Papers by Author: Thomas Frank

Abstract: Conventional MOSFETs and Hall-bar MOSFETs are fabricated side by side by over-oxidation of N-implanted or N-/Al-coimplanted 4H-SiC layers. It is demonstrated that the N-/Al-coimplanted MOSFETs possess a positive threshold voltage at room temperature and reach high values of the channel mobility. The effective electron mobility and Hall mobility in Hall-bar MOSFETs are 31 cm2/Vs and 150 cm2/Vs, respectively, indicating a high density of interface traps in spite of the excellent high mobility values.

Abstract: 3C-SiC/SiO2-capacitors are fabricated by over-oxidation of an implanted Gaussian nitrogen (N) profile and investigated by conductance spectroscopy. A double peak structure is observed in the conductance spectra indicating two types of traps, which change their charge state at identical time constant, however, which are located at different energy positions in the bandgap of 3C-SiC. The experimental G/w-V and C-V characteristics are simulated and the existence of two types of traps is verified in the framework of a theoretical model.

Abstract: An alternative oxidation technique is developed and built up, which provides monatomic oxygen during the whole oxidation process. The set-up consists of a tungsten lamp furnace and a microwave-plasma. A number of different gases can be introduced into the oxidation quartz tube. In addition, an Al2O3-layer is deposited on a part of the oxide layers by atomic layer chemical vapor deposition (ALCVD). First oxidation runs result in encouraging low values of the density of interface states Dit and in the flatband voltage UFB. It turns out that with the present experimental conditions, the comparison of MOS capacitors fabricated with different dielectric layers favors gate dielectrics grown in O2/N2-ambient.

Abstract: In n-type 4H-SiC, over-oxidation of an implanted surface-near, Gaussian nitrogen-profile results in MOS capacitors, which possess a distinctly reduced density of interface states Dit and an undesirable large negative flatband voltage UFB. Their values are determined by the implantation parameters and the thickness of the oxide layer. The negative flatband voltage can strongly be compensated in the case that a Gaussian aluminum-profile is co-implanted prior to the oxidation. Depending on the conditions of the Al implantation, UFB can be controlled within a wide range. Secondary ion mass spectrometry analyses reveal that the implanted N and Al atoms are mobile in the oxide layer during the oxidation process and are partly accumulated at the SiC/SiO2 interface.

Abstract: A near-surface Gaussian nitrogen (N) profile is implanted into the Si- or C-face of n-/ptype 4H-SiC epilayers prior to a standard oxidation process. The corresponding MOS capacitors are investigated by conductance and internal photoemission spectroscopy. The effect of N-implantation on the density of interface traps Dit is studied and a model is proposed, which consistently explains the observed results.

Abstract: Hall effect investigations taken on Si+-/N+-, C+-/N+- or Ne+-/N+-co-implanted 4H-SiC layers and deep level transient spectroscopy investigations taken on Si+-implanted 4H-SiC layers provide experimental evidence for an electrically neutral defect complex formed during the annealing process at temperatures between 1400°C and 1700°C. This defect complex consumes nitrogen donors and an intrinsic Si containing defect species (interstitial Si or Si-antisite) or Cvacancies. At our present knowledge, we favor an (NX-SiY)-complex.

Abstract: The aim of the present work is to grow 3C-SiC on (0001) 6H-SiC seeds using the Physical Vapour Transport (PVT) method and to study the electrical and structural properties of the grown material. Photoluminescence (PL)-mappings reveal that the overgrown layer consists predominantly of the 3C-SiC polytype and capacitance-voltage (C-V) measurements result in a net nitrogen donor concentration of 1x1016cm-3. Transmission Electron Microscopy (TEM) observations also confirm that the overgrown layer is of the 3C-SiC polytype having the cubic [111] crystallographic direction parallel to the c-axis of the 6H-SiC substrate. In some cases, twin crystals of 3C-SiC are formed immediately after the interface and, in a few cases, small 6H-SiC inclusions are observed in the cubic film having the same orientation as the substrate. The film near the substrate/overgrown interface shows a high density of defects such as dislocations and stacking faults (SF’s), which propagate into the overgrown layer. Finally although there is a rapid decrease of the defect density within the first 60 µm from the interface, the SF density remains almost constant within the last 100 µm below the surface.