Papers by Author: Leonard C. Feldman

Abstract: Temperature dependent capacitance-voltage (C-V) and constant capacitance transient spectroscopy (CCDLTS) measurements have been performed to investigate the role of N in improving the transport properties of 4H-SiC MOS transistors. The higher channel mobility in the N pre-implanted transistors is due at least in part to activation of a small fraction of the implanted N near the SiO₂/SiC interface as donors in SiC during oxidation, thus reducing the effects of interface trapping. In addition, the absence of oxidation-induced near-interface defects, which were observed in NO-annealed capacitors, may contribute to the improved mobility in N pre-implanted transistors.

Abstract: Phosphorous passivation of the SiO2/4H-SiC interface lowers the interface trap density and increases the field effect mobility for n-channel MOSFETs to twice the value of 30-40cm2/V-s obtained using standard NO nitridation. Passivation using P2O5 introduced with an SiP2O7 planar diffusion source (PDS) converts the oxide layer to phosphosilicate glass (PSG) which is a polar material. BTS (bias‐temperature‐stress) measurements with MOS capacitors and FETs show that the benefits of reduced interface trap density and increased mobility are offset by unstable flat band and threshold voltages. This instability can be removed by etching away the PSG oxide and depositing a replacement SiO2 layer. However, trap densities for etched MOS capacitors are "NO-like" (i.e., higher), which would lead one to expect a lower mobility if MOSFETs are fabricated with the PSG / etch / deposited oxide process.

Abstract: We report on the benefits and the shortcomings of the NO annealing process following observations made on capacitors and transistors with various nitrogen densities at the SiO2/SiC interface. While NO annealing leads to a progressively lower interface state density and higher inversion mobility, consistent with Coulomb-limited transport, MOSFET properties are still limited by the relatively poor interface quality. Moreover, NO induces a large amount of hole traps in the oxide. We establish that these properties are not related to the oxidation rate and we discuss them in terms of the nitrogen content.

Abstract: The electrical properties of the SiC/SiO2 interface resulting from oxidation of the n-type 6H-SiC polytype were studied by hi-lo CV, temperature dependent CV and constant capacitance deep level transient spectroscopy (CCDLTS) techniques. Several trap species differing in energy and capture cross section were identified. A trap distribution at 0.5 eV below the 6H-SiC conduction band energy and a shallower density of states in both the 6H and 4H polytyes are passivated by post-oxidation NO annealing. However, other ultra-shallow and deeper defect distributions remain after nitridation. The latter may originate from semiconductor traps.

Abstract: The SiO2/SiC interface is characterized for carbon accumulation using the carbon isotope 13C as a marker layer combined with secondary ion mass spectroscopy (SIMS). SiC was epitaxially grown using an isotopically enriched propane source and subsequently oxidized to a thickness required to consume the entire 13C layer. Mass specific depth profiles through the oxide film yield residual carbon concentrations at or below 3x1011 cm-2. The depth resolution of SIMS and natural abundance of 13C in the bulk SiC film limit sensitivity but allow us to set a limit of 2.5x1014 cm-2 carbon build up at or near the interface.

Abstract: We report on the effect of nitridation on the negative and positive charge buildup in SiC gate oxides during carrier injection. We observe that the incorporation of nitrogen at the SiO2/SiC interface can enhance the reliability of the interface by suppressing the generation of interface states upon electron injection but that it can also degrade the oxide by creating additional hole traps. We relate these phenomena to the passivation of atomic-level defects by nitrogen.

Abstract: Post-oxidation anneals that introduce nitrogen at the SiO2/4H-SiC interface have been most effective in reducing the large interface trap density near the 4H-SiC conduction band-edge for (0001) Si face 4H-SiC. Herein, we report the effect of nitridation on interfaces created on the (11 20) a-face and the (0001) C-face of 4H-SiC. Significant reductions in trap density (from >1013 cm-2 eV-1 to ~ 1012 cm-2 eV-1 at EC-E ~0.1 eV) were observed for these different interfaces, indicating the presence of substantial nitrogen susceptible defects for all crystal faces. Annealing nitridated interfaces in hydrogen results in a further reduction of trap density (from ~1012 cm-2 eV-1 to ~5 x 1011 cm-2 eV-1 at EC-E ~0.1 eV). Using sequential anneals in NO and H2, maximum field effect mobilities of ~55 cm-2 V-1s-1 and ~100 cm-2 V-1s-1 have been obtained for lateral MOSFETs fabricated on the (0001) and (11 20) faces, respectively. These electronic measurements have been correlated to the interface chemical composition.

Abstract: Nickel ohmic contacts to p-type epitaxial and heavily implanted 4H-SiC are described. Room and elevated temperature results are presented. Elevated temperature measurements of specific contact resistance are compared to theoretical calculations. The calculations require the acceptor doping concentration and the contact’s barrier height. Epitaxial material has a known acceptor value thereby allowing the barrier height to be deduced by requiring agreement between the calculated and measured values of the contact resistance. Calculations of the contact resistance for implanted material use the barrier height from the epitaxial results along with a variable activated acceptor doping concentration which is adjusted to give agreement with measured room temperature specific contact resistances. Specific contact resistances as low as 7x10-6 ohm-cm2 fabricated on the Si face have been obtained to epitaxial 4H p-type material whereas contacts to implanted material result in much larger contact resistance values of 4x10-5 ohm-cm2. These results, when compared to theoretical calculations, indicate that activated acceptor doping concentrations in heavily implanted material are on the order of 2% of the implant concentration.

Abstract: Silicon has been the semiconductor of choice for microelectronics largely because of the unique properties of its native oxide (SiO2) and the Si/SiO2 interface. For high-temperature and/or high-power applications, however, one needs a semiconductor with a wider energy gap and higher thermal conductivity. Silicon carbide has the right properties and the same native oxide as Si. However, in the late 1990’s it was found that the SiC/SiO2 interface had high interface trap densities, resulting in poor electron mobilities. Annealing in hydrogen, which is key to the quality of Si/SiO2 interfaces, proved ineffective. This paper presents a synthesis of theoretical and experimental work by the authors in the last six years and parallel work in the literature. High-quality SiC/SiO2 interfaces were achieved by annealing in NO gas and monatomic H. The key elements that lead to highquality Si/SiO2 interfaces and low-quality SiC/SiO2 interfaces are identified and the role of N and H treatments is described. More specifically, optimal Si and SiC surfaces for oxidation are identified and the atomic-scale processes of oxidation and resulting interface defects are described. In the case of SiC, we conclude that excess carbon at the SiC/SiO2 interface leads to a bonded Si-C-O interlayer with a mix of fourfold- and threefold-coordinated C and Si atoms. The threefold coordinated atoms are responsible for the high interface trap density and can be eliminated either by H-passivation or replacement by N. Residual Si-Si bonds, which are partially passivated by H and N remain the main limitation. Perspectives for the future for both Si- and SiC-based MOSFETs are discussed.

Abstract: The effect of thermal treatments in nitric oxide (NO) on the paramagnetic defects at the 4H-SiC/SiO2 interface are analyzed by EPR in oxidized porous samples. The results on ultrathin thermal oxides show that the NO treatment at 1000°C is insufficient for an efficient reduction of the two dominant paramagnetic interface defects: PbC centers and carbon clusters. From the NRA and XPS analysis of bulk samples treated under the same conditions we attribute the weak effect to the low nitrogen concentration of only 1% at the interface.