Epitaxial Growth of High‑Quality SrGe₂ Thin Films on Ge Substrates via Reactive Deposition Epitaxy

Abstract

Semiconductor strontium digermanide (SrGe₂) exhibits a large absorption coefficient in the near‑infrared, making it a compelling material for multijunction solar cells. In this study we report the first synthesis of SrGe₂ thin films on Ge (100), (110), and (111) substrates using reactive deposition epitaxy (RDE). We demonstrate that film morphology is highly sensitive to substrate temperature (300–700 °C) and crystallographic orientation. A single‑oriented SrGe₂ film was obtained on Ge(110) at 500 °C, offering a pathway to integrate SrGe₂ into high‑efficiency thin‑film photovoltaic devices.

Background

Alkaline‑earth silicides have long attracted interest for applications in solar cells, thermoelectrics, and optoelectronics [1–3]. Although germanides are less explored, theoretical predictions suggest they possess promising electrical and optical characteristics [10–16]. SrGe₂, a representative alkaline‑earth germanide, crystallizes in a BaSi₂‑type orthorhombic structure (space group: Pnma, no. 62, Z = 8) [12–16]. It is an indirect‑bandgap semiconductor with a gap of ~0.82 eV and an absorption coefficient of 7.8 × 10⁵ cm⁻¹ at 1.5 eV—higher than that of Ge (4.5 × 10⁵ cm⁻¹)—making it ideal for the bottom cell of tandem architectures. Thin‑film fabrication on arbitrary substrates could enable high‑efficiency, low‑cost tandem solar cells.

Our prior work successfully produced high‑quality BaSi₂ thin films on Si (111) and Si (001) substrates via a two‑step method: reactive deposition epitaxy (RDE) followed by molecular beam epitaxy (MBE) [17–20]. The resulting (100)-oriented BaSi₂ films exhibited long minority‑carrier lifetimes and large diffusion lengths, leading to a 9.9 % conversion efficiency in a p‑BaSi₂/n‑Si heterojunction solar cell— the highest ever reported for semiconducting silicides [23]. Motivated by these achievements and the analogous crystal structure of SrGe₂, we applied the RDE/MBE approach to SrGe₂ on Ge substrates.

Experimental

A molecular beam epitaxy (MBE) system (base pressure 5 × 10⁻⁷ Pa) equipped with a standard Knudsen cell for Sr and an electron‑beam evaporation source for Si was employed. Sr was deposited onto Ge (100), (110), and (111) substrates at substrate temperatures (T_sub) ranging from 300 to 700 °C. Prior to deposition, the Ge wafers were cleaned with a 1.5 % HF solution for 2 min followed by a 7 % HCl solution for 5 min. Deposition parameters were: 0.7 nm min⁻¹ for 120 min on Ge(001), 1.4 nm min⁻¹ for 30 min on Ge(011), and 1.3 nm min⁻¹ for 60 min on Ge(111). After Sr deposition, a 5‑nm amorphous Si capping layer was deposited at room temperature to protect the reactive Sr–Ge surface from oxidation. Film crystallinity was assessed via reflection high‑energy electron diffraction (RHEED) and X‑ray diffraction (XRD; Rigaku Smart Lab, Cu K_α radiation). Surface morphology was examined by scanning electron microscopy (SEM; Hitachi SU‑8020) and transmission electron microscopy (TEM; FEI Tecnai Osiris, 200 kV) equipped with energy‑dispersive X‑ray spectroscopy (EDX) and high‑angle annular dark‑field STEM (HAADF‑STEM) (probe diameter ~1 nm).

Results and Discussion

Figure 1 displays the RHEED and θ–2θ XRD patterns for all samples after Sr deposition. Streaky or spotted RHEED patterns confirm epitaxial growth of Sr–Ge compounds. For Ge(100) substrates, Sr₅Ge₃ peaks appear across all temperatures, while SrGe emerges only at 600–700 °C. SrGe₂ is detected only at 300 °C, yielding preferential [100]-oriented SrGe₂ and [220]-oriented Sr₅Ge₃. Higher temperatures enhance the Ge(200) substrate signal, indicating increased Sr–Ge coverage (Fig. 2).

On Ge(110), only SrGe₂ (411) and substrate peaks are observed from 300–600 °C. The 500 °C sample shows the strongest SrGe₂ (411) intensity, signifying a single‑composition, highly oriented SrGe₂ film. Ge(111) substrates yield SrGe₂ peaks at all temperatures: [110]-oriented SrGe₂ at 300, 400, 500, and 700 °C, with broad peaks at 300 and 400 °C; multi‑oriented SrGe₂ at 500 and 600 °C; and minor Sr₅Ge₃ (220) at 400, 500, and 700 °C. These morphologies reflect the interplay between substrate surface energy and Sr/Ge flux balance [24].

RHEED and θ–2θ XRD patterns of the samples after Sr deposition. The crystal orientation of the Ge substrate is a–e (100), f–j (110), and k–o (111). T_sub ranges from 300 to 700 °C for each substrate. Peaks corresponding to SrGe₂ are highlighted in red.

SEM images of the samples after Sr deposition. The crystal orientation of the Ge substrate is a–e (100), f–j (110), and k–o (111). T_sub ranges from 300 to 700 °C for each substrate. Arrows indicate crystal directions of the Ge substrates.

Figure 2 reveals that at 300 °C, the surface is largely covered by Sr–Ge compounds. At 400–600 °C, distinct patterns reflecting the substrate symmetry appear: twofold for Ge(100), onefold for Ge(110), and threefold for Ge(111). Such symmetry is characteristic of epitaxial Sr–Ge growth on Si substrates [1, 25] and corroborates the RHEED observations. At 700 °C, dot‑like patterns suggest rapid Sr migration or evaporation due to high temperature. These results explain the streaky or spotted RHEED patterns in Fig. 1.

We examined the Ge(110) sample grown at 500 °C in detail. A 100‑nm amorphous Si overlayer protected the film from oxidation. HAADF‑STEM (Fig. 3a) and EDX mapping (Fig. 3b) confirm that the Sr–Ge compound covers nearly the entire surface. The magnified HAADF‑STEM (Fig. 3c) shows that the Sr–Ge layer infiltrates the Ge substrate, a hallmark of RDE growth [17, 18]. The compositional line scan (Fig. 3d) reveals a 1:2 Sr:Ge ratio, confirming SrGe₂ formation. These observations corroborate the XRD and RHEED data.

HAADF‑STEM and EDX characterization of the SrGe₂ thin film grown on the Ge(110) substrate at 500 °C. a HAADF‑STEM image. b EDX elemental map from the region shown in panel a. c Magnified HAADF‑STEM image. d Elemental composition profile obtained by a STEM‑EDX line scan along the arrow in panel (c).

Bright‑field and dark‑field TEM images (Fig. 4a–c) reveal that SrGe₂ is epitaxially aligned with the Ge substrate but exhibits two in‑plane orientations. The lattice image (Fig. 4d) shows two SrGe₂ domains (A and B) separated by a grain boundary. The SAED pattern (Fig. 4e) confirms the coexistence of both domains, each with Ge(111) planes parallel to SrGe₂ (220) planes. No dislocations or stacking faults were observed beyond the grain boundary, indicating high crystalline quality. Thus, RDE growth on Ge(110) yields defect‑free SrGe₂ films.

TEM characterization of the SrGe₂ thin film grown on the Ge(110) substrate at 500 °C. a Bright‑field TEM image. b, c Dark‑field TEM images using the SrGe₂ {220} plane reflection shown in each diffraction pattern. d High‑resolution lattice image showing SrGe₂ crystals. e SAED pattern showing the SrGe₂ 〈113〉 zone axis, taken from the region including SrGe₂ crystals and the Ge substrate.

Conclusions

We have successfully fabricated SrGe₂ thin films on Ge substrates using reactive deposition epitaxy. Film morphology is highly dependent on substrate temperature and crystallographic orientation. While Ge(100) and Ge(111) yield multi‑oriented SrGe₂ or secondary Sr–Ge phases, Ge(110) produces a single‑oriented, defect‑free SrGe₂ film at 500 °C. TEM analysis confirms the absence of interfacial dislocations, underscoring the high crystalline quality of the films. Ongoing work focuses on characterizing electrical and optical properties and extending the approach to Si and glass substrates for incorporation into near‑infrared absorption layers of multijunction solar cells.

Abbreviations

EDX:

Energy‑dispersive X‑ray spectrometer

HAADF‑STEM:

High‑angle annular dark‑field scanning transmission electron microscopy

MBE:

Molecular beam epitaxy

RDE:

Reactive deposition epitaxy

RHEED:

Reflection high‑energy electron diffraction

SEM:

Scanning electron microscopy

TEM:

Transmission electron microscopy

T_sub:

Substrate temperature

XRD:

X‑ray diffraction