Papers by Keyword: Carrier Lifetime

Abstract: Temperature dependent microwave detected photoconductivity MDP and time-resolved photoluminescence TRPL were employed to investigate the carrier lifetime in CVD grown 4H-SiC epilayers of different thickness. The minority carrier lifetime may be found from both theMDP and defect PL decay at room temperature for all epilayers, whereas the near bandedge emission (NBE) decay is much faster for thin epilayers (<17 μm) due to the substrate proximity and only follows the minority carrier lifetime for thicker samples at lower excess carrier concentrations.

Abstract: Carrier lifetime in low carrier concentration 4H-SiC epitaxial layers grown on the C-face was enhanced by using carbon implantation and post annealing. The measured carrier lifetime increased with the thickness of the epitaxial layer and was 11.4 µs for the 150 µm thick epitaxial layer. The internal carrier lifetime was estimated as 21 µs from the dependence of the measured carrier lifetime on the epitaxial layer thickness. This value is almost comparable to the reported values of the internal carrier lifetime for the layers grown on the Si-face.

Abstract: For high voltage SiC bipolar devices, carrier lifetime is an important parameter, and for optimization of device performance, we need to control distribution of the carrier lifetime in a wafer. So far, there have been limited systems for depth-resolved carrier lifetime measurements without cross sectional cut. In this study, we adopted a free carrier absorption technique and made local overlapping of the probe laser light with excitation laser light to develop depth-resolved carrier lifetime measurements. We named the developed system a microscopic FCA system and demonstrated measurement results for samples with and without intentional carrier lifetime distribution.

Abstract: An investigation into the increased leakage currents and reduced blocking voltages associated with 1450°C lifetime enhancement oxidation for the 4H-SiC p-GTOs is presented. Roughening of the 4H-SiC surface due to localized crystallization of SiO₂, or crystobalite formation, during the high temperature oxidation was identified as one of the main causes of this issue. A factor of 30 difference in permeability to O₂ between amorphous SiO₂ and crystobalite caused uneven oxidation, which resulted in significant roughness. This roughness, placed at the metallurgical junction between the gate and the drift layer, where the E-field is greatest, is believed to be responsible for the premature breakdown characteristics. A 2-step lifetime enhancement process, which moves this roughness to the lower E-field region of the device was introduced to alleviate this issue. A 15 kV 4H-SiC p-GTO with the 2-step lifetime enhancement process demonstrated a significant reduction in V_F over the 1300°C oxidized devices, without any impact on blocking characteristics.

Abstract: 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.

Abstract: Epitaxial growth of 4H-SiC with intentional V or Ti doping was performed to obtain short minority carrier lifetimes, using VCl₄ or TiCl₄ as the doping sources. The doping efficiencies and quality of the epilayers were compared for H₂+SiH₄+C₃H₈ and H₂+SiH₄+C₃H₈+HCl gas systems. The addition of V or Ti in highly N-doped epilayer demonstrated very short minority carrier lifetimes of 20-30 ns at 250°C.

Abstract: An epitaxial growth technique for 4H-SiC with B doping was developed to control the carrier lifetimes of the epilayers. A linear relationship was observed between the B doping concentration and the flow rate of tri-ethyl-boron, which was used as the B doping source. A room temperature photoluminescence spectrum of a N-and B-doped epilayer showed a broad B-related peak at 2.37 eV instead of a band-edge luminescence, which indicates that the carrier recombination path was changed by the B doping. The minority carrier lifetime decreased (< 30 ns at 250°C) with increasing B doping concentration. The thermal stability of the short carrier lifetime was compared with a conventional carrier lifetime reduction method, namely an electron irradiation technique. After thermal annealing at 1700°C, the carrier lifetime of the electron irradiated epilayer recovered while that of the B-doped epilayer remained, indicating that the carrier lifetime controlled by the B doping technique was more stable against the thermal processes.

Abstract: The impact of the lifetime enhancement process using high temperature thermal oxidation method on 4H-SiC P-GTOs was investigated. 15 kV 4H-SiC P-GTOs with 140 μm thick drift layers, with and without 1450°C lifetime enhancement oxidation (LEO) process, were compared. The LEO process increased the average carrier lifetime in p-type epi layer from 0.9 μs to 6.25 μs, and it was observed that the effectiveness of the lifetime enhancement process was very sensitive to the doping concentration. The device with the LEO process showed a significant reduction in forward voltage drop and a substantially lower holding current, as expected from the carrier lifetime measurements. However, a slight reduction in blocking capability was also observed from the devices treated with LEO process. The common emitter current gain (β) of the wide base test NPN BJT was approximately 10X higher for the wafer with LEO process.

Abstract: Over 150 μm thick epilayers of 4H-SiC with long carrier lifetime have been grown with a chlorinated growth process. The carrier lifetime have been determined by time resolved photoluminescence (TRPL), the lifetime varies a lot between different areas of the sample. This study investigates the origins of lifetime variations in different regions using deep level transient spectroscopy (DLTS), low temperature photoluminescence (LTPL) and a combination of KOH etching and optical microscopy. From optical microscope images it is shown that the area with the shortest carrier lifetime corresponds to an area with high density of structural defects.

Abstract: We present an extended model for the simulation of the effective minority carrier lifetime in 4H-SiC epiwafers after optical excitation. This multilayer model uses measured values (such as doping profile, point defect concentration and capture cross sections, epilayer thickness) as input parameters. The bulk lifetime and the diffusion constant are calculated from the actual time dependent excess carrier profiles, resulting in more realistic transients having different decay regimes than in other models. This enables a better understanding of optical lifetime measurements.