Impact of barrier width on topological insulator phase in InN/InGaN quantum wells with moderate strain

We present a theoretical study demonstrating that in InN/InGaN quantum wells, the topological insulator phase depends largely not only on the quantum well width, but also on the width of the barriers. We show that for structures with a large width of the barriers equal to 200 nm, the topological insulator exists only when the quantum well width is less than 4.5 nm. For quantum wells with widths of 4.5 nm, we obtain a unique topological phase transition from the normal insulator phase to the nonlocal topological semimetal via the Weyl semimetal phase. Decreasing the width of the barriers from 200 to 20 nm results in a large increase in the bulk energy gap in the topological insulator phase, which can greatly facilitate experimental verification of the topological insulator in InN/InGaN quantum wells. We reveal that this effect originates from increasing the built-in electric field in the barriers, which remarkably decreases the penetration of the conduction band wavefunction in the barrier. We also demonstrate that the bulk energy gap in the topological insulator phase is larger in the free-standing structures than in the structures grown on the substrates with the same In content as the barriers.