Florin Ciobanu

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.

Al2O3 prepared by atomic layer deposition as gate dielectric on 6H-SiC(0001)

Al2O3 films were deposited as alternative gate dielectric on hydrogen-terminated 6H-SiC(0001) by atomic layer chemical vapor deposition and characterized by photoelectron spectroscopy (PES) and admittance measurements. The PES results indicate an abrupt interface free of significant Si–suboxide contributions where the Al2O3 layer is connected to SiC by bridging oxygen atoms. The admittance measurements yield an interface state density which is lower than that of the thermally formed oxide and show in particular no increase toward the conduction band edge. Furthermore, a nearly symmetrical band alignment of Al2O3 on 6H-SiC with offsets of 2.2 and 1.8 eV is determined for the valence and conduction bands, respectively. This makes Al2O3 a serious competitor to thermal oxides as gate insulator in SiC devices.

Low Density of Interface States in n-Type 4H-SiC MOS Capacitors Achieved by Nitrogen Implantation

A surface-near Gaussian nitrogen (N) profile is implanted into n-type 4H-SiC epilayers prior to a standard oxidation process. Depending on the depth of the oxidized layer and on the implanted N concentration, the density of interface states DIT determined in corresponding 4H-SiC MOS capacitors decreases to a minimum value of approx. 1010 cm-2eV-1 in the investigated energy range (EC-(0.1 eV to 0.6 eV)), while the flat-band voltage increases to negative values due to generated fixed positive charges. A thin surface-near layer, which is highly N-doped during the chemical vapour deposition growth, leads to a reduction of DIT only close to the conduction band edge.

SiC MATERIAL PROPERTIES

This chapter briefly summarizes device-relevant material properties of the wide bandgap semiconductor silicon carbide. The polytypes 4H-, 6H- and 3C-SiC are predominantly considered. These SiC polytypes can reproducibly be grown as single crystals; they have superior electronic and thermal material properties. The conductivity type can be adjusted by shallow donors and acceptors. Special sections are related to the diffusion of dopants, to the impurity conduction, to the minority carrier lifetime and to the different types of traps generated at the interface of thermally grown SiC/SiO2 structures.

Traps at the Interface of 3C-SiC/SiO2-MOS-Structures

Conductance method and admittance spectroscopy are employed to monitor t raps at the interface of differently processed 3Cand 4H-SiC/SiO 2 MOS capacitors. It is demonstrated that oxidation of 3C-SiC under NO-ambient leads to low values of the densi ty of interface states in the entire SiC band gap.

SiC/SiO2 Interface States: Properties and Models

Properties of defects encountered at the oxidized surfaces of silicon carbide (SiC) suggest their origin to be different from the dangling-bond-type defects commonly observed in the oxidized silicon. Among different models of these SiC/oxide interface states advanced during the past decade, two have received substantial experimental support. This first one is the “carbon cluster” model, which ascribes the traps with energy levels in the SiC bandgap to inclusions of elemental carbon formed during the SiC surface treatment and subsequent oxidation. The second model invokes intrinsic defects of SiO2 to account for the high density of interface states in the energy range close to the conduction band of SiC. Achievements in reducing the SiC/SiO2 defect density are discussed.

Electrical and Optical Characterization of SiC

Three topics are reported in this paper: (a) the determination of a temperaturedependent Hall scattering factor for holes r H,h(T) in 4H-SiC, (b) the detection of shallow Al-related defect centers in Al-doped, p-type 6H-/3C-SiC; these defects are generated either by i mplantation of any ion species or by an oxidation process and (c) the observation of absorption lines in infra red (IR) spectra, which are due to phosphorus donors in 6H-SiC.

Traps at the SiC/SiO 2 -Interface

The density of interface states Dit at SiC/SiO 2 interfaces of different SiC polytypes (4H-, 6H- and 15R-SiC) is monitored and the origin of these states is discussed. The hydrogenation behavior of interface states in the temperature range from 250°C to 1000°C is studied by C-V and G-V investigations. The strong increase of Dit close to the 4H-SiC conduction band is attributed to defects located in the oxide (so-called “Near Interface Traps”).

Nitrogen Implantation - An Alternative Technique to Reduce Traps at SiC/SiO2-Interfaces

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.