Experimental Study of Photochemical Deposition in a Condensed Phase on a Cryogenic Substrate

我们研究了凝聚相中的光化学沉积。在实验中，用KrF和ArF准分子激光辐照低温衬底上的液态硅烷（SiH_4）。用拉曼散射、扫描电子显微镜、透射电子显微镜和透射电子衍射对沉积膜进行了表征。该膜由许多棒状晶体硅（c-Si）产物组成。它们的直径和高度分别为400-500 nm和500-700 nm。值得注意的是，棒状c-Si已经在低温衬底上生长。因此，我们认为液-固界面反应在结晶生长过程中起着重要的作用。换句话说，从c-Si表面的生长是必要的或结晶。我们提出以下沉积机制。即（1）SiH_4分子在双光子激发下解离为SiH_3自由基和氢原子。(2)产生的原子氢从“相邻”分子中提取氢原子，并形成氢分子H_3。(3)剩余的自由基联合收割机并形成Si-Si键。类似的反应相继发生，并且生成的硅化氢（Si_xH_y）沉积在衬底上。在第二阶段，形成的晶核（Si_xH_y）通过单光子吸收生长为c-Si。一般来说，随着分子变得更大，其吸收边向更长的波长移动。因此，大尺寸的硅氢化合物容易被激发，并通过单光子吸收过程与“邻近的”SiH_4反应。该技术解决了光化学气相沉积技术中的一般问题，例如在光学窗口上的沉积、基底的热损伤和气相中的低沉积速率。

We have studied photochemical deposition in a condensed phase. In the experiment, liquid silane (SiH_4) on a cryogenic substrate is irradiated with KrF and ArF excimer laser. The deposition film is characterized by Raman scattering, scanning electron microscopy, transmission electron microscopy and transmission electron diffraction. The film is composed of many rod-like crystalline silicon (c-Si) products. Their diameter and height are 400-500 nm and 500-700 nm, respectively. It is remarkable that the rod-like c-Si has grown on the cryogenic substrate. Therefore, we consider that the liquid-solid interface reaction plays an important role in the crystallization growth. In other words, the growth from the c-Si surface is necessary or crystallization. We propose the following deposition mechanism. Namely, (1) SiH_4 molecule photoexcited by a two-photon absorption would be dissociated into a SiH_3 radical and an atomic hydrogen. (2) A created atomic hydrogen extracts a hydrogen atom from the "neighboring" molecule, and forms a hydrogen molecule H_3. (3) The remaining radicals combine and form a Si-Si bond. A similar reaction occurs successively and the generated hydrogen silicides (Si_xH_y) are deposited on the substrate. In the second stage, the formed nuclei (Si_xH_y) grow to be c-Si through the one-photon absorption. Generally speaking, as a molecule becomes larger, its absorption edge shifts to longer wavelength. Therefore, the large hydrogen silicides are excited and react easily with "neighboring" SiH_4 through the one-photon absorption process. This technique solves general problems in photochemical vapor deposition techniques, such as deposition on the optical window, thermal damages of the substrate and the low deposition rate in the gas phase.