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Absorption and emission modulation in a MoS2-GaN (0001) heterostructure by interfacephonon-exciton couplingPoudel, Yuba; Slawinska, Jagoda; Gopal, Priya; Seetharaman, Sairaman; Hennighausen,Zachariah; Kar, Swastik; D'souza, Francis; Nardelli, Marco Buongiorno; Neogi, ArupPublished in:Photonics research

DOI:10.1364/PRJ.7.001511

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Citation for published version (APA):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 byinterface 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 aMoS2–GaN (0001) heterostructure by interfacephonon–exciton couplingYUBA POUDEL,1 JAGODA SŁAWINSKA,1 PRIYA GOPAL,1 SAIRAMAN SEETHARAMAN,2 ZACHARIAH HENNIGHAUSEN,3

SWASTIK KAR,3 FRANCIS D’SOUZA,2 MARCO BUONGIORNO NARDELLI,1 AND ARUP NEOGI1,*1Department of Physics, University of North Texas, Denton, Texas 76203, USA2Department of Chemistry, University of North Texas, Denton, Texas 76203, USA3Department 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 emittersdue to their close proximity in the lattice constants. The surface properties of a polar semiconductor such as GaNare 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 monolayerMoS2–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 theenergy levels and the excitonic states in the semiconductors and the coupling of the electronic states with phononsresult in the modification of carrier relaxation rates. Modulation of the excitonic absorption states in MoS2 ismeasured 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 significantred-shift of the C excitonic band and faster dephasing of carriers in MoS2. However, optical excitation at energyhigher than the bandgap of both semiconductors slows down the dephasing of carriers and energy exchange at theinterface. Enhanced and blue-shifted PL emission is observed in MoS2. GaN band-edge emission is reduced inintensity at room temperature due to increased phonon-induced scattering of carriers in the GaN layer. Ourresults demonstrate the relevance of interface coupling between the semiconductors for the development of opticaland electronic applications. © 2019 Chinese Laser Press

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

1. INTRODUCTION

Gallium nitride (GaN) is extensively studied for optoelectronicapplications such as light emitters, high electron mobility tran-sistors, and photodetectors [1–3]. The broadband light emitterscan 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 requiredto generate red or near-infrared light decreases the efficiency oflight 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-3Dheterostructures based on GaN interfaced with transition metal

dichalcogenides (TMDs), which have a lattice constant similarto GaN [7]. The TMDs monolayers have high quantum effi-ciency of light emission in the red wavelength regime becausetheir 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 rangecovering the lower ultraviolet to the near-infrared range. Theinterface in such a hybrid-layered material critically influencesthe electronic transport and carrier transfer. The band offset,interface phonons, the proximity of defect states of the twomaterial system, and the interaction of excitons in the 2D semi-conductor with that in the bulk III-V semiconductor 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 regionand has multiple excitonic states across the visible to the ultra-violet (UV) wavelength range [8,9]. GaN absorbs in the UVregion, and a simultaneous interband transition of carriers canbe generated with UV excitation. The extent of UV absorptionin MoS2 is much higher than that in GaN and the absorptionfrom the MoS2–GaN heterostructure in the UV-visible regionis higher than that in GaN and MoS2 [10]. Furthermore, theGaN emission band lies near the Γ point of MoS2 that has ahigh density of states [8]. The overlap of these electronic tran-sitions can facilitate energy transfer at the interface betweenthe two semiconductors. The optical properties of monolayerMoS2 are 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 semiconductor or its hybrid structurewith plasmons is extensively studied because of the potentialapplications due to the tunable electrical and optical propertiesof these materials [13–16]. The relative position of the energylevels determines the transport or the relaxation of the opticallygenerated carriers across the interface in the MoS2–GaN het-erostructure [15,17]. The excitation at the C exciton state,which is close to the Γ point, results in a charge transfer withoutany momentum change [11,18]. Optically dressed phononssignificantly influence the dynamics of the excitonic states inMoS2- [19] or graphene- [20] based semiconductor nanostruc-tures. In these recent works, the plasmonically induced dressedphonon states were utilized for coherent coupling of excitonsand photons. Metal nanoparticles were used to drive the pho-nons for the enhanced light–matter interaction that resulted incoherent exciton–plasmon coupling. However, these phononrelated effects can be induced by a polar dielectric surface toinfluence 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 therelaxation of optically generated carriers in nitrides. The driftof 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 ofoptically excited carriers in GaN [21]. The interaction of pho-nons with electrons in a MoS2–GaN heterostructure canmodulate the dynamics of the excitonic states in MoS2 [22].The coupling of phonons with the carriers or excitons affectsthe charge carrier mobility and causes the broadening of theemission 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 MoS2 and 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 theMoS2–GaN (0001) heterostructure is calculated using densityfunctional theory (DFT), which revealed a type I band align-ment consistent with our experiments and previously reported

results [15,22]. The valence band maxima and the conductionband minima are almost entirely contributed by MoS2 states.We observed the coupling of optically generated carriers withthe phonon modes and transfer of energy and charge across theinterface between the semiconductors. As a result, a significantchange in the position, amplitude, and linewidth of the exci-tonic absorption state of MoS2 at the Γ point as well as signifi-cantly changed dephasing of carriers at all excitonic bandsoccurs. The PL emission from MoS2 is enhanced in intensityand blue-shifted, but the PL emission from GaN is reduced inintensity 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 aMoS2 monolayer and GaN (0001) sur-face employing the Quantum ESPRESSO package [24,25]. Weused generalized gradient approximation (GGA-PBEsol) forthe exchange-correlation function along with ACBN0 [26],a novel pseudo-hybrid Hubbard density functional approachthat 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 basisof 80 Ry. The heterostructure was constructed by stacking theMoS2 on the Ga-terminated GaN slab containing 12 Ga-Nbilayers and fixing the in-plane lattice constant to the calculatedbulk 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 werepassivated by pseudohydrogens. We have considered severalstacking configurations and found the most stable one shownin Figs. 1(a) and 1(b) with theMoS2–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 orderto determine a correct interlayer distance between MoS2 andGaN in each structure. The Brillouin zone sampling at theDFT level was performed following the Monkhorst–Packscheme using a 10 × 10 × 1 k-points grid further increasedto 16 × 16 × 1 in the projected density of states calculations.The electronic band-structure plots in the form of MoS2 andGaN projected density of states (k, E ) maps were obtained us-ing the GREEN package [30–32] as a post-processing tool; forthese reasons the structure was recalculated self-consistentlywith the SIESTA code [33] using similar values of all theparameters (e.g., exchange-correlation functional, k-pointsmeshes). The optical properties were calculated employingthe PAOFLOW code [34].

3. EXPERIMENT

A bulk GaN-monolayer MoS2 vertical heterostructure is syn-thesized by fabricating monolayerMoS2 over the commerciallyavailable 4.5 μm thick silicon-doped GaN film on a doubleside polished sapphire substrate (MSE supplies) by the chemi-cal vapor deposition (CVD) method [35]. The equilibrium ab-sorption spectra of the individual semiconductors are measuredusing 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 ofMoS2 and thedecay kinetics were studied with optical pump-probe spectros-copy. A 100 fs Ti:sapphire oscillator seeded optical parametricamplifier 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 thebehavior of the excited sample is studied with white-light probepulses. The difference in absorption of the probe pulses (ΔA) inthe presence and absence of pump pulses is measured at differ-ent delay times by varying the distance traveled by the pumpand probe pulses.

The PL emission properties of the III–V semiconductor arestudied using a home-built PL setup. An ultraviolet (UV) ex-citation line at 3.82 eV from a He-Cd laser source is used toexcite the sample. The emitted signal from the sample is col-lected using a pair of UV collimating lenses in a reflectinggeometry. The emitted signal is filtered using a 3.76 eV edgelong-pass filter and then detected using a CCD spectrometer.The emission characteristics of MoS2 are studied with a home-built micro-PL system. The setup consists of a home-builtupright epifluorescent microscope fitted with 2.25 eV edgedichroic beam splitter. A laser line with an energy of 2.33 eVis focused onto the sample through a 100× microscope objec-tive. The emitted signal from the sample is collected by thesame objective lens, and separated from the reflected and the

scattered excitation signal passing through a dichroic mirror.The emitted signal from the monolayerMoS2 is further filteredwith a 2.10 eV edge long-pass filter and detected with anAD111 photomultiplier tube (PMT)-based spectrometer.

4. RESULTS AND DISCUSSION

The structural and electronic details of the interface determinethe optical properties of the heterostructure. The theoreti-cal model based on first principles calculations will be dis-cussed before presenting the experimental observations. TheMoS2–GaN (0001) interface is modeled assuming the latticematching between the monolayer MoS2 and the GaN crystals.Several stacking configurations were considered as shown inFig. 6 in Appendix A, but our calculations clearly favoredthe structure shown in Figs. 1(a) and 1(b) with S and Mo atomsaligned with the topmost Ga and N, respectively. The analysisof relative band-edge positions in Fig. 1(c) indicated that in allthe configurations the band gap is significantly reduced withrespect to GaN, in agreement with the previous studies [10,15].The most stable structure reveals a band gap of predominantlyof MoS2 origin, which is further confirmed by the electronicband structure in Fig. 1(d). Importantly, the type I band align-ment is robust against the structural details; in particular, theband offsets hardly change between four different interfaceconfigurations.

Fig. 1. Geometry and electronic structure of theMoS2–GaN heterostructure calculated from the first principles. (a) Top view and (b) side view ofthe most stable interface structure (II). The unit cell is marked as a blue dashed parallelogram in panel (a). The dashed bonds in (b) denote theirmostly van der Waals character. (c) Relative band edge positions of bulk GaN and four interface structures. The bands of the isolated MoS2monolayer 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 showthe MoS2 layers with lateral dimensions extending over 5 μmgrown on GaN as shown in Figs. 8(a) and 8(b) in Appendix A.The Raman and the absorption spectra provide the static op-tical characteristics of the heterostructure formed. The Ramanmodes ofMoS2, GaN, and theMoS2–GaN interface are shownin Figs. 2(a), 2(b), and 2(c), respectively. The active Ramanmodes E1

2g and A1g are observed at 384 cm−1 and 404 cm−1,respectively, for MoS2 on a quartz substrate. However, thesemodes are slightly blue-shifted, and the energy difference be-tween these Raman modes reduced to 19 cm−1 in the hetero-structure. The Raman characteristics combined with the AFMcharacteristics demonstrate that MoS2 consists of a singleatomic layer. The reduced spacing between the MoS2 Ramanmodes on GaN substrate illustrates that the MoS2 layer is lessstrained in GaN compared to the quartz substrate. The Ramanmodes are centered at 575 cm−1 and 738 cm−1 and represent,respectively, the E2 and A1 longitudinal optical (LO) phononmodes of GaN. An additional Raman mode is observed in themonolayer at 419 cm−1, which has been previously reportedas high-order harmonic frequency of an acoustic phonon ofGaN and is not an active Raman mode [22,36,37]. ThisRaman mode is coupled with the transverse acoustic phononmode (XA) of MoS2 and generates a new mode at 598 cm−1

[38]. The broad Raman mode centered at 454 cm−1 is the com-bination of the second-order longitudinal acoustic (2LA) modeand optical mode A2u [39]. Moreover, the new Raman modeat 636 cm−1 is the surface optical (SO) phonon of GaN [38].The substantially modified Raman characteristics demonstrateinterlayer electron-phonon coupling. The large blue-shift ofthe A1 Raman mode of GaN in the heterostructure is attributedto the interaction of the LO phonon with free charge carriersand the change in carrier density due to interface coupling [40].The broadening of the A1 Raman mode and relative increasein intensity in the interface are attributed to the overlappingwith the overtone of the E1

2g Raman mode of MoS2 [41].The steady-state absorption spectrum of MoS2 on quartz

substrate consists of A and B excitonic bands centered at1.85 eV and 2.03 eV, respectively, and a broad band centeredat 2.92 eV. In the presence of the GaN layer, the A and B ex-citonic bands show negligible shift; however, the C excitonicband significantly red-shifted to 2.73 eV as shown in Fig. 2(d).The C excitonic state is located near the Γ point and is iden-tified with large density of states due to band nesting [8].The inset shows the absorption band in GaN. The absorptionspectrum obtained from the DFT simulations shown inFig. 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 MoS2 on quartz, GaN, and MoS2–GaN interface, respectively, showing the active Raman modes. Thereare various Raman modes activated at the MoS2–GaN interface. (d) Steady-state absorption spectrum of MoS2 on quartz (black) and MoS2–GaNinterface (red). (e) The imaginary part of permittivity of a freestanding MoS2 layer (black) and MoS2–GaN interface (red). The insets in (d) and(e) show the corresponding spectrum of GaN.

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band edge. The calculated as well as experimentally measuredspectra show the enhanced absorption in the visible regionin the MoS2–GaN heterostructure consistent with reportedresults [10]. MoS2 has a high-energy absorption band in theUV region [8,42]. The extent of UV absorption in MoS2 islarger compared to GaN [10]. The optical pump energy at2.33 eV induces an interband transition at the K point, andby choosing an excitation at 3.54 eV, the interband transitionof carriers is generated in both semiconductors at the Γ point.

The transient absorption characteristics of MoS2 are influ-enced due to coupling between the interfacial phonons andcarriers and the carrier transfer at the interface are shown inFigs. 3 and 4. The transient absorption spectrum of MoS2on 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, and2.91 eV, respectively, as shown in Fig. 3(a). The dependence ofthe transient absorption spectrum of MoS2 on the GaN layeron 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 increase in pump fluence. However, the C excitonicband appears when the pump fluence exceeds the thresholdpower density. The existence of the C excitonic band with2.33 eV excitation has been reported to be due to many bodyeffects in MoS2 [43–46]. Band nesting close to the Γ pointcontributes to the higher amplitude of the C excitonic band.The transient absorption spectrum of MoS2 is significantlymodified due to the electronic states of GaN. The C excitonicband shows a large red-shift to 2.76 eV; however, there is aslight change in the position of the A and B excitonic bands.The optically excited carriers inMoS2 with the pump pulse arecoupled to GaN phonons. The contribution of GaN in theband structure near the Γ point as shown in Fig. 1(d) also il-lustrates the coupling of GaN with the electronic transitions atthe interface. The exciton–phonon coupling and the chargetransfer 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 interbandtransition of carriers in both MoS2 and GaN is achieved at the

Fig. 3. (a) Transient absorption spectrum ofMoS2 showing the effect of the GaN layer and the effect of excitation energy on excitonic absorptionbands. The black, red, and blue colors represent theMoS2 on quartz with 2.33 eV pump excitation,MoS2 on GaN with 2.33 eV pump excitation,and MoS2 on GaN with 3.54 eV pump excitation, respectively. The inset shows the schematics of the interface phonon coupling and the chargetransfer at the interface. (b) Power dependence of the transient absorption spectrum ofMoS2 on GaN with 2.33 eV. The black, red, blue, and pinkcolors represent the spectrum at pump fluence of 93.75, 187.5, 281, and 375 μJ∕cm2, respectively.

Fig. 4. (a) Decay kinetics showing the recovery of probe absorptionat the (a) A, (b) B, and (c) C excitonic bands of MoS2 showing theeffect of the GaN layer and the effect of excitation energy on excitonicabsorption bands. The black, red, and blue colors represent the MoS2on quartz with 2.33 eV pump excitation,MoS2 on GaN with 2.33 eVpump excitation, and MoS2 on 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 theamplitude of the C excitonic band. The type I band alignmentat the MoS2–GaN interface shown by the DFT calculationsfacilitates the transfer of the photoexcited electrons in GaNto the conduction band in MoS2 with 3.54 eV excitation.The hot phonons generated in GaN have a lifetime of about5 ps and cause slow energy relaxation of hot carriers [21,47].Thus, the carriers as well as the phonons in the GaN layerinteract with the MoS2 layer across the interface, which alsochanges the transient absorption characteristics of MoS2. TheA and B excitonic absorption states are slightly red-shifted, butthe C excitonic band is broadened, and the peak is red-shiftedto 2.68 eV. The scattering of carriers in MoS2 due to interfacephonons causes the spectral broadening of the C excitonicband. The carriers transferred to the MoS2 layer relax fromtheir Γ point to the A and B excitonic states in the presenceof the interface phonons and presumably increase the excitondensity in the MoS2–GaN heterostructure compared to the2.33 eV excitation.

The comparison of the decay kinetics of the A, B, and Cexcitonic states for the three cases is shown in Figs. 4(a),4(b), and 4(c), respectively. The decay kinetics are representedby the equation y � P

A�i exp�− x

ti� � y0, where A

�i represents

the respective amplitude of lifetime t i and y0 represents theresidual absorption. ForMoS2 on quartz, a biexponential decayof carriers from the excitonic states is observed with finiteresidual absorption in the 10 ps time window we considered.In the presence of a GaN layer, we observed an additional fastdecay component with a lifetime of the order 200 fs with2.33 eV optical excitation that results in the substantially fastrecovery of probe absorption at the excitonic bands of MoS2.This fast decay component represents the carrier dephasing dueto coupling of electronic transitions in MoS2 with a GaNlayer, which is consistent with the reported results [22]. Theresidual absorption is significantly reduced due to enhanced de-phasing in the presence of a GaN layer. The recovery of probeabsorption gradually slows down at the B and C excitonic statescompared to the A excitonic state because of slow cooling of hotcarriers at a higher energy state [43]. The faster recovery of theabsorption of the probe at the A excitonic band (Fig. 9 inAppendix A) at higher input pump fluence is consistent withthe formation of the C excitonic band due to many body effectsinvolving A and B excitons. However, the electrostatic screen-ing at a high density of the optically generated carriers, chargetransfer from GaN to MoS2 at the Γ point, and the slower en-ergy relaxation due to hot interfacial phonons in GaN causea slower recovery of the probe absorption and higher residualabsorption at 3.54 eV optical excitation.

The PL emission characteristics of the heterostructure arepresented in Fig. 5. Figure 5(a) shows the combined emissionband of the heterostructure at room temperature (RT). Theemission spectrum consists of a GaN emission band centeredat 3.4 eV and a weak defect band centered at 2.25 eV measuredwith 3.82 eV optical excitation as well as aMoS2 emission bandcentered at 1.87 eV measured with 2.33 eV excitation. With anoptical excitation at 2.33 eV, the carriers inMoS2 are excited atthe 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 differentcompared to the corresponding emission bands of the GaNlayer and MoS2 on a quartz substrate. The emission spectrumof MoS2 on 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 inFig. 5(c) is significantly different compared to the MoS2 emis-sion in a quartz substrate. In the presence of a GaN layer, theemission 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 withthe GaN layer [49] causes the conversion of trions into excitonsin MoS2 that results in the substantially reduced contributionof 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 onthe MoS2 layer, and charge transfer across the interface inthe presence of a GaN layer [9,15,49,50]. The exciton bandis blue-shifted by 65 meV, whereas the trion band is blue-shifted by 21 meV in the presence of a GaN layer. The higherblue-shift of the exciton band over the trion band is attributedto the stronger dependence of emission characteristics of an ex-citon compared to a trion [51].

The optical excitation with energy greater than the bandgapof GaN causes an interband transition of carriers in both semi-conductors at the Γ point. Excitation in this region enhancesthe charge transfer between the semiconductors withoutmomentum change [11,18]. In the GaN layer, the opticallyexcited carriers can relax to the GaN band edges at the Γ pointand recombine to generate PL emission. In addition, electronsin the GaN layer can transfer to the conduction band ofMoS2 at the Γ point. In the MoS2 layer, carriers are excitedto high-energy states due to optical excitation. The excitedcarriers at deep levels, especially holes, have fairly low proba-bility to be scattered to the A and B excitonic states at theK point [52]. Also, the excitation at higher energy increasesthe probability of intervalley scattering of carriers [51]. Thus,the radiative recombination of carriers at the K point is highlyreduced. Therefore, no emission is observed in MoS2 with3.82 eV excitation. The intensity of GaN emission bands isreduced in the heterostructure at RT. However, the GaN emis-sion is recovered at a lower temperature of 30 K. The reducedintensity of GaN emission in MoS2–GaN at RT is assigned toincrease LO phonon-induced non-radiative recombinationat RT. The decrease in intensity of defect band emission inGaN is due to reabsorption by the MoS2 layer. The LOphonon modes of GaN are observed at low temperature, asshown in Figs. 5(e) and 5(f ), where 0,1,2,3 represent thezeroth, first, second, and third phonon replica, respectively.The Huang–Rhys (H-R) factor is calculated using the equa-tion Sn � �n� 1� I n�1

In, where In represents the intensity of

the nth-order phonon replica [53]. The H-R factor calculatedfrom the zeroth and first phonon replica increases from 0.51 inthe GaN sample to 0.65 in the MoS2–GaN sample, whichillustrates the enhanced coupling of carriers with phonons inthe heterostructure.

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5. CONCLUSION

In summary, we reported the change in the transient absorptioncharacteristics 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 phononmodes and the energy exchange at the interface. The originand activation of new Raman modes in the heterostructure in-dicate the electron–phonon coupling between GaN andMoS2.The optical excitation with 2.33 eV causes the interband tran-sition of carriers in MoS2. The coupling of optically excitedcarriers in MoS2 with phonons and the exchange of carrierswith GaN across the interface at the Γ point significantlychange the transient absorption characteristics in MoS2 and

result in the enhanced and blue-shifted PL emission fromMoS2. Optical excitation with an energy greater than thebandgap of GaN generates interband transitions in both MoS2and GaN near the Γ point. Excitation at this energy induces thecoupling of carriers in MoS2 with the hot phonons in GaN,which slows down the relaxation of carriers in MoS2 and theinterlayer carrier transfer as well as the intervalley scattering inMoS2. The LO phonon-induced scattering of carriers reducesthe intensity of band-edge PL emission in GaN at RT. We be-lieve that our study will be helpful to understand the energy andcarrier transfer across the interface, which is crucial to improvethe device performance in optoelectronic and light-harvestingapplications based on a MoS2–GaN heterostructure [54].

Fig. 5. PL emission spectrum. (a) The emission spectrum heterostructure showing the MoS2 and GaN emission bands. PL emission band ofMoS2 on (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) andMoS2–GaN (red) heterostructures. The inset shows the defect band emission afterheterostructure formation. PL emission characteristics at 30 K showing the LO phonon replica of GaN in (e) GaN and (f ) MoS2–GaN.

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APPENDIX A

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

Funding. Office of Naval Research (ONR-MURIN000141310635); 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 thefunding 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 atthe University of North Texas and the Texas AdvancedComputing 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) Heightprofile of MoS2 layer along the line in (a).

Fig. 9. Power-dependent recovery of probe absorption at the A ex-citonic band of MoS2 on GaN. The black, red, blue, and pink colorsrepresent the spectrum at pump fluence of 93.75, 187.5, 281, and375 μJ∕cm2, respectively.

Fig. 6. Top and side views of different stacking configurations I–IVof the MoS2–GaN heterostructure. The numbers in the bottom arethe energy differences with respect to the most stable structure II.

Fig. 7. Calculated energy band alignment diagram of 2D MoS2and bulk GaN. The valence band levels are aligned with respect tothe vacuum, and the valence band offset is calculated by choosingas a reference the most stable MoS2–GaN heterostructure (model IIin Fig. 1).

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