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STM studies on quantum wire structures in air and liquid helium
Superlattices and Microstructures, Vol. 20, No. 3, 1996
STM studies on quantum wire structures in air and liquid helium J. Smoliner, C. Eder, G. Strasser Institut f¨ur Festk¨orperelektronik, Floragasse 7, A-1040 Wien, Austria
¨ hm G. Bo Walter Schottky Institut, TU M¨unchen, D- 85748, Garching, Germany
G. Weimann Frauenhoferinstitut f¨ur angewandte Festk¨orperphysik, Tullastraße 72, D-79108 Freiburg, Germany
(Received 20 May 1996) In this work, low temperature scanning tunneling microscopy (STM) studies on quantum wires are reported, which were fabricated by laser holography and wet chemical etching. Inverted heterostructures with thin and highly doped cap layers were used as substrates in order to keep the total tunneling barrier as small as possible. Current–voltage curves were measured on the wires and in the depleted areas between them. Between the wires, significant current is only observed for electrons which tunnel from the GaAs valence band into the STM tip, whereas symmetric current–voltage curves are observed on the wires. This behavior is ascribed to the influence of surface depletion and thus, a comparison of current imaging spectroscopy data taken at 300 K and in liquid helium directly yields the edge depletion width of the quantum wires. c 1996 Academic Press Limited
Scanning tunneling microscopy [1] (STM) is widely used to investigate surface effects on various materials. Single GaAs–AlGaAs heterostructure interfaces were first investigated by Salemink et al. [2], who observed the electron depletion and confinement region at the GaAs–AlGaAs interface directly in real space. Similar experiments were even carried out in air by Tanimoto et al. [3]. Due to the difficulties of finding single interfaces on cleaved surfaces, further work was mainly carried out on multilayer systems such as GaAs– AlGaAs multi quantum wells [4]. Also multilayers of pn-junctions were investigated on Silicon [5, 6] and GaAs–AlGaAs heterostructures [7]. On both materials the current voltage characteristics are clearly different if recorded in a p-doped or n-doped region [6]. Surface passivation was found to give better results since it helps to keep the interfaces clean and reduces tip-induced band bending effects [8, 9]. Using appropriate surface passivation, such experiments can also be carried out in air, where a special imaging method was used to decouple the electronic contrast from the topography [10]. Until now, STM studies on quantum wires were mainly carried out by cross sectional techniques on directly grown quantum wires. In this paper, STM and current imaging tunneling spectroscopy (CITS) studies are 0749–6036/96/070261 + 05 $18.00/0
c 1996 Academic Press Limited
262
Superlattices and Microstructures, Vol. 20, No. 3, 1996 I(V)
STM tip A
B
z(Å) 138 0 25000
Au-Au/Ge-Ni contact
1DEG
1DEG
1DEG
1DEG
0 GaAs
AlGaAs
n+-GaAs GaAs
y(Å)
2DEG
x(Å)
depleted
0 25000
2DEG
Fig. 1. A, Sample geometry and self consistently calculated conduction band profiles for the etched and non etched areas of the sample. B, 3D topographic image of a quantum wire.
reported on quantum wires, which were fabricated by wet chemical etching. Comparing the CITS and surface profiles taken at 4.2 K, the edge depletion width on the quantum wires can be determined directly. The samples for this experiment consist of an unintentionally p-doped GaAs layer grown on a semi˚ doped AlGaAs (d = 50 A, ˚ ND = insulating substrate followed by an undoped spacer (d = 200 A), 18 −3 ˚ ˚ 3×10 cm , x = 35%), another spacer (d = 250 A) and n-doped GaAs (d = 800 A, N D = 1.2×1015 cm−3 ). ˚ N D = 6.2 × 1018 cm−3 ). At the upper GaAs–AlGaAs The GaAs cap layer was highly n-doped (d = 150 A, interface, a two-dimensional (2D) electron gas is formed. At T = 4.2 K, the electron mobility in this 2D electron gas is µ = 70 000 cm2 Vs−1 at an electron concentration of n = 3.5 × 1011 cm−2 . On the samples, ˚ deep into large photoresist gratings having a period of 3 µm and a wire width of 1 µm were etched 100 A the GaAs. Thus, quantum wires are formed at the upper GaAs–AlGaAs interface. Note that for technical ˚ AlGaAs barrier between reasons, a second 2D channel also exists at the lower interface. However, the 500 A the channels is too thick for tunneling processes, and thus, a direct charge transfer between the upper and lower channel can be neglected. Figure 1A shows a schematic view of the sample geometry and a self consistently calculated conduction band profile for the etched and non etched areas of the sample. Due to the highly doped cap layer in the non etched areas, the surface depletion depth and therefore the tunneling barrier between the surface and the quantum wires is extremely small. In the etched areas however, the whole layer between the surface and the AlGaAs is depleted. ˚ high Figure 1B shows a three-dimensional topographic image of a single quantum wire. The wire is 100 A and the etched areas are relatively smooth. Local current voltage curves were traced directly on the wires and in the surface depleted areas between the wires. Figure 2A shows the data measured at 300 K. On the wires, a tunneling current is observed for both bias directions. Between the wires, however, a tunneling current is only observed for negative sample bias. To explain this behavior, we first consider the consistently calculated conduction band profiles in the non etched regions of the sample. As shown earlier [11], the resistance between the surface and the upper electron layer is small compared to the tunneling resistance across the vacuum barrier. Thus, electrons injected from
Superlattices and Microstructures, Vol. 20, No. 3, 1996 A 4 3
T = 300 K
263 B
Non-etched areas
0.4 0 I (nA) 25000
I (nA)
2 1 0
Etched areas
–1
y(Å)
–2 –3 –4 –2 –1.5 –1 –0.5 0 0.5 Sample bias (V)
0 1
1.5
2
x(Å)
0 25000
Fig. 2. A, Local IV curves traced directly on the wires and in the etched areas of the sample (T = 300 K). V. B, 3D CITS image measured at 300 K.
the STM into the sample will be collected in the upper electron layer. Depending on the size of the applied voltage, the charge transfer between the tip and the wires happens via surface states and subsequent tunneling through the surface space charge layer or by direct injection into the conduction band. For negative sample bias, electrons predominantly from the valence band of the semiconductor will tunnel through the vacuum barrier. Recombination of holes with free electrons from the sub surface electron layer via surface states prevents a charging of the sample. In the etched regions of the sample, however, the resistance of the upper GaAs layer is much larger than in the non etched areas since the low doped GaAs layer is completely surface depleted here. Thus, the voltage which is applied between the wires and the tip will drop partially in the depleted regions of the sample and also across the vacuum barrier. Further, the transition region between the depleted areas and the wires forms an internal n− –n+ diode. As a consequence, the tip Fermi level can not be raised above the Schottky barrier at the sample surface for positive sample bias because the voltage drop in the sample is too large. Moreover, the n− –n+ diode is biased in reverse direction, which also rules out a carrier transfer between the tip and the wires. For negative sample bias, however, the n− –n+ diode is biased in forward direction and thus, electrons are allowed to flow from the wire regions into the depleted areas. These carriers can reach the sample surface and therefore, a tunneling current is observed for negative sample bias only. For more details about this mechanism see Ref. [12]. For a quantitative analysis of this behavior, current imaging tunneling spectroscopy was applied. For this measurement, the sample topography was scanned at negative bias (V = −3 V), where tunneling current is observed in all areas of the sample. Simultaneously, the current is also measured at positive bias (V = +3 V) at each point of the sample. The 3D view of the corresponding current image taken at 300 K is shown in Fig. 2B. The quantum wire is clearly resolved and everywhere in the etched areas beside the wire, the current is zero. The structures on the wire are purely noise and not correlated with the topography. All the measurements above were also carried out in liquid helium. Qualitatively, the IV curves show the same behavior as at T = 300 K, which means that they are symmetric on the wires and asymmetric in the etched areas between the wires. Figure 3 shows the topographic profile and the corresponding CITS profiles measured at T = 300 K and T = 4.2 K. Due to the reduced scanning range at T = 4.2 K, the profiles were only measured across the edge of the quantum wire. To obtain a better signal to noise ratio, the profiles were averaged along the wire. As one can see in Fig. 3, the topographic profiles and the CITS profiles are shifted with respect to each other. At 300 K, the relative shift is in the order of 0.1 µm. At 4.2 K, however, the relative shift is approximately ≈ 0.35 µm and thus the current onset is clearly shifted into an area on top of the wire.
264
Superlattices and Microstructures, Vol. 20, No. 3, 1996 0.4
120
0.35
100
0.25 0.2
60 40
(1) (2)
(3)
End of scanning range at T = 4.2K
I (nA)
Height (Å)
0.3 80
0.15 0.1
20
T = 4.2K
0.05 0
0 0
0.5
1 x(µm)
1.5
2
Fig. 3. Topographic profile (curve 1) and current imaging tunneling spectroscopy (CITS) profiles taken at T = 300 K (curve 2) and T = 4.2 K (curve 3). Note that the scanning range reduces from 10 µm × 10 µm to 2 µm × 2 µm if the temperature is reduced to T = 4.2 K.
As shown above, tunneling into the sample is not possible in areas where the upper electron channel is depleted. Thus, difference between the topographic and CITS profile in first approximation is a direct measure for the edge depletion width on our quantum wires. The larger value for the edge depletion width at low temperatures is explained by the carrier freezeout in the doped AlGaAs. As known from unstructured samples, the electron concentration in a 2D channel decreases with decreasing temperature, which is due to a reduction of the effective concentration of ionized donors in the AlGaAs barrier. A lower concentration of ionized donors, however, leads to larger depletion lengths and thus, also the edge depletion width of the quantum wires will increase at low temperatures. In summary, STM studies were performed on wet chemically etched quantum wires. To keep the barrier between the wire and the surface as small as possible, the wires were fabricated on inverted GaAs–AlGaAs heterostructures with highly doped GaAs cap layer. Current–voltage curves measured directly on the wires and in the depleted areas between the wires, display a completely different behavior. On the wires, tunneling is possible for both bias directions, in depleted areas, however, an internal diode inhibits tunneling for positive sample bias. To evaluate this behavior more quantitatively, CITS images were measured at 300 K and in liquid helium. At low temperatures, a comparison of the current and topographic profiles directly yields the edge depletion width of the quantum wire. Acknowledgements—This work was sponsored by Fonds zur F¨orderung der wissenschaftlichen Forschung (FWF) project No. P11285-PHY and Gesellschaft f¨ur Mikroelektronik (GMe). The authors are grateful to E. Gornik for continuous support.
References [1] G. Binnig, G. Rohrer, Ch. Gerber and E. Weibel, Phys. Rev. Lett. 49, 57 (1982). [2] H. W. M. Salemink, H. P. Meier, R. Ellialtioglu, J. W. Gerritsen and P. R. M. Muralt, Appl. Phys. Lett. 54, 1112 (1989). [3] M. Tanimoto and Y. Nakano, J. Vac. Sci. Technol. A8, 553 (1990). [4] H. Salemink and O. Albrektsen, J. Vac. Sci. Technol. B9, 779 (1991). [5] S. Kordic, E. J. van Loenen and A. J. Walker, Appl. Phys. Lett. 59, 3154 (1991).
Superlattices and Microstructures, Vol. 20, No. 3, 1996
265
[6] Sumio Hosaka, Shigeyuki Hosoki, Keiji Takata, Katsudata Horiuchi and Nobuyoshi Natsukai, Appl. Phys. Lett. 53, 487 (1988). [7] H. W. M. Salemink, M. B. Johnson, O. Albrektsen and P. Konraad, Solid State Electron. 37, 1053 (1994). [8] S. Gwo, K J. Chao and C. K. Shih, Appl. Phys. Lett. 64, 493 (1994). [9] S. Gwo, K. J. Chao, C. K. Shih, K. Sadra and B. G. Streetman, J. Vac. Sci. Technol. B11, 1509 (1993). [10] T. Pinnington, S. N. Patitsas, C. Lavoie, A. Sanderson and T. Tiedje, J. Vac. Sci. Technol. B11, 908 (1993). [11] J. Smoliner, Semicond. Sci. Technol. 11, 1 (1996). [12] C. Eder, J. Smoliner, G. B¨ohm and G. Weimann, Submitted to Semicond. Sci. Technol. (1996).
266
Superlattices and Microstructures, Vol. 20, No. 3, 1996
STM studies on quantum wire structures in air and liquid helium J. Smoliner, C. Eder, G. Strasser Institut f¨ur Festk¨orperelektronik, Floragasse 7, A-1040 Wien, Austria
¨ hm G. Bo Walter Schottky Institut, TU M¨unchen, D- 85748, Garching, Germany
G. Weimann Frauenhoferinstitut f¨ur angewandte Festk¨orperphysik, Tullastraße 72, D-79108 Freiburg, Germany
(Received 20 May 1996) In this work, low temperature scanning tunneling microscopy (STM) studies on quantum wires are reported, which were fabricated by laser holography and wet chemical etching. Inverted heterostructures with thin and highly doped cap layers were used as substrates in order to keep the total tunneling barrier as small as possible. Current–voltage curves were measured on the wires and in the depleted areas between them. Between the wires, significant current is only observed for electrons which tunnel from the GaAs valence band into the STM tip, whereas symmetric current–voltage curves are observed on the wires. This behavior is ascribed to the influence of surface depletion and thus, a comparison of current imaging spectroscopy data taken at 300 K and in liquid helium directly yields the edge depletion width of the quantum wires. c 1996 Academic Press Limited
Scanning tunneling microscopy [1] (STM) is widely used to investigate surface effects on various materials. Single GaAs–AlGaAs heterostructure interfaces were first investigated by Salemink et al. [2], who observed the electron depletion and confinement region at the GaAs–AlGaAs interface directly in real space. Similar experiments were even carried out in air by Tanimoto et al. [3]. Due to the difficulties of finding single interfaces on cleaved surfaces, further work was mainly carried out on multilayer systems such as GaAs– AlGaAs multi quantum wells [4]. Also multilayers of pn-junctions were investigated on Silicon [5, 6] and GaAs–AlGaAs heterostructures [7]. On both materials the current voltage characteristics are clearly different if recorded in a p-doped or n-doped region [6]. Surface passivation was found to give better results since it helps to keep the interfaces clean and reduces tip-induced band bending effects [8, 9]. Using appropriate surface passivation, such experiments can also be carried out in air, where a special imaging method was used to decouple the electronic contrast from the topography [10]. Until now, STM studies on quantum wires were mainly carried out by cross sectional techniques on directly grown quantum wires. In this paper, STM and current imaging tunneling spectroscopy (CITS) studies are 0749–6036/96/070261 + 05 $18.00/0
c 1996 Academic Press Limited
262
Superlattices and Microstructures, Vol. 20, No. 3, 1996 I(V)
STM tip A
B
z(Å) 138 0 25000
Au-Au/Ge-Ni contact
1DEG
1DEG
1DEG
1DEG
0 GaAs
AlGaAs
n+-GaAs GaAs
y(Å)
2DEG
x(Å)
depleted
0 25000
2DEG
Fig. 1. A, Sample geometry and self consistently calculated conduction band profiles for the etched and non etched areas of the sample. B, 3D topographic image of a quantum wire.
reported on quantum wires, which were fabricated by wet chemical etching. Comparing the CITS and surface profiles taken at 4.2 K, the edge depletion width on the quantum wires can be determined directly. The samples for this experiment consist of an unintentionally p-doped GaAs layer grown on a semi˚ doped AlGaAs (d = 50 A, ˚ ND = insulating substrate followed by an undoped spacer (d = 200 A), 18 −3 ˚ ˚ 3×10 cm , x = 35%), another spacer (d = 250 A) and n-doped GaAs (d = 800 A, N D = 1.2×1015 cm−3 ). ˚ N D = 6.2 × 1018 cm−3 ). At the upper GaAs–AlGaAs The GaAs cap layer was highly n-doped (d = 150 A, interface, a two-dimensional (2D) electron gas is formed. At T = 4.2 K, the electron mobility in this 2D electron gas is µ = 70 000 cm2 Vs−1 at an electron concentration of n = 3.5 × 1011 cm−2 . On the samples, ˚ deep into large photoresist gratings having a period of 3 µm and a wire width of 1 µm were etched 100 A the GaAs. Thus, quantum wires are formed at the upper GaAs–AlGaAs interface. Note that for technical ˚ AlGaAs barrier between reasons, a second 2D channel also exists at the lower interface. However, the 500 A the channels is too thick for tunneling processes, and thus, a direct charge transfer between the upper and lower channel can be neglected. Figure 1A shows a schematic view of the sample geometry and a self consistently calculated conduction band profile for the etched and non etched areas of the sample. Due to the highly doped cap layer in the non etched areas, the surface depletion depth and therefore the tunneling barrier between the surface and the quantum wires is extremely small. In the etched areas however, the whole layer between the surface and the AlGaAs is depleted. ˚ high Figure 1B shows a three-dimensional topographic image of a single quantum wire. The wire is 100 A and the etched areas are relatively smooth. Local current voltage curves were traced directly on the wires and in the surface depleted areas between the wires. Figure 2A shows the data measured at 300 K. On the wires, a tunneling current is observed for both bias directions. Between the wires, however, a tunneling current is only observed for negative sample bias. To explain this behavior, we first consider the consistently calculated conduction band profiles in the non etched regions of the sample. As shown earlier [11], the resistance between the surface and the upper electron layer is small compared to the tunneling resistance across the vacuum barrier. Thus, electrons injected from
Superlattices and Microstructures, Vol. 20, No. 3, 1996 A 4 3
T = 300 K
263 B
Non-etched areas
0.4 0 I (nA) 25000
I (nA)
2 1 0
Etched areas
–1
y(Å)
–2 –3 –4 –2 –1.5 –1 –0.5 0 0.5 Sample bias (V)
0 1
1.5
2
x(Å)
0 25000
Fig. 2. A, Local IV curves traced directly on the wires and in the etched areas of the sample (T = 300 K). V. B, 3D CITS image measured at 300 K.
the STM into the sample will be collected in the upper electron layer. Depending on the size of the applied voltage, the charge transfer between the tip and the wires happens via surface states and subsequent tunneling through the surface space charge layer or by direct injection into the conduction band. For negative sample bias, electrons predominantly from the valence band of the semiconductor will tunnel through the vacuum barrier. Recombination of holes with free electrons from the sub surface electron layer via surface states prevents a charging of the sample. In the etched regions of the sample, however, the resistance of the upper GaAs layer is much larger than in the non etched areas since the low doped GaAs layer is completely surface depleted here. Thus, the voltage which is applied between the wires and the tip will drop partially in the depleted regions of the sample and also across the vacuum barrier. Further, the transition region between the depleted areas and the wires forms an internal n− –n+ diode. As a consequence, the tip Fermi level can not be raised above the Schottky barrier at the sample surface for positive sample bias because the voltage drop in the sample is too large. Moreover, the n− –n+ diode is biased in reverse direction, which also rules out a carrier transfer between the tip and the wires. For negative sample bias, however, the n− –n+ diode is biased in forward direction and thus, electrons are allowed to flow from the wire regions into the depleted areas. These carriers can reach the sample surface and therefore, a tunneling current is observed for negative sample bias only. For more details about this mechanism see Ref. [12]. For a quantitative analysis of this behavior, current imaging tunneling spectroscopy was applied. For this measurement, the sample topography was scanned at negative bias (V = −3 V), where tunneling current is observed in all areas of the sample. Simultaneously, the current is also measured at positive bias (V = +3 V) at each point of the sample. The 3D view of the corresponding current image taken at 300 K is shown in Fig. 2B. The quantum wire is clearly resolved and everywhere in the etched areas beside the wire, the current is zero. The structures on the wire are purely noise and not correlated with the topography. All the measurements above were also carried out in liquid helium. Qualitatively, the IV curves show the same behavior as at T = 300 K, which means that they are symmetric on the wires and asymmetric in the etched areas between the wires. Figure 3 shows the topographic profile and the corresponding CITS profiles measured at T = 300 K and T = 4.2 K. Due to the reduced scanning range at T = 4.2 K, the profiles were only measured across the edge of the quantum wire. To obtain a better signal to noise ratio, the profiles were averaged along the wire. As one can see in Fig. 3, the topographic profiles and the CITS profiles are shifted with respect to each other. At 300 K, the relative shift is in the order of 0.1 µm. At 4.2 K, however, the relative shift is approximately ≈ 0.35 µm and thus the current onset is clearly shifted into an area on top of the wire.
264
Superlattices and Microstructures, Vol. 20, No. 3, 1996 0.4
120
0.35
100
0.25 0.2
60 40
(1) (2)
(3)
End of scanning range at T = 4.2K
I (nA)
Height (Å)
0.3 80
0.15 0.1
20
T = 4.2K
0.05 0
0 0
0.5
1 x(µm)
1.5
2
Fig. 3. Topographic profile (curve 1) and current imaging tunneling spectroscopy (CITS) profiles taken at T = 300 K (curve 2) and T = 4.2 K (curve 3). Note that the scanning range reduces from 10 µm × 10 µm to 2 µm × 2 µm if the temperature is reduced to T = 4.2 K.
As shown above, tunneling into the sample is not possible in areas where the upper electron channel is depleted. Thus, difference between the topographic and CITS profile in first approximation is a direct measure for the edge depletion width on our quantum wires. The larger value for the edge depletion width at low temperatures is explained by the carrier freezeout in the doped AlGaAs. As known from unstructured samples, the electron concentration in a 2D channel decreases with decreasing temperature, which is due to a reduction of the effective concentration of ionized donors in the AlGaAs barrier. A lower concentration of ionized donors, however, leads to larger depletion lengths and thus, also the edge depletion width of the quantum wires will increase at low temperatures. In summary, STM studies were performed on wet chemically etched quantum wires. To keep the barrier between the wire and the surface as small as possible, the wires were fabricated on inverted GaAs–AlGaAs heterostructures with highly doped GaAs cap layer. Current–voltage curves measured directly on the wires and in the depleted areas between the wires, display a completely different behavior. On the wires, tunneling is possible for both bias directions, in depleted areas, however, an internal diode inhibits tunneling for positive sample bias. To evaluate this behavior more quantitatively, CITS images were measured at 300 K and in liquid helium. At low temperatures, a comparison of the current and topographic profiles directly yields the edge depletion width of the quantum wire. Acknowledgements—This work was sponsored by Fonds zur F¨orderung der wissenschaftlichen Forschung (FWF) project No. P11285-PHY and Gesellschaft f¨ur Mikroelektronik (GMe). The authors are grateful to E. Gornik for continuous support.
References [1] G. Binnig, G. Rohrer, Ch. Gerber and E. Weibel, Phys. Rev. Lett. 49, 57 (1982). [2] H. W. M. Salemink, H. P. Meier, R. Ellialtioglu, J. W. Gerritsen and P. R. M. Muralt, Appl. Phys. Lett. 54, 1112 (1989). [3] M. Tanimoto and Y. Nakano, J. Vac. Sci. Technol. A8, 553 (1990). [4] H. Salemink and O. Albrektsen, J. Vac. Sci. Technol. B9, 779 (1991). [5] S. Kordic, E. J. van Loenen and A. J. Walker, Appl. Phys. Lett. 59, 3154 (1991).
Superlattices and Microstructures, Vol. 20, No. 3, 1996
265
[6] Sumio Hosaka, Shigeyuki Hosoki, Keiji Takata, Katsudata Horiuchi and Nobuyoshi Natsukai, Appl. Phys. Lett. 53, 487 (1988). [7] H. W. M. Salemink, M. B. Johnson, O. Albrektsen and P. Konraad, Solid State Electron. 37, 1053 (1994). [8] S. Gwo, K J. Chao and C. K. Shih, Appl. Phys. Lett. 64, 493 (1994). [9] S. Gwo, K. J. Chao, C. K. Shih, K. Sadra and B. G. Streetman, J. Vac. Sci. Technol. B11, 1509 (1993). [10] T. Pinnington, S. N. Patitsas, C. Lavoie, A. Sanderson and T. Tiedje, J. Vac. Sci. Technol. B11, 908 (1993). [11] J. Smoliner, Semicond. Sci. Technol. 11, 1 (1996). [12] C. Eder, J. Smoliner, G. B¨ohm and G. Weimann, Submitted to Semicond. Sci. Technol. (1996).
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Superlattices and Microstructures, Vol. 20, No. 3, 1996