Formation of Si quantum dots in nanocrystalline silicon

Abstract

Amorphous Si (a-Si) thin films deposited on Si(110) substrates were crystallized by using rapid thermal annealing. From structural investigations using X-ray diffraction (XRD) and cross-sectional transmission electron microscopy (TEM) we find that this crystallization is a two step process. In a first step single crystalline needles having a thickness of 1–2 nm grow in 〈111〉 direction towards the thin film surface starting from the a-Si Si interface. In a second process these needles induce growth of rectangular shaped Si crystallites (Si quantum dots) with 2–3 nm length and 3–5 nm width in the space between the needles. This process is driven by the difference in density between the a-Si regions and the Si- needles ( ϱ(a-Si) > ϱ(Si)) and consequently leads to stretching of the a-Si regions. Strain relief is carried out consecutively by transition of these amorphous regions into the nanocrystalline phase. Room-temperature photoluminescence (PL) using the 337 nm line of an N 2 laser for excitation shows intense blue light emission from the nanocrystalline thin films. The luminescence band between 2.6 and 3.2 eV consists of distinct peaks. Time resolved PL yields decay time constants τ 1 = 170–250 ps and τ 2 = 500–800 ps depending on the spatial positions of PL excitation across the sample surface. The blue light emission from the nanocrystalline thin films is explained by quantum size effects in the Si nanocrystallites which actually are Si quantum dots.