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Molecular beam evaporation-grown indium oxide and indium aluminium films for low-temperature gas sensors
Sensors and Actuators B 66 Ž2000. 85–87 www.elsevier.nlrlocatersensorb
Molecular beam evaporation-grown indium oxide and indium aluminium films for low-temperature gas sensors R. Winter ) , K. Scharnagl, A. Fuchs, T. Doll, I. Eisele UniÕersitat Fakultat ¨ der Bundeswehr Munchen, ¨ ¨ f ur ¨ Elektrotechnik ET9, Institut f ur ¨ Physik, Werner-Heisenberg-Weg 39, D-85577 Neubiberg, Germany Accepted 22 July 1999
Abstract Indium oxide and indium aluminium oxide thin films for low-temperature work function sensors were grown on silicon substrates by molecular beam evaporation ŽMBE.. Depending on the substrate temperature and the oxygen partial pressure during film growth, different stoichiometry and morphology were obtained. After depositing, the films were annealed in oxygen. The films were characterized by AES, RBS, XRD and SEM. The stoichiometry of the layers varies between pure In 2 O 3 and a composition of pure indium and In 2 O 3. All the films are polycrystalline with a grain size up to 300 nm. Thicker films form a porous layer. The indium aluminium oxide films show no crystalline phases. Both as-grown and annealed films were tested for NO 2 , CO, CO 2 , NH 3 , Cl 2 and O 3 at room temperature and 1308C by Kelvin probe measurements in dry and humid air. In 2 O 3 films show a high sensitivity to NO 2 , CO and CO 2 . A higher sensitivity for the porous layers is obtained due to the greater surface. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Indium oxide; Work function; Thin films
1. Introduction In 2 O 3 films became an interesting material for sensor application during the last years. Films were deposited by sputtering w1,2x techniques or by sol–gel-deposition w3,4x. Molecular beam evaporation ŽMBE. is a new approach for thin film deposition of indium oxide films which offers the opportunity to control the stoichiometry and morphology by varying the process parameters. Furthermore, one can deposit doped oxide films by coevaporation.
2. Experimental 2.1. Sample preparation The films were grown on ²100:-pq-silicon substrates by MBE in an UHV-chamber. The natural oxide on the
) Corresponding author. Fax: [email protected]
q49-89-6004-3877;
e-mail:
silicon was removed by thermal desorption at 9008C for 5 min. High-purity indium Ž99.995%. was evaporated with an electron beam evaporater ŽLeybold ESV2.. Aluminium was deposited by thermal evaporation. The oxygen partial pressure during the deposition was either 1.5 = 10y6 or 1.5 = 10y5 mbar. The substrate temperature ranged from room temperature up to 5008C. After deposition, the films were studied by AES, RBS, XRD and SEM. The substrate temperature during film deposition influences the morphology of the layers. Films grown at room temperature are smooth, whereas above 2008C, one gets a grainy film. The grain sizes range from 100 nm for 2008C substrate temperature to 300 nm for 5008C ŽFigs. 1 and 2.. Thick films Ž) 500 nm. grown at higher temperatures form a porous film. Indium has three stable oxides: In 2 O, InO and In 2 O 3 w5x. In 2 O is not stable in ultrahigh vacuum. The stoichiometry was determined by AES and RBS. All three oxides were found, but only two phases were detected by XRD: tetragonal indium and cubic In 2 O 3 . The stoichiometry of the films is mainly influenced by the oxygen partial pressure during deposition: for the lower oxygen partial pressure, one gets irregular crystallized indium with some
0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 9 9 . 0 0 2 9 8 - 1
R. Winter et al.r Sensors and Actuators B 66 (2000) 85–87
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Fig. 1. SEM-picture of In 2 O 3 films grown at 3008C. Fig. 3. CPD for indium oxide at 1308C in dry air.
In 2 O 3 , whereas samples prepared with the higher oxygen partial pressure consist of nearly pure In 2 O 3 . The crystallinity is better for films grown at lower temperatures. After annealing in oxygen at 8008C for 2 h, all samples consist of In 2 O 3 . The Al x In yO films shows now crystalline phases. 2.2. Gas measurements
Furthermore, ozone measurements were carried out at 50, 100 and 300 ppb O 3 .
3. Results and discussion 3.1. Indium oxide
The work function measurements were carried out in a continuous flow gas mix system by Kelvin probe measurements. The reference electrode was the gold electrode of the Kelvin probe. The difference between the work function of the films and the gold electrode, the contact potential difference ŽCPD., was recorded by a computer. The samples were mounted on a heater and measured at 308C and 1308C. Before the measurements, the samples were exposed to synthetic air for 2 h. Afterwards, the samples were exposed three times to a measurement cycle of 40 min test gas and 40 min synthetic air for each test gas. Test gases were 100 ppm NO 2 , 1000 ppm CO, 5000 ppm CO 2 , 100 ppm SO 2 , 70 ppm NH 3 and 10 ppm Cl 2 .
3.1.1. Influence of the morphology Smooth, grainy and porous In 2 O 3 films with grain sizes from 100 to 300 nm were examined. The films are sensitive to NO 2 , CO, CO 2 and SO 2 at 308C and 1308C and to Cl 2 at 1308C. The sensitivity to NH 3 is low ŽFig. 3.. The porous films show a higher sensitivity to all test gases except NH 3 . The increase of the CPD could be explained with the greater surface of these films. A possible explanation for the opposite behaviour to NH 3 is an over-compensation effect of the Kelvin probe gold electrode. For NH 3 , one would expect a negative CPD change. If the work function change of the film is smaller than of the reference electrode, the resulting signal is positive and an increase of the work function change of the film leads to a
Fig. 2. SEM-picture of In 2 O 3 films grown at 5008C.
Fig. 4. CPD for indium oxide with elemental indium at 308C in dry air.
R. Winter et al.r Sensors and Actuators B 66 (2000) 85–87
87
morphology and stoichiometry of the films. Increasing the surface of the indium oxide Žgrainy or porous films. leads to an unspecific increase in signal heights. The sensitivity at room temperature can be improved by elemental indium in the films. A very interesting feature of the MBE technique is the possibility to add other metals to the layers in different controllable amounts. Doping the layers with aluminium leads to a significant lowering of the cross sensitivity to chlorine, whereas the sensitivities to the other test gases are not affected.
References Fig. 5. Influence of the aluminium content of In x Al yO to the contact potential difference at 1308C in dry air.
smaller CPD change. The difference in signal heights between smooth and grainy films is small. 3.1.2. Influence of the stoichiometry Films with different amount of elemental indium were studied. At room temperature, the films with elemental indium show higher signals to all test gases except NH 3 . Especially the sensitivity to Cl 2 and SO 2 is increased. Fig. 4 shows this effect for a smooth film of 100 nm thickness. The effect is more significant for films with higher indium content. The higher sensitivity is possibly caused by catalytic activation on the elemental indium. At higher temperatures Ž1308C., these films were not stable. 3.2. Indium aluminium oxide Indium aluminium oxide films with different aluminium content were tested at room temperature. The aluminium content mainly influences the sensitivity to Cl 2 . For higher aluminium content, the sensitivity to chlorine is suppressed. No significant change of the sensitivity to NO 2 , CO, CO 2 , SO 2 and NH 3 can be observed. Fig. 5 shows the results for two 100 nm films. Compared to pure indium oxide films and indium oxide films with elemental indium, the sensitivities to CO, CO 2 and SO 2 are lowered. 3.3. Ozone measurements Indium oxide films were tested for ozone at room temperature in dry and humid air. The CPD for 300 ppb O 3 is 170 mV. No differences in signal heights for dry and humid air Ž30% r.h.. were observed. The desorption times are in the range of a few hours and increase in dry air.
4. Conclusions MBE-grown indium oxide and indium aluminium oxide films offer a wide range to influence the sensitivity by
w1x I. Hamberg, C.G. Granqvist, Evaporated Sn-doped In 2 O 3 films: basic optical properties and applications to energy efficient windows, Journal of Applied Physics 60 Ž11. Ž1986. R123–R159. w2x T. Doll, A. Fuchs, I. Eisele, S. Gropelli, G. Sberveglieri, Conductivity and work function ozone sensors based on indium oxide, Sensors and Actuators, B 49 Ž1998. 63–67. w3x A. Gurlo, M. Ivanovskaya, A. Pfau, U. Weimar, W. Gopel, Sol–gel ¨ prepared In 2 O 3 thin films, Thin Solid Films 307 Ž1997. 288–293. w4x W. Wlodarski, H.-T. Sun, A. Gurlo, W. Gopel, Sol–gel prepared ¨ In 2 O 3 thin films for ozone sensing, Proceedings of Transducers ŽChicago. 97 Ž1997. 573–576. w5x Gmelin’s Handbook of Inorganic Chemistry, Vol. 37, Springer Verlag, Berlin, 1974, p. 64.
Biographies
Ronny Winter received his diploma in physics from the Universitat ¨ Leipzig in the field of high-temperature superconductivity in 1994. He is currently working towards his PhD thesis at the Universitat ¨ der Bundeswehr Munchen. His interests include chemical sensors based on metal ¨ oxides. Klaus Scharnagl obtained his diploma in physics from the Technische Universitat in 1998. Now he is working towards his PhD thesis ¨ Munchen ¨ at the Universitat His subjects include chemi¨ der Bundeswehr Munchen. ¨ cal sensors especially for H 2 -sensing. Alexander Fuchs received his diploma in physics from the Technische Universitat in 1994. He is currently working towards his PhD ¨ Munchen ¨ thesis at the Universitat in the field of mi¨ der Bundeswehr Munchen ¨ crosystem technology and chemical sensors. Theodor Doll studied music and physics in Munich and was then at KWS, Germany in R&D of environmental diagnostic systems. In 1991, he went to the Universitat where he obtained ¨ der Bundeswehr Munchen ¨ the Dr.ing. from the faculty of electrical engineering. Since 1995, he is working as assistant professor in the field of MEMS, surface chemistry and nanotechnology. Ignaz Eisele obtained the Dr.rer.nat. from the Technische Universitat ¨ Munchen in 1972. From 1972 until 1980, he worked at Siemens Research ¨ Laboratories in Munich and in 1980, he became a professor of material science and in 1994, of microsystem technology at the Universitat ¨ der Bundeswehr Munchen. ¨
Molecular beam evaporation-grown indium oxide and indium aluminium films for low-temperature gas sensors R. Winter ) , K. Scharnagl, A. Fuchs, T. Doll, I. Eisele UniÕersitat Fakultat ¨ der Bundeswehr Munchen, ¨ ¨ f ur ¨ Elektrotechnik ET9, Institut f ur ¨ Physik, Werner-Heisenberg-Weg 39, D-85577 Neubiberg, Germany Accepted 22 July 1999
Abstract Indium oxide and indium aluminium oxide thin films for low-temperature work function sensors were grown on silicon substrates by molecular beam evaporation ŽMBE.. Depending on the substrate temperature and the oxygen partial pressure during film growth, different stoichiometry and morphology were obtained. After depositing, the films were annealed in oxygen. The films were characterized by AES, RBS, XRD and SEM. The stoichiometry of the layers varies between pure In 2 O 3 and a composition of pure indium and In 2 O 3. All the films are polycrystalline with a grain size up to 300 nm. Thicker films form a porous layer. The indium aluminium oxide films show no crystalline phases. Both as-grown and annealed films were tested for NO 2 , CO, CO 2 , NH 3 , Cl 2 and O 3 at room temperature and 1308C by Kelvin probe measurements in dry and humid air. In 2 O 3 films show a high sensitivity to NO 2 , CO and CO 2 . A higher sensitivity for the porous layers is obtained due to the greater surface. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Indium oxide; Work function; Thin films
1. Introduction In 2 O 3 films became an interesting material for sensor application during the last years. Films were deposited by sputtering w1,2x techniques or by sol–gel-deposition w3,4x. Molecular beam evaporation ŽMBE. is a new approach for thin film deposition of indium oxide films which offers the opportunity to control the stoichiometry and morphology by varying the process parameters. Furthermore, one can deposit doped oxide films by coevaporation.
2. Experimental 2.1. Sample preparation The films were grown on ²100:-pq-silicon substrates by MBE in an UHV-chamber. The natural oxide on the
) Corresponding author. Fax: [email protected]
q49-89-6004-3877;
e-mail:
silicon was removed by thermal desorption at 9008C for 5 min. High-purity indium Ž99.995%. was evaporated with an electron beam evaporater ŽLeybold ESV2.. Aluminium was deposited by thermal evaporation. The oxygen partial pressure during the deposition was either 1.5 = 10y6 or 1.5 = 10y5 mbar. The substrate temperature ranged from room temperature up to 5008C. After deposition, the films were studied by AES, RBS, XRD and SEM. The substrate temperature during film deposition influences the morphology of the layers. Films grown at room temperature are smooth, whereas above 2008C, one gets a grainy film. The grain sizes range from 100 nm for 2008C substrate temperature to 300 nm for 5008C ŽFigs. 1 and 2.. Thick films Ž) 500 nm. grown at higher temperatures form a porous film. Indium has three stable oxides: In 2 O, InO and In 2 O 3 w5x. In 2 O is not stable in ultrahigh vacuum. The stoichiometry was determined by AES and RBS. All three oxides were found, but only two phases were detected by XRD: tetragonal indium and cubic In 2 O 3 . The stoichiometry of the films is mainly influenced by the oxygen partial pressure during deposition: for the lower oxygen partial pressure, one gets irregular crystallized indium with some
0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 9 9 . 0 0 2 9 8 - 1
R. Winter et al.r Sensors and Actuators B 66 (2000) 85–87
86
Fig. 1. SEM-picture of In 2 O 3 films grown at 3008C. Fig. 3. CPD for indium oxide at 1308C in dry air.
In 2 O 3 , whereas samples prepared with the higher oxygen partial pressure consist of nearly pure In 2 O 3 . The crystallinity is better for films grown at lower temperatures. After annealing in oxygen at 8008C for 2 h, all samples consist of In 2 O 3 . The Al x In yO films shows now crystalline phases. 2.2. Gas measurements
Furthermore, ozone measurements were carried out at 50, 100 and 300 ppb O 3 .
3. Results and discussion 3.1. Indium oxide
The work function measurements were carried out in a continuous flow gas mix system by Kelvin probe measurements. The reference electrode was the gold electrode of the Kelvin probe. The difference between the work function of the films and the gold electrode, the contact potential difference ŽCPD., was recorded by a computer. The samples were mounted on a heater and measured at 308C and 1308C. Before the measurements, the samples were exposed to synthetic air for 2 h. Afterwards, the samples were exposed three times to a measurement cycle of 40 min test gas and 40 min synthetic air for each test gas. Test gases were 100 ppm NO 2 , 1000 ppm CO, 5000 ppm CO 2 , 100 ppm SO 2 , 70 ppm NH 3 and 10 ppm Cl 2 .
3.1.1. Influence of the morphology Smooth, grainy and porous In 2 O 3 films with grain sizes from 100 to 300 nm were examined. The films are sensitive to NO 2 , CO, CO 2 and SO 2 at 308C and 1308C and to Cl 2 at 1308C. The sensitivity to NH 3 is low ŽFig. 3.. The porous films show a higher sensitivity to all test gases except NH 3 . The increase of the CPD could be explained with the greater surface of these films. A possible explanation for the opposite behaviour to NH 3 is an over-compensation effect of the Kelvin probe gold electrode. For NH 3 , one would expect a negative CPD change. If the work function change of the film is smaller than of the reference electrode, the resulting signal is positive and an increase of the work function change of the film leads to a
Fig. 2. SEM-picture of In 2 O 3 films grown at 5008C.
Fig. 4. CPD for indium oxide with elemental indium at 308C in dry air.
R. Winter et al.r Sensors and Actuators B 66 (2000) 85–87
87
morphology and stoichiometry of the films. Increasing the surface of the indium oxide Žgrainy or porous films. leads to an unspecific increase in signal heights. The sensitivity at room temperature can be improved by elemental indium in the films. A very interesting feature of the MBE technique is the possibility to add other metals to the layers in different controllable amounts. Doping the layers with aluminium leads to a significant lowering of the cross sensitivity to chlorine, whereas the sensitivities to the other test gases are not affected.
References Fig. 5. Influence of the aluminium content of In x Al yO to the contact potential difference at 1308C in dry air.
smaller CPD change. The difference in signal heights between smooth and grainy films is small. 3.1.2. Influence of the stoichiometry Films with different amount of elemental indium were studied. At room temperature, the films with elemental indium show higher signals to all test gases except NH 3 . Especially the sensitivity to Cl 2 and SO 2 is increased. Fig. 4 shows this effect for a smooth film of 100 nm thickness. The effect is more significant for films with higher indium content. The higher sensitivity is possibly caused by catalytic activation on the elemental indium. At higher temperatures Ž1308C., these films were not stable. 3.2. Indium aluminium oxide Indium aluminium oxide films with different aluminium content were tested at room temperature. The aluminium content mainly influences the sensitivity to Cl 2 . For higher aluminium content, the sensitivity to chlorine is suppressed. No significant change of the sensitivity to NO 2 , CO, CO 2 , SO 2 and NH 3 can be observed. Fig. 5 shows the results for two 100 nm films. Compared to pure indium oxide films and indium oxide films with elemental indium, the sensitivities to CO, CO 2 and SO 2 are lowered. 3.3. Ozone measurements Indium oxide films were tested for ozone at room temperature in dry and humid air. The CPD for 300 ppb O 3 is 170 mV. No differences in signal heights for dry and humid air Ž30% r.h.. were observed. The desorption times are in the range of a few hours and increase in dry air.
4. Conclusions MBE-grown indium oxide and indium aluminium oxide films offer a wide range to influence the sensitivity by
w1x I. Hamberg, C.G. Granqvist, Evaporated Sn-doped In 2 O 3 films: basic optical properties and applications to energy efficient windows, Journal of Applied Physics 60 Ž11. Ž1986. R123–R159. w2x T. Doll, A. Fuchs, I. Eisele, S. Gropelli, G. Sberveglieri, Conductivity and work function ozone sensors based on indium oxide, Sensors and Actuators, B 49 Ž1998. 63–67. w3x A. Gurlo, M. Ivanovskaya, A. Pfau, U. Weimar, W. Gopel, Sol–gel ¨ prepared In 2 O 3 thin films, Thin Solid Films 307 Ž1997. 288–293. w4x W. Wlodarski, H.-T. Sun, A. Gurlo, W. Gopel, Sol–gel prepared ¨ In 2 O 3 thin films for ozone sensing, Proceedings of Transducers ŽChicago. 97 Ž1997. 573–576. w5x Gmelin’s Handbook of Inorganic Chemistry, Vol. 37, Springer Verlag, Berlin, 1974, p. 64.
Biographies
Ronny Winter received his diploma in physics from the Universitat ¨ Leipzig in the field of high-temperature superconductivity in 1994. He is currently working towards his PhD thesis at the Universitat ¨ der Bundeswehr Munchen. His interests include chemical sensors based on metal ¨ oxides. Klaus Scharnagl obtained his diploma in physics from the Technische Universitat in 1998. Now he is working towards his PhD thesis ¨ Munchen ¨ at the Universitat His subjects include chemi¨ der Bundeswehr Munchen. ¨ cal sensors especially for H 2 -sensing. Alexander Fuchs received his diploma in physics from the Technische Universitat in 1994. He is currently working towards his PhD ¨ Munchen ¨ thesis at the Universitat in the field of mi¨ der Bundeswehr Munchen ¨ crosystem technology and chemical sensors. Theodor Doll studied music and physics in Munich and was then at KWS, Germany in R&D of environmental diagnostic systems. In 1991, he went to the Universitat where he obtained ¨ der Bundeswehr Munchen ¨ the Dr.ing. from the faculty of electrical engineering. Since 1995, he is working as assistant professor in the field of MEMS, surface chemistry and nanotechnology. Ignaz Eisele obtained the Dr.rer.nat. from the Technische Universitat ¨ Munchen in 1972. From 1972 until 1980, he worked at Siemens Research ¨ Laboratories in Munich and in 1980, he became a professor of material science and in 1994, of microsystem technology at the Universitat ¨ der Bundeswehr Munchen. ¨