Epitaxial PbGeSe thin films and their photoluminescence in the mid-wave infrared

PbSe is a narrow bandgap IV-VI compound semiconductor with application in mid-wave infrared optoelectronics, thermoelectrics, and quantum devices. Alkaline earth or rare earth elements such as Sr and Eu can substitute Pb to widen the bandgap of PbSe in heterostructure devices, but they come with challenges such as deteriorating optical and electronic properties, even in dilute concentrations due to their dissimilar atomic nature. We substitute Pb instead with column-IV Ge and assess the potential of rocksalt phase PbGeSe as a wider bandgap semiconductor in thin films grown by molecular beam epitaxy on GaAs substrates. Low sticking of GeSe adatoms requires synthesis temperatures below 260 °C to incorporate Ge, but this yields poor structural and compositional uniformity as determined by X-ray diffraction. Consequently, as-grown films in the range Pb0.94Ge0.06Se to Pb0.83Ge0.17Se (6-17% Ge) show much less bandgap widening in photoluminescence than prior work on bulk crystals using absorption. We observe that post-growth rapid thermal annealing at temperatures of 375-450 °C improves the crystal quality and recovers bandgap widening. Rapid interdiffusion of Ge during annealing, however, remains a challenge in harnessing such PbGeSe materials for compositionally sharp heterostructures. Annealed 17%-Ge films emit light at 3-3.1 um with minimal shift in wavelength versus temperature. These samples are wider in bandgap than PbSe films by 55 meV at room temperature and the widening increases to 160 meV at 80 K, thanks to sharply different dependence of bandgap on temperature in PbSe and PbGeSe.

💡 Research Summary

This paper investigates the feasibility of using germanium‑substituted lead selenide (PbGeSe) as a wider‑bandgap material for mid‑wave infrared (MWIR) applications, aiming to replace highly mismatched alkaline‑earth (Sr) or rare‑earth (Eu) dopants with a more chemically compatible group‑IV element. Epitaxial PbGeSe films were grown by molecular beam epitaxy (MBE) on arsenic‑capped GaAs (001) substrates that first received a 12 nm PbSe buffer layer. Because GeSe has a much higher vapor pressure than PbSe, the growth temperature had to be kept below 260 °C to achieve any Ge incorporation. A systematic study of growth temperature (195–260 °C) showed that lower temperatures improve Ge uptake but also lead to surface roughening and reduced crystalline quality, as evidenced by increasingly spotty RHEED patterns.

Two strategies were employed to vary the Ge content between 6 % and 17 %: (1) at a fixed temperature of 195 °C, the GeSe beam equivalent pressure (BEP) was increased, yielding samples P1–P3 with 6, 8, and 11 % Ge; (2) at 210 °C, both GeSe and PbSe BEPs were scaled up to increase the overall growth rate, producing samples R1–R3 with 7, 13, and 17 % Ge. X‑ray diffraction reciprocal‑space maps confirmed that the lattice constant contracts linearly with Ge concentration, indicating successful alloy formation.

Photoluminescence (PL) measurements of the as‑grown films revealed weak emission and a surprisingly small blue‑shift of the peak wavelength with increasing Ge, suggesting that Ge incorporation was non‑uniform and that defect‑related non‑radiative recombination dominated. To recover optical quality, the authors applied rapid thermal annealing (RTA) under a SiO₂ cap in nitrogen, exploring temperatures from 375 °C to 450 °C for 60 s. Annealing dramatically improved PL intensity and induced a clear, composition‑dependent blue‑shift. The 17 % Ge film annealed at 450 °C shifted from ~3.3 µm to ~3.0 µm and displayed the strongest emission. Further extending the anneal time did not yield additional benefits, indicating that a brief high‑temperature pulse is sufficient.

After annealing, the bandgap widening became approximately linear with Ge content, delivering ~21 meV (6 % Ge), ~42 meV (11 % Ge), and ~55 meV (17 % Ge) at room temperature—values comparable to bulk absorption studies (≈4 meV per percent Ge). Temperature‑dependent PL showed that the bandgap of PbGeSe varies much less with temperature than pure PbSe, resulting in an additional 160 meV widening at 80 K. This distinct temperature coefficient could be advantageous for low‑temperature infrared detectors.

However, the annealing process also caused rapid Ge interdiffusion, blurring compositional gradients and posing a challenge for fabricating sharp heterostructures such as quantum wells or barriers. Moreover, biaxial strain from the GaAs substrate contributes an offset to the measured bandgap values, complicating direct comparison with unstrained bulk data.

In summary, the work demonstrates that low‑temperature MBE growth combined with optimized rapid thermal annealing can produce epitaxial PbGeSe films with improved crystallinity and a tunable bandgap up to 55 meV wider than PbSe at room temperature. The material shows minimal wavelength shift with temperature, making it a promising candidate for PbSe‑based heterostructures in MWIR photodetectors and emitters. Future efforts should focus on controlling Ge diffusion, managing strain, and integrating these alloys into device architectures to fully exploit their advantageous band alignment and reduced Auger recombination characteristics.