Spin-Gapless Semiconductors Show Promise in Spintronic Devices

July 08, 2020 by Antonio Anzaldua Jr.

The relatively new semiconductor—spin-gapless semiconductors—bridges zero‐gap materials and half‐metals while having spin-polarized electrons and holes.

Conventional electronics and information technology are based on the electron’s charge and bandgap. In contrast, the emerging field of spintronics relies on the spin of electrons (their intrinsic angular momentum) in addition to their charge.

Researchers at the University of Wollongong (UoW) recently completed an extensive study of spin-gapless semiconductors (SGS), a new class of materials that bridges semiconductors and half-metals. These researchers began experimenting with materials that would prevent wasted energy dissipation from electrical conduction. 


The band structures of parabolic and Dirac type SGS materials with spin-orbital coupling

Dirac and parabolic SGS materials' band structures along with spin-orbital coupling. Image used courtesy of Centre for Future Low-Energy Electronics Technologies (Fleet)


Experts in this field claim that SGS will allow researchers to better develop ultra-fast, ultra-low power spintronics.


Origins of Spin Gapless Semiconductors

Spin-gapless semiconductors were first proposed by Professor Xiaolin Wang, Director of Institute for superconducting and electronic materials at UoW in 2008. At the onset of their research, Professor Wang and his team had been trying to find suitable materials for ultra-fast spintronic applications. The team’s goal was to operate a spintronic device with no wasted dissipation of energy from electrical conduction. 

Important to Wang's research was an understanding of different materials' band structures.

For instance, in silicon, the conduction and valence band are separated by a small gap. With small threshold energy, it can boost electrons into the material’s conduction band. For conductors, a material needs a small gap between valence and conduction band to allow electrons to flow easily. In insulators, materials are separated by a larger bandgap, preventing the flow of electrons.


Valence (lower) bands and conduction (upper) bands in metal, insulator, or semiconductor.

Valence (lower) bands and conduction (upper) bands in metal, insulator, or semiconductor. Image used courtesy of Centre for Future Low-Energy Electronics Technologies (Fleet)


In the study, Professor Wang and his team discovered that a spin-gapless semiconductor had both the conduction and valence band together at the edge. This meant that no threshold energy was needed to move electrons. The new material was thus sensitive to external influences. SGS bridges zero‐gap materials and half‐metals while having fully spin-polarized electrons and holes.


Branches of SGS: Dirac Dispersion and Parabolic Dispersion

Earlier this month, Professor Wang and his team at UoW began pulling further away from standard silicon-based semiconductors by looking at a Dirac and three sub-types of parabolic SGSs in different systems. SGS band structure falls into two categories of energy dispersion: Dirac linear dispersion and parabolic dispersion. 

SGS categorized as Dirac dispersion allows the electron’s effective mass to be eliminated. Dirac-type spin gapless semiconductors can achieve dissipationless charge transport at the sample edge by separating out fully spin-polarized charges using an applied external field or internal magnetization.

Parabolic subtypes present gapless behavior for both conduction and valence bands but spin in different directions. However, spintronic devices need only one spin orientation.


How SGS May Impact Spintronics

Spintronic devices quickly transport electrons without dissipating energy. These devices can also manipulate charge and become controllable once external energy is applied. To meet these requirements, Professor Wang sought ways to eliminate the mass of charged particles and to make those massless charges fully electron spin-polarized.

Professor Wang shared his thoughts on how SGS will impact spintronics: “Potential applications of SGSs in next-generation spintronic devices [include] low-electronics, and optoelectronics with high speed and low energy consumption.” 

Because SGS allows for twice as many degrees of freedom to manipulate, spintronic devices have the potential for much more efficient data storage and transfer. Researchers select the most useful Dirac SGSs material based on Manganese oxide shaped in honeycomb-like lattices. 


Manganese oxide shaped in honeycomb-like lattices

Manganese oxide shaped in honeycomb-like lattices. Image used courtesy of the Centre for Future Low-Energy Electronics Technologies (Fleet)

Professor Wang proposed there are two key selection criteria for massless, dissipationless spintronics: namely, that they are ferromagnetic, and they have suitable lattices to create the necessary band structure.


Will SGS Hold Up Against Silicon and Other Materials?

In the past decade, a large number of Dirac or parabolic type SGSs have been predicted by density functional theory, and some parabolic SGSs have been experimentally demonstrated in both monolayer and bulk materials. A number of potential candidates for Dirac-type SGS have been reviewed in Wang’s latest work.

In semiconductor circuits that use silicon as a base, electrons scatter off imperfections and structure, dissipating energy in the process. It is this wasted energy that makes up the bulk of the 5% of global electricity being consumed in information technology (IT) server farms and data centers. 

While it is too early to determine whether SGS will be a notable competitor to silicon or other semiconductors, engineers may see an eventual increase in SGS-based materials in spintronic devices. This search for Dirac SGSs for massless and dissipationless spintronics may stimulate interest in other material candidates as well.