Lateral electrical transport, optical properties and photocurrent measurements in two-dimensional arrays of silicon nanocrystals embedded in SiO2
© Gardelis et al; licensee Springer. 2011
Received: 22 December 2010
Accepted: 16 March 2011
Published: 16 March 2011
In this study we investigate the electronic transport, the optical properties, and photocurrent in two-dimensional arrays of silicon nanocrystals (Si NCs) embedded in silicon dioxide, grown on quartz and having sizes in the range between less than 2 and 20 nm. Electronic transport is determined by the collective effect of Coulomb blockade gaps in the Si NCs. Absorption spectra show the well-known upshift of the energy bandgap with decreasing NC size. Photocurrent follows the absorption spectra confirming that it is composed of photo-generated carriers within the Si NCs. In films containing Si NCs with sizes less than 2 nm, strong quantum confinement and exciton localization are observed, resulting in light emission and absence of photocurrent. Our results show that Si NCs are useful building blocks of photovoltaic devices for use as better absorbers than bulk Si in the visible and ultraviolet spectral range. However, when strong quantum confinement effects come into play, carrier transport is significantly reduced due to strong exciton localization and Coulomb blockade effects, thus leading to limited photocurrent.
Silicon nanocrystals (Si NCs) embedded in dielectric matrices such as silicon dioxide or silicon nitride have unique electrical and optical properties which are determined by quantum size and Coulomb blockade effects [1–3]. A significant consequence of the quantum size effect is the bandgap opening with decreasing NC size [4, 5]. This unique property of the Si NCs can be exploited in order to build absorbers for photovoltaic applications [6–12]. A fundamental problem with the existing silicon (Si) photovoltaics is that a significant part of the solar cell spectrum in the ultraviolet region, i.e., at energies much higher than the bandgap of silicon, is absorbed creating hot electrons and holes which relax to their respective band edges, losing their energy as heat through electron-phonon scattering and subsequent phonon emission. This effect poses a limit to the conversion efficiency of the cell. One way to increase the conversion efficiency beyond this limit is to use a tandem cell, i.e., a stack of absorber layers with different bandgaps to cover a larger range of the solar spectrum than a single bandgap absorber layer. These structures belong to the third generation of solar cells and are predicted to have an energy conversion efficiency limit of 60% .
The growth of very thin nanocrystalline (nc) Si films with thickness from 5 to 30 nm by low-pressure chemical vapor deposition (LPCVD) was reported by the authors previously . Films grown by this method have columnar structures and consist of a high density of Si NCs with a very narrow size distribution and arranged in a two-dimensional (2-D) array configuration . The size of the NCs in the z-direction is homogeneous in the whole film and equal to the film thickness, whereas in the x-y plane their size does not vary significantly with film thickness. In this work, we have grown similar nc-Si films on quartz substrates, in a range of thicknesses between 10 and 30 nm using LPCVD and subsequently oxidized them in order to form films containing Si NCs of controlled sizes embedded in a SiO2 matrix. The aim of this work is to use such films as absorbers in photovoltaic devices. We investigated their electrical and optical properties and measured photocurrent. We found that the electrical transport properties of the films were determined by tunneling of carriers through the SiO2 barriers between the Si NCs at low temperatures, whereas at higher temperatures by thermionic emission over these barriers. We also observed Coulomb blockade effects which persisted even above room temperature for the films containing the smaller Si NCs. In the case of the smaller NCs, photocurrent measurements as a function of energy showed similar dependence as that of the absorption, revealing strong absorption and photocurrent generation in visible and ultraviolet. Photoluminescence was observed only in the film which contained the smallest Si NCs (with sizes less than 2 nm), which were well isolated from each other. By comparing the photoluminescence and absorption spectra obtained from this film, we confirm the existence of an energy shift between photoluminescence (PL) and absorption, known as the Stokes shift. In addition, the PL energy is red-shifted compared with the corresponding energy predicted from the quantum confinement effect due to a pinning of the bandgap at Si NCs/SiO2 interfaces [15–24]. In this film, no photocurrent was observed.
where α(λ) is the absorption coefficient and x is the film thickness. Photoluminescence was excited by the 457.9-nm line of an Ar ion laser.
Results and discussion
The as-grown films had columnar structures and consisted of Si NCs, in a 2-D array configuration, with sizes in the z-direction equal to the film thickness. The lateral dimension (x-y plane) of the Si NCs was in all films between 12 and 13 nm with a narrow size Gaussian distribution which did not vary much with film thickness . Thermal oxidation reduced the thickness of the films and hence the vertical dimension of the Si NCs within the films. Oxidation has also reduced, to some extent, the lateral size of the Si NCs giving rise to thin SiO2 tunnel barriers between adjacent Si NCs.
Electrical transport measurements
Electrical measurements under illumination
In summary, we have investigated systematically the electrical, optical, and photocurrent properties of very thin films on quartz containing Si NCs in a 2-D configuration, with sizes in the range between less than 2 and 20 nm, embedded in a silicon dioxide matrix. Strong Coulomb blockade effects in the electric transport were observed, particularly in the films containing the smaller Si NCs. Absorption measurements showed an energy upshift of the energy bandgap of the Si NCs with decreasing size. Photocurrent spectra followed absorption, revealing that photocurrent is indeed due to electron hole generation within the Si NCs. Moreover, in films containing very small Si NCs (sizes <2 nm), separated by SiO2 barriers, strong quantum confinement effects were observed. Excitons generated by light absorption within the Si NCs were strongly localized, and no photocurrent was measured. In these films, exciton recombination by light emission was more probable than non-radiative recombination, resulting in light emission at room temperature. This systematic study confirms that Si NCs are interesting for use as better ultraviolet absorbers than bulk Si in photovoltaic devices. However, when strong confinement comes into play in the Si NCs, one should consider strong localization effects of the photo-generated excitons that result in the absence of photocurrent.
This work was financially supported by the EU research project FP6-ICT-I3 ANNA, contract no. 026134.
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