Atthawut Chanthaphan

Synchrotron x-ray photoelectron spectroscopy study on thermally grown SiO2/4H-SiC(0001) interface and its correlation with electrical properties

The correlation between atomic structure and the electrical properties of thermally grown SiO2/4H-SiC(0001) interfaces was investigated by synchrotron x-ray photoelectron spectroscopy together with electrical measurements of SiC-MOS capacitors. We found that the oxide interface was dominated by Si-O bonds and that there existed no distinct C-rich layer beneath the SiC substrate despite literature. In contrast, intermediate oxide states in Si core-level spectra attributable to atomic scale roughness and imperfection just at the oxide interface increased as thermal oxidation progressed. Electrical characterization of corresponding SiC-MOS capacitors also indicated an accumulation of both negative fixed charges and interface defects, which correlates well with the structural change in the oxide interface and provides insight into the electrical degradation of thermally grown SiC-MOS devices.

Investigation of unusual mobile ion effects in thermally grown SiO2 on 4H-SiC(0001) at high temperatures

Generation and elimination of mobile ions in thermally grown SiO2 on 4H-SiC(0001) were systematically investigated by electrical measurements of MOS capacitors. In contrast to a SiO2/Si system, intrinsic positive mobile ions were found to exist in as-oxidized SiO2/SiC structures, leading to significant instability of SiC-MOS devices. Post-oxidation annealing in Ar ambient mostly eliminates the mobile ions, but they are generated again by subsequent high-temperature hydrogen annealing despite the improved interface quality. The density of the mobile ions was estimated to be several 1012 cm−2. Possible physical origins of the mobile ions are discussed on the basis of the experimental findings.

Study of SiO2/4H-SiC interface nitridation by post-oxidation annealing in pure nitrogen gas

An alternative and effective method to perform interface nitridation for 4H-SiC metal-oxide-semiconductor (MOS) devices was developed. We found that the high-temperature post-oxidation annealing (POA) in N2 ambient was beneficial to incorporate a sufficient amount of nitrogen atoms directly into thermal SiO2/SiC interfaces. Although N2-POA was ineffective for samples with thick thermal oxide layers, interface nitridation using N2-POA was achieved under certain conditions, i.e., thin SiO2 layers ( 1350°C). Electrical characterizations of SiC-MOS capacitors treated with high-temperature N2-POA revealed the same evidence of slow trap passivation and fast trap generation that occurred in NO-treated devices fabricated with the optimized nitridation conditions.

Improved bias-temperature instability characteristics in SiC metal-oxide-semiconductor devices with aluminum oxynitride dielectrics

Significant improvement of bias-temperature instability characteristics in SiC-based metal-oxide-semiconductor (MOS) devices was demonstrated with high-permittivity aluminum oxynitride (AlON) dielectrics deposited on thin thermal oxides. AlON/SiO2 stacked dielectrics were found to be beneficial not only for reducing gate leakage current but also for suppressing diffusion of positively charged ions, leading to stable SiC-MOS characteristics even under strong electric fields and high temperatures. Unlike the prompt electric-field-induced ion migration in thermally grown and sputter-deposited SiO2 dielectrics, the ion drift for the stacked gate dielectrics was confined within the thin SiO2 underlayers owing to low ion diffusivity in AlON layers. Impacts of mobile ions on interface properties of SiC-MOS devices and effects of intentional ion trapping within the AlON layers were also systematically investigated.

Understanding and controlling bias-temperature instability in SiC metal-oxide-semiconductor devices induced by unusual generation of mobile ions

Unusual behavior of bias-temperature instabilities in SiC metal-oxide-semiconductor (MOS) devices is studied. Electrical measurements of SiC-MOS capacitors are used to investigate details of self-generated mobile ions in thermal oxides on 4H-SiC(0001) substrates, such as their polarity, density, distribution, and impact on interface properties. It is found that positive bias-temperature stress (BTS) accumulates self-generated positive mobile ions at the bottom SiO2/SiC interface with an areal density of several 1012 cm−2, and that they induce additional electron trap formation at the interface. Using this knowledge, we demonstrate effective removal of the positive mobile ions with a combination of negative BTS and subsequent etching of the oxide surface.

Cathodoluminescence study of radiative interface defects in thermally grown SiO2/4H-SiC(0001) structures

Radiative defects in thermally grown SiO2/4H-SiC(0001) structures and their location in depth were investigated by means of cathodoluminescence spectroscopy. It was found that while luminescence peaks ascribed to oxygen vacancy and nonbridging oxygen hole centers were observed both from thermal oxides grown on (0001) Si-face and C-face surfaces as with thermal oxides on Si, intense yellow luminescence at a wavelength of around 600 nm was identified only from the oxide interface on the Si-face substrate regardless of the oxide thickness and dopant type. Possible physical origins of the radiative centers localized near an oxide interface of a few nm thick are discussed on the basis of visible light emission from Si backbone structures.

Impact of Interface Defect Passivation on Conduction Band Offset at SiO2/4H-SiC Interface

The change in energy band alignment of thermally grown SiO2/4H-SiC(0001) structures due to an interface defect passivation treatment was investigated by means of synchrotron radiation photoelectron spectroscopy (SR-PES) and electrical characterization. Although both negative fixed charge and interface state density in SiO2/SiC structures were effectively reduced by high-temparature hydrogen gas annealing (FGA), the conduction band offset (ΔEc) at the SiO2/SiC interface was found to be decreased by about 0.1 eV after FGA. In addition, a subsequent vacuum annealing to induce hydrogen desorption from the interface resulted in not only a slight degradation in interface property but also a partial recovery of ΔEc value. These results indicate that the hydrogen passivation of negatively charged defects near the thermally grown SiO2/SiC interface causes the reduction in conduction band offset. Therefore, the tradeoff between interface quality and conduction band offset for thermally grown SiO2/SiC MOS structure needs to be considered for developing SiC MOS devices.

Improvement of SiO2/4H-SiC Interface Quality by Post-Oxidation Annealing in N2 at High-Temperatures

The efficient and practical method for SiO2/4H-SiC interface improvement using post-oxidation annealing (POA) in pure N2 ambient was studied by means of x-ray photoelectron spectroscopy (XPS) analysis and electrical characterization. SiC-MOS capacitors with slope-shaped thermal oxides were used to investigate optimal conditions for interface nitridation. It was found that the amount of nitrogen atoms incorporated into the interfaces increased when raised the annealing temperature up to 1400°C, and thin oxide (< 30 nm) was used. Furthermore, N2-POA at 1400°C was proven to be very promising as equivalent to NO-POA in terms of reduced interface state density of SiC-MOS devices.

Degradation of SiO2/SiC Interface Properties due to Mobile Ions Intrinsically Generated by High-Temperature Hydrogen Annealing

The impact of mobile ions intrinsically generated in thermally grown SiO2 by high-temperature forming gas annealing (FGA) on the SiO2/4H-SiC interface properties was studied by means of electrical characterization of SiC metal-oxide-semiconductor (MOS) capacitors. Unlike Si devices, mobile ions located at the interfaces were found to cause a remarkable stretch-out of capacitance-voltage (C-V) curve near the accumulation condition, and the degree of stretch-out was more pronounced with increasing probe frequency. This suggests that the interface states with a long emission time constant are formed near the conduction band edge due to the mobile ions. To clarify this unusual phenomenon, several characterization techniques to evaluate interface state densities (Dit), including Terman, conductance, and C-ψs methods, were employed. The Dit values estimated for SiO2/SiC interfaces with mobile ions were a few times as large as those without mobile ions.

Cathodoluminescence Study of SiO2/4H-SiC Structures Treated with High-Temperature Post-Oxidation Annealing

The radiative defect centers in thermally-grown SiO2/4H-SiC structures with high-temperature post-oxidation annealing (POA) in various ambient gas, i.e. Ar, H2, and NOx, were examined using cathodoluminescence (CL) measurement. It was found that radiative centers with an extremely high luminescent efficiency were remained at the SiO2/SiC interfaces after Ar-POA and FGA. Thus, these defect centers are very stable against high-temperature annealing and reducing ambient. In contrast, NOx-POA significantly reduced amounts of the radiative defects that might be related to channel mobility improvement in SiC-MOSFETs.