Epitaxial Interdigitated Back Contact (IBC) solar cell test platform for novel light trapping schemes

An Interdigitated Back Contact (IBC) solar cell is being developed for evaluation of emerging light trapping schemes of silicon nanowire arrays on pyramidal textured surfaces. The front surface of the baseline IBC cell design was optimized with a thin film coating considering both antireflection and passivation to reduce surface recombination. Addition of a front surface field (FSF) was shown to improve the surface passivation of the cell. PC2D simulations of the baseline device predict an efficiency of 17.4%. Silicon nanowire arrays and hybrid structures of silicon nanowires on pyramids were successfully fabricated. Hemispherical reflectance measurements show that a weighted average reflectance of just 1.89% was achieved. With adequate surface passivation, these highly-effective antireflective structures could result in a power conversion efficiency increase compared to traditional light trapping methods when incorporated into the IBC cell

Low temperature silicon nitride waveguides for multilayer platforms

Several 3D multilayer silicon photonics platforms have been proposed to provide densely integrated structures for complex integrated circuits. Amongst these platforms, great interest has been given to the inclusion of silicon nitride layers to achieve low propagation losses due to their capacity of providing tight optical confinement with low scattering losses in a wide spectral range. However, none of the proposed platforms have demonstrated the integration of active devices. The problem is that typically low loss silicon nitride layers have been fabricated with LPCVD which involves high processing temperatures (<1000 ºC) that affect metallisation and doping processes that are sensitive to temperatures above 400ºC. As a result, we have investigated ammonia-free PECVD and HWCVD processes to obtain high quality silicon nitride films with reduced hydrogen content at low temperatures. Several deposition recipes were defined through a design of experiments methodology in which different combinations of deposition parameters were tested to optimise the quality and the losses of the deposited layers. The physical, chemical and optical properties of the deposited materials were characterised using different techniques including ellipsometry, SEM, FTIR, AFM and the waveguide loss cut-back method. Silicon nitride layers with hydrogen content between 10-20%, losses below 10dB/cm and high material quality were obtained with the ammonia-free recipe. Similarly, it was demonstrated that HWCVD has the potential to fabricate waveguides with low losses due to its capacity of yielding hydrogen contents <10% and roughness <1.5nm

Junction formation with HWCVD and TCAD model of an epitaxial back-contact solar cell

In this paper, we present morphological and electrical characteristics of a junction formed of Si p-type films deposited on an n-type silicon wafer using a hot wire chemical vapor deposition (HWCVD) tool. We describe the fabrication process and study the influence of diborane flow and postprocess annealing in improving junction characteristics. Our morphological studies undertaken using atomic force microscopy show that the initial deposition suffered from voids rather than being a uniform film; however, this improves significantly under our annealing treatment. The improvement in morphology was observed in the electrical characteristics, with estimated Voc doubling and rectification of the junction improving by several orders of magnitude. Fitting of the current-voltage curves to a two-diode model showed that increasing the diborane flow in the process helps reduce the saturation current and ideality factors, while increasing the shunt resistance. Electrochemical capacitance-voltage (ECV) and quasi-steady-state photoconductance measurements are used to characterize the deposited films further. A solar cell device with a silicon epitaxy emitter is modeled using industry-standard 3-D modeling tools and input parameters from experimental data, and the impact of defects is studied. A potential efficiency approaching 25% is shown to be feasible for an optimized device

Hot-wire chemical vapour deposition for silicon nitride waveguides

In this work, we demonstrate the use of HWCVD as an alternative technique to grow SiN layers for photonic waveguides at temperatures <400ºC. In particular, the effect of the ammonia flow and the filament temperature on the material structure, optical properties and propagation losses of the deposited films was investigated. SiN layers with good thickness uniformity, roughness as low as 0.61nm and H concentration as low as 10.4×1021 atoms/cm3 were obtained. Waveguides fabricated on the studied materials exhibited losses as low as 7.1 and 12.3 dB/cm at 1310 and 1550nm respectively

Hot-wire polysilicon waveguides with low deposition temperature

We fabricated and measured the optical loss of polysilicon waveguides deposited using Hot-Wire Chemical Vapour Deposition (HWCVD) at a temperature of 240°C. A polysilicon film 220 nm thick was deposited on top of a 2000 nm thick PECVD silicon dioxide. The crystalline volume fraction of the polysilicon film was measured by Raman spectroscopy to be 91%. The optical propagation losses of 400, 500, and 600 nm waveguides were measured to be 16.9, 15.9, and 13.5 dB/cm, respectively, for transverse electric (TE) mode at the wavelength of 1550 nm. Scattering loss is expected to be the major contributor to the propagation loss

Design, modeling, fabrication, and evaluation of thermoelectric generators with hot-wire chemical vapor deposited polysilicon as thermoelement material

This paper presents the design, modeling, fabrication, and evaluation of thermoelectric generators (TEGs) with p-type polysilicon deposited by hot-wire chemical vapor deposition (HWCVD) as thermoelement material. A thermal model is developed based on energy balance and heat transfer equations using lumped thermal conductances. Several test structures were fabricated to allow characterization of the boron-doped polysilicon material deposited by HWCVD. The film was found to be electrically active without any post-deposition annealing. Based on the tests performed on the test structures, it is determined that the Seebeck coefficient, thermal conductivity, and electrical resistivity of the HWCVD polysilicon are 113 µV/K, 126 W/mK, and 3.58 × 10-5 µm, respectively. Results from laser tests performed on the fabricated TEG are in good agreement with the thermal model. The temperature values derived from the thermal model are within 2.8% of the measured temperature values. For a 1W laser input, an open-circuit voltage and output power of 247 mV and 347 nW, respectively, were generated. This translates to a temperature difference of 63°C across the thermoelements. This paper demonstrates that HWCVD, which is a cost-effective way of producing solar cells, can also be applied in the production of TEGs. By establishing that HWCVD polysilicon can be an effective thermoelectric material, further work on developing photovoltaic-thermoelectric (PV-TE) hybrid microsystems that are cost-effective and better performing can be explored

Optimisation of ex-situ annealing process for epitaxial silicon emitters via Hot Wire CVD

A refined Hot Wire Chemical Vapour Deposition (HWCVD) process for fabricating boron doped silicon emitters using a shortened anneal time and temperature is presented. We are able to crystallise our grown films with a post-deposition annealing treatment of 2 minutes at 800°C, a significant improvement from previous work using several hours at 1000°C. Direct implications of higher annealing temperatures on the film quality is discussed. In addition, the potential for in situ annealing using a higher deposition temperature is presented, with future works aiming to find and apply the optimum deposition temperature for epitaxial silicon with HWCVD

Vertically coupled a-Si:H multimode interference waveguides for multi-layer silicon photonics platform

We successfully demonstrated low temperature fabrication process of vertical MMI a:Si-H waveguides for multi-layer photonic integrated circuit. Measured MMI loss of 1.97dB/MMI and vertical light coupling of TE polarization at 1550nm wavelength have been achieved

p⁺ polycrystalline silicon growth via hot wire chemical vapour deposition for silicon solar cells

Hot wire chemical vapour deposition (HWCVD) is explored as a way of growing boron-doped silicon for photovoltaic devices. Deposition temperatures are measured using a custom-built monitoring system for two different filament configurations within the HWCVD tool. A refined fabrication process is presented, using an altered filament arrangement, that currently provides a maximum deposition temperature of 535 °C, for growing boron-doped silicon films via HWCVD, with the inclusion of a short post-deposition anneal at 800 °C for 2 min. Transmission electron microscopy reveals improvements in interfacial quality, as well as larger grains, present after post-annealing treatments. In addition, re-crystallisation of as-deposited amorphous Si films under a short anneal is confirmed using Raman spectroscopy. The enhancements in morphology translate to a boost in current rectification based on dark current-voltage measurements. This is further supported by secondary-ion mass spectrometry analysis, presenting p+ properties with uniform doping in the 1021 cm−3 region