Band-gap states of tungsten in silicon carbide ͑polytypes 4H, 6H, and 15R͒ are investigated by deep-level transient spectroscopy ͑DLTS͒ and admittance spectroscopy on n-type SiC. Doping with W is done by ion implantation and annealing. To establish a definite chemical identification of band-gap states, the radioactive isotope 178 W is used as a tracer: band-gap states involving a 178 W isotope are uniquely identified by their decreasing concentration during the nuclear transmutation of 178 W to Hf. In addition, conventional doping studies with stable W isotopes are performed. Within the part of the band gap accessible by DLTS on n-type SiC, there is one tungsten-related deep level with a large capture cross section (10 Ϫ12 cm 2 ) for electrons. In the polytypes 4H, 6H, and 15R, its energy is 1.43, 1.16, and 1.14 eV below the conduction-band edge (E C ), respectively. The polytype dependence of this level position directly reflects the conduction-band offset. In the 4H polytype, an additional level close to the conduction band (E C Ϫ0.17 eV) exists that is absent in the other polytypes because of their smaller band gap. Due to the acceptorlike deep band-gap state, tungsten is a good candidate for a compensating center to produce semi-insulating SiC.
One Be-related deep level in the band gap of 4H-SiC was identified by radiotracer deep level transient spectroscopy (DLTS). The radioactive isotope Be7 was recoil implanted into p-type as well as n-type 4H-SiC for these radiotracer experiments. DLTS spectra were taken repeatedly during the elemental transmutation of Be7 to Li7. In the case of p-type 4H-SiC, they exhibit one peak of time-dependent height. Its concentration decreases with the halflife of the nuclear decay of Be7 (T1/2=53.3 d). Thus, this level at 1.06 eV above the valence band edge is identified as Be-related. In n-type 4H-SiC, neither Be- nor Li-correlated deep levels have been found in the investigated part of the band gap within the measurement accuracy.
Impurity atom cluster and nanocrystal formation in Er-implanted hexagonal SiC were studied using TEM and HAADF-STEM. Short interstitial loops were initially observed to form in the as-implanted layers. After annealing at 1600 degrees C extended matrix defects (wide interstitial loops and voids), Er atom clusters and nanocrystals grew. The wide interstitial loops act as strong sinks capturing diffusing dopants that gather first in lines, then planes, and finally in three-dimensional ErSi2 nanocrystals. The unstrained nanocrystals have a hill-like shape and only two polarity-dependent orientations with respect to the matrix. One-, two-, and three-dimensional Er atom clusters were also identified. For the case of Ge implantation, again the wide interstitial loops act as sinks for the implanted Ge, representing the seeds of the nanocrystal.
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