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Optical properties of zinc oxynitride thin films
Thin Solid Films 317 Ž1998. 322–325
Optical properties of zinc oxynitride thin films Masanobu Futsuhara a
a,b,)
, Katsuaki Yoshioka b, Osamu Takai
a
Department of Materials Processing Engineering, Nagoya UniÕersity, Chikusa-ku, Nagoya 464-01, Japan b Paint Design Institute, Nippon Paint, Shinagawa-ku, Tokyo 140, Japan
Abstract Optical properties of zinc oxynitride ŽZn xO y N z . films are investigated. Zn x N yOz films are deposited onto glass substrates from a ZnO target in N2 –Ar mixtures by reactive rf magnetron sputtering. Structure and chemical bonding states are studied by X-ray diffraction and X-ray photoelectron spectroscopy, respectively. Transmission spectra are measured with a double beam spectrometer. Absorption coefficients are calculated from the transmission spectra with film thickness, and their dependence on photon energy is examined to determine optical band gap Ž Eg .. Eg decreases from 3.26 to 2.30 eV with increasing nitrogen concentration in the films. Zn x N yOz is a promising optically functional material that can act in the visible range. q 1998 Elsevier Science S.A. Keywords: Zinc; Oxynitride; Optical band gap; Sputtering
1. Introduction Various zinc compounds are actively investigated because of their significant properties. For example, impurity doped ZnO is a promising material for transparent, conductive films because of its high transparency in the visible range and high electron conductivity w1,2x. Recently, zinc ternary oxides such as Zn x Sn yOz and Zn x In yOz systems have drawn much attention as new materials for transparent and conductive films w3,4x. However, the Zn xO y N z system has been scarcely studied. Zn 3 N2 has been determined to be an n-type semiconductor with direct gap of 1.23 eV w5x. On the other hand, the optical band gap Ž Eg . of ZnO is determined to be around 3.2 eV. We can, therefore, expect to alter the optical band gap from 1.23 to 3.2 eV by varying the composition of Zn xO y N z films. ZnO is well known as a photocatalyst with high activity w6x. It is, however, necessary to irradiate the ultra-violet rays that have higher energy than Eg of ZnO to make ZnO chemically active. Hence, a material that can act in the visible range is required to use the sunlight more efficiently. Zn xO y N z films are promising for this purpose. In this paper, we report on the optical properties of Zn xO y N z films. Structure was studied by X-ray diffraction ŽXRD.. Chemical bonding states and composition of Zn xO y N z films are measured by X-ray photoelectron spec-
)
Corresponding author.
0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 7 . 0 0 6 4 6 - 9
troscopy ŽXPS.. The variation in Eg relates to the changes in microstructure and chemical composition. 2. Experimental details Zn xO y N z films were prepared on borosilicate glass substrates by reactive rf magnetron sputtering. A sputtering chamber was evacuated below 5 = 10y4 Pa with an oil diffusion pump and a rotary pump. Before the deposition, a target was etched by Ar ions for 30 min to remove contamination on the target. Ar–N2 mixtures were used for sputtering gases, and N2 concentration varied from 0 to 75%. Ar and N2 gases were introduced into the chamber through each mass flow controller. The total flow rate was regulated at 20 sccm during deposition. A glass substrate mounted on a substrate holder, and the substrate temperature was kept constant at 423 K by a heater set behind the substrate holder. A sintered ZnO disc was used for the target. The target-substrate distance was 70 mm, and rf power was 100 W. Film thickness was measured with a stylus profilometer at 300–1000 nm. Optical properties were measured with a double beam spectrometer. A clean borosilicate glass was used for a reference. Absorption coefficients Ž a . were calculated from transmission spectra with film thickness. Structures of the films was determined by XRD with Cu K a radiation. Chemical bonding states and chemical compositions of the films were analyzed by XPS. Pressure in an analytical chamber was kept better than 1 = 10y7 Pa. A
M. Futsuhara et al.r Thin Solid Films 317 (1998) 322–325
323
Mg anode was used for an X-ray source. Since the samples were exposed in air before the measurements, the binding energy of the C 1s peak was used for a reference binding energy, which was assumed to be 285.0 eV. Ar ion etching was carried out during 2 min with an Ar ion gun attached to the XPS system. The energy of Ar ions was adjusted to 2 keV during etching. After this, the chemical composition was determined from the ratio of each peak area to total peak area for Zn 2p, N 1s and O 1s peaks.
3. Results and discussion 3.1. Structure and chemical bonding states XRD analysis characterized the Zn x N yOz films to be polycrystalline with ZnO structure because all the diffraction peaks observed could be identified to ZnO peaks and no Zn 3 N2 peaks were observed w7,8x. This suggested that nitrogen atoms substituted to oxygen sites in the lattice of ZnO crystal. Fig. 1 indicates X-ray diffraction patterns of the films deposited at various N2 concentrations in 2 u between 30 and 40 degrees. Fig. 1a–d correspond to diffraction patterns of the films prepared at the N2 concentrations of 0, 20, 50 and 75%, respectively. A strong and sharp 002 peak is observed in Fig. 1a, which indicates that the ZnO film exhibits a preferred 001 orientation with c-axis perpendicular to the substrate. With increasing N2 concentration, the diffraction angle of the 002 peak shifts to lower diffraction angle side. The d-spacing value was
Fig. 1. X-ray diffraction patterns of films prepared at various N2 concentrations; Ža. 0%, Žb. 20%, Žc. 50% and Žd. 75%.
Fig. 2. O 1s and N 1s narrow scan spectra of the films prepared at various N2 concentrations; Ža. 0%, Žb. 20%, Žc. 50% and Žd. 75%.
determined from the diffraction angle of the 002 peak. The ˚ with N2 d-spacing value increases from 2.611 to 2.639 A concentration as shown in Fig. 1. The microstructure depends on N2 concentration in sputtering gas. Chemical bonding states of the films were examined by XPS analysis. Wide scan spectra of the films were measured in the binding energy between 0 and 1200 eV. Photoelectron Zn 2p, Zn 3s, Zn 3p, 3d, N 1s, O 1s and C 1s peaks, and Auger Zn LMM, O KLL and C KLL peaks were observed. No other peaks originated from other elements were observed. Fig. 2 shows N 1s and O 1s narrow scan spectra of Zn x N yOz films prepared at various N2 concentrations. Each O 1s peak is divided into two components. The peak at 530.5 eV is attributed to Zn–O bonds because this value is in good agreement with previous values w9,10x. Each N 1s peak consists of two components. The N 1s peak at 396.2 eV shifts 2.6 eV toward lower binding energy side from the N 1s peak for free amine Ž398.8 eV.. This shows that N–Zn bonds are formed. XPS analysis reveals that N–Zn and O–Zn bonds coexist in the films. The component at 532.0 eV in the O 1s spectra is attributed to the formation of O–H bonds, and the component in 398.0 eV for the N 1s spectra indicates the formation of N–H bonds. These components did not vanish after Ar ion etching. This phenomenon may, therefore,
324
M. Futsuhara et al.r Thin Solid Films 317 (1998) 322–325
Fig. 3. Relationship between chemical composition of Zn x O y N z films and N2 concentration in sputtering gas.
result from strong affinity between Zn xO y N z and water molecules. The chemical composition in the film is shown in Fig. 3 as a function of N2 concentration. Nitrogen concentration in films increases with N2 concentration, and reaches to about 10 at% at the N2 concentration of 75%. On the other hand, oxygen concentration decreases with N2 concentration, and reaches about 40% at the N2 concentration of 75%. Zinc concentration in the films is approximately constant with increasing N2 concentration. Nitrogen and oxygen concentrations are almost saturated at the N2 concentration of 50%. From the results of XRD and XPS measurements, it is found that the nitrogen concentration in Zn xO y N z films increases up to about 10%, and the variation of chemical composition causes the change in microstructure of the Zn xO y N z films. 3.2. Optical properties Fig. 4 shows transmission spectra of the Zn xO y N z films prepared at various N2 concentrations. In all the spectra, sharp absorption is observed and fundamental absorption
Fig. 5. Optical band gap and nitrogen concentration in films vs. N2 concentration in sputtering gas.
edge shifts toward shorter wavelength side with increasing nitrogen concentration in the films. Absorption coefficients were calculated from the transmission spectra with film thickness. Dependence of the absorption coefficient on photon energy was examined to determine Eg of the Zn xO y N z films. The dependence obeyed the direct transition equation. Fig. 5 indicates the relationship between Eg and the N2 concentration in sputtering gas. Eg decreased from 3.26 to 2.30 eV with increasing the N2 concentration. In this figure, nitrogen concentrations in films are also plotted against the N2 concentration in sputtering gas. The Eg value decreases with increasing nitrogen concentration in Zn xO y N z films. The decrease in Eg is probably related to the difference in ionicity between Zn–O and Zn–N bonds. According to the Pauling theory w11x, ionicity in a single bond increases with the difference in values of electron negativity between two elements formed the single bond. The electron negativity of O Ž3.5. is larger than that of N Ž3.0., which indicates that the Zn–O bond has larger ionicity than the Zn–N bond. The decrease in Eg is probably attributed to the decrease in ionicity due to the formation of Zn–N bonds.
4. Conclusion
Fig. 4. Transmission spectra of the films prepared at various N2 concentrations; Ža. 0%, Žb. 20%, Žc. 50% and Žd. 75%.
Zn xO y N z films were prepared by reactive rf magnetron sputtering. The Zn xO y N z films were determined to be polycrystalline with ZnO structure, and their microstructure depended on the nitrogen concentration in the films. The nitrogen concentration in the Zn xO y N z film was controllable by changing N2 concentration in sputtering gas. The Zn xO y N z films shows n-type conducting behavior with direct transition. By increasing nitrogen concentration in films, optical band gap decreased from 3.26 to 2.30 eV. Zn x N yOz is found to be a promising optically functional material which can act in the visible range.
M. Futsuhara et al.r Thin Solid Films 317 (1998) 322–325
References w1x T. Minami, H. Nanto, S. Takata, J. Appl. Phys. Lett. 41 Ž1982. 958. w2x M. Kadota, T. Kasanami, M. Minakata, Jpn. J. Appl. Phys. 32 Ž1993. 2341. w3x T. Minami, H. Sonohara, S. Takahata, S. Sato, Jpn. J. Appl. Phys. 33 Ž1994. 1693. w4x H. Enoki, T. Nakamura, J. Echigoya, Phys. Status Solidi A 129 Ž1992. 181. w5x M. Futsuhara, K. Yoshioka, O. Takai, Proc., 3rd Asia–Pacific Sympo. on Plasma Sci. and Technol., 2 Ž1996. 511.
325
w6x J. Dewald, J. Phys. Chem. Solids 14 Ž1960. 155. w7x Powder Diffraction File compiled by the Joint Committee on Powder Diffraction, Card No. 36-1451. w8x Powder Diffraction File compiled by the Joint Committee on Powder Diffraction, Card No. 35-0762. w9x B.R. Strohmeier, D.M. Hercules, J. Catal. 86 Ž1984. 266. w10x S.W. Gaarenstroom, N. Winograd, J. Chem. Phys. 67 Ž1977. 3500. w11x L. Pauling, The Nature of the Chemical Bond, Chap. 3, Cornell University, Ithaca, New York, 1960.
Optical properties of zinc oxynitride thin films Masanobu Futsuhara a
a,b,)
, Katsuaki Yoshioka b, Osamu Takai
a
Department of Materials Processing Engineering, Nagoya UniÕersity, Chikusa-ku, Nagoya 464-01, Japan b Paint Design Institute, Nippon Paint, Shinagawa-ku, Tokyo 140, Japan
Abstract Optical properties of zinc oxynitride ŽZn xO y N z . films are investigated. Zn x N yOz films are deposited onto glass substrates from a ZnO target in N2 –Ar mixtures by reactive rf magnetron sputtering. Structure and chemical bonding states are studied by X-ray diffraction and X-ray photoelectron spectroscopy, respectively. Transmission spectra are measured with a double beam spectrometer. Absorption coefficients are calculated from the transmission spectra with film thickness, and their dependence on photon energy is examined to determine optical band gap Ž Eg .. Eg decreases from 3.26 to 2.30 eV with increasing nitrogen concentration in the films. Zn x N yOz is a promising optically functional material that can act in the visible range. q 1998 Elsevier Science S.A. Keywords: Zinc; Oxynitride; Optical band gap; Sputtering
1. Introduction Various zinc compounds are actively investigated because of their significant properties. For example, impurity doped ZnO is a promising material for transparent, conductive films because of its high transparency in the visible range and high electron conductivity w1,2x. Recently, zinc ternary oxides such as Zn x Sn yOz and Zn x In yOz systems have drawn much attention as new materials for transparent and conductive films w3,4x. However, the Zn xO y N z system has been scarcely studied. Zn 3 N2 has been determined to be an n-type semiconductor with direct gap of 1.23 eV w5x. On the other hand, the optical band gap Ž Eg . of ZnO is determined to be around 3.2 eV. We can, therefore, expect to alter the optical band gap from 1.23 to 3.2 eV by varying the composition of Zn xO y N z films. ZnO is well known as a photocatalyst with high activity w6x. It is, however, necessary to irradiate the ultra-violet rays that have higher energy than Eg of ZnO to make ZnO chemically active. Hence, a material that can act in the visible range is required to use the sunlight more efficiently. Zn xO y N z films are promising for this purpose. In this paper, we report on the optical properties of Zn xO y N z films. Structure was studied by X-ray diffraction ŽXRD.. Chemical bonding states and composition of Zn xO y N z films are measured by X-ray photoelectron spec-
)
Corresponding author.
0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 7 . 0 0 6 4 6 - 9
troscopy ŽXPS.. The variation in Eg relates to the changes in microstructure and chemical composition. 2. Experimental details Zn xO y N z films were prepared on borosilicate glass substrates by reactive rf magnetron sputtering. A sputtering chamber was evacuated below 5 = 10y4 Pa with an oil diffusion pump and a rotary pump. Before the deposition, a target was etched by Ar ions for 30 min to remove contamination on the target. Ar–N2 mixtures were used for sputtering gases, and N2 concentration varied from 0 to 75%. Ar and N2 gases were introduced into the chamber through each mass flow controller. The total flow rate was regulated at 20 sccm during deposition. A glass substrate mounted on a substrate holder, and the substrate temperature was kept constant at 423 K by a heater set behind the substrate holder. A sintered ZnO disc was used for the target. The target-substrate distance was 70 mm, and rf power was 100 W. Film thickness was measured with a stylus profilometer at 300–1000 nm. Optical properties were measured with a double beam spectrometer. A clean borosilicate glass was used for a reference. Absorption coefficients Ž a . were calculated from transmission spectra with film thickness. Structures of the films was determined by XRD with Cu K a radiation. Chemical bonding states and chemical compositions of the films were analyzed by XPS. Pressure in an analytical chamber was kept better than 1 = 10y7 Pa. A
M. Futsuhara et al.r Thin Solid Films 317 (1998) 322–325
323
Mg anode was used for an X-ray source. Since the samples were exposed in air before the measurements, the binding energy of the C 1s peak was used for a reference binding energy, which was assumed to be 285.0 eV. Ar ion etching was carried out during 2 min with an Ar ion gun attached to the XPS system. The energy of Ar ions was adjusted to 2 keV during etching. After this, the chemical composition was determined from the ratio of each peak area to total peak area for Zn 2p, N 1s and O 1s peaks.
3. Results and discussion 3.1. Structure and chemical bonding states XRD analysis characterized the Zn x N yOz films to be polycrystalline with ZnO structure because all the diffraction peaks observed could be identified to ZnO peaks and no Zn 3 N2 peaks were observed w7,8x. This suggested that nitrogen atoms substituted to oxygen sites in the lattice of ZnO crystal. Fig. 1 indicates X-ray diffraction patterns of the films deposited at various N2 concentrations in 2 u between 30 and 40 degrees. Fig. 1a–d correspond to diffraction patterns of the films prepared at the N2 concentrations of 0, 20, 50 and 75%, respectively. A strong and sharp 002 peak is observed in Fig. 1a, which indicates that the ZnO film exhibits a preferred 001 orientation with c-axis perpendicular to the substrate. With increasing N2 concentration, the diffraction angle of the 002 peak shifts to lower diffraction angle side. The d-spacing value was
Fig. 1. X-ray diffraction patterns of films prepared at various N2 concentrations; Ža. 0%, Žb. 20%, Žc. 50% and Žd. 75%.
Fig. 2. O 1s and N 1s narrow scan spectra of the films prepared at various N2 concentrations; Ža. 0%, Žb. 20%, Žc. 50% and Žd. 75%.
determined from the diffraction angle of the 002 peak. The ˚ with N2 d-spacing value increases from 2.611 to 2.639 A concentration as shown in Fig. 1. The microstructure depends on N2 concentration in sputtering gas. Chemical bonding states of the films were examined by XPS analysis. Wide scan spectra of the films were measured in the binding energy between 0 and 1200 eV. Photoelectron Zn 2p, Zn 3s, Zn 3p, 3d, N 1s, O 1s and C 1s peaks, and Auger Zn LMM, O KLL and C KLL peaks were observed. No other peaks originated from other elements were observed. Fig. 2 shows N 1s and O 1s narrow scan spectra of Zn x N yOz films prepared at various N2 concentrations. Each O 1s peak is divided into two components. The peak at 530.5 eV is attributed to Zn–O bonds because this value is in good agreement with previous values w9,10x. Each N 1s peak consists of two components. The N 1s peak at 396.2 eV shifts 2.6 eV toward lower binding energy side from the N 1s peak for free amine Ž398.8 eV.. This shows that N–Zn bonds are formed. XPS analysis reveals that N–Zn and O–Zn bonds coexist in the films. The component at 532.0 eV in the O 1s spectra is attributed to the formation of O–H bonds, and the component in 398.0 eV for the N 1s spectra indicates the formation of N–H bonds. These components did not vanish after Ar ion etching. This phenomenon may, therefore,
324
M. Futsuhara et al.r Thin Solid Films 317 (1998) 322–325
Fig. 3. Relationship between chemical composition of Zn x O y N z films and N2 concentration in sputtering gas.
result from strong affinity between Zn xO y N z and water molecules. The chemical composition in the film is shown in Fig. 3 as a function of N2 concentration. Nitrogen concentration in films increases with N2 concentration, and reaches to about 10 at% at the N2 concentration of 75%. On the other hand, oxygen concentration decreases with N2 concentration, and reaches about 40% at the N2 concentration of 75%. Zinc concentration in the films is approximately constant with increasing N2 concentration. Nitrogen and oxygen concentrations are almost saturated at the N2 concentration of 50%. From the results of XRD and XPS measurements, it is found that the nitrogen concentration in Zn xO y N z films increases up to about 10%, and the variation of chemical composition causes the change in microstructure of the Zn xO y N z films. 3.2. Optical properties Fig. 4 shows transmission spectra of the Zn xO y N z films prepared at various N2 concentrations. In all the spectra, sharp absorption is observed and fundamental absorption
Fig. 5. Optical band gap and nitrogen concentration in films vs. N2 concentration in sputtering gas.
edge shifts toward shorter wavelength side with increasing nitrogen concentration in the films. Absorption coefficients were calculated from the transmission spectra with film thickness. Dependence of the absorption coefficient on photon energy was examined to determine Eg of the Zn xO y N z films. The dependence obeyed the direct transition equation. Fig. 5 indicates the relationship between Eg and the N2 concentration in sputtering gas. Eg decreased from 3.26 to 2.30 eV with increasing the N2 concentration. In this figure, nitrogen concentrations in films are also plotted against the N2 concentration in sputtering gas. The Eg value decreases with increasing nitrogen concentration in Zn xO y N z films. The decrease in Eg is probably related to the difference in ionicity between Zn–O and Zn–N bonds. According to the Pauling theory w11x, ionicity in a single bond increases with the difference in values of electron negativity between two elements formed the single bond. The electron negativity of O Ž3.5. is larger than that of N Ž3.0., which indicates that the Zn–O bond has larger ionicity than the Zn–N bond. The decrease in Eg is probably attributed to the decrease in ionicity due to the formation of Zn–N bonds.
4. Conclusion
Fig. 4. Transmission spectra of the films prepared at various N2 concentrations; Ža. 0%, Žb. 20%, Žc. 50% and Žd. 75%.
Zn xO y N z films were prepared by reactive rf magnetron sputtering. The Zn xO y N z films were determined to be polycrystalline with ZnO structure, and their microstructure depended on the nitrogen concentration in the films. The nitrogen concentration in the Zn xO y N z film was controllable by changing N2 concentration in sputtering gas. The Zn xO y N z films shows n-type conducting behavior with direct transition. By increasing nitrogen concentration in films, optical band gap decreased from 3.26 to 2.30 eV. Zn x N yOz is found to be a promising optically functional material which can act in the visible range.
M. Futsuhara et al.r Thin Solid Films 317 (1998) 322–325
References w1x T. Minami, H. Nanto, S. Takata, J. Appl. Phys. Lett. 41 Ž1982. 958. w2x M. Kadota, T. Kasanami, M. Minakata, Jpn. J. Appl. Phys. 32 Ž1993. 2341. w3x T. Minami, H. Sonohara, S. Takahata, S. Sato, Jpn. J. Appl. Phys. 33 Ž1994. 1693. w4x H. Enoki, T. Nakamura, J. Echigoya, Phys. Status Solidi A 129 Ž1992. 181. w5x M. Futsuhara, K. Yoshioka, O. Takai, Proc., 3rd Asia–Pacific Sympo. on Plasma Sci. and Technol., 2 Ž1996. 511.
325
w6x J. Dewald, J. Phys. Chem. Solids 14 Ž1960. 155. w7x Powder Diffraction File compiled by the Joint Committee on Powder Diffraction, Card No. 36-1451. w8x Powder Diffraction File compiled by the Joint Committee on Powder Diffraction, Card No. 35-0762. w9x B.R. Strohmeier, D.M. Hercules, J. Catal. 86 Ž1984. 266. w10x S.W. Gaarenstroom, N. Winograd, J. Chem. Phys. 67 Ž1977. 3500. w11x L. Pauling, The Nature of the Chemical Bond, Chap. 3, Cornell University, Ithaca, New York, 1960.