eLearning

Cheng Cen,
Stefan Thiel,
Jochen Mannhart,
Jeremy Levy

Electronic confinement at nanoscale dimensions remains a central means of science and
technology. We demonstrate nanoscale lateral confinement of a quasi–two-dimensional electron
gas at a lanthanum aluminate–strontium titanate interface. Control of this confinement using an
atomic force microscope lithography technique enabled us to create tunnel junctions and field-effect
transistors with characteristic dimensions as small as 2 nanometers. These electronic devices can be
modified or erased without the need for complex lithographic procedures. Our on-demand
nanoelectronics fabrication platform has the potential for widespread technological application.

Controlling electronic confinement in the
solid state is increasingly challenging as
the dimensionality and size scale are
reduced. Bottom-up approaches to nanoelectron-
ics use self-assembly and templated synthesis;
examples include junctions between self-assembled
molecule layers (1, 2), metallic and semicon-
ducting quantum dots, carbon nanotubes (3–6),
nanowires, and nanocrystals (7, 8). Top-down
approaches retain the lithographic design motif
used extensively at micrometer and submicro-
meter scales and make use of tools such as
electron-beam lithography, atomic force micros-
copy (AFM) (9), nanoimprint lithography (10),
dip-pen nanolithography (11), and scanning
tunneling microscopy (12). Among the top-down
approaches, those that begin from modulation-
doped semiconductor heterostructures have led
to profound scientific discoveries (13, 14).
The interface between polar and nonpolar
semiconducting oxides displays remarkable
properties reminiscent of modulation-doped semi-
conductors (15–21). When the thickness of the
polar insulator (e.g., LaAlO3) exceeds a critical
value (dc = 3 unit cell), because of the polarization
discontinuity at the interface, the potential
difference across LaAlO3 will generate a “polar-
ization catastrophe” and induce the formation of a
quasi–two-dimensional electron gas (q-2DEG) at
the interface joining the two insulators (17). In
addition to the key role played by the polar dis-
continuity, there is evidence that, when present,
oxygen vacancies in the SrTiO3 also contribute to
the formation of the electron gas (22, 23).
We focus on LaAlO3-SrTiO3 heterostructures.
Because of the large conduction-band offset
between LaAlO3 and SrTiO3, the q-2DEG is
confined largely within the first few unit cells of
SrTiO3 (22, 24), with very little penetration into
the LaAlO3 layer (25). Electric fields have been
used to control the metal-insulator transition at
room temperature (17) and the superconductor-
insulator transition at cryogenic temperatures (21).
Further in-plane confinement of the q-2DEG has
been achieved by lithographically modulating the
thickness of the crystalline LaAlO3 layer (26).
Control over the metal-insulator transition at
scales of <4 nm was demonstrated by means of
a conducting AFMprobe (24). This latter method
forms the basis for the results reported below.