Optical properties and Zeeman spectroscopy of niobium in silicon carbide

The optical signature of niobium in the low-temperature photoluminescence spectra of three common polytypes of SiC (4H, 6H, and 15R) is observed and confirms the previously suggested concept that Nb occupies preferably the Si-C divacancy with both Si and C at hexagonal sites. Using this concept we propose a model considering a Nb-bound exciton, the recombination of which is responsible for the observed luminescence. The exciton energy is estimated using first-principles calculation and the result is in very good agreement with the experimentally observed photon energy in 4H SiC at low temperature. The appearance of six Nb-related lines in the spectra of the hexagonal 4H and 6H polytypes at higher temperatures is tentatively explained on the grounds of the proposed model and the concept that the Nb center can exist in both ${\mathrm{C}}_{1h}$ and ${\mathrm{C}}_{3v}$ symmetries. The Zeeman splitting of the photoluminescence lines is also recorded in two different experimental geometries and the results are compared with theory based on phenomenological Hamiltonians. Our results show that Nb occupying the divacancy at the hexagonal site in the studied SiC polytypes behaves like a deep acceptor.

Figure 1

PL spectra of 4H-, 6H-, and 15R-SiC showing the Nb-associated lines at $T=5\mathrm{K}$. The inset summarizes the results of the doping experiment discussed in text. The weak contribution of the 15R and 4H Nb-related lines in the spectrum of the 6H sample, as well as of the Nb line of 6H in the 4H spectrum are due to the presence of microscopic polytype inclusions in these samples. In particular, the 15R sample was obtained from a macroscopic inclusion in the 6H polytype. (Some data is adapted with permission from Ref. [18].)

Figure 2

(a) Temperature dependence of the polarized PL spectra associated with Nb in 4H-SiC. (b) Polarized spectra of the Nb-related spectrum in 6H-SiC at 27 K. The lowermost curve in both panels is the corresponding PLE spectrum. One unpolarized spectrum per polytype is also shown, chosen so as to illustrate the appearance as a shoulder of the very weak $\mathrm{N}{\mathrm{b}}_5$ line. (Some data is adapted with permission from Ref. [18].)

Figure 3

Schematic presentation of the energy levels discussed in text. (a) Energy levels of Nb in different charge states. The splitting of the neutral charge state is due to Jahn-Teller distortion of the defect. The down- (up-) arrows represent the processes discussed in text which contribute to (decrease) the recombination energy emitted as a photon. (b) Suggested structure of the ground and excited states of Nb in the ASV configuration. The selection rules for optical transitions are also depicted; the overstricken transitions are allowed by group theory but require spin flip, making them much less probable. The inset illustrates schematically the bonding in the neutral charge state of Nb in the ASV configuration.

Figure 4

Zeeman splitting of the Nb-related lines recorded with two different polarizations in magnetic field $\mathbf{B}\parallel \mathbf{c}$ up to 5 T. Parts (a) and (b) of the figure refer to right- and left-hand circular polarizations, respectively. Note the scale changes in the high-energy region of the spectra. (Data is adapted with permission from Ref. [18].)

Figure 5

Illustration of the Zeeman splitting of the Nb-related lines in magnetic field $\mathbf{B}\perp \mathbf{c}$ up to 10 T at three different temperatures as denoted in each panel. Different temperatures are used in order to distinguish high- from low-temperature lines in the spectra. The zero-field positions of the lines are denoted in each panel. (Some data is adapted with permission from Ref. [18].)

Figure 6

Splitting of the energy levels in magnetic field $\mathbf{B}\parallel \mathbf{c}$ [panels (a) and (b)] and $\mathbf{B}\perp \mathbf{c}$ [panels (c) and (d)] calculated using the $j$ model [panels (a) and (c)] and the $ls$ model [panels (b) and (d)] for the excited states, and the $j$ model for the ground state with small arbitrary splitting of 0.1 meV. Note that all transitions denoted with arrows in part (a) occur at energy shifts very close to $\pm \mu B_{\parallel }$ from the zero-field line position (exactly $\pm \mu B_{\parallel }$ if the ground-state zero-field splitting is zero). For the case $\mathbf{B}\perp \mathbf{c}$, only transitions used in Fig. 7 are denoted and enumerated to comply with the discussion in text.

Figure 7

Comparison of all experimental data with the phenomenological models for the cases (a) $\mathbf{B}\parallel \mathbf{c}$ and (b) $\mathbf{B}\parallel \mathbf{c}$. (Most of the spectra are shown in Figs. 4 and 5.) The symbols in the panels represent the experimental line positions. The full lines are the theoretically calculated line positions using the $j$ model for the ground and excited states. The dashed lines in part (a) are obtained by shifting the full lines from $\mathrm{N}{\mathrm{b}}_0$ to $\mathrm{N}{\mathrm{b}}_3$, demonstrating the same splitting of these lines. The dotted lines in part (b) represent transitions denoted $5^{\prime }$ and $6^{\prime }$ in Fig. 6, which are calculated using the $ls$ model for the excited state and the $j$ model for the ground state, and are given for comparison. Note that three different symbols are used to denote the data from the spectra measured at the three different temperatures in the case $\mathbf{B}\perp \mathbf{c}$, and the enumeration of the theoretical lines follows the enumeration of the transitions in Fig. 6.