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Structural and photoluminescence properties of thin alumina films on silicon, fabricated by electrochemistry
Materials Science and Engineering B101 (2003) 65 /70 www.elsevier.com/locate/mseb
Structural and photoluminescence properties of thin alumina films on silicon, fabricated by electrochemistry M. Kokonou a, A.G. Nassiopoulou a,*, A. Travlos b a
IMEL/NCSR Demokritos, P.O.Box 60228, 15310 Aghia Paraskevi, Athens, Greece b Institute of Materials Science, NCSR Demokritos, Greece
Abstract Alumina thin films on a silicon substrate were fabricated by anodisation of Aluminium films, deposited by electron gun evaporation, in different acid aqueous solutions. The thickness of the initial Al film was changed in the range of 50 /500 nm and its composition was either pure aluminium or aluminium with 1% silicon. The structure and properties of the obtained alumina films were extensively investigated and they were found to depend strongly on the thickness and composition of the initial aluminium layer, as well as on the acid aqueous solution used. One important result obtained from the optical characterization of the alumina films was the very bright photoluminescence (PL), which also depended on the acid aqueous solution used and the alumina film thickness. The obtained results will be discussed in detail. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Silicon; Nanocrystals; Alumina; Anodization; Photoluminescence
1. Introduction Anodic alumina films have received significant attention in recent years due to their interesting dielectric, optical and other properties, which make them very attractive for various applications. The possibility to grow porous alumina with regular vertical pores of controlled sizes on a silicon substrate opens important new perspectives for potential applications in electronic, optoelectronic and magnetic nanodevices. In this respect, it is very essential to fully understand the mechanism and the kinetics of growth of the films and the different parameters which influence their structure (anodising conditions, anodisation solution, etc.). However, while a lot of work has been devoted to the anodisation of bulk aluminium in order to get an alumina film on its surface (see Ref. [1] and references therein), very little work has been published on alumina films on a silicon substrate [2 /7]. Different methods of film growth were in general used in the above references, * Corresponding author. Tel.: /3-10-6503-123; fax: /3-10-6511723. E-mail address: [email protected] (A.G. Nassiopoulou).
as for example reactive rf sputtering [4], low pressure metal/organic chemical vapor deposition [5,6], electron beam evaporation [7] or electrochemistry [2,3]. In this paper we investigate the growth of thin alumina films (thickness below 0.5 mm) on a silicon substrate by using anodisation in different acid aqueous solutions. The structure and properties of the obtained alumina films were found to depend on the electrochemical solution used and the thickness and composition of the initial film. The obtained films were measured for their photoluminescence (PL) properties, which also depended on film thickness and the anodisation conditions. The obtained results will be discussed below.
2. Experimental results and discussion 2.1. Sample preparation Al thin films were deposited on p-type (100) Si wafers using electron gun evaporation from a target composed either of pure Al or an alloy of Al(1% Si) [8]. Prior to Al deposition, the Si wafers were cleaned in an H2O2:H2SO4 solution and an ohmic contact was formed
0921-5107/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5107(02)00653-0
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on their backside. Al films of five different thicknesses, in the range of 15 /500 nm, were deposited and the samples were then anodised in different solutions at room temperature. Anodisation was carried out under constant dc voltage and three different acid aqueous solutions were used as follows: (a) in 10 wt.% sulfuric acid aqueous solution, under a constant voltage of 20 V, with the limit current density set at 100 mA cm 2; (b) in 6 wt.% oxalic acid aqueous solution, under a constant voltage of 50 V, with the limit current density set again at 100 mA cm 2; and (c) in 1 wt.% citric acid aqueous solution, with the value of the constant voltage depending in this case on the thickness of the Al films: 30 V for a thickness of 15 nm, 50 V for 30 nm, 50 or 70 V for 50 nm, 70 V for 100 nm and 100 V for 500 nm. The limit current density was set at 20 mA cm 2. During the process, the current density reached rapidly the limit value; it was stabilized at 20 mA cm 2 in sulfuric and oxalic acids, and at 5mA/cm2 in citric acid and it then
started to decrease. The voltage supply was intentionally cut when the current density reached the 10% of the stabilized value for one set of samples and when it reached the 25% of the stabilized value for another set of samples. After anodisation, the samples were rinsed in deionized water and dried in N2 gas. 2.2. TEM cross-sectional observation The samples were characterized systematically by transmission electron microscopy (TEM). Films anodised in the three different acid solutions (sulfuric, citric and oxalic acids) were examined. Other parameters that were changed included film thickness and the composition of the Al target material used in the electron gun evaporator. Either pure aluminium or an alloy of aluminium with 1% silicon was used. Fig. 1 shows an alumina film, about 120 nm thick, obtained by anodisation in sulfuric acid of an alumi-
Fig. 1. TEM images of porous nanocrystalline alumina film, about 120 nm thick, obtained by anodization of an Al (1% Si) film in 10% wt sulfuric acid. In (a) is a bright field micrograph, while in (b) a dark field image is shown, where we can identify silicon nanocrystals embedded in the nanocystalline alumina matrix.
Fig. 2. Dark field TEM images of an alumina film, obtained by anodization of a 50 nm thick Al (1% Si) film in 1 wt.% citric acid aquous solution. The alumina film is nanocrystalline and it is composed of Al2O3 (a) and Si (b) nanocrystals.
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Fig. 3. Polycrystalline, porous anodic alumina layer formed by anodization of a 500 nm thick Al(1% Si) layer in 6% wt oxalic acid aquous solution. In (a) a bright field micrograph is shown, where we see that the film is porous and nanocrystalline. The pores are randomly oriented. In (b) a dark field micrograph of the alumina nanocrystals is shown. Their size is in the range of 5 /15 nm. In (c) a dark field micrograph of silicon nanocrystals embedded in the porous alumina layer is shown. A very high density of silicon nanocrystals of uniform sizes in the range of 0.7 /1 nm are identified. They are uniformly distributed in the alumina matrix.
nium film, 50 nm thick, obtained from a target of Al(1% Si). Fig. 1a is a bright field micrograph, which shows that the obtained alumina film is porous and amorphous. The pores were randomly oriented. During TEM observation, some alumina nanocrystals were identified
67
in the amorphous matrix. Fig. 1b is a dark field image from the same area of the sample as in Fig. 1a, which shows the existence of some silicon nanocrystals embedded in the porous alumina layer. The same Al (1% Si) film as above, anodised in citric, instead of sulphuric acid, gives a film of quite different structural characteristics. The obtained alumina film was not in this case porous and it was composed of a mixture of Al2O3 (Fig. 2a) and Si (Fig. 2b) nanocrystals. Anodisation in oxalic acid of Al(1% Si) films gives also nanocrystalline alumina with silicon nanocrystals embedded in it. An example is given in Fig. 3, where the initial Al(1% Si) film was 500 nm thick. In (a) a bright field micrograph of the alumina film is shown. The film is porous and nanocrystalline and its thickness is /49 nm. In (b) and (c) we see dark field images of alumina and silicon nanocrystals, respectively. The size of the alumina nanocrystals is in the range of 5 /15 nm, while that of silicon nanocrystals is in the range of 0.7 /1 nm. The silicon nanocrystals are uniformly distributed in the alumina matrix and their density is surprisingly high (white spots in the micrograph of Fig. 3c). Anodisation of pure aluminium films, without any percentage of silicon, under the same conditions as those used in the anodisation of Al(1% Si), gives alumina films with very different structural characteristics. An example is shown in Fig. 4 for the case of anodisation in citric acid, of an Al film, 50 nm thick. Fig. 4a shows a bright field micrograph of the alumina film, which is nanocrystalline and porous. The pores are randomly distributed through the film. In contrast to the case of the Al(1% Si) film, in the case of the pure Al film, anodised under the same conditions as the Al (1% Si), it was difficult to anodise the whole Al layer, as shown in Fig. 4a, where the alumina film was 50 nm thick, while a remaining non-anodised Al film, 22 nm thick, is seen between the silicon substrate and the Al2O3 layer. In Fig. 4b a dark field image is shown, where we identify the Al2O3 nanocrystals. Anodisation of a pure Al film, 500 nm thick, in oxalic acid, gives an alumina film with very different structural characteristics than all the other films described above, as seen in Fig. 5. The film is in this case amorphous and it contains regular cylindrical pores, with a diameter of about 30 nm, grown vertically in the direction of film growth (perpendicular to the substrate). The pores lie on vertical pillars, formed on the silicon substrate in the same direction as the pores and composed of aluminium crystals. Between these pillars and the pores a nonporous layer is seen. The amorphous porous alumina layer is 560 nm thick. 2.3. Photoluminescence Photoluminescence spectra in the visible range (500 / 850 nm) were measured using a Jobin Yvon spectro-
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Fig. 4. Bright field (a) and dark field (b) TEM images of an alumina film obtained by anodization of a 50 nm thick pure Al film, without any Si, in 1% wt citric acid aquous solution. The alumina film is porous and nanocrystalline (crystal planes were identified during TEM observation, although nanocrystals were not clearly seen). A remaining Al film is also seen between silicon substrate and alumina.
meter. The excitation source was an Argon laser line at 457.9 nm and the excitation power varied from 0.16 to 22 mV. For an excitation power below 14 mW a chopper was used to cut off the noise. All measurements were carried out at room temperature. Samples before anodisation showed no photoluminescence (PL) in the above mentioned wavelength range. Samples anodised in sulfuric acid showed weak PL, while those anodised in citric and oxalic acids, showed a highly efficient broad PL band in the visible range, with a maximum in the range of 550/580 nm. This PL band was very stable with time. The thinner samples showed a maximum at smaller wavelengths than thicker samples. An example of PL spectra is shown in Fig. 6. In general, PL intensity was found to depend on the excitation power, the aluminium film thickness before anodisation and the composition of the initial aluminium film (pure or with 1% Si). Fig. 7 shows comparative PL spectra from alumina films from anodisation of either a pure
Fig. 5. Bright field micrograph of an alumina layer formed after anodisation of a 500 nm pure Al film, in 6% wt oxalic acid aquous solution. The film is porous and amorphous. The pores are cylindrical, their diameter is about 30 nm and they form an ordered array. At the interface with the silicon substrate the pores are interrupted by the amorphous material and Al cyllindrical pillars are seen on the silicon substrate in the direction of the pores.
aluminium film (open symbols, (a)) or a film of Al with 1% Si (dark symbols, (b)). We see that for thicker alumina films (thickness of few hundreds of nm), the alumina films from pure Al give PL of higher intensity compared to alumina films from Al (1% Si). This is
Fig. 6. PL spectra of an alumina film (a) obtained from anodization of a pure Al film anodized in 1% citric acid, for 5 different values of thickness (b) obtained from anodization of 3 50 nm Al (1% Si) films in three different acid aqueous solutions. The excitation was an Ar laser line at 457.9 nm and the excitation power was 20 mW.
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Fig. 7. PL spectra of alumina films, anodised in aqueous citric acid 1% wt. Spectra (a) correspond to alumina fabricated from anodisation of a pure Al thin film and (b) to alumina from anodisation of an alloy of Al (1% Si).
inversed in the case of alumina films which are thinner than 100 nm. This trend was systematically observed in all samples examined. The observed PL does not seem to have its origin in silicon nanocrystals embedded in the alumina matrix, as proposed by Ong and Zhu [9], although some contribution may be attributed to them. This is supported by the fact that the alumina films with silicon nanocrystals embedded in them show even weaker PL than without silicon nanocrystals when the film thickness is below 100 nm. PL from crystalline alumina is in general assigned to optical transitions from defect states [10], the most common being the F center (corresponding to an oxygen vacancy with one trapped electron). PL emission at 420 nm is obtained from alumina when an electron recombines with the F center. From porous crystalline alumina, fabricated by anodisation of an aluminium foil in 0.3 M oxalic acid (thickness of alumina in the range of 50 /100 nm) [11], a broad PL band in the 400/600 nm range has been obtained, with a maximum at about 450 nm, slightly red-shifted compared to the peak from bulk crystalline alumina. This red shift has been attributed to an increase in the density of oxygen vacancies in the alumina membrane, or to internal stresses at the interface with the remaining aluminium. The PL observed from our samples shows a maximum at about 560 nm, significantly red-shifted compared to the emission of bulk crystalline alumina. However we believe that the origin of the luminescence is the same in both cases.The observed red-shift may be explained if we take into account different factors, as for example the excitation wavelength used, which may limit optical pumping to one part of the luminescent band, the nanocrystalline nature of our alumina films, which causes band gap opening, and possible internal stresses within the material. A more detailed study is on going in order to elucidate these points.
3. Conclusion Aluminium thin films, either pure or containing 1% silicon, were deposited on a silicon substrate by electron gun evaporation and they were then anodised in different acid aqueous solutions (citric, oxalic or sulfuric) in order to obtain alumina thin films. The structure of the final film depended strongly on the initial aluminium film composition (pure Al or Al(1% Si)) and thickness, as well as on the anodisation solution. Anodisation of aluminium films of thickness higher than 0.5 mm in oxalic acid resulted in porous alumina with regular cylindrical pores in the direction of the anodisation current. Thinner aluminium films anodised under the same conditions resulted in nanocrystalline alumina. If the initial aluminium film contained 1% Si, a high density of silicon nanocrystals, embedded in the nanocrystalline alumina, was formed. If the anodisation solution was changed (citric or sulfuric instead of oxalic acid aqueous solution), the obtained alumina films were different. Very bright broad photoluminescence with a maximum at 550/580 nm was obtained from films anodised in oxalic or citric acid aqueous solutions, while samples anodised in sulfuric acid showed weaker PL. The intensity of the emitted PL depended strongly on film thickness. Further experiments are in progress in order to elucidate the origin of PL, which is most probably attributed to optical transitions involving the F defect state.
References [1] G.E. Thomson, Thin Solid Films 297 (1997) 192 /201. [2] J.H. Wu, X.L. Wu, N. Tang, Y.F. Mei, X.M. Bao, Appl. Phys. A 72 (2001) 735. [3] S. Lazarouk, S. Katsouba, A. Demianovich, V. Stanovski, S. Voitech, V. Vysotski, V. Ponomar, Microl. Eng. 50 (2000) 321.
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[4] Y. Zhao, Y. Qian, W. Yu, Z. Chen, Thin Solid Films 286 (1996) 45. [5] W. Koh, S.J. Ku, Y. Kim, Thin Solid Films 304 (1997) 222. [6] D.H. Kuo, B.Y. Chung, R.J. Wu, Thin Solid Films 398 (2001) 35. [7] J.C. Pivin, N.V. Gaponenko, I. Molchan, R. Kudrawiec, J. Misiewicz, L. Bruja, G.E. Thomson, P. Skeldon, J. Alloy Compounds 341 (2002) 272.
[8] M. Kokonou, S. Lazarouk, A.G. Nassiopoulou, A. Travlos, G. Kaltsas, D. Pagonis, PSST /2002, Abstract Book, Tenerife, Spain. [9] P.P. Ong, Y. Zhu, Physica E 15 (2002) 118. [10] B.D. Evans, M. Stapelbroek, Phys. Rev. B. 18 (12) (1978) 7089. [11] J.Y. Du, W.L. Cai, C.M. Mo, J. Chen, L.D. Zhang, X.G. Zhu, Appl. Phys. Lett. 74 (20) (1999) 2951.
Structural and photoluminescence properties of thin alumina films on silicon, fabricated by electrochemistry M. Kokonou a, A.G. Nassiopoulou a,*, A. Travlos b a
IMEL/NCSR Demokritos, P.O.Box 60228, 15310 Aghia Paraskevi, Athens, Greece b Institute of Materials Science, NCSR Demokritos, Greece
Abstract Alumina thin films on a silicon substrate were fabricated by anodisation of Aluminium films, deposited by electron gun evaporation, in different acid aqueous solutions. The thickness of the initial Al film was changed in the range of 50 /500 nm and its composition was either pure aluminium or aluminium with 1% silicon. The structure and properties of the obtained alumina films were extensively investigated and they were found to depend strongly on the thickness and composition of the initial aluminium layer, as well as on the acid aqueous solution used. One important result obtained from the optical characterization of the alumina films was the very bright photoluminescence (PL), which also depended on the acid aqueous solution used and the alumina film thickness. The obtained results will be discussed in detail. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Silicon; Nanocrystals; Alumina; Anodization; Photoluminescence
1. Introduction Anodic alumina films have received significant attention in recent years due to their interesting dielectric, optical and other properties, which make them very attractive for various applications. The possibility to grow porous alumina with regular vertical pores of controlled sizes on a silicon substrate opens important new perspectives for potential applications in electronic, optoelectronic and magnetic nanodevices. In this respect, it is very essential to fully understand the mechanism and the kinetics of growth of the films and the different parameters which influence their structure (anodising conditions, anodisation solution, etc.). However, while a lot of work has been devoted to the anodisation of bulk aluminium in order to get an alumina film on its surface (see Ref. [1] and references therein), very little work has been published on alumina films on a silicon substrate [2 /7]. Different methods of film growth were in general used in the above references, * Corresponding author. Tel.: /3-10-6503-123; fax: /3-10-6511723. E-mail address: [email protected] (A.G. Nassiopoulou).
as for example reactive rf sputtering [4], low pressure metal/organic chemical vapor deposition [5,6], electron beam evaporation [7] or electrochemistry [2,3]. In this paper we investigate the growth of thin alumina films (thickness below 0.5 mm) on a silicon substrate by using anodisation in different acid aqueous solutions. The structure and properties of the obtained alumina films were found to depend on the electrochemical solution used and the thickness and composition of the initial film. The obtained films were measured for their photoluminescence (PL) properties, which also depended on film thickness and the anodisation conditions. The obtained results will be discussed below.
2. Experimental results and discussion 2.1. Sample preparation Al thin films were deposited on p-type (100) Si wafers using electron gun evaporation from a target composed either of pure Al or an alloy of Al(1% Si) [8]. Prior to Al deposition, the Si wafers were cleaned in an H2O2:H2SO4 solution and an ohmic contact was formed
0921-5107/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5107(02)00653-0
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on their backside. Al films of five different thicknesses, in the range of 15 /500 nm, were deposited and the samples were then anodised in different solutions at room temperature. Anodisation was carried out under constant dc voltage and three different acid aqueous solutions were used as follows: (a) in 10 wt.% sulfuric acid aqueous solution, under a constant voltage of 20 V, with the limit current density set at 100 mA cm 2; (b) in 6 wt.% oxalic acid aqueous solution, under a constant voltage of 50 V, with the limit current density set again at 100 mA cm 2; and (c) in 1 wt.% citric acid aqueous solution, with the value of the constant voltage depending in this case on the thickness of the Al films: 30 V for a thickness of 15 nm, 50 V for 30 nm, 50 or 70 V for 50 nm, 70 V for 100 nm and 100 V for 500 nm. The limit current density was set at 20 mA cm 2. During the process, the current density reached rapidly the limit value; it was stabilized at 20 mA cm 2 in sulfuric and oxalic acids, and at 5mA/cm2 in citric acid and it then
started to decrease. The voltage supply was intentionally cut when the current density reached the 10% of the stabilized value for one set of samples and when it reached the 25% of the stabilized value for another set of samples. After anodisation, the samples were rinsed in deionized water and dried in N2 gas. 2.2. TEM cross-sectional observation The samples were characterized systematically by transmission electron microscopy (TEM). Films anodised in the three different acid solutions (sulfuric, citric and oxalic acids) were examined. Other parameters that were changed included film thickness and the composition of the Al target material used in the electron gun evaporator. Either pure aluminium or an alloy of aluminium with 1% silicon was used. Fig. 1 shows an alumina film, about 120 nm thick, obtained by anodisation in sulfuric acid of an alumi-
Fig. 1. TEM images of porous nanocrystalline alumina film, about 120 nm thick, obtained by anodization of an Al (1% Si) film in 10% wt sulfuric acid. In (a) is a bright field micrograph, while in (b) a dark field image is shown, where we can identify silicon nanocrystals embedded in the nanocystalline alumina matrix.
Fig. 2. Dark field TEM images of an alumina film, obtained by anodization of a 50 nm thick Al (1% Si) film in 1 wt.% citric acid aquous solution. The alumina film is nanocrystalline and it is composed of Al2O3 (a) and Si (b) nanocrystals.
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Fig. 3. Polycrystalline, porous anodic alumina layer formed by anodization of a 500 nm thick Al(1% Si) layer in 6% wt oxalic acid aquous solution. In (a) a bright field micrograph is shown, where we see that the film is porous and nanocrystalline. The pores are randomly oriented. In (b) a dark field micrograph of the alumina nanocrystals is shown. Their size is in the range of 5 /15 nm. In (c) a dark field micrograph of silicon nanocrystals embedded in the porous alumina layer is shown. A very high density of silicon nanocrystals of uniform sizes in the range of 0.7 /1 nm are identified. They are uniformly distributed in the alumina matrix.
nium film, 50 nm thick, obtained from a target of Al(1% Si). Fig. 1a is a bright field micrograph, which shows that the obtained alumina film is porous and amorphous. The pores were randomly oriented. During TEM observation, some alumina nanocrystals were identified
67
in the amorphous matrix. Fig. 1b is a dark field image from the same area of the sample as in Fig. 1a, which shows the existence of some silicon nanocrystals embedded in the porous alumina layer. The same Al (1% Si) film as above, anodised in citric, instead of sulphuric acid, gives a film of quite different structural characteristics. The obtained alumina film was not in this case porous and it was composed of a mixture of Al2O3 (Fig. 2a) and Si (Fig. 2b) nanocrystals. Anodisation in oxalic acid of Al(1% Si) films gives also nanocrystalline alumina with silicon nanocrystals embedded in it. An example is given in Fig. 3, where the initial Al(1% Si) film was 500 nm thick. In (a) a bright field micrograph of the alumina film is shown. The film is porous and nanocrystalline and its thickness is /49 nm. In (b) and (c) we see dark field images of alumina and silicon nanocrystals, respectively. The size of the alumina nanocrystals is in the range of 5 /15 nm, while that of silicon nanocrystals is in the range of 0.7 /1 nm. The silicon nanocrystals are uniformly distributed in the alumina matrix and their density is surprisingly high (white spots in the micrograph of Fig. 3c). Anodisation of pure aluminium films, without any percentage of silicon, under the same conditions as those used in the anodisation of Al(1% Si), gives alumina films with very different structural characteristics. An example is shown in Fig. 4 for the case of anodisation in citric acid, of an Al film, 50 nm thick. Fig. 4a shows a bright field micrograph of the alumina film, which is nanocrystalline and porous. The pores are randomly distributed through the film. In contrast to the case of the Al(1% Si) film, in the case of the pure Al film, anodised under the same conditions as the Al (1% Si), it was difficult to anodise the whole Al layer, as shown in Fig. 4a, where the alumina film was 50 nm thick, while a remaining non-anodised Al film, 22 nm thick, is seen between the silicon substrate and the Al2O3 layer. In Fig. 4b a dark field image is shown, where we identify the Al2O3 nanocrystals. Anodisation of a pure Al film, 500 nm thick, in oxalic acid, gives an alumina film with very different structural characteristics than all the other films described above, as seen in Fig. 5. The film is in this case amorphous and it contains regular cylindrical pores, with a diameter of about 30 nm, grown vertically in the direction of film growth (perpendicular to the substrate). The pores lie on vertical pillars, formed on the silicon substrate in the same direction as the pores and composed of aluminium crystals. Between these pillars and the pores a nonporous layer is seen. The amorphous porous alumina layer is 560 nm thick. 2.3. Photoluminescence Photoluminescence spectra in the visible range (500 / 850 nm) were measured using a Jobin Yvon spectro-
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Fig. 4. Bright field (a) and dark field (b) TEM images of an alumina film obtained by anodization of a 50 nm thick pure Al film, without any Si, in 1% wt citric acid aquous solution. The alumina film is porous and nanocrystalline (crystal planes were identified during TEM observation, although nanocrystals were not clearly seen). A remaining Al film is also seen between silicon substrate and alumina.
meter. The excitation source was an Argon laser line at 457.9 nm and the excitation power varied from 0.16 to 22 mV. For an excitation power below 14 mW a chopper was used to cut off the noise. All measurements were carried out at room temperature. Samples before anodisation showed no photoluminescence (PL) in the above mentioned wavelength range. Samples anodised in sulfuric acid showed weak PL, while those anodised in citric and oxalic acids, showed a highly efficient broad PL band in the visible range, with a maximum in the range of 550/580 nm. This PL band was very stable with time. The thinner samples showed a maximum at smaller wavelengths than thicker samples. An example of PL spectra is shown in Fig. 6. In general, PL intensity was found to depend on the excitation power, the aluminium film thickness before anodisation and the composition of the initial aluminium film (pure or with 1% Si). Fig. 7 shows comparative PL spectra from alumina films from anodisation of either a pure
Fig. 5. Bright field micrograph of an alumina layer formed after anodisation of a 500 nm pure Al film, in 6% wt oxalic acid aquous solution. The film is porous and amorphous. The pores are cylindrical, their diameter is about 30 nm and they form an ordered array. At the interface with the silicon substrate the pores are interrupted by the amorphous material and Al cyllindrical pillars are seen on the silicon substrate in the direction of the pores.
aluminium film (open symbols, (a)) or a film of Al with 1% Si (dark symbols, (b)). We see that for thicker alumina films (thickness of few hundreds of nm), the alumina films from pure Al give PL of higher intensity compared to alumina films from Al (1% Si). This is
Fig. 6. PL spectra of an alumina film (a) obtained from anodization of a pure Al film anodized in 1% citric acid, for 5 different values of thickness (b) obtained from anodization of 3 50 nm Al (1% Si) films in three different acid aqueous solutions. The excitation was an Ar laser line at 457.9 nm and the excitation power was 20 mW.
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Fig. 7. PL spectra of alumina films, anodised in aqueous citric acid 1% wt. Spectra (a) correspond to alumina fabricated from anodisation of a pure Al thin film and (b) to alumina from anodisation of an alloy of Al (1% Si).
inversed in the case of alumina films which are thinner than 100 nm. This trend was systematically observed in all samples examined. The observed PL does not seem to have its origin in silicon nanocrystals embedded in the alumina matrix, as proposed by Ong and Zhu [9], although some contribution may be attributed to them. This is supported by the fact that the alumina films with silicon nanocrystals embedded in them show even weaker PL than without silicon nanocrystals when the film thickness is below 100 nm. PL from crystalline alumina is in general assigned to optical transitions from defect states [10], the most common being the F center (corresponding to an oxygen vacancy with one trapped electron). PL emission at 420 nm is obtained from alumina when an electron recombines with the F center. From porous crystalline alumina, fabricated by anodisation of an aluminium foil in 0.3 M oxalic acid (thickness of alumina in the range of 50 /100 nm) [11], a broad PL band in the 400/600 nm range has been obtained, with a maximum at about 450 nm, slightly red-shifted compared to the peak from bulk crystalline alumina. This red shift has been attributed to an increase in the density of oxygen vacancies in the alumina membrane, or to internal stresses at the interface with the remaining aluminium. The PL observed from our samples shows a maximum at about 560 nm, significantly red-shifted compared to the emission of bulk crystalline alumina. However we believe that the origin of the luminescence is the same in both cases.The observed red-shift may be explained if we take into account different factors, as for example the excitation wavelength used, which may limit optical pumping to one part of the luminescent band, the nanocrystalline nature of our alumina films, which causes band gap opening, and possible internal stresses within the material. A more detailed study is on going in order to elucidate these points.
3. Conclusion Aluminium thin films, either pure or containing 1% silicon, were deposited on a silicon substrate by electron gun evaporation and they were then anodised in different acid aqueous solutions (citric, oxalic or sulfuric) in order to obtain alumina thin films. The structure of the final film depended strongly on the initial aluminium film composition (pure Al or Al(1% Si)) and thickness, as well as on the anodisation solution. Anodisation of aluminium films of thickness higher than 0.5 mm in oxalic acid resulted in porous alumina with regular cylindrical pores in the direction of the anodisation current. Thinner aluminium films anodised under the same conditions resulted in nanocrystalline alumina. If the initial aluminium film contained 1% Si, a high density of silicon nanocrystals, embedded in the nanocrystalline alumina, was formed. If the anodisation solution was changed (citric or sulfuric instead of oxalic acid aqueous solution), the obtained alumina films were different. Very bright broad photoluminescence with a maximum at 550/580 nm was obtained from films anodised in oxalic or citric acid aqueous solutions, while samples anodised in sulfuric acid showed weaker PL. The intensity of the emitted PL depended strongly on film thickness. Further experiments are in progress in order to elucidate the origin of PL, which is most probably attributed to optical transitions involving the F defect state.
References [1] G.E. Thomson, Thin Solid Films 297 (1997) 192 /201. [2] J.H. Wu, X.L. Wu, N. Tang, Y.F. Mei, X.M. Bao, Appl. Phys. A 72 (2001) 735. [3] S. Lazarouk, S. Katsouba, A. Demianovich, V. Stanovski, S. Voitech, V. Vysotski, V. Ponomar, Microl. Eng. 50 (2000) 321.
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