經由分子束磊晶技術，高品質氧化釓和氧化釔奈米薄膜成功地磊晶在(0001)h晶面的氮化鎵基板上。儘管晶格之間存在15-17%的不匹配，氧化釓和氧化釔磊晶薄膜仍保有非常佳的均勻性和良好的結晶性。
磊晶成長過程中，利用臨場反射式高能電子繞射的圖形，有助於了解表面結構和型態。並且，使用同步輻射高亮度、高解析度之光源進行Ｘ光繞射、Ｘ光反射率的實驗，可有效地深入研究超薄磊晶氧化物薄膜之結構特性。此外藉由C-V和J-E的特性曲線去探討磊晶薄膜之電性特徵。
在分子束磊晶沉積氧化釓和氧化釔初期，磊晶薄膜具有六重對稱之六方晶結構，薄膜之(0001)h平面與(0001)h晶向之氮化鎵基板相同，同時在平面上則維持[11-20]h//氮化鎵[11-20]h的結晶方向關係。隨氧化物薄膜厚度增加，六方晶相轉換成單斜晶相，而主要的結晶平面由(0001)h轉變成(-201)m，而[11-20]h晶向轉變成[020]m。在這一系列的成長過程中，由臨場反射式高能電子繞射和高解晰X光繞射實驗的觀測下，定義3到4奈米為相變化之臨界厚度。我們使用層狀結構的模型來合理解釋這兩種相似結構之原子排列，相轉變發生的原因可能來自於磊晶成長過程中導入的應變場。伴隨著厚度效應的發生，吾人觀察到由六方晶相主導和單斜晶相主導之兩種不同結構的形貌。

High-quality nano-thick Gd2O3 and Y2O3 epitaxial films have been grown on GaN(0001)h substrate by molecular beam epitaxy (MBE). The epi-layer of R2O3 (where R denotes Gd and Y) still exhibit remarkably uniform thickness and highly structural perfection despite a large lattice mismatch of 15-17% with the substrate.
Structural and morphological investigation were carried out by in-situ reflection high energy electron diffraction (RHEED)、synchrotron x-ray diffraction (XRD) and x-ray reflectivity (XRR). The electric properties were studied by measuring their C-V and J-V characteristics.
The initial stage of the Gd2O3 and Y2O3 epitaxial growth corresponds to a hexagonal phase with 6-fold symmetry. The hetero-structure follows an epitaxial relationship R2O3(0001)h [11-20]h // GaN(0001)h [11-20]h. With increasing layer thickness, the structure of the R2O3 film changes from hexagonal with (0001)h normal to monoclinic with (-201)m normal and the in-plane alignment with GaN[11-20]h changes from [11-20]h to [020]m. We concluded that approximate 3-4 nm is the critical thickness characterized by in-situ RHEED and ex-situ HRXRD. We utilized layer-like structural feature to nicely account for the phase transformation between the two phases. Overall, the strain filed might be responsible for the thickness dependent hexagonal to monoclinic phase transformation during MBE growth.

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