Siegmund Greulich-Weber

EPR and ENDOR Investigations of Shallow Impurities in SiC Polytypes

Investigations of nitrogen donors in 6H-, 4H- and 3C-SiC using conventional electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and optical detection of EPR and ENDOR as well as optical absorption and emission spectroscopy are reviewed and critically discussed. An attempt is presented to interpret the experimentally found large differences in hyperfine interactions of the 14N nuclei on the various inequivalent sites in the different polytypes of SiC in terms of valley–orbit splittings and “central-cell corrections” in the framework of the effective mass theory (EMT). P-doping by neutron transmutation in 6H-SiC resulted in various P-related EPR spectra previously associated with shallow P donors and P–vacancy complexes. In analogy to the new interpretation of the N donor spectra in various polytypes, it is proposed that all P-related spectra found hitherto in 6H-SiC are due to isolated P donors in ground and excited EMT states. A detailed discussion is presented of the electronic structure of B acceptors, as determined by EPR and in particular by ENDOR investigations: The B atom itself has only very little unpaired hole density, while the hole resides mainly on a neighbouring relaxed C atom B acceptors have a rather “deep” character and pronounced dynamical properties. A discussion of the present understanding of the so-called deep B centre (D centre) is also given. In contrast to B, the Al acceptor behaves as expected from the effective mass theory. It shows, however, two optical absorption bands identified by optical detection of EPR which are related to an ionization transition to the valence band and another transition, probably to a V impurity.

On the microscopic structures of shallow donors in 6H SiC: studies with EPR and ENDOR

Abstract Nitrogen and phosphorus in 6H-SiC were investigated with electron paramagnetic resonance (EPR) at 10 GHz and at 140 GHz and with electron nuclear double resonance (ENDOR). The phosphorus defects were prepared by neutron transmutation of 30 Si in 6H-SiC. We observed two sets of EPR spectra due to two different phosphorus-related defects. Both EPR spectra exhibit a strong temperature dependence. It is proposed, that one of these EPR spectra is due to the isolated shallow phosphorus donor on Si sites and the other due to a P-vacancy-pair defect. The electronic structures of the shallow donors P and N are discussed using effective mass theory. It is shown that the shallow P donor on the hexagonal site has not yet been observed.

Electrical detection of electron paramagnetic resonance: New possibilities for the study of point defects

An investigation of the possibilities to measure electron paramagnetic resonance (EPR) with electrical detection (EDEPR) by measuring the microwave or radio frequency‐induced change of the photoconductivity of various bulk Si samples containing shallow and deep level defects is presented. It was found that an electron‐hole recombination mechanism via a donor (D0)‐acceptor (A0) pair explains the observations and may be necessary if EDEPR is to be detected. A qualitative expression for the EDEPR signal intensity is presented. EDEPR offers sensitivity several orders of magnitude better than normal EPR. As few as 107 shallow P donors could be observed. In addition, EDEPR can be measured with spatial resolution allowing defect mapping.

ENDOR investigation of the microscopic structure of the boron acceptor in 6H-SiC

The boron acceptor in 6H-SiC was investigated using electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR). The hyperfine interactions with 11B could be determined precisely for the two quasi-cubic and the hexagonal sites. The microscopic model suggested from the EPR and ENDOR results is as follows: boron occupies a silicon site and is a negatively charged ligand to an adjacent carbon atom on which most of the unpaired spin density is located. At low temperatures the symmetry of the two quasi-cubic site defects is monoclinic, while the hexagonal site defect has C3V symmetry about the hexagonal axis. At about 50 K the two quasi-cubic site defects experience a thermally activated motion of the hole at the adjacent carbon about the hexagonal crystal axis and the apparent defect symmetry also becomes C3V. The boron acceptor should be regarded as a boron-induced carbon acceptor.

Electrical detection of electron nuclear double resonance in silicon

Electrical detection of electron nuclear double resonance (EDENDOR) is demonstrated using shallow P donors in silicon. The EDENDOR spectra are compared with conventional ENDOR spectra. With EDENDOR, both the 31P hyperfine as well as 29Si superhyperfine interactions could be resolved. The sensitivity of EDENDOR seems higher compared to conventional ENDOR, especially for the 29Si ligands. EDENDOR is particularly suitable for the study of the microscopic structure of point defects in semiconductors, especially for detection of paramagnetic centers that are present in low concentrations and in small volumes.

EPR and ENDOR investigations of B acceptors in 3C-, 4H- and 6H-silicon carbide

The shallow boron acceptors in 3C-, 4H- and 6H-SiC were investigated with electron paramagnetic resonance (EPR) at high frequency (142 GHz) providing a precise knowledge of the electronic g tensors. The hyperfine (hf) interactions of the boron acceptor on the hexagonal site and the quasi-cubic site in 4H-SiC were determined precisely with electron nuclear double resonance (ENDOR). The hf interactions and superhyperfine (shf) interactions with surrounding and neighbours were interpreted within a semiempirical analysis. The microscopic model suggested from the EPR and ENDOR results and from the semiempirical analysis is as follows: the shallow boron acceptors have the same electronic structure in 6H-, 4H- and 3C-SiC and have to be viewed as B-induced C acceptors. The hole is located in the connection line between and the adjacent C. Depending on the microwave frequency used in the measurements the hole at the quasi-cubic site defects experiences a thermally activated motion about the hexagonal crystal axis at high temperatures.

Magnetic circular dichroism of a vanadium impurity in 6H-silicon carbide

The magnetic circular dichroism of the absorption (MCDA) of VSi4+ in 6H-silicon carbide shows a strong temperature dependence. Within a crystal field approach the MCDA and g-factors of the hexagonal site defect could be explained, whereas the situation is more complicated for both quasi-cubic site defects owing to a dynamical Jahn-Teller effect.

Nitrogen Donor Aggregation in 4H-SiC: g-Tensor Calculations

The microscopic origin of the Nx EPR-lines observed in heavily nitrogen doped 4H-SiC and 6H-SiC is discussed with the help of EPR parameters calculated from first principles. Based on the symmetry of the g-tensors we propose a model with distant NC donor pairs on inequivalent lattice sites which are coupled to S=1 centers but with nearly vanishing zero-field splittings, giving rise to an essentially S=1/2 like spectrum. The proposed aggregation in neutral donor pairs can contribute to the saturation of the free concentration observed in heavily nitrogen doped SiC.

Experimental verification of apparent negative refraction in low-epsilon material in the microwave regime

We analyze the effect of negative refraction in dielectric photonic crystals with almost circular equifrequency surfaces experimentally and theoretically. We found experimental evidence for negative refraction in the microwave regime in low-epsilon material and suggest an improved interpretation. The apparent negative refraction originates from Bragg refraction and its combination with noncoupling bands.

Further evidence for the B−SI-C+ model of the boron acceptor in 6H silicon carbide from a theoretical analysis of the hyperfine interactions

A semiempirical analysis of the sizes and signs of the small boron hyperfine interaction constants as determined previously by electron nuclear double resonance in boron-doped 6H silicon carbide shows that the B − SI -C + model of this centre can account for the highly unusual feature of small isotropic and anisotropic boron hyperfine constants that are positive and negative, respectively.