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InSb thin films grown on GaAs substrate and their magneto-resistance effect
Journal of Crystal Growth 227–228 (2001) 619–624
InSb thin films grown on GaAs substrate and their magneto-resistance effect Atsushi Okamoto*, Arata Ashihara, Takayuki Akaogi, Ichiro Shibasaki Asahikasei Corporation, 2-1 Samejima, Fuji, Shizuoka 416-8501, Japan
Abstract InSb thin films were grown on GaAs (1 0 0) substrates by molecular beam epitaxy (MBE). N-type impurity (Sn and Si) doping of InSb was investigated for the purpose of reducing its temperature dependence of resistivity. It was found that stable Sn-doped InSb thin films with large electron mobility (4.5 104 cm2/V s) and sheet electron concentration (7.2 1012 cm2) can be fabricated. Magneto-resistance (MR) devices using these films show a large enough MR effect and an extremely small temperature dependence of resistance. # 2001 Elsevier Science B.V. All rights reserved. Keywords: A1. Doping; A3. Molecular beam epitaxy; B1. Antimonides
1. Introduction Devices with Magneto-resistance (MR) effect have recently been found to be useful for application to various kinds of contactless sensors required for electronic systems. Bulk single crystal InSb has the largest electron mobility (7.8 104 cm2/V s). However, this bulk material is not suitable for producing a low cost small device. A single-crystal InSb thin film grown on GaAs substrate is expected to have large electron mobility, which may produce a large MR effect useful for magnetic sensor applications. InSb thin film having large electron mobility has a large MR effect, which is good for producing MR elements having high sensitivity to magnetic fields [1].
*Corresponding author. Tel.: +81-545-62-3401; fax: +81545-62-3419. E-mail address: [email protected] (A. Okamoto).
The main aim of our study was to grow InSb thin films on a suitable substrate having high electron mobility and high sheet resistance, to investigate their MR effect and related characteristics for application to practical MR effect devices. Molecular beam epitaxy (MBE) was used, to grow InSb thin films directly on GaAs (1 0 0) substrates, despite the large lattice mismatch (14%). We investigated the crystal quality and the MR effect of the films. We also studied the growth of a Sn-doped InSb thin film. The band-gap energy of InSb is only 0.17 eV, so, at around room temperature the electron concentration increases rapidly with the temperature. Therefore, the problem with using InSb MR elements in practical applications is that the large temperature dependence of the resistivity (2%/8C) limits the ambient temperature and thus limits the application. To obtain stable operation in a wide temperature range, a small temperature coefficient for resistivity of InSb thin film active layer is required.
0022-0248/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 7 8 4 - 9
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A. Okamoto et al. / Journal of Crystal Growth 227–228 (2001) 619–624
To solve this problem we have been investigating in situ impurity doping of InSb thin films. We have already reported the growth and the effect of in situ Si doping of InSb [2], and that the temperature dependence of resistivity has been lowered for Si-doped InSb thin film. We started an investigation of Sn doping [3]. In this paper, in situ Sn doping of InSb thin films is discussed as well as a comparison with Si doping. We found several merits of Sn doping over Si doping. The MR effect and the temperature dependence of resistivity in the InSb thin films were also studied. Finally, Sn-doped InSb thin film was used to produce MR elements followed by a study of the basic properties of the elements.
2. MBE growth MBE was used to grow 1.0-micron-thick InSb films directly on semi-insulating GaAs (1 0 0) substrates. Elemental In and Sb4 were used as growth sources, elemental Si and Sn as dopant sources, and elemental As4 as a pre-growth source. The growth of undoped InSb was investigated first. After keeping the GaAs substrate at 6508C for 5 min under As4 vapor, InSb thin films were grown at the rate of 1.0 mm/h and at the beam flux
ratio Sb/In of 10–20. The substrate temperature was varied to obtain high electron mobility films. The electrical properties, electron mobility, and electron concentration at room temperature (300 K) were determined from Hall measurements. The typical electron mobility obtained for an undoped InSb thin film with 1.0 mm thickness was 54,000 cm2/V s and the electron concentration was 2 1016/cm3 at substrate temperatures between 4208C and 4608C. A large thickness dependence of the electrical properties was also found [2]. The surface morphology of the obtained film was very smooth as shown in Fig. 1(a), which is important for the device fabrications. To study the effect of the large lattice mismatch, we investigated the structure of the interface between InSb and GaAs by using transmission electron microscopy (TEM). The photographs are shown in Fig. 1(b), and (c). A large threading dislocation density (3 109 cm2) was observed, which might have been caused by the large lattice mismatch between InSb and GaAs. Moreover, the lattice mismatch was clearly observed in the lattice image of the interface, as shown in Fig. 1(c). Next, we grew in situ Sn-doped InSb thin films, then compared their characteristics with the Si-doped and undoped ones. The concentration of Si or Sn atoms in InSb can be easily controlled
Fig. 1. Surface morphology of an InSb film (a), and cross-section of InSb/GaAs interface structures [(b) whole image and (c) lattice image].
A. Okamoto et al. / Journal of Crystal Growth 227–228 (2001) 619–624
by adjusting the temperature of the K-cell measured by thermocouple. The K-cell temperature of Sn is 4008C lower than that of Si. The substrate temperature dependence of the electric properties of InSb thin films is shown in Figs. 2 and 3. The K-cell temperature was kept constant at 11008C for Si and 7008C for Sn respectively. But the substrate temperature dependence of Si-doped InSb was quite different compared to that of the Sn-doped or undoped
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InSb. The electron concentration of Si-doped InSb decreased markedly with an increase of the substrate temperature. The reason why a larger substrate temperature dependence was observed for Si-doped InSb might be that the strong heat radiation from a high temperature Si source caused some change of the ratio of activated electron concentration to the doped Si concentration. The electron concentration of Sn-doped InSb does not depend on the substrate temperature, which seems to indicate that the ratio of activated electron concentration to doped-Sn concentration is stable. The electron mobility of InSb thin films with various electron concentration is shown in Fig. 4. The electron mobility of doped InSb films was lower than that of undoped InSb films. The electron mobility of films doped with Sn was higher than that of films doped with Si having the same electron concentration. This may indicate that the activation ratio of Sn as an n-type carrier is larger than that of Si. We also found that the properties of Sn-doped InSb films are easier to be controlled than Si-doped ones, so it can be expected that Sn-doped InSb is more suitable for MR device applications.
Fig. 2. Substrate temperature dependence of electron concentration of 1.0-mm-thick InSb films.
Fig. 3. Substrate temperature dependence of electron mobility of 1.0-mm-thick InSb films.
Fig. 4. Relation between electron mobility and electron concentration of 1.0-mm-thick InSb films.
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3. The measurement of magneto-resistance effect We studied the MR effect of the InSb thin films (including undoped, Si-doped, and Sn-doped) grown on GaAs substrates. MR devices with structures similar to that shown in Fig. 5 were fabricated using the following fabrication process. First, the InSb layer was etched by a wet process to form a rectangularshaped mesa pattern. Next, short bar electrodes and terminal electrodes were formed on the InSb layer by the sequential deposition of Cu/Ni/Au metal layers and followed by a lift-off process using a photolithographic method. Then, a silicon nitride thin film as a passivation layer covering the whole wafer, was formed by plasma-assisted chemical vapor deposition, and then windows for the terminal electrodes were formed by etching the silicon nitride layer. Thus MR elements were formed on the GaAs substrate. After the wafer was cut into separate MR chips, each chip was placed on a lead frame island, then the terminal electrodes and lead were bonded and connected by Au wire. Finally, each chip was packaged in an epoxy resin by using the standard transfer molding process to fabricate the packaged MR device. The MR effect of the samples was measured. The amount of the effect depends on the geometrical factor L=W, where W is the width of the current path, and L is the distance between the two
Fig. 5. Schematic of an MR device.
shortbar electrodes of the MR devices. Fig. 6 shows the MR effect of InSb films when L=W was 0.24 at a magnetic flux density from 0 to 0.5 T. At a low field (under 0.1 T) the resistance change (dR=R, where R is the input resistance of the MR element) for each film is proportional to the square of the magnetic field and to the square of the electron mobility of the film. The resistance change is larger for an undoped InSb film than for a doped one because the electron mobility of the undoped InSb is higher. Moreover, the resistance change depended only on the electron mobility of the film, regardless of whether the donor impurity is Si or Sn. To examine the importance of temperature dependence of resistance, we plotted the ratio of the sheet resistance Rs at 508C and 1008C for various kinds of InSb thin films as shown in Fig. 7. The figure shows that the Rs ratio seems to be only determined by the electron concentration of the InSb thin films. There was no difference between the temperature dependence of the resistivity of a Si-doped InSb film and that of a Sn-doped InSb film having the same carrier concentration. By using 1=r ¼ enm, where r is the resistivity, n is the carrier concentration, m is the mobility, the temperature coefficient of r is expressed as, dr=r dT ¼ dn=n dT dm=m dT:
Fig. 6. Resistance change of MR elements (L=W ¼ 0:24) using various InSb films. m is electron mobility at room temperature.
A. Okamoto et al. / Journal of Crystal Growth 227–228 (2001) 619–624
Fig. 7. Relation between the electron concentration and the ratio Rs(1008C)/Rs(508C) of 1.0-mm-thick InSb films. (Rs is sheet resistance at each temperature.)
This equation shows that the temperature coefficient of resistivity only depends on n, dn=dT, m, and dm=dT, and it is independent of the doping impurity element. The result shown in Fig. 7 can be confirmed by using the above relation. One of the most important practical results of this work is the reduction of the temperature dependence of InSb by n-type impurity doping. The undoped InSb thin film has a large temperature coefficient of 2%/8C for resistivity. This causes serious problems for practical application of MR devices. Figs. 4 and 7 show us the beautiful results of impurity doping. The very small temperature coefficient of resistance of InSb thin film with a higher electron mobility of more than 30,000 cm2/V s still produced a large MR effect that is applicable to high-sensitivity MR devices. Temperature dependence of the resistance of fabricated MR elements with L=W ¼ 0:24 for undoped, Si-doped, and Sn-doped InSb thin films was studied. The resistance of MR elements for undoped InSb was found to be much more temperature dependent than the resistances of MR elements for InSb doped with Si or Sn. The temperature dependence of the resistance was reduced drastically by Si or Sn doping. In Fig. 8,
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Fig. 8. Temperature dependence of normalized resistance of MR elements for various InSb films. (m is the electron mobility, and Ns is the sheet electron concentration at room temperature.)
the temperature dependence of resistance of an InSb thin film MR element is shown for easy understanding of the above discussion.
4. Conclusions We found conditions under which InSb thin films with high electron mobility can be grown on a GaAs substrate and also found that there are several reasons for choosing Sn doping over Si doping: (1) The electron mobility of Sn-doped InSb is greater than that of Si-doped InSb for the same electron concentration. (2) Cell temperature can be lowered about 4008C. (3) The ratio of activated electron concentration to doped impurity concentration in Sn-doped films is stable. (4) The electron concentration of Sn-doped InSb does not depend on the growth temperature. The MR effects of InSb (undoped, Si-doped, and Sn-doped) depend only on the mobility of the films. The temperature dependence of the resistivity decreased drastically with increased electron concentration, regardless of whether the donor impurity was Si or Sn. The MR elements with Si-doped or Sn-doped InSb thin films were stable and highly sensitive to a magnetic field over a wide temperature range, i.e.
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between 508C and 1508C. We expect to see new applications for InSb MR devices in the future.
References [1] T. Yoshida, A. Okamoto, S. Muramatsu, N. Kuze, I. Shibasaki, Proceedings of the International Conference
on Solid-State Sensors and Actuators, Chicago, 1997, p. 417. [2] A. Okamoto, T. Yoshida, S. Muramatsu, I. Shibasaki, J. Crystal Growth 201/202 (1999) 765. [3] A. Okamoto, A. Ashihara, I. Shibasaki, Proceedings of the Tenth International Conference on Solid-State Sensors and Actuators, Sendai, 1999, p. 514.
InSb thin films grown on GaAs substrate and their magneto-resistance effect Atsushi Okamoto*, Arata Ashihara, Takayuki Akaogi, Ichiro Shibasaki Asahikasei Corporation, 2-1 Samejima, Fuji, Shizuoka 416-8501, Japan
Abstract InSb thin films were grown on GaAs (1 0 0) substrates by molecular beam epitaxy (MBE). N-type impurity (Sn and Si) doping of InSb was investigated for the purpose of reducing its temperature dependence of resistivity. It was found that stable Sn-doped InSb thin films with large electron mobility (4.5 104 cm2/V s) and sheet electron concentration (7.2 1012 cm2) can be fabricated. Magneto-resistance (MR) devices using these films show a large enough MR effect and an extremely small temperature dependence of resistance. # 2001 Elsevier Science B.V. All rights reserved. Keywords: A1. Doping; A3. Molecular beam epitaxy; B1. Antimonides
1. Introduction Devices with Magneto-resistance (MR) effect have recently been found to be useful for application to various kinds of contactless sensors required for electronic systems. Bulk single crystal InSb has the largest electron mobility (7.8 104 cm2/V s). However, this bulk material is not suitable for producing a low cost small device. A single-crystal InSb thin film grown on GaAs substrate is expected to have large electron mobility, which may produce a large MR effect useful for magnetic sensor applications. InSb thin film having large electron mobility has a large MR effect, which is good for producing MR elements having high sensitivity to magnetic fields [1].
*Corresponding author. Tel.: +81-545-62-3401; fax: +81545-62-3419. E-mail address: [email protected] (A. Okamoto).
The main aim of our study was to grow InSb thin films on a suitable substrate having high electron mobility and high sheet resistance, to investigate their MR effect and related characteristics for application to practical MR effect devices. Molecular beam epitaxy (MBE) was used, to grow InSb thin films directly on GaAs (1 0 0) substrates, despite the large lattice mismatch (14%). We investigated the crystal quality and the MR effect of the films. We also studied the growth of a Sn-doped InSb thin film. The band-gap energy of InSb is only 0.17 eV, so, at around room temperature the electron concentration increases rapidly with the temperature. Therefore, the problem with using InSb MR elements in practical applications is that the large temperature dependence of the resistivity (2%/8C) limits the ambient temperature and thus limits the application. To obtain stable operation in a wide temperature range, a small temperature coefficient for resistivity of InSb thin film active layer is required.
0022-0248/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 7 8 4 - 9
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A. Okamoto et al. / Journal of Crystal Growth 227–228 (2001) 619–624
To solve this problem we have been investigating in situ impurity doping of InSb thin films. We have already reported the growth and the effect of in situ Si doping of InSb [2], and that the temperature dependence of resistivity has been lowered for Si-doped InSb thin film. We started an investigation of Sn doping [3]. In this paper, in situ Sn doping of InSb thin films is discussed as well as a comparison with Si doping. We found several merits of Sn doping over Si doping. The MR effect and the temperature dependence of resistivity in the InSb thin films were also studied. Finally, Sn-doped InSb thin film was used to produce MR elements followed by a study of the basic properties of the elements.
2. MBE growth MBE was used to grow 1.0-micron-thick InSb films directly on semi-insulating GaAs (1 0 0) substrates. Elemental In and Sb4 were used as growth sources, elemental Si and Sn as dopant sources, and elemental As4 as a pre-growth source. The growth of undoped InSb was investigated first. After keeping the GaAs substrate at 6508C for 5 min under As4 vapor, InSb thin films were grown at the rate of 1.0 mm/h and at the beam flux
ratio Sb/In of 10–20. The substrate temperature was varied to obtain high electron mobility films. The electrical properties, electron mobility, and electron concentration at room temperature (300 K) were determined from Hall measurements. The typical electron mobility obtained for an undoped InSb thin film with 1.0 mm thickness was 54,000 cm2/V s and the electron concentration was 2 1016/cm3 at substrate temperatures between 4208C and 4608C. A large thickness dependence of the electrical properties was also found [2]. The surface morphology of the obtained film was very smooth as shown in Fig. 1(a), which is important for the device fabrications. To study the effect of the large lattice mismatch, we investigated the structure of the interface between InSb and GaAs by using transmission electron microscopy (TEM). The photographs are shown in Fig. 1(b), and (c). A large threading dislocation density (3 109 cm2) was observed, which might have been caused by the large lattice mismatch between InSb and GaAs. Moreover, the lattice mismatch was clearly observed in the lattice image of the interface, as shown in Fig. 1(c). Next, we grew in situ Sn-doped InSb thin films, then compared their characteristics with the Si-doped and undoped ones. The concentration of Si or Sn atoms in InSb can be easily controlled
Fig. 1. Surface morphology of an InSb film (a), and cross-section of InSb/GaAs interface structures [(b) whole image and (c) lattice image].
A. Okamoto et al. / Journal of Crystal Growth 227–228 (2001) 619–624
by adjusting the temperature of the K-cell measured by thermocouple. The K-cell temperature of Sn is 4008C lower than that of Si. The substrate temperature dependence of the electric properties of InSb thin films is shown in Figs. 2 and 3. The K-cell temperature was kept constant at 11008C for Si and 7008C for Sn respectively. But the substrate temperature dependence of Si-doped InSb was quite different compared to that of the Sn-doped or undoped
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InSb. The electron concentration of Si-doped InSb decreased markedly with an increase of the substrate temperature. The reason why a larger substrate temperature dependence was observed for Si-doped InSb might be that the strong heat radiation from a high temperature Si source caused some change of the ratio of activated electron concentration to the doped Si concentration. The electron concentration of Sn-doped InSb does not depend on the substrate temperature, which seems to indicate that the ratio of activated electron concentration to doped-Sn concentration is stable. The electron mobility of InSb thin films with various electron concentration is shown in Fig. 4. The electron mobility of doped InSb films was lower than that of undoped InSb films. The electron mobility of films doped with Sn was higher than that of films doped with Si having the same electron concentration. This may indicate that the activation ratio of Sn as an n-type carrier is larger than that of Si. We also found that the properties of Sn-doped InSb films are easier to be controlled than Si-doped ones, so it can be expected that Sn-doped InSb is more suitable for MR device applications.
Fig. 2. Substrate temperature dependence of electron concentration of 1.0-mm-thick InSb films.
Fig. 3. Substrate temperature dependence of electron mobility of 1.0-mm-thick InSb films.
Fig. 4. Relation between electron mobility and electron concentration of 1.0-mm-thick InSb films.
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3. The measurement of magneto-resistance effect We studied the MR effect of the InSb thin films (including undoped, Si-doped, and Sn-doped) grown on GaAs substrates. MR devices with structures similar to that shown in Fig. 5 were fabricated using the following fabrication process. First, the InSb layer was etched by a wet process to form a rectangularshaped mesa pattern. Next, short bar electrodes and terminal electrodes were formed on the InSb layer by the sequential deposition of Cu/Ni/Au metal layers and followed by a lift-off process using a photolithographic method. Then, a silicon nitride thin film as a passivation layer covering the whole wafer, was formed by plasma-assisted chemical vapor deposition, and then windows for the terminal electrodes were formed by etching the silicon nitride layer. Thus MR elements were formed on the GaAs substrate. After the wafer was cut into separate MR chips, each chip was placed on a lead frame island, then the terminal electrodes and lead were bonded and connected by Au wire. Finally, each chip was packaged in an epoxy resin by using the standard transfer molding process to fabricate the packaged MR device. The MR effect of the samples was measured. The amount of the effect depends on the geometrical factor L=W, where W is the width of the current path, and L is the distance between the two
Fig. 5. Schematic of an MR device.
shortbar electrodes of the MR devices. Fig. 6 shows the MR effect of InSb films when L=W was 0.24 at a magnetic flux density from 0 to 0.5 T. At a low field (under 0.1 T) the resistance change (dR=R, where R is the input resistance of the MR element) for each film is proportional to the square of the magnetic field and to the square of the electron mobility of the film. The resistance change is larger for an undoped InSb film than for a doped one because the electron mobility of the undoped InSb is higher. Moreover, the resistance change depended only on the electron mobility of the film, regardless of whether the donor impurity is Si or Sn. To examine the importance of temperature dependence of resistance, we plotted the ratio of the sheet resistance Rs at 508C and 1008C for various kinds of InSb thin films as shown in Fig. 7. The figure shows that the Rs ratio seems to be only determined by the electron concentration of the InSb thin films. There was no difference between the temperature dependence of the resistivity of a Si-doped InSb film and that of a Sn-doped InSb film having the same carrier concentration. By using 1=r ¼ enm, where r is the resistivity, n is the carrier concentration, m is the mobility, the temperature coefficient of r is expressed as, dr=r dT ¼ dn=n dT dm=m dT:
Fig. 6. Resistance change of MR elements (L=W ¼ 0:24) using various InSb films. m is electron mobility at room temperature.
A. Okamoto et al. / Journal of Crystal Growth 227–228 (2001) 619–624
Fig. 7. Relation between the electron concentration and the ratio Rs(1008C)/Rs(508C) of 1.0-mm-thick InSb films. (Rs is sheet resistance at each temperature.)
This equation shows that the temperature coefficient of resistivity only depends on n, dn=dT, m, and dm=dT, and it is independent of the doping impurity element. The result shown in Fig. 7 can be confirmed by using the above relation. One of the most important practical results of this work is the reduction of the temperature dependence of InSb by n-type impurity doping. The undoped InSb thin film has a large temperature coefficient of 2%/8C for resistivity. This causes serious problems for practical application of MR devices. Figs. 4 and 7 show us the beautiful results of impurity doping. The very small temperature coefficient of resistance of InSb thin film with a higher electron mobility of more than 30,000 cm2/V s still produced a large MR effect that is applicable to high-sensitivity MR devices. Temperature dependence of the resistance of fabricated MR elements with L=W ¼ 0:24 for undoped, Si-doped, and Sn-doped InSb thin films was studied. The resistance of MR elements for undoped InSb was found to be much more temperature dependent than the resistances of MR elements for InSb doped with Si or Sn. The temperature dependence of the resistance was reduced drastically by Si or Sn doping. In Fig. 8,
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Fig. 8. Temperature dependence of normalized resistance of MR elements for various InSb films. (m is the electron mobility, and Ns is the sheet electron concentration at room temperature.)
the temperature dependence of resistance of an InSb thin film MR element is shown for easy understanding of the above discussion.
4. Conclusions We found conditions under which InSb thin films with high electron mobility can be grown on a GaAs substrate and also found that there are several reasons for choosing Sn doping over Si doping: (1) The electron mobility of Sn-doped InSb is greater than that of Si-doped InSb for the same electron concentration. (2) Cell temperature can be lowered about 4008C. (3) The ratio of activated electron concentration to doped impurity concentration in Sn-doped films is stable. (4) The electron concentration of Sn-doped InSb does not depend on the growth temperature. The MR effects of InSb (undoped, Si-doped, and Sn-doped) depend only on the mobility of the films. The temperature dependence of the resistivity decreased drastically with increased electron concentration, regardless of whether the donor impurity was Si or Sn. The MR elements with Si-doped or Sn-doped InSb thin films were stable and highly sensitive to a magnetic field over a wide temperature range, i.e.
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A. Okamoto et al. / Journal of Crystal Growth 227–228 (2001) 619–624
between 508C and 1508C. We expect to see new applications for InSb MR devices in the future.
References [1] T. Yoshida, A. Okamoto, S. Muramatsu, N. Kuze, I. Shibasaki, Proceedings of the International Conference
on Solid-State Sensors and Actuators, Chicago, 1997, p. 417. [2] A. Okamoto, T. Yoshida, S. Muramatsu, I. Shibasaki, J. Crystal Growth 201/202 (1999) 765. [3] A. Okamoto, A. Ashihara, I. Shibasaki, Proceedings of the Tenth International Conference on Solid-State Sensors and Actuators, Sendai, 1999, p. 514.