Blair R. Tuttle

Silicon carbide: A unique platform for metal-oxide-semiconductor physics

A sustainable energy future requires power electronics that can enable significantly higher efficiencies in the generation, distribution, and usage of electrical energy. Silicon carbide (4H-SiC) is one of the most technologically advanced wide bandgap semiconductor that can outperform conventional silicon in terms of power handling, maximum operating temperature, and power conversion efficiency in power modules. While SiC Schottky diode is a mature technology, SiC power Metal Oxide Semiconductor Field Effect Transistors are relatively novel and there is large room for performance improvement. Specifically, major initiatives are under way to improve the inversion channel mobility and gate oxide stability in order to further reduce the on-resistance and enhance the gate reliability. Both problems relate to the defects near the SiO2/SiC interface, which have been the focus of intensive studies for more than a decade. Here we review research on the SiC MOS physics and technology, including its brief history, the state-of-art, and the latest progress in this field. We focus on the two main scientific problems, namely, low channel mobility and bias temperature instability. The possible mechanisms behind these issues are discussed at the device physics level as well as the atomic scale, with the support of published physical analysis and theoretical studies results. Some of the most exciting recent progress in interface engineering for improving the channel mobility and fundamental understanding of channel transport is reviewed.

Microscopic theory of hydrogen in silicon devices

Incorporation of hydrogen has a strong effect on the characteristics of silicon devices. A fundamental understanding of the microscopic mechanisms is required in order to monitor and control the behavior of hydrogen. First-principles calculations have been instrumental in providing such understanding. We first outline the basic principles that govern the interaction between hydrogen and silicon, followed by an overview of recent first-principles results for hydrogen interactions with silicon. We show that H/sub 2/ molecules are far less inert than previously assumed. We then discuss results for motion of hydrogen through the material, as relating to diffusion and defect formation. We also discuss the enhanced stability of Si-D compared to Si-H bonds, which may provide a means of suppressing defect generation. We present a microscopic mechanism for hydrogen-hydrogen exchange, and examine the metastable /spl equiv/SiH/sub 2/ complex formed during the exchange process. Throughout, we highlight issues relevant for hydrogen in amorphous silicon (used in solar cells, sensors and displays) and in Si-SiO/sub 2/ structures (used in integrated circuits). The broader impact of first-principles calculations on computational electronics will also be discussed.

Atomic state and characterization of nitrogen at the SiC/SiO2 interface

We report on the concentration, chemical bonding, and etching behavior of N at the SiC(0001)/SiO2 interface using photoemission, ion scattering, and computational modeling. For standard NO processing of a SiC MOSFET, a sub-monolayer of nitrogen is found in a thin inter-layer between the substrate and the gate oxide (SiO2). Photoemission shows one main nitrogen related core-level peak with two broad, higher energy satellites. Comparison to theory indicates that the main peak is assigned to nitrogen bound with three silicon neighbors, with second nearest neighbors including carbon, nitrogen, and oxygen atoms. Surprisingly, N remains at the surface after the oxide was completely etched by a buffered HF solution. This is in striking contrast to the behavior of Si(100) undergoing the same etching process. We conclude that N is bound directly to the substrate SiC, or incorporated within the first layers of SiC, as opposed to bonding within the oxide network. These observations provide insights into the chemistr...

A Quantitative Model for ELDRS and

A physics-based TCAD model for enhanced low-dose-rate sensitivity in linear bipolar devices is developed. Quantitative agreement is found with measured data over a wide range of dose rates and H2 concentrations. Analysis of the degradation effects of individual defect types, the implementation of which has been informed by first principles calculations, provides insights into the mechanisms behind enhanced low-dose-rate effects in different hydrogen environments. The effects of initial defect concentration and location and the energetics of the defect-related reactions are explored. Conclusions are drawn about the roles of molecular hydrogen and hydrogenated defects in the radiation response of these devices.

{\rm H}_{2}

Interface-trap buildup and annealing are examined as a function of temperature, dose rate, and H2 concentration using a physics-based model. The roles of a number of common oxide defects in radiation-induced interface-trap buildup are evaluated under various conditions. Defects near the interface play a significant role in determining interface-trap buildup by trapping protons as proton concentration and temperature increase.

Degradation Effects in Irradiated Oxides Based on First Principles Calculations

A model for radiation-induced interface-trap buildup distinguishes among the contributions of hydrogen dimerization, electron recombination, and electric field mechanisms, quantitatively explaining time-dependent and true dose rate effects in irradiated bipolar isolation oxides. Hydrogen dimerization is the dominant ELDRS mechanism for devices exposed to medium H2 concentrations (1% per volume), whereas H2 cracking dominates as H2 concentration is increased further. Electron recombination mechanisms contribute at high dose rates ( >; 100 rad(SiO2)/s), but are not the dominant ELDRS mechanism at dose rates lower than 100 rad(SiO2)/s).

The Effects of Proton-Defect Interactions on Radiation-Induced Interface-Trap Formation and Annealing

Both enhanced interface trap buildup and interface trap annealing can occur during elevated temperature irradiation (ETI), depending on the temperature, total dose, and dose rate. In this paper we describe mechanisms that govern the rate-limiting processes of interface trap buildup and annealing during ETI. Hydrogenated oxygen vacancies can facilitate hydrogen dimerization at elevated temperatures. This results in the removal of protons that can create interface traps, so degradation is suppressed. Hydrogen dimerization becomes more competitive with degradation mechanisms as the concentration of protons near the interface increases and/or as temperature increases.

Mechanisms Separating Time-Dependent and True Dose-Rate Effects in Irradiated Bipolar Oxides

Oxidation is widely used to fabricate complex materials and structures, controlling the properties of both the oxide and its interfaces. It is commonly assumed that the majority diffusing species in the oxide is the dominant oxidant, as is for Si oxidation. It is not possible, however, to account for the experimental data of SiC oxidation using such an assumption. We report first-principles calculations of the pertinent atomic-scale processes, account for the observations, and demonstrate that, for Si-face SiC, interface bonding dictates that atomic oxygen, the minority diffusing species, is the dominant oxidant.

Mechanisms of Interface Trap Buildup and Annealing During Elevated Temperature Irradiation

The authors present an overview of recent results for hydrogen interactions with amorphous silicon (a-Si), based on first-principles calculations. They review the current understanding regarding molecular hydrogen, and show that H{sub 2} molecules are far less inert than previously assumed. They then discuss results for motion of hydrogen through the material, as relating to diffusion and defect formation. They present a microscopic mechanism for hydrogen-hydrogen exchange, and examine the metastable {triple_bond}SiH{sub 2} complex formed during the exchange process. They also discuss the enhanced stability of Si-D compared to Si-H bonds, which may provide a means of suppressing light-induced defect generation.

Competing atomic and molecular mechanisms of thermal oxidation—SiC versus Si

We use density functional calculations to elucidate the effects of strain on the electronic properties of 4H-SiC. Both compressive and tensile uniaxial strain result in a smaller energy gap and splitting of the conduction band valleys. Compared to compressive strain, tensile strain results in larger valley splitting and larger changes to the electron effective masses. For strain larger than 1.5%, in one hexagonal direction, the important conductivity mass can be reduced by more than 50%. For biaxial tensile strain, we also observe effective mass changes similar to the uniaxial results.