Absorption and emission modulation in a MoS₂–GaN (0001) heterostructure by interface phonon–exciton coupling

Abstract

______________________________________________________________________________________________________ Semiconductor heterostructures based on layered two-dimensional transition metal dichalcogenides (TMD) interfaced to gallium nitride (GaN) are excellent material systems to realize broadband light emitters and absorbers. The surface properties of the polar semiconductor, such as GaN are dominated by interface phonons, thus the optical properties of the vertical heterostructure depend strongly on the interface exciton-phonon coupling. The origin and activation of different Raman modes in the heterostructure due to coupling between interfacial phonons and optically generated carriers in a monolayer MoS2-GaN (0001) heterostructure was observed. This coupling strongly influences the nonequilibrium absorption properties of MoS2 and the emission properties of both semiconductors. Density functional theory (DFT) calculations were performed to study the band alignment of the interface, which revealed a type-I heterostructure. The optical excitation with interband transition in MoS2 at K-point strongly modulates the C excitonic band in MoS2. The overlap of absorption and emission bands of GaN with the absorption bands of MoS2 induces the energy and charge transfer across the interface with an optical excitation at Γ-point. A strong modulation of the excitonic absorption states is observed in MoS2 on GaN substrate with transient optical pump-probe spectroscopy. The interaction of carriers with phonons and defect states leads to the enhanced and blue shifted emission in MoS2 on GaN substrate. Our results demonstrate the relevance of interface coupling between phonons and carriers for the development of optical and electronic applications. ______________________________________________________________________________________________________

Plasmon-Induced Enhanced Light Emission and Ultrafast Carrier Dynamics in a Tunable Molybdenum Disulfide-Gallium Nitride Heterostructure

“…MoS 2 semiconductors also have enhanced electron density of states at the Г point at 3.01 eV, closer to the exciton absorption states of bulk GaN. The interface of the hybrid semiconductor system results in a type II heterojunction estimated from DFT calculations [ 5 ]. The hybrid GaN/MoS 2 semiconductor has a conduction band offset of 0.23 eV [ 19 ].…”

“…A single or few monolayers of MoS 2 exhibit direct excitonic transitions in the red wavelength [ 4 ]. The hybrid integration of monolayer MoS 2 on bulk GaN has shown PL emission over a broad range of the visible spectrum covering red, yellow, and ultraviolet wavelengths [ 5 ]. The hybrid integration of this material system results in an increased density of states within the bandgap energy regime of GaN due to an overlap of the high-density electronic state of MoS 2 with the defect bands in GaN [ 6 ].…”

Charge Redistribution in Mg-Doped p-Type MoS2/GaN Photodetectors

“…With the rapid enhancement in their integration, low power consumption, and miniaturized electric applications, MoS 2 /GaN heterojunction photodetectors have paved the way for valley optoelectronic devices and wide spectral detection. − However, traditional MoS 2 /GaN is a n–n heterojunction due to the existence of natural S and N vacancy defects, limiting their application in electronic devices. , Furthermore, it has been challenging for MoS 2 /GaN n–n heterojunctions to meet the growing demand for enhanced photogenerated carrier transfer and built-in electric field. Typically, to improve the performance of electronic devices, the electronic and optoelectronic properties of materials are optimized by adjusting the electron concentration and the type of carriers. − Subsequently, the contact resistance of the devices decreases, and an optimal band offset and strong internal electric field are obtained.…”

Defect-Enhanced Exciton–Exciton Annihilation in Monolayer Transition Metal Dichalcogenides at High Exciton Densities

“…As for monolayer WS 2 , one zone boundary edge phonon LA­(M) at 176 cm –1 and its intense overtone at 352 cm –1 appeared near the Raman mode E 2g 1 (356 cm –1 ), whereas the frequency difference between E 2g 1 and A 1g (419 cm –1 ) was 63 cm –1 . As for WSe 2 , in-plane mode E 2g 1 and out-of-plane mode A 1g were at 251 and 260 cm –1 , respectively; however, the main reason for concluding that WSe 2 is a monolayer material is the absence of Raman peaks at B 2g 1 (308 cm –1 ), owing to the interaction between adjacent layers induced by van der Waals forces instead of the difference between the values of E 2g 1 and A 1g . − Besides, the AFM results also prove that the four selected 2D materials are monolayer materials with a thickness less than 1 nm in Figure S2, which is consistent with the conclusion obtained using Raman spectroscopy. , The absorption and PL spectra of the four monolayer materials are shown in Figure . For monolayer 2D materials with a direct bandgap, the minimum value of the conduction band and the maximum value of the valence band are located at the binary indexed corners K + and K – of the 2D hexagonal Brillouin zone, and there are obvious PL peaks for the four materials. , The enhanced space confinement effect of the exciton wave function and the weakened dielectric screening effect make the binding energy of the exciton approximately 400 meV, and the lack of inversion symmetry and spin–orbit coupling effect result in valley-contrasting strong spin splitting of the valence and conduction bands. , These effects lead to strong spectrally distinguishable exciton absorption characteristics in the absorption spectra and make the emission peaks of A and B exciton peaks distinguished in the PL spectra of MoS 2 and MoSe 2 .…”

“…58−60 Besides, the AFM results also prove that the four selected 2D materials are monolayer materials with a thickness less than 1 nm in Figure S2, which is consistent with the conclusion obtained using Raman spectroscopy. 61,62 The absorption and PL spectra of the four monolayer materials are shown in Figure 2. For monolayer 2D materials with a direct bandgap, the minimum value of the conduction band and the maximum value of the valence band are located at the binary indexed corners K + and K − of the 2D hexagonal Brillouin zone, and there are obvious PL peaks for the four materials.…”