Papers by Keyword: Carrier Lifetime

Abstract: In this work we report the results of a set of experiments carried out to assess the ability of recombination lifetime measurements for the detection of palladium contamination in silicon. Palladium is found to be a very effective recombination center, so recombination lifetime measurements are a very sensitive method to detect palladium in silicon. The surface segregation of palladium was monitored by the reduction of its recombination activity in the silicon volume. The palladium segregation at the wafer surface was checked by selective etching, and by Transmission Electron Microscopy (TEM) and Energy Dispersive X-ray (EDX) analysis.After validating recombination lifetime measurements for palladium detection, we use these measurements to define suitable approaches to the prevention of palladium contamination of silicon devices. The efficiency of a diffusion barrier layer (silicon nitride) and of decontamination by wet cleaning are tested.

Abstract: Thick multi-layer 4H-SiC epitaxial growth was investigated for very high-voltage Si-face p-channel insulated gate bipolar transistors (p-IGBTs). The multi-layer included n⁺ buffer, p⁺ field stop, and thick p^- drift layers. Two processes were employed to enhance the carrier lifetime of the p^- drift layer: carbon ion implantation/annealing and hydrogen annealing, and the enhanced carrier lifetime was confirmed by the open-circuit voltage decay measurement. Using the grown thick multi-layer 4H-SiC, simple pin diodes were fabricated instead of p-IGBTs to demonstrate efficient conductivity modulation in the thick p^- drift layer. While the on-state voltage was high at room temperature, it decreased significantly at elevated temperatures, and attained 3.5 V at 100 A/cm² at 200°C for the diode with the carrier lifetime enhancement processes, indicating sufficient conductivity modulation.

Abstract: Ultrahigh-voltage SiC flip-type n-channel implantation and epitaxial (IE)-IGBTs were developed, and the static and dynamic performance was investigated. A large device (8 mm × 8mm) with a blocking voltage greater than 16 kV was achieved, and an on-state current of 20 A was obtained at the low on-state voltage (V_on) of 4.8 V. R_onA_diff was 23 mΩ·cm² at V_on = 4.8 V. In order to evaluate the switching characteristics of the IE-IGBT, ultrahigh-voltage power modules were assembled. A chopper circuit configuration was used to evaluate the switching characteristics of the IE-IGBT. Smooth turn-off waveforms were successfully obtained at V_CE = 6.5 kV and I_CE = 60 A in the temperature range from room temperature to 250°C.

Abstract: A series of aluminium doped (from 2×10¹⁶ to 8×10¹⁹ cm^-3) 4H-SiC epitaxial layers were mainly studied by Low Temperature Photoluminescence and time-resolved optical pump-probe techniques to determine the concentration of aluminium, its activation ratio, the doping related carrier lifetime, hole mobility and excess carrier diffusion length.

Abstract: The electronic quality of a Physical Vapour Transport (PVT) grown 15R-SiC crystal at different stages of growth was assessed by time-resolved optical pump-probe techniques. The measured differential transmittivity (DT) kinetics for the layers corresponding to the initial, middle and final stages of growth revealed clear differences in the decay of the DT signal, indicating a decreasing concentration of traps at the later stages of the crystal growth. The estimated trap concentration in the initial layer was N_T ≈ 10¹⁹ cm^-3, while it decreased down to less than 2×10¹⁸ cm^-3 in the top layer. The injection dependence of the diffusion coefficient at room temperature confirmed the gradual decrease of N_T in the layers corresponding to later stages of growth. Accordingly, the bipolar diffusion coefficient in the middle and the top layer was D_a ≈ 2 cm²/s, while D_a = 0.9 cm²/s was measured in the layer closest to the seed.

Abstract: Three different turn-off strategies are presented for 6.5 kV class SiC Thyristors. A cathode current of 62 A (284 A/cm²) is successfully turned off by applying a reverse bias of ≈ 30 V to the cathode of a SiC Thyristor. The minimum turn-off time (t_q) for the Thyristor using this forced commutation technique is investigated as a function of cathode current density, reverse gate current and the dV/dt of the re-applied forward (blocking) bias. The Anode Switched Thyristor (AST) turn-off mode is demonstrated at a maximum cathode voltage of 3600 V and 14.5 A (199 A/cm²) of cathode current. A cathode current of 5.5 A (75 A/cm²) is successfully turned off by the gate turn off (GTO) or hard turn-off mode at different temperatures up to 200°C. The high-level lifetime (t_HL) in the thick p-layer is extracted from the hard turn-off waveforms.

Abstract: Investigation of excess carrier dynamics in a 15R-SiC bulk layer grown by physical vapour transport (PVT) on 15R-SiC substrate has been carried out using pump-probe techniques: an interband carrier injection by a picosecond laser pulse and measuring the induced absorption and diffraction of a probe beam. For this task, differential transmittivity (DT) and light induced transient grating (LITG) techniques were used. Room temperature carrier lifetime varied in the 3 ns 8 ns range at excess carrier densities above ΔN₀ = 7×10¹⁷ cm^-3 and was ascribed to the recovery time of optically recharged carrier traps, and their activation energy of E_a = 75 meV was determined. The presence of recharged traps caused the injection-dependence of the diffusion coefficient D, whereby its value dropped below 0.1 cm²/s at ΔN₀ < 1×10¹⁸ cm^-3 and gradually increased up to 0.7 cm²/s at higher injections. At elevated temperatures (300 K < T < 700 K), when the traps are thermally activated, the diffusivity increased up to ~ 1.5 cm²/s and was independent on ΔN₀. The overgrown layer parameters were comparable to those of the used 15R PVT seed.

Abstract: The deep levels ON₁ and ON_2a/b introduced by oxidation into 4H-SiC are characterized via standard DLTS and via filling pulse dependent DLTS measurements. Separation of the closely spaced ON_2a/b defect is achieved by using a higher resolution correlation function (Gaver-Stehfest 4) and apparent energy level, apparent electron capture cross section and filling pulse measurement derived capture cross sections are given.

Abstract: Carrier lifetime measurements and wafer mappings have been done on several different 4H SiC wafers to compare two different measurement techniques, time-resolved photoluminescence and microwave induced photoconductivity decay. The absolute values of the decay time differ with a factor of two, as expected from recombination and measurement theory. Variations within each wafer are comparable with the two techniques. Both techniques are shown to be sensitive for substrate quality and distribution of extended defects.

Abstract: Free carrier lifetimes and diffusion coefficients were determined in 6H-SiC bulk crystals grown by PVT on 6H-and 4H-SiC seeds varying the temperature from 300 K to 650 K and at excess carrier densities ΔN₀ from 10¹⁷ cm^-3 to 10¹⁹ cm^-3. Carrier generation was achieved by using a single or two-photon absorption of picosecond pulses at 351 and 532 nm, respectively. Fast and slow recombination transients revealed the decay time of free carriers and the presence of deep acceptor traps. The thermal trap activation energy E_a = 0.33 eV was determined in the 6H/4H sample and ascribed to the boron, while the presence of deeper traps is suggested in the 6H/6H sample. At room temperature and reaching conditions of trap saturation regime (ΔN₀ 10¹⁹ cm³), both crystals revealed the bipolar diffusion coefficient D_a 4 cm²/s. For comparison, we also determined the photoelectrical parameters in commercial 6H-SiC crystals grown by the Lely and PVT techniques.