Ab initio Study Of The Aspects Of AsxP1−x Alloys And SiC/GeC Bilayer Heterostructure

Speaker: Kazi J. Tasnim (PhD Defense)

Abstract: Due to the unique structural and electronic properties along with the practical applications in current and near future, investigations about the group-IV graphene-like two-dimensional (2D) sheets have been accelerated. Among them, 2D SiC and GeC sheets are polar materials with in-plane charge transfer from Si (Ge) to C atoms. An interesting question is how the electrostatic force, triggered by such in-plane charge transfer, plays a role in stabilizing the vertical heterostructures formed by 2D SiC and GeC sheets beyond vdW interaction. To answer this question, we have systematically investigated, the effects of the electrostatic interaction between layers of 2D SiC/GeC bilayer heterostructure with different stacking and out-of-plane species ordering within the framework of density functional theory. The results illustrated that, in addition to the weak vdW interaction, the electrostatic interlayer interaction can also induce the pi - pi orbital hybridization between adjacent layers, with the strong hybridization in the cases of Si-C and C-Ge species orderings but the weak hybridization between layers for C-C ordering. In particular, in the cases of either Si-C or C-Ge species orderings, the attractive electrostatic interlayer interaction stabilizes the inter-layer distance and the vdW interaction makes the system attain a lower cohesive energy. On the contrary, in the case of the C-C species ordering, the vdW interaction mostly controls the inter-layer distance and the repulsive electrostatic inter-layer force has less effect in here. The ways how the layers are stacked and how the species are ordered in the structure can greatly affect the band structure of the hybrid 2D SiC/GeC bilayer heterostructure. When the hybrid system is stacked in a specific way (e.g., AA stacking with Si-C/C-Ge ordering or AB stacking with C-C ordering), it shows an indirect band gap nature. On the other hand, a direct band gap behavior can be achieved under different stacking configurations (e.g., AB stacking with Si-C or C-Ge or Si-Ge ordering, or AA stacking with C-C/Si-Ge ordering). This means that the band gap of the 2D polar hybrid heterostructure can be adjusted depending on how the layers are arranged and how the species are ordered. Additionally, the SiC/GeC hybride structure has a weak type-II band alignment characteristic. These findings indicate that the hybrid structure holds promise for applications in light-emitting diodes and laser emitters.

The structural anisotropy in the principal axes (i.e., along the armchair and the zigzag directions) in black phosphorus (bP) leads to a wide range of applications in nanoelectronics, energy application, etc. However, its high reactivity in the air limits its practical applications. One approach to overcome this problem is to make alloys of bP with other materials with similar structural properties. Black arsenic (b-As) is a good candidate and black As_xP_1-x alloys have attracted experimentalists to synthesize. Since the most stable phase for As is gray As with the β phase,  and the most stable phase for P is the bP with the α phase, it is crucial to understand the whole aspects of the AsxP1-x alloys including (i) their structural and electronic properties in the β and α phases, (ii) the energetics between two phases, and (iii) the effect of Li intercalation and high pressure on their structural properties. In the second part of my research project, we have conducted a comprehensive theoretical study to deeply understand the aspects of AsxP1-x alloys. We have found that (1) structurally, the black As_xP_1-x alloys are stabilized in anisotropic puckered structures with the AB stacking, keeping the α phase as in bP. The gray AsxP1-x alloys, on the other hand, are stabilized in buckled structures with ABC stacking, keeping the  β phase as in the gray As.  The distribution of As and P atoms in the black AsxP1-x alloys prefers to form armchair As-As and P-P bonds, aligning along zigzag direction. But the distribution of As and P atoms in gray As_xP_1-x alloys prefers to form in-plane As-P bonds, instead of As-As or P-P bonds; (2) electronically, alloying widens the band gap compared to pure bP and b-As in black As_xP_1-x alloys, making the materials semiconducting with tunable direct-indirect nature, while the semi-metallic feature in the gray As and blue P is still kept in gray AsxP1-x alloys; (3) there is a critical As concentration, below it black As_xP_1-x alloys are energetically more favorable, but above it gray As_xP_1-x alloys are more favorable; (4) a local structural transformation or phase segregation was found in black As_xP_1-x alloys under either Li intercalation or the high pressure, providing pathways for structural phase transitions and leading to the new type of materials.