was clearly observed from the wurtzite ZnO region, and the d-spacings for (1010) and (0001) planes of ZnO were mentioned in Figure 4a. The lattice image from the ZnSnN2 at the interface region was not as clear as those from ZnO and the upper region of ZnSnN2, but the d-spacings for (0001) and (1010) planes of ZnSnN2 could still be determined to be around 5.47 and 2.90 Å, respectively. The unclear and disordered


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lattice image from ZnSnN2 at the interface region implied that there was a transition period in the initial growth of ZnSnN2 on the ZnO layer. The thickness for the transition period was estimated to be about 2 nm based on the HRTEM image (Figure 4a). The upper region of ZnSnN2 in Figure 4b illustrated a well-ordered crystal structure of pseudo-wurtzite ZnSnN2. As shown in Figure 4c, the observed RHEED patterns after 5 min of the growth start of ZnSnN2 on ZnO, corresponding to around 2 nm ZnSnN2 thickness, were less clear than those at the end of growth in Figure 1. Regardless of the slight disordering at the very initial growth stage of ZnSnN2 on ZnO, all the results from RHEED, XRD, and TEM from the ZnSnN2 films in this study evidenced that the ZnSnN2 growth on ZnO buffer follows a pseudo-wurtzite structure rather than an orthorhombic structure.11,15,36 The optical transmission spectra of the ZnSnN2 films with ZnO buffer are shown in Figure 5a. The observed transmission data are composed of both ZnO buffer and ZnSnN2 layers. In order to estimate the transmittance of ZnSnN2 layer, the transmittance spectrum for a ZnO layer with the same thickness as the buffer layer is given in Figure 5a. The transmittance of the ZnO layer with wavelengths over 450 nm was higher than 90 %, and it had an absorption edge at 385 nm. Therefore, we can expect that the observed transmission spectra over 450 nm almost corresponded to those from ZnSnN2. The spectra of the samples grown at 450 °C, 550 °C, and 650 °C were almost similar. However, the one of the 350 °C sample is a little bit difference that might be the result of the larger Zn:Sn ratio and thickness of this sample. The transmittance was increased with the increase of growth temperature and the decrease of Zn:Sn ratio. The variation of the optical bandgap of the ZnSnN2 films versus growth temperature was determined from the absorption spectra (Figure 5b). Assuming that ZnSnN2 has a direct bandgap, the direct optical bandgap (Eg) was estimated using the formula: αhν= A(hν-Eg)1/2; where the absorption



coefficient α is determined from transmittance measurements, A is a constant, and hν is the incident photon energy. The plot of (αhν)2 versus photon energy (hν) approximately exhibited a linear behavior, which is consistent with the characteristics of the direct bandgap. The determined optical bandgaps were 1.85, 2.0, 2.05, and 2.15 eV for the growth temperatures of 350 °C, 450 °C, 550 °C, and 650 °C, respectively. The Zn:Sn ratio decreased when the growth temperature was increased. Thus, an increase in bandgaps accompanied by the decrease in Zn:Sn ratio was observed. However, it is also possible that the changes in bandgaps are not resulting from the variations in Zn:Sn ratio only and other factors might have played a role. Fioretti et al. have demonstrated that the optical bandgaps of polycrystalline ZnSnN2 films depend on the free carrier density and the ZnSnN2 films without a detectable free carrier has an optical bandgap around 1.0 eV.31 On the other hand, Veal et al. have indicated the optical bandgaps of single crystal wurtzite ZnSnN2 films depend more on their degree of cation disorder rather than on their free electron density,13 reporting their values to be between 1.33 and 2.38 eV. Additionally, they have suggested that the fundamental bandgap of ZnSnN2 can vary between 1.0 and 2.0 eV, depending also on the degree of cation disorder.13 Further investigation is needed to conclude the bandgap of this material. The morphologies of the ZnSnN2 films grown at 350 - 650 °C were evaluated by AFM for 10 × 10 µm2 and 2 × 2 µm2 scanning area (Figure 6a). No excess metallic coverage, which generally resulted in micrometer-order large size droplets, was observed from the surface morphology evaluated by AFM. The ZnSnN2 films manifested uniform granular surface morphologies. The RMS roughness values were 0.74 nm, 0.94 nm, 0.75 nm, 0.90 nm for ZnSnN2 films grown at 350 °C, 450 °C, 550 °C, 650 °C, respectively. The uncertainty in the RMS roughness is less than 0.15 nm by measurements on several regions. The larger grain sizes of the


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ZnSnN2 films grown at 450 °C and 550 °C may be related with the better crystallinity of the corresponding ZnSnN2 films. All the grown ZnSnN2 films were very smooth and had a light brown color apparent to the naked eyes, as shown in the photograph depicted in Figure 6b. Room-temperature Hall measurement results of the four samples in total thickness of two-layer structure, free electron densities, and carrier motilities were plotted as solid symbols in Figure 7. The free electron densities were 4.45 1018, 4.19 1019, 5.781019, 7.02 1019 cm-3 and carrier motilities were 16.2, 15.3, 29.0, 27.4 cm2/Vs in the four samples grown at 350 °C, 450 °C, 550 °C, 650 °C, respectively. In a separate, same growth condition, single layer ZnO film with a thickness of 100 nm, the corresponding values are 7.981018 cm-3 and 17.5 cm2/Vs. We assumed that the conductivity and mobility were uniform in each of the ZnO and ZnSnN2 layers, and we ignored the possible different properties of the ZnO/ZnSnN2 interface and other unknown factors. Using basic two-layer model,37 the estimated free electron densities and carrier mobilities in only ZnSnN2 layer grown at 350 °C, 450 °C, 550 °C, 650 °C will be 3.3 1017, 3.52 1019, 5.011019, 6.23 1019 cm-3, and 14.1, 12.6, 29, 27.5 cm2/Vs, respectively. Despite the crystallinity and grown temperature, a trend can be observed in the free electron densities as they increase when the Zn:Sn ratio decreases, but no clear trend can be deduced in the variations of the carrier mobility of ZnSnN2 layer. This is in line with the study of the polycrystalline Znrich ZnSnN2 film reported by Fioretti et al., in which the trend has been ascribed to the effect of the excess Zn on the disordered cation sub-lattice, which would compensate these anion sources of degenerate carrier density.31 The estimated free electron density in ZnSnN2 film grown at 350 °C was two order of magnitude lower than that in other samples and even one order of magnitude lower than the lowest reported free electron density in polycrystalline ZnSnN2 film.31 The reason is not clear yet and further investigations to explain the reasons are needed.



CONCLUSIONS In summary, single crystal ZnSnN2 films have been grown on (0001) sapphire substrates by using ZnO buffer. The ZnSnN2 film grown without the ZnO buffer showed amorphous-like disordered characteristics. On the other hand, all the ZnSnN2 films grown with the ZnO buffer at growth temperatures from 350 to 650 °C showed a pseudo-wurtzite structure and did not manifest the formation of an orthorhombic structure. Based on RHEED, XRD, and TEM analyses, the epitaxial relationships between Al2O3 substrate, ZnO buffer, and ZnSnN2 film were determined to be [1120] ZnSnN2 // [1120] ZnO // [1010] Al2O3 and [0001] ZnSnN2 // [0001] ZnO // [0001] Al2O3. Furthermore, the bandgaps of ZnSnN2 films could be tuned from 1.85 to 2.15 eV simply by increasing the growth temperatures from 350 to 650 °C. High crystallinity ZnSnN2 films were obtained for the films grown at 450 °C and 550 °C. The FWHM values of (0002) omega rocking curves were as small as 0.52° and 0.71°, respectively. All the grown ZnSnN2 films had uniform and flat surface with the RMS roughness less than 1 nm. The Hall measurement results showed that the electron density were higher in the samples grown at higher growth temperature and with higher Zn:Sn ratio. By applying ZnO as a buffer, we demonstrated a simple but useful approach to grow high quality single crystal ZnSnN2 films.

ACKNOWLEDGMENT


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This work was supported by the research fund of Chungnam National University (Grant No. 2015- 1799-01) and by the National Research Foundation of Korea (NRF) grant funded by the Korea government (Grant No. 2013R1A1A2061251).



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FIGURE CAPTIONS Figure 1. RHEED patterns observed along the [1100] and the [1120] azimuths of sapphire for (a) Al2O3 substrate, (b) ZnO buffer layer, and ZnSnN2 films grown at (c) 350 °C, (d) 450 °C, (e) 550 °C, and (f) 650 °C. (g) Schematic drawing for the determined epitaxial relationships between ZnO and ZnSnN2. (h) RHEED patterns observed along the [1100] and the [1120] azimuths of sapphire for the ZnSnN2 films grown directly on Al2O3 substrate without the ZnO buffer at 450 °C. (i) Cross-sectional TEM images of the four samples.

Figure 2. (a) XRD θ-2θ measurement results (peaks with closed square marks are from the sapphire substrate), (b) XRD phi scan of {1101} ZnSnN2 and {1101} ZnO reflections, and (c) (0002) XRD omega rocking curves for ZnO buffer and ZnSnN2 from the samples grown at 450 °C and 550 °C.



Figure 3. (a) Cross-sectional bright-field TEM micrograph observed at [1100] zone axis of sapphire substrate from the ZnSnN2 sample grown at 450 °C. (b) STEM EDS element mapping results showing the sample structure. (c) Corresponding SAD pattern from the ZnO/Al2O3 interface region mentioned by a circle in (a). (d) Corresponding SAD pattern from the ZnSnN2/ZnO interface region mentioned by a circle in (a).

Figure 4. (a) HRTEM micrograph at the ZnSnN2/ZnO interface region and (b) upper ZnSnN2 region from the sample grown at 450 °C. (c) RHEED patterns from the ZnSnN2 films grown at 350 °C, 450 °C, 550 °C and 650 °C at the very initial stage of growth (after 5 min growth).

Figure 5. (a) Transmission spectra of all samples and a separate ZnO film with thickness of 100 nm. (b) Plot of (αhν)2 versus photon energy (hν) from the optical absorption spectra to determine the optical bandgaps for the ZnSnN2 films grown at 350 °C, 450 °C, 550 °C, and 650 °C.

Figure 6. AFM images of the ZnSnN2 films grown at 350 °C - 650 °C evaluated on 10 × 10 µm2 (a) and 2 × 2 µm2 (b) area. (c) Photograph of corresponding samples.

Figure 7. Free electron density and carrier mobility for all samples measured by roomtemperature Hall measurement (closed square and triangle) and calculated values (open square and triangle) for ZnSnN2 layer grown at 350 °C, 450 °C, 550 °C, and 650 °C.


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TOC-SYNOPSIS PAGE For Table of Contents Use Only Epitaxial growth of bandgap tunable ZnSnN2 films on (0001) Al2O3 substrates by using a ZnO buffer Duc Duy Le,† Trong Si Ngo, † Jung-Hoon Song,‡ and Soon-Ku Hong†,* †

Department of Materials Science and Engineering, Chungnam National University, 99 Daehakro, Youseong-gu, Daejeon 34134, Republic of Korea ‡

Department of Physics, Kongju National University, 56 Gongjudaehak-ro, Shinkwan-dong, Kongju, Chungcheongnam-do 32588, Republic of Korea

*Corresponding author Soon-Ku Hong



Synopsis: By applying ZnO as a buffer, we demonstrated a simple but useful approach to grow high quality single crystal ZnSnN2 films on (0001) sapphire substrate. (a) Photograph of a sample with ZnSnN2 film grown at 450 °C, (b) cross-sectional STEM EDS element mapping result showing the sample structure, (c) HRTEM micrograph observed at [1120] zone axis of the single crystal ZnSnN2.


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Figure 1. RHEED patterns observed along the [11 ̅00] and the [112 ̅0] azimuths of sapphire for (a) Al2O3 substrate, (b) ZnO buffer layer, and ZnSnN2 films grown at (c) 350 °C, (d) 450 °C, (e) 550 °C, and (f) 650 °C. (g) Schematic drawing for the determined epitaxial relationships between ZnO and ZnSnN2. (h) RHEED patterns observed along the [11 ̅00] and the [112 ̅0] azimuths of sapphire for the ZnSnN2 films grown directly on Al2O3 substrate without the ZnO buffer at 450 °C. (i) Cross-sectional TEM images of the four samples. 176x176mm (300 x 300 DPI)



Figure 2. (a) XRD θ-2θ measurement results (peaks with closed square marks are from the sapphire substrate), (b) XRD phi scan of {11 ̅01} ZnSnN2 and {11 ̅01} ZnO reflections, and (c) (0002) XRD omega rocking curves for ZnO buffer and ZnSnN2 from the samples grown at 450 °C and 550 °C. 195x142mm (300 x 300 DPI)


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Figure 3. (a) Cross-sectional bright-field TEM micrograph observed at [11 ̅00] zone axis of sapphire substrate from the ZnSnN2 sample grown at 450 °C. (b) STEM EDS element mapping results showing the sample structure. (c) Corresponding SAD pattern from the ZnO/Al2O3 interface region mentioned by a circle in (a). (d) Corresponding SAD pattern from the ZnSnN2/ZnO interface region mentioned by a circle in (a). 186x190mm (300 x 300 DPI)



Figure 4. (a) HRTEM micrograph at the ZnSnN2/ZnO interface region and (b) upper ZnSnN2 region from the sample grown at 450 °C. (c) RHEED patterns from the ZnSnN2 films grown at 350 °C, 450 °C, 550 °C and 650 °C at the very initial stage of growth (after 5 min growth). 248x99mm (300 x 300 DPI)


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Figure 5. (a) Transmission spectra of all samples and a separate ZnO film with thickness of 100 nm. (b) Plot of (αhν)2 versus photon energy (hν) from the optical absorption spectra to determine the optical bandgaps for the ZnSnN2 films grown at 350 °C, 450 °C, 550 °C, and 650 °C. 91x152mm (300 x 300 DPI)



Figure 6. AFM images of the ZnSnN2 films grown at 350 °C - 650 °C evaluated on 10 × 10 µm2 (a) and 2 × 2 µm2 (b) area. (c) Photograph of corresponding samples. 244x167mm (300 x 300 DPI)


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Figure 7. Free electron density and carrier mobility for all samples measured by room-temperature Hall measurement (closed square and triangle) and calculated values (open square and triangle) for ZnSnN2 layer grown at 350 °C, 450 °C, 550 °C, and 650 °C. 165x182mm (300 x 300 DPI)


Epitaxial Growth of Bandgap Tunable ZnSnN2 Films on (0001) Al2O3

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