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Infrared spectroscopy of polycrystalline ZnO and ZnO:N thin films
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Journal of Crystal Growth 281 (2005) 297–302 www.elsevier.com/locate/jcrysgro
Infrared spectroscopy of polycrystalline ZnO and ZnO:N thin films B.M. Keyes, L.M. Gedvilas, X. Li, T.J. Coutts National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401, USA Received 2 November 2004; accepted 16 April 2005 Available online 31 May 2005 Communicated by R. Fornari
Abstract Polycrystalline zinc oxide (ZnO) and nitrogen-doped zinc oxide (ZnO:N) films, about 1 mm thick, were grown by metalorganic chemical vapor deposition on crystalline silicon substrates. Infrared absorption measurements reveal a complex growth chemistry resulting in the presence of carbon, oxygen, and nitrogen-related functional groups not seen in single-crystal material. Noteworthy changes in the absorbance spectra that occur with the incorporation of nitrogen include a group of strongly absorbing bands around 1800 cm 1 and a band at 3020 cm 1 attributable to a N–H bond. These atomic configurations, in addition to the observed O–H bands around 3400 cm 1, provide insight into the difficulties of creating p-type ZnO through the incorporation of nitrogen. r 2005 Elsevier B.V. All rights reserved. PACS: 71.55.Gs; 78.30.Fs; 81.15.Gh Keywords: A1. Characterization; A1. Impurities; A1. FTIR-spectroscopy; A3. Metalorganic chemical vapor deposition; B1. Oxides; B1. Zinc compounds
1. Introduction Zinc oxide (ZnO) is an optically transparent material of technological importance for, among other things, its use as a transparent conducting oxide in flat-panel displays and solar cells, and its potential as a blue-light-emitting diode. Full use of Corresponding author. Tel.: +1303 384 6695;
fax: +1303 384 6605. E-mail address: [email protected] (B.M. Keyes).
ZnO in these applications requires the ability to control the majority-carrier type and concentration. Although the growth of n-type ZnO is straightforward, p-type doping has proven significantly more difficult. Theoretical calculations of the electrical behavior of defects, impurities, and single atomic species such as nitrogen predict difficulty in attaining p-type ZnO [1–5]. Of the available impurities, hydrogen is of great importance because it tends to be present in most growth environments and is
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.04.053
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predicted to incorporate in ZnO as a donor or to passivate acceptor species [6–8]. Despite these predicted difficulties, evidence shows that p-type ZnO can be grown using a single intentionally added acceptor species, with nitrogen being the most common [9]. Although a variety of nitrogen sources and growth techniques have resulted in p-type ZnO:N, the creation of p-type ZnO:N is not routine. Specific growth and post-processing conditions are particularly important in realizing stable p-type material. Although discussion of the ZnO carrier concentration often refers to the presence and influence of hydrogen [9], the dominant hydrogen-related complexes in ZnO:N have not been determined experimentally. The measurement of the local vibrational modes (LVMs) provides valuable insight into the determination of impurity complexes within a material. Ultimately, this can lead to a better understanding of the role these impurities play in influencing or controlling properties such as the majority-carrier type and concentration. Fourier transform infrared (FTIR) spectroscopy is capable of observing these LVMs and has been successfully applied to the investigation of hydrogen in nitrogen-free ZnO. These studies on as-grown [10] and hydrogen-treated [6,10–12] single-crystal material report infrared absorption ranging from about 3300 to 3600 cm 1 attributable to O–H complexes that depends on the sample, treatment, or both. This wide range of observed frequencies, in addition to conflicting polarization-dependent data [6,10,12], suggest that the bonding configuration of O–H in ZnO is not completely unique or understood. In all these cases, including this work, the presence of O–H, combined with the predicted doping behavior, indicate that hydrogen is acting, in part, as an unintentional donor species [6,7]. To our best knowledge, there are no reports of hydrogen-related LVMs in ZnO extrinsically doped with nitrogen. This situation is technologically important because of the role that hydrogen may play in the ability to create stable p-type ZnO:N. In this letter, we use FTIR spectroscopy to investigate the LVMs of polycrystalline nitrogen-doped ZnO:N thin films. These data are interpreted through reference to existing literature on experimental and theoretical studies. We also
include data on undoped polycrystalline ZnO thin films for comparison.
2. Experimental procedure Polycrystalline films, 0.5 to 1 mm thick, were grown by metalorganic chemical vapor deposition (MOCVD) using diethylzinc, O2, and NO precursors on single-crystal silicon substrates. The nitrogen concentration of the ZnO:N samples is about 2 at%. Carrier concentrations for these growth conditions are typically nX1016 cm 3 and pp1013 cm 3 for the ZnO and ZnO:N material, respectively. Note that the majority hole concentration is well below the concentration of nitrogen incorporated into the film. Details of the growth process and other film properties can be found elsewhere [8,13,14]. The infrared absorption modes of these films were obtained from FTIR transmittance measurements taken at room temperature. The films were grown on parallelogram-shaped single-crystal silicon substrates and mounted in an attenuated total reflectance (ATR) accessory attached to a Nicolet Magna-IR 550 spectrometer. The transmittance was measured relative to an uncoated silicon substrate, and the absorbance spectra displayed here are calculated by taking the logarithm of 1/transmittance. The multiple bounces within the ATR substrate increase the effective thickness of the thin film by a factor of about 20 and allow us to observe the relatively small hydrogen-related peaks. This enhanced path length is essential for detecting hydrogen in device-related material, because current measurements of hydrogen in ZnO are limited to millimetre-thick single-crystal samples.
3. Results and discussion The FTIR absorbance spectrum from 2600 to 3800 cm 1 of the undoped polycrystalline ZnO material is shown in Fig. 1. This spectral region encompasses several important stretch modes involving hydrogen bonded to carbon, nitrogen, and oxygen [15,16]. From these data, it is apparent
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Fig. 1. FTIR absorbance spectra of polycrystalline ZnO and Gaussian peaks fit to the data.
Fig. 2. FTIR absorbance spectra of polycrystalline ZnO and Gaussian peaks fit to the data.
that hydrogen is unintentionally incorporated into an MOCVD-grown polycrystalline ZnO film. This is also confirmed by secondary-ion mass spectroscopy measurements that reveal hydrogen located throughout the bulk of the sample. The broad asymmetrical absorption region that peaks around 3400 cm 1 is likely due to the O–H bonds [15,16]. Theoretical calculations [6,7] predict O–H vibrations in ZnO ranging from 3216 to 3644 cm 1, depending on the configuration and number of hydrogen atoms in the complex. The broad absorption band at 3400 cm 1 encompasses the calculated frequencies of O–H on the surface (3497 cm 1), in the anti-bonding configuration (3352 cm 1), and associated with zinc vacancies (3216 and 3228 cm 1). Because of the asymmetrical and broad shape of this absorption band, the exact configuration(s) cannot be determined from these data. The absorbance spectrum of the undoped ZnO material also exhibits a series of smaller, sharper absorption peaks at 2857, 2925, and 2960 cm 1, typical of C–H stretch frequencies. The presence of both carbon and hydrogen in the diethylzinc precursor and the detection of both impurities in the bulk of the thin film [13] are reasons to expect carbon–hydrogen (C–H) modes in the FTIR spectrum of this material. These specific peaks correlate well with the observed frequencies of the C–H2 symmetric stretch (2855710 cm 1), C–H2 asymmetric stretch (2926710 cm 1), and C–H3
asymmetric stretch (2962710 cm 1) of saturated hydrocarbons, respectively [15,16]. Similar C–Hn peaks have also been observed in single-crystal ZnO [17]. The FTIR absorbance spectrum from 1100 to 2100 cm 1 of the polycrystalline ZnO material is shown in Fig. 2. Absorption bands in this spectral region are often associated with hydrogen-related bending modes, stretch modes of hydrogen bonded to heavier elements (e.g., zinc), and various carbon, oxygen, and nitrogen-related stretch modes not involving hydrogen. For this material, these absorption bands represent an appreciable amount of the total infrared absorption. This is in contrast to existing FTIR literature on crystalline ZnO, where no mention is made of absorption peaks in this spectral region. In this spectral region, the undoped ZnO material exhibits dominant peaks ranging from 1180 to 1970 cm 1. Gaussian peaks fit to the absorbance data are displayed in Fig. 2. The lowest energy peaks at 1180 and 1220 cm 1 are in the region associated with C–O bonds, possibly the result of carbon impurities. The series of peaks (1320, 1395, 1410, 1560, 1605, and 1641 cm–1) in the region extending from 1300 to 1650 cm 1 are likely due to the bending modes of O–H bonds. The possibility of a wide variety of O–H bending modes is consistent with the broad asymmetrical absorption band attributed to O–H stretching vibrations (see Fig. 1).
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Specific possibilities for the large bands around 1395 and 1560 cm 1 are O–H bonds bound to the thin film (e.g., as in zinc hydroxide [18]) and adsorbed water, respectively. One other possibility is that the two dominant absorption bands are from the CO2 stretch modes of a zinc acetate-like structure [15,16]. These CO2 stretch modes can exhibit relatively strong absorption, with the symmetric stretch between 1360 and 1450 cm 1 and the asymmetric stretch between 1540 and 1650 cm 1, also in agreement with these data. Finally, the 1942 and 1971 cm 1 peaks are higher than expected for bending modes and are likely due to stretching vibrations. The only clear candidate for a hydrogen-related stretch mode in this spectral region is the Zn–H bond. Apart from being energetically unfavorable [6], the expected vibrational frequencies are considerably lower at 1500 to 1700 cm 1 for hydrogen in the bulk [6] or 1710 cm 1 for adsorbed hydrogen [19,20]. Of the non-hydrogen-related possibilities for these two peaks, the carbonyl group (C–O) is the most probable due to its wide range of possible absorption frequencies. Specifically, when bonded to a metal, the metal carbonyl group will exhibit strong absorption in the spectral region extending from 1700 to 2200 cm 1 [15]. Although the definite assignment of the absorption peaks below 2500 cm 1 to specific structural modes is problematic, there is definitely a complicated chemistry involved in MOCVD-grown polycrystalline ZnO that is not present in single-crystal material. The addition of nitrogen results in several changes in the infrared absorbance spectrum of MOCVD-grown polycrystalline ZnO:N. Fig. 3 contains the absorbance spectrum of ZnO:N over the spectral range extending from 2800 to 3600 cm 1, along with Gaussian fits to the data. Comparison with the data for undoped ZnO (Fig. 1) reveals that, with the incorporation of nitrogen, the absorption band in the O–H region (3410 cm–1) becomes narrower and more symmetrical, perhaps indicative of a single dominant atomic configuration. The addition of nitrogen also results in a broad band at 3000 cm 1 originating from two Gaussian absorption peaks centered at 2970 and 3020 cm 1. The peak at 2970 cm 1 in the nitrogen-doped
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Fig. 3. FTIR absorbance spectra of polycrystalline ZnO:N and Gaussian peaks fit to the data.
ZnO:N is likely due to a C–Hn mode. The 3020 cm 1 peak is higher in frequency than the expected C–H stretch modes and lower than any observed [6,10–12] or calculated [6,7] O–H stretch mode in ZnO. This makes the nitrogen–hydrogen (N–H) bond a likely candidate for this observed mode. The assignment of this peak to a N–H mode is further supported by recent theoretical calculations of the formation energy and vibrational frequency for various N–H configurations in ZnO:N [8,21]. From these theoretical calculations, the most energetically favorable N–H bond corresponds to hydrogen situated in an antibonding configuration (ABN). The predicted absorption frequency of this configuration (3070 cm 1) agrees well with the observed mode at 3020 cm 1. Finally, this (AB)N complex is predicted to be a neutral complex, representing the passivation of a nitrogen acceptor, and is clearly detrimental to the intended purpose of creating a p-type material by incorporating nitrogen in a hydrogen-containing environment. The FTIR absorbance spectrum from 1100 to 2100 cm 1 of the polycrystalline ZnO:N material, along with Gaussian peaks fit to the data, is shown in Fig. 4. As with the ZnO absorbance (Fig. 2), and in contrast to existing FTIR literature on crystalline ZnO, the absorption bands in this region represent an appreciable amount of the total infrared absorption. This is particularly true with the nitrogen-doped film, where it should be noted that the full-scale absorbance is more than a
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Fig. 4. FTIR absorbance spectra of polycrystalline ZnO:N and Gaussian peaks fit to the data.
factor of ten larger than that for the undoped ZnO sample. The lowest energy peaks at 1185 and 1225 cm 1 are similar in energy to the low-energy peaks of ZnO and, as before, are in the region associated with C–O bonds, possibly the result of carbon impurities. The region extending from 1300 to 1650 cm 1 exhibits only two of the peaks (1320 and 1410 cm 1) seen in the ZnO material. The reduction in observed absorption bands in this region is consistent with the observed reduction in the range of O–H stretch frequencies that occurs with the incorporation of nitrogen (Fig. 3 vs. Fig. 1) and the resulting elimination of associated O–H bending modes. The largest change in the absorption bands located in this spectral region resulting from the incorporation of nitrogen occurs over the subrange extending from 1700 to 2000 cm 1. With the addition of nitrogen, absorption in this region increases in magnitude and is composed of peaks centered at 1710, 1800, 1840, and 2000 cm 1. The peak at 1710 cm 1 is consistent with the surface Zn–H stretch mode [19,20]. The peaks in the 1800 to 2000 cm 1 region are noteworthy because of their large size, implying a relatively strong absorption coefficient, high concentration, or both. Possible atomic configurations consistent with the known elements present in this material and detectable by FTIR include CO, CN, and NObased functional groups. One bond arrangement
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consistent with a strong absorption in this spectral region is the C–O bond of the metal carbonyl group. This is also a candidate for the absorption bands around 1950 cm 1 in the undoped ZnO. The changes in width, strength, and location that occur with the incorporation of nitrogen could be the result of differences in growth chemistry or structural and atomic environment [13,14]. It is also possible that one or more of the large absorption bands around 1800 cm 1 are directly related to the presence of nitrogen. Adsorbed [22] and free nitric oxide (NO) exhibit absorption in the 1800 to 1900 cm 1 spectral region, although significant adsorption is not expected for the plain ZnO surface [22]. Interestingly, recent theoretical work shows that C, N, and O can form substitutional diatomic molecules on the oxygen site in ZnO [23]. The calculated absorption frequencies for substitutional NO are shifted down from the free molecule frequencies to below those observed here. On the other hand, although organic CN bonds typically absorb above or below the 1800 to 2000 cm 1 spectral region [15,16], substituted (NC)O is predicted to absorb within this region [23]. Additional support for the presence of CN also comes from recent X-ray photoemission spectroscopy measurements on similar material [24]. Finally, these diatomic molecular complexes are predicted to incorporate as donors, effectively replacing the desired NO acceptor species.
4. Conclusion In conclusion, the use of a silicon ATR substrate allows for the FTIR characterization of MOCVDgrown polycrystalline ZnO and ZnO:N films as thin as 0.5 mm. These data reveal a complex growth chemistry resulting in the incorporation of carbon, oxygen, and nitrogen-related functional groups not seen in single-crystal material. Of significance is the creation of a new atomic configuration(s) that strongly absorb in the region of 1800 to 2000 cm 1. Possible causes for this absorption are the metal carbonyl group and substitutional diatomic molecules. The effect of carbonyls on the material properties of the material are unknown. The possibility of a substitutional diatomic species
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is particularly interesting because of the predicted charge state and the resulting elimination of the desired NO acceptor [23]. These data also provide the first substantial evidence of hydrogen incorporation in nitrogen-doped ZnO through the observation of C–H, O–H, and N–H bonds. The observed O–H band is influenced by the presence of nitrogen and is predicted to contribute a donor state to the material [6,7]. The observed N–H bond represents the passivation of a nitrogen acceptor [8]. These configurations, in addition to the possibility of diatomic molecular donor complexes, are detrimental to the creation of p-type ZnO and help explain the relatively low majority hole concentration or even the creation of n-type material that can occur with the incorporation of nitrogen. Controlling the balance between these hydrogen- and nitrogen-related complexes and any acceptor states, either through growth or processing conditions, is vital to the repeatable and controlled creation of p-type ZnO.
Acknowledgements This work was completed at the National Renewable Energy Laboratory under US Department of Energy Contract no. DE-AC3699GO10337. The authors are also indebted to S.B. Zhang, S. Limpijumnong, and C.L. Perkins for helpful discussions.
References [1] S.B. Zhang, S.-H. Wei, A. Zunger, Phys. Rev. B 63 (2001) 75205.
[2] S.B. Zhang, S.-H. Wei, A. Zunger, J. Appl. Phys. 83 (1998) 3192. [3] A. Zunger, Appl. Phys. Lett. 83 (2003) 57. [4] T. Yamamoto, H. Katayama-Yoshida, Jpn. J. Appl. Phys. 38 (1999) L166. [5] L.G. Wang, A. Zunger, Phys. Rev. Lett. 90 (2003) 256401. [6] E.V. Lavrov, J. Weber, F. Bo¨rrnert, C.G. Van de Walle, R. Helbig, Phys. Rev. B 66 (2002) 165205. [7] C.G. Van de Walle, Phys. Rev. Lett. 85 (2000) 1012. [8] X. Li, S. Limpijumnong, B.M. Keyes, S. Asher, C.L. Perkins, S.B. Zhang, S.-H. Wei, T.J. Coutts, Appl. Phys. Lett. 86 (2005) 122107. [9] D.C. Look, B. Claflin, Phys. Stat. Sol. (b) 3 (2004) 624. [10] C.H. Seager, S.M. Myers, J. Appl. Phys. 94 (2003) 2888. [11] S.J. Jokela, M.D. McCluskey, K.G. Lynn, Physica B 340–342 (2003) 221. [12] M.D. McCluskey, S.J. Jokela, K.K. Zhuravlev, P.J. Simpson, K.G. Lynn, Appl. Phys. Lett. 81 (2002) 3807. [13] X. Li, Y. Yan, T.A. Gessert, C.L. Perkins, D. Young, C. DeHart, M. Young, T.J. Coutts, J. Vac. Sci. Technol. A 21 (2003) 1342. [14] X. Li, Y. Yan, T.A. Gessert, C. DeHart, C.L. Perkins, D. Young, T.J. Coutts, Electrochem. and Sol. St. Lett. 6 (2003) C56. [15] N.B. Colthup, L.H. Daly, S.E. Wiberly, Introduction to Infrared and Raman Spectroscopy, third ed., Academic Press, New York, 1990. [16] B. Smith, Infrared Spectral Interpretation, CRC Press, Boca Raton, FL, 1999. [17] N.H. Nickel, K. Fleischer, Phys. Rev. Lett. 90 (2003) 197402. [18] S. Rajendran, N. Palaniswamy, Phys. Chem./0103008 (2000). [19] R.P. Eischens, W.A. Pliskin, M.J.D. Low, J. Catalysis 1 (1962) 180. [20] D. Scarano, S. Bertarione, G. Spoto, A. Zecchina, C. Otero Area´n, Thin Solid Films 400 (2001) 50. [21] C.G. Van de Walle, unpublished, 2004. [22] F. Boccuzzi, E. Guglielminotti, Surf. Sci. 271 (1992) 149. [23] S. Limpijumnong, X. Li, S.-H. Wei, S.B. Zhang, Appl. Phys. Lett. 86 (2005), accepted for publication. [24] C.L. Perkins, S.-H. Lee, X. Li, S. Asher, T.J. Coutts, J. Appl. Phys. 97 (2005) 034907.
Journal of Crystal Growth 281 (2005) 297–302 www.elsevier.com/locate/jcrysgro
Infrared spectroscopy of polycrystalline ZnO and ZnO:N thin films B.M. Keyes, L.M. Gedvilas, X. Li, T.J. Coutts National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401, USA Received 2 November 2004; accepted 16 April 2005 Available online 31 May 2005 Communicated by R. Fornari
Abstract Polycrystalline zinc oxide (ZnO) and nitrogen-doped zinc oxide (ZnO:N) films, about 1 mm thick, were grown by metalorganic chemical vapor deposition on crystalline silicon substrates. Infrared absorption measurements reveal a complex growth chemistry resulting in the presence of carbon, oxygen, and nitrogen-related functional groups not seen in single-crystal material. Noteworthy changes in the absorbance spectra that occur with the incorporation of nitrogen include a group of strongly absorbing bands around 1800 cm 1 and a band at 3020 cm 1 attributable to a N–H bond. These atomic configurations, in addition to the observed O–H bands around 3400 cm 1, provide insight into the difficulties of creating p-type ZnO through the incorporation of nitrogen. r 2005 Elsevier B.V. All rights reserved. PACS: 71.55.Gs; 78.30.Fs; 81.15.Gh Keywords: A1. Characterization; A1. Impurities; A1. FTIR-spectroscopy; A3. Metalorganic chemical vapor deposition; B1. Oxides; B1. Zinc compounds
1. Introduction Zinc oxide (ZnO) is an optically transparent material of technological importance for, among other things, its use as a transparent conducting oxide in flat-panel displays and solar cells, and its potential as a blue-light-emitting diode. Full use of Corresponding author. Tel.: +1303 384 6695;
fax: +1303 384 6605. E-mail address: [email protected] (B.M. Keyes).
ZnO in these applications requires the ability to control the majority-carrier type and concentration. Although the growth of n-type ZnO is straightforward, p-type doping has proven significantly more difficult. Theoretical calculations of the electrical behavior of defects, impurities, and single atomic species such as nitrogen predict difficulty in attaining p-type ZnO [1–5]. Of the available impurities, hydrogen is of great importance because it tends to be present in most growth environments and is
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.04.053
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predicted to incorporate in ZnO as a donor or to passivate acceptor species [6–8]. Despite these predicted difficulties, evidence shows that p-type ZnO can be grown using a single intentionally added acceptor species, with nitrogen being the most common [9]. Although a variety of nitrogen sources and growth techniques have resulted in p-type ZnO:N, the creation of p-type ZnO:N is not routine. Specific growth and post-processing conditions are particularly important in realizing stable p-type material. Although discussion of the ZnO carrier concentration often refers to the presence and influence of hydrogen [9], the dominant hydrogen-related complexes in ZnO:N have not been determined experimentally. The measurement of the local vibrational modes (LVMs) provides valuable insight into the determination of impurity complexes within a material. Ultimately, this can lead to a better understanding of the role these impurities play in influencing or controlling properties such as the majority-carrier type and concentration. Fourier transform infrared (FTIR) spectroscopy is capable of observing these LVMs and has been successfully applied to the investigation of hydrogen in nitrogen-free ZnO. These studies on as-grown [10] and hydrogen-treated [6,10–12] single-crystal material report infrared absorption ranging from about 3300 to 3600 cm 1 attributable to O–H complexes that depends on the sample, treatment, or both. This wide range of observed frequencies, in addition to conflicting polarization-dependent data [6,10,12], suggest that the bonding configuration of O–H in ZnO is not completely unique or understood. In all these cases, including this work, the presence of O–H, combined with the predicted doping behavior, indicate that hydrogen is acting, in part, as an unintentional donor species [6,7]. To our best knowledge, there are no reports of hydrogen-related LVMs in ZnO extrinsically doped with nitrogen. This situation is technologically important because of the role that hydrogen may play in the ability to create stable p-type ZnO:N. In this letter, we use FTIR spectroscopy to investigate the LVMs of polycrystalline nitrogen-doped ZnO:N thin films. These data are interpreted through reference to existing literature on experimental and theoretical studies. We also
include data on undoped polycrystalline ZnO thin films for comparison.
2. Experimental procedure Polycrystalline films, 0.5 to 1 mm thick, were grown by metalorganic chemical vapor deposition (MOCVD) using diethylzinc, O2, and NO precursors on single-crystal silicon substrates. The nitrogen concentration of the ZnO:N samples is about 2 at%. Carrier concentrations for these growth conditions are typically nX1016 cm 3 and pp1013 cm 3 for the ZnO and ZnO:N material, respectively. Note that the majority hole concentration is well below the concentration of nitrogen incorporated into the film. Details of the growth process and other film properties can be found elsewhere [8,13,14]. The infrared absorption modes of these films were obtained from FTIR transmittance measurements taken at room temperature. The films were grown on parallelogram-shaped single-crystal silicon substrates and mounted in an attenuated total reflectance (ATR) accessory attached to a Nicolet Magna-IR 550 spectrometer. The transmittance was measured relative to an uncoated silicon substrate, and the absorbance spectra displayed here are calculated by taking the logarithm of 1/transmittance. The multiple bounces within the ATR substrate increase the effective thickness of the thin film by a factor of about 20 and allow us to observe the relatively small hydrogen-related peaks. This enhanced path length is essential for detecting hydrogen in device-related material, because current measurements of hydrogen in ZnO are limited to millimetre-thick single-crystal samples.
3. Results and discussion The FTIR absorbance spectrum from 2600 to 3800 cm 1 of the undoped polycrystalline ZnO material is shown in Fig. 1. This spectral region encompasses several important stretch modes involving hydrogen bonded to carbon, nitrogen, and oxygen [15,16]. From these data, it is apparent
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Fig. 1. FTIR absorbance spectra of polycrystalline ZnO and Gaussian peaks fit to the data.
Fig. 2. FTIR absorbance spectra of polycrystalline ZnO and Gaussian peaks fit to the data.
that hydrogen is unintentionally incorporated into an MOCVD-grown polycrystalline ZnO film. This is also confirmed by secondary-ion mass spectroscopy measurements that reveal hydrogen located throughout the bulk of the sample. The broad asymmetrical absorption region that peaks around 3400 cm 1 is likely due to the O–H bonds [15,16]. Theoretical calculations [6,7] predict O–H vibrations in ZnO ranging from 3216 to 3644 cm 1, depending on the configuration and number of hydrogen atoms in the complex. The broad absorption band at 3400 cm 1 encompasses the calculated frequencies of O–H on the surface (3497 cm 1), in the anti-bonding configuration (3352 cm 1), and associated with zinc vacancies (3216 and 3228 cm 1). Because of the asymmetrical and broad shape of this absorption band, the exact configuration(s) cannot be determined from these data. The absorbance spectrum of the undoped ZnO material also exhibits a series of smaller, sharper absorption peaks at 2857, 2925, and 2960 cm 1, typical of C–H stretch frequencies. The presence of both carbon and hydrogen in the diethylzinc precursor and the detection of both impurities in the bulk of the thin film [13] are reasons to expect carbon–hydrogen (C–H) modes in the FTIR spectrum of this material. These specific peaks correlate well with the observed frequencies of the C–H2 symmetric stretch (2855710 cm 1), C–H2 asymmetric stretch (2926710 cm 1), and C–H3
asymmetric stretch (2962710 cm 1) of saturated hydrocarbons, respectively [15,16]. Similar C–Hn peaks have also been observed in single-crystal ZnO [17]. The FTIR absorbance spectrum from 1100 to 2100 cm 1 of the polycrystalline ZnO material is shown in Fig. 2. Absorption bands in this spectral region are often associated with hydrogen-related bending modes, stretch modes of hydrogen bonded to heavier elements (e.g., zinc), and various carbon, oxygen, and nitrogen-related stretch modes not involving hydrogen. For this material, these absorption bands represent an appreciable amount of the total infrared absorption. This is in contrast to existing FTIR literature on crystalline ZnO, where no mention is made of absorption peaks in this spectral region. In this spectral region, the undoped ZnO material exhibits dominant peaks ranging from 1180 to 1970 cm 1. Gaussian peaks fit to the absorbance data are displayed in Fig. 2. The lowest energy peaks at 1180 and 1220 cm 1 are in the region associated with C–O bonds, possibly the result of carbon impurities. The series of peaks (1320, 1395, 1410, 1560, 1605, and 1641 cm–1) in the region extending from 1300 to 1650 cm 1 are likely due to the bending modes of O–H bonds. The possibility of a wide variety of O–H bending modes is consistent with the broad asymmetrical absorption band attributed to O–H stretching vibrations (see Fig. 1).
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Specific possibilities for the large bands around 1395 and 1560 cm 1 are O–H bonds bound to the thin film (e.g., as in zinc hydroxide [18]) and adsorbed water, respectively. One other possibility is that the two dominant absorption bands are from the CO2 stretch modes of a zinc acetate-like structure [15,16]. These CO2 stretch modes can exhibit relatively strong absorption, with the symmetric stretch between 1360 and 1450 cm 1 and the asymmetric stretch between 1540 and 1650 cm 1, also in agreement with these data. Finally, the 1942 and 1971 cm 1 peaks are higher than expected for bending modes and are likely due to stretching vibrations. The only clear candidate for a hydrogen-related stretch mode in this spectral region is the Zn–H bond. Apart from being energetically unfavorable [6], the expected vibrational frequencies are considerably lower at 1500 to 1700 cm 1 for hydrogen in the bulk [6] or 1710 cm 1 for adsorbed hydrogen [19,20]. Of the non-hydrogen-related possibilities for these two peaks, the carbonyl group (C–O) is the most probable due to its wide range of possible absorption frequencies. Specifically, when bonded to a metal, the metal carbonyl group will exhibit strong absorption in the spectral region extending from 1700 to 2200 cm 1 [15]. Although the definite assignment of the absorption peaks below 2500 cm 1 to specific structural modes is problematic, there is definitely a complicated chemistry involved in MOCVD-grown polycrystalline ZnO that is not present in single-crystal material. The addition of nitrogen results in several changes in the infrared absorbance spectrum of MOCVD-grown polycrystalline ZnO:N. Fig. 3 contains the absorbance spectrum of ZnO:N over the spectral range extending from 2800 to 3600 cm 1, along with Gaussian fits to the data. Comparison with the data for undoped ZnO (Fig. 1) reveals that, with the incorporation of nitrogen, the absorption band in the O–H region (3410 cm–1) becomes narrower and more symmetrical, perhaps indicative of a single dominant atomic configuration. The addition of nitrogen also results in a broad band at 3000 cm 1 originating from two Gaussian absorption peaks centered at 2970 and 3020 cm 1. The peak at 2970 cm 1 in the nitrogen-doped
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)
Fig. 3. FTIR absorbance spectra of polycrystalline ZnO:N and Gaussian peaks fit to the data.
ZnO:N is likely due to a C–Hn mode. The 3020 cm 1 peak is higher in frequency than the expected C–H stretch modes and lower than any observed [6,10–12] or calculated [6,7] O–H stretch mode in ZnO. This makes the nitrogen–hydrogen (N–H) bond a likely candidate for this observed mode. The assignment of this peak to a N–H mode is further supported by recent theoretical calculations of the formation energy and vibrational frequency for various N–H configurations in ZnO:N [8,21]. From these theoretical calculations, the most energetically favorable N–H bond corresponds to hydrogen situated in an antibonding configuration (ABN). The predicted absorption frequency of this configuration (3070 cm 1) agrees well with the observed mode at 3020 cm 1. Finally, this (AB)N complex is predicted to be a neutral complex, representing the passivation of a nitrogen acceptor, and is clearly detrimental to the intended purpose of creating a p-type material by incorporating nitrogen in a hydrogen-containing environment. The FTIR absorbance spectrum from 1100 to 2100 cm 1 of the polycrystalline ZnO:N material, along with Gaussian peaks fit to the data, is shown in Fig. 4. As with the ZnO absorbance (Fig. 2), and in contrast to existing FTIR literature on crystalline ZnO, the absorption bands in this region represent an appreciable amount of the total infrared absorption. This is particularly true with the nitrogen-doped film, where it should be noted that the full-scale absorbance is more than a
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1.2
Absorabance
1.0 0.8 0.6 0.4 0.2 0.0 1200
1400 1600 1800 Wavenumbers (cm-1)
2000
Fig. 4. FTIR absorbance spectra of polycrystalline ZnO:N and Gaussian peaks fit to the data.
factor of ten larger than that for the undoped ZnO sample. The lowest energy peaks at 1185 and 1225 cm 1 are similar in energy to the low-energy peaks of ZnO and, as before, are in the region associated with C–O bonds, possibly the result of carbon impurities. The region extending from 1300 to 1650 cm 1 exhibits only two of the peaks (1320 and 1410 cm 1) seen in the ZnO material. The reduction in observed absorption bands in this region is consistent with the observed reduction in the range of O–H stretch frequencies that occurs with the incorporation of nitrogen (Fig. 3 vs. Fig. 1) and the resulting elimination of associated O–H bending modes. The largest change in the absorption bands located in this spectral region resulting from the incorporation of nitrogen occurs over the subrange extending from 1700 to 2000 cm 1. With the addition of nitrogen, absorption in this region increases in magnitude and is composed of peaks centered at 1710, 1800, 1840, and 2000 cm 1. The peak at 1710 cm 1 is consistent with the surface Zn–H stretch mode [19,20]. The peaks in the 1800 to 2000 cm 1 region are noteworthy because of their large size, implying a relatively strong absorption coefficient, high concentration, or both. Possible atomic configurations consistent with the known elements present in this material and detectable by FTIR include CO, CN, and NObased functional groups. One bond arrangement
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consistent with a strong absorption in this spectral region is the C–O bond of the metal carbonyl group. This is also a candidate for the absorption bands around 1950 cm 1 in the undoped ZnO. The changes in width, strength, and location that occur with the incorporation of nitrogen could be the result of differences in growth chemistry or structural and atomic environment [13,14]. It is also possible that one or more of the large absorption bands around 1800 cm 1 are directly related to the presence of nitrogen. Adsorbed [22] and free nitric oxide (NO) exhibit absorption in the 1800 to 1900 cm 1 spectral region, although significant adsorption is not expected for the plain ZnO surface [22]. Interestingly, recent theoretical work shows that C, N, and O can form substitutional diatomic molecules on the oxygen site in ZnO [23]. The calculated absorption frequencies for substitutional NO are shifted down from the free molecule frequencies to below those observed here. On the other hand, although organic CN bonds typically absorb above or below the 1800 to 2000 cm 1 spectral region [15,16], substituted (NC)O is predicted to absorb within this region [23]. Additional support for the presence of CN also comes from recent X-ray photoemission spectroscopy measurements on similar material [24]. Finally, these diatomic molecular complexes are predicted to incorporate as donors, effectively replacing the desired NO acceptor species.
4. Conclusion In conclusion, the use of a silicon ATR substrate allows for the FTIR characterization of MOCVDgrown polycrystalline ZnO and ZnO:N films as thin as 0.5 mm. These data reveal a complex growth chemistry resulting in the incorporation of carbon, oxygen, and nitrogen-related functional groups not seen in single-crystal material. Of significance is the creation of a new atomic configuration(s) that strongly absorb in the region of 1800 to 2000 cm 1. Possible causes for this absorption are the metal carbonyl group and substitutional diatomic molecules. The effect of carbonyls on the material properties of the material are unknown. The possibility of a substitutional diatomic species
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is particularly interesting because of the predicted charge state and the resulting elimination of the desired NO acceptor [23]. These data also provide the first substantial evidence of hydrogen incorporation in nitrogen-doped ZnO through the observation of C–H, O–H, and N–H bonds. The observed O–H band is influenced by the presence of nitrogen and is predicted to contribute a donor state to the material [6,7]. The observed N–H bond represents the passivation of a nitrogen acceptor [8]. These configurations, in addition to the possibility of diatomic molecular donor complexes, are detrimental to the creation of p-type ZnO and help explain the relatively low majority hole concentration or even the creation of n-type material that can occur with the incorporation of nitrogen. Controlling the balance between these hydrogen- and nitrogen-related complexes and any acceptor states, either through growth or processing conditions, is vital to the repeatable and controlled creation of p-type ZnO.
Acknowledgements This work was completed at the National Renewable Energy Laboratory under US Department of Energy Contract no. DE-AC3699GO10337. The authors are also indebted to S.B. Zhang, S. Limpijumnong, and C.L. Perkins for helpful discussions.
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