The piezotronic and piezophototronic properties of orthorhombic

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The piezotronic and piezophototronic properties of orthorhombic ZnSnN2 fabricated using Zn–Sn3N4 composition spreads through combinatorial reactive sputtering Cheng-Hsuan Kuo, and Kao-Shuo Chang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00586 • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 11, 2017

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Crystal Growth & Design

The piezotronic and piezophototronic properties of orthorhombic ZnSnN2 fabricated using Zn–Sn3N4 composition spreads through combinatorial reactive sputtering

Cheng-Hsuan Kuo1, and Kao-Shuo Chang1,2,* 1

Department of Materials Science & Engineering, National Cheng Kung University.

`No.1, University Road, Tainan City 70101, Taiwan. 2

Promotion Center for Global Materials Research, National Cheng Kung University.

No.1, University Road, Tainan City 70101, Taiwan. *

e-mail: [email protected]; Tel.: +886-6-2757575 ext. 62922.

ABSTRACT This paper presents a combinatorial methodology for fabricating orthorhombic ZnSnN2 (ZTN) by using Zn–Sn3N4 composition spreads. This study is the first to verify the substantial piezotronic and piezophototronic features of ZTN on the basis of asymmetric current–voltage (I–V) characteristics. Regarding the piezophototronic effect, at a -5 V bias, the current density was enhanced up to 2.5 times when the applied pressure increased from 0.625 to 2.5 GPa. Schottky barrier height variations at S1 and S2 were calculated under a pressure of 2.5 GPa and were observed to have 1

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increased and decreased by approximately 1.0 and 22 mV, respectively. The results clarified the I–V behavior and also supported the proposed energy-band structure evolution of piezotronic and piezophototronic ZTN. In addition, ZTN formation was verified through X-ray photoelectron spectroscopy and X-ray diffraction. A deconvolution algorithm was employed to validate the ratio of orthorhombic ZTN (Pna21) (approximately 30%). In addition, UV–vis spectrometry revealed that the energy bandgap of ZTN was approximately 2.0 eV.

Keywords: Combinatorial reactive sputtering, Zn–Sn3N4 composition spread, orthorhombic ZnSnN2, piezotronic properties, piezophototronic properties.

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1. Introduction Charge carrier transport modulated by the stress-induced piezopotential that is built in piezoelectric materials has been applied to various devices such as photocatalysts [1–3], sensors [4, 5], transistors [5, 6], light emitting diodes (LEDs) [5, 7], solar cells [8], switches [9], and photodetectors [10]. Among these materials, piezoelectric nitrides represent a promising system for exploration because they exhibit various remarkable properties [11, 12]. However, compared with piezoelectric oxides, they have received little attention from researchers [13]. A crucial feature of widely used piezoelectric nitrides, such as III-nitrides (AlN, GaN, and InN) [14, 15], is that their energy bandgaps (Eg) vary substantially [12]. According to our review of relevant literature, the Eg of InxGa1-xN can be tuned by varying the ratio of In to Ga [16]. Moreover, InGaN can be applied to blue LEDs with multiple GaN–InGaN barriers [17]. However, synthesizing high-quality InGaN films is critical because of the substantial lattice mismatch that can occur between InN and GaN (up to approximately 11%) [18]. Another drawback is that both In and Ga are scarce. Therefore, researchers have been investigating wurtzite Zn–IV–N2 as a novel alternative to III-nitrides because the optical and electrical properties of wurtzite Zn– IV-N2 are equivalent or superior to those of III-nitrides.

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According to literature reports, the Eg of Zn–IV–N2 materials also varies extensively, from approximately 1.7 eV for ZnSnN2 to approximately 4.5 eV for ZnSiN2 [19, 20]. In addition, the lattice mismatch among Zn–IV–N2 materials is insignificant compared with that among III-nitrides. This feature indicates that the strain-induced polarization in the junctions among Zn–IV–N2 materials is reduced [21]. ZnSiN2 can be fabricated through various approaches [22–27]. However, there remains considerable inconsistency between experimental results and theoretical calculations because different fabrication methods usually lead to dissimilar induced strains (lattice distortions) in ZnGeN2. Another Zn–IV–N2 compound is ZnSnN2 (ZTN), which comprises common, nontoxic elements; moreover, superior Eg tunability can be achieved by controlling the cation disorder [28, 29], which enables the coverage of the entire visible region of the solar spectrum. In addition, the low formation energy of donor defects yields an intrinsic n-type ZTN [30], in which substantially high carrier concentrations are observable (up to 1020 cm−3) [24]. However, the association between the Eg and carrier concentrations is inconsistent because of the Burstein–Moss effect [30, 31]. Orthorhombic (without inversion operation) and monoclinic structures are possible lattice structures of ZTN [32]. However, ZTN-related studies remain limited, primarily because of the difficulties in fabricating ZTN, particularly orthorhombic 4


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ZTN [31]. Chen et al. applied the first principles investigation to study the phase stability and defect physics of ZTN by considering its formation energy [30], reporting that synthesizing single-phase ZTN is highly challenging because of the narrow chemical potential region of the constituent elements. On the basis of experimental and theoretical approximations, Lahourcade et al. realized that wurtzite ZTN is the most stable structure, in which the Zn and Sn atoms were arranged to form orthorhombic Pna21 and Pmc21 space groups, although the formation energy of a zincblende-derived structure was also low [32]. ZTN fabrication through physical approaches had not been reported until recently. In 2013, Quayle et al. synthesized ZTN by using a plasma-assisted vapor–liquid–solid technique to study the lattice parameters and Eg of ZTN [19]. In 2014, Feldberg et al. employed plasma-assisted molecular beam epitaxy to explore the growth dynamics of crystalline ZTN [28]. In 2015, Fioretti et al. used a combinatorial radio-frequency (RF) cosputtering approach to fabricate thin ZTN films on glass substrates by using Zn and Sn metal targets to explore the potential application of the ZTN films in photovoltaics [31]. They highlighted a tunable carrier density as a function of the cation composition, in which a low carrier density of 1.8 × 1018 cm−3 and high mobility of 8.3 cm2 V−1 s−1 were obtained. In addition, Deng et al. successfully synthesized polycrystalline ZTN films on quartz and polyethylene terephthalate substrates through DC magnetron sputtering 5



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at room temperature [33]. In 2016, Kawamura et al. synthesized single crystalline ZTN through a high-pressure (>5.5 GPa) metathesis reaction [34]. That year, Qin et al. also fabricated polycrystalline ZTN films through magnetron sputtering to construct Si/ZnSnN2 p-n junctions [24]. Although ZTN has been explored and applied to various electronic fields [24, 29, 32], research on orthorhombic ZTN remains challenging and studies investigating the piezoelectric properties of ZTN are scant. Furthermore, extended piezo-related features, such as piezotronic and piezophototronic properties [35, 36], are yet to be investigated systemically. In the present study, orthorhombic ZTN was fabricated using a novel combinatorial methodology and its piezoelectric properties were studied. Using a combinatorial methodology is an efficient strategy for producing a uniform set of samples on a single substrate in a single process [37, 38]. The samples are then screened systemically for specific applications. Compared with conventional manufacturing methods, this methodology saves a considerable amount of time, resources, and manpower. In this study, this technique was employed to create a Zn– Sn3N4 composition spread to rapidly determine the potential composition ranges exhibiting piezo-related properties. Compared with the aforementioned studies, six unexplored aspects were investigated: 1) developing a facile and straightforward 6


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approach to facilitate ZTN synthesis by coupling similar-thickness wedges of natural Sn3N4 and artificial Zn gradients (Fig. 1); 2) examining the coupling between Zn and Sn3N4 at various thicknesses and cycles; 3) ascertaining the properties of orthorhombic ZTN (Pna21) through X-ray diffraction (XRD), UV–vis, and electrical measurements; 4) studying piezo-related properties of ZTN according to its current– voltage (I–V) characteristics; 5) determining the Schottky barrier height variations of ZTN while under stress; and 6) differentiating the impurity phases of Sn and Sn3N4 from ZTN and studying their influences on the electrical performance of ZTN. Our results elucidate the substantial piezotronic and piezophototronic properties of ZTN (Pna21).

2. Experimental methods Combinatorial reactive sputtering, which involves a stainless steel moving shutter, was the basis for synthesizing Zn–Sn3N4 composition spreads on fluorine-doped SnO2 (FTO)/glass substrates. The substrates were cleaned ultrasonically by using acetone, alcohol, and then isopropyl alcohol for 5 min each. A background pressure of approximately 1 × 10−6 Torr was applied. Before deposition, a 5-min presputtering process was performed using pure Ar plasma (working pressure, 15 mTorr; working power, 50 W) to remove surface contaminants from the Zn and Sn targets. The 7



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working distance between the target and substrate was approximately 7 cm. Zn–Sn3N4 composition spreads were synthesized by coupling natural Sn3N4 and artificial Zn thickness gradients. A natural Sn3N4 gradient was achieved by optimizing the angles of the Sn-target sputtering gun, in which a reactive RF mode (working power, 50 W; working pressure, 20 mTorr) was applied. The gas ratios of Ar:N2 (24:3, 27:3, 28:3, 29:3, 33:3, and 35:3) and the substrate temperature (400°C–500°C) were tuned to optimize the Sn3N4 crystallinity. The schematic in Fig. 1(a) shows that the desirable thickness of the natural Sn3N4 gradient decreases from the left to the right of the FTO substrate. The figure also shows an artificial gradient layer of Zn that was sputtered on top of the Sn3N4 layer by using a programmable moving shutter [Fig. 1(b)]. Layers with various thicknesses were obtained by adjusting the shutter moving speed. A Zn target was sputtered using pure Ar plasma under the following conditions: working power, DC 50 W; working pressure, 20 mTorr; and deposition temperature, 500°C. The gradient was also generated to follow the Sn3N4 gradient to maximize the formation of ZTN. To ensure effective coupling between the Zn and Sn3N4, the thickness of their thickness gradients was controlled by modulating the deposition time (e.g., varying from 12 min to 1 h for Sn3N4 and from 3 to 7 min for Zn). The suitable Zn–Sn3N4 composition spread was found to be 7 min of deposition for the artificial Zn gradient on top of 15 min of deposition for the natural Sn3N4 gradient. To 8


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further enhance the ZTN formation, two approaches were applied: 1) repeating the suitable Zn–Sn3N4 composition spread once [Fig. 1(c)], and 2) increasing the deposition time of the artificial Zn gradient to 12 min [Fig. 1 (d)]. For easy characterization, various sample sizes were fabricated. For instance, six small precut pieces (4 × 24 mm) of FTO/glass substrate, indexed as piece 1 (thick side) to piece 6 (thin side) [Fig. 1(c)], were deposited for the XRD measurement. A single large piece (24 × 24 mm) of FTO/glass substrate was deposited for X-ray photoelectron spectroscopy (XPS), UV–vis, and I–V analysis. An HCl solution (pH ≅ 2) was used as the etchant to remove metal Sn from the samples (Sn + HCl → SnCl2 + H2). The process yielded the formation of highly crystalline ZTN because of its high tolerance to acidic solutions [34]. The process was performed at 40°C at two etching times (depending on the etching rates of the different samples). Etching was performed for 10 min for the six small pieces of FTO/glass and 30 min for the single large piece of FTO/glass. Numerous Zn–Sn3N4 composition spreads were reliably fabricated for various characterizations. Glancing angle XRD (D8 DISCOVER, Bruker AXS Gmbh, Germany) was performed to analyze the sample crystallinity. Field-emission scanning electron microscopy (FE-SEM, JEOL JSM-7001, USA) was used to examine the morphology. XPS (PHI 5000 VersaProbe, ULVAC-PHI, Inc. Japan) was conducted to 9



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identify the chemical states of constituent materials. A UV–vis spectrometer was employed to measure the optical properties; a Tauc plot was applied to identify the Eg of the samples. The piezotronic properties of ZTN were determined using a special probe design that comprised a semiconductor parameter analyzer (HP-4145B, National Instruments Corporation, USA) and two tungsten (W) electrical probes. One probe, integrated with a sensitive stress reader, simultaneously supplied the voltage and stress to the samples. The other probe was grounded and contacted with the bottom electrode of the FTO. The probe area was approximately 7.85 × 10−7 cm2. Moreover, the piezophototronic effect was characterized when 15-W visible irradiation was applied.

3. Results and Discussion Ordered and disordered cation configurations lead to orthorhombic and monoclinic ZTN, respectively [28, 29]. Basically, ZTN can be synthesized through two reactions: 2Zn + Sn3N4 → Sn + 2ZnSnN2

(1)

Sn + Zn3N2 → 2Zn + ZnSnN2

(2)

where the defects of ZnSn2− and SnZn2+ are predominant in Reactions (1) and (2), respectively. The formation energy of SnZn2+ is markedly lower than that of ZnSn2− 10


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[30], indicating that Reaction (2) is prone to form monoclinic ZTN because a substantial amount of SnZn2+ is favorable to forming a disordered defect configuration. By contrast, orthorhombic ZTN is preferentially formed in Reaction (1) because little or moderate amounts of ZnSn2− favors the formation of an ordered defect configuration; in addition, the noncentrosymmetric feature of orthorhombic ZTN enables piezoelectric polarization induction. Thus, the strategy of Zn-doped Sn3N4 [Reaction (1)] was adopted in this study. To further enhance the efficiency of exploring orthorhombic ZTN, combinatorial Zn–Sn3N4 composition spreads were fabricated. Fabricating the constituent compound of nanostructured Sn3N4 in a Zn–Sn3N4 composition spread is crucial. Our previous study [39] indicated that a high working pressure enables the deposition of species with low mobility, thus leading to the formation of nanorod-like structures with observable separation and excellent alignment normal to the substrate. Thus, in this study, a working pressure of 20 mTorr was applied. However, to compensate the deterioration of the Sn3N4 crystallinity at such a high working pressure, the N2/Ar ratios and substrate temperature were tuned. Among the various manufacturing conditions, the Sn3N4 sample fabricated at Ar:N2 = 35:3 and 500°C for 1 h [black curve, Fig. 2(a)] exhibited desirable crystallinity (JCPDS 01-070-3184). The corresponding top-view and side-view SEM images are 11



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presented in Fig. 2(b) and (c), showing rod-like Sn3N4. The resulting thickness was approximately 480 nm. To couple Sn3N4 with Zn effectively to form ZTN, the thickness of Sn3N4 should be minimized while sustaining its favorable crystallinity. Thus, the deposition time of Sn3N4 was studied from 12 min to 1 h. The results revealed that only the 15-min sample fulfilled the aforementioned requirement [red curve, Fig. 2(a)]. Thus, the conditions (gas ratios of Ar:N2, 35:3; deposition temperature, 500°C; working pressure, 20 mTorr; and deposition time, 15 min) were determined as favorable for fabricating Sn3N4 with a natural thickness gradient. The XRD results of the Zn–Sn3N4 composition spreads fabricated, as shown in Fig. 1(c), through 10 min of HCl(aq) etching, are presented in Fig. 3(a). In general, Sn3N4 [(220), (311), (222), (400), and (422)] was observed on all pieces; however, weak signals were identified on pieces 1 and 2. The ZTN peak at approximately 34.8° [(201)/(121)] and Sn peaks at approximately 32° [(101)], 44° [(220)], and 45° [(211)] were observed on pieces 1 and 2. No Zn was observable. These observations indicated that the desirable ratios of Zn and Sn3N4 on pieces 1 and 2 drove the formation of ZTN [Reaction (1)]. The residual Sn observed on pieces 1 and 2 was attributed to the protection exerted by ZTN. Although Sn did not exhibit piezoelectricity, its high conductivity influenced the electrical characterization, which is illustrated in Fig. 7(b). The XRD results of the samples fabricated according to the strategy depicted in Fig. 12


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1(d) are presented in Fig. 3(b), which shows similar diffraction patterns. However, ZTN was readily observed on pieces 1–3, and the peak intensity at approximately 34.8° [(201)/(121)] was more pronounced than that of the aforementioned sample [Fig. 3(a)]. Thus, these samples were examined to ensure the synthesis of orthorhombic ZTN. The synthesis of orthorhombic ZTN is challenging [28, 40]. To confirm that the ZTN synthesis was successful, the XRD results of piece 1 were further analyzed using a deconvolution algorithm. Fig. 4(a) presents the partial XRD patterns (32°–35°) of monoclinic ZTN (black line) and two potential orthorhombic structures of ZTN [Pna21 (blue line) and Pmc21 (red line)] obtained using theoretical calculations [32]. Sn3N4 (orange line) was also plotted for comparison. The green line denotes an overlap of Pna21, Pmc21, and monoclinic ZTN phases. Fig. 4(b) presents the deconvolution results of the characteristic peak at approximately 32.8°. Comparing the two peak locations with those shown in Fig. 4(a) reveals that the red and blue dashed lines are associated with Sn3N4 and orthorhombic ZTN (Pna21), respectively. No monoclinic ZTN formation was observed. The calculated ratio of ZTN (Pna21)/Sn3N4 was approximately 30%/70%. Thus, the synthesis of orthorhombic ZTN was validated. Fig. 5 presents the corresponding optical property obtained through UV–vis 13



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spectroscopy. The inset presents the spectrum of the optical density [absorption coefficient (α) × film thickness] as a function of incident-light wavelengths by using an absorption mode. The sharp decay below 400 nm was attributed to the UV absorption by the FTO/glass. Because ZTN is a direct Eg semiconductor [30], the Tauc plot of (αhv)2 versus hν was used to derive the Eg of the sample. The figure shows two observable linear regions. The corresponding slopes of the lines extrapolated to the x-axis were associated with the Eg of ZTN (approximately 2.0 eV, green dashed line) and Sn3N4 (approximately 2.3 eV, red dashed line); these values are consistent with reports in the literature [28, 30, 32, 40, 41]. To study the evolution of chemical states of the constituent elements and electrical characteristics of the yielded phases across the sample area, a single large piece of Zn (12 min)–Sn3N4 (15 min) composition spreads was deposited [Fig. 6(a)]) and etched at 40°C for 30 min. Four locations from location 1 (thick side) to location 4 (thin side) are depicted [Fig. 6(a)]. The sample is ideal for XPS and I–V measurements because location 1 contained minor Sn, substantial Sn3N4, and discernible ZTN, and location 4 contained no observable Sn or ZTN and detectable amounts of Sn3N4, as indicated by the XRD results (not shown). Fig. 6(b) presents the XPS results of location 1 (red curve) and location 4 (green curve). A deconvolution algorithm was applied to location 1 to identify the chemical states of Sn 3d5/2. The weak deconvoluted peak 14


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(blue dashed line) centered at approximately 484.8 eV was attributed to Sn0, indicating the presence of minor metal Sn; the strong peak (black dashed line) centered at approximately 486.6 eV was associated with Sn4+, which was contributed by Sn3N4 and ZTN. However, the peak centered at approximately 486.9 eV (green curve) at location 4 was associated with Sn3N4 only; no Sn0 was observed. Thus, the red peak shift of Sn4+ from 486.9 to 486.6 eV resulted from the ZTN formation, which is consistent with the XRD analysis. However, because N 1s and Zn 2p3/2 signals did not vary substantially between locations 1 and 4, only location 1 is shown. Fig. 6(c) shows the N 1s spectrum, in which the respective peaks centered at approximately 396 and 399 eV were attributed to the Sn–N bond (red dashed line) and Sn–N–O bond (blue dashed line), which may have resulted from the oxidation of sample surfaces. The Zn 2p3/2 spectrum [Fig. 6(d)] presents the peak centered at approximately 1022 eV, indicating the chemical state of Zn2+. The piezotronic and piezophototronic effects [35, 36] have been effectively applied to various electronic devices [36]. The fundamental mechanism is based on charge carrier transport modulated by variation in the Schottky barrier height. To examine these properties of ZTN, a facile I–V measurement was employed. Figure 7 presents the results from location 1 [Fig. 6(a)]. As discussed previously, location 1 consisted of Sn, Sn3N4, and ZTN; therefore, various positions on this location were 15



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investigated and the effects of Sn and Sn3N4 on the electrical properties of ZTN were studied. The measurement configuration is illustrated in the inset of Fig. 7(a), in which the probe supplying both stress and bias is in direct contact with the sample (S2); the grounded probe is in indirect contact with the sample through the FTO substrate (S1), which was achieved by scratching off a small portion of the sample on the FTO surface. To ensure that the sample was discarded completely, a four-point probe was used to monitor the resistance of the highly conductive exposed FTO surface. Fig. 7(a) presents typical I–V characteristics that were observed at location 1. The asymmetric I–V characteristics under pressures of 1.25 GPa (blue line) and 2.5 GPa (orange line) suggested that a single Schottky contact was formed at S1. According to the I–V features, the Sn3N4 phase was probed because 1) Sn3N4 was the predominant phase on location 1, as discussed previously; 2) the threshold current densities (JD), indicated by the pink double-headed arrow, did not vary substantially as a function of pressures, suggesting no piezotronic effect (Sn3N4 exhibits no piezoelectricity); and 3) minor JD values were obtained and no Ohmic-like behavior was observed, implying that no Sn was involved. By contrast, Fig. 7(b) presents the I–V characteristics from a different position that contained a mixture of Sn3N4 and Sn. The curves show a similar trend to that in Fig. 7(a), indicating the function of the predominant Sn3N4 matrix. 16


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However, certain variations that were noted were attributed to the effectiveness of minor Sn: 1) JD values were substantially higher at the applied voltage range compared with those in Fig. 7(a), and 2) an Ohmic-like behavior was observed in the positive voltage range. These I–V characteristics were inconsistent, indicating the presence of minor metal Sn, and most of the Sn was embedded into the Sn3N4 matrix or distributed nonuniformly on location 1. In addition, the threshold JD (also denoted by the pink double-headed arrow) did not vary as a function of pressure at 1.25 GPa (blue line) and 2.5 GPa (orange line), and enhancement of the JD values was limited when visible irradiation was applied (dashed lines). These features suggested that Sn exhibited no piezotronic or piezophototronic effects. However, substantial changes in the I–V characteristics were observed for the region containing ZTN [Fig. 7(c)]. The asymmetric I–V characteristics indicated that back-to-back Schottky contacts (S1 and S2) were formed (almost negligible JD at a positive applied bias). No effects of Sn and S3N4 were observed. The observed threshold JD (also associated with the Schottky barrier height) variation as a function of pressure implied the presence of a piezotronic effect (solid lines) that was directly contributed by ZTN. Thus, the Sn3N4 matrix was coated with ZTN for the probed region [inset of Fig. 7(c)]. To clearly illustrate the turn-on voltage variation (∆V), an enlarged plot is shown for the bias range of −2.5 to −3.5 V. As shown in the figure, the 17



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turn-on voltage at −2.75 V under 0.625 GPa (black line) changed to −2.5 V when the pressure was increased to 2.5 GPa (orange line). The high turn-on voltage might be attributed to the existence of Sn3N4 [inset of Fig. 7(c)]. Although the ZTN was directly probed, average I-V characteristics were measured. Furthermore, when a negative bias was applied, the JD values were negatively enhanced as the applied pressure was increased from 0.625 to 2.5GPa (solid lines). When visible irradiation was applied, the piezophototronic characteristics of ZTN were observed (dashed lines) because a more negatively enhanced JD as a function of applied stresses was achieved; in addition, the JD measured at the negative bias range exhibited the same trend as that observed for the piezotronic effect. Although the measured JD was not high, the piezotronic or piezophototronic effects were substantial. For example, the JD value at a bias of −5 V was enhanced 2.5 times when the applied pressure was increased from 0.625 to 2.5 GPa (red vertical double-headed arrow) for the piezophototronic effect, and a 50% increase in JD was observed at 2.5 GPa with and without illumination (orange vertical double-headed arrow). The piezoresistive effect was not considered because of the features of the exhibited I-V characteristics on the basis of our previous study [42]. The JD values may be increased by modulating the alignment, morphology, and ZTN quantities on the substrate. The aforementioned distinctive I–V characteristics [Fig. 7(a)–(c)] provided an alternative approach for distinguishing the 18


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piezotronic or piezophototronic ZTN from Sn and Sn3N4 and were also consistent with the XRD and XPS results. Investigation of orthorhombic ZTN based piezotronic devices is still ongoing. A simple energy band structure evolution [Fig. 7(d)], including before (top) and after applying pressure (middle) and additional supplying bias (bottom), was employed to illustrate the I–V characteristics of ZTN. Because the work function of ZTN (Φ ZTN) remains unknown, ΦZTN was assumed to approximate that of Sn3N4 (ΦSn3N4; 4.7 eV) [43]. S1 represents the contact between FTO and Sn3N4, and S2 denotes the contact between ZTN and W. Thus, the Schottky barrier height at S1 is higher than that at S2 because of the larger Φ of the commercial FTO substrate (4.92– 5.12 eV) compared with that of W (4.55 eV) [top, Fig. 7(d)]. When stress was applied to ZTN and a positive piezopotential was induced, the Schottky barrier height at S2 was reduced [dashed red line; middle, Fig. 7(d)], which facilitated electron (e−) transport from S2 to S1. When a negative bias (up to −5 V) was additionally supplied at S2, the e− energy increased [orange line; bottom, Fig. 7(d)]. Consequently, the e− flow from S2 to S1 was further enhanced. This characteristic represents the piezotronic effect. By contrast, when a positive bias (up to 5 V) and stress were applied simultaneously at S2, the e− flow from S1 to S2 was limited because of an increase in the essential Schottky barrier height at S1. 19



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The Schottky barrier height variations (S1 and S2) under stress were calculated to further elucidate the I–V characteristics of ZTN [Fig. 7(c)]. On the basis of thermionic emission–diffusion theory, the JD for a Schottky diode is expressed as

J = J e − 1 = A∗∗ T e

e − 1

(3)

where JS is the saturation current, A** represents the effective Richardson constant, T is the absolute temperature, q denotes the electronic charge, Φ is the barrier height, k represents Boltzmann constant, and V is the applied bias. When the reversed bias (Vr) is greater than 3kT/q, V, and

, the electrical field E is simplified to be

proportional to V1/2. Thus, ln Jr can be deduced to be linearly proportional to V1/4 for Schottky contact. The details were explained in our previous study [44]. The analysis of ln (JD) as a function of V1/4 for S1 is shown in Fig. 8(a); a linear relationship was observed at a bias ranging from approximately 4.2 V (V1/4 ≅ 1.43 V) to 5 V (V1/4 ≅ 1.5 V) under various pressures. A similar trend was observed for S2 at a negative bias range. Thus, the Schottky behavior at S1 and S2 was validated. In addition, the Schottky barrier height variation (∆Φ) at S1 and S2 under various pressures can be determined using Eq. (3). Because A∗∗ T e

!

is constant, ∆Φ between two

different pressures (P′ and P0) can be expressed as follows [44]:

∆Φ = ∆Φ"# − ∆Φ"$ =

() + *

ln '(

)*!

,

(4) 20


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where P0 is a reference pressure (0.625 GPa) and P′ is varied from 1.25 to 2.5 GPa. Thus, ∆Φ"# − ∆Φ"$ at S1 was determined at −4.9 V under various P′ values [Fig. 8(c)]. As shown in the figure, ∆Φ"# − ∆Φ"$ increased by approximately 1.0 mV at 2.5 GPa. A similar analysis was employed for S2, in which ∆Φ"# − ∆Φ"$ was reduced by approximately 22 mV at 2.5 GPa. The calculated results clearly justified the proposed model [Fig. 7(d)] and the exhibited piezotronic effect. Although orthorhombic ZTN was reliably fabricated, the acquired best condition might not be suitable for scale-up because of nonuniform distribution of deposited species.

4. Conclusions This is the first study to demonstrate an effective combinatorial Zn–Sn3N4 composition spread technique through reactive sputtering to exploit the piezotronic and piezophototronic properties of ZTN. The XRD results revealed that ZTN was formed readily on pieces 1–3 of the Zn–Sn3N4 composition spread (12 min of deposition of the artificial Zn gradient on top of 15 min of deposition for the natural Sn3N4 gradient) [Fig. 1 (d)]. Orthorhombic ZTN was obtained using a deconvolution algorithm of the characteristic peak at approximately 32.8°. No monoclinic ZTN was observed; the calculated ratio of ZTN (Pna21)/Sn3N4 was approximately 30%/70%. 21



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The Eg of ZTN was deduced as approximately 2.0 eV by using a UV–vis spectrometer. In addition, ZTN formation was verified through XPS, which indicated a peak shift of Sn4+ from 486.9 to 486.6 eV. Furthermore, the piezotronic and piezophototronic effects of ZTN were demonstrated using I–V characteristics, in which the JD values at a bias of −5 V were enhanced 2.5 times when the applied pressure was increased from 0.625 to 2.5 GPa for the piezophototronic effect. The evolution of a simple energy band structure was proposed to illustrate the I–V characteristics of orthorhombic ZTN. On the basis of thermionic emission–diffusion theory, the Schottky behavior was validated and Schottky barrier height variations were observed to have increased by approximately 1.0 mV at S1 and decreased by approximately 22 mV at S2 (both under 2.5 GPa). The results clearly justify the proposed energy band evolution and also greatly support the exhibited piezotronic effect.

Acknowledgments This study was partially supported by the Ministry of Science and Technology, Taiwan (MOST 104-2221-E-006-025).

22


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Determination of the basic optical parameters of ZnSnN2, Optics Letters 2015, 40, 1282-1285. [34] Kawamura, F.; Yamada, N.; Imai, M.; Taniguchi, T. Synthesis of ZnSnN2 crystals via a high-pressure metathesis reaction, Cryst. Res. Technol. 2016, 51, 220-224. [35] Wang, Z. L. Piezotronic and Piezophototronic Effects, J. Phys. Chem. Lett. 2010, 1, 1388-1393. [36] Wang, Z. L. Progress in Piezotronics and Piezo-Phototronics, Adv. Mater. 2012, 24, 4632-4646. [37] Chang, K.-S.; Lu, W.-C.; Wu, C.-Y.; Feng, H.-C. High-throughput identification of higher-k dielectrics from an amorphous N2-doped HfO2–TiO2 library, Journal of Alloys and Compounds 2014, 615, 386-389. [38] Xiang, X. D.; Sun, X.; Briceno, G.; Lou, Y.; Wang, K.-A.; Chang, H.; Wallace-Freedman, W. G.; Chen, S.-W.; Schultz, P. G. A Combinatorial Approach to Materials Discovery, Science 1995, 268, 1738-1740. [39] Lin, Z.-A.; Lu, W.-C.; Wu, C.-Y.; Chang, K.-S. Facile fabrication and tuning of TiO2 nanoarchitectured morphology using magnetron sputtering and its applications to photocatalysis, Ceramics International 2014, 40, 15523-15529. [40] Feldberg, N.; Aldous, J. D.; Linhart, W. M.; Phillips, L. J.; Durose, K.; Stampe, P. A.; Kennedy, R. J.; Scanlon, D. O.; Vardar, G.; Field III, R. L.; Jen, T. Y.; Goldman, R. 28


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Nanowire Arrays and Piezophotocatalytic Applications, J. Am. Ceram. Soc.

2016, 99, 2593-2600.

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Figure Captions Fig. 1. Fabrication of Zn–Sn3N4 composition spreads. (a) Schematic of a natural Sn3N4 thickness gradient. (b) Schematic of an artificial Zn thickness gradient. (c) Two cycles of Zn–Sn3N4 composition spreads. (d) 12-min deposition of an artificial Zn gradient on top of a natural Sn3N4 thickness gradient. Fig. 2. Fabrication of nanostructured Sn3N4. (a) X-ray diffraction (XRD) results of 1-h (black curve) and 15-min (red curve) depositions. Top (b) and side (c) view scanning electron microscopy (SEM) images. Fig. 3. X-ray diffraction (XRD) results of two Zn–Sn3N4 composition spreads fabricated on six pieces of FTO substrates after 10 min of HCl(aq) etching. (a) Two cycles of Zn–Sn3N4 composition spreads. (b) 12-min deposition of an artificial Zn gradient on top of a natural Sn3N4 thickness gradient. Fig. 4. Verification of orthorhombic ZTN formation. (a) Partial X-ray diffraction (XRD) patterns (32°–35°) of ZTN Pna21 (blue line) and Pmc21 (red line) and monoclinic ZTN (black line) obtained from [30]. Sn3N4 (orange line) is also plotted for comparison. (b) A deconvolution algorithm employed on piece 1 of the sample fabricated using the strategy in Fig. 1(d). (c) Calculated ratio of ZTN(Pna21)/Sn3N4. Fig. 5. Tauc plot of the sample (piece 1) fabricated using the strategy in Fig. 1(d). The inset shows the absorption spectrum obtained using a UV–vis spectrometer. 30


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Fig. 6. X-ray photoelectron spectroscopy (XPS) results. (a) Schematic of the composition spread deposited on a single large piece of FTO/glass substrate. (b) Sn 3d5/2 spectra of location 1 (red curve) and location 4 (green curve). A deconvolution algorithm was applied to location 1. (c) N 1s spectrum of location 1. (d) Zn 2p3/2 spectrum of location 1. Fig. 7. Current–voltage (I–V) characteristics exhibited by various phases obtained from location 1 of the sample [Fig. 6(a)]. (a) Sn3N4. The inset illustrates the measurement configuration. (b) Mixture of Sn3N4 and Sn. (c) ZTN. (d) Energy band structure evolution. Fig. 8. Schottky behavior at S1 (a) and S2 (b). Schottky barrier height variation at S1 (c) and S2 (d).

31



"For Table of Contents Use Only" The piezotronic and piezophototronic properties of orthorhombic ZnSnN2 fabricated using Zn–Sn3N4 composition spreads through combinatorial reactive sputtering Cheng-Hsuan Kuo1, and Kao-Shuo Chang1,2,*

0.2

ZTN + Sn3N4 Current density (A/cm2)

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0 -5

-3

-1

1

1.25 GPa -0.2 2.5 GPa 1.25 GPa + light 2.5 GPa + light

-0.4

S2

∆Φ Φp’-p0

3

5

Zn(12min) Sn3N4(15min) FTO 4 1 2 3 Pressure probe Ground probe S2

-0.6

S1

ZTN

Sn3N4

FTO Glass

-0.8 Voltage (V)

Synopsis: A combinatorial methodology was applied to fabricate orthorhombic ZnSnN2 and to verify its piezotronic and piezophototronic features. The energy-band structure evolution was proposed.

32


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Fig. 1. Fabrication of Zn–Sn3N4 composition spreads. (a) Schematic of a natural Sn3N4 thickness gradient. (b) Schematic of an artificial Zn thickness gradient. (c) Two cycles of Zn–Sn3N4 composition spreads. (d) 12-min deposition of an artificial Zn gradient on top of a natural Sn3N4 thickness gradient. 254x190mm (300 x 300 DPI)



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Fig. 2. Fabrication of nanostructured Sn3N4. (a) X-ray diffraction (XRD) results of 1-h (black curve) and 15min (red curve) depositions. Top (b) and side (c) view scanning electron microscopy (SEM) images. 254x190mm (300 x 300 DPI)


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Fig. 3. X-ray diffraction (XRD) results of two Zn–Sn3N4 composition spreads fabricated on six pieces of FTO substrates after 10 min of HCl(aq) etching. (a) Two cycles of Zn–Sn3N4 composition spreads. (b) 12-min deposition of an artificial Zn gradient on top of a natural Sn3N4 thickness gradient. 254x190mm (300 x 300 DPI)



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Fig. 4. Verification of orthorhombic ZTN formation. (a) Partial X-ray diffraction (XRD) patterns (32°–35°) of ZTN Pna21 (blue line) and Pmc21 (red line) and monoclinic ZTN (black line) obtained from [30]. Sn3N4 (orange line) is also plotted for comparison. (b) A deconvolution algorithm employed on piece 1 of the sample fabricated using the strategy in Fig. 1(d). (c) Calculated ratio of ZTN(Pna21)/Sn3N4. 254x190mm (300 x 300 DPI)


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Fig. 5. Tauc plot of the sample (piece 1) fabricated using the strategy in Fig. 1(d). The inset shows the absorption spectrum obtained using a UV–vis spectrometer. 254x190mm (300 x 300 DPI)



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Fig. 6. X-ray photoelectron spectroscopy (XPS) results. (a) Schematic of the composition spread deposited on a single large piece of FTO/glass substrate. (b) Sn 3d5/2 spectra of location 1 (red curve) and location 4 (green curve). A deconvolution algorithm was applied to location 1. (c) N 1s spectrum of location 1. (d) Zn 2p3/2 spectrum of location 1. 254x190mm (300 x 300 DPI)


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Fig. 7. Current–voltage (I–V) characteristics exhibited by various phases obtained from location 1 of the sample [Fig. 6(a)]. (a) Sn3N4. The inset illustrates the measurement configuration. (b) Mixture of Sn3N4 and Sn. (c) ZTN. (d) Energy band structure evolution. 254x190mm (300 x 300 DPI)



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Fig. 8. Schottky behavior at S1 (a) and S2 (b). Schottky barrier height variation at S1 (c) and S2 (d). 254x190mm (300 x 300 DPI)


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The piezotronic and piezophototronic properties of orthorhombic

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