Papers by Keyword: DLTS

Abstract: We demonstrate our new local deep level spectroscopy system improved for more accurate analysis of trap states at SiO₂/4H-SiC interfaces. Full waveforms of the local capacitance transient with the amplitude of attofarads and the time scale of microseconds were obtained and quantitatively analyzed. The local energy distribution of interface state density in the energy range of E_C − E_it = 0.31–0.38 eV was obtained. Two-dimensional mapping of the interface states showed inhomogeneous contrasts with the lateral spatial scale of several hundreds of nanometers, suggesting that the physical origin of the trap states at SiO₂/SiC interfaces is likely to be microscopically clustered.

Abstract: In this contribution, we report on the electrical characterization of point defects in 4H-SiC p⁺in diodes. Ten electrically active levels have been detected in the base region of the devices, by employing Deep Level Transient Spectroscopy (DLTS) and Minority Carrier Transient Spectroscopy (MCTS). Of these ten levels, six are majority carrier traps, in the 0.1-1.7 eV energy range below the conduction band edge, and four were minority carrier traps located in the 0.13-0.4 eV energy range above the valence band edge. We found that, during DLTS measurements, both majority and minority carrier traps can be detected and we explain this by considering the behavior of the quasi-Fermi levels. At last, we studied the impact of proton irradiation on the minority charge carrier lifetime.

Abstract: Germanium (Ge) doping of 4H silicon carbide (SiC) has recently attracted attention because a conductivity-enhancing effect was reported. In this work, we report on an experimental and theoretical approach to elucidate this effect. Ge and tin (Sn) – a second candidate of group IV elements – have been implanted into n-type 4H-SiC. Despite the expected isoelectric nature of Ge and Sn, a more efficient annealing of implantation-induced defects was observed compared to noble gas implantation with identical simulated initial implantation damage. In particular, a strong reduction of the prominent Z_1/2 defect was observed. Density functional theory calculations under equilibrium conditions show that Ge is mainly incorporated on a substitutional silicon lattice site without creating new charge transition levels in the bandgap. The low abundance of other Ge-related defects suggests that kinetic mechanisms should be responsible for the observed effect of group IV doping.

Abstract: The carbon vacancy (V_C) is a major point defect in high-purity 4H-SiC epitaxial layers limiting the minority charge carrier lifetime. In layers grown by chemical vapor deposition techniques, the V_C concentration is typically in the range of 10¹² cm^-3 and after device processing at temperatures approaching 2000 °C, it can be enhanced by several orders of magnitude. In the present contribution, we show that the cooling rate after high-temperature processing has a profound influence on the resulting V_C concentration where a slow rate promotes elimination of V_C. Further, isochronal annealing of as-grown and as-oxidized epi-layers protected by a carbon-cap was undertaken between 800 °C and 1600 °C. The results reveal that thermodynamic equilibrium of V_C is established rather rapidly at moderate temperatures, reaching a V_C concentration of only a few times 10¹¹ cm^-3 after 40 min at 1500 °C. Hence, the concept of eliminating V_C’s by annealing at moderate temperatures under C-rich equilibrium conditions shows great promise and enables re-annealing of high-temperature processed wafers, in contrast to the procedures commonly used today to eliminate V_C. In-diffusion of carbon interstitials and out-diffusion of V_C’s are discussed as the kinetics processes establishing the thermodynamic equilibrium

Abstract: The carbon vacancy (V_C) is the major charge carrier lifetime limiting-defect in 4H-SiC epitaxial layers and it is readily formed during elevated heat treatments. Here we describe two ways for controlling the V_C concentration in 4H-SiC epi-layer using different annealing procedures. One set of samples was subjected to high temperature processing at 1950 °C for 3 min, but then different cooling rates were applied. A significant reduction of the V_C concentration was demonstrated by the slow cooling rate. In addition, elimination of the V_C’s was also established by annealing a sample, containing high V_C concentration, at 1500 °C for a sufficiently long time. Both procedures clearly demonstrate the need for maintaining thermodynamic equilibrium during cooling.

Abstract: The evolution of irradiation-induced and hydrogen-related defects in n-type silicon in the temperature range 0 – 300 °C has been studied by deep level transient spectroscopy (DLTS) and minority carrier transient spectroscopy (MCTS). Implantation of a box-like profile of hydrogen was performed into the depletion region of a Schottky diode to undertake the DLTS and MCTS measurements. Proportionality between the formation of two hydrogen-related deep states and a decrease of the vacancy-oxygen center concentration was found together with the appearance of new hydrogen-related energy levels.

Abstract: A deep level transient spectroscopy (DLTS) study on n- and p-type diluted Si_1-xGe_x alloys (x=0, 0.011, 0.026, 0.046, and 0.070) is presented. Defect levels of several carbon-hydrogen (CH) complexes are observed. The high-resolution Laplace-DLTS technique allows us to detect configurations of defects which contain different numbers of Ge atoms in the first and second-nearest neighbourhood of the CH complexes. The electrical properties of the defects will be analysed and their origin will be discussed.

Abstract: Defect structure, electrical properties and defect-related luminescence (DRL) of light emitting diodes (LED) with the active defect-rich region produced by oxygen implantation and a subsequent multistep annealing of silicon wafers were investigated. It was found that defect-rich regions possess an embedded positive charge in both n-and p-type of the samples whose origin was ascribed to oxygen precipitates (OP). The presence of that charge in the implanted region of p-based LED gave rise to the apparent conductivity type conversion and to a significant increase of free electron concentration in n-based LEDs. A significant difference in the shape and in the excitation dependence of luminescence spectra as well as in the properties of DLTS signals was found between p-and n-type samples. From an analysis of the obtained data the DRL band centered at 0.79 eV was ascribed to small OPs segregated at dislocations whose filling with the holes hinders optical transitions via dislocation-related states at 0.805 eV and the broad DRL band at energies higher than 0.81 eV was ascribed to large OPs.

Abstract: The carbon vacancy (V_C) is a minority carrier lifetime controlling defect in 4H-SiC and it is formed during high temperature treatment. In this study, we have performed heat treatment on two sets of n-type 4H-SiC epitaxial samples. The first set was isothermally treated at 1850 °C to follow the evolution of V_C as a function of time. The V_C concentration is not affected by changing the duration. Samples of the other set were treated at 1950 °C for 10 min, but with different cooling rates and a reduction of the V_C concentration was indeed demonstrated by lowering the cooling rate. The V_C concentration in the slow-cooled sample is about 2 times less than in the fast-cooled one, reflecting a competition between equilibrium conditions and the cooling rate.

Abstract: We have investigated the electrical properties of germanium-implanted n-type 4H-SiC epitaxial layers. Deep level transient spectroscopy (DLTS) was employed in order to study the influence of germanium ions on implantation-induced point defects. In particular, we observe a decrease of the concentration of Z_1/2 defect with increasing dose of implanted germanium.