Search

Inelastic X-ray scattering (IXS) on an In0.17 Ga0.83As/ GaAs0.8 P0.2 multiple quantum well (MQW) superlattice has been conducted to investigate the potential for phonon bottlenecks in low dimensional materials. This work shows that the measured spectra are in good agreement with an adiabatic bond charge model prediction and back-folded phonon modes make large contributions to the broadening of peaks observed in the spectra. The high-lying mode at 45 meV in the MQW is attributed to vibrations of Ga and P and confirmed by both experiment and theory. The acoustic phonons have a dominant contribution from the Ga and As components, and the contribution from InAs is small and only visible at around 29.7 meV. Low energy optical modes resulting from back-folding might be a key to increased electron-phonon coupling in the material system. The suitability of utilizing IXS as a means to investigate phonon modes in low dimensional materials is also discussed.

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.

We have investigated the ultrafast carrier dynamics in a 1 μm bulk In0.265Ga0.735N thin film grown using energetic neutral atom-beam lithography/epitaxy molecular beam epitaxy. Cathodoluminescence and X-ray diffraction experiments are used to observe the existence of indium-rich domains in the sample. These domains give rise to a second carrier population and bi-exponential carrier cooling is observed with characteristic lifetimes of 1.6 and 14 ps at a carrier density of 1.3 × 1016 cm−3. A combination of band-filling, screening, and hot-phonon effects gives rise to a two-fold enhanced mono-exponential cooling rate of 28 ps at a carrier density of 8.4 × 1018 cm−3. This is the longest carrier thermalization time observed in bulk InGaN alloys to date.

Though Ge crystallization has been widely studied, few works investigate metal-induced crystallization of ultrathin Ge films. For 2 nm Ge films in oxide matrix, crystallization becomes challenging due to easy oxidation and low mobility of Ge atoms. Introducing metal atoms may alleviate these problems, but the functions and the behaviours of metal atoms need to be clarified. This paper investigates the crystallization dynamics of a multilayer structure 1.9 nm Ge/0.5 nm Al/1.5 nm Al2O3 under rapid thermal annealing (RTA). The functions of metal atoms, like effective anti-oxidation, downshifting Raman peaks, and incapability to decrease crystallization temperature, are found and explained. The metal behaviours, such as inter-diffusion and defect generation, are supported with direct evidences, Al-Ge nanobicrystals, and Al cluster in Ge atoms. With these understandings, a two-step RTA process achieves high-quality 2 nm nanocrystal Ge films with Raman peak at 298 cm−1 of FWHM 10.3 cm−1 and atomic smooth interf...

In this work, an industrially viable manufacturing process—rapid thermal annealing (RTA) of Molybdenum back contact is proposed and investigated to improve the performance of sputtered Cu2ZnSnS4 (CZTS) solar cells. The RTA process was found to facilitate Na diffusion from soda lime glass to Mo as well as CZTS and improve the crystallinity of the Mo film. Consequently, the surface morphology of the subsequently deposited CZTS absorbers is improved, which results in significant enhancement of open circuit voltage, short-circuit current density, fill factor, and conversion efficiency.

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.

The rates of charge carrier relaxation by phonon emission are of substantial importance in the field of hot carrier solar cell, primarily in investigation of mechanisms to slow down hot carrier cooling. In this work, energy and momentum resolved deformation potentials relevant to electron-phonon scattering are computed for wurtzite InN and GaN as well as an InN/GaN multiple quantum well (MQW) superlattice using ab-initio methods. These deformation potentials reveal important features such as discontinuities across the electronic bandgap of the materials and variations over tens of eV. The energy dependence of the deformation potential is found to be very similar for wurtzite nitrides despite differences between the In and Ga pseudopotentials and their corresponding electronic band structures. Charge carrier relaxation by this mechanism is expected to be minimal for electrons within a few eV of the conduction band edge. However, hole scattering at energies more accessible to excitation by solar radiation is possible between heavy and light hole states. Moderate reductions in overall scattering rates are observed in MQW relative to the bulk nitride materials.

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...

Modulation doping, an effective way to dope quantum dots (QDs), modifies the confinement energy levels in the QDs. We present a self-consistent full multi-grid solver to analyze the effect of modulation doping on the confinement energy levels in large-area structures containing Si QDs in SiO2 and Si3N4 dielectrics. The confinement energy was found to be significantly lower when QDs were in close proximity to dopant ions in the dielectric. This effect was found to be smaller in Si3N4, while smaller QDs in SiO2 were highly susceptible to energy reduction. The energy reduction was found to follow a power law relationship with the QD size.

The performance of hot carrier solar cells has been analyzed using a model incorporating the relaxation times of the relevant electronic processes. The variations in the maximum efficiency as well as the carrier statistics are demonstrated according to different carrier-carrier scattering rates. The significant dependance of cell performances on Coulomb scattering properties is quantitatively demonstrated and explained by carrier depletion, asymmetric electron and hole statistics, and unmatched transmission properties for carrier extraction. The inter-relation between the electronic properties of the absorber material and the contact properties implies that an integrated design with both parts is required.

The hot carrier solar cell is a third generation photovoltaic device that extracts photo-generated carriers before they thermalise. In this work, the efficiency of a hot carrier solar cell with a 50 nm indium nitride (InN) absorber layer has been calculated, taking into account the realistic transport properties of energy selective contacts. The cell performance has been modeled considering the carrier extraction through contacts as ballistic. A potential practical implementation of a hot carrier solar cell, with contacts based on an InXGa1−XN/InN/InXGa1−XN quantum well structure, has been proposed, with calculated maximum efficiency of 37.15% under 1000 suns.

The performance of an InN based hot carrier solar cell with a bulk InN absorber has been evaluated using an innovative approach that takes into account absorber energy-momentum dispersion relations, energy conservation, Auger recombination and impact ionization mechanisms simultaneously. The non ideality of the energy selective filters has also been included in the model. In order to obtain practical achievable values of conversion efficiency, the actual thermalisation velocity of hot carriers in InN has been measured using time resolved photoluminescence. Results of the computations shown limiting efficiencies of 24% for 1000 suns and 36.2% for maximal concentration.

Highly conductive thin films composed of Ge nanocrystals (Ge-NCs) embedded in SiO2 matrix were prepared using a low thermal budget process. P-type behavior in the undoped films was attributed to the hole accumulation caused by acceptor-like surface states. Quantitative analysis has demonstrated that the density of surface states was sufficient to cause the measured electrical conductivity. The charge transport mechanism was identified as thermally activated charge hopping from one NC to its nearest neighbor. Postgrowth rapid thermal annealing dramatically increased the conductivities and this was tentatively explained by the modification of Ge-NCs’ surface structure and reduction in defects in SiO2 matrix.

A simple and silicon process-compatible technique is reported for the synthesis of Ge nanocrystals (Ge-ncs) at low temperatures below 400 °C, which is much lower than the typical growth temperatures. The Ge-ncs were found to form only within a temperature window between 350 and 420 °C. The underlying mechanism has been explained by a competitive process between Volmer–Weber growth and oxidation reaction. We further implemented this technique in the fabrication of multilayered Ge-ncs which exhibited controllable crystallite size with high crystallization quality. The low temperature technique developed in this work would allow production of Ge-ncs and relative devices on low cost substrates, such as glass.

We fabricated p-i-n diodes by sputtering alternating layers of silicon dioxide and silicon rich oxide with a nominal atomic ratio O/Si=0.7 onto quartz substrates with in situ boron for p-type and phosphorus for n-type doping. After crystallization, dark and illuminated I-V characteristics show a diode behavior with an open circuit voltage of 373 mV. Due to the thinness of the layers and their corresponding high resistivity, lateral current flow results in severe current crowding. This effect is taken into account when extracting the electronic bandgap based on temperature dependent diode I-V measurements.

Heterojunctions (HJs) were fabricated from p-type Si nanocrystals (Si NCs) embedded in a SiC matrix on an n-type crystalline Si substrate. Transmission electron microscopy revealed that Si NCs are clearly established, with sizes in the range of 3–5nm. The HJ diodes showed a good rectification ratio of 1.0×104 at ±1.0V at 298K. The ideality factor, junction built-in potential, and open-circuit voltage are ∼1.24, 0.72V, and 0.48V, respectively. Measurement of temperature-dependent I-V curves in forward conduction suggests that, in the medium voltage range, junction interface recombination can be described as the dominant current transport mechanism.