Ulrike Grossner

The effect of doping and growth stoichiometry on the core structure of a threading edge dislocation in GaN

Density-functional-theory calculations have been performed to study the effect of doping and growth stoichiometry on the core structure of a threading edge dislocation in GaN. Four candidate structures were examined and their formation energies were found to depend strongly on Fermi level and growth stoichiometry. A structure having gallium vacancies at the dislocation core is predicted to be most stable in n-type material grown under nitrogen-rich conditions, while a structure without vacancies is most stable in p-type material grown under these conditions. In material grown under gallium-rich conditions, a structure having nitrogen vacancies at the dislocation core is predicted to be most stable in p-type material, whereas a variety of core structures should be present in n-type material. Edge dislocations are predicted to behave as electron traps in n-type material and may act as hole traps in p-type material depending on the growth conditions.

Al+ Implanted 4H-SiC p+-i-n Diodes: Evidence for Post-Implantation-Annealing Dependent Defect Activation

Two families of Al+ implanted vertical p+in diodes that have been processed all by identical steps except the post implantation annealing one have been characterized with current voltage measurements from -100 to +5V at different temperatures. Analysis of the static forward current voltage characteristics shows two different ideality factor regions, which are distinct for each family. The reverse current voltage characteristics reveals corresponding two different activation energies. These are assumed to be correlated to the Z1/2 defect for the one case and another one with an activation energy of 0.25eV.

Ni-Al-Ti ohmic contacts on Al implanted 4H-SiC

This study shows that an Al-Ti bilayer with an Al to Ti atomic ratio suitable forohmic contacts on p-type 4H-SiC can be covered by a Ni film during the high temperature alloying process, without altering the ohmic nature, while eliminating a detrimental contact morphology caused by the presence of molten Al-Si during alloying. On 1×1020 cm-3 Al-implanted 4H-SiC layer, the RT specific contact resistance of this Ni-Al-Ti contact measured by TLM-C method is (3 ± 1)×10-6 Ωcm2.

SiC Device Manufacturing: How Processing Impacts the Material and Device Properties

Studies in the literature have shown how the different processing steps can have an impact on the electronic properties of SiC devices. In this contribution, we will review the importance of preserving the crystalline integrity of SiC epilayers through the major processing steps like etching, implantation and oxidation. It will be shown that the major cause for SiC device failures, e.g bipolar degradation and low field effect mobility, is the presence of carbon-related defects like the carbon vacancy (VC) and carbon interstitials (Ci). At last, the different techniques devised to reduce the presence of these harmful defects will also be reviewed.

Highly accurate virtual dynamic characterization of discrete SiC power devices

Optimized low-inductive layouting of the package interconnections and external PCBs and bus-bars are necessary to benefit from Silicon Carbide (SiC) power devices, which allow inherently very fast switching transitions. In this paper, a comprehensive modeling procedure for highly accurate virtual dynamic characterization of discrete SiC power devices is described taking into account the 3D geometry of the internal and external interconnections of package as input. The modeling requirements are discussed on an example of a commercial 1.2 kV, 80 mΩ SiC Power MOSFET in a standard TO-247 package (Cree C2M0080120D). The software tools, Simplorer, Saber, Q3D and LTSpice, commonly used for modeling and simulation of power modules, are evaluated with respect to their modeling capabilities for SiC devices.

Electronic band structure of the buried SiO2/SiC interface investigated by soft x-ray ARPES

The electronic structure of the SiO2/SiC (0001) interface, buried below SiO2 layers with a thickness from 2 to 4 nm, was explored using soft X-ray angle-resolved photoemission spectroscopy with photon energies between 350 and 1000 eV. The measurements have detected the characteristic k-dispersive energy bands of bulk Silicon Carbide (SiC) below the SiO2 layer without any sign of additional dispersive states, up to an estimated instrumental sensitivity of ≈5 × 109 cm2 eV. This experimental result supports the physical picture that the large density of interface traps observed in macroscopic measurements results from dangling bonds randomized by the SiO2 rather than from Shockley-Tamm surface derived states extending into the bulk SiC.

1950°C Annealing of Al+ Implanted 4H-SiC: Sheet Resistance Dependence on the Annealing Time

This study shows that, after annealing at 1950°C, a 1×1020 cm-3 Al+ implanted 4H-SiC material shows a decreasing resistivity with increasing annealing time in the range 5-25 min. After this, the resistivity remains constant up to an annealing time of 40 min. The estimated minimum time to gain the thermal equilibrium in this implanted material at 1950°C is 12 min. Electrical characterization has been performed in the 20-680 K temperature range.

Analysis of thin thermal oxides on (0001) SiC epitaxial layers

In this study, electrical properties of MOS capacitors with varying oxide thicknesses have been investigated. The oxide growth was performed at 1050 °C without any further post-oxidation annealing steps resulting in oxide thicknesses between 2 nm and 32 nm. Capacitance-Voltage measurements revealed a decreasing density of interface defects for increasing oxide thickness suggesting a deterioration of the interface at the initial stage of the growth.

Influence of Annealing on the Al2O3/4H-SiC Interface

Summarizing, after a post-deposition annealing at 1000°C, correlation of XPS, SIMS and HRTEM data yields a scenario where the SiO x layer, occurring after ozone cleaning and Al 2 O 3 deposition, breaks up and transforms into islands of SiO 2 , which is thermodynamically very stable, at the interface. As a result, a rather rough interface region evolves and excess of pure Si appears in the Al 2 O 3 film. Moreover, a pronounced accumulation of H takes place in the rough interface region and this may at least partly be responsible for the low density of shallow electron states reported for annealed Al 2 O 3 /4H-SiC structures.

A more accurate electromagnetic modeling of WBG power modules

A major requirement for further development of wide-band gap (WBG) power devices and their applications is the optimization of packages and PCB layouts to enable fast-switching capabilities. Electromagnetic modelling allows the prediction of parasitic inductances, capacitances, and resistances of the current paths within power modules, which cannot be easily approached in measurements. As a result, electromagnetic-circuit-coupled modeling enables the optimization of package layouts and interconnections before manufacturing actual power modules. The accuracy and limitations of present numerical techniques for three-dimensional (3D) electromagnetic modeling of power modules is still neither well understood nor verified. This paper presents the extraction of parasitics of power semiconductor packages using two electromagnetic modelling approaches. The first approach is based on a well-established 3D electromagnetic quasi-static solver, ANSYS Q3D Extractor. For the second approach, a numerical solver based on the Partial Element Equivalent Circuit (PEEC) method is developed and assessed in terms of modelling accuracy required by fast switching WBG-based power converters. The PEEC method is presented as a promising numerical technique, which can potentially be used to overcome the limitations of the EM modeling based on the ANSYS Q3D Extractor.