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Lateral electron transport through single InAs quantum dots grown by molecular beam epitaxy
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Physica E 21 (2004) 423 – 425 www.elsevier.com/locate/physe
Lateral electron transport through single InAs quantum dots grown by molecular beam epitaxy M. Junga;∗ , K. Hirakawaa; b a Institute
b Core
of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan Research for Evolutional Science and Technology, Japan Science and Technology Corporation, Japan
Abstract The transport properties of single InAs quantum dots (QDs) grown by molecular beam epitaxy have been investigated by metallic leads with nanogaps. It was found that the uncapped InAs QDs grown on the GaAs surfaces show metallic conductivities, indicating that even the exposed QDs are not depleted. On the contrary, it was found that no current 3ows through the exposed wetting layers. For the case of the QDs covered with GaAs capping layers, clear Coulomb gaps and Coulomb staircases have been observed at 4:2 K. ? 2003 Elsevier B.V. All rights reserved. PACS: 73.21.La; 73:23: − b; 73.23.Hk; 73.63.Kv Keywords: InAs quantum dot; Single electron tunneling; Coulomb blockade
The electronic and optical properties of self-assembled InAs quantum dots (QDs) have attracted a great amount of interest during the past several years. Research has been directed towards both their fundamental physics and device applications, such as QD memory devices, lasers, and infrared photodetectors. Although electron transport through QDs has been investigated, it has been di>cult to probe them at the single dot level due to the lack of spatial resolution. A few di?erent approaches have been tried to investigate the electrical properties of single InAs QD. In particular, a scanning tunneling microscopy (STM) has been utilized to probe InAs nanocrystal suspended on the gold surface [1]. Although STM is a very powerful tool to investigate the electronic structures, ∗
Corresponding author. Tel./fax: +81-3-5452-6261. E-mail address: [email protected] (M. Jung).
the major obstacle is the lack of mechanical and electrical 3exibility. Other works have been reported on the tunneling transport in a vertical quantum dot structures, in which a layer of InAs QDs was located in the center of a GaAs quantum well [2,3]. This structure has a drawback of rather small compatibility with further device applications. In this work, the transport properties of single InAs QDs grown by molecular beam epitaxy have been investigated by metallic leads with nanogaps. It was found that the uncapped InAs QDs grown on the GaAs surfaces show metallic conductivities, indicating that even the exposed QDs are not depleted. On the contrary, it was found that no current 3ows through the exposed wetting layers. For the case of the QDs covered with GaAs capping layers, clear Coulomb gaps and Coulomb staircases have been observed at 4:2 K. InAs QDs were grown by molecular beam epitaxy on semi-insulating (1 0 0) GaAs. After a growth of a
1386-9477/$ - see front matter ? 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2003.11.199
424
M. Jung, K. Hirakawa / Physica E 21 (2004) 423 – 425
300-nm-thick GaAs bu?er layer at 600◦ C, the InAs QDs were grown at 500◦ C. In order to investigate electrical properties of single QDs, we grew two series of samples. In one kind of samples, InAs QDs were grown on GaAs surfaces without GaAs capping layers. In the other type of samples, QDs were capped with thin GaAs layers. As shown in Figs. 1(a) and (b), we have characterized the size of InAs QDs with atomic force micrograph (AFM) and scanning electron microscopic (SEM). The average base diameter and height of the grown QDs were about 30 nm and
Fig. 1. (a) AFM image of InAs QDs. (b) SEM image of InAs QDs. (c) AFM image of uncapped surface (2 m × 2 m Jeld). (d) The surface morphology of capped InAs QDs with 32 MLs of GaAs. Image shows a 2 m × 2 m Jeld.
10 nm, respectively. The AFM images (2 m×2 m) shown in Figs. 1(c) and (d) illustrate the evolution of the surface morphology during the overgrowth with 0 –32 MLs of GaAs. From the image of the uncapped sample (Fig. 1(c)), the areal density of the QDs were determined to be ∼ 4 × 1010 cm−2 . As shown in Fig. 1(d), the surface morphology becomes 3at after the deposition of 32 MLs of GaAs capping layer. Typical SEM images of fabricated nanogap samples are presented in Figs. 2(a) and (b). Nanogaps with 10 –20 nm have been fabricated by standard electron beam lithography. The poly(methylmethacrylate) (PMMA) Jlm with a thickness of 130 nm was spin-coated on the GaAs surface. The PMMA-coated wafer was baked at 170◦ C for 60 min. After e-beam exposure, the pattern was developed in a 1:2 mixture of methyl-isobutyl-ketone and isopropanol. The metal electrodes were formed by successively depositing 5 nm NiCr and 25 nm Au and lift-o? in hot acetone. As seen in Fig. 2(a), nanogap metallic electrodes with ∼ 20 nm separation were formed. Although there was a stochastic uncertainty, SEM images conJrmed that approximately 10% of the fabricated nano-junctions touches single InAs QDs. Typical SEM images of fabricated nanogap samples are presented in Figs. 2(a) and (b). As seen in Fig. 2(a), nanogap electrodes with 20 nm separation, which touches only one uncapped InAs QD, were formed. Fig. 3 shows current versus voltage (I –V ) curves measured at 4:2 K for a sample fabricated on uncapped InAs QDs. Quasi-ohmic I –V characteristics with resistance ∼ 4 kM were observed, indicating that even the InAs QDs exposed on GaAs surfaces are
Fig. 2. (a) SEM image of typical samples fabricated on a wafer without a GaAs capping layer and (b) with a capping layer. An average nanogap width was 10 –20 nm.
M. Jung, K. Hirakawa / Physica E 21 (2004) 423 – 425
425
60.0 30.0
T = 4.2 K
25.0
Current (nA)
Current (µA)
30.0
4.0
0.0
2.0 20.0 0.0
15.0 10.0
-2.0
-30.0
Conductance (nS)
ID - VD without GaAs cap layer T = 4.2 K Through InAs QD Through InAs wetting layer
5.0 -4.0 -0.3
-60.0 -0.2
-0.1
0.0
0.1
Fig. 3. Current–voltage curves measured at 4:2 K on a nanogap sample fabricated on uncapped InAs quantum dots (solid line). No current 3ows through InAs wetting layers (dashed line).
not depleted. The measured resistance is much smaller than the quantum resistance e2 =h ∼ 26 kM and, consequently, obscures the Coulomb blockade e?ect. On the contrary, no current 3ows through the wetting layers. The wetting layer, which is a monolayer of InAs, is likely to be oxidized in air together with a few layers of GaAs underneath. Hence, the wetting layer region of the sample is likely to show properties similar to that of the oxidized GaAs surface [4]. In order to observe single electron tunneling effect, a tunneling barrier was inserted between the QDs and the nanogap electrodes. Fig. 4 shows the I –V curve measured at 4:2 K for a typical nanogap sample fabricated on InAs QDs capped with a 32 ML-thick GaAs layer grown at 300◦ C (see Fig. 1(d)). With the low-temperature grown tunneling barrier, the tunneling resistance was increased up to 10 MM. As seen in the Jgure, the I –V curve exhibits clear Coulomb blockade and Coulomb staircase characteristics. The right-hand side of Fig. 4 shows the tunneling conductance obtained by numerical di?erentiation of the I – V curve. Since the present sample does not have a gate electrode and, hence, the spectroscopic information on the conductance is rather limited, it is di>cult to assign the conductance peaks to the shell structure in the dot. However, judging from the multiple peaks seen in the conductance spectrum, it is likely that the present QD with a diameter of ∼ 30 nm contains shell levels higher than “p”-states.
-0.1
0.0
0.1
0.2
0.0 0.3
Voltage (V)
0.2
Voltage (V)
-0.2
Fig. 4. Current–voltage curve (left) and tunneling conductance spectrum (right) measured at 4:2 K on a nanogap sample fabricated on QDs covered with a 32 ML-thick GaAs capping layer grown at 300◦ C.
In summary, we have investigated lateral single electron transport using InAs QDs. We have found that the uncapped InAs QDs grown on the GaAs surfaces show metallic conductivities, indicating that even the exposed SAQDs are not depleted. On the contrary, it was found that no current 3ows through the exposed InAs wetting layers. For the case of the QDs covered with GaAs capping layers grown at 300◦ C, the tunneling resistance was enhanced up to ∼ 100 MM and a clear Coulomb staircase has been observed at 4:2 K. The authors thank H. Sakaki and Y. Arakawa for their continuous encouragement. This work was supported by SORST of the Japan Science and Technology Corporation, the Grant-in-Aid from Japan Society for the Promotion of Science (No. 13555104 and 15206037), and the Grant-in-Aid for COE research (No. 12CE2004) and IT program from MEXT. References [1] U. Banin, Y.W. Cao, D. Katz, O. Millo, Nature 400 (1999) 542. [2] D.G. Austing, S. Tarucha, P.C. Main, M. Henini, S.T. Stoddart, L. Eaves, Appl. Phys. Lett. 75 (1999) 671. [3] R.J.A. Hill, A. Patane, P.C. Main, L. Eaves, B. Gustafson, M. Henini, S. Tarucha, D.G. Austing, Appl. Phys. Lett. 79 (2001) 3275. [4] I. Tanaka, I. Kamiya, H. Sakaki, N. Qureshi, S.J. Allen, P.M. Petro?, Appl. Phys. Lett. 74 (1999) 844.
Physica E 21 (2004) 423 – 425 www.elsevier.com/locate/physe
Lateral electron transport through single InAs quantum dots grown by molecular beam epitaxy M. Junga;∗ , K. Hirakawaa; b a Institute
b Core
of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan Research for Evolutional Science and Technology, Japan Science and Technology Corporation, Japan
Abstract The transport properties of single InAs quantum dots (QDs) grown by molecular beam epitaxy have been investigated by metallic leads with nanogaps. It was found that the uncapped InAs QDs grown on the GaAs surfaces show metallic conductivities, indicating that even the exposed QDs are not depleted. On the contrary, it was found that no current 3ows through the exposed wetting layers. For the case of the QDs covered with GaAs capping layers, clear Coulomb gaps and Coulomb staircases have been observed at 4:2 K. ? 2003 Elsevier B.V. All rights reserved. PACS: 73.21.La; 73:23: − b; 73.23.Hk; 73.63.Kv Keywords: InAs quantum dot; Single electron tunneling; Coulomb blockade
The electronic and optical properties of self-assembled InAs quantum dots (QDs) have attracted a great amount of interest during the past several years. Research has been directed towards both their fundamental physics and device applications, such as QD memory devices, lasers, and infrared photodetectors. Although electron transport through QDs has been investigated, it has been di>cult to probe them at the single dot level due to the lack of spatial resolution. A few di?erent approaches have been tried to investigate the electrical properties of single InAs QD. In particular, a scanning tunneling microscopy (STM) has been utilized to probe InAs nanocrystal suspended on the gold surface [1]. Although STM is a very powerful tool to investigate the electronic structures, ∗
Corresponding author. Tel./fax: +81-3-5452-6261. E-mail address: [email protected] (M. Jung).
the major obstacle is the lack of mechanical and electrical 3exibility. Other works have been reported on the tunneling transport in a vertical quantum dot structures, in which a layer of InAs QDs was located in the center of a GaAs quantum well [2,3]. This structure has a drawback of rather small compatibility with further device applications. In this work, the transport properties of single InAs QDs grown by molecular beam epitaxy have been investigated by metallic leads with nanogaps. It was found that the uncapped InAs QDs grown on the GaAs surfaces show metallic conductivities, indicating that even the exposed QDs are not depleted. On the contrary, it was found that no current 3ows through the exposed wetting layers. For the case of the QDs covered with GaAs capping layers, clear Coulomb gaps and Coulomb staircases have been observed at 4:2 K. InAs QDs were grown by molecular beam epitaxy on semi-insulating (1 0 0) GaAs. After a growth of a
1386-9477/$ - see front matter ? 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2003.11.199
424
M. Jung, K. Hirakawa / Physica E 21 (2004) 423 – 425
300-nm-thick GaAs bu?er layer at 600◦ C, the InAs QDs were grown at 500◦ C. In order to investigate electrical properties of single QDs, we grew two series of samples. In one kind of samples, InAs QDs were grown on GaAs surfaces without GaAs capping layers. In the other type of samples, QDs were capped with thin GaAs layers. As shown in Figs. 1(a) and (b), we have characterized the size of InAs QDs with atomic force micrograph (AFM) and scanning electron microscopic (SEM). The average base diameter and height of the grown QDs were about 30 nm and
Fig. 1. (a) AFM image of InAs QDs. (b) SEM image of InAs QDs. (c) AFM image of uncapped surface (2 m × 2 m Jeld). (d) The surface morphology of capped InAs QDs with 32 MLs of GaAs. Image shows a 2 m × 2 m Jeld.
10 nm, respectively. The AFM images (2 m×2 m) shown in Figs. 1(c) and (d) illustrate the evolution of the surface morphology during the overgrowth with 0 –32 MLs of GaAs. From the image of the uncapped sample (Fig. 1(c)), the areal density of the QDs were determined to be ∼ 4 × 1010 cm−2 . As shown in Fig. 1(d), the surface morphology becomes 3at after the deposition of 32 MLs of GaAs capping layer. Typical SEM images of fabricated nanogap samples are presented in Figs. 2(a) and (b). Nanogaps with 10 –20 nm have been fabricated by standard electron beam lithography. The poly(methylmethacrylate) (PMMA) Jlm with a thickness of 130 nm was spin-coated on the GaAs surface. The PMMA-coated wafer was baked at 170◦ C for 60 min. After e-beam exposure, the pattern was developed in a 1:2 mixture of methyl-isobutyl-ketone and isopropanol. The metal electrodes were formed by successively depositing 5 nm NiCr and 25 nm Au and lift-o? in hot acetone. As seen in Fig. 2(a), nanogap metallic electrodes with ∼ 20 nm separation were formed. Although there was a stochastic uncertainty, SEM images conJrmed that approximately 10% of the fabricated nano-junctions touches single InAs QDs. Typical SEM images of fabricated nanogap samples are presented in Figs. 2(a) and (b). As seen in Fig. 2(a), nanogap electrodes with 20 nm separation, which touches only one uncapped InAs QD, were formed. Fig. 3 shows current versus voltage (I –V ) curves measured at 4:2 K for a sample fabricated on uncapped InAs QDs. Quasi-ohmic I –V characteristics with resistance ∼ 4 kM were observed, indicating that even the InAs QDs exposed on GaAs surfaces are
Fig. 2. (a) SEM image of typical samples fabricated on a wafer without a GaAs capping layer and (b) with a capping layer. An average nanogap width was 10 –20 nm.
M. Jung, K. Hirakawa / Physica E 21 (2004) 423 – 425
425
60.0 30.0
T = 4.2 K
25.0
Current (nA)
Current (µA)
30.0
4.0
0.0
2.0 20.0 0.0
15.0 10.0
-2.0
-30.0
Conductance (nS)
ID - VD without GaAs cap layer T = 4.2 K Through InAs QD Through InAs wetting layer
5.0 -4.0 -0.3
-60.0 -0.2
-0.1
0.0
0.1
Fig. 3. Current–voltage curves measured at 4:2 K on a nanogap sample fabricated on uncapped InAs quantum dots (solid line). No current 3ows through InAs wetting layers (dashed line).
not depleted. The measured resistance is much smaller than the quantum resistance e2 =h ∼ 26 kM and, consequently, obscures the Coulomb blockade e?ect. On the contrary, no current 3ows through the wetting layers. The wetting layer, which is a monolayer of InAs, is likely to be oxidized in air together with a few layers of GaAs underneath. Hence, the wetting layer region of the sample is likely to show properties similar to that of the oxidized GaAs surface [4]. In order to observe single electron tunneling effect, a tunneling barrier was inserted between the QDs and the nanogap electrodes. Fig. 4 shows the I –V curve measured at 4:2 K for a typical nanogap sample fabricated on InAs QDs capped with a 32 ML-thick GaAs layer grown at 300◦ C (see Fig. 1(d)). With the low-temperature grown tunneling barrier, the tunneling resistance was increased up to 10 MM. As seen in the Jgure, the I –V curve exhibits clear Coulomb blockade and Coulomb staircase characteristics. The right-hand side of Fig. 4 shows the tunneling conductance obtained by numerical di?erentiation of the I – V curve. Since the present sample does not have a gate electrode and, hence, the spectroscopic information on the conductance is rather limited, it is di>cult to assign the conductance peaks to the shell structure in the dot. However, judging from the multiple peaks seen in the conductance spectrum, it is likely that the present QD with a diameter of ∼ 30 nm contains shell levels higher than “p”-states.
-0.1
0.0
0.1
0.2
0.0 0.3
Voltage (V)
0.2
Voltage (V)
-0.2
Fig. 4. Current–voltage curve (left) and tunneling conductance spectrum (right) measured at 4:2 K on a nanogap sample fabricated on QDs covered with a 32 ML-thick GaAs capping layer grown at 300◦ C.
In summary, we have investigated lateral single electron transport using InAs QDs. We have found that the uncapped InAs QDs grown on the GaAs surfaces show metallic conductivities, indicating that even the exposed SAQDs are not depleted. On the contrary, it was found that no current 3ows through the exposed InAs wetting layers. For the case of the QDs covered with GaAs capping layers grown at 300◦ C, the tunneling resistance was enhanced up to ∼ 100 MM and a clear Coulomb staircase has been observed at 4:2 K. The authors thank H. Sakaki and Y. Arakawa for their continuous encouragement. This work was supported by SORST of the Japan Science and Technology Corporation, the Grant-in-Aid from Japan Society for the Promotion of Science (No. 13555104 and 15206037), and the Grant-in-Aid for COE research (No. 12CE2004) and IT program from MEXT. References [1] U. Banin, Y.W. Cao, D. Katz, O. Millo, Nature 400 (1999) 542. [2] D.G. Austing, S. Tarucha, P.C. Main, M. Henini, S.T. Stoddart, L. Eaves, Appl. Phys. Lett. 75 (1999) 671. [3] R.J.A. Hill, A. Patane, P.C. Main, L. Eaves, B. Gustafson, M. Henini, S. Tarucha, D.G. Austing, Appl. Phys. Lett. 79 (2001) 3275. [4] I. Tanaka, I. Kamiya, H. Sakaki, N. Qureshi, S.J. Allen, P.M. Petro?, Appl. Phys. Lett. 74 (1999) 844.