| Silicon carbide (SiC) semiconductor has excellent physical properties, such as a wide band-gap (Eg), large critical breakdown field, high electron saturation velocity, and high thermal conductivity, all of which make this material a "successor" of silicon in the power semiconductor devices field. SiC MOSFETs have excellent properties, as a kind of important power control devices, such as low on resistance, high input impedance, the characteristics of the switch speed, high blocking voltage, which make them have potential use in energy, rail transit, industrial electronics, and automotive electronics. However, SiC MOSFETs have several limitations, such as extremely low inversion channel mobility, poor reliability, and high power consumption, which are mainly attributed to the following reasons: ? the high density of interface traps (D,,) at the SiO2/SiC interface, which can trap and scatter carriers in the inversion channel, thereby rendering a low inversion channel mobility; and ? high specific contact resistance (pc), which can lead to high power consumption and self-heating effect, thereby deteriorating the device properties and leading to poor thermal reliability. As key techniques for preparing SiC MOSFETs, passivation interface traps and improvement metal/SiC Ohmic contact have become research hotspots. Compared with Si, SiC presents much more complex surface states, such as dangling bonds, contaminants, adsorbates, and rough structure. Consequently, the electrical properties of the SiO2/SiC interface and metal/SiC contact are strongly dependent upon the original SiC surface. Therefore, a new SiC passivation technique for improving the electrical properties of the SiO2/SiC interface and metal/SiC contact must be developed.In this work, a low-temperature, low-damage electron cyclotron resonance (ECR) microwave hydrogen-nitrogen mixed plasma (HNP) treatment was employed to passivate the n-type 4H-SiC surface. The properties improvements in the SiO2/SiC interface and the involved mechanisms were studied using HNP, while those in the TiC/SiC Ohmic contact and the involved mechanisms were studied using H plasma treatment (HP) and HNP. The main research contents and results are as outlined follows:(1) A low-temperature, low-damage electron ECR microwave HNP treatment was proposed to passivate the SiC surface. The effects and mechanism of HNP treatment on the structural, chemical, and electronic properties of the surface were investigated by reflection high energy electron diffraction (RHEED), atomic force microscope (AFM), and X-ray photoelectron spectroscopy (XPS) measurements. The RHEED and AFM results indicated that a smooth, ordered, and unreconstructed 4H-SiC surface could be obtained by HNP treatment at 400? for 8 min in the flow rates of 60 seem and 6 sccm for H2 and N2, respectively. The XPS results indicated that the contaminants and adsorbates were removed from the surface, and the surface Fermi level was de-pinned. The calculated surface states density (Ds) was as low as 5.47 × 1010 cm-2·eV-1. The band diagrams provided further insights into the passivation mechanism of the SiC surfaces by the simultaneous effects of H and N. The dangling bonds were replaced by Si-H bonding and anti-bonding states below the valance band (Ev) or above the conduction band (Ec) without any contribution to Ds. The Si-N and SiOxNy bonds introduced a trap level in the bandgap (-0.5 eV above Ev) that could act only as a donor trap, which had less contribution to Ds. Compared with HP treatment, HNP treatment was insensitive to treatment time and was more controllable when passivating SiC surfaces. This process not only overcome the drawbacks of lattice damage and surface reconstruction that are caused by traditional processes but also produces a chemically and electronically passivated SiC surface. An ideal surface is beneficial for improving the properties of the SiO2/SiC interface and metal/SiC contact.(2) An ECR microwave HNP pretreatment for the 4H-SiC surface combined with HNP post-oxidation annealing (POA) was proposed to improve the SiO2/SiC interface properties. The effects on the SiO2/SiC interface properties and the involved mechanism were investigated by current-voltage (I-V), capacitance-voltage (C-V), AFM, XPS, and secondary ion mass spectrometer (SIMS) measurements. The I-V and C-V results indicated that HNP surface pretreatment effectively enhanced the oxide insulating properties and reduced the density of interface traps (Dit). Combined with POA, the oxide insulating properties (as high as 11.1 MV/cm) and SiC MOS reliability were further enhanced, whereas Dit was further decreased because of the passivation of the formed defects after oxidation. The AFM and XPS results indicated that decreased Dit was closely correlated with interface flattening because of surface flattening and surface state (contaminants, adsorbates, and dangling bonds) reduction. The SIMS results revealed that the depth profile of C with HNP surface pretreatment was significantly steep near the interface (from 4.5 nm to 3.7 nm), which indicated that the distribution of C became narrow, and that C was effectively suppressed by H and N atoms during oxidation. By combining the effects of HNP surface pretreatment and HNP POA, the distribution of C was further reduced (3.1 nm) by the further passivation of the formed defects after oxidation. H and N passivation shifted the defects from the shallow level to the deep level, or no levels in Eg, thereby decreasing Dit in the upper half of SiC Eg. This process provides a new passivation of the interface states by the simultaneous effects of HNP surface pretreatment and HNP POA. The correlation among passivation, SiC surface properties, SiO2/SiC interface properties, and defect levels was established. This result can help explore new interface passivation processes and improve the performance and reliability of SiC MOSFET.(3) An ECR microwave HP/HNP surface pretreatment for 4H-SiC surface combined with N2 POA was proposed for improving the TiC/SiC contact properties. The effects on the TiC/SiC contact properties and the involved mechanism were investigated by ?-?, AFM, XPS, and X-ray diffraction (XRD) measurements. The ?-? results indicated that moderately doped (1 × 1018 cm-3) SiC surfaces were passivated by HP/HNP surface pretreatment to form ideal TiC/SiC Ohmic contacts. After HP surface pretreatment, pc reached as low as 1.5× 10-5 ?·cm2 after low-temperature annealing (600?). The Ohmic behavior was mainly attributed to the low barrier height (q?Bn) at the TiC/SiC contact interface. The AFM, XPS, and XRD results indicated that q?Bn was reduced by the release of pinned Fermi level via surface flattening and SiC surface states reduction after HPT as well as by the generation of donor-type carbon vacancies that reduced the depletionlayer width for electron tunneling after annealing. However, annealing at 800? degraded the Ohmic properties principally because of the partial hydrogen desorption from the SiC surface that increased Ds and the formation of complex interface phases with high q?Bn-The interface band structures were analyzed to elucidate the mechanism of Ohmic contact formations. These processe s avoid traditional heavy doping substrate and high-temperature annealing, thereby helping improve the stability of SiC devices, simplify their processes, and reduce their production cost. |
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