Gerhard Pensl

Intrinsic SiC/SiO2 Interface States

The energy distribution of electron states at SiC/SiO 2 interfaces produced by oxidation of various (3C, 4H, 6H) SiC polytypes is studied by electrical analysis techniques and internal photoemission spectroscopy. A similar distribution of interface traps over the SiC bandgap is observed for different polytypes indicating a common nature of interfacial defects. Carbon clusters at the SiC/SiO 2 interface and near-interfacial defects in the SiO 2 are proposed to be responsible for the dominant portion of interface traps, while contributions caused by dopant-related defects and dangling bonds at the SiC surface are not observed.

Deep Defect Centers in Silicon Carbide Monitored with Deep Level Transient Spectroscopy

Electrical data obtained from deep level transient spectroscopy investigations on deep defect centers in the 3C, 4H, and 6H SiC polytypes are reviewed. Emphasis is put on intrinsic defect centers observed in as-grown material and subsequent to ion implantation or electron irradiation as well as on defect centers caused by doping with or implantation of transition metals (vanadium, titanium, chromium, and scandium).

Electrical and optical characterization of SiC

Abstract We review the currentuse of electrical and optical methods to study the semiconducting properties of SiC. More specifically we treat Hall measurements, deep-level transient spectroscopy, infrared absorption and luminescence. Some very recent results, not yet available in the literature, on donor and acceptor levels in 3C-SiC, 4H-SiC and 6H-SiC are discussed.

Band offsets and electronic structure of SiC/SiO2 interfaces

The electronic structure of SiC/SiO2 interfaces was studied for different SiC polytypes (3C, 4H, 6H, 15R) using internal photoemission of electrons from the semiconductor into the oxide. The top of the SiC valence band is located 6 eV below the oxide conduction band edge in all the investigated polytypes, while the conduction band offset at the interface depends on the band gap of the particular SiC polytype. In the energy range up to 1.5 eV above the top of the SiC valence band, interface states were found. Their electron spectrum is similar to that of sp2‐bonded carbon clusters in diamond‐like a‐C:H films suggesting the presence of elemental carbon at the SiC/SiO2 interfaces.

Doping of SiC by Implantation of Boron and Aluminum

Experimental studies on aluminum (Al) and boron (B) implantation in 4H/6H SiC are reported; the implantation is conducted at room temperature or elevated temperatures (500 to 700 °C). Both Al and B act as shallow acceptors in SiC. The ionization energy of these acceptors, the hole mobility and the compensation in the implanted layers are obtained from Hall effect investigations. The degree of electrical activity of implanted Al/B atoms is determined as a function of the annealing temperature. Energetically deep centers introduced by the Al + /B + implantation are investigated. The redistribution of implanted Al/B atoms subsequent to anneals and extended lattice defects are monitored. The generation of the B-related D-center is studied by coimplantation of Si/B and C/B, respectively.

Nitrogen donors in 4H‐silicon carbide

Hall‐effect and infrared‐absorption measurements are performed on n‐type 4H‐SiC samples to investigate the energy positions of the ground state and the excited states of the nitrogen donor in the 4H polytype of silicon carbide. Two electrically active levels (Hall effect) and three series of absorption lines (infrared spectra) are assigned to two nitrogen donor species which substitute on the two inequivalent lattice sites (h,k) in 4H‐SiC. Valley‐orbit splitting of the ground‐state level of the nitrogen donors on hexagonal sites (h) is found to be equal to ΔEvo(h)=7.6 meV. It is shown that the energy position of excited states of both nitrogen donors can be calculated by the effective‐mass approximation by assuming anisotropic effective masses m⊥=0.18m0 and m∥=0.22m0. The influence of the two inequivalent lattice sites on the values of ionization energy and valley orbit splitting of the nitrogen donor ground‐state levels is discussed.

Hall effect and infrared absorption measurements on nitrogen donors in 6H‐silicon carbide

Hall effect and infrared absorption measurements of n‐type silicon carbide of the 6H polytype are employed to investigate the energy position of the ground state and excited states of the nitrogen donor. A donor model is proposed that assigns four series of absorption lines to electronic transitions of three donor species residing at three inequivalent lattice sites (h,k1,k2). A valley‐orbit splitting of 12.6 meV is determined for donors on the hexagonal site h. For 2p0, 2p±, 3p0, and 3p± excited states, the effective‐mass approximation is found to hold within experimental errors assuming a transverse and longitudinal effective electron mass of m⊥=(0.24±0.01) m0 and m∥=(0.34±0.02) m0, respectively.

Mechanisms responsible for improvement of 4H-SiC/SiO2 interface properties by nitridation

An analysis of fast and slow traps at the interface of 4H–SiC with oxides grown in O2, N2O, and NO reveals that the dominant positive effect of nitridation is due to a significant reduction of the slow electron trap density. These traps are likely to be related to defects located in the near-interfacial oxide layer. In addition, the analysis confirms that the fast interface states related to clustered carbon are also reduced by nitridation.

Band alignment and defect states at SiC/oxide interfaces

Comparative analysis of the electronic structure of thermally oxidized surfaces of silicon and silicon carbide indicates that in both cases the fundamental (bulk-band-related) spectrum of electron states is established within less than 1 nm distance from the interface plane. The latter suggests an abrupt transition from semiconductor to insulator. However, a large density of interface traps is observed in the oxidized SiC, which are mostly related to the clustering of elemental carbon during oxide growth and to the presence of defects in the near-interfacial oxides. Recent advancements in reducing the adverse effect of these traps suggest that the SiC oxidation technology has not reached its limits yet and fabrication of functional SiC/oxide interfaces is possible.

Boron-related deep centers in 6H-SiC

Abstract6H-silicon carbide layers are grown by a liquid phase epitaxy (LPE) process. The layers are doped with boron either by ion implantation or during the LPE process from a B-doped silicon melt. Deep-level transient spectroscopy (DLTS), admittance spectroscopy and photoluminescence (PL) are used to investigate deep impurity centers. Two electrically active defect centers are detected: the isolated boron acceptor at EB=Ev+0.3eV and the boron-related D-center at ED=Ev+0.58eV. The yellow luminescence observed in these layers is proposed to be due to pair recombination via D-center and nitrogen donor. Formation and origin of the D-center are discussed.