Paul G. Snyder

Variable angle of incidence spectroscopic ellipsometry: Application to GaAs‐AlxGa1−xAs multiple heterostructures

The sensitivity of spectroscopic ellipsometry data to multilayer model parameters is shown to be a strong function of the angle of incidence. A quantitative study of sensitivity versus angle of incidence is performed for a GaAs‐AlxGa1−xAs‐GaAs substrate structure, showing that maximum sensitivity to layer thicknesses and AlGaAs composition occurs near the wavelength‐dependent principal angle. These results are verified by experimental measurements on two molecular‐beam epitaxy grown samples. New spectral features, not found in previous ellipsometry studies of similar structures, are also reported.The sensitivity of spectroscopic ellipsometry data to multilayer model parameters is shown to be a strong function of the angle of incidence. A quantitative study of sensitivity versus angle of incidence is performed for a GaAs‐AlxGa1−xAs‐GaAs substrate structure, showing that maximum sensitivity to layer thicknesses and AlGaAs composition occurs near the wavelength‐dependent principal angle. These results are verified by experimental measurements on two molecular‐beam epitaxy grown samples. New spectral features, not found in previous ellipsometry studies of similar structures, are also reported.

InP optical constants between 0.75 and 5.0 eV determined by variable‐angle spectroscopic ellipsometry

Using variable‐angle spectroscopic ellipsometry (VASE) InP optical constants for photon energies have been determined in the range from 0.75 to 5.0 eV, which includes the fundamental gap at 1.35 eV. Above 1.5 eV the results are consistent with previously measured pseudovalues from an oxide‐stripped sample when a very thin residual overlayer is accounted for. They are also shown to be compatible with previously published prism measurements of refractive index below the band gap. Real and imaginary parts of the dielectric function are shown to be Kramers–Kronig (KK) self‐consistent above the gap, and the KK analysis was used to extend the dielectric function below the measurement range to 0.5 eV. The assumptions underlying biased fitting of VASE data and the importance of variable‐angle measurements were investigated. The detection and significance of systematic errors for general VASE data analysis were also investigated, especially with regard to fit parameter confidence limits.

Modeling AlxGa1−xAs optical constants as functions of composition

Three models for the dielectric function ex(hν) of AlxGa1−xAs are reviewed. All are based on measured optical constants at discrete compositions. The validity of each model near critical point energies, and otherwise, is evaluated. Only the energy‐shift model is appropriate over the entire available spectrum (1.5–6.0 eV), including the band‐gap (E0) region.

Temperature dependence of optical properties of GaAs

Pseudodielectric functions 〈e〉=〈e1〉+i〈e2〉 of GaAs were measured by spectroscopic ellipsometry (SE), in the range of 1.6–4.45 eV, at temperatures from room temperature (RT) to ∼610 °C. A very clean, smooth surface was obtained by first growing an epitaxial layer of GaAs on a GaAs substrate and immediately capping it with a protective layer of arsenic. The cap prevented surface oxidation during transport to the measurement chamber, where it was evaporated under ultrahigh vacuum at ∼350 °C. Room‐temperature SE results from this surface are in good agreement with those in the literature obtained by wet‐chemical etching. A quantitative analysis of the 〈e〉 spectrum was made using the harmonic‐oscillator approximation (HOA). It is shown by the HOA that the E1 and E1+Δ1 energy‐band critical points shift downward ∼300 meV as temperature increases from RT to ∼610 °C. An algorithm was developed, using the measured optical constants at a number of fixed temperatures, to compute the dielectric function spectrum at an ...

Characterization of pulsed laser deposited zinc oxide

Abstract The pulsed laser deposition of zinc oxide films (ZnO) has been studied as a function of laser wavelength, and substrate temperature. Optical emission spectroscopy of the laser produced plume was used to characterize the deposition process. The deposited films were characterized by X-ray diffractometry, Auger electron spectroscopy, and scanning electron microscopy. Highly textured (002) ZnO films deposited at substrate temperatures of 300 °C with laser wavelengths of 532 nm and 248 nm. However, the energy fluence of 248 nm radiation controls the degree of texturing, allowing highly textured films to be deposited at room temperature.

Determination of AlAs optical constants by variable angle spectroscopic ellipsometry and a multisample analysis

Using variable angle spectroscopic ellipsometry, optical constants for AlAs (1.4–5.0 eV) are presented which are simultaneously compatible with measured data from four different samples. The below‐gap index values are compatible with published prism measured values. The second derivative spectrum are compatible with published values above the direct band gap. The AlAs spectra is Kramers–Kronig self‐consistent over the measured range and is compatible with published values from 0.6 to 1.4 eV. The optical constants for thin (<50 A) GaAs caps on AlAs are shown to be different from bulk GaAs values and require special consideration when fitting ellipsometric data. For the thin GaAs caps, the E1 and E1+Δ1 critical‐point structure is shifted to higher energies as previously observed for GaAs quantum wells. Bulk AlAs optical constants are shown to be different from those of a thin (∼20 A) AlAs barrier layer embedded in GaAs. The thin barrier layer exhibits a highly broadened critical‐point structure. This barrie...

Fundamentals and applications of variable angle spectroscopic ellipsometry

Abstract Being able to accurately select the angle of incidence and spectral range for measurements makes ellipsometry a very powerful tool for materials, surface and interface analysis. The technique is fast, totally computer automated and can be performed at atmospheric pressure. It is totally non-invasive and is sensitive to fractions of atom layer thicknesses. We illustrate the power of variable angle spectroscopic ellipsometry with three examples: optical coatings; surface roughness and wetting; electric field effects in semiconductor studies.

Optical constants of GaxIn1−xP lattice matched to GaAs

The optical constants of Ga0.51In0.49P have been determined from 0.8 to 5.0 eV using variable‐angle spectroscopic ellipsometry measurements at room temperature. The metal‐organic vapor‐phase‐epitaxy‐grown samples were x‐ray analyzed to confirm lattice matching to the GaAs substrate. The effects of the native oxide were numerically removed from the data to determine the intrinsic optical constants. This is important because the optical constants reported become generally useful for modeling multiple‐layer structures. A Kramers–Kronig analysis was used to reduce interference‐related fluctuations in the below‐gap refractive index. Near the band edge a mathematical form for excitonic absorption was included. Critical point energies were extracted using a numerical second‐derivative fitting algorithm.

Variable angle spectroscopic ellipsometry: A non-destructive characterization technique for ultrathin and multilayer materials

Abstract In the most commonly used form of ellipsometry, a monochromatic collimated linearly polarized light beam is directed at an angle ϕ to the normal of a sample under study. The specularly reflected beam is, in general, elloptically polarized, and the state of polarization is analyzed using a second polarizer and photodetector. A rotating analyzer automated spectroscopic ellipsometer has been constructed in our laboratory. The angle of incidence can be set over a wide range of angles, with a precision and repeatibility of ±0.01°. A computer controls the monochromator, the azimuth of a stepper-motor-driven polarizer, a shutter, and the digitization of the detector signal. There are several other schemes used for acquiring ellipsometric data, and these are discussed in several sources. The purpose of this short review is to show how variable angle of incidence spectroscopic ellipsometry can be used as a powerful non-destructive tool for analysis of surfaces and multilayer films.

Studies of thin strained InAs, AlAs, and AlSb layers by spectroscopic ellipsometry

The optical constants for thin layers of strained InAs, AlAs, and AlSb have been investigated by spectroscopic ellipsometry and multi‐sample analyses. These materials are important for high‐speed resonant tunneling diodes in the AlAs/InAs/In0.53Ga0.47As and AlSb/InAs material systems. Understanding the optical properties for these thin layers is important for developing in situ growth control using spectroscopic ellipsometry. Ex situ room‐temperature measurements were made on multiple samples. The resulting fitted optical constants are interpreted as apparent values because they are dependent on the fit model and sample structure. These apparent optical constants for very thin layers can be dependent on thickness and surrounding material, and are generally applicable only for layers found in a similar structural context. The critical point features of optical constants for the strained layers and for the thin unstrained cap layers were found to differ from bulk values, and three principle effects (strain, quantum confinement, and thin‐barrier critical‐point broadening) have been identified as responsible. Of these three, the broadening of the E1 and E1+Δ1 critical points for thin barrier material is the newest and most pronounced. This thin barrier effect is shown to be a separate effect from strain, and is also observable for the AlAs/GaAs system.The optical constants for thin layers of strained InAs, AlAs, and AlSb have been investigated by spectroscopic ellipsometry and multi‐sample analyses. These materials are important for high‐speed resonant tunneling diodes in the AlAs/InAs/In0.53Ga0.47As and AlSb/InAs material systems. Understanding the optical properties for these thin layers is important for developing in situ growth control using spectroscopic ellipsometry. Ex situ room‐temperature measurements were made on multiple samples. The resulting fitted optical constants are interpreted as apparent values because they are dependent on the fit model and sample structure. These apparent optical constants for very thin layers can be dependent on thickness and surrounding material, and are generally applicable only for layers found in a similar structural context. The critical point features of optical constants for the strained layers and for the thin unstrained cap layers were found to differ from bulk values, and three principle effects (strain,...