Parallel nanogap fabrication with nanometre size control
using III-V semiconductor epitaxial technology.
Iván Fernández-Martínez, Yolanda González and Fernando Briones.
Instituto de Microelectrónica de Madrid, IMM-CNM-CSIC, Isaac Newton 8 PTM, 28760 Tres Cantos,
Madrid, Spain.
AUTHOR EMAIL ADDRESS: ivan@imm.cnm.csic.es
ABSTRACT
A nanogap fabrication process using strained epitaxial III-V beams is reported. The process is
highly reproducible, allows parallel fabrication and nanogap size control. The beams are fabricated from
MBE grown (GaAs/GaP)/AlGaAs strained heterostructures, standard e-beam lithography, and wet
etching. During the wet etching process, the relaxation of the accumulated stress at the epitaxial
heterostructure produces a controlled beam breakage at the previously defined beam notch. After the
breakage, the relaxed strain is proportional to the beam length, allowing nanogap size control. The
starting structure is similar to a mechanically adjustable break junction but the stress causing the
breakage is, in this case, built-in the beam. This novel technique should be useful for molecular-scale
electronics devices.
1
INTRODUCTION
A reproducible and full-wafer compatible fabrication process for adjustable contact electrodes
separated several nanometres is a major technological challenge. Until now, several approaches have
been developed [1-6] to achieve a controlled parallel process for patterning multiple nanogaps with
controlled sizes. In this letter a novel method for parallel nanogap fabrication with highly accurate gap
size control is presented. This method takes advantage of the thickness control at monolayer level of the
Molecular-Beam Epitaxy (MBE) technique to grow multilayer heterostructures. The developed process
consists of a MBE growth of a strained epitaxial (GaAs/GaP)/AlGaAs heterostructure followed by the
fabrication of free-standing GaAs/GaP beams using standard semiconductor processing techniques. The
fabricated GaAs/GaP beams relax their accumulated stress during the substrate release process. We use
this relaxation process to achieve coplanar nano-separated electrodes with nanometre resolution.
EXPERIMENTAL DETAILS: SAMPLE GROWTH
All samples used in this work were grown on GaAs (001) substrates. The samples consist of an
initial 100 nm thick GaAs buffer layer grown at substrate temperature (Ts) of 580 ºC by conventional
MBE. At this point Ts is decreased at Ts = 450 ºC and the rest of the layers are grown by atomic layer
MBE (ALMBE) [7]. This growth technique has shown its capability for obtaining flat and abrupt
interfaces in III–V heteroepitaxial systems [8]. Follow the GaAs buffer layer, 1 µm thick Al0.75Ga0.25As
undoped layer was grown. This Al0.75Ga0.25As acts as a sacrificial layer, i.e., can be selectively wet
etched [9]. At the top of this sacrificial layer 10 periods of (n-type) (GaAs)56/(GaP)1 superlattice (SL)
were grown. The heterostructure is finished by a highly Si doped 56 monolayer (ML) thick GaAs layer.
In the following we will refer to this layer as pseudo GaAsP layer for simplicity. Notice that the 10 ML
of GaP are symmetrically distributed through the pseudo GaAsP layer. This pseudo GaAsP layer grows
2
under a strong in-plane tensile accumulated stress due to the fact that the in-plane equilibrium lattice
parameter of GaP is 3.6% smaller than that of the GaAs. As we embedded individual GaP layers in
GaAs, it is ensured that no misfit dislocations are formed [10]. A schematic picture of the grown
heterostructure is shown in Fig.1.
Figure 1) Schematic pictures of the grown (pseudoGaAsP)/AlGaAs heterostructure on GaAs (001)
substrate. 10 MLs of GaP are introduced symmetrically in the pseudo GaAsP top layer.
EXPERIMENTAL DETAILS: DEVICE FABRICATION
Once the heteroepitaxy is grown, we use the following process to fabricate the beams. In the first step,
a beam with a narrow neck (constriction or notch) is defined by standard e-beam lithography. This
constriction is patterned with an asymmetric shape and it is not centered with respect to the beam. The
structure is patterned aligning the beams along the [110] or [1-10] crystallographic directions which are
contained in (1-10) and (110) planes respectively, i.e., the easiest GaAs cleavage planes. The length of
the patterned structure (L1) is tailored from 4 to 10m and the width (t) of the constriction is kept fixed
at 400nm. To transfer the resist mask pattern to the pseudo GaAsP layer we choose an isotropic wet
3
chemical etching using a H3PO4:H2O2:DI water solution (8:4:68). Precise control of the wet etching
process allows narrow and clean constrictions. After this wet chemical etching, the constriction width
decreases to around 300nm. An oxygen plasma was used to remove the resist mask. A schematic picture
of the device after these processes is shown in Fig 2.a). Finally, the beam is released by using a selective
wet etching of the Al0.75Ga0.25As sacrificial layer with an HF solution (1.5% in water). Cyclohexane
sublimation in an N2 atmosphere at –10ºC has been used to avoid beam collapse or stiction by surface
tension during etch medium removal and drying.
a)
Pseudo GaAsP
Al0.75Ga0.25 As
L1
GaAs (001)
Al0.75Ga0.25 As
t
Pseudo GaAsP
b)
dgap
1
2
Pseudo GaAsP
Al0.75Ga0.25 As
L1
Lundercut
GaAs (001)
Lundercut
dgap
1
2
4
Figure 2 a) Schematic pictures of the pattern transferred to the pseudo GaAsP layer. A beam of length
L1 with a constriction of width t is patterned using standard e-beam lithography and a first wet chemical
etching using H3PO4:H2O2:DI in water solution. b) Schematic pictures of the epitaxial break junction
formed after wet selective etching of the Al0.75Ga0.25As layer. A cantilever of length L1 stays freestanding and contracts due to the elastic strain relaxation during the release process. The direction of
the accumulated elastic stresses (1, 2) are shown by the black arrows. A cleavage in the
nanoconstriction formed during elastic strain relaxation that gives rise to a nano-scale gap (d). The
length Lundercut also contribute to the electrode separation.
Once the entire pseudo GaAsP layer is released, it stays freely suspended, as shown in Fig 2.b).
During the release process, the formed cantilevers contract to achieve the final strain state that
minimizes the system free-energy of the pseudo GaAsP layer. This contraction gives rise to a relaxation
distance (d) which value is calculated using a force-balanced model for strain sharing, between the GaP
and GaAs layers. For an ideal elastic system, the relaxation distance can be expressed as:
d  L
a 0,GaAs  a 0,GaAsP
a 0,GaAs
 L
a 0,GaAs  a 0,GaP
a 0,GaAs

1
nGaAs M GaAs
1

nGaP M GaP
(1)
where L is the length of the total etched structure (L = L1 + 2Lundercut), a0,GaAs is the GaAs equilibrium
lattice parameter and a0,GaAsP is the lattice parameter of the released pseudo GaAsP structure. a0,GaP is
the GaP equilibrium lattice parameter, n and M are the corresponding number of MLs and the biaxial
elastic moduli (M=1/(c11+c12), where c11 and c12 are the elastic constants of the corresponding material).
Then, the relaxation distance is turned into a nanogap due to the presence of a “notch” (constriction) at
the end of the previously defined patterned beam. Fig 2.b). In that constriction, a cleavage is produced
during the in-plane cantilever contraction at the release process, giving rise to two atomically flat
surfaces (cleaved (110) or (1-10) planes) that define a nano-scale gap. The asymmetric shape and the
5
position of the constriction with respect to the beam creates a non uniform lateral strain that assists the
cleavage and consequent nanogap formation. There is a critical length LC to produce a breakage. Below
this value (L<LC), the beam has not stored enough elastic energy to produce a breakage at the beam
nanoconstriction, and the nanogap is not formed. If the length of the structure L is larger than LC, the
cleavage is produced and the equation (1) is valid. For our fabricated device 300nm nanoconstriction
width, the experimental value of LC is between 3.5 m
and 4.0 m, increasing for wider
nanoconstrictions. The calculated elastic energy per unit area stored in each GaP ML is 1.12N/m. As
shown below, in this work we obtain experimental results demonstrating that 10 GaP MLs store enough
elastic energy to break an 300nm width x 200 nm thick semiconductor constriction. The approach
applied in this fabrication process is similar to that used for obtaining mechanically controllable break
junctions (MCBJ) [11] In that device, a notched metal wire is mechanically broken using a piezoelectric
actuator, obtaining a controllable tunneling gap. In our case, e-beam lithography is used to define a
semiconductor “notch” (constriction) at the end of a patterned beam.
A scanning electron microscope (SEM) image of the final processed device, and the nanogap formed
after breaking the constriction are shown in Fig.3). Fig.3.a) shows a top view of the entire device. The
suspended structure is colored in light gray. The GaAs substrate and the non-etched heteroepitaxial layer
are colored in blue and red respectively. From this image, we first can extract the etching rate (1m/min)
of the Al0.75Ga0.25As layer, measuring the length of the undercuting. A SEM image of a constriction with
t = 300nm width is shown on Fig.3.b). Notice the asymmetric shape and the position of the constriction
with respect to the beam. The nanogap formed by the cantilever contraction during the pseudo GaAsP
layer release due to the relaxation of the in-plane accumulated stress (1, 2) is clearly observed on fig.
3c). In this figure the structure has been tilted 90 degrees in order to observe the nanogap details. The
direction of the contraction is represented in Fig.3.a) with two black arrows. An important feature is that
the nanogap consists of atomically flat and strictly parallel surfaces formed by the (110) or (1-10)
cleavage planes.
6
a)
b)
2
2
t
1
1
L1
L undercuting
c)
d
Figure 3) Scanning electron microscope image of the fabricated structures a) Top view of the device
with the suspended structures colored in light gray. GaAs substrate is colored in blue, and un-etched
heteroepitaxy is colored in red. The direction of the contraction due to the relaxation of elastic
accumulated stress (1 and 2) are shown by the black arrows. The undercutting of the Al0.75Ga0.25As
sacrificial layer selective wet etched is also shown. b) Top view of the defined beam constriction, t
denotes the width. c) Side view of the nanogap
(90 degrees SEM tilted image) formed at the
constriction after release.
An array of four cantilevers with the same constriction width and different beam lengths (from 4 to 10
m) has been designed and fabricated. The obtained array is shown on Fig. 4.a). The formed nanogaps
for different cantilever lengths are shown on Fig. 4.b). Fig. 4.c) shows the nanogap size d as a function
of the beam length L1. The data obtained at low nanogap sizes was quite hard due to our SEM resolution
limit .A linear relation is obtained, as expected from eq. (1). These results clearly show that the nanogap
formation mechanism developed in this work allows us to control the nanogap size by changing the III-
7
V heteroepitaxial beam length, getting nanogap sizes as low as 5 nm.
a)
Beam Length
I
b)
4m
II
III
IV
10m
I:d  5nm
c)
II:d  15nm
75
d (nm)
60
45
30
15
0
3 4 5 6 7 8 9 10
Beam length (m)
III:d  50nm
IV:d  70nm
Figure 4) a) Scanning electron microscope image of a four-junction array in which the length of the
cantilever is designed from 4 to 10 m. The device is tilted 45 degrees during the image acquisition. b)
Side view of the different nanogaps that correspond to different beam lengths. The device is tilted 90
degrees during the image acquisition. c) Nanogap size d as a function of the beam length L1.
We have made some electrical measurements of the open circuit leakage current, obtaining a
value of 220nA, at a fixed voltage of 1.5V. This value is typical from low temperature AlGaAs growth.
As it has been previously shown [5], this leakage current can be reduced to the pA regime by means
oxidation of the AlGaAs surfaces, making the devices developed in this work very promising for its
application in molecular electronics.
CONCLUSIONS
8
A novel parallel nanogap fabrication process using strained epitaxial GaAs/GaP beams is reported.
This method combines ALMBE growth of the (GaAs/GaP)/AlGaAs heterostructures with standard ebeam lithography and wet etching procedures to fabricate free-standing structures. A relaxation of the
accumulated stress while the GaAs/GaP beams are released from the substrate is used to achieve nanoseparate electrodes. The formed nanogap consists of atomically flat and strictly parallel surfaces formed
by the (110) or (1-10) cleavage planes. The process is highly reproducible, allows parallel fabrication
and nanogap size control. Moreover, III-V epitaxial semiconductor growth permits a precise doping
control in each epitaxial layer. This is interesting to implement mechanical adjustability of the beams or
electrostatic gating by applying a voltage between the substrate and the pseudo GaAsP top layer.
ACKNOWLEDGMENTS
The author IFM would like to thank MEC for the FPI financial support. J.P.Silveira and J.L.CostaKrämer are gratefully acknowledged for the fruitful discussions and the stimulating support.
REFERENCES
[] Champagne A R, Pasupathy A N and Ralph D C 2005 NanoLetters 5 305
[2] Park H, Lim A K L, Park J, Alivisatos A P and McEuen P L 1999 Appl. Phys. Lett. 75 301
[3] Reed M A, Zhou C, Muller C J, Burgin T P and Tour J M 1997 Science 278 252
[4] Qin L, Park S, Huang L and Mirkin C A 2005 Science 309 113
[5] Krahne R, Yacoby A, Shtrikman H, Bar-Joseph I, Dadosh T and Sperling J 2002 Appl. Phys. Lett.
81 730. Maruccio G, Marzo P, Krahne R, Passaseo A, Cingolani R and Rinaldi R 2007 Small 3 1184
[6] Johnston D E, Strachan D R and Johnson A T 2007 NanoLetters 7 2774
9
[7] Briones F, González L and Ruiz A 1989 Appl. Phys. A 49 729.
[8] Mazuelas A, Ruiz A, Ponce F and Briones F. 1993 J. Phys. D 26 A167
[9] The use of MBE-grown structures that includes a sacrificial layer to fabricate freely suspended
nanometre scale semiconductor structures (NEMS) is called surface nanomachining, and it has been
extensively used. See for example: Ekinci K L 2005 Rev. Sci. Instrum, 76 061101
[10] Mazuelas A, González L, Ponce F A, Tapfer L and Briones F. 1993 Journal of Crystal Growth
131 465. Castrillo P, Armelles G, Silveira J P, Briones F and Barbolla J 1997 Appl. Phys. Lett. 71 1353
[11] Muller C J, van Ruitenbeek J M and de Jongh L J 1992 Physica C 191 485.
10