Infrared dielectric response, index of refraction, and absorption of germanium-tin alloys with tin contents up to 27% deposited by molecular beam epitaxy

Abstract

The dielectric spectral response of Ge1-xSnx thin film alloys with relatively high Sn contents (0.15 ≤ x ≤ 0.27) and thickness from 42 to 132 nm was characterized by variable angle spectroscopic ellipsometry over the wavelength range from 0.190 to 6 μm. The Ge1-xSnx thin films were deposited on Ge substrates by molecular beam epitaxy using an electron-beam source for Ge to achieve a substrate temperature below 150 °C to prevent the surface segregation of Sn. From the measured dielectric function, the complex refractive index was calculated indicating an increase in the real index with the Sn content at mid-infrared wavelengths. The ellipsometry revealed that the band structure critical point energies red-shifted with the increasing Sn content. The optical absorption coefficient was calculated from the imaginary index and showed a strong absorption into, and beyond, the mid-infrared with the increasing Sn content.

Synthesis and Fundamental Studies of Si-Compatible (Si)GeSn and GeSn Mid-IR Systems with Ultrahigh Sn Contents

“…This issue can be addressed by fine-tuning the growth parameters or using multiple compositionally graded buffer layers, 18 but these methods introduce additional complexity into the process and seem to lose effectiveness beyond 15% Sn. On the other hand, while higher Sn contents have been achieved using the molecular beam epitaxy (MBE) method 19 or sputtering epitaxy, 20 the introduction of active dopants and transferability to a Si-based system remain problematic using this technique.…”

Mid-infrared (3–8 μm) Ge1−ySny alloys (0.15 < y < 0.30): Synthesis, structural, and optical properties

“…This is due to the fact that the standard Ge-buffer technology loses its effectiveness for accommodating the lattice mismatch, causing a deterioration in materials quality that can even lead to epitaxial breakdown. 1 Attempts to circumvent the quality issues to achieve alloys with y >> 0.15 are based on lowering the growth temperature to about 150 ℃, [2][3][4][5] or using complex buffer layers with intermediate compositions. [6][7][8] However, very few reports have been published on band gaps from such samples, 4,8,9 and the few results available are difficult to compare due to unknown or large strains present, compositional uncertainties, different methodologies for extracting the band gaps, and sample complexity.…”

Short-range order in GeSn alloy

“…Interestingly, despite of this concern, high-Sn content GeSn alloys have been recently synthesized through chemical-vapor deposition 16,34,35 and molecular beam epitaxy. 36 Although strain gradient was found to play an important role in incorporating Sn, 35 we conceive that an additional main factor contributing to stability could well be related to the identified lower-than-ideal Sn-Sn coordination number, because a depletion of Sn atoms through a replacement by Ge atoms from their nearest neighbors could greatly reduce the possibility of local gathering of Sn atoms, which is a prerequisite of Sn segregation.…”

“…%. 16 As recent advance in synthesis enables the growth of high-Sn content alloys far beyond Sn's solubility in Ge and Si, 7,9,10,16,[34][35][36] a fundamental question emerges as to how random these group IV alloys truly are.…”