Structural and optical characterization of two-dimensional arrays of Si nanocrystals embedded in SiO2 for photovoltaic applications

Abstract

We report on the structural and optical characterization of two-dimensional arrays of silicon nanocrystals (SiNCs) suitable for photovoltaic applications. Single and multiple SiNC layers were grown on quartz by low pressure chemical vapor deposition of Si and subsequent thermal oxidation steps. The single SiNC layers consisted of one SiNC layer embedded in two silicon dioxide (SiO2) layers, whereas the multi-layered structure consisted of five SiNC layers of equal thickness separated by SiO2 layers. SiNC layers with thicknesses ranging from 2 to 25 nm were investigated. A thorough structural characterization of the films was carried out by combining grazing incidence x-ray diffraction, x-ray reflectivity, and transmission electron microscopy (TEM). Both XRD and TEM measurements revealed that the SiNC layers were polycrystalline in nature and composed of SiNCs, separated by grain boundaries, with their vertical size equal to the SiNC layer and their lateral size characterized by a narrow size distribution. The high resolution TEM (HRTEM) images showed that oxidation of the SiNC layers proceeded by consumption of Si from their top surface, without any detectable oxidation at the grain boundaries. Only in the case of the thinnest investigated SiNC layer (2 nm), the SiNCs were well separated by SiO2 tunnel barriers. From transmission and reflection optical measurements, energy band gaps of the SiNCs were estimated. These results were correlated with the sizes of the SiNCs obtained by HRTEM. A shift of the estimated band gaps with decreasing SiNC size was observed. This was consistent with quantum size effects in the SiNCs. The film containing the smallest SiNCs (2 nm in the growth direction), besides a significant shift of the absorption edge to higher energies, showed light emission at room temperature which is due to radiative recombination of photo-generated carriers in localized SiNCs separated by SiO2 tunnel barriers.

Colloidal Solutions with Silicon Nanocrystals: Structural and Optical Properties

“…This can be attributed to the fact that the sizes of larger crystals are due to the agglomerations of small crystals, thus producing a preferential orientation as a consequence of this agglomeration. When we carry out the measurement of the smallest crystal dimensions, in a certain way we measure the crystal dimensions individually; therefore, we can say that we are measuring the true natural orientation of the nanocrystals which is possibly because of the way of preparation of the colloidal solution samples, contrary to other works where they have different orientation tendencies of the crystals such as (111) [60] or an orientation (220) [4]. An important feature which is noteworthy to emphasize is the intensity of the diffraction peaks; such intensity is attributed to the thickness formed by the silicon nanocrystals on the substrate [4].…”

“…Other interesting result is related to the emission efficiency of light in the visible range, which is linked to the energy band gap of the Si-nc, which in turn depends strongly on the size of the Si-ncs, and when they have lesser size than the Bohr exciton radius in the silicon, the spatial confining of the carriers in the nanocrystal is greater, which gives rise to a strong overlap of the wave functions in the k-space in both electrons and holes. Therefore, the nanocrystals can absorb and emit light with different energies by controlling only their size [4]. Silicon nanocrystals have been widely exploited in electronics and other areas, due to their low toxicity, and they exhibit the ability to be doped in order to become either an n-type or p-type material [5]; this fact allows generating new technologies such as sensors [6][7][8], biosensors [9][10][11][12], magnetic materials [13], photodiodes [14], Bragg reflectors [15], photonic applications [16][17][18][19][20][21], nonvolatile storage devices [22][23][24], solar cells of third generation [25][26][27][28][29] as well as tandem solar cells [30][31][32][33][34][35], among other devices [35,36].…”

“…These photons make a more efficient conversion of the solar energy into electric energy. These two ways proposed require a careful control of the size of the nanocrystals [4].…”

Impact of sublayer thickness and annealing on silicon nanostructures formation in a-Si:H/a-SiNx:H superlattices for photovoltaics

“…tandem solar cells) containing size-controlled silicon nanocrystals (Si-NCs) as an absorber. When the diameter of the Si-NCs is smaller than the Bohr radius (~ 5.29 nm), the quantum confinement effect starts to contribute to the extraction and generation of the excitons [3][4][5]. This very active field of research has been since the 1990s with the observation of strong light emission in the visible range from porous silicon at room temperature [6,7].…”

Narrow band photocurrent response from partially phase separated a-SiNx:H thin films

“…Processing the as-deposited material with suitable post deposition treatments like furnace annealing, 8 swift heavy ion irradiation 9 and rapid thermal annealing 3,8,10 leads to the precipitation of Si QDs in silicon nitride matrix. Though, so far, the stress has been on achieving phase separated Si QDs in Si 3 N 4 matrix; recent theoretical 11 and experimental reports [12][13][14] suggest that isolated QDs may not be very attractive for applications where charge transport is desired. It is also mentioned that conduction may be possible only if the QDs are closely spaced (via tunnelling), or, as we will discuss in the present report, if the matrix is partially phase separated 7 wherein, the excess Si provides the necessary conduction pathways.…”