Optical selection rules in hexagonal Ge polytypes and their lifting by symmetry perturbation

TL;DR

This study investigates the optical properties of hexagonal germanium polytypes, revealing symmetry-based selection rules that limit light emission, and demonstrates how atomic substitutions can lift these restrictions to enhance optoelectronic performance.

Contribution

The paper provides a comprehensive theoretical analysis of hexagonal Ge polytypes' optical properties and shows how symmetry perturbations can significantly improve their light emission capabilities.

Findings

4H-Ge has a parity-forbidden optical transition leading to very long radiative lifetimes.

Symmetry perturbations via Si substitution increase optical matrix elements and reduce lifetimes.

Complete optical characterization including excitonic effects up to 5 eV is provided.

Abstract

Hexagonal germanium polytypes have emerged as promising direct-gap semiconductors for silicon-integrated optoelectronics, yet their optical properties remain largely unexplored beyond the well-studied 2H phase. We present a comprehensive theoretical study of optical properties of hexagonal 2H-, 4H-, and 6H-Ge polytypes through ab initio calculations of quasiparticle band structures, dipole transition matrix elements, and solution of the Bethe-Salpeter equation. While all three polytypes exhibit direct band gaps of increasing size from 2H to 6H, we reveal that the fundamental optical transition in 4H-Ge is parity-forbidden due to matching band parities at the valence and conduction band edges. This selection rule results in a radiative lifetime seven orders of magnitude longer than in 2H- and 6H-Ge, severely limiting light emission capabilities. To demonstrate that the selection rule can be lifted, we introduce controlled symmetry perturbations by substituting single Ge atoms with Si in each unit cell, breaking the crystal symmetry. This perturbation increases the optical matrix elements by up to two orders of magnitude and reduces radiative lifetimes for all perturbed polytypes. We also compute absorption coefficients and frequency-dependent dielectric tensors for both light polarizations, including excitonic effects up to 5 eV, providing complete optical characterization of ideal and symmetry-perturbed hexagonal Ge systems relevant for optoelectronic applications.