Applied Surface Science 316 (2014) 62–65
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Realization of Ag-S codoped p-type ZnO thin ﬁlms Tian Ning Xu a,b,∗ , Xiang Li a , Zhong Lu a , Yong Yue Chen b , Cheng Hua Sui a , Hui Zhen Wu b a b
Department of Science, Zhijiang College of Zhejiang University of Technology, Hangzhou, Zhejiang 310024, People’s Republic of China Department of Physics, State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou, Zhejiang 310027, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 6 March 2014 Received in revised form 13 June 2014 Accepted 24 July 2014 Available online 1 August 2014 Keywords: ZnO p-type Ag S Homojunction
a b s t r a c t Ag-S codoped ZnO ﬁlms have been grown on quartz substrates by e-beam evaporation at low temperature (100 ◦ C). The effects of Ag2 S content on the structural and electrical properties of the ﬁlms were investigated. The results showed that 2 wt% Ag2 S doped ﬁlms exhibited p-type conduction, with a resistivity of 0.0347 cm, a Hall mobility of 9.53 cm2 V−1 s−1 , and a hole concentration of 1.89 × 1019 cm−3 at room temperature. The X-ray photoelectron spectroscopy measurements showed that Ag and S have been incorporated into the ﬁlms. To further conﬁrm the p-type conduction of Ag-S codoped ZnO ﬁlms, a ZnO:(Ag, S)/i-ZnO/ZnO:Al homojunction was fabricated and rectifying behaviors of which was measured. High electrical performance and low growth temperature indicate that Ag2 S is a promising dopant to fabricate p-type Ag-S codoped ZnO ﬁlms. © 2014 Elsevier B.V. All rights reserved.
1. Introduction ZnO has a wide and direct bandgap (3.37 eV) and large exciton binding energy (60 meV) at room temperature. These excellent properties make it a promising material for short wavelength optoelectronic device applications, such as ultraviolet (UV) lasers and light-emitting diodes [1–3]. However, the doping asymmetry problem or p-type doping difﬁculty hinders the potential applications of ZnO materials [4,5]. Great efforts have been devoted to fabricate p-type ZnO materials with different dopants, including group I elements (Li, Na) [6,7] and group V elements (N, P, As) [8–11]. The group IB element such as Ag has also attracted much attention because it can act as an acceptor in ZnO, if incorporated in substitutional Zn site. The ﬁrst-principle calculations indicate that the formation energy for Ag on the substitutional site is lower than that on the interstitial site, which suggests Ag could be an effective dopant for producing p-type ZnO . Experimentally, H.S. Kang et al. have fabricated p-type ZnO:Ag ﬁlms by pulsed laser deposition . In their reports, the as-grown ﬁlms show poor electrical properties with high resistivity (34–54 cm), low hole concentration (1016 –1017 cm−3 ) and small hole mobility (0.29–2.32 cm2 V−1 s−1 ). The latest experimental results indicate that it is difﬁcult to improve the electrical properties of p-type ZnO:Ag ﬁlms only by monodoped Ag [14,15]. The main reasons can be attributed to the
∗ Corresponding author. Tel.: +86 57187313644; fax: +86 571 87953885. E-mail address: [email protected]
(T.N. Xu). http://dx.doi.org/10.1016/j.apsusc.2014.07.156 0169-4332/© 2014 Elsevier B.V. All rights reserved.
limited acceptor solubility, high ionization energy of the Ag acceptor and various self-compensating defects . To overcome the above doping problem, Ag-S codoping concept has been reported. L.J. Sun et al. revealed that Ag-S codoping in ZnO generated AgZn nSO defect complexes, and the acceptor ionization energy of which is much lower than that of monodoped Ag . J.C. Li et al. have fabricated the Ag-S codoped ZnO ﬁlms with metal Ag wires and ZnS thin wafers as dopants . Their results also conﬁrmed the AgZn nSO acceptor had lower formation energy, and suggested S alloying in ZnO may increase the solid solubility of Ag . However, the electrical properties of Ag-S codoped ZnO ﬁlms reported by J.C. Li el al are weak. Metal Ag wires and ZnS used as dopants may be not effective to generate AgZn -nSO defects. Moreover, other unnecessary defects such as Ag interstitial may be introduced into the ﬁlms. Theoretical calculation indicates if Ag is introduced initially in an interstitial site, it is difﬁcult to transform to a substitutional site . As a result, new dopant (Ag2 S) should be explored to fabricated p-type Ag-S codoped ZnO with low resistivity, high hole concentration and large hole mobility, by exploiting the merits of Ag-S codoping sufﬁciently. In this paper, the Ag-S codoped p-type ZnO ﬁlms were grown on quartz substrates by e-beam evaporation using Ag2 S as dopant. The effects of Ag2 S content in target on structural and electrical properties of the ﬁlms were investigated. The chemical states of Ag and S in Ag-S codoped ZnO ﬁlms were characterized by a Xray photoemission spectrometer. The p-type conduction of Ag-S codoped ZnO ﬁlm was further conﬁrmed by a rectifying ZnO:(Ag, S)/i-ZnO/ZnO:Al homojunction.
T.N. Xu et al. / Applied Surface Science 316 (2014) 62–65
Fig. 1. (a) The XRD pattern of ZnO:(Ag, S) ﬁlms with different Ag2 S contents. (b) The dependence of (0 0 2) peak position and FWHM of ZnO:(Ag, S) ﬁlms on Ag2 S content.
2. Experiments The targets in this experiment were synthesized using high purity of ZnO (99.99%) and Ag2 S (99.99%) powders. The nominal concentrations of Ag2 S in the targets were 0.5 wt%, 1 wt% and 2 wt%, respectively. The mixed powders were pressed into ﬂat cylinders and then sintered into ceramic targets. The Ag-S codoped ZnO ﬁlms were deposited on quartz substrates by e-beam evaporation . Before deposition, the chamber was evacuated down to 2.5 × 10−3 Pa, and then the working pressure was keeping at about 8.5 × 10−3 Pa. The substrate temperature was 100 ◦ C and no oxygen gas was introduced into the champer during deposition. These growth conditions are inclined to grow Ag-S codoped ZnO with Zn-rich characteristic, which is helpful to obtain p-type ZnO . The ﬁlm thickness was measured to be about 1.0 m by an XP-1 Stylus Proﬁlometer. The crystal structure of the ZnO:(Ag, S) ﬁlms was characterized by X-ray diffraction (XRD, PANalytial X’PRO) ˚ source. The surface morphologies with a Cu K␣ ( = 1.54056 A) were examined by ﬁeld emission scanning electron microscopy (FE-SEM, HITACHIS-4800). The electrical properties were measured by Hall measurements in the van der Pauw conﬁguration (BIORAD HL5500PC) in which indium electrodes were used for ohmic contacts. The chemical composition of the ﬁlms was identiﬁed by X-ray photoelectron spectroscopy (XPS, Kratos AXIS Ultra DLD). The ZnO:(Ag, S)/i-ZnO/ZnO:Al homojunction was made and the current–voltage (I–V) characteristics were measured by a Keithley 2612 system. 3. Results and discussion Fig. 1(a) shows the XRD patterns of undoped and Ag-S codoped ZnO ﬁlms with different Ag2 S contents. It can be seen that the
Ag-S codoped ZnO ﬁlms are polycrystalline and have a hexagonal wurtzite structure. Except the diffraction peaks related with hexagonal ZnO, a peak at 43.0◦ is observed for all three samples. It deviates from (2 0 0) Ag2 S peak (PDF-14-0072) at 43.4◦ and (2 0 0) Ag peak (PDF-42-0874) at 43.8◦ . Furthermore, the same peak were also observed for Ag monodoped ZnO ﬁlms grown at 100 ◦ C with Ag2 O dopant. It vanished for samples grown at high temperature (200 ◦ C). Therefore, it is difﬁcult to denote as Ag2 S or Ag peak simply and may need further investigation. The (0 0 2) peak position and full width at half maximum (FWHM) values of Ag-S codoped ZnO ﬁlms vs. Ag2 S content is illustrated in Fig. 1(b). The (0 0 2) peak position of undoped ZnO is 34.49◦ and then decreases to 34.37◦ for Ag-S codoped ZnO with 2 wt% Ag2S. The (0 0 2) diffraction angle shift originates from the substitution of Zn2+ and O2− by Ag+ and S2− , respectively. The larger ionic radius of Ag1+ and S2− incorporated into ZnO increase the lattice constant. The dependence of FWHM on Ag2 S content indicates Ag-S codoped ZnO ﬁlm with 2 wt% Ag2 S has better crystal quality than those ZnO ﬁlms doped with 1 wt% or 0.5 wt% Ag2 S. Table 1 summarizes the electrical properties of Ag-S codoped ZnO ﬁlms measured by Hall-effect measurements in the van der Pauw conﬁguration. It can be seen that sample A and B show n-type conductivity, while sample C, D, E and F exhibit p-type conductivity. This may be caused by the fact that the amount of AgZn acceptor in the ﬁlm grown with low Ag2 S content is insufﬁcient to compensate the intrinsic donor defects such as VO and Zni . As a result, the ﬁlm exhibited n-type conduction. As the Ag2 S content increases to 2 wt%, more AgZn acceptor defects form, and the amount of the AgZn acceptor is larger than that of native donor defects, leading to p-type conduction. The Hall results are supported by the XRD data. Sample C possesses relatively better electrical properties, with a resistivity of 0.0347 cm, a Hall mobility of 9.53 cm2 V−1 s−1 , and a hole concentration of 1.89 × 1019 cm−3 at room temperature. The experimental errors are estimated within 5% for the Hall mobility. To investigate the reproducibility of p-type Ag-S codoped ZnO ﬁlms, the growth condition of sample C was used to grow new Ag-S codoped ZnO ﬁlms (sample D, E and F). Once again, p-type conductivity with low resistivity, high hole concentration and hall mobility was measured from sample D, E and F. Therefore, the ptype conductivity with high hall mobility is reproducible, although the electrical properties have small difference between sample C and sample D, E and F. To investigate the stability of p-type AgS codoped ZnO ﬁlms, the electrical properties of sample C were retested after a year later. The retested results still exhibit p-type feature. To identify the chemical bonding states of Ag and S in the Ag-S codoped ZnO ﬁlms, the XPS measurement is carried out. The XPS data have been calibrated according to the C 1s peak at 284.6 eV. Fig. 2(a) shows the XPS spectrum of the Ag 3d peak for the AgS codoped ZnO ﬁlm. The inset shows the S 2p peak is located at 167.4 eV in the Ag-S codoped ZnO ﬁlm. This result has never been reported. It is known that the binding energies of S 2p in S element, Ag2 S and ZnS are 164.0 eV , 161.0 eV  and 162.1 eV , respectively. As a result, the S doped in ZnO could not be in the forms of S element, Ag2 S or ZnS. By considering the fact that Ag, Zn, S and O have been detected by XPS, S doped in ZnO substitutes for O site and forms AgZn -nSO complex. A possible sketch of Ag-S codoped ZnO structure is plotted in the inset of Fig. 2(b). This structure is suggested by the ﬁrst-principle calculations . Ag and S codoped in ZnO substitute Zn site and O site, respectively. Therefore, Ag-O bond and Ag-S bond are expected in the ﬁlm. Fig. 2(b) shows the asymmetric Ag 3d5/2 peak can be separated into two peaks by Lorentz ﬁtting, which are located at 367.8 eV and 368.2 eV. This result is similar to the work reported by L.J. Sun et al., and they attributed the former peak to Ag-O bond and the latter peak to AgS bond . The XPS data indicate our experiment can codop Ag
T.N. Xu et al. / Applied Surface Science 316 (2014) 62–65
Table 1 The electrical properties of the Ag-S codoped ZnO ﬁlms. sample A B C D E F
Ag2 S content (%) 0.5 1.0 2.0 2.0 2.0 2.0
Resistivity ( cm)
Carrier concentration (cm−3 )
Mobility (cm2 V−1 s−1 )
0.2427 0.4789 0.0347 0.0222 0.0298 0.0230
2.49 × 10 4.59 × 1018 1.89 × 1019 1.48 × 1019 2.43 × 1019 9.49 × 1018
1.03 2.84 9.53 9.10 8.61 8.52
n n p p p p
and S in ZnO effectively by using Ag2 S dopant. In other hand, the contents of Ag, S, Zn and O in ZnO ﬁlm detected by XPS are 1.30 at%, 0.23 at%, 51.92 at% and 46.55 at%, respectively. Compared to 2 wt% Ag2 S in ZnO target (if convert to atomic concentration for Ag and S are 1.34 at% and 0.67%), Ag content in ﬁlm is consistent with that in target, and S content is less than that in target. It indicates that a little amount of Ag2 S suffers thermal decomposition during the target sintering and ﬁlm growth process. Moreover, the Ag-S codoped ZnO ﬁlm exhibits Zn-rich characteristic. The Zn-rich condition is helpful to form AgZn -nSO complex acceptor . To further conﬁrm the p-type conduction in Ag-S codoped ZnO ﬁlms, ZnO:(Ag, S)/i-ZnO/ZnO:Al homojunction was fabricated. The sketch and a photo of the p-i-n homojunction structure are illustrated in the inset in Fig. 3(a). As shown in the photo, the Ag-S codoped ZnO (the black spots) is opaque and Al-doped ZnO is transparent. While depositing Ag-S codoped ZnO on Al-doped ZnO, a template with eleven holes was used. Fig. 3(a) shows the I–V charac-
Fig. 3. (a) I–V characteristics of the ZnO:(Ag, S)/i-ZnO/ZnO:Al homojunction. The insets show its schematic diagram and real photo. (b) The I–V curves of the probe contact to the p-type Ag-S codoped ZnO layer and n-type ZnO:Al layer.
teristics of the one of ZnO homojunctions. The device exhibits clear rectifying behavior. The same behavior was also observed from the rest ZnO homojunctions in the sample. To avoid the rectiﬁcation from Schottky contact between probe and ZnO ﬁlms, the I–V curves of probe on both p-type ZnO layer and n-type ZnO layer were also measured and plotted in Fig. 3(b). The good linear I–V dependences are observed, which indicates the contacts between probe on both p-type ZnO layer and n-type ZnO layer are Ohmic. Considering the high carrier concentration (1019 cm−3 for hole concentration and 1020 cm−3 for electron concentration), the Ohmic contact without metal electrode is comprehensible. 4. Conclusions
Fig. 2. (a) XPS spectrum of Ag 3d core levels of Ag-S codoped ZnO thin ﬁlm with 2 wt% Ag2S. The XPS spectrum of S 2p levels of the sample is shown in the inset. (b) Ag 3d5/2 peak ﬁtting curves and possible sketch of Ag-S and Ag-O bond in Ag-S codoped ZnO ﬁlm is shown in the inset.
In summary, p-type Ag-S codoped ZnO ﬁlms have been fabricated on quartz substrates with ZnO ceramic targets containing different Ag2 S contents. XRD and Hall measurements show the ﬁlms grown with 2% Ag2 S have better crystal quality and exhibit ptype conduction with a resistivity of 0.0347 cm, a Hall mobility
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of 9.53 cm2 V−1 s−1 , and a hole concentration of 1.89 × 1019 cm−3 at room temperature. XPS measurement indicates that Ag and S have been incorporated into the ZnO ﬁlms and forms AgZn -SO complex acceptor. A homojunction based on ZnO:(Ag, S)/i-ZnO/ZnO:Al exhibits obvious rectifying characteristics. The reproducible and improved p-type conduction of Ag-S codoped ZnO ﬁlms suggests that Ag2 S is a promising dopant for fabricating p-type ZnO with high electrical property. Acknowledgments This work was supported by the Natural Science Foundation of Zhejiang Province (No. Y1110549) and Research Foundation of Education Bureau of Zhejiang Province (No. Y200803369). References  Ü. Özgür, Ya. I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Do˘gan, V. Avrutin, S.-J. Cho, H. Morkoc, A comprehensive review of ZnO materials and devices, J. Appl. Phys. 98 (2005) 0413011–041301103.  Z.K. Tang, G.K.L. Wong, P. Yu, M. Kavasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Room-temperature ultraviolet laser emission from self-assembled ZnO microcrystallite thin ﬁlms, Appl. Phys. Lett. 72 (1998) 3270–3272.  T. Aoki, Y. Hatanaka, D.C. Look, ZnO diode fabricated by excimer-laser doping, Appl. Phys. Lett. 76 (2000) 3257–3258.  S.B. Zhang, S.K. Wei, A. Zunger, A phenomenological model for systematization and prediction of doping limits in II-VI and I-III-VI2 compounds, J. Appl. Phys. 83 (1998) 3192–3196.  Y.F. Yan, J.B. Li, S.H. Wei, M.M. Al-Jassim, Possible approach to overcome the doping asymmetry in wideband gap semiconductors, Phys. Rev. Lett. 98 (2007) 1355061–1355064.  Y.J. Zeng, Z.Z. Ye, W.Z. Xu, D.Y. Li, J.G. Lu, L.P. Zhu, B.H. Zhao, Dopant suource choice for formation of p-type ZnO:Li acceptor, Appl. Phys. Lett. 88 (2006) 0621071–0621073.  S.S. Lin, Robust low resistivity p-type ZnO:Na ﬁlms after ultraviolet illumination:The elimination of grain boundaries, Appl. Phys. Lett. 101 (2012) 1221091–1221094.  J.G. Reynolds, C.L. Reynolds Jr., A. Mohanata, J.F. Muth, J.E. Rowe, H.O. Everitt, D.E. Aspnes, Shallow acceptor complexes in p-type ZnO, Appl. Phys. Lett. 102 (2013) 1521141–1521145.
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