The properties of metal-oxide semiconductor (MOS) capacitors with different chemical treatments have been examined in this study. A MOS capacitor consists of an Al2O3/n-GaN/AlN buffer/Si substrate. Four chemical treatments, containing organic solvents, oxygen plasma and BCl3 plasma, dilute acidic and alkali solvents, and hydroflfluoric acid, were used to reduce the metal ions, native oxides, and organic contaminants. The n-GaN surface was treated with these chemical treatments before Al2O3 was grown on the treated n-GaN surface to reduce the interface state trap density (Dit). The value of Dit was calculated using the capacitance–voltage curve at 1 MHz. The Dit of a u-GaN surface was modifified using various solutions, which further inflfluenced the contact properties of GaN.
Surface cleaning treatments are the foundation of a semiconductor device fabrication process. Surface cleaning significantly affects the epitaxial defects, metal contact resistance/stability , and overall device quality of GaN-based devices. Evaluating surface cleanliness requires considering the electrical properties of the device, structure, and interface state trap of the surface. Moreover, a surface treatment is used to remove the native oxides, organic contaminants, metal ions, particulates, residual species, and weaknesses in atomic bonding.
Recently, AlGaN/GaN high electron mobility transistors (HEMTs) were demonstrated for use in power electronic devices. In an HEMT device, a high saturation current, low leakage current, and high transconductance are necessary. Therefore, a low-resistance ohmic contact and low interface state trap density (Dit) must be obtained for an HEMT device. Interface states may cause various operational stability and reliability drawbacks in GaN-based HEMTs such as threshold voltage instability and current collapse phenomena. A surface treatment not only improves the device performance but also enhances the ohmic contact characteristics of GaN with metals.
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The chemical bonding states on the GaN surface were characterized using X-ray photoelectron spectroscopy (XPS) with a monochromate Al Kα X-ray (energy; 1486.6 eV). The shift in the XPS spectra was calibrated using a charge neutralization gun because of surface charge accumulation by emitting photoelectrons. The angle between the incident photons and the detected photoelectrons was set at 45◦ , which is sensitive to an analysis of surface chemical states.
In summary, chemical treatments were successfully used to reduce the Dit and improve the surface quality. The lowest Dit of 8.30 × 1011 cm−2 of the MOS capacitor was obtained when the sample was treated with treatment 4 because treatment 4 consisted of an alkaline solution, acidic solution, and diluted HCl, and was used to remove organic contaminants, metal ions, and native oxides. The chemical solution treatment not only reduced the contamination but also introduced the donor density to change the ohmic contact property of n-GaN with metal material.
