Lingfeng Wu

Passivation effects in B doped self-assembled Si nanocrystals

Doping of semiconductor nanocrystals has enabled their widespread technological application in optoelectronics and micro/nano-electronics. In this work, boron-doped self-assembled silicon nanocrystal samples have been grown and characterised using Electron Spin Resonance and photoluminescence spectroscopy. The passivation effects of boron on the interface dangling bonds have been investigated. Addition of boron dopants is found to compensate the active dangling bonds at the interface, and this is confirmed by an increase in photoluminescence intensity. Further addition of dopants is found to reduce the photoluminescence intensity by decreasing the minority carrier lifetime as a result of the increased number of non-radiative processes.

Electroluminescence from Si nanocrystal/c-Si heterojunction light-emitting diodes

Silicon nanocrystals have shown attractive properties for photonic and photovoltaic applications. We demonstrate all-Si light-emitting diodes based on boron-doped Si nanocrystal/c-Si p-n heterojunction structure, which show electroluminescence in the visible/infrared regions. The electroluminescencespectra of these diodes can be modified by changing the quantum confining barriers from SiO2 to Si3N4. Our results are an important demonstration of electroluminescence from boron-doped Si nanocrystals—a wide band gap absorber material for third generation photovoltaics.

Determination of active doping in highly resistive boron doped silicon nanocrystals embedded in SiO2 by capacitance voltage measurement on inverted metal oxide semiconductor structure

We investigate the Capacitance-Voltage (CV) measurement to study the electrically active boron doping in Si nanocrystals (ncSi) embedded in SiO2. The ncSi thin films with high resistivity (200–400 Ω cm) can be measured by using an inverted metal oxide semiconductor (MOS) structure (Al/ncSi (B)/SiO2/Si). This device structure eliminates the complications from the effects of lateral current flow and the high sheet resistance in standard lateral MOS structures. The characteristic MOS CV curves observed are consistent with the effective p-type doping. The CV modeling method is presented and used to evaluate the electrically active doping concentration. We find that the highly boron doped ncSi films have electrically active doping of 1018–1019 cm−3 despite their high resistivity. The saturation of doping at about 1.4 × 1019 cm−3 and the low doping efficiency less than 5% are observed and discussed. The calculated effective mobility is in the order of 10−3 cm2/V s, indicating strong impurity/defect scattering e...

In-situ high resolution transmission electron microscopy observation of silicon nanocrystal nucleation in a SiO2 bilayered matrix

Solid-state nucleation of Si nanocrystals in a SiO2 bilayered matrix was observed at temperatures as low as 450 °C. This was achieved by aberration corrected high-resolution transmission electron microscopy (HRTEM) with real-time in-situ heating up to 600 °C. This technique is a valuable characterization tool especially with the recent interest in Si nanostructures for light emitting devices, non-volatile memories, and third-generation photovoltaics which all typically require a heating step in their fabrication. The control of size, shape, and distribution of the Si nanocrystals are critical for these applications. This experimental study involves in-situ observation of the nucleation of Si nanocrystals in a SiO2 bilayered matrix fabricated through radio frequency co-sputtering. The results show that the shapes of Si nanocrystals in amorphous SiO2 bilayered matrices are irregular and not spherical, in contrast to many claims in the literature. Furthermore, the Si nanocrystals are well confined within the...

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.

Investigation in feasibility of Molybdenum as a back contact layer for Silicon based quantum dot solar cells

A vertical structure with a back contact layer is suggested for silicon quantum dots (Si QDs) solar cells to overcome the current crowding effect arising from the high lateral resistance in the emitter layer of the existing mesa-structured Si QDs solar cells on quartz substrates. Molybdenum (Mo) is widely used as the back contact layer in CIGS solar cells due to its high electrical conductivity, good optical reflectance and chemical stability. This paper will focus on the feasibility of Mo as a back contact layer deposited between a quartz substrate and a sputtered silicon rich oxide (SRO) bilayers structure to obtain a fully vertical Si QDs solar cell. In this structure, the desired previously mentioned electrical and optical properties of the Mo thin film have to be maintained during and after a high temperature annealing process. This high temperature process is unavoidable in this structure as it is required to form the Si QDs. This paper aims to study factors that have impacts on critical properties of the Mo thin films processed in contact with Si and SiO2 at high temperatures. Characterizations including film thickness, microstructure, sheet resistance and optical reflectance measurements are also performed. Furthermore, interfacial properties between the Mo layer and the upper SRO bilayers are investigated.

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.