As graphene sparked the burst of studies in two-dimensional (2D) materials, a plethora of candidate 2D systems has been found and widely applied in opto-electronic devices. Due to the weakly bounded van der Waals (vdW) interaction between the layers, 2D materials are feasible to be exfoliated and recombined into many types of vdW heterostructures without the constraint of lattice mismatching^[1]. In addition, the electronic properties can be flexibly tuned by the stacking orders^[2], from intrinsic multilayer graphene^[3–5] to hybrid heterostructure compositions^[6], e.g.,  the transition metal dichalcogenides family MoS2/WSe2^[7–9] and graphene/boron nitride (BN)^[10]. Since the electronic band structure can be dramatically different at the vicinity of the surface (in contrast to the bulk state), the surface state of the vdW materials can be promising to be optimized for efficient opto-electronic devices^[11–13].

Indium selenide, including InSe and In2Se3, is such a III–VI semiconductor that has distinguishing opto-electronic properties with a high photocurrent generation rate^[14], high electron mobility^[15,16], and anisotropic thermal conductivity^[17–19]. A direct bandgap in the near-infrared (IR) range with a tunable bandgap close to 1.4 eV (for bulk) is found in In2Se3^[20]. In addition, abundant phases have been identified in In2Se3^[21–24], and in-plane (IP) and out-of-plane (OOP) ferroelectricity has been reported in α phases^[25–27], while antiferroelectricity has been found in the β′ phase^[28]. Both α and β′ phases are room temperature phases that are friendly to the manufacture of opto-electronic and memory devices.

In this work, we focus on the study of the opto-electronic properties of α-In2Se3 in terahertz (THz) emission and photocarrier behaviors, which is significant in improving the THz modulation efficiency by ferroelectrics. We find the intrinsic relationship between THz emission efficiency and Auger carrier dynamics in a series of different thickness samples. We propose the surface state plays a role in the surge current acceleration as well as the ultrafast Auger carrier recombination, which is enlightening in the fabrication of ferroelectric α-In2Se3 thin film for ideal THz performance.

The 2D layered In2Se3 is grown on mica substrates by the chemical vapor deposition (CVD) method using In2Se3 (99.99%, Alfa Aesar) powder source as the precursor and an Ar/H2 mixture as the carrier gas. The ceramic boat, filled with the In2Se3 powder source, is placed in the center of a 1-inch (2.54 cm) transparent quartz tube. A freshly cleaved piece of fluorophlogopite mica is placed in the downstream about 10–15 cm away from the boat of the In2Se3 and serves as the substrate. The quartz tube is purged with Ar gas at a rate of 500 sccm (standard cubic centimeters per minute) for 10 min before heating, for the purpose of cleaning the gas line and removing oxygen. The temperature of the In2Se3 powder source is heated up to 800°C at a heating rate of 26°C/min, and then kept for 5–15 min for the growth in the atmosphere of ∼3sccmH2 gas and ∼50sccm Ar gas. The structure and characterization details can be found in Fig. 1.

THz emission spectroscopy is performed on the reflective pump-probe basis. The femtosecond amplifier pulses, characterized by a pulse width of 100 fs, a pulse energy of 4 mJ, a repetition rate of 1 kHz, and a central wavelength of 800 nm, are focused with a 45° incident angle, as indicated by Fig. 2(a). The polarization of the pulse can be continuously tuned with a half-wave plate (HWP) characterized by the angle (θ) with respect to the P polarization. The THz radiation is emitted by the free electrons excited by the pumping pulse with energy of 1.55 eV, which covers the bandgap of intrinsic α-In2Se3. The time-domain spectroscopy is characterized by a probe pulse assisted by the electro-optical sampling method in (110) oriented ZnTe nonlinear crystal.

The ultrafast carrier dynamics of α-In2Se3 is investigated by the reflective optical pump-probe method, using a femtosecond amplifier laser system with a central wavelength of 800 nm, energy of 0.7 mJ, duration of 35 fs, and repetition rate of 10 kHz. The output pulse is split into two beams using a beam splitter. The strong pump beam is frequency doubled to 400 nm (3.10 eV) using a 200 µm β-barium borate (BBO) crystal and modulated by an optical chopper. The weak probe beam travels through a computer-controlled delay stage. As shown in Fig. 3(a), the two beams are focused and overlapped on the sample surface, and the reflected probe beam is monitored by a silicon balanced photodetector connected with a lock-in amplifier.

In this paper, we grow the sample using the CVD method by different thicknesses and prepare a bulk sample with finely polished (001) surface as a reference. We report that the THz emission pattern varies strongly with sample thickness, and the dynamics of the excited carriers indicate that the surface depletion field plays an important role in longitudinal photocurrent transfer.

α-In2Se3 exhibits a rhombohedral R3m crystal structure (point group 3m), which does not allow centrosymmetry and therefore supports the presence of spontaneous polarization from both the IP and OOP ferroelectric phase^[27,29]. Figure 1(a) illustrates the 3R stacking of the α-In2Se3 with quintuple layers via vdW interaction. The Raman spectrum shows two characteristic broad peaks centralized at 109.1cm−1 and 196.2cm−1, representing the less coherent A(LO + TO) and A(LO) phonon modes. However, the previous literature reports that β-In2Se3 features a 110cm−1 peak for the A(LO + TO) mode^[29]. Thus, we infer the multilayer nanoflakes (>100nm) could exhibit a mixture phase with both α and β phases. An atomic force microscopy (AFM) image in Fig. 1(c) indicates the edge thickness of the large triangular piece of α-In2Se3 approaches 1.28 nm and a relatively uniform surface morphology, while thicker area within the largest triangular piece grows symmetrically around the center by subtriangular stackings.

The efficiency of THz generation from wide bandgap semiconductors (>1eV) is normally not as high as that of narrow bandgap semiconductors (<1eV) due to their smaller electron mobility. However, α-In2Se3 exhibits a rather high electron mobility up to 2.5cm2V−1s−1^[30,31], which could yield a large THz signal. As shown in Fig. 2(b), the dynamical range and peak field (peak–peak value) in 40 nm thin film α-In2Se3 are 3 times higher than that in bulk, indicating an intriguing carrier dynamics associated with surface effect. Normally, the THz emission mechanism for narrowband semiconductors is intimately connected to the nonlinear optical properties, such as optical rectification in zinc–blende type of materials^[32], photo-Dember effects in doped semiconductors^[33,34], photon-drag effects, and surface depletion current surge in 2D systems^[35–37].

The general THz radiation mechanism induced by ultrashort pulses can be summarized by an elegant formula: ETHz∝∂2P∂t2+∂J∂t, where P represents the nonlinear polarization that is symmetry-allowed, while J is the transient photocurrent influenced by external or internal fields. The contribution to THz radiation from the first section arises from the induced polarization associated with the crystalline symmetry, while the THz radiation from the second section can reflect the intrinsic carrier dynamics. Akin to the 2H-MoS2 monolayer results^[38], there is also a competition between optical rectification and surface current surge in α-In2Se3, which can be evidenced by Figs. 2(c) and 2(d), where incident polarization dependence in Fig. 2(c) exhibits twofold symmetry, and negligible azimuthal dependence in Fig. 2(d) indicates the large surge current contribution.

By a simple mathematical derivation on the nonlinear second-order susceptibility tensor for point group 3m, which can refer to the supplementary information of a reference of MoS2^[39] that shares the same point group as α-In2Se3, the incident polarization dependence of the THz radiation amplitude can be simplified as ETHz∝Acosθ+Bsinθ+Ccos(2θ)+Dsin(2θ)+E,(1)where A, B, C, D, and E are coefficients associated with the nonzero components of the second-order susceptibility tensor d22, d15, d31, and d33. Likewise, the azimuthal angle dependence of the THz amplitude can be written as ETHz∝cosαin(d22sin(3ϕ)−2d15)+sinαin(d31+d33).(2)Here the optimized incident angle αin is found at 45°. The measuring geometry is shown in Fig. 2(a); the incident polarization dependence is obtained by fixing the THz polarization along the x axis through a wire grid polarizer (WGP) while rotating the linear polarization angle of the incident beam through an HWP. By fitting the THz amplitude signal with Eq. (1), an explicit twofold period on the θ can well describe the optical rectification process.

In addition to the optical rectification occurring in the broken surface symmetry, a large signal from the surge current cannot be ignored. The contribution of the surge current part can be explained by two possibilities: (1) the photoexcited carriers accelerated by the surface depletion field; (2) the diffusion velocity difference from electrons and holes, which is also called the photo-Dember effect. Here we tend to take the first type of photocurrent surge by surface depletion field, since the mobility of electrons and holes in the α-In2Se3 is comparable to MoS2 with the same order. On the other hand, the large current shift in Fig. 2(c) from zero base line (dashed) indicated by the arrow suggests the existence of the depletion field, since the static component E=−2cosαind31+sinαind33 in Eq. (1) prevails at a 45° incidence. Moreover, we demonstrate two types of fitting curves to the azimuthal-dependent map according to the Eq. (2), as shown in the dashed and solid red lines. By considering the threefold component in Eq. (2) (labeled as Fitting 1), the relation of d22/(d31+d33)<10 is valid by a least-mean-square type of fitting, suggesting the static component dominates and exhibits a nearly isotropic pattern in azimuthal dependence, depicted by the solid red curve when taking d22 to be negligible (labeled as Fitting 2).

After examining the THz properties from the α-In2Se3, it is inevitable that we should investigate the photoexcited carrier dynamics, whose behavior can largely modify the THz radiation and can further assist in discriminating the different mechanisms found for bulk and thin-film states. It is significant to understand why a higher THz generation efficiency is found for films rather than bulk, which can provide guidance for THz sensor design. The carrier dynamics is depicted by the pump-probe reflectivity measurements, as shown in Fig. 3. The free carriers are generated by second harmonic generation (SHG) light in the UV region while being probed by IR. The reflectivity of the IR probe changes due to the transient generation and relaxation of the carrier density induced by photoexcitation. Figure 3(b) describes the reflectivity change from the bulk α-In2Se3 surface with a fine-polished (001) plane. The differences in reflectivity, represented by ΔR, increase with the pump fluence. Meanwhile, the signals after the peaks also drop more quickly at higher pumping fluence, indicating increased decaying rates. Similar features are also found in the thin-film samples of subplots Figs. 3(c)–3(e). For the smaller than 5 nm samples, as shown in subplot Fig. 3(f), the increment of the reflectivity is negligible, while a dramatic change of carrier bleaching is observed in all the pumping series.

Multiple-carrier recombination can roughly be described by different types of Auger processes, the decay time of which becomes shorter as the pump fluence increases. On the one hand, when one electron transfers the energy to another electron by recombining with a hole, it is normally called three-carrier Auger recombination. On the other hand, when an exciton is induced by photoexcitation (electron–hole pair), the energy can transform to another exciton, known as bimolecular Auger recombination. As reported in a related study by Tao et al.^[40], the dominant recombination type can gradually switch from three-carrier to bimolecular while the sample thickness increases. However, in our analysis, we use a double-exponential fitting approach to extrapolate the recombination process, which could potentially affect the THz emission.

Figure 4 shows that the fluence-dependent decay rates from both bulk (left columns) and thin-film (right columns) states of α-In2Se3 are obtained by double-exponential fittings by the following equation: ΔR(t)R=A1e−tτ1+A2e−tτ2,(3)where A1 (A2) and τ1 (τ2) represent the amplitude and time constant of the short (long) relaxation procedures. The decaying parts are normalized by the maximum of ΔR/R and are shown in Figs. 4(a) and 4(b). The relative ratios of fast (A1′) and slow (A2′) procedures are modulated by the pump fluence, as shown in the inset of Fig. 4(c) for bulk and Fig. 4(d) for thin film. The decaying procedure is dominated by long decay in the case of low pump fluence. The equilibrium pumping point of A1′ and A2′ for bulk is larger than that of the thin film, indicating a large amount of conduction-band carriers are found in bulk, as the total carrier density is induced by higher-power pumping. The fast decay time τ1 is around 4 and 2 ps for the thin film and bulk sample, respectively. Here we infer the fast decaying part to be three-carrier Auger process, which is believed to reside around 1 to 6.5 ps^[40], while the slower decaying part behaves like the bimolecular process. The decaying time constants τ from all Auger processes drop while increasing the pump fluence, as shown in Figs. 4(e) and 4(f) (the exception is the first point of τ1 where a single exponential curve gives the best fitting). This is not surprising, since both the three-carrier and bimolecular processes comply with the relation^[40] of τβ=1Cβn(0)β−1, where β=2,3 represents the bimolecular or three-carrier case and n(0) is the transient carrier density induced by optical pumping.

When reducing the sample thickness to sub-40 nm, in contrast to the bulk case, the surface state may assist in trapping one hole and recombining it with an electron in the conduction band while transferring the energy to another free electron. This phenomenon has been reported in ZnO and CdSe nanocrystals^[41,42]. The three-carrier Auger recombination has a higher probability of occurring in thinner materials, while the bimolecular case becomes more common in bulk samples when the surface-to-volume ratio decreases. According to the measurements in Fig. 4, the faster three-carrier Auger process dominates in thin film when increasing the pumping energy, which likely accounts for the high intensity THz generation in the thin film. Another possible explanation may arise from band structure deformation while decreasing from bulk to nanoflake. Assuming that the bandgap difference is negligible under the case of strong pumping^[20], the same number of free carriers moves dramatically differently in bulk and thin-film bands. For the bulk case, more free carriers move within the conduction band with slow group velocity (as indicated by the high proportion of bimolecular Auger behavior). However, the free electrons moving in discretized conduction energy levels in thin film may drop to the next energy level by the meV energy difference with a much higher group velocity, in contrast to the smoothing conduction band movement in bulk. This may explain why the current surge by a large surface depletion field is found in thin film, as indicated by the enhanced THz field emission.

We report the study of THz emission from an interesting vdW material α-In2Se3 and its carrier properties in thin-film and bulk states. We find the THz emission efficiency is increased by 3 times when reducing the thin-film thickness to sub-40 nm, in contrast to the bulk state. We further investigate the carrier dynamical difference between bulk and thin-film states by pump-probe reflectivity measurements and extrapolate the key decaying parameters of the free carriers induced by pumping. We argue that the three-carrier Auger process tends to dominate the electron–hole pair recombination more in thin film than in bulk, which provides a faster recombination in less than 4 ps. This phenomenon could be accompanied with a fast current surge induced by the surface depletion field in thin film, which enhances the efficiency of THz emission. Our study provides a new strategy for studying THz radiation through carrier analysis and can be exploited to investigate other vdW families.