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Y128- and Y36-cut single-crystalline lithium niobate (LN) thin films are fabricated by the crystal-ion-slicing (CIS) technique onto LN substrates. The conditions for the successful exfoliation of submicron-thick LN thin films are independent of the wafer orientation used in the present work. Wafer bonding using benzocyclobutene (BCB) is adopted to transfer LN thin films onto substrates, instead of the generally used hydrophilic bonding, which does not need a strict surface polishing process before the bonding. A noncontact polishing method involving low-energy Ar+ irradiation is adopted to treat the sliced LN thin films. The atomic force microscopy result shows that the surface roughness of the LN thin film is reduced from 10.6 to 6.4 nm.

An AlGaN/GaN metal-insulator-semiconductor heterostructure field-effect transistor (MISHFET) with about 40 nm Al 2 O 3 for both surface passivation and gate dielectric has been investigated and compared with the regular metal-semiconductor heterostructure field-effect transistor (MESHFET). The output characteristic measurements have shown that the MISHFET yielded 34% increase of the saturation drain current compared to the MESHFET. The Hall effect measurements of AlGaN/GaN two-dimensional electron gas (2DEG) coated with Al 2 O 3 thin films indicated an increase of mobility and density of 2DEG, and thus a decrease of the parasitic series resistance. The XRD analysis of the AlGaN/GaN heterostructure showed that strain was introduced into the AlGaN barrier layer with Al 2 O 3 coating. The energy band calculations showed that the biaxial tensile stress should possibly be the main mechanism for the performance improvement of the MISHFET.

Sodium beta-alumina (SBA) is deposited on AlGaN/GaN by using a co-deposition process with sodium and Al 2 O 3 as the precursors. The X-ray diffraction (XRD) spectrum reveals that the deposited thin film is amorphous. The binding energy and composition of the deposited thin film, obtained from the X-ray photoelectron spectroscopy (XPS) measurement, are consistent with those of SBA. The dielectric constant of the SBA thin film is about 50. Each of the capacitance-voltage characteristics obtained at five different frequencies shows a high-quality interface between SBA and AlGaN. The interface trap density of metal-insulator-semiconductor high-electron-mobility transistor (MISHEMT) is measured to be (3.5∼9.5)×10 10 cm −2 •eV −1 by the conductance method. The fixed charge density of SBA dielectric is on the order of 2.7×10 12 cm −2 . Compared with the AlGaN/GaN metal-semiconductor heterostructure high-electronmobility transistor (MESHEMT), the AlGaN/GaN MISHEMT usually has a threshold voltage that shifts negatively. However, the threshold voltage of the AlGaN/GaN MISHEMT using SBA as the gate dielectric shifts positively from −5.5 V to −3.5 V. From XPS results, the surface valence-band maximum (VBM-E F ) of AlGaN is found to decrease from 2.56 eV to 2.25 eV after the SBA thin film deposition. The possible reasons why the threshold voltage of AlGaN/GaN MISHEMT with the SBA gate dielectric shifts positively are the influence of SBA on surface valence-band maximum (VBM-E F ), the reduction of interface traps and the effects of sodium ions, and/or the fixed charges in SBA on the two-dimensional electron gas (2DEG).