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Comparative photoluminescence study on p-type and n-type ZnO films codoped by nitrogen and aluminium
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Optical Materials 30 (2008) 1422–1426 www.elsevier.com/locate/optmat
Comparative photoluminescence study on p-type and n-type ZnO films codoped by nitrogen and aluminium Haiping Tang a
a,b
, Zhizhen Ye b, Haiping He
b,*
Department of Mechanical and Electrical Engineering, Baoji University of Arts and Sciences, Baoji, Shannxi 721007, PR China b State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China Received 1 June 2007; received in revised form 21 August 2007; accepted 27 August 2007 Available online 10 October 2007
Abstract We report on the temperature-dependent photoluminescence (PL) properties of n-type and p-type ZnO films codoped with N and Al. For the n-type film, the dominant emission at low temperature is exciton bound to neutral donors, while for the p-type film it is exciton bound to neutral acceptor at 3.33 eV. Four defect or impurity levels, including N acceptor, residual acceptor, and two doping-induced unknown deep acceptors, were identified. The energy level of the N acceptor was determined to be 0.23 eV. Excitation energy dependence of the PL was also investigated. It was found that at high excitation energy, the formation of exciton was suppressed by the formation of D+Aeh complexes. Ó 2007 Elsevier B.V. All rights reserved. PACS: 71.55.Gs; 61.72.Vv Keywords: Zinc oxide; Photoluminescence; p-type
1. Introduction ZnO is a wide band gap (3.37 eV) semiconductor with a very large exciton binding energy of 60 meV, which, in principle, allows the exciton-governed luminescence at short wavelengths to be dominant at room temperature. It has attracted considerable interest due to its potential applications in short wavelength optoelectronic devices [1,2]. However, a major obstacle for the application of ZnO is the difficulty in achieving good and reliable p-type material [3]. Recently, steady progress in doping ZnO with p-type has been made via various techniques [4–11], and electroluminescence from ZnO-based pn junctions and light-emission diodes (LEDs) has been reported [6,12,13]. Among the doping methods, the donor–acceptor codoping method has proving very promising. Most recent theoretical study [14] on Ga–N codoping revealed that by codoping *
Corresponding author. Tel./fax: +86 571 87952625. E-mail address: [email protected] (H. He).
0925-3467/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2007.08.006
Ga and N a fully occupied impurity band can be formed above the valence band maximum, which significantly lowers the ionization energy of the N acceptors. Recently, a new method of generating p-type ZnO by S incorporation also shows shallower acceptor state [15]. However, there are still many deficiencies in the p-type layer when compared with n-type ZnO. Such deficiencies include radiative and nonradiative defects and/or impurities, which degrade the near band edge luminescence. To better understand and improve the film’s optical properties and meet the application of ZnO in short wavelength spectral range, therefore, it is essential to collect information of these defects and impurities. Photoluminescence (PL) spectroscopy is a very sensitive tool for characterizing radiative defects. PL from p-type ZnO doped with group-V elements has been investigated. In particular, for N-doped ZnO, low temperature PL has been used to determine the acceptor level by studying the recombination of bound excitons (BX) and donor–acceptor pair (DAP). In ZnO material, the knowledge of donors
H. Tang et al. / Optical Materials 30 (2008) 1422–1426
and acceptors, especially of acceptors, is rather poor. Up to eleven excitonic recombinations where excitons bind to neutral donors and/or acceptors have been observed, however, the chemical nature of the donor and acceptor species remained to be determined [16]. The situation appears more complicated for the codoped case. A few theoretical works [17–20] dealing with the thermodynamics of defects, impurities, and complexes in N-doped ZnO have been published. Defects, impurities, and complexes that may act as deep level centers have been proposed. However, few experimental data about these species have been reported. In previous studies [5,21], we have demonstrated that N can be effectively incorporated into ZnO and reasonable ptype conduction can be achieved via Al–N co-doping technique. In this work, we focused on the PL properties of the ZnO:N:Al films. Four acceptor levels were derived from the temperature-dependent PL analysis. We found that the energy level located at about 0.23 eV above the valence band is most likely attributed to N acceptor. 2. Experimental ZnO:N:Al films were grown on a Si substrate at 500 °C by DC reactive magnetron sputtering method using AlxZn1x alloy as the target and N2O + O2 as the dopant gas. Details of the sputtering parameters were described elsewhere [21]. The x value in AlxZn1x alloy targets was chosen to be 0.004 and 0.01 to obtain different Al content in the films. Hall and I–V measurements indicated that the film with Al content of 0.4 and 1 at% is p-type and n-type, respectively [21]. PL spectra were recorded on a FluoRoLOG-3-TAU (France, Jobin Yvon) fluorescence spectrometer, using Xe lamp as the source of excitation light. The measuring temperature various from 10 to 300 K. Low temperatures were obtained by CCS-335 (American, Janis) equipment. 3. Results and discussion Fig. 1 shows the temperature-dependent PL spectra of the two ZnO:N:Al films. Both of them are broad and show five PL bands centered at 3.33–3.36, 3.24, 3.17, 3.04, and 2.85 eV, respectively. At low temperatures, the PL spectra are dominated by the 3.17 and 3.04 eV bands. As the temperatures increases, the 3.04 eV emission becomes dominant for the 0.4 at% Al-doped sample, while for the 1 at% Al-doped sample it is the 3.17 eV band. At 250– 260 K, the 3.33 eV bands for the 0.4 at% Al-doped sample disappear suddenly. For the 1 at% Al-doped case, however, the 3.36 eV band generally converts to emission band with peak energy of 3.28 eV and the PL spectra is a mixture of the 3.17 and 3.28 eV bands at room temperature. The band at 3.36 and 3.33 eV in n- and p-type samples can be attributed to recombination of D0X and A0X in ZnO, respectively. It is well known that the PL of ZnO is dominated by BX lines at low temperature. By far in ZnO, more than ten donors bound exciton (D0X) and
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acceptor bound exciton (A0X) lines have been reported in the range of 3.315–3.372 eV. The temperature dependence of the peak energy of BX band for the two samples was plotted in Fig. 2. The reported [22] temperature dependence of free exciton (FX) in ZnO was also given as a reference. As the temperature increases, the peak energy gradually decreases and close to the energy of FX at 160–200 K, indicating a conversion of BX to FX at elevated temperature. Thus the room temperature PL for the 1 at% Al-doped sample (Fig. 1b) is composed of the 3.17 eV band and the free exciton band. The 3.24 eV band is weak and is most likely related to residual acceptor in ZnO. Meyer et al. [16] have reported DAP line at 3.22 eV in undoped ZnO and ascribed it to unknown residual acceptor. The emission near 3.05 eV has been assigned to recombination of localized carries [23]. The peak position of the 3.17 band varies less than 10 meV in the temperature range of 10–300 K, whereas the band gap decreases by about 70 meV. This weak temperature dependence is typical for emissions related to defect level. Thus it was proposed that the band is due to DAP recombination. It is well known that the recombination probability of DAP is proportional to the concentration of neutral acceptors. The dominance of the 3.17 eV band in 1 at% Al-doped sample is indicative of larger concentration of the related neutral acceptor. In p-type semiconductor, the Fermi level is close to the acceptor level, which results in low concentration of neutral acceptors. In contrast, the Fermi level is close to the donor level in n-type semiconductor thus leading to high concentration of neutral acceptors. Because the 0.4 at% Al-doped sample is p-type while the 1 at% Al-doped one is n-type, we suggest that the 3.17 eV band is related to N acceptor. The acceptor level can be derived from D0 A0 ¼ Eg ðED þ EA Þ þ e2 =4pr
ð1Þ
where Eg, ED, and EA are the band gap, donor, and acceptor energies, respectively, and r is the pair separation. We take the low temperature band gap of 3.437 eV and thus the room temperature value of 3.377 eV. Thus EA levels related to 3.17 eV bands are evaluated to be 0.23 eV. The value of 0.23 eV is close to the reported [4,16] N acceptor in ZnO, 0.16–0.20 eV. Fig. 3 shows the PL spectra excited with various excitation energies for the n-type sample. It was found that the peak energy of D0X is almost constant while that of the 3.17 eV bands decrease with increasing excitation energy. The maximal redshift is as large as 40 meV. The redshift of D0A0 energy at higher excitation energy can be explained as follows: at higher excitation energy, the free carriers possess higher energy and they can go longer spatial distance before losing energy and being trapped by donor/acceptor centers. This leads to more dispersive spatial distribution of the trapped carriers. In other words, the pair separation increases with excitation energy, thus leading to lower recombination energy. The redshift of the peak
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H. Tang et al. / Optical Materials 30 (2008) 1422–1426
Fig. 1. Temperature-dependent PL spectra of (a) p-type and (b) n-type ZnO:N:Al films. The DAP emission involving unknown and N acceptor is labeled as DAXP and DANP, respectively.
Fig. 2. Peak energy of bound excitons and DANP as a function of temperature. The solid line represents data for free exciton (Ref. [21]).
Fig. 3. PL spectra for the n-type film under different excitation energy.
H. Tang et al. / Optical Materials 30 (2008) 1422–1426
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supports the assignment of the 3.17 eV peak to DAP recombination. The emission around 2.8 eV is obviously different from the commonly observed green band (2.5 eV) in ZnO. Tsukazaki et al. [6] has also reported a very broad emission centered at 2.8 eV in p-type ZnO:N film. Similarly, Reshchikov et al. [24] observed a broad PL band around 2.9 eV in GaN. Both of them assumed that the broad emission is due to DAP recombination. Fig. 4 shows the thermal quenching characteristics of the 2.85 eV bands obtained from Fig. 1b. The integrated PL intensity was obtained by Gaussian fit to the spectra. From Fig. 4 the activation energy for the 2.85 eV band was calculated to be 57 meV by fitting the data [23]. The PL quenching could be attributed to thermal release of trapped electrons from the donor level to the conduction band. Thus the obtained activation energies can be assigned to the thermal ionization energy of shallow donors. It was reported that the energy level of AlZn donor in bulk ZnO is located at about 51 meV below the conduction band [16], which is close to the activation energy. Therefore, we suggest the shallow donor involved in the 2.85 eV emission is most likely AlZn. The energy level of the deep acceptor responsible for the band can then be derived from Eq. (1). For a rough estimation, the last term in Eq. (1) is set to 0.03 eV according to Look et al. [4]. By using ED of 0.05 and 0.03 eV, the obtained acceptor level for the 2.85 eV bands is about 0.59 eV. The energy level of the residual acceptor related to the 3.24 eV emission can also be estimated to be 0.17–0.20 eV. Because our samples are doped and grown by sputtering method, native defects like vacancies may be induced. To assess the sample quality, we have performed XPS measurements (not shown). The results revealed that both samples are Zn-rich, with Zn/O ratios of 1.1. Therefore, oxygen vacancies (VO) should exist in the samples while
zinc vacancies (VZn) should be unlikely because the formation energy of VZn is very large (5.5 eV) under Zn-rich conditions [25]. However, it is rather difficult to identify emission involving VO because there are still considerable disputes on it [26]. While many researchers assign the green band around 2.5 eV to VO, there also a lot of work (see Ref. [26] and therein) disagree it. In a recent work, Janotti and Van de Walle [27] predicted that VO is a deep donor, which luminescence at energies of 1.5 and 0.7 eV. In our PL spectra, however, neither the emission around 2.5 eV nor the ones at 1.5 eV and 0.7 eV were clearly observed. It should also be noted that hydrogen inclusion can be a major contribution to the donor states [28]. The I4 excitonic line at 3.3628 eV has been assigned to the H donors [16]. In our samples, due to the degraded crystal quality and broadened PL line width resulting from doping, we were unable to resolve this line from the spectra. However, we have confirmed the presence of H in our Al–N codoped samples by infrared absorption in a previous work [29]. From above results, one can identify several acceptor levels in ZnO:N:Al films. It was then clear that there are at least four competing radiative paths for the photoexcited carriers in our Al–N co-doped samples, that is, to form bound excitons or to form D+Aeh complexes. Fig. 5 shows the peak intensity ratio of DAP emissions to BX as a function of excitation energy. It was found that the intensity of BX decreases with increasing excitation energy. This decrease is accompanied by an increase in the DAP intensities. It implies that the formation of exciton is suppressed at high-energy excitation and it allows more chances for the excited carriers to bind with the donor and acceptor centers forming D+Aeh complexes. The microscopic nature of the deep acceptors with binding energy of 0.59 eV is still unclear. While residual deep acceptor impurities and interface states in the grain bound-
Fig. 4. Integrated intensity of the 2.85 eV emission as a function of the reciprocal temperature. An activation energy of 57 meV was obtained by fitting.
Fig. 5. Peak intensity ratio of the defect or impurity-related emissions to bound exciton as a function of excitation energy for the n-type film. LC represent emission related to localized carrier at 3.04 eV.
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aries [30] cannot be ruled out, it is also very likely induced by doping. In N-doped ZnO, many deep level complexes can be formed as predicted by theoretical calculations [18–20]. While the defect dynamics in N-doped ZnO has been investigated, the report on both theoretical and experimental study on N–Al codoping is few. Actually, the case of N–Al codoping is much more complicated because of the presence of Al. For example, it was predicted [31] that substitutional Al and N atoms could form donor–acceptor complex due to the strong bond strength of the Al–N bond. Such complex may act as a deep level and give rise to emission with lower energy. Further investigation should be conducted to clarify the actual origin of the 2.85 eV emission. 4. Conclusions In summary, PL from n- and p-type ZnO:N:Al films has been investigated. The PL spectra of the p-type film show A0X band at about 3.33 eV. Several defect or impurityrelated emissions at 3.24, 3.17, 3.04, and 2.85 eV were observed. For p-type film with 0.4 at% of Al the PL peaked at 3.04 eV, while for n-type film with 1 at% of Al the peak energy is about 3.17 eV. We suggest the 3.17 eV emission is related to the N acceptor with energy level of 0.23 eV. Thermal quenching of the 2.85 eV band gives the activation energy of 57 meV, which corresponding to the binding energy of AlZn donors in the films. Excitation energy dependence of the PL revealed that, at high excitation energy, the DAP energies show a redshift and the formation of exciton was suppressed by the formation of D+Aeh complexes. Acknowledgement This work was supported by NSFC under Grant No. 50532060. References [1] Z.K. Tang, G.K.L. Wong, P. Yu, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Appl. Phys. Lett. 72 (1998) 3270. [2] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Tao, S. Koyama, M.Y. Shen, T. Goto, Appl. Phys. Lett. 70 (1997) 2230.
[3] D.C. Look, C. Coskun, B. Claflin, G.C. Farlow, Physica B 340 (2003) 32. [4] D.C. Look, D.C. Reynolds, C.W. Litton, R.L. Jones, D.B. Eason, G. Cantwell, Appl. Phys. Lett. 81 (2002) 1830. [5] J.G. Lu, Z.Z. Ye, F. Zhuge, Y.J. Zeng, B.H. Zhao, L.P. Zhu, Appl. Phys. Lett. 85 (2004) 3134. [6] A. Tsukazaki, A. Ohtomo, T. Onuma, M. Ohtani, T. Makino, M. Sumiya, K. Ohtani, S.F. Chichibu, S. Fuke, Y. Segawa, H. Ohno, H. Koinuma, M. Kawasaki, Nat. Mat. 4 (2005) 42. [7] C.C. Lin, S.Y. Chen, S.Y. Cheng, H.Y. Lee, Appl. Phys. Lett. 84 (2004) 5040. [8] T. Aoki, Y. Hatanaka, D.C. Look, Appl. Phys. Lett. 76 (2000) 3257. [9] K.K. Kim, S.H. Kim, D.H. Hwang, J.H. Lim, S.J. Park, Appl. Phys. Lett. 83 (2003) 63. [10] Y.R. Ryu, T.S. Lee, H.W. White, Appl. Phys. Lett. 83 (2003) 87. [11] D.C. Look, G.M. Renlund, R.H. Burgener, J.R. Sizelove, Appl. Phys. Lett. 85 (2004) 5269. [12] W.Z. Xu, Z.Z. Ye, Y.J. Zeng, L.P. Zhu, B.H. Zhao, L. Jiang, J.G. Lu, H.P. He, S.B. Zhang, Appl. Phys. Lett. 88 (2006) 173506. [13] Y.R. Ryu, T.S. Lee, J.A. Lubguban, H.W. White, B.J. Kim, Y.S. Park, C.J. Youn, Appl. Phys. Lett. 88 (2006) 241108. [14] Y.F. Yan, J.B. Li, S.H. Wei, M.M. Al-Jassim, Phys. Rev. Lett. 98 (2007) 135506. [15] C. Persson, C. Platzer-Bjo¨rkman, J. Malmstro¨m, T. To¨rndahl, M. Edoff, Phys. Rev. Lett. 97 (2006) 146403. [16] B.K. Meyer, H. Alves, D.M. Hofmann, W. Kriegseis, D. Forster, F. Bertram, J. Chresten, A. Hoffmann, M. Strabburg, M. Dworzak, U. Haboeck, A.V. Rodina, Phys. Status Solidi. B 241 (2004) 231. [17] E.C. Lee, Y.S. Kim, Y.G. Jin, K.J. Chang, Phys. Rev. B 64 (2001) 085120. [18] S. Limpijumnong, X.N. Li, S.H. Wei, S.B. Zhang, Appl. Phys. Lett. 86 (2005) 211910. [19] L.G. Wang, A. Zunger, Phys. Rev. Lett. 90 (2003) 256401. [20] C.H. Park, S.B. Zhang, S.H. Wei, Phys. Rev. B 66 (2002) 073202. [21] F. Zhuge, L.P. Zhu, Z.Z. Ye, D.W. Ma, J.G. Lu, J.Y. Huang, F.Z. Wang, Z.G. Ji, S.B. Zhang, Appl. Phys. Lett. 87 (2005) 092103. [22] L.J. Wang, N.C. Giles, J. Appl. Phys. 94 (2003) 973. [23] 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. [24] M.A. Reshchikov, F. Shahedipour, R.Y. Korotkov, B.W. Wessels, M.P. Ulmer, J. Appl. Phys. 87 (2000) 3351. [25] A.F. Kohan, G. Ceder, D. Morgan, C.G. Van de Walle, Phys. Rev. B 61 (2000) 15019. ¨ zgu¨r, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dog˘an, ¨.O [26] U V. Avrutin, S.J. Cho, H. Morkoc, J. Appl. Phys. 98 (2005) 041301. [27] A. Janotti, C.G. Van de Walle, Appl. Phys. Lett. 87 (2005) 122102. [28] C.G. Van de Walle, J. Neugebauer, Nature 423 (2003) 626. [29] H.P. He, F. Zhuge, Z.Z. Ye, L.P. Zhu, B.H. Zhao, J.Y. Huang, J. Phys. D. 39 (2006) 2339. [30] J.M. Carlsson, H.S. Domingos, P.D. Bristowe, B. Hellsing, Phys. Rev. Lett. 91 (2003) 165506. [31] T. Yamamoto, Thin Solid Films 420 (2002) 100.
Optical Materials 30 (2008) 1422–1426 www.elsevier.com/locate/optmat
Comparative photoluminescence study on p-type and n-type ZnO films codoped by nitrogen and aluminium Haiping Tang a
a,b
, Zhizhen Ye b, Haiping He
b,*
Department of Mechanical and Electrical Engineering, Baoji University of Arts and Sciences, Baoji, Shannxi 721007, PR China b State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China Received 1 June 2007; received in revised form 21 August 2007; accepted 27 August 2007 Available online 10 October 2007
Abstract We report on the temperature-dependent photoluminescence (PL) properties of n-type and p-type ZnO films codoped with N and Al. For the n-type film, the dominant emission at low temperature is exciton bound to neutral donors, while for the p-type film it is exciton bound to neutral acceptor at 3.33 eV. Four defect or impurity levels, including N acceptor, residual acceptor, and two doping-induced unknown deep acceptors, were identified. The energy level of the N acceptor was determined to be 0.23 eV. Excitation energy dependence of the PL was also investigated. It was found that at high excitation energy, the formation of exciton was suppressed by the formation of D+Aeh complexes. Ó 2007 Elsevier B.V. All rights reserved. PACS: 71.55.Gs; 61.72.Vv Keywords: Zinc oxide; Photoluminescence; p-type
1. Introduction ZnO is a wide band gap (3.37 eV) semiconductor with a very large exciton binding energy of 60 meV, which, in principle, allows the exciton-governed luminescence at short wavelengths to be dominant at room temperature. It has attracted considerable interest due to its potential applications in short wavelength optoelectronic devices [1,2]. However, a major obstacle for the application of ZnO is the difficulty in achieving good and reliable p-type material [3]. Recently, steady progress in doping ZnO with p-type has been made via various techniques [4–11], and electroluminescence from ZnO-based pn junctions and light-emission diodes (LEDs) has been reported [6,12,13]. Among the doping methods, the donor–acceptor codoping method has proving very promising. Most recent theoretical study [14] on Ga–N codoping revealed that by codoping *
Corresponding author. Tel./fax: +86 571 87952625. E-mail address: [email protected] (H. He).
0925-3467/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2007.08.006
Ga and N a fully occupied impurity band can be formed above the valence band maximum, which significantly lowers the ionization energy of the N acceptors. Recently, a new method of generating p-type ZnO by S incorporation also shows shallower acceptor state [15]. However, there are still many deficiencies in the p-type layer when compared with n-type ZnO. Such deficiencies include radiative and nonradiative defects and/or impurities, which degrade the near band edge luminescence. To better understand and improve the film’s optical properties and meet the application of ZnO in short wavelength spectral range, therefore, it is essential to collect information of these defects and impurities. Photoluminescence (PL) spectroscopy is a very sensitive tool for characterizing radiative defects. PL from p-type ZnO doped with group-V elements has been investigated. In particular, for N-doped ZnO, low temperature PL has been used to determine the acceptor level by studying the recombination of bound excitons (BX) and donor–acceptor pair (DAP). In ZnO material, the knowledge of donors
H. Tang et al. / Optical Materials 30 (2008) 1422–1426
and acceptors, especially of acceptors, is rather poor. Up to eleven excitonic recombinations where excitons bind to neutral donors and/or acceptors have been observed, however, the chemical nature of the donor and acceptor species remained to be determined [16]. The situation appears more complicated for the codoped case. A few theoretical works [17–20] dealing with the thermodynamics of defects, impurities, and complexes in N-doped ZnO have been published. Defects, impurities, and complexes that may act as deep level centers have been proposed. However, few experimental data about these species have been reported. In previous studies [5,21], we have demonstrated that N can be effectively incorporated into ZnO and reasonable ptype conduction can be achieved via Al–N co-doping technique. In this work, we focused on the PL properties of the ZnO:N:Al films. Four acceptor levels were derived from the temperature-dependent PL analysis. We found that the energy level located at about 0.23 eV above the valence band is most likely attributed to N acceptor. 2. Experimental ZnO:N:Al films were grown on a Si substrate at 500 °C by DC reactive magnetron sputtering method using AlxZn1x alloy as the target and N2O + O2 as the dopant gas. Details of the sputtering parameters were described elsewhere [21]. The x value in AlxZn1x alloy targets was chosen to be 0.004 and 0.01 to obtain different Al content in the films. Hall and I–V measurements indicated that the film with Al content of 0.4 and 1 at% is p-type and n-type, respectively [21]. PL spectra were recorded on a FluoRoLOG-3-TAU (France, Jobin Yvon) fluorescence spectrometer, using Xe lamp as the source of excitation light. The measuring temperature various from 10 to 300 K. Low temperatures were obtained by CCS-335 (American, Janis) equipment. 3. Results and discussion Fig. 1 shows the temperature-dependent PL spectra of the two ZnO:N:Al films. Both of them are broad and show five PL bands centered at 3.33–3.36, 3.24, 3.17, 3.04, and 2.85 eV, respectively. At low temperatures, the PL spectra are dominated by the 3.17 and 3.04 eV bands. As the temperatures increases, the 3.04 eV emission becomes dominant for the 0.4 at% Al-doped sample, while for the 1 at% Al-doped sample it is the 3.17 eV band. At 250– 260 K, the 3.33 eV bands for the 0.4 at% Al-doped sample disappear suddenly. For the 1 at% Al-doped case, however, the 3.36 eV band generally converts to emission band with peak energy of 3.28 eV and the PL spectra is a mixture of the 3.17 and 3.28 eV bands at room temperature. The band at 3.36 and 3.33 eV in n- and p-type samples can be attributed to recombination of D0X and A0X in ZnO, respectively. It is well known that the PL of ZnO is dominated by BX lines at low temperature. By far in ZnO, more than ten donors bound exciton (D0X) and
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acceptor bound exciton (A0X) lines have been reported in the range of 3.315–3.372 eV. The temperature dependence of the peak energy of BX band for the two samples was plotted in Fig. 2. The reported [22] temperature dependence of free exciton (FX) in ZnO was also given as a reference. As the temperature increases, the peak energy gradually decreases and close to the energy of FX at 160–200 K, indicating a conversion of BX to FX at elevated temperature. Thus the room temperature PL for the 1 at% Al-doped sample (Fig. 1b) is composed of the 3.17 eV band and the free exciton band. The 3.24 eV band is weak and is most likely related to residual acceptor in ZnO. Meyer et al. [16] have reported DAP line at 3.22 eV in undoped ZnO and ascribed it to unknown residual acceptor. The emission near 3.05 eV has been assigned to recombination of localized carries [23]. The peak position of the 3.17 band varies less than 10 meV in the temperature range of 10–300 K, whereas the band gap decreases by about 70 meV. This weak temperature dependence is typical for emissions related to defect level. Thus it was proposed that the band is due to DAP recombination. It is well known that the recombination probability of DAP is proportional to the concentration of neutral acceptors. The dominance of the 3.17 eV band in 1 at% Al-doped sample is indicative of larger concentration of the related neutral acceptor. In p-type semiconductor, the Fermi level is close to the acceptor level, which results in low concentration of neutral acceptors. In contrast, the Fermi level is close to the donor level in n-type semiconductor thus leading to high concentration of neutral acceptors. Because the 0.4 at% Al-doped sample is p-type while the 1 at% Al-doped one is n-type, we suggest that the 3.17 eV band is related to N acceptor. The acceptor level can be derived from D0 A0 ¼ Eg ðED þ EA Þ þ e2 =4pr
ð1Þ
where Eg, ED, and EA are the band gap, donor, and acceptor energies, respectively, and r is the pair separation. We take the low temperature band gap of 3.437 eV and thus the room temperature value of 3.377 eV. Thus EA levels related to 3.17 eV bands are evaluated to be 0.23 eV. The value of 0.23 eV is close to the reported [4,16] N acceptor in ZnO, 0.16–0.20 eV. Fig. 3 shows the PL spectra excited with various excitation energies for the n-type sample. It was found that the peak energy of D0X is almost constant while that of the 3.17 eV bands decrease with increasing excitation energy. The maximal redshift is as large as 40 meV. The redshift of D0A0 energy at higher excitation energy can be explained as follows: at higher excitation energy, the free carriers possess higher energy and they can go longer spatial distance before losing energy and being trapped by donor/acceptor centers. This leads to more dispersive spatial distribution of the trapped carriers. In other words, the pair separation increases with excitation energy, thus leading to lower recombination energy. The redshift of the peak
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Fig. 1. Temperature-dependent PL spectra of (a) p-type and (b) n-type ZnO:N:Al films. The DAP emission involving unknown and N acceptor is labeled as DAXP and DANP, respectively.
Fig. 2. Peak energy of bound excitons and DANP as a function of temperature. The solid line represents data for free exciton (Ref. [21]).
Fig. 3. PL spectra for the n-type film under different excitation energy.
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supports the assignment of the 3.17 eV peak to DAP recombination. The emission around 2.8 eV is obviously different from the commonly observed green band (2.5 eV) in ZnO. Tsukazaki et al. [6] has also reported a very broad emission centered at 2.8 eV in p-type ZnO:N film. Similarly, Reshchikov et al. [24] observed a broad PL band around 2.9 eV in GaN. Both of them assumed that the broad emission is due to DAP recombination. Fig. 4 shows the thermal quenching characteristics of the 2.85 eV bands obtained from Fig. 1b. The integrated PL intensity was obtained by Gaussian fit to the spectra. From Fig. 4 the activation energy for the 2.85 eV band was calculated to be 57 meV by fitting the data [23]. The PL quenching could be attributed to thermal release of trapped electrons from the donor level to the conduction band. Thus the obtained activation energies can be assigned to the thermal ionization energy of shallow donors. It was reported that the energy level of AlZn donor in bulk ZnO is located at about 51 meV below the conduction band [16], which is close to the activation energy. Therefore, we suggest the shallow donor involved in the 2.85 eV emission is most likely AlZn. The energy level of the deep acceptor responsible for the band can then be derived from Eq. (1). For a rough estimation, the last term in Eq. (1) is set to 0.03 eV according to Look et al. [4]. By using ED of 0.05 and 0.03 eV, the obtained acceptor level for the 2.85 eV bands is about 0.59 eV. The energy level of the residual acceptor related to the 3.24 eV emission can also be estimated to be 0.17–0.20 eV. Because our samples are doped and grown by sputtering method, native defects like vacancies may be induced. To assess the sample quality, we have performed XPS measurements (not shown). The results revealed that both samples are Zn-rich, with Zn/O ratios of 1.1. Therefore, oxygen vacancies (VO) should exist in the samples while
zinc vacancies (VZn) should be unlikely because the formation energy of VZn is very large (5.5 eV) under Zn-rich conditions [25]. However, it is rather difficult to identify emission involving VO because there are still considerable disputes on it [26]. While many researchers assign the green band around 2.5 eV to VO, there also a lot of work (see Ref. [26] and therein) disagree it. In a recent work, Janotti and Van de Walle [27] predicted that VO is a deep donor, which luminescence at energies of 1.5 and 0.7 eV. In our PL spectra, however, neither the emission around 2.5 eV nor the ones at 1.5 eV and 0.7 eV were clearly observed. It should also be noted that hydrogen inclusion can be a major contribution to the donor states [28]. The I4 excitonic line at 3.3628 eV has been assigned to the H donors [16]. In our samples, due to the degraded crystal quality and broadened PL line width resulting from doping, we were unable to resolve this line from the spectra. However, we have confirmed the presence of H in our Al–N codoped samples by infrared absorption in a previous work [29]. From above results, one can identify several acceptor levels in ZnO:N:Al films. It was then clear that there are at least four competing radiative paths for the photoexcited carriers in our Al–N co-doped samples, that is, to form bound excitons or to form D+Aeh complexes. Fig. 5 shows the peak intensity ratio of DAP emissions to BX as a function of excitation energy. It was found that the intensity of BX decreases with increasing excitation energy. This decrease is accompanied by an increase in the DAP intensities. It implies that the formation of exciton is suppressed at high-energy excitation and it allows more chances for the excited carriers to bind with the donor and acceptor centers forming D+Aeh complexes. The microscopic nature of the deep acceptors with binding energy of 0.59 eV is still unclear. While residual deep acceptor impurities and interface states in the grain bound-
Fig. 4. Integrated intensity of the 2.85 eV emission as a function of the reciprocal temperature. An activation energy of 57 meV was obtained by fitting.
Fig. 5. Peak intensity ratio of the defect or impurity-related emissions to bound exciton as a function of excitation energy for the n-type film. LC represent emission related to localized carrier at 3.04 eV.
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aries [30] cannot be ruled out, it is also very likely induced by doping. In N-doped ZnO, many deep level complexes can be formed as predicted by theoretical calculations [18–20]. While the defect dynamics in N-doped ZnO has been investigated, the report on both theoretical and experimental study on N–Al codoping is few. Actually, the case of N–Al codoping is much more complicated because of the presence of Al. For example, it was predicted [31] that substitutional Al and N atoms could form donor–acceptor complex due to the strong bond strength of the Al–N bond. Such complex may act as a deep level and give rise to emission with lower energy. Further investigation should be conducted to clarify the actual origin of the 2.85 eV emission. 4. Conclusions In summary, PL from n- and p-type ZnO:N:Al films has been investigated. The PL spectra of the p-type film show A0X band at about 3.33 eV. Several defect or impurityrelated emissions at 3.24, 3.17, 3.04, and 2.85 eV were observed. For p-type film with 0.4 at% of Al the PL peaked at 3.04 eV, while for n-type film with 1 at% of Al the peak energy is about 3.17 eV. We suggest the 3.17 eV emission is related to the N acceptor with energy level of 0.23 eV. Thermal quenching of the 2.85 eV band gives the activation energy of 57 meV, which corresponding to the binding energy of AlZn donors in the films. Excitation energy dependence of the PL revealed that, at high excitation energy, the DAP energies show a redshift and the formation of exciton was suppressed by the formation of D+Aeh complexes. Acknowledgement This work was supported by NSFC under Grant No. 50532060. References [1] Z.K. Tang, G.K.L. Wong, P. Yu, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Appl. Phys. Lett. 72 (1998) 3270. [2] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Tao, S. Koyama, M.Y. Shen, T. Goto, Appl. Phys. Lett. 70 (1997) 2230.
[3] D.C. Look, C. Coskun, B. Claflin, G.C. Farlow, Physica B 340 (2003) 32. [4] D.C. Look, D.C. Reynolds, C.W. Litton, R.L. Jones, D.B. Eason, G. Cantwell, Appl. Phys. Lett. 81 (2002) 1830. [5] J.G. Lu, Z.Z. Ye, F. Zhuge, Y.J. Zeng, B.H. Zhao, L.P. Zhu, Appl. Phys. Lett. 85 (2004) 3134. [6] A. Tsukazaki, A. Ohtomo, T. Onuma, M. Ohtani, T. Makino, M. Sumiya, K. Ohtani, S.F. Chichibu, S. Fuke, Y. Segawa, H. Ohno, H. Koinuma, M. Kawasaki, Nat. Mat. 4 (2005) 42. [7] C.C. Lin, S.Y. Chen, S.Y. Cheng, H.Y. Lee, Appl. Phys. Lett. 84 (2004) 5040. [8] T. Aoki, Y. Hatanaka, D.C. Look, Appl. Phys. Lett. 76 (2000) 3257. [9] K.K. Kim, S.H. Kim, D.H. Hwang, J.H. Lim, S.J. Park, Appl. Phys. Lett. 83 (2003) 63. [10] Y.R. Ryu, T.S. Lee, H.W. White, Appl. Phys. Lett. 83 (2003) 87. [11] D.C. Look, G.M. Renlund, R.H. Burgener, J.R. Sizelove, Appl. Phys. Lett. 85 (2004) 5269. [12] W.Z. Xu, Z.Z. Ye, Y.J. Zeng, L.P. Zhu, B.H. Zhao, L. Jiang, J.G. Lu, H.P. He, S.B. Zhang, Appl. Phys. Lett. 88 (2006) 173506. [13] Y.R. Ryu, T.S. Lee, J.A. Lubguban, H.W. White, B.J. Kim, Y.S. Park, C.J. Youn, Appl. Phys. Lett. 88 (2006) 241108. [14] Y.F. Yan, J.B. Li, S.H. Wei, M.M. Al-Jassim, Phys. Rev. Lett. 98 (2007) 135506. [15] C. Persson, C. Platzer-Bjo¨rkman, J. Malmstro¨m, T. To¨rndahl, M. Edoff, Phys. Rev. Lett. 97 (2006) 146403. [16] B.K. Meyer, H. Alves, D.M. Hofmann, W. Kriegseis, D. Forster, F. Bertram, J. Chresten, A. Hoffmann, M. Strabburg, M. Dworzak, U. Haboeck, A.V. Rodina, Phys. Status Solidi. B 241 (2004) 231. [17] E.C. Lee, Y.S. Kim, Y.G. Jin, K.J. Chang, Phys. Rev. B 64 (2001) 085120. [18] S. Limpijumnong, X.N. Li, S.H. Wei, S.B. Zhang, Appl. Phys. Lett. 86 (2005) 211910. [19] L.G. Wang, A. Zunger, Phys. Rev. Lett. 90 (2003) 256401. [20] C.H. Park, S.B. Zhang, S.H. Wei, Phys. Rev. B 66 (2002) 073202. [21] F. Zhuge, L.P. Zhu, Z.Z. Ye, D.W. Ma, J.G. Lu, J.Y. Huang, F.Z. Wang, Z.G. Ji, S.B. Zhang, Appl. Phys. Lett. 87 (2005) 092103. [22] L.J. Wang, N.C. Giles, J. Appl. Phys. 94 (2003) 973. [23] 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. [24] M.A. Reshchikov, F. Shahedipour, R.Y. Korotkov, B.W. Wessels, M.P. Ulmer, J. Appl. Phys. 87 (2000) 3351. [25] A.F. Kohan, G. Ceder, D. Morgan, C.G. Van de Walle, Phys. Rev. B 61 (2000) 15019. ¨ zgu¨r, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dog˘an, ¨.O [26] U V. Avrutin, S.J. Cho, H. Morkoc, J. Appl. Phys. 98 (2005) 041301. [27] A. Janotti, C.G. Van de Walle, Appl. Phys. Lett. 87 (2005) 122102. [28] C.G. Van de Walle, J. Neugebauer, Nature 423 (2003) 626. [29] H.P. He, F. Zhuge, Z.Z. Ye, L.P. Zhu, B.H. Zhao, J.Y. Huang, J. Phys. D. 39 (2006) 2339. [30] J.M. Carlsson, H.S. Domingos, P.D. Bristowe, B. Hellsing, Phys. Rev. Lett. 91 (2003) 165506. [31] T. Yamamoto, Thin Solid Films 420 (2002) 100.