Optical and Electronic Properties of Epitaxial Germanium

Optical characterization, modeling, and strain-dependent transitions in epitaxial Ge and GeSn thin films.

Overview

This research examines the

  • optical and electronic behavior of epitaxial Ge and GeSn thin films for photonics and quantum devices, using spectroscopic ellipsometry, and critical-point (CP) analysis.
  • How strain, defects, buffer layer thickness, and substrate orientation modulate the dielectric response and band-structure transitions.

1️⃣ Optical Constants and Critical-Point Transitions in Biaxially Tensile-Strained Ge

Phys. Rev. Applied 23, 024037 (2025)
DOI: 10.1103/PhysRevApplied.23.024037

  • Comprehensive ellipsometry modeling of biaxially strained epitaxial Ge films grown by molecular beam epitaxay (MBE).
  • Strain-induced shifts in Critical Point transitions were extracted via second-derivative CP analysis, linking strain and film thickness to dielectric evolution.
Energy band diagram alteration in strained Ge films. © American Physical Society, License RNP/25/OCT/097415.

Key Findings:

  • Extracted full (n, k) and (ε₁, ε₂) spectra for strained Ge.
  • Developed a multi-layer optical model including surface and oxide effects.
  • Established strain–band-structure correlation for Si-compatible photonics.

2️⃣ Impact of Defects, Buffer-Layer Thickness, and Substrate Orientation on the Optical Properties of Epitaxial Ge

Phys. Rev. Applied 25, 024061 (2026)
DOI: 10.1103/s26s-23sh

  • Investigated how defect density, buffer-layer thickness, and substrate orientation govern the optical constants and dielectric response of epitaxial Ge films.
  • Combined spectroscopic ellipsometry with critical-point analysis to connect growth and structural parameters to band-structure transitions.
Dependence of the epitaxial-Ge optical response on defects, buffer-layer thickness, and substrate orientation. © American Physical Society.

Key Findings:

  • Quantified how buffer-layer thickness and substrate orientation tune the critical-point energies and broadening.
  • Linked defect density to changes in the (ε₁, ε₂) spectra and sub-gap absorption.
  • Provided growth-condition guidance for Si-compatible Ge photonics.

Methods

  • Spectroscopic Ellipsometry (0.4–4.5 eV)
  • Critical-Point Analysis
  1. Optical constants and critical-point transitions in biaxially tensile-strained epitaxial thin films of germanium
    Rutwik Joshi, Nina Hong, Neha Singh, Muntasir Mahdi, and Mantu K. Hudait Phys. Rev. Applied 23, 024037 (2025)
    DOI: 10.1103/PhysRevApplied.23.024037
  2. Impact of defects, buffer-layer thickness, and substrate orientation on the optical properties of epitaxial germanium
    Muntasir Mahdi, Nina Hong, Neha Singh, and Mantu K. Hudait Phys. Rev. Applied 25, 024061 (2026)
    DOI: 10.1103/s26s-23sh