Keita Nomoto

Pulsed KrF excimer laser dopant activation in nanocrystal silicon in a silicon dioxide matrix

We demonstrate that a pulsed KrF excimer laser (λ = 248 nm, τ = 22 ns) can be used as a post-furnace annealing method to greatly increase the electrically active doping concentration in nanocrystal silicon (ncSi) embedded in SiO2. The application of a single laser pulse of 202 mJ/cm2 improves the electrically active doping concentration by more than one order of magnitude while also improving the conductivity. It is confirmed that there is no film ablation or significant change in ncSi structure by atomic force microscopy and micro-Raman spectroscopy. We propose that the increase in free-carrier concentration is the result of interstitial P/B dopant activation, which are initially inside the Si crystallites. Evidence of mobility limited carrier transport and degenerate doping in the ncSi are measured with temperature-dependent conductivity.

Defect-Induced Luminescence Quenching vs. Charge Carrier Generation of Phosphorus Incorporated in Silicon Nanocrystals as Function of Size

Phosphorus doping of silicon nanostructures is a non-trivial task due to problems with confinement, self-purification and statistics of small numbers. Although P-atoms incorporated in Si nanostructures influence their optical and electrical properties, the existence of free majority carriers, as required to control electronic properties, is controversial. Here, we correlate structural, optical and electrical results of size-controlled, P-incorporating Si nanocrystals with simulation data to address the role of interstitial and substitutional P-atoms. Whereas atom probe tomography proves that P-incorporation scales with nanocrystal size, luminescence spectra indicate that even nanocrystals with several P-atoms still emit light. Current-voltage measurements demonstrate that majority carriers must be generated by field emission to overcome the P-ionization energies of 110–260 meV. In absence of electrical fields at room temperature, no significant free carrier densities are present, which disproves the concept of luminescence quenching via Auger recombination. Instead, we propose non-radiative recombination via interstitial-P induced states as quenching mechanism. Since only substitutional-P provides occupied states near the Si conduction band, we use the electrically measured carrier density to derive formation energies of ~400 meV for P-atoms on Si nanocrystal lattice sites. Based on these results we conclude that ultrasmall Si nanovolumes cannot be efficiently P-doped.

Characterisation of active dopants in boron-doped self-assembled silicon nanostructures

Doping of silicon nanocrystals has become an important topic due to its potential to enable the fabrication of environmentally friendly and cost-effective optoelectronic and photovoltaic devices. However, doping of silicon nanocrystals has been proven difficult and most of the structural and electronic properties are still not well understood. In this work, the intrinsic and boron-doped self-assembled silicon nanocrystals were prepared and mainly characterised by the transient current method to study the behaviour of charge carriers in these materials. Our experiments quantified the amount of electrically active boron dopants that contributed to charge transport. From this, the boron doping efficiency in the nanocrystal superlattice was estimated.

Accurate analysis of the size distribution and crystallinity of boron doped Si nanocrystals via Raman and PL spectra

A narrow size distribution of quantum dots (QDs) is needed for their application in photovoltaics but collection of such information is difficult. This paper demonstrates the application of Raman spectroscopy as a characterisation tool to extract the size distribution and crystalline fraction of Si QD samples fabricated through the sputter-anneal method. Measured Raman spectra of Si QD materials are de-convoluted into four components according to their origins and Raman scattering by Si QD cores is described by a modified one phonon confinement model, while other components are reproduced with Gaussian functions. Through fitting of Raman spectra, Si QD size distributions and Si crystalline fractions are obtained. The results are compared with the values extracted from PL modelling on a series of B doped Si QD samples. The good consistency between the values extracted by these two methods confirms the validity of the Raman model. The result confirms that Si crystallization has been suppressed by B doping as the average Si QD size and Si crystalline fraction are reduced with increased B doping level.

Boron-Incorporating Silicon Nanocrystals Embedded in SiO 2 : Absence of Free Carriers vs. B-Induced Defects

Boron (B) doping of silicon nanocrystals requires the incorporation of a B-atom on a lattice site of the quantum dot and its ionization at room temperature. In case of successful B-doping the majority carriers (holes) should quench the photoluminescence of Si nanocrystals via non-radiative Auger recombination. In addition, the holes should allow for a non-transient electrical current. However, on the bottom end of the nanoscale, both substitutional incorporation and ionization are subject to significant increase in their respective energies due to confinement and size effects. Nevertheless, successful B-doping of Si nanocrystals was reported for certain structural conditions. Here, we investigate B-doping for small, well-dispersed Si nanocrystals with low and moderate B-concentrations. While small amounts of B-atoms are incorporated into these nanocrystals, they hardly affect their optical or electrical properties. If the B-concentration exceeds ~1 at%, the luminescence quantum yield is significantly quenched, whereas electrical measurements do not reveal free carriers. This observation suggests a photoluminescence quenching mechanism based on B-induced defect states. By means of density functional theory calculations, we prove that B creates multiple states in the bandgap of Si and SiO2. We conclude that non-percolated ultra-small Si nanocrystals cannot be efficiently B-doped.

Microstructure analysis of silicon nanocrystals formed from silicon rich oxide with high excess silicon: Annealing and doping effects

Thin films consisting of silicon nanocrystals fabricated by high silicon content in silicon rich oxide show unique properties of decreasing resistivity and increasing light absorption while maintaining quantum confinement effects. With that said, the effect of the annealing temperature and doping element on the microscopic structure of silicon nanocrystals (Si NCs) and the film are still under research. In this study, individual intrinsic, boron-, and phosphorus-doped films are annealed at various temperatures, and their structural properties are analyzed via atom probe tomography together with glancing incidence x-ray diffraction, Raman spectroscopy (Raman), transmission electron microscopy (TEM), and energy filtered TEM. In addition, photoluminescence (PL) is performed and linked with their microstructural properties. The Si NC growth is confirmed at annealing temperatures of 1000 °C and 1100 °C. The microstructure of the Si NCs in the whole film is dramatically changed by increasing the annealing tempe...

All-silicon tandem solar cells: Practical limits for energy conversion and possible routes for improvement

Silicon nanocrystals (Si NCs) embedded in a dielectric matrix is regarded as one of the most promising materials for the third generation photovoltaics, owing to their tunable bandgap that allows fabrication of optimized tandem devices. Previous work has demonstrated fabrication of Si NCs based tandem solar cells by sputter-annealing of thin multi-layers of silicon rich oxide and SiO2. However, these device efficiencies were much lower than expected given that their theoretical values are much higher. Thus, it is necessary to understand the practical conversion efficiency limits for these devices. In this article, practical efficiency limits of Si NC based double junction tandem cells determined by fundamental material properties such as minority carrier, mobility, and lifetime are investigated. The practical conversion efficiency limits for these devices are significantly different from the reported efficiency limits which use Shockley-Queisser assumptions. Results show that the practical efficiency limi...

Temperature dependent electroluminescence from all-Si-nanocrystal p-i-n diodes grown on dielectric substrates

In this work, we demonstrate for the first time the electroluminescence (EL) from a mesa isolated p-i-n diode based on silicon nanocrystals (Si NCs) embedded in a SiO2 matrix fabricated on a dielectric substrate. The structure fabricated on a dielectric substrate ensures that the EL signal originates entirely from the Si NC material. A small offset between the EL (1.28 eV) and photoluminescence (PL) (1.33 eV) peak energies has been observed at room temperature. We attribute this discrepancy to the different subset of light-emitting Si NCs in EL and PL. A model classifying Si NCs into connected NCs and isolated NCs is proposed. Atom probe tomography is employed to visualize the existence of isolated NCs and connected NCs. This model has been further studied using temperature dependent EL and PL, where a blue-shift of peak energy is observed as the temperature is increased. The blue-shift is attributed to the temperature dependent transport between the two subsets of NCs and the quenching of the PL emission...

Structural, optical, and electrical properties of silicon nanocrystals fabricated by high silicon content silicon-rich oxide and silicon dioxide bilayers

Intrinsic, boron (B)-doped, and phosphorus (P)-doped silicon nanocrystals (Si NCs) formed from an excess Si concentration of 40 at. % were investigated to study their structural, optical, and electrical properties. Atom probe tomography (APT) revealed that the size and arrangement of Si NCs were different in each sample. A strong blue shift in photoluminescence spectra for the intrinsic and B-doped Si NCs was correlated with the volume fraction of small Si NCs. The lower resistivity of the B-doped sample than the P-doped one was explained by the percolation of Si NCs through the film.

High Si content SRO/SiO2 bilayer superlattices with boron and phosphorus doping for next generation Si quantum dot photovoltaics

Si quantum dots or interchangeably Si nanocrystals are promising materials for all-Si tandem solar cells in next generation photovoltaics. Si QDs in this study were fabricated by annealing sputtered Si rich oxide/SiO2 (SRO/SiO2) bilayer superlattice thin-films. Advantages of high Si content SRO layers include lower resistivity and higher light absorption cross-sections which are more suitable for photovoltaic devices. However, theoretically high Si content SRO produces greater size distribution and larger Si nanocrystals which in this case slightly lowers the bandgap towards that of crystalline Si. This study investigates the properties of high Si content SRO/SiO2 bilayer superlattice thin-films and the effect of boron and phosphorus doping.