Formation and nano-scale characterization of ultrahigh-density nanodot superlattices

半导体纳米结构由于其量子约束效应而引起材料光电特性的变化，引起了人们的广泛关注。我们发现，在厚度为~0.3nm的超薄SiO_2薄膜覆盖的Si表面上生长出了尺寸为~5 nm、密度为~10^<12> cm^<-2>的超小型锗纳米点。沉积在表面的Ge原子通过SiO_2+Ge→SiO↑+ GeO↑的化学反应与超薄SiO_2膜反应形成Ge纳米点成核位。由于在成核位点上的吸附能比在SiO_2膜上的吸附能大，沉积的Ge原子主要被成核位点捕获。这使得形成的锗纳米点尺寸超小且均匀，密度超高。Ge纳米点呈球形，与stranski - krstanov生长形成的Ge岛不同。在锗沉积过程中，通过改变衬底温度可以在衬底上形成外延或非外延的锗纳米点。超薄SiO_2技术还可用于形成硅纳米点、β-FeSi_2纳米点或纳米岛等不同种类的纳米结构。利用扫描隧道光谱研究了单个锗纳米点的电子特性。随着点尺寸的减小，Ge纳米点的能带隙增大到~1.4 eV，尺寸从7 nm变化到2 nm，这可以解释为Ge纳米点中的载流子量子约束效应。在室温下，我们还观察到了以离散隧道电流波动为特征的锗纳米点上的库仑封锁效应。离散波动可以用纳米点中的单电子陷阱和单电子逸出到硅衬底来解释。我们进一步研究了锗纳米点嵌入硅薄膜的堆叠结构的光学性质。高温退火后的结构在0.8 eV （~1.5 μm）左右的光子能量下产生了强烈的光致发光和电致发光。0.8 eV左右的光子能量广泛用于光通信。我们还开发了阴极发光和电致发光系统，使用扫描隧道显微镜来研究单个纳米点的光学特性。少

Semiconductor nanostructures have attracted much interest due to their quantum-confinement effects, which can cause changes in material opto-electronic properties. We found that ultra-small Ge nanodots with the size of ~5 nm and ultra-high density of ~10^<12> cm^<-2> grew on Si surfaces covered with ultrathin SiO_2 films (~0.3nm thickness). Deposited Ge atoms on the surfaces reacted with the ultrathin SiO_2 films by the chemical reaction of SiO_2+Ge→SiO↑ + GeO↑ to form Ge nanodot nucleation sites. The following deposited Ge atoms were mainly captured by the nucleation sites due to the larger adsorption energy on the nucleation sites than that on the SiO_2 films. This resulted in Ge nanodot formation with ultra-small and uniform size and with ultra-high density. Ge nanodots had spherical shape which was different from that of the Ge island formed by the Stranski-Krastanov growth. Epitaxial or non-epitaxial Ge nanodots could be formed on the substrates by changing substrate temperatures … More during Ge deposition. The ultrathin SiO_2 technolgy could also been applied to form different kinds of nanostructures such as Si nanodots and β-FeSi_2 nanodots or nanoislands.We investigated electronic properties of individual Ge nanodots by scanning tunneling spectroscopy. With decreasing the dot size, the energy band gap of Ge nanodots increased to ~1.4 eV with the size change from 7 to 2 nm, which could be explained by the carrier quantum-confinement effect in Ge nanodots. We also observed the Coulomb-blockade effect featured by discrete tunneling current fluctuation on Ge nanodots at room temperature. The discrete fluctuation was explained by a single electron trap in a nanodot and a single electron escape into Si substrate. We further investigated the optical properties of the stacked structures in which Ge nanodots were embedded in Si films. Intense photoluminescence and electroluminescence were observed at photon energies around 0.8 eV (~1.5 μm) from the structures after high-temperature annealing. The photon energies around 0.8 eV are widely used for the optical communication.We also developed cathode-luminescence and electroluminescence systems using scanning tunneling microscopy to investigate optical properties of an individual nanodot. Less