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Register a new userAbsorption and emission modulation in MoS2-GaN (0001) heterostructure by interface phonon-exciton coupling
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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 non-equilibrium 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 {Gamma}-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.
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Hybrid heterostructures based on bulk GaN and two-dimensional (2D) materials offer novel paths toward nanoelectronic devices with engineered features. Here, we study the electronic properties of a mixed-dimensional heterostructure composed of intrinsic n-doped MoS2 flakes transferred on p-doped GaN(0001) layers. Based on angle-resolved photoemission spectroscopy (ARPES) and high resolution X-ray photoemission spectroscopy (HR-XPS), we investigate the electronic structure modification induced by the interlayer interactions in MoS2/GaN heterostructure. In particular, a shift of the valence band with respect to the Fermi level for MoS2/GaN heterostructure is observed; which is the signature of a charge transfer from the 2D monolayer MoS2 to GaN. ARPES and HR-XPS revealed an interface dipole associated with local charge transfer from the GaN layer to the MoS2 monolayer. Valence and conduction band offsets between MoS2 and GaN are determined to be 0.77 and -0.51 eV, respectively. Based on the measured work functions and band bendings, we establish the formation of an interface dipole between GaN and MoS2 of 0.2 eV.
The ability to perform efficient electrical spin injection from ferromagnetic metals into two-dimensional semiconductor crystals based on transition metal dichalcogenide monolayers is a prerequisite for spintronic and valleytronic devices using these materials. Here, the hcp Co(0001)/MoS2 interface electronic structure is investigated by first-principles calculations based on the density functional theory. In the lowest energy configuration of the hybrid system after optimization of the atomic coordinates, we show that interface sulfur atoms are covalently bound to one, two or three cobalt atoms. A decrease of the Co atom spin magnetic moment is observed at the interface, together with a small magnetization of S atoms. Mo atoms also hold small magnetic moments which can take positive as well as negative values. The charge transfers due to covalent bonding between S and Co atoms at the interface have been calculated for majority and minority spin electrons and the connections between these interface charge transfers and the induced magnetic properties of the MoS2 layer are discussed. Band structure and density of states of the hybrid system are calculated for minority and majority spin electrons, taking into account spin-orbit coupling. We demonstrate that MoS2 bound to the Co contact becomes metallic due to hybridization between Co d and S p orbitals. For this metallic phase of MoS2, a spin polarization at the Fermi level of 16 % in absolute value is calculated, that could allow spin injection into the semiconducting MoS2 monolayer channel. Finally, the symmetry of the majority and minority spin electron wave functions at the Fermi level in the Co-bound metallic phase of MoS2 and the orientation of the border between the metallic and semiconducting phases of MoS2 are investigated, and their impact on spin injection into the MoS2 channel is discussed.
Atomically thin layer transition metal dichalcogenides have been intensively investigated for their rich optical properties and potential applications in nano-electronics. In this work, we study the incoherent optical phonon and exciton population dynamics in monolayer WS2 by time-resolved spontaneous Raman scattering spectroscopy. Upon excitation of the exciton transition, both the Stokes and anti-Stokes optical phonon scattering strength exhibit a large reduction. Based on the detailed balance, the optical phonon population is retrieved, which shows an instant build-up and a relaxation lifetime of around 4 ps at an exciton density E12 cm-2. The corresponding optical phonon temperature rises by 25 K, eventually, after some 10s of picoseconds, leading to a lattice heating by only around 3 K. The exciton relaxation dynamics extracted from the transient vibrational Raman response shows a strong excitation density dependence, signaling an important bi-molecular contribution to the decay. The exciton relaxation rate is found to be (70 ps)-1 and exciton-exciton annihilation rate 0.1 cm2s-1. These results provide valuable insight into the thermal dynamics after optical excitation and enhance the understanding of the fundamental exciton dynamics in two-dimensional transition metal materials.
The electron-phonon coupling strength in the spin-split valence band maximum of single-layer MoS$_2$ is studied using angle-resolved photoemission spectroscopy and density functional theory-based calculations. Values of the electron-phonon coupling parameter $lambda$ are obtained by measuring the linewidth of the spin-split bands as a function of temperature and fitting the data points using a Debye model. The experimental values of $lambda$ for the upper and lower spin-split bands at K are found to be 0.05 and 0.32, respectively, in excellent agreement with the calculated values for a free-standing single-layer MoS$_2$. The results are discussed in the context of spin and phase-space restricted scattering channels, as reported earlier for single-layer WS$_2$ on Au(111). The fact that the absolute valence band maximum in single-layer MoS$_2$ at K is almost degenerate with the local valence band maximum at $Gamma$ can potentially be used to tune the strength of the electron-phonon interaction in this material.
The strain field of a dislocation emerging at a free surface is partially relaxed to ensure stress free boundary conditions. We show that this relaxation strain at the outcrop of edge threading dislocations in GaN{0001} gives rise to a piezoelectric volume charge. The electric field produced by this charge distribution is strong enough to dissociate free excitons at distances over 100 nm from the dislocation line. We evaluate the impact of this effect on cathodoluminescence images of dislocations.
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