Engineering n-Type GeBi Thin Films by Molecular Beam Epitaxy: Crystalline Control and Infrared/THz Optical Performance

Background

The expansion of dense wavelength division multiplexing (DWDM) into the L‑band (1.56–1.62 µm) demands optoelectronic detectors that can operate across the full 1.44–1.93 µm range. Existing Ge‑based alloys such as GeSi and GeSn have achieved cut‑off wavelengths up to 1.6 µm, leaving the 2 µm barrier unmet for applications in mid‑infrared sensing and free‑space communication.

GeBi offers a compelling alternative: by alloying Bi into the Ge lattice, the band‑gap can be engineered toward longer wavelengths while preserving n‑type conductivity. Prior work on GeSn and GeBi has shown the potential for high‑performance detectors in the 1.3–1.6 µm range, but the cut‑off wavelength remains limited. This study demonstrates that GeBi films grown by MBE can reach a 2.3 µm cut‑off, effectively bridging the gap between C‑band and L‑band detection.

Experimental Procedures

GeBi films were deposited on p‑type Si (100) wafers using a custom MBE chamber operated at a base pressure of 4 × 10⁻⁹ to 5 × 10⁻¹⁰ Torr. Ge and Bi were sourced from effusion cells heated to 1200 °C and 400–550 °C, respectively, to achieve a controlled flux ratio. The substrate temperature was maintained at 150 °C, yielding growth rates between 1.66 and 2.50 nm min⁻¹. Detailed growth parameters are tabulated in Table 1.

Structural characterization employed grazing‑incidence XRD, SEM (JMS6490LV, JEOL), AFM (300 HV, SEIKO), and Raman spectroscopy (LabRAM HR, Edinburgh Instruments). Optical absorption and transmittance were measured with a Lambda 75 UV/VIS/NIR spectrometer and a Fourier‑transform far‑infrared spectrometer. THz transmission was obtained via time‑domain spectroscopy (THz-TDS).

Results and Discussion

XRD Analysis
Figure 1 shows that all samples exhibit clear GeBi diffraction peaks. At low Bi (x = 0.020), the film is oriented along (014). As Bi increases to x = 0.102, the (104) peak diminishes while the (012) peak emerges. For x ≥ 0.183, the (012) orientation dominates, indicating a strain‑driven reorientation due to the larger lattice constant of Bi.

XRD patterns of Ge_1‑xBi_x films with Bi contents from 2 % to 22.2 %.

SEM Morphology
Figure 2 illustrates the surface evolution. At 2 % Bi, the film is smooth (Fig. 2a). At 10.2 % Bi, isolated Bi‑rich dots (33–42 nm) appear (Fig. 2b). Beyond 18.3 % Bi, secondary phases (amorphous Bi, Ge) coexist with the primary GeBi phase, and grain sizes reach up to ~1 µm (Fig. 2c,d). Excess Bi beyond its solubility limit segregates at grain boundaries, creating surface roughness.

SEM images of GeBi films: (a) 2 %; (b) 10.2 %; (c) 18.3 %; (d) 20.3 %.

AFM Surface Roughness
Figure 3 shows AFM topographies. The root‑mean‑square (RMS) roughness increases sharply with Bi, reaching ~15 nm at 20.3 % Bi. The increased roughness is consistent with Bi segregation observed in SEM.

AFM images of GeBi films: (a) 2 %; (b) 10.2 %; (c) 18.3 %; (d) 20.3 %.

Raman Spectroscopy
Figure 4 displays the Ge‑Bi vibrational mode near 190 cm⁻¹. With increasing Bi, the peak strengthens and shifts to higher wavenumbers, indicating lattice strain amplification and successful Bi incorporation.

Raman spectra of GeBi films with varying Bi contents.

Near‑Infrared Optical Properties
Figure 5a shows that reflectance decreases across 1014–2500 nm as Bi increases, implying higher absorption. The absorption valley at 1932–1938 nm (indirect band‑gap) diminishes and vanishes for Bi > 20 %. Direct‑band absorption around 1446–1452 nm similarly weakens with high Bi. Figure 5b demonstrates a transmission inflection near 1020 nm, attributed to the Si substrate’s band‑edge. Overall, higher Bi reduces reflectance and adjusts transmittance in the NIR, improving detector responsivity.

Reflectance (a) and transmission (b) spectra of GeBi films across the NIR band.

Far‑Infrared and THz Response
Figure 6a and 6b reveal a stable absorption window between 4–15 µm, while Figure 6c shows that absorption increases from 9.3 % to 22.6 % as Bi rises from 2 % to 10.2 %. Beyond 10 % Bi, absorption peaks near 1–7.5 µm and declines at longer wavelengths, reflecting surface roughness and Bi‑phase formation. THz transmittance (Figure 7) drops by ~10 % when Bi increases from 2 % to 10.2 %, but recovers slightly at 18.3–22.2 % Bi, indicating tunable THz absorption that could be harnessed for modulators.

Transmission (a), reflectance (b), and absorption (c) spectra of GeBi films in the far‑IR band.

THz transmission spectra of GeBi films with different Bi contents.

Conclusion

We have successfully grown n‑type Ge_1‑xBi_x thin films (x = 0–0.222) on p‑Si(100) substrates via low‑temperature MBE. XRD and SEM confirm that Bi incorporation dictates crystallographic orientation and microstructure, while AFM and Raman evidence lattice strain modulation. Optical studies demonstrate that moderate Bi doping lowers NIR reflectance, extends the cut‑off to 2.3 µm, and tunes THz transmittance, positioning GeBi as a strong candidate for broadband infrared detectors and THz modulators.

Abbreviations

AFM:

Atomic‑force microscopy

MBE:

Molecular beam epitaxy

SEM:

Scanning electron microscope

THz:

Terahertz

XRD:

X‑ray diffraction