Phase Transformation of Molecular Beam Epitaxy-Grown Nanometer-Thick Gd₂O₃ and Y₂O₃ on GaN

Abstract

High quality nanometer-thick Gd₂O₃ and Y₂O₃ (rare-earth oxide, R₂O₃) films have been epitaxially grown on GaN (0001) substrate by molecular beam epitaxy (MBE). The R₂O₃ epi-layers exhibit remarkable thermal stability at 1100 °C, uniformity, and highly structural perfection. Structural investigation was carried out by in situ reflection high energy electron diffraction (RHEED) and ex-situ X-ray diffraction (XRD) with synchrotron radiation. In the initial stage of epitaxial growth, the R₂O₃ layers have a hexagonal phase with the epitaxial relationship of R₂O₃ (0001)(H)<1120>(H)//GaN(0001)(H)<1120>(H). With the increase in R₂O₃ film thickness, the structure of the R₂O₃ films changes from single domain hexagonal phase to monoclinic phase with six different rotational domains, following the R₂O₃ (201)(M)[020](M)//GaN(0001)(H)<1120>(H) orientational relationship. The structural details and fingerprints of hexagonal and monoclinic phase Gd₂O₃ films have also been examined by using electron energy loss spectroscopy (EELS). Approximate 3-4 nm is the critical thickness for the structural phase transition depending on the composing rare earth element.

Single-Crystal Y2O3 Epitaxially on GaAs(001) and (111) Using Atomic Layer Deposition

“…Previous study on MBE-Y 2 O 3 grown on GaN(0001) showed that Y 2 O 3 can exist in hexagonal phase as the film thickness $\le$3 nm and its crystalline structure resembles that of the cubic phase in many aspects [36]. It is risky to identify the phase by the specular reflections alone and thus essential to examine the positions of the off-normal reflections.…”

Influence of nanostructure formation on the crystal structure and morphology of epitaxially grown Gd2O3 on Si(001)

“…Therefore, it is probable that the transformation to the monoclinic phase occurs when hexagonal islands grow further until the stress is reduced to the value where the monoclinic phase is exhausted. Such a hexagonal to monoclinic phase transition is reported for the growth of Gd 2 O 3 on GaN and SiC (Chang et al, 2013;Fissel et al, 2006a). Moreover, a pressure-induced sizedependent phase transition is also found in many other nanosized materials, such as Hf, ZrO, CdTe, CdS or CdSe (Xiong et al, 2013;Garvie, 1965;Wu et al, 2008;Haase & Alivisatos, 1992;Schroeder & Persans, 1996;Chen et al, 1997).…”

“…Thin films of Gd 2 O 3 were found to show a thicknessdependent change of the crystal phase from hexagonal to monoclinic structure during epitaxial growth on GaN, SiC and GaAs (Chang et al, 2013;Fissel et al, 2006a;Chiang et al, 2014). The epitaxial growth of Gd 2 O 3 on Si(111) at a low temperature results in a non-cubic structure (Moellers et al, 2017).…”

Investigation of the excitations of plasmons and surface exciton polaritons in monoclinic gadolinium sesquioxide by electron energy-loss spectroscopy and plasmon spectroscopic imaging

“…It was found that 10 nm-thick Gd 2 O 3 lms minimize the effect of moisture absorption on the electrical performance. 8,11 However, technologies beyond the 16 nm CMOS require the combination of high-carrier-mobility semiconductors and high k gate dielectric for further reducing the EOT to less than 1 nm. 11 The thickness of the m-Gd 2 O 3 lms on various semiconductor substrates is restricted in the range of 4 to 8 nm when considering an EOT value less than 1 nm.…”

“…8,11 However, technologies beyond the 16 nm CMOS require the combination of high-carrier-mobility semiconductors and high k gate dielectric for further reducing the EOT to less than 1 nm. 11 The thickness of the m-Gd 2 O 3 lms on various semiconductor substrates is restricted in the range of 4 to 8 nm when considering an EOT value less than 1 nm. Furthermore, the restricted thickness makes it even more challenging to investigate the electronic excitations in m-Gd 2 O 3 thin lms and exclude the effects of interface plasmons generated in the heterostructures.…”