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Properties of N-doped ZnO thin films in annealing process
Solid State Communications 143 (2007) 562–565 www.elsevier.com/locate/ssc
Properties of N-doped ZnO thin films in annealing process Yinzhu Zhang a , Jianguo Lu a,b,∗ , Lanlan Chen a,c , Zhizhen Ye a a State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, China b International Innovation Center, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8520, Japan c Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China
Received 22 February 2007; received in revised form 11 June 2007; accepted 29 June 2007 by R. Phillips Available online 12 July 2007
Abstract N-doped ZnO (ZnO:N) thin films were prepared by magnetron sputtering using NH3 as the N-doping source. The as-grown ZnO:N films showed high resistivity ∼103 cm due to the hydrogen passivation effect. The properties of ZnO:N films under various annealing conditions (e.g., temperature, environment, and duration) were systematically studied with the aim of achieving p-type conductivity. The lowest room-temperature resistivity was found to be 7.73 cm for p-type ZnO films annealed at 500 ◦ C for 10 min in N2 , with the hole concentration of 9.36 × 1017 cm−3 and Hall mobility of 0.86 cm2 V−1 s−1 . Optical absorption spectra were performed to analyze the behaviors of hydrogen and nitrogen in p-type doping of ZnO. The NO –H complexes were largely present in as-grown ZnO films, which could be dissociated by thermal annealing resulting in activated N acceptors. Thus, the p-type conductivity was achieved in annealed ZnO:N films. A hydrogen-assisted nitrogen-acceptor doping mechanism was proposed as an answer for the achievement of p-type ZnO. c 2007 Elsevier Ltd. All rights reserved.
PACS: 61.72.Vv; 71.55.Gs; 81.05.Dz; 81.15.Cd; 81.40.Ef Keywords: A. Thin films; B. Thermal annealing; C. Impurities in semiconductors; D. Electrical properties
1. Introduction In the past decades, researches on wide-bandgap semiconductors have led to the commercialization of devices based on group-III nitrides. As an alternative to GaN, ZnO is attracting substantial attention. ZnO has a strong exciton binding energy of 60 meV, much larger than that of GaN (25 meV) and the thermal energy at room temperature (26 meV), which can ensure an efficient exciton emission at room temperature under low excitation energy. As a consequence, ZnO is recognized as a promising material for application to optoelectronic devices in the blue–UV region. However, this material has largely failed to live up to its potential, because it is difficult to produce low-resistivity p-type ZnO. Recently, thanks to the considerable efforts of researchers [1–24], steady progress has been made in this field. Among the possible p-type doping agents, N is
∗ Corresponding author at: International Innovation Center, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8520, Japan. Tel.: +81 75 383 3076. E-mail address: [email protected] (J. Lu).
c 2007 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2007.07.004
regarded as the most effective one considering its size, energy level, and compensation effects [4]. But there still remains a problem for using N as the p-type dopant; that is, N is not very soluble in ZnO. Theoretically, the N solubility could be greatly enhanced by forming NO –H complexes in ZnO co-doped with H impurities [13]. As previously discussed in the context of Mg-doped GaN [25,26], H has the beneficial effect of achieving p-type doping. Whether this type of dopant engineering also works in the case of ZnO will depend on the behaviors of H and N during a post-growth annealing process. In this regard, the knowledge of properties of N-doped ZnO (ZnO:N) in annealing process is very necessary. Up to now, however, the study of this issue has been rather limited [5,6]. Researches on phosphorus-doped ZnO (ZnO:P) revealed that the p-type conductivity could be realized by thermal annealing at 700–800 ◦ C in an O2 ambient [20–22]. In contrast, previous reports on the annealing treatment of ZnO:N films in an O2 ambient did not show a definitive result [5,6]. The details of conductivity in annealed ZnO:N have still been unclear so far. In this work, we demonstrate a systematical study of the
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annealing effect on properties of ZnO:N films under various conditions with the aim of achieving p-type conductivity. The behaviors of H and N in ZnO films are analyzed to answer for the achievement of p-type conductivity. 2. Experiments ZnO:N thin films were prepared on α-Al2 O3 (0001) substrates by rf reactive magnetron sputtering. The frequency and power of the rf generator were 13.56 MHz and 120 W, respectively. A ceramic disc of ZnO (99.99% purity) was used as the target. NH3 –O2 gas mixture (99.99% purity for both cases) was used as the sputtering ambient under a constant total pressure of 5 Pa with the NH3 partial pressure of 70%. The films were deposited at 350 ◦ C for 30 min and naturally cooled down to room temperature. The thus obtained samples were annealed in N2 (105 Pa), O2 (105 Pa), and vacuum (10−2 Pa) environments. The annealing temperatures ranged from 200 to 900 ◦ C and the time was in the range from 5 to 60 min. The electrical properties of films were measured in the Van der Pauw configuration using a BIO-RAD HL5500 system. The insulating sapphire substrates assured that Hall-effect measurements were performed without influence from substrate condition [14,17]. The optical absorption spectra were recorded on a Lambda 20 UV–visible spectrophotometer and a FTIR8900 Fourier transform infrared spectrometer (FTIR). All the measurements were carried out at room temperature. 3. Results and discussion Fig. 1 shows the electrical properties of ZnO:N films annealed in N2 for 10 min at different temperatures. The as-grown sample exhibits a high resistivity of about 1.58 × 103 cm. No evident change is observed at annealing temperatures ≤300 ◦ C (region I). Because the sample is highly resistive, the carrier type cannot be definitely determined by Hall-effect measurements. When the annealing temperatures are in the range 400–600 ◦ C (region II), the p-type conductivity is obtained. The lowest room-temperature resistivity is found to be 7.73 cm at 500 ◦ C, with the hole concentration of 9.36 × 1017 cm−3 and Hall mobility of 0.86 cm2 V−1 s−1 . At the annealing temperatures ≥700 ◦ C (region III), however, the samples are of n-type conductivity. In this study ammonia was used as the N-doping source, which would result in hydrogen in the growth environment [7,8]. In this case, it is theoretically suggested that the N− O and H+ charged defects will combine to form the neutral NO –H defect complexes in ZnO, thereby compensating the N-related acceptors [13]. This hydrogen passivation effect is believed to be the reason for the high resistivity of as-grown samples. The most energetically favorable NO –H bond corresponds to hydrogen situated in an antibond configuration (ABN ) [13]. According to previous reports, the enhancement of N solubility and the suppression of compensating interstitials could be expected by the formation of NO –H complexes [7,13]. When the ZnO film is annealed at appropriate temperatures, NO –H complexes may be dissociated and the resultant hydrogen is
Fig. 1. Electrical properties of ZnO:N films annealed for 10 min in N2 ambient at different temperatures. The results illustrated at 25 ◦ C refer to those of the asgrown sample. Open symbols show p-type conductivity. Semi-closed symbols show ambiguous carrier type; that is, the samples are of high resistivity. Closed symbols show n-type conductivity.
annealed out from the film. The thermal stability of hydrogen is considerably lower in ZnO than in GaN [27], so it can be more easily removed from the ZnO film. Then, the N impurities are activated and can behave as effective acceptors responding to the achievement of p-type conductivity. This process may thus be named hydrogen-assisted nitrogen-acceptor doping mechanism, which is similar to that for achieving p-type conductivity in Mg-doped GaN [25,26]. The absence of p-type conductivity at high annealing temperatures is possibly due to the outdiffusion of N impurities and/or the formation N2 -on-O substitution ((N2 )O ) donors after the NO –H complexes are dissociated, since a single NO substitution is not very stable in ZnO [9]. In addition, the outdiffusion of oxygen and/or segregation of zinc may also occur at high annealing temperatures producing native defects such as oxygen vacancy (VO ), zinc interstitial (Zni ), and zinc antisite (ZnO ), which are believed to act as donors [28]. To support our analyses, the optical absorption spectra of ZnO:N films were investigated. Fig. 2 shows the FTIR absorbance spectra in the wavenumber range 2800–3500 cm−1 . This spectral region includes several important local stretch modes involving hydrogen bonded to carbon, nitrogen, and oxygen. For as-grown ZnO:N film, the broad asymmetrical band around 3392 cm−1 is most likely due to the O–H bonds [13,18]. Theoretical calculations suggest that the O–H vibrations in ZnO are in the range from 3216 to 3644 cm−1 depending on the configuration and number of hydrogen atoms in the complex [29,30]. With annealing treatment, this band becomes narrower and more symmetrical peak appears at 3404 cm−1 , which could be due to the surface mode of O–H bands [30]. The absorption peak at 2896 cm−1 is due to the C–Hn stretch frequencies, similar to that observed in single-crystal ZnO [31]. The absorption band around 3040 cm−1 is presumably related to the NO –H bonds. Recent theoretical calculations suggested that the predicted absorption
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Fig. 3. Optical absorption spectra of the as-grown and annealed ZnO:N films. The annealing of the film was carried out at 500 ◦ C for 10 min in N2 .
Fig. 2. FTIR absorbance spectra of the as-grown and annealed ZnO:N films. The annealing of the film was carried out at 500 ◦ C for 10 min in N2 .
frequency of a NO –H bond in the ABN configuration is around 3070 cm−1 [13], in good agreement with the observed stretchmode frequency at 3040 cm−1 . This neutral NO –H complex represents the passivation of a nitrogen acceptor, as mentioned above. When the ZnO:N film is annealed in N2 at 500 ◦ C, all the three peaks at 2896, 3040, and 3392 cm−1 are reduced. In particular, the 3040 cm−1 peak, derived from NO –H stretch mode, almost completely disappears. This indicates that the C–Hn , NO –H, and O–H bands are dissociated by thermal annealing. For the introduced N impurities, since the NO –H complexes are dissociated, these N atoms are activated and behave as effective acceptors. Thus, p-type ZnO could be obtained. Fig. 3 shows the optical absorption spectra of the as-grown and annealed (at 500 ◦ C) ZnO:N films. Compared with the as-grown sample, a near-edge absorption is present in the annealed one. As mentioned above, the N atoms are dissociated from NO –H complexes and serve as effective acceptors in the annealed film. Thus, the origin of this absorption may be related to the N acceptor states in the bandgap of ZnO. Garces et al. [3] demonstrated that the N impurities could be introduced in ZnO by thermal annealing in N2 . In his report the near-edge absorption was also observed from the electron paramagnetic resonance (EPR) and optical absorption spectra [3]. The nearedge absorption may represent transitions from singly ionized N acceptors to shallow donors and the conduction band or from singly ionized N acceptors to deep levels such as transitionmetal impurities or intrinsic defects. Therefore, the near-edge absorption implies that the activated N acceptors are present in the annealed ZnO:N film. In this regard, the absence of nearedge absorption in the as-grown film presumably suggests that the N impurities are inactive in this case, which is believed to be the result of the hydrogen passivation effect. The observations presented here further confirm our aforementioned analyses. To make a systematic investigation of annealing effect on properties of ZnO:N films, the samples were also annealed
Fig. 4. Carrier concentrations of ZnO:N films annealed for 10 min in O2 (a) and vacuum (b) environments at different temperatures. The results illustrated at 25 ◦ C refer to those of the as-grown sample.
in O2 and vacuum environments for 10 min at temperatures ranged from 200 to 900 ◦ C. Fig. 4(a) shows the carrier concentrations of annealed ZnO:N films in O2 , as suggested by Hall-effect measurements. Similar to annealing in N2 , the p-type conductivity could also be realized in a temperature window when the ZnO:N film is annealed in O2 . But the difference between them is evident. As compared with the former, the annealing temperature window (500–600 ◦ C) becomes narrower for the latter, and moreover, the observed hole concentration is only in the 1016 cm−3 order, which is much lower than that (1017 cm−3 ) of annealed samples in N2 . It seems that N2 is more suitable for producing p-type ZnO than O2 in an annealing process. The detailed explanations for this difference are unfeasible at present, but we can consider several possible reasons. Firstly, the outdiffusion of N impurities may be more evident in O2 than in N2 during the annealing process. As mentioned above, Garces et al. [3] demonstrated that annealing a ZnO sample in N2 ambient could even produce NO acceptor dopants. In this regard, the presence of N2 in annealing environment may prevent N impurities from diffusing out from the ZnO:N film. Secondly, the hydrogen annealed out from the
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ranged from 200 to 900 ◦ C, duration from 5 to 60 min, and environments including N2 , O2 , and vacuum. The p-type conductivity of ZnO could be obtained under appropriate annealing conditions. The dissociation of NO –H complexes and the creation of activated N acceptors in ZnO were identified by optical absorption spectra. A hydrogen-assisted nitrogenacceptor doping mechanism was proposed to produce p-type ZnO. We hope this study will provide an insight into the fundamental understanding of the p-type doping of ZnO. References
Fig. 5. Carrier concentrations of ZnO:N films annealed at 500 ◦ C in N2 ambient for different durations.
body of the film may exist in the surface of the film combining with oxygen to form an O–H bond. This surface mode of an O–H bond has been observed in the FTIR spectrum of the film annealed in N2 , as shown in Fig. 2. In an O2 ambient there certainly will be much more abundant O–H bonds formed in the surface of the film. It was argued that the O–H bond, in fact, can be regarded as a new type of donor dopant atom in ZnO, the addition of the proton turning the oxygen into an element behaving much like fluorine [29]. Thus, an n-type layer is expected to form in the surface of the annealed p-type film. This n-type surface conductivity may exert a great influence on the final result of Hall-effect measurements [32]. Fig. 4(b) shows the carrier concentrations of ZnO:N films annealed in vacuum, as suggested by Hall-effect measurements. It is found that the annealing in vacuum only induces highresistivity or n-type samples. No p-type conductivity was obtained, which is very different from the ZnO:N film annealed in N2 or O2 . The absence of p-type conductivity for annealed films in vacuum is possibly due to the serious outdiffusion of N impurities from the film and the creation of large amounts of native defect donors such as VO , Zni , and ZnO in ZnO. At an annealing temperature of 900 ◦ C, the ZnO film shows strong n-type conductivity with the electron concentration of 9.78 × 1019 cm−3 . Fig. 5 displays the carrier concentrations of ZnO:N samples annealed for various durations at 500 ◦ C in N2 ambient. When the annealing time is in the range from 5 to 20 min, the p-type conductivity could be achieved. It seems that a rapid thermal annealing (e.g., 10 min) is necessary for achieving good p-type ZnO. Annealing for a long time does not lead to positive results. When the annealing time exceeds 20 min, only high-resistivity or n-type samples are obtained. This observation can also be interpreted by the intensified tendency of outdiffusion of N impurities, formation of (N2 )O donors, and creation of native defect donors with increasing annealing duration. 4. Conclusions In summary, we have systematically studied the properties of ZnO:N films in annealing processes, with temperatures
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Properties of N-doped ZnO thin films in annealing process Yinzhu Zhang a , Jianguo Lu a,b,∗ , Lanlan Chen a,c , Zhizhen Ye a a State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, China b International Innovation Center, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8520, Japan c Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China
Received 22 February 2007; received in revised form 11 June 2007; accepted 29 June 2007 by R. Phillips Available online 12 July 2007
Abstract N-doped ZnO (ZnO:N) thin films were prepared by magnetron sputtering using NH3 as the N-doping source. The as-grown ZnO:N films showed high resistivity ∼103 cm due to the hydrogen passivation effect. The properties of ZnO:N films under various annealing conditions (e.g., temperature, environment, and duration) were systematically studied with the aim of achieving p-type conductivity. The lowest room-temperature resistivity was found to be 7.73 cm for p-type ZnO films annealed at 500 ◦ C for 10 min in N2 , with the hole concentration of 9.36 × 1017 cm−3 and Hall mobility of 0.86 cm2 V−1 s−1 . Optical absorption spectra were performed to analyze the behaviors of hydrogen and nitrogen in p-type doping of ZnO. The NO –H complexes were largely present in as-grown ZnO films, which could be dissociated by thermal annealing resulting in activated N acceptors. Thus, the p-type conductivity was achieved in annealed ZnO:N films. A hydrogen-assisted nitrogen-acceptor doping mechanism was proposed as an answer for the achievement of p-type ZnO. c 2007 Elsevier Ltd. All rights reserved.
PACS: 61.72.Vv; 71.55.Gs; 81.05.Dz; 81.15.Cd; 81.40.Ef Keywords: A. Thin films; B. Thermal annealing; C. Impurities in semiconductors; D. Electrical properties
1. Introduction In the past decades, researches on wide-bandgap semiconductors have led to the commercialization of devices based on group-III nitrides. As an alternative to GaN, ZnO is attracting substantial attention. ZnO has a strong exciton binding energy of 60 meV, much larger than that of GaN (25 meV) and the thermal energy at room temperature (26 meV), which can ensure an efficient exciton emission at room temperature under low excitation energy. As a consequence, ZnO is recognized as a promising material for application to optoelectronic devices in the blue–UV region. However, this material has largely failed to live up to its potential, because it is difficult to produce low-resistivity p-type ZnO. Recently, thanks to the considerable efforts of researchers [1–24], steady progress has been made in this field. Among the possible p-type doping agents, N is
∗ Corresponding author at: International Innovation Center, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8520, Japan. Tel.: +81 75 383 3076. E-mail address: [email protected] (J. Lu).
c 2007 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2007.07.004
regarded as the most effective one considering its size, energy level, and compensation effects [4]. But there still remains a problem for using N as the p-type dopant; that is, N is not very soluble in ZnO. Theoretically, the N solubility could be greatly enhanced by forming NO –H complexes in ZnO co-doped with H impurities [13]. As previously discussed in the context of Mg-doped GaN [25,26], H has the beneficial effect of achieving p-type doping. Whether this type of dopant engineering also works in the case of ZnO will depend on the behaviors of H and N during a post-growth annealing process. In this regard, the knowledge of properties of N-doped ZnO (ZnO:N) in annealing process is very necessary. Up to now, however, the study of this issue has been rather limited [5,6]. Researches on phosphorus-doped ZnO (ZnO:P) revealed that the p-type conductivity could be realized by thermal annealing at 700–800 ◦ C in an O2 ambient [20–22]. In contrast, previous reports on the annealing treatment of ZnO:N films in an O2 ambient did not show a definitive result [5,6]. The details of conductivity in annealed ZnO:N have still been unclear so far. In this work, we demonstrate a systematical study of the
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annealing effect on properties of ZnO:N films under various conditions with the aim of achieving p-type conductivity. The behaviors of H and N in ZnO films are analyzed to answer for the achievement of p-type conductivity. 2. Experiments ZnO:N thin films were prepared on α-Al2 O3 (0001) substrates by rf reactive magnetron sputtering. The frequency and power of the rf generator were 13.56 MHz and 120 W, respectively. A ceramic disc of ZnO (99.99% purity) was used as the target. NH3 –O2 gas mixture (99.99% purity for both cases) was used as the sputtering ambient under a constant total pressure of 5 Pa with the NH3 partial pressure of 70%. The films were deposited at 350 ◦ C for 30 min and naturally cooled down to room temperature. The thus obtained samples were annealed in N2 (105 Pa), O2 (105 Pa), and vacuum (10−2 Pa) environments. The annealing temperatures ranged from 200 to 900 ◦ C and the time was in the range from 5 to 60 min. The electrical properties of films were measured in the Van der Pauw configuration using a BIO-RAD HL5500 system. The insulating sapphire substrates assured that Hall-effect measurements were performed without influence from substrate condition [14,17]. The optical absorption spectra were recorded on a Lambda 20 UV–visible spectrophotometer and a FTIR8900 Fourier transform infrared spectrometer (FTIR). All the measurements were carried out at room temperature. 3. Results and discussion Fig. 1 shows the electrical properties of ZnO:N films annealed in N2 for 10 min at different temperatures. The as-grown sample exhibits a high resistivity of about 1.58 × 103 cm. No evident change is observed at annealing temperatures ≤300 ◦ C (region I). Because the sample is highly resistive, the carrier type cannot be definitely determined by Hall-effect measurements. When the annealing temperatures are in the range 400–600 ◦ C (region II), the p-type conductivity is obtained. The lowest room-temperature resistivity is found to be 7.73 cm at 500 ◦ C, with the hole concentration of 9.36 × 1017 cm−3 and Hall mobility of 0.86 cm2 V−1 s−1 . At the annealing temperatures ≥700 ◦ C (region III), however, the samples are of n-type conductivity. In this study ammonia was used as the N-doping source, which would result in hydrogen in the growth environment [7,8]. In this case, it is theoretically suggested that the N− O and H+ charged defects will combine to form the neutral NO –H defect complexes in ZnO, thereby compensating the N-related acceptors [13]. This hydrogen passivation effect is believed to be the reason for the high resistivity of as-grown samples. The most energetically favorable NO –H bond corresponds to hydrogen situated in an antibond configuration (ABN ) [13]. According to previous reports, the enhancement of N solubility and the suppression of compensating interstitials could be expected by the formation of NO –H complexes [7,13]. When the ZnO film is annealed at appropriate temperatures, NO –H complexes may be dissociated and the resultant hydrogen is
Fig. 1. Electrical properties of ZnO:N films annealed for 10 min in N2 ambient at different temperatures. The results illustrated at 25 ◦ C refer to those of the asgrown sample. Open symbols show p-type conductivity. Semi-closed symbols show ambiguous carrier type; that is, the samples are of high resistivity. Closed symbols show n-type conductivity.
annealed out from the film. The thermal stability of hydrogen is considerably lower in ZnO than in GaN [27], so it can be more easily removed from the ZnO film. Then, the N impurities are activated and can behave as effective acceptors responding to the achievement of p-type conductivity. This process may thus be named hydrogen-assisted nitrogen-acceptor doping mechanism, which is similar to that for achieving p-type conductivity in Mg-doped GaN [25,26]. The absence of p-type conductivity at high annealing temperatures is possibly due to the outdiffusion of N impurities and/or the formation N2 -on-O substitution ((N2 )O ) donors after the NO –H complexes are dissociated, since a single NO substitution is not very stable in ZnO [9]. In addition, the outdiffusion of oxygen and/or segregation of zinc may also occur at high annealing temperatures producing native defects such as oxygen vacancy (VO ), zinc interstitial (Zni ), and zinc antisite (ZnO ), which are believed to act as donors [28]. To support our analyses, the optical absorption spectra of ZnO:N films were investigated. Fig. 2 shows the FTIR absorbance spectra in the wavenumber range 2800–3500 cm−1 . This spectral region includes several important local stretch modes involving hydrogen bonded to carbon, nitrogen, and oxygen. For as-grown ZnO:N film, the broad asymmetrical band around 3392 cm−1 is most likely due to the O–H bonds [13,18]. Theoretical calculations suggest that the O–H vibrations in ZnO are in the range from 3216 to 3644 cm−1 depending on the configuration and number of hydrogen atoms in the complex [29,30]. With annealing treatment, this band becomes narrower and more symmetrical peak appears at 3404 cm−1 , which could be due to the surface mode of O–H bands [30]. The absorption peak at 2896 cm−1 is due to the C–Hn stretch frequencies, similar to that observed in single-crystal ZnO [31]. The absorption band around 3040 cm−1 is presumably related to the NO –H bonds. Recent theoretical calculations suggested that the predicted absorption
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Fig. 3. Optical absorption spectra of the as-grown and annealed ZnO:N films. The annealing of the film was carried out at 500 ◦ C for 10 min in N2 .
Fig. 2. FTIR absorbance spectra of the as-grown and annealed ZnO:N films. The annealing of the film was carried out at 500 ◦ C for 10 min in N2 .
frequency of a NO –H bond in the ABN configuration is around 3070 cm−1 [13], in good agreement with the observed stretchmode frequency at 3040 cm−1 . This neutral NO –H complex represents the passivation of a nitrogen acceptor, as mentioned above. When the ZnO:N film is annealed in N2 at 500 ◦ C, all the three peaks at 2896, 3040, and 3392 cm−1 are reduced. In particular, the 3040 cm−1 peak, derived from NO –H stretch mode, almost completely disappears. This indicates that the C–Hn , NO –H, and O–H bands are dissociated by thermal annealing. For the introduced N impurities, since the NO –H complexes are dissociated, these N atoms are activated and behave as effective acceptors. Thus, p-type ZnO could be obtained. Fig. 3 shows the optical absorption spectra of the as-grown and annealed (at 500 ◦ C) ZnO:N films. Compared with the as-grown sample, a near-edge absorption is present in the annealed one. As mentioned above, the N atoms are dissociated from NO –H complexes and serve as effective acceptors in the annealed film. Thus, the origin of this absorption may be related to the N acceptor states in the bandgap of ZnO. Garces et al. [3] demonstrated that the N impurities could be introduced in ZnO by thermal annealing in N2 . In his report the near-edge absorption was also observed from the electron paramagnetic resonance (EPR) and optical absorption spectra [3]. The nearedge absorption may represent transitions from singly ionized N acceptors to shallow donors and the conduction band or from singly ionized N acceptors to deep levels such as transitionmetal impurities or intrinsic defects. Therefore, the near-edge absorption implies that the activated N acceptors are present in the annealed ZnO:N film. In this regard, the absence of nearedge absorption in the as-grown film presumably suggests that the N impurities are inactive in this case, which is believed to be the result of the hydrogen passivation effect. The observations presented here further confirm our aforementioned analyses. To make a systematic investigation of annealing effect on properties of ZnO:N films, the samples were also annealed
Fig. 4. Carrier concentrations of ZnO:N films annealed for 10 min in O2 (a) and vacuum (b) environments at different temperatures. The results illustrated at 25 ◦ C refer to those of the as-grown sample.
in O2 and vacuum environments for 10 min at temperatures ranged from 200 to 900 ◦ C. Fig. 4(a) shows the carrier concentrations of annealed ZnO:N films in O2 , as suggested by Hall-effect measurements. Similar to annealing in N2 , the p-type conductivity could also be realized in a temperature window when the ZnO:N film is annealed in O2 . But the difference between them is evident. As compared with the former, the annealing temperature window (500–600 ◦ C) becomes narrower for the latter, and moreover, the observed hole concentration is only in the 1016 cm−3 order, which is much lower than that (1017 cm−3 ) of annealed samples in N2 . It seems that N2 is more suitable for producing p-type ZnO than O2 in an annealing process. The detailed explanations for this difference are unfeasible at present, but we can consider several possible reasons. Firstly, the outdiffusion of N impurities may be more evident in O2 than in N2 during the annealing process. As mentioned above, Garces et al. [3] demonstrated that annealing a ZnO sample in N2 ambient could even produce NO acceptor dopants. In this regard, the presence of N2 in annealing environment may prevent N impurities from diffusing out from the ZnO:N film. Secondly, the hydrogen annealed out from the
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ranged from 200 to 900 ◦ C, duration from 5 to 60 min, and environments including N2 , O2 , and vacuum. The p-type conductivity of ZnO could be obtained under appropriate annealing conditions. The dissociation of NO –H complexes and the creation of activated N acceptors in ZnO were identified by optical absorption spectra. A hydrogen-assisted nitrogenacceptor doping mechanism was proposed to produce p-type ZnO. We hope this study will provide an insight into the fundamental understanding of the p-type doping of ZnO. References
Fig. 5. Carrier concentrations of ZnO:N films annealed at 500 ◦ C in N2 ambient for different durations.
body of the film may exist in the surface of the film combining with oxygen to form an O–H bond. This surface mode of an O–H bond has been observed in the FTIR spectrum of the film annealed in N2 , as shown in Fig. 2. In an O2 ambient there certainly will be much more abundant O–H bonds formed in the surface of the film. It was argued that the O–H bond, in fact, can be regarded as a new type of donor dopant atom in ZnO, the addition of the proton turning the oxygen into an element behaving much like fluorine [29]. Thus, an n-type layer is expected to form in the surface of the annealed p-type film. This n-type surface conductivity may exert a great influence on the final result of Hall-effect measurements [32]. Fig. 4(b) shows the carrier concentrations of ZnO:N films annealed in vacuum, as suggested by Hall-effect measurements. It is found that the annealing in vacuum only induces highresistivity or n-type samples. No p-type conductivity was obtained, which is very different from the ZnO:N film annealed in N2 or O2 . The absence of p-type conductivity for annealed films in vacuum is possibly due to the serious outdiffusion of N impurities from the film and the creation of large amounts of native defect donors such as VO , Zni , and ZnO in ZnO. At an annealing temperature of 900 ◦ C, the ZnO film shows strong n-type conductivity with the electron concentration of 9.78 × 1019 cm−3 . Fig. 5 displays the carrier concentrations of ZnO:N samples annealed for various durations at 500 ◦ C in N2 ambient. When the annealing time is in the range from 5 to 20 min, the p-type conductivity could be achieved. It seems that a rapid thermal annealing (e.g., 10 min) is necessary for achieving good p-type ZnO. Annealing for a long time does not lead to positive results. When the annealing time exceeds 20 min, only high-resistivity or n-type samples are obtained. This observation can also be interpreted by the intensified tendency of outdiffusion of N impurities, formation of (N2 )O donors, and creation of native defect donors with increasing annealing duration. 4. Conclusions In summary, we have systematically studied the properties of ZnO:N films in annealing processes, with temperatures
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