Within Reststrahlen bands of polar semiconductors, surface phonon–plasmon coupling is of great interest in infrared nanophotonics. Here, we demonstrate an active long-wavelength infrared device of graphene integrated with an AlN/SiC polar heterostructure. As a low-loss dielectric design, the subwavelength structure device takes advantage of interfacial photogating effect on electrostatic doping of the graphene and the interfaced SiC, and the tunable spectral behavior is originated from the hybridization of the doping-dependent surface phonon–plasmon resonances. This finding provides a steady-state manipulating method to the surface modes for the low-loss nanophotonic devices on SiC platform, and the graphene Fermi level tunable to cross the Dirac point in a steady response even makes the intrinsic graphene photodetectors feasible.

Polar dielectric crystals usually exhibit highly intriguing but unusual optical properties from their negative real part of permittivity within spectral regions between the transverse (TO) and longitudinal (LO) optical phonons, referred to as Reststrahlen bands.1,2 Over the spectral ranges, this metal-like behavior enables different optical phonon modes to be excited not only as propagating surface phonon polaritons (SPhPs)3 but also as localized epsilon-near-zero mode and magnetic polaritons (MPs).4–6 All these modes originate from collective vibrations of polar crystal lattice atoms, instead of free carriers in metals and semiconductors. Analogous to the surface plasmons (SPs) on the conductors, the way to excite SPhPs is also predominant and interesting for the capability of subwavelength field confinement, and it shows greater potential in enhanced light–matter interactions for much lower electromagnetic loss than the plasmonics. Up until now, the SPhP modes of polar crystals have been employed for various infrared and THz-wave applications, such as light manipulation,7,8 sensing,9 and detecting.10 In particular, those nanophotonic devices on SiC platform are more attractive. For negative real permittivity of SiC crystals, the third-generation semiconductors exhibit high reflection in the Reststrahlen band from 797 to 973 cm−1 (wavenumber hereinafter), within the important long-wavelength infrared (LWIR) window of atmosphere. Here, the information transmission of molecular vibrations and blackbody radiation is transparent and frequently used for material phase and spatial target identifications, respectively. With a strong field confinement, the surface phonon technology has been developed for a variety of thermal radiation applications,11–14 and the polar crystals are potential in active photonic devices as well.15,16

Controllable interplay of different electromagnetic resonant modes is a distinct recent interest in optical metasurfaces, especially the surface phonon–plasmon coupling.17,18 Here, the light–matter interactions are manipulated with more flexibility. Despite its attraction for infrared sensing and signaling techniques, how to modify the coupling system is a matter. Recently, more and more efforts have been devoted to the graphene-integrated systems, where hybrid surface plasmon–phonon modes can be manipulated by means of electrostatic doping.19–21 As one of the two-dimensional-layered materials, graphene has attracted much attention due to its tunable conductivity and high carrier mobility.22 If an active integrated photonic device is developed on the SiC platform, a wide low-loss dip of the infrared transmission is available from the substrate and its epitaxial layer of III-nitride; unfortunately, the graphene electrostatic doping will be limited by the SiC material itself. The wide bandgap semiconductor has an extremely low intrinsic carrier concentration and thermal generation rate of minority. For example, at a temperature of 300 K, 4H-SiC has the intrinsic value of 6.7 × 10−11 cm−3,23 much lower than the silicon of 1.4 × 1010 cm−3, but it can be raised by UV photon injection to generate excess carriers. It is worth noting that the use of interfacial photogating effect was reported to change the channel conductivity of graphene field-effect transistors (GFETs).24 Traditionally, it is applicable to graphene photodetectors of high speed and high responsivity and also has potential in ultraweak light signal probing.25,26 Possible for an active photonic device to have an identical doping mechanism, the photogating effect can also meet the requirement for modulating infrared transmission.

Here, we demonstrate an active LWIR nanophotonic device in a metal/graphene/AlN/SiC stacked structure, where hybrid surface phonon–plasmon resonances are manipulated by the electrostatic doping of the graphene and the interfaced SiC. With the assistance of interfacial photogating effect under UV-light pump, it is much easier to create an inversion layer on the SiC surface and even to tune the graphene Fermi energy level across the Dirac point. This work provides an efficient approach to manipulating the surface phonon and plasmon polaritons for the infrared nanophotonic devices on the SiC platform and even opens a path for developing intrinsic graphene photodetectors.

Figure 1(a) shows the schematic diagram of the device. It has a stacked structure of metal grating and graphene on an AlN/SiC polar semiconductor heterostructure. An N+-type (0001) 4H-SiC wafer doped in ∼1 × 1019 cm−3 is selected as the substrate for the epitaxial growth of 6.5 μm-thick N-type SiC with the carrier concentration of ∼1 × 1016 cm−3 and following 100 nm Al-faced AlN layers. The monolayer graphene sheet is transferred onto the AlN surface and then patterned by oxygen plasma etching for electric isolation. Figure 1(b) shows the Raman spectra of the stacked multilayers. Except of the graphene 2D mode at 2684 cm−1, they are overlapped seriously. Fortunately, the G mode can be extracted if the AlN/SiC spectrum is considered as the background.27 The inset shows the result of background subtraction, and we locate the G mode at 1593 cm−1. From its intensity ratio of 1/1.8 to the 2D mode, the graphene is proven to be a monolayer.28 In addition, the D mode cannot be traced yet, indicating its high quality after the transferring process.29 Au: 80 nm/Cr: 20 nm, the source/drain electrodes, and the two-dimensional metal grating arrays in a square lattice are defined by electron beam lithography and lift-off techniques. Finally, as shown in Fig. 1(c), the device is packaged in a ceramic chip carrier for characterization.

FIG. 1.

(a) Schematic diagram of the graphene-integrated infrared nanophotonic device based on interfacial photogating effect and electric-gated field effect, where the conductive substrate of N+-type SiC works as the gate. (b) Raman spectra sampled on the graphene/AlN/SiC and AlN/SiC. (c) Optical image of a prototype device. Inset: polarization-insensitive square grating arrays in the period of 5.0 µm and the metal width of 4.5 µm, where the scale bar is 10 µm.

FIG. 1.

(a) Schematic diagram of the graphene-integrated infrared nanophotonic device based on interfacial photogating effect and electric-gated field effect, where the conductive substrate of N+-type SiC works as the gate. (b) Raman spectra sampled on the graphene/AlN/SiC and AlN/SiC. (c) Optical image of a prototype device. Inset: polarization-insensitive square grating arrays in the period of 5.0 µm and the metal width of 4.5 µm, where the scale bar is 10 µm.

Close modal

Besides the electric-gated mechanism, usually applied for the GFET optical modulators,30,31 our device works on the interfacial photogating effect. It is front-illuminated by an off-the-shelf LED working at a central wavelength of 365 nm. Based upon the size of light beam much larger than the device, we assume that the illuminance is uniform. Due to the bandgap being smaller than the photonic energy, the UV-photon injection can generate excess carriers to significantly raise the minority density of holes in the SiC layer. The carrier holes in the depletion region are separated from the electrons and swept to the AlN/SiC interface by the built-in electric field of the heterojunction; as a result, the graphene Fermi level can be slightly lifted. Here, we adopt this interfacial photogating effect as an assistance to control the SiC and the graphene electrostatic doping. Consequently, for an increase in carrier density, the gate equivalent capacitance and graphene channel conductance are easy to change under the control of the gate bias, and even an inversion layer will appear on the SiC surface if the applied bias is positive and larger than a threshold.

Figure 2 shows the band diagrams of the device when worked under different conditions. At a gate bias of VG = 0, the graphene in the dark state under the pump power density of PUV = 0 has an initial Fermi energy level at EFG,1. Under the condition of thermal equilibrium, between the AlN/SiC interface, the carrier diffusion leads to upward bending of the SiC conduction-band (Ec) and valence-band (Ev) edges and a built-in field inside the SiC layer. Both spontaneous and piezoelectric polarizations of AlN in tensile strain create equivalent forward biases to reduce the built-in potential height. However, this case is different from a graphene/AlGaAs/GaAs structure with two-dimensional electron gas confined.32 From the photon injection, the incremental holes at the interface slightly raise the bright-stated graphene Fermi level to EFG,2. At VG > 0, a larger reverse bias makes the bending upward more seriously and further raises the dark-stated Fermi level to EFG,3, while in the bright state, it climbs to a much higher value of EFG,4, and even this climbing process will be across the Dirac point if the graphene is P-type conductive initially.

FIG. 2.

Energy-band diagrams of the graphene/AlN/SiC heterostructure. (a) and (b) At VG = 0, the photogenerated carrier of holes (in circles) is swept to the AlN/SiC interface by the built-in field of the heterojunction, slightly to raise the graphene Fermi level from (a) EFG,1 to (b) EFG,2 by electrostatic doping. (c) and (d) If a reverse-biased gate voltage of VG > 0 is applied, more holes aggregate at the interface, and the dark- and bight-stated levels are induced to rise to (c) EFG,3 and (d) EFG,4, respectively. 1/Cg = 1/Ca + 1/Cs, the gate capacitance (Cg) is a series one of the AlN layer (Ca) and the SiC depletion layer (Cs). For the case of strong inversion, the low-frequency gate capacitance, Cg,max = Ca.

FIG. 2.

Energy-band diagrams of the graphene/AlN/SiC heterostructure. (a) and (b) At VG = 0, the photogenerated carrier of holes (in circles) is swept to the AlN/SiC interface by the built-in field of the heterojunction, slightly to raise the graphene Fermi level from (a) EFG,1 to (b) EFG,2 by electrostatic doping. (c) and (d) If a reverse-biased gate voltage of VG > 0 is applied, more holes aggregate at the interface, and the dark- and bight-stated levels are induced to rise to (c) EFG,3 and (d) EFG,4, respectively. 1/Cg = 1/Ca + 1/Cs, the gate capacitance (Cg) is a series one of the AlN layer (Ca) and the SiC depletion layer (Cs). For the case of strong inversion, the low-frequency gate capacitance, Cg,max = Ca.

Close modal

The SiC depletion-layer capacitance (Cs) in series with the AlN-layer one (Ca) accounts for most of the heterojunction capacitance when it is reverse-biased, and the junction capacitance is equivalent to the gate capacitance (Cg). As mentioned above, the SiC minority concentration and thermal generation rate are extremely low, and thus, an inversion layer is hardly formed at its surface. To increase the gate bias up, the depletion region will continually widen, and the dark-stated SiC layer is always deeply depleted. For the device in a bright state of PUV > 0, the case is different. The photo-generated holes will fill in the deep-depleted well33 to easily form an inversion layer of holes as the reverse bias increases, resulting in a large low-frequency gate capacitance up to a maximum of Cg,max. However, owing to the concentration of holes not to follow an applied ac signal, the high-frequency capacitance is small yet.

Figure 3(a) shows the bias dependence of the gate capacitance. No matter what the state is, at the high frequency, Cg decreases with an increasing VG, indicating that the SiC layer depleted all long. However, it increases with the pump illuminance, attributed to more carrier injection in unit time. The largest capacitance cannot be observed in quasi-static test, but the device transfer characteristic in Fig. 3(b) confirms an increase in the gate capacitance. For the device in the different state, distinct features can be found here. In the dark state, with an increasing VG, the drain current (IDS) of graphene channel monotonously reduces, but it almost can stay at the same level even if a large positive gate bias is applied. For this case of SiC in deep depletion, Cg is too small to induce the graphene electrostatic doping remarkable, not to mention creating a SiC inversion layer. Due to a larger Cg, the graphene Fermi level in the bright state is tunable in a large range, even across the Dirac point. Thus, to turn around the graphene conductive type becomes easy if assisted by the interfacial photogating effect. Notably, the electric transfer behavior is similar to that of conventional GFETs. Based on the Dirac point found at VG,Dirac ≈ 18.6 V, the initial graphene after being transferred is P-type conductive. Corresponding to the Fermi level at EFG,2, the bright-stated graphene chemical potential of μFG,2 ≈ −0.34 eV is calculated.

FIG. 3.

(a) CgVG curves measured at 100 kHz for the device under different pump power density (PUV). Regime I: depletion; II: deep-depletion at the high frequency. (b) At a source–drain bias of VDS = 50 mV, the device transfer characteristic curves and gate leakage current (IG) are under the normal incident pump with PUV = 0 and 37 mW/cm2. At VG > 0, IG < 6 nA is maintained. In addition, the leakage current is sharply increased at a larger reverse-biased voltage due to the dielectric soft breakdown.

FIG. 3.

(a) CgVG curves measured at 100 kHz for the device under different pump power density (PUV). Regime I: depletion; II: deep-depletion at the high frequency. (b) At a source–drain bias of VDS = 50 mV, the device transfer characteristic curves and gate leakage current (IG) are under the normal incident pump with PUV = 0 and 37 mW/cm2. At VG > 0, IG < 6 nA is maintained. In addition, the leakage current is sharply increased at a larger reverse-biased voltage due to the dielectric soft breakdown.

Close modal
For the dispersion calculations of material and structure, the monolayer graphene is modeled as a conductive surface whose conductivity is calculated by the Kubo formula.34 At an optical angular frequency (ω), the permittivity of anisotropic SiC and AlN along the direction parallel or perpendicular to the crystal C-axis (j = || or ⊥) is written as35,
εjω=ε,j1+ωLO,j2ωTO,j2ωTO,j2ω2iωγjNe2ε0mj*m01ω2+iωΓ.
(1)
Except of the high-frequency value (ε), the first term in Eq. (1) is contributed by optical phonons, where e is the elementary charge, ωLO/ωTO is the LO/TO phonon frequency, and γ is the damping rate. The second term is due to the free-carrier contribution, where m0 is the electron mass and N, m*, and Γ are the carrier concentration, relative effective mass, and damping rate, respectively. Excluding the experimental carrier concentration, the parameter values of SiC and AlN are taken from Refs. 35 and 36, respectively.

As shown in Fig. 4, the structure dispersions of AlN/SiC and graphene/AlN/SiC are described by false-color maps of the imaginary reflecting coefficient (rp) under p-polarized light incidence.37 It can be explained by the reported mode evolution with the thickness change of AlN film on the SiC substrate.38 The LO-frequencies of AlN locate inside the SiC Reststrahlen band. If the AlN thickness is thinner than 50 nm, its ENZ mode strongly interacts with the SiC SPhP supported at the AlN/SiC interface, to form an upper branch and a lower branch. Otherwise, the interaction is obviously weakened with increasing the thickness. For example, that it is 100 nm, as shown in Fig. 4(a), the upper branch turns back to the original pure SiC SPhP, while a bare AlN SPhP evolves from the lower one after largely decoupled from the SiC SPhP. Figures 4(b)4(d) present the dispersion of the new collective modes, hybrid surface phonon–plasmon polaritons (SPhP-SPG), that arise from the coupling of graphene plasmons and the SiC or AlN SPhP in the graphene/AlN/SiC multilayer and, thus, can be manipulated by tuning the graphene chemical potential. In addition, similar to traditional metal–insulator–metal configurations, multi-order MP modes between the heterostructure and the top metal will be supported by the SiC SPhP if coupled with the metal plasmons,39 and thus, they are called SPhP-SPn M (n = 1, 3, 5, …, the MP-mode order) modes here. Due to the frequencies close to the AlN LO-frequencies, there should exist different degree of the interactions between the SPhP-SPn M modes and the supported SPhP-SPG mode by the AlN.

FIG. 4.

Dispersion curves calculated for (a) AlN/SiC and (b)–(d) the graphene/AlN/SiC multilayers, of which the AlN thickness is 100 nm, and the graphene has different chemical potentials (μFG) of (b) 0.1 eV, (c) 0.3 eV, and (d) 0.6 eV.

FIG. 4.

Dispersion curves calculated for (a) AlN/SiC and (b)–(d) the graphene/AlN/SiC multilayers, of which the AlN thickness is 100 nm, and the graphene has different chemical potentials (μFG) of (b) 0.1 eV, (c) 0.3 eV, and (d) 0.6 eV.

Close modal

Figure 5(a) shows the experimental and simulated infrared reflecting spectra of the device, and the simulation is carried out by the three-dimensional finite difference time domain method. A slight difference exists between the experimental and the simulating spectra, attributable to the actual device having non-ideal geometry or material growth. At a frequency within the Reststrahlen band, the negative SiC permittivity enables the hybrid surface phonon–plasmon polaritons to be stimulated. The SPhP-SPG mode supported by the SiC is observed at 952.8 cm−1; as shown in Fig. 5(b), its E-field distribution indicates that the enhanced near field is confined at the AlN/SiC interface and around the graphene in the slots of metal grating. In addition, three lower SPhP-SPn M modes are confirmed to be excited in the metal/AlN/SiC configuration, and their E-field distributions are distinctly different from the SPhP-SPG mode. For the lowest-ordered mode, as shown in Fig. 5(c), its electromagnetic energy is mainly confined within the AlN dielectric layer, and the same is true for the SPhP-SP5 M mode. However, for the SPhP-SP3 M mode depicted in Fig. 5(d), there exists a part of electromagnetic energy confined around the graphene in the slots of metal grating, which is attributed to the stronger interaction between the SPhP-SP3 M mode and the AlN-supported SPhP-SPG mode, as mentioned above. Furthermore, central frequencies of the hybrid modes are dependent on the permittivity of interfaced SiC and graphene. Through tuning the carrier density of the graphene and the SiC surface, one can manipulate the device reflecting spectrum.

FIG. 5.

(a) In the initial state of the graphene with chemical potential μFG,2 = −0.34 eV, simulated (black solid line) and experimental (red dashed line) reflectance spectra of the device. For three localized polariton resonances: (b) SPhP-SPG, (c) SPhP-SP1 M, and (d) SPhP-SP3 M, the normalized E-field intensity distributions are along the y axis, normal to the device surface.

FIG. 5.

(a) In the initial state of the graphene with chemical potential μFG,2 = −0.34 eV, simulated (black solid line) and experimental (red dashed line) reflectance spectra of the device. For three localized polariton resonances: (b) SPhP-SPG, (c) SPhP-SP1 M, and (d) SPhP-SP3 M, the normalized E-field intensity distributions are along the y axis, normal to the device surface.

Close modal

Figure 6 shows the changes in the device reflecting spectrum and mode frequency with the applied gate bias. Except for the SPhP-SPG mode in which the 2DEG involves, all the SPhP-SPM modes in the dark state change little as the bias increases, and the reflecting spectrum nearly keeps constant. In the bright state, however, the device shows the spectral tunability under the control of the gate bias. The SPhP-SPn M modes are blueshifted all the while. Example for the gate bias increasing from VG = −4 to 30 V, limited by the AlN dielectric breakdown, the change in the SPhP-SP1 M mode is 6.8 cm−1 only, and it reaches 8.2 cm−1 for another polarization-sensitive device in a longer period. From the sharp-increased leakage current at a larger reverse-biased gate voltage, we estimate the dielectric strength ∼3 MV/cm, much lower than the reported >7 MV/cm.40 There is reason to think if the AlN layer is improved in the crystal quality or doped with O atoms,41 the frequency tuning ranges for the SPhP-SPMn modes can be further increased with the strength. Meanwhile, dependent on the graphene conductivity, the SPhP-SPG mode starts to be redshifted until approaching the Dirac point, and then, it turns around to be blueshifted. A tuning range of 3.9 cm−1 is achieved, and it can also be broadened by improving the AlN dielectric strength or lowering the initial graphene Fermi level.

FIG. 6.

Under the conditions: (a) dark state of PUV = 0 and (b) bright state of PUV = 193 mW/cm2 at a pump incident angle 80°, or illuminance ∼33.5 mW/cm2. The gate bias dependence of [(a) and (b)] the experimental reflecting spectrum and the central frequency of (c) SPhP-SP1 M and (d) SPhP-SPG modes.

FIG. 6.

Under the conditions: (a) dark state of PUV = 0 and (b) bright state of PUV = 193 mW/cm2 at a pump incident angle 80°, or illuminance ∼33.5 mW/cm2. The gate bias dependence of [(a) and (b)] the experimental reflecting spectrum and the central frequency of (c) SPhP-SP1 M and (d) SPhP-SPG modes.

Close modal

Different from the pulse pump technique used for tuning the surface phonon mode of SiC nanostructure42 and inevitable transient-state response, our device provides with a substituted method of steady-state response. The surface phonon mode is manipulated by the electrostatic doping combined with the continuous wave pump. By the heterojunction built-in field and gate field effects, the carrier holes are separated from electrons to achieve highly localized carriers in the graphene and the interfaced SiC. This method can generate excess carriers in the wide bandgap semiconductor with a much lower carrier injection level. It seems feasible for a practical device. Moreover, after the pump is switched off, the tuning state under the gate field control is steady for a long time, and the refresh speed is proportional to the pump light power density in case the gate field is switched off. In particular, this behavior is also potential for the photodetector application.

In summary, the active infrared device of graphene integrated with the AlN/SiC heterostructure is demonstrated. From the surface phonon–plasmon coupling, the hybrid polariton modes associated with the electrostatic doping of the graphene and the interfaced SiC are controllable on the interfacial photogating effect. Notably, the steady-state responses of the hybrid modes can be fulfilled with a low carrier injection level. In addition, the mode tuning range can be further boosted by improving the crystal quality of AlN or the initial chemical potential of graphene. This work provides an active tuning method of steady-state response for the hybrid system on the SiC platform. For the use of optical phonons in dielectrics, it also exploits a path toward low-loss nanophotonic devices and intrinsic graphene photodetectors of high responsibility.

This work was supported by the National Natural Science Foundation of China (NSFC) (Grant Nos. 62275086, 61675080, and 61735018).

The authors have no conflicts to disclose.

Ye Zhang: Conceptualization (lead); Data curation (lead); Formal analysis (lead); Investigation (lead); Methodology (lead); Software (equal); Validation (lead); Visualization (equal); Writing – original draft (lead). Xiangyu Gao: Formal analysis (equal); Investigation (equal); Supervision (equal). Hui Xia: Formal analysis (supporting); Investigation (equal); Software (supporting); Visualization (supporting). Junjie Mei: Methodology (supporting); Software (equal); Supervision (supporting). Zihui Cui: Visualization (supporting); Writing – review & editing (supporting). Jianjun Lai: Supervision (supporting); Writing – review & editing (supporting). Changhong Chen: Funding acquisition (lead); Project administration (lead); Resources (lead); Supervision (lead); Writing – review & editing (lead).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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