Accelerated Carrier Recombination by Grain Boundary/Edge Defects in MBE Grown Transition Metal Dichalcogenides

Defect-carrier interaction in transition metal dichalcogenides (TMDs) play important roles in carrier relaxation dynamics and carrier transport, which determines the performance of electronic devices. With femtosecond laser time-resolved spectroscopy, we investigated the effect of grain boundary/edge defects on the ultrafast dynamics of photoexcited carrier in MBE grown MoTe2 and MoSe2. We found that, comparing with exfoliated samples, carrier recombination rate in MBE grown samples accelerates by about 50 times. We attribute this striking difference to the existence of abundant grain boundary/edge defects in MBE grown samples, which can serve as effective recombination centers for the photoexcited carriers. We also observed coherent acoustic phonons in both exfoliated and MBE grown MoTe2, indicating strong electron-phonon coupling in this materials. Our measured sound velocity agrees well with previously reported result of theoretical calculation. Our findings provide useful reference for the fundamental parameters: carrier lifetime and sound velocity, reveal the undiscovered carrier recombination effect of grain boundary/edge defects, both of which will facilitate the defect engineering in TMD materials for high speed opto-electronics.

Transition metal dichalcogenides (TMDs), a family of layered materials, have attracted tremendous interest in recent years due to their unique properties at two-dimensional scale, such as direct band gap in monolayer 1 , stable exciton 2 , strong spin-valley coupling 3 , and immunity to short channel effects 4 . One potential application of TMDs is nano-and flexible optoelectronics, in which the dynamics of the photoexcited carriers/excitons plays an essential role in determining the device performance and functionality. Thus, measuring and understanding the dynamics of photoexcited carriers in TMDs is very important to realize novel electronic devices. Currently, there are mainly three ways to produce 2D TMDs: Mechanical exfoliation, Chemical vapor deposition (CVD), and Molecular beam epitaxy (MBE) growth. While the most abundant defects in all three kinds of TMDs are chalcogen vacancies, CVD and MBE TMDs samples show considerable amount of other defect species, such as grain boundary/edge (GB/E) sites and impurities 5 . Typically, defects can have significant impacts on the carrier dynamics. For example, chalcogen vacancies can induce mid-gap states which can give rise to radiative bound excitons, reducing the intrinsic PL intensity 6 ; While oxygen impurities occupying chalcogen vacancy sites can eliminate the mid-gap states 7 , oxygen impurities taking up molybdenum vacancy sites keep those mid-gap states and play the role as effective carrier trappers 8, 9 . Comparing with exfoliated and CVD synthesized samples, studies on the photoexcited carrier dynamics in MBE grown TMDs are rare. Our previous work has shown that one of the main structural defects in MBE grown TMDs are the GB/E sites 10 . These GB/E defects can hinder carrier transport by introducing localized charge-carrier states. However, the effect of these defects on the carrier relaxation dynamics remains unknown. In this letter, we utilize femtosecond laser pump-probe spectroscopy to investigate the photoexcited carrier dynamics in MBE grown MoSe2 and MoTe2 thin films. Comparing with exfoliated samples, the excited carrier decay rates in MBE grown samples are about 50 times faster, as revealed by transient reflection signals, which suggests that the GB/E defects in MBE samples can serve as effective recombination centers for the photoexcited electron-hole pairs. Coherent acoustic phonons in both MBE grown and exfoliated MoTe2 thin film have also been observed, from which the phonon sound velocities are extracted that agree with the theoretical value.
Hexagonal MoSe2 and MoTe2 thin films (5 nm) are grown by MBE on sapphire substrate. Details of the growth process and the structural, spectroscopic, and electrical characterization of the samples can be found in our previous work 10 . Figure 1a and 1b show the transmission electron microscopy (TEM) images of the MBE samples 10 . The main feature of the MBE samples is the presence of a large amount of nanometer-size grains, and thus, plenty of the GB/E sites. The origin of these small size grains is still not well known, one possible reason could be that the large vapor pressure difference between Mo and the chalcogen species adversely narrows the growth window, thereby making the film prone to chalcogen defects and reduced grain dimensions. Similar grain structures have been observed in MBE MoSe2 reported by other groups 11,12 . In contrast, exfoliated TMD samples have much larger grain size, and thus much fewer GB/E defects [13][14][15][16] . As shown in Figure 1c 14 , no obvious GB/E can be seen in exfoliated MoSe2 within an area comparable to Figure 1b.
The large difference in the density of GB/E sites between MBE grown and exfoliated samples provides us a good physical model to study the effect of boundary defects on the photoexcited carrier dynamics.
For comparison, we have also mechanically exfoliated MoSe2 and MoTe2 thick flakes onto Si and sapphire substrates with scotch tape, respectively. We've measured the transient differential reflection signals (∆R/R0, where R and R0 are the reflectivities after and before the pump excitation, respectively) in both MBE grown and exfoliated samples with femtosecond laser pump-probe spectroscopy and compared the carrier relaxation rates in these samples. Our laser pulses are generated from a Ti: Sapphire oscillator operating at 80 MHz repetition rate, with about 100fs pulse width (FWHM), 800 nm central wavelength, and 30 μm diameter of laser spots on the sample surface. Details of our experimental setup can be found in Ref. 8. Figure 2a shows the ∆R/R0 signals in exfoliated MoTe2 measured at different pump fluences.
Besides the sharp rise at the beginning due to the excitation, the signals consist of another small rise after the excitation and then a slow decay component. Noticeable pulse-like signals are observed at the beginning and around 750ps later (echo), which are related to coherent acoustic phonons (strain pulse) generated by pump laser and will be discussed later. Multilayer MoTe2 has an energy difference of 1.16eV at the K point in the momentum space 17 . In our experiment, the pump and probe photon energies are identical (around 1.55eV), much higher than the energy gap at the K point. Therefore, after excitation, the photoexcited electrons/holes will not stay at the pumped (also probed) energy level but quickly relax to the bottom/top states of conduction/valence band through carrier-carrier scattering (carrier thermalization process to reach a temperature defined Fermi distribution) and carrier-phonon scattering (carrier cooling process to reach an equilibrium temperature between carrier and phonon systems), respectively 18 . During these processes, the excess energy of carriers is transferred to the phonon system, generating coherent acoustic phonons and increasing the lattice temperature. After the ultrafast (fs to ps time scale) carrier thermalization 18 , phase space filling effect 19 at the probed energy level should be rather weak due to a very small or even no occupation at that level. Hence, unlike the resonantly-probed case, the reflection change ∆R in the non-resonantly probed case here should not be sensitive to the absorption change (imaginary part of complex refractive index) induced by Pauli blocking of the phase filling effect, but mainly dominated by the real part of complex reflective index change induced by the thermalized carriers instead 9 . Therefore, the small increase of ∆R/R0 signal after the sharp rise can be attributed to the refractive index change (real part) induced by a slight lattice temperature increase, and the decay of ∆R/R0 signal actually reflects the recombination process of the thermalized electron-hole pairs. Figure 2a shows that the peak of ∆R/R0 signal is proportional to the pump fluence, and the signals measured at different pump fluences overlap when normalized (as shown in the inset), which indicates that the pump fluences used in our experiments are small enough that the ∆R/R0 value is linear with the excited carrier density 20 .  coherent acoustic phonons (strain pulse) generated by the pump laser and will be discussed later. Similar to that in the exfoliated sample, the decay is from the decreasing of the excited carrier density due to electron-hole pair recombination. The magnitude of ∆R/R0 signals is also proportional to the pump fluence, and the normalized signals overlap as shown in the inset, indicating that the pump fluence is low enough to ensure a linear relation between the value of ∆R/R0 signals and the excited carrier density plus the phonon vibration amplitude 20 .
The comparison of ∆R/R0 signals in exfoliated and MBE MoTe2 is shown in Figure 2c. A gigantic difference in the signal decay rates is observed. Based on the above signal analysis, the non-oscillating part of the transient ∆R/R0 signals (not considering the echoes and oscillations from acoustic phonons) in exfoliated and MBE MoTe2 after carrier thermalization (fs to ps process) can be described with the following expression: The first, the second and the third term on the right-hand side of Eq. (1) represents the lattice temperature change, the carrier cooling process, and the carrier recombination, respectively. , , and are the signal amplitude for lattice temperature change, carrier cooling, and carrier recombination, respectively, Obviously, the huge difference in carrier lifetime should come from the distinction in structural properties between the two samples: either the dimension (thickness) or the structural defects. The MBE sample has a thickness of 5 nm (7 atomic layers). It has been reported that multilayer MoS2 with thickness larger than 5 atomic layers have carrier lifetimes close to that of the bulk MoS2 21 , which suggests that the difference in structural defects but not the thickness should be responsible for the observed lifetime difference here.
Generally, chalcogen vacancy is the most abundant defect species in TMD materials 22  These defects can typically introduce energy levels in the bandgap and serve as efficient recombination centers that can capture the photoexcited electrons and holes at similar rates, with timescale usually much shorter than the intrinsic lifetime 23 . Therefore, the carrier recombination process can be accelerated by the GB/E defects and the effective carrier lifetime is thus shortened dramatically, as demonstrated in Figure   2c. Previously, the defect assisted electron-hole recombination has been observed 24 . However, the previous study could not identify specific type of defect as recombination center. Here, through comparing the carrier relaxation dynamics in exfoliated and MBE grown samples with large difference in the density of specific defects, we suggest that the GB/E defects are responsible for the observed lifetime reduction in MBE grown MoTe2.
To confirm whether the recombination-center nature of GB/E defects is universal in MBE grown TMDs, we conducted similar measurements of the differential reflection signal ∆R/R0 in exfoliated and MBE grown MoSe2, as shown in Figure 3a

X-ray photoelectron spectroscopy (XPS) of exfoliated MoTe2 flakes
The elemental composition and chemical stoichiometry were investigated by Omicron Multiprobe X-ray photoelectron spectroscopy (XPS) setup. All XPS spectra were acquired at room temperature using monochromatic Al-Kα (hν = 1486.7 eV) X-ray radiation source 1 . The background pressure during measurements was below 3 × 10 -10 mbar. The XPS spectra of the exfoliated MoTe2 flake is shown in Figure  s1.

Thickness of the exfoliated MoTe2
The thickness of our exfoliated MoTe2 flake is determined by Atomic force microscope (AFM). The results are shown in Figure s2