Divacancies near or at lattice defects in SiC, the PL5–PL7 photoluminescence centers, are known to have more favorable optical and spin properties for applications in quantum technology compared to the usual divacancies. These centers were previously predicted to be divacancies near stacking faults. Using electron paramagnetic resonance, we observe PL5, PL6, and four other divacancy-like centers, labeled PLa–PLd, in electron-irradiated high-purity semi-insulating (HPSI) 4H-SiC. From the observed fine-structure D-tensors, we show that these centers including PL6, which has so far been believed to be an axial center, all have C1h symmetry. Among these, PLa, PLc, and PLd are basal divacancies and PL5 and PL6 are slightly deviated from axial symmetry, while PLb is different from others with the principal Dzz axis of the D-tensor aligning at ∼34° off the c-axis. We show that these modified divacancies are only detected in one type of HPSI materials but not in commercial n- and p-type substrates or n-type pure epitaxial layers irradiated by electrons regardless of surface treatments which are known to create stacking faults.

Silicon carbide (SiC) has been developed for power device applications for many years.1 Since the last decade, SiC has emerged as a promising material for applications in quantum technology,2–4 especially for scalable quantum communication networks since it hosts various spin-active photoluminescence (PL) centers, which can be used for fabrication of single photon sources emitting light near and at telecom wavelengths with low transmission losses in fiber networks,5,6 and has so far been the only wide bandgap semiconductor with advantages of industrial wafer scale, CMOS compatible technology, and established nanofabrication technique.

Among various PL centers in SiC, the neutral divacancy (VCVSi0), i.e., an uncharged complex consisting of a C vacancy (VC) and a nearest Si vacancy (VSi), and the negative Si vacancy (VSi) have been most studied centers since their well-controlled fabricated single emitters with single spins having long coherence times and can be optically7,8 and electrically9 controlled. The neutral divacancy is a spin S = 1 center. Its four different configurations in 4H-SiC, namely, two axial centers with both vacancies occupying the hexagonal (hh) and quasi-cubic (kk) lattice and two basal centers (hk) and (kh), have been identified by electron paramagnetic resonance (EPR).10 Their PL emissions in the spectral range of 1078–1132 nm with zero-phonon lines (ZPL) PL1, PL2, PL3, and PL4 corresponding to the configurations, hh, kk, hk, and kh, have been identified in 4H-SiC.2 In addition to PL1–PL4, two more ZPLs, PL5 and PL6 at 1041.9 and 1037.7 nm, respectively, were observed by Koehl and co-workers.2 Based on zero-field optically detected magnetic resonance (ODMR) measurements, PL5 and PL6 were assigned to basal and axial centers, respectively. In a later study by Falk and co-workers, PL5 was reassigned to axial divacancy.11 Also, in their study of n-type 4H-SiC layers implanted with C ions, another divacancy-like center PL7 was observed by ODMR and categorized a basal divacancy, but fine-structure parameters of this center have not been determined. The PL of PL7 is still unknown.

Unlike the usual divacancies PL1–PL4 centers, which are not detected at room temperature and exhibit PL quenching,12–14 the PL5–PL7 centers have stable PL emissions at room temperature.2,11 Moreover, coherent control of spins by ODMR is possible even above 550 K and temperature sensing up to 450 K have been demonstrated.15 These defects show much higher ODMR contrast even at room temperature (the change of PL intensity under resonant conditions is up to ∼30% for PL716) compared to the usual divacancies PL1–PL4 (typically a few percentages). This is a huge advantage in coherent control of spins using ODMR.

The observation of PL5–PL7 in C-implanted materials indicates the involvement of lattice damages or defects.11 Recently, these defects have been suggested by calculations to be divacancies near stacking faults.17 This proposed model of divacancies near and at a 6H polytypic inclusion in the 4H polytype could explain the robustness against photoionization and room temperature stability. Since PL5 was previously suggested to be a basal center without information on the principal Dzz axis of the D-tensor,2 theoretical modeling assigned it to a basal vacancy.17 The assignment of these defects to axial or basal configurations requires detailed experimental data on the fine-structure D-tensor which have so far been lacking.

In this work, we use EPR and PL to study these modified divacancies in 4H-SiC. For increasing the concentration of these defects to levels detectable by EPR, we increase their concentration in high-purity semi-insulating (HPSI) 4H-SiC by electron irradiation. In addition to PL5 and PL6, we observe four other divacancy-like centers. Their spin-Hamiltonian parameters have been determined. We show that electron irradiation does not create these modified divacancies in commercial n- and p-type substrates or pure epitaxial layers regardless of surface treatments which are known to induce stacking faults.

The starting material used in this study is commercial HPSI 4H-SiC grown by physical vapor transport (PVT) from Cree. The samples are cut from a 4 in. wafer with chemical mechanical polish (CMP) surface. To check the model of divacancies near stacking faults, we use different starting materials, namely, commercial n- and p-type substrates with the as-sliced backside and mechanically polished front side using diamond papers with grain sizes of 1–3 μm, and n-type pure (free carrier concentration in the low 1013 cm−3 range) freestanding 4H-SiC epitaxial layers (∼250 μm thick) grown by chemical vapor deposition (CVD) with the substrate removed by mechanical polishing. The surface treatment such as mechanical polishing18–21 or wafer slicing22 is known to create stacking faults. Samples are then irradiated by 2 MeV with fluences between 1 × 1018 and 1 × 1019 cm−2 at room temperature and subsequently annealed in N2-gas flow ambient at temperatures in the range of 800–1020 °C to form divacancies.

EPR measurements are performed on a Bruker cw-E500 ESR spectrometer operating at X band frequency (∼9.4 GHz) equipped with a liquid-He flow cryostat that can regulate the sample temperature between 4 and 300 K. For above-bandgap illumination in EPR experiments, we use a Xenon lamp (150 W) with short-wave pass filters to let light with wavelengths shorter than 470 nm go through. The PL measurements are performed at low temperatures using a closed-cycle cryostat S50 from Montana, which can regulate the sample temperature between 3.5 and 295 K. For excitation at wavelengths 940 nm or shorter, a tunable Ti-sapphire Mira 900 laser has been used. The PL emission is dispersed by a Jobin Yvon HR460 monochromator equipped with a 300 grooves/mm grating blazed at 1000 nm and detected by an InGaAs multi-channel detector with optimum sensitivity in the spectral range of the neutral divacancy (1000–1200 nm) and resolution of ∼1 Å. Some PL measurements are performed at 2 K using a liquid-He bath cryostat.

The PL5 and PL6 PL center with the ZPL at 1041.9 and 1037.7 nm, respectively, is always observed by PL in as-grown HPSI 4H-SiC from Cree but have not so far been detected by EPR due to low concentrations. Electron irradiation and annealing can increase the concentration of the divacancy but will move the Fermi level from the (+|0) donor level of VC at ∼EC − 1.5 eV23,24,25 up to the (+|0) donor level of the C antisite-vacancy pair (CSiVC) at ∼EC − 1.1 eV (or ∼EV + 2.2 eV),23,26 converting VCVSi0 (at ∼EV + 1.1 eV) to VCVSi (at ∼EV + 2.0 eV).14 This occurs when the (0|−) acceptor level of the Si vacancy (at ∼EV + 1.25 eV)14 is not enough to compensate the (+|0) donor level of CSiVC. However, the relative concentration between VSi and CSiVC can be changed since they have different annealing behaviors. Although VSi becomes mobile at ∼700 °C, it is annealed out gradually and still survives after annealing at ∼1200 °C,27 while CSiVC is annealed out rapidly at temperatures above 1000 °C.28 We have found that with increasing the concentration of VSi by increasing the dose of irradiation to 4 × 1018 cm−2 and annealing at ∼1020 °C, VSi can compensate the (+|0) donor level of CSiVC and pull the Fermi level back to the (+|0) donor level of VC, stabilizing the divacancy in the neutral charge state in equilibrium. Figure 1(a) shows the EPR spectrum in this material after electron irradiation and annealing measured at room temperature in dark for the magnetic field along the c-axis (B||c). As indicated in the figure, the dominating signals are from the four configurations of the neutral divacancy PL1–PL4 and the negative Si vacancy, VSi.27 In addition, there are seven weaker pairs of lines from spin S = 1 centers with two of them appearing close to the PL1 and PL2 lines. We will prove later that these two centers are the previously reported PL5 and PL6 centers.2 There are other pairs of lines close to basal-divacancy lines PL3 and PL4. Since no parameters related to the PL7 center being available, we do not label these new EPR centers following the literature, i.e., using PL with increasing number. These lines are, therefore, labeled as PLa–PLd as indicated in Figs. 1(a) and 1(b). In addition, there is a new pair of weak lines appearing near the PLa lines, labeled as PLa′ and indicated by arrows in the figure. Due to too weak signals, we could follow the angular dependence of these lines only at some angles and the data are not enough for determination of its parameters and, hence, not included in the analysis. We also check EPR under above-bandgap excitation and do not observe noticeable changes in the intensity of these EPR lines.

FIG. 1.

EPR spectra in electron-irradiated HPSI 4H-SiC from Cree measured at room temperature in dark for (a) B||c and (b) Bc. (a) Strong signals PL1–PL4 of VCVSi0 and other pairs of weak lines from PL5, PL6, and five new centers PLa, PLa′, PLb–PLd. Three at the center of the spectrum belong to VSi.27 The upper insets in (a) show parts of the spectrum in extended magnetic field scale with signals from PL1, PL2, and the modified divacancies PL5 and PL6. The lower inset in (a) shows a part of the spectrum measured for B at ∼15° off the c-axis indicating the splitting of both PL5 and PL6 lines. The splitting seen with some lines, e.g., for PLb in (b) is caused by a small misorientation (∼1°) of the rotation plane of the magnetic field from the (1−100) plane.

FIG. 1.

EPR spectra in electron-irradiated HPSI 4H-SiC from Cree measured at room temperature in dark for (a) B||c and (b) Bc. (a) Strong signals PL1–PL4 of VCVSi0 and other pairs of weak lines from PL5, PL6, and five new centers PLa, PLa′, PLb–PLd. Three at the center of the spectrum belong to VSi.27 The upper insets in (a) show parts of the spectrum in extended magnetic field scale with signals from PL1, PL2, and the modified divacancies PL5 and PL6. The lower inset in (a) shows a part of the spectrum measured for B at ∼15° off the c-axis indicating the splitting of both PL5 and PL6 lines. The splitting seen with some lines, e.g., for PLb in (b) is caused by a small misorientation (∼1°) of the rotation plane of the magnetic field from the (1−100) plane.

Close modal

The angular dependences of EPR lines from the modified divacancies PL5, PL6, and PLa–PLd with the magnetic field rotating in the (1−100) plane are shown in Fig. 2. An EPR center in a hexagonal lattice has six possible orientations, which become equivalent when the magnetic field is along the c-axis, regardless of their symmetry. Therefore, resonances correspond to these six orientations coincide and give rise to one EPR line for each transition, i.e., two lines corresponding to allowed transitions |−1⟩ ↔ |0 > and |0⟩ ↔ |+1⟩ of these spin S = 1 centers. For an axial center with C3v symmetry, no splitting of the line should occur when rotating the magnetic field away from the c-axis. However, for a center with C1h symmetry, some orientations become inequivalent when the magnetic field is not along the c-axis and the line will split. With the magnetic field rotating in the (1−100) plane, the line at B||c will split into three, each corresponding to two equivalent orientations.

FIG. 2.

Angular dependences of resonance lines from PLa–PLd centers with the magnetic field rotating in the (1−100) plane. The angles 0° and 90° correspond to the B||[0001] and B||[11−20] directions, respectively. The curves show the simulations using the spin-Hamiltonian parameters given in Table I including a small misorientation of the rotation plane of ∼1° with the (1−100) plane to account for small splitting of the lines that can be seen at intermediate angles for PLa–PLd.

FIG. 2.

Angular dependences of resonance lines from PLa–PLd centers with the magnetic field rotating in the (1−100) plane. The angles 0° and 90° correspond to the B||[0001] and B||[11−20] directions, respectively. The curves show the simulations using the spin-Hamiltonian parameters given in Table I including a small misorientation of the rotation plane of ∼1° with the (1−100) plane to account for small splitting of the lines that can be seen at intermediate angles for PLa–PLd.

Close modal
TABLE I.

Spin-Hamiltonian parameters of the modified divacancy centers PL5, PL6, and PLa–PLd in 4H-SiC at room temperature. The angle θ of the principal Dzz axis of the D-tensor is given in degrees with θ = 0 corresponding to the [0001] direction of the c-axis. The D and E parameters are given in GHz and MHz, respectively.

ParametersgD (GHz)E (MHz)θ (deg)
PL5 2.0034 1.328 13.0 
PL6 2.0037 1.357 6.0 1.5 
PLa 2.0040 1.289 −75.0 70 
PLb 2.0035 0.750 70.0 34 
PLc 2.0042 1.424 94.9 70 
PLd 2.0040 1.289 104.9 70 
ParametersgD (GHz)E (MHz)θ (deg)
PL5 2.0034 1.328 13.0 
PL6 2.0037 1.357 6.0 1.5 
PLa 2.0040 1.289 −75.0 70 
PLb 2.0035 0.750 70.0 34 
PLc 2.0042 1.424 94.9 70 
PLd 2.0040 1.289 104.9 70 

In our study, the sample with the c-plane (0001) surface is cut with the length along the [1−100] direction. However, the sample was mounted not exactly along the vertical rod of the sample holder. Due to this misalignment, the sample was not rotated exactly in the (1−100) plane but slightly inclined. As a result, all the orientations become inequivalent at certain angles, and three lines split into six. At angles close to the c-axis, the splitting is small and may not be seen in the experiments but becomes visible when the angle between the magnetic field and the c-axis increases as can be seen in the angular dependences.

We notice that the splitting of the PL6 and PL5 lines is clearly observed at angles of 5° and 15° off the c-axis, respectively, as shown in the insets of Fig. 1(a) and their angular dependences, indicating that the symmetry of both centers is not axial (C3v) but orthorhombic, C1h. Thus, as shown from the observed angular dependences, the center PL5, PL6, and PLa–PLd all have C1h symmetry.

The obtained angular dependences can be described by the following spin-Hamiltonian:
H = μ B g B S + S D S .
(1)
Here, μB is the Bohr magneton, the electron spin is S = 1, g is the g-tensor, and D is the second rank fine-structure tensor, which is often decomposed into two fine-structure parameters, D = 3Dzz/2 and E = (Dxx − Dyy)/2, representing the contribution to the zero-field splitting (ZFS) caused by axial and orthorhombic fields, respectively. The symmetry of all these centers is constrained to C1h.

We find that all the angular dependences can be fitted with isotropic g-values and the splitting of lines is caused by spin–spin interaction. The misorientations in different planes are also included in the fits, which show that the rotation plane is inclined by ∼1° with the (1−100) plane. The parameters obtained from the best fits to the experimental data using spin-Hamiltonian Eq. (1) are given in Table I. The simulations of angular dependences of these centers using obtained parameters are plotted as curves in Fig. 2.

In zero-field ODMR measurements, resonance lines are expected to appear at frequencies (D ± E).2 For an axial center, E = 0 and only a single ODMR resonance line should be detected, and its frequency corresponds to the D value. The ZFS will be the largest in the direction of the c-axis and is equal to 2D. If E ≠ 0, two ODMR lines are expected, and the symmetry of the center is lower to C1h or C1 in the hexagonal lattice. Koehl and co-workers2 observed a single ODMR line for PL6 and assigned it to an axial divacancy. For PL5, two resonances were observed at 1.3559 and 1.3689 GHz. This gives D = 1.3624 GHz and E = 6.5 MHz. A later study from the same group gives D = 1.373 GHz and E = 16.5 MHz.11 Our D and E values for PL5 determined at room temperature are smaller (D = 1.328 GHz and E = 13 MHz) due to the temperature dependence of the ZFS, which occurs for all the divacancies, e.g., for PL1 and PL2, the D value is ∼4.5 G (∼12.6 MHz) larger at 34 K compared to that at 293 K.

With a small value E of 6 MHz as we determined for PL6, the resonances at (D ± E) may not be resolved in zero-field ODMR experiments, which often have much broader linewidths (typically a few tens of MHz) compared to EPR (<1 G or ∼2.08 MHz). Moreover, the principal Dzz axis of the D-tensor is only 1° and 1.5° off the c-axis for PL5 and PL6, respectively. This very small deviation from the axial symmetry makes PL5 and PL6 appearing as axial centers. This may explain the assignment of PL5–PL6 to axial centers in previous studies.11 Based on their similar fine-structure parameters, we assign the two largest splitting pairs of lines in our EPR spectrum to the previously reported PL5 and PL6 centers.2 Thus, our data clearly indicate that both PL5 and PL6 centers have C1h symmetry but with a very small deviation from axial symmetry.

It is noticed that in zero-field ODMR measurements, the E value can be determined but not the direction of the dipole–dipole interaction between spins, i.e., the principal Dzz axis of the D-tensor. Therefore, the PL5 center with E ≠ 0 has been specified as a basal divacancy,2 although E ≠ 0 only means the principal Dzz axis of the D-tensor is not along the c-axis but can take any angles between 0° and 90°, not necessary to be along the direction of a Si–C basal bond (∼70°) as the PL3 and PL4 basal divacancies. Thus, PL5 and PL6 should be considered axial divacancies disturbed by lattice defects.

The PLa, PLc, and PLd centers have fine-structure parameters (D, E, and the angle θ = 70°) rather similar to that of PL3 and PL4. It is noticed that the PLc center has almost double intensity compared to other modified divacancies, and a hyperfine structure due to the interaction between the electron and the nuclear spins of 29Si in the next nearest neighbor shell, similar to that of the divacancies,10 could be detected. The PLa, PLc, and PLd centers may be related to the PL3 and PL4 basal divacancies being disturbed by lattice defects. Unlike other modified basal divacancies, the PLb center has a much smaller D value and a completely different direction of the Dzz axis (θ = 34°). However, its signals at B||c show a similar hyperfine structure as for the divacancies.

We notice that for n-type substrate with the concentration of the N donor of ∼1 × 1017 cm−3, electron irradiation with doses up to 4 × 1018 cm−2, the divacancy is in the negative charge state and the EPR observation of VCV0Si requires illumination. However, in the sample irradiated to a dose of 1 × 1019 cm−2, EPR of VCV0Si can be detected in darkness, indicating that the Fermi level locates below the (0|−) level of the divacancy and can be at the (0|−) level of VSi at ∼ EV + 1.25 eV. Figure 3(a) shows the EPR spectrum in an electron-irradiated and annealed n-type substrate measure at 293 K for B||c. The signals from four configurations of the neutral divacancies PL1–PL4 and the negative C vacancy (VC) and its 29Si hyperfine structure29 are detected. In addition, a pair of sharp lines from an unknown defect appears close to the PL3 lines as indicated by arrows. These lines have a specific hyperfine structure different from that of the divacancies. No signals of the modified divacancies PL5, PL6, and PLa–PLd could be detected. In the n-type 4H-SiC CVD layer irradiated with an electron dose of 4 × 1018 cm−2 and annealed at 820 °C, the Fermi level is still at the (+|0) level of CSiVC and the EPR detection of VCV0Si requires illumination. Figure 3(b) shows the EPR spectrum in an electron-irradiated and annealed n-type epitaxial 4H-SiC layer measured at 34 K under above-bandgap illumination with light from a Xenon lamp. As can be seen in the figure, the spectrum shows strong signals of PL1–PL4 divacancies, the negative Si vacancy, and the EI4 center, which is a complex between a C antisite-vacancy pair and a C vacancy in the third neighbor site in the neutral charge state,30 but no signals of the modified divacancies.

FIG. 3.

(a) EPR spectrum in electron-irradiated (1 × 1019 cm−2) and annealed (850 °C) n-type substrate measured at 293 K in dark for B||c shows strong signals of the neutral divacancies PL1–PL4 and the negative C vacancy, VC, with its 29Si hyperfine lines29 but no signals of modified divacancies. A weak pair of lines indicated by red arrows belongs to an unknown defect with an electron spin S = 1. (b) EPR spectrum in n-type pure free-standing 4H-SiC epilayer irradiated with 2 MeV-electrons to a dose of 4 × 1018 cm−2 and annealed at 820 °C, measured at 34 K for B||c under illumination with light from a Xenon lamp, showing strong signals of PL1–PL4, the EI-4 center30 and the negative Si vacancy.27 No signals of modified divacancies PL5, PL6, and PLa–PLd have been detected.

FIG. 3.

(a) EPR spectrum in electron-irradiated (1 × 1019 cm−2) and annealed (850 °C) n-type substrate measured at 293 K in dark for B||c shows strong signals of the neutral divacancies PL1–PL4 and the negative C vacancy, VC, with its 29Si hyperfine lines29 but no signals of modified divacancies. A weak pair of lines indicated by red arrows belongs to an unknown defect with an electron spin S = 1. (b) EPR spectrum in n-type pure free-standing 4H-SiC epilayer irradiated with 2 MeV-electrons to a dose of 4 × 1018 cm−2 and annealed at 820 °C, measured at 34 K for B||c under illumination with light from a Xenon lamp, showing strong signals of PL1–PL4, the EI-4 center30 and the negative Si vacancy.27 No signals of modified divacancies PL5, PL6, and PLa–PLd have been detected.

Close modal

We also check EPR of as-grown and electron-irradiated HPSI 4H-SiC grown by high-temperature CVD (HTCVD) from Norstel, and electron-irradiated commercial p-type substrates but do not observe any signals from PL5, PL6, and PLa–PLd centers (the spectra are not shown).

The above samples have also been measured by PL at low temperatures (2–4 K) and parts of the spectra containing the ZPLs of PL5, PL6, and PL3–PL4 are shown in Fig. 4 (for clarity, the region of the ZPLs PL1, PL2, and the phonon sideband is not shown). Figure 4(a) shows the PL spectrum in as-grown HPSI 4H-SiC from Cree. In addition to strong ZPLs PL3 and PL4, the ZPLs PL5 and PL6 are clearly observed. After electron irradiation (dose: 1 × 1018 cm–2) and annealing, the PL5 and PL6 ZPLs reduce in intensity, while other two ZPLs, PL5′ and PL6′, which weakly appear in the as-grown sample, increase [Fig. 4(b)]. The PL5′ and PL6′ ZPLs belong to unknown defects and have previously been reported.14 There are also several weak ZPLs of unknown origin appearing in the spectral range between PL3 and PL4. Figure 4(c) shows the PL spectrum in a p-type 4H-SiC substrate after electron irradiation (dose: 2 × 1018 cm–2) and annealing. This sample shows strong ZPLs of the neutral divacancies but no signals of PL5 and PL6. In irradiated and the annealed n-type epitaxial layer, the ZPLs PL5 and PL6 are also not detected as can be seen in Fig. 4(d). We also notice that the ZPLs of PL5 and PL6 centers are not detected in as-grown and irradiated HPSI 4H-SiC from Norstel. In irradiated n-type substrates, we also observed the ZPLs of PL5′ and PL6′ but no signals from PL5 and PL6 similar to the results reported in Ref. 14. In HPSI materials from Cree irradiated with an electron dose of 1 × 1017 cm–2 [Fig. 4(b)], the Fermi level is moved to the (+|0) level of CSiVC23 and the divacancy is in the negative charge state. The 940-nm excitation has the photon energy of ∼1.32 eV, which is just above the energy threshold that can move electrons from the (0|−) acceptor level of VCVSi at ∼EV + 2.0 eV14 to the CB to activate the neutral charge state. However, only a part of the total concentration of the divacancy can be activated to the neutral charge state by optical excitation and contribute to the PL signal. The competition in recombination between a large number of defects created by irradiation and the divacancy can have a stronger impact on the PL intensity than its concentration, leading to a drastic decrease of the PL emissions of the divacancies as observed for the PL5 and PL6 ZPLs in the irradiated HPSI material in Fig. 4(b).

FIG. 4.

Low-temperature (2–4 K) PL spectra of several 4H-SiC samples with strong divacancy lines in the region of the PL5 and PL6 lines and the PL3 and PL4 lines. (a) As-grown and (b) electron-irradiated (dose: 1 × 1018 cm–2) HPSI material from Cree; (c) p-type electron-irradiated (2 × 1018 cm–2) substrate; (d) n-type electron-irradiated (4 × 1018 cm–2) epitaxial layer. PL5–PL6 are observed only in the HPSI material from Cree. The PL5´ and PL6´ lines prominent in the irradiated and annealed HPSI sample are of unknown origin.14 The weak line labeled R in (b) denotes the Raman LO mode. The excitation wavelength is 940 nm for (a), (b), and (d) and 785 nm for (c). The irradiated samples are annealed at 800 °C for (b) and (c) and at 820 °C for (d). Although irradiation creates more divacancies, the competition in recombination from a large number of defects created by electron irradiation and the wrong charge state of the divacancies in the irradiated HPSI sample can have a stronger impact on the PL intensity, leading to a drastic decrease of the PL5 and PL6 ZPLs in (b) (see text).

FIG. 4.

Low-temperature (2–4 K) PL spectra of several 4H-SiC samples with strong divacancy lines in the region of the PL5 and PL6 lines and the PL3 and PL4 lines. (a) As-grown and (b) electron-irradiated (dose: 1 × 1018 cm–2) HPSI material from Cree; (c) p-type electron-irradiated (2 × 1018 cm–2) substrate; (d) n-type electron-irradiated (4 × 1018 cm–2) epitaxial layer. PL5–PL6 are observed only in the HPSI material from Cree. The PL5´ and PL6´ lines prominent in the irradiated and annealed HPSI sample are of unknown origin.14 The weak line labeled R in (b) denotes the Raman LO mode. The excitation wavelength is 940 nm for (a), (b), and (d) and 785 nm for (c). The irradiated samples are annealed at 800 °C for (b) and (c) and at 820 °C for (d). Although irradiation creates more divacancies, the competition in recombination from a large number of defects created by electron irradiation and the wrong charge state of the divacancies in the irradiated HPSI sample can have a stronger impact on the PL intensity, leading to a drastic decrease of the PL5 and PL6 ZPLs in (b) (see text).

Close modal

Thus, both EPR and PL measurements indicate that electron irradiation does not create the modified divacancy centers in materials other than HPSI 4H-SiC from Cree regardless of their surface treatments which are known to create stacking faults.

The model of point defects near stacking faults has previously been proposed for the Si antisite in 4H-SiC, which can be created by electron irradiation.31 It is expected that our n- and p-type substrates with as-sliced and mechanical-polish surfaces or free-standing epilayers with the substrate removed by mechanical polishing will contain higher densities of stacking faults compared to a commercial HPSI 4H-SiC wafer of production grade that has been CMP polished and is ready for CVD growth of power devices. If the PL5–PL7 centers are divacancies being created near or at stacking faults and can be induced by electron irradiation to a concentration detectable by EPR in the HPSI material from Cree, they are expected to be formed in all our studied samples. The absence of these modified divacancies in other electron-irradiated materials suggests that other lattice defects/damages apart from stacking faults may be involved in these defects.

In summary, we have observed six spin S = 1 EPR centers in electron-irradiated HPSI 4H-SiC material from Cree. These centers have C1h symmetry and isotropic g-values in the range 2.0034–2.0042 close to that of the neutral divacancies PL1–PL4 (g ∼ 2.0040). Two of these centers show very small deviations from axial symmetry with the principal Dzz axis of the D-tensor being ∼1.0°–1.5° from the c-axis. Comparing their fine-structure parameters D and E with the previously reported modified divacancies, we assign these two centers to PL5 and PL6. Among the other four centers PLa–PLd, three of which, PLa, PLc, and PLd are basal paired centers and have D and E values similar to that of PL3 and PL4 divacancies, while the PLb center has a different principal Dzz axis (aligning ∼34° off the c-axis) and a smaller D value (slightly more than a half of D values of other divacancy centers). Based on the formation and the obtained spin-Hamiltonian parameters, these new EPR centers are suggested to be divacancies at or near lattice defects.

We notice that these modified divacancies are not detected by EPR or PL in commercial n- and p-type substrates or n-type pure epitaxial layers irradiated by electrons regardless of surface treatments, which are known to create stacking faults. Our results provide information on fine-structure parameters, spin–spin interaction, and the formation of these centers, which can be useful for defect modeling and identification.

Support from the EU H2020 project QuanTELCO (Grant No. 862721) for N.T.S. and I.G.I.; the Knut and Alice Wallenberg Foundation (Grant No. KAW 2018.0071) for N.T.S., D.S, and I.G.I.; and AFM (CeNano grant 2021) for D.S. is acknowledged. T.O. thanks for the support from the Japan Society for the Promotion of Science JSPS KAKENHI (grant Nos. 20H00355 and 21H04553).

The authors have no conflicts to disclose.

N. T. Son: Conceptualization (lead); Data curation (lead); Formal analysis (lead); Funding acquisition (equal); Investigation (lead); Methodology (lead); Resources (equal); Supervision (lead); Validation (equal); Writing – original draft (lead); Writing – review and editing (lead). D. Shafizadeh: Data curation (equal); Investigation (equal); Validation (equal); Writing – original draft (equal). T. Ohshima: Investigation (supporting); Methodology (supporting); Resources (equal); Validation (equal); Writing – original draft (supporting). I. G. Ivanov: Conceptualization (supporting); Data curation (supporting); Formal analysis (supporting); Funding acquisition (equal); Investigation (equal); Methodology (supporting); Resources (equal); Validation (equal); Writing – original draft (supporting); Writing – review and editing (supporting).

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

1.
J. A.
Cooper
,
M. R.
Melloch
,
R.
Singh
,
A.
Agarwal
, and
J. W.
Palmour
, “
Status and prospects for SiC power MOSFETs
,”
IEEE Trans. Electron Devices
49
,
658
(
2002
).
2.
W. F.
Koehl
,
B. B.
Buckley
,
F. J.
Heremans
,
G.
Calusine
, and
D. D.
Awschalom
, “
Room temperature coherent control of defect spin qubits in silicon carbide
,”
Nature
479
,
84
(
2011
).
3.
D. D.
Awschalom
,
R.
Hanson
,
J.
Wrachtrup
, and
B. B.
Zhou
, “
Quantum technologies with optically interfaced solid-state spins
,”
Nat. Photonics
12
,
516
(
2018
).
4.
M.
Atatüre
,
D.
Englund
,
N.
Vamivakas
,
S.-Y.
Lee
, and
J.
Wrachtrup
, “
Material platforms for spin-based photonic quantum technologies
,”
Nat. Rev. Mater.
3
,
38
(
2018
).
5.
A.
Lohrmann
,
B. C.
Johnson
,
J. C.
McCallum
, and
S.
Castelletto
, “
A review on single photon sources in silicon carbide
,”
Rep. Prog. Phys.
80
,
034502
(
2017
).
6.
N. T.
Son
,
C. P.
Anderson
,
A.
Bourassa
,
K. C.
Miao
,
C.
Babin
,
M.
Widmann
,
M.
Niethammer
,
J.
Ul Hassan
,
N.
Morioka
,
I. G.
Ivanov
,
F.
Kaiser
,
J.
Wrachtrup
, and
D. D.
Awschalom
, “
Developing silicon carbide for quantum spintronics
,”
Appl. Phys. Lett.
116
,
190501
(
2020
).
7.
D. J.
Christle
,
A. L.
Falk
,
P.
Andrich
,
P. V.
Klimov
,
J. U.
Hassan
,
N. T.
Son
,
E.
Janzén
,
T.
Ohshima
, and
D. D.
Awschalom
, “
Isolated electron spins in silicon carbide with millisecond coherence times
,”
Nat. Mater.
14
,
160
(
2015
).
8.
M.
Widmann
,
S.-Y.
Lee
,
T.
Rendler
,
N.
Tien Son
,
H.
Fedder
,
S.
Paik
,
L.
Yang
,
N.
Zhao
,
S.
Yang
,
I.
Booker
,
A.
Denisenko
,
M.
Jamali
,
S. A.
Momenzadeh
,
I.
Gerhardt
,
T.
Ohshima
,
A.
Gali
,
E.
Janzén
, and
J.
Wrachtrup
, “
Coherent control of single spins in silicon carbide at room temperature
,”
Nat. Mater.
14
,
164
(
2015
).
9.
M.
Niethammer
,
M.
Widmann
,
T.
Rendler
,
N.
Morioka
,
Y.-C.
Chen
,
R.
Stöhr
,
J.
Ul Hassan
,
S.
Onoda
,
T.
Ohshima
,
S.-Y.
Lee
,
A.
Mukherjee
,
J.
Isoya
,
N. T.
Son
, and
J.
Wrachtrup
, “
Coherent electrical readout of defect spins in silicon carbide by photo-ionization at ambient conditions
,”
Nat. Commun.
10
,
5569
(
2019
).
10.
N. T.
Son
,
P.
Carlsson
,
J.
Ul Hassan
,
E.
Janzén
,
T.
Umeda
,
J.
Isoya
,
A.
Gali
,
M.
Bockstedte
,
N.
Morishita
,
T.
Ohshima
, and
H.
Itoh
, “
Divacancy in 4H-SiC
,”
Phys. Rev. Lett.
96
,
055501
(
2006
).
11.
A. L.
Falk
,
B. B.
Buckley
,
G.
Calusine
,
W. F.
Koehl
,
V. V.
Dobrovitski
,
A.
Politi
,
C. A.
Zorman
,
P. X.-L.
Feng
, and
D. D.
Awschalom
, “
Polytype control of spin qubits in silicon carbide
,”
Nat. Commun.
4
,
1819
(
2013
).
12.
D. A.
Golter
and
C. W.
Lai
, “
Optical switching of defect charge states in 4H-SiC
,”
Sci. Rep.
7
,
13406
(
2017
).
13.
G.
Wolfowicz
,
C. P.
Anderson
,
A. L.
Yeats
,
S. J.
Whiteley
,
J.
Niklas
,
O. G.
Poluektov
,
F. J.
Heremans
, and
D. D.
Awschalom
, “
Optical charge state control of spin defects in 4H-SiC
,”
Nat. Commun.
8
,
1876
(
2017
).
14.
B.
Magnusson
,
N. T.
Son
,
A.
Csore
,
A.
Gällström
,
T.
Ohshima
,
A.
Gali
, and
I. G.
Ivanov
, “
Excitation properties of the divacancy in 4H-SiC
,”
Phys. Rev. B
98
,
195202
(
2018
).
15.
F.-F.
Yan
,
J.-F.
Wang
,
Q.
Li
,
Z.-D.
Cheng
,
J.-M.
Cui
,
W.-Z.
Liu
,
J.-S.
Xu
,
C.-F.
Li
, and
G.-C.
Guo
, “
Coherent control of defect spins in silicon carbide above 550 K
,”
Phys. Rev. Appl.
10
,
144042
(
2018
).
16.
Q.
Li
,
J.-F.
Wang
,
F.-F.
Yan
,
J.-Y.
Zhou
,
H.-F.
Wang
,
H.
Liu
,
L.-P.
Guo
,
X.
Zhou
,
A.
Gali
,
Z.-H.
Liu
,
Z.-Q.
Wang
,
K.
Sun
,
G.-P.
Guo
,
J.-S.
Tang
,
H.
Li
,
L.-X.
You
,
J.-S.
Xu
,
C.-F.
Li
, and
G.-C.
Guo
, “
Room-temperature coherent manipulation of single-spin qubits in silicon carbide with a high readout contrast
,”
Nat. Sci. Rev.
9
,
nwab122
(
2022
).
17.
V.
Ivády
,
J.
Davidsson
,
N.
Delegan
,
A. L.
Falk
,
P. V.
Klimov
,
S. J.
Whiteley
,
S. O.
Hruszkewycz
,
M. V.
Holt
,
F. J.
Heremans
,
N. T.
Son
,
D. D.
Awschalom
,
I. A.
Abrikosov
, and
A.
Gali
, “
Stabilization of point-defect spin qubits by quantum wells
,”
Nat. Commun.
10
,
5607
(
2019
).
18.
S.
Ushio
,
T.
Fujimoto
,
H.
Tsuge
,
M.
Katsuno
,
S.
Sato
,
K.
Tani
,
H.
Hirano
, and
T.
Yano
, “
Formation of double stacking faults from polishing scratches on 4H-SiC (0001) substrate
,”
Mater. Sci. Forum
778–780
,
390
(
2014
).
19.
R. S.
Okojie
,
X.
Huang
,
M.
Dudley
,
M
,
Zhang
, and
P.
Pirouz
, “
Process-Induced Deformations and Stacking Faults in 4H-SiC
,”
MRS Online Proceedings Library
911
,
702
(
2005
).
20.
M.
Katsuno
,
M.
Nakabayashi
,
T.
Fujimoto
,
N.
Ohtani
,
H.
Yashiro
,
H.
Tsuge
,
T.
Aigo
,
T.
Hoshino
, and
K.
Tatsumi
, “
Stacking Fault Formation in Highly Nitrogen-Doped 4H-SiC Substrates with Different Surface Preparation Conditions
,”
Mater. Sci. Forum
600-603
,
341
(
2009
).
21.
S.
Tsukimoto
,
T.
Ise
,
G.
Maruyama
,
S.
Hashimoto
,
T.
Sakurada
,
J.
Senzaki
,
T.
Kato
,
K.
Kojima
, and
H.
Okumura
, “
Local Strain Distribution and Microstructure of Grinding-Induced Damage Layers in SiC Wafer
,”
J. Electronic Materials
47
,
6722
(
2018
).
22.
Y.
Ishikawa1
,
Y.-Z.
Yao
,
Y.
Sugawara
,
K.
Sato
,
Y.
Okamoto
,
N.
Hayashi
,
B.
Dierre
,
K.
Watanabe
, and
T.
Sekiguchi
, “
Comparison of slicing-induced damage in hexagonal SiC by wire sawing with loose abrasive, wire sawing with fixed abrasive, and electric discharge machining
,”
J. J. Appl. Phys.
53
,
071301
(
2014
).
23.
H.
Nakane
,
M.
Kato
,
Y.
Ohkouchi
,
X. T.
Trinh
,
I. G.
Ivanov
,
T.
Ohshima
, and
N. T.
Son
, “
Deep levels related to the carbon antisite-vacancy pair in 4H-SiC
,”
J. Appl. Phys.
130
,
065703
(
2021
).
24.
N. T.
Son
,
X. T.
Trinh
,
L. S.
Løvlie
,
B. G.
Svensson
,
K.
Kawahara
,
J.
Suda
,
T.
Kimoto
,
T.
Umeda
,
J.
Isoya
,
T.
Makino
,
T.
Ohshima
, and
E.
Janzén
, “
Negative-U System of Carbon Vacancy in 4H-SiC
,”
Phys. Rev. Lett.
109
,
187603
(
2012
).
25.
I. D.
Booker
,
E.
Janzén
,
N. T.
Son
,
J.
Hassan
,
P.
Stenberg
, and
E. Ö.
Svenbjörnsson
, “
Donor and double donor transitions of the carbon vacancy related EH6/7 deep level in 4H-SiC
,”
J. Appl. Phys.
119
,
235703
(
2016
).
26.
N. T.
Son
,
P.
Stenberg
,
V.
Jokubavicius
,
H.
Abe
,
T.
Ohshima
,
J.
Ul Hassan
, and
I. G.
Ivanov
, “
Energy levels and charge state control of the carbon antisite-vacancy defect in 4H-SiC
,”
Appl. Phys. Lett.
114
,
212105
(
2019
).
27.
N. T.
Son
,
P.
Stenberg
,
V.
Jokubavicius
,
T.
Ohshima
,
J.
Ul Hassan
, and
I. G.
Ivanov
, “
Ligand hyperfine interactions at silicon vacancies in 4H-SiC
,”
J. Phys.: Condens. Matter
31
,
195501
(
2019
).
28.
T.
Umeda
,
J.
Isoya
,
T.
Ohshima
,
N.
Morishita
,
H.
Itoh
, and
A.
Gali
, “
Identification of positively charged carbon antisite-vacancy pairs in 4H-SiC
,”
Phys. Rev. B
75
,
245202
(
2007
).
29.
X. T.
Trinh
,
K.
Szász
,
T.
Hornos
,
K.
Kawahara
,
J.
Suda
,
T.
Kimoto
,
A.
Gali
,
E.
Janzén
, and
N. T.
Son
, “
Negative-U carbon vacancy in 4H-SiC: Assessment of charge correction schemes and identification of the negative carbon vacancy at the quasicubic site
,”
Phys. Rev. B
88
,
235209
(
2013
).
30.
P.
Carlsson
,
N. T.
Son
,
A.
Gali
,
J.
Isoya
,
N.
Morishita
,
T.
Ohshima
,
B.
Magnusson
, and
E.
Janzén
, “
EPR and ab initio calculation study on the EI4 center in 4H- and 6H-SiC
,”
Phys. Rev. B
82
,
235103
(
2010
).
31.
A.
Lohrmann
,
N.
Iwamoto
,
Z.
Bodrog
,
S.
Castelletto
,
T.
Ohshima
,
T. J.
Karle
,
A.
Gali
,
S.
Prawer
,
J. C.
McCallum
, and
B. C.
Johnson
, “
Single-photon emitting diode in silicon carbide
,”
Nat. Commun.
6
,
7783
(
2015
).
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