JAIST Repository
https://dspace.jaist.ac.jp/Title

Diameter dependence of current‑voltage
characteristics of ultrasmall area AlSb‑InAs
resonant tunneling diodes with diameters down to
20nm

Author(s)Nomoto, K.; Taira, K.; Suzuki, T.; Hase, I.;
Hiroshima, H.; Komuro, M.

Citation

Applied Physics Letters, 70(15): 2025‑2027

Issue Date

1997‑04‑14

Type

Journal Article

Text version

publisher

URL

http://hdl.handle.net/10119/4502

RightsCopyright 1997 American Institute of Physics.
This article may be downloaded for personal use
only. Any other use requires prior permission ofthe author and the American Institute of Physics.
The following article appeared in K.Nomoto,
K.Taira, T.Suzuki, I.Hase, H.Hiroshima, andK.Komuro, Applied Physics Letters, 70(15), 2025‑
2027 (1997) and may be found at
http://link.aip.org/link/?APPLAB/70/2025/1

Description

Japan Advanced Institute of Science and Technology

Diameter dependence of current–voltage characteristics of ultrasmall area
AlSb–InAs resonant tunneling diodes with diameters down to 20 nm
K. Nomoto,a) K. Taira, T. Suzuki, and I. HaseSony Corporation Research Center, Hodogaya-ku, Yokohama 240, Japan

H. Hiroshima and M. Komuro
Electrotechnical Laboratory, Ministry of International Trade and Industry, Tsukuba-city, Ibaraki, 305,
Japan

~Received 7 October 1996; accepted for publication 10 February 1997!
We have studied the current–voltage characteristics I(V) of ultrasmall area AlSb–InAs resonant
tunneling diodes ~RTDs! with diameters down to 20 nm. Resonant tunneling peaks were observed
for all the diodes at room temperature. The peak-to-valley ratio reduces with the decreasing
diameter of the RTD. We found from the diameter dependence of the valley current that thereduction is due to a contribution of the thermally activated surface current to the valley current. ForRTDs with diameters less than 100 nm, we observed fine structures around zero bias at 4 K. They
can be attributed to tunneling through zero-dimensional states confined by a RTD sidewall.
© 1997 American Institute of Physics. @S0003-6951~97!04015-1#
Reducing the area of vertical semiconductor resonant
tunneling diodes ~RTDs! is of much interest, because thereduction makes it possible ~i! to study single-electron tunneling and the Coulomb blockade effect,1–3 ~ii! to increase
the device density on a chip using RTD,4 and ~iii! to reducethe capacitance that determines the cutoff frequency.5 Up todate, most of the research on small area RTDs has employedan AlGaAs–GaAs material system. However, when the diameter of the AlGaAs–GaAs RTD is reduced to less than
100 nm, the contact layers of the RTD are pinched off by thesurface depletion layer induced by the surface Fermi-level
pinning in the middle of the band gap of GaAs. For InAs, on
the other hand, the surface Fermi level is located just below
or above the conduction-band edge and no surface depletion
layer exists.6 Therefore, InAs-based RTDs are expected tohave no diameter limitation. Although InAs-based RTDs
have been studied for high-frequency application,5 and tounderstand the basic physics of tunneling affected by bandstructure,7 no attempt to reduce the diameter has been reported. In this letter, we report I(V) characteristics of symmetric AlSb–InAs double-barrier RTDs with diameters
down to 20 nm which is, to our knowledge, the smallest
diameter of a RTD that has ever been reported. We will first
focus on the diameter dependence of the peak current, thevalley current, and the ratio of these currents. There is adebate about the origin of the increasing valley current density observed for smaller area AlGaAs–GaAs RTDs.2,8 We
confirm that for AlSb–InAs RTDs, the surface leakage
current is responsible for most of the increase in thevalley current density. We also report fine structures due totunneling through zero-dimensional ~0D! states observed indI(V)/dV curve at 4 K.
The device layer was grown by molecular beam epitaxy
at 500 °C on a ~001!-oriented 131018 cm23 S-doped
n-InAs substrate. The layer sequence was as follows: ~i! 600
nm n-InAs bottom contact layer (n5131017 cm23 ),~ii! 10nm InAs spacer layer, ~iii! 5 nm AlSb barrier, ~iv! 6 nm InAs
well, ~v! 5 nm AlSb barrier, ~vi! 10 nm InAs spacer layer,a!

Electronic mail: knomoto@src.sony.co.jp

Appl. Phys. Lett. 70 (15), 14 April 1997~vii! 100 nm n-InAs contact layer (n5831017 cm23 ), and~viii! 330 nm n 1 -InAs top contact layer (n5131018
cm23 ). Silicon was used as the dopant. Layers ~ii!–~vi! wereundoped. Using electron-beam lithography with a JEOL 50
keV electron-beam exposure system, JBX-6000SA, andpolymethylmethacrylate resist, nonalloy AuGe/Au Ohmic
metallization dots with diameters between 2000 and 20 nm
were defined on n 1 -InAs top contact. The Ohmic metalserved as an etch mask for SiCl4 /He reactive ion etching
defining the mesas in the epitaxial structure. The diode diameters were measured using a scanning electron microscope. To make contact to the top of the mesa, a planarizing
and insulating photoresist was spun on the as-etched sample
and then etched back by O2 plasma etching to expose theFIG. 1. I(V) curves at room temperature of three RTDs with diameters f of20, 80, and 300 nm. Inset: Schematic band diagram of the RTD investigated.
The 2D-subband energy is obtained from the calculation of the transmission
probability for zero bias.

0003-6951/97/70(15)/2025/3/$10.00© 1997 American Institute of Physics

Copyright ©2001. All Rights Reserved.

2025

FIG. 3. I(V) and dI(V)/dV curves observed at 4 K of the RTD with adiameter of 80 nm. Fine structures observed at approximately the same
absolute voltage are marked with the same letter.

FIG. 2. Peak current ~s!, valley current ~d!, and resonant current ~n! atroom temperature ~a! and 4 K ~b! as a function of RTD diameters. The
best-fitted lines I RT , I v , and their sum I p 5I RT1I v are also shown. Inset:
Peak-to-valley ratio as a function of diameters. Solid line is calculated using(I RT1I v )/I v .

Ohmic metal. A gold contact pad was then evaporated over
the top of the mesa.
Figure 1 shows I(V)curves of diodes with diameters of20, 80, and 300 nm. The voltage is applied to the top contact
with respect to the grounded substrate. Current peaks areobserved at room temperature for all RTDs. The peaks at;60.25 V are caused by resonant tunneling through thelowest two-dimensional ~2D! subband in the InAs quantum
well. The resonant voltage decreases with decreasing diodediameter, which may be due to a stray series resistance. The‘‘bumps’’ observed at 650 mV for all the diodes are interpreted as the tunneling through an InAs G-point quasiboundstate confined by AlSb X-point barriers.7
In Fig. 2, we show the diameter dependence of the peakcurrent I p , the valley current I v , and the resonant tunnelingcurrent defined by I RT5I p 2I v at positive bias at room temperature and at 4 K. We observed the same characteristics fornegative bias. We found no pinch off diameter for InAs
RTDs. The current component I RT is well fitted by the function I RT5J RT( p f 2 /4), where J RT52.13106 A/m2 irrespective of temperature. Therefore, I RT is attributed to a resonanttunneling-current component. On the other hand, valley
current I v is fitted by the sum of a bulk current and a surface
current, I v 5J v ( p f 2 /4)1J s ( p f ), where J v 51.23106
2026(9.13104 ) A/m2 and J s 54.031021 (5.731022 ) A/m atroom temperature ~4 K!. From this, we know that ~1! theresonant-tunneling-current density does not reduce with decreasing the diode diameter, ~2! not only a bulk current butalso a surface current contributes to the valley current, and~3! both the bulk and the surface current are thermally activated. The peak-to-valley ( P/V) ratio decreases with decreasing the diode diameter, as shown in the inset of Fig. 2.
For AlGaAs–GaAs RTDs, this tendency has been alreadyobserved.2,8 This is due to the increasing valley current density. There is, however, a debate about the origin of the increasing; the weakened lateral-momentum-conservation ruledue to the Heisenberg uncertainty principle2 and surfacerelated phenomena8 are topics of discussion. For InAs–AlSb
RTDs, we conclude from the results ~1!, ~2!, and ~3! that thethermally activated surface current makes the major contribution towards the increasing valley current density. At thediode sidewall surface, AlSb is damaged by both oxidation
and bombardment of ion etchant during the fabrication process, which would cause the midband-gap surface defect levels. Because InAs has an electron accumulating layer at thesurface, the hopping through such surface defect levels
would contribute to the thermally activated surface current.
Surface–phonon scattering can also cause such a surface current.
Figure 3 shows I(V)and dI(V)/dV curves at 4 K for a
RTD with a diameter of 80 nm, which is the smallestdiameter RTD that we successfully measured at 4 K. We
observed fine structures around zero bias. The fine structuresare symmetric in voltage. We have not observed such structures at room temperature or for RTDs with diameters larger
than 200 nm. The pattern of the fine structure was reproducible for different samples with the same diameter. We consider that the fine structure is attributed to tunneling through0D states in an InAs well confined by a RTD sidewall. The
spacing between the fine structures is 4–10 mV, which corresponds to 2–5 meV in energy. If we assume that the lateralpotential is a square well potential with infinite barrier height
and the well width f580 nm, the energy spacing becomesAppl. Phys. Lett., Vol. 70, No. 15, 14 April 1997

Copyright ©2001. All Rights Reserved.

Nomoto et al.

* f 2 )52.5 meV. This value is in good agree;\ 2 p 2 /(2m InAs
ment with the experimental results. The irregular peak spacing suggests that the lateral potential is not a parabolic potential. This would be because the electrons in InAs areconfined not by the surface depletion layer, but abruptly in
an insulator.
In summary, the diameter dependence of I(V) characteristics of AlSb–InAs RTDs with diameter down to 20 nm has
been studied. The peak-to-valley ratio reduces with decreasing diode diameter. The diameter dependence of the valley
current indicates that the reduction is due to the thermally
activated surface current. We also observed fine structures
around zero bias at 4 K. The sample dependence and thespacing in voltage suggest that they are caused by tunnelingthrough the 0D state confined by an insulator at the diodesidewall.
The authors would like to thank T. Shimada for her technical assistance. The authors are also grateful to J. Westwater

for his help in the preparation of the manuscript. This work
was performed under the management of FED as a part ofthe MITI R&D program ~Quantum Functional Devices
Project! supported by NEDO.
1

M. A. Reed, J. N. Randall, R. J. Aggarwal, R. J. Matyi, T. M. Moore, andA. E. Wetsel, Phys. Rev. Lett. 60, 535 ~1988!.
2
B. Su, V. J. Goldman, and J. E. Chunningham, Phys. Rev. B 46, 7644
~1992!.
3
K. Nomoto, T. Suzuki, K. Taira, and I. Hase, Jpn. J. Appl. Phys. 1 33,L1142 ~1994!; Phys. Rev. B 55, 2523 ~1997!.
4For a review, see Physics of Quantum Electron Devices, edited by F.
Capasso ~Springer, New York, 1990!.
5L. F. Lio, R. Beresford, and W. I. Wang, Appl. Phys. Lett. 53, 2320~1988!.
6
A. G. Milnes and A. Y. Polyakov, Mater. Sci. Eng. B 18, 237 ~1993!.
7
R. E. Carnahan, M. A. Maldonado, K. P. Martin, A. Nogaret, R. J. Higgins, L. A. Cury, D. K. Maude, J. C. Portal, J. F. Chen, and A. Y. Cho,Appl. Phys. Lett. 62, 1385 ~1993!.
8T. Schmidt, M. Tewordt, R. J. Haug, K. v. Kliting, B. Shönherr, P. Grambow, A. Förster, and H. Lüth, Appl. Phys. Lett. 68, 838 ~1996!.

Appl. Phys. Lett., Vol. 70, No. 15, 14 April 1997

Copyright ©2001. All Rights Reserved.

Nomoto et al.

2027