2D Heterostructures

One of the most urgent problems of current electronics industry is related to the size of the elements used in the building of the various devices. All our computers, phones, etc are fabricated using complex, engineered structures generally composed by several layers of materials with different properties.

Society demands that these structures meet, among others, two critical criteria:

– On the one hand, they must be small. Laptops, smart phones, etc are the result of the search for more and more compact devices.

– At the same time, they must become more and more powerful, i.e. perform more operations and quicker. That is, we want smaller structures at no expense of the power of the device.

Gordon E. Moore, one of the co-founders of Intel, empirically stated in 1965 that the number of transistors (the basic electronic element these devices are based off of) per unit area would double every year. It is thanks to the efforts of the scientific community and industry that this has been possible, thus achieving our original goal: the miniaturization of devices at almost no expense. Currently, the largest number of transistors in a commercially available single-chip processor is 7.2 billion in 456 mm2, with transistor sizes in the range of tens of nanometres (1 nm = 1 millionth of a mm), which is basically just some tens of atoms.

However, it is possible that things might get too small. Indeed, transistors, as well as any other electronic devices, are based on the principle by which the motion of electrons can be controlled by tuning some energy levels inside the materials involved. When, however, things are very small, we enter the realm of Quantum Mechanics. This is where the phenomenon of tunnelling might occur: electrons going through a material which should be forbidden to them due to a lack of sufficient energy.

As a comparison, imagine the following. In classical physics, if you have a metallic ball and throw it against a wall, usually the metallic ball will bounce back. However, if you throw it with sufficient energy, it might go through the wall. In quantum physics, though, throwing a ball at low energy implies that the ball has also a non-zero chance of traversing the wall, even though this is classically forbidden.

You can therefore now see the problem: if things get too small we cannot control the motion of electrons anymore by using the energetics of the materials, because these electrons will be able to go through them even in those cases where we don’t want them to.

This is where the 2D materials come in. To build a proper semiconductor structure, at least the following elements are needed: a metal, a semiconductor, and an insulator. As it happens, the realm of 2D materials are capable of providing us with all those three elements: graphene acting as a metal, transition metal dichalcogenides (TMDs) acting as semiconductors, and hexagonal boron nitride (hBN) acting as an insulator.

 

2dheterostructure
Representation of different types of 2D materials and how they can be stacked one on top of the other. Nature 499, 419 (2013)

 

These materials have the advantage of being only 1-3 atoms thick, thus reducing the current size limit of electronic elements by a factor of 10.

The combination of these materials require, however, proper engineering to build so-called 2D heterostructures. You can imagine them as placing Lego bricks one on top of the other to achieve complex structures such as quantum light-emitting diodes1 (LEDs which emit single photons; see post of Luca Sortino to know more about this).

Therefore, the arising of the use of 2D heterostructures has opened the door to many new and exciting physics.

 

1. Nat Comun 7, 12978 (2016)

 

By Alejandro Rodriguez, PhD Student of the Quantum Information and Nanoscale Metrology group of Prof. Atature at the University of Cambridge, UK.

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