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Transport characterization of GaAs quantum dots connected with quantum wires fabricated by selective area metalorganic vapor phase epitaxy
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Solid-State Electronics Vol. 42, No. 7±8, pp. 1227±1231, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0038-1101/98 $19.00 + 0.00 S0038-1101(98)00009-4
TRANSPORT CHARACTERIZATION OF GaAs QUANTUM DOTS CONNECTED WITH QUANTUM WIRES FABRICATED BY SELECTIVE AREA METALORGANIC VAPOR PHASE EPITAXY K. KUMAKURA, J. MOTOHISA and T. FUKUI Research Center for Interface Quantum Electronics (RCIQE), Hokkaido University, North 13 West 8, Sapporo, 060, Japan AbstractÐWe fabricated novel quantum nanostructures where the quantum dots are connected with quantum wires using metalorganic vapor phase epitaxy (MOVPE) on (001) GaAs masked substrates. In particular a GaAs single electron transistor was successfully fabricated and its transport properties were investigated. We prepared two devices which have arti®cially designed two- or three-prominences in the channel region. These prominences produced a quantum dot connecting with quantum wires in applying the gate voltage. By comparing the electrical properties of the two devices, we discussed a model for formation of quantum dot and tunneling barriers in the channel. # 1998 Elsevier Science Ltd. All rights reserved
1. INTRODUCTION
Semiconductor quantum wires (QWRs) and quantum dots (QDs) have attracted much attention for new electrical and optical device applications, such as single electron transistors (SETs) or QD lasers. One of the most promising method to realize such structures is to form them directly by crystal growth. For example, high-density In(Ga)As QD structures are reported using self-organized growth[1±3], where three dimensional islands of In(Ga)As are automatically formed on GaAs substrate to accommodate strain and surface energy. Although this method is indeed eective to fabricate isolated QDs and QD lasers using these QDs are demonstrated[1,2], it has disadvantages in position and size control. Selective area metalorganic vapor phase epitaxy (SA-MOVPE), which has been applied to the fabrication of optical integrated circuits[4], is also a promising and versatile technique for the fabrication of quantum nano-structures[5], and uniform QD structures can be formed by adjusting the growth conditions as well as substrate orientation and mask pattern, for example, GaAs (001)[6±8] and GaAs (111)B[5] substrates. Furthermore, we can expect exciting possibilities to realize novel quantum functional devices, such as SETs and electron wave interference devices (EWIDs), since it is easy to connect well-arranged QDs and QWRs, as we have demonstrated in ``QDnetwork'' in the previous study[9]. In our previous study[10,11], we have reported on the fabrication of a QWR and a SET by SAMOVPE of GaAs/AlGaAs modulation doped structures and their transport properties. SET was realized by a designed masked substrate in which three
prominences were attached to the channel region to form QD and tunneling barriers. In this paper, we will describe the transport characterization of two devices fabricated by SA-MOVPE, which have twoor three-prominences in the channel of narrow two dimensional electron gas (2DEG). By comparing the transport properties of such two devices, we will discuss a model for the formation of QD, QWRs and tunneling barriers in the channel. 2. FABRICATION PROCESS OF THE DEVICES
We grew GaAs/AlGaAs selectively doped double heterostructure on masked GaAs substrates by SAMOVPE. Growth was carried out using a lowpressure (0.1 atm) horizontal, RF-heated, quartz reactor system. The detailed growth condition for SA-MOVPE is described in Ref.[11]. The typical mobility and sheet electron concentration of 2DEG grown on a planar substrate were 70,000 cm2/Vs and 8 1011 cmÿ2 at 77 K, respectively. A schematic illustration of the mask pattern for device structure (device A, B) is shown in Fig. 1(a) and (b). The pattern for device B is the same one as in the previous reports. The pattern is formed on semi-insulating (001) GaAs coated with 40-nm-thick SiNx layer by electron beam lithography and wet chemical etching. The wire-like opening with two- [Fig. 1(a)] or three- [Fig. 1(b)] prominences aligned in the [110] direction corresponds to channel region of the devices. Figure 1(c) shows SEM image after the selective growth and (d) schematic illustration of the device structure for device B. The wire region consists of {111}B, and {110} facet sidewalls are formed near
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Fig. 1. Schematic illustration of SiNx masked substrate of (a) device A and (b) device B for selective area MOVPE, (c) SEM image after the selective growth on the masked substrate and (d) schematic illustration of the device structure for device B.
the prominences, whereas (001) plane appeared on the top portion of the whole structure. Similar facet sidewall structures and top surface were formed for device A. The channel width in wire region is measured to be about 400 nm by SEM observation, and its thickness is about 18 nm which is estimated from the peak position of cathodeluminescence measurement[10]. The narrow 2DEG channel becomes quasi-1DEG channel in applying the negative gate voltage near the pinch o[11]. Since the prominences in the channel introduce variation in its width, dots, wires and tunneling barriers can be
formed by applying gate bias around the prominences. After the growth of these structures, source and drain Ge/Au/Ni ohmic electrodes were formed on the wide 2DEG regions by a lift-o process followed by alloying at 3508C for 5 min in N2 atmosphere. Next, a Schottky gate of Al electrode was formed by a lift-o process on the prominent region. Since no growth occurs on {111}B facets, both edges of the narrow 2DEG channel are exposed to air. Therefore, the Schottky gate has direct contact at the edges of the narrow 2DEG,
Transport characterization of GaAs quantum dots
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and controls the electric ®eld perpendicular to the edges. The gate length of the devices was 2 mm for both devices. 3. TRANSPORT PROPERTIES
Figure 2(a) shows ID±VG characteristics for device A, measured with various source-drain bias VDS (VDS=0.2, 0.5, 1.0 mV) at 1.9 K. Conductance oscillations were observed at each drain bias, though the detailed structures of the peaks varied mainly due to the broadening of each peak with increasing VDS. ID±VG characteristics for device B is shown in Fig. 2(b). Here the device is the same one as used in the previous study[11]. The present data was obtained a few months after the measurements described in the previous study. There are shifts in the threshold voltage and some discrepancies in the ®ne structures, but very similar conductance oscillations were observed. If we compare the conductance oscillation in two devices, it is more clear, and sharper peaks were seen in device B. In addition, in ID±VDS characteristics for device A, clear Coulomb gaps and their modulation by changing the gate voltage were not observed before the
Fig. 3. ID±VDS characteristics at 1.9 K of (a) device A and (b) device B.
Fig. 2. ID±VG characteristics measured with various VDS conditions at 1.9 K of (a) device A and (b) device B.
device was completely pinch o, as shown in Fig. 3(a). We think this is because the dot size is too large or tunneling barriers are not well de®ned to observe the Coulomb gap at this measurement temperature. On the other hand, ID±VDS characteristics for device B shows clear Coulomb blockade eect, and modulation of Coulomb gaps by gate voltage were seen near the pinch-o voltage as shown in Fig. 3(b). The maximum Coulomb gap D in the present measurement is about 10 mV. Following a simple model of Coulomb blockade, the total capacitance CS of the dot can be estimated to be about 16 aF from D = e/CS, and the radius R of the dot is estimated to be about 17 nm, using the formula CS=8ee0R. These values are almost the same as the previous results of the same device[11], and indicates the excellent reproducibility and stability of the device. In Fig. 4, the magnetic ®eld B dependence of ID± VG characteristics for device B is shown. We can see the peaks become sharper with increasing B. In addition, the amplitude of conductance peaks and peak positions near the pinch o voltage change in a complicated manner as a function of B. Similar behavior in the magnetic ®eld is reported in single
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Fig. 4. Magnetic ®eld dependence of ID±VG characteristics of device B.
when the depletion layer from the surface and both edges of narrow 2DEG is expanded by applying the negative gate voltage, potential energy modulation was formed due to variation of eective channel width. Therefore, in the case of small gate voltage, the conductance oscillation was thought to be the result of electron wave interference, similar to Si quantum dots[13]. Near the pinch-o voltage, due to three prominences arranged as shown in Fig. 1(c), a small dot, wires and tunneling barriers can be formed as schematically shown in Fig. 5. These results suggest that three prominences arranged in the channel region play an important role for the large and clear oscillations associated with Coulomb blockade eect. 5. CONCLUSIONS
electron charging eect[12]. These complicated dependence on B is presumably due to the interplay of Coulomb interaction, change of electron con®nement due to B in a dot and change of the transmission probability through the barriers, although the qualitative and quantitative understanding is not clear in our devices at present.
4. POSSIBLE MODEL FOR FORMATION OF QUANTUM DOT
Each structure showed clear conductance oscillation near the pinch-o gate voltage. However, details of the conductance oscillation, ID±VDS characteristics, and magnetic ®eld dependence of ID±VG characteristics are completely dierent. This results from the dierence of the designed channel shape of the device structures and quantum dot formation in two devices. For device A, since two-prominences are aligned on opposite side of the channel, it is likely to form a large dot unless suciently large gate bias is applied. In addition, tunneling barriers which are necessary to observe the Coulomb blockade eect in QDs, might not be suf®ciently formed. On the other hand, for device B,
Fig. 5. A model for formation of a quantum dot and tunneling barriers.
We have demonstrated GaAs QD connected with GaAs QWRs by SA-MOVPE, and applied these structures to GaAs SETs. Their transport properties were investigated at low temperature. The results of transport measurements of SETs showed Coulomb blockade type conductance oscillations near the pinch-o voltage. ID±VG characteristics were also investigated in magnetic ®eld B, and it was found that conductance peaks become more sharper with increasing B. The amplitude of conductance peaks and peak positions near the pinch o voltage showed complicated dependence on B, which also suggest the existence of Coulomb blockade eect. AcknowledgementsÐThe authors wish to thank Professor H. Hasegawa for fruitful discussions and encouragement, and greatly acknowledge S. Hara, T. Umeda, Y. Oda for technical assistance in MOVPE growth, and Dr. S. Kasai, H. Okada, M. Akabori for helpful discussions. One of the authors (K. K.) would like to thank the Japan Society for the Promotion of Science for the partial ®nancial support.
REFERENCES
1. Temmyo, J., Kuramochi, E., Sugo, M., Nishiya, T., NoÈtzel, R. and Tamamura, T., Electron. Lett., 1995, 31, 209. 2. Bimberg, D., Ledentsov, N. N., Grundmann, M., Kirstaedter, N., Schmidt, O. G., Mao, M. H., Ustinov, V. M., Egorov, A.Yu., Zhukov, A. E., Kopev, P. S., Alferov, Zh.I., Ruvimov, S. S., GoÈsele, U. and Heydenreich, J., Jpn. J. Appl. Phys., 1996, 35, 1311. 3. Leonard, D., Krishnamurthy, M., Reaves, C. M., Denbaars, S. P. and Petro, P. M., Appl. Phys. Lett., 1993, 63, 3203. 4. Aoki, M., Sano, H., Suzuki, M., Takahashi, M., Uomi, K. and Takai, A., Electron. Lett., 1991, 27, 2138. 5. Fukui, T., Ando, S., Tokura, Y. and Toriyama, T., Appl. Phys. Lett., 1991, 58, 2018. 6. Nagamune, Y., Tsukamoto, S., Nishioka, M. and Arakawa, Y., J. Crystal Growth, 1993, 126, 707. 7. Kumakura, K., Nakakoshi, K., Motohisa, J., Fukui, T. and Hasegawa, H., Jpn. J. Appl. Phys., 1995, 34, 4387.
Transport characterization of GaAs quantum dots 8. Lebens, J. A., Tsai, C. S., Vahala, K. J. and Kuech, T. F., Appl. Phys. Lett., 1990, 56, 2642. 9. Kumakura, K., Motohisa, J. and Fukui, T., J. Crystal Growth, 1997, 170, 700. 10. Kumakura, K., Motohisa, J. and Fukui, T., Collected Abstract of 8th Annual Conference on Modulated Semiconductor Structures, 1997, p. 139.
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11. Kumakura, K., Motohisa, J. and Fukui, T., Physica E. Submitted. 12. Weis, J., Haug, R. J., Klitzing, K.v. and Ploog, K., Phys. Rev. B, 1992, 46, 12837. 13. Leobandung, E., Guo, L., Wang, Y. and Chou, S. Y., Appl. Phys. Lett., 1995, 67, 938.
Solid-State Electronics Vol. 42, No. 7±8, pp. 1227±1231, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0038-1101/98 $19.00 + 0.00 S0038-1101(98)00009-4
TRANSPORT CHARACTERIZATION OF GaAs QUANTUM DOTS CONNECTED WITH QUANTUM WIRES FABRICATED BY SELECTIVE AREA METALORGANIC VAPOR PHASE EPITAXY K. KUMAKURA, J. MOTOHISA and T. FUKUI Research Center for Interface Quantum Electronics (RCIQE), Hokkaido University, North 13 West 8, Sapporo, 060, Japan AbstractÐWe fabricated novel quantum nanostructures where the quantum dots are connected with quantum wires using metalorganic vapor phase epitaxy (MOVPE) on (001) GaAs masked substrates. In particular a GaAs single electron transistor was successfully fabricated and its transport properties were investigated. We prepared two devices which have arti®cially designed two- or three-prominences in the channel region. These prominences produced a quantum dot connecting with quantum wires in applying the gate voltage. By comparing the electrical properties of the two devices, we discussed a model for formation of quantum dot and tunneling barriers in the channel. # 1998 Elsevier Science Ltd. All rights reserved
1. INTRODUCTION
Semiconductor quantum wires (QWRs) and quantum dots (QDs) have attracted much attention for new electrical and optical device applications, such as single electron transistors (SETs) or QD lasers. One of the most promising method to realize such structures is to form them directly by crystal growth. For example, high-density In(Ga)As QD structures are reported using self-organized growth[1±3], where three dimensional islands of In(Ga)As are automatically formed on GaAs substrate to accommodate strain and surface energy. Although this method is indeed eective to fabricate isolated QDs and QD lasers using these QDs are demonstrated[1,2], it has disadvantages in position and size control. Selective area metalorganic vapor phase epitaxy (SA-MOVPE), which has been applied to the fabrication of optical integrated circuits[4], is also a promising and versatile technique for the fabrication of quantum nano-structures[5], and uniform QD structures can be formed by adjusting the growth conditions as well as substrate orientation and mask pattern, for example, GaAs (001)[6±8] and GaAs (111)B[5] substrates. Furthermore, we can expect exciting possibilities to realize novel quantum functional devices, such as SETs and electron wave interference devices (EWIDs), since it is easy to connect well-arranged QDs and QWRs, as we have demonstrated in ``QDnetwork'' in the previous study[9]. In our previous study[10,11], we have reported on the fabrication of a QWR and a SET by SAMOVPE of GaAs/AlGaAs modulation doped structures and their transport properties. SET was realized by a designed masked substrate in which three
prominences were attached to the channel region to form QD and tunneling barriers. In this paper, we will describe the transport characterization of two devices fabricated by SA-MOVPE, which have twoor three-prominences in the channel of narrow two dimensional electron gas (2DEG). By comparing the transport properties of such two devices, we will discuss a model for the formation of QD, QWRs and tunneling barriers in the channel. 2. FABRICATION PROCESS OF THE DEVICES
We grew GaAs/AlGaAs selectively doped double heterostructure on masked GaAs substrates by SAMOVPE. Growth was carried out using a lowpressure (0.1 atm) horizontal, RF-heated, quartz reactor system. The detailed growth condition for SA-MOVPE is described in Ref.[11]. The typical mobility and sheet electron concentration of 2DEG grown on a planar substrate were 70,000 cm2/Vs and 8 1011 cmÿ2 at 77 K, respectively. A schematic illustration of the mask pattern for device structure (device A, B) is shown in Fig. 1(a) and (b). The pattern for device B is the same one as in the previous reports. The pattern is formed on semi-insulating (001) GaAs coated with 40-nm-thick SiNx layer by electron beam lithography and wet chemical etching. The wire-like opening with two- [Fig. 1(a)] or three- [Fig. 1(b)] prominences aligned in the [110] direction corresponds to channel region of the devices. Figure 1(c) shows SEM image after the selective growth and (d) schematic illustration of the device structure for device B. The wire region consists of {111}B, and {110} facet sidewalls are formed near
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K. Kumakura et al.
Fig. 1. Schematic illustration of SiNx masked substrate of (a) device A and (b) device B for selective area MOVPE, (c) SEM image after the selective growth on the masked substrate and (d) schematic illustration of the device structure for device B.
the prominences, whereas (001) plane appeared on the top portion of the whole structure. Similar facet sidewall structures and top surface were formed for device A. The channel width in wire region is measured to be about 400 nm by SEM observation, and its thickness is about 18 nm which is estimated from the peak position of cathodeluminescence measurement[10]. The narrow 2DEG channel becomes quasi-1DEG channel in applying the negative gate voltage near the pinch o[11]. Since the prominences in the channel introduce variation in its width, dots, wires and tunneling barriers can be
formed by applying gate bias around the prominences. After the growth of these structures, source and drain Ge/Au/Ni ohmic electrodes were formed on the wide 2DEG regions by a lift-o process followed by alloying at 3508C for 5 min in N2 atmosphere. Next, a Schottky gate of Al electrode was formed by a lift-o process on the prominent region. Since no growth occurs on {111}B facets, both edges of the narrow 2DEG channel are exposed to air. Therefore, the Schottky gate has direct contact at the edges of the narrow 2DEG,
Transport characterization of GaAs quantum dots
1229
and controls the electric ®eld perpendicular to the edges. The gate length of the devices was 2 mm for both devices. 3. TRANSPORT PROPERTIES
Figure 2(a) shows ID±VG characteristics for device A, measured with various source-drain bias VDS (VDS=0.2, 0.5, 1.0 mV) at 1.9 K. Conductance oscillations were observed at each drain bias, though the detailed structures of the peaks varied mainly due to the broadening of each peak with increasing VDS. ID±VG characteristics for device B is shown in Fig. 2(b). Here the device is the same one as used in the previous study[11]. The present data was obtained a few months after the measurements described in the previous study. There are shifts in the threshold voltage and some discrepancies in the ®ne structures, but very similar conductance oscillations were observed. If we compare the conductance oscillation in two devices, it is more clear, and sharper peaks were seen in device B. In addition, in ID±VDS characteristics for device A, clear Coulomb gaps and their modulation by changing the gate voltage were not observed before the
Fig. 3. ID±VDS characteristics at 1.9 K of (a) device A and (b) device B.
Fig. 2. ID±VG characteristics measured with various VDS conditions at 1.9 K of (a) device A and (b) device B.
device was completely pinch o, as shown in Fig. 3(a). We think this is because the dot size is too large or tunneling barriers are not well de®ned to observe the Coulomb gap at this measurement temperature. On the other hand, ID±VDS characteristics for device B shows clear Coulomb blockade eect, and modulation of Coulomb gaps by gate voltage were seen near the pinch-o voltage as shown in Fig. 3(b). The maximum Coulomb gap D in the present measurement is about 10 mV. Following a simple model of Coulomb blockade, the total capacitance CS of the dot can be estimated to be about 16 aF from D = e/CS, and the radius R of the dot is estimated to be about 17 nm, using the formula CS=8ee0R. These values are almost the same as the previous results of the same device[11], and indicates the excellent reproducibility and stability of the device. In Fig. 4, the magnetic ®eld B dependence of ID± VG characteristics for device B is shown. We can see the peaks become sharper with increasing B. In addition, the amplitude of conductance peaks and peak positions near the pinch o voltage change in a complicated manner as a function of B. Similar behavior in the magnetic ®eld is reported in single
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K. Kumakura et al.
Fig. 4. Magnetic ®eld dependence of ID±VG characteristics of device B.
when the depletion layer from the surface and both edges of narrow 2DEG is expanded by applying the negative gate voltage, potential energy modulation was formed due to variation of eective channel width. Therefore, in the case of small gate voltage, the conductance oscillation was thought to be the result of electron wave interference, similar to Si quantum dots[13]. Near the pinch-o voltage, due to three prominences arranged as shown in Fig. 1(c), a small dot, wires and tunneling barriers can be formed as schematically shown in Fig. 5. These results suggest that three prominences arranged in the channel region play an important role for the large and clear oscillations associated with Coulomb blockade eect. 5. CONCLUSIONS
electron charging eect[12]. These complicated dependence on B is presumably due to the interplay of Coulomb interaction, change of electron con®nement due to B in a dot and change of the transmission probability through the barriers, although the qualitative and quantitative understanding is not clear in our devices at present.
4. POSSIBLE MODEL FOR FORMATION OF QUANTUM DOT
Each structure showed clear conductance oscillation near the pinch-o gate voltage. However, details of the conductance oscillation, ID±VDS characteristics, and magnetic ®eld dependence of ID±VG characteristics are completely dierent. This results from the dierence of the designed channel shape of the device structures and quantum dot formation in two devices. For device A, since two-prominences are aligned on opposite side of the channel, it is likely to form a large dot unless suciently large gate bias is applied. In addition, tunneling barriers which are necessary to observe the Coulomb blockade eect in QDs, might not be suf®ciently formed. On the other hand, for device B,
Fig. 5. A model for formation of a quantum dot and tunneling barriers.
We have demonstrated GaAs QD connected with GaAs QWRs by SA-MOVPE, and applied these structures to GaAs SETs. Their transport properties were investigated at low temperature. The results of transport measurements of SETs showed Coulomb blockade type conductance oscillations near the pinch-o voltage. ID±VG characteristics were also investigated in magnetic ®eld B, and it was found that conductance peaks become more sharper with increasing B. The amplitude of conductance peaks and peak positions near the pinch o voltage showed complicated dependence on B, which also suggest the existence of Coulomb blockade eect. AcknowledgementsÐThe authors wish to thank Professor H. Hasegawa for fruitful discussions and encouragement, and greatly acknowledge S. Hara, T. Umeda, Y. Oda for technical assistance in MOVPE growth, and Dr. S. Kasai, H. Okada, M. Akabori for helpful discussions. One of the authors (K. K.) would like to thank the Japan Society for the Promotion of Science for the partial ®nancial support.
REFERENCES
1. Temmyo, J., Kuramochi, E., Sugo, M., Nishiya, T., NoÈtzel, R. and Tamamura, T., Electron. Lett., 1995, 31, 209. 2. Bimberg, D., Ledentsov, N. N., Grundmann, M., Kirstaedter, N., Schmidt, O. G., Mao, M. H., Ustinov, V. M., Egorov, A.Yu., Zhukov, A. E., Kopev, P. S., Alferov, Zh.I., Ruvimov, S. S., GoÈsele, U. and Heydenreich, J., Jpn. J. Appl. Phys., 1996, 35, 1311. 3. Leonard, D., Krishnamurthy, M., Reaves, C. M., Denbaars, S. P. and Petro, P. M., Appl. Phys. Lett., 1993, 63, 3203. 4. Aoki, M., Sano, H., Suzuki, M., Takahashi, M., Uomi, K. and Takai, A., Electron. Lett., 1991, 27, 2138. 5. Fukui, T., Ando, S., Tokura, Y. and Toriyama, T., Appl. Phys. Lett., 1991, 58, 2018. 6. Nagamune, Y., Tsukamoto, S., Nishioka, M. and Arakawa, Y., J. Crystal Growth, 1993, 126, 707. 7. Kumakura, K., Nakakoshi, K., Motohisa, J., Fukui, T. and Hasegawa, H., Jpn. J. Appl. Phys., 1995, 34, 4387.
Transport characterization of GaAs quantum dots 8. Lebens, J. A., Tsai, C. S., Vahala, K. J. and Kuech, T. F., Appl. Phys. Lett., 1990, 56, 2642. 9. Kumakura, K., Motohisa, J. and Fukui, T., J. Crystal Growth, 1997, 170, 700. 10. Kumakura, K., Motohisa, J. and Fukui, T., Collected Abstract of 8th Annual Conference on Modulated Semiconductor Structures, 1997, p. 139.
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11. Kumakura, K., Motohisa, J. and Fukui, T., Physica E. Submitted. 12. Weis, J., Haug, R. J., Klitzing, K.v. and Ploog, K., Phys. Rev. B, 1992, 46, 12837. 13. Leobandung, E., Guo, L., Wang, Y. and Chou, S. Y., Appl. Phys. Lett., 1995, 67, 938.