Materials Science Forum Vols. 717-720

Abstract: In unintentionally Nb-doped 4H-SiC grown by high-temperature chemical vapor deposition (HTCVD), an electron paramagnetic resonance (EPR) center with C_1h symmetry and an electron spin S=1/2 was observed. The spectrum shows a hyperfine structure consisting of ten equal-intensity hyperfine (hf) lines which is identified as due to the hf interaction between the electron spin and the nuclear spin of ⁹³Nb. An additional hf structure due to the interaction with two equivalent Si neighbors was also observed. Ab initio supercell calculations of Nb in 4H-SiC suggest that Nb may form complex with a C-vacancy (V_C) resulting in an asymmetric split-vacancy (ASV) defect, Nb_Si-V_C. Combining results from EPR and supercell calculations, we assign the observed Nb-related EPR center to the hexagonal-hexagonal configuration of the AVS defect in the neutral charge state, (Nb_Si-V_C)⁰.

Abstract: Structural disorder and lattice recovery of high dose, manganese implanted, semi-insulating, 4H-SiC have been studied by secondary ion mass spectrometry, Rutherford backscattering in channeling directions, visible-to-near infrared optical spectroscopy as well as with transmission electron microscopy. After heat treatment at 1400 and 1600 °C, a substantial rearrangement of manganese is observed in the implanted region. However, the crystal has not been fully recovered. More disorder remains in the [11 3] compared to the [0001] channel direction. Stacking faults, voids and 3C inclusions are observed in the implanted region. A Mn containing phase has most likely formed.

Abstract: Diffusion of transition metals in 4H-SiC has been investigated by secondary ion mass spectroscopy using epilayers and substrates implanted with titanium (Ti), chromium (Cr), iron (Fe), or nickel (Ni). In the epilayers, Cr, Fe, and Ni atoms have diffused by argon (Ar) annealing at 1780°C for 30 min. In n⁺ substrates, the diffusivity of the metals is smaller than that in the epilayers, and only Ni has diffused by the annealing. By the Ar or helium implantation following the implantation of transition metals, diffusion of transition metals can be successfully suppressed.

Abstract: An annealing study, in the 100-1400 C temperature range ,was carried out on Cl-implanted n- or p-type 4H-SiC epilayers. The electrical characterization of the epilayers shows the rise of several deep levels and the role of Cl, on both carrier concentration and defects' microscopic structure, is discussed in the light of theoretical results obtained by density functional calculations performed on a 64-atom cubic SiC supercell.

Abstract: Nitrogen-boron doped 6H-SiC epilayers grown on low off-axis 6H-SiC substrates have been characterized by photoluminescence and Raman spectroscopy. The photoluminescence results show that a doping larger than 10¹⁸ cm^-3 is favorable to observe the luminescence and addition of nitrogen is resulting in an increased luminescence. A dopant concentration difference larger than 4x10¹⁸ cm^-3 is proposed to achieve intense photoluminescence. Raman spectroscopy further confirmed the doping type and concentrations for the samples. The results indicate that N-B doped SiC is being a good wavelength converter in white LEDs applications.

Abstract: A global minima search weighted and non-weighted least squares algorithm has been employed for a comparative study of various possible models to describe electrical properties of high dose Al implanted 4H-SiC. A wide range of experimental data has been taken from literature to demonstrate validation of the model. It was found that a single activation energy level, a temperature dependent Hall scattering factor and a degeneracy factor equal to 4 lead to a satisfactory fitting of experimental Hall data up to an Al substitutional density of 1020cm 3 in 4H SiC.

Abstract: It has been clarified that Z_1/2 center, a well known deep level as a lifetime killer, can be reduced to the concentration below 10¹¹ cm^-3 by thermal oxidation or C⁺ implantation plus Ar annealing. In this study, the authors investigate the trap-reduction phenomena systematically (experimentally), and propose a model to analyze the phenomena. Furthermore, prediction of the defect distributions is realized by solving a diffusion equation in accordance with the trap reduction model. This analytical model can explain almost all experimental data: oxidation-temperature dependence, oxidation-time dependence, and initial-Z_1/2-concentration dependence of the defect reduction. Based on these results, the authors accomplish to eliminate the Z_1/2 center to a depth of 100 μm in the sample with a relatively high initial-Z_1/2-concentration of 10¹³ cm^-3 by thermal oxidation at 1400°C for 16.5 h.

Abstract: In this paper the impact of high temperature annealing on the formation of intrinsic defects in 4H-SiC such as Z_1/2 and EH_6/7 was examined. Therefore, three epitaxial layers with various initial concentrations of the Z_1/2- and EH_6/7-centers (10¹¹ – 10¹³ cm^-3) were investigated. It turns out that depending on the initial defect concentration the high temperature annealing leads to a monotone increase of the Z_1/2- and EH_6/7-concentration in a temperature range from 1600 to 1750°C. For a defined temperature above these values, the resulting defect concentration is independent of the sample’s initial values. Beside the growth conditions themselves such as C/Si ratio the thermal post-growth processing has a severe impact on the carrier lifetime which must be taken into account during device fabrication.

Abstract: Deep Level Transient Spectroscopy (DLTS) and Double-correlated DLTS (DDLTS) measurements have been conducted on Schottky contacts fabricated on n-type 4H-SiC epilayers using different contact metals in order to separate the EH₆- and EH₇-centers, which usually appear as a broad double peak in DLTS spectra. The activation energy of EH₆ (E_C - E_T(EH₆) = 1.203 eV) turns out to be independent of the electric field. As a consequence, EH₆ is acceptor-like according to the missing Poole-Frenkel effect. Therefore, it can be excluded that the EH₆-center and the prominent acceptor-like Z_1/2-center belong to different charge states of the same microscopic defect as theoretically suggested. It is proposed that EH₆ is a complex containing a carbon vacancy and another component available at high concentrations. The activation energy of EH₇ (E_C - E_T(EH₇) = 1.58 eV) has been evaluated indirectly by fitting the DLTS spectra of the EH_6/7 double peak taking the previously determined parameters of EH₆ into account.

Abstract: Isolated point defects possessing high spin ground state and below-band-gap excitation may play a key role in realizing solid state quantum bits in semiconductors which are the basic building blocks of quantum computers. Silicon vacancy in silicon carbide provides these features making it a feasible candidate in this special and emerging field of science. However, it has been not clarified what is the exact nature of the luminescence of silicon vacancy detected in hexagonal polytypes. This is the first crucial step needed to understand this basic defect in silicon carbide. We report density functional theory based calculations on silicon vacancy defect. Based on the obtained results we identify the silicon vacancy related photoluminescence signals with the negatively charged defect.