2 February 2018
Fine tuning boron content in nitride alloys
© Semiconductor Today Magazine / Juno PublishiPicture: Disco’s DAL7440 KABRA laser saw.
On the basis that controlling the electronic properties at the interface between materials can help in the quest for improvements in computer memory, King Abdullah University of Science and Technology (KAUST) has shown that varying the atomic composition of boron nitride (BN)-based alloys enables the tuning of electric polarization.
Some materials exhibit spontaneous polarization, even without an external electric field. Such materials have potential uses in computer memory, but this application requires a material system in which the polarization is controllable. Visiting Student Research Program (VSRP) student Kaikai Liu, his supervisor Xiaohang Li and coworkers investigated one approach to polarization engineering at the interface between BN-based alloys.
Spontaneous polarization is strongly dependent on the structure and composition of the atomic crystal. Piezo-electric materials can change polarization when physically deformed.
The KAUST team used software (the Vienna ab-initio Simulation Package) to investigate electronic properties of the ternary alloys boron aluminum nitride and boron gallium nitride, looking at how they change as boron replaces 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,” says Liu.
The team showed that the spontaneous polarization changes very nonlinearly with increasing boron content, contradicting previous studies that assume a linear relationship.
The reason for this nonlinearity is attributed to volume deformation of the alloy’s wurtzite crystal structure. The nonlinear change in the piezoelectric polarization is less pronounced, but evident. This arises because of the large difference in atomic spacing between boron nitride and both aluminum nitride and gallium nitride. Furthermore, boron aluminum nitride or boron gallium nitride can become non-piezoelectric when the boron content is more than 87% and 74%, respectively.
The work shows that a large range of spontaneous and piezoelectric polarization constants could be made available simply by changing the boron content. This could be useful for developing optical and electronic junction devices formed at the interface between conventional nitride semiconductors and either boron aluminum nitride or boron gallium nitride.
“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.