- Email: [email protected]
GZO tri-layer films by introducing H2 into sputtering atmosphere
Superlattices and Microstructures 140 (2020) 106456
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Improvement of transparent conductive properties of GZO/Cu/ GZO tri-layer films by introducing H2 into sputtering atmosphere B.L. Zhu a, *, J.M. Ma a, K. Lv a, C.J. Wang a, J. Wu a, Z.H. Gan a, J. Liu a, X.W. Shi b a
The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan, 430081, People’s Republic of China Key Laboratory of Material Physics of Ministry of Education, Zhengzhou University, Zhengzhou, 450052, People’s Republic of China
b
A R T I C L E I N F O
A B S T R A C T
Keywords: TCO/metal/TCO tri-layer films Cu films Hydrogenated ZnO-based films Transparent conductive properties Figure of merit (FOM) Bandgap (Eg)
For preparation of transparent conductive oxide (TCO)/metal/TCO tri-layer films, Cu and Gadoped ZnO (GZO) films were respectively used as the metal and TCO layers and were depos ited by magnetron sputtering at room temperature. The effect of film thickness on transparent conductive properties of single Cu films was first investigated in order to better understand the properties of tri-layer films. The results show that the resistivity, sheet resistance (Rs), average transmittance in visible light range (TVis), and figure of merit (FOM) of Cu films decrease with increasing film thickness. The investigations of effect of H2 flux on the transparent conductive properties of GZO films show that hydrogenated GZO (HGZO) films have lower resistivity and higher TVis than those of GZO films. On basis of the investigation of single Cu and GZO films, the GZO/Cu/GZO (GcG) and HGZO/Cu/HGZO (HcH) tri-layer films were prepared, and their trans parent conductive properties were investigated as a function of Cu and GZO (or HGZO) layer thickness. The results show that these two kinds of tri-layer films have the best FOM at Cu layer thickness of 7 nm and GZO (or HGZO) layer thickness of 40 nm. Compared with GcG tri-layer films, HcH tri-layer films have better FOM, indicating that introducing H2 into sputtering at mosphere plays a significant role on the transparent conductive properties of the tri-layer films. In addition, bandgap (Eg) of single GZO and tri-layer films was discussed as a function of carrier concentration in the paper.
1. Introduction Transparent conductive films (TCFs) are characterized by the simultaneous achievement of a high electrical conductivity and high optical transparency in visible light range. With the explosive development of optoelectronic devices including sensor, thin film transistor (TFT), thin-film solar cells, light-emitting diodes, flat-panel displays, touch screens and electrochromics, TCFs have become an essential component and play vital roles in these optoelectronic devices. Several classes of TCFs have been developed, including transparent conductive oxide (TCO) film, ultrathin metal film (UMF), metal nanowire network, metal grid, carbon nanotube (CNT), graphene and conducting polymer [1–6]. The TCO films, typically represented by indium tin oxide (In2O3:Sn, ITO) films, dominate the current use of TCFs in optoelectronic devices. However, limited availability of indium makes the ITO films expensive. Furthermore, high performance TCO films can be obtained usually at higher substrate temperatures and brittleness of oxide is unfavorable for
* Corresponding author. E-mail address: [email protected] (B.L. Zhu). https://doi.org/10.1016/j.spmi.2020.106456 Received 6 October 2019; Received in revised form 14 February 2020; Accepted 17 February 2020 Available online 18 February 2020 0749-6036/© 2020 Elsevier Ltd. All rights reserved.
Superlattices and Microstructures 140 (2020) 106456
B.L. Zhu et al.
Table 1 The process parameters of film preparation. Parameter
Film type Cu
Base pressure (Pa) Target-substrate distance (mm) Substrate temperature (oC) Sputtering pressure (Pa) Ar flux (sccm) H2 flux (sccm) DC sputtering power of Cu target (W) RF sputtering power of GZO target (W) Sputtering time of Cu target (s) Sputtering time of GZO target (s) Thickness of Cu films (or layers) (nm) Thickness of GZO or HGZO films (or layers) (nm)
3 � 10 65 RT 0.8 95 – 30 – 5–20 – 3–13 –
–3
GZO or HGZO
GcG
3 � 10 65 RT 0.8 95 0–20 – 150 – 600 – 100–150
3 � 10 65 RT 0.8 95 – 30 150 5–20 80–240 3~13 20–60
–3
HcH –3
3 � 10–3 65 RT 0.8 95 20 30 150 5–20 120–360 3~13 20–60
fabrication of flexible optoelectronic devices. By controlling film thickness, UMFs with high conductivity and optical transparency can be achieved, but they are very sensitive to heat and humidity, which induces rapid degradation of the optoelectronic properties. The properties of the films based on metal nanowires or grids are comparable to those of ITO films, and the film can be produced cost efficiently on a large scale via solution processing or lithography. However, they display a high surface roughness or haze which affects the properties of the device. Although the CNTs and graphene have extremely high conductivity, transparent conductive properties of the films based on graphene and CNTs cannot compete with those of metal-based and TCO films due to structural defects and high junction resistances. In addition, the preparation of the films based on CNTs or graphene on a large scale is relatively difficult and the films also exhibit a high surface roughness. The conducting polymers have problem of lower conductivity or easily degradable nature. It can be seen that single material cannot satisfy multifunctional requirements for TCFs, and thus composite materials such as TCO/ metal/TCO, CNTs/polymer and metal/polymer have been also developed [1,4,7,8]. In TCO/metal/TCO tri-layer films, metal layer is the key element for obtaining low resistance of the films, and the top and bottom TCO layers can preserve metal layer from oxidation, suppress the light reflection from the metal layer and also act as conducting layer. Because this structure can achieve high trans mittance and low resistivity, TCO/metal/TCO tri-layer films have been studied intensively in recent years with use of various metal and TCO materials. Among the most common metal materials, Al is cheap but it has high optical loss and relatively high resistivity; Au has high heat and chemical stability but it is very expensive; Ag has lowest resistivity and optical loss but it is also expensive; and Cu has comparable electrical properties, less expensive but inferior transmittance compared with Ag [9–11]. In2O3, SnO2, ZnO and their doped oxide films are common TCO film candidates. Among these TCO films, ZnO and doped ZnO films have been widely investigated due to their favorable characteristic of abundance in nature, non-toxicity and stability in hydrogen plasma. Especially, group-III el ements, such as Al, Ga, In and B, doped ZnO films have comparable transparent conductive properties with ITO films [12,13]. However, as similar as ITO films, Al, Ga, In or B doped ZnO films cannot obtain high transparent conductive properties at substrate temperature of room temperature (RT), which is disadvantageous to obtain high performance tri-layer structured TCFs. Previous studies have indicated that the transparent conductive properties of ZnO-based films can be significantly improved by introducing H2 into deposition atmosphere at substrate temperature of RT [14,15]. Unfortunately, hydrogenated ZnO-based films are seldom used to fabricate tri-layer structured TCFs [16]. In this study, Cu films were used as the metal layers, and transparent conductive properties of single Cu films were investigated as a function of film thickness in order to better understand the properties of tri-layer structured TCFs. Considering that Ga dopant is less reactive with oxygen and covalent bond length of Ga–O (1.92 Å) is more close to that of Zn–O (1.97 Å) compared with other group-III elements [17], Ga-doped ZnO (GZO) films were used as TCO layers. The hydrogenated GZO (HGZO) films were prepared by intro ducing H2 into deposition atmosphere, and their transparent conductive properties were investigated as a function of H2 flux to obtain optimized H2 flux. Finally, GZO/Cu/GZO (GcG) and HGZO/Cu/HGZO (HcH) tri-layer films were prepared, and their transparent conductive properties were investigated as a function of the thickness of Cu and GZO (or HGZO) layers. The bandgap (Eg) of the GZO and tri-layer films was also investigated in the paper. The results indicate that single Cu film cannot easily obtain highly transparent conducive properties due to its significantly reduced transmittance with increasing film thickness. By introducing appropriate flux of H2 into deposition atmosphere, the transparent conducive properties of GZO films can be significantly improved. At the optimized thickness of Cu and GZO (or HGZO) layers, GcG and HcG tri-layer films can be achieved with better transparent conductive properties than those of single Cu, GZO or HGZO films. Compared with GcG tri-layer films, better transparent conductive properties of HcH tri-layer films indicate that introduction of H2 into deposition atmosphere is an effective method to improve the properties of ZnO-based tri-layer films. The Eg of the films is found to be related to the carrier concentration and can be explained by Burstein-Moss effect and many-body effect. 2. Experimental procedures All the films were deposited on a thin plate of soda-lime-silica glass in a magnetron sputtering system. Before deposition, the substrates were cleaned in alcohol, acetone and distilled water successively in an ultrasonic cleaner for 15 min. The purity of GZO (3 wt 2
Superlattices and Microstructures 140 (2020) 106456
B.L. Zhu et al.
Fig. 1. (a) The transmittance spectra of different thicknesses of Cu films. The ρ, n and μ (b) and Rs, Tvis and FOM (c) of Cu films as a function of film thickness.
% Ga2O3) and Cu targets was higher than 99.99%, and their size was φ60 mm � 6 mm. The process parameters of film preparation are shown in Table 1. The GcG and HcH tri-layer films were prepared by depositing oxide layer, metal layer and oxide layer on glass substrates consecutively without a vacuum break, and the thickness of top and bottom GZO or HGZO layers was kept same. The Cu films were firstly deposited for longer time (3–15 min) and thus their thickness could be easily measured by interference microscope. According the film thickness as a function of deposition time, the deposition rate was obtained (0.67 nm/s), and then the Cu films or mid-layer with different thicknesses (3–13 nm) were deposited by controlling deposition time (5–25 s). Interference microscope (SC57-6JA) was used to determine the thickness of the films. The crystalline structure of the films was analyzed by X-ray diffraction (XRD; D8 Advance, Bruker Axs) with Cu Kα1 incident radiation in θ-2θ Bragg-Brentano geometry. The electrical properties of the films, including resistivity (ρ), sheet resistance (Rs), carrier concentration (n) and mobility (μ) were determined by Hall Effect measurement using Van der Pauw method at RT. The transmission spectra of the films were obtained from UV–visible spectrophotometer (UV–2102PC, Unico) in the wavelength range of 300–900 nm. The average transmittance in visible light range (Tvis) of the films was obtained in the range of 400–800 nm. The Eg of the films can be estimated by using the Tauc method [18], i. e., αhν ¼ A(hν-Eg)1/2, where α is absorption coefficient, A is a constant, and hν is the photon energy. For a given film thickness (t), the α of the films can be obtained using relation of T ¼ (1-R)2 exp (-αt), where T is the transmittance of the film and R is the reflectance [19]. Since the reflectance is negligible and insignificant near the absorption edge [19], this relation simplifies to the following relation: α ¼ -lnT/t. In order to evaluate the transparent conductive properties of the films synthetically, figure of merit (FOM) of the films can be defined as T10 vis/Rs [20]. The higher FOM indicates that the films possess high transmittance and low resistivity simultaneously.
3
Superlattices and Microstructures 140 (2020) 106456
B.L. Zhu et al.
(caption on next page) 4
Superlattices and Microstructures 140 (2020) 106456
B.L. Zhu et al.
Fig. 2. The transmittance spectra (a) and (αhν)2 versus hν plots (b) of GZO films prepared at different H2 fluxes. The insert in (b) shows the Eg of GZO films as a function of H2 flux. The ρ, n and μ (c) and Rs, Tvis and FOM (d) of GZO films as a function of H2 flux.
Fig. 3. The XRD patterns of GZO films prepared with different H2 fluxes. The position (2θ), full width at half maximum (FWHM) and crystallite size (D) of the diffraction peaks are marked in the figure.
3. Results and discussion 3.1. Single Cu films Fig. 1 shows the properties of the Cu films as a function of film thickness. As shown in Fig. 1(a), the transmittance spectra of Cu films show a peak at about 600 nm, and transmittance decreases with increasing thickness of Cu films. As can be seen from Fig. 1(b), the carrier concentration and mobility of the film increase, and its resistivity decreases with increasing film thickness. Furthermore, a great decrease in resistivity can be observed when the film thickness increases from 5 to 7 nm. From Fig. 1(c), it can be found that the Rs, Tvis and FOM decrease with increasing thickness of Cu film. At Cu film thickness of 3 nm, the film displays maximum FOM of 1.37 � 10 3 Ω 1, with Rs of 47.8 Ω/sq. and Tvis of 76.12%. In previous studies [21,22], the Cu film with thickness of 8 nm had TVis of 64% and Rs of 15 Ω/sq. When the thickness of top Ni layer was fixed at 1 nm, the percolation threshold of Cu layer of Cu–Ni bi-layer films, i.e., the thickness corresponding to which the layer becomes continuous, was found to be between 5.5 and 6.5 nm. Furthermore, bi-layer film had TVis of 63% and Rs of 30 Ω/sq. at Cu layer thickness of 5 nm, and it had TVis of 57% and Rs of 15 Ω/sq. at Cu layer thickness of 7 nm. Although the Rs in these reports is lower, the reduced TVis results in the FOM of the films (2.41 � 10 4- 7.69 � 10 4 Ω 1) being lower than our reported value. According to the 3D (Volmer-Weber) growth mode [23,24], a great decrease in resistivity as film thickness increases from 5 to 7 nm indicates that the continuous Cu film is formed at film thickness of 7 nm. After the formation of continuous Cu films, i.e., the thickness of Cu film is above 7 nm, the conductive properties of Cu film enhance continuously due to the improvement of film crystallinity decreasing the grain boundary scattering [25,26]. In previous studies [27–29], the maxima of transmission spectra had been observed at about 600 nm in Cu films, and the origination could be attributed to the excitation of electrons from the d-band to the Fermi surface. With increasing Cu film thickness, the carrier concentration of the film increases, as mentioned above. Thus, there are more bound electrons available for excitation, leading to a drop in transmittance. On the other hand, reflectance of Cu film increases with increase in Cu film thickness due to formation of continuous films. Thus, the transmittance of Cu film decreases with increasing film thickness. Furthermore, the highest FOM is obtained at Cu film thickness of 3 nm due to the great decrease in transmittance with increasing Cu thickness. 3.2. Single GZO and HGZO films Fig. 2 shows the properties of GZO films as a function of H2 flux. From Fig. 2(a), it can be observed that all the transmittance spectra of the films in the visible light range are flat, and the UV absorption edge of the films shifts to shorter wavelength with increasing H2 flux. The Eg value of the films obtained from (αhν)2 versus hν curve firstly increases and then tends to constant with increasing H2 flux, as shown in Fig. 2(b). As shown in Fig. 2(c), carrier concentration of the film increases, its carrier mobility tends to first increase and then decrease, and its resistivity decreases with increasing H2 flux. It is should be noted that there are relatively large changes in carrier concentration and resistivity when H2 flux increases from 0 to 5 sccm. From Fig. 2(d), the Rs greatly decreases but Tvis and FOM greatly increase as H2 flux increases from 0 to 5 sccm, after that they tend to constant values with further increasing H2 flux. At H2 flux of 20 sccm, Rs, Tvis and FOM of the film are about 315 Ω/sq., 93.5%, and 1.6 � 10 3 Ω 1, respectively. When H2 is introduced into deposition atmosphere, the increase in carrier concentration of GZO films can be attributed to the 5
Superlattices and Microstructures 140 (2020) 106456
B.L. Zhu et al.
Fig. 4. The broadening of the Eg of GZO films as a function of n2/3.
following two factors: (i) H is doped into interstitial positions of the lattice and acts as a shallow donor in ZnO [14,15]; and (ii) the substitution of Ga atoms for Zn atoms is enhanced [30]. With H2 flux increasing from 0 to 5 sccm, the amount of incorporated H in GZO films increases, and thus the carrier concentration of the films greatly increases. However, the incorporated H in GZO films tends to saturate as the H2 flux increases from 5 to 20 sccm, which results in the slight increase of carrier concentration. On the other hand, H plasma will be formed during sputtering when H2 is introduced into deposition atmosphere, and its etching effect on GZO films can reduce the film thickness [15]. Due to the decrease of film thickness, the (002) diffraction peak intensity of GZO films gradually decreases with increasing H2 flux, as shown in Fig. 3. The crystallite size of the films can be calculated according to Scherrer equation [31]. It can be seen from Fig. 3 that the crystallite size of all the films is smaller (4.73–8.35 nm), indicating that the crystal quality of the films is poor due to the low substrate temperature (RT). Furthermore, the carrier mobility of all the films is lower [3.45–5.39 cm2/(V⋅s)] due to poor crystal quality. When H2 flux increases from 0 to 5 sccm, the increase in the carrier mobility of GZO films can be attributed to the increase of crystallize size decreasing the grain-boundary scattering. But when the H2 flux is above 5 sccm, decrease of carrier mobility can be ascribed to the fact that further increased carrier concentration results in ionized impurity scattering [15,32]. Due to the improvement of film crystallinity, passivation of the grain boundary and intrinsic defect and decrease of film thickness after introducing H2 into deposition atmosphere [15,16], the transmittance of GZO film increases with increasing H2 flux. Finally, the increase of TVis and decrease of Rs result in higher FOM values for HGZO films. In addition, the larger low energy tail is observed in Tauc plot for the film deposited using 20 sccm H2, which could be attributed to impurities [33]. As discussed above, more H is doped into interstitial positions of the ZnO lattice and more Zn atoms are replaced by Ga atoms with increasing H2 flux. These impurities could form the level in the forbidden band. The transitions via low-level impurities are responsible for lower energy absorption region [33]. With regard to Eg of the films, it can be discussed according to the change in carrier concentration. From Fig. 2(b) and (c), it can be found that the Eg of GZO films increases with increase in carrier concentration. Compared to the Eg of pure ZnO (3.28 eV), the broadening values of the Eg of GZO film can be obtained, and they are shown as a function of n2/3 in Fig. 4. It can be seen that the broadening of the Eg of GZO films is proportional to n2/3, indicating that this broadening is due to the Burstein-Moss effect, as suggested by other reports [15,34]. According to the Burstein-Moss effect [35,36], the increase of carrier concentration will cause shift of Fermi level towards a higher energy and the lowest states in the conduction band (in an n-type semiconductor) are filled with electrons. Thus, interband transitions correspond to the excitation of valence band electrons into empty states above the minimum of the conduction band, leading to a broadening of the Eg. 3.3. GcG and HcH tri-layer films Fig. 5 shows the properties of GcG and HcH tri-layer films as a function of Cu layer thickness when the thickness of top and bottom GZO or HGZO layers is kept at 40 nm. As can be seen from Fig. 5 (a) and (b), similar to Cu films, the transmittance spectra of tri-layer films also show a peak at about 600 nm. For both types of tri-layer film, the UV absorption edge can be observed and it shifts to longer wavelength with increasing Cu layer thickness. From Fig. 5 (c) and (d), the Eg shows a decreased trend with increasing Cu layer thickness. As shown in Fig. 5(e), the carrier concentration and mobility of the tri-layer film increase, and its resistivity decreases with increasing thickness of Cu layer. As shown in Fig. 5(f), the Rs decreases, but the TVis first increases slightly and then decreases with increasing Cu layer thickness. As Cu layer thickness increases from 3 to .7 nm, it should be noted that the Rs of films shows a relatively large decrease and Tvis slightly increases. The FOM of the films first increases and then decreases with increasing Cu layer thickness, and the maximum FOM is achieved at Cu layer thickness of 7 nm. Fig. 6 shows the properties of GcG and HcH tri-layer films as a function of GZO or HGZO layer thickness at the optimized Cu layer thickness of 7 nm. From Fig. 6 (a) and (b), the transmittance spectra of tri-layer films with different GZO or HGZO layer thicknesses also show a peak at about 600 nm. For both types of tri-layer films, the UV absorption edge shifts to longer wavelength with increasing GZO 6
Superlattices and Microstructures 140 (2020) 106456
B.L. Zhu et al.
Fig. 5. The transmittance spectra (a, b) and (αhν)2 versus hν plots (c, d) of GcG (a, c) and HcH (b, d) tri-layer films with different thicknesses of Cu layer. The inserts in (c) and (d) show the Eg of tri-layer films as a function of Cu layer thickness. The ρ, n and μ (e) and Rs, Tvis and FOM (f) of tri-layer films as a function of Cu layer thickness.
or HGZO layer thickness. From Fig. 6 (c) and (d), the Eg first increases and then slightly decreases for GcG tri-layer films but it increases for HcH tri-layer films with increasing GZO or HGZO layer thickness, respectively. As shown in Fig. 6(e), the carrier concentration of the tri-layer film decreases, its carrier mobility is maintained around a certain value with irregular changes, and the resistivity 7
Superlattices and Microstructures 140 (2020) 106456
B.L. Zhu et al.
Fig. 6. The transmittance spectra (a, b) and (αhν)2 versus hν plots (c, d) of GcG (a, c) and HcH (b, d) tri-layer films with different thicknesses of GZO or HGZO layer. The inserts in (c) and (d) show the Eg of tri-layer films as a function of GZO or HGZO layer thickness. The ρ, n and μ (e) and Rs, Tvis and FOM (f) of tri-layer films as a function of GZO or HGZO layer thickness.
increases with increasing thickness of GZO or HGZO layer thickness. As shown in Fig. 6 (f), the Rs is almost invariable and Tvis and FOM first increase and then decrease with increasing GZO or HGZO layer thickness. Maximum Tvis and FOM are obtained at GZO or HGZO layer thickness of 40 nm. At the same Cu layer and oxide layer thickness, the results shown in Figs. 5 and 6 indicate that HcH tri-layer films have sharper 8
Superlattices and Microstructures 140 (2020) 106456
B.L. Zhu et al.
Fig. 7. (a) The comparison in Rs of single Cu film and tri-layer films. The insert in (a) shows the schematic representation of the electrical behavior of TCO/metal/TCO tri-layer films. Schematic energy band diagrams of GZO/Cu (b) and HGZO/Cu (c) systems before and after contact.
9
Superlattices and Microstructures 140 (2020) 106456
B.L. Zhu et al.
transmission peaks and higher Eg than those of GcG tri-layer films. For the conductive properties of the films, HcH tri-layer films have lower resistivity, higher carrier concentration and mobility than those of GcG tri-layer films. Furthermore, HcH tri-layer films have better transparent conductive properties than GcG tri-layer films, namely, lower Rs, higher Tvis and higher FOM values. In present study, 40 nm HGZO/7 nm Cu/40 nm HGZO film displays maximum FOM of 7.89 � 10 3 Ω 1, with Rs of 18 Ω/sq. and Tvis of 82.3%. For 40 nm GZO/7 nm Cu/40 nm GZO films, FOM, Rs, and Tvis are about 4.91 � 10 3 Ω 1, 23 Ω/sq. and 80.4%, respectively. In previous GcG tri-layer films reported by Gong et al. [11], the 10 nm GZO/10 nm Cu/10 nm GZO had the highest FOM (2.68 � 10 3 Ω 1). Wang et al. investigated three kinds of multilayer structures (AZO/Cu, AZO/Cu/AZO, and Cu/AZO) [37], and found that the 40 nm AZO/7 nm Cu bi-layer film displayed the highest FOM of 4.82 � 10 3 Ω 1, with Rs of 21.7 Ω/sq. and TVis of 80% (400–800 nm). It can be found that the transparent conductive properties obtained in our study are superior to these reports. However, Yang and Song et al. reported that 40 nm AZO/8 nm Cu/40 nm AZO tri-layer film displayed the FOM of 1.94 � 10 2 Ω 1, with Rs of 9 Ω/sq and TVis of 84% [38,39]. The results of these reports are superior to ours, which may be due to the fact that the Cu layer is prepared with different parameters, but the detailed reason needs to be further studied. The TCO/metal/TCO tri-layer structured films are usually considered as a parallel circuit of three resistors [7,40,41], as shown in insert of Fig. 7(a). According to this model, the Rs of tri-layer films should be smaller than that of single Cu films. However, compared with Rs of single Cu films, the Rs of HcH tri-layer films is close but that of GcG tri-layer films is higher, as shown in Fig. 7(a). This result indicates that the TCO layer also affects the conductive properties of tri-layer films in addition to metal layer. In fact, the conductive properties of tri-layer films should consider the carrier transfer between the TCO and metal layers [11,42], which is not considered in parallel circuit model. The studies have indicated that the work function (Φ) values of the metals are usually lower than those of TCOs [43–45]. When metal layer is in contact with TCO layer, the electron will transfer from metal to TCO layer in order to keep ther modynamic equilibrium, leading to the same Fermi level throughout the junction. The transfer of electron will result in an accu mulation type contact between the metal and TCO layers and bending downward of semiconductor band at the contact. In this study, the schematic energy band diagrams of GZO/Cu and HGZO/Cu systems before and after contact are shown in Fig. 7(b) and (c), respectively. As shown in Fig. 7(b), electrons transfer from Cu layer to GZO layer when GZO layer is in contact with Cu layer, which results in weakening of the conductive properties of Cu layer in tri-layer films. Thus, the Rs of GcG tri-layer films is higher than that of single Cu films. However, the work function of HGZO film is smaller than that of GZO film due to its higher carrier concentration [46, 47], and thus relatively smaller amount of electrons transfer from Cu layer to HGZO layer when HGZO layer is in contact with Cu layer, as shown in Fig. 7(c). This implies that Cu layer in HcH tri-layer films keeps higher conductive properties and thus their Rs is close to that of single Cu films. On the other hand, the Rs of both types of tri-layer films is obviously lower than that of single GZO or HGZO film due to electrons injection from the Cu layer into the GZO or HGZO layer. After electron transfer from metal into TCO layer, the carrier concentration (n) of TCO/metal/TCO tri-layer films can be estimated Meta l by the relation of n ffi dMetalNþ2�d [48], where NMetal is the carrier concentration of metal layer and dMeatl and dTCO are the thickness of TCO
metal layer and TCO layer, respectively. Since the variation of dMetal is smaller and the NMetal increases with increasing dMetal, carrier concentration of tri-layer films increases with increasing Cu layer thickness but it decreases with increasing GZO or HGZO layer thickness. As discussed above, Cu layer in HcH tri-layer films keeps higher conductive properties, i.e., has higher carrier concentration, and thus the carrier concentration of HcH tri-layer films is higher than that of GcG tri-layer films at the same Cu and GZO (or HGZO) layer thickness. The carrier mobility of tri-layer films is related to the carrier mobility of TCO and metal layers [42,43]. Obviously, the carrier mobility of Cu layer is dominative due to its smaller resistance, and it increases with increasing layer thickness. Thus, carrier mobility of GcG or HcH tri-layer films increases with increasing Cu layer thickness at the same GZO or HGZO layer thickness, but it is maintained around a certain value with increasing GZO (or HGZO) films at the fixed Cu layer thickness. For the higher carrier mobility of HcH tri-layer films than that of GcG tri-layer film, it can be attributed to the higher carrier mobility of HGZO layer than that of GZO layer. According to the relations of 1/ρ ¼ nqμ (q is electron charge) and Rs ¼ ρ/t, it is easy to understand the changes in resistivity and Rs of tri-layer films with the Cu layer thickness, the oxide layer thickness, and the oxide type. It is noteworthy that relatively larger decreases in resistivity and Rs are observed in both types of tri-layer films as Cu layer thickness increases from 3 to 7 nm, which in dicates the continuous Cu layer is formed at Cu layer thickness of 7 nm, as suggested in single Cu films. Usually, the TVis of TCO/metal/TCO tri-layer structured films shows a trend of first increase and then decrease with increasing metal layer thickness, and the highest TVis is obtained once the continuous metal layer is formed [44,48,49]. In this study, TVis of tri-layer films as a function of Cu layer thickness indicates that continuous Cu layer is formed at Cu layer thickness of 7 nm. Theoretical and experimental results have indicated that suitable thickness of TCO layer can play a role in the antireflection layer to block the reflected light from a metal layer in the visible range, and thus the tri-layer films can be achieved the highest TVis [50–52]. Usually, the suitable thickness of TCO layer is related to the refractive index and extinction coefficient of TCO and metal layers. In this study, suitable thickness of GZO or HGZO layer is obtained at 40 nm, which is basically consistent with previous reports [7]. Due to obtaining of highest TVis and relatively lower Rs, highest FOM is achieved at Cu layer thickness of 7 nm and GZO or HGZO layer thickness of 40 nm. Basically, the TVis of HcH tri-layer films is higher than that of GcG tri-layer films, which may be related to higher TVis of HGZO layer. Furthermore, HcH tri-layer films display higher TVis and lower Rs than those of GcG tri-layer films, and thus their FOM is higher. On a whole, the Eg of tri-layer films shows a decreased trend with increase in carrier concentration from Figs. 5(c)–(e) and 6(c)-(e). Compared to single GZO film (3.29 eV) and HGZO film (3.69 eV), the narrowing values of Eg of tri-layer films can be obtained, and they are shown as a function of n1/3 in Fig. 8. It can be seen that the narrowing of Eg of tri-layer films is roughly proportional to n1/3, indicating that this narrowing is due to the many-body effect, as suggested by some researchers [34,53,54]. The many-body effect is mutual exchange and Coulomb interactions between the added free electrons in the conduction band and electron impurity scattering [55]. 10
Superlattices and Microstructures 140 (2020) 106456
B.L. Zhu et al.
Fig. 8. The narrowing of the Eg of tri-layer films as a function of n1/3.
4. Conclusions The resistivity, Rs, TVis and FOM of single Cu films decrease with increasing film thickness from 3 to 13 nm, and the continuous film is formed at thickness of 7 nm. With increase of H2 flux, the resistivity and Rs of single GZO films decrease but their TVis and FOM increase. For both GcG and HcH tri-layer films, their resistivity and Rs decrease but the TVis first increases and then decreases with increasing Cu layer thickness. With increasing GZO or HGZO layer thickness, resistivity of GcG and HcH tri-layer films increases, Rs keeps constant, and TVis first increases and then decreases. When Cu layer thickness is 7 nm and GZO (or HGZO) layer thickness is 40 nm, two kinds of tri-layer films have the best FOM. Compared with GcG tri-layer films, HcH tri-layer films have better FOM, indicating that introducing H2 in sputtering atmosphere is an effective method to improve the transparent conductive properties tri-layer films. With increasing H2 flux, carrier concentration of GZO film increases and thus its Eg increases due to the Burstein-Moss effect. With increasing Cu layer thickness or decreasing GZO (or HGZO) layer thickness, carrier concentration of tri-layer films increases, which results in decreased trend of the Eg due to the many-body effect. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement B.L. Zhu: Conceptualization, Methodology, Writing - original draft. J.M. Ma: Investigation, Validation. K. Lv: Investigation. C.J. Wang: Investigation. J. Wu: Formal analysis. Z.H. Gan: Resources. J. Liu: Writing - review & editing. X.W. Shi: Writing - review & editing. Acknowledgments This work was supported by the National Nature Science Foundation of China (Grant No. 50902105). References [1] A.I. Hofmann, E. Cloutet, G. Hadziioannou, Materials for transparent electrodes: from metal oxides to organic alternatives, Adv. Electron. Mater. 4 (2018) 1700412, https://doi.org/10.1002/aelm.201700412. [2] A. Kumar, C.W. Zhou, The race to replace tin-doped indium oxide: which material will win? ACS Nano 4 (2010) 11–14, https://doi.org/10.1021/nn901903b. [3] S. Sharma, S. Shriwastava, S. Kumar, K. Bhatt, C.C. Tripathi, Alternative transparent conducting electrode materials for flexible optoelectronic devices, OptoElectron. Rev. 26 (2018) 223–235, https://doi.org/10.1016/j.opelre.2018.06.004. [4] L.X. He, S.C. Tjong, Nanostructured transparent conductive films: fabrication, characterization and applications, Math. Sci. Eng. R 109 (2016) 1–101, https:// doi.org/10.1016/j.mser.2016.08.002. [5] W.R. Cao, J. Li, H.Z. Chen, J.G. Xue, Transparent electrodes for organic optoelectronic devices: a review, J. Photon. Energy 4 (2014), 040990, https://doi.org/ 10.1117/1.JPE.4.040990. [6] M. Morales-Masis, S.D. Wolf, R. Woods-Robinson, J.W. Ager, C. Ballif, Transparent electrodes for efficient optoelectronics, Adv. Electron. Mater. 3 (2017) 1600529, https://doi.org/10.1002/aelm.201600529. [7] C. Guillen, J. Herrero, TCO/metal/TCO structures for energy and flexible electronics, Thin Solid Films 520 (2011) 1–17, https://doi.org/10.1016/j. tsf.2011.06.091. [8] L. Cattin, J.C. Bernede, M. Morsli, Toward indium-free optoelectronic devices: dielectric/metal/dielectric alternative transparent conductive electrode in organic photovoltaic cells, Phys. Status Solidi A 210 (2013) 1047–1061, https://doi.org/10.1002/pssa.201228089.
11
Superlattices and Microstructures 140 (2020) 106456
B.L. Zhu et al.
[9] J. Lou, M. Bao, B.J. Ye, H.M. Weng, H.J. Du, Z.B. Zhang, The design of transparent conductive ZnO/metal/ZnO multilayers for incidence from 0� to 70� , J. Opt. A-Pure Appl. Opt. 11 (2009), 085501 https://doi.org/10.1088/1464-4258/11/8/085501. [10] H. Zhou, J. Xie, M.F. Mai, J. Wang, X.Q. Shen, S.Y. Wang, L.H. Zhang, K. Kisslinger, H.Q. Wang, J.X. Zhang, Y. Li, J.H. Deng, S.M. Ke, X.R. Zeng, High-quality AZO/Au/AZO sandwich film with ultralow optical loss and resistivity for transparent flexible electrodes, ACS Appl. Mater. Interfaces 10 (2018) 16160–16168, https://doi.org/10.1021/acsami.8b00685. [11] L. Gong, J.G. Lu, Z.Z. Ye, Conductive Ga doped ZnO/Cu/Ga doped ZnO thin films prepared by magnetron sputtering at room temperature for flexible electronics, Thin Solid Films 519 (2011) 3870–3874, https://doi.org/10.1016/j.tsf.2011.01.396. [12] T. Minami, Present status of transparent conducting oxide thin-film development for indium-tin-oxide (ITO) substitutes, Thin Solid Films 516 (2008) 5822–5828, https://doi.org/10.1016/j.tsf.2007.10.063. [13] S.C. Dixon, D.O. Scanlon, C.J. Carmalt, I.P. Parkin, n-Type doped transparent conducting binary oxides: an overview, J. Mater. Chem. C 4 (2016) 6946–6961, https://doi.org/10.1039/c6tc01881e. [14] B.L. Zhu, J. Wang, S.J. Zhu, J. Wu, R. Wu, D.W. Zeng, C.S. Xie, Influences of hydrogen introduction on structure and properties of ZnO thin films during sputtering and post-annealing, Thin Solid Films 519 (2011) 3809–3815, https://doi.org/10.1016/j.tsf.2011.01.187. [15] B.L. Zhu, J. Wang, S.J. Zhu, J. Wu, D.W. Zeng, C.S. Xie, Investigation of structural, electrical, and optical properties for Al-doped ZnO films deposited at different substrate temperatures and H2 ratios, J. Electrochem. Soc. 159 (2012) H536–H544, https://doi.org/10.1149/2.022206jes. [16] G.H. Yan, Y. Yuan, W.L. Chen, R.J. Hong, Influence of H2 flow ratio on the photoelectric properties of hydrogenated AZO thin films with embedded silver layer, Mater. Lett. 185 (2016) 272–274, https://doi.org/10.1016/j.matlet.2016.09.004. [17] H.J. Ko, Y.F. Chen, S.K. Hong, H. Wenisch, T. Yao, D.C. Look, Ga-doped ZnO films grown on GaN templates by plasma-assisted molecular-beam epitaxy, Appl. Phys. Lett. 77 (2000) 3761–3763, https://doi.org/10.1063/1.1331089. [18] B.D. Viezbicke, S. Patel, B.E. Davis, D.P. Birnie, Evaluation of the Tauc method for optical absorption edge determination: ZnO thin films as a model system, Phys. Status Solidi B 252 (8) (2015) 1700–1710, https://doi.org/10.1002/pssb.201552007. [19] E. Senadım, H. Kavak, R. Esen, The effect of annealing on structural and optical properties of ZnO thin films grown by pulsed filtered cathodic vacuum arc deposition, J. Phys.-Condens. Mat. 18 (2006) 6391–6400, https://doi.org/10.1088/0953-8984/18/27/021. [20] G. Haacke, New Figure of merit for transparent conductors, J. Appl. Phys. 47 (1976) 4086–4089, https://doi.org/10.1063/1.323240. [21] D.S. Ghosh, R. Betancur, T.L. Chen, V. Pruneri, JordiMartorell, Semi-transparent metal electrode of Cu-Ni as a replacement of an ITO in organic photovoltaic cells, Sol. Energ. Mat. Sol. C. 95 (2011), https://doi.org/10.1016/j.solmat.2010.12.040, 1228-123. [22] S. Cheylan, D.S. Ghosh, D. Krautz, T.L. Chen, V. Pruneri, Organic light-emitting diode with indium-free metallic bilayer as transparent anode, Org. Electron. 12 (2011) 818–822, https://doi.org/10.1016/j.orgel.2011.02.015. [23] C.T. Campbell, Metal films and particles on oxide surfaces: structural, electronic and chemisorptive properties, J. Chem. Soc. Faraday. Trans. 92 (1996) 1435–1445, https://doi.org/10.1039/ft9969201435. [24] N. Kaiser, Review of the fundamentals of thin-film growth, Appl. Optic. 41 (2002) 3053–3060, https://doi.org/10.1364/AO.41.003053. [25] J.W. Lim, M. Isshiki, Electrical resistivity of Cu films deposited by ion beam deposition: effects of grain size, impurities, and morphological defect, J. Appl. Phys. 99 (2006), 094909, https://doi.org/10.1063/1.2194247. [26] W. Zhang, S.H. Brongersma, T. Clarysse, et al., Surface and grain boundary scattering studied in beveled polycrystalline thin Cu films, J. Vac. Sci. Technol. B 22 (2004) 1830–1833, https://doi.org/10.1116/1.1771666. [27] A. Axelevitch, B. Gorenstein, G. Golan, Investigation of optical transmission in thin metal films, Phys. Proc. 32 (2012) 1–13, https://doi.org/10.1016/j. phpro.2012.03.510. [28] A. Belosludtsev, N. Kyzas, Real-time in-situ investigation of copper ultrathin films growth, Mater. Lett. 232 (2018) 216–219, https://doi.org/10.1016/j. matlet.2018.08.127. [29] H. Ehrenreich, H.R. Philipp, Optical properties of Ag and Cu, Phys. Rev. 128 (1962) 1622–1629, https://doi.org/10.1103/PhysRev.128.1622. [30] H. Akazawa, Hydrogen induced electric conduction in undoped ZnO and Ga-doped ZnO thin films: creating native donors via reduction, hydrogen donors, and reactivating extrinsic donors, J. Vac. Sci. Technol., A 32 (2014), 051511, https://doi.org/10.1116/1.4892777. [31] B.L. Zhu, D.W. Zeng, J. Wu, W.L. Song, C.S. Xie, Synthesis and gas sensitivity of In-doped ZnO nanoparticles, J. Mater. Sci. Mater. Electron. 14 (2003) 521–526, https://doi.org/10.1023/A:1023989304943. [32] W.M. Kim, Y.H. Kim, J.S. Kim, J. Jeong, Y.J. Baik, J.K. Park, K.S. Lee, T.Y. Seong, Hydrogen in polycrystalline ZnO thin films, J. Phys. D Appl. Phys. 43 (2010) 365406, https://doi.org/10.1088/0022-3727/43/36/365406. [33] S.S. Lin, J.L. Huang, Effect of thickness on the structural and optical properties of ZnO films by r.f. magnetron sputtering, Surf. Coating. Technol. 185 (2004) 222–227, https://doi.org/10.1016/j.surfcoat.2003.11.014. [34] J.G. Lu, S. Fujita, T. Kawaharamura, H. Nishinaka, Y. Kamada, T. Ohshima, Z.Z. Ye, Y.J. Zeng, Y.Z. Zhang, L.P. Zhu, H.P. He, B.H. Zhao, Carrier concentration dependence of band gap shift in n-type ZnO:Al films, J. Appl. Phys. 101 (2007), 083705, https://doi.org/10.1063/1.2721374. [35] E. Burstein, Anomalous optical absorption limit in InSb, Phys. Rev. 93 (1954) 632–633, https://doi.org/10.1103/PhysRev.93.632. [36] T.S. Moss, The interpretation of the properties of indium antimonide, Proc. Phys. Soc. B 67 (1954) 775–782, https://doi.org/10.1088/0370-1301/67/10/306. [37] Y.P. Wang, J.G. Lu, X. Bie, Z.Z. Ye, X. Li, D. Song, X.Y. Zhao, W.Y. Ye, Transparent conductive and near-infrared reflective Cu-based Al-doped ZnO multilayer films grown by magnetron sputtering at room temperature, Appl. Surf. Sci. 257 (2011) 5966–5971, https://doi.org/10.1016/j.apsusc.2011.01.068. [38] T.L. Yang, Z.S. Zhang, S.M. Song, Y.H. Li, M.S. Lv, Z.C. Wu, S.H. Han, Structural, optical and electrical properties of AZO/Cu/AZO tri-layer films prepared by radio frequencymagnetron sputtering and ion-beam sputtering, Vacuum 83 (2) (2009) 257–260, https://doi.org/10.1016/j.vacuum.2008.05.029. [39] S.M. Song, T.L. Yang, M.S. Lv, Y.H. Li, Y.Q. Xin, L.L. Jiang, Z.C. Wu, S.H. Han, Effect of Cu layer thickness on the structural, optical and electrical properties of AZO/Cu/AZO tri-layer films, Vacuum 85 (2010) 39–44, https://doi.org/10.1016/j.vacuum.2010.03.008. [40] A. Kloppel, W. Kriegseisa, B.K. Meyer, A. Scharmann, C. Daube, J. Stollenwerk, J. Trube, Dependence of the electrical and optical behaviour of ITO-silver-ITO multilayers on the silver properties, Thin Solid Films 365 (2000) 139–146, https://doi.org/10.1016/S0040-6090(99)00949-9. [41] M. Bender, W. Seelig, C. Daube, H. Frankenberger, B. Ocker, J. Stollenwerk, Dependence of film composition and thicknesses on optical and electrical properties of ITO-metal-ITO multilayers, Thin Solid Films 326 (1998) 67–71, https://doi.org/10.1016/S0040-6090(98)00520-3. [42] K. Sivaramakrishnan, N.D. Theodore, J.F. Moulder, T.L. Alford, The role of copper in ZnO/Cu/ZnO thin films for flexible electronics, J. Appl. Phys. 106 (2009), 063510, https://doi.org/10.1063/1.3213385. [43] A. Indluru, T.L. Alford, Effect of Ag thickness on electrical transport and optical properties of indium tin oxide-Ag-indium tin oxide multilayers, J. Appl. Phys. 105 (2009) 123528, https://doi.org/10.1063/1.3153977. [44] S.H. Yu, W.F. Zhang, L.X. Li, D. Xu, H.L. Dong, Y.X. Jin, Optimization of SnO2/Ag/SnO2 tri-layer films as transparent composite electrode with high figure of merit, Thin Solid Films 552 (2014) 150–154, https://doi.org/10.1016/j.tsf.2013.11.109. [45] K.M. Shafi, R. Vinodkumar, R.J. Bose, V.N. Uvais, V.P.M. Pillai, Effect of Cu on the microstructure and electrical properties of Cu/ZnO thin films, J. Alloys Compd. 551 (2013) 243–248, https://doi.org/10.1016/j.jallcom.2012.10.032. [46] J.A. Sans, J.F. Sanchez-Royo, A. Segura, G. Tobias, E. Canadell, Chemical effects on the optical band-gap of heavily doped ZnO:MIII (M¼Al,Ga, In):An investigation by means of photoelectron spectroscopy, optical measurements under pressure, and band structure calculations, Phys. Rev. B 79 (2009) 195105, https://doi.org/10.1103/PhysRevB.79.195105. [47] J.J. Jia, A. Takasaki, N. Oka, Y. Shigesato, Experimental observation on the Fermi level shift in polycrystalline Al-doped ZnO films, J. Appl. Phys. 112 (2012), 013718, https://doi.org/10.1063/1.4733969. [48] S.H. Yu, L.X. Li, X.S. Lyu, W.F. Zhang, Preparation and investigation of nano-thick FTO/Ag/FTO multilayer transparent electrodes with high figure of merit, Sci. Rep. 6 (2016) 20399, https://doi.org/10.1038/srep20399. [49] A. Bingel, M. Steglich, P. Naujok, R. Muller, U. Schulz, N. Kaiser, A. Tunnermann, Influence of the ZnO:Al dispersion on the performance of ZnO:Al/Ag/ZnO:Al transparent electrodes, Thin Solid Films 616 (2016) 594–600, https://doi.org/10.1016/j.tsf.2016.09.032.
12
Superlattices and Microstructures 140 (2020) 106456
B.L. Zhu et al.
[50] S. Kim, J.L. Lee, Design of dielectric/metal/dielectric transparent electrodes for flexible electronics, J. Photon. Energy 2 (2012), 021215, https://doi.org/ 10.1117/1.JPE.2.021215. [51] H.K. Lee, J.Y. Na, Y.J. Moon, T.Y. Seong, S.K. Kim, Design of near-unity transmittance dielectric/Ag/ITO electrodes for GaN-based light-emitting diodes, Curr. Appl. Phys. 15 (2015) 833–838, https://doi.org/10.1016/j.cap.2015.04.044. [52] D. Ebner, M. Bauch, T. Dimopoulos, High performance and low cost transparent electrodes based on ultrathin Cu layer, Optic Express 25 (2017) A240–A252, https://doi.org/10.1364/OE.25.00A240. [53] W.M. Kim, J.S. Kim, J.H. Jeong, J.K. Park, Y.J. Baik, T.Y. Seong, Analysis of optical band-gap shift in impurity doped ZnO thin films by using nonparabolic conduction band parameters, Thin Solid Films 531 (2013) 430–435, https://doi.org/10.1016/j.tsf.2013.01.078. [54] J. Kumar, A.K. Srivastava, Band gap narrowing in zinc oxide-based semiconductor thin films, J. Appl. Phys. 115 (2014) 134904, https://doi.org/10.1063/ 1.4870709. [55] B.E. Sernelius, K.F. Berggren, Z.C. Jin, I. Hamberg, C.G. Granqvist, Band-gap tailoring of ZnO by means of heavy Al doping, Phys. Rev. B 37 (1988) 10244–10248, https://doi.org/10.1103/PhysRevB.37.10244.
13
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Improvement of transparent conductive properties of GZO/Cu/ GZO tri-layer films by introducing H2 into sputtering atmosphere B.L. Zhu a, *, J.M. Ma a, K. Lv a, C.J. Wang a, J. Wu a, Z.H. Gan a, J. Liu a, X.W. Shi b a
The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan, 430081, People’s Republic of China Key Laboratory of Material Physics of Ministry of Education, Zhengzhou University, Zhengzhou, 450052, People’s Republic of China
b
A R T I C L E I N F O
A B S T R A C T
Keywords: TCO/metal/TCO tri-layer films Cu films Hydrogenated ZnO-based films Transparent conductive properties Figure of merit (FOM) Bandgap (Eg)
For preparation of transparent conductive oxide (TCO)/metal/TCO tri-layer films, Cu and Gadoped ZnO (GZO) films were respectively used as the metal and TCO layers and were depos ited by magnetron sputtering at room temperature. The effect of film thickness on transparent conductive properties of single Cu films was first investigated in order to better understand the properties of tri-layer films. The results show that the resistivity, sheet resistance (Rs), average transmittance in visible light range (TVis), and figure of merit (FOM) of Cu films decrease with increasing film thickness. The investigations of effect of H2 flux on the transparent conductive properties of GZO films show that hydrogenated GZO (HGZO) films have lower resistivity and higher TVis than those of GZO films. On basis of the investigation of single Cu and GZO films, the GZO/Cu/GZO (GcG) and HGZO/Cu/HGZO (HcH) tri-layer films were prepared, and their trans parent conductive properties were investigated as a function of Cu and GZO (or HGZO) layer thickness. The results show that these two kinds of tri-layer films have the best FOM at Cu layer thickness of 7 nm and GZO (or HGZO) layer thickness of 40 nm. Compared with GcG tri-layer films, HcH tri-layer films have better FOM, indicating that introducing H2 into sputtering at mosphere plays a significant role on the transparent conductive properties of the tri-layer films. In addition, bandgap (Eg) of single GZO and tri-layer films was discussed as a function of carrier concentration in the paper.
1. Introduction Transparent conductive films (TCFs) are characterized by the simultaneous achievement of a high electrical conductivity and high optical transparency in visible light range. With the explosive development of optoelectronic devices including sensor, thin film transistor (TFT), thin-film solar cells, light-emitting diodes, flat-panel displays, touch screens and electrochromics, TCFs have become an essential component and play vital roles in these optoelectronic devices. Several classes of TCFs have been developed, including transparent conductive oxide (TCO) film, ultrathin metal film (UMF), metal nanowire network, metal grid, carbon nanotube (CNT), graphene and conducting polymer [1–6]. The TCO films, typically represented by indium tin oxide (In2O3:Sn, ITO) films, dominate the current use of TCFs in optoelectronic devices. However, limited availability of indium makes the ITO films expensive. Furthermore, high performance TCO films can be obtained usually at higher substrate temperatures and brittleness of oxide is unfavorable for
* Corresponding author. E-mail address: [email protected] (B.L. Zhu). https://doi.org/10.1016/j.spmi.2020.106456 Received 6 October 2019; Received in revised form 14 February 2020; Accepted 17 February 2020 Available online 18 February 2020 0749-6036/© 2020 Elsevier Ltd. All rights reserved.
Superlattices and Microstructures 140 (2020) 106456
B.L. Zhu et al.
Table 1 The process parameters of film preparation. Parameter
Film type Cu
Base pressure (Pa) Target-substrate distance (mm) Substrate temperature (oC) Sputtering pressure (Pa) Ar flux (sccm) H2 flux (sccm) DC sputtering power of Cu target (W) RF sputtering power of GZO target (W) Sputtering time of Cu target (s) Sputtering time of GZO target (s) Thickness of Cu films (or layers) (nm) Thickness of GZO or HGZO films (or layers) (nm)
3 � 10 65 RT 0.8 95 – 30 – 5–20 – 3–13 –
–3
GZO or HGZO
GcG
3 � 10 65 RT 0.8 95 0–20 – 150 – 600 – 100–150
3 � 10 65 RT 0.8 95 – 30 150 5–20 80–240 3~13 20–60
–3
HcH –3
3 � 10–3 65 RT 0.8 95 20 30 150 5–20 120–360 3~13 20–60
fabrication of flexible optoelectronic devices. By controlling film thickness, UMFs with high conductivity and optical transparency can be achieved, but they are very sensitive to heat and humidity, which induces rapid degradation of the optoelectronic properties. The properties of the films based on metal nanowires or grids are comparable to those of ITO films, and the film can be produced cost efficiently on a large scale via solution processing or lithography. However, they display a high surface roughness or haze which affects the properties of the device. Although the CNTs and graphene have extremely high conductivity, transparent conductive properties of the films based on graphene and CNTs cannot compete with those of metal-based and TCO films due to structural defects and high junction resistances. In addition, the preparation of the films based on CNTs or graphene on a large scale is relatively difficult and the films also exhibit a high surface roughness. The conducting polymers have problem of lower conductivity or easily degradable nature. It can be seen that single material cannot satisfy multifunctional requirements for TCFs, and thus composite materials such as TCO/ metal/TCO, CNTs/polymer and metal/polymer have been also developed [1,4,7,8]. In TCO/metal/TCO tri-layer films, metal layer is the key element for obtaining low resistance of the films, and the top and bottom TCO layers can preserve metal layer from oxidation, suppress the light reflection from the metal layer and also act as conducting layer. Because this structure can achieve high trans mittance and low resistivity, TCO/metal/TCO tri-layer films have been studied intensively in recent years with use of various metal and TCO materials. Among the most common metal materials, Al is cheap but it has high optical loss and relatively high resistivity; Au has high heat and chemical stability but it is very expensive; Ag has lowest resistivity and optical loss but it is also expensive; and Cu has comparable electrical properties, less expensive but inferior transmittance compared with Ag [9–11]. In2O3, SnO2, ZnO and their doped oxide films are common TCO film candidates. Among these TCO films, ZnO and doped ZnO films have been widely investigated due to their favorable characteristic of abundance in nature, non-toxicity and stability in hydrogen plasma. Especially, group-III el ements, such as Al, Ga, In and B, doped ZnO films have comparable transparent conductive properties with ITO films [12,13]. However, as similar as ITO films, Al, Ga, In or B doped ZnO films cannot obtain high transparent conductive properties at substrate temperature of room temperature (RT), which is disadvantageous to obtain high performance tri-layer structured TCFs. Previous studies have indicated that the transparent conductive properties of ZnO-based films can be significantly improved by introducing H2 into deposition atmosphere at substrate temperature of RT [14,15]. Unfortunately, hydrogenated ZnO-based films are seldom used to fabricate tri-layer structured TCFs [16]. In this study, Cu films were used as the metal layers, and transparent conductive properties of single Cu films were investigated as a function of film thickness in order to better understand the properties of tri-layer structured TCFs. Considering that Ga dopant is less reactive with oxygen and covalent bond length of Ga–O (1.92 Å) is more close to that of Zn–O (1.97 Å) compared with other group-III elements [17], Ga-doped ZnO (GZO) films were used as TCO layers. The hydrogenated GZO (HGZO) films were prepared by intro ducing H2 into deposition atmosphere, and their transparent conductive properties were investigated as a function of H2 flux to obtain optimized H2 flux. Finally, GZO/Cu/GZO (GcG) and HGZO/Cu/HGZO (HcH) tri-layer films were prepared, and their transparent conductive properties were investigated as a function of the thickness of Cu and GZO (or HGZO) layers. The bandgap (Eg) of the GZO and tri-layer films was also investigated in the paper. The results indicate that single Cu film cannot easily obtain highly transparent conducive properties due to its significantly reduced transmittance with increasing film thickness. By introducing appropriate flux of H2 into deposition atmosphere, the transparent conducive properties of GZO films can be significantly improved. At the optimized thickness of Cu and GZO (or HGZO) layers, GcG and HcG tri-layer films can be achieved with better transparent conductive properties than those of single Cu, GZO or HGZO films. Compared with GcG tri-layer films, better transparent conductive properties of HcH tri-layer films indicate that introduction of H2 into deposition atmosphere is an effective method to improve the properties of ZnO-based tri-layer films. The Eg of the films is found to be related to the carrier concentration and can be explained by Burstein-Moss effect and many-body effect. 2. Experimental procedures All the films were deposited on a thin plate of soda-lime-silica glass in a magnetron sputtering system. Before deposition, the substrates were cleaned in alcohol, acetone and distilled water successively in an ultrasonic cleaner for 15 min. The purity of GZO (3 wt 2
Superlattices and Microstructures 140 (2020) 106456
B.L. Zhu et al.
Fig. 1. (a) The transmittance spectra of different thicknesses of Cu films. The ρ, n and μ (b) and Rs, Tvis and FOM (c) of Cu films as a function of film thickness.
% Ga2O3) and Cu targets was higher than 99.99%, and their size was φ60 mm � 6 mm. The process parameters of film preparation are shown in Table 1. The GcG and HcH tri-layer films were prepared by depositing oxide layer, metal layer and oxide layer on glass substrates consecutively without a vacuum break, and the thickness of top and bottom GZO or HGZO layers was kept same. The Cu films were firstly deposited for longer time (3–15 min) and thus their thickness could be easily measured by interference microscope. According the film thickness as a function of deposition time, the deposition rate was obtained (0.67 nm/s), and then the Cu films or mid-layer with different thicknesses (3–13 nm) were deposited by controlling deposition time (5–25 s). Interference microscope (SC57-6JA) was used to determine the thickness of the films. The crystalline structure of the films was analyzed by X-ray diffraction (XRD; D8 Advance, Bruker Axs) with Cu Kα1 incident radiation in θ-2θ Bragg-Brentano geometry. The electrical properties of the films, including resistivity (ρ), sheet resistance (Rs), carrier concentration (n) and mobility (μ) were determined by Hall Effect measurement using Van der Pauw method at RT. The transmission spectra of the films were obtained from UV–visible spectrophotometer (UV–2102PC, Unico) in the wavelength range of 300–900 nm. The average transmittance in visible light range (Tvis) of the films was obtained in the range of 400–800 nm. The Eg of the films can be estimated by using the Tauc method [18], i. e., αhν ¼ A(hν-Eg)1/2, where α is absorption coefficient, A is a constant, and hν is the photon energy. For a given film thickness (t), the α of the films can be obtained using relation of T ¼ (1-R)2 exp (-αt), where T is the transmittance of the film and R is the reflectance [19]. Since the reflectance is negligible and insignificant near the absorption edge [19], this relation simplifies to the following relation: α ¼ -lnT/t. In order to evaluate the transparent conductive properties of the films synthetically, figure of merit (FOM) of the films can be defined as T10 vis/Rs [20]. The higher FOM indicates that the films possess high transmittance and low resistivity simultaneously.
3
Superlattices and Microstructures 140 (2020) 106456
B.L. Zhu et al.
(caption on next page) 4
Superlattices and Microstructures 140 (2020) 106456
B.L. Zhu et al.
Fig. 2. The transmittance spectra (a) and (αhν)2 versus hν plots (b) of GZO films prepared at different H2 fluxes. The insert in (b) shows the Eg of GZO films as a function of H2 flux. The ρ, n and μ (c) and Rs, Tvis and FOM (d) of GZO films as a function of H2 flux.
Fig. 3. The XRD patterns of GZO films prepared with different H2 fluxes. The position (2θ), full width at half maximum (FWHM) and crystallite size (D) of the diffraction peaks are marked in the figure.
3. Results and discussion 3.1. Single Cu films Fig. 1 shows the properties of the Cu films as a function of film thickness. As shown in Fig. 1(a), the transmittance spectra of Cu films show a peak at about 600 nm, and transmittance decreases with increasing thickness of Cu films. As can be seen from Fig. 1(b), the carrier concentration and mobility of the film increase, and its resistivity decreases with increasing film thickness. Furthermore, a great decrease in resistivity can be observed when the film thickness increases from 5 to 7 nm. From Fig. 1(c), it can be found that the Rs, Tvis and FOM decrease with increasing thickness of Cu film. At Cu film thickness of 3 nm, the film displays maximum FOM of 1.37 � 10 3 Ω 1, with Rs of 47.8 Ω/sq. and Tvis of 76.12%. In previous studies [21,22], the Cu film with thickness of 8 nm had TVis of 64% and Rs of 15 Ω/sq. When the thickness of top Ni layer was fixed at 1 nm, the percolation threshold of Cu layer of Cu–Ni bi-layer films, i.e., the thickness corresponding to which the layer becomes continuous, was found to be between 5.5 and 6.5 nm. Furthermore, bi-layer film had TVis of 63% and Rs of 30 Ω/sq. at Cu layer thickness of 5 nm, and it had TVis of 57% and Rs of 15 Ω/sq. at Cu layer thickness of 7 nm. Although the Rs in these reports is lower, the reduced TVis results in the FOM of the films (2.41 � 10 4- 7.69 � 10 4 Ω 1) being lower than our reported value. According to the 3D (Volmer-Weber) growth mode [23,24], a great decrease in resistivity as film thickness increases from 5 to 7 nm indicates that the continuous Cu film is formed at film thickness of 7 nm. After the formation of continuous Cu films, i.e., the thickness of Cu film is above 7 nm, the conductive properties of Cu film enhance continuously due to the improvement of film crystallinity decreasing the grain boundary scattering [25,26]. In previous studies [27–29], the maxima of transmission spectra had been observed at about 600 nm in Cu films, and the origination could be attributed to the excitation of electrons from the d-band to the Fermi surface. With increasing Cu film thickness, the carrier concentration of the film increases, as mentioned above. Thus, there are more bound electrons available for excitation, leading to a drop in transmittance. On the other hand, reflectance of Cu film increases with increase in Cu film thickness due to formation of continuous films. Thus, the transmittance of Cu film decreases with increasing film thickness. Furthermore, the highest FOM is obtained at Cu film thickness of 3 nm due to the great decrease in transmittance with increasing Cu thickness. 3.2. Single GZO and HGZO films Fig. 2 shows the properties of GZO films as a function of H2 flux. From Fig. 2(a), it can be observed that all the transmittance spectra of the films in the visible light range are flat, and the UV absorption edge of the films shifts to shorter wavelength with increasing H2 flux. The Eg value of the films obtained from (αhν)2 versus hν curve firstly increases and then tends to constant with increasing H2 flux, as shown in Fig. 2(b). As shown in Fig. 2(c), carrier concentration of the film increases, its carrier mobility tends to first increase and then decrease, and its resistivity decreases with increasing H2 flux. It is should be noted that there are relatively large changes in carrier concentration and resistivity when H2 flux increases from 0 to 5 sccm. From Fig. 2(d), the Rs greatly decreases but Tvis and FOM greatly increase as H2 flux increases from 0 to 5 sccm, after that they tend to constant values with further increasing H2 flux. At H2 flux of 20 sccm, Rs, Tvis and FOM of the film are about 315 Ω/sq., 93.5%, and 1.6 � 10 3 Ω 1, respectively. When H2 is introduced into deposition atmosphere, the increase in carrier concentration of GZO films can be attributed to the 5
Superlattices and Microstructures 140 (2020) 106456
B.L. Zhu et al.
Fig. 4. The broadening of the Eg of GZO films as a function of n2/3.
following two factors: (i) H is doped into interstitial positions of the lattice and acts as a shallow donor in ZnO [14,15]; and (ii) the substitution of Ga atoms for Zn atoms is enhanced [30]. With H2 flux increasing from 0 to 5 sccm, the amount of incorporated H in GZO films increases, and thus the carrier concentration of the films greatly increases. However, the incorporated H in GZO films tends to saturate as the H2 flux increases from 5 to 20 sccm, which results in the slight increase of carrier concentration. On the other hand, H plasma will be formed during sputtering when H2 is introduced into deposition atmosphere, and its etching effect on GZO films can reduce the film thickness [15]. Due to the decrease of film thickness, the (002) diffraction peak intensity of GZO films gradually decreases with increasing H2 flux, as shown in Fig. 3. The crystallite size of the films can be calculated according to Scherrer equation [31]. It can be seen from Fig. 3 that the crystallite size of all the films is smaller (4.73–8.35 nm), indicating that the crystal quality of the films is poor due to the low substrate temperature (RT). Furthermore, the carrier mobility of all the films is lower [3.45–5.39 cm2/(V⋅s)] due to poor crystal quality. When H2 flux increases from 0 to 5 sccm, the increase in the carrier mobility of GZO films can be attributed to the increase of crystallize size decreasing the grain-boundary scattering. But when the H2 flux is above 5 sccm, decrease of carrier mobility can be ascribed to the fact that further increased carrier concentration results in ionized impurity scattering [15,32]. Due to the improvement of film crystallinity, passivation of the grain boundary and intrinsic defect and decrease of film thickness after introducing H2 into deposition atmosphere [15,16], the transmittance of GZO film increases with increasing H2 flux. Finally, the increase of TVis and decrease of Rs result in higher FOM values for HGZO films. In addition, the larger low energy tail is observed in Tauc plot for the film deposited using 20 sccm H2, which could be attributed to impurities [33]. As discussed above, more H is doped into interstitial positions of the ZnO lattice and more Zn atoms are replaced by Ga atoms with increasing H2 flux. These impurities could form the level in the forbidden band. The transitions via low-level impurities are responsible for lower energy absorption region [33]. With regard to Eg of the films, it can be discussed according to the change in carrier concentration. From Fig. 2(b) and (c), it can be found that the Eg of GZO films increases with increase in carrier concentration. Compared to the Eg of pure ZnO (3.28 eV), the broadening values of the Eg of GZO film can be obtained, and they are shown as a function of n2/3 in Fig. 4. It can be seen that the broadening of the Eg of GZO films is proportional to n2/3, indicating that this broadening is due to the Burstein-Moss effect, as suggested by other reports [15,34]. According to the Burstein-Moss effect [35,36], the increase of carrier concentration will cause shift of Fermi level towards a higher energy and the lowest states in the conduction band (in an n-type semiconductor) are filled with electrons. Thus, interband transitions correspond to the excitation of valence band electrons into empty states above the minimum of the conduction band, leading to a broadening of the Eg. 3.3. GcG and HcH tri-layer films Fig. 5 shows the properties of GcG and HcH tri-layer films as a function of Cu layer thickness when the thickness of top and bottom GZO or HGZO layers is kept at 40 nm. As can be seen from Fig. 5 (a) and (b), similar to Cu films, the transmittance spectra of tri-layer films also show a peak at about 600 nm. For both types of tri-layer film, the UV absorption edge can be observed and it shifts to longer wavelength with increasing Cu layer thickness. From Fig. 5 (c) and (d), the Eg shows a decreased trend with increasing Cu layer thickness. As shown in Fig. 5(e), the carrier concentration and mobility of the tri-layer film increase, and its resistivity decreases with increasing thickness of Cu layer. As shown in Fig. 5(f), the Rs decreases, but the TVis first increases slightly and then decreases with increasing Cu layer thickness. As Cu layer thickness increases from 3 to .7 nm, it should be noted that the Rs of films shows a relatively large decrease and Tvis slightly increases. The FOM of the films first increases and then decreases with increasing Cu layer thickness, and the maximum FOM is achieved at Cu layer thickness of 7 nm. Fig. 6 shows the properties of GcG and HcH tri-layer films as a function of GZO or HGZO layer thickness at the optimized Cu layer thickness of 7 nm. From Fig. 6 (a) and (b), the transmittance spectra of tri-layer films with different GZO or HGZO layer thicknesses also show a peak at about 600 nm. For both types of tri-layer films, the UV absorption edge shifts to longer wavelength with increasing GZO 6
Superlattices and Microstructures 140 (2020) 106456
B.L. Zhu et al.
Fig. 5. The transmittance spectra (a, b) and (αhν)2 versus hν plots (c, d) of GcG (a, c) and HcH (b, d) tri-layer films with different thicknesses of Cu layer. The inserts in (c) and (d) show the Eg of tri-layer films as a function of Cu layer thickness. The ρ, n and μ (e) and Rs, Tvis and FOM (f) of tri-layer films as a function of Cu layer thickness.
or HGZO layer thickness. From Fig. 6 (c) and (d), the Eg first increases and then slightly decreases for GcG tri-layer films but it increases for HcH tri-layer films with increasing GZO or HGZO layer thickness, respectively. As shown in Fig. 6(e), the carrier concentration of the tri-layer film decreases, its carrier mobility is maintained around a certain value with irregular changes, and the resistivity 7
Superlattices and Microstructures 140 (2020) 106456
B.L. Zhu et al.
Fig. 6. The transmittance spectra (a, b) and (αhν)2 versus hν plots (c, d) of GcG (a, c) and HcH (b, d) tri-layer films with different thicknesses of GZO or HGZO layer. The inserts in (c) and (d) show the Eg of tri-layer films as a function of GZO or HGZO layer thickness. The ρ, n and μ (e) and Rs, Tvis and FOM (f) of tri-layer films as a function of GZO or HGZO layer thickness.
increases with increasing thickness of GZO or HGZO layer thickness. As shown in Fig. 6 (f), the Rs is almost invariable and Tvis and FOM first increase and then decrease with increasing GZO or HGZO layer thickness. Maximum Tvis and FOM are obtained at GZO or HGZO layer thickness of 40 nm. At the same Cu layer and oxide layer thickness, the results shown in Figs. 5 and 6 indicate that HcH tri-layer films have sharper 8
Superlattices and Microstructures 140 (2020) 106456
B.L. Zhu et al.
Fig. 7. (a) The comparison in Rs of single Cu film and tri-layer films. The insert in (a) shows the schematic representation of the electrical behavior of TCO/metal/TCO tri-layer films. Schematic energy band diagrams of GZO/Cu (b) and HGZO/Cu (c) systems before and after contact.
9
Superlattices and Microstructures 140 (2020) 106456
B.L. Zhu et al.
transmission peaks and higher Eg than those of GcG tri-layer films. For the conductive properties of the films, HcH tri-layer films have lower resistivity, higher carrier concentration and mobility than those of GcG tri-layer films. Furthermore, HcH tri-layer films have better transparent conductive properties than GcG tri-layer films, namely, lower Rs, higher Tvis and higher FOM values. In present study, 40 nm HGZO/7 nm Cu/40 nm HGZO film displays maximum FOM of 7.89 � 10 3 Ω 1, with Rs of 18 Ω/sq. and Tvis of 82.3%. For 40 nm GZO/7 nm Cu/40 nm GZO films, FOM, Rs, and Tvis are about 4.91 � 10 3 Ω 1, 23 Ω/sq. and 80.4%, respectively. In previous GcG tri-layer films reported by Gong et al. [11], the 10 nm GZO/10 nm Cu/10 nm GZO had the highest FOM (2.68 � 10 3 Ω 1). Wang et al. investigated three kinds of multilayer structures (AZO/Cu, AZO/Cu/AZO, and Cu/AZO) [37], and found that the 40 nm AZO/7 nm Cu bi-layer film displayed the highest FOM of 4.82 � 10 3 Ω 1, with Rs of 21.7 Ω/sq. and TVis of 80% (400–800 nm). It can be found that the transparent conductive properties obtained in our study are superior to these reports. However, Yang and Song et al. reported that 40 nm AZO/8 nm Cu/40 nm AZO tri-layer film displayed the FOM of 1.94 � 10 2 Ω 1, with Rs of 9 Ω/sq and TVis of 84% [38,39]. The results of these reports are superior to ours, which may be due to the fact that the Cu layer is prepared with different parameters, but the detailed reason needs to be further studied. The TCO/metal/TCO tri-layer structured films are usually considered as a parallel circuit of three resistors [7,40,41], as shown in insert of Fig. 7(a). According to this model, the Rs of tri-layer films should be smaller than that of single Cu films. However, compared with Rs of single Cu films, the Rs of HcH tri-layer films is close but that of GcG tri-layer films is higher, as shown in Fig. 7(a). This result indicates that the TCO layer also affects the conductive properties of tri-layer films in addition to metal layer. In fact, the conductive properties of tri-layer films should consider the carrier transfer between the TCO and metal layers [11,42], which is not considered in parallel circuit model. The studies have indicated that the work function (Φ) values of the metals are usually lower than those of TCOs [43–45]. When metal layer is in contact with TCO layer, the electron will transfer from metal to TCO layer in order to keep ther modynamic equilibrium, leading to the same Fermi level throughout the junction. The transfer of electron will result in an accu mulation type contact between the metal and TCO layers and bending downward of semiconductor band at the contact. In this study, the schematic energy band diagrams of GZO/Cu and HGZO/Cu systems before and after contact are shown in Fig. 7(b) and (c), respectively. As shown in Fig. 7(b), electrons transfer from Cu layer to GZO layer when GZO layer is in contact with Cu layer, which results in weakening of the conductive properties of Cu layer in tri-layer films. Thus, the Rs of GcG tri-layer films is higher than that of single Cu films. However, the work function of HGZO film is smaller than that of GZO film due to its higher carrier concentration [46, 47], and thus relatively smaller amount of electrons transfer from Cu layer to HGZO layer when HGZO layer is in contact with Cu layer, as shown in Fig. 7(c). This implies that Cu layer in HcH tri-layer films keeps higher conductive properties and thus their Rs is close to that of single Cu films. On the other hand, the Rs of both types of tri-layer films is obviously lower than that of single GZO or HGZO film due to electrons injection from the Cu layer into the GZO or HGZO layer. After electron transfer from metal into TCO layer, the carrier concentration (n) of TCO/metal/TCO tri-layer films can be estimated Meta l by the relation of n ffi dMetalNþ2�d [48], where NMetal is the carrier concentration of metal layer and dMeatl and dTCO are the thickness of TCO
metal layer and TCO layer, respectively. Since the variation of dMetal is smaller and the NMetal increases with increasing dMetal, carrier concentration of tri-layer films increases with increasing Cu layer thickness but it decreases with increasing GZO or HGZO layer thickness. As discussed above, Cu layer in HcH tri-layer films keeps higher conductive properties, i.e., has higher carrier concentration, and thus the carrier concentration of HcH tri-layer films is higher than that of GcG tri-layer films at the same Cu and GZO (or HGZO) layer thickness. The carrier mobility of tri-layer films is related to the carrier mobility of TCO and metal layers [42,43]. Obviously, the carrier mobility of Cu layer is dominative due to its smaller resistance, and it increases with increasing layer thickness. Thus, carrier mobility of GcG or HcH tri-layer films increases with increasing Cu layer thickness at the same GZO or HGZO layer thickness, but it is maintained around a certain value with increasing GZO (or HGZO) films at the fixed Cu layer thickness. For the higher carrier mobility of HcH tri-layer films than that of GcG tri-layer film, it can be attributed to the higher carrier mobility of HGZO layer than that of GZO layer. According to the relations of 1/ρ ¼ nqμ (q is electron charge) and Rs ¼ ρ/t, it is easy to understand the changes in resistivity and Rs of tri-layer films with the Cu layer thickness, the oxide layer thickness, and the oxide type. It is noteworthy that relatively larger decreases in resistivity and Rs are observed in both types of tri-layer films as Cu layer thickness increases from 3 to 7 nm, which in dicates the continuous Cu layer is formed at Cu layer thickness of 7 nm, as suggested in single Cu films. Usually, the TVis of TCO/metal/TCO tri-layer structured films shows a trend of first increase and then decrease with increasing metal layer thickness, and the highest TVis is obtained once the continuous metal layer is formed [44,48,49]. In this study, TVis of tri-layer films as a function of Cu layer thickness indicates that continuous Cu layer is formed at Cu layer thickness of 7 nm. Theoretical and experimental results have indicated that suitable thickness of TCO layer can play a role in the antireflection layer to block the reflected light from a metal layer in the visible range, and thus the tri-layer films can be achieved the highest TVis [50–52]. Usually, the suitable thickness of TCO layer is related to the refractive index and extinction coefficient of TCO and metal layers. In this study, suitable thickness of GZO or HGZO layer is obtained at 40 nm, which is basically consistent with previous reports [7]. Due to obtaining of highest TVis and relatively lower Rs, highest FOM is achieved at Cu layer thickness of 7 nm and GZO or HGZO layer thickness of 40 nm. Basically, the TVis of HcH tri-layer films is higher than that of GcG tri-layer films, which may be related to higher TVis of HGZO layer. Furthermore, HcH tri-layer films display higher TVis and lower Rs than those of GcG tri-layer films, and thus their FOM is higher. On a whole, the Eg of tri-layer films shows a decreased trend with increase in carrier concentration from Figs. 5(c)–(e) and 6(c)-(e). Compared to single GZO film (3.29 eV) and HGZO film (3.69 eV), the narrowing values of Eg of tri-layer films can be obtained, and they are shown as a function of n1/3 in Fig. 8. It can be seen that the narrowing of Eg of tri-layer films is roughly proportional to n1/3, indicating that this narrowing is due to the many-body effect, as suggested by some researchers [34,53,54]. The many-body effect is mutual exchange and Coulomb interactions between the added free electrons in the conduction band and electron impurity scattering [55]. 10
Superlattices and Microstructures 140 (2020) 106456
B.L. Zhu et al.
Fig. 8. The narrowing of the Eg of tri-layer films as a function of n1/3.
4. Conclusions The resistivity, Rs, TVis and FOM of single Cu films decrease with increasing film thickness from 3 to 13 nm, and the continuous film is formed at thickness of 7 nm. With increase of H2 flux, the resistivity and Rs of single GZO films decrease but their TVis and FOM increase. For both GcG and HcH tri-layer films, their resistivity and Rs decrease but the TVis first increases and then decreases with increasing Cu layer thickness. With increasing GZO or HGZO layer thickness, resistivity of GcG and HcH tri-layer films increases, Rs keeps constant, and TVis first increases and then decreases. When Cu layer thickness is 7 nm and GZO (or HGZO) layer thickness is 40 nm, two kinds of tri-layer films have the best FOM. Compared with GcG tri-layer films, HcH tri-layer films have better FOM, indicating that introducing H2 in sputtering atmosphere is an effective method to improve the transparent conductive properties tri-layer films. With increasing H2 flux, carrier concentration of GZO film increases and thus its Eg increases due to the Burstein-Moss effect. With increasing Cu layer thickness or decreasing GZO (or HGZO) layer thickness, carrier concentration of tri-layer films increases, which results in decreased trend of the Eg due to the many-body effect. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement B.L. Zhu: Conceptualization, Methodology, Writing - original draft. J.M. Ma: Investigation, Validation. K. Lv: Investigation. C.J. Wang: Investigation. J. Wu: Formal analysis. Z.H. Gan: Resources. J. Liu: Writing - review & editing. X.W. Shi: Writing - review & editing. Acknowledgments This work was supported by the National Nature Science Foundation of China (Grant No. 50902105). References [1] A.I. Hofmann, E. Cloutet, G. Hadziioannou, Materials for transparent electrodes: from metal oxides to organic alternatives, Adv. Electron. Mater. 4 (2018) 1700412, https://doi.org/10.1002/aelm.201700412. [2] A. Kumar, C.W. Zhou, The race to replace tin-doped indium oxide: which material will win? ACS Nano 4 (2010) 11–14, https://doi.org/10.1021/nn901903b. [3] S. Sharma, S. Shriwastava, S. Kumar, K. Bhatt, C.C. Tripathi, Alternative transparent conducting electrode materials for flexible optoelectronic devices, OptoElectron. Rev. 26 (2018) 223–235, https://doi.org/10.1016/j.opelre.2018.06.004. [4] L.X. He, S.C. Tjong, Nanostructured transparent conductive films: fabrication, characterization and applications, Math. Sci. Eng. R 109 (2016) 1–101, https:// doi.org/10.1016/j.mser.2016.08.002. [5] W.R. Cao, J. Li, H.Z. Chen, J.G. Xue, Transparent electrodes for organic optoelectronic devices: a review, J. Photon. Energy 4 (2014), 040990, https://doi.org/ 10.1117/1.JPE.4.040990. [6] M. Morales-Masis, S.D. Wolf, R. Woods-Robinson, J.W. Ager, C. Ballif, Transparent electrodes for efficient optoelectronics, Adv. Electron. Mater. 3 (2017) 1600529, https://doi.org/10.1002/aelm.201600529. [7] C. Guillen, J. Herrero, TCO/metal/TCO structures for energy and flexible electronics, Thin Solid Films 520 (2011) 1–17, https://doi.org/10.1016/j. tsf.2011.06.091. [8] L. Cattin, J.C. Bernede, M. Morsli, Toward indium-free optoelectronic devices: dielectric/metal/dielectric alternative transparent conductive electrode in organic photovoltaic cells, Phys. Status Solidi A 210 (2013) 1047–1061, https://doi.org/10.1002/pssa.201228089.
11
Superlattices and Microstructures 140 (2020) 106456
B.L. Zhu et al.
[9] J. Lou, M. Bao, B.J. Ye, H.M. Weng, H.J. Du, Z.B. Zhang, The design of transparent conductive ZnO/metal/ZnO multilayers for incidence from 0� to 70� , J. Opt. A-Pure Appl. Opt. 11 (2009), 085501 https://doi.org/10.1088/1464-4258/11/8/085501. [10] H. Zhou, J. Xie, M.F. Mai, J. Wang, X.Q. Shen, S.Y. Wang, L.H. Zhang, K. Kisslinger, H.Q. Wang, J.X. Zhang, Y. Li, J.H. Deng, S.M. Ke, X.R. Zeng, High-quality AZO/Au/AZO sandwich film with ultralow optical loss and resistivity for transparent flexible electrodes, ACS Appl. Mater. Interfaces 10 (2018) 16160–16168, https://doi.org/10.1021/acsami.8b00685. [11] L. Gong, J.G. Lu, Z.Z. Ye, Conductive Ga doped ZnO/Cu/Ga doped ZnO thin films prepared by magnetron sputtering at room temperature for flexible electronics, Thin Solid Films 519 (2011) 3870–3874, https://doi.org/10.1016/j.tsf.2011.01.396. [12] T. Minami, Present status of transparent conducting oxide thin-film development for indium-tin-oxide (ITO) substitutes, Thin Solid Films 516 (2008) 5822–5828, https://doi.org/10.1016/j.tsf.2007.10.063. [13] S.C. Dixon, D.O. Scanlon, C.J. Carmalt, I.P. Parkin, n-Type doped transparent conducting binary oxides: an overview, J. Mater. Chem. C 4 (2016) 6946–6961, https://doi.org/10.1039/c6tc01881e. [14] B.L. Zhu, J. Wang, S.J. Zhu, J. Wu, R. Wu, D.W. Zeng, C.S. Xie, Influences of hydrogen introduction on structure and properties of ZnO thin films during sputtering and post-annealing, Thin Solid Films 519 (2011) 3809–3815, https://doi.org/10.1016/j.tsf.2011.01.187. [15] B.L. Zhu, J. Wang, S.J. Zhu, J. Wu, D.W. Zeng, C.S. Xie, Investigation of structural, electrical, and optical properties for Al-doped ZnO films deposited at different substrate temperatures and H2 ratios, J. Electrochem. Soc. 159 (2012) H536–H544, https://doi.org/10.1149/2.022206jes. [16] G.H. Yan, Y. Yuan, W.L. Chen, R.J. Hong, Influence of H2 flow ratio on the photoelectric properties of hydrogenated AZO thin films with embedded silver layer, Mater. Lett. 185 (2016) 272–274, https://doi.org/10.1016/j.matlet.2016.09.004. [17] H.J. Ko, Y.F. Chen, S.K. Hong, H. Wenisch, T. Yao, D.C. Look, Ga-doped ZnO films grown on GaN templates by plasma-assisted molecular-beam epitaxy, Appl. Phys. Lett. 77 (2000) 3761–3763, https://doi.org/10.1063/1.1331089. [18] B.D. Viezbicke, S. Patel, B.E. Davis, D.P. Birnie, Evaluation of the Tauc method for optical absorption edge determination: ZnO thin films as a model system, Phys. Status Solidi B 252 (8) (2015) 1700–1710, https://doi.org/10.1002/pssb.201552007. [19] E. Senadım, H. Kavak, R. Esen, The effect of annealing on structural and optical properties of ZnO thin films grown by pulsed filtered cathodic vacuum arc deposition, J. Phys.-Condens. Mat. 18 (2006) 6391–6400, https://doi.org/10.1088/0953-8984/18/27/021. [20] G. Haacke, New Figure of merit for transparent conductors, J. Appl. Phys. 47 (1976) 4086–4089, https://doi.org/10.1063/1.323240. [21] D.S. Ghosh, R. Betancur, T.L. Chen, V. Pruneri, JordiMartorell, Semi-transparent metal electrode of Cu-Ni as a replacement of an ITO in organic photovoltaic cells, Sol. Energ. Mat. Sol. C. 95 (2011), https://doi.org/10.1016/j.solmat.2010.12.040, 1228-123. [22] S. Cheylan, D.S. Ghosh, D. Krautz, T.L. Chen, V. Pruneri, Organic light-emitting diode with indium-free metallic bilayer as transparent anode, Org. Electron. 12 (2011) 818–822, https://doi.org/10.1016/j.orgel.2011.02.015. [23] C.T. Campbell, Metal films and particles on oxide surfaces: structural, electronic and chemisorptive properties, J. Chem. Soc. Faraday. Trans. 92 (1996) 1435–1445, https://doi.org/10.1039/ft9969201435. [24] N. Kaiser, Review of the fundamentals of thin-film growth, Appl. Optic. 41 (2002) 3053–3060, https://doi.org/10.1364/AO.41.003053. [25] J.W. Lim, M. Isshiki, Electrical resistivity of Cu films deposited by ion beam deposition: effects of grain size, impurities, and morphological defect, J. Appl. Phys. 99 (2006), 094909, https://doi.org/10.1063/1.2194247. [26] W. Zhang, S.H. Brongersma, T. Clarysse, et al., Surface and grain boundary scattering studied in beveled polycrystalline thin Cu films, J. Vac. Sci. Technol. B 22 (2004) 1830–1833, https://doi.org/10.1116/1.1771666. [27] A. Axelevitch, B. Gorenstein, G. Golan, Investigation of optical transmission in thin metal films, Phys. Proc. 32 (2012) 1–13, https://doi.org/10.1016/j. phpro.2012.03.510. [28] A. Belosludtsev, N. Kyzas, Real-time in-situ investigation of copper ultrathin films growth, Mater. Lett. 232 (2018) 216–219, https://doi.org/10.1016/j. matlet.2018.08.127. [29] H. Ehrenreich, H.R. Philipp, Optical properties of Ag and Cu, Phys. Rev. 128 (1962) 1622–1629, https://doi.org/10.1103/PhysRev.128.1622. [30] H. Akazawa, Hydrogen induced electric conduction in undoped ZnO and Ga-doped ZnO thin films: creating native donors via reduction, hydrogen donors, and reactivating extrinsic donors, J. Vac. Sci. Technol., A 32 (2014), 051511, https://doi.org/10.1116/1.4892777. [31] B.L. Zhu, D.W. Zeng, J. Wu, W.L. Song, C.S. Xie, Synthesis and gas sensitivity of In-doped ZnO nanoparticles, J. Mater. Sci. Mater. Electron. 14 (2003) 521–526, https://doi.org/10.1023/A:1023989304943. [32] W.M. Kim, Y.H. Kim, J.S. Kim, J. Jeong, Y.J. Baik, J.K. Park, K.S. Lee, T.Y. Seong, Hydrogen in polycrystalline ZnO thin films, J. Phys. D Appl. Phys. 43 (2010) 365406, https://doi.org/10.1088/0022-3727/43/36/365406. [33] S.S. Lin, J.L. Huang, Effect of thickness on the structural and optical properties of ZnO films by r.f. magnetron sputtering, Surf. Coating. Technol. 185 (2004) 222–227, https://doi.org/10.1016/j.surfcoat.2003.11.014. [34] J.G. Lu, S. Fujita, T. Kawaharamura, H. Nishinaka, Y. Kamada, T. Ohshima, Z.Z. Ye, Y.J. Zeng, Y.Z. Zhang, L.P. Zhu, H.P. He, B.H. Zhao, Carrier concentration dependence of band gap shift in n-type ZnO:Al films, J. Appl. Phys. 101 (2007), 083705, https://doi.org/10.1063/1.2721374. [35] E. Burstein, Anomalous optical absorption limit in InSb, Phys. Rev. 93 (1954) 632–633, https://doi.org/10.1103/PhysRev.93.632. [36] T.S. Moss, The interpretation of the properties of indium antimonide, Proc. Phys. Soc. B 67 (1954) 775–782, https://doi.org/10.1088/0370-1301/67/10/306. [37] Y.P. Wang, J.G. Lu, X. Bie, Z.Z. Ye, X. Li, D. Song, X.Y. Zhao, W.Y. Ye, Transparent conductive and near-infrared reflective Cu-based Al-doped ZnO multilayer films grown by magnetron sputtering at room temperature, Appl. Surf. Sci. 257 (2011) 5966–5971, https://doi.org/10.1016/j.apsusc.2011.01.068. [38] T.L. Yang, Z.S. Zhang, S.M. Song, Y.H. Li, M.S. Lv, Z.C. Wu, S.H. Han, Structural, optical and electrical properties of AZO/Cu/AZO tri-layer films prepared by radio frequencymagnetron sputtering and ion-beam sputtering, Vacuum 83 (2) (2009) 257–260, https://doi.org/10.1016/j.vacuum.2008.05.029. [39] S.M. Song, T.L. Yang, M.S. Lv, Y.H. Li, Y.Q. Xin, L.L. Jiang, Z.C. Wu, S.H. Han, Effect of Cu layer thickness on the structural, optical and electrical properties of AZO/Cu/AZO tri-layer films, Vacuum 85 (2010) 39–44, https://doi.org/10.1016/j.vacuum.2010.03.008. [40] A. Kloppel, W. Kriegseisa, B.K. Meyer, A. Scharmann, C. Daube, J. Stollenwerk, J. Trube, Dependence of the electrical and optical behaviour of ITO-silver-ITO multilayers on the silver properties, Thin Solid Films 365 (2000) 139–146, https://doi.org/10.1016/S0040-6090(99)00949-9. [41] M. Bender, W. Seelig, C. Daube, H. Frankenberger, B. Ocker, J. Stollenwerk, Dependence of film composition and thicknesses on optical and electrical properties of ITO-metal-ITO multilayers, Thin Solid Films 326 (1998) 67–71, https://doi.org/10.1016/S0040-6090(98)00520-3. [42] K. Sivaramakrishnan, N.D. Theodore, J.F. Moulder, T.L. Alford, The role of copper in ZnO/Cu/ZnO thin films for flexible electronics, J. Appl. Phys. 106 (2009), 063510, https://doi.org/10.1063/1.3213385. [43] A. Indluru, T.L. Alford, Effect of Ag thickness on electrical transport and optical properties of indium tin oxide-Ag-indium tin oxide multilayers, J. Appl. Phys. 105 (2009) 123528, https://doi.org/10.1063/1.3153977. [44] S.H. Yu, W.F. Zhang, L.X. Li, D. Xu, H.L. Dong, Y.X. Jin, Optimization of SnO2/Ag/SnO2 tri-layer films as transparent composite electrode with high figure of merit, Thin Solid Films 552 (2014) 150–154, https://doi.org/10.1016/j.tsf.2013.11.109. [45] K.M. Shafi, R. Vinodkumar, R.J. Bose, V.N. Uvais, V.P.M. Pillai, Effect of Cu on the microstructure and electrical properties of Cu/ZnO thin films, J. Alloys Compd. 551 (2013) 243–248, https://doi.org/10.1016/j.jallcom.2012.10.032. [46] J.A. Sans, J.F. Sanchez-Royo, A. Segura, G. Tobias, E. Canadell, Chemical effects on the optical band-gap of heavily doped ZnO:MIII (M¼Al,Ga, In):An investigation by means of photoelectron spectroscopy, optical measurements under pressure, and band structure calculations, Phys. Rev. B 79 (2009) 195105, https://doi.org/10.1103/PhysRevB.79.195105. [47] J.J. Jia, A. Takasaki, N. Oka, Y. Shigesato, Experimental observation on the Fermi level shift in polycrystalline Al-doped ZnO films, J. Appl. Phys. 112 (2012), 013718, https://doi.org/10.1063/1.4733969. [48] S.H. Yu, L.X. Li, X.S. Lyu, W.F. Zhang, Preparation and investigation of nano-thick FTO/Ag/FTO multilayer transparent electrodes with high figure of merit, Sci. Rep. 6 (2016) 20399, https://doi.org/10.1038/srep20399. [49] A. Bingel, M. Steglich, P. Naujok, R. Muller, U. Schulz, N. Kaiser, A. Tunnermann, Influence of the ZnO:Al dispersion on the performance of ZnO:Al/Ag/ZnO:Al transparent electrodes, Thin Solid Films 616 (2016) 594–600, https://doi.org/10.1016/j.tsf.2016.09.032.
12
Superlattices and Microstructures 140 (2020) 106456
B.L. Zhu et al.
[50] S. Kim, J.L. Lee, Design of dielectric/metal/dielectric transparent electrodes for flexible electronics, J. Photon. Energy 2 (2012), 021215, https://doi.org/ 10.1117/1.JPE.2.021215. [51] H.K. Lee, J.Y. Na, Y.J. Moon, T.Y. Seong, S.K. Kim, Design of near-unity transmittance dielectric/Ag/ITO electrodes for GaN-based light-emitting diodes, Curr. Appl. Phys. 15 (2015) 833–838, https://doi.org/10.1016/j.cap.2015.04.044. [52] D. Ebner, M. Bauch, T. Dimopoulos, High performance and low cost transparent electrodes based on ultrathin Cu layer, Optic Express 25 (2017) A240–A252, https://doi.org/10.1364/OE.25.00A240. [53] W.M. Kim, J.S. Kim, J.H. Jeong, J.K. Park, Y.J. Baik, T.Y. Seong, Analysis of optical band-gap shift in impurity doped ZnO thin films by using nonparabolic conduction band parameters, Thin Solid Films 531 (2013) 430–435, https://doi.org/10.1016/j.tsf.2013.01.078. [54] J. Kumar, A.K. Srivastava, Band gap narrowing in zinc oxide-based semiconductor thin films, J. Appl. Phys. 115 (2014) 134904, https://doi.org/10.1063/ 1.4870709. [55] B.E. Sernelius, K.F. Berggren, Z.C. Jin, I. Hamberg, C.G. Granqvist, Band-gap tailoring of ZnO by means of heavy Al doping, Phys. Rev. B 37 (1988) 10244–10248, https://doi.org/10.1103/PhysRevB.37.10244.
13