Creation and identification of the two spin states of dicarbon antisite defects in $4H\text{-SiC}$

This paper deals with the positive identification by low-temperature photoluminescence microspectroscopy of the two spin states of the dicarbon antisites in $4H\text{-SiC}$. The defects are created by high-dose electron irradiation at room temperature or by subsequent exposure to intense 325 nm radiation at temperatures up to $1300°\text{C}$. Identification was achieved by their formation and annealing characteristics, by the energies of their local vibrational modes, by the nature of their splitting in ${}^{13}\text{C}$ isotope enriched samples, and by comparison with published results of ab initio local density approximation calculations. Four related but different forms of this defect have been predicted, two with $S=0$ and two with $S=1$, and their calculated properties are consistent with the experimental results presented here. The excitation processes for the optical centers within the irradiated region are quite unusual. For a 488 nm laser excitation, both spin states of the defect are observed by up-conversion. For a 325 nm excitation, the optical centers are only observed at the periphery of the high-dose irradiated regions after the sample has been exposed to an intense 325 nm beam. In this case, the optical centers are mainly in the $S=0$ state. The centers are eliminated by annealing in the range of $800–950°\text{C}$.

Figure 1

Comparison of PL spectra of the 463 nm triplet obtained at different excitation wavelengths as follows: (a) 488 nm, (b) 457.9 nm, and (c) 325 nm. The dip in intensity near 488 nm in (a) is caused by the holographic notch filter used to attenuate the laser signal. Peaks $R$ are caused by Raman scattering. The spectra were acquired at $\sim 7\text{K}$. The inset shows splitting of the M2 LVM that is occasionally observed.

Figure 2

Full spectrum of the 463 nm triplet using 325 nm excitation with the specimen at $\sim 7\text{K}$. M1 and M2 are the lower- and higher-energy LVMs. $n\text{M}1$ and $n\text{M}2$ refer to $n\text{th}$ order harmonics.

Figure 3

Polarized 463 nm ZPL spectra from a chosen area of a sample mounted edge on to the incident 488 nm laser beam (on the {0110} face). Sample at $\sim 7\text{K}$.

Figure 4

(Color online) 457.9–nm-excited spectrum of the 463 nm triplet with the laser incident along [0001] on the Si surface of the ${}^{13}\text{C}$-enriched sample at $\sim 7\text{K}$. The insets show deconvolutions of the ZPLs (left), the lower-energy (M1) LVMs (mid), and the higher-energy (M2) LVMs (right).

Figure 5

457.9-nm-excited polarized spectra of the 463 nm triplet with the laser incident on the ${}^{13}\text{C}$-enriched sample edge on to the beam. The sample was at $\sim 7\text{K}$.

Figure 6

(Color online) Results of an attempt to simulate the complex form of the M2 spectrum in Fig. 8. In (a), the envelope function is shown together with the three individual triplets (each with 1:4:4 intensity ratios) originating from each of the three ZPLs [indicated by (1), (2), and (3)]. (b) shows the result of simulation in comparison with the experimental result.

Figure 7

(a) Increase in intensity of 463.2 nm emission with time of exposure to 325 nm beam at 10% power level; sample temperature $\sim 7\text{K}$. (b) Comparison of line scans with $15\mu \text{m}$ spacing across two electron-irradiated areas of a specimen (extending over the regions indicated by the double-headed arrows), one irradiated at 300 kV (lower) and the other at 250 kV (upper). The lower set of line scans was obtained first, using 1% of the power of the 325 nm laser ($\sim 0.03\text{mW}$ into a $5\mu \text{m}$ spot); the upper set was obtained later from the same region at 25% power. Note the appearance of the 463 nm triplet in the upper set of line scans and its absence from the lower set. Other more minor changes may also be observed and they are discussed in the associated paper (Ref. 1). The ZPLs in the wavelength range of 429–435 nm are the so-called alphabet lines. The spectra were acquired at $\sim 7\text{K}$.

Figure 8

(Color online) Integrated intensity maps of various optical centers obtained at 1% 325 nm laser power after exposure to an intense (%) 325 nm beam (5 min per point) stepped along a radial line (indicated by ${}^{\ast }$) through the circular electron-irradiated area of $300\mu \text{m}$ diameter and extending outside it. Maps labeled (a)–(d) are for the alphabet lines with this designation. The irradiated area (partially shown) is at the left side of each map (see diagram). The pixel size is $20\times 20\mu {\text{m}}^2$. The enhancements and diminutions survived annealing to $800°\text{C}$. The spectra were recorded at $\sim 7\text{K}$.

Figure 9

(Color online) Temperature dependence of the ZPL integrated intensities as a function of reciprocal temperature.

Figure 10

Line scan across a 190 kV electron-irradiated region of a $4H\text{-SiC}$ sample annealed at $1300°\text{C}$, with the spectra being excited with 10% 325 nm laser power and the sample at $\sim 7\text{K}$. This line scan shows the coexistence of the 463 nm triplet with other optical centers having LVMs [the ZPL at 414 nm is mentioned in the associated paper (Ref. 1)] and also with the well-known $D_1$ center. Note that in this example, the 463 nm PL extends across the center of the irradiated region with 325 nm excitation. The spacing of the points along the line was $15\mu \text{m}$.