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Realization of p-type conduction in (ZnO)1−x(AlN)x thin films grown by RF magnetron sputtering
Journal of Alloys and Compounds 478 (2009) 54–58
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Realization of p-type conduction in (ZnO)1−x (AlN)x thin films grown by RF magnetron sputtering K.P. Bhuvana a , J. Elanchezhiyan b , N. Gopalakrishnan a , B.C. Shin b , T. Balasubramanian a,∗ a b
Thin films Laboratory, Department of Physics, National Institute of Technology, Tiruchirappalli 620 015, India Electronic Ceramics Center, Dong Eui University, Busan-614-714, South Korea
a r t i c l e
i n f o
Article history: Received 27 July 2008 Received in revised form 5 December 2008 Accepted 10 December 2008 Available online 24 December 2008 PACS: 71.55.Gs 73.61.Ga Keywords: p-Type ZnO RF magnetron sputtering Codoping
a b s t r a c t A different approach for codoping in ZnO using AlN as dopant (codopant) has been attempted to realize p-ZnO by RF magnetron sputtering. The (ZnO)1−x (AlN)x [AlN codoped ZnO] films of different doping concentrations (0.5, 1, 2 and 4 mol%) grown on Si(1 0 0) substrates have been subjected to X-ray diffraction (XRD) and Hall measurements to investigate their structural and electrical properties, respectively. XRD results reveal that all the films are constituted in wurtzite structure with the preferential orientation of (0 0 2) diffraction plane. It has been observed that the c-axis lattice constant is higher than unstressed bulk value for 0.5 and 1 mol% AlN doped ZnO films which support the incorporation of N atoms into the film. The Hall measurements show that the (ZnO)1−x (AlN)x films with 0.5 and 1 mol% of AlN exhibit p-type conduction with the carrier concentration of 9.797 × 1018 /cm3 and 2.415 × 1019 /cm3 , respectively. The grain size observed through XRD is comparable to that observed through FESEM. The incorporation of nitrogen into the film upon doping of AlN is confirmed by Fourier transformed infrared spectroscopy (FTIR) and energy dispersive spectroscopy (EDS). © 2008 Elsevier B.V. All rights reserved.
1. Introduction ZnO is a promising material for optoelectronic applications because of its wide band gap (3.37 eV) and large excitonic binding energy of 60 meV [1]. ZnO has been expected to be a suitable material for light emitting diode (LED) and laser diode (LD) in UV or blue spectral region, because of its ultraviolet (UV) excitonic emission at room temperature [2–5]. The development of ZnO-based optoelectronic devices has suffered from one major disadvantage: the lack of good and reproducible p-type conduction [6]. For the practical application of ZnO as optoelectronic device, both n-type and p-type conductions are necessary. High quality n-type can be achieved by doping with column III elements such as Ga, Al and In [7,8]. However, p-type doping is difficult due to various reasons such as deep acceptor level, low solubility of acceptor dopant and native donor defects like zinc interstitial (Zni ) and oxygen vacancy (Vo ) [9]. In order to achieve p-type conduction in ZnO, these defects have to be suppressed or eliminated. Several reports have suggested that p-type conduction in zinc oxide can be achieved by codoping method (simultaneously doped with donor (d) and acceptor (a)). The codoping method causes the formation of the complexes a–d–a which accelerates the acceptors into the films. Theoretical calculations of the electronic band
structure predict that nitrogen is the best candidate for producing a shallow acceptor level in ZnO [10]. Yamamoto and Yoshida [11] have proposed a codoping method using acceptors (N) and donors (Ga or Al or In) to increase the solubility of nitrogen concentration in ZnO thin films. Furthermore, doping with Al and N shows better results than doping with Ga and N since the Al–N and Al–O bonds are stronger than Ga–N and Ga–O bonds [9,12]. To this date, ZnO films have been deposited by various techniques such as metal organic chemical vapour deposition (MOCVD) [13,14], molecular beam epitaxy (MBE) [15,16] and pulsed laser deposition (PLD) [17,18]. Many reports have been made for Al–N codoped ZnO films to realize p-type conduction in ZnO using Al2 O3 and N2 , N2 O/NH3 gases [19,20] as a source of Al and N, respectively. There are several drawbacks in using N2 and N2 O gases as a source of nitrogen doping like toxicity and the need for cracking N2 or N2 O into N. In order to avoid such difficulties, we have reported a different idea of doping nitrogen simultaneously with aluminium by directly doping the AlN into ZnO. In this paper we have presented a novel idea to realize p-type conduction in ANZO (AlN doped ZnO) films on n-type Si (1 0 0) substrate by RF magnetron sputtering technique. To the authors’ knowledge except Kobayashi et al. [21,22], no reports available on AlN doped ZnO. 2. Experimental
∗ Corresponding author. Tel.: +91 431 2501801x3603; fax: +91 431 2500133. E-mail address: [email protected] (T. Balasubramanian). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.12.010
Thin films of (ZnO)1−x (AlN)x have been deposited on n-type Si (1 0 0) substrate by RF magnetron sputtering system using AlN doped targets as a source material.
K.P. Bhuvana et al. / Journal of Alloys and Compounds 478 (2009) 54–58
55
Prescribed amount of AlN (purity, 99.99%) and host ZnO (purity, 99.99%) powders have been thoroughly mixed for 10 h by ball milling and pressed in a form of cylindrical pallets. All the pallets have been sintered at 950 ◦ C for 6 h at slow ramping and cooling rate. Pallets thus prepared have been used as targets (dia = 50 mm and thickness = 2 mm) for sputtering. Prior to the deposition of film, the substrates have been cleaned ultrasonically by successive rinsing in ethanol, acetone and distilled water respectively for 10 min in each solution. The cleaned substrate has been placed at a distance of 5 cm from the target. The chamber has been evacuated to a base pressure of 8 × 10−6 mbar before introducing Ar (sputtering gas) and O2 (reactive gas). The sputtering process has been carried out for 30 min. The sputtering pressure (Ar + O2 ), RF power and substrate temperature are 0.2 mbar and 450 ◦ C, respectively, maintained throughout the growth process. The thickness of the films has been measured by filmetrics (F20) and is found to be around 120 nm. The crystalline structure and the quality of the films have been investigated by powder X-ray diffractometer (Rigaku) using Cu K␣ radiation ( = 1.540562 Å). Fourier transformed infrared spectra (FTIR) of (ZnO)1−x (AlN)1−x films have been obtained by PerkinElmer system. The EDS spectrum has been carried out by FEI-Quanta 200F while FESEM images are recorded in Quanta FEG. Hall measurements of the films have been carried out using HMS 3000 (Ekopia) system. All the measurements have been carried out at room temperature.
3. Results and discussion 3.1. XRD studies Fig. 1 shows the XRD pattern of pure and (ZnO)1−x (AlN)x films grown on Si (1 0 0) substrate using RF magnetron sputtering. All the films are in polycrystalline form of wurtzite structure with the preferential orientation of (0 0 2) diffraction plane. The full width half maximum (FWHM) of (0 0 2) peak and crystallite size plotted as a function of AlN concentration has been shown in Fig. 2. At the lower concentration of AlN (≤1 mol%), the FWHM increases (crystallite size decreases) and reaches a maximum for 1 mol% and then again decreases (crystallite size increases) for further addition of AlN (2 and 4 mol%). Fig. 3 shows the variation of the d-spacing value with respect to dopant concentration. It is observed that the d-spacing value increases for 0.5 and 1 mol% (ZnO)1−x (AlN)x films because of the replacement of N3− ions on O2− site. However, for higher dopant concentrations (2 and 4 mol%) d-spacing value decreases compared
Fig. 2. FWHM of (0 0 2) and grain size as a function of AlN concentration.
to bulk ZnO due to the replacement of Al3+ on Zn2+ site. In the former case, the ionic radius of N3− ion is larger than O2− and hence the replacement of larger ion on smaller ionic site increases the d spacing. In the later case (Al3+ in Zn2+ ), the ionic radius of Al3+ is smaller than Zn2+ and hence the replacement of Al3+ on Zn2+ site decreases the d spacing. Therefore it is expected that more N ions have been incorporated at lower concentration and more Al ions have been incorporated at higher concentration of AlN. Moreover the (ZnO)1−x (AlN)x film with 1 mol% has the lattice constant closer to that of the bulk. 1 mol% AlN doped ZnO has the closest dspacing value as well as the highest hole concentration (from Hall measurements) suggesting the best codoping. Zeng et al. [23] has proposed that the best codoping effect can be realized when the Al–N codoped ZnO have the d-spacing value closest to the nominally undoped ZnO. 3.2. Hall measurements The electrical properties of pure and (ZnO)1−x (AlN)x films have been examined by Hall measurements at room temperature. To realize the reliability and repeatability of the conduction of the films, these measurements have been carried out several times and concordant results were observed. The carrier concentration as a function of AlN concentration has been plotted and shown
Fig. 1. XRD pattern of pure ZnO and ANZO films for different doping levels.
Fig. 3. Variation of the 2 value of (0 0 2) plane with AlN concentration.
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K.P. Bhuvana et al. / Journal of Alloys and Compounds 478 (2009) 54–58
Fig. 5. FTIR spectra of ANZO films.
3.3. FTIR analysis
Fig. 4. (a) Carrier concentration, Hall mobility and resistivity as a function of AlN concentration. (b) Conc. of AlN versus the electrical parameters (Hall mobility and resistivity).
in Fig. 4a. It is worth to note that the ZnO films doped with 0.5 and 1 mol% of AlN exhibit p-type conduction with hole concentrations of 9.797 × 1018 /cm3 and 2.415 × 1019 /cm3 , respectively. However, when AlN is greater than 1 mol%, the hole concentrations start to decrease and n-type conduction is observed for 2 and 4 mol% with the electron concentrations of 3.706 × 1018 and 5.625 × 1019 /cm3 , respectively. The XRD result also strongly supports our observation of p-type conduction at lower concentration (due to the more of N incorporation) and n-type conduction of higher concentration (due to more of Al incorporation). The corresponding mobilities 1.275 and 0.2887 cm2 /V s for p-conduction have been observed for 0.5 and 1 mol% of AlN, respectively. The Hall mobility is found to be very small for 1 mol% and this might be due to the grain boundary scattering which is the major mechanism in Al–N codoped ZnO as proposed by Zhu et al. [24]. XRD results also reveal that for 1 mol% (ZnO)1−x (AlN)x film, the FWHM is high which means very small grain size leads to large number of grain boundaries. As discussed by Gu et al. [25], the Al atoms are inactive at the amorphous state (lower grain size) and hence it cannot liberate the electrons on replacing the Zn2+ ion by Al3+ in ZnO lattice. Hence, N is active enough to release holes in those films. This is the reason for p-type conduction in (ZnO)0.095 (AlN)0.005 and (ZnO)0.99 (AlN)0.01 films on Si(1 0 0). As the grain size increases, Al becomes more active and it liberates electrons which is the source for the increase of electron concentration and n-type conduction.
In order to confirm the presence of nitrogen in the film the FTIR measurements were carried out. Fig. 5 shows the FTIR spectra of (ZnO)1−x (AlN)x films. All the (ZnO)1−x (AlN)x films show the absorption peak at around 613 cm−1 which corresponds to Al–O bonds as assigned by Jang et al. [26]. The absorption peak of Al–O bond is intensive for 4 mol% (ZnO)1−x (AlN)x film than rest of the ANZO films of lower concentration indicating the high density of Al–O bonds in the film. This again confirms the more incorporation of Al than N at 4 mol% (ZnO)1−x (AlN)x film. The absorptions at around 860 and 910 cm−1 are attributed to transverse mode (TO) and the A1 and E1 longitudinal optical (LO) phonon modes of crystalline Al–N bond, respectively [27,28]. It should be noted that the density (intensity) of the Al–N bonds are higher for 0.5 and 1 mol% (ZnO)1−x (AlN)x films as compared with the other composition. This
Fig. 6. EDS spectrum of 1 mol% AlN doped ZnO.
K.P. Bhuvana et al. / Journal of Alloys and Compounds 478 (2009) 54–58
could be the reason for getting p-ZnO as observed in the Hall measurement because of the more incorporation of the N in the films. This has a good agreement with our XRD results. There is a peak around 2357 cm−1 , which is not attributed to any of the possible bonds such as Zn–O, Al–O, Al–N. With an observation of similar peak at around 2150 cm−1 in AlN films by Sanchez et al. [29] has reported that this corresponds to the multiple impurity bond of nitrogen. The presence of nitrogen has also confirmed already by comparing the properties of Al doped ZnO and AlN doped ZnO films and reported recently [30].
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(Al&N) is found in the spectrum to imply the high quality of the film. 3.5. FESEM analysis Fig. 7 shows the field emission secondary electron microscopy (FESEM) pictures of 2 and 4 mol% doped ZnO films. The grain sizes observed from the FESEM pictures are more or less equal to the grain size measured from XRD. The small difference might be due to the domains found in the FESEM picture formed by the aggregation of nano-size crystallites.
3.4. EDS analysis 4. Conclusion In addition to FTIR analysis, energy dispersive spectroscopy (EDS) analysis has been carried out to confirm the presence of N. Fig. 6 shows the EDS spectrum of the best codoped film (i.e. 1% AlN doped ZnO) as evidence from XRD and Hall measurements in which the presence of N is confirmed. It is worth to note that no other element except the host elements (Zn&O) and the dopants
A simple approach, AlN has been used as the codopant in the ZnO to realize p-ZnO. The AlN doped (codoped) ZnO films grown on Si (1 0 0) substrate by RF sputtering have been examined by XRD, Hall effect, FTIR, EDS and FESEM analysis. It has been observed that the films with 0.5 and 1 mol% of AlN exhibit the ptype conduction with the hole concentrations of 9.797 × 1018 and 2.415 × 1019 /cm3 , respectively. The XRD, Hall measurements, FTIR and EDS analysis support each other in establishing the formation of p-conduction in (ZnO)1−x (AlN)x films. Hence, it is suggested that AlN can used as the best codopant for the fabrication of optoelectronic devices. Acknowledgement This work was supported by Technical Education Quality Improvement Programme (TEQIP), Government of India for providing the financial support. References
Fig. 7. FESEM images of (a) 2 mol% and (b) 4 mol% AlN doped ZnO films.
[1] Z. Yan, Z.T. Song, W.L. Liu, Q. Wan, F.M. Zhang, S.L. Feng, Thin Solid Films 492 (2005) 203–206. [2] Z.K. Tang, G.K.L. Wong, P. Yu, et al., Appl. Phys. Lett. 72 (1998) 3270. [3] D.M. Bagnell, Y.F. Chen, Z. Zhu, et al., Appl. Phys. Lett. 70 (1997) 2230. [4] K. Minegishi, Y. Koiwai, Y. Kikuchi, et al., Jpn. J. Appl. Phys. 36 (1997) L1453. [5] Z. Fu, B. Lin, G. Liao, J. Cryst. Growth 193 (1998) 316. [6] Z.-H. Zhang, Z.-Z. Ye, D.-W. Ma, L.-P. Zhu, T. Zhou, B.-H. Zhao, Z.-G. Fei, Mater. Lett. 59 (2005) 2732–2734. [7] P. Nunes, E. Fortunato, P. Tonello, F. Braz Fernandes, P. Vilarinho, R. Martins, Vacuum 64 (2002) 281. [8] Y. Morinaga, K. Sakuragi, N. Fujimura, T. Ito, J. Cryst. Growth 174 (1997) 691. [9] C. Zhang, X. Li, J. Bian, W. Yu, X. Gao, Solid State Commun. 132 (2004) 75–78. [10] A Kobayashi, O.F. Sankey, J.D. Dow, Phys. Rev. B 28 (1983) 946. [11] T. Yamamoto, H.K. Yoshida, Physica B 302–303 (2001) 155. [12] L.G. Wang, A. Zunger, Phys. Rev. Lett. 90 (2003) 256401. [13] K.H. Bang, D.K. Hwang, M.C. Jeong, K.S. Sohn, J.M. Myoung, Solid State Commun. 126 (2003) 623. [14] C.R. Gorla, N.W. Emanetoglu, S. Liang, W.E. Mayo, Y. Lu, J. Appl. Phys. 85 (1999) 2595. [15] A. Kumano, A. Ashrafi, A. Ueta, A.I. Suemune, J. Cryst. Growth 214–215 (2000) 280. [16] A.B.M. Alamamun Ashrafi, A. Ueta, A. Avramescu, H. Kumano, I. Suemune, Y.W. Ok, T.-Y. Seong, Appl. Phys. Lett. 76 (2000) 550. [17] Y.R. Ryu, S. Zhu, D.C. Look, J.M. Wrobel, H.M. Jeong, H.W. White, J. Cryst. Growth 216 (2000) 330–334. [18] F.K. Shan, B.C. Shin, S.W. Jang, Y.S. Yu, J. Eur. Ceram. Soc. 24 (2004) 1015– 1018. [19] G. Yuan, Z. Ye, L. Zhu, Y. Zeng, J. Huang, Q. Qian, J. Lu, Mater. Lett. 58 (2004) 3741–3744. [20] H.P. He, Z.Z. Ye, F. Zhuge, Y.J. Zeng, L.P. Zhu, B.H. Zhao, J.Y. Huang, Z. Chen, Solid State Commun. 138 (2006) 542–545. [21] K. Kobayashi, Y. Kondo, Y. Tomita, Y. Maeda, S. Matsushima, Appl. Surf. Sci. 253 (2007) 5035. [22] K. Kobayashi, Y. Kondo, Y. Tomita, Y. Maeda, S. Matsushima, Thin Solid Films 516 (2008) 4894–4898. [23] Y.-J. Zeng, Z.-Z. Ye, J.-G. Lu, L.-P. Zhu, D.-Y. Li, B.-H. Zhao, J.-Y. Huang, Appl. Surf. Sci. 249 (2005) 203–207. [24] Q.Y. Zhu, Z.Z. Ye, G.D. Yuan, J.Y. Huang, L.P. Zhu, B.H. Zhao, J.G. Lu, Appl. Surf. Sci. 253 (2006) 1903–1906. [25] X. Gu, L. Zhu, Z. Ye, Q. Ma, H. He, Y. Zhang, B. Zhao, Solar Energy Mater. Solar Cells 92 (2008) 343.
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[26] K. Jang, K. Lee, J. Kim, S. Hwang, J. Lee, S.K. Dhungel, S. Jung, J. Yi, Mater. Sci. Semicond. Process. 9 (2006) 1137–1141. [27] C. Cibert, F. Tétard, P. Djemia, C. Champeaux, A. Catherinota, D. Tetard, Superlattic Microstruct. 36 (2004) 409–416. [28] C. Ristoscu, C. Ducu, G. Socol, F. Craciunoiu, I.N. Mihailescu, Appl. Surf. Sci. 248 (2005) 411–415.
[29] G. Sanchez, A. Wu, P. Tristant, C. Tixier, B. Soulestin, J. Desmaison, A. Bologn Alles, Thin Solid Films 516 (2008) 4868–4875. [30] K.P. Bhuvana, J. Elanchezhiyan, N. Gopalakrishnan, T. Balasubramanian, Appl. Surf. Sci. 255 (2008) 2026–2029.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Realization of p-type conduction in (ZnO)1−x (AlN)x thin films grown by RF magnetron sputtering K.P. Bhuvana a , J. Elanchezhiyan b , N. Gopalakrishnan a , B.C. Shin b , T. Balasubramanian a,∗ a b
Thin films Laboratory, Department of Physics, National Institute of Technology, Tiruchirappalli 620 015, India Electronic Ceramics Center, Dong Eui University, Busan-614-714, South Korea
a r t i c l e
i n f o
Article history: Received 27 July 2008 Received in revised form 5 December 2008 Accepted 10 December 2008 Available online 24 December 2008 PACS: 71.55.Gs 73.61.Ga Keywords: p-Type ZnO RF magnetron sputtering Codoping
a b s t r a c t A different approach for codoping in ZnO using AlN as dopant (codopant) has been attempted to realize p-ZnO by RF magnetron sputtering. The (ZnO)1−x (AlN)x [AlN codoped ZnO] films of different doping concentrations (0.5, 1, 2 and 4 mol%) grown on Si(1 0 0) substrates have been subjected to X-ray diffraction (XRD) and Hall measurements to investigate their structural and electrical properties, respectively. XRD results reveal that all the films are constituted in wurtzite structure with the preferential orientation of (0 0 2) diffraction plane. It has been observed that the c-axis lattice constant is higher than unstressed bulk value for 0.5 and 1 mol% AlN doped ZnO films which support the incorporation of N atoms into the film. The Hall measurements show that the (ZnO)1−x (AlN)x films with 0.5 and 1 mol% of AlN exhibit p-type conduction with the carrier concentration of 9.797 × 1018 /cm3 and 2.415 × 1019 /cm3 , respectively. The grain size observed through XRD is comparable to that observed through FESEM. The incorporation of nitrogen into the film upon doping of AlN is confirmed by Fourier transformed infrared spectroscopy (FTIR) and energy dispersive spectroscopy (EDS). © 2008 Elsevier B.V. All rights reserved.
1. Introduction ZnO is a promising material for optoelectronic applications because of its wide band gap (3.37 eV) and large excitonic binding energy of 60 meV [1]. ZnO has been expected to be a suitable material for light emitting diode (LED) and laser diode (LD) in UV or blue spectral region, because of its ultraviolet (UV) excitonic emission at room temperature [2–5]. The development of ZnO-based optoelectronic devices has suffered from one major disadvantage: the lack of good and reproducible p-type conduction [6]. For the practical application of ZnO as optoelectronic device, both n-type and p-type conductions are necessary. High quality n-type can be achieved by doping with column III elements such as Ga, Al and In [7,8]. However, p-type doping is difficult due to various reasons such as deep acceptor level, low solubility of acceptor dopant and native donor defects like zinc interstitial (Zni ) and oxygen vacancy (Vo ) [9]. In order to achieve p-type conduction in ZnO, these defects have to be suppressed or eliminated. Several reports have suggested that p-type conduction in zinc oxide can be achieved by codoping method (simultaneously doped with donor (d) and acceptor (a)). The codoping method causes the formation of the complexes a–d–a which accelerates the acceptors into the films. Theoretical calculations of the electronic band
structure predict that nitrogen is the best candidate for producing a shallow acceptor level in ZnO [10]. Yamamoto and Yoshida [11] have proposed a codoping method using acceptors (N) and donors (Ga or Al or In) to increase the solubility of nitrogen concentration in ZnO thin films. Furthermore, doping with Al and N shows better results than doping with Ga and N since the Al–N and Al–O bonds are stronger than Ga–N and Ga–O bonds [9,12]. To this date, ZnO films have been deposited by various techniques such as metal organic chemical vapour deposition (MOCVD) [13,14], molecular beam epitaxy (MBE) [15,16] and pulsed laser deposition (PLD) [17,18]. Many reports have been made for Al–N codoped ZnO films to realize p-type conduction in ZnO using Al2 O3 and N2 , N2 O/NH3 gases [19,20] as a source of Al and N, respectively. There are several drawbacks in using N2 and N2 O gases as a source of nitrogen doping like toxicity and the need for cracking N2 or N2 O into N. In order to avoid such difficulties, we have reported a different idea of doping nitrogen simultaneously with aluminium by directly doping the AlN into ZnO. In this paper we have presented a novel idea to realize p-type conduction in ANZO (AlN doped ZnO) films on n-type Si (1 0 0) substrate by RF magnetron sputtering technique. To the authors’ knowledge except Kobayashi et al. [21,22], no reports available on AlN doped ZnO. 2. Experimental
∗ Corresponding author. Tel.: +91 431 2501801x3603; fax: +91 431 2500133. E-mail address: [email protected] (T. Balasubramanian). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.12.010
Thin films of (ZnO)1−x (AlN)x have been deposited on n-type Si (1 0 0) substrate by RF magnetron sputtering system using AlN doped targets as a source material.
K.P. Bhuvana et al. / Journal of Alloys and Compounds 478 (2009) 54–58
55
Prescribed amount of AlN (purity, 99.99%) and host ZnO (purity, 99.99%) powders have been thoroughly mixed for 10 h by ball milling and pressed in a form of cylindrical pallets. All the pallets have been sintered at 950 ◦ C for 6 h at slow ramping and cooling rate. Pallets thus prepared have been used as targets (dia = 50 mm and thickness = 2 mm) for sputtering. Prior to the deposition of film, the substrates have been cleaned ultrasonically by successive rinsing in ethanol, acetone and distilled water respectively for 10 min in each solution. The cleaned substrate has been placed at a distance of 5 cm from the target. The chamber has been evacuated to a base pressure of 8 × 10−6 mbar before introducing Ar (sputtering gas) and O2 (reactive gas). The sputtering process has been carried out for 30 min. The sputtering pressure (Ar + O2 ), RF power and substrate temperature are 0.2 mbar and 450 ◦ C, respectively, maintained throughout the growth process. The thickness of the films has been measured by filmetrics (F20) and is found to be around 120 nm. The crystalline structure and the quality of the films have been investigated by powder X-ray diffractometer (Rigaku) using Cu K␣ radiation ( = 1.540562 Å). Fourier transformed infrared spectra (FTIR) of (ZnO)1−x (AlN)1−x films have been obtained by PerkinElmer system. The EDS spectrum has been carried out by FEI-Quanta 200F while FESEM images are recorded in Quanta FEG. Hall measurements of the films have been carried out using HMS 3000 (Ekopia) system. All the measurements have been carried out at room temperature.
3. Results and discussion 3.1. XRD studies Fig. 1 shows the XRD pattern of pure and (ZnO)1−x (AlN)x films grown on Si (1 0 0) substrate using RF magnetron sputtering. All the films are in polycrystalline form of wurtzite structure with the preferential orientation of (0 0 2) diffraction plane. The full width half maximum (FWHM) of (0 0 2) peak and crystallite size plotted as a function of AlN concentration has been shown in Fig. 2. At the lower concentration of AlN (≤1 mol%), the FWHM increases (crystallite size decreases) and reaches a maximum for 1 mol% and then again decreases (crystallite size increases) for further addition of AlN (2 and 4 mol%). Fig. 3 shows the variation of the d-spacing value with respect to dopant concentration. It is observed that the d-spacing value increases for 0.5 and 1 mol% (ZnO)1−x (AlN)x films because of the replacement of N3− ions on O2− site. However, for higher dopant concentrations (2 and 4 mol%) d-spacing value decreases compared
Fig. 2. FWHM of (0 0 2) and grain size as a function of AlN concentration.
to bulk ZnO due to the replacement of Al3+ on Zn2+ site. In the former case, the ionic radius of N3− ion is larger than O2− and hence the replacement of larger ion on smaller ionic site increases the d spacing. In the later case (Al3+ in Zn2+ ), the ionic radius of Al3+ is smaller than Zn2+ and hence the replacement of Al3+ on Zn2+ site decreases the d spacing. Therefore it is expected that more N ions have been incorporated at lower concentration and more Al ions have been incorporated at higher concentration of AlN. Moreover the (ZnO)1−x (AlN)x film with 1 mol% has the lattice constant closer to that of the bulk. 1 mol% AlN doped ZnO has the closest dspacing value as well as the highest hole concentration (from Hall measurements) suggesting the best codoping. Zeng et al. [23] has proposed that the best codoping effect can be realized when the Al–N codoped ZnO have the d-spacing value closest to the nominally undoped ZnO. 3.2. Hall measurements The electrical properties of pure and (ZnO)1−x (AlN)x films have been examined by Hall measurements at room temperature. To realize the reliability and repeatability of the conduction of the films, these measurements have been carried out several times and concordant results were observed. The carrier concentration as a function of AlN concentration has been plotted and shown
Fig. 1. XRD pattern of pure ZnO and ANZO films for different doping levels.
Fig. 3. Variation of the 2 value of (0 0 2) plane with AlN concentration.
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K.P. Bhuvana et al. / Journal of Alloys and Compounds 478 (2009) 54–58
Fig. 5. FTIR spectra of ANZO films.
3.3. FTIR analysis
Fig. 4. (a) Carrier concentration, Hall mobility and resistivity as a function of AlN concentration. (b) Conc. of AlN versus the electrical parameters (Hall mobility and resistivity).
in Fig. 4a. It is worth to note that the ZnO films doped with 0.5 and 1 mol% of AlN exhibit p-type conduction with hole concentrations of 9.797 × 1018 /cm3 and 2.415 × 1019 /cm3 , respectively. However, when AlN is greater than 1 mol%, the hole concentrations start to decrease and n-type conduction is observed for 2 and 4 mol% with the electron concentrations of 3.706 × 1018 and 5.625 × 1019 /cm3 , respectively. The XRD result also strongly supports our observation of p-type conduction at lower concentration (due to the more of N incorporation) and n-type conduction of higher concentration (due to more of Al incorporation). The corresponding mobilities 1.275 and 0.2887 cm2 /V s for p-conduction have been observed for 0.5 and 1 mol% of AlN, respectively. The Hall mobility is found to be very small for 1 mol% and this might be due to the grain boundary scattering which is the major mechanism in Al–N codoped ZnO as proposed by Zhu et al. [24]. XRD results also reveal that for 1 mol% (ZnO)1−x (AlN)x film, the FWHM is high which means very small grain size leads to large number of grain boundaries. As discussed by Gu et al. [25], the Al atoms are inactive at the amorphous state (lower grain size) and hence it cannot liberate the electrons on replacing the Zn2+ ion by Al3+ in ZnO lattice. Hence, N is active enough to release holes in those films. This is the reason for p-type conduction in (ZnO)0.095 (AlN)0.005 and (ZnO)0.99 (AlN)0.01 films on Si(1 0 0). As the grain size increases, Al becomes more active and it liberates electrons which is the source for the increase of electron concentration and n-type conduction.
In order to confirm the presence of nitrogen in the film the FTIR measurements were carried out. Fig. 5 shows the FTIR spectra of (ZnO)1−x (AlN)x films. All the (ZnO)1−x (AlN)x films show the absorption peak at around 613 cm−1 which corresponds to Al–O bonds as assigned by Jang et al. [26]. The absorption peak of Al–O bond is intensive for 4 mol% (ZnO)1−x (AlN)x film than rest of the ANZO films of lower concentration indicating the high density of Al–O bonds in the film. This again confirms the more incorporation of Al than N at 4 mol% (ZnO)1−x (AlN)x film. The absorptions at around 860 and 910 cm−1 are attributed to transverse mode (TO) and the A1 and E1 longitudinal optical (LO) phonon modes of crystalline Al–N bond, respectively [27,28]. It should be noted that the density (intensity) of the Al–N bonds are higher for 0.5 and 1 mol% (ZnO)1−x (AlN)x films as compared with the other composition. This
Fig. 6. EDS spectrum of 1 mol% AlN doped ZnO.
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could be the reason for getting p-ZnO as observed in the Hall measurement because of the more incorporation of the N in the films. This has a good agreement with our XRD results. There is a peak around 2357 cm−1 , which is not attributed to any of the possible bonds such as Zn–O, Al–O, Al–N. With an observation of similar peak at around 2150 cm−1 in AlN films by Sanchez et al. [29] has reported that this corresponds to the multiple impurity bond of nitrogen. The presence of nitrogen has also confirmed already by comparing the properties of Al doped ZnO and AlN doped ZnO films and reported recently [30].
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(Al&N) is found in the spectrum to imply the high quality of the film. 3.5. FESEM analysis Fig. 7 shows the field emission secondary electron microscopy (FESEM) pictures of 2 and 4 mol% doped ZnO films. The grain sizes observed from the FESEM pictures are more or less equal to the grain size measured from XRD. The small difference might be due to the domains found in the FESEM picture formed by the aggregation of nano-size crystallites.
3.4. EDS analysis 4. Conclusion In addition to FTIR analysis, energy dispersive spectroscopy (EDS) analysis has been carried out to confirm the presence of N. Fig. 6 shows the EDS spectrum of the best codoped film (i.e. 1% AlN doped ZnO) as evidence from XRD and Hall measurements in which the presence of N is confirmed. It is worth to note that no other element except the host elements (Zn&O) and the dopants
A simple approach, AlN has been used as the codopant in the ZnO to realize p-ZnO. The AlN doped (codoped) ZnO films grown on Si (1 0 0) substrate by RF sputtering have been examined by XRD, Hall effect, FTIR, EDS and FESEM analysis. It has been observed that the films with 0.5 and 1 mol% of AlN exhibit the ptype conduction with the hole concentrations of 9.797 × 1018 and 2.415 × 1019 /cm3 , respectively. The XRD, Hall measurements, FTIR and EDS analysis support each other in establishing the formation of p-conduction in (ZnO)1−x (AlN)x films. Hence, it is suggested that AlN can used as the best codopant for the fabrication of optoelectronic devices. Acknowledgement This work was supported by Technical Education Quality Improvement Programme (TEQIP), Government of India for providing the financial support. References
Fig. 7. FESEM images of (a) 2 mol% and (b) 4 mol% AlN doped ZnO films.
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