Exploration of exciton dynamics in GaTe nanoflakes via temperature- and power-dependent time-resolved photoluminescence spectra

GaTe nanoflakes have been receiving much research attention recently due to their applications in optoelectronic devices, such as anisotropic non-volatile memory, solar cells, and high-sensitivity photodetectors from the ultraviolet to the visible region. Further applications, however, have been impeded due to the limited understanding of their exciton dynamics. In this work we perform temperatureand power-dependent time-resolved photoluminescence (PL) spectra to comprehensively investigate the exciton dynamics of GaTe nanoflakes. Temperaturedependent PL measurements manifest that spectral profiles of GaTe nanoflakes change dramatically from cryogenic to room temperature, where the bound exciton and donor-to-acceptor pair transition normally disappear above 100 K, while the charged exciton survives to room temperature. The lifetimes of these excitons and their evolution vs temperature have been uncovered by time-resolved PL spectra. Further measurements reveal the entirely different power-dependent exciton behaviors of GaTe nanoflakes between room and cryogenic temperatures. The underlying mechanisms have been proposed to explore the sophisticated exciton dynamics within GaTe nanoflakes. Our results offer a more thorough understanding of the exciton dynamics of GaTe nanoflakes, enabling further progress in engineering GaTe-based applications, such as photodetectors, light-emitting diodes, and nanoelectronics. © 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement


Introduction
Very recently, two-dimensional III-VI group layered materials with the chemical formula of MX (M = In, Ga; X = S, Se, Te) have received considerable attention [1,2], due to their unique features, such as wide light absorption range [3], suitable energy band gaps [4], large nonlinear coefficient [5], high performance in carrier mobility and on/off ratio [6], as well as good light responsiveness in visible-ultraviolet light [7]. These superiorities make them tremendous application prospects in photovoltaic devices [8], optoelectronic devices [9], nonlinear optics [10], and microelectronics [6]. Furthermore, compared to molybdenum and tungsten chalcogenides emitting light efficiently only in films with a single unit cell thickness [11], III-VI group materials are bright light emitters in a range of thicknesses [12]. This may relax the stringent fabrication requirements and show more competition for novel heterostructured devices in light-emitting diodes.
Gallium telluride (GaTe) is one kind of III-VI group layered semiconductor material. The main components of the III-VI group, including GaS, GaSe, InS, and InSe, are crystallizing in a four-sheet sublayer stacking pattern (X-M-M-X) with each sublayer of hexagonal structure. Different from these materials, bulk GaTe appears as a distorted form of the MX structures, belonging to the monoclinic lattice system with a space group of C 3 2h [13,14]. In this case, two-thirds of Ga-Ga bonds are perpendicular to the layer plane and one-third are almost in the layer plane [15]. Interestingly, the newest study demonstrated the thickness-induced structural phase transformation of layered GaTe between monoclinic and hexagonal structures [16]. Furthermore, Ho group also reported the coexistence of monoclinic and hexagonal phases in multilayered GaTe, which emerged two-color photoluminescence (PL) emission [17]. This phenomenon can be partly derived from the 2H (two-layer hexagonal) crystal structure of GaSe 1−x S x series solids [18]. Consequently, the complicated crystal structure of GaTe emerges unique optical and electrical properties, as compared with other III-IV layered materials. Taking anisotropy as an example, electrical conductivity anisotropy between x and y directions of GaTe nanoflakes can be gate tuned from several folds to over 10 3 , yielding an anisotropic non-volatile memory behavior in GaTe nanoflakes [6]. Similarly, only x-polarized transition is allowed in the band edge of multilayer GaTe, rather than both x-and y-polarized transitions coexisting in the other MX layered semiconductors [19]. On the other aspect, GaTe nanoflakes have a direct bandgap of about 1.67 eV, making GaTe extremely valuable in the fields of optoelectronic devices [7], radiation detectors [20], and solar cells [21]. The photodetectors based on GaTe nanoflakes exhibit a remarkable photoresponsivity better than 10 4 A/W and response speed faster than 6 ms, which are better than that of graphene, MoS 2 , and other layered materials. Theoretical calculations predict that GaTe-graphene heterostructures can combine the intense response to visible light as well as the high energy radiation photoabsorption of GaTe, with the ballistic transport properties of graphene, to extended applications in solar cells and radiation detectors [9]. Although GaTe nanoflakes have been widely used in photodetectors, comprehensive investigations on exciton dynamics of GaTe nanoflakes are still limited, which will be helpful to improve the photoresponsivity and sensitivity of the optoelectronic devices.
In this work, we perform temperature-and power-dependent time-resolved PL spectra to reveal the exciton dynamics of GaTe nanoflakes. We attribute the asymmetric spectra profiles of GaTe nanoflakes at room temperature to the combination of neutral excitons (X 0 ) and charged excitons (X T ), with the dissociation energy of X T about 13 meV. PL spectra under cryogenic conditions have been attributed to the combination of donor-to-acceptor transition (X DAP ), shallow-acceptor-bound exciton (X SAB ), and X T , where both X DAP and X SAB rapidly disappear with the increase of temperature and quickly saturate with the increase of excitation power. The lifetimes of X DAP and X SAB are determined to be around 100 ns and 10 ns, which shorten with the increase in temperature. The underlying mechanisms have been proposed to explore the sophisticated exciton dynamics within GaTe nanoflakes.

Sample preparation and characterization
GaTe nanoflakes were prepared by mechanical exfoliation from bulk crystal, purchasing from SxiCarbon Technology (Shenzhen), and then transferred on a SiO 2 /Si substrate. The morphology of GaTe nanoflakes was characterized by optical microscope (OLYMPUS TH4-200) and atomic force microscope (AFM, Flex-Axi). And the Raman spectra were recoeded by a commercial Raman spectrometer (LabRAM HR, Horiba) using the 532 nm laser excitation.

Optical setup
The optical experiments at both room temperature and cryogenic conditions were performed by using a home-built scanning confocal microscope. The experimental schematic has been illustrated in the Appendix (Fig. 5). Particularly, a 532 nm continuum-wave laser (CW, MSL-FN-532, CNI) was used to carry out PL spectra. A pulse laser at the wavelength of 532 nm with a pulse width of about 100 ps was used to perform the time-resolved PL spectra, at the repetition frequency of 5 MHz (Sc-Pro, Yangtze Soton Laser). The laser beam was focused by an objective (60×, NA=0.82, NIRLT-APO, Attocube) with the lateral dimension of the focal spot of about 600 nm. The GaTe nanoflakes were mounted in a cryogenic vacuum chamber (Montana Instruments), of which the temperature can be varied from 4 K to room temperature. PL imaging of GaTe nanoflakes was created through a 4-f system with a fast-steering scanner (FSM-300-M-03, Newport). PL intensity and its spectrum were synchronously recorded, by using a single-photon detector (SPCM-AQR-15, PerkinElmer) and a monochromator equipped with a cooled charge-coupled device (CCD, DU920P, Andor), respectively.

Results and discussion
Abundant GaTe nanoflakes with the thickness ranging from several to hundreds of nanometers were prepared by mechanical exfoliation and transferred on a SiO 2 /Si substrate. We find that PL spectra of ultrathin GaTe nanoflakes (less than 20 nm) show dramatic variations from sample to sample at cryogenic conditions, as they are most likely related to crystal defects and degradation [12]. Further works are needed to clarify this phenomenon. We have confirmed that GaTe nanoflakes with a thickness larger than 30 nm emerge similar exciton behaviors. Here we present a typical result of a GaTe nanoflake with a thickness of about 100 nm. Figure 1(a) shows the optical imaging of the GaTe nanoflake, the relative homogenous contrast color indicates that the basal plane of GaTe is flat and uniform. The thickness of the nanoflake has been explored by AFM characterization, as shown in Fig. 1(b). The height of the cross line has been presented in the inset. Raman spectroscopy were further used to characterize the sample, as shown in Fig. 1(c). The 520 cm −1 phonon mode from the Si substrate was used for calibration. The Raman peaks at 105, 111, 159, 173, 205, 266, and 281 cm −1 can be assigned to the representative vibration modes of the monoclinic phase of GaTe crystal, consisting reasonably well with previous works [22]. Figure 1(d) presents PL spectra of the GaTe nanoflake at room temperature. To our knowledge, GaTe materials with different thicknesses normally emerge asymmetric spectral profiles [19,23,24], however, a clear explanation is still lacking. The PL spectra can be reasonably fitted by two Lorentzian curves. Thus, two possibilities might be responsible for the asymmetric shape. In one aspect, we can assign the two peaks to the neutral exciton (X 0 ) and charged exciton (trion, X T ), as the mature treatment in transition metal dichalcogenides [25,26]. X T possibly results from unintentional doping. In another aspect, the two peaks can be attributed to the optical transition of the monoclinic and hexagonal phase of GaTe nanoflakes, inspired by the very recent works concerning on phase transformation of GaTe and PL spectra of GaSe [16,27]. However, further experimental and theoretical considerations about the later assignment are still under investigation. Here, we tentatively attribute the two components to X 0 and X T .
We firstly perform the temperature-dependent PL spectra and time-resolved PL measurements of GaTe nanoflakes, which can provide important information on exciton dynamics of semiconductor materials. Figure 2(a) presents the evolution of PL spectra as a function of temperature. We can find that at cryogenic conditions (ranging from 4 K to 90 K), GaTe emerges multi-peaks with the energies of about 1.757 eV, 1.701 eV, and 1.594 eV [as shown in Fig. 2(b)]. These results are somewhat different from previous works about GaTe bulk crystal [15,28,29] or recent work about ribbon-like multilayer GaTe [19]. The low-energy peak with broadening feature can be attributed to the donor-acceptor pair transition (X DAP ), the middle one can be assigned to the shallow-acceptor-bound exciton (X SAB ), as suggested by Ho et al. [19]. Based on the Photoluminescence (PL) spectra of the GaTe nanoflake, which is fit to two Lorentzian curves. The component with the blue shadow is assigned to trion emission (X T ), and the other to neutral exciton emission (X 0 ). assignment at room temperature mentioned above, we suggest the high-energy peak to X T , rather than free exciton (X 0 ). Otherwise, the new-formed peak at the high-energy tail is intricate and unassigned. As the temperature increases from 4 K to room temperature, the PL intensity of both X DAP and X SAB decays rapidly and disappears at about 100 K, due to thermal ionization. On the other aspect, even though the PL intensity of X T decreases, it still survives at room temperature. The redshift of X T peak energy can be well fitted by Varshni semiempirical relationship E(T) = E(0) − αT 2 /(β + T), yielding the transition energy E(0) to be 1.765(6) eV, the strength of exciton-phonon interaction α to be 0.5(2) meV/K, and the parameter β to be 189(26) K, respectively. The values of α and β are agreement rationally good with the relevant works [19], while the transition energy is slight redshift, as compared with the reported value of ∼1.79 eV [28,29]. Two possible reasons may be responsible for the energy variation. One is the difference in the thickness and quality of GaTe materials, the other is the diversity of the trion dissociation energy ∆E (the energy difference between X 0 and X T ). Beyond the redshift of peak energy, the full width at half maximum (FWHM) of X T exhibits significantly broadening with the increase of temperature, which can attribute to the increasing probability of collision between exciton and longitudinal optical phonons [30]. Figure 2(d) presents the time-resolved PL intensity as a function of sample temperature. Notably, when the temperature is higher than 110 K, PL traces of the GaTe nanoflake are close to instrument response function (IRF), while for the traces below 110 K, PL decays can be fitted with a triple exponential function I(t) = ∑︁ 3 i=1 A i · e −t/τ i , as the solid lines shown in Fig. 2(d). These behaviors are in agreement favorable with the evolution of their spectra under different temperatures. The fitted lifetimes of each component and their relative ratios have been listed in Table 1 and Fig. 2(e). We can find that both lifetimes and their ratios [ Fig. 2(e)] show dramatic change. Taking the component with the shortest lifetime as an example, its lifetime changes from a few nanoseconds to sub-nanoseconds and the ratio varies from 4% (30 K) to 100% (110 K). These results hint the complicated exciton dynamics of GaTe nanoflakes under different temperatures. Combining the evolution of PL spectra and lifetimes, the short value (between subto several nanoseconds) can be attributed to the lifetime of X T , the middle one (in the region of 5 ns and 21 ns) to that of X SAB , and the long lifetime (16 ns to 177 ns) to that of X DAP. Although time-resolved PL of the GaTe nanoflake presents sophisticated behaviors, their lifetimes become shorter gradually with the increasing temperature, originating from the enhanced interaction between the exciton with the longitudinal optical phonons. The excitation power dependence of PL spectra can provide further insights into the physical origins of the corresponding PL peaks and the potential many-body effect within the materials. Figure 3 depicts PL spectra of the GaTe nanoflake at room temperature under different excitation powers. To our surprise, PL intensity displays a tendency to saturate rapidly and even emerges slight quenching behavior [the inset in Fig. 3(a)], indicating the presence of exciton-exciton annihilation (EEA) under relative high-power excitation. Furthermore, the saturation of X T is faster than that of X 0 [ Fig. 3(d)], indicating the limited free charges within this GaTe nanoflake. Further experiments with high temporal resolution (such as transient absorption or reflective spectrum [31][32][33]) are needed to explore the EEA and many-body effect within GaTe nanoflakes. On the other hand, beyond expected power-induced broadening [ Fig. 3(c)], the change in the PL spectral profiles can be ignored. Thus, PL spectra are deconvoluted by two peaks [as processing in Fig. 1(d)] for the full power region and assigned to X 0 and X T , respectively. We can find that under the low excitation power, the energy peaks of X 0 and X T , as well as their difference ∆E, almost stand at a plateau level. In this case, the trion dissociation energy ∆E can be determined to be about 13 meV, close to the reported thermal energy of bound exciton within GaTe nanoflakes [19]. Under the high excitation power, X 0 emerges dramatic blueshift, while X T presents redshift, as shown in Fig. 3(b). These phenomena might originate from the combination of the EEA effect and band-structure renormalizations under high carrier concentrations [34]. Excitation power-dependent PL spectra of the GaTe nanoflake under the temperature of 4 K are more complicated, emerging significant change in their spectra together with PL intensity, as shown in Fig. 4 (PL spectra in the logarithmic plot can be found in the Appendix, Fig. 6). At low power excitation, GaTe presents three main peaks, which can be assigned to X T , X SAB , and X DAP [the inset of Fig. 4(a)], as we discussed above. Since X SAB and X DAP are associated with excitons bound to the donor or acceptor levels, their PL intensities are expected to saturate at high excitation power when these levels are fully populated with excitons. Consistent with this expectation, both X SAB and X DAP exhibit a sub-linear power dependence and saturate rapidly. In contrast, the PL intensity of X T almost scales linearly (with the power exponent of 1.07), without a significant sign of saturation in this power region. As a result, the overall PL spectra are mostly dominated by X SAB and X DAP at low power excitation, while X T becomes observable and predominant at high excitation intensities, as the spectral weight shown in Fig. 4(d). The solid lines shown in Figs. 4(c) and 4(d) are empirical fitting curves, with the formulas and parameters listed in Tables 2 and 3. During the evolution of PL intensity, their peak energies also manifest considerable change, as depicted in Fig. 4(b). With the increase of excitation power, both X T and X DAP present persistent redshift, coinciding with the variation of X T at room temperature [ Fig. 3(b)]. Whereas the evolution of X SAB is sophisticated, redshift at relatively low power excitation and then recovering to the initial value when the power intensities are further increased.  Tables 2 and 3.

Conclusion
In conclusion, we perform temperature-and power-dependent PL spectra of GaTe nanoflakes to investigate their exciton dynamics. By analyzing the temperature-dependent measurements, we propose that the charged exciton (X T ), donor-acceptor pair transition (X DAP ), and shallowacceptor-bound exciton (X SAB ) dominate PL spectra of GaTe nanoflakes at cryogenic conditions under low power excitation, while PL spectra at room temperature are mainly attributed to X T and neutral exciton (X 0 ). The dissociation energy of X T is extracted to be around 13 meV at low power excitation. The lifetimes of X T , X DAP , and X SAB and their evolution have been demonstrated by time-resolved PL traces. Power-dependent measurements reveal the sophisticated behaviors of excitons within GaTe nanoflakes. The overall PL intensity at room temperature manifests saturation rapidly and even quenches slightly at high power excitation. While only X DAP and X SAB at cryogenic conditions tend to saturate with the increase of power intensity, X T emerges a linear power dependence. The exploration of exciton behaviors within GaTe nanoflakes has been proposed and discussed. The presented results offer a comprehensive insight into the exciton dynamics of GaTe nanoflakes which may aid further applications in GaTe based photodetectors, light-emitting diodes, and solar cells.

Appendix
Note: The time-resolved PL spectra were fitted via triple exponential function: τ 1 , τ 2 , and τ 3 are the lifetimes of X T , X SAB , and X DAP . The values in the brackets represent the corresponding standard derivations.