Ulf Lindefelt

Ionization rates and critical fields in 4H silicon carbide

Epitaxial p-n diodes in 4H SiC are fabricated showing a good uniformity of avalanche multiplication and breakdown. Peripheral breakdown is overcome using the positive angle beveling technique. Photomultiplication measurements were performed to determine electron and hole ionization rates. For the electric field parallel to the c-axis impact ionization is strongly dominated by holes. A hole to electron ionization coefficient ratio of up to 50 is observed. It is attributed to the discontinuity of the conduction band of 4H SiC for the direction along the c axis. Theoretical values of critical fields and breakdown voltages in 4H SiC are calculated using the ionization rates obtained.

Relativistic band structure calculation of cubic and hexagonal SiC polytypes

A full-potential band structure calculation, within the local density approximation to the density functional theory, has been performed for the polytypes 3C, 2H, 4H, and 6H of SiC. The calculated effective electron masses are found to be in very good agreement with experimental values. The electron-optical phonon coupling has been estimated and the polaron masses are calculated to be 3%–13% larger than the corresponding bare masses. The effective electron masses of the second lowest conduction band minima are also presented and the calculated energy difference between the two lowest minima in 4H–SiC is only 0.12 eV. The lowest conduction band in 6H–SiC is found to be very flat and to have a double-well-like minimum along the ML line. The top of the valence bands has been parametrized according to the k⋅p approximation, whereupon the effective hole masses have been determined. The spin-orbit interaction was found to have a strong influence on the value of the effective hole masses. Furthermore, total and ...

A 4.5 kV 6H silicon carbide rectifier

Reactive ion etched silicon carbide mesa pin diodes with voltage blocking capabilities as high as 4.5 kV have been fabricated from 6H–SiC epitaxial layers. The epitaxial structure was grown by chemical vapor deposition on an n+ substrate giving a low‐doped 45 μm thick n− active base layer and a 1.5 μm thick high‐doped p+ emitter layer on top. A high minority carrier lifetime of 0.43 μs in the n− active base layer provides good on‐state properties with a typical forward voltage drop of 6 V at 100 A/cm2.

Doping-induced band edge displacements and band gap narrowing in 3C–, 4H–, 6H–SiC, and Si

Models for doping-induced band edge displacements and band gap narrowing in both n-type and p-type 3C–, 4H–, and 6H–SiC are presented for the first time. For comparison, Si has also been considered. The models constitute an extension of the theory of Jain and Roulston [Solid State Electron. 34, 453 (1991)] and take into account the three different electron effective mass components associated with hexagonal lattices. Furthermore, a more careful treatment of minority carrier correlation energy has been made, applying a two-band model for the dielectric function of a hole gas in the plasmon-pole approximation. The results for the band edge displacements are expressed in simple analytical form as functions of doping concentration.

Auger recombination in 4H-SiC: Unusual temperature behavior

The band-to-band Auger recombination in 4H-SiC material is studied using a time-resolved photoinduced absorption technique. The Auger recombination coefficient is derived from the kinetics of electron-hole plasma in heavily doped n-type 4H-SiC and in low-doped 4H-SiC epitaxial layers in the temperature interval 300–565 K. Within this range, its value decreases from γ3=(7±1)×10−31u2009cm6u2009s−1 to γ3=(4±1)×10−32u2009cm6u2009s−1. The observed pronounced reduction of Auger recombination rate with temperature is correlated to temperature dependent threshold energy of Auger process.

Cubic polytype inclusions in 4H–SiC

Multiple stacking faults in 4H–SiC, leading to narrow 3C polytype inclusions along the hexagonal c direction, have been studied using an ab initio supercell approach with 96 atoms per supercell. The number of neighboring stacking faults considered is two, three, and four. The wave functions and the two-dimensional energy bands, located in the band gap and associated with the narrow inclusions, can be reconciled with a planar quantum-well model with quantum-well depth equal to the conduction band offset between 3C– and 4H–SiC. We show that the existence of the electronic dipole moment due to the spontaneous polarization leads to a clear asymmetry of the bound wave functions inside the quantum well, and that the perturbation associated with the change in the dipole moment caused by the 3C–like inclusion accounts for the appearance of very shallow localized states at the valence band edge. We have also calculated the stacking fault energies for successive stacking faults. It is found that the stacking fault ...

Heat generation in semiconductor devices

A general and practical model for heat generation that can be used in the heat conduction equation for nonisothermal semiconductor device simulations is presented. The model is developed for cubic semiconductors with position‐dependent, multivalley, and multiband band structures, Fermi‐Dirac statistics and an accurate treatment of electron‐hole scattering. Starting from the Boltzmann transport equations for electrons, holes, and phonons, an internal energy balance equation, consistent with basic thermodynamic principles, is derived. By applying the linear phenomenological equations of irreversible thermodynamics and Onsager’s relations, the energy balance equation is reformulated into a heat conduction equation, and a heat generation source term is identified. The Peltier coefficients appearing in the model are analyzed, and thermal boundary conditions are given. The influence from optical effects, in particular photon reabsorption, is also discussed. Finally, physical insight into the mechanisms governin...

BAND GAP NARROWING IN N-TYPE AND P-TYPE 3C-, 2H-, 4H-, 6H-SIC, AND SI

Doping-induced energy shifts of the conduction band minimum and the valence band maximum have been calculated for n-type and p-type 3C-, 2H-, 4H-, 6H-SiC, and Si. The narrowing of the fundamental band gap and of the optical band gap are presented as functions of ionized impurity concentration. The calculations go beyond the common parabolic treatments of the ground state energy dispersion by using energy dispersion and overlap integrals from band structure calculations. The nonparabolic valence band curvatures influence strongly the energy shifts especially in p-type materials. The utilized method is based on a zero-temperature Green’s function formalism within the random phase approximation with local field correction according to Hubbard. We have parametrized the shifts of the conduction and the valence bands and made comparisons with recently published results from a semi-empirical model.

Theoretical study of planar defects in silicon carbide

We report on a theoretical investigation of extended planar defects in 3C-, 4H-, 6H-, and 15R-SiC which can be formed without breaking any bonds, covering a wide range of planar defects: twin boundaries, stacking faults, and polytype inclusions. Their electronic structures have been intensively studied using an ab initio supercell approach based on the density functional theory. Stacking fault energies are also calculated using both the supercell method and the axial next-nearest-neighbour Ising model. We discuss the electronic properties and energies of these defects in terms of the geometrical differences of stacking patterns.

Temperature dependence of avalanche breakdown for epitaxial diodes in 4H silicon carbide

The temperature dependence of avalanche breakdown is investigated for uniform and microplasma-related breakdown in epitaxial 4H SiC p-n junctions. P-n mesa diodes fabricated with positive angle beveling and oxide passivation can withstand temperatures of up to 300–400u2009°C in the breakdown regime. Uniform avalanche breakdown in 4H silicon carbide appears to have a positive temperature coefficient, in contrast to the 6H polytype, where the temperature coefficient is negative. The influence of deep levels on avalanche breakdown in epitaxial diodes is of minor importance for uniform breakdown, but appears to be significant for breakdown through microplasmas. A negative temperature coefficient for the avalanche breakdown voltage can be observed even for 4H SiC if the breakdown is dominated by microplasmas.