Photoluminescence efficiency of MBE-grown MoSe$_2$ monolayers featuring sharp excitonic lines and diverse grain structures

TL;DR

This work quantifies photoluminescence efficiency in MBE-grown MoSe$_2$ monolayers on h-BN, showing that measured brightness is strongly modulated by optical interferences and the Purcell effect from the photonic environment. By correcting for these effects on a spot-by-spot basis and accounting for monolayer coverage, the authors reveal an intrinsic brightness that scales with coverage rather than grain size, indicating negligible diffusion and edge contributions for nanometer-scale grains. A transfer-matrix model ties the observed intensity variations to h-BN thickness and local refractive environment, providing a framework to compare disparate samples fairly. A higher Se flux during growth improves PL yield, pointing to Se vacancies and Mo adatoms as major non-radiative loss channels and highlighting defect engineering as a key lever for optimizing MBE-grown TMD optoelectronics.

Abstract

Recent studies have demonstrated that using h-BN as a substrate for the growth of transition metal dichalcogenides can significantly reduce excitonic linewidths. However, many other optical parameters still require optimization. In this work, we present a detailed study of the low-temperature photoluminescence efficiency of MBE-grown MoSe$_2$ monolayers on h-BN substrates, comparing them to state-of-the-art exfoliated monolayers encapsulated in h-BN. We demonstrate that a quantitative comparison between samples requires accounting for interference effects and Purcell enhancement or suppression of the emission. By accounting for these effects in both photoluminescence and Raman signals, we show that the overall intrinsic luminescence efficiency is proportional to the sample coverage. Consequently, we find that exciton diffusion and edge effects are negligible in spectroscopy of MBE-grown samples, even for nanometer-sized crystals.

Figures (9)

• Figure 1: The picture shows AFM height maps of four samples grown in separate MBE processes. The maps provide the coverage factor of a monolayer, and information regarding morphology, flakes orientation, size or overgrowth with the bilayer on top of the monolayer. The sides of the square maps are all 800 nm in dimension.
• Figure 2: Representative spectra of the photoluminescence (T=10 K) and the Raman scattering (at room temperature) of all investigated MBE--samples and the reference exfoliated MoSe$_{\textrm{2}}$ monolayer encapsulated from the top and the bottom with h--BN flakes. The spectra were normalised to similar maximum intensity for easier shape comparison. In both cases, the laser used for PL excitation and Raman scattering was green $\lambda = 532$ nm laser.
• Figure 3: Raman scattering and PL spectra measured on the same sample with 60% ML coverage. The spectra were measured from flakes with varying h--BN thickness, as indicated near each spectrum. The interference effects significantly influence the spectral emission intensity but not its shape. The scale is linear but for clarity, the spectra were shifted verically. The integrated data from all flakes for both panels is presented in Fig. 5.
• Figure 4: The graph presents measured and calculated by transfer--matrix formalism reflectivity from the h--BN flakes with thickness 13.5 nm and 491 nm, and with MoSe$_2$ on its top. The reflectivity spectra are normalised by the reflectivity measured from just a simple substrate of Si and SiO$_2$ of the same oxide thickness. The lower intensity of the experimental data might be caused by an additional scattering factor due to the unevenness of h--BN flakes.
• Figure 5: The upper panel (a) shows the data points of measured PL intensity (integrated spectrum) vs the thickness of the h--BN flake at which position (spot) the measurement was taken. The dashed line depicts the theoretical calculation obtained by the transfer matrix method averaged by the objective numerical aperture. The lower panel (b) analogically presents the data point for the Raman scattering for line 241 cm$^\textrm{-1}$.