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    题名: First-principles study in wurtzite InN bulk, thin film, and nanobelt
    作者: 周若芸;Chou,Jo-Yun
    贡献者: 物理學系
    关键词: first-principles;wurtzite;structure properties
    日期: 2014-07-22
    上传时间: 2014-10-15 14:48:20 (UTC+8)
    出版者: 國立中央大學
    摘要: 氮化銦是三五族的半導體材料,它具有比其它以氮為基底的半導體還要狹窄的能隙(大約0.7-0.9 eV) 。
    我們運用第一原理中的LDA(local density approximation)搭配Quantum Espresso軟體計算氮化銦在塊材、切面為±(0001)的薄膜與切面為±(0001)和±(11 ̅00)的奈米帶在維度變化時的晶格常數,進而分析氮化銦在極性面(0001)與非極性面(11 ̅00)原子結構的變化。我們探討在±(0001)的薄膜上,由於各個原子陰電性不同使表面電荷轉移,導致銦與氮兩端因為表面氫或表面接氧而有不同的結構變化。另外,我們發現當為裸面的薄膜時,氮化銦薄膜的臨界厚度為12層,而層數小於12層時,薄膜會由sp3鍵結的纖鋅礦轉變為sp2的石墨結構。有趣的是,在表面均接氫的奈米帶中,我們發現由於銦與氮陰電性的不同,造成在非極性面上鍵結傾斜的情形。
    為了解決LDA在計算能隙時低估的問題,我們加入了PSF(pseudofunction method)與PSFEXX(matrix method for the calculation of nonlocal exchange potential)方法,利用直接去計算非局域的交換能來修正並計算出與實驗值(0.7-0.8eV)符合的能隙(0.7eV)。我們認為在低維度的氮化銦中,由於表面態較多,若能運用此方法去精準計算出其能隙與能帶結構,那將會成為分析結構現象的重要角色。;InN is an important compound in group-III nitride semiconductors and has attracted much attention due to its debated narrow energy gap (0.7-0.9eV). Recently, Hu et al. proposed a single crystalline InN nanobelts, and then Dong et. al. observed the X-ray absorption (XAS) and the X-ray emission (XES) spectra of InN thin film and nanobelt.
    We first employed the first-principles calculation, i.e. Quantum Espresso, with the local-density approximation (LDA) to study the structural properties of InN bulk, thin film in wurtzite (0001) direction, and nanobelt enclosed by wurtzite ±(0001) and ±(11 ̅00) planes. For the InN thin film, the saturation effect of H-saturated, O-saturated, and bare surface on the polar (0001) plane are considered. The subtle surface charge transfer, due to the different electronegativities between ions, gives rise to the opposite In- and N-terminated surface geometry between H- and O-saturated thin films. Moreover, our DFT+LDA calculation suggests that the critical thickness of bare InN polar thin film is 6-bilayer, since we observed the structural transfer from the wurtzite structure with sp3 bond geometry in 6-bilayer case into the graphitic structure with sp2 bond geometry in 4-bilayer case. Interesting, for the H-saturated InN nanobelt, we observe the tilted interfacial In-N bonds, due to the smaller (larger) electronegativity between In (N) and H ions.
    In order to resolve the underestimate of energy gaps of N-based semiconductors, the newly proposed matrix method for the nonlocal exchange potential with the pseudofunction (PSFEXX) method is employed and then gives excellent agreement between calculated 0.7eV (6.0eV) and experimental 0.7-0.8eV (6.2eV). We strongly believe that this PSFEXX method may provide more reliable structural and electronic properties in low-dimensional InN materials, where those misleading surface states near the zero or negative energy gap of InN by LDA or GGA calculation can be efficiently corrected and then play a significant role on the structural properties.
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