As third-generation wide-bandgap semiconductors, silicon carbide (SiC) crystals have excellent characteristics1^,2 such as high stiffness, high thermal conductivity, large electron saturation mobility, and large breakdown electric field3 and are widely used in electric and optoelectronic devices for harsh environments.4^–8 The SiC semiconductors have fruitful optically active point defects,9 long spin coherence times,10 and special nuclear spin characteristics,11 so they are well suited for quantum computing and sensing. They are also superior in biomedical fields due to their low density and excellent biocompatibility.12^–14 The SiC semiconductors are useful in integrated quantum and nonlinear photonics.15^–17 Semiconductor nanowires (NWs) are unique nanostructures with flexibility in device design. Precisely tunable NW arrays have competent applications. For example, the vertically aligned silicon NWs with frequency-dependent reflectance act as sensors.18 The III–V semiconductor NWs show excellent infrared lasing.19^–21 The SiC NWs have high flexibility and stretchability, high specific surface areas, good field-emission properties, excellent stability, and favorable biocompatibility.22^–24 They have promising applications in high-temperature electric devices,25 energy storage,26 field emitters,27 sensors,28 phototransistors,29 and exploration of light–matter interaction.30 The inherent excellent characteristics and optically active defects of SiC allow applications in optical microresonators17^,31^,32 and photonic crystals.33^,34 The applications of SiC NWs in photonics are only in infancy; how to prepare large-scale SiC NWs that can be naturally integrated into bulk SiC photonic devices remains a challenge. Herein, we study the single-root forest-like SiC NWs fabricated with the anodization method. They exhibit broadband luminescence covering the red–green–blue region. It exhibits as evenly-spaced Fabry–Pérot cavity modes, with a very short picosecond lifetime caused by the quantum-electrodynamic Purcell effect. The luminescence exhibits abrupt enhancement at an elevated temperature. The polytype shows a significant influence on both electronic and luminescence properties of such SiC NWs.

The electrochemical etching is a favorable top–down approach to achieve nanoscale SiC.35^,36 The Si- and C-terminated surfaces have distinct etching rates, and the etched SiC tends to have anisotropic morphology at low etching currents.37 We fabricate the SiC NWs using electrochemical etching of bulk 4H- or 6H-SiC single crystal at a current density of 50mA/cm2. The electrochemical dissolution of SiC occurs through anodic oxidation and requires participation of holes; hence, UV light illumination was employed to advance the etching by generating photo-generated holes.2^,38 The scanning electron microscope (SEM) reveals that after peeling off the topmost surface of the etched crystal, a large number of forest-like SiC NWs were exposed [Figs. 1(a) and 1(d)]. The diameters of the SiC NWs depend on the preparation methods: they could be 40 nm for the catalyst-assisted synthesized cubic SiC NWs,39 25 nm for the SiC NWs prepared using the thermal evaporation method,40 and 50 to 200 nm for those prepared through carbothermic reduction of silica and bamboo.41 The diameters of the current 4H-SiC NWs range around 42.9 nm [Fig. 1(a)]. The atomic force microscopy reveals their homogeneous morphologies [Fig. 1(b)] and an average diameter of 37.8 nm. The SEM shows that the diameters of the 6H-SiC NWs range around 31.9 nm [Fig. 1(d)]. The atomic force microscope (AFM) shows that they are uniform [Fig. 1(e)], with an average diameter of 30.3 nm. The difference between the AFM and SEM results is because one cannot ensure the same region was measured. Both the 4H-SiC single crystal and NWs exhibit the strongest X-ray diffraction (XRD) peak at 2θ=35.6deg [Fig. 1(c)] arising from the (004) planes, indicating that bulk SiC grows layer by layer of (004) planes and most etching-yielded SiC NWs stretch around the ⟨001⟩ orientation.41 The 6H-SiC single crystal and NWs exhibit a strong XRD peak at 35.5 deg belonging to the (006) planes of 6H-SiC [Fig. 1(f)]. The hexagonal 4H- and 6H-SiC crystals usually have Si-terminated (001) and C-terminated (001¯) polar surfaces.42

The side-view image of the 4H-SiC NW forest exhibits an apparent Fabry–Pérot cavity structure [Fig. 1(g)]. The cavity length is ∼3.40μm, and the average length of the SiC NWs is ∼1.05μm. As shown by the experimental scheme [Fig. 1(h)], the intact SiC layers sandwich the NW layer and act as upper and lower reflection mirrors of the Fabry–Pérot cavity. This structure is confirmed by the consistency of the experimentally and theoretically derived cavity optical path (Supplementary Material). Figures 2(a)–2(d) show the photoluminescence (PL) spectra of the 4H- and 6H-SiC NW forests measured at different sites [Figs. 2(e) and 2(f)] and under 532 nm laser excitation. Each spectrum comprises a series of discrete and sharp emission lines of equal spacing [Fig. 2(g)], which indicates that the spontaneous emission of the SiC NWs is limited by the cavity resonance modes. Figure S1 in the Supplementary Material shows the standard deviation of the full width at half-maximum averaged over all cavity-mode subpeaks of each PL spectrum. The PL of different sites on the same sample shows deviating inter-mode frequency difference and cavity-mode linewidths [Figs. 2(a)–2(d)], suggesting different cavity lengths at different sites. The longitudinal modes of the Fabry–Pérot cavity follow 2∑inili=mλ,43 where ∑inili is the total optical path of the cavity, m is a positive integer, and λ is the wavelength of light in free space. The energy difference between the adjacent cavity-mode lines follows ΔE=hc2∑inili, where h is the Planck constant. The refractive index satisfies the Cauchy equation: n(λ)=A+(B/λ2)×106,44 where A and B are two constants and the wavelength λ takes the units of nanometers. We have A=2.5610, B=0.0340 for 4H-SiC, which gives a refractive index of 2.67 at 571 nm. The SiC NW forests excited under a 325 nm laser also exhibited cavity-mode PL.

Note that both 4H- and 6H-SiC single crystals exhibit merely faint PL under either 325 or 532 nm laser excitation (Figs. S2 and S3 in the Supplementary Material). This is because as indirect-bandgap semiconductors, their carrier radiative recombination requires participation of phonons to satisfy the conservation of momentum and thus is inefficient as a (electron–photon–phonon) three-body process.12 In practice, the observed PL should arise from some defects45^,46 because the energies of the emitted photons are much lower than the bulk bandgaps. The luminescence of 4H-SiC is slightly stronger than that of 6H-SiC, implying that 4H-SiC contains more color centers. The 4H-SiC and 6H-SiC single crystals emit green and red light, respectively. Then, what are the origins of the broadband emissions of the SiC NW forests? The luminescence of the SiC NWs is sensitive to their surface structures.47^–51 The quantum confinement effect that plays an important role in the PL of the colloidal SiC quantum dots2^,12^,52 is unobservable in such SiC NWs because they have diameters that are much bigger than the exciton Bohr radius.12 The PL spectra of the 4H- and 6H-SiC NWs are much broader than that of the SiC single crystals (Figs. S2 and S4 in the Supplementary Material). The energies of photons emitted by the SiC NWs are much lower than the bulk bandgaps, confirming the origins of defects in their PL. The PL intensity of the SiC NWs is much higher than that of bulk crystals. The 4H-SiC and 6H-SiC NWs separately show luminescence enhancement of over three times and over 13 times (Fig. S5 in the Supplementary Material). It supports the surface-defect-origin of the PL of the NWs because they have much higher surface-to-volume ratios compared with bulk crystals. This is consistent with the previous viewpoint that the luminescence of porous SiC stems from surface defects.2^,53 The Gaussian fits of the PL spectra of the 4H- and 6H-SiC NWs show blue, green, yellow, and red subbands (Fig. S4 in the Supplementary Material). The blue PL of the colloidal SiC quantum dots54^–58 was supposed to arise from C═O-related surface-defect states.55 The laser irradiation readily breaks the unstable C═O groups, and this explains the weak blue PL in the SiC NWs. By contrast, the strong intensity of the green and red emissions suggests that their related surface groups are much more robust against laser irradiation. The 510 nm green emission of the carbon dots was supposed to arise from C─O─C surface states.58 Similarly, the green emission of the SiC NWs may also stem from C─O─C surface states59 or from silicon dimer-related states.54 The PL of porous silicon at ∼590nm stems from Si═O surface states,60 and the calculation indicates that they induce yellow luminescence of the SiC quantum dots.61 All these suggest that the yellow–red PL of the SiC NWs arises from the surface Si═O color centers. The SiC NW forests also exhibit red PL [under 532 nm (202.8μW) laser excitation] (Fig. S6 in the Supplementary Material) peaked at ∼650nm that may arise from carriers trapped on some more complex surface double bonds; the conjugation of a (C═C or C═O) double bond to another (e.g., ester) group usually causes red shift of related light absorption by over 30 to 40 nm,62 as confirmed in the case of carbon dots.63 In practice, the high-resolution transmission electron microscope (TEM) reveals that there is a thick (∼2.49nm) amorphous layer on the surface of the SiC NW [Fig. 3(a)]. The high-angle annular dark field (HAADF) microscopy and energy dispersive X-ray spectroscopy (EDS) element mapping [Fig. 3(c) and Fig. S7 in the Supplementary Material] show a strong O signal, which covers the same area as that of Si and C. This supports the existence of oxygen-related groups on the surfaces of the SiC NWs. The thickness of the amorphous shells of the SiC NWs in the NW forest deviates from each other [Fig. 3(a)]. The high-resolution TEM reveals that SiC NWs in the same forest have somewhat deviating crystallographic orientation along the long NW axis [Figs. 3(a) and 3(b)]. This discriminates such etching-yielded SiC NWs from the SiC NWs grown using the bottom–up method, which usually shares the same orientation of growth, such as ⟨001⟩ orientation, which is energetically favorable.2

To gain a deeper understanding of the luminescent properties of the 4H- and 6H-SiC NWs, we further measured the low-temperature PL of the SiC NWs obtained under 325-nm xenon-light excitation. Both 4H- and 6H-SiC NWs exhibit blue and green emissions [Figs. 4(a) and 4(d)]. The blue band is much more intense. Based on the Gaussian fits of the PL spectra, we obtain the plot of the PL peak wavelength versus temperature [Figs. 4(b) and 4(e)]. Unlike most conventional semiconductors with bandgaps that vary with temperature and thus their PL shifts with temperature,64^,65 each of the three emission bands of the SiC NWs remains fixed when the temperature varies, suggesting their origins of defects because of the weak temperature dependence of the surface-defect energy levels. This is consistent with the PL result under 325 nm laser excitation. There are a few studies on how the temperature affects the PL of SiC. The B-doped SiC exhibited a blue shift of PL with increasing temperature.66 The intensities of the three PL subbands of the 4H- and 6H-SiC NWs have slightly different temperature dependence [Figs. 4(c) and 4(f)]. When the temperature increases from 80 to 200 K, the PL peak intensities of the 4H- and 6H-SiC NWs only fluctuate slightly. However, the PL increases abruptly at 200 K and then increases slowly again from 200 to 300 K. This intriguing abrupt PL enhancement at ∼200K may be ascribed to the breaking of the surface groups that act as nonradiative transition centers. For the 4H-SiC NWs, the intensities of the 450 and 540 nm peaks increase with temperature, and that of the 400 nm peak remains nearly unchanged. All three subbands of the 6H-SiC NWs become stronger with increasing temperature. The different temperature dependence of the 400 nm band for the 4H- and 6H-SiC NWs may be attributed to their different surface structures.

Quantum electrodynamics predicts that the microcavity can highly shorten (or prolong) the spontaneous emission lifetime of the embedded emitter in it if the latter is in resonance or detuning with the cavity mode. Time-resolved PL spectrum of the 4H-SiC NWs (emission: 576 nm, excitation: 400 nm) [Fig. 5] satisfies the well stretched-exponential law I(t)=I0exp[−(tτ)1/h]+C,67^–69 where I0 is the initial PL intensity, τ is a constant measuring the relaxation time, h measures the distribution range of the lifetimes, and C represents the background signal. The fitting gives τ and h for two decay curves (Table S1 in the Supplementary Material). The average lifetime follows τave=h×τ×Γ(h) (Γ: gamma function). The Ti: sapphire femtosecond laser can penetrate into the deep layer of the etched SiC wafer, reach and excite the interior NWs within the cavity [site a1 in Fig. 2(e)], resulting in their cavity-mode PL decay [Fig. 5] with a very short average lifetime (τcav) of 200 ps. By contrast, the faint light coming from the NanoLED can only excite the surface-exposed NWs, leading to their free-space mode PL decay with a much longer lifetime (τfree) of 1670 ps. According to the Purcell effect as predicted by quantum electrodynamics, the near-resonance photon density of states can be highly improved in a microcavity, and this leads to enhanced spontaneous emission.70 The linewidth of the Fabry–Pérot cavity resonance mode [Fig. 2(a)] of the 4H-SiC NWs follows c2∑inili1−(R1R2)1/2π(R1R2)1/4,71 where c is the speed of light, and R1 and R2 are the reflectivities of two cavity surfaces. For simplicity, assume an air–SiC interface on both upper and lower surfaces [the actual cavity contains a thin film beneath the SiC NW layer, which offers a film–air interface to act as the lower cavity mirror, although the film length is small, ∼0.4μm, see Fig. 1(g)]. The linewidth of the 2.17-eV peak is calculated to be 0.034 eV (Supplementary Material). The spontaneous emission rate in the case of weak light–matter coupling is proportional to the local photon density of states according to Fermi’s golden rule.72 As a result, the Purcell factor, that is, the ratio between the transition rates of the cavity mode and free-space mode, follows Fp=1/τcav1/τfree=3λc34π2n3QVm,73 where λc is the free-space wavelength, n is the refractive index of the medium surrounding the emitter, the cavity quality factor Q≈υcΔυc=63.8, as evaluated using the linewidth 0.034 eV for the 2.17 eV peak, and Vm is the mode volume.74^–76 We get Fp=1/τcav1/τfree=8.35 using the experimentally measured lifetimes (for simplicity, the quantum efficiency is assumed to be constant, which would bring error but not qualitatively change the conclusion). Vm is assumed to be Vm=πd2H4, where H is the cavity length, and d is the cavity-mode beam diameter. We simplify the shape of the NW as a cylinder and assume that the mode-limited spontaneous emission from the single SiC NW has a mode cross section close to its diameter [42.9 nm, Fig. 1(h)]. The refractive index of the SiC NWs (2.67 at 571 nm) is significantly greater than unity of air. The waveguide effect renders the light field more confined in the NWs, leading to a reduction in the effective mode volume. On the other hand, the average inter-NW distance is much bigger than the average NW diameter, which limits the influence of the multi-NW coupling effect. These suggest that the approximation of the mode cross-section size by the NW diameter is a reasonable although simplified assumption. The Purcell factor is calculated to be 8.6 using Fp=3λc34π2n3QVm. The measured inaccuracy in cavity length affects the value of Fp, resulting in a difference between the calculated and experimental values. The big Purcell factor indicates that the spontaneous emission of the SiC NWs is significantly enhanced in the cavity than in free space.

We further study the electronic structures of the single-root 4H- and 6H-SiC NW forests, which play important roles in band alignment-meaningful heterostructure dielectric photonic devices and field electron emitters. The work function of the sample is calculated from the contact potential difference (CPD) measured using the Kelvin probe force microscope (KPFM). CPD refers to the potential difference between the sample and the AFM tip: VCPD=ϕtip−ϕsamplee,77 where ϕsample and ϕtip are work functions of the sample and tip, and e is the elementary charge. Figures 6(a) and 6(b) show the CPD maps of the 4H- and 6H-SiC NWs. The CPD values are uniform across the whole measured region. The average CPD of the 4H-SiC NWs is 595.2 mV, and that of the 6H-SiC NWs is 487.8 mV [Fig. 6(c)]. This indicates that the work function of the former is smaller, and its Fermi energy is higher. The ultraviolet photoelectron spectroscope (UPS) (He I energy: 21.22 eV) of the 4H- and 6H-SiC NWs offers more information concerning the electronic structures [Figs. 6(d) and 6(e)]. The difference between the He I energy and the intercept of the linear fit of the higher energy side of the UPS spectrum gives a work function of 3.78 and 4.37 eV for the 4H- and 6H-SiC NWs, respectively. The 4H-SiC NWs have a smaller work function, which is consistent with the KPFM result. The intercept of the linear fit of the UPS spectrum on the lower energy side gives a valence band maximum (VBM) energy of −2.46 and −2.52eV (relative to Fermi energy) for the 4H- and 6H-SiC NWs, respectively. The SiC NWs have no apparent quantum confinement effect.12 Therefore, they and SiC single crystal share nearly the same bandgap. The bandgap of 4H-SiC is 3.28 eV, and that of 6H-SiC is 3.08 eV. Based on these parameters, we present the energy band diagrams for the 4H- and 6H-SiC NWs [Fig. 6(f)], which indicates that they are n-type semiconductors, being identical to the case of pristine bulk crystals. The electron affinity energy of the 4H-SiC NWs is 2.96 eV, and that of the 6H-SiC NWs is bigger (3.81 eV). This difference suggests that the 4H-SiC NWs are suited for applications in high-frequency and high-power devices, whereas the 6H-SiC NWs are suitable for constructing high-pressure and high-temperature devices.78

Finally, note that the experiments show that all the measured parameters unavoidably have some small range of sample-to-sample variability; however, these do not change the qualitative conclusions. In addition, note that the theoretical simulation of the cavity mode inside the NWs using methods such as the finite-difference time-domain method could give more accurate results than the mode calculation starting from theoretical equations and geometric parameters.

We have studied the unique photophysical properties of the structurally and electronically interconnected forest-like SiC NWs. They exhibit Fabry–Pérot cavity mode spontaneous emission, which is significantly enhanced owing to the vacuum fluctuation-induced Purcell effect. The PL shows abrupt enhancement at an elevated temperature of 200 K. The polytype has a significant influence on the electronic properties of the SiC NWs. Considering their excellent mechanical properties, strong resistance to irradiation, and superior electric properties, these monolithic SiC NW forests with cavity-mode-limited spontaneous emissions may have unique applications in field electron emission, sensors, and electromagnetic microwave absorption. They may also be applied in microscale and nanoscale integrated photonic devices as integratable and SiC chip-compatible micro light emitters and in semiconductor–metal integrated optoelectronic devices. The implementation of micro- and nano-laser arrays is also conceivable considering the wide-region tunable emission wavelengths of such SiC NWs.

Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant No. 12274076).

Xueli Sun is a postgraduate student at the Key Laboratory of Quantum Materials and Devices of Ministry of Education and the School of Physics, Southeast University. Her research focuses on silicon carbide nanowires and microcavities.

Qin Ling is a PhD candidate at the Key Laboratory of Quantum Materials and Devices of Ministry of Education and the School of Physics, Southeast University. Her research focuses on optical properties of chalcopyrite quantum dots.

Ruonan Miao is a PhD candidate at the Key Laboratory of Quantum Materials and Devices of Ministry of Education and the School of Physics, Southeast University. Her research focuses on optical properties of copper-related semiconductors.

Huaxin Wu is a PhD candidate at the Key Laboratory of Quantum Materials and Devices of Ministry of Education and the School of Physics, Southeast University. His research focuses on optical and thermal properties of microplatelets.

Jiyang Fan is currently a professor of physics at the Key Laboratory of Quantum Materials and Devices of Ministry of Education and the School of Physics, Southeast University. He received his BS degree from the Shandong University, MS degree from the Institute of Theoretical Physics, Chinese Academy of Sciences, and PhD from the Nanjing University. His research interest mainly lies in the field of semiconductor nanophotonics.