First-Principles Calculations on the Thermodynamic and Electronic Properties of Defective Semiconductors and Semiconductor Alloys

Abstract

The continuously growing computing power and the advent of advanced computational algorithms have enabled the atomistic simulations of materials at an unprecedented accuracy and scalability. In particular, first-principles calculations based on density functional theory (DFT) provide exceptional predictive capability in studying solid-state materials. In this dissertation, I applied DFT calculations to study a wide range of semiconductors that show great promise in electronic and optoelectronic applications, with a focus on the thermodynamics of point defects in semiconductors and computational discovery of novel semiconductor alloys.

Point defects are pervasive in semiconductors and it is crucial to understand their impact on the properties. Efficient p-type doping of ultrawide bandgap nitride semiconductors has been a long-standing challenge. By calculating the formation energy and charge-transition levels for intrinsic defects and extrinsic Mg dopant, I discover two avenues towards an enhanced p-doping efficiency for AlN and Al-rich AlGaN alloy. The first approach is to engineer the N chemical potentials to be N-rich, which dramatically reduces the formation energy for Mg dopant by about 2 eV and increases the formation energy of compensating N vacancy by almost 3 eV. Another method is to engineer the position of the Fermi level away from the valence band of AlGaN alloy by the formation of Ga/AlGaN Schottky junction during non-equilibrium growth. This leads to an extremely small formation energy of 0.4 eV for Mg dopant. These two approaches can be generalized to a broader range of ultrawide bandgap semiconductors.

Using the same methodology, the electronic properties and defect physics of LaN and Cu2O are investigated. Contrast to previous claim that LaN is a semimetal, I find that LaN has a direct bandgap of 0.62 eV and is an intrinsic n-type semiconductor. The origin of its electrical conductivity is likely due to the formation of N vacancy and substitutional O impurities. The dominant intrinsic defects of Cu2O are the Cu simple vacancy and the Cu split vacancy. However, their large ionization energies lead to low hole concentration. I survey a variety of candidate elements for p-type doping and find that N, S, and Mg are effective p-dopants.

Another part of this dissertation studies the thermodynamic and electronic properties of emerging semiconductors and semiconductor alloys. II-IV-N2 materials exhibit unique properties due to disordering on the cation sublattice. The short-range disorder around the N atom has large impact on the thermodynamic stability. Using ZnSnN2 as a model system, I find that the formation energy decreases with respect to an increased fraction of octet-rule conserving Zn2Sn2 motifs. Interestingly, the bandgap of ZnSnN2 is linearly proportional to the square of the long-range order parameter, which enables alloy-free bandgap engineering. In some cases, configurational disorder can improve the thermodynamic stability by maximizing the entropy of the system. Using high-throughput DFT calculations, I investigate the stability over the entire composition space for GeSnPbSSeTe high entropy chalcogenide alloys (HEC). At the growth temperature of 900 K, 99% of the HEC compositions are stabilized by the large entropy. The bandgap calculations and the transport measurement by my collaborators show that equimolar HEC is an ambipolarly dopable semiconductor.

The computational studies presented here demonstrate the importance of point defects in controlling the properties of semiconductors and pave the way for the adoption of heterovalent ternary nitrides and high entropy chalcogenide alloys in a wide range of functional applications.