- Email: [email protected]
Nb-doped SrTiO3 Schottky junction
__ __ \E!
Jg
ELSEVIER
PHYSICA E Physica C 282-287
(1997) 2501-2502
_
Electrical Properties of a BaI.,K,BiO,/Nb-doped SrTiO, Schottky Junction Seiji Suzuki, Tetsuya Yamamoto, Hiroshi Suzuki, Kenichi Kawaguchi, Kazuhiko Takahashi, and Yorinobu Yoshisato Tsukuba Research Center, SANYOElectric Co., Ltd. 2-l Koyadai, Tsukuba, Ibaraki 305, Japan Current-voltage properties for a Bal.,KBiOJNb-doped SrTia (0.01 wt%) Schottky junction have been measured and investigated over a temperature range from 5.3 to 300 K. Excellent rectification properties of the junction were observed, with high reproducibility. The barrier height Vu decreased and the ideality factor n increased with decreasing temperature. The modified Richardson plot of saturation current density showed a straight-line dependence. These results may be explained by the existence of interfacial layer or the inhomogeneity of the Schottky barrier height. 1. Introduction We have been studying a low-energy type superconducting-base transistor (LSBT) using Bal..lGBiG (BKBO) as a base layer and Nb-doped SrTiO, (STNO) as a collector layer.[l,2] LSBT is expected to have great potential for achieving transistor operation with low power dissipation. We have developed a reproducible process for fabricating BKBO/STNO (base/collector) Schottky junctions with high rectification properties, and investigated the room-temperature properties of the junctions.[3] For the development of LSBT, investigation of low temperature properties of the junctions are important. Here we report on the temperature dependence of the current-voltage characteristics of the BKBO/STNO junction. 2. Experimental BKBO/STNO heterojunctions were fabricated by depositing epitaxial BKBO thin films onto the single-crystal STNO (110) substrates. The nominal Nb concentration in raw materials was 0.01 wt%. Before deposition, the substrate was annealed in flowing 02 at 1100°C for an hour to improve the surface condition. The annealed substrate was thermally cleaned in a BKBO deposition chamber, and a BKBO epitaxial thin film was grown at 400°C using the RF-magnetron sputtering method. Critical temperature of the film was about 20 K. The film was patterned into square junctions by conventional photolithography and ion-milling. Junctions areas were ranged from 0.2x0.2 to 0.5x0.5 mm’.[3,4] Current-voltage measurements were performed over a temperature range from 5.3 to3OOK. *Correspondingauthor. Fax: +81 298 37 2836 E-mail: [email protected] 0921-4534/97/$17.00 0 Elsevier Science B.V. All rights reserved PI1 SO921-4534(97)01353-l
3. Results and Discussion
Figure 1 shows typical semi-log plots of the current-density vs. voltage (log J-V) characteristics of the BKBO/STNO(O.Ol wt%) junctions at various temperatures in the forward bias region. Suaightline behavior in the forward current was observed above 30 K. The reverse current was lower than 10d A.cm-2 even at a bias of -5 V in the temperature range we measured. These excellent rectification properties were observed, as well as high reproducibility. The forward current at 5.3 K was larger than that at 30 K, suggesting that the currenttransport mechanism was changed between 5.3 and 30 K. We shall, for the moment, confine ourselves to the properties of the temperatures above 30 K. The barrier height VW(~) and the ideality factor n(T) obtained from the log J-V characteristics using the thermionic-emission theory are summarized in Table I. As is evident from Table I, the value of if&“) decreased and the value of n(T) increased with decrease in temperature. Figure 2 shows the temperature dependence of nkeT/q (the inverse slope of a log J-V) for identifying the current-transport mechanisms. The deviation from the temperature independent n = 1 becomes large with decrease in temperature. This behavior does not correspond to any current-transport mechanisms listed in the original categorization by Saxena.[S] Figure 3 shows the regular and modified Richardson plots of the saturation current density. By changing the abscissa of the Richardson plot from l/T to l/nT, it is possible to obtain a straight-line as shown in Fig. 3. The barrier height at 0 K obtained from the slope of this straight line was 1.92 V. It is notable that this value is close to the barrier height estimated from the capacitance-voltage measurement, 2.0 V, and also close to the values of n(T)xVdT). These results indicate that there may be some imperfection in the junction interface. One possible reason is the
S. Suzuki et al./Physica C 282-287 (1997) 2501-2502
2502
lE+OO lE-01
0 300K 6
lE-02
8 2
x ZOOK A 100K
lE-03 o 50K
2 2 8
0 30K
lE-04
OXA P
,"X A0
5.3K
v
ox
lE-05
g
:,A
f "
:
lE-06
x
0
lE-07
A
ox
l/l orl/nTIK'l]
0”
0
lE-06 0.5
0
1
1.5
2
2.5
Voltage [VI
Figure 1 Typical log J-V characteristics of BKBO/STNO(O.Ol wt%) Schottky junction at various temperatures. The junction size is 300 pm square. 30
I
0
I
,
I’
I’ /’ z
I
0
20
,
I’
,’ I
L 0 I
,/’
,
5
I
0
10
I
0
/ /’
I’
n=l
8’ ,’
/’ 10
0
bT4
20
30
[meal
Figure 2 Temperature dependence of nkeT/q (the inverse slope of a log J-V) for identifying the current-transport mechanisms.
Figure 3 Regular (a) and modified plot of the saturation current density.
(0)
Richardson
existence of the interfacial layer with low dielectric constant between BKBO and STN0.[3] Other authors have reported that the Schottky-barrierheight inhomogeneity offer an excellent explanation of the various dependencies of n on temperatureJ61 To clarify the current transportation mechanism through the junction, more detailed studies are being carried out. 4. Summary Current-voltage characteristics of the BKBO /STNO(O.Ol wt%) Schottky junction has been investigated over a temperature range from 5.3 to 300 K. Excellent rectification properties of the junction were observed, with high reproducibility. The barrier height VW decreased and ideality factor n increased with decreasing in temperature. The origin of the temperature dependence of VW and n are not clear at the present time. The existence of an interfacial layer or Schottky-barrier-height inhomogeneity may explain the temperature dependence of the electrical properties of the BKBO/STNO junction. This work was supported management of FED.
by NED0
under the
REFERENCES Table1 The values temperatures. TlKl V,(T)
[VI
n(T)
of Vm and n at various
300
200
100
50
30
1.73
I s7
1.12
0.74
0.52
1.12
1.23
1.74
2.6 1
3.71
1. T.Yamamoto et al., Jpn.J.Appl.Phys 30, 3911 (1991) et al., Jpn.J.Appl.Phys 32, 783 (1993) 3. SSuzuki et al., submitted to J.Appl.Phys. 4. M.Iyori et al, Jpn.J.Appl.Phys 32, 1946 (1993) 2. H.Suzuki
5. A.N.Sa_xena, Surf.Sci. 13, 151 (1969) 6. for example, J. Werner et al., J.Appl.Phys., 69, 1522 (1991): R.T.Tung, Phys.Rev.B45, 13509 (1992)
Jg
ELSEVIER
PHYSICA E Physica C 282-287
(1997) 2501-2502
_
Electrical Properties of a BaI.,K,BiO,/Nb-doped SrTiO, Schottky Junction Seiji Suzuki, Tetsuya Yamamoto, Hiroshi Suzuki, Kenichi Kawaguchi, Kazuhiko Takahashi, and Yorinobu Yoshisato Tsukuba Research Center, SANYOElectric Co., Ltd. 2-l Koyadai, Tsukuba, Ibaraki 305, Japan Current-voltage properties for a Bal.,KBiOJNb-doped SrTia (0.01 wt%) Schottky junction have been measured and investigated over a temperature range from 5.3 to 300 K. Excellent rectification properties of the junction were observed, with high reproducibility. The barrier height Vu decreased and the ideality factor n increased with decreasing temperature. The modified Richardson plot of saturation current density showed a straight-line dependence. These results may be explained by the existence of interfacial layer or the inhomogeneity of the Schottky barrier height. 1. Introduction We have been studying a low-energy type superconducting-base transistor (LSBT) using Bal..lGBiG (BKBO) as a base layer and Nb-doped SrTiO, (STNO) as a collector layer.[l,2] LSBT is expected to have great potential for achieving transistor operation with low power dissipation. We have developed a reproducible process for fabricating BKBO/STNO (base/collector) Schottky junctions with high rectification properties, and investigated the room-temperature properties of the junctions.[3] For the development of LSBT, investigation of low temperature properties of the junctions are important. Here we report on the temperature dependence of the current-voltage characteristics of the BKBO/STNO junction. 2. Experimental BKBO/STNO heterojunctions were fabricated by depositing epitaxial BKBO thin films onto the single-crystal STNO (110) substrates. The nominal Nb concentration in raw materials was 0.01 wt%. Before deposition, the substrate was annealed in flowing 02 at 1100°C for an hour to improve the surface condition. The annealed substrate was thermally cleaned in a BKBO deposition chamber, and a BKBO epitaxial thin film was grown at 400°C using the RF-magnetron sputtering method. Critical temperature of the film was about 20 K. The film was patterned into square junctions by conventional photolithography and ion-milling. Junctions areas were ranged from 0.2x0.2 to 0.5x0.5 mm’.[3,4] Current-voltage measurements were performed over a temperature range from 5.3 to3OOK. *Correspondingauthor. Fax: +81 298 37 2836 E-mail: [email protected] 0921-4534/97/$17.00 0 Elsevier Science B.V. All rights reserved PI1 SO921-4534(97)01353-l
3. Results and Discussion
Figure 1 shows typical semi-log plots of the current-density vs. voltage (log J-V) characteristics of the BKBO/STNO(O.Ol wt%) junctions at various temperatures in the forward bias region. Suaightline behavior in the forward current was observed above 30 K. The reverse current was lower than 10d A.cm-2 even at a bias of -5 V in the temperature range we measured. These excellent rectification properties were observed, as well as high reproducibility. The forward current at 5.3 K was larger than that at 30 K, suggesting that the currenttransport mechanism was changed between 5.3 and 30 K. We shall, for the moment, confine ourselves to the properties of the temperatures above 30 K. The barrier height VW(~) and the ideality factor n(T) obtained from the log J-V characteristics using the thermionic-emission theory are summarized in Table I. As is evident from Table I, the value of if&“) decreased and the value of n(T) increased with decrease in temperature. Figure 2 shows the temperature dependence of nkeT/q (the inverse slope of a log J-V) for identifying the current-transport mechanisms. The deviation from the temperature independent n = 1 becomes large with decrease in temperature. This behavior does not correspond to any current-transport mechanisms listed in the original categorization by Saxena.[S] Figure 3 shows the regular and modified Richardson plots of the saturation current density. By changing the abscissa of the Richardson plot from l/T to l/nT, it is possible to obtain a straight-line as shown in Fig. 3. The barrier height at 0 K obtained from the slope of this straight line was 1.92 V. It is notable that this value is close to the barrier height estimated from the capacitance-voltage measurement, 2.0 V, and also close to the values of n(T)xVdT). These results indicate that there may be some imperfection in the junction interface. One possible reason is the
S. Suzuki et al./Physica C 282-287 (1997) 2501-2502
2502
lE+OO lE-01
0 300K 6
lE-02
8 2
x ZOOK A 100K
lE-03 o 50K
2 2 8
0 30K
lE-04
OXA P
,"X A0
5.3K
v
ox
lE-05
g
:,A
f "
:
lE-06
x
0
lE-07
A
ox
l/l orl/nTIK'l]
0”
0
lE-06 0.5
0
1
1.5
2
2.5
Voltage [VI
Figure 1 Typical log J-V characteristics of BKBO/STNO(O.Ol wt%) Schottky junction at various temperatures. The junction size is 300 pm square. 30
I
0
I
,
I’
I’ /’ z
I
0
20
,
I’
,’ I
L 0 I
,/’
,
5
I
0
10
I
0
/ /’
I’
n=l
8’ ,’
/’ 10
0
bT4
20
30
[meal
Figure 2 Temperature dependence of nkeT/q (the inverse slope of a log J-V) for identifying the current-transport mechanisms.
Figure 3 Regular (a) and modified plot of the saturation current density.
(0)
Richardson
existence of the interfacial layer with low dielectric constant between BKBO and STN0.[3] Other authors have reported that the Schottky-barrierheight inhomogeneity offer an excellent explanation of the various dependencies of n on temperatureJ61 To clarify the current transportation mechanism through the junction, more detailed studies are being carried out. 4. Summary Current-voltage characteristics of the BKBO /STNO(O.Ol wt%) Schottky junction has been investigated over a temperature range from 5.3 to 300 K. Excellent rectification properties of the junction were observed, with high reproducibility. The barrier height VW decreased and ideality factor n increased with decreasing in temperature. The origin of the temperature dependence of VW and n are not clear at the present time. The existence of an interfacial layer or Schottky-barrier-height inhomogeneity may explain the temperature dependence of the electrical properties of the BKBO/STNO junction. This work was supported management of FED.
by NED0
under the
REFERENCES Table1 The values temperatures. TlKl V,(T)
[VI
n(T)
of Vm and n at various
300
200
100
50
30
1.73
I s7
1.12
0.74
0.52
1.12
1.23
1.74
2.6 1
3.71
1. T.Yamamoto et al., Jpn.J.Appl.Phys 30, 3911 (1991) et al., Jpn.J.Appl.Phys 32, 783 (1993) 3. SSuzuki et al., submitted to J.Appl.Phys. 4. M.Iyori et al, Jpn.J.Appl.Phys 32, 1946 (1993) 2. H.Suzuki
5. A.N.Sa_xena, Surf.Sci. 13, 151 (1969) 6. for example, J. Werner et al., J.Appl.Phys., 69, 1522 (1991): R.T.Tung, Phys.Rev.B45, 13509 (1992)