Polishing-related optical anisotropy of semi-insulating GaAs studied by reflectance difference spectroscopy

The strong in-plane optical anisotropy of (001) semi-insulating GaAs, which comes from the submicron region under the surface, has been observed by reflectance difference spectroscopy. The optical anisotropy can be explained by the anisotropic strain that is introduced by the asymmetric distribution of 60 degrees dislocations during surface polishing. The simulated spectra reproduce the line shape of the experimental ones. The simulations show that the anisotropic strain is typically about 2.3x10(-4). (C) 2000 American Institute of Physics. [S0021-8979(00)01315-3].

Evolution of wetting layers in InAs/GaAs quantum-dot system studied by reflectance difference spectroscopy

The wetting layers (WL) in InAs/GaAs quantum-dot system have been studied by reflectance difference spectroscopy (RDS), in which two structures related to the heavy-hole (HH) and light-hole (LH) transitions in the WL have been observed. The evolution and segregation behaviors of WL during Stranski-Krastanow (SK) growth mode have been studied from the analysis of the WL-related optical transition energies. It has been found that the segregation coefficient of Indium atoms varies linearly with the InAs amount in WL. In addition, the effect of the growth temperature on the critical thickness for InAs island formation has also been studied. The critical thickness defined by the appearance of InAs dots, which is determined by AFM, shows a complex variation with the growth temperature. However, the critical thickness determined by RDS is almost constant in the range of 510-540 degrees C.

In-plane optical anisotropy in GaAsN/GaAs single-quantum well investigated by reflectance-difference spectroscopy

The interface properties of GaNxAs1-x/GaAs single-quantum well is investigated at 80 K by reflectance difference spectroscopy. Strong in-plane optical anisotropies (IPOA) are observed. Numerical calculations based on a 4 band K . P Hamiltonian are performed to analyze the origin of the optical anisotropy. It is found that the IPOA can be mainly attributed to anisotropic strain effect, which increases with the concentration of nitrogen. The origin of the strain component epsilon(xy) is also discussed.

Reflectance-difference spectroscopy study of the Fermi-level position of low-temperature-grown GaAs

The results of a reflectance-difference spectroscopy study of GaAs grown on (100) GaAs substrates by low-temperature molecular-beam epitaxy (LT-GaAs) are presented. In-plane optical anisotropy resonances which come from the linear electro-optic effect produced by the surface electric field are observed. The RDS line shape of the resonances clearly shows that the depletion region of LT-GaAs is indeed extremely narrow (much less than 200 Angstrom). The surface potential is obtained from the RDS resonance amplitude without the knowledge of space-charge density. The change of the surface potential with post-growth annealing temperatures reflects a complicated movement of the Fermi level in LT-GaAs. The Fermi level still moves for samples annealed at above 600 degrees C, instead of being pinned to the As precipitates. This behavior can be explained by the dynamic properties of defects in the annealing process.

Investigation of GaAs/AlGaAs interfaces by reflectance-difference spectroscopy

The in-plane optical anisotropy of several GaAs/AlGaAs quantum well samples with different well widths has been measured at room temperature by reflectance-difference spectroscopy (RDS). The RDS line shapes are found to be similar in all the samples examined here, which dominantly consist of two peak-like signals corresponding to 1HH-->1E and 1LH-->1E transition. As the well width is decreased, or the 1 ML InAs layer is inserted at one interface, the intensity of the anisotropy increases quickly. Our detail analysis shows that the anisotropy mainly arises from the anisotropic interface roughness. The results demonstrate that the RDS technique is sensitive to the interface structures.

The two- to three-dimensional growth transition of InAs/GaAs epitaxy layer studied by reflectance difference spectroscopy

In this work, we have adopted reflectance difference spectroscopy to study the evolution of InAs layer grown at different temperatures in GaAs matrix. Associated with the two- to three-dimensional growth transition of InAs layer, the transition energies and the in-plane optical anisotropy of InAs wetting layer exhibit abrupt changes. This provides a new way to decide the critical thickness h(c) for the growth transition. The obtained h(c)s are compared with those determined by atomic force microscope measurement, and discrepancy is found at high temperatures. The origin of the difference is clarified and the variations in hc with temperature are further discussed. (C) 2010 American Institute of Physics. [doi:10.1063/1.3494043]

Study on the thermal stability of InN by in-situ laser reflectance system

The thermal stability of InN in the growth environment in metalorganic chemical vapor deposition was systematically investigated in situ by laser reflectance system and ex situ by morphology characterization, X-ray diffraction and X-ray photoelectron spectroscopy. It was found that InN can withstand isothermal annealing at temperature as high as 600 degrees C in NH3 ambient. While in N-2 atmosphere, it will decompose quickly to form In-droplets at least at the temperature around 500 degrees C, and the activation energy of InN decomposition was estimated to be 2.1 +/- 0.1 eV. Thermal stability of InN when annealing in NH3 ambient during temperature altering would be very sensitive to ramping rate and NH3 flow rate, and InN would sustain annealing process at small ramping rate and sufficient supply of reactive nitrogen radicals. Whereas In-droplets formation was found to be the most frequently encountered phenomenon concerning InN decomposition, annealing window for conditions free of In-droplets was worked out and possible reasons related are discussed. In addition, InN will decompose in a uniform way in the annealing window, and the decomposition rate was found to be in the range of 50 and 100 nm/h. Hall measurement shows that annealing treatment in such window will improve the electrical properties of InN. (c) 2005 Elsevier B.V. All rights reserved.

Detection of indium segregation effects in InGaAs/GaAs quantum wells using reflectance-difference spectrometry

The influence of the Indium segregation on the interface asymmetry in InGaAs/GaAs quantum wells have been studied by reflectance-difference spectroscopy (RDS). It is found that the anisotropy of the 2H1E (2HH --> 1E) transition is very sensitive to the degree of the interface asymmetry. Calculations taking into account indium segregation yield good agreement with the observed anisotropy structures. It demonstrates that the anisotropy intensity ratio of the 1L1E (1LH --> 1E) and 2H1E transitions measured by RDS can be used to characterize the interface asymmetry. (C) 2002 Elsevier Science B.V. All rights reserved.

Interference effects in differential reflectance spectra of the GaAs epilayers grown on Si substrate

We report the observation of oscillating features in differential reflectance spectra from the GaAs epilayer grown on Si substrate in the energy range both below and above the fundamental band gap. It is demonstrated that the oscillating features are due to the difference in the interference between two neighboring areas of the sample. The interference arises from two light beams reflected from different interfaces of the sample. The calculated spectra in the nonabsorption region are in good agreement with measured data. It is shown that the interference effect can be used as a sensitive method to characterize the inhomogeneity of the semiconductor heterostructures. (C) 1998 American Institute of Physics. [S0021-8979(98)08723-4].

A New Discrimination Method of Maize Seed Varieties Based on Near-Infrared Spectroscopy

A new discrimination method for the maize seed varieties based on the near-infrared spectroscopy was proposed. The reflectance spectra of maize seeds were obtained by a FT-NIR spectrometer (12 000-4 000 cm(-1)). The original spectra data were preprocessed by first derivative method. Then the principal component analysis (PCA) was used to compress the spectra data. The principal components with the cumulate reliabilities more than 80% were used to build the discrimination models. The model was established by Psi-3 neuron based on biomimetic pattern recognition (BPR). Especially, the parameter of the covering index was proposed to assist to discriminating the variety of a seed sample. The authors tested the discrimination capability of the model through four groups of experiments. There were 10, 18, 26 and 34 varieties training the discrimination models in these experiments, respectively. Additionally, another seven maize varieties and nine wheat varieties were used to test the capability of the models to reject the varieties not participating in training the models. Each group of the experiment was repeated three times by selecting different training samples at random. The correct classification rates of the models in the four-group experiments were above 91. 8%. The correct rejection rates for the varieties not participating in training the models all attained above 95%. Furthermore, the performance of the discrimination models did not change obviously when using the different training samples. The results showed that this discrimination method can not only effectively recognize the maize seed varieties, but also reject the varieties not participating in training the model. It may be practical in the discrimination of maize seed varieties.

Detection of indium segregation effects in InGaAs/GaAs quantum wells using reflectance-difference spectrometry

The influence of the Indium segregation on the interface asymmetry in InGaAs/GaAs quantum wells have been studied by reflectance-difference spectroscopy (RDS). It is found that the anisotropy of the 2H1E (2HH --> 1E) transition is very sensitive to the degree of the interface asymmetry. Calculations taking into account indium segregation yield good agreement with the observed anisotropy structures. It demonstrates that the anisotropy intensity ratio of the 1L1E (1LH --> 1E) and 2H1E transitions measured by RDS can be used to characterize the interface asymmetry. (C) 2002 Elsevier Science B.V. All rights reserved.

Internal Friction Of Bend-Deformed Nanocrystalline Nickel By Mechanical Spectroscopy

Internal friction of nanocrystalline nickel is investigated by mechanical spectroscopy from 360 K to 120 K. Two relaxation peaks are found when nanocrystalline nickel is bent up to 10% strain at room temperature and fast cooling. However, these two peaks disappear when the sample is annealed at room temperature in vacuum for ten days. The occurrence and disappearance of the two relaxation peaks can be explained by the interactions of partial dislocations and point defects in nanocrystalline materials.

Electrochemical Impedance Spectroscopy Of Peo Coating On Aluminum Alloy In Nacl Solution

Various Plasma Electrolytic Oxidation (PEO) ceramic coatings were prepared on LY12 aluminum alloy by adjusting the concentration of sodium silicate solution. Optical microscope (OM), XRD and EIS were used to study their morphology, composition and anti corrosion behavior in NaCl solution. Increasing concentration of sodium silicate leads to the increase of the total coating thickness while too high and too low concentration lead to the decrease of inner dense layer. The main composition of PEO coatings prepared in 20, 40 and above 60g/L concentration solution are correspondingly alumina, alumina with mullite, and amorphous phase. The corrosion resistance is determined by the inner dense layer. Increasing the thickness of inner dense layer can improve the anti-corrosion performance. PEO coating's corrosion resistance in acidic, alkaline and neutral NaCl solution is proved and the corrosion mechanism involved is also discussed.

Curie Transition Of Nc Nickel By Mechanical Spectroscopy And Magnetization Study

Mechanical spectroscopy measurement is performed to study the internal friction of nanocrystalline ( NC) nickel with an average grain size of 23 nm from room temperature to 610 K. An internal friction peak is observed at about 550 K, which corresponds to the Curie transition process of the NC nickel according to the result of magnetization test. Moreover, the fact that the Curie temperature of NC nickel is lower than that of coarse-grained nickel is explained by an analytical model based on the weakening of cohesive energy.

Surface-enhanced resonance Raman scattering spectroscopy of single R6G molecules

Surface-enhanced resonance Raman scattering (SERRS) of Rhodamine 6G (R6G) adsorbed on colloidal silver clusters has been studied. Based on the great enhancement of the Raman signal and the quench of the fluorescence, the SERRS spectra of R6G were recorded for the samples of dye colloidal solution with different concentrations. Spectral inhomogeneity behaviours from single molecules in the dried sample films were observed with complementary evidences, such as spectral polarization, spectral diffusion, intensity fluctuation of vibrational lines and even "breathing" of the molecules. Sequential spectra observed from a liquid sample with an average of 0.3 dye molecules in the probed volume exhibited the expected Poisson distribution for actually measuring 0, 1 or 2 molecules. Difference between the SERRS spectra of R6G excited by linearly and circularly polarized light were experimentally measured.