In the quest to improve computer memory, controlling the electronic properties at the interface between materials - varying the atomic composition of boron-nitride-based alloys to fine tune its polarization, for example - is key, believe researchers from Saudi Arabia.
When an electric field is applied to a single atom, the centre of the mass cloud of negatively-charged electrons shifts away from the positively-charged nucleus it is surrounding. In a crystaline solid, these so-called electric dipoles of all atoms combine to create an effect known as electric polarization.
Some materials demonstrate spontaneous polarization even without an external electrical field, and it is these materials which may have potential uses in improving computer memory. However, for this to be useful, the application requires a material system in which polarization is controllable. Spontaneous polarization is strongly dependent on the structure and composition of the atomic crystal, and some materials – known as piezo electrics – can change polarization when physically deformed.
Researchers from the King Abdullah University of Science and Technology (KAUST) have been investigating one approach to polarization engineering at the interface between boron-nitride based alloys, which show promise for use in optical and electronic interface devices. The researchers have shown that varying boron content enables tuning of the electric polarization at the interface between boron aluminum nitride and boron gallium nitride alloys.
Using software called the Vienna ab initio Simulation Package, visiting student KaiKai Liu, his supervisor Xiaohang Li and their colleagues investigated the electronic properties of ternary alloys boron aluminum nitride and boron gallium nitride. They focussed on how the electronic properties change as boron replaces the aluminum and gallium atoms respectively.
We calculated the spontaneous polarization and piezoelectric constants of boron nitride alloys within a newly proposed theoretical framework and the impact of the polarization at junctions of these two materials.
The team demonstrated that the spontaneous polarization changes nonlinearly with increasing boron content; a finding contradicting previous studies which had assumed the relationship would be linear.
The reason for this nonlinearity has been attributed to the volume deformation of the alloy's unusual atomic structure, known as wurtzite – a hexagonal crystal structure. The wurtzite's structure is non-centrosymmetric, meaning it lacks inversion symmetry. Because of this, wurtzite crystals demonstrate piezoelectricity and pyroelectricity - a property of certain crystals which are electrically polarized naturally - which centrosymmetric crystals lack.
"The spontaneous polarization constants show moderate nonlinearity due to the volume deformation and the dipole moment difference between the hexagonal and wurtzite structures. The piezoelectric constants exhibit significant bowing because of the large lattice difference between binary alloys" the researchers write in Applied Physical Letters. "The large range of SP and PZ constants of BxAl1-xN (BAlN) and BxGa1-xN (BGaN) can be beneficial for the compound semiconductor device development."
The piezoelectric polarization nonlinear change is less pronounced, but it is evident. It arises because of the large difference in atomic spacing between boron nitride and both aluminum nitride and gallium nitride. If the boron content is more than 74% for boron gallium nitride and 87% for boron aluminum nitride, they can become nonpiezoelectric.
The work shows that, by simply changing the boron content, a large range of spontaneous and piezoelectric polarization constant could be made available. This could be beneficial when developing electronic and optical junction devices, formed at the interface between either boron aluminum nitride or boron gallium nitride and the conventional nitride semiconductors.
"Our next step will be to experimentally test the proposed junctions, which our theory predicts could have much better device performance than current approaches," says Liu.
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