Extraction of intrinsic response from S-parameters of laser diodes

We describe a new method for extracting the intrinsic response of a laser diode from S-parameters measured using a calibrated vector network analyzer. The experimental results obtained using the new method are compared with those obtained using the optical modulation method and the frequency response subtraction method. Good agreement has been obtained, confirming the new method validity and accuracy. The new method has the advantages of obtaining the intrinsic characteristics of a laser diode with conventional measurements using a network analyzer.

Influence of growth parameters of frequency-radio plasma nitrogen source on extending emission wavelengths from 1.31 mu m to 1.55 mu m GaInNAs/GaAs quantum wells grown by molecular-beam epitaxy

High (42.5%) indium content GaInNAs/GaAs quantum wells with room temperature emission wavelength from 1.3 mu m to 1.5 mu m range were successfully grown by Radio Frequency Plasma Nitrogen source assisted Molecular Beam Epitaxy. The growth parameters of plasma power and N-2 How rate were optimized systematically to improve the material quality. Photoluminescence and transmission electron microscopy measurements showed that the optical and crystal quality of the 1.54 mu m GaInNAs/GaAs QWs was kept as comparable as that in 1.31 mu m.

A model for scattering due to interface roughness in finite quantum wells

A model for scattering due to interface roughness in finite quantum wells (QWs) is developed within the framework of the Boltzmann transport equation and a simple and explicit expression between mobility limited by interface roughness scattering and barrier height is obtained. The main advantage of our model is that it does not involve complicated wavefunction calculations, and thus it is convenient for predicting the mobility in thin finite QWs. It is found that the mobility limited by interface roughness is one order of amplitude higher than the results derived by assuming an infinite barrier, for finite barrier height QWs where x = 0.3. The mobility first decreases and then flattens out as the barrier confinement increases. The experimental results may be explained with monolayers of asperity height 1-2, and a correlation length of about 33 angstrom. The calculation results are in excellent agreement with the experimental data from AlxGa1-xAs/GaAs QWs.

A novel method of determining semiconductor parameters in EBIC and SEBIV modes of SEM

We present a novel method for determining semiconductor parameters such as diffusion length L, lifetime tau and surface recombination velocity S of minority carriers by employing scanning electron microscopy (SEM). This new method is applicable to both electron beam induced current (EBIC and surface electron beam induced voltage (SEBIV) modes in SEM. The quantitative descriptions for EBIC and SEBIV signals are derived. The parameters L, S and tau can be directly extracted from the expressions for EBIC or SEBIV signals and their relaxation characteristics in experiment. As an example, the values of L, S and tau for n-p junction and p-Si crystal are determined by using the novel method in EBIC or SEBIV mode. The carrier diffusion length of a p-Si crystal is determined to be 8.74 mum in SEBIV mode. It is very close to the normal diffusion length of 7.41 mum of this sample. The novel method is proved to be very helpful for the quantitative characterization of semiconductor materials and devices. Especially, the SEBIV mode in SEM shows great potential for investigating semiconductor structures nondestructively.

Surface roughness and high density of cubic twins and hexagonal inclusions in cubic GaN epilayers

Surface roughness and its correlation with the polarity of internal hexagonal inclusions and cubic twins have been investigated by atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The surface roughness resulted from large amount of strips, which prolonged in [1 (1) over bar0] direction with small size in [110] or [110] direction. The sidestep of each strip is just the top of high density of hexagonal inclusions or cubic microtwins. Moreover, XRD shows that the amount of hexagonal inclusions and cubic microtwins measured in [110] direction are twice or more as much as in [110] direction. Therefore, it is hexagonal inclusions, cubic twins and their distributive polarity that is responsible to the surface characteristics of cubic GaN epilayers.

Photoluminescence properties of SiGe/Si single wells with fluctuating structural parameters

Photoluminescence properties of SiGe/Si single wells with fluctuating structural parameters are studied. Four SiGe/Si single wells have been grown on Si(001) at 750 degrees C by disilane and solid Ge molecular beam epitaxy with varied disilane cracking-temperatures. Intense NP and TO-phonon replicas are detected up to 70 K in the photoluminescence spectra and the activation energy of the thermal quenching of the photoluminescence is 28 +/- 4 meV. The high growth temperature and purposeful introduction of fluctuation of structural parameters may be responsible for the improvement of the thermal quenching property.

The dependence of growth rate of GaN buffer layer on growth parameters by metalorganic vapor-phase epitaxy

The growth rate of GaN buffer layers on sapphire grown by metalorganic vapor-phase epitaxy (MOVPE) in an atmospheric pressure, two-channel reactor was studied. The growth rate, as measured using laser reflectance, was found to be dependent on growth temperature, molar flow rate of the sources tin this case, trimethylgallium and ammonia) and the input configuration of sources into the reactor. A model of the GaN buffer layer growth process by MOVPE is proposed to interpret the experimental evidence. (C) 1998 Elsevier Science B.V. All rights reserved.

Subtraction of S-parameters for adiabatic small-signal modulation characteristics of laser diode - art. no. 60201V

An extended subtraction method of scattering parameters for characterizing laser diode is introduced in this paper. The intrinsic small-signal response can be directly extracted from the measured transmission coefficients of laser diode by the method. However the chip temperature may change with the injection bias current due to thermal effects, which causes inaccurate intrinsic response by our method. Therefore, how to determine the chip temperature and keep the laser chip adiabatic is very critical when extracting the intrinsic response. To tackle these problems, the dependence of the lasing wavelength of the laser diode on the chip temperature is investigated, and an applicable measurement setup which keeps the chip temperature stable is presented. The scattering parameters of laser diode are measured on diabatic and adiabatic conditions, and the extracted intrinsic responses for both conditions are compared. It is found that the adiabatic intrinsic responses are evidently superior to those without thermal consideration. The analysis indicates that inclusion of thermal effects is necessary to acquire accurate intrinsic response.

Dispersion characteristics of nanometer-scaled silicon rib waveguides

We investigate the dispersion properties of nanometer-scaled silicon waveguides with channel and rib cross section around the optical fiber communication wavelength and systematically study their relationship with the key structural parameters of the waveguide. The simulation results show that the introduction of an extra degree of freedom in the rib depth enables the rib waveguide more flexible in engineering the group velocity dispersion (GVD) compared with the channel waveguide. Besides, we get the structural parameters of the waveguides that can realize zero-GVD at 1550 nm.

Stretchable Graphene: A Close Look at Fundamental Parameters through Biaxial Straining

Tunable biaxial stresses, both tensile and compressive, are applied to a single layer graphene by utilizing piezoelectric actuators. The Gruneisen parameters for the phonons responsible for the D, G, 2D and 2D' peaks are studied. The results show that the D peak is composed of two peaks, unambiguously revealing that the 2D peak frequency (omega(2D)) is not exactly twice that of the D peak (omega(D)). This finding is confirmed by varying the biaxial strain of the graphene, from which we observe that the shift of omega(2D)/2 and omega(D) are different. The employed technique allows a detailed study of the interplay between the graphene geometrical structures and its electronic properties.

Effect of pulse heating parameters on the microscale bubble dynamics at a microheater surface

We study the effects of pulse heating parameters on the micro bubble behavior of a platinum microheater (100 mu m x 20 mu m) immersed in a methanol pool. The experiment covers the heat fluxes of 10-37 MW/m(2) and pulse frequencies of 25-500 Hz. The boiling incipience is initiated at the superheat limit of methanol, corresponding to the homogeneous nucleation. Three types of micro boiling patterns are identified. The first type is named as the bubble explosion and regrowth, consisting of a violent explosive boiling and shrinking, followed by a slower bubble regrowth and subsequent shrinking, occurring at lower heat fluxes. The second type, named as the bubble breakup and attraction, consists of the violent explosive boiling, bubble breakup and emission, bubble attraction and coalescence process, occurring at higher heat fluxes than those of the first type. The third type, named as the bubble size oscillation and large bubble formation, involves the initial explosive boiling, followed by a short periodic bubble growth and shrinking. Then the bubble continues to increase its size, until a constant bubble size is reached which is larger than the microheater length. 

Boundary conditions at the solid-liquid surface over the multiscale channel size from nanometer to micron

The boundary condition at the solid surface is one of the important problems for the microfluidics. In this paper we study the effects of the channel sizes on the boundary conditions (BC), using the hybrid computation scheme adjoining the molecular dynamics (MD) simulations and the continuum fluid mechanics. We could reproduce the three types of boundary conditions (slip, no-slip and locking) over the multiscale channel sizes. The slip lengths are found to be mainly dependent on the interfacial parameters with the fixed apparent shear rate. The channel size has little effects on the slip lengths if the size is above a critical value within a couple of tens of molecular diameters. We explore the liquid particle distributions nearest the solid walls and found that the slip boundary condition always corresponds to the uniform liquid particle distributions parallel to the solid walls, while the no-slip or locking boundary conditions correspond to the ordered liquid structures close to the solid walls. The slip, no-slip and locking interfacial parameters yield the positive, zero and negative slip lengths respectively. The three types of boundary conditions existing in "microscale" still occur in "macroscale". However, the slip lengths weakly dependent on the channel sizes yield the real shear rates and the slip velocity relative to the solid wall traveling speed approaching those with the no-slip boundary condition when the channel size is larger than thousands of liquid molecular diameters for all of the three types of interfacial parameters, leading to the quasi-no-slip boundary conditions.