A new class of 2D semiconductor has been developed by researchers in the US. The free-standing quantum membranes, which are made of indium arsenide (InAs) and are just a few layers thick, have properties that are in stark contrast to the semiconductor layers used in conventional transistors. In addition to the fundamental-science implications, the new work could help us better understand how so-called structurally confined semiconductor devices function.

Electrons in nanometre-thick semiconductor films behave differently from those in thicker samples because their motion is effectively constrained to two dimensions. These novel properties have been put to work in high-performance electronics and quantum optics, while more recently scientists have turned their attention to even thinner materials such as graphene because of these materials' even more intriguing properties.

Now, Ali Javey of the University of California, Berkeley and colleagues have developed another class of 2D semiconductors dubbed quantum membranes (QMs), which have band structures that can be tuned from bulk to 2D simply by changing the thickness of the material. These QMs are made of indium arsenide (InAs), which is a "III–V" compound semiconductor.

The tuneable properties of QMs rely on an effect called quantum confinement, whereby the electronic and optical properties of an object change as it becomes smaller. The effect usually kicks in at about 10 nm or less, and it occurs because electrons and holes are squeezed into a space that approaches a critical quantum measurement, called the exciton Bohr radius. This is the average distance between an electron and hole in a bulk sample of the semiconductor.

Free-standing membrane

Unlike conventional compound-semiconductor quantum-well structures that can only be created on certain substrates, InAs QMs are confined at both the top and bottom surfaces by either an insulator layer and/or a vacuum. This means that they are free standing and can be placed on a variety of substrates. What is more, the structure of the QM is such that the active semiconductor layer in the material can be placed in direct contact with the gate stack in a field-effect transistor (FET) – something that could allow them to be used in high-performance devices.

Javey's team employed an epitaxial-layer transfer technique that involves growing 5–50 nm thick InAs on GaSb/AlGaSb substrates. The top InAs layer is then patterned to the desired shape and structure, and this is followed by a selective wet etch of the sacrificial AlGaSb layer. The InAs layer is then picked up and transferred onto a substrate such as Si/SiO2 or CaF2.

"The result is a free-standing InAs QM on a substrate of our choice that is bonded through Van der Waals interactions without the constraints of the original growth substrate," Javey told physicsworld.com. "This allows us to explore the fundamental material and device properties of QMs with varying thicknesses."

Thanks to optical-absorption studies, the researchers succeeded in mapping out the sub-band energies of InAs QMs while varying the thickness of the structures. They also explored the effect of quantized sub-bands on the electrical properties of FETs made from the QMs and found that electron mobility in the material did not depend on the applied field, except at very high fields. Such behaviour is very different to that seen in conventional metal–oxide–semiconductor FETs (MOSFETs).

High carrier mobility

"This study reveals the basic transport physics and performance limitations in QM field-effect transistors (QMFETs) by studying the quantum-confinement effect in transport properties such as effective mobility and quantum resistance, thus providing guidelines for future device designs, especially for III–V nanoelectronics," says Javey. "III–V devices on silicon substrates are particularly attractive candidates for replacing conventional silicon MOSFETs, thanks to their high carrier mobility, which can enable high-speed, low-power devices."

The team now plans to further explore various other fundamental material properties in these semiconductors and discover how good III–V QMFETs really are when compared with state-of-the-art silicon MOSFETs.

The work is detailed in Nano Letters.