Temperature‑Dependent Band Gap and Decomposition of MBE‑Grown MoSe₂ Ultrathin Films

Background

Two‑dimensional transition‑metal dichalcogenides (TMDs) have become a vibrant research area due to their unique electronic, optical, and valley‑related phenomena [1–7]. Semiconducting TMDs typically exhibit a direct band gap at the K point in the monolayer limit, giving rise to pronounced excitonic transitions [8–17] that enhance optoelectronic performance [18–25]. MoSe₂ is especially attractive, with a direct gap of 1.55 eV that aligns closely with the Shockley–Queisser optimum for single‑junction photovoltaics [26–30]. Modifying the band gap through temperature control or partial oxidation opens avenues for tunable light‑absorption devices [31, 32]. Yet, high‑temperature stability of MoSe₂ films has not been systematically explored, largely because producing large‑area, single‑crystalline ultrathin films is challenging.

Growth techniques such as chemical vapor deposition (CVD), pulsed laser deposition, and molecular‑beam epitaxy (MBE) have advanced the synthesis of TMD films [5, 33–35]. While CVD can yield uniform films, grain sizes often remain limited; metal‑organic CVD has improved this but still produces polycrystalline films [36]. In contrast, MBE reliably produces epitaxial, atomically flat films with precise thickness control via in‑situ reflection high‑energy electron diffraction (RHEED) [1]. This makes MBE ideal for fundamental studies of temperature‑dependent properties.

In this work, we report on the high‑temperature optical and stoichiometric behavior of MBE‑grown MoSe₂ ultrathin films. Using spectroscopic ellipsometry, we track the band gap across a wide temperature range and directly observe the decomposition process through surface crystallinity and composition measurements.

Methods

MoSe₂ films were epitaxially deposited on graphenized 6H‑SiC substrates in a home‑built MBE chamber with a base pressure of 1 × 10⁻¹⁰ Torr. Bilayer graphene was prepared by annealing the SiC at 1300 °C for ~5 min following established protocols [1]. The lattice mismatch between MoSe₂ and graphene is ~0.3 %. Molybdenum and selenium were evaporated using an e‑beam source and an effusion cell, respectively, at a substrate temperature of 250 °C and a growth rate of 0.1 ML min⁻¹. Post‑annealing at 600 °C for 30 min followed, and RHEED monitored the surface during growth (18 kV).

Film crystallinity was confirmed by high‑resolution X‑ray diffraction (HRXRD, Bruker D8 Discover). Optical reflection was measured with two spectroscopic ellipsometers (J.A. Woollam V‑VASE): one in air and one in a separate UHV chamber. Stoichiometry was analyzed by time‑of‑flight medium‑energy ion‑scattering spectroscopy (TOF‑MEIS, KMAC MEIS‑K120) using a 100.8 keV He⁺ beam. Thickness estimates employed bulk densities of 3.21 g cm⁻³ for SiC and 6.98 g cm⁻³ for MoSe₂.

Results and discussion

Three MoSe₂ films with nominal thicknesses of 1, 2.5, and 16 ML were fabricated on graphene/SiC. RHEED patterns (Fig. 1a–c) confirm epitaxial growth: sharp streaks for the 1 and 2.5 ML films and a weaker, rounded pattern for the 16 ML film, indicative of increased in‑plane disorder at higher thicknesses. HRXRD of the 16 ML sample (Fig. 1d) shows only (00n) reflections, evidencing c‑axis ordering despite surface disorder.

Room‑temperature ellipsometry spectra (Fig. 2) display the characteristic A (~1.5 eV) and B (~1.7 eV) excitonic peaks for all thicknesses, consistent with prior measurements on exfoliated and CVD‑grown MoSe₂ [38, 44]. The A‑peak energy shows minimal thickness dependence, confirming the direct‑gap nature of monolayers. Extracted optical band gaps (E_g(300 K)) increase sharply from 1.54 eV (2.5 ML) to 2.18 eV (1 ML), reflecting the direct‑to‑indirect transition (Table 1).

Temperature‑dependent ellipsometry (Fig. 3) reveals a continuous red‑shift of excitonic features up to a decomposition temperature (T_dec) that rises from 700 °C (1 ML) to 725 °C (16 ML). Above T_dec, spectra become featureless, indicating loss of crystalline order. The T_dec values are markedly lower than bulk MoSe₂ (≈1200 °C in air, 980 °C in UHV) [46, 47], underscoring the thermal fragility of ultrathin layers.

The A‑exciton peak shifts linearly with temperature up to T_dec (Fig. 4b), mirroring behavior observed in exfoliated monolayers [26]. Optical band gaps derived from Tauc plots also decrease linearly across the entire temperature range (Fig. 4c). Fitting with the Huang–Rhys vibronic model yields E_g(0) between 1.5 and 2.32 eV and a coupling parameter S of 3–4, with a fixed phonon energy of 11.6 meV [26]. These parameters differ from those obtained for exciton energies in monolayers but align with values reported for bulk III‑V semiconductors (e.g., GaAs, GaP) [48–51], indicating that thermal expansion dominates the band‑gap contraction above 150 K [53].

To probe the decomposition mechanism, RHEED and TOF‑MEIS were performed on 2‑ML films annealed at 600, 720, and 850 °C (Fig. 5). RHEED shows streaky patterns at 600 °C, additional spots at 720 °C, and complete loss of diffraction at 850 °C. TOF‑MEIS depth profiles reveal a stoichiometric Mo:Se ratio of 1:2 at 600 °C, whereas at 720 °C the ratio drops to 1:1.7 and the film thickness increases to ~1.6 nm, indicating selenium loss and surface roughening. Thus, MoSe₂ begins to disorder and decompose above 720 °C, leaving a disordered molybdenum layer at 850 °C. These observations provide a concrete basis for designing high‑temperature processing routes for metal chalcogenide devices.

Conclusions

We have characterized the temperature‑dependent optical properties and decomposition behavior of MBE‑grown MoSe₂ ultrathin films. The band gap decreases linearly with temperature, well described by the Huang–Rhys vibronic model, while the decomposition temperature increases with film thickness. These findings establish critical thermal limits for MoSe₂‑based electronic and optoelectronic devices and provide a foundation for high‑temperature fabrication strategies of related metal chalcogenides.

Abbreviations

CVD:

Chemical vapor deposition;

E_g(0):

Band gap at 0 K;

E_g(300 K):

Band gap at 300 K;

HRXRD:

High‑resolution X‑ray diffraction;

MBE:

Molecular beam epitaxy;

RHEED:

Reflection high‑energy electron diffraction;

T_dec:

Decomposition temperature;

TMD:

Transition metal dichalcogenide;

TOF‑MEIS:

Time‑of‑flight medium‑energy ion‑scattering spectroscopy;