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

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Absorption and emission modulation in a MoS2-GaN (0001) heterostructure by interface

phonon-exciton coupling

Poudel, Yuba; Slawinska, Jagoda; Gopal, Priya; Seetharaman, Sairaman; Hennighausen,

Zachariah; Kar, Swastik; D'souza, Francis; Nardelli, Marco Buongiorno; Neogi, Arup

Published in:

Photonics research DOI:

10.1364/PRJ.7.001511

IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below.

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Publication date: 2019

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Poudel, Y., Slawinska, J., Gopal, P., Seetharaman, S., Hennighausen, Z., Kar, S., D'souza, F., Nardelli, M. B., & Neogi, A. (2019). Absorption and emission modulation in a MoS2-GaN (0001) heterostructure by interface phonon-exciton coupling. Photonics research, 7(12), 1511-1520.

https://doi.org/10.1364/PRJ.7.001511

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Absorption and emission modulation in a

MoS

2

–GaN (0001) heterostructure by interface

,

1 AND

A

RUP

N

EOGI1,

*

1_{Department of Physics, University of North Texas, Denton, Texas 76203, USA}

2_{Department of Chemistry, University of North Texas, Denton, Texas 76203, USA} 3_{Department of Physics, Northeastern University, Boston, Massachusetts 02115, USA}

*Corresponding author: arup@unt.edu

Received 26 September 2019; revised 30 October 2019; accepted 30 October 2019; posted 30 October 2019 (Doc. ID 378848); published 1 December 2019

Semiconductor heterostructures based on layered two-dimensional transition metal dichalcogenides (TMDs) in-terfaced to gallium nitride (GaN) are excellent material systems to realize broadband light absorbers and emitters due to their close proximity in the lattice constants. The surface properties of a polar semiconductor such as GaN are dominated by interface phonons, and thus the optical properties of the vertical heterostructure are in-fluenced by the coupling of these carriers with phonons. The activation of different Raman modes in the hetero-structure caused by the coupling between interfacial phonons and optically generated carriers in a monolayer MoS2–GaN (0001) heterostructure is observed. Different excitonic states in MoS2 are close to the interband

en-ergy state of intraband defect state of GaN. Density functional theory (DFT) calculations are performed to de-termine the band alignment of the interface and revealed a type-I heterostructure. The close proximity of the energy levels and the excitonic states in the semiconductors and the coupling of the electronic states with phonons result in the modification of carrier relaxation rates. Modulation of the excitonic absorption states in MoS2 is

measured by transient optical pump-probe spectroscopy and the change in emission properties of both semi-conductors is measured by steady-state photoluminescence (PL) emission spectroscopy. There is significant red-shift of the C excitonic band and faster dephasing of carriers inMoS2. However, optical excitation at energy

higher than the bandgap of both semiconductors slows down the dephasing of carriers and energy exchange at the interface. Enhanced and blue-shifted PL emission is observed inMoS2. GaN band-edge emission is reduced in

intensity at room temperature due to increased phonon-induced scattering of carriers in the GaN layer. Our results demonstrate the relevance of interface coupling between the semiconductors for the development of optical and electronic applications. © 2019 Chinese Laser Press

https://doi.org/10.1364/PRJ.7.001511

1. INTRODUCTION

Gallium nitride (GaN) is extensively studied for optoelectronic applications such as light emitters, high electron mobility tran-sistors, and photodetectors [1–3]. The broadband light emitters can be constructed based on ternary compounds such as in-dium gallium nitride (InGaN) formed from III-V bulk semi-conductors, which offer tunable band gaps from 3.4 eV (GaN) to 0.64 eV (InN) [4,5]. However, the high In content required to generate red or near-infrared light decreases the efficiency of light generation due to In segregation and a higher defect den-sity resulting from the lattice mismatch with the substrate [6]. These problems can be overcome by developing novel 2D-3D heterostructures based on GaN interfaced with transition metal

dichalcogenides (TMDs), which have a lattice constant similar to GaN [7]. The TMDs monolayers have high quantum effi-ciency of light emission in the red wavelength regime because their direct band gaps are significantly narrower than in GaN. The heterostructure of monolayer molybdenum disulphide (MoS2) interfaced with GaN constitutes an ideal material

sys-tem for efficient light emitters over a broad wavelength range covering the lower ultraviolet to the near-infrared range. The interface in such a hybrid-layered material critically influences the electronic transport and carrier transfer. The band offset, interface phonons, the proximity of defect states of the two material system, and the interaction of excitons in the 2D semi-conductor with that in the bulk III-V semisemi-conductor are

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expected to modify the carrier mobility and absorption charac-teristics in addition to the emission properties of the materials. MoS2 has a high exciton binding energy in the red region

and has multiple excitonic states across the visible to the ultra-violet (UV) wavelength range [8,9]. GaN absorbs in the UV region, and a simultaneous interband transition of carriers can be generated with UV excitation. The extent of UV absorption in MoS2is much higher than that in GaN and the absorption

from the MoS2–GaN heterostructure in the UV-visible region

is higher than that in GaN and MoS2 [10]. Furthermore, the

GaN emission band lies near theΓ point of MoS2 that has a

high density of states [8]. The overlap of these electronic tran-sitions can facilitate energy transfer at the interface between the two semiconductors. The optical properties of monolayer MoS2are significantly affected by the semiconductor substrate,

and the relaxation of the excited carriers depends on the exci-tation energy [11,12]. The heterostructure of a 2D semicon-ductor with a bulk semiconsemicon-ductor or its hybrid structure with plasmons is extensively studied because of the potential applications due to the tunable electrical and optical properties of these materials [13–16]. The relative position of the energy levels determines the transport or the relaxation of the optically generated carriers across the interface in the MoS2–GaN

het-erostructure [15,17]. The excitation at the C exciton state, which is close to theΓ point, results in a charge transfer without any momentum change [11,18]. Optically dressed phonons significantly influence the dynamics of the excitonic states in MoS2- [19] or graphene- [20] based semiconductor

nanostruc-tures. In these recent works, the plasmonically induced dressed phonon states were utilized for coherent coupling of excitons and photons. Metal nanoparticles were used to drive the pho-nons for the enhanced light–matter interaction that resulted in coherent exciton–plasmon coupling. However, these phonon related effects can be induced by a polar dielectric surface to influence the excitonic properties of 2D materials.

GaN is a polar semiconductor widely used in optoelectron-ics. Its surface properties strongly depend on the termination (gallium or nitrogen) during the growth. The surface and inter-face phonon related processes are dominant mechanisms for the relaxation of optically generated carriers in nitrides. The drift of optically generated hot electrons in GaN produces a non-equilibrium phonon population in momentum space, so-called “hot phonons.” These phonons slow down the relaxation of optically excited carriers in GaN [21]. The interaction of pho-nons with electrons in a MoS2–GaN heterostructure can

modulate the dynamics of the excitonic states in MoS2 [22].

The coupling of phonons with the carriers or excitons affects the charge carrier mobility and causes the broadening of the emission bands that is crucial for the development of the broad-band lasers and LEDs [23].

In this study, we report the modification of the non-equilibrium absorption characteristics of MoS2and the

photo-luminescence (PL) emission properties in 2D MoS2–GaN

(0001) vertical heterostructure induced by the different cou-plings at the interface. The most stable interface of the MoS2–GaN (0001) heterostructure is calculated using density

functional theory (DFT), which revealed a type I band align-ment consistent with our experialign-ments and previously reported

results [15,22]. The valence band maxima and the conduction band minima are almost entirely contributed by MoS2 states.

We observed the coupling of optically generated carriers with the phonon modes and transfer of energy and charge across the interface between the semiconductors. As a result, a significant change in the position, amplitude, and linewidth of the exci-tonic absorption state of MoS2at theΓ point as well as

signifi-cantly changed dephasing of carriers at all excitonic bands occurs. The PL emission from MoS2 is enhanced in intensity

and blue-shifted, but the PL emission from GaN is reduced in intensity due to coupling of electrons with longitudinal optical (LO) phonons and reabsorption of the emitted light by MoS2.

2. THEORETICAL CALCULATIONS

We performed DFT calculations for van der Waals (vdW) het-erostructures based on a MoS2monolayer and GaN (0001)

sur-face employing the Quantum ESPRESSO package [24,25]. We used generalized gradient approximation (GGA-PBEsol) for the exchange-correlation function along with ACBN0 [26], a novel pseudo-hybrid Hubbard density functional approach that ensured the accurate value of the GaN band gap. The ion– electron interaction was treated with the projector augmented-wave pseudopotentials [27] from the PSlibrary database [28] while the wave functions were expanded in a plane-wave basis of 80 Ry. The heterostructure was constructed by stacking the MoS2 on the Ga-terminated GaN slab containing 12 Ga-N

bilayers and fixing the in-plane lattice constant to the calculated bulk value of the substrate (3.18 Å, 1 Å = 0.1 nm). The dan-gling bonds of nitrogen at the bottom side of the slab were passivated by pseudohydrogens. We have considered several stacking configurations and found the most stable one shown in Figs.1(a)and1(b)with the MoS2–GaN distance of

approx-imately 2.32 Å in line with the previous results [10]. A semi-empirical vdW correction (DFT-D2) [29] was added in order to determine a correct interlayer distance between MoS2 and

GaN in each structure. The Brillouin zone sampling at the DFT level was performed following the Monkhorst–Pack scheme using a 10 × 10 × 1 k-points grid further increased to 16 × 16 × 1 in the projected density of states calculations. The electronic band-structure plots in the form of MoS2 and

GaN projected density of states (k, E) maps were obtained us-ing the GREEN package [30–32] as a post-processing tool; for these reasons the structure was recalculated self-consistently with the SIESTA code [33] using similar values of all the parameters (e.g., exchange-correlation functional, k-points meshes). The optical properties were calculated employing the PAOFLOW code [34].

3. EXPERIMENT

A bulk GaN-monolayer MoS2 vertical heterostructure is

syn-thesized by fabricating monolayer MoS2over the commercially

available 4.5 μm thick silicon-doped GaN film on a double side polished sapphire substrate (MSE supplies) by the chemi-cal vapor deposition (CVD) method [35]. The equilibrium ab-sorption spectra of the individual semiconductors are measured using a spectrophotometer with a white-light source and a pho-tomultiplier detector. Raman characteristics of the heterostruc-ture are studied with a high-performance micro-Raman

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spectrometer equipped with an Olympus BX51 microscope us-ing an optical excitation of 2.33 eV.

The non-equilibrium absorption spectrum of MoS2and the

decay kinetics were studied with optical pump-probe spectros-copy. A 100 fs Ti:sapphire oscillator seeded optical parametric amplifier laser was used as source for pump and probe pulses. Excitations at 2.33 eV and 3.54 eV are used as pump energies. The sample is first excited with the pump pulses, and the behavior of the excited sample is studied with white-light probe pulses. The difference in absorption of the probe pulses (ΔA) in the presence and absence of pump pulses is measured at differ-ent delay times by varying the distance traveled by the pump and probe pulses.

The PL emission properties of the III–V semiconductor are studied using a home-built PL setup. An ultraviolet (UV) ex-citation line at 3.82 eV from a He-Cd laser source is used to excite the sample. The emitted signal from the sample is col-lected using a pair of UV collimating lenses in a reflecting geometry. The emitted signal is filtered using a 3.76 eV edge long-pass filter and then detected using a CCD spectrometer. The emission characteristics of MoS2are studied with a

home-built micro-PL system. The setup consists of a home-home-built upright epifluorescent microscope fitted with 2.25 eV edge dichroic beam splitter. A laser line with an energy of 2.33 eV is focused onto the sample through a 100× microscope objec-tive. The emitted signal from the sample is collected by the same objective lens, and separated from the reflected and the

scattered excitation signal passing through a dichroic mirror. The emitted signal from the monolayer MoS2is further filtered

with a 2.10 eV edge long-pass filter and detected with an AD111 photomultiplier tube (PMT)-based spectrometer. 4. RESULTS AND DISCUSSION

The structural and electronic details of the interface determine the optical properties of the heterostructure. The theoreti-cal model based on first principles theoreti-calculations will be dis-cussed before presenting the experimental observations. The MoS₂–GaN (0001) interface is modeled assuming the lattice matching between the monolayer MoS2and the GaN crystals.

Several stacking configurations were considered as shown in Fig. 6 in Appendix A, but our calculations clearly favored the structure shown in Figs.1(a)and1(b)with S and Mo atoms aligned with the topmost Ga and N, respectively. The analysis of relative band-edge positions in Fig.1(c)indicated that in all the configurations the band gap is significantly reduced with respect to GaN, in agreement with the previous studies [10,15]. The most stable structure reveals a band gap of predominantly of MoS2 origin, which is further confirmed by the electronic

band structure in Fig.1(d). Importantly, the type I band align-ment is robust against the structural details; in particular, the band offsets hardly change between four different interface configurations.

Fig. 1. Geometry and electronic structure of the MoS2–GaN heterostructure calculated from the first principles. (a) Top view and (b) side view of

the most stable interface structure (II). The unit cell is marked as a blue dashed parallelogram in panel (a). The dashed bonds in (b) denote their mostly van der Waals character. (c) Relative band edge positions of bulk GaN and four interface structures. The bands of the isolated MoS2

monolayer are not aligned, and the panel indicates only the value of the band gap. More details are provided in Fig. 7 in Appendix A. (d) Electronic structure of the interface calculated within the semi-infinite surface model and projected on MoS₂(red) and GaN states (blue).

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The atomic force microscopy (AFM) characteristics show the MoS2 layers with lateral dimensions extending over 5μm

grown on GaN as shown in Figs.8(a)and8(b)in Appendix A. The Raman and the absorption spectra provide the static op-tical characteristics of the heterostructure formed. The Raman modes of MoS2, GaN, and the MoS2–GaN interface are shown

in Figs. 2(a), 2(b), and 2(c), respectively. The active Raman modes E1_2g and A1g are observed at 384 cm−1 and 404 cm−1,

respectively, for MoS2 on a quartz substrate. However, these

modes are slightly blue-shifted, and the energy difference be-tween these Raman modes reduced to 19 cm−1in the hetero-structure. The Raman characteristics combined with the AFM characteristics demonstrate that MoS2 consists of a single

atomic layer. The reduced spacing between the MoS2 Raman

modes on GaN substrate illustrates that the MoS2layer is less

strained in GaN compared to the quartz substrate. The Raman modes are centered at 575 cm−1and 738 cm−1and represent, respectively, the E2 and A1 longitudinal optical (LO) phonon

modes of GaN. An additional Raman mode is observed in the monolayer at 419 cm−1, which has been previously reported as high-order harmonic frequency of an acoustic phonon of GaN and is not an active Raman mode [22,36,37]. This Raman mode is coupled with the transverse acoustic phonon mode (XA) of MoS2 and generates a new mode at 598 cm−1

[38]. The broad Raman mode centered at 454 cm−1is the com-bination of the second-order longitudinal acoustic (2LA) mode and optical mode A2u[39]. Moreover, the new Raman mode

at 636 cm−1is the surface optical (SO) phonon of GaN [38]. The substantially modified Raman characteristics demonstrate interlayer electron-phonon coupling. The large blue-shift of the A1Raman mode of GaN in the heterostructure is attributed

to the interaction of the LO phonon with free charge carriers and the change in carrier density due to interface coupling [40]. The broadening of the A1 Raman mode and relative increase

in intensity in the interface are attributed to the overlapping with the overtone of the E12g Raman mode of MoS2 [41].

The steady-state absorption spectrum of MoS2 on quartz

substrate consists of A and B excitonic bands centered at 1.85 eV and 2.03 eV, respectively, and a broad band centered at 2.92 eV. In the presence of the GaN layer, the A and B ex-citonic bands show negligible shift; however, the C exex-citonic band significantly red-shifted to 2.73 eV as shown in Fig.2(d). The C excitonic state is located near theΓ point and is iden-tified with large density of states due to band nesting [8]. The inset shows the absorption band in GaN. The absorption spectrum obtained from the DFT simulations shown in Fig.2(e)predicts that the imaginary part of the dielectric func-tion of the MoS2–GaN heterostructure decreases at the GaN

Fig. 2. (a), (b), (c) Raman spectrum of MoS₂on quartz, GaN, and MoS₂–GaN interface, respectively, showing the active Raman modes. There are various Raman modes activated at the MoS₂–GaN interface. (d) Steady-state absorption spectrum of MoS₂on quartz (black) and MoS₂–GaN interface (red). (e) The imaginary part of permittivity of a freestanding MoS2layer (black) and MoS2–GaN interface (red). The insets in (d) and

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band edge. The calculated as well as experimentally measured spectra show the enhanced absorption in the visible region in the MoS2–GaN heterostructure consistent with reported

results [10]. MoS2 has a high-energy absorption band in the

UV region [8,42]. The extent of UV absorption in MoS2 is

larger compared to GaN [10]. The optical pump energy at 2.33 eV induces an interband transition at the K point, and by choosing an excitation at 3.54 eV, the interband transition of carriers is generated in both semiconductors at theΓ point. The transient absorption characteristics of MoS2 are

influ-enced due to coupling between the interfacial phonons and carriers and the carrier transfer at the interface are shown in Figs. 3 and 4. The transient absorption spectrum of MoS2

on a quartz substrate with an optical excitation of 2.33 eV con-sists of A, B, and C excitonic bands centered at 1.85, 1.99, and 2.91 eV, respectively, as shown in Fig.3(a). The dependence of the transient absorption spectrum of MoS2 on the GaN layer

on the power density of the pump has been shown in Fig.3(b). The amplitudes of the A and B excitonic bands gradually in-crease with inin-crease in pump fluence. However, the C excitonic band appears when the pump fluence exceeds the threshold power density. The existence of the C excitonic band with 2.33 eV excitation has been reported to be due to many body effects in MoS2 [43–46]. Band nesting close to the Γ point

contributes to the higher amplitude of the C excitonic band. The transient absorption spectrum of MoS2 is significantly

modified due to the electronic states of GaN. The C excitonic band shows a large red-shift to 2.76 eV; however, there is a slight change in the position of the A and B excitonic bands. The optically excited carriers in MoS2with the pump pulse are

coupled to GaN phonons. The contribution of GaN in the band structure near theΓ point as shown in Fig.1(d) also il-lustrates the coupling of GaN with the electronic transitions at the interface. The exciton–phonon coupling and the charge transfer across the interface as shown in the inset in Fig.3(a)

result in changes in the non-equilibrium absorption spectra [22]. With a pump pulse excitation at 3.54 eV, the interband transition of carriers in both MoS2and GaN is achieved at the

Fig. 3. (a) Transient absorption spectrum of MoS₂showing the effect of the GaN layer and the effect of excitation energy on excitonic absorption bands. The black, red, and blue colors represent the MoS₂on quartz with 2.33 eV pump excitation, MoS₂on GaN with 2.33 eV pump excitation, and MoS2on GaN with 3.54 eV pump excitation, respectively. The inset shows the schematics of the interface phonon coupling and the charge

transfer at the interface. (b) Power dependence of the transient absorption spectrum of MoS2on GaN with 2.33 eV. The black, red, blue, and pink

colors represent the spectrum at pump fluence of 93.75, 187.5, 281, and 375μJ∕cm2, respectively.

Fig. 4. (a) Decay kinetics showing the recovery of probe absorption at the (a) A, (b) B, and (c) C excitonic bands of MoS₂ showing the effect of the GaN layer and the effect of excitation energy on excitonic absorption bands. The black, red, and blue colors represent the MoS2

on quartz with 2.33 eV pump excitation, MoS₂on GaN with 2.33 eV pump excitation, and MoS₂on GaN with 3.54 eV pump excitation, respectively.

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Γ point. In MoS2, the generated hot carriers relax to the C

ex-citonic band at a significantly slow rate [43], which reduces the amplitude of the C excitonic band. The type I band alignment at the MoS2–GaN interface shown by the DFT calculations

facilitates the transfer of the photoexcited electrons in GaN to the conduction band in MoS2 with 3.54 eV excitation.

The hot phonons generated in GaN have a lifetime of about 5 ps and cause slow energy relaxation of hot carriers [21,47]. Thus, the carriers as well as the phonons in the GaN layer interact with the MoS2 layer across the interface, which also

changes the transient absorption characteristics of MoS2. The

A and B excitonic absorption states are slightly red-shifted, but the C excitonic band is broadened, and the peak is red-shifted to 2.68 eV. The scattering of carriers in MoS2due to interface

phonons causes the spectral broadening of the C excitonic band. The carriers transferred to the MoS2 layer relax from

theirΓ point to the A and B excitonic states in the presence of the interface phonons and presumably increase the exciton density in the MoS2–GaN heterostructure compared to the

2.33 eV excitation.

The comparison of the decay kinetics of the A, B, and C excitonic states for the three cases is shown in Figs. 4(a),

4(b), and4(c), respectively. The decay kinetics are represented by the equationy PA_i exp−_tx

i y0, whereA

i represents

the respective amplitude of lifetime t_i and y₀ represents the residual absorption. For MoS2on quartz, a biexponential decay

of carriers from the excitonic states is observed with finite residual absorption in the 10 ps time window we considered. In the presence of a GaN layer, we observed an additional fast decay component with a lifetime of the order 200 fs with 2.33 eV optical excitation that results in the substantially fast recovery of probe absorption at the excitonic bands of MoS2.

This fast decay component represents the carrier dephasing due to coupling of electronic transitions in MoS2 with a GaN

layer, which is consistent with the reported results [22]. The residual absorption is significantly reduced due to enhanced de-phasing in the presence of a GaN layer. The recovery of probe absorption gradually slows down at the B and C excitonic states compared to the A excitonic state because of slow cooling of hot carriers at a higher energy state [43]. The faster recovery of the absorption of the probe at the A excitonic band (Fig. 9 in Appendix A) at higher input pump fluence is consistent with the formation of the C excitonic band due to many body effects involving A and B excitons. However, the electrostatic screen-ing at a high density of the optically generated carriers, charge transfer from GaN to MoS2at theΓ point, and the slower

en-ergy relaxation due to hot interfacial phonons in GaN cause a slower recovery of the probe absorption and higher residual absorption at 3.54 eV optical excitation.

The PL emission characteristics of the heterostructure are presented in Fig.5. Figure5(a)shows the combined emission band of the heterostructure at room temperature (RT). The emission spectrum consists of a GaN emission band centered at 3.4 eV and a weak defect band centered at 2.25 eV measured with 3.82 eV optical excitation as well as a MoS2emission band

centered at 1.87 eV measured with 2.33 eV excitation. With an optical excitation at 2.33 eV, the carriers in MoS2are excited at

the K point [48]. The carriers tend to relax to the A and B

excitonic states at the K point in MoS2. The emission

charac-teristics from the heterostructure are significantly different compared to the corresponding emission bands of the GaN layer and MoS2 on a quartz substrate. The emission spectrum

of MoS2on a quartz substrate consists of exciton and trion

re-combination bands centered at 1.80 eV and 1.78 eV with con-tributions of 53% and 47%, respectively, as shown in Fig.5(b). The emission spectrum from MoS2 on a GaN layer shown in

Fig.5(c)is significantly different compared to the MoS2

emis-sion in a quartz substrate. In the presence of a GaN layer, the emission band in MoS2 is enhanced in intensity and

signifi-cantly blue-shifted. The excitation energy at 2.33 eV is reso-nant to the defect band in GaN. The charge exchange with the GaN layer [49] causes the conversion of trions into excitons in MoS2 that results in the substantially reduced contribution

of trion recombination in the PL emission process. The en-hanced and blue-shifted PL emission is attributed to the in-creased absorption in the visible region, reduced strain on the MoS2 layer, and charge transfer across the interface in

the presence of a GaN layer [9,15,49,50]. The exciton band is shifted by 65 meV, whereas the trion band is blue-shifted by 21 meV in the presence of a GaN layer. The higher blue-shift of the exciton band over the trion band is attributed to the stronger dependence of emission characteristics of an ex-citon compared to a trion [51].

The optical excitation with energy greater than the bandgap of GaN causes an interband transition of carriers in both semi-conductors at theΓ point. Excitation in this region enhances the charge transfer between the semiconductors without momentum change [11,18]. In the GaN layer, the optically excited carriers can relax to the GaN band edges at theΓ point and recombine to generate PL emission. In addition, electrons in the GaN layer can transfer to the conduction band of MoS2 at theΓ point. In the MoS2 layer, carriers are excited

to high-energy states due to optical excitation. The excited carriers at deep levels, especially holes, have fairly low proba-bility to be scattered to the A and B excitonic states at the K point [52]. Also, the excitation at higher energy increases the probability of intervalley scattering of carriers [51]. Thus, the radiative recombination of carriers at the K point is highly reduced. Therefore, no emission is observed in MoS2 with

3.82 eV excitation. The intensity of GaN emission bands is reduced in the heterostructure at RT. However, the GaN emis-sion is recovered at a lower temperature of 30 K. The reduced intensity of GaN emission in MoS2–GaN at RT is assigned to

increase LO phonon-induced non-radiative recombination at RT. The decrease in intensity of defect band emission in GaN is due to reabsorption by the MoS2 layer. The LO

phonon modes of GaN are observed at low temperature, as shown in Figs. 5(e) and 5(f ), where 0,1,2,3 represent the zeroth, first, second, and third phonon replica, respectively. The Huang–Rhys (H-R) factor is calculated using the equa-tion S_n n 1In1

In, where In represents the intensity of

thenth-order phonon replica [53]. The H-R factor calculated from the zeroth and first phonon replica increases from 0.51 in the GaN sample to 0.65 in the MoS2–GaN sample, which

illustrates the enhanced coupling of carriers with phonons in the heterostructure.

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5. CONCLUSION

In summary, we reported the change in the transient absorption characteristics of monolayer MoS2 and the modified PL

emis-sion characteristics in a monolayer MoS2–GaN (0001)

hetero-structure due to the coupling of carriers with the phonon modes and the energy exchange at the interface. The origin and activation of new Raman modes in the heterostructure in-dicate the electron–phonon coupling between GaN and MoS2.

The optical excitation with 2.33 eV causes the interband tran-sition of carriers in MoS2. The coupling of optically excited

carriers in MoS2 with phonons and the exchange of carriers

with GaN across the interface at the Γ point significantly change the transient absorption characteristics in MoS2 and

result in the enhanced and blue-shifted PL emission from MoS2. Optical excitation with an energy greater than the

bandgap of GaN generates interband transitions in both MoS2

and GaN near theΓ point. Excitation at this energy induces the coupling of carriers in MoS2 with the hot phonons in GaN,

which slows down the relaxation of carriers in MoS2 and the

interlayer carrier transfer as well as the intervalley scattering in MoS2. The LO phonon-induced scattering of carriers reduces

the intensity of band-edge PL emission in GaN at RT. We be-lieve that our study will be helpful to understand the energy and carrier transfer across the interface, which is crucial to improve the device performance in optoelectronic and light-harvesting applications based on a MoS2–GaN heterostructure [54].

Fig. 5. PL emission spectrum. (a) The emission spectrum heterostructure showing the MoS₂and GaN emission bands. PL emission band of MoS2on (b) quartz substrate and (c) GaN substrate. The fitted peaks represent the contribution due to trion (blue) and exciton (cyan)

recombi-nation. (d) GaN band-edge emission from the GaN (black) and MoS2–GaN (red) heterostructures. The inset shows the defect band emission after

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APPENDIX A

The appendix section includes the atomic force microscopy im-age and the different stacking configurations of MoS2–GaN

heterostructure.

Funding. Office of Naval Research (ONR-MURI N000141310635); National Science Foundation (NSF-EFRI # 1741677, NSF EECCS 1351424); AMMPI (Seed Grant); University of North Texas (COS Seed Grant).

Acknowledgment. A. N. and F. D. acknowledge the funding from UNT-AMMPI. J. S., P. G., and M. B. N. ac-knowledge support by ONR-MURI N000141310635. A. N. acknowledges the support from the NSF-EFRI project. S. K. acknowledges the support from NSF EECCS project. Finally, we acknowledge the High Performance Computing Center at the University of North Texas and the Texas Advanced Computing Center at the University of Texas, Austin. Disclosures. The authors declare no conflicts of interest.

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Fig. 8. (a) AFM image of MoS₂ on GaN substrate. (b) Height profile of MoS2layer along the line in (a).

Fig. 9. Power-dependent recovery of probe absorption at the A ex-citonic band of MoS2on GaN. The black, red, blue, and pink colors

represent the spectrum at pump fluence of 93.75, 187.5, 281, and 375 μJ∕cm2_{, respectively.}

Fig. 6. Top and side views of different stacking configurations I–IV of the MoS2–GaN heterostructure. The numbers in the bottom are

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