Levels In 4H-SiC Research Articles

Dual configuration of shallow acceptor levels in 4H-SiC

Acceptor dopants in 4H-SiC exhibit energy levels that are located deeper in the band gap than the thermal energy at room temperature (RT), resulting in incomplete ionization at RT. Therefore, a comprehensive understanding of the defect energetics and how the impurities are introduced into the material is imperative. Herein, we study impurity related defect levels in 4H-SiC epitaxial layers (epi-layers) grown by chemical vapor deposition (CVD) under various conditions using minority carrier transient spectroscopy (MCTS). We find two trap levels assigned to boron impurities, B and D, which are introduced to varying degrees depending on the growth conditions. A second acceptor level that was labeled X in the literature and attributed to impurity related defects is also observed. Importantly, both the B and X levels exhibit fine structure revealed by MCTS measurements. We attribute the fine structure to acceptor impurities at hexagonal and pseudo-cubic lattice sites in 4H-SiC, and tentatively assign the X peak to Al based on experimental findings and density functional theory calculations.

Effect of quantum confinement on the defect-induced localized levels in 4H-SiC(0001)/SiO2 systems

In the present study, we characterize the nature of interface states in silicon carbide (SiC) metal–oxide–semiconductor (MOS) systems by analyzing the electrical characteristics of MOS field effect transistors (MOSFETs) based on the results of numerical calculations. In the calculation, the potential distributions and energy sub-bands were calculated by solving Poisson and Schrödinger equations, respectively. As a result, we demonstrate that the defect-induced localized levels in the bandgap are subjected to quantum confinement at the inversion layer, leading to the increase in their energy levels. The result implies that the conventional interface defects (e.g., near-interface oxide traps), which create defect states at certain energy levels measured from the vacuum level, are unlikely to be the major origin of the interface states in SiC MOS systems. The interface state density is almost uniquely determined by the oxide formation process (as oxidation or interface nitridation) and independent of the acceptor concentration (3 × 1015–1 × 1018 cm−3). It is also suggested that the drain current decrease observed in heavily doped MOSFETs is mainly due to the decrease in the drift mobility rather than that in the free carrier density.

Density and energy level of a deep-level Mg acceptor in 4H-SiC

Reliably determining the densities and energy levels of deep-level dominant acceptors in heavily doped wide-band-gap semiconductors has been a topic of recent discussion. In these discussions, the focus is on both Hall scattering factors for holes and distribution functions for acceptors. Mg acceptor levels in 4H-SiC seem to be deep, and so here the electrical properties of Mg-implanted 4H-SiC layers are studied by measuring Hall effects. The obtained Hall scattering factors are not reliable because they drop to less than 0.5 at high measurement temperatures. Moreover, the Fermi–Dirac distribution function is unsuitable for examining Mg acceptors because the obtained acceptor density is much higher than the concentration of implanted Mg atoms. However, by using a distribution function that includes the influence of the excited states of a deep-level acceptor, the density and energy level of Mg acceptors can be reliably determined.

Identification of Structures of the Deep Levels in 4H-SiC

We investigated annealing behavior of the carrier lifetime and the deep levels in electron irradiated n-type and semi-insulating 4H-SiCs. We observed two peaks for each sample by photo induced current transient spectroscopy (PICTS) measurements, and their heights depended on annealing temperature. By comparing the annealing behavior of the peak height with reported temperature dependence of concentrations of various defects, we speculated that the observed peaks originate from either VSiVC, CSiVC or EI4.

Effect of the Schottky Barrier Height on the Detection of Midgap Levels in 4H-SiC by Deep Level Transient Spectroscopy

The effect of the Schottky barrier height on the detection of the midgap defects EH6 and EH7 in 4H-SiC by deep level transient spectroscopy (DLTS) is systematically studied. The results show that the DLTS peak height - and as a consequence the observed defect concentration - increases with the increasing barrier height and saturates above 1.5 eV for EH6 and above 1.7 eV for EH7, while below 1.1 eV the DLTS peak height completely disappears. A model is applied, which determines the position of the quasi-Fermi level in the space charge region as a function of the barrier height and of the reverse bias applied and which explains the variation of the DLTS peak height.

Effect of the Schottky barrier height on the detection of midgap levels in 4H-SiC by deep level transient spectroscopy

The effect of the Schottky barrier height on the detection of the concentration of midgap defects using deep level transient spectroscopy (DLTS) is experimentally and theoretically studied for EH6 and EH7 defects in 4H-SiC. In this special case, the DLTS signal height for EH6 and EH7 increases with increasing barrier height and saturates at values above 1.5 and 1.7eV, respectively. Below 1.1eV, the DLTS peak completely disappears for both defects. The experimental data are explained by a theoretical model. The course of the quasi-Fermi level in the space charge region is calculated as a function of the reverse current through it, which is determined by the barrier height, and the reverse bias applied.

Influence of Cooling Rate after High Temperature Annealing on Deep Levels in High-Purity Semi-Insulating 4H-SiC

The influence of different cooling rates on deep levels in 4H-SiC after high temperature annealing has been investigated. The samples were heated from room temperature to 2300°C, followed by a 20 minutes anneal at this temperature. Different subsequent cooling sequences down to 1100°C were used. The samples have been investigated using photoluminescence (PL) and IV characteristics. The PL intensities of the silicon vacancy (VSi) and UD-2, were found to increase with a faster cooling rate.

Optical cross sections of deep levels in 4H-SiC

We have characterized deep levels in 4H-SiC epilayers grown by cold wall chemical vapor deposition by the deep level transient spectroscopy (DLTS) and the optical-capacitance-transient spectroscopy (O-CTS). Four kinds of DLTS peaks were detected in the epilayers. Three of them are identified as the Z1∕2, EH6∕7, and RD1∕2 centers, while the other one has never been reported previously, and was named the NB center. On the basis of these DLTS data we have estimated the thermal ionization energies. The classical optical ionization energies of these centers, which are given by the sums of thermal ionization energies and Franck-Condon shifts, were estimated via fittings of the measured optical cross sections from O-CTS data by means of a sufficiently general theoretical model. Temperature dependences of nonradiative multiphonon carrier capture cross sections for the Z1∕2 and NB centers were roughly estimated in terms of parametrical dependences on thermal ionization energies and Franck-Condon shifts.

Effect of high-temperature processing on the creation of boron-related deep levels in 4H-SiC

High-temperature processing was applied to SiC boron-doped epitaxial layers, epitaxial layers grown on boron-rich substrates, and boron-implanted samples in order to establish conditions favorable for the formation of boron-related centers (D-centers) that introduce a deep level in SiC bandgap. Photoluminescence (PL) was used to detect and compare the formation of the D-center after different processing steps. It was confirmed that the presence of lattice defects was required for achieving significant boron diffusion and D-center formation. In addition, epitaxial growth on the top of a boron-implanted sample yielded efficient boron in-diffusion in the growing layer, which resulted in a material compensated with D-centers. The temperature dependence of the D-center formation efficiency was investigated.

Diffusion length and junction spectroscopy analysis of low-temperature annealing of electron irradiation-induced deep levels in 4H-SiC

The effects of low-temperature annealing in 8.2MeV electron-irradiated 4H-SiC Schottky diodes were investigated. Deep-level transient spectroscopy and minority-carrier diffusion length (Ld) measurements were carried out on not-irradiated samples and on irradiated samples before and after thermal treatments up to T=450°C. We found that several deep levels in the upper half band gap (S1 with enthalpy ET=0.27eV, S2 with ET=0.35eV, S4 with ET=0.71eV, and S5 with ET=0.96eV) anneal out or modify at temperature values lower or equal to T=450°C, whereby their progressive annealing out is accompanied by a net increase of Ld, up to 50% of the value in the as-irradiated sample. We drew some conclusions regarding the microscopic nature of the defects related to the deep levels, according to their annealing behavior.

Low temperature annealing of electron irradiation induced defects in 4H-SiC

Low temperature annealing of electron irradiation-induced deep levels in 4H-SiC is reported. The major deep level transient spectroscopy peak S2 associated with the energy level at Ec−0.39eV disappears in the temperature range 360–400K, and some rearrangement of the peak S3, associated with the defect Z1∕Z2 with energy level at Ec−0.5∕Ec−0.65eV occurs in the temperature interval 400–470K. A net free charge carrier concentration increase goes along with the disappearance of peak S2 at Ec−0.39eV, whereas the charge collection efficiency of the diode does not experience any significant change. An interpretation of the annealing of peak S2 on a microscopic scale is given.

Boron Centers in 4H-SiC

The origin of the deep boron related acceptor in SIC is subject to a lot of controversy. Based on ENDOR investigations, a B-Si+V-C model was suggested, while PL studies indicated the acceptor on the carbon sublattice. Our former ab initio LDA molecular cluster calculation showed that in the B-Si+V-C complex the carbon vacancy acts as the acceptor. Now, ah initio LDA supercell calculations have been carried out for boron-related complexes to calculate the occupation levels in 4H-SiC. It has been found that the 0/- level for the B-Si+V-C complex lies in the upper half of the gap, therefore it can be disregarded as the origin of the deep boron-related acceptor level. Investigating other feasible boron-related complexes, B-Si+Si-C appears to be the best candidate.

Effect of C/B Sequential Implantation on the B Acceptors in 4H-SiC

ABSTRACTWe have systematically investigated the effect of C/B sequential implantation on the B-related acceptors and deep levels in 4H-SiC using thermal admittance spectroscopy. With increasing concentration of co-implanted C, the density of deep levels was found to start decreasing and to be completely suppressed for the C and B ratio of 1:1. Moreover, the density and ionization energy of B acceptors were seen to start increasing and lowering, respectively, with increasing C concentration. However, we found that excess C-content leads to the formation of a new complex defect. The C-V results also support the expected increase in the total hole concentration with increasing concentration of the co-implanted C-atoms, which is followed by a decrease in the concentration under C-rich condition. This is in reasonable agreements with the behavior of the B acceptors and deep defect levels. Therefore, the concentration of co-implanted C-atoms is considered to be very sensitive to the formation of the B acceptor levels.

Nitrogen Donor Concentrations and Its Energy Levels in 4H-SiC Uniquely Determined by a New Graphical Method Based on Hall-Effect Measurement

This is the first report on the experimental results using the precise determination proposed in our previous papers. By means of this analysis, the concentrations and energy levels of dopants in a semiconductor can be uniquely determined using the temperature dependence n(T) of the majority-carrier concentration obtained experimentally from the Hall-effect measurement. In other words, the concentration and energy level of each dopant can be evaluated using the corresponding peak value and temperature of n(T)/kT. In nitrogen (N)-doped 4H-SiC, the concentration and energy level of the shallow donor are 6.45 ×1015 cm-3 and EC-0.0653 eV, respectively, and those of the deep donor are 3.04 ×1016 cm-3 and EC-0.124 eV, respectively, where EC is the bottom of the conduction band. The acceptor concentration is 6.14 ×1013 cm-3. These obtained values are found to be quite reliable.

Shallow and deep levels in n-type 4H-SiC

The nitrogen levels in 4H-SiC have been determined using thermal admittance spectroscopy. The values of Ec−0.053 eV for nitrogen at the hexagonal site and Ec−0.10 eV for nitrogen at the quasicubic site agree with those reported using other techniques. The deep levels in 4H-SiC were studied using optical admittance spectroscopy. The optical admittance spectrum showed, besides the conductance peak corresponding to band to band transitions, four other conductance peaks. These peaks correspond to photoexcitation of carriers from the defect levels to the conduction band. It is inferred from a comparison with 6H-SiC that the conductance peak b4 is due to excitation of electrons from the vanadium donor at Ec−1.73 eV. The photoconductance build up transients of the Ec−1.73 eV level are described fully by one exponential term. This suggests that only one center contributed to the observed conductance. The decay kinetics of persistent photoconductance due to the Ec−1.73 eV level follow the stretched exponential form. The potential barrier against recapture of photoexcited carriers was determined to be 18 meV for the vanadium donor level in 4H-SiC.

Thermal And Optical Admittance Spectroscopy Studies Of Defects In 15r-Sic

AbstractNitrogen is the common n-type dopant of the various polytypes of silicon carbide. The nitrogen levels in 4H-SiC (at EC-53 meV and EC-100 meV) and in 6H-SiC (at EC-89 meV, EC- 100 meV, and EC-125 meV) have been studied in detail by temperature dependent Hall effect measurements, electron spin resonance (ESR), and thermal admittance spectroscopy. Until now, such detailed studies of the nitrogen levels in 15R-SiC have not been carried out.Lely-grown 15R samples were used in these studies. The net carrier concentrations (ND-NA), determined by room temperature CV measurements, ranged from 1 × 1018 to 3 × 1018 cm−3. The nitrogen levels in 15R-SiC were studied using thermal admittance spectroscopy. Optical admittance spectroscopy (OAdS) was used to study the deeper defects in this polytype. It was found that optical transitions to the conduction band were inhibited in the heavily doped material.