Marc Túnica : Doping in hexagonal-diamond Si-based semiconductors

The emergence of hexagonal-diamond (2H) group-IV semiconductors has opened new opportunities for next-generation optoelectronics. Unlike their cubic-diamond (3C) counterparts, whose indirect band gaps limit optical emission, 2H polytypes exhibit distinct electronic structures: 2H-Si presents a lower indirect gap (0.95 eV), while 2H-Ge becomes a pseudo-direct-gap semiconductor with a reduced band gap (0.3 eV).

Furthermore, 2H-Si₁ ₓGeₓ alloys enable continuous band-gap tuning from indirect (2H-Si) ₋ to direct (2H-Ge), covering emission wavelengths in the technologically relevant 1.3–1.8 μm range. Photoluminescence studies reveal that increasing Ge content shifts the conduction-band minimum toward Γ, resulting in a direct and optically allowed transition for Ge concentrations above 0.65. These properties position 2H group-IV materials as promising candidates for CMOS-compatible mid-infrared emitters. However, their full technological potential requires a deeper understanding of doping mechanisms, impurity thermodynamics, and dopant–defect interactions—areas that remain largely unexplored.

This thesis investigates dopant incorporation, segregation, and extended defect interactions in hexagonal Si-based semiconductors using Density Functional Theory. It addresses three central research questions: (i) How does the hexagonal crystal phase modify the structural and electronic behavior of conventional p-type and n-type impurities? (ii) How do extreme boron concentrations affect the structural stability of 2H-SiGe alloys? (iii) How do dopants interact with extended planar defects, particularly the ubiquitous I₃- type basal stacking fault?

The results demonstrate that group-III impurities are generally more stable in the 2H phase, reflecting their preference for local threefold coordination inherent to the hexagonal lattice. Charged acceptors exhibit enhanced stability and slightly shallower chargetransition levels in 2H-Si compared to 3C-Si. Neutral donors show no strong phase preference, whereas positively charged donors favor the cubic structure, leading to shallower donor levels in 3C-Si than in 2H-Si. These findings highlight the critical role of local bonding and crystal symmetry in shaping impurity energetics across polytypes.

Regarding hyperdoping, the thesis shows that ultra-high boron concentrations in 2H-SiGe alloys do not compromise structural or thermodynamic stability, even under heavy doping. This confirms the energetic feasibility of hyperdoping in the hexagonal alloy, suggesting potential for superconductivity in metastable 2H-SiGe, analogous to effects observed in highly doped 3C-Si, Ge, and SiGe. 

Finally, the analysis of dopant interactions with the I₃ basal stacking fault reveals that neutral and negatively charged p-type dopants preferentially occupy lattice sites away from the fault, a tendency significantly weaker for neutral or positively charged n-type dopants and isovalent impurities. This segregation behavior arises from a complex interplay of electrostatic ionization, local steric distortions, and symmetry breaking at the defect, pointing out the critical influence of local defect environments on dopant stability and distribution in 2H-Si.

Overall, this thesis establishes a comprehensive theoretical framework for understanding doping phenomena in hexagonal group-IV semiconductors. The results support the rational design of optimized 2H-Si, 2H-Ge, and 2H-SiGe materials and nanostructures, provide insights into their behavior under technologically relevant doping conditions, and guide future experimental advancements in optoelectronic and quantum-material applications