Surface fingerprints of individual silicon nanocrystals in laser-annealed Si/SiO2 superlattice: Evidence of nanoeruptions of laser-pressurized silicon

Abstract

Picosecond and nanosecond laser annealing and simulation of amorphous silicon thin films for solar cell applications J. Appl. Phys. 115, 043108 (2014); 10.1063/1.4863402Structural and electrical studies of ultrathin layers with Si0.7Ge0.3 nanocrystals confined in a SiGe/SiO2 superlattice J.Silicon nanocrystals prepared by continuous-wave laser annealing of a free-standing Si/SiO 2 superlattice are studied for the first time by using methods of surface analysis (scanning electron microscopy and atomic force microscopy). The surface topology and composition are compared with transmission electron microscopy images that show a projection through the whole film, allowing us to discriminate silicon nanocrystals located near the film surface. These nanocrystals have an unusual pear-like shape with the thinner part sticking out of the laser-illuminated surface. The non-spherical shape of these nanocrystals is explained by eruption of silicon pressurized at the stage of crystallization from the melt phase. This hypothesis is supported by the micro-Raman spectra which show low stress near the surface features, in contrast to the neighbouring regions having high compressive stress. V C 2012 American Institute of Physics. [http://dx.

Optical and Structural Properties of Si Nanocrystals in SiO2 Films

“…In this region, no Si-related peak is detected by Raman spectroscopy, the absorption is very low, and no PL is observed ( Figure 12 ). In agreement, for 0.5-μm-thick Si/SiO 2 SL (2-nm-thick Si and SiO 2 layers), no Si clusters are detected in bright-field scanning TEM (BF-STEM) images, and energy-dispersive X-ray spectroscopy (EDS) indicates an increase of x in the central region compared to the pristine film [ 141 , 142 ]. Quantitative analysis based on the EDS measurements is not possible because quantification using low-energy X-ray lines such as oxygen K alpha is not feasible.…”

“…An optical photograph of a typical laser-annealed area on a free-standing film and a Raman map from the same area are presented in Figure 11 . Three regions can be distinguished: (i) central region, (ii) ring around the central region, and (iii) pristine film outside the irradiated area [ 140 , 141 , 142 ].…”

“…In the ring around the central region, the EFTEM, BF-STEM, and TEM images evidence large Si-nc embedded in amorphous SiO 2 ( Figure 13 ) [ 140 , 141 , 142 ]. The maximum Si-nc diameters (~200 nm) for the 0.5-μm-thick Si/SiO 2 SLs [ 141 ] are on average two times larger than those detected for the 0.5-μm-thick free-standing SiO x films ( x ~ 1.7–1.8) [ 140 ]. This difference is reasonably explained by a larger Si content in Si/SiO 2 SLs ( x ~ 0.7 from the deposition conditions) compared to the SiO x films ( x ~ 1.7–1.8).…”

“…For laser-annealed 0.5-μm-thick Si/SiO 2 SLs, surface analysis (SEM and AFM) has been used to discriminate Si-nc located near the film surface [ 141 ]. In the tilt TEM images ( Figure 13 c), these “surface Si-nc” have an unusual pear-like shape with the thinner part presumably sticking out of the laser-illuminated surface.…”

“…Position 1 corresponds to the surface Si-nc shown in Figure 13 . Reproduced with permission from [ 141 ]. Copyright 2012, AIP Publishing LLC.…”