Thermoelectric properties of AlInN alloys, grown by metalorganic vapor phase epitaxy (MOVPE), with In-contents (x)
from 11 % up to 21.34% were characterized and analyzed at room temperature. The thermoelectric figure of merit (Z*T)
values of the n-Al1-xInxN alloys were measured as high as 0.391 up to 0.532 at T = 300 K. The use of high In-content (x
= 21.34%) AlInN alloys leads to significant reduction in thermal conductivity [κ = 1.62 W/(mK)] due to the increased
alloy scattering, however, the optimized thermoelectric material was obtained for AlInN alloy with In-content of 17%
attributed to its large power factor.
The thermoelectric properties of n-type wurtzite AlInN alloys are investigated by simulating the electron and
phonon scatterings. The electrical conductivity, Seebeck coefficient and electronic thermal conductivity are obtained
by considering all major electron scatterings. The lattice thermal conductivity is simulated by considering phonon
scatterings. The simulation provides useful guideline in material optimization for thermoelectricity.
Novel staggered InGaN quantum wells (QWs) and type-II InGaN-GaNAs QWs with improved momentum matrix
element lead to improved internal quantum efficiency for green-emitting light-emitting diodes (LEDs). Approaches for
enhancing internal quantum efficiency, light extraction efficiency, and efficiency-droop in nitride LEDs are discussed.
The thermal conductivity of high-quality narrow-bandgap (0.77eV) InN grown on GaN on sapphire substrate by pulsed-
MOVPE method was measured and analyzed. To accurately extract the thermal conductivities of GaN and InN films
grown on sapphire substrate, 2D multilayer thermal diffusion model and extended 3ω slope technique are employed. The
thermal conductivity of sapphire substrate measured is 41 W/(mK). The thermal conductivity of undoped GaN film is
measured as 108 W/(mK). High-quality pulsed-MOVPE grown InN film exhibits thermal conductivity of 126 W/(mK),
which is higher in comparison to the previously-reported value of porous InN ceramics 45 W/(mK), yet lower than the
theoretical value 176 W/(mK) based on phonon scattering.
We present the band structure calculation of dilute-As GaNAs alloys (from 0% to 6.25% As) by employing the densityfunctional
theory that adopts the local density approximation. Our studies indicate that the GaNAs shows a direct
bandgap property. A small incorporation of As into the GaN alloy leads to the a significant decrease in the energy gap,
which allows direct band gap transition covering from 3.47 eV (0% As) down to 1.93eV (6.25% As). The finding
implies the dilute-As GaNAs alloy as an excellent candidate for the active material for optoelectronics that covers the
entire visible spectral regime. The carrier effective masses of dilute-As GaNAs alloys are also presented.
The thermoelectric properties of AlGaN and InGaN semiconductors are analyzed. In our analysis, the thermal conductivities, electrical conductivities, Seebeck coefficients, and figure of merits (Z*T) of AlGaN and InGaN semiconductors are computed. The electron transports in AlGaN and InGaN alloys are analyzed by solving Boltzmann transport equation, taking into account the dominant mechanisms of energy-dependent electron scatterings. Virtual crystal model is implemented to simulate the lattice thermal conductivity from phonon scattering for both AlGaN and InGaN alloys. The calculated Z*T is as high as 0.15 for optimized InGaN alloy at temperature around 1000 K. For optimized AlGaN composition, the Z*T of 0.06-0.07 can be achieved. The improved thermoelectric performance of ternary alloys over binary alloys can be attributed to the reduced lattice thermal conductivity.
The growths of droplet-free narrow-bandgap InN semiconductors on Ga-polar and N-polar GaN templates on c-plane sapphire substrates were performed by pulsed-MOVPE growth techniques. Under the optimum In-polar InN growth conditions, the carrier mobility and n-type carrier concentration were measured as 681 cm2/(V.sec) and 1.5×1019 cm-3, respectively. The room-temperature photoluminescence measurements of optimized In-polar grown by pulsed-MOVPE technique resulted in peak wavelength at 0.76 eV. The growth of N-polar InN grown on the N-polar GaN template is discussed and compared to that of the In-polar InN.
Improvement of light extraction efficiency of InGaN light emitting diodes (LEDs) using polydimethylsiloxane (PDMS)
concave microstructures arrays was demonstrated. The size effect of the concave microstructures on the light extraction
efficiency of III-Nitride LEDs was studied. Depending on the size of the concave microsturctures, ray tracing
simulations show that the use of PDMS concave microstructures arrays can lead to increase in light extraction efficiency
of InGaN LEDs by 1.4 to 1.9 times. Experiments utilizing 1.0 μm PDMS concave microstructures arrays demonstrated
1.60 times improvement in light extraction, which is consistent with simulated improvement of 1.63 times. The
enhancement in light extraction efficiency is attributed to increase in effective photon escape cone due to PDMS concave
microstructures arrays, and reduced Fresnel reflection within the photon escape cone due to the grading of refractive
index change between GaN / PDMS / air interface.
We demonstrated and analyzed 480-nm emitting III-Nitride LEDs using SiO2/polystyrene (PS) microlens arrays,
deposited via rapid-convective-deposition. Output power of MOCVD-grown InGaN QW LEDs with SiO2/PS microlens
exhibited improvement of 219%. Numerical simulation of the light extraction efficiency optimization of III-Nitride
LEDs with SiO2/PS microlens was carried out using Monte Carlo ray tracing including 3D self-consistent photon-carrier
interaction. The light extraction efficiency of the LEDs with microlens array is optimized for the PS layer thickness and
the SiO2 microspheres diameter. The simulations show good agreement with experiments, indicating the use of SiO2/PS
microlens leads to increased photon escape cone.
This paper presents a novel method of wavefront measurement for adaptive optic systems by using a photonic crystal. In adaptive optics the wavefront shape of the incident wave is measured and used to set a reconfigurable mirror array to compensate for the aberration, reduce distortion and improve image and beam quality. In a 2D approach, the tilted wavefront at each pixel enters a V-shape structure of waveguides in a photonic crystal. By measuring and comparing the output power in the two waveguides, we can determine the tilted angle of the incident light at that pixel. This method can also be applied in a 3D approach.
Surface micromachining processes based on III-V compound semiconductors are presented in this paper, in order to develop Micro-Opto-Electro-Mechanical systems (MOEMS). By fabricating micro cantilevers composed of seven InP/Air gap pairs, the major techniques of the surface micromachining are studied, including non-selective and selective etching, rinsing and drying. A severe problem of the sticking phenomena during rinsing and drying is avoided by the implementation of the Critical Point Drying (CPD) method.
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