Teresa Cole

Effects of depolarization of polarimetric components on null ellipsometry

Abstract Depolarization of optical components degrades the performance of a polarimeter, especially in the short wavelength region. Expressions for the light intensity in a null ellipsometer (NE) and a polarizer-sample-analyzer (PSA) system with depolarization errors in all components are given. Errors in Ψ and Δ using NE are mainly caused by depolarization of the compensator. A sample consisting of a thin sapphire slab 0.8 mm thick was tested. Δ measured by NE were shown to be accurate even for sample depolarization as large as 0.3, while those measured by rotating-analyzer ellipsometry (RAE) were seriously affected by sample depolarization. Methods for measuring depolarization were devised. The measured depolarization increases with the slit-width of the monochromator and the aperture size of irises and decreases with increasing wavelength. Depolarization spectra measured using NE, RAE and transmittance agree with one another.

Characterization for imperfect polarizers under imperfect conditions.

The principles for measuring the extinction ratio and transmittance of a polarizer are formulated by use of the principal Mueller matrix, which includes both polarization and depolarization. The extinction ratio is about half of the depolarization, and the contrast is the inverse of the extinction ratio. Errors in the extinction ratio caused by partially polarized incident light and the misalignment of polarizers can be corrected by the devised zone average method and the null method. Used with a laser source, the null method can measure contrasts for very good polarizers. Correct algorithms are established to deduce the depolarization for three comparable polarizers calibrated mutually. These methods are tested with wire-grid polarizers used in the 3-5-microm wavelength region with a laser source and also a lamp source. The contrasts obtained from both methods agree.

Investigation of interfacial roughness of InxGa1−xAs epitaxial layers on GaAs and InP substrates by soft x‐ray reflectivity

The grazing incidence x‐ray reflectivity is a nondestructive and sensitive technique for probing the depth profile of electron density in layered structures. This method has been utilized in the soft x‐ray regime to determine the roughness of interfaces, and the epilayer thickness in InxGa1−xAs/InP and InxGa1−xAs/GaAs heterostructures, for x=0.57 and x=0.60, grown by molecular beam epitaxy. By fitting the experimental results to our model, assuming uncorrelated interfacial roughness, we conclude that the top surface roughness does not depend on the type of the substrate or presence of stress in the epilayer, and is always smaller than interfacial roughness. The main factors which control the interfacial roughness are the quality of substrate and/or growth conditions rather than strain or lattice mismatch.

Surfactant effects of thallium in the epitaxial growth of indium arsenide on gallium arsenide(001)

We report on the role of thallium as a surfactant in the growth of InAs on GaAs(001) by molecular beam epitaxy. Thallium suppresses the transition from two-dimensional to three-dimensional growth modes for temperatures below 440 °C without incorporating significantly into the bulk InAs lattice. It extends the range of temperatures at which InAs may be grown, and appears to improve the crystalline quality of growth at 520 °C. Preliminary results indicate that at low temperature (280 °C) thallium does not prolong pseudomorphic growth beyond the normal critical layer thickness, but rather may form interfacial InTlAs with a greater concentration of thallium than has been achieved to date.

Characterization of infrared polarizers

The three independent characteristics of a polarizer are transmittance, extinction ratio, and cross-polarized scattering. Extinction ratio, depolarization, incoherent near-specular scattering (NSS), and contrasts are mutually dependent. Spectra of transmittance, extinction ratio, depolarization, and contrast for wiregrid polarizers were measured in the 3- to 5-micrometers wavelength region using the zones average method developed in this paper. Good polarizers are hard to find in this region. By putting two polarizers in a series with the polarization axes parallel to each other, the extinction ratio can be improved greatly by the ratio of total NSS to cross-polarized NSS of the rear polarizer.

A photoemission study of coherently strained InxGa1−xAs/InP(100)

Abstract Coherently strained 8 nm thick films of In 0.30 Ga 0.70 As (tensional strain = 1.7%) and In 0.75 Ga 0.25 As (compressional strain = −1.5%) were grown by molecular beam epitaxy on InP(100), on which In 0.53 Ga 0.47 As is the lattice-matched composition. Samples were studied with synchrotron-radiation photoemission measurements of the d-core levels of each ion. The results show (i) shifts of the core-level energies of surface ions consistent with previous results on As-rich (100) III–V surfaces, and (ii) significant intrinsic broadening of the core levels, especially the In 4d. Comparison of the two compositions showed (i) an increase in the binding energies of the Ga 3d and In 4d peak by 0.17 eV at the higher In concentration, and (ii) a constant separation of the In 4d from the Ga 3d of (1.80 ± 0.03) eV. A rigid-ion model of the alloy-semiconductor surface successfully predicts the observed halfwidths. The model also predicts that surface rumpling occurs in the As-terminated cation layer: The In ions are pushed out beyond the Ga ions. This rumpling has the effect of increasing the shift of the 4d-core binding energy for the “surface” In atoms.

Infrared-Wavelength Modulation Spectra Of InGaAs Grown By MBE And LPE

We present the infrared wavelength-modulation spectra of molecular-beam-epitaxy- (MBE) and liquid-phase-epitaxy- (LPE) grown Ini_xGaxAs (0.45 < x < 0.50) epilayers on (100) InP substrates. The transmittance, reflectance, and their wavelength derivatives are measured in the neighborhood of the band gap at room temperature. The dependence of band gap on strain is presented and analyzed. It is shown that epilayers partially relax the interfacial strain according to their thickness, giving different dependences of band gap on alloy composition. The results are analyzed by introducing a normalized fractional-strain parameter and applying Hookes-law elasticity and deformation potential theories.

Experimental And Theoretical X-Ray Absorption Near-Edge Study Of III-V Compound Semiconductor Materials

We report experimental and theoretical near-edge x-ray absorption fine structure (NEXAFS) spectra of clean and arsenic-capped gallium arsenide and the pseudo-binary alloy indium-gallium arsenide. Experimental data were obtained using synchrotron-radiation total-photoelectron-yield spectroscopy from the Ga(M2,3), As(M2,3), In(N2,3), and In(M4,5) edges. In addition, both C(K) and 0(K) NEXAFS spectra, and photon-stimulated ion-desorption mass spectra were obtained to assess and monitor the sample cleanliness. The samples studied were grown by molecular-beam epitaxy at China Lake and capped with arsenic for protection during transit to the Stanford Synchrotron-Radiation Laboratory. We have found by monitoring the As edges that heating the samples to 300 or 350°C completely removes the arsenic cap. Also, we find that after evaporation of the As cap, the NEXAFS spectra are identical for capped and uncapped samples. Theoretical calculations of the arsenic NEXAFS spectra were performed using a full multiple-scattering theory. The inputs to the calculations were ab initio phase shifts, calculated using pseudopotentials and a model geometry. Theoretical calculations of the arsenic NEXAFS spectra of In0.53Ga0.47As crystals are reported also. Here, the calculations are performed by creating many model crystals that have the appropriate stoichiometry and averaging the resulting spectra.