Abdelatif Jaouad

Fabrication of silicon nitride waveguides for visible-light using PECVD: a study of the effect of plasma frequency on optical properties

This paper presents work aimed at optimizing the fabrication of silicon nitride Si(x)N(y) thin-film visible-light planar waveguides using plasma-enhanced chemical vapour deposition (PECVD). The effects of plasma frequency, precursor gas ratio, and thermal annealing in relation to waveguide optical properties (refractive index, propagation losses) are studied. Experimental results over a wide range of precursor gas ratios show convincingly that waveguides fabricated using low-frequency PECVD have lower propagation losses in the visible range compared to waveguides of equal refractive index fabricated with high-frequency PECVD.

Antireflection Coating Design for Triple-Junction III–V/Ge High-Efficiency Solar Cells Using Low Absorption PECVD Silicon Nitride

The design of antireflection coating (ARC) for multijunction solar cells is challenging due to the broadband absorption and the need for current matching of each subcell. Silicon nitride, which is deposited by plasma-enhanced chemical vapor deposition (PECVD) using standard conditions, is widely used in the silicon wafer solar cell industry but typically suffers from absorption in the short-wavelength range. We propose the use of silicon nitride deposited by low-frequency PECVD (LFSiN) optimized for high refractive index and low optical absorption as a part of the ARC design for III–V/Ge triple-junction solar cells. This material can also act as a passivation/encapsulation coating. Simulations show that the SiO

Optimized Pre-Treatment Process for MOS-GaN Devices Passivation

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Five-volt vertically-stacked, single-cell GaAs photonic power converter

/LFSiN double-layer ARC can be very effective in reducing the reflection losses over the wavelength range of the limiting subcell for top subcell-limited, as well as middle subcell-limited, triple-junction solar cells. We also demonstrate that the structure’s performance is stable over expected variations in the layer parameters (thickness and refractive index) in the vicinity of the optimal values.

AlGaAs tunnel junction for high efficiency multi-junction solar cells: Simulation and measurement of temperature-dependent operation

In this letter, we present an effective GaN surface passivation process, which was developed by optimizing the surface chemical pretreatment prior to the PECVD- SiOx deposition. It is demonstrated that the electronic properties of the GaN/SiOx interface are drastically influenced by the surface preparation conditions. Among the used chemicals, we found that KOH/HCl leads to the best GaN/SiOx interface quality. MOS capacitors fabricated using this pretreatment have shown a near ideal capacitance-voltage characteristics, with a good surface potential modulation, small flatband voltage shift, low hysteresis, and no significant frequency dispersion. Using this optimized passivation process, AlGaN/GaN-based MOS-high electron mobility transistors (HEMTs) were fabricated. Electrical characterizations have shown up to four orders of magnitude lower gate leakage current and three orders of magnitude lower off-state current compared with the reference Schottky gate HEMT.

Advances with vertical epitaxial heterostructure architecture (VEHSA) phototransducers for optical to electrical power conversion efficiencies exceeding 50 percent

The high-efficiency conversion of photonic power into electrical power is of broad-range applicability to many industries due to its electrical isolation from the surrounding environment and immunity to electromagnetic interference which affects the performance and reliability of sensitive electronics. A photonic power converter, or phototransducer, can absorb several watts of infrared laser power transmitted through a multimode fiber and convert this to electrical power for remote use. To convert this power into a useful voltage, we have designed, simulated, and fabricated a photovoltaic phototransducer that generates >5 V using a monolithic, lattice-matched, vertically-stacked, single-cell device that eliminates complex fabrication and assembly steps. Experimental measurements have demonstrated a conversion efficiency of up to 60.1% under illumination of ~11 W/cm2 at a wavelength of 835 nm, while simulations indicate that efficiencies reaching 70% should be realistically achievable using this novel design.

Electrothermal Mapping of AlGaN/GaN HEMTs Using Microresistance Thermometer Detectors

AlGaAs tunnel junctions are shown to be well-suited to concentrated photovoltaics where temperatures and current densities can be dramatically higher than for 1-sun flat-panel systems. Detailed comparisons of AlGaAs/AlGaAs tunnel junction experimental measurements over a range of temperatures expected during device operation in concentrator systems are presented. Experimental and simulation results are compared in an effort to decouple the tunnel junction from the overall multi-junction solar cell. The tunnel junction resistance is experimentally studied as a function of the temperature to determine its contribution to overall efficiency of the solar cell. The current-voltage behavior of the isolated TJ shows that as the temperature is increased from 25°C to 85°C, the resistance decreases from ~4.7×10-4 ¿·cm2 to ~0.3×10-4 ¿·cm2 for the operational range of a multi-junction solar cell under concentration.

Enhanced photocarrier extraction mechanisms in ultra-thin photovoltaic GaAs n/p junctions

A monolithic compound semiconductor phototransducer optimized for narrow-band light sources was designed for and has achieved conversion efficiencies exceeding 50%. The III-V heterostructure was grown by MOCVD, based on the vertical stacking of a number of partially absorbing GaAs n/p junctions connected in series with tunnel junctions. The thicknesses of the p-type base layers of the diodes were engineered for optimal absorption and current matching for an optical input with wavelengths centered in the 830 nm to 850 nm range. The device architecture allows for improved open-circuit voltage in the individual base segments due to efficient carrier extraction while simultaneously maintaining a complete absorption of the input photons with no need for complicated fabrication processes or reflecting layers. Progress for device outputs achieving in excess of 12 V is reviewed in this study.

Effect of parameter variations on the current-voltage behavior of AlGaAs tunnel junction models

Self-heating effects in AlGaN/GaN high-electron mobility transistors (HEMTs) can notably reduce electron mobility and produce reliability concerns. Electrothermal characterization and appropriate thermal management are required to address this situation. This letter presents the measurement of channel temperature (T_ch) of GaN HEMTs in multiple bias conditions with a good accuracy. The measurements are executed using the integrated microresistance thermometer detector (μRTD) technique in AlGaN/GaN HEMTs on SiC and sapphire substrates. The integrated Ti/Pt μRTD sensor with linear resistance-temperature characteristic is used to obtain an I_ds-V_ds-T_ch map for each device. Thermal resistances are compared for similar operation conditions, obtaining RTH = 34.7 °C · W^-1 for the HEMT on SiC and RTH = 157.2 °C · W^-1 for the HEMT on sapphire.

Through cell vias contacts for multijunction solar cells

PV devices with active areas of ~3:4 mm2 were fabricated and tested with top electrodes having different emitter gridline spacings with active area shadowing values between 0% and 1.8%. As expected, the thicker n/p junctions exhibit hindered photocarrier extraction, with low fill factor (FF) values, for devices prepared with sparse gridline designs. However, this study clearly demonstrates that for thin n/p junctions photocarrier extraction can still be efficient (FF > 80%) even for devices with no gridlines, which we explain using a TCAD model. The electric field profiles of devices with and without hindered photocarrier extraction are also discussed.