GeSiSn Thin Films with Nanoislands: Strain‑Driven Morphology, Superstructure Control, and Mid‑IR Photoluminescence

Abstract

This study establishes how the two‑dimensional to three‑dimensional (2D‑3D) critical transition thickness in GeSiSn films depends on Sn composition when the Ge fraction is fixed. Growth at 150 °C and Sn contents up to 16 % reveal that the transition thickness shrinks as Sn rises, confirming the strain‑driven onset of island formation. Phase diagrams describing the evolution of surface superstructures during Sn deposition on Si(100) and Ge(100) are presented, enabling precise monitoring of Sn coverage via RHEED and preventing Sn segregation. By carefully tuning growth temperatures, we fabricated multilayer stacks comprising pseudomorphic GeSiSn layers and nanoisland arrays with densities up to 1.8 × 10¹² cm⁻². Notably, a double‑domain (10 × 1) superstructure appears during Si overgrowth on GeSiSn and transforms to a (2 × 1) structure upon mild annealing. Photoluminescence from these periodic structures spans 0.6–0.85 eV (1.45–2 µm). Band‑diagram calculations for Ge_0.315Si_0.65Sn_0.035 heterostructures indicate that the emission originates from inter‑band transitions between the X valley of Si (or Δ₄ valley of GeSiSn) and the heavy‑hole subband of GeSiSn.

Background

Silicon photonics has successfully produced waveguides, photodetectors, and modulators, yet efficient light emission remains elusive because Si possesses an indirect bandgap. Adding Sn to Ge, Si, or GeSi alloys offers a pathway to a direct‑gap IV‑group semiconductor. In GeSn, the Γ–L valley separation narrows, and a direct bandgap emerges at ≈9 % Sn in the cubic lattice; this threshold drops below 6 % under tensile strain and rises above 11 % under compressive strain ^[4–6]. Sn incorporation also lowers the bandgap, extending operation into the mid‑infrared—critical for optical interconnects, fiber‑optic links, sensors, and energy‑conversion devices ^[7]. Consequently, Ge–Si–Sn materials have attracted growing research attention.

Despite significant progress in producing high‑quality GeSn epitaxy ^[8–9], challenges persist: Sn precipitation and segregation during growth or oxidation ^[10–12]. Non‑equilibrium techniques such as MBE and CVD suppress these defects, and strategies like reduced growth temperature, strain engineering, or ternary alloying with Si mitigate Sn precipitation ^[13–16]. However, detailed morphological and surface‑structure data for single‑crystal GeSiSn films remain scarce, hindering the design of nanoheterostructures with controlled strain and bandgap.

This work addresses these gaps by presenting multilayer superlattices that combine pseudomorphic GeSiSn layers and nanoislands without dislocations, enabling bandgap tuning across a broad infrared range. We provide new 2D‑3D transition thickness data for fixed Ge content and Sn ranging from 0 to 16 %, phase diagrams for Sn growth on Si(100) and Ge(100), and photoluminescence characterization of the resulting periodic structures. Band‑diagram calculations using model‑solid theory elucidate the observed emission mechanisms.

Experimental

All samples were grown in an ultrahigh‑vacuum MBE chamber (10⁻⁷–10⁻⁸ Pa) on Si(100) substrates using the Katun C system. Si was evaporated from an electron‑beam source; Ge and Sn were supplied by Knudsen cells. The GeSiSn growth rate spanned 0.015–0.05 nm/s, while the substrate temperature ranged from 150–450 °C. Sn concentrations varied from 0 to 20 % by adjusting flux ratios. Both single GeSiSn layers and multilayer stacks (GeSiSn/Si heterojunctions) were fabricated. In the multilayers, a GeSiSn layer was followed by a 10‑nm Si cap grown at 400–500 °C. RHEED monitored surface evolution in real time; intensity profiles along crystallographic rods were extracted from video footage to pinpoint the 2D‑3D transition thickness. STM (Omicron‑Riber) examined surface morphology, and photoluminescence (PL) employed an ACTON 2300i monochromator with an InGaAs detector (1.1–2.2 µm). A 532 nm Nd:YAG laser excited the samples.

Results and Discussion

GeSiSn films were deposited at 150 °C with Sn contents from 0 to 16 %. Strain accumulation due to lattice mismatch triggered a 2D‑3D transition at a critical thickness that depends on Sn fraction. Figure 1 illustrates the RHEED‑based determination for Ge_0.6Si_0.28Sn_0.12, where the abrupt intensity rise marks the transition.

2D-3D transition moment determination during the GeSiSn film growth: a RHEED pattern from the Si(100)-(2 × 1) surface before the Ge_0.6Si_0.28Sn_0.12 growth is shown, b the space‑time intensity distribution of the vertical profile in the gray scale and the intensity dependence of the horizontal profile on the deposited Ge_0.6Si_0.28Sn_0.12 film thickness. The profiles are indicated by the arrows in (a) and (b), and c the final RHEED pattern after the 1.91‑nm‑thick Ge_0.6Si_0.28Sn_0.12 deposition

The critical thicknesses for all Sn compositions (0–16 %) at 150 °C are plotted in Figure 2. As Sn increases, the lattice mismatch with Si grows from 2.5 % to 5.6 %, causing the transition to occur at thinner films. This trend confirms the strain‑driven nature of island nucleation.

The critical 2D‑3D transition thickness dependences on the GeSiSn film composition at several fixed values of Ge content, Sn content from 0 to 16%, and at the growth temperature of 150 °C

Using these diagrams, we fabricated multilayer structures that either preserve pseudomorphic GeSiSn layers or intentionally introduce nanoislands. However, Sn segregation during Si overgrowth can generate undesirable surface reconstructions. Phase diagrams of Sn on Si(100) and Ge(100) (Figure 3) map the evolution of surface superstructures across 100–750 °C, allowing us to identify the exact Sn coverage during growth. The double‑domain (10 × 1) superstructure, observed during Si deposition on GeSiSn, disappears after a brief anneal at 400–500 °C, reverting to the (2 × 1) structure characteristic of clean Si.

Phase diagrams of the change of the superstructure during growth: a Sn on Si(100) and b Sn on Ge(100)

STM imaging of Ge_0.75Si_0.2Sn_0.05 surfaces (Figure 5) reveals nanoisland arrays with densities up to 1.8 × 10¹² cm⁻² and average diameters of 4–9 nm, depending on the growth period. Higher Sn fractions during successive Si layers increase island density while reducing size, consistent with the observed shift from (2 × 1) to c(8 × 4) surface reconstructions. The optimal growth window for dense, uniform islands is 150–250 °C, where RHEED oscillations confirm two‑dimensional wetting‑layer growth before island nucleation.

STM images of the Ge_0.75Si_0.2Sn_0.05 surface with the scan size of 400 nm × 400 nm: a the Ge_0.75Si_0.2Sn_0.05 surface in the first period, b the Ge_0.75Si_0.2Sn_0.05 surface in the fifth period; the distribution histograms for the number of islands on the size of the base for the Ge_0.75Si_0.2Sn_0.05 film: c in the first period (the Ge_0.75Si_0.2Sn_0.05 film thickness equals 1.78 nm) and d in the fifth period (the Ge_0.75Si_0.2Sn_0.05 film thickness equals 1.89 nm)

TEM analysis of a 25‑nm‑period Ge_0.5Si_0.45Sn_0.05/Si superlattice (Figure 6) confirms the absence of threading dislocations and the sharpness of interfaces, validating the pseudomorphic nature of the layers. X‑ray diffraction corroborates the intended compositions, matching the set flux ratios within experimental uncertainty.

a TEM image from the multilayer structure including the Ge_0.5Si_0.45Sn_0.05 heterotransition with the 25‑nm period. b High‑resolution TEM image from the same structure

Photoluminescence spectra of the multilayer structures (Figure 7) display peaks at 0.78, 0.69, and 0.65 eV, corresponding to 1.59, 1.80, and 1.90 µm. The emission shifts to lower energies and increases in intensity with higher Sn content (3.5–6 %). Band‑diagram calculations using model‑solid theory show that the PL arises from inter‑band transitions between the Si X valley (or the Δ₄ valley of GeSiSn) and the heavy‑hole subband of GeSiSn. The calculations also reveal that increasing Sn depth widens the quantum‑well potential, enhancing radiative recombination.

The photoluminescence spectra from multilayer periodic structures with the 3.5, 4.5, and 6% Sn content in the pseudomorphic GeSiSn layers

Band‑diagram modeling of the Si/Ge_0.315Si_0.65Sn_0.035/Si heterostructure (Figure 8) confirms the inter‑band origin of the PL and quantifies the strain‑induced band offsets. The calculated band edges agree with the experimental peak positions, underscoring the validity of the model.

The Si/Ge_0.315Si_0.65Sn_0.035/Si heterocomposition band diagram

Conclusions

We have quantified the 2D‑3D critical thickness for GeSiSn films with fixed Ge content and Sn ranging from 0 to 16 % at 150 °C, revealing a clear strain‑driven reduction with increasing Sn. Phase diagrams for Sn on Si(100) and Ge(100) provide a practical tool for monitoring and suppressing Sn segregation during multilayer growth. The fabricated superlattices feature pseudomorphic GeSiSn layers or nanoisland arrays with densities up to 1.8 × 10¹² cm⁻². The first observation of a (10 × 1) double‑domain superstructure during Si overgrowth on GeSiSn, which transforms to (2 × 1) upon mild annealing, demonstrates the sensitivity of surface reconstructions to Sn coverage. Photoluminescence from these periodic structures spans 0.6–0.8 eV (1.45–2 µm), and band‑diagram analysis attributes the emission to X‑ or Δ₄‑valley to heavy‑hole transitions. Future work will explore higher Sn fractions to push emission beyond 2 µm.

Abbreviations

CVD:

Chemical vapor deposition

MBE:

Molecular beam epitaxy

ML:

Monolayer

PL:

Photoluminescence

RHEED:

Reflection high energy electron diffraction

STM:

Scanning tunneling microscopy

TEM:

Transmission electron microscopy