Laser induced deposition of nanocrystalline Si with preferred crystallographic orientation

Laser induced chemical vapor deposition (LCVD) has been utilized for formation of thin films on various substrates. The quality of the films depends on the irradiation and process parameters. Under high beam energy and repetition rate powder particles can be obtained instead of continuous film. In the present work we studied the conditions required for obtaining Si powder particles composed of nanocrystalline grains having uniform size and spherical shape. The Si was deposited from a SiH4/Ar gas mixture on a thermally grown amorphous Si-oxide layer on a single crystalline Si(100) substrate by using an excimer laser. The laser was operated at 193 nm with a pulse duration of 24 ns. It was found that at a laser energy density between 60 and 300 mJ/cm2, a repetition rate above 20 Hz, and a total pressure above 60 Torr, powdered Si was formed. SEM studies of the deposited Si show large clusters (1–10 μm) of Si powder particles. The clusters have characteristic shapes depending on their size. Microstructural studies by analytical and high resolution transmission electron microscopy show that the Si particles consist of nanocrystalline grains having perfect circular shape and narrow size distribution. In addition, the grains have a preferred crystallographic orientation even though deposited on amorphous Si-oxide film. The microstructural observations of the Si powder particles are explained by homogeneous nucleation of nano-size Si grains in the gas phase, followed by agglomeration of the nano-size grains to clusters in the gas phase, and finally deposition of the Si clusters, while conserving the initial size and microstructure, on the substrate and formation of Si powder particles.