Highly conductive and transparent films of ultra-thin p-type nickel silicide films have been prep... more Highly conductive and transparent films of ultra-thin p-type nickel silicide films have been prepared by RF magnetron sputtering of nickel on silicon substrates followed by rapid thermal annealing in an inert environment in the temperature range 400–600 °C. The films are uniform throughout the wafer with thicknesses in the range of 3–6 nm. The electrical and optical properties are presented for nickel silicide films with varying thickness. The Drude–Lorentz model and Fresnel equations were used to calculate the dielectric properties, sheet resistance, absorption and transmission of the films. These ultrathin nickel silicide films have excellent optoelectronic properties for p-type contacts with optical transparencies up to 80% and sheet resistance as low as ~0.15 µΩ cm. Furthermore, it was shown that the use of a simple anti-reflection (AR) coating can recover most of the reflected light approaching the values of a standard Si solar cell with the same AR coating. Overall, the combination of ultra-low thickness, high transmittance, low sheet resistance and ability to recover the reflected light by utilizing standard AR coating makes them ideal for utilization in silicon based photovoltaic technologies as a p-type transparent conductor.
Group-IIIb-transition-metal-alloyed wurtzite Group-IIIa-nitride (IIIb-IIIa-N) thin films have hig... more Group-IIIb-transition-metal-alloyed wurtzite Group-IIIa-nitride (IIIb-IIIa-N) thin films have higher piezoelectric characteristics than binary IIIa-N for a broad range of applications in photonic, electronic, sensing, and energy harvesting systems. We perform theoretical thermodynamic analysis for the deposition and epitaxial growth of Y-alloyed GaN and AlN films by a newly introduced growth technique of hybrid chemical vapor deposition (HybCVD), which can overcome the limitations of the conventional techniques. We investigate the equilibrium vapor pressures in the source zones to determine the dominant precursors of cations for the input of the mixing zone. Then, we study the driving force for the vapor-solid phase reactions of cation precursors in the growth zone to calculate the relationship between the solid composition of YxGa1−xN and YxAl1−xN and the relative amount of input precursors (Y vs. GaCl and AlCl3) in different deposition conditions, such as temperature, V/III precur...
Silver nanowire (Ag NWs) networks with high transparency and low resistivity are widely used as p... more Silver nanowire (Ag NWs) networks with high transparency and low resistivity are widely used as promising candidates for the replacement of indium tin oxide (ITO)-based transparent conducting oxides (TCOs) in light-emitting diodes (LEDs). However, LEDs with Ag NW electrodes are less efficient than those with ITO electrodes because of their low electrical properties, such as high contact resistance and strong absorption in the visible region. In this work, we tried to improve the efficiency of LEDs with transparent conducting electrodes of Ag NWs networks via localized surface plasmons (LSPs) by adopting silver nanoparticles. We studied the effect of the thickness of the p-GaN layer on surface plasmon coupling. When a 45 nm thick p-GaN layer was used, the internal quantum efficiency was improved by LSP coupling between a dipole of QW and Ag NW/NP, and the light extraction was improved because the NPs afforded a leakage mode and acted as scattering centers.
2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC) (A Joint Conference of 45th IEEE PVSC, 28th PVSEC & 34th EU PVSEC), 2018
III-V compound semiconductors are the best photovoltaic solar cell (SC) materials for highest con... more III-V compound semiconductors are the best photovoltaic solar cell (SC) materials for highest conversion efficiencies. However, they are expensive and lack in mechanical flexibility and manufacturing scalability. We developed a new PV platform where nearly single-crystalline III-V thin films were directly deposited on inexpensive polycrystalline metal tapes. While a promising efficiency of ~7.5% was achieved for flexible single junction SCs on this platform, the open circuit voltage of SC suffers from minority carrier recombination at grainboundary sites according to a simulation study. A bulk passivation treatment is conducted on the SC devices which resulted in more than 350mV increase for Voc.
2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC) (A Joint Conference of 45th IEEE PVSC, 28th PVSEC & 34th EU PVSEC), 2018
We have demonstrated GaAs single junction solar cells on epi-ready low-cost flexible metal tape a... more We have demonstrated GaAs single junction solar cells on epi-ready low-cost flexible metal tape and studied the impact of passivation on base thickness of the fabricated device. In our process, single-crystalline-like germanium films are grown on the flexible metal tape over which epitaxial (Al)GaAs semiconductor thin films are grown by metal organic chemical vapor deposition (MOCVD). The grown device architecture was modified with different GaAs base thickness. Passivation using citric acid for GaAs with thinner base of 378nm resulted in reduction of leakage current by two orders of magnitude more than that with thicker base of 1137nm. Photon conversion efficiency greater than 6% with open circuit voltage $\mathrm{(V_{OC}})$ of 560mV, short circuit current density $\mathrm{(J_{SC}})$ of 16.8mA/cm}2 and fill factor (FF) of 69% was observed under A.M 1.5 (1 sun) after surface passivation and anti-reflection coating (ARC). Further improvement in device efficiency is achieved with new ...
2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC) (A Joint Conference of 45th IEEE PVSC, 28th PVSEC & 34th EU PVSEC), 2018
High-quality epitaxial and single-crystalline-like GaAs thin films grown by metal-organic chemica... more High-quality epitaxial and single-crystalline-like GaAs thin films grown by metal-organic chemical vapor deposition (MOCVD) on ion-beam textured buffers on flexible metal substrates have been reported earlier. Here we report optimization of lattice-matched InGaP on GaAs for the growth of 2J solar cells. The grown InGaP films were investigated by highresolution X-ray diffraction (HRXRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Further, the flexible InGaP films were used to grow doublejunction GaAs solar cells. The 1J GaAs solar cells resulted in 11 % efficiency as reported earlier, show promise for low-cost, flexible, lightweight and high-performance III-V multijunction solar cells which can be a potentially game-changing technology in the photovoltaic market.
The undoped BAlN electron-blocking layer (EBL) is investigated to replace the conventional AlGaN ... more The undoped BAlN electron-blocking layer (EBL) is investigated to replace the conventional AlGaN EBL in light-emitting diodes (LEDs). Numerical studies of the impact of variously doped EBLs on the output characteristics of LEDs demonstrate that the LED performance shows heavy dependence on the p-doping level in the case of the AlGaN EBL, while it shows less dependence on the p-doping level for the BAlN EBL. As a result, we propose an undoped BAlN EBL for LEDs to avoid the p-doping issues, which a major technical challenge in the AlGaN EBL. Without doping, the proposed BAlN EBL structure still possesses a superior capacity in blocking electrons and improving hole injection compared with the AlGaN EBL having high doping. Compared with the Al0.3Ga0.7N EBL with a doping concentration of 1 × 1020 cm−3, the undoped BAlN EBL LED still shows lower droop (only 5%), compatible internal quantum efficiency (2% enhancement), and optical output power (6% enhancement). This study provides a feasib...
Gallium nitride (GaN) has emerged as one of the most attractive base materials for next-generatio... more Gallium nitride (GaN) has emerged as one of the most attractive base materials for next-generation high-power and high-frequency electronic devices. Recent efforts have focused on realizing vertical power device structures such as in situ oxide, GaN interlayer based vertical trench metal–oxide–semiconductor field-effect transistors (OG-FETs). Unfortunately, the higher-power density of GaN electronics inevitably leads to considerable device self-heating which impacts device performance and reliability. Halide vapor-phase epitaxy (HVPE) is currently the most common approach for manufacturing commercial GaN substrates used to build vertical GaN transistors. Vertical device structures consist of GaN layers of diverse doping levels. Hence, it is of crucial importance to measure and understand how the dopant type (Si, Fe, and Mg), doping level, and crystal quality alter the thermal conductivity of HVPE-grown bulk GaN. In this work, a steady-state thermoreflectance (SSTR) technique was use...
GaN-based high-power wide-bandgap semiconductor electronics and photonics have been considered as... more GaN-based high-power wide-bandgap semiconductor electronics and photonics have been considered as promising candidates to replace conventional devices for automotive applications due to high energy conversion efficiency, ruggedness, and superior transient performance. However, performance and reliability are detrimentally impacted by significant heat generation in the device active area. Therefore, thermal management plays a critical role in the development of GaN-based high-power electronic and photonic devices. This paper presents a comprehensive review of the thermal management strategies for GaN-based lateral power/RF transistors and light-emitting diodes (LEDs) reported by researchers in both industry and academia. The review is divided into three parts: (1) a survey of thermal metrology techniques, including infrared thermography, Raman thermometry, and thermoreflectance thermal imaging, that have been applied to study GaN electronics and photonics; (2) practical thermal manag...
Nickel silicides are widely used as contact materials for electronic devices based on silicon (Si... more Nickel silicides are widely used as contact materials for electronic devices based on silicon (Si). However, they have been predominantly fabricated by annealing separate Ni and Si phases which leads to phase and structural complexity. In this letter, direct epitaxial growth of a single-phase nickel disilicide (NiSi2) thin film by sputter deposition of NiSi2 is achieved on low-cost and flexible Hastelloy tapes which offers a promising route to fabricate low-cost, flexible electronic devices. Biaxially textured titanium nitride (TiN) is applied as the seeding layer and the diffusion barrier under NiSi2. An epitaxial relationship of (001)⟨100⟩NiSi2 ǁ (001)⟨110⟩TiN is observed with an extra-large lattice mismatch (∼10.3%) between NiSi2 and TiN. Both the bonding similarity and the passivation effect by hydrogen promote the epitaxial growth of NiSi2 on TiN. The flat and smooth NiSi2 thin film consists of grains with a size of 50–100 nm. An epitaxially grown Si film on NiSi2 further demon...
Highly conductive and transparent films of ultra-thin p-type nickel silicide films have been prep... more Highly conductive and transparent films of ultra-thin p-type nickel silicide films have been prepared by RF magnetron sputtering of nickel on silicon substrates followed by rapid thermal annealing in an inert environment in the temperature range 400–600 °C. The films are uniform throughout the wafer with thicknesses in the range of 3–6 nm. The electrical and optical properties are presented for nickel silicide films with varying thickness. The Drude–Lorentz model and Fresnel equations were used to calculate the dielectric properties, sheet resistance, absorption and transmission of the films. These ultrathin nickel silicide films have excellent optoelectronic properties for p-type contacts with optical transparencies up to 80% and sheet resistance as low as ~0.15 µΩ cm. Furthermore, it was shown that the use of a simple anti-reflection (AR) coating can recover most of the reflected light approaching the values of a standard Si solar cell with the same AR coating. Overall, the combination of ultra-low thickness, high transmittance, low sheet resistance and ability to recover the reflected light by utilizing standard AR coating makes them ideal for utilization in silicon based photovoltaic technologies as a p-type transparent conductor.
Group-IIIb-transition-metal-alloyed wurtzite Group-IIIa-nitride (IIIb-IIIa-N) thin films have hig... more Group-IIIb-transition-metal-alloyed wurtzite Group-IIIa-nitride (IIIb-IIIa-N) thin films have higher piezoelectric characteristics than binary IIIa-N for a broad range of applications in photonic, electronic, sensing, and energy harvesting systems. We perform theoretical thermodynamic analysis for the deposition and epitaxial growth of Y-alloyed GaN and AlN films by a newly introduced growth technique of hybrid chemical vapor deposition (HybCVD), which can overcome the limitations of the conventional techniques. We investigate the equilibrium vapor pressures in the source zones to determine the dominant precursors of cations for the input of the mixing zone. Then, we study the driving force for the vapor-solid phase reactions of cation precursors in the growth zone to calculate the relationship between the solid composition of YxGa1−xN and YxAl1−xN and the relative amount of input precursors (Y vs. GaCl and AlCl3) in different deposition conditions, such as temperature, V/III precur...
Silver nanowire (Ag NWs) networks with high transparency and low resistivity are widely used as p... more Silver nanowire (Ag NWs) networks with high transparency and low resistivity are widely used as promising candidates for the replacement of indium tin oxide (ITO)-based transparent conducting oxides (TCOs) in light-emitting diodes (LEDs). However, LEDs with Ag NW electrodes are less efficient than those with ITO electrodes because of their low electrical properties, such as high contact resistance and strong absorption in the visible region. In this work, we tried to improve the efficiency of LEDs with transparent conducting electrodes of Ag NWs networks via localized surface plasmons (LSPs) by adopting silver nanoparticles. We studied the effect of the thickness of the p-GaN layer on surface plasmon coupling. When a 45 nm thick p-GaN layer was used, the internal quantum efficiency was improved by LSP coupling between a dipole of QW and Ag NW/NP, and the light extraction was improved because the NPs afforded a leakage mode and acted as scattering centers.
2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC) (A Joint Conference of 45th IEEE PVSC, 28th PVSEC & 34th EU PVSEC), 2018
III-V compound semiconductors are the best photovoltaic solar cell (SC) materials for highest con... more III-V compound semiconductors are the best photovoltaic solar cell (SC) materials for highest conversion efficiencies. However, they are expensive and lack in mechanical flexibility and manufacturing scalability. We developed a new PV platform where nearly single-crystalline III-V thin films were directly deposited on inexpensive polycrystalline metal tapes. While a promising efficiency of ~7.5% was achieved for flexible single junction SCs on this platform, the open circuit voltage of SC suffers from minority carrier recombination at grainboundary sites according to a simulation study. A bulk passivation treatment is conducted on the SC devices which resulted in more than 350mV increase for Voc.
2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC) (A Joint Conference of 45th IEEE PVSC, 28th PVSEC & 34th EU PVSEC), 2018
We have demonstrated GaAs single junction solar cells on epi-ready low-cost flexible metal tape a... more We have demonstrated GaAs single junction solar cells on epi-ready low-cost flexible metal tape and studied the impact of passivation on base thickness of the fabricated device. In our process, single-crystalline-like germanium films are grown on the flexible metal tape over which epitaxial (Al)GaAs semiconductor thin films are grown by metal organic chemical vapor deposition (MOCVD). The grown device architecture was modified with different GaAs base thickness. Passivation using citric acid for GaAs with thinner base of 378nm resulted in reduction of leakage current by two orders of magnitude more than that with thicker base of 1137nm. Photon conversion efficiency greater than 6% with open circuit voltage $\mathrm{(V_{OC}})$ of 560mV, short circuit current density $\mathrm{(J_{SC}})$ of 16.8mA/cm}2 and fill factor (FF) of 69% was observed under A.M 1.5 (1 sun) after surface passivation and anti-reflection coating (ARC). Further improvement in device efficiency is achieved with new ...
2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC) (A Joint Conference of 45th IEEE PVSC, 28th PVSEC & 34th EU PVSEC), 2018
High-quality epitaxial and single-crystalline-like GaAs thin films grown by metal-organic chemica... more High-quality epitaxial and single-crystalline-like GaAs thin films grown by metal-organic chemical vapor deposition (MOCVD) on ion-beam textured buffers on flexible metal substrates have been reported earlier. Here we report optimization of lattice-matched InGaP on GaAs for the growth of 2J solar cells. The grown InGaP films were investigated by highresolution X-ray diffraction (HRXRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Further, the flexible InGaP films were used to grow doublejunction GaAs solar cells. The 1J GaAs solar cells resulted in 11 % efficiency as reported earlier, show promise for low-cost, flexible, lightweight and high-performance III-V multijunction solar cells which can be a potentially game-changing technology in the photovoltaic market.
The undoped BAlN electron-blocking layer (EBL) is investigated to replace the conventional AlGaN ... more The undoped BAlN electron-blocking layer (EBL) is investigated to replace the conventional AlGaN EBL in light-emitting diodes (LEDs). Numerical studies of the impact of variously doped EBLs on the output characteristics of LEDs demonstrate that the LED performance shows heavy dependence on the p-doping level in the case of the AlGaN EBL, while it shows less dependence on the p-doping level for the BAlN EBL. As a result, we propose an undoped BAlN EBL for LEDs to avoid the p-doping issues, which a major technical challenge in the AlGaN EBL. Without doping, the proposed BAlN EBL structure still possesses a superior capacity in blocking electrons and improving hole injection compared with the AlGaN EBL having high doping. Compared with the Al0.3Ga0.7N EBL with a doping concentration of 1 × 1020 cm−3, the undoped BAlN EBL LED still shows lower droop (only 5%), compatible internal quantum efficiency (2% enhancement), and optical output power (6% enhancement). This study provides a feasib...
Gallium nitride (GaN) has emerged as one of the most attractive base materials for next-generatio... more Gallium nitride (GaN) has emerged as one of the most attractive base materials for next-generation high-power and high-frequency electronic devices. Recent efforts have focused on realizing vertical power device structures such as in situ oxide, GaN interlayer based vertical trench metal–oxide–semiconductor field-effect transistors (OG-FETs). Unfortunately, the higher-power density of GaN electronics inevitably leads to considerable device self-heating which impacts device performance and reliability. Halide vapor-phase epitaxy (HVPE) is currently the most common approach for manufacturing commercial GaN substrates used to build vertical GaN transistors. Vertical device structures consist of GaN layers of diverse doping levels. Hence, it is of crucial importance to measure and understand how the dopant type (Si, Fe, and Mg), doping level, and crystal quality alter the thermal conductivity of HVPE-grown bulk GaN. In this work, a steady-state thermoreflectance (SSTR) technique was use...
GaN-based high-power wide-bandgap semiconductor electronics and photonics have been considered as... more GaN-based high-power wide-bandgap semiconductor electronics and photonics have been considered as promising candidates to replace conventional devices for automotive applications due to high energy conversion efficiency, ruggedness, and superior transient performance. However, performance and reliability are detrimentally impacted by significant heat generation in the device active area. Therefore, thermal management plays a critical role in the development of GaN-based high-power electronic and photonic devices. This paper presents a comprehensive review of the thermal management strategies for GaN-based lateral power/RF transistors and light-emitting diodes (LEDs) reported by researchers in both industry and academia. The review is divided into three parts: (1) a survey of thermal metrology techniques, including infrared thermography, Raman thermometry, and thermoreflectance thermal imaging, that have been applied to study GaN electronics and photonics; (2) practical thermal manag...
Nickel silicides are widely used as contact materials for electronic devices based on silicon (Si... more Nickel silicides are widely used as contact materials for electronic devices based on silicon (Si). However, they have been predominantly fabricated by annealing separate Ni and Si phases which leads to phase and structural complexity. In this letter, direct epitaxial growth of a single-phase nickel disilicide (NiSi2) thin film by sputter deposition of NiSi2 is achieved on low-cost and flexible Hastelloy tapes which offers a promising route to fabricate low-cost, flexible electronic devices. Biaxially textured titanium nitride (TiN) is applied as the seeding layer and the diffusion barrier under NiSi2. An epitaxial relationship of (001)⟨100⟩NiSi2 ǁ (001)⟨110⟩TiN is observed with an extra-large lattice mismatch (∼10.3%) between NiSi2 and TiN. Both the bonding similarity and the passivation effect by hydrogen promote the epitaxial growth of NiSi2 on TiN. The flat and smooth NiSi2 thin film consists of grains with a size of 50–100 nm. An epitaxially grown Si film on NiSi2 further demon...
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