EPR spectrum of donors in 6H SiC in a broad temperature range

An EPR study of donors in 6H SiC crystals with an uncompensated donor concentration (N _D−N _A) of 2×10¹⁸ to 1×10¹⁶ cm⁻³ performed in the temperature range 4.2 to 160 K at frequencies of 9 and 140 GHz showed that 6H n-SiC samples have two donor states in the gap. One of them originates from nitrogen occupying three inequivalent lattice sites with ionization energies of 150 and 80 meV, and the second is connected with a structural defect lying deeper in the gap than nitrogen. The temperature dependences of donor EPR line intensities have been found to deviate from the Curie law. The observed EPR line-intensity peaks of donors are produced in a temperature-driven successive redistribution of donor electrons between the donor levels. The temperature dependences of EPR line intensities obtained from samples with low donor concentrations were used to determine the valley-orbit splitting of nitrogen in cubic sites. Fiz. Tverd. Tela (St. Petersburg) 40, 1824–1828 (October 1998)     

EPR spectra of the excited nitrogen state in 6H-SiC

A study is reported of donor EPR spectra in compensated 6H-SiC crystals with donor concentrations (N _D -N _A ) varied from 8×10¹⁷ to 5×10¹⁶ cm^?3, performed within a temperature interval from 77 to 170 K at a frequency of 37 GHz. A second paramagnetic state of nitrogen in silicon carbide has been found to exist, and it is associated with its excited 1S(E) state becoming paramagnetic after thermal ionization of the donor electrons from the 1S(A ₁) to 1S(E) level. The EPR spectrum of nitrogen in the 1S(E) state is a single line with an anisotropic width because of the unresolved hyperfine structure. A light-induced charge transfer between the ground, 1S(A ₁), and excited, 1S(E), nitrogen states has been observed. The valley-orbit splitting and the energy required to ionize donor electrons from the 1S(E) to higher lying excited states have been determined for the cubic nitrogen sites. The parameters of a structural defect, characteristic of n-type 6H-SiC compensated crystals, have been established.     

A Homebuilt ESE Spectrometer on the Basis of a High-Power Q-Band Microwave Bridge

We present a Q-band spectrometer which was built recently at the Institute of Physical Chemistry of the University of Stuttgart. It allows us to perform the field-sweep electron spin echo (ESE), pulsed electron–nuclear double resonance (ENDOR), relaxation and electron spin echo envelope modulation experiments both at room and low (down to 1.5 K) temperatures. The spectrometer consists of an electromagnet, digital field controller, pulsed microwave bridge, probehead, cryostat, radio frequency unit, pulse programmer and data acquisition electronics. The Q-band microwave bridge with 10.8 W output power is based on a two-stage IMPATT-diode pulse amplifier. The commercial Varian electromagnet system is controlled by a 24-bit home-built digital controller. The external devices are interfaced to the two PCs via GPIB and LAN. The spectrometer control software was developed in Visual C++. It consists of two programs running synchronously on the control PCs. The spectrometer is equipped with a cylindrical TE₀₁₁ cavity constructed both for ESE and for pulsed ENDOR. The cavity fits into a liquid He cryostat thus allowing low-temperature experiments. An 8-bit data acquisition digitizer is used to collect the echo signals, and the PBESR-PRO-400 digital word generator orchestrates the pulse experiments and sets pulse sequences of the microwave bridge. The spectrometer performance is demonstrated on nitrogen impurities in a polycrystalline synthetic diamond, on silver clusters supported on NaA zeolite and electron-irradiated tooth enamel. Authors' address: Igor Tkach, Institute of Physical Chemistry, University of Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany     

Influence of Dye Molecules on the Polarization of Ferroelectric Vinylidene Fluoride Copolymer

Crystallography Reports - Electrical, electromechanical, and structural–optical properties of films of vinylidene fluoride copolymer with tetrafluoroethylene TFE, doped with Rhodamine 6G dye,...     

Kinetics of the behavior of photosensitive impurities and defects in high-purity semi-insulating silicon carbide

The kinetics of the behavior of photosensitive impurities and defects in the high-purity semi-insulating material 4H-SiC has been studied both theoretically and experimentally using electron paramagnetic resonance (EPR) under photoexcitation and optical admittance spectroscopy. The rate equations describing the processes of recombination, trapping, and ionization of nonequilibrium charge carriers bound dynamically to shallow donors and acceptors (nitrogen and boron), as well as of charge carrier transfer from the shallow nitrogen donor to deep levels of intrinsic defects, have been solved. A comparison of the calculations with the experimental curves plotting the decay of admittance conductance and EPR signal intensities due to nitrogen and boron after termination of photoexcitation has revealed that the probabilities of hole trapping by an ionized acceptor and the rate of ionization of a neutral boron acceptor are two orders of magnitude higher than those of similar processes in a system of donor levels. The latter is dominated by cascade electron transitions between levels in the band gap, as well as by electron-hole recombination.