Tang Yidan

Influences of high-temperature annealing on atomic layer deposited Al2O3/4H-SiC

High-temperature annealing of the atomic layer deposition (ALD) of Al2O3 films on 4H-SiC in O2 atmosphere is studied with temperature ranging from 800 °C to 1000 °C. It is observed that the surface morphology of Al2O3 films annealed at 800 °C and 900 °C is pretty good, while the surface of the sample annealed at 1000 °C becomes bumpy. Grazing incidence X-ray diffraction (GIXRD) measurements demonstrate that the as-grown films are amorphous and begin to crystallize at 900 °C. Furthermore, C—V measurements exhibit improved interface characterization after annealing, especially for samples annealed at 900 °C and 1000 °C. It is indicated that high-temperature annealing in O2 atmosphere can improve the interface of Al2O3/SiC and annealing at 900 °C would be an optimum condition for surface morphology, dielectric quality, and interface states.

Improved adhesion and interface ohmic contact on n-type 4H-SiC substrate by using Ni/Ti/Ni

The Ni/Ti/Ni multilayer ohmic contact properties on a 4H-SiC substrate and improved adhesion with the Ti/Au overlayer have been investigated. The best specific contact resistivity of 3.16 × 10−5 Ωcm2 was obtained at 1050 °C. Compared with Ni/SiC ohmic contact, the adhesion between Ni/Ti/Ni/SiC and the Ti/Au overlayer was greatly improved and the physical mechanism under this behavior was analyzed by using Raman spectroscopy and X-ray energy dispersive spectroscopy (EDS) measurement. It is shown that a Ti-carbide and Ni-silicide compound exist at the surface and there is no graphitic carbon at the surface of the Ni/Ti/Ni structure by Raman spectroscopy, while a large amount of graphitic carbon appears at the surface of the Ni/SiC structure, which results in its bad adhesion. Moreover, the interface of the Ni/Ti/Ni/SiC is improved compared to the interface of Ni/SiC.

Charge trapping behavior and its origin in Al2O3/SiC MIS system

Charge trapping behavior and its origin in Al2O3/SiC MOS structure are investigated by analyzing the capacitance–voltage (C–V) hysteresis and the chemical composition of the interface. The C–V hysteresis is measured as a function of oxide thickness series for an Al2O3/SiC MIS capacitor. The distribution of the trapped charges, extracted from the C–V curves, is found to mainly follow a sheet charge model rather than a bulk charge model. Therefore, the electron injection phenomenon is evaluated by using linear fitting. It is found that most of the trapped charges are not distributed exactly at the interface but are located in the bulk of the Al2O3 layers, especially close to the border. Furthermore, there is no detectable oxide interface layer in the x-ray photoelectron spectroscope (XPS) and transmission electron microscope (TEM) measurements. In addition, Rutherford back scattering (RBS) analysis shows that the width of the Al2O3/SiC interface is less than 1 nm. It could be concluded that the charge trapping sites in Al2O3/SiC structure might mainly originate from the border traps in Al2O3 film rather than the interface traps in the interfacial transition layer.

Effect of annealing on the characteristics of Ti/Al ohmic contacts to p-Type 4H-SiC

Ti/Al contacts deposited on p-type epilayer doped with Al at 2×1019 cm-3 are reported. The current-voltage curves of Ti/Al contacts annealed at different temperatures from 800 to 1000 °C were measured, which provided the specific contact resistances (SCRs) of 6.59×10-5 Ω/cm2 and 7.81×10-5Ω/cm2 after annealing at 900°C for 5min and 950°C for 2min, respectively. The microstructures of Ti/Al contact on P-type 4H–SiC were investigated by X-ray diffraction (XRD). The results of XRD show that the phases of Ti3SiC2 was formed at the metal/SiC interface after annealing, which could be effective to ohmic contacts on P-type 4H-SiC. The quantitative phase analysis were also discussed, which show that the phase composition of Ti3SiC2is key factor for low resistance to P-type 4H–SiC. Moreover, simulations proved that the gradual Ti3SiC2ISL reduces or eliminates the effective barrier height at the metal/Ti3SiC2/p-type and may also contribute to low contact resistivity.

Compact 2×2 Multi-Mode Interference Couplers with Uneven Splitting-Ratios Based on Silicon Nanowires

Two types of uneven splitting-ratio 2×2 multi-mode interference (MMI) couplers based on silicon nanowires are designed, fabricated and characterized. The splitting ratios are 85:15 and 72:28, respectively. The devices have compact sizes and low excess losses. The footprints of the rectangular MMI region are only about 3μm×18μm and 3μm×14μm, and the minimum excess losses (ELs) are 1.30 dB and 0.82 dB. The measured splitting-ratios are consistent with the designed values. Based on their performance, these 2×2 MMI couplers are suitable candidates for the coupling section of microring resonators where a large resonance bandwidth is required for high speed signal processing. The uneven splitting capability also provides a convenient way to further optimize the Q factor and the bandwidth of the resonator.