J. C. Woo

GaAs-based near-infrared omnidirectional reflector

We introduce a compound-semiconductor-based omnidirectional reflector. A four-layer-pair stack of GaAs/AlAs was grown epitaxially using molecular-beam epitaxy, and was then converted to a GaAs/Al2O3 multilayer stack by selective oxidation of the AlAs layers. The resultant one-dimensional photonic crystal exhibited omnidirectional reflection properties in near-IR wavelength range below 1 μm. Reflectance spectra measured at various incidence angles and polarizations were observed to be in good agreement with theoretically simulated results.

Semiconductor microlenses fabricated by one-step wet etching

We fabricated refractive semiconductor microlenses using a diffusion-limited chemical etching technique based an Br/sub 2/ solution. The simple one-step wet etching process produced high-quality microlenses of GaAs and InP, the two most popular compound semiconductor materials used in optoelectronics. A spherical GaAs microlens with a nominal lens diameter of 30 /spl mu/m exhibited a radius of curvature and focal length of 91 and 36 /spl mu/m, respectively. The surface roughness, examined by atomic force microscopy (AFM), was measured to be below /spl plusmn/10 /spl Aring/. This microlens fabrication method should be readily applicable due to the simplicity in processing and the high-quality results.

Probing strained InGaN/GaN nanostructures with ultrashort acoustic phonon wave packets generated by femtosecond lasers

Large amplitude time-domain oscillations are detected in InxGa1−xN/GaN structures via femtosecond differential reflectivity spectroscopy. The oscillation amplitude increases with increasing indium fraction and abruptly disappears at a critical time that depends on GaN thickness. We show that spatially localized, coherent acoustic phonon wave packets are generated via the photoexcited carriers and propagate into the samples modulating the reflectivity. Our results show that a system with strong built-in strain can be a very effective source for ultrafast acoustic phonon wave packets which can be used as a powerful probe for nanoscale structures.

Energy-level engineering of self-assembled quantum dots by using AlGaAs alloy cladding layers

The ground-state energy level of an InAs quantum dot (QD) system can be changed from 1070 to 700 nm by changing the aluminum composition in the AlxGa1−xAs matrix. For all the QDs, the lattice-mismatched strains are the same as that of InAs/GaAs QDs, so that QDs are easily formed. Photoluminescence signals from the structures were strong at low temperature and stayed relatively high at room temperature. The results suggest that these highly strained QDs in the alloy matrices could be an excellent choice for the energy-level engineering of QDs.

Interplay between carrier and impurity concentrations in annealed Ga1- xMnxAs : Intrinsic anomalous hall effect

Investigating the scaling behavior of annealed Ga1-xMnxAs anomalous Hall coefficients, we note a universal crossover regime where the scaling behavior changes from quadratic to linear. Furthermore, measured anomalous Hall conductivities in the quadratic regime when properly scaled by carrier concentration remain constant, spanning nearly a decade in conductivity as well as over 100 K in T_[C] and comparing favorably to theoretically predicated values for the intrinsic origins of the anomalous Hall effect. Both qualitative and quantitative agreements strongly point to the validity of new equations of motion including the Berry phase contributions as well as the tunability of the anomalous Hall effect.

Spin polarization of self-assembled CdSe quantum dots in ZnMnSe

Nearly complete spin polarization (SP) of carriers was observed in photoluminescence (PL) experiments on CdSe self-assembled quantum dots (QDs) embedded in ZnMnSe at 5 K in magnetic fields above 1 T. The redshift of the PL from CdSe QDs in the presence of the magnetic field was less than that of ZnMnSe, but still sufficiently large to account for the high SP. At low fields, however, the SP of carriers in CdSe QDs is observed to be smaller than in the surrounding ZnMnSe. This, along with time-resolved PL data, suggests that the SP in CdSe QDs is mainly due, not to the injection of SP carriers from ZnMnSe, but rather due to the Zeeman splitting of internal QD states.

Statistical Noise Analysis of CMOS Image Sensors in Dark Condition

The statistical noise analysis of the CMOS image sensors in the dark condition has been performed with a newly developed 3-D technology computer-aided design framework. The noise histograms of the correlated double sampling output, due to the random distribution of the oxide traps in the source follower MOSFET, have been evaluated. In this framework, the random telegraph signal noise is accurately characterized in the device level, and the numerical efficiency for the statistical analysis is achieved by employing the Greens function method based on the drift-diffusion model. As an application, one million samples of the source follower MOSFET have been simulated, and the effect of the channel width, the channel length, and the oxide trap density on the noise histogram has been investigated.

Statistical analysis of random telegraph noise in CMOS image sensors

We propose a statistical method to predict the dark random readout noise in CMOS image sensors. First, we calculate the dark random noise originated from oxide traps present in the source-follower MOSFET. Statistical variation in the dark noise is associated with the random variation of the oxide defects in the CMOS image sensor cells in both the energy and space domain. Considering the effect of the correlated double sampling, we define the dark random noise as the standard deviation in the time domain and analyze the effect of the MOSFET width and length variations and temperature on its dark random noise.

Evidence of metallic clustering in annealed Ga1−xMnxAs from atypical scaling behavior of the anomalous Hall coefficient

The anomalous Hall coefficient (Rs) and longitudinal resistivity (ρxx) scaling relationship (RS=cρxxn) for a series of annealed Ga1−xMnxAs epilayers (x≈0.055) are investigated. As-grown samples exhibit a scaling parameter n∼1 and samples annealing near the optimal annealing temperature, n∼2. For annealing temperatures far above the optimum they observe scaling with n>3, which is similar to the behavior of certain inhomogeneous systems. Optical spectroscopy measurements also reveal an enhancement of the absorption coefficient in these samples for photons of energy around 1eV. These atypical behaviors are characteristic of spherical resonance from metallic inclusions.

Bimodal roughness of heterointerface in quantum wells analyzed by photoluminescence excitation spectroscopy

Photoluminescence excitation (PLE) studies were performed on GaAs‐Al0.25Ga0.75As quantum wells (QWs) with fractional monolayer differences. The quantized PLE peaks and their submonolayer shifts clearly show that the heterointerface of thin QWs prepared by growth‐interrupted molecular beam epitaxy has islands which extend out a lateral dimension larger than 100 A, but they themselves have the microroughness smaller than 30 A. The result of this work using exciton as the probe provides a clear evidence supporting the bimodal roughness model.