超薄鐵銥合金的成分比例與結構研究

Investigations of the compositional ratio and structure for ultrathin FeIr alloy

李亞倫(Ya-Lun Li)

指導教授 ： 蔡志申

國立臺灣師範大學/理學院/物理學系/碩士(2011年)

摘要

本論文主要研究鐵超薄膜在銥(111)基底上的成長模式、表面結構、化學偏移及合金成分比例。樣品製備與實驗均在超高真空環境下進行，並透過低能量電子繞射與歐傑電子能譜進行實驗觀測。在室溫300 K鐵超薄膜的成長方面，我們首先以歐傑電子能譜觀察一系列不同厚度之鐵薄膜，發現鐵薄膜在銥單晶上的化學偏移與塊材電負度所預期的結果有相反的趨勢。當鐵薄膜厚度超過2 ML時，其L1M1M2歐傑電子動能隨厚度增加而下降，銥N1N2N7歐傑電子動能隨厚度增加而上升，介面效應仍然明顯；厚度超過4 ML時，鐵L1M1M2歐傑電子動能變化趨於平緩，介面效應減弱，此時樣品的化學狀態以塊材鐵為主。從室溫300 K鐵超薄膜成長之低能量電子繞射實驗結果發現，當鐵薄膜厚度超過5.8 ML時，鐵原子主要是以bcc(110)在fcc(111)上的Kurdjumov-Sachs (KS)模式進行磊晶；當厚度小於1.8 ML時，鐵原子則以基底fcc(111)的方式進行磊晶。鐵超薄膜樣品加熱退火至800 K時，我們從歐傑電子能譜的強度分析可以得到穩定的鐵銥成分比例為1:3；化學偏移的分析發現銥N1N2N7歐傑電子動能比起乾淨銥單晶有下降的趨勢，因此排除鐵原子退吸附的可能；在低能量電子繞射實驗結果中，電子入射動能120 eV時可以發現清楚的(2×2)亮點。由以上三個實驗結果我們推測鐵銥形成規則合金FeIr3，最後透過氬離子濺射實驗進行深度分析，發現實驗所得之濺射效率與FeIr3模型的計算結果相差3%，顯示鐵銥確實形成規則合金FeIr3。另一方面，在低能量電子繞射實驗結果中，電子入射動能75 eV時，可以發現鐵銥合金表面上存在有鐵的兩種結構：bcc(110) KS與bcc(111) (3/2×3/2)R20°。當鐵超薄膜樣品厚度大於5.8 ML時，此兩種結構會同時存在於加熱退火後的FeIr3合金表面；當厚度小於1.8 ML時，合金表面將只剩下bcc(111)結構。

關鍵字

鐵 ； 銥 ； 合金 ； 表面結構 ； 成分比例

並列摘要

The study of Fe/Ir(111) system was discussed in this thesis. The growth mode, surface structure, chemical shift and the proportion of alloy composition was investigated using the low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Sample preparation and experiments were carried out under ultrahigh vacuum condition. For the growth of ultrathin iron films at room temperature (300 K), at first we explored a series of different thickness of iron films by Auger electron spectroscopy. We found that the trend of chemical shift between iron film and a Ir(111) substrate is opposite to the trend of electronegativity between iron and iridium bulk. When the thickness of iron film is between 2 and 4 ML, the kinetic energy of Fe L1M1M2 Auger electron decreases and that of Ir N1N2N7 Auger electron increases with the thickness increased. It showed that the interface effect is still evident. When the thickness of iron film exceeds 4 ML, the change of the kinetic energy of Fe L1M1M2 Auger electron goes flatten. It showed the weakening interface effect and the chemical state of the sample based on iron bulk state. From LEED experiments, when the thickness of iron film exceeds 5.8 ML, the arrangement of iron atoms is mainly the Kurdjumov-Sachs (KS) orientation corresponding to epitaxial growth of bcc(110) on fcc(111). When the thickness of iron film is less than 1.8 ML, the arrangement of iron atoms is mainly pseudomorphic growth.
When the sample of Fe/Ir(111) is annealed to 800 K, the intensity from the Auger electron spectroscopy analysis of iron and iridium shows a stable composition ratio of 1:3. The chemical shift analysis shows that the kinetic energy of Ir N1N2N7 Auger electron is less than that of clean Ir(111) thus excluding the possibility of the desorption of iron atoms. The incident electron of LEED where the kinetic energy is 120 eV can be used to resolve the clear (2×2) diffraction pattern. From the above three experiment results, we infer that the formation of ordered FeIr3 alloy. From the sputtering depth profiling measurements, the sputter efficiencies fall within 3% of the value of the theoretical calculation from the FeIr3 model. On the other hand, the incident electron of LEED with kinetic energy of 75 eV can be used to observe the two structures of iron which present on the surface of FeIr3 alloy: bcc(110) KS and bcc (111) (3/2×3/2) R20°. When the thickness of iron film is greater than 5.8 ML, two structures were observed after FeIr3 alloy formation. When the thickness of iron film is less than 1.8 ML, only bcc (111) structure exists on the FeIr3 surface.

並列關鍵字

Fe ； Ir ； alloy ； surface structure ； compositional ratio

參考文獻

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