Jen Inn Chyi

Material properties of compositional graded InxGa1−xAs and InxAl1−xAs epilayers grown on GaAs substrates

The residual strain, crystallographic tilt, and surface topography of InxGa1−xAs and InxAl1−xAs (0<x<0.3) epilayers grown on GaAs substrates are investigated. The residual strain of the InxAl1−xAs grown on graded InyAl1−yAs is shown to be strongly dependent on the thickness of the underlying‐graded buffer layers and is larger than that of the InGaAs of the same structure. The crystallographic tilt of the InGaAs epilayers with respect to GaAs substrate is found to be strongly dependent on the growth temperature as well as the layer structure of the underlying buffer layer, while that of InAlAs is insensitive to these two factors. This behavior is attributed to the different roughness of the growth front between these two material systems and is consistent with the observation by atomic force microscopy.

Nanostructures and carrier localization behaviors of green-luminescence InGaN/GaN quantum-well structures of various silicon-doping conditions

The results of photoluminescence (PL), detection-energy-dependent photoluminescence excitation (DEDPLE), excitation-energy-dependent photoluminescence (EEDPL), and strain state analysis (SSA) of three InGaN/GaN quantum-well (QW) samples with silicon doping in the well, barrier and an undoped structure are compared. The SSA images show strongly clustering nanostructures in the barrier-doped sample and relatively weaker composition fluctuations in the undoped and well-doped samples. Differences in silicon doping between the samples give rise to the differences in DEDPLE and EEDPL spectra, as a result of the differences in carrier localization. In addition, the PL results provide us clues for speculating that the S-shaped PL peak position behavior is dominated by the quantum-confined Stark effect in an undoped InGaN/GaN QW structure.

Transport in a gated Al0.18Ga0.82N/GaN electron system

We have investigated the low-temperature transport properties of front-gated Al0.18Ga0.82N/GaN heterostructures. At zero gate voltage, the Hall mobility increases with decreasing temperature (20u200aK⩽T⩽190u200aK) due to a reduction in phonon scattering. For T⩽20u200aK, the mobility decreases with decreasing temperature. This is due to weak localization in a weakly disordered two-dimensional system. By changing the applied gate voltage, we can vary the carrier density n from 3.11×1012 to 6.95×1012u200acm−2 in our system. The carrier density shows a linear dependence on the applied gate voltage, consistent with a simple parallel-plate capacitor model. The average distance between the GaN electron system and the AlGaN/GaN interface is estimated to be 240u200aA. At high carrier densities (n>4.65×1012u200acm−2), the measured mobility (μ) is found to be a decreasing function of carrier density as μ∼n−0.31. Loss of mobility with increasing carrier density is dominated by interface roughness scattering. At low carrier densities (n 4.65×1012u200acm−2), the measured mobility (μ) is found to be a decreasing function of carrier density as μ∼n−0.31. Loss of mobility with increasing carrier density is dominated by interface roughness scattering. At low carrier densities (n<4.24...

Beyond the Debye length in high ionic strength solution: direct protein detection with field-effect transistors (FETs) in human serum

In this study, a new type of field-effect transistor (FET)-based biosensor is demonstrated to be able to overcome the problem of severe charge-screening effect caused by high ionic strength in solution and detect proteins in physiological environment. Antibody or aptamer-immobilized AlGaN/GaN high electron mobility transistors (HEMTs) are used to directly detect proteins, including HIV-1 RT, CEA, NT-proBNP and CRP, in 1X PBS (with 1%BSA) or human sera. The samples do not need any dilution or washing process to reduce the ionic strength. The sensor shows high sensitivity and the detection takes only 5u2009minutes. The designs of the sensor, the methodology of the measurement, and the working mechanism of the sensor are discussed and investigated. A theoretical model is proposed based on the finding of the experiments. This sensor is promising for point-of-care, home healthcare, and mobile diagnostic device.

High sensitivity cardiac troponin I detection in physiological environment using AlGaN/GaN High Electron Mobility Transistor (HEMT) Biosensors

In this study, we report the development of a high sensitivity assay for the detection of cardiac troponin I using electrical double layer gated high field AlGaN/GaN HEMT biosensor. The unique gating mechanism overcomes the drawback of charge screening seen in traditional FET based biosensors, allowing detection of target proteins in physiological solutions without sample processing steps. Troponin I specific antibody and aptamer are used as receptors. The tests carried out using purified protein solution and clinical serum samples depict high sensitivity, specificity and wide dynamic range (0.006-148ng/mL). No additional wash or sample pre-treatment steps are required, which greatly simplifies the biosensor system. The miniaturized HEMT chip is packaged in a polymer substrate and easily integrated with a portable measurement unit, to carry out quantitative troponin I detection in serum samples with < 2µl sample volume in 5min. The integrated prototype biosensor unit demonstrates the potential of the method as a rapid, inexpensive, high sensitivity CVD biomarker assay. The highly simplified protocols and enhanced sensor performance make our biosensor an ideal choice for point of care diagnostics and personal healthcare systems.

Effect of column III vacancy on arsenic precipitation in low-temperature grown III–V arsenides

Separately grown p-type, intrinsic, and n-type GaAs at low temperatures as well as a combined p-i-n structure have been used to study the formation of As precipitates upon annealing at 800u2009°C. For the separate structures, least precipitates have been noticed in the n-type material. In contrast, the highest density of precipitates appears in the n region for the p-i-n structure. In addition, an obvious band depleted of precipitates, exists in the intrinsic region near the n-i interface. A general vacancy model, including Fermi level effect and crystal bonding strength (thermodynamic factor), has been developed to explain the current results as well as to predict As precipitation in various low temperature grown III–V heterostructures.

Excitation Density and Temperature Dependent Photoluminescence of InGaAs Self-Assembled Quantum Dots

In this paper, we shall report on the excitation density and temperature dependent photoluminescence produced by discrete energy levels from InGaAs self-assembled quantum dots. While increasing the photoexcitation density, five peaks originating from discrete energy levels of quantum dot and wetting layer are observed. By deconvoluting these spectra using multiple Gaussian fit, the intensity of each state is saturated following its degeneracy. We describe the lateral confinement of quantum dots using a parabolic potential model. The saturated values are in good agreement with the degeneracy of this potential type. From the temperature dependent photoluminescence, we observed the thermally activated quenching of each state. Our results suggest that the wetting layer acts as a barrier to the carrier thermallization processes offering a two dimensional path for inter-dot coupling.

Epitaxial AlN Thin Film Surface Acoustic Wave Devices Prepared on GaN/Sapphire Using Low-Temperature Helicon Sputtering System

High-quality epitaxial AlN films have been deposited on GaN/sapphire using helicon sputtering at a temperature of 300 °C. The surface acoustic wave (SAW) characteristics, in terms of insertion loss, stopband rejection, and electromechanical coupling coefficient, of SAW devices fabricated on AlN/GaN/sapphire are much superior than those fabricated on GaN/sapphire. The investigation of environmental effects, including temperature and relative humidity, shows that the ambient stability can be improved with the deposition of an AlN film on GaN/sapphire. An oscillator fabricated using an AlN/GaN/sapphire-based SAW device was developed for application in sensors. The composite structure of AlN on GaN may bring about the development of high-frequency components, which integrate and utilize their semiconducting, optoelectronic, and piezoelectric properties.

Improved electroluminescence of InAs quantum dots with strain reducing layer

The effects of the arrangement between InAs quantum dots and InGaAs strain-reducing layer (SRL) on the optical properties of quantum dot light emitting diodes are investigated. Electroluminescence wavelength longer than 1.3 μm is obtained as InAs quantum dots (QDs) are covered with a thin InGaAs SRL. For the same sample, the full-width at half-maximum of the ground state emission peak is as narrow as 19 meV at low injection current, and less than 40 meV even at saturation condition. It is also found that the slope efficiency of the diode is higher than that of the other samples in the linear region and its light output saturation level is higher because of the higher density of QDs.

Room-Temperature Operation of In0.5Ga0.5 As Quantum Dot Lasers Grown on Misoriented GaAs Substrates by Molecular Beam Epitaxy.

We have investigated the characteristics of self-assembled In0.5Ga0.5As quantum dot lasers grown on three types of (100) GaAs substrates, i.e. 0°, 4° and 15°-tilted toward the (111)Ga plane. The lasing wavelength at room temperature is 1018 nm, 1015 nm, and 1030 nm for the 0°, 4° and 15°-0ff samples, respectively. Due to the better quantum confinement and arrangement of the quantum dots, the 4o-off samples exhibit a lower threshold current of 47 mA at room temperature, compared to 73 and 65 mA for the 0° and 15°-off samples, respectively. For the same reason, the characteristic temperature obtained between 150 and 300 K is 117 K for the 4°-off samples, compared to 113 K and 100 K for the 0° and 15°-off samples, respectively.