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Effect of nanowires in microporous structures on the thermoelectric properties of oxidized Sb-doped ZnO film
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
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Original Article
Effect of nanowires in microporous structures on the thermoelectric properties of oxidized Sb-doped ZnO film Guojian Lia, Lin Xiaoa,b, Shiying Liua,b, Huimin Wanga,b, Yang Gaoa,b, Qiang Wanga, a b
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Key Laboratory of Electromagnetic Processing of Materials (Ministry of Education), Northeastern University, Shenyang 110819, China School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Sb-doped ZnO film Oxidization growth Thermoelectric material P-type conduction
Sb-doped ZnO thermoelectric films with microporous structures are fabricated by oxidizing evaporated Zn-Sb thin films in a leaf-like surface. High magnetic field (HMF) and Sb are employed to tune the formation of nanowires and nanorods in the microporous films and conduction type. Nanowire is formed in the film with Sb content of 3.0% and nanorod is formed with 4.6% Sb with the absence of HMF. P-type ZnO films with a wuterzite are formed. The resistivity of the films decreases by two orders of magnitude by increasing Sb content. The resistivity of films decreases 45% and 80% by forming nanowires and nanorods, respectively. The power factor of the nanorod structures increases by two orders of magnitude by comparison with others and reaches to 52.6 μW/m K2. This indicates that the nanorod structures with a higher Sb content are easy to obtain stable ptype semiconductor with a higher power factor.
1. Introduction Thermoelectric materials can convert heat to electricity through the Seebeck effect [1]. They play a key role in solving the problem of waste heat recovery. However, the materials are not widely used because of the low efficiency [2]. It is an extremely challenging task to explore materials with a high thermoelectric performance, which is characterized by a dimensionless figure of merit ZT (=S2σT/κ), where σ is the electrical conductivity, S is the Seebeck coefficient, T is the absolute temperature, and κ is the thermal conductivity [3]. The most effective way to improve the ZT value is to independently optimize the thermal conductivity and the electrical conductivity. The independent optimization of σ and κ is possible in theory because different length scales are associated with phonons and electric charges [4]. Size effect in nanostructure provides an opportunity to significantly increase the efficiency and the ZT value of thermoelectric materials [5–7]. Among various methods, nanosized porosity is a more effective method to reduce the thermal conductivity by taking advantage of the phonon scattering on the porosity boundaries [8]. The thermal conductivity of the nanoporous Bi thin films is ∼6 times smaller than that of bulk materials at room temperature [9]. The thermal conductivity reduced by two orders of magnitude compared with the bulk value when the porosity is up to 64% [10]. The ZT value of the Bi-Te-Sb thin film was enhanced to 1.8 at room temperature by arranging the 20 nm nanopores with an average distance of 50 nm [11]. Although nanosized
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porosity is an efficient method in reducing the thermal conductivity and increasing the Seebeck coefficient, the electrical conductivity sometimes decreases so significantly that the porosity degrades the ZT value [12]. Thus, it is necessary to explore one method to improve the electrical conductivity in the porous thin films in order to achieve much greater ZT value. In this study, nanowires have been formed in porous structures to improve the electrical conductivity of thermoelectric thin films. P-type Sb-doped ZnO thin films were selected as the research objective. This is because ZnO is a low cost, nontoxic and high stable thermoelectric materials [13,14]. In addition, it is generally recognized that homo p-n junction is very important in obtaining high-efficient thermoelectric devices because no additional barrier is needed to overcome the majority charge carriers in forward direction [15]. The intrinsic ZnO is ntype conductivity due to the existence of many donor impurities, such as oxygen vacancies, zinc interstitials, and hydrogen. Thus, the dopant of acceptor impurity in ZnO to form p-type materials is more difficult [16]. The porous structures in the Sb-doped ZnO thin films were formed by oxidizing as-deposited Zn-Sb thin films. This is because the nanowires and porosity can be controlled by tuning the oxidization processes [17]. Because Lorenz force of the high magnetic field (HMF) can suppress the growth of nanowires [18], HMF was employed to tune the existence of nanowires in the porous structures during the growth of the ZnO films in order to define the effect of nanowires on the
Corresponding author. E-mail address: [email protected] (Q. Wang).
https://doi.org/10.1016/j.jeurceramsoc.2017.11.022 Received 19 May 2017; Received in revised form 7 November 2017; Accepted 9 November 2017 0955-2219/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: Li, G., Journal of the European Ceramic Society (2017), http://dx.doi.org/10.1016/j.jeurceramsoc.2017.11.022
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Fig. 1. Schematic diagrams of porous Sb-doped ZnO thin films with (b) and without (c) nanowires prepared by oxidizing as-deposited Zn-Sb (a) thin films.
temperatures increasing, as shown in Fig. 2(b). These morphologies can meet the requirement of oxidization growth of the porous structures. The morphologies of the films have a significant change after the oxidization. The surface of the Sb-doped ZnO films oxidized from the Zn320 °C-Sb465 °C films without HMF (Zn320 °C-Sb465 °C-0T Sbdoped ZnO) was formed by irregular particles. This morphology is one of the typical porous structures in films [20]. The irregular particles are formed by many spherical particles with diameters in the range of 10 ∼ 110 nm. Nanowires exist among these particles. The diameter of the nanowires is about 20 nm. The maximum length is about 3.5 μm. In addition, the Zn350 °C-Sb500 °C-0T Sb-doped ZnO film was formed by nanorods. This is different from that of the Zn320 °C-Sb465 °C case. More detailed reasons should be studied further. However, it can deduce that the structure is strongly related to the surface morphology and the Sb content. The sizes of the rods are uniform. Their diameter and length are respectively 25 and 50 nm. After the HMF was applied, there is no nanowire in the Zn320 °C-Sb465 °C-12T Sb-doped ZnO film. The nanorods in the Zn350 °C-Sb500 °C-12T film disappeared and changed to the structure as the Zn320 °C-Sb465 °C-12T case. However, there are a few nanowires in the films. The structures with and without nanowires in the porous films were formed by the oxidization method with the application of HMF. Zn is easy to be oxidized to ZnO nanowire at the apex. The formation mechanism of these morphologies with a high Sb content is discussed as follows. Previous studies indicate that the oxidation process was divided into three steps [21]. The first is the oxidation of the surface Zn layer. The second is the segregation of the inside metal Zn atoms to the surface. The third is the oxidization of segregated Zn atoms. In this study, the inside Zn atoms segregate to the surface, adsorb oxygen and become ZnO nanowire. However, if Zn atoms segregate to the apex of the surface, it can grow to form nanowire. Similarly, the Sb atoms also segregate to the apex, however, Sb is difficult to become nanowire. This means that the ZnO nanowire did not grow because of the existence of the segregated Sb atoms. That’s to say, the dopant atoms obstacle the adsorption of oxygen and can suppress the growth at the apex. Thus, the nanowires change to nanorods with the Sb content increasing. This leads to the morphology variation with a higher Sb content. In addition, the HMF suppresses the growth of long nanowires. In the fabrication of ZnO by thermal oxidation of metallic Zn-Sb, oxygen in the air is first adsorbed onto the Zn atoms. The neutral Zn atoms then lose electrons and become ions. Finally, the released electrons reduce oxygen molecules to oxygen ions at the metal-oxide interface. Nanowires are formed by the diffusion of oxygen and Zn ion. Application of a 12 T magnetic field causes the diffusion of oxygen ions to be affected by the Lorentz force (qv × B). This force suppresses the growth of ZnO nanowires [18]. In addition, the results of EDS elemental mapping indicate that the irregular particles are formed by Sb-doped ZnO. The compositions were measured by using EDS and XPS, as shown in Table 1. There is a little difference between the EDS and XPS results due to their different measurement accuracies. However, the trends of the composition are similar with changing fabrication conditions. The Sb content increases by the source temperatures increasing. The
thermoelectric properties. Finally, the effect of the nanowires in the microporous structures on the electrical conductivity and thermoelectric parameters was examined. 2. Experimental details As-deposited Zn-Sb thin films were prepared on Si (100) or quartz plates by molecular beam vapor deposition, which was described in detail in our previous study [19]. The substrates were ultrasonically cleaned in acetone and alcohol separately for 15 min and were dried by an argon gun under ultra-high pressure. Work pressure is about 1.0 × 10−4 Pa. Rough surface of the as-deposited films is necessary to achieve porous structures in the oxidized thin film [18]. The as-deposited Zn-Sb thin films in a leaf-like surface morphology were first prepared on the heated substrate (200 °C), as shown in Fig. 1(a). This kind of structure can be formed because the lower melting points of Zn (419 °C) and Sb (631 °C). The thicknesses of all the as-deposited films are around 400 nm. The thickness and Sb content were tuned by the source temperatures. Two source temperatures were selected. The films were grown for 65 min at the source temperatures of Zn (320 °C) and Sb (465 °C) and that for 24 min with the source temperatures of Zn (350 °C) and Sb (500 °C). The films with different source temperatures are respectively named as Zn320 °C-Sb465 °C films and Zn350 °CSb500 °C films. High-purity Zn (99.999%) and Sb (99.999%) particles with a diameter of about 3 mm were used as the source materials. Finally, the as-deposited Zn-Sb thin films were oxidized in open air for 3 h at 400 °C with a heating rate of 5 °C/min in a heat-treatment furnace under a superconducting magnet (JMTD-12T-100). The porous Sbdoped ZnO thin films were obtained by furnace cooling with and without 12 T HMF. The direction of the magnetic field was upward and perpendicular to the substrate. In this study, the formation of nanowires can connect the porous structures, which will increase the channels of the carriers transport. Furthermore, the nanowires always have a low resistivity because of the single crystal structure. Thus, the nanowire formation in porous structures improves the electrical conductivity. Surface morphology was observed using field emission scanning electron microscopy (FESEM; SUPPA 35, Carl Zeiss Inc., Germany). Composition and chemical states were determined by energy-dispersive X-ray spectroscopy (EDS; Inca, Oxford Instruments, Abingdon, UK) and by X-ray photoemission spectroscopy (XPS; ESCALAB 250Xi, Thermo Scientific, USA). Phase formation of the films was examined by X-ray diffraction (XRD; DMAX 2400, Rigaku, Japan) with a grazing incidence of 1° in 2θ mode with monochromatic Cu Kα1 radiation (λ = 0.154056 nm). Electrical resistivity and Seebeck coefficient were measured using Seebeck coefficient/electrical resistance measuring system (ZEM-3, Ulvac-riko Inc., Japan). 3. Results and discussion Fig. 2 gives the surface morphologies of the as-deposited Zn-Sb films and the oxidized Sb-doped ZnO films. It can be seen that the surface morphologies of the as-deposited films are formed by many leaf-like particles. The amount of the leaf-like particles reduced with the source 2
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Fig. 2. Surface morphologies of the as-deposited ZnSb films (a)(b) and the oxidized Sb-doped ZnO films without (c)(d) and with the application of 12 T magnetic field (e)(f). The left column is Zn320 °CSb465 °C films and the right column is Zn350 °CSb500 °C films.
the results of the Zn320 °C-Sb465 °C-0T film, only the TEM images of the Zn320 °C-Sb465 °C-0T and Zn350 °C-Sb500 °C-0T Sb-doped ZnO films were shown. Continuous rings were seen in the selected area electron diffraction of the films. This indicates the polycrystalline structure of the films. In addition, only ZnO rings exist and they are corresponding to the facets of (100), (002), (101), (102), (110), and (103). The facets agree well with the XRD results. However, no rings of Sb2O3 and Zn-Sb related compounds appear in the result due to the limited diffraction area. From the cross-section TEM images, the structures are similar and are formed by the continuous layer and surface particles. The layer with a thickness of about 140 nm was stacked by spherical grains with a diameter of 15 nm. For the Zn320 °CSb465 °C-0T film, the interplanar spacing is consistent with the ZnO results. Meanwhile, there are single crystalline nanowires on the surface. For the Zn350 °C-Sb500 °C-0T film, the nanorods appear on the surface area. In the observed area, the width and length are 20 and 50 nm, respectively. Each nanorod only includes one grain. The nanorod contents are higher than the nanowires in the Zn320 °CSb465 °C-0T film. These results are similar with the SEM results. Thermoelectric parameters of resistivity, Seebeck coefficient, and power factor of the Sb-doped ZnO films under different fabrication conditions were measured using ZEM-3, as shown in Fig. 5. The resistivity of all the films decreases with measurement temperature increasing and it shows a semiconducting behavior. The resistivity of the Zn350 °C-Sb500 °C films decreases by two orders of magnitude by comparison with that of the Zn320 °C-Sb465 °C films. Two possible reasons contribute to this. One is that the Sb content in the Zn350 °CSb500 °C films is higher than that of in the Zn320 °C-Sb465 °C films. More Sb ions provide more carriers. The other is that the surface irregular particles are compact in the Zn350 °C-Sb500 °C films and the carriers are easy to migrate. In addition, compared with the films without nanowires in the Zn320 °C-Sb465 °C films, the resistivity of films with nanowires decreases by 45%. The existence of nanorods decreases resistivity by 80% in the Zn350 °C-Sb500 °C films. The difference in resistivity is mainly due to the crystallinity difference based on the XRD results. The resistivity variation of the films with different
average Sb contents of the Zn320 °C-Sb465 °C and Zn350 °C-Sb500 °C films are about 3.0% and 4.6%, respectively. HMF has no significant effect on the compositions. The uniformity of elemental distributions by using oxidization method in preparing doped ZnO has been proved in our previous study [18]. XRD was used to analyze the phase formation of the as-deposited films and the oxidized doped ZnO films, as shown in Fig. 3. The standard peaks of Zn, ZnO, Zn4Sb3, SbZn, and Sb2O3 are also shown in the figure in order to clarify the phase formations in the films. As far as the as-deposited Zn-Sb films were concerned, the peaks’ positions of the films are similar. The intensity of the Zn350 °C-Sb500 °C film is a little stronger than that of the Zn320 °C-Sb465 °C film because higher source temperatures are easy to enhance the crystallinity of the films. When the source temperature is higher, the evaporated atoms will carry higher energy. This energy will increase the nucleation and growth ability, which results in the enhancement of crystallinity. Meanwhile, most of the peaks are consistent with the standard peaks of wurtzite Zn. Other peaks are attributed to Zn4Sb3. These results indicate that the asdeposited films were mainly formed by the wurtzite Zn and a small amount of the Zn4Sb3. After the films were oxidized, the peak intensity of the Zn320 °C-Sb465 °C-0T film is much stronger than that of other films. The possible reason is much lower degree of crystallinity or smaller grain size in other films. More details should be studied further. However, the peaks positions of all the films are similar. They are consistent with the standard peaks of wurtzite ZnO. No significant shift occurs for the diffraction angle. This result is attributed to a small lattice mismatch between ionic radii of Sb (0.076 nm) and Zn (0.074 nm) [22]. It indicates that Sb ions may systematically substitute Zn ions in the films. Additionally, several weak peaks appeared, such as 42.03°, 47.07°, and 54.48°. Zn- and Sb-based compounds are similar to the compounds of SbZn, Zn4Sb3, and Sb2O3. A small amount of Zn4Sb3 and SbZn exists in the oxidized films because the Zn-Sb compounds are not easy to be oxidized. Thus, the best way to dope Sb is to suppress the formation of the Zn-Sb compounds in the as-deposited films. Cross-section TEM images were used to clarify the internal structures, as shown in Fig. 4. Since the results of 12 T films are similar with
Table 1 Atomic percents of Sb, Zn and O in the as-deposited and Sb-doped ZnO films measured by EDS (left) and XPS (right). at.%
As-deposited film
Sb-doped ZnO
Zn320 °C-Sb465 °C
Zn350 °C-Sb500 °C
Zn320 °C-Sb465 °C 0T
Sb Zn O Method
9.2 90.8 – EDS
12.3 87.7 – XPS
14.8 85.2 – EDS
12.5 87.5 – XPS
Zn350 ºC-Sb500 ºC 12 T
3.1 37.4 59.5 EDS
3
3.2 45.4 51.4 XPS
2.6 36.0 61.4 EDS
0T 3.2 52.1 44.7 XPS
4.0 35.6 60.4 EDS
12 T 4.8 49.8 45.4 XPS
4.0 33.4 62.6 EDS
5.6 43.1 51.3 XPS
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Fig. 3. XRD results of the as-deposited Zn-Sb films (a) and the Sb-doped ZnO films (b). Standard peaks of Zn, ZnO, Zn4Sb3, SbZn, and Sb2O3 are shown in the figure.
high temperature. The transformation from p-type to n-type is considered to be due to the excitation of n-type carrier with the increase of temperature. In the sample of low Sb concentration, accepter concentration is lower and is easy to excite n-type carrier at high temperature. The S decreases significantly with the application of HMF. This means that the higher Sb content is inclined to obtain p-type semiconductor by using oxidization method. HMF inhibited the nanorods growth [18]. Thus, nanowires with 4.6% Sb oxidized under HMF become thin and long. For the Sb-doped ZnO，Seebeck coefficient decreased with the decrease of the nanowire diameter [25]. For materials containing both electrons and holes, Seebeck coefficient S can be expressed as an average S value of the individual bands [26]. The total coefficient is given by Stotal = (σeSe + σhSh)/(σe + σh), where the subscripts e and h respectively refer to electrons and holes; σ is the electric conductivity. The Zn350 °C-Sb500 °C-0T film is p-type conductivity. According to the XPS results, when films were oxidized under HMF, the Sb5+ content (hole concentration) increases, which leads to the decrease of the Sh value. Meanwhile, the decrease of carrier concentration diminishes the σh, which decreasing the Stotal. The power factor (PF) of the p-type Zn350 °C-Sb500 °C films is much higher than that of the ntype Zn320 °C-Sb465 °C films. The former PF increases by two orders of magnitude by comparison with the latter. This indicates that the nanorod structure with a higher Sb content can easily obtain p-type
morphologies such as nanowires and nanorods was analyzed according to previous studies [21,23]. High surface-to-volume ratio of nanobelts elevated the number of surface atoms, which enhanced the scattering (hence decreasing the carrier mean free path), and thus reducing conductivity. Meanwhile, nanowires have also been shown to support correlated electron transport. Both two factors change the resistivity of the films. However, the effect of the nanowires on the electrical mobility is needed to be studied further in the case of the films with same crystallinity. In addition, the resistivity shows weak temperature dependence in this study, particularly films grown at 500 °C. This case may be related to the porous structures. The resistivity of the Zn350 °CSb500 °C-0T film reduces to 4.13 × 10−4 Ω m from 5.34 × 10−4 Ω m with the increase of temperature to 250 °C from 50 °C. The reduction of resistivity is ∼22.6%, which is similar with the result of the Ag-doped ZnO [24]. The Seebeck coefficients decrease as measurement temperatures increase. The positive Seebeck coefficient of the Zn350 °C-Sb500 °C films indicates that the conductivity of the p-type films. Furthermore, the Seebeck coefficient decreases by 76.5% with the application of HMF. However, the variation of Seebeck coefficients of the Zn320 °CSb465 °C films is different from the case of the Zn350 °C-Sb500 °C films in terms of temperature. The positive S at low temperature changes to negative. This means that the conductivity type changes to n-type at a
Fig. 4. Cross-section TEM (left) and HRTEM (right) images of the Zn320 °C-Sb465 °C-0T (a)(b) and Zn350 °C-Sb500 °C-0T (c)(d) oxidized Sb-doped ZnO films. The insert images are selected area electron diffraction.
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Fig. 5. Thermoelectric parameters of resistivity (a), Seebeck coefficient (b), and power factor (c) of the Sb-doped ZnO films with different fabrication conditions.
to that of O 1s., which is different from the results of other studies [29–31]. The overlap of binding energies between Sb 3d5/2 and O 1 s is attributed to the formation of Sb-O bonding indicating the substitution of Sb into the ZnO lattice. The difference in binding energy between Sb 3d5/2 and 3d3/2 is 9.3 eV. This is different from the pure Sb (9.35 eV). XRD results showed that small amount of Zn-Sb compound exists in the film, resulting in the decrease of binding energy of Sb 3d [32]. In addition, the peak which is attributed to the oxidation states of Sb appears at the binding energy of 539.6 eV according to previous study [33]. However, it can be seen that no peak of Sb oxide exists in the Zn320 °CSb465 °C-12T and Zn350 °C-Sb500 °C-0T films. This means that HMF plays different roles in the oxidization growth of the Zn-Sb films with different surface morphologies.
semiconductor with a higher PF. Nanorods may play an important role in the enhancement of power factor. However, the real reason should be studied further in the future. If quantum confinement improves the thermopower, both nanorod structures and nanowire structures should show same effect. The main difference between the high and low Sb contents is the amount of nanostructure in the films. The film was mainly formed by nanorods with a higher Sb content, while the nanowires only exist in the porosity in the low Sb content. Thus, the amount of nanostructure may result in the enhancement of the thermopower. As far as the effect of the second phase on the thermoelectric properties was concerned, the Zn-Sb phase, especially the β-phase Zn4Sb3 will certainly affect the thermoelectric properties because power factors are higher than that of ZnO. However, the content of the Zn-Sb phase is very small in this study. It is difficult to define the influence of Zn-Sb phase on the thermoelectric properties in details. We repeatedly measure the power factor of the same sample one year later. The results of the power factor of the Zn350 °C-Sb500 °C-0T film are shown in Fig. 5(c). It can be seen that the measurement of seven times are similar. After the sample is deposited in an open air for one year, values decrease due to the change of the morphologies of nanorods. However, sample values are similar and are much higher than those of other three films. That is to say, the sample has a good thermal stability. XPS measurements were carried out to investigate the chemical states of elements inside the Sb-doped ZnO lattice structure. We measured the existence of ZnO in the depth. The ZnO distributes uniformly inside the film in the depth. Fig. 6 shows the XPS spectra of the Sbdoped ZnO films at the etching time of 1200 s. The Zn 2p peaks split into the Zn 2p1/2 and Zn 2p3/2 lines at 1044.7 and 1021.7 eV, respectively. The difference in binding energy between these two peaks is 23.0 eV which agreed well with the standard value of ZnO (23.0 eV) [27]. This means that ZnO was mainly formed in the films. The electronic states of Sb 3d5/2, O 1 s and Sb 3d3/2 are observed at 527.6, 530.6 and 536.9 eV, respectively. The binding energy of O 1 s is similar with other results [28]. However, the binding energy of Sb 3d5/2 is not close
4. Conclusions This study found that microporous structures were fabricated by oxidizing evaporated Zn-Sb thin films in a leaf-like surface. The formation of nanowires or nanorods and conduction type were affected by the application of HMF and the change of Sb contents. 3.0% Sb formed nanowires and 4.6% Sb for nanorods with the absence of HMF. The films with 4.6% Sb remained p-type conduction with the rise of temperature. However, the p-type transformed to n-type with 3.0% Sb as temperature increases because the Sb3+ ion became Sb5+ ion. The resistivity decreased by two orders of magnitude with the increase of Sb content from 3.0% to 4.6%. The resistivity of films decreased by 45% and 80% by forming nanowires and nanorods, respectively. The power factor of nanorod structures increased by two orders of magnitude by comparison with others and reached to 52.6 μW/m K2. This study gives a method to obtain stable p-type semiconductor with a higher power factor.
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Fig. 6. Zn 2p spectra (a), O 1 s and Sb 3d (b) of the Sb-doped ZnO films with different fabrication conditions of (I) Zn320 °C-Sb465 °C-0T; (II) Zn320 °CSb465 °C-12T; (III) Zn350 °C-Sb500 °C-0T; and (IV) Zn350 °C-Sb500 °C-12T.
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This work was supported by the National Natural Science Foundation of China, China (Grant Nos. 51425401 and 51690161) and the Fundamental Research Funds for the Central Universities (Grant Nos. N160905001 and N160907001), and General Project of Science and Technology Education Department of Liaoning Province (No. L2014091). References [1] V. Zlatic, A.C. Hewson, Properties and Application of Thermoelectric Materials, Springer Dordrecht, Netherlands, 2008. [2] G. Chen, M.S. Dresselhaus, G. Dresselhaus, J.P. Fleurial, T. Caillat, Recent developments in thermoelectric materials, Int. Mater. Rev. 48 (1) (2003) 45–66. [3] G.J. Snyder, E.S. Toberer, Complex thermoelectric materials, Nat. Mater. 7 (2008) 105–114. [4] C. Wood, Materials for thermoelectric energy conversion, Rep. Prog. Phys. 51 (4) (1988) 459–539. [5] P. Pichanusakorn, P. Bandaru, Nanostructured thermoelectrics, Mater. Sci. Eng. R 67 (2010) 19–63. [6] K.P. Ghatak, S. Bhattacharya, Thermoelectric Power in Nanostructured Materials, Springer-Verlag, Berlin Heidelberg, 2010. [7] R. Venkatasubramanian, E. Silvola, T. Colpitts, B. O’Quinn, Thin-film thermoelectric devices with high room-temperature figures of merit, Nature 413 (2001) 597–602. [8] J.K. Yu, S. Mitrovic, D. Tham, J. Varghese, J.R. Heath, Reduction of thermal conductivity in phononic nanomesh structures, Nat. Nanotech. 5 (2010) 718–721. [9] N.W. Park, W.Y. Lee, T.H. Park, D.J. Kim, S.H. Cho, S.Y. Lee, S.K. Lee, Temperaturedependent thermal conductivity of nanoporous Bi thin films by controlling pore size and porosity, J. Alloy. Compd. 639 (2015) 289–295. [10] G.H. Tang, C. Bi, B. Fu, Thermal conduction in nano-porous silicon thin film, J. Appl. Phys. 114 (18) (2013) 184302. [11] M.S. Kashiwagi, K. Hirata, Y. Harada, K. Zheng, M. Miyazaki, C. Hahiro Adachi, Enhanced figure of merit of porous thin film of bismuth antimony telluride, Appl. Phys. Lett. 98 (2011) 023114. [12] H. Lee, D. Vashaee, D.Z. Wang, M.S. Dresselhaus, Z.F. Ren, G. Chen, Effects of nanoscale porosity on thermoelectric properties of SiGe, J.Appl. Phys. 107 (9) (2010) 094308. [13] S. Walia, S. Balendhran, H. Nili, S. Zhuiykov, G. Rosengarten, Q.H. Wang, M. Bhaskaran, S. Sriram, M.S. Strano, K. Kalantar-zadeh, Transition metal oxides –thermoelectric propertie, Prog. Mater. Sci. 58 (2013) 1443–1489. [14] K.P. Ong, D.J. Singh, P. Wu, Analysis of the thermoelectric properties of n-type ZnO, Phys. Rev. B 83 (2011) 115110. [15] F. Schuster, B. Laumer, R.R. Zamani, C. Magen, J.R. Morante, J. Arbiol, M. Stutzmann, p-GaN/n-ZnO heterojunction nanowires: optoelectronic properties and the role of interface polarity, ACS Nano 8 (5) (2014) 4376–4384.
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Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Original Article
Effect of nanowires in microporous structures on the thermoelectric properties of oxidized Sb-doped ZnO film Guojian Lia, Lin Xiaoa,b, Shiying Liua,b, Huimin Wanga,b, Yang Gaoa,b, Qiang Wanga, a b
⁎
Key Laboratory of Electromagnetic Processing of Materials (Ministry of Education), Northeastern University, Shenyang 110819, China School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Sb-doped ZnO film Oxidization growth Thermoelectric material P-type conduction
Sb-doped ZnO thermoelectric films with microporous structures are fabricated by oxidizing evaporated Zn-Sb thin films in a leaf-like surface. High magnetic field (HMF) and Sb are employed to tune the formation of nanowires and nanorods in the microporous films and conduction type. Nanowire is formed in the film with Sb content of 3.0% and nanorod is formed with 4.6% Sb with the absence of HMF. P-type ZnO films with a wuterzite are formed. The resistivity of the films decreases by two orders of magnitude by increasing Sb content. The resistivity of films decreases 45% and 80% by forming nanowires and nanorods, respectively. The power factor of the nanorod structures increases by two orders of magnitude by comparison with others and reaches to 52.6 μW/m K2. This indicates that the nanorod structures with a higher Sb content are easy to obtain stable ptype semiconductor with a higher power factor.
1. Introduction Thermoelectric materials can convert heat to electricity through the Seebeck effect [1]. They play a key role in solving the problem of waste heat recovery. However, the materials are not widely used because of the low efficiency [2]. It is an extremely challenging task to explore materials with a high thermoelectric performance, which is characterized by a dimensionless figure of merit ZT (=S2σT/κ), where σ is the electrical conductivity, S is the Seebeck coefficient, T is the absolute temperature, and κ is the thermal conductivity [3]. The most effective way to improve the ZT value is to independently optimize the thermal conductivity and the electrical conductivity. The independent optimization of σ and κ is possible in theory because different length scales are associated with phonons and electric charges [4]. Size effect in nanostructure provides an opportunity to significantly increase the efficiency and the ZT value of thermoelectric materials [5–7]. Among various methods, nanosized porosity is a more effective method to reduce the thermal conductivity by taking advantage of the phonon scattering on the porosity boundaries [8]. The thermal conductivity of the nanoporous Bi thin films is ∼6 times smaller than that of bulk materials at room temperature [9]. The thermal conductivity reduced by two orders of magnitude compared with the bulk value when the porosity is up to 64% [10]. The ZT value of the Bi-Te-Sb thin film was enhanced to 1.8 at room temperature by arranging the 20 nm nanopores with an average distance of 50 nm [11]. Although nanosized
⁎
porosity is an efficient method in reducing the thermal conductivity and increasing the Seebeck coefficient, the electrical conductivity sometimes decreases so significantly that the porosity degrades the ZT value [12]. Thus, it is necessary to explore one method to improve the electrical conductivity in the porous thin films in order to achieve much greater ZT value. In this study, nanowires have been formed in porous structures to improve the electrical conductivity of thermoelectric thin films. P-type Sb-doped ZnO thin films were selected as the research objective. This is because ZnO is a low cost, nontoxic and high stable thermoelectric materials [13,14]. In addition, it is generally recognized that homo p-n junction is very important in obtaining high-efficient thermoelectric devices because no additional barrier is needed to overcome the majority charge carriers in forward direction [15]. The intrinsic ZnO is ntype conductivity due to the existence of many donor impurities, such as oxygen vacancies, zinc interstitials, and hydrogen. Thus, the dopant of acceptor impurity in ZnO to form p-type materials is more difficult [16]. The porous structures in the Sb-doped ZnO thin films were formed by oxidizing as-deposited Zn-Sb thin films. This is because the nanowires and porosity can be controlled by tuning the oxidization processes [17]. Because Lorenz force of the high magnetic field (HMF) can suppress the growth of nanowires [18], HMF was employed to tune the existence of nanowires in the porous structures during the growth of the ZnO films in order to define the effect of nanowires on the
Corresponding author. E-mail address: [email protected] (Q. Wang).
https://doi.org/10.1016/j.jeurceramsoc.2017.11.022 Received 19 May 2017; Received in revised form 7 November 2017; Accepted 9 November 2017 0955-2219/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: Li, G., Journal of the European Ceramic Society (2017), http://dx.doi.org/10.1016/j.jeurceramsoc.2017.11.022
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Fig. 1. Schematic diagrams of porous Sb-doped ZnO thin films with (b) and without (c) nanowires prepared by oxidizing as-deposited Zn-Sb (a) thin films.
temperatures increasing, as shown in Fig. 2(b). These morphologies can meet the requirement of oxidization growth of the porous structures. The morphologies of the films have a significant change after the oxidization. The surface of the Sb-doped ZnO films oxidized from the Zn320 °C-Sb465 °C films without HMF (Zn320 °C-Sb465 °C-0T Sbdoped ZnO) was formed by irregular particles. This morphology is one of the typical porous structures in films [20]. The irregular particles are formed by many spherical particles with diameters in the range of 10 ∼ 110 nm. Nanowires exist among these particles. The diameter of the nanowires is about 20 nm. The maximum length is about 3.5 μm. In addition, the Zn350 °C-Sb500 °C-0T Sb-doped ZnO film was formed by nanorods. This is different from that of the Zn320 °C-Sb465 °C case. More detailed reasons should be studied further. However, it can deduce that the structure is strongly related to the surface morphology and the Sb content. The sizes of the rods are uniform. Their diameter and length are respectively 25 and 50 nm. After the HMF was applied, there is no nanowire in the Zn320 °C-Sb465 °C-12T Sb-doped ZnO film. The nanorods in the Zn350 °C-Sb500 °C-12T film disappeared and changed to the structure as the Zn320 °C-Sb465 °C-12T case. However, there are a few nanowires in the films. The structures with and without nanowires in the porous films were formed by the oxidization method with the application of HMF. Zn is easy to be oxidized to ZnO nanowire at the apex. The formation mechanism of these morphologies with a high Sb content is discussed as follows. Previous studies indicate that the oxidation process was divided into three steps [21]. The first is the oxidation of the surface Zn layer. The second is the segregation of the inside metal Zn atoms to the surface. The third is the oxidization of segregated Zn atoms. In this study, the inside Zn atoms segregate to the surface, adsorb oxygen and become ZnO nanowire. However, if Zn atoms segregate to the apex of the surface, it can grow to form nanowire. Similarly, the Sb atoms also segregate to the apex, however, Sb is difficult to become nanowire. This means that the ZnO nanowire did not grow because of the existence of the segregated Sb atoms. That’s to say, the dopant atoms obstacle the adsorption of oxygen and can suppress the growth at the apex. Thus, the nanowires change to nanorods with the Sb content increasing. This leads to the morphology variation with a higher Sb content. In addition, the HMF suppresses the growth of long nanowires. In the fabrication of ZnO by thermal oxidation of metallic Zn-Sb, oxygen in the air is first adsorbed onto the Zn atoms. The neutral Zn atoms then lose electrons and become ions. Finally, the released electrons reduce oxygen molecules to oxygen ions at the metal-oxide interface. Nanowires are formed by the diffusion of oxygen and Zn ion. Application of a 12 T magnetic field causes the diffusion of oxygen ions to be affected by the Lorentz force (qv × B). This force suppresses the growth of ZnO nanowires [18]. In addition, the results of EDS elemental mapping indicate that the irregular particles are formed by Sb-doped ZnO. The compositions were measured by using EDS and XPS, as shown in Table 1. There is a little difference between the EDS and XPS results due to their different measurement accuracies. However, the trends of the composition are similar with changing fabrication conditions. The Sb content increases by the source temperatures increasing. The
thermoelectric properties. Finally, the effect of the nanowires in the microporous structures on the electrical conductivity and thermoelectric parameters was examined. 2. Experimental details As-deposited Zn-Sb thin films were prepared on Si (100) or quartz plates by molecular beam vapor deposition, which was described in detail in our previous study [19]. The substrates were ultrasonically cleaned in acetone and alcohol separately for 15 min and were dried by an argon gun under ultra-high pressure. Work pressure is about 1.0 × 10−4 Pa. Rough surface of the as-deposited films is necessary to achieve porous structures in the oxidized thin film [18]. The as-deposited Zn-Sb thin films in a leaf-like surface morphology were first prepared on the heated substrate (200 °C), as shown in Fig. 1(a). This kind of structure can be formed because the lower melting points of Zn (419 °C) and Sb (631 °C). The thicknesses of all the as-deposited films are around 400 nm. The thickness and Sb content were tuned by the source temperatures. Two source temperatures were selected. The films were grown for 65 min at the source temperatures of Zn (320 °C) and Sb (465 °C) and that for 24 min with the source temperatures of Zn (350 °C) and Sb (500 °C). The films with different source temperatures are respectively named as Zn320 °C-Sb465 °C films and Zn350 °CSb500 °C films. High-purity Zn (99.999%) and Sb (99.999%) particles with a diameter of about 3 mm were used as the source materials. Finally, the as-deposited Zn-Sb thin films were oxidized in open air for 3 h at 400 °C with a heating rate of 5 °C/min in a heat-treatment furnace under a superconducting magnet (JMTD-12T-100). The porous Sbdoped ZnO thin films were obtained by furnace cooling with and without 12 T HMF. The direction of the magnetic field was upward and perpendicular to the substrate. In this study, the formation of nanowires can connect the porous structures, which will increase the channels of the carriers transport. Furthermore, the nanowires always have a low resistivity because of the single crystal structure. Thus, the nanowire formation in porous structures improves the electrical conductivity. Surface morphology was observed using field emission scanning electron microscopy (FESEM; SUPPA 35, Carl Zeiss Inc., Germany). Composition and chemical states were determined by energy-dispersive X-ray spectroscopy (EDS; Inca, Oxford Instruments, Abingdon, UK) and by X-ray photoemission spectroscopy (XPS; ESCALAB 250Xi, Thermo Scientific, USA). Phase formation of the films was examined by X-ray diffraction (XRD; DMAX 2400, Rigaku, Japan) with a grazing incidence of 1° in 2θ mode with monochromatic Cu Kα1 radiation (λ = 0.154056 nm). Electrical resistivity and Seebeck coefficient were measured using Seebeck coefficient/electrical resistance measuring system (ZEM-3, Ulvac-riko Inc., Japan). 3. Results and discussion Fig. 2 gives the surface morphologies of the as-deposited Zn-Sb films and the oxidized Sb-doped ZnO films. It can be seen that the surface morphologies of the as-deposited films are formed by many leaf-like particles. The amount of the leaf-like particles reduced with the source 2
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Fig. 2. Surface morphologies of the as-deposited ZnSb films (a)(b) and the oxidized Sb-doped ZnO films without (c)(d) and with the application of 12 T magnetic field (e)(f). The left column is Zn320 °CSb465 °C films and the right column is Zn350 °CSb500 °C films.
the results of the Zn320 °C-Sb465 °C-0T film, only the TEM images of the Zn320 °C-Sb465 °C-0T and Zn350 °C-Sb500 °C-0T Sb-doped ZnO films were shown. Continuous rings were seen in the selected area electron diffraction of the films. This indicates the polycrystalline structure of the films. In addition, only ZnO rings exist and they are corresponding to the facets of (100), (002), (101), (102), (110), and (103). The facets agree well with the XRD results. However, no rings of Sb2O3 and Zn-Sb related compounds appear in the result due to the limited diffraction area. From the cross-section TEM images, the structures are similar and are formed by the continuous layer and surface particles. The layer with a thickness of about 140 nm was stacked by spherical grains with a diameter of 15 nm. For the Zn320 °CSb465 °C-0T film, the interplanar spacing is consistent with the ZnO results. Meanwhile, there are single crystalline nanowires on the surface. For the Zn350 °C-Sb500 °C-0T film, the nanorods appear on the surface area. In the observed area, the width and length are 20 and 50 nm, respectively. Each nanorod only includes one grain. The nanorod contents are higher than the nanowires in the Zn320 °CSb465 °C-0T film. These results are similar with the SEM results. Thermoelectric parameters of resistivity, Seebeck coefficient, and power factor of the Sb-doped ZnO films under different fabrication conditions were measured using ZEM-3, as shown in Fig. 5. The resistivity of all the films decreases with measurement temperature increasing and it shows a semiconducting behavior. The resistivity of the Zn350 °C-Sb500 °C films decreases by two orders of magnitude by comparison with that of the Zn320 °C-Sb465 °C films. Two possible reasons contribute to this. One is that the Sb content in the Zn350 °CSb500 °C films is higher than that of in the Zn320 °C-Sb465 °C films. More Sb ions provide more carriers. The other is that the surface irregular particles are compact in the Zn350 °C-Sb500 °C films and the carriers are easy to migrate. In addition, compared with the films without nanowires in the Zn320 °C-Sb465 °C films, the resistivity of films with nanowires decreases by 45%. The existence of nanorods decreases resistivity by 80% in the Zn350 °C-Sb500 °C films. The difference in resistivity is mainly due to the crystallinity difference based on the XRD results. The resistivity variation of the films with different
average Sb contents of the Zn320 °C-Sb465 °C and Zn350 °C-Sb500 °C films are about 3.0% and 4.6%, respectively. HMF has no significant effect on the compositions. The uniformity of elemental distributions by using oxidization method in preparing doped ZnO has been proved in our previous study [18]. XRD was used to analyze the phase formation of the as-deposited films and the oxidized doped ZnO films, as shown in Fig. 3. The standard peaks of Zn, ZnO, Zn4Sb3, SbZn, and Sb2O3 are also shown in the figure in order to clarify the phase formations in the films. As far as the as-deposited Zn-Sb films were concerned, the peaks’ positions of the films are similar. The intensity of the Zn350 °C-Sb500 °C film is a little stronger than that of the Zn320 °C-Sb465 °C film because higher source temperatures are easy to enhance the crystallinity of the films. When the source temperature is higher, the evaporated atoms will carry higher energy. This energy will increase the nucleation and growth ability, which results in the enhancement of crystallinity. Meanwhile, most of the peaks are consistent with the standard peaks of wurtzite Zn. Other peaks are attributed to Zn4Sb3. These results indicate that the asdeposited films were mainly formed by the wurtzite Zn and a small amount of the Zn4Sb3. After the films were oxidized, the peak intensity of the Zn320 °C-Sb465 °C-0T film is much stronger than that of other films. The possible reason is much lower degree of crystallinity or smaller grain size in other films. More details should be studied further. However, the peaks positions of all the films are similar. They are consistent with the standard peaks of wurtzite ZnO. No significant shift occurs for the diffraction angle. This result is attributed to a small lattice mismatch between ionic radii of Sb (0.076 nm) and Zn (0.074 nm) [22]. It indicates that Sb ions may systematically substitute Zn ions in the films. Additionally, several weak peaks appeared, such as 42.03°, 47.07°, and 54.48°. Zn- and Sb-based compounds are similar to the compounds of SbZn, Zn4Sb3, and Sb2O3. A small amount of Zn4Sb3 and SbZn exists in the oxidized films because the Zn-Sb compounds are not easy to be oxidized. Thus, the best way to dope Sb is to suppress the formation of the Zn-Sb compounds in the as-deposited films. Cross-section TEM images were used to clarify the internal structures, as shown in Fig. 4. Since the results of 12 T films are similar with
Table 1 Atomic percents of Sb, Zn and O in the as-deposited and Sb-doped ZnO films measured by EDS (left) and XPS (right). at.%
As-deposited film
Sb-doped ZnO
Zn320 °C-Sb465 °C
Zn350 °C-Sb500 °C
Zn320 °C-Sb465 °C 0T
Sb Zn O Method
9.2 90.8 – EDS
12.3 87.7 – XPS
14.8 85.2 – EDS
12.5 87.5 – XPS
Zn350 ºC-Sb500 ºC 12 T
3.1 37.4 59.5 EDS
3
3.2 45.4 51.4 XPS
2.6 36.0 61.4 EDS
0T 3.2 52.1 44.7 XPS
4.0 35.6 60.4 EDS
12 T 4.8 49.8 45.4 XPS
4.0 33.4 62.6 EDS
5.6 43.1 51.3 XPS
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Fig. 3. XRD results of the as-deposited Zn-Sb films (a) and the Sb-doped ZnO films (b). Standard peaks of Zn, ZnO, Zn4Sb3, SbZn, and Sb2O3 are shown in the figure.
high temperature. The transformation from p-type to n-type is considered to be due to the excitation of n-type carrier with the increase of temperature. In the sample of low Sb concentration, accepter concentration is lower and is easy to excite n-type carrier at high temperature. The S decreases significantly with the application of HMF. This means that the higher Sb content is inclined to obtain p-type semiconductor by using oxidization method. HMF inhibited the nanorods growth [18]. Thus, nanowires with 4.6% Sb oxidized under HMF become thin and long. For the Sb-doped ZnO，Seebeck coefficient decreased with the decrease of the nanowire diameter [25]. For materials containing both electrons and holes, Seebeck coefficient S can be expressed as an average S value of the individual bands [26]. The total coefficient is given by Stotal = (σeSe + σhSh)/(σe + σh), where the subscripts e and h respectively refer to electrons and holes; σ is the electric conductivity. The Zn350 °C-Sb500 °C-0T film is p-type conductivity. According to the XPS results, when films were oxidized under HMF, the Sb5+ content (hole concentration) increases, which leads to the decrease of the Sh value. Meanwhile, the decrease of carrier concentration diminishes the σh, which decreasing the Stotal. The power factor (PF) of the p-type Zn350 °C-Sb500 °C films is much higher than that of the ntype Zn320 °C-Sb465 °C films. The former PF increases by two orders of magnitude by comparison with the latter. This indicates that the nanorod structure with a higher Sb content can easily obtain p-type
morphologies such as nanowires and nanorods was analyzed according to previous studies [21,23]. High surface-to-volume ratio of nanobelts elevated the number of surface atoms, which enhanced the scattering (hence decreasing the carrier mean free path), and thus reducing conductivity. Meanwhile, nanowires have also been shown to support correlated electron transport. Both two factors change the resistivity of the films. However, the effect of the nanowires on the electrical mobility is needed to be studied further in the case of the films with same crystallinity. In addition, the resistivity shows weak temperature dependence in this study, particularly films grown at 500 °C. This case may be related to the porous structures. The resistivity of the Zn350 °CSb500 °C-0T film reduces to 4.13 × 10−4 Ω m from 5.34 × 10−4 Ω m with the increase of temperature to 250 °C from 50 °C. The reduction of resistivity is ∼22.6%, which is similar with the result of the Ag-doped ZnO [24]. The Seebeck coefficients decrease as measurement temperatures increase. The positive Seebeck coefficient of the Zn350 °C-Sb500 °C films indicates that the conductivity of the p-type films. Furthermore, the Seebeck coefficient decreases by 76.5% with the application of HMF. However, the variation of Seebeck coefficients of the Zn320 °CSb465 °C films is different from the case of the Zn350 °C-Sb500 °C films in terms of temperature. The positive S at low temperature changes to negative. This means that the conductivity type changes to n-type at a
Fig. 4. Cross-section TEM (left) and HRTEM (right) images of the Zn320 °C-Sb465 °C-0T (a)(b) and Zn350 °C-Sb500 °C-0T (c)(d) oxidized Sb-doped ZnO films. The insert images are selected area electron diffraction.
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Fig. 5. Thermoelectric parameters of resistivity (a), Seebeck coefficient (b), and power factor (c) of the Sb-doped ZnO films with different fabrication conditions.
to that of O 1s., which is different from the results of other studies [29–31]. The overlap of binding energies between Sb 3d5/2 and O 1 s is attributed to the formation of Sb-O bonding indicating the substitution of Sb into the ZnO lattice. The difference in binding energy between Sb 3d5/2 and 3d3/2 is 9.3 eV. This is different from the pure Sb (9.35 eV). XRD results showed that small amount of Zn-Sb compound exists in the film, resulting in the decrease of binding energy of Sb 3d [32]. In addition, the peak which is attributed to the oxidation states of Sb appears at the binding energy of 539.6 eV according to previous study [33]. However, it can be seen that no peak of Sb oxide exists in the Zn320 °CSb465 °C-12T and Zn350 °C-Sb500 °C-0T films. This means that HMF plays different roles in the oxidization growth of the Zn-Sb films with different surface morphologies.
semiconductor with a higher PF. Nanorods may play an important role in the enhancement of power factor. However, the real reason should be studied further in the future. If quantum confinement improves the thermopower, both nanorod structures and nanowire structures should show same effect. The main difference between the high and low Sb contents is the amount of nanostructure in the films. The film was mainly formed by nanorods with a higher Sb content, while the nanowires only exist in the porosity in the low Sb content. Thus, the amount of nanostructure may result in the enhancement of the thermopower. As far as the effect of the second phase on the thermoelectric properties was concerned, the Zn-Sb phase, especially the β-phase Zn4Sb3 will certainly affect the thermoelectric properties because power factors are higher than that of ZnO. However, the content of the Zn-Sb phase is very small in this study. It is difficult to define the influence of Zn-Sb phase on the thermoelectric properties in details. We repeatedly measure the power factor of the same sample one year later. The results of the power factor of the Zn350 °C-Sb500 °C-0T film are shown in Fig. 5(c). It can be seen that the measurement of seven times are similar. After the sample is deposited in an open air for one year, values decrease due to the change of the morphologies of nanorods. However, sample values are similar and are much higher than those of other three films. That is to say, the sample has a good thermal stability. XPS measurements were carried out to investigate the chemical states of elements inside the Sb-doped ZnO lattice structure. We measured the existence of ZnO in the depth. The ZnO distributes uniformly inside the film in the depth. Fig. 6 shows the XPS spectra of the Sbdoped ZnO films at the etching time of 1200 s. The Zn 2p peaks split into the Zn 2p1/2 and Zn 2p3/2 lines at 1044.7 and 1021.7 eV, respectively. The difference in binding energy between these two peaks is 23.0 eV which agreed well with the standard value of ZnO (23.0 eV) [27]. This means that ZnO was mainly formed in the films. The electronic states of Sb 3d5/2, O 1 s and Sb 3d3/2 are observed at 527.6, 530.6 and 536.9 eV, respectively. The binding energy of O 1 s is similar with other results [28]. However, the binding energy of Sb 3d5/2 is not close
4. Conclusions This study found that microporous structures were fabricated by oxidizing evaporated Zn-Sb thin films in a leaf-like surface. The formation of nanowires or nanorods and conduction type were affected by the application of HMF and the change of Sb contents. 3.0% Sb formed nanowires and 4.6% Sb for nanorods with the absence of HMF. The films with 4.6% Sb remained p-type conduction with the rise of temperature. However, the p-type transformed to n-type with 3.0% Sb as temperature increases because the Sb3+ ion became Sb5+ ion. The resistivity decreased by two orders of magnitude with the increase of Sb content from 3.0% to 4.6%. The resistivity of films decreased by 45% and 80% by forming nanowires and nanorods, respectively. The power factor of nanorod structures increased by two orders of magnitude by comparison with others and reached to 52.6 μW/m K2. This study gives a method to obtain stable p-type semiconductor with a higher power factor.
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Fig. 6. Zn 2p spectra (a), O 1 s and Sb 3d (b) of the Sb-doped ZnO films with different fabrication conditions of (I) Zn320 °C-Sb465 °C-0T; (II) Zn320 °CSb465 °C-12T; (III) Zn350 °C-Sb500 °C-0T; and (IV) Zn350 °C-Sb500 °C-12T.
Acknowledgements
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