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High Quality III-nitride and III–oxide Prepared by Atomic Layer Deposition and its Applications in Electronic and Optical Devices
atomic layer deposition (ALD),plasma,gallium nitride (GaN),aluminum nitride (AlN),gallium oxide (Ga2O3),buffer layer,light emitting diodes (LED),high electron mobility transistor (HEMT),
|Publication Year :||2016|
接著經由研究發現，在適當退火條件下，可以在藍寶石基板上長出高品質氮化鎵薄膜，並可以將其作為高溫GaN磊晶的緩衝層。此緩衝層在高溫磊晶過程中會將因晶格不匹配而產生的缺陷侷限在靠近基板約10 nm區域，同時使薄膜上層保持良好的結晶品質，藉此大幅降低GaN磊晶層中的缺陷密度至2.2 × 105 cm−2。此高品質磊晶層在進一步成長InGaN/GaN發光二極體結構時，可以有效降低非輻射復合的機率，進而提高二極體發光的效率。
最後部分討論將ALD成長之氧化鎵薄膜應用於AlGaN/GaN HEMT。藉由調整製程參數成長出高品質之氧化鎵薄膜，使之作為HEMT的介電層以及鈍化層。此氧化鎵具有高均勻度以及高包覆度的特性，同時與GaN可形成良好的介面，因此可以有效提升HEMT元件特性。與傳統HEMT比較，在加入此氧化層後，元件的IV特性及次臨限擺幅（Subthreshold Swing, S.S.）均明顯優化，同時使用此氧化層可以大幅降低元件之漏電流並提升元件的崩潰電壓，元件的特性具有明顯優化。
Nowadays, atomic layer deposition (ALD) becomes a promising technique to obtain uniform films in nanoscale which is widely applied in optical and electrical devices. Result from the self-limiting behavior and layer-by layer growth, films grown by the ALD offer several benefits such as easy and precise thickness control, low defect density, excellent step coverage and conformality, good reproducibility, high uniformity over a large area, and low deposition temperature. In this dissertation, the characteristics of GaN and AlN films grown by ALD and its application in light emitting diodes (LED) and high electron mobility transistor (HEMT) was demonstrated.
At first, the effects of modifying the ALD parameters to characteristics of GaN films were studied. The self-limiting growth of GaN was manifested by the saturation of the deposition rate with the doses of TEG and NH3 and the linear dependence between film thickness and ALD cycle. It was found that the increase in the growth temperature leads to the rise of nitrogen content and improved crystallinity of GaN thin films, from amorphous at a low deposition temperature of 200oC to polycrystalline hexagonal structures at a high growth temperature of 500oC.
Moreover, we reported InGaN/GaN LEDs with ultralow threading dislocation (TD) density and improved efficiency on a sapphire substrate, on which a near strain-free GaN compliant buffer layer was grown by ALD. In general, the large lattice mismatch between GaN epilayer and the substrate results to high density of defects in it, and then causes a major obstacle for the further improvement of solid-state lighting and high-power electronics. However, this “compliant” buffer layer is capable of relaxing strain due to the absorption of misfit dislocations in a region within ~10 nm from the interface, leading to a high quality overlying GaN epilayer with an unusual TD density as low as 2.2 × 105 cm−2. The ultra low TD density in GaN resulted in the improvement of efficiency and performance of InGaN/GaN LED on it.
Atomic layer epitaxy of nitride ultrathin films was realized by ALD. The X-ray diffraction reveals high crystalline quality of the nitride ultrathin epilayer with a thickness of only a few tens of nm. The high-resolution transmission electron microscopy also indicates the high-quality single-crystal hexagonal phase of the nitride epitaxial layer.
Finally, films of gallium oxide (Ga2O3) were prepared through remote plasma atomic layer deposition (RP-ALD) using triethylgallium and oxygen plasma. The chemical composition and optical properties of the Ga2O3 thin films were investigated; the saturation growth displayed a linear dependence with respect to the number of ALD cycles. These uniform ALD films showed excellent uniformity and smooth interface between Ga2O3 and GaN. Then an ALD Ga2O3 film was used as the gate dielectric and surface passivation layer in a metal–oxide–semiconductor high-electron-mobility transistor (MOS-HEMT), which exhibited device characteristics superior to that of a corresponding conventional Schottky gate HEMT. Under similar bias conditions, the gate leakage currents of the MOS-HEMT were two orders of magnitude lower than those of the conventional HEMT. The subthreshold swing (SS) and breakdown voltage of the MOS-HEMT were 110 mV decade–1 lower and 44 V higher compared to conventioal HEMT. Those indicate that Ga2O3 is a good candidate for the gate dielectric and passivation layer of AlGaN/GaN HEMT
|Appears in Collections:||材料科學與工程學系|
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