Impacts of Point Defects on Shallow Doping in Cubic Boron Arsenide: A First Principles Study

Technical Summary: Impacts of Point Defects on Shallow Doping in Cubic Boron Arsenide

Problem Statement
Cubic boron arsenide (BAs) has emerged as a promising material for advanced electronics due to its exceptional thermal conductivity and ambipolar mobility. However, the practical application of BAs is hindered by the difficulty in effectively controlling p-type and n-type doping. While intrinsic defects and impurities are known to exist in BAs samples and influence thermal properties, their specific impact on electronic properties—particularly the formation and stability of shallow impurity states required for effective doping—remains incompletely understood. Recent studies suggest that impurities like Si, C, and H may induce unintended p-type behavior, yet a comprehensive investigation into how common point defects interact with intended shallow dopants has been lacking.

Methodology
This study employs first-principles calculations based on Density Functional Theory (DFT) to investigate the thermodynamic and electronic interactions between point defects and shallow dopants in BAs.

• Computational Framework: Calculations were performed using the Vienna ab initio Simulation Package (VASP) with the projector augmented-wave (PAW) method. The generalized gradient approximation (GGA) formulated by the PBEsol functional was utilized to balance accuracy and computational cost.
• System Configuration: A $3 \times 3 \times 3$ supercell (216 atoms) of the conventional cubic cell was used for defect calculations, with a fixed lattice parameter of 4.778 Å. A $2 \times 2 \times 2$ $\Gamma$-centered k-point mesh and a plane-wave cutoff energy of 700 eV were applied.
• Defect and Dopant Selection: The study focused on four shallow dopants identified in previous literature: p-type ($Be_B$, $Si_{As}$) and n-type ($Si_B$, $Se_{As}$). Common impurities (C, O, Si) and intrinsic defects were analyzed. The most favorable configurations for single defects and nearest-neighbor defect pairs were selected based on formation energies.
• Analysis Metrics: Key properties computed included formation energies ($E_f$), electronic density of states (DOS), and charge density isosurfaces. The formation energy of a dopant in the presence of a defect ($E^I_f(D)$) was calculated to determine if defects facilitate or hinder dopant formation. The nature of doping (shallow vs. deep) was assessed by examining the position of the Fermi level ($E_F$) relative to band edges and the spatial delocalization of charge density near the valence band maximum (VBM) or conduction band minimum (CBM).

Key Contributions and Results
The study systematically evaluated the formation energies and electronic characteristics of selected dopants in the presence of various point defects.

1. Identification of Favorable Defects:

   • Among impurities (C, O, Si), the most favorable point defects were identified as $C_{As}$, $O_{BOAs}$ (an O-O-As complex), $Si_{As}$, $C_{As}Si_B$, and $OBSiAs$. All exhibited formation energies below 1.5 eV.
   • Among intrinsic defects, the antisite pair $As_B B_{As}$ was found to have a formation energy comparable to the most favorable impurity defects.
   • Notably, the formation energy of $O_{BOAs}$ was calculated to be negative (-0.53 eV) under the PBEsol functional, suggesting high stability, though this value was revised to 0.10 eV using the HSEsol functional.

2. Impact on p-type Doping ($Be_B$ and $Si_{As}$):

   • Most point defects, including the charge-neutral $As_B B_{As}$ and $C_{As}Si_B$, as well as acceptor-like defects ($C_{As}$, $Si_{As}$), preserved the shallow p-type nature of $Be_B$ and $Si_{As}$.
   • The O impurity, specifically in the form of $O_{BOAs}$, was found to be detrimental. It created a localized defect trap state at the top of the VBM, disrupting the shallow acceptor state.
   • For $Si_{As}$, $O_{BOAs}$ formed a trap state in the middle of the band gap, which is also detrimental.

3. Impact on n-type Doping ($Si_B$ and $Se_{As}$):

   • n-type doping proved more sensitive to defects. The presence of acceptor-like impurities ($C_{As}$ and $Si_{As}$) shifted the Fermi level out of the conduction band, effectively destroying the n-type character.
   • Donor-like defects ($O_{BOAs}$, $OBSiAs$) formed localized trap states within the band gap, further compromising n-type conductivity.
   • Only the charge-neutral defects, $As_B B_{As}$ and $C_{As}Si_B$, were found to preserve the n-type shallow state, maintaining the Fermi level slightly above the CBM with moderate spatial delocalization.

4. Beneficial Role of Intrinsic Defects:

   • Contrary to the detrimental effects of many impurities, the intrinsic antisite defect pair $As_B B_{As}$ was found to benefit both p-type and n-type doping, preserving the shallow nature of all four dopants investigated.

Significance and Claims
The paper claims that its findings provide a critical thermodynamic analysis of the interaction between point defects and shallow dopants in BAs. The primary significance lies in the identification of specific impurity constraints and the beneficial role of intrinsic defects:

• Doping Limitations: The study highlights that n-type doping in BAs is particularly challenging, requiring not only the selection of appropriate dopants but also the rigorous removal of C, Si, and O impurities to prevent the formation of compensating defects or trap states.
• Strategic Development: The identification of $As_B B_{As}$ as a beneficial intrinsic defect suggests that controlling the concentration of this specific antisite pair could be a viable strategy for enhancing both p- and n-type doping efficiency.
• Impurity Control: The results indicate that while p-type doping is relatively robust against most impurities (except O), n-type doping is highly vulnerable. Consequently, the development of BAs-based electronics requires strict control over O, C, and Si concentrations, particularly for n-type applications.

The authors conclude that these insights into defect-dopant interactions can pave the way for the strategic development of BAs-based electronics by guiding the synthesis and processing conditions necessary to achieve effective doping.