Preparation of Regular Amorphous Multilayers

Molecular beam epitaxy, as a mature technology, was employed for engineering amorphous/amorphous MLs to atomic precision at the thickness required. Si/SiO2 periodic MLs were grown [45-47] using the following deposition procedure. A thin Si layer is deposited on substrate and then it is taken out of the vacuum chamber and submitted to 600 s exposure to ultraviolet ozone. This procedure is repeated to create the necessary number of periods. Later, a-Si/SiO2 periodic MLs have been prepared with electron beam evaporation [48-53] or rf sputtering [54, 55] and in-situ plasma oxidation. Plasma-activated gas cell has been used as the oxygen source [48-50]. Plasma-enhanced chemical vapor deposition of a-Si and layer-by-layer plasma oxidation have been used to fabricate a-Si:H/SiO2 MLs [56]. The thickness of a-Si:H layers has been controlled by the duration time of plasma oxidation of the silicon layers initially deposited. Nanocrystalline Si layers have been grown on SiO2 by rapid thermal chemical vapor deposition [57].

Regular a-MLs have also been prepared by a consecutive deposition of two materials applying a variety of rather simple and cheaper set-ups, as compared to the MBE normally used for crystalline MLs. Glow-discharge [13, 58, 59], electron beam evaporation [60], plasma- and photochemically enhanced chemical vapor deposition [15-17, 61-65] techniques, and reactive and magnetron sputtering [66-71] have been applied for preparation of a-Si:H- and a-Ge:H-based a-MLs. Physical vapor deposition [23, 26-28, 30, 72-81], laser ablation [20, 82-85], and electron beam evaporation [86] techniques have been used for fabrication of chalco-genide a-MLs. When one applies a nonepitaxial technique for the preparation of a multilayer structure, each layer of this structure (having a thickness between 2 and 20 nm) can be prepared either by one-step (continuous) material deposition or applying a step-by-step approach. Experiments on preparation of a-MLs based on a-Si:H have shown [87] that multilayers prepared by applying the step-by-step deposition of each layer resulted in smoother interfaces and a better periodicity than in the case of continuous deposition of the layers. This approach has also been applied for preparation of selenium-based a-MLs and, thus, high-quality chalco-genide MLs have been produced with smooth and parallel layers [77, 78, 88].

There are few works on layer-by-layer production of nano-crystalline/amorphous MLs. Low-pressure chemical vapor deposition has been used [89-91] to deposit a nano-crystalline nc-Si layer. By controlled, high-temperature thermal oxidation, part of this layer was oxidized to form SiO2, thus forming a Si/SiO2 bilayer. This procedure was repeated to build a multilayer structure. Regular multilayer structures of nc-CdSe/SiOx (Fig. 2), nc-CdSe/GeS2, nc-Se/SiOx, and nc-CdSe/ZnSe have also been produced by consecutive thermal evaporation of each material of the respective pair [92-98].

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