One of the main challenges in realizing 4H–SiC (silicon carbide)-based bipolar devices is the improvement of minority carrier lifetime in as-grown epitaxial layers. Although Z1/2 has been identified as the dominant carrier lifetime limiting defect, we report on B-related centers being another dominant source of recombination and acting as lifetime limiting defects in 4H–SiC epitaxial layers. Combining time-resolved photoluminescence (TRPL) measurement in near band edge emission and 530 nm, deep level transient spectroscopy, and minority carrier transient spectroscopy (MCTS), it was found that B related deep levels in the lower half of the bandgap are responsible for killing the minority carriers in n-type, 4H–SiC epitaxial layers when the concentration of Z1/2 is already low. The impact of these centers on the charge carrier dynamics is investigated by correlating the MCTS results with temperature-dependent TRPL decay measurements. It is shown that the influence of shallow B acceptors on the minority carrier lifetime becomes neutralized at temperatures above ∼422 K. Instead, the deep B related acceptor level, known as the D-center, remains active until temperatures above ∼570 K. Moreover, a correlation between the deep level concentrations, minority carrier lifetimes, and growth parameters indicates that intentional nitrogen doping hinders the formation of deep B acceptor levels. Furthermore, tuning growth parameters, including growth temperature and C/Si ratio, is shown to be crucial for improving the minority carrier lifetime in as-grown 4H–SiC epitaxial layers.

The carbon vacancy (VC) is perhaps the most prominent point defect in silicon carbide (SiC) and it is an efficient charge carrier lifetime killer in high-purity epitaxial layers of 4H-SiC. The VC concentration needs to be controlled and minimized for optimum materials and device performance, and an approach based on post-growth thermal processing under C-rich ambient conditions is presented. It utilizes thermodynamic equilibration and after heat treatment at 1500 °C for 1 h, the VC concentration is shown to be reduced by a factor ~25 relative to that in as-grown state-of-the-art epi-layers. Concurrently, a considerable enhancement of the carrier lifetime occurs throughout the whole of >40 µm thick epi-layers.

In this study we have grown thick 4H-SiC epitaxial layers with different n-type doping levels in the range 1E15 cm-3 to mid 1E18 cm-3, in order to investigate the influence on carrier lifetime. The epilayers were grown with identical growth conditions except the doping level on comparable substrates, in order to minimize the influence of other parameters than the n-type doping level. We have found a drastic decrease in carrier lifetime with increasing n-type doping level. Epilayers were further characterized with low temperature photoluminescence and deep level transient spectroscopy.

With the aim of determining the concentration quenching mechanism of Eu2+ doped Ca2BO3Cl, a series of phosphors with a varied Eu2+ concentration (Ca2-xBO3Cl:xEu(2+)) was synthesized by the solid state reaction method. The phase structure was determined by X-ray diffraction. Photoluminescence (PL) measurements showed broad excitation and emission signatures of the allowed f-d transition of Eu2+ ions. The PL emission intensity was found to be increased by increasing the concentration of Eu2+ ions up to x=0.03 and then decreased as a result of the concentration quenching effect. The lifetime of the emission from the Eu2+ ions was measured and the decrease in the lifetime with increasing Eu2+ concentration confirmed that non-radiative energy transfer occurred between Eu2+ ions. From the luminescence data, the value of the critical transfer distance was calculated as 1.5 nm and the corresponding concentration quenching mechanism was verified to be a dipole-dipole interaction. (C) 2015 Elsevier Ltd. All rights reserved.

Efficient energy transfer was demonstrated in the SrF2:Eu2+, Pr3+ phosphor synthesized by the co-precipitation method. Results obtained with X-ray diffraction (XRD), scanning electron microscopy (SEM), x-ray spectroscopy (XPS), photoluminescence (PL) and decay curves proposed the UV-Vis energy transfer process. The energy transfer process between the Eu2+ and Pr3+ ions in SrF2 was investigated to evaluate the potential of the Eu2+ ion as a sensitizer for the Pr3+ ion. The results proposed that Eu2+ could be a good sensitizer for absorbing the UV photons and efficiently enhancing the Pr3+ emission intensity. The energy transfer process was effective until concentration quenching for the Pr3+ ions occurred. The concentration quenching was attributed to cross-relaxation between the Pr3+ ions. (C) 2016 Author(s).

4H-SiC epilayers with very smooth surfaces were grown with high growth rates on 4° off-cut substrates using standard silane/propane chemistry. Specular surfaces with RMS values below 0.2 nm are presented for epilayers grown with growth rates up to 30 μm/h using horizontal hot-wall chemical vapor deposition, with up to 100 μm thickness. Optimization of in-situ etching conditions and C/Si ratio are presented.

Degradation-free ultrahigh-voltage (> 10 kV) PiN diodes using on-axis 4H-SiC with low forward voltage drop (V-F = 3.3 V at 100 A/cm(2)) and low differential on-resistance (R-ON = 3.4 m Omega.cm(2)) are fabricated, measured, and analyzed by device simulation. The devices show stable on-state characteristics over a broad temperature range up to 300 degrees C. They show no breakdown up to 10 kV, i.e., the highest blocking capability for 4H-SiC devices using on-axis to date. The minority carrier lifetime (tau(P)) is measured after epitaxial growth by time resolved photoluminescence (TRPL) technique at room temperature. The tau(P) is measured again after device fabrication by open circuit voltage decay (OCVD) up to 500 K.

4H-SiC epilayers were grown on 2 degrees off-cut substrates using standard silane/propane chemistry, with the aim of characterizing in-grown stacking faults. The stacking faults were analyzed with low temperature photoluminescence spectroscopy, room temperature photoluminescence mappings, room temperature cathodoluminescence and synchrotron white beam X-ray topography. At least three different types of in-grown stacking faults were observed, including double Shockley stacking faults, triple Shockley stacking faults and bar-shaped stacking faults. Those stacking faults are all previously found in 4 degrees and 8 degrees off-cut epilayers; however, the geometrical size is larger in epilayers grown on 2 degrees off-cut substrates due to lower off-cut angle. The stacking faults were formed close to the epilayer/substrate interface during the epitaxial growth. (C) 2015 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim

SrF2:Eu,Ce3+ nanophosphors were successfully synthesized by the hydrothermal method during down-shifting investigations for solar cell applications. The phosphors were characterized by X-ray diffraction (XRD), scanning Auger nanoprobe, time of flight-secondary ion mass spectrometry (TOF-SIMS), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectroscopy. XRD showed that the crystallite size calculated with Scherrers equation was in the nanometre scale. XPS confirmed the formation of the matrix and the presence of the dopants in the SrF2 host. The PL of the nanophosphor samples were studied using different excitation sources. The phenomenon of energy transfer from Ce3+ to Eu2+ has been demonstrated.

We report on optical studies of exciton localization and recombination kinetics in two single 2.2 nm thick AlxGa1-xN/Alx+0.1Ga0.9-xN quantum well structures (x = 0.55 and 0.6) grown by plasma assisted molecular beam epitaxy on a c-sapphire substrate. Strong localization potential inherent for both the quantum well and barrier regions results in merging of the quantum well and barrier emission spectra into a single broad line centered at 285 nm (x = 0.55) and 275 nm (x = 0.6). Time-resolved photoluminescence measurements revealed surprising temperature stability of the photoluminescence decay time constant (approximate to 400 ps) relevant to the recombination of the quantum well localized excitons. This observation implies nearly constant quantum efficiency of the quantum well emission in the whole range from 4.6 to 300 K.