- Email: [email protected]
Laser synthesis of nanostructures based on transition metal oxides
Applied Surface Science 252 (2006) 4449–4452 www.elsevier.com/locate/apsusc
Laser synthesis of nanostructures based on transition metal oxides S.A. Mulenko a,*, V.P. Mygashko b a
Institute for Metal Physics NAS of Ukraine, 36 Academician Vernadsky Blvd., UA-03680, Kiev 142, Ukraine b Kiev Taras Shevchenko University, Radiophysics Department, 2 Glushkov Blvd., UA-03127, Kiev 127, Ukraine Received 3 May 2005; accepted 11 July 2005
Abstract Nanostructures based on iron oxides in the form of thin films were synthesized while laser chemical vapor deposition (LCVD) of elements from iron carbonyl vapors (Fe(CO)5) under the action of Ar+ laser radiation (lL = 488 nm) on the Si substrate surface with power density about 102 W/ cm2 and vapor pressure 666 Pa. Analysis of surface morphology and relief of the deposited films was carried out with scanning electron microscopy (SEM) and atomic force microscopy (AFM). This analysis demonstrated their cluster structure with average size no more than 100 nm. It was found out that the thicker the deposited film, the larger sizes of clusters with more oxides of higher oxidized phases were formed. The film thickness (d) was 10 and 28 nm. The deposited films exhibited semiconductor properties in the range 170–340 K which were stipulated by oxide content with different oxidized phases. The width of the band gap Eg depends on oxide content in the deposited film and was varied in the range 0.30–0.64 eV at an electrical field of 1.6 103 V/m. The band gap Eg was varied in the range 0.46–0.58 eV at an electrical field of 45 V/m. The band gap which is stipulated by impurities in iron oxides Ei was varied in the range 0.009–0.026 eV at an electrical field of 1.6 103 V/m and was varied in the range 0–0.16 eV at an electrical field 45 V/m. These narrow band gap semiconductor thin films displayed of the quantum dimensional effect. # 2006 Published by Elsevier B.V. PACS: 81.16.Mk; 73.22f Keywords: Laser deposition; Thin films; Quantum dimensional effect
1. Introduction It is known [1–4], the transition metal oxides such as Fe2 O3x (0 x 1), MoOx (2 x 3) exhibit semiconductor, electrochromic and photochromic properties. These properties make these oxides as functional materials for optoelectronics devices. In the present work, the laser chemical vapor deposition (LCVD) method was used for the synthesis of narrow band gap semiconductor thin films based on iron oxides. As it was shown [5,6], iron oxides were formed while oxidizing iron atoms with atomic oxygen released due to the dissociative chemisorption process of CO molecules adsorbed on Fe surface. So it is needed to find out the influence of oxide content
* Corresponding author. Tel.: +380 44 424 10 05; fax: +380 44 424 25 61. E-mail address: [email protected] (S.A. Mulenko). 0169-4332/$ – see front matter # 2006 Published by Elsevier B.V. doi:10.1016/j.apsusc.2005.07.110
of deposited films and their morphology on electrical properties of these films. 2. Experimental The Ar+ laser tuned at 488 nm line was used in LCVD method for film deposition from iron carbonyl (Fe(CO)5) vapors on the Si substrate surface. Thin films with the thickness (d) 28 nm and less were deposited in our experiment. The reactor with the Si substrate was pumped down to 0.133 Pa before filling it with iron carbonyl vapors. The laser radiation was focused on the Si substrate surface with a laser power density of about 102 W/cm2 during 10 min and less. The vapor pressure was 666 Pa while film deposition. The maximum possible heating of the Si substrate surface was about 2 K in local point when exposure time (texp) was longer than 10 min at film thickness more than 28 nm. The substrate temperature was
4450
S.A. Mulenko, V.P. Mygashko / Applied Surface Science 252 (2006) 4449–4452
measured by thermocouple while laser irradiation of the Si substrate surface. Removing the reactor with the Si substrate under the laser beam gave the possibility to obtain the homogeneous film with the size 0.6 mm 6.0 mm. The thickness of deposited films was measured by ion etching with steps of 1 nm and optical interference microscopy method. Maximum growth rate of film thickness was about 8 102 nm/s and it decreased to 102 nm/s when the increasing of exposure time was up to 20 min. The morphology of the deposited film was studied by scanning electron microscopy (SEM), using a JEM 2000 FX II apparatus. The content analysis of the deposited film was carried out by auger electron spectroscopy (AES) using a JEOL JAMP-10S AUGER MICROPROBE apparatus. This analysis revealed the presence of iron (Fe), carbon (C) and oxygen (O) along the film thickness with steps of 1 nm depth profiling. The relief of the deposited film was investigated by atomic force microscopy (AFM), using a Nanoscope ‘‘Dimention 3000’’ in tapping mode. The direct current (dc) electrical resistance of the Si sample and of the Si samples with the deposited films was measured by using a two-probe technique. A silver coating formed ohmic contacts with samples. Temperature measurements of the electrical resistance of samples were carried out in the ranges 340–170 and 290–220 K. From these data, the geometrical size of the sample and of the deposited film, the specific conductivity (s) was calculated in the above temperature range. 3. Results As it was shown [5,6], the mechanism of Fe(CO)5 molecules dissociation under the action of Ar+ laser radiation was photochemical in conditions of our experiment. The photon energy of Ar+ laser radiation is enough to break no more than two Fe–C bonds in Fe(CO)5 molecules [7]. Therefore, the primary process of photochemical dissociation of Fe(CO)5 molecules under the action of Ar+ laser radiation will be the following: FeðCOÞ5 þ Ehn ! FeðCOÞ3 þ 2CO:
Fig. 1. Morphology of the deposited film from Fe(CO)5 vapors on the Si substrate surface under the action of Ar+ laser radiation: (a) texp ffi 3 min, d ffi 10 nm; (b) texp ffi 10 min, d ffi 28 nm.
The longer the exposure time of the Si substrate surface to Ar+ laser radiation, the more oxides with higher oxidized phases were formed in the deposited film. It was proved by Xray photoelectron spectroscopy (XPS) analysis of the deposited film that the increasing of exposure time from 2 min to 8 min resulted in a shift of Fe 2p3/2 peak towards higher binding energy: from 709.9 to 711.5 eV [10]. The SEM analysis of the deposited film surface demonstrated that its cluster structure had an average size less than 100 nm and depended upon exposure time of the Si substrate surface
(1)
Atomic iron could be formed only in secondary process as the result of the interaction of radicals Fe(CO)3 with precursor molecules Fe(CO)5 on the Si substrate surface. Atomic iron is very active and can be oxidized very easy with atomic oxygen to have been released from the dissociation of CO molecules. Under the catalytic action of iron atoms on CO molecules adsorbed on Fe, C–O binding energy reduces from 11 eV in gas phase to 4.4 eV, due to the dissociative chemisorption process [8,9] and even to 1.0 eV [6]. Therefore, the dissociation process of adsorbed CO molecules is photochemical under the action of Ar+ laser radiation in conditions of this experiment and can be expressed by the following process: xCO þ nhnL ! xC þ xO;
(2)
where x = 8 for Fe(CO) decomposition on the Si substrate surface; n = 2, 1 is a number of Ar+ laser photons. Atomic iron was oxidized with atomic oxygen up to Fe2O3x (0 x 1).
Fig. 2. Atomic force microscopy made for 10 nm film thickness: scan size is 1.0 mm; data scale (height) is 20.0 nm.
S.A. Mulenko, V.P. Mygashko / Applied Surface Science 252 (2006) 4449–4452
4451
field of 1.6 103 V/m. The specific conductivity for the 10 nm thick film varied in the range 16.5–2.0 V1 cm1 at the same temperature range and at the same electrical field. All determined values in expression (3) at different applied electrical field for two films with different thickness are presented in Table 1. 4. Discussion From experimental data to have been obtained for two samples with different thickness one can see the influence of exposure time of the substrate surface to laser radiation. The longer exposure time of the substrate surface to laser radiation, the thicker the deposited film with more iron oxides with higher oxidized phases was being formed. Stoichiometry of formed iron oxides can be expressed as Fe2O3x (0 x 1). So the width of the band gap Eg depends upon oxides content in the deposited film. The more iron oxides with higher oxidized phases in the deposited film, the more width of the band gap Eg for the obtained semiconductor materials at two different applied electrical fields. As it is seen from experimental data, the more applied electrical field, the more value of the specific conductivity for films with different thickness. From the other hand the width of the band gap Ei depends upon the film thickness too. But in this case the thicker the deposited film, the less the width of the band gap Ei at two different applied electrical fields. The specific conductivity depends upon the surface morphology of the deposited film. As it is seen from Figs. 1a and b and 3, the larger an average size of clusters, the more specific conductivity of the deposited film can be reached. While heating, the specific conductivity of the film with lower thickness and smaller average size of clusters is lower than for the film with larger thickness, due to the increasing of the electron–phonon scattering for the film with more metallic properties. Inversely proportional dependence of the width of the band gap Ei upon the film thickness raised to the second power at different applied electrical fields is the display of the quantum dimensional effect for thin films of narrow band gap semiconductors. The kinetic energy of charge carriers is quantized along the normal (z) to the substrate surface in the limit of the film thickness and can be expressed by the following relation [12]:
Fig. 3. The temperature dependence of the specific conductivity (s) of the deposited film at an electrical field of 45 V/m: (1) d ffi 28 nm and (2) d ffi 10 nm; (1) and (2) are experimental dots; solid lines are theoretical lines.
(Fig. 1a and b). The relief of the deposited film made by AFM is shown in Fig. 2. As it is seen in Fig. 3, the temperature dependence of the specific conductivity for two films with thickness of 28 and 10 nm shows a typical semiconductor behavior. As it is known [11], s is inversely proportional to an effective mass (m*) of charge carriers. This effective mass is a function of applied electrical field. But there is a range of an electrical field where this mass does not depend upon this field. The range of this electrical field for our samples was less than 90 V/m. So the specific conductivity was measured at two meanings of an electrical field, i.e. at 45 and 1.6 103 V/m. The temperature dependence of the specific conductivity can be approximated by the following expression: Eg Ei s ¼ s g exp þ s i exp ; (3) 2kT kT where Eg is the band gap of iron oxides for intrinsic conductivity and Ei is the band gap assigned for impurities in iron oxides such as iron atoms. The specific conductivity for the 28 nm thick film varied in the range 2.3–0.5 V1 cm1 when the temperature was varied in the range 290–220 K at an electrical field of 45 V/m (Fig. 3(1)). The specific conductivity for the 10 nm thick film varied in the range 0.42– 0.05 V1 cm1 at the same temperature range and at the same electrical field (Fig. 3(2)). The specific conductivity for the 28 nm thick film varied in the range 25–2.0 V1 cm1 when the temperature was varied in the range 340–170 K at an electrical
EzðnÞ ¼
ðnhÞ2 ; 8mz d2
(4)
where n = 1, 2, 3, . . . and h is the Planck constant. The display of dimensional quantization is the increasing of the band gap Ei
Table 1 Determined values (Eg, Ei, sg, si) for two films at different applied electrical field Applied electrical field (V/m)
Film thickness (nm) 28
1.6 103 4.5 10
10 1
Eg (eV)
Ei (eV)
sg (V
0.64 0.58
0.009 0.000
1.14 106 1.9 105
1
cm )
si (V 3.2 0.5
1
1
cm )
Eg (eV)
Ei (eV)
sg (V1 cm1)
si (V1 cm1)
0.30 0.46
0.026 0.16
1.6 10 3 3.8 10 3
10 24
4452
S.A. Mulenko, V.P. Mygashko / Applied Surface Science 252 (2006) 4449–4452
of the thin film while decreasing its thickness. Moreover, the display of the quantum dimensional effect is sharper at lower applied electrical field owing to lower broadening by an electrical field of energy levels stipulated with impurities.
Acknowledgement
5. Conclusions
References
The specific conductivity of thin films deposited from Fe(CO)5 vapors which based on iron oxides with stoichiometric content Fe2O3x (0 x 1) demonstrated typical semiconductor temperature trend with the width of the band gap for intrinsic conductivity Eg less than 1.0 eV. The more oxides with higher oxidized phases in the deposited film, the more the width of the band gap Eg at different applied electrical field. The display of the quantum dimensional effect is sharper at lower applied electrical field, i.e. the lower applied electrical field, the more the band gap assigned for impurities in iron oxides Ei. Electrical properties of deposited films from Fe(CO)5 vapors depend upon their cluster structure. The more size of clusters, the more the specific conductivity of semiconductor films based on iron oxides at the temperature about 300 K. It was shown that LCVD method is up-to-date method for the synthesis of narrow band gap semiconductor thin films which can be proposed for IR detectors and sensors.
This work was supported in part by INTAS grant (Project No. 04-78-7124).
[1] S.A. Mulenko, V.S. Ovechko, Proc. EUROPTO, SPIE 3822 (1999) 46. [2] A. Donnadien, D. Davazoglou, A. Abdellaoui, Thin Solid Films 164 (1988) 333. [3] C. Julien, A. Khelfa, O.M. Hussain, G.A. Nazri, J. Cryst. Growth 156 (1995) 235. [4] Z. Hassain, J. Mater. Res. 16 (2001) 2695. [5] S.A. Mulenko, Proc. SPIE 5121 (2003) 280. [6] S.A. Mulenko, Proc. SPIE 5399 (2004) 171. [7] J.T. Yardley, B. Gitlin, G. Natanson, A.M. Rosan, J. Chem. Phys. 74 (1981) 370. [8] D.W. Moon, S.L. Bernasck, J.P. Lu, J.L. Gland, D.J. Dwyer, Surf. Sci. 184 (1987) 99. [9] X. Xu, J.I. Steinfeld, Appl. Surf. Sci. 45 (1990) 281. [10] I. Voicu, R. Alexandrescu, R. Cireasa, I. Morjan, D.C. Dumitras, S. Mulenko, A. Pogorelyi, A. Andrei, J. Phys. IV 5 (1995) 679. [11] C.W. Wert, R.M. Thomson, Physics of Solids, McGraw-Hill Book Company, New York, 1964. [12] V.F. Kiselev, S.N. Kozlov, A.V. Zoteev, Osnovy Fiziki Poverhnosti Tverdogo Tela, M. Moskovski Universitet, 1999.
Laser synthesis of nanostructures based on transition metal oxides S.A. Mulenko a,*, V.P. Mygashko b a
Institute for Metal Physics NAS of Ukraine, 36 Academician Vernadsky Blvd., UA-03680, Kiev 142, Ukraine b Kiev Taras Shevchenko University, Radiophysics Department, 2 Glushkov Blvd., UA-03127, Kiev 127, Ukraine Received 3 May 2005; accepted 11 July 2005
Abstract Nanostructures based on iron oxides in the form of thin films were synthesized while laser chemical vapor deposition (LCVD) of elements from iron carbonyl vapors (Fe(CO)5) under the action of Ar+ laser radiation (lL = 488 nm) on the Si substrate surface with power density about 102 W/ cm2 and vapor pressure 666 Pa. Analysis of surface morphology and relief of the deposited films was carried out with scanning electron microscopy (SEM) and atomic force microscopy (AFM). This analysis demonstrated their cluster structure with average size no more than 100 nm. It was found out that the thicker the deposited film, the larger sizes of clusters with more oxides of higher oxidized phases were formed. The film thickness (d) was 10 and 28 nm. The deposited films exhibited semiconductor properties in the range 170–340 K which were stipulated by oxide content with different oxidized phases. The width of the band gap Eg depends on oxide content in the deposited film and was varied in the range 0.30–0.64 eV at an electrical field of 1.6 103 V/m. The band gap Eg was varied in the range 0.46–0.58 eV at an electrical field of 45 V/m. The band gap which is stipulated by impurities in iron oxides Ei was varied in the range 0.009–0.026 eV at an electrical field of 1.6 103 V/m and was varied in the range 0–0.16 eV at an electrical field 45 V/m. These narrow band gap semiconductor thin films displayed of the quantum dimensional effect. # 2006 Published by Elsevier B.V. PACS: 81.16.Mk; 73.22f Keywords: Laser deposition; Thin films; Quantum dimensional effect
1. Introduction It is known [1–4], the transition metal oxides such as Fe2 O3x (0 x 1), MoOx (2 x 3) exhibit semiconductor, electrochromic and photochromic properties. These properties make these oxides as functional materials for optoelectronics devices. In the present work, the laser chemical vapor deposition (LCVD) method was used for the synthesis of narrow band gap semiconductor thin films based on iron oxides. As it was shown [5,6], iron oxides were formed while oxidizing iron atoms with atomic oxygen released due to the dissociative chemisorption process of CO molecules adsorbed on Fe surface. So it is needed to find out the influence of oxide content
* Corresponding author. Tel.: +380 44 424 10 05; fax: +380 44 424 25 61. E-mail address: [email protected] (S.A. Mulenko). 0169-4332/$ – see front matter # 2006 Published by Elsevier B.V. doi:10.1016/j.apsusc.2005.07.110
of deposited films and their morphology on electrical properties of these films. 2. Experimental The Ar+ laser tuned at 488 nm line was used in LCVD method for film deposition from iron carbonyl (Fe(CO)5) vapors on the Si substrate surface. Thin films with the thickness (d) 28 nm and less were deposited in our experiment. The reactor with the Si substrate was pumped down to 0.133 Pa before filling it with iron carbonyl vapors. The laser radiation was focused on the Si substrate surface with a laser power density of about 102 W/cm2 during 10 min and less. The vapor pressure was 666 Pa while film deposition. The maximum possible heating of the Si substrate surface was about 2 K in local point when exposure time (texp) was longer than 10 min at film thickness more than 28 nm. The substrate temperature was
4450
S.A. Mulenko, V.P. Mygashko / Applied Surface Science 252 (2006) 4449–4452
measured by thermocouple while laser irradiation of the Si substrate surface. Removing the reactor with the Si substrate under the laser beam gave the possibility to obtain the homogeneous film with the size 0.6 mm 6.0 mm. The thickness of deposited films was measured by ion etching with steps of 1 nm and optical interference microscopy method. Maximum growth rate of film thickness was about 8 102 nm/s and it decreased to 102 nm/s when the increasing of exposure time was up to 20 min. The morphology of the deposited film was studied by scanning electron microscopy (SEM), using a JEM 2000 FX II apparatus. The content analysis of the deposited film was carried out by auger electron spectroscopy (AES) using a JEOL JAMP-10S AUGER MICROPROBE apparatus. This analysis revealed the presence of iron (Fe), carbon (C) and oxygen (O) along the film thickness with steps of 1 nm depth profiling. The relief of the deposited film was investigated by atomic force microscopy (AFM), using a Nanoscope ‘‘Dimention 3000’’ in tapping mode. The direct current (dc) electrical resistance of the Si sample and of the Si samples with the deposited films was measured by using a two-probe technique. A silver coating formed ohmic contacts with samples. Temperature measurements of the electrical resistance of samples were carried out in the ranges 340–170 and 290–220 K. From these data, the geometrical size of the sample and of the deposited film, the specific conductivity (s) was calculated in the above temperature range. 3. Results As it was shown [5,6], the mechanism of Fe(CO)5 molecules dissociation under the action of Ar+ laser radiation was photochemical in conditions of our experiment. The photon energy of Ar+ laser radiation is enough to break no more than two Fe–C bonds in Fe(CO)5 molecules [7]. Therefore, the primary process of photochemical dissociation of Fe(CO)5 molecules under the action of Ar+ laser radiation will be the following: FeðCOÞ5 þ Ehn ! FeðCOÞ3 þ 2CO:
Fig. 1. Morphology of the deposited film from Fe(CO)5 vapors on the Si substrate surface under the action of Ar+ laser radiation: (a) texp ffi 3 min, d ffi 10 nm; (b) texp ffi 10 min, d ffi 28 nm.
The longer the exposure time of the Si substrate surface to Ar+ laser radiation, the more oxides with higher oxidized phases were formed in the deposited film. It was proved by Xray photoelectron spectroscopy (XPS) analysis of the deposited film that the increasing of exposure time from 2 min to 8 min resulted in a shift of Fe 2p3/2 peak towards higher binding energy: from 709.9 to 711.5 eV [10]. The SEM analysis of the deposited film surface demonstrated that its cluster structure had an average size less than 100 nm and depended upon exposure time of the Si substrate surface
(1)
Atomic iron could be formed only in secondary process as the result of the interaction of radicals Fe(CO)3 with precursor molecules Fe(CO)5 on the Si substrate surface. Atomic iron is very active and can be oxidized very easy with atomic oxygen to have been released from the dissociation of CO molecules. Under the catalytic action of iron atoms on CO molecules adsorbed on Fe, C–O binding energy reduces from 11 eV in gas phase to 4.4 eV, due to the dissociative chemisorption process [8,9] and even to 1.0 eV [6]. Therefore, the dissociation process of adsorbed CO molecules is photochemical under the action of Ar+ laser radiation in conditions of this experiment and can be expressed by the following process: xCO þ nhnL ! xC þ xO;
(2)
where x = 8 for Fe(CO) decomposition on the Si substrate surface; n = 2, 1 is a number of Ar+ laser photons. Atomic iron was oxidized with atomic oxygen up to Fe2O3x (0 x 1).
Fig. 2. Atomic force microscopy made for 10 nm film thickness: scan size is 1.0 mm; data scale (height) is 20.0 nm.
S.A. Mulenko, V.P. Mygashko / Applied Surface Science 252 (2006) 4449–4452
4451
field of 1.6 103 V/m. The specific conductivity for the 10 nm thick film varied in the range 16.5–2.0 V1 cm1 at the same temperature range and at the same electrical field. All determined values in expression (3) at different applied electrical field for two films with different thickness are presented in Table 1. 4. Discussion From experimental data to have been obtained for two samples with different thickness one can see the influence of exposure time of the substrate surface to laser radiation. The longer exposure time of the substrate surface to laser radiation, the thicker the deposited film with more iron oxides with higher oxidized phases was being formed. Stoichiometry of formed iron oxides can be expressed as Fe2O3x (0 x 1). So the width of the band gap Eg depends upon oxides content in the deposited film. The more iron oxides with higher oxidized phases in the deposited film, the more width of the band gap Eg for the obtained semiconductor materials at two different applied electrical fields. As it is seen from experimental data, the more applied electrical field, the more value of the specific conductivity for films with different thickness. From the other hand the width of the band gap Ei depends upon the film thickness too. But in this case the thicker the deposited film, the less the width of the band gap Ei at two different applied electrical fields. The specific conductivity depends upon the surface morphology of the deposited film. As it is seen from Figs. 1a and b and 3, the larger an average size of clusters, the more specific conductivity of the deposited film can be reached. While heating, the specific conductivity of the film with lower thickness and smaller average size of clusters is lower than for the film with larger thickness, due to the increasing of the electron–phonon scattering for the film with more metallic properties. Inversely proportional dependence of the width of the band gap Ei upon the film thickness raised to the second power at different applied electrical fields is the display of the quantum dimensional effect for thin films of narrow band gap semiconductors. The kinetic energy of charge carriers is quantized along the normal (z) to the substrate surface in the limit of the film thickness and can be expressed by the following relation [12]:
Fig. 3. The temperature dependence of the specific conductivity (s) of the deposited film at an electrical field of 45 V/m: (1) d ffi 28 nm and (2) d ffi 10 nm; (1) and (2) are experimental dots; solid lines are theoretical lines.
(Fig. 1a and b). The relief of the deposited film made by AFM is shown in Fig. 2. As it is seen in Fig. 3, the temperature dependence of the specific conductivity for two films with thickness of 28 and 10 nm shows a typical semiconductor behavior. As it is known [11], s is inversely proportional to an effective mass (m*) of charge carriers. This effective mass is a function of applied electrical field. But there is a range of an electrical field where this mass does not depend upon this field. The range of this electrical field for our samples was less than 90 V/m. So the specific conductivity was measured at two meanings of an electrical field, i.e. at 45 and 1.6 103 V/m. The temperature dependence of the specific conductivity can be approximated by the following expression: Eg Ei s ¼ s g exp þ s i exp ; (3) 2kT kT where Eg is the band gap of iron oxides for intrinsic conductivity and Ei is the band gap assigned for impurities in iron oxides such as iron atoms. The specific conductivity for the 28 nm thick film varied in the range 2.3–0.5 V1 cm1 when the temperature was varied in the range 290–220 K at an electrical field of 45 V/m (Fig. 3(1)). The specific conductivity for the 10 nm thick film varied in the range 0.42– 0.05 V1 cm1 at the same temperature range and at the same electrical field (Fig. 3(2)). The specific conductivity for the 28 nm thick film varied in the range 25–2.0 V1 cm1 when the temperature was varied in the range 340–170 K at an electrical
EzðnÞ ¼
ðnhÞ2 ; 8mz d2
(4)
where n = 1, 2, 3, . . . and h is the Planck constant. The display of dimensional quantization is the increasing of the band gap Ei
Table 1 Determined values (Eg, Ei, sg, si) for two films at different applied electrical field Applied electrical field (V/m)
Film thickness (nm) 28
1.6 103 4.5 10
10 1
Eg (eV)
Ei (eV)
sg (V
0.64 0.58
0.009 0.000
1.14 106 1.9 105
1
cm )
si (V 3.2 0.5
1
1
cm )
Eg (eV)
Ei (eV)
sg (V1 cm1)
si (V1 cm1)
0.30 0.46
0.026 0.16
1.6 10 3 3.8 10 3
10 24
4452
S.A. Mulenko, V.P. Mygashko / Applied Surface Science 252 (2006) 4449–4452
of the thin film while decreasing its thickness. Moreover, the display of the quantum dimensional effect is sharper at lower applied electrical field owing to lower broadening by an electrical field of energy levels stipulated with impurities.
Acknowledgement
5. Conclusions
References
The specific conductivity of thin films deposited from Fe(CO)5 vapors which based on iron oxides with stoichiometric content Fe2O3x (0 x 1) demonstrated typical semiconductor temperature trend with the width of the band gap for intrinsic conductivity Eg less than 1.0 eV. The more oxides with higher oxidized phases in the deposited film, the more the width of the band gap Eg at different applied electrical field. The display of the quantum dimensional effect is sharper at lower applied electrical field, i.e. the lower applied electrical field, the more the band gap assigned for impurities in iron oxides Ei. Electrical properties of deposited films from Fe(CO)5 vapors depend upon their cluster structure. The more size of clusters, the more the specific conductivity of semiconductor films based on iron oxides at the temperature about 300 K. It was shown that LCVD method is up-to-date method for the synthesis of narrow band gap semiconductor thin films which can be proposed for IR detectors and sensors.
This work was supported in part by INTAS grant (Project No. 04-78-7124).
[1] S.A. Mulenko, V.S. Ovechko, Proc. EUROPTO, SPIE 3822 (1999) 46. [2] A. Donnadien, D. Davazoglou, A. Abdellaoui, Thin Solid Films 164 (1988) 333. [3] C. Julien, A. Khelfa, O.M. Hussain, G.A. Nazri, J. Cryst. Growth 156 (1995) 235. [4] Z. Hassain, J. Mater. Res. 16 (2001) 2695. [5] S.A. Mulenko, Proc. SPIE 5121 (2003) 280. [6] S.A. Mulenko, Proc. SPIE 5399 (2004) 171. [7] J.T. Yardley, B. Gitlin, G. Natanson, A.M. Rosan, J. Chem. Phys. 74 (1981) 370. [8] D.W. Moon, S.L. Bernasck, J.P. Lu, J.L. Gland, D.J. Dwyer, Surf. Sci. 184 (1987) 99. [9] X. Xu, J.I. Steinfeld, Appl. Surf. Sci. 45 (1990) 281. [10] I. Voicu, R. Alexandrescu, R. Cireasa, I. Morjan, D.C. Dumitras, S. Mulenko, A. Pogorelyi, A. Andrei, J. Phys. IV 5 (1995) 679. [11] C.W. Wert, R.M. Thomson, Physics of Solids, McGraw-Hill Book Company, New York, 1964. [12] V.F. Kiselev, S.N. Kozlov, A.V. Zoteev, Osnovy Fiziki Poverhnosti Tverdogo Tela, M. Moskovski Universitet, 1999.