D. M. Schaadt

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.

Reduction of reverse-bias leakage current in Schottky diodes on GaN grown by molecular-beam epitaxy using surface modification with an atomic force microscope

The characteristics of dislocation-related leakage current paths in an AlGaN/GaN heterostructure grown by molecular-beam epitaxy and their mitigation by local surface modification have been investigated using conductive atomic force microscopy. When a voltage is applied between the tip in an atomic force microscope (AFM) and the sample, a thin insulating layer is formed in the vicinity of the leakage paths where current is observed. As the insulating layer reaches a thickness of 2–3 nm, the leakage current is blocked and subsequent growth of the layer is prevented. Although conductive screw or mixed dislocations are observed, dislocations with a screw component that do not conduct current are also apparent. The reverse-bias leakage current is reduced by a factor of two in a large-area diode fabricated on an area modified in this manner with an AFM compared to typical diodes fabricated on unmodified areas with comparable series resistances, confirming that dislocation-related leakage current paths are a ma...

Charge storage in Co nanoclusters embedded in SiO2 by scanning force microscopy

Scanning force microscopy was used to study localized charge deposition and subsequent transport in Co nanoclusters embedded in SiO2 deposited on an n-type Si substrate. Co nanoclusters were charged by applying a bias voltage pulse between tip and sample, and electrostatic force microscopy was used to image charged areas, to determine quantitatively the amount of stored charge, and to characterize the discharging process. Charge was deposited controllably and reproducibly within areas ∼20–50 nm in radius, and an exponential decay in the peak charge density was observed. Longer decay times were measured for positive than for negative charge; this is interpreted as a consequence of the Coulomb-blockade energy associated with single-electron charging of the Co nanoclusters.

Lateral variations in threshold voltage of an AlxGa1−xN/GaN heterostructure field-effect transistor measured by scanning capacitance spectroscopy

Local dC/dV spectroscopy performed in a scanning capacitance microscope (SCM) was used to map, quantitatively and with high spatial resolution (∼50u200anm), lateral variations in the threshold voltage of an AlxGa1−xN/GaN heterostructure field-effect transistor epitaxial layer structure. Scanning capacitance and the associated threshold voltage images show small round features less than 150 nm in diameter with a corresponding shift in threshold voltage of about 1.5–2 V, and larger features several microns in size with a corresponding shift in threshold voltage of approximately 1 V. The small features in the SCM and threshold voltage images are consistent with the presence of charged threading dislocations, while the variations in threshold voltage over large areas could be a result of thickness and/or composition variations in the AlxGa1−xN layer.

Reverse-bias leakage current reduction in GaN Schottky diodes by electrochemical surface treatment

An electrochemical surface treatment has been developed that decreases the reverse-bias leakage current in Schottky diodes fabricated on GaN grown by molecular-beam epitaxy (MBE). This treatment suppresses current flow through localized leakage paths present in MBE-grown GaN, while leaving other diode characteristics, such as the Schottky barrier height, largely unaffected. A reduction in leakage current of three orders of magnitude was observed for Schottky diodes fabricated on the modified surface compared to diodes fabricated on the unmodified surface for reverse-bias voltages as large as -20 V. In addition to suppressing reverse-bias leakage, the surface treatment was found to improve substantially the ideality factor of the modified surface diodes compared to that of unmodified surface diodes, suggesting that such a surface modification process could be useful for a variety of GaN-based electronic devices

Scanning Kelvin probe microscopy of surface electronic structure in GaN grown by hydride vapor phase epitaxy

Scanning Kelvin probe microscopy is used to image surface potential variations in GaN (0001) grown by hydride vapor phase epitaxy. The influence of finite probe tip size on these measurements is analyzed, suggesting that significant differences between measured and actual surface potential variations may exist. Experimentally, localized regions in which the surfacework function increases by ∼0.1–0.2 V are observed, indicating a shift in the Fermi level toward the valence band; these are attributed to the presence of negatively charged threading dislocations. The magnitudes of the observed variations in surface potential are comparable to those reported in the literature, and compare favorably with those predicted on the basis of a model in which the dislocation is represented as a filled line of acceptor states and the interaction between the sample and a probe tip of finite size is considered. In this model, the finite size of the probe tip is found to exert a substantial influence on the degree to which the full variation in surface potential is observed in scanning Kelvin probe measurements.

Origin and microscopic mechanism for suppression of leakage currents in Schottky contacts to GaN grown by molecular-beam epitaxy

Dislocation-related conduction paths in n-type GaN grown by molecular-beam epitaxy and a mechanism for local suppression of current flow along these paths are analyzed using conductive atomic force microscopy, scanning Auger spectroscopy, and macroscopic current–voltage measurements. Application of an electric field at the GaN surface in an ambient atmospheric environment is shown to lead to local formation of gallium oxide in the immediate vicinity of the conduction paths, resulting in the strong suppression of subsequent current flow. Current–voltage measurements for Schottky diodes in which local conduction paths have been suppressed in this manner exhibit reverse-bias leakage currents reduced by two to four orders of magnitude compared to those in Schottky diodes not subjected to any surface modification process. These results demonstrate that the dislocation-related current leakage paths are the dominant source of leakage current in Schottky contacts to n-type GaN grown by molecular-beam epitaxy, and elucidate the nature of a microscopic process for their suppression.

Light trapping in thin-film solar cells via scattering by nanostructured antireflection coatings

The use of nanostructured TiO2 layers fabricated on thin-film solar cells to provide, simultaneously, both antireflection functionality and light trapping via scattering of long-wavelength photons into guided optical modes is demonstrated and analyzed in thin-film quantum-well solar cells. Nanosphere lithography is used for fabrication of periodic arrays of subwavelength-scale TiO2 structures, and separation of active device layers from their epitaxial growth substrate and integration with the nanostructured TiO2 layer enables increased optical absorption via coupling to both Fabry-Perot resonances and guided lateral propagation modes in the semiconductor. The nanostructured TiO2 layer is shown to act as a graded-index coating at optical wavelengths and simultaneously to scatter incident light into guided optical modes within the device. The dependence of these effects on angle of incidence is also analyzed.

Miscut-angle dependence of perpendicular magnetic anisotropy in thin epitaxial CoPt3 films grown on vicinal MgO

The effect of vicinal substrates on the growth-induced perpendicular magnetic anisotropy of epitaxial CoPt3 films has been studied. A small (2°, 4°, or 10°) miscut angle of the vicinal substrate causes the crystallographic axes of the sample to be tilted along the miscut direction. The magnitude of the perpendicular anisotropy is unaffected by the presence of substrate steps produced by the miscut angle, while an additional, in-plane anisotropy develops with a larger miscut angle. Effects of the steps are seen in magnetic force microscopic images of domain wall pinning.

Quantitative analysis of nanoscale electronic properties in an AlxGa1−xN/GaN heterostructure field-effect transistor structure

Local dC/dV spectroscopy performed in a scanning capacitance microscope is used to map, quantitatively and with high spatial resolution, lateral variations in the threshold voltage of an AlxGa1−xN/GaN heterostructure field-effect transistor epitaxial layer structure. Theoretical analysis and numerical simulations are used to quantify charge concentrations, the corresponding threshold voltage shifts, and the influence of the measurement apparatus on these results. High-resolution scanning capacitance and the associated threshold voltage images reveal round features <150 nm in diameter within which a shift in threshold voltage of about 1.5–2 V is measured. Theoretical analysis and numerical simulations indicate that these features are consistent with the presence of charged threading dislocations with a linear charge density of ∼107u200ae/cm−1 that cause localized partial or full depletion of carriers from the two-dimensional electron gas. Large-scale scanning capacitance images reveal variations in contrast ov...