E. T. Yu

Enhanced semiconductor optical absorption via surface plasmon excitation in metal nanoparticles

Surface plasmon resonances in metallic nanoparticles are of interest for a variety of applications due to the large electromagnetic field enhancement that occurs in the vicinity of the metal surface, and the dependence of the resonance wavelength on the nanoparticle’s size, shape, and local dielectric environment. Here we report an engineered enhancement of optical absorption and photocurrent in a semiconductor via the excitation of surface plasmon resonances in spherical Au nanoparticles deposited on the semiconductor surface. The enhancement in absorption within the semiconductor results in increased photocurrent response in Si pn junction diodes over wavelength ranges that correspond closely to the nanoparticle plasmon resonance wavelengths as determined by measurements of extinction spectra. These observations suggest a variety of approaches for improving the performance of devices such as photodetectors, imaging arrays, and photovoltaics.

Improved performance of amorphous silicon solar cells via scattering from surface plasmon polaritons in nearby metallic nanoparticles

An engineered enhancement in short-circuit current density and energy conversion efficiency in amorphous silicon p-i-n solar cells is achieved via improved transmission of electromagnetic radiation arising from forward scattering by surface plasmon polariton modes in Au nanoparticles deposited above the amorphous silicon film. For a Au nanoparticle density of ∼3.7×108cm−2, an 8.1% increase in short-circuit current density and an 8.3% increase in energy conversion efficiency are observed. Finite-element electromagnetic simulations confirm the expected increase in transmission of electromagnetic radiation at visible wavelengths, and suggest that substantially larger improvements should be attainable for higher nanoparticle densities.

Photocurrent spectroscopy of optical absorption enhancement in silicon photodiodes via scattering from surface plasmon polaritons in gold nanoparticles

Experimental characterization and finite-element numerical simulations of the electromagnetic interaction between Au nanoparticles positioned atop a Si pn junction photodiode and incident electromagnetic plane waves have been performed as a function of wavelength. The presence of the Au nanoparticles is found to lead to increased electromagnetic field amplitude within the semiconductor, and consequently increased photocurrent response, over a broad range of wavelengths extending upward from the nanoparticle surface plasmon polariton resonance wavelength. At shorter wavelengths, a reduction in electromagnetic field amplitude and a corresponding decrease in photocurrent response in the semiconductor are observed. Numerical simulations reveal that these different behaviors are a consequence of a shift in the phase of the nanoparticle polarizability near the surface plasmon polariton wavelength, leading to interference effects within the semiconductor that vary strongly with wavelength. These observations hav...

Measurement of piezoelectrically induced charge in GaN/AlGaN heterostructure field-effect transistors

Electron concentration profiles have been obtained for AlxGa1−xN/GaN heterostructure field-effect transistor structures. Analysis of the measured electron distributions demonstrates the influence of piezoelectric effects in coherently strained layers on III-V nitride heterostructure device characteristics. Characterization of a nominally undoped Al0.15Ga0.85N/GaN transistor structure reveals the presence of a high sheet carrier density in the GaN channel which may be explained as a consequence of piezoelectrically induced charges present at the Al0.15Ga0.85N/GaN interface. Measurements performed on an Al0.15Ga0.85N/GaN transistor structure with a buried Al0.15Ga0.85N isolation layer indicate a reduction in electron sheet concentration in the transistor channel and accumulation of carriers below the Al0.15Ga0.85N isolation layer, both of which are attributable to piezoelectric effects.

Analysis of leakage current mechanisms in Schottky contacts to GaN and Al0.25Ga0.75N∕GaN grown by molecular-beam epitaxy

Temperature-dependent current-voltage measurements combined with conductive atomic force microscopy and analytical modeling have been used to assess possible mechanisms of reverse-bias leakage current flow in Schottky diodes fabricated from GaN and Al0.25Ga0.75N∕GaN structures grown by molecular-beam epitaxy. Below 150K, leakage current is nearly independent of temperature, indicating that conduction is dominated by tunneling transport. At higher temperatures, leakage current in both GaN and Al0.25Ga0.75N∕GaN diode structures is well described by a Frenkel-Poole emission model. Based on the inferred emission barrier heights and the observation that room-temperature leakage current is dominated by the presence of highly conductive dislocations, it is suggested that the key carrier transport process is emission of electrons from a trap state near the metal-semiconductor interface into a continuum of states associated with each conductive dislocation. In this model for leakage current flow, the emission barr...

Metal and dielectric nanoparticle scattering for improved optical absorption in photovoltaic devices

The influence of electromagnetic scattering by Au and silica nanoparticles placed atop silicon photovoltaic devices on absorption and photocurrent generation has been investigated. The nanoparticles produce substantial increases in power transmission into the semiconductor and consequently photocurrent response from ∼500to>1000nm. Increases in power conversion efficiency under simulated solar irradiation of up to 8.8% are observed experimentally, and numerical simulations provide quantitatively accurate predictions of these observed enhancements. Additional simulations indicate that these concepts can be applied to a broad range of photovoltaic device structures, including those based on low-index materials for which conventional antireflection coatings are problematic.

Analysis of reverse-bias leakage current mechanisms in GaN grown by molecular-beam epitaxy

Temperature-dependent current–voltage measurements have been used to determine the reverse-bias leakage current mechanisms in Schottky diodes fabricated on GaN grown by molecular-beam epitaxy, and two dominant mechanisms are clearly identified. The first mechanism is field-emission tunneling from the metal into the semiconductor, which is dominant at low temperatures and which, at higher temperatures, becomes significant for large reverse-bias voltages. The second mechanism, presumed to be associated with dislocation-related leakage current paths, is observed to have an exponential temperature dependence and becomes significant above approximately 275 K. The temperature dependence of the second mechanism is consistent with either one-dimensional variable-range-hopping conduction along the dislocation or trap-assisted tunneling.

Schottky barrier engineering in III–V nitrides via the piezoelectric effect

A method for enhancing effective Schottky barrier heights in III–V nitride heterostructures based on the piezoelectric effect is proposed, demonstrated, and analyzed. Two-layer GaN/AlxGa1−xN barriers within heterostructure field-effect transistor epitaxial layer structures are shown to possess significantly larger effective barrier heights than those for AlxGa1−xN, and the influence of composition, doping, and layer thicknesses is assessed. A GaN/Al0.25Ga0.75N barrier structure optimized for heterojunction field-effect transistors is shown to yield a barrier height enhancement of 0.37 V over that for Al0.25Ga0.75N. Corresponding reductions in forward-bias current and reverse-bias leakage are observed in current–voltage measurements performed on Schottky diodes.

Gate leakage current mechanisms in AlGaN/GaN heterostructure field-effect transistors

Gate leakage currents in AlGaN/GaN heterostructure field-effect transistor (HFET) structures with conventional and polarization-enhanced barriers have been studied. Comparisons of extensive gate leakage current measurements with two-dimensional simulations show that vertical tunneling is the dominant mechanism for gate leakage current in the standard-barrier HFET and that the enhanced-barrier structure suppresses this mechanism in order to achieve a reduced leakage current. An analytical model of vertical tunneling in a reverse-biased HFET gate-drain diode is developed to evaluate the plausibility of this conclusion. The model can be fit to the measured data, but suggests that additional leakage mechanisms such as lateral tunneling from the edge of the gate to the drain or defect-assisted tunneling also contribute to the total leakage current. The vertical tunneling current mechanism is shown to be more significant to the gate leakage current in III–V nitride HFETs than in HFETs fabricated in other III–V ...

A silicon-based photocathode for water reduction with an epitaxial SrTiO3 protection layer and a nanostructured catalyst

The rapidly increasing global demand for energy combined with the environmental impact of fossil fuels has spurred the search for alternative sources of clean energy. One promising approach is to convert solar energy into hydrogen fuel using photoelectrochemical cells. However, the semiconducting photoelectrodes used in these cells typically have low efficiencies and/or stabilities. Here we show that a silicon-based photocathode with a capping epitaxial oxide layer can provide efficient and stable hydrogen production from water. In particular, a thin epitaxial layer of strontium titanate (SrTiO3) was grown directly on Si(001) by molecular beam epitaxy. Photogenerated electrons can be transported easily through this layer because of the conduction-band alignment and lattice match between single-crystalline SrTiO3 and silicon. The approach was used to create a metal-insulator-semiconductor photocathode that, under a broad-spectrum illumination at 100 mW cm(-2), exhibits a maximum photocurrent density of 35 mA cm(-2) and an open circuit potential of 450 mV; there was no observable decrease in performance after 35 hours of operation in 0.5 M H2SO4. The performance of the photocathode was also found to be highly dependent on the size and spacing of the structured metal catalyst. Therefore, mesh-like Ti/Pt nanostructured catalysts were created using a nanosphere lithography lift-off process and an applied-bias photon-to-current efficiency of 4.9% was achieved.