Correlation between optical and electrical properties in In0.52Al0.48As/InxGa1-xAs metamorphic high-electron-mobility-transistor structures on GaAs substrates

4.2 K photoluminescence (PL) and 77 K standard Hall-effect measurements were performed for In0.52Al0.48As/InxGa1-xAs metamorphic high-electron-mobility-transistor (HEMT) structures grown on GaAs substrates with different indium contents in the InxGa1-xAs well or different Si delta-doping concentrations. It was found that electron concentrations increased with increasing PL intensity ratio of the "forbidden" transition (the second electron subband to the first heavy-hole subband) to the sum of the "allowed" transition (the first electron subband to the first heavy-hole subband) and the forbidden transition. And electron mobilities decreased with increasing product of the average full width at half maximum of allowed and forbidden transitions and the electron effective mass in the InxGa1-xAs quantum well. These results show that PL measurements are a good supplemental tool to Hall-effect measurements in optimization of the HEMT layer structure. (c) 2006 American Institute of Physics.

Distributed mode coupling in microring channel drop filters

The authors present the observation of wide transmission dips in a microring channel drop filter by two-dimensional finite-difference time-domain simulation. The authors show that distributed mode coupling between the input waveguide and the resonator results in the oscillations of the coupling efficiency and the envelope of transmission spectra with wavelength. The critical coupling as the light just passing through the coupling region is important for optimizing related devices. If the width of the input waveguide is different from that of the ring resonator, the phenomenon can be greatly reduced. (c) 2006 American Institute of Physics.

Theoretical analysis about the influence of channel layer thickness on the 2D electron gas and its distribution in InP-based high-electron-mobility transistors

The principle of high-electron-mobility transistor (HEMT) and the property of two-dimensional electron gas (2DEG) have been analyzed theoretically. The concentration and distribution of 2DEG in various channel layers are calculated by numerical method. Variation of 2DEG concentration in different subband of the quantum well is discussed in detail. Calculated results show that sheet electron concentration of 2DEG in the channel is affected slightly by the thickness of the channel. But the proportion of electrons inhabited in different subbands can be affected by the thickness of the channel. When the size of channel lies between 20-25 nm, the number of electrons occupying the second subband reaches the maximum. This result can be used in parameter design of materials and devices.

Raman evidence for atomic correlation between the two constituent tubes in double-walled carbon nanotubes

Four well-resolved peaks with very narrow linewidths were found in the D-band and G'-band features of double-walled carbon nanotubes (DWNTs). This fact implies the occurrence of additional van Hove singularities (vHSs) in the joint density of states (JDOS) of DWNTs, which is consistent with theoretical calculations. According to their peak frequencies and theoretical analysis, the two outer peaks can be deduced to originate from a strong coupling between the two constituent tubes of commensurate DWNTs and the two inner peaks were curvature-related and assigned to originate from the two tubes with a weak coupling. This observation and elucidation constitute the first Raman evidence for atomic correlation and the resulting electronic structure change of the two constituent tubes in DWNTs. This result opens the possibility of predicting and modifying the electronic properties of DWNTs for their electronic applications.

Shape and spatial correlation control of InAs-InAlAs-InP(001) nanostructure superlattices

The control of shape and spatial correlation of InAs-InAlAs-InP(001) nanostructure superlattices has been realized by changing the As overpressure during the molecular-beam epitaxy (MBE) growth of InAs layers. InAs quantum wires (QWRs) are obtained under higher As overpressure (1x10(-5) Torr), while elongated InAs quantum dots (QDs) are formed under lower As overpressure (5x10(-6) or 2.5x10(-6) Torr). Correspondingly, spatial correlation changes from vertical anti-correlation in QWR superlattices to vertical correlation in QD superlattices, which is well explained by the different alloy phase separation in InAlAs spacer layers triggered by the InAs nanostrcutures. It was observed that the alloy phase separation in QD superlattices could extend a long distance along the growth direction, indicating the vertical correlation of QD superlattices can be kept in a wide range of spacer layer thickness.

Accurate mode analysis of ultraviolet-written channel waveguides

The transfer matrix method combined with the effective index method is adopted to model the silica-based channel waveguide patterned by UV writing. The effective indexes of the graded index channel waveguides with different dimension are calculated. The maximal error of the effective index is less than 3 x 10(-5). By this method, the number of the guided mode and the dimension range to guide certain modes can be obtained easily. Finally, the dimension range to guide a single mode is presented. (c) 2005 Society of Photo-Optical Instrumentation Engineers.

Correlation effects for semiconducting single-wall carbon nanotubes: A density matrix renormalization group study

In this paper we report the applicability of the density matrix renormalization group (DMRG) approach to the cylindrical single wall carbon nanotube (SWCN) for the purpose of its correlation effect. By applying the DMRG approach to the t+U+V model, with t and V being the hopping and Coulomb energies between the nearest neighboring sites, respectively, and U the on-site Coulomb energy, we calculate the phase diagram for the SWCN with chiral numbers (n(1)=3, n(2)=2), which reflects the competition between the correlation energy U and V. Within reasonable parameter ranges, we investigate possible correlated ground states, the lowest excitations, and the corresponding correlation functions in which the connection with the excitonic insulator is particularly addressed.

Electron transport properties of MM-HEMT with varied channel indium contents

Transport properties of two-dimensional electron gas (2DEG) are crucial to metamorphic high-electron-mobility transistors (MM-HEMT). We have investigated the variations of subband electron mobility and concentration versus temperature from Shubnikov-de Hass oscillations., and variable temperature Hall measurements. The results indicate that the electrical performance is the best when the In content is 0.65 in the channel for MM-HEMT. When the In content exceeds 0.65, a large lattice mismatch will cause dislocations and result in the decrease of mobility and the fall of performance in materials and devices.

Correlation of structural and optical investigation of InGaAs/InAlAs quantum cascade laser

Double X-ray diffraction has been used to investigate InGaAs/InAlAs quantum cascade (QC) laser grown on InP substrate by molecule beam epitaxy, by means of which, excellent lattice matching, the interface smoothness, the uniformity of the thickness and the composition of the epilayer are disclosed. What is more, these results are in good agreement with designed value. The largest lattice mismatch is within 0.18% and the intersubband absorption wavelength between two quantized energy levels is achieved at about lambda = 5.1 mum at room temperature. At 77 K, the threshold density of the QC laser is less than 2.6 kA/cm(2) when the repetition rate is 5 kHz and the duty cycle is 1%. (C) 2003 Elsevier Science B.V. All rights reserved.

Correlation between Er3+ emission and the microstructure of a-SiOx : H < Er > films

Photoluminescence (PL) from Er-implanted hydrogenated amorphous silicon suboxide (a-SiOX:H(x<2.0)) films was measured. Two luminescence bands with maxima at lambda congruent to 750 nm and lambda congruent to 1.54mum, ascribed to the a-SiOX:H intrinsic emission and Er3+ emission, were observed. Peak intensities of the two bands follow the same trend as a function of annealing temperature from 300 to 1000degreesC. Micro-Raman results indicate that the a-SiOX:H films are a mixture of two phases, an amorphous SiOX matrix and amorphous silicon (a-Si) domains embedded there in. FTIR spectra confirm that hydrogen effusion from a-SiOX:H films occurs during annealing. Hydrogen effusion leads to a reconstruction of the microstructure of a-Si domains, thus having a strong influence on Er3+ emission. Our study emphasizes the role of a-Si domains on Er3+ emission in a-SiOX:H films.

Correlation of calibration equations for test fixtures

This paper begins from the thru-short-open (TSO) and thru-line-match (TLM) methods to investigate the correlation of the calibration equations of these two methods, The relations among the measurements with the corresponding standards are obtained. It is found that the line standard with zero length can be used instead of ideal open and short, in case that two test fixtures are symmetrical. For asymmetrical fixtures, the measurements with the standards line, open and short are related at certain frequencies, and the matched load can be replaced by the line standards. The relations established are used to test short and match standards and analyze the freqPuency limits of the TSO method, Good agreement between theory and experiment is obtained, It is found that the TSO method becomes very poor when the insertion phase of the thru standard is near n pi/4, and this method has a lower frequency limit. The TLM method is found unsuitable for calibrating asymmetrical fixtures.

Hydrogen contaminant and its correlation with background electrons in GaN

GaN epilayers grown by molecular beam epitaxy using NH3 as the nitrogen source were found to contain hydrogen. We further notice that the background electron concentration in GaN can be correlated with the amount of hydrogen contaminant. X-ray photoelectron spectroscopy (XPS) measurements of the N Is peak reveal that hydrogen is bound to nitrogen. This will make the corresponding Ga atom see insufficient N counterpart, as can be inferred from the XPS Ga 3d spectrum. We then think that nitrogen in the lattice terminated by hydrogen is an effective nitrogen vacancy and hence a donor accounting for the background electrons.

Growth and characterization of InGaAs/InAlAs/InP high-electron-mobility transistor structures towards high channel conductivity

High-quality InGaAs/InAlAs/InP high-electron-mobility transistor (HEMT) structures with lattice-matched or pseudomorphic channels have been grown by molecular-beam epitaxy (MBE). The purpose of this work is to enhance the channel conductivity by changing the epitaxial structure and growth process. With the use of pseudomorphic step quantum-well channel, the highest channel conductivity is achieved at x = 0.7, the corresponding electron mobilities are as high as 12300 (300 K) and 61000 cm(2)/V.s (77 K) with two-dimensional electron gas (2DEG) density of 3.3 x 10(12) cm(-2). These structures are comprehensively characterized by Hall measurements, photoluminescence, double crystal X-ray diffraction and transmission electron microscopy. Strong room-temperature luminescence is observed, demonstrating the high optical quality of the samples. We also show that decreasing the In composition in the InyAl1-yAs spacer is very effective to increase the 2DEG density of PHEMT structures. (C) 1998 Published by Elsevier Science B.V. All rights reserved.

Correlation Between Lattice Strain and Energy Gap Bowing of AlxGa1-xN Epitaxial Thin Films

The correlation between the energy band-gap of AlxGa1-xN epitaxial thin films and lattice strain was investigated using both High Resolution X-ray Diffraction (HRXRD) and Spectroscopic Ellipsometry (SE). The Al fraction, lattice relaxation, and elastic lattice strain were determined for all AlxGa1-xN epilayers, and the energy gap as well. Given the type of intermediate layer, a correlation trend was found between energy band-gap bowing parameter and lattice mismatch, the higher the lattice mismatch is, the smaller the bowing parameter (b) will be.

Design and Fabrication of a Photonic Crystal Channel Drop Filter Based on an Asymmetric Silicon-on-Insulator Slab

We report on the design and fabrication of a photonic crystal (PC) channel drop filter based on an asymmetric silicon-on-insulator (SOI) slab. The filter is composed of two symmetric stick-shape micro-cavities between two single-line-defect (W1) waveguides in a triangular lattice, and the phase matching condition for the filter to improve the drop efficiency is satisfied by modifying the positions and radii of the air holes around the micro-cavities. A sample is then fabricated by using electron beam lithography (EBL) and inductively coupled plasma (ICP) etching processes. The measured 0 factor of the filter is about 1140, and the drop efficiency is estimated to be 73% +/- 5% by fitting the transmission spectrum.