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InAs quantum dot field effect transistors
Superlattices and Microstructures, Vol. 25, No. 1/2, 1999 Article No. spmi.1998.0688 Available online at http://www.idealibrary.com on
InAs quantum dot field effect transistors G. Y USA† Institute of Industrial Science, The University of Tokyo, Japan
H. S AKAKI‡ Japan Science and Technology Corporation, Japan
(Received 26 October 1998) We have studied single electron and hole storage in self-assembled InAs quantum dots (QDs) embedded in GaAs/n-AlGaAs field effect transistors (QD-FETs). We prepared two types of QD-FETs. A single electron and a photo-generated single hole can be stored in each QD in Type 1. In the new Type II, single-electron discharge processes can be controlled by a surface gate voltage (Vg ) as well as single-electron storage processes. We demonstrate possible application to novel photo devices and quantum dot memory devices. c 1999 Academic Press
Key words: InAs quantum dots, FET, MBE.
1. Introduction Quantum dot structures (QDs) [1] with zero-dimensionality have become a subject of research in device application, as these structures allow low threshold lasers, novel single-electron devices and detectors. Selfassembled QDs grown on lattice-mismatched substrates have shown to be promising QD fabrications because of their easiness and cleanliness. In this paper, we suggest two different types of quantum dot FETs (QD-FETs) and demonstrate their operation and possible device applications [2, 3].
2. Sample structures We prepared two types of selectively doped GaAs/n-AlGaAs FETs, with embedded InAs self-assembled QDs grown by molecular beam epitaxy (MBE), see Figs 1A and B. For both types, we first grew 200 nm GaAs buffer layers and superlattice buffers containing 11 periods of a 20 nm Al0.25 Ga0.75 As and 2 nm GaAs. The concentration of Si δ-doping is of 1 × 1012 cm−2 [4]. The type 1 QD-FET is based on an inverted HEMT structure (T1-A). We grew a 70 nm-thick undoped Al0.25 Ga0.75 As spacer. The InAs dot layer is separated by a 200 nm-thick GaAs layer from the 2D channel. For this type, electrons flow into the dot layer by applying a gate voltage (Vg ). Type II QD-FETs are based on quantum well HEMT structures (T2-A, B). InAs dot layers are embedded in AlGaAs barrier layers and separated from the QW by 5 ∼ 80 nm-thick AlGaAs. The average base diameter, height, and density of the QDs on both GaAs and AlGaAs are 20 nm, 5 nm, and 5–10 × 1010 cm−2 , † E-mail: [email protected]; Web site: http:// www.iis.u-tokyo.ac.jp/∼yusa ‡ Also at: Institute of Industrial Science, The University of Tokyo, Japan
0749–6036/99/020247 + 04
$30.00/0
c 1999 Academic Press
248
Superlattices and Microstructures, Vol. 25, No. 1/2, 1999 A, Type I
Delta-doped Si
InAs dot layer
2DEG
GaAs GaAs buffer
AlGaAs/GaAs superlattice
Wd = 200 nm
600 nm
200 nm
B, Type II
InAs dot layer Delta-doped Si
GaAs buffer
Gate
AlGaAs/GaAs superlattice
AlGaAs Gate
Wd = 2–80 nm 2DEG
Fig. 1. Schematic potential diagram of two types of InAs quantum dot FETs. The InAs dot layer is embedded into the GaAs layer. A, Type I; and in an AlGaAs barrier B, Type II, respectively.
respectively. For each type, we prepared a reference FET without an InAs dot layer, T1-B for Type I and T2-C for Type II. These samples were processed into Hall bars with 100 nm-thick aluminium (A1) gate electrodes.
3. Results and discussion 3.1. Type I First, we measured the electron concentration Ns of T1-A and T1-B by the Hall effect by scanning Vg . The results at 13 K are plotted as a function of Vg in Fig. 2A in the dark and B, after light illumination. In Fig. 2A, Ns of both T1-A and T1-B increase linearly until Vg reaches 0.8 V (up-scan). When Vg exceeds 0.8 V, the gate leakage current between the channel and the gate begins to flow in T1-B. In contrast, Ns of T1-A tends to saturate for Vg > 0.8 V. Then, the potential in the GaAs layer is lowered by the positive Vg and electrons can be easily injected into the QDs. Stored electrons in QDs raise the potential and block the leakage current. Once the QDs are fully occupied with electrons at Vg = 1.5 V, the leakage current starts to flow. In the down-scan of Vg from 1.5 V, the Ns –Vg characteristics of sample T1-A are shifted by 5 × 1010 cm−2 . The number of stored electrons Nelect in the QDs is estimated to be 7.5 × 1010 cm−2 by Gauss’ law. As this number is close to the density of the QDs, each QD seems to capture one electron each. The barrier is so thick that the trapped electrons cannot be discharged by applying negative Vg and, hence, the second up-scan agrees with the first down-scan. Next, we studied the effect of illuminating the sample by near infrared (NIR) light from a titanium-sapphire laser. The photon energy E NIR of the laser was 1.5 eV. The samples were illuminated for several tenths of a second at Vg = −0.5 V. During this process, the channel resistance decreased and reached their steady-state values. Then, the laser was switched off to let the system reach another steady state. We then measured Ns scanning Vg . The result of sample T1-A is shown by a solid line in Fig. 2B. As this graph shows, after NIR illumination, the Ns –Vg characteristics of the up-scan are shifted to the upper side by 5 × 1010 cm−2 . Note that the amount of the Ns shift due to the illumination agrees with that due to electron storage. This suggests that the same number of positive charges is stored in the QDs after illumination. As the number of stored positive charges in the QDs agrees reasonably well with the number of stored electrons, a single hole can be stored in a QD.
Superlattices and Microstructures, Vol. 25, No. 1/2, 1999 A, Dark
2
13 K Ns (1011cm–2)
249
T1 - B
1.5 T1 - A
1 0.5 0
– 1.0 – 0.5
0.0 0.5 Vg (V)
1.0
1.5
2.0
B, Light
Ns (1011cm–2)
2 1.5
T1 - A
1 0.5 0
T1 - B – 1.0 – 0.5
0.0 0.5 Vg (V)
1.0
1.5
2.0
Fig. 2. Electron concentration (Ns ) in 2D channel as a function of surface gate voltage (Vg ) measured by Hall. A, In the dark: electrons are trapped in the QDs; B, after light illumination: holes and electrons are stored in the up-scan and the down-scan of Vg , respectively. 12
N (1010cm–2)
10
T2-A (Wd = 80 nm) T2-B (Wd = 10 nm)
8 6 4 4.2 K
2 0 –0.4
–0.2
0.0
Cooled down at –0.4V 0.2 0.4 0.6 0.8 1.0
Fig. 3. Ns –Vg characteristics measured by the Hall effect at 4.2 K, for the T2-A and B. The samples are cooled down at Vg = −0.4 V.
3.2. Type II In Type I, once an electron is stored in a QD, activation processes are required in order to discharge the electron. Next, we describe a new type of QD-FET, in which electron trap and discharge processes are fully controlled by Vg . We prepared two Type II QD-FETs, T2-A and T2-B, in which the dot layers were separated by 10–80 nm of AlGaAs barriers. Ns for sample T2-A and B is shown in Fig. 3. In the second Vg up-scan of T2-A, Ns –Vg characteristics agree with the first up-scan very well, which suggests that trapped electrons in the dots are discharged at a Vg of between 0.1–0.5 V. The ground state in the QW channel appears to resonate with the excited states of the QDs or with the InAs wetting layer state, which facilitates the electron tunneling from
250
Superlattices and Microstructures, Vol. 25, No. 1/2, 1999
the QW to the QDs. A tunneled electron is easily relaxed into a QD ground state. At a Vg lower than 0.5 V, as the barrier effect is lowered by the electric field near the 2D channel, the states in the QDs can resonate with external states, which facilitates the electron tunneling from the QDs to the QW. These two different resonance processes may cause the hysteresis in Ns –Vg characteristics. In T2-B, the InAs dot layer is located so close to the QW that the Vg cannot lever the QD states efficiently and, hence, the electrons stay in the QDs at a Vg of −0.2 V.
4. Conclusions In summary, we demonstrated two types of QD-FETs using GaAs/n-AlGaAs heterostructures and InAs quantum dots. First, we demonstrated single electron and single hole storage in the QDs. This type of FET may be used for novel photodetectors, photon-storage devices, and single-electron memory devices controlled by gate voltage and light illumination. In regard to the Type II FET, we demonstrated a single-electron storage and discharge action fully controlled by the gate voltage. Acknowledgements—One of the authors, GY, would like to thank the Research Fellowships of Japan Society for the Promotion of Science for Young Scientists for the partial financial support. Part of this work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, and Culture, Japan.
References [1] [2] [3] [4]
}H. Sakaki, Surf. Sci. 267, 623 (1992). }G. Yusa and H. Sakaki, Electron. Lett. 32, 491 (1996). }G. Yusa and H. Sakaki, Appl. Phys. Lett. 70, 345 (1997). }H. Sakaki, G. Yusa, T. Someya, Y. Ohno, T. Noda, H. Akiyama, Y. Kadoya, and H. Noge, Appl. Phys. Lett. 67, 3444 (1995). [5] }S. Tiwari, F. Rana, H. Hanafi, A. Hartstein, E. F. Crabbe, and K. Chan, Appl. Phys. Lett. 68, 1377 (1996).
InAs quantum dot field effect transistors G. Y USA† Institute of Industrial Science, The University of Tokyo, Japan
H. S AKAKI‡ Japan Science and Technology Corporation, Japan
(Received 26 October 1998) We have studied single electron and hole storage in self-assembled InAs quantum dots (QDs) embedded in GaAs/n-AlGaAs field effect transistors (QD-FETs). We prepared two types of QD-FETs. A single electron and a photo-generated single hole can be stored in each QD in Type 1. In the new Type II, single-electron discharge processes can be controlled by a surface gate voltage (Vg ) as well as single-electron storage processes. We demonstrate possible application to novel photo devices and quantum dot memory devices. c 1999 Academic Press
Key words: InAs quantum dots, FET, MBE.
1. Introduction Quantum dot structures (QDs) [1] with zero-dimensionality have become a subject of research in device application, as these structures allow low threshold lasers, novel single-electron devices and detectors. Selfassembled QDs grown on lattice-mismatched substrates have shown to be promising QD fabrications because of their easiness and cleanliness. In this paper, we suggest two different types of quantum dot FETs (QD-FETs) and demonstrate their operation and possible device applications [2, 3].
2. Sample structures We prepared two types of selectively doped GaAs/n-AlGaAs FETs, with embedded InAs self-assembled QDs grown by molecular beam epitaxy (MBE), see Figs 1A and B. For both types, we first grew 200 nm GaAs buffer layers and superlattice buffers containing 11 periods of a 20 nm Al0.25 Ga0.75 As and 2 nm GaAs. The concentration of Si δ-doping is of 1 × 1012 cm−2 [4]. The type 1 QD-FET is based on an inverted HEMT structure (T1-A). We grew a 70 nm-thick undoped Al0.25 Ga0.75 As spacer. The InAs dot layer is separated by a 200 nm-thick GaAs layer from the 2D channel. For this type, electrons flow into the dot layer by applying a gate voltage (Vg ). Type II QD-FETs are based on quantum well HEMT structures (T2-A, B). InAs dot layers are embedded in AlGaAs barrier layers and separated from the QW by 5 ∼ 80 nm-thick AlGaAs. The average base diameter, height, and density of the QDs on both GaAs and AlGaAs are 20 nm, 5 nm, and 5–10 × 1010 cm−2 , † E-mail: [email protected]; Web site: http:// www.iis.u-tokyo.ac.jp/∼yusa ‡ Also at: Institute of Industrial Science, The University of Tokyo, Japan
0749–6036/99/020247 + 04
$30.00/0
c 1999 Academic Press
248
Superlattices and Microstructures, Vol. 25, No. 1/2, 1999 A, Type I
Delta-doped Si
InAs dot layer
2DEG
GaAs GaAs buffer
AlGaAs/GaAs superlattice
Wd = 200 nm
600 nm
200 nm
B, Type II
InAs dot layer Delta-doped Si
GaAs buffer
Gate
AlGaAs/GaAs superlattice
AlGaAs Gate
Wd = 2–80 nm 2DEG
Fig. 1. Schematic potential diagram of two types of InAs quantum dot FETs. The InAs dot layer is embedded into the GaAs layer. A, Type I; and in an AlGaAs barrier B, Type II, respectively.
respectively. For each type, we prepared a reference FET without an InAs dot layer, T1-B for Type I and T2-C for Type II. These samples were processed into Hall bars with 100 nm-thick aluminium (A1) gate electrodes.
3. Results and discussion 3.1. Type I First, we measured the electron concentration Ns of T1-A and T1-B by the Hall effect by scanning Vg . The results at 13 K are plotted as a function of Vg in Fig. 2A in the dark and B, after light illumination. In Fig. 2A, Ns of both T1-A and T1-B increase linearly until Vg reaches 0.8 V (up-scan). When Vg exceeds 0.8 V, the gate leakage current between the channel and the gate begins to flow in T1-B. In contrast, Ns of T1-A tends to saturate for Vg > 0.8 V. Then, the potential in the GaAs layer is lowered by the positive Vg and electrons can be easily injected into the QDs. Stored electrons in QDs raise the potential and block the leakage current. Once the QDs are fully occupied with electrons at Vg = 1.5 V, the leakage current starts to flow. In the down-scan of Vg from 1.5 V, the Ns –Vg characteristics of sample T1-A are shifted by 5 × 1010 cm−2 . The number of stored electrons Nelect in the QDs is estimated to be 7.5 × 1010 cm−2 by Gauss’ law. As this number is close to the density of the QDs, each QD seems to capture one electron each. The barrier is so thick that the trapped electrons cannot be discharged by applying negative Vg and, hence, the second up-scan agrees with the first down-scan. Next, we studied the effect of illuminating the sample by near infrared (NIR) light from a titanium-sapphire laser. The photon energy E NIR of the laser was 1.5 eV. The samples were illuminated for several tenths of a second at Vg = −0.5 V. During this process, the channel resistance decreased and reached their steady-state values. Then, the laser was switched off to let the system reach another steady state. We then measured Ns scanning Vg . The result of sample T1-A is shown by a solid line in Fig. 2B. As this graph shows, after NIR illumination, the Ns –Vg characteristics of the up-scan are shifted to the upper side by 5 × 1010 cm−2 . Note that the amount of the Ns shift due to the illumination agrees with that due to electron storage. This suggests that the same number of positive charges is stored in the QDs after illumination. As the number of stored positive charges in the QDs agrees reasonably well with the number of stored electrons, a single hole can be stored in a QD.
Superlattices and Microstructures, Vol. 25, No. 1/2, 1999 A, Dark
2
13 K Ns (1011cm–2)
249
T1 - B
1.5 T1 - A
1 0.5 0
– 1.0 – 0.5
0.0 0.5 Vg (V)
1.0
1.5
2.0
B, Light
Ns (1011cm–2)
2 1.5
T1 - A
1 0.5 0
T1 - B – 1.0 – 0.5
0.0 0.5 Vg (V)
1.0
1.5
2.0
Fig. 2. Electron concentration (Ns ) in 2D channel as a function of surface gate voltage (Vg ) measured by Hall. A, In the dark: electrons are trapped in the QDs; B, after light illumination: holes and electrons are stored in the up-scan and the down-scan of Vg , respectively. 12
N (1010cm–2)
10
T2-A (Wd = 80 nm) T2-B (Wd = 10 nm)
8 6 4 4.2 K
2 0 –0.4
–0.2
0.0
Cooled down at –0.4V 0.2 0.4 0.6 0.8 1.0
Fig. 3. Ns –Vg characteristics measured by the Hall effect at 4.2 K, for the T2-A and B. The samples are cooled down at Vg = −0.4 V.
3.2. Type II In Type I, once an electron is stored in a QD, activation processes are required in order to discharge the electron. Next, we describe a new type of QD-FET, in which electron trap and discharge processes are fully controlled by Vg . We prepared two Type II QD-FETs, T2-A and T2-B, in which the dot layers were separated by 10–80 nm of AlGaAs barriers. Ns for sample T2-A and B is shown in Fig. 3. In the second Vg up-scan of T2-A, Ns –Vg characteristics agree with the first up-scan very well, which suggests that trapped electrons in the dots are discharged at a Vg of between 0.1–0.5 V. The ground state in the QW channel appears to resonate with the excited states of the QDs or with the InAs wetting layer state, which facilitates the electron tunneling from
250
Superlattices and Microstructures, Vol. 25, No. 1/2, 1999
the QW to the QDs. A tunneled electron is easily relaxed into a QD ground state. At a Vg lower than 0.5 V, as the barrier effect is lowered by the electric field near the 2D channel, the states in the QDs can resonate with external states, which facilitates the electron tunneling from the QDs to the QW. These two different resonance processes may cause the hysteresis in Ns –Vg characteristics. In T2-B, the InAs dot layer is located so close to the QW that the Vg cannot lever the QD states efficiently and, hence, the electrons stay in the QDs at a Vg of −0.2 V.
4. Conclusions In summary, we demonstrated two types of QD-FETs using GaAs/n-AlGaAs heterostructures and InAs quantum dots. First, we demonstrated single electron and single hole storage in the QDs. This type of FET may be used for novel photodetectors, photon-storage devices, and single-electron memory devices controlled by gate voltage and light illumination. In regard to the Type II FET, we demonstrated a single-electron storage and discharge action fully controlled by the gate voltage. Acknowledgements—One of the authors, GY, would like to thank the Research Fellowships of Japan Society for the Promotion of Science for Young Scientists for the partial financial support. Part of this work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, and Culture, Japan.
References [1] [2] [3] [4]
}H. Sakaki, Surf. Sci. 267, 623 (1992). }G. Yusa and H. Sakaki, Electron. Lett. 32, 491 (1996). }G. Yusa and H. Sakaki, Appl. Phys. Lett. 70, 345 (1997). }H. Sakaki, G. Yusa, T. Someya, Y. Ohno, T. Noda, H. Akiyama, Y. Kadoya, and H. Noge, Appl. Phys. Lett. 67, 3444 (1995). [5] }S. Tiwari, F. Rana, H. Hanafi, A. Hartstein, E. F. Crabbe, and K. Chan, Appl. Phys. Lett. 68, 1377 (1996).