Low-dimensional II-VI oxide-based semiconductor nanostructure photodetectors for light sensing are described.
Depending on the absorption edge and energy bandgap of the nanostructured materials, the detection wavelength range
can be controlled. The physical properties of the fabricated nanostructures are investigated. The p-n junction property of
n-ZnO and p-CuO nanostructures is obtained. This growth of the ZnO nanorod arrays on CuO nanostructures may be
useful for photodetection applications. The NiO/ZnO nanostructures are also synthesized. Metal-semiconductor-metal
(MSM) type photodetectors are fabricated by integrating the oxide-based (i.e., ZnO and CuO) semiconductor
nanostructures. Using the solution-based ZnO seed layer, the UV MSM type photodetectors with the vertically-aligned
ZnO nanorod arrays are also fabricated. Their photoresponse characteristics are evaluated in a specific spectral range.
The growth and structural/optical properties of metal-oxide semiconductor nanostructures by a simple, low-cost, and
large-scalable fabrication method were studied. These nanostructures were applied to energy and optoelectronic devices,
such as piezoelectric nanogenerators and photodetective sensors, to improve the device performance. The morphologies
and crystallinity of the fabricated nanostructures were observed from scanning electron microscope/transmission
electron microscope images, respectively. The piezoelectric output current and photoresponse property were
characterized by manufacturing the nanogenerators and photodetectors with prepared metal oxide nanostructures. These
results can provide a fundamental understanding of the mechanisms for improving the device performance in
applications of metal-oxide semiconductor nanostructures for energy and sensing devices.
We reported the controlled growth and optical properties of porous nanocolumnar structures by the oblique angle
deposition (OAD) method using an electron-beam evaporation system. Distributed Bragg reflectors, graded refractive
index films, and antireflective coatings were studied to apply to various devices including vertical cavity surface
emitting lasers, light-emitting diodes, solar cells, and sensors. The characteristics of the fabricated devices were
measured. These results suggest that the tailoring of optical properties by a simple OAD technique is very promising for
optical and optoelectronic device applications.
We fabricated a variety of periodic and random III-V, IV, and II-IV semiconductor nanostructures based on various
materials, such as Si, GaAs, ZnO, sapphire, etc., by top-down and bottom-up processes for optoelectronic device
applications. The periodic arrays of nanostructures were fabricated by the dry etching after forming nanopatterns using
the laser interference lithography or monolayer assembly of silica nanospheres. The random nanostructures were formed
by a thermal dewetting process of continuous metal thin films and a subsequent dry etching process. Also, the porous
films with nanocolumns were formed by a glancing angle deposition. The ZnO nanostructures were fabricated by the
hydrothermal growth and electrochemical deposition. The optical properties of the fabricated nanostructures were
measured, together with theoretical analyses using the rigorous coupled-wave analysis method. To improve the device
performance, these semiconductor nanostructures were applied to the devices such as light emitting diodes, solar cells,
and sensors.
We fabricate various ZnO (zinc oxide) nanostructures, such as nanorods, nanotips and nanoflowers, as well as ZnO
subwavelength grating structures for applications in optoelectronic devices such as solar cells, light emitting diodes, and
biosensors. The optical properties are theoretically analyzed using the rigorous coupled-wave analysis method. The
fabricated ZnO nanostructures are of wurzite crystal structure. The reflection and absorption characteristics depend
strongly on the shape and geometry of Zn nanostructures. The ZnO nanostructures with Au (or Ag) particles, based on
surface plasmons, are also investigated.
This study reports broadband antireflective subwavelength structures (SWS) on various semiconductor materials for
near-infrared detector applications. Two fabrication methods are proposed, i.e., a lenslike shape transfer and an overall
dry etch process of Ag nanoparticles. These methods provide relatively simple, fast, inexpensive process steps, which is
applicable for practical device applications. The fabricated SWS showed extremely lower reflectance spectra compared
to that of flat surface in the near-IR range, indicating good agreement with the simulation results. We also propose
amorphous silicon SWS on InGaAs photodetector to enhance the absorption efficiency.
We report the subwavelength antireflection structures in various semiconductor materials such as Si, ZnO, and GaP/light
emitting diode (LED) structure for LED and solar cell applications in the visible and near-infrared wavelength region,
together with the rigorous coupled wave analysis simulation. Subwavelength structures are fabricated by holographic
lithography and dry etching, effectively suppressing the surface reflection. To enhance the absorption efficiency over a
wide-angle broadband range of incident light, the thin-film crystalline Si solar cells with subwavelength structure, which
reduce the surface reflection, are studied. The improvement of light intensity is achieved for the fabricated LEDs with a
subwavelength structure compared to the conventional LEDs due to a strongly reduced internal reflection at the
semiconductor/air interface.
We studied the thermal characteristics and analysis of InGaAs/InAlAs quantum cascade lasers (QCLs) in terms of
internal temperature distribution, heat flux, and thermal conductance from the heat transfer simulation. The heat source
densities were obtained from threshold power densities measured experimentally for QCLs under room-temperature
continuous-wave operation. The use of a thick electroplated Au around the laser ridges helps increase the heat removal
from devices. The two-dimensional anisotropic heat dissipation model was used to analyze the thermal behaviors inside
the device. The simulation results were also compared with those estimated from experimental data.
We studied the impurity-free vacancy diffusion (IFVD) of In0.2Ga0.8As/GaAs multi-quantum well (MQW) structures for advanced optoelectronic devices and their integration. To gain deeper insight in IFVD, the influence of the plasma enhanced chemical vapor deposition parameters, such as SiH4 flow rate, deposition temperature, deposition pressure and rf power, of SiOx and SiNx capping layers on the band gap energy shift induced by IFVD was investigated. We observed that the magnitude of the blue shift increases with the decrease of SiH4 flow rate for SiOx and SiNx capping layers. The blue shift of the PL peak energy increases greatly with the increase of deposition pressure and slightly with the decrease of deposition temperature. The influence of rf power was found to be negligible. All these dependences are related to the porosity in the dielectric capping layers in the QW intermixing. We fabricated the wavelength shifted ridge-waveguide InGaAs/GaAs MQW lasers with 967, 946 and 927 nm emission wavelengths that have undergone IFVD using SiO2 capping layers at different annealing temperatures of 850, 900 and 950 °C, respectively. Also, multi-wavelength InGaAs/GaAs MQW lasers by the area-selective IFVD using different stoichiometric SiOx capping layers were studied. The lasing wavelength difference of about 31 nm is obtained between the ridge-waveguide laser diodes fabricated with the MQWs that had undergone the same thermal treatments using the SiOx film provided with SiH4 flow rates of 20 and 300 sccm.
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