Microchips of the future: Suitable insulators are still missing

Date:
March 9, 2021
Source:
Vienna University of Technology
Summary:
2D semiconductors (such as graphene) could revolutionize
electronics: They can be used to produce extremely small
transistors. However, in order to make a transistor, insulators are
required too. So extremely thin insulating materials are needed
as well. New results show: The materials used until now are not
the way to go.



FULL STORY ==========================================================================
For decades, there has been a trend in microelectronics towards ever
smaller and more compact transistors. 2D materials such as graphene
are seen as a beacon of hope here: they are the thinnest material
layers that can possibly exist, consisting of only one or a few atomic
layers. Nevertheless, they can conduct electrical currents -- conventional silicon technology, on the other hand, no longer works properly if the
layers become too thin.


========================================================================== However, such materials are not used in a vacuum; they have to be
combined with suitable insulators -- in order to seal them off from
unwanted environmental influences, and also in order to control the flow
of current via the so-called field effect. Until now, hexagonal boron
nitride (hBN) has frequently been used for this purpose as it forms an excellent environment for 2D materials.

However, studies conducted by TU Wien, in cooperation with ETH Zurich,
the Russian Ioffe Institute and researchers from Saudi Arabia and Japan,
now show that, contrary to previous assumptions, thin hBN layers are not suitable as insulators for future miniaturised field-effect transistors,
as exorbitant leakage currents occur. So if 2D materials are really to revolutionise the semiconductor industry, one has to start looking for
other insulator materials.

The study has now been published in the scientific journal "Nature Electronics." The supposedly perfect insulator material "At first glance, hexagonal boron nitride fits graphene and two-dimensional materials better
than any other insulator," says Theresia Knobloch, first author of the
study, who is currently working on her dissertation in Tibor Grasser's
team at the Institute of Microelectronics at TU Wien. "Just like the
2D semiconducting materials, hBN consists of individual atomic layers
that are only weakly bonded to each other." As a result, hBN can easily
be used to make atomically smooth surfaces that do not interfere with
the transport of electrons through 2D materials. "You might therefore
think that hBN is the perfect material -- both as a substrate on which
to place thin-film semiconductors and also as a gate insulator needed
to build field-effect transistors," says Tibor Grasser.

Small leakage currents with big effects A transistor can be compared
to a water tap -- only instead of a stream of water, electric current
is switched on and off. As with a water tap, it is very important for
a transistor that nothing leaks out of the valve itself.

This is exactly what the gate insulator is responsible for in the
transistor: It isolates the controlling electrode, via which the current
flow is switched on and off, from the semiconducting channel itself,
through which the current then flows. A modern microprocessor contains
about 50 billion transistors -- so even a small loss of current at the
gates can play an enormous role, because it significantly increases the
total energy consumption.

In this study, the research team investigated the leakage currents that
flow through thin hBN layers, both experimentally and using theoretical calculations. They found that some of the properties that make hBN such
a suitable substrate also significantly increase the leakage currents
through hBN. Boron nitride has a small dielectric constant, which
means that the material interacts only weakly with electric fields. In consequence, the hBN layers used in miniaturised transistors must only
be a few atomic layers thick so that the gate's electric field can
sufficiently control the channel. At the same time, however, the leakage currents become too large in this case, as they increase exponentially
when reducing the layer thickness.

The search for insulators "Our results show that hBN is not suitable as a
gate insulator for miniaturised transistors based on 2D materials," says
Tibor Grasser. "This finding is an important guide for future studies,
but it is only the beginning of the search for suitable insulators for
the smallest transistors. Currently, no known material system can meet
all the requirements, but it is only a matter of time and resources until
a suitable material system is found." "The problem is complex, but this
makes it all the more important that many scientists devote themselves
to the search for a solution, because our society will need small,
fast and, above all, energy-efficient computer chips in the future,"
Theresia Knobloch is convinced.

========================================================================== Story Source: Materials provided by Vienna_University_of_Technology. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Theresia Knobloch, Yury Yu. Illarionov, Fabian Ducry, Christian
Schleich,
Stefan Wachter, Kenji Watanabe, Takashi Taniguchi, Thomas Mueller,
Michael Waltl, Mario Lanza, Mikhail I. Vexler, Mathieu Luisier,
Tibor Grasser. The performance limits of hexagonal boron nitride
as an insulator for scaled CMOS devices based on two-dimensional
materials.

Nature Electronics, 2021; 4 (2): 98 DOI: 10.1038/s41928-020-00529-x ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/03/210309114317.htm

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