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Deposition of thin films of different oxides of copper by RF reactive sputtering and their characterization
Vacuum 57 (2000) 377}385
Deposition of thin "lms of di!erent oxides of copper by RF reactive sputtering and their characterization S. Ghosh *, D.K. Avasthi , P. Shah, V. Ganesan, A. Gupta, D. Sarangi, R. Bhattacharya, W. Assmann Nuclear Science Centre, Aruna Asaf Ali Road, Post Box 10502, New Delhi 110067, India Inter University Consortium for DAE facilities, Indore 452001, India National Physical Laboratory, New Delhi 110012, India Sektion Physik, Universita( t Mu( enchen, 85748 Garching, Germany Received 26 October 1999
Abstract Thin "lms of Cu O and CuO are deposited by RF reactive sputtering at di!erent substrate temperatures. Crystalline phases are identi"ed by grazing angle X-ray di!raction (GAXRD). It shows that Cu O phase is prominent at the substrate temperature corresponding to 303C (no deliberate heating of the substrate) and 1503C. CuO phase is obtained at a substrate temperature of 3003C. The band gap of the "lms are found by optical absorption method. Surface morphology of the "lms are characterized by atomic force microscopy (AFM). Stoichiometric analysis is done by elastic recoil detection analysis (ERDA) technique with highenergy heavy ion beam. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Copper oxide; Reactive sputtering; Grazing angle X-ray di!raction; Atomic force microscopy; Elastic recoil detection analysis; Thornton zone model
1. Introduction Di!erent oxides of copper are of interest because of their emerging applications in low-cost solar cell technology. Apart from its low cost, the other unique features are that (1) it is nontoxic, (2) it has a theoretical solar cell e$ciency of 18%, (3) Cu is available in abundance and the (4) formation of oxide layer is simple [1]. Di!erent Cu O-based heterojunction solar cells are studied earlier [2] with promising results. Application of CuO/Cu as solar thermal selective absorber is suggested by Roos et. al. [3]. Another interest of copper oxide systems lies in high ¹ materials. A study of copper * Corresponding author. 0042-207X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 1 5 1 - 2
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oxides attracts attention due to their presence as constituent of di!erent high ¹ superconductors. Deposition of these "lms by oxidation of copper foils has been reported recently [4]. Reactive sputtering is known for a long time for the deposition of compound "lms from the elemental targets. Deposition conditions play a major role in the nature and properties of the "lms. We, therefore attempted the formation of Cu O and CuO by reactive sputtering. In the present work, di!erent phases of oxides of copper are prepared by varying the substrate temperature. The properties of the prepared cuprous and cupric oxide "lms related to surface morphology is examined by atomic force microscopy. The band gap of the "lms are determined by optical absorption. The stoichiometry of the "lms are measured by ERDA. 2. Experimental Di!erent oxide phases of copper are prepared in thin "lm form by an indigenously developed [5] RF reactive sputtering system consisting of a stainless-steel vacuum chamber of 40 cm diameter evacuated by a di!usion pump (500 l/s) backed by rotary pump (200 l/min). After achieving an ultimate base pressure of 4;10\ T, high-purity oxygen gas is introduced in the chamber controlled by MKS mass #ow controller and kept at 7.5 sccm. A highly pure (5 N) Cu disk of 10 cm diameter "xed with water-cooled cathode is powered by an RF generator of frequency 13.56 MHz. The RF is capacitively coupled to the cathode and the power applied to the cathode is about 200 W. The pressure during deposition is about 5;10\ T. The schematic of the deposition system is shown in Fig. 1. Films are deposited in the oxygen plasma environment in three runs. During deposition, substrate temperature was kept at 30 (no deliberate heating of the substrate), 150 and
Fig. 1. Schematic of the RF sputtering thin "lm coating unit system developed at Nuclear Science Centre.
S. Ghosh et al. / Vacuum 57 (2000) 377}385
379
3003C in the "rst, second and third runs, respectively. Deposition time for the "lms are kept at 15 min. Thickneses of the "lms deposited on the #oat glass substrate are measured using a Talystep pro"lometer (Rank, Taylor and Holson, UK). The rate of deposition of the "lms are calculated by dividing the thickness by the time of deposition. Crystalline phases of the "lms were analyzed by grazing angle X-ray di!raction method with Cu K radiation in Bragg Brantano geometry. The a optical transmission of the "lms are measured using a SHIMATZU 3101 PC UV-VIS spectrophotometer in a wavelength range of 300}1200 nm. Surface morphology of the "lms are examined with atomic force microscope (Digital instrument NANOSCOPE-E). Grain size and root mean square surface roughness are determined from AFM images within an area of 500 nm;500 nm. For the grain size determination, length and breadth of each grain along XY plane is calculated and the area is determined. This area is equated to pr to get the diameter which is considered as the size of that single grain. The e!ective grain size is calculated by taking the average of 5}6 grains in each AFM frame. The root mean square surface roughness is calculated with the support of the software. From the optical transmission data absorption coe$cient values of the "lms are calculated and optical band gap of the "lms are determined by standard tauc's plot technique [6]. Cu and O contents of the "lms deposited at 30 and 1503C are obtained by ERDA [7] using 200 MeV Ag beam at NSC Pelletron accelerator. Recoils of Cu and O are detected by *E!E detector telescope [8] consisting of an ionization chamber and a solid-state surface barrier detector. Similar experiment is done at Munich Tandem accelerator with 210 MeV I beam on the "lm deposited at 3003C with a large area (6.3 msr) *E!E telescope detector [9].
3. Results and discussion The coating thickness of the "lms are 120, 172.5 and 210 nm corresponding to the substrate temperature 30, 150 and 3003C, respectively. From thickness and deposition time, the deposition rates of the "lms are found to be 8.0, 11.5, 14.0 nm/min for substrate temperature corresponding to 30, 150 and 3003C, respectively. It is known that the formation of oxide phase during reactive sputtering occurs almost exclusively near the substrate and the reaction rate increases which leads to a higher deposition [10]. Therefore, the increase in deposition rate with the substrate temperature as observed by us is expected. GAXRD result shows that the "lms are polycrystalline and indicates that di!erent oxide phases are formed depending on the di!erent substrate temperature. Prominent phase observed is of Cu O, when the substrate is kept at 30 and 1503C. The XRD pattern of these samples are shown in Figs. 2 and 3. The [1 1 1] phase become much stronger when the substrate is kept at 1503C. CuO phases [!1 1 1] and [2 0 0] are obtained at a substrate temperature of 3003C as shown in Fig. 4. The e!ect of higher substrate temperature for the formation of CuO phase can be understood by the following reaction: 2Cu O#O P4CuO. (1) At lower temperature, "rst Cu O phase is formed. When the temperature rises and reaches 2003C and above, Cu O starts reacting with O and form CuO phase. From thermodynamic consider ations, Gibbs free energy [11] of the reaction (1) comes around !3.73 kcal/mol at about 2003C. Therefore, the formation of CuO phase at a temperature of 3003C can be easily explained by this
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Fig. 2. XRD pattern of the "lm with substrate at room temperature.
Fig. 3. XRD pattern of the "lm with substrate at 1503C.
reaction. In the literature [4], it has been reported that during the oxidation of Cu, initially Cu O and then CuO phase starts forming due to more oxidization. Atomic force microscopy shows that substrate temperature plays an important role for the surface morphology of the "lms. It has been observed for the "lm deposited at 303C that, 8}10 small grains of size &40 nm diameter agglomerate together and make a big grain of size &120 nm. Surface roughness of the "lm is &3.2 nm. With rise in temperature, bigger grains start developing and the grain size becomes 160 nm at 1503C temperature. Surface roughness of this "lm decreases by 8% as compared to the previous case. The grain size decreases to 90 nm at 3003C. Surface roughness of this "lm is about 2.1 nm. The change in the surface morphology of the "lms deposited at 30 and 1503C can be understood by Thornton zone model [12]. This model predicts three structural zones as a function of ¹/¹ , where ¹ is the substrate temperature and ¹ is the
S. Ghosh et al. / Vacuum 57 (2000) 377}385
381
Fig. 4. XRD pattern of the "lm with substrate at 3003C.
coating material melting point. The zones are divided in the following manner: Zone 1. ¹/¹ (0.3,
Zone 2. 0.3(¹/¹ (0.5 and
Zone 3. 0.5(¹/¹ (1.0.
In this case, the ¹/¹ value for the "lms are 0.024, 0.121 and 0.226 corresponding to substrate
temperature 30, 150 and 3003C, considering the ¹/¹ value of Cu O and CuO from the standard
data book [11]. Films are grown in zone 1 regime where the main process is shadowing (a simple geometric interaction between the roughness of the growing surface and the angular directions of the arriving sputtered atoms). According to the model, in this regime crystallite size increases with ¹/¹ . The increase in grain size in the "lm deposited at 1503C with respect to the "lm deposited at
303C matches well with the theoretical predictions. However, in the case of the "lm deposited at 3003C, the grain size decreases as compared to the "lms grown at 303C and 1503C, despite the increase in ¹/¹ value. The reduction in grain size can be attributed to the change in crystallo graphic phase from Cu O to CuO. Therefore Thornton model is not applicable for the "lm grown at 3003C. Previously Jian Li et al. [13] have also shown that due to the phase change from Cu O to CuO at about 300}4003C the grain size decreases appreciably. Surface morphology (AFM pictures) of these "lms are shown in Figs. 5}7 corresponding to 30, 150 and 3003C, respectively. The absorption coe$cient (a) of the "lms are determined by the following relation: a"!1/d (Log ¹/¹ ) where, d is the "lm thickness, ¹ is the transmission at di!erent wavelength and ¹ is a constant which depends on the refractive index of the substrate (n ) as well as the refractive index of the "lm 1 (n ) by the relation $ ¹ "16n n /[(1#n )(n #n )]. $ 1 $ 1 $ In the present case, refractive index values of substrate and "lms are obtained using an ellipsometer (Rudloph Model 43603-200e) with an angle of incidence of 703 at 546 nm wavelength. The intercept
382
S. Ghosh et al. / Vacuum 57 (2000) 377}385
Fig. 5. AFM image of the "lm with substrate at room temperature.
Fig. 6. AFM image of the "lm with substrate at 1503C.
S. Ghosh et al. / Vacuum 57 (2000) 377}385
383
Fig. 7. AFM image of the "lm with substrate at 3003C.
Fig. 8. Photon energy versus absorption coe$cient of the "lm deposited at 3003C temperature.
on the X-axis of the plot of (ahl) versus hl gives the value of the optical band gap. A typical plot of the absorption coe$cient versus the photon energy is shown in Fig. 8 for the "lm deposited at 3003C. Optical band-gap values of the "lms deposited at 30 and 1503C are 1.75, 2.04 and 1.47 eV, respectively (in Table 1). Heavy ion ERDA with gaseous *E!E detector telescope revealed that
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S. Ghosh et al. / Vacuum 57 (2000) 377}385
Table 1 The deposition rate, crystalline phases, grain size, roughness, band gap and stoichiometry of the "lms prepared at di!erent substrate temperature Substrate temp (3C)
Deposition rate (nm/min)
Crystalline phase
Grain size (nm)
Roughness (nm)
Band Gap (eV)
Cu/O ratio
30
8.0
Cu O
3.2
1.71
2.2 : 1.0
150 300
11.5 14.0
Cu O CuO
45 (small) 140 (Large) 160 90
2.9 2.1
2.04 1.47
2.1 : 1.0 1.2 : 1.0
Fig. 9. ERDA spectrum of recoils of Cu of CuO by 210 MeV I beam.
the "lms are slightly oxygen de"cient. Stoichiometry of the "lms are shown in Table 1. Typical ERDA spectra of Cu and O of the CuO "lm are shown in Figs. 9 and 10. In these spectra, channels (in X-axis) represent the energy of the recoil Cu and O, respectevly. Recoil energy is converted to the depth scale as shown in the "gures. Counts (in >-axis) represent the number of recoil which represents the concentrations of Cu and O. It has been observed that the "lms are O de"cient but are close to the required stoichiometry. Among the Cu O phases formed at 30 and 1503C, the stoichiometry of the "lms at 1503C is close to ideal. Its band-gap value is also close to the previously reported value of 2.2 eV [4]. Using the areal density of Cu and O of the "lms and the thickness obtained by ERDA the speci"c gravity or density q of the "lms (gm/cm) are determined using the following relation: o"N M/N t where, N is the areal density (atoms/cm), t is the "lm thickness, M is the molecular weight and N is the Avogadro's number. The typical density as determined for the Cu O and CuO "lms are &6.4 gm/cm and 4.9 gm/cm, respectively. These results are in agreement with the TRIM95 simulation code.
S. Ghosh et al. / Vacuum 57 (2000) 377}385
385
Fig. 10. ERDA spectrum of recoils of O of CuO by 210 MeV I beam.
4. Conclusion Copper oxide "lms have been prepared by RF reactive sputtering keeping substrate at 30, 150 and 3003C, respectively. Deposition rates are found to be strongly in#uenced by the substrate temperature. Di!erent phases of copper oxides are found at di!erent temperatures of deposition. CuO phase is obtained in the "lms prepared at a substrate temperature of 3003C. Optical transmission and the band gap of the "lms are reduced with an increase in the substrate temperature. AFM images show that the phase transformation from Cu O to CuO is accompanied by a reduction in grain size. Stoichiometric analysis of the "lms by ERDA shows that the "lms are O de"cient.
References [1] Olsen LC, Addis FW, Miller W. Sol Cells 1982}1983;7:247. [2] Trivich U, Wang EY, Komp RJ, Kakar AS. Proceedings of the 13th IEEE Photovoltaic Special Conference, Washington. New York: IEEE, 1978. p. 174. [3] Roos A, Chibuye T, Karlson B. Sol Energy Mater 1983;7:453. [4] Musa AO, Akomolafe T, Karter MJ. Sol Energy Mater 1998;51:305. [5] Shiva Kumar VV, Venkatraman S, Ajith Kumar BP, Avasthi DK, Sarangi D, Bhattacharya R, Gupta A, Proceedings of National Symposium on Vacuum Science and Technology. 1997. cp-19. [6] Tauc J, Grigorovici R, Vancu A. Phys State Sol 1966;15:627. [7] Lecuyer J, Brassard C, Cardinal C, Chabbal J, Deschenes L, Labrie JP, Therrault B, Martel JG, St-Jacques R. J Appl Phys 1976;47:381. [8] Avasthi DK, Hui SK, Subramanyam ET, Mehta BR. Vacuum 1996;47:1061. [9] Assmann W, Huber H, Steinhausen Ch, Dobler M, Weidinger A. Nucl Instr and Meth B 1994;89:242. [10] Maissel LI, Glang R. Hand book of thin "lm technology. New York: Mc Graw-Hill, 1983. p. 4}27. [11] Weast RI, Handbook of chemistry and physics, 67th ed., Florida: Chemical Rubber Company Press, 1986}1987. [12] Thornton JA. Annu Rev Mater Sci 1977;7:239. [13] Li J, Vizkelethy G, Revesz P, Mayer JW. J Appl Phys 1991;69:1021.
Deposition of thin "lms of di!erent oxides of copper by RF reactive sputtering and their characterization S. Ghosh *, D.K. Avasthi , P. Shah, V. Ganesan, A. Gupta, D. Sarangi, R. Bhattacharya, W. Assmann Nuclear Science Centre, Aruna Asaf Ali Road, Post Box 10502, New Delhi 110067, India Inter University Consortium for DAE facilities, Indore 452001, India National Physical Laboratory, New Delhi 110012, India Sektion Physik, Universita( t Mu( enchen, 85748 Garching, Germany Received 26 October 1999
Abstract Thin "lms of Cu O and CuO are deposited by RF reactive sputtering at di!erent substrate temperatures. Crystalline phases are identi"ed by grazing angle X-ray di!raction (GAXRD). It shows that Cu O phase is prominent at the substrate temperature corresponding to 303C (no deliberate heating of the substrate) and 1503C. CuO phase is obtained at a substrate temperature of 3003C. The band gap of the "lms are found by optical absorption method. Surface morphology of the "lms are characterized by atomic force microscopy (AFM). Stoichiometric analysis is done by elastic recoil detection analysis (ERDA) technique with highenergy heavy ion beam. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Copper oxide; Reactive sputtering; Grazing angle X-ray di!raction; Atomic force microscopy; Elastic recoil detection analysis; Thornton zone model
1. Introduction Di!erent oxides of copper are of interest because of their emerging applications in low-cost solar cell technology. Apart from its low cost, the other unique features are that (1) it is nontoxic, (2) it has a theoretical solar cell e$ciency of 18%, (3) Cu is available in abundance and the (4) formation of oxide layer is simple [1]. Di!erent Cu O-based heterojunction solar cells are studied earlier [2] with promising results. Application of CuO/Cu as solar thermal selective absorber is suggested by Roos et. al. [3]. Another interest of copper oxide systems lies in high ¹ materials. A study of copper * Corresponding author. 0042-207X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 1 5 1 - 2
378
S. Ghosh et al. / Vacuum 57 (2000) 377}385
oxides attracts attention due to their presence as constituent of di!erent high ¹ superconductors. Deposition of these "lms by oxidation of copper foils has been reported recently [4]. Reactive sputtering is known for a long time for the deposition of compound "lms from the elemental targets. Deposition conditions play a major role in the nature and properties of the "lms. We, therefore attempted the formation of Cu O and CuO by reactive sputtering. In the present work, di!erent phases of oxides of copper are prepared by varying the substrate temperature. The properties of the prepared cuprous and cupric oxide "lms related to surface morphology is examined by atomic force microscopy. The band gap of the "lms are determined by optical absorption. The stoichiometry of the "lms are measured by ERDA. 2. Experimental Di!erent oxide phases of copper are prepared in thin "lm form by an indigenously developed [5] RF reactive sputtering system consisting of a stainless-steel vacuum chamber of 40 cm diameter evacuated by a di!usion pump (500 l/s) backed by rotary pump (200 l/min). After achieving an ultimate base pressure of 4;10\ T, high-purity oxygen gas is introduced in the chamber controlled by MKS mass #ow controller and kept at 7.5 sccm. A highly pure (5 N) Cu disk of 10 cm diameter "xed with water-cooled cathode is powered by an RF generator of frequency 13.56 MHz. The RF is capacitively coupled to the cathode and the power applied to the cathode is about 200 W. The pressure during deposition is about 5;10\ T. The schematic of the deposition system is shown in Fig. 1. Films are deposited in the oxygen plasma environment in three runs. During deposition, substrate temperature was kept at 30 (no deliberate heating of the substrate), 150 and
Fig. 1. Schematic of the RF sputtering thin "lm coating unit system developed at Nuclear Science Centre.
S. Ghosh et al. / Vacuum 57 (2000) 377}385
379
3003C in the "rst, second and third runs, respectively. Deposition time for the "lms are kept at 15 min. Thickneses of the "lms deposited on the #oat glass substrate are measured using a Talystep pro"lometer (Rank, Taylor and Holson, UK). The rate of deposition of the "lms are calculated by dividing the thickness by the time of deposition. Crystalline phases of the "lms were analyzed by grazing angle X-ray di!raction method with Cu K radiation in Bragg Brantano geometry. The a optical transmission of the "lms are measured using a SHIMATZU 3101 PC UV-VIS spectrophotometer in a wavelength range of 300}1200 nm. Surface morphology of the "lms are examined with atomic force microscope (Digital instrument NANOSCOPE-E). Grain size and root mean square surface roughness are determined from AFM images within an area of 500 nm;500 nm. For the grain size determination, length and breadth of each grain along XY plane is calculated and the area is determined. This area is equated to pr to get the diameter which is considered as the size of that single grain. The e!ective grain size is calculated by taking the average of 5}6 grains in each AFM frame. The root mean square surface roughness is calculated with the support of the software. From the optical transmission data absorption coe$cient values of the "lms are calculated and optical band gap of the "lms are determined by standard tauc's plot technique [6]. Cu and O contents of the "lms deposited at 30 and 1503C are obtained by ERDA [7] using 200 MeV Ag beam at NSC Pelletron accelerator. Recoils of Cu and O are detected by *E!E detector telescope [8] consisting of an ionization chamber and a solid-state surface barrier detector. Similar experiment is done at Munich Tandem accelerator with 210 MeV I beam on the "lm deposited at 3003C with a large area (6.3 msr) *E!E telescope detector [9].
3. Results and discussion The coating thickness of the "lms are 120, 172.5 and 210 nm corresponding to the substrate temperature 30, 150 and 3003C, respectively. From thickness and deposition time, the deposition rates of the "lms are found to be 8.0, 11.5, 14.0 nm/min for substrate temperature corresponding to 30, 150 and 3003C, respectively. It is known that the formation of oxide phase during reactive sputtering occurs almost exclusively near the substrate and the reaction rate increases which leads to a higher deposition [10]. Therefore, the increase in deposition rate with the substrate temperature as observed by us is expected. GAXRD result shows that the "lms are polycrystalline and indicates that di!erent oxide phases are formed depending on the di!erent substrate temperature. Prominent phase observed is of Cu O, when the substrate is kept at 30 and 1503C. The XRD pattern of these samples are shown in Figs. 2 and 3. The [1 1 1] phase become much stronger when the substrate is kept at 1503C. CuO phases [!1 1 1] and [2 0 0] are obtained at a substrate temperature of 3003C as shown in Fig. 4. The e!ect of higher substrate temperature for the formation of CuO phase can be understood by the following reaction: 2Cu O#O P4CuO. (1) At lower temperature, "rst Cu O phase is formed. When the temperature rises and reaches 2003C and above, Cu O starts reacting with O and form CuO phase. From thermodynamic consider ations, Gibbs free energy [11] of the reaction (1) comes around !3.73 kcal/mol at about 2003C. Therefore, the formation of CuO phase at a temperature of 3003C can be easily explained by this
380
S. Ghosh et al. / Vacuum 57 (2000) 377}385
Fig. 2. XRD pattern of the "lm with substrate at room temperature.
Fig. 3. XRD pattern of the "lm with substrate at 1503C.
reaction. In the literature [4], it has been reported that during the oxidation of Cu, initially Cu O and then CuO phase starts forming due to more oxidization. Atomic force microscopy shows that substrate temperature plays an important role for the surface morphology of the "lms. It has been observed for the "lm deposited at 303C that, 8}10 small grains of size &40 nm diameter agglomerate together and make a big grain of size &120 nm. Surface roughness of the "lm is &3.2 nm. With rise in temperature, bigger grains start developing and the grain size becomes 160 nm at 1503C temperature. Surface roughness of this "lm decreases by 8% as compared to the previous case. The grain size decreases to 90 nm at 3003C. Surface roughness of this "lm is about 2.1 nm. The change in the surface morphology of the "lms deposited at 30 and 1503C can be understood by Thornton zone model [12]. This model predicts three structural zones as a function of ¹/¹ , where ¹ is the substrate temperature and ¹ is the
S. Ghosh et al. / Vacuum 57 (2000) 377}385
381
Fig. 4. XRD pattern of the "lm with substrate at 3003C.
coating material melting point. The zones are divided in the following manner: Zone 1. ¹/¹ (0.3,
Zone 2. 0.3(¹/¹ (0.5 and
Zone 3. 0.5(¹/¹ (1.0.
In this case, the ¹/¹ value for the "lms are 0.024, 0.121 and 0.226 corresponding to substrate
temperature 30, 150 and 3003C, considering the ¹/¹ value of Cu O and CuO from the standard
data book [11]. Films are grown in zone 1 regime where the main process is shadowing (a simple geometric interaction between the roughness of the growing surface and the angular directions of the arriving sputtered atoms). According to the model, in this regime crystallite size increases with ¹/¹ . The increase in grain size in the "lm deposited at 1503C with respect to the "lm deposited at
303C matches well with the theoretical predictions. However, in the case of the "lm deposited at 3003C, the grain size decreases as compared to the "lms grown at 303C and 1503C, despite the increase in ¹/¹ value. The reduction in grain size can be attributed to the change in crystallo graphic phase from Cu O to CuO. Therefore Thornton model is not applicable for the "lm grown at 3003C. Previously Jian Li et al. [13] have also shown that due to the phase change from Cu O to CuO at about 300}4003C the grain size decreases appreciably. Surface morphology (AFM pictures) of these "lms are shown in Figs. 5}7 corresponding to 30, 150 and 3003C, respectively. The absorption coe$cient (a) of the "lms are determined by the following relation: a"!1/d (Log ¹/¹ ) where, d is the "lm thickness, ¹ is the transmission at di!erent wavelength and ¹ is a constant which depends on the refractive index of the substrate (n ) as well as the refractive index of the "lm 1 (n ) by the relation $ ¹ "16n n /[(1#n )(n #n )]. $ 1 $ 1 $ In the present case, refractive index values of substrate and "lms are obtained using an ellipsometer (Rudloph Model 43603-200e) with an angle of incidence of 703 at 546 nm wavelength. The intercept
382
S. Ghosh et al. / Vacuum 57 (2000) 377}385
Fig. 5. AFM image of the "lm with substrate at room temperature.
Fig. 6. AFM image of the "lm with substrate at 1503C.
S. Ghosh et al. / Vacuum 57 (2000) 377}385
383
Fig. 7. AFM image of the "lm with substrate at 3003C.
Fig. 8. Photon energy versus absorption coe$cient of the "lm deposited at 3003C temperature.
on the X-axis of the plot of (ahl) versus hl gives the value of the optical band gap. A typical plot of the absorption coe$cient versus the photon energy is shown in Fig. 8 for the "lm deposited at 3003C. Optical band-gap values of the "lms deposited at 30 and 1503C are 1.75, 2.04 and 1.47 eV, respectively (in Table 1). Heavy ion ERDA with gaseous *E!E detector telescope revealed that
384
S. Ghosh et al. / Vacuum 57 (2000) 377}385
Table 1 The deposition rate, crystalline phases, grain size, roughness, band gap and stoichiometry of the "lms prepared at di!erent substrate temperature Substrate temp (3C)
Deposition rate (nm/min)
Crystalline phase
Grain size (nm)
Roughness (nm)
Band Gap (eV)
Cu/O ratio
30
8.0
Cu O
3.2
1.71
2.2 : 1.0
150 300
11.5 14.0
Cu O CuO
45 (small) 140 (Large) 160 90
2.9 2.1
2.04 1.47
2.1 : 1.0 1.2 : 1.0
Fig. 9. ERDA spectrum of recoils of Cu of CuO by 210 MeV I beam.
the "lms are slightly oxygen de"cient. Stoichiometry of the "lms are shown in Table 1. Typical ERDA spectra of Cu and O of the CuO "lm are shown in Figs. 9 and 10. In these spectra, channels (in X-axis) represent the energy of the recoil Cu and O, respectevly. Recoil energy is converted to the depth scale as shown in the "gures. Counts (in >-axis) represent the number of recoil which represents the concentrations of Cu and O. It has been observed that the "lms are O de"cient but are close to the required stoichiometry. Among the Cu O phases formed at 30 and 1503C, the stoichiometry of the "lms at 1503C is close to ideal. Its band-gap value is also close to the previously reported value of 2.2 eV [4]. Using the areal density of Cu and O of the "lms and the thickness obtained by ERDA the speci"c gravity or density q of the "lms (gm/cm) are determined using the following relation: o"N M/N t where, N is the areal density (atoms/cm), t is the "lm thickness, M is the molecular weight and N is the Avogadro's number. The typical density as determined for the Cu O and CuO "lms are &6.4 gm/cm and 4.9 gm/cm, respectively. These results are in agreement with the TRIM95 simulation code.
S. Ghosh et al. / Vacuum 57 (2000) 377}385
385
Fig. 10. ERDA spectrum of recoils of O of CuO by 210 MeV I beam.
4. Conclusion Copper oxide "lms have been prepared by RF reactive sputtering keeping substrate at 30, 150 and 3003C, respectively. Deposition rates are found to be strongly in#uenced by the substrate temperature. Di!erent phases of copper oxides are found at di!erent temperatures of deposition. CuO phase is obtained in the "lms prepared at a substrate temperature of 3003C. Optical transmission and the band gap of the "lms are reduced with an increase in the substrate temperature. AFM images show that the phase transformation from Cu O to CuO is accompanied by a reduction in grain size. Stoichiometric analysis of the "lms by ERDA shows that the "lms are O de"cient.
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