Absorption, structural, and electrical properties of Ge films prepared by ion-beam-assisted deposition Jian Leng,1,2,* Li Zhao,1 Yiqin Ji,2 Huasong Liu,2 and Kewen Zhuang2 1
Department of Physics and Surface Physics National Key Laboratory, Fudan University, Shanghai 200433, China
2
Tianjin Key Laboratory of Optical Thin Films, Tianjin Jinhang Institute of Technical Physics, Tianjin 300192, China *Corresponding author: [email protected] Received 27 August 2013; accepted 14 September 2013; posted 9 October 2013 (Doc. ID 196478); published 12 November 2013
Effects of ion energy on the optical, microstructure, and electrical properties of Ge films prepared by ion-beam-assisted deposition were investigated. The absorption edge is found to shift toward a longer wavelength when the ion energy increases. 150 eV ion bombardment energy could help to reduce absorption in the infrared spectrum, elevating 2% of film transmittance. Diffraction intensity decreases with bombardment ion energy indicates that the crystallinity of Ge film is degenerated. Electrical property has been analyzed through Hall measurement. The resistivity of sample prepared with 300 eV ion energy drops substantially from 477 to 137 Ω cm, and it changes slowly with further increase of ion bombardment energy. © 2013 Optical Society of America OCIS codes: (310.1860) Deposition and fabrication; (310.6860) Thin films, optical properties; (310.6870) Thin films, other properties; (240.2130) Ellipsometry and polarimetry. http://dx.doi.org/10.1364/AO.53.000A48
1. Introduction
Nanocrystals of indirect-gap semiconductor materials, such as Si and Ge, have attracted enormous attention due to their quantum-confined electronic states. Germanium (Ge) has been widely used in infrared optical systems because of its high refractive index and wide transparent region in the infrared band, and it also could be applied in novel integrated optoelectronics and microelectronics devices. It has a small effective mass and high mobility as compared to Si. Microstructure of Ge nanocrystals determines the optical, structural, and electrical properties [1,2]. Several techniques are being used to fabricate Ge nanocrystals, such as electron-beam evaporation [3,4], plasma-enhanced chemical vapor deposition [5,6], r.f.-magnetron sputtering [7,8], and pulsed laser deposition [9].
1559-128X/14/040A48-04$15.00/0 © 2014 Optical Society of America A48
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As reported by Martinu and Poitras [10], using ion bombardment during the deposition process could significantly impact the optical properties of the coatings by increasing the film density and reducing defect levels and could particularly improve mechanical properties, increasing firmness of the film. So ion-beam-assisted deposition (IBAD) has become one of the preferred methods to produce high-quality thin films [11]. In this article, we report on the microstructure and optical properties of germanium films prepared on m-ZnS and BK7 substrates by IBAD. 2. Experiments
Ge thin films were deposited on Φ40 mm × 1 mm BK7 glass, Φ25 mm × 2 mm m-ZnS substrates in the same preparation process. The equipment used for Ge film deposition was an IBAD equipped with a Kaufman ion source made by Veeco-Ion Technology. The diameter of the ion source is 16 mm, and the maximum power of the source is 600 W. The ionsource beam was directed off-center to the rotating
substrate holder; the values of the ion-beam voltage, V i , which determines the ion energy Ei, were between 50 and 1200 V; the ion-beam current, I i , was fixed between 60 and 600 mA. During deposition, Ar gas flow was kept at 18 SCCM (cubic centimeters per minute at STP) and 7 SCCM for ion source and neutralization, respectively. Prior to deposition, the substrate surfaces were precleaned using Ar ion bombardment for 5 min in order to further reduce contamination on the substrate surfaces. The precursor for evaporation was Ge slices with the purity of 99.99%, and deposition parameters are listed in Table 1. During the deposition, the substrate was rotated at 20 rpm and the distance between the substrate and the evaporation source were 1300 mm in order to ensure uniformity. The growth rate and thickness of films were measured by a quartz crystal monitor (IC-5; Inficon, Inc.), optical transmittance of the Ge films was traced at room temperature by a Perkin Elmer UV/VIS/NIR Lambda 900 spectrophotometer in the wavelength range 190–2600 nm. Infrared optical properties were characterized by postdeposition infrared ellipsometry measurements (IRSE-5E; SOPRA) and FTIR spectrometer (VERTEX 70; BRUKER CO., Inc.). The crystal structure was characterized by x-ray diffractometry (XRD) (MXP18; Mac Science Ltd.) using Cu K radiation, with a scanning speed of 4/min and an incident angle of 4°. Electrical resistivity was measured by Hall measurement. 3. Results and Discussions
Figure 1 shows transmittance of the as-deposited Ge films on m-ZnS with different ion energy as a function of wavelength. The behavior of transmittance of Ge films is almost similar in the wavelength range 600–2500 nm. However, the peak values of transmittance are lower than that of ZnS substrate, indicating optical absorption of Ge films in this spectrum, and it is also shown that the transmittance decreases with the increase of assisted ion energy. In addition, the sudden/ rapid fall at wavelengths below 1300 nm is probably due to the absorption edge, which is found to shift toward a longer wavelength when the ion energy increases. The spectral absorption coefficient α is calculated from the spectral transmittance curves using the relation [12] 1 1 ; α ln t T Table 1.
Fig. 1. Transmittance property of samples prepared with different ion energy.
where t is the thickness of the film, and T is the transmittance at a given wavelength. T decreases with the increase of ion energy in the shortwave spectral region, as shown in Fig. 1, which means that the spectral absorption in the shortwave region increases with ion energy. The absorption coefficient α in the vicinity of the bandgap energy also follows the Urbach’s rule, which predicts [13] α α0 exphν∕Ee ;
(2)
where α0 is a constant, Ee is the Urbach energy describing the energy width of the band edge, and hν is the incident photon energy. It can be concluded from Eqs. (1) and (2) that with the increase of assisted ion energy, the absorption coefficient α in the vicinity of the bandgap energy increases, and the energy width of the band edge decreases accordingly. The shift in the absorption edge is known as Burstein–Moss effect [14], which could be attributed to the increase in free carriers [3]. Figure 2 shows the variations in resistivity of Ge films with the ion energy. For the film deposited without ion bombardment resistivity is 477 Ω cm, and when the energy increases up to 300 eV, the resistivity drops substantially to 137 Ω cm,
(1)
Fundamental Fabrication Conditions of Ge Films
Ion energy (eV) Ion beam current (mA) Base pressure (torr) Growth rate (Å/s) Substrate temperature (°C) Thickness of Ge film (nm)
0–600 0–300 5 × 10−6 5 180 400
Fig. 2. Resistivity of Ge films as a function of ion energy. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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and it decrease slowly with the assisted ion-energy continuous increase. And the decrease in resistivity can be explained by the increase in carrier concentration, consistent with the change of bandgap energy. As shown in Fig. 3, the transmittance property of the sample is improved while the increment of ion energy is low. Although the thickness of all samples monitored by a quartz crystal oscillator was kept the same as 400 nm, it was different because of different deposition parameters, measured by spectroscopic ellipsometry. Ge film on ZnS substrate is chosen 1∕2λ optical thickness around 3.7 μm, so the extreme transmittance value could reflect film absorption. The transmittance peak of the sample prepared without ion-beam bombardment is 72.8%, while the sample prepared with 150 eV ion energy is 75.0%. But with the continuous increase of ion energy, transmittance decreases obviously. The sample prepared with 300 eV IBAD has the maximum transmittance of 73.1%, and transmittance peak of the sample prepared with the ion energy of 600 eV is 71.7%. In the same way, film extinctive coefficient (k) of samples with lower IBAD energy is less. So it can be concluded that certain extent ion-beam bombardment could improve Ge film transmittance, while high ion-beam energy will cause the opposite effect. Figure 4 shows the relationship between the refractive index (n) and ion energy. While ion bombardment energy is below 300 eV, n increases with the ion energy. For example, n of the sample prepared with 300 eV IBAD is 4.38 at 8 μm, 0.11 larger than sample deposited without ion-beam bombardment, and that’s because the ion beam could lead to the collapse of pores in Ge film [15]. But with the continuous increase of ion energy, the refractive index begins to decrease, and that maybe because a high-energy ion beam could cause anti-sputtering disarrange Ge film structure, leading to more pores. Figure 5 shows the XRD results of Ge films, and it can be seen that all the films are polycrystalline, and the films grow along (1 1 1) preferred orientations. X-ray diffraction peaks are broad indicating that the present films are composed of nanoparticles
Fig. 4. Refractive index of samples prepared with different ion energy.
Fig. 5. XRD patterns for Ge films.
[16,17]. Present results are in good agreement with those found in JCPDS card [18]. It is noted that the diffraction intensity decreases with bombardment ion energy, which could be illustrated by comparing diffraction peak height. This indicates the crystallinity of Ge film is degenerated. Ge particles have higher energy while evaporated with higher ionenergy bombardment, so they could move more freely on the surface, disrupting Ge film formation tendency, and this would reduce film diffraction intensity. 4. Conclusion
Fig. 3. Transmittance property of samples with different ion energy. A50
APPLIED OPTICS / Vol. 53, No. 4 / 1 February 2014
Ion-beam-assisted deposition was adopted to deposit Ge thin films with different parameters. With the increase of assisted ion energy, the absorption edge is found to shift toward a longer wavelength. Transmittance decreases with the increase of assisted ion energy over 300 eV because of the increase of extinctive coefficient. Electrical resistivity decreases abruptly from 477 to 96 Ω cm with the increase in ion energy because of the rise in carrier concentration. High-energy ion bombardment could reduce film crystallinity by affecting film-growing tendency.
References 1. C. Bostedt, T. van Buuren, T. M. Willey, N. Franco, L. J. Terminello, C. Heske, and T. Moller, “Strong quantumconfinement effects in the conduction band of germanium nanocrystals,” Appl. Phys. Lett. 84, 4056–4058 (2004). 2. Q.-C. Zhang, J. C. Kelly, and M. J. Kenny, “Germanium implanted with high dose oxygen and its optical properties,” Nucl. Instrum. Methods Phys. Res. B 47, 257–262 (1990). 3. A. F. Khan, M. Mehmood, A. M. Rana, and T. Muhammad, “Effect of annealing on structural, optical and electrical properties of nanostructured Ge thin films,” Appl. Surf. Sci. 256, 2031–2037 (2010). 4. Q. Wan, C. L. Lin, W. L. Liu, and T. H. Wang, “Structural and electrical characteristics of Ge nanoclusters embedded in Al2O3 gate dielectric,” Appl. Phys. Lett. 82, 4708–4710 (2003). 5. C. Li, J. Xu, T. Lin, W. Li, Sh. X. Li, and K. J. Chen, “Structural, electronic and optical properties of ultra-thin hydrogenated amorphous germanium films,” Chinese J. Lumin. 32, 1165– 1170 (2011). 6. Z. He, J. Xu, W. Li, K. Chen, and D. Feng, “Crystallization and oxidation process of nc-Ge in a-SiO2 matrix from a-Si:H/aGe:H multilayers,” J. Non-Cryst. Solids 266–269, 1025–1028 (2000). 7. Y. X. Jie, A. T. S. Wee, C. H. A. Huan, W. X. Sun, Z. X. Shen, and S. J. Chua, “Raman and photoluminescence properties of Ge nanocrystals in silicon oxide matrix,” Mater Sci. Eng. B 107, 8–13 (2004). 8. S. K. Ray and K. Das, “Luminescence characteristics of Ge nanocrystals embedded in SiO2 matrix,” Opt. Mater. 27, 948–952 (2005). 9. Y. Zhu and P. P. Ong, “Preparation and photoluminescence of thin films of Ge nanoparticles embedded in Al2O3 matrices,” J. Phys. Condens. Mat. 13, 4075–4080 (2001).
10. L. Martinu and D. Poitras, “Plasma deposition of optical films and coatings: a review,” J. Vac. Sci. Technol. A 18, 2619–2645 (2000). 11. Y. J. Park, K. M. A. Sobahan, and C. K. Hwangbo, “Optical and structural properties of Fe2O3 thin films prepared by ion-beam assisted deposition,” Surf. Coat. Technol. 203, 2646–2650 (2009). 12. I. K. E. Zawawi, N. R. Khalil, and M. A. Mahdy, “Impact of various irradiation photon energies and gamma doses on the optical properties of ZnSe nanostructure thin film,” J. Mater. Sci. Mater. Electron. 23, 520–527 (2012). 13. N. Chopra, A. Mansingh, and G. K. Chadha, “Electrical, optical, and structural properties of amorphous V2O5TeO2 blown films,” J. Non-Cryst. Solids 126, 194–201 (1990). 14. V. Bhosle, A. Tiwari, and J. Narayan, “Metallic conductivity and metal-semiconductor transition in Ga-doped ZnO,” Appl. Phys. Lett. 88, 1–3 (2006). 15. J. Leng, Zh. N. Yu, Y. Q. Li, D. P. Zhang, X. Y. Liao, and W. Xue, “Optical and electrical properties of Y2O3 thin films prepared by ion beam assisted deposition,” Appl. Surf. Sci. 256, 5832–5836 (2010). 16. P. Caldelas, A. G. Rolo, M. J. M. Gomes, E. Alves, A. R. Ramos, O. Conde, S. Yerci, and R. Turan, “Raman and XRD studies of Ge nanocrystals in alumina films grown by RF-magnetron sputtering,” Vacuum 82, 1466–1469 (2008). 17. A. F. Khan, M. Mehmood, A. M. Rana, and M. T. Bhatti, “Effect of annealing on electrical resistivity of rf-magnetron sputtered nanostructured SnO2 thin films,” Appl. Surf. Sci. 255, 8562–8565 (2009). 18. The Joint Committee on Powder Diffraction Standards (JCPDS), Card No. 03-065- 9209, by International Centre for Diffraction Data (ICDD), Swarthmore, Pennsylvania, USA.
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Department of Physics and Surface Physics National Key Laboratory, Fudan University, Shanghai 200433, China
2
Tianjin Key Laboratory of Optical Thin Films, Tianjin Jinhang Institute of Technical Physics, Tianjin 300192, China *Corresponding author: [email protected] Received 27 August 2013; accepted 14 September 2013; posted 9 October 2013 (Doc. ID 196478); published 12 November 2013
Effects of ion energy on the optical, microstructure, and electrical properties of Ge films prepared by ion-beam-assisted deposition were investigated. The absorption edge is found to shift toward a longer wavelength when the ion energy increases. 150 eV ion bombardment energy could help to reduce absorption in the infrared spectrum, elevating 2% of film transmittance. Diffraction intensity decreases with bombardment ion energy indicates that the crystallinity of Ge film is degenerated. Electrical property has been analyzed through Hall measurement. The resistivity of sample prepared with 300 eV ion energy drops substantially from 477 to 137 Ω cm, and it changes slowly with further increase of ion bombardment energy. © 2013 Optical Society of America OCIS codes: (310.1860) Deposition and fabrication; (310.6860) Thin films, optical properties; (310.6870) Thin films, other properties; (240.2130) Ellipsometry and polarimetry. http://dx.doi.org/10.1364/AO.53.000A48
1. Introduction
Nanocrystals of indirect-gap semiconductor materials, such as Si and Ge, have attracted enormous attention due to their quantum-confined electronic states. Germanium (Ge) has been widely used in infrared optical systems because of its high refractive index and wide transparent region in the infrared band, and it also could be applied in novel integrated optoelectronics and microelectronics devices. It has a small effective mass and high mobility as compared to Si. Microstructure of Ge nanocrystals determines the optical, structural, and electrical properties [1,2]. Several techniques are being used to fabricate Ge nanocrystals, such as electron-beam evaporation [3,4], plasma-enhanced chemical vapor deposition [5,6], r.f.-magnetron sputtering [7,8], and pulsed laser deposition [9].
1559-128X/14/040A48-04$15.00/0 © 2014 Optical Society of America A48
APPLIED OPTICS / Vol. 53, No. 4 / 1 February 2014
As reported by Martinu and Poitras [10], using ion bombardment during the deposition process could significantly impact the optical properties of the coatings by increasing the film density and reducing defect levels and could particularly improve mechanical properties, increasing firmness of the film. So ion-beam-assisted deposition (IBAD) has become one of the preferred methods to produce high-quality thin films [11]. In this article, we report on the microstructure and optical properties of germanium films prepared on m-ZnS and BK7 substrates by IBAD. 2. Experiments
Ge thin films were deposited on Φ40 mm × 1 mm BK7 glass, Φ25 mm × 2 mm m-ZnS substrates in the same preparation process. The equipment used for Ge film deposition was an IBAD equipped with a Kaufman ion source made by Veeco-Ion Technology. The diameter of the ion source is 16 mm, and the maximum power of the source is 600 W. The ionsource beam was directed off-center to the rotating
substrate holder; the values of the ion-beam voltage, V i , which determines the ion energy Ei, were between 50 and 1200 V; the ion-beam current, I i , was fixed between 60 and 600 mA. During deposition, Ar gas flow was kept at 18 SCCM (cubic centimeters per minute at STP) and 7 SCCM for ion source and neutralization, respectively. Prior to deposition, the substrate surfaces were precleaned using Ar ion bombardment for 5 min in order to further reduce contamination on the substrate surfaces. The precursor for evaporation was Ge slices with the purity of 99.99%, and deposition parameters are listed in Table 1. During the deposition, the substrate was rotated at 20 rpm and the distance between the substrate and the evaporation source were 1300 mm in order to ensure uniformity. The growth rate and thickness of films were measured by a quartz crystal monitor (IC-5; Inficon, Inc.), optical transmittance of the Ge films was traced at room temperature by a Perkin Elmer UV/VIS/NIR Lambda 900 spectrophotometer in the wavelength range 190–2600 nm. Infrared optical properties were characterized by postdeposition infrared ellipsometry measurements (IRSE-5E; SOPRA) and FTIR spectrometer (VERTEX 70; BRUKER CO., Inc.). The crystal structure was characterized by x-ray diffractometry (XRD) (MXP18; Mac Science Ltd.) using Cu K radiation, with a scanning speed of 4/min and an incident angle of 4°. Electrical resistivity was measured by Hall measurement. 3. Results and Discussions
Figure 1 shows transmittance of the as-deposited Ge films on m-ZnS with different ion energy as a function of wavelength. The behavior of transmittance of Ge films is almost similar in the wavelength range 600–2500 nm. However, the peak values of transmittance are lower than that of ZnS substrate, indicating optical absorption of Ge films in this spectrum, and it is also shown that the transmittance decreases with the increase of assisted ion energy. In addition, the sudden/ rapid fall at wavelengths below 1300 nm is probably due to the absorption edge, which is found to shift toward a longer wavelength when the ion energy increases. The spectral absorption coefficient α is calculated from the spectral transmittance curves using the relation [12] 1 1 ; α ln t T Table 1.
Fig. 1. Transmittance property of samples prepared with different ion energy.
where t is the thickness of the film, and T is the transmittance at a given wavelength. T decreases with the increase of ion energy in the shortwave spectral region, as shown in Fig. 1, which means that the spectral absorption in the shortwave region increases with ion energy. The absorption coefficient α in the vicinity of the bandgap energy also follows the Urbach’s rule, which predicts [13] α α0 exphν∕Ee ;
(2)
where α0 is a constant, Ee is the Urbach energy describing the energy width of the band edge, and hν is the incident photon energy. It can be concluded from Eqs. (1) and (2) that with the increase of assisted ion energy, the absorption coefficient α in the vicinity of the bandgap energy increases, and the energy width of the band edge decreases accordingly. The shift in the absorption edge is known as Burstein–Moss effect [14], which could be attributed to the increase in free carriers [3]. Figure 2 shows the variations in resistivity of Ge films with the ion energy. For the film deposited without ion bombardment resistivity is 477 Ω cm, and when the energy increases up to 300 eV, the resistivity drops substantially to 137 Ω cm,
(1)
Fundamental Fabrication Conditions of Ge Films
Ion energy (eV) Ion beam current (mA) Base pressure (torr) Growth rate (Å/s) Substrate temperature (°C) Thickness of Ge film (nm)
0–600 0–300 5 × 10−6 5 180 400
Fig. 2. Resistivity of Ge films as a function of ion energy. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
A49
and it decrease slowly with the assisted ion-energy continuous increase. And the decrease in resistivity can be explained by the increase in carrier concentration, consistent with the change of bandgap energy. As shown in Fig. 3, the transmittance property of the sample is improved while the increment of ion energy is low. Although the thickness of all samples monitored by a quartz crystal oscillator was kept the same as 400 nm, it was different because of different deposition parameters, measured by spectroscopic ellipsometry. Ge film on ZnS substrate is chosen 1∕2λ optical thickness around 3.7 μm, so the extreme transmittance value could reflect film absorption. The transmittance peak of the sample prepared without ion-beam bombardment is 72.8%, while the sample prepared with 150 eV ion energy is 75.0%. But with the continuous increase of ion energy, transmittance decreases obviously. The sample prepared with 300 eV IBAD has the maximum transmittance of 73.1%, and transmittance peak of the sample prepared with the ion energy of 600 eV is 71.7%. In the same way, film extinctive coefficient (k) of samples with lower IBAD energy is less. So it can be concluded that certain extent ion-beam bombardment could improve Ge film transmittance, while high ion-beam energy will cause the opposite effect. Figure 4 shows the relationship between the refractive index (n) and ion energy. While ion bombardment energy is below 300 eV, n increases with the ion energy. For example, n of the sample prepared with 300 eV IBAD is 4.38 at 8 μm, 0.11 larger than sample deposited without ion-beam bombardment, and that’s because the ion beam could lead to the collapse of pores in Ge film [15]. But with the continuous increase of ion energy, the refractive index begins to decrease, and that maybe because a high-energy ion beam could cause anti-sputtering disarrange Ge film structure, leading to more pores. Figure 5 shows the XRD results of Ge films, and it can be seen that all the films are polycrystalline, and the films grow along (1 1 1) preferred orientations. X-ray diffraction peaks are broad indicating that the present films are composed of nanoparticles
Fig. 4. Refractive index of samples prepared with different ion energy.
Fig. 5. XRD patterns for Ge films.
[16,17]. Present results are in good agreement with those found in JCPDS card [18]. It is noted that the diffraction intensity decreases with bombardment ion energy, which could be illustrated by comparing diffraction peak height. This indicates the crystallinity of Ge film is degenerated. Ge particles have higher energy while evaporated with higher ionenergy bombardment, so they could move more freely on the surface, disrupting Ge film formation tendency, and this would reduce film diffraction intensity. 4. Conclusion
Fig. 3. Transmittance property of samples with different ion energy. A50
APPLIED OPTICS / Vol. 53, No. 4 / 1 February 2014
Ion-beam-assisted deposition was adopted to deposit Ge thin films with different parameters. With the increase of assisted ion energy, the absorption edge is found to shift toward a longer wavelength. Transmittance decreases with the increase of assisted ion energy over 300 eV because of the increase of extinctive coefficient. Electrical resistivity decreases abruptly from 477 to 96 Ω cm with the increase in ion energy because of the rise in carrier concentration. High-energy ion bombardment could reduce film crystallinity by affecting film-growing tendency.
References 1. C. Bostedt, T. van Buuren, T. M. Willey, N. Franco, L. J. Terminello, C. Heske, and T. Moller, “Strong quantumconfinement effects in the conduction band of germanium nanocrystals,” Appl. Phys. Lett. 84, 4056–4058 (2004). 2. Q.-C. Zhang, J. C. Kelly, and M. J. Kenny, “Germanium implanted with high dose oxygen and its optical properties,” Nucl. Instrum. Methods Phys. Res. B 47, 257–262 (1990). 3. A. F. Khan, M. Mehmood, A. M. Rana, and T. Muhammad, “Effect of annealing on structural, optical and electrical properties of nanostructured Ge thin films,” Appl. Surf. Sci. 256, 2031–2037 (2010). 4. Q. Wan, C. L. Lin, W. L. Liu, and T. H. Wang, “Structural and electrical characteristics of Ge nanoclusters embedded in Al2O3 gate dielectric,” Appl. Phys. Lett. 82, 4708–4710 (2003). 5. C. Li, J. Xu, T. Lin, W. Li, Sh. X. Li, and K. J. Chen, “Structural, electronic and optical properties of ultra-thin hydrogenated amorphous germanium films,” Chinese J. Lumin. 32, 1165– 1170 (2011). 6. Z. He, J. Xu, W. Li, K. Chen, and D. Feng, “Crystallization and oxidation process of nc-Ge in a-SiO2 matrix from a-Si:H/aGe:H multilayers,” J. Non-Cryst. Solids 266–269, 1025–1028 (2000). 7. Y. X. Jie, A. T. S. Wee, C. H. A. Huan, W. X. Sun, Z. X. Shen, and S. J. Chua, “Raman and photoluminescence properties of Ge nanocrystals in silicon oxide matrix,” Mater Sci. Eng. B 107, 8–13 (2004). 8. S. K. Ray and K. Das, “Luminescence characteristics of Ge nanocrystals embedded in SiO2 matrix,” Opt. Mater. 27, 948–952 (2005). 9. Y. Zhu and P. P. Ong, “Preparation and photoluminescence of thin films of Ge nanoparticles embedded in Al2O3 matrices,” J. Phys. Condens. Mat. 13, 4075–4080 (2001).
10. L. Martinu and D. Poitras, “Plasma deposition of optical films and coatings: a review,” J. Vac. Sci. Technol. A 18, 2619–2645 (2000). 11. Y. J. Park, K. M. A. Sobahan, and C. K. Hwangbo, “Optical and structural properties of Fe2O3 thin films prepared by ion-beam assisted deposition,” Surf. Coat. Technol. 203, 2646–2650 (2009). 12. I. K. E. Zawawi, N. R. Khalil, and M. A. Mahdy, “Impact of various irradiation photon energies and gamma doses on the optical properties of ZnSe nanostructure thin film,” J. Mater. Sci. Mater. Electron. 23, 520–527 (2012). 13. N. Chopra, A. Mansingh, and G. K. Chadha, “Electrical, optical, and structural properties of amorphous V2O5TeO2 blown films,” J. Non-Cryst. Solids 126, 194–201 (1990). 14. V. Bhosle, A. Tiwari, and J. Narayan, “Metallic conductivity and metal-semiconductor transition in Ga-doped ZnO,” Appl. Phys. Lett. 88, 1–3 (2006). 15. J. Leng, Zh. N. Yu, Y. Q. Li, D. P. Zhang, X. Y. Liao, and W. Xue, “Optical and electrical properties of Y2O3 thin films prepared by ion beam assisted deposition,” Appl. Surf. Sci. 256, 5832–5836 (2010). 16. P. Caldelas, A. G. Rolo, M. J. M. Gomes, E. Alves, A. R. Ramos, O. Conde, S. Yerci, and R. Turan, “Raman and XRD studies of Ge nanocrystals in alumina films grown by RF-magnetron sputtering,” Vacuum 82, 1466–1469 (2008). 17. A. F. Khan, M. Mehmood, A. M. Rana, and M. T. Bhatti, “Effect of annealing on electrical resistivity of rf-magnetron sputtered nanostructured SnO2 thin films,” Appl. Surf. Sci. 255, 8562–8565 (2009). 18. The Joint Committee on Powder Diffraction Standards (JCPDS), Card No. 03-065- 9209, by International Centre for Diffraction Data (ICDD), Swarthmore, Pennsylvania, USA.
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