Olof Kordina

Deep level defects in electron-irradiated 4H SiC epitaxial layers

Deep level defects in electron-irradiated 4H SiC epitaxial layers grown by chemical vapor deposition were studied using deep level transient spectroscopy. The measurements performed on electron-irradiated p+n junctions in the temperature range 100–750 K revealed several electron traps and one hole trap with thermal ionization energies ranging from 0.35 to 1.65 eV. Most of these defects were already observed at a dose of irradiation as low as ≈5×1013 cm-2. Dose dependence and annealing behavior of the defects were investigated. For two of these electron traps, the electron capture cross section was measured. From the temperature dependence studies, the capture cross section of these two defects are shown to be temperature independent.

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.

In situ substrate preparation for high-quality SiC chemical vapour deposition

Abstract In situ preparation of 4H and 6H silicon carbide substrate surfaces in hydrogen and hydrogen-propane etching systems has been studied. The etching of on-axis(0001) 6H-SiC substrates resulted in regular straight terraces and one unit high steps. The etching of on-axis (0001) 4H-SiC substrates resulted in broad terraces interrupted by large step formations. The 4H- and 6H-SiC (0001) off-axis substrates (3.5° towards 〈112¯0〉 yield smooth etched surfaces with the exception of stripe-like defects on the 4H polytype which are shown to be related to stacking-faults. The stacking faults are suggested to be a cause for step-bunching and surface roughening. Hydrogen-etching prior to growth has been shown to improve the epitaxial layer quality both concerning defect formation and step-bunching.

Growth of SiC by "Hot-Wall" CVD and HTCVD

A reactor concept for the growth of high-quality epitaxial SiC films has been investigated. The reactor concept is based on a hot-wall type susceptor which, due to the unique design, is very power efficient. Four different susceptors are discussed in terms of quality and uniformity of the grown material. The films are grown using the silane–propane–hydrogen system on off-axis (0001) 6H- and 4H-SiC substrates. Layers with doping levels in the low 1014 cm—3 showing strong free exciton emission in the photoluminescence spectra may readily be grown reproducibly in this system. The quality of the grown layers is also confirmed by the room temperature minority carrier lifetimes in the microsecond range and the optically detected cyclotron resonance data which give mobilities in excess of 100000 cm2/Vs at 6 K. Finally, a brief description will be given of the HTCVD technique which shows promising results in terms of high quality material grown at high growth rates.

High temperature chemical vapor deposition of SiC

A growth process has been investigated for the epitaxial growth of silicon carbide. The technique can simply be described as chemical vapor deposition (CVD) at high temperatures, hence the name high temperature CVD (HTCVD). The growth process however, differs greatly from that of the CVD process due to the significant sublimation and etch rates at the extreme growth temperatures (1800–2300°C). The grown rates obtained with the HTCVD are in the order of several tens of μm/h to 0.5 mm/h. The purity and crystallinity of the growth layers are outstanding showing strong free exciton related photoluminescence.

Nitrogen doping concentration as determined by photoluminescence in 4H– and 6H–SiC

Low‐temperature photoluminescence (PL) spectroscopy is used for determination of the nitrogen doping concentration in noncompensated 4H– and 6H–SiC by comparing the intensity of nitrogen‐bound exciton (BE) lines to that of the free exciton (FE), the latter being used as an internal reference. The results are compared with a previous work performed for the case of 6H–SiC only. A line‐fitting procedure with the proper line shapes is used to determine the contribution of the BE and FE lines in the PL spectrum. The ratio of the BE zero‐phonon lines (R0 and S0 in 6H, Q0 in 4H) to the FE most intensive phonon replica around 77 meV exhibits very well a direct proportional dependence on the doping as determined by capacitance–voltage (C–V) measurements for both polytypes. The use of fitting procedure which takes into account the real line shapes, the influence of the spectrometer transfer function, and the structure of the PL spectrum in the vicinity of the FE replica allows us determination of the N‐doping conce...

Electron effective masses in 4H SiC

Results from optically detected cyclotron resonance (ODCR) studies of electron effective masses in 4H SiC are reported. ODCR measurements were performed on high‐purity n‐type 4H SiC epitaxial layers grown by chemical vapor deposition at both X band (9.23 GHz) and Q band (35.05 GHz) microwave frequencies. Electron effective masses in 4H SiC were directly determined as m⊥*=0.42m0 and m∥*=0.29m0. A scattering time in the basal plane τ⊥≊4.3×10−11 s, and hence, the corresponding electron mobility μ⊥≊1.8×105 cm2/V s, was obtained from a fit of the ODCR line shape.

The minority carrier lifetime of n‐type 4H‐ and 6H‐SiC epitaxial layers

The minority carrier lifetime has been measured on n‐type 6H‐ and 4H‐SiC epitaxial layers. We observe inherently longer lifetimes in 4H layers compared to 6H‐SiC layers. A value as high as 2.1 μs has been measured at room temperature in 4H‐SiC, however, large variations may be observed over the surface. The lifetime increases with temperature and at a typical operating temperature of a device the lifetime is close to 5 μs. The lifetime appears to be correlated with the morphology of the epitaxial film showing that the lifetime limiting defect may be related to a crystalline imperfection. A strong correlation can also be seen with the thickness of the epitaxial layers.

Electron effective masses and mobilities in high‐purity 6H–SiC chemical vapor deposition layers

The first observation of cyclotron resonance in 6H‐SiC by optically detected cyclotron resonance (ODCR) spectroscopy at X‐band microwave frequency is reported. High purity undoped, n‐type 6H‐SiC layers grown by chemical vapor deposition (CVD), with residual doping concentrations in the 1014–1015 cm−3 range, were investigated. Effective mass values were determined as m*⊥=(0.42±0.02)m0 and m*∥=(2.0±0.2)m0. From the fit of the ODCR line shape, a remarkably high mobility at 6 K was deduced: μ⊥≊1.1×105 cm2/V s for electrons in the basal plane. The anisotropy of the effective mass and the carrier mobility is discussed in comparison with previously reported data.

Chloride-based CVD growth of silicon carbide for electronic applications.

Chloride-Based CVD Growth of Silicon Carbide for Electronic Applications Henrik Pedersen,* Stefano Leone, Olof Kordina, Anne Henry, Shin-ichi Nishizawa, Yaroslav Koshka, and Erik Janz en Department of Physics, Chemistry and Biology, Link€oping University, SE-581 83 Link€oping, Sweden National Institute of Advanced Industrial Science and Technology (AIST), Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan Department of Electrical and Computer Engineering, Mississippi State University, Mississippi State, Mississippi 39762, United States