No abstract

The temperature dependence of the Hall hole density and the Hall mobility data of heavy doped p-type 4H-SiC(Al) materials obtained by Al+ ion implantation have been analysed in the frame of the charge neutrality condition and the relaxation time approximation. Samples with implanted Al concentrations in the range 1019–1020 cm−3 and 1950 °C/5 min conventional annealing have been taken into account. The reliability of the calculation has been critically discussed by focusing the attention on both the validity limits of the models for the impurity scattering mechanisms and the adopted Hall factor. By introducing empirical mass anisotropy factors, reasons were given in favour of a generalized use of the unique experimental evaluation of the Hall factor reported by the literature for p-type 4H-SiC, assessed for an Al acceptor density in the range of 1.8 × 1015 cm−3–2 × 1018 cm−3. The simultaneous fits of the Hall hole density and mobility data indicate an electrical activation of the Al impurities of the order or higher than 70% and a compensation of about 10% of the Al acceptors.

The carbon vacancy (VC) is a major point defect in high-purity 4H-SiC epitaxial layers limiting the minority charge carrier lifetime. In layers grown by chemical vapor deposition techniques, the VC concentration is typically in the range of 1012 cm−3, and after device processing at temperatures approaching 2000 °C, it can be enhanced by several orders of magnitude. In the present study, both as-grown layers and a high-temperature processed one have been annealed at 1500 °C and the VC concentration is demonstrated to be strongly reduced, exhibiting a value of only a few times 1011 cm−3 as determined by deep-level transient spectroscopy measurements. The value is reached already after annealing times on the order of 1 h and is evidenced to reflect thermodynamic equilibrium under C-rich ambient conditions. The physical processes controlling the kinetics for establishment of the VC equilibrium are estimated to have an activation energy below ∼3 eV and both in-diffusion of carbon interstitials and out-diffusion of VC's are discussed as candidates. This concept of VC elimination is flexible and readily integrated in a materials and device processing sequence.

The CUPID Collaboration is designing a tonne-scale, background-free detector to search for double beta decay with sufficient sensitivity to fully explore the parameter space corresponding to the inverted neutrino mass hierarchy scenario. One of the CUPID demonstrators, CUPID-Mo, has proved the potential of enriched Li$$_{2}$$ 2 $$^{100}$$ 100 MoO$$_4$$ 4 crystals as suitable detectors for neutrinoless double beta decay search. In this work, we characterised cubic crystals that, compared to the cylindrical crystals used by CUPID-Mo, are more appealing for the construction of tightly packed arrays. We measured an average energy resolution of ($$6.7\pm 0.6$$ 6.7 ± 0.6 ) keV FWHM in the region of interest, approaching the CUPID target of 5 keV FWHM. We assessed the identification of $$\alpha $$ α particles with and without a reflecting foil that enhances the scintillation light collection efficiency, proving that the baseline design of CUPID already ensures a complete suppression of this $$\alpha $$ α -induced background contribution. We also used the collected data to validate a Monte Carlo simulation modelling the light collection efficiency, which will enable further optimisations of the detector.

As-grown and pre-oxidized silicon carbide (SiC) samples of polytype 4H have been annealed at temperatures up to 1950 C for 10 min duration using inductive heating, or at 2000 C for 30 s using microwave heating. The samples consisted of a n-type high-purity epitaxial layer grown on 4 off-axis h0001i n þ-substrate and the evolution of the carbon vacancy (V C) concentration in the epitaxial layer was monitored by deep level transient spectroscopy via the characteristic Z 1/2 peak. Z 1/2 appears at $0.7 eV below the conduction band edge and arises from the doubly negative charge state of V C. The concentration of V C increases strongly after treatment at temperatures ! 1600 C and it reaches almost 10 15 cm À3 after the inductive heating at 1950 C. A formation enthalpy of $5.0 eV is deduced for V C , in close agreement with recent theoretical predictions in the literature, and the entropy factor is found to be $5 k (k denotes Boltzmann's constant). The latter value indicates substantial lattice relaxation around V C , consistent with V C being a negative-U system exhibiting considerable Jahn-Teller distortion. The microwave heated samples show evidence of non-equilibrium conditions due to the short duration used and display a lower content of V C than the inductively heated ones. Finally, concentration-versus-depth profiles of V C favour formation in the "bulk" of the epitaxial layer as the prevailing process and not a Schottky type process at the surface. V