S. D. Yoo

Variation of GaN valence bands with biaxial stress and quantification of residual stress

Low-temperature reflectance data on epitaxial GaN thin-film samples covering the widest range of tensile and compressive stress (−3.8–3.5 kbar) thus far explicitly show the nonlinear behavior of the B–A and C–A splittings versus the energy of the A exciton. Lineshape ambiguities that hindered previous interpretations have been resolved with reciprocal-space analysis, allowing us to obtain band parameters such as ΔSO=17.0±1meV with increased confidence.

Trends in residual stress for GaN/AlN/6H–SiC heterostructures

We discuss trends in residual stress as a function of film thickness, growth temperature, and substrate orientation for GaN/AlN/6H–SiC heterostructures. Films are mostly compressive for samples less than about 0.7 μm thick, are tensile up to about 2 μm, then abruptly become less tensile with stress values near 1 kbar thereafter. We interpret this as a successive relief of lattice mismatch and thermal stresses culminating in a catastrophic relief by unknown mechanisms at moderate thicknesses. These data indicate that relaxation processes in these heterostructures are not as well understood as previously supposed.

Spectroscopic ellipsometric characterization of undoped ZnTe films grown on GaAs

We report highly accurate dielectric function data for ZnTe. These data were made possible by the high quality of the heteroepitaxial material and the development of a chemical etching procedure for producing abrupt surfaces on ZnTe; they provided the first observation of the E0+Δ0 structure in ZnTe by spectroscopic ellipsometry and evidence for several contributions to the E2 structure. Accurate critical point energies were obtained by Fourier analysis.

Optical properties of AlxGa1−xP (0⩽x⩽0.52) alloys

We report optical properties of AlxGa1−xP (0⩽x⩽0.52) alloys grown by gas source molecular-beam epitaxy on S-doped GaP(001) substrates. Room-temperature pseudodielectric function spectra from 1.5 to 6.0 eV were obtained by spectroscopic ellipsometry. By applying the parabolic-band critical point model to numerically calculated second energy derivatives of these spectra, we obtained accurate room-temperature values of the E1, E0′, E2, and E2′ critical point energies and their dependence on composition x.

Elimination of endpoint-discontinuity artifacts in the analysis of spectra in reciprocal space

Reciprocal-space analysis offers several advantages for determining critical point parameters in optical and other spectra, for example the separation of baseline effects, information, and noise in low-, medium-, and high-index Fourier coefficients, respectively. However, endpoint-discontinuity artifacts can obscure much of the information when segments are isolated for analysis. We developed a procedure for eliminating these artifacts and recovering buried information by minimizing in the white-noise region the mean-square deviation between the Fourier coefficients of the data and those of low-order polynomials, then subtracting the resulting coefficients from the data over the entire range. We find that spectral analysis is optimized if no false data are used, i.e., when the number of points transformed equals the number of actual data points in the segment. Using fractional differentiation we develop a simple derivation of the variation of the reciprocal-space coefficients with index n for Lorentzian a...

Ellipsometric studies of Cd1−xMgxTe (0⩽x⩽0.5) alloys

The determination of the above band gap optical properties of zincblende Cd1−xMgxTe (0⩽x⩽0.5) ternary alloys are reported on. Using the parabolic-band critical point model, room-temperature critical point energies of the E0, E0+Δ0, E1, E1+Δ1, E2, and E0′ interband transitions from numerically calculated second energy derivatives of ellipsometric spectra were obtained. The presence of two distinct structures in the E2 feature for x>0 was also observed.

In situ monitoring of MOVPE growth by combined spectroscopic ellipsometry and reflectance-difference spectroscopy

Comprehensive characterization of epitaxial growth by metal organic vapor phase epitaxy (MOVPE) requires a combination of thin-film, near-surface-, and surface-sensitive techniques to determine layer thicknesses and compositions, composition of the most recently deposited material, and surface chemistry, respectively. These data can be obtained non-destructively by spectroscopic ellipsometry (SE) and reflectance-difference (-anisotropy) spectroscopy (RDS/RAS). Here we describe the first unified optical system, basically a rotating-polarizer ellipsometer (RPE) integrated into a modified commercial rotating-sample MOVPE reactor, that performs both SE and RDS simultaneously with a single optical path. Data are obtained in parallel from 240 to 840 nm with a high-speed 16-bit photodiode array (PDA) at a repetition rate greater than 2 Hz and a precision of ±0.0001. We provide examples of its use, and show in particular that GaP intermixes with Si during the initial stages of heteroepitaxy. Capabilities of the presented configuration and its potential for future investigations are discussed.

Real-time optical characterization of heteroepitaxy by organometallic chemical vapor deposition

Heteroepitaxy of GaP on Si(100) and GaAs(100) is investigated under organometallic chemical vapor deposition conditions using combined spectroscopic ellipsometry (SE) and non-normal-incidence reflectance-difference (-anisotropy) spectroscopy. Real-time monitoring greatly assists in identifying optimum starting surfaces for heteroepitaxy since prolonged exposure to PH3 results in roughening of Si(100) and GaAs(100) surfaces, in agreement with previous work. Real-time SE data of GaP on Si indicate that under our conditions GaP and Si interpenetrate as optically identifiable materials over the first 75 A, suggesting that either trimethylgallium or a reaction by-product can act as a catalyst for the formation of Si{111} facets.

Analysis of optical spectra by Fourier methods

Abstract Results of a systematic investigation of reciprocal-space analysis, applied to optical spectroscopy, are reported. Filtering procedures for removing baseline and noise artifacts are more effective in reciprocal- than in real-space. Also, correlations among parameters are reduced and the functional dependence of real-space lineshapes need be known a priori only in very general terms. We apply reciprocal-space analysis to achieve accurate values of critical point energies for the E 1 and E 1 +Δ 1 transitions of GaAs from ellipsometric spectra and to locate critical point energies in low-temperature reflectance data of GaN. We show that data may easily be corrected for systematic artifacts such as the monochromator slit width, an optimum slit width can be defined, and the degree of improvement needed to achieve a particular level of performance can be predicted.

Dielectric function of epitaxial ZnSe films

We examine various ZnSe spectra to obtain that which best represents the dielectric response e of ZnSe. The measured evolution of pseudodielectric function 〈e〉 data with chemical etching shows that the natural overlayer on ZnSe can be modeled accurately only if we assume that it contains amorphous Se. Hence previous assumptions made in correcting 〈e〉 mathematically are not correct, and data obtained on stripped samples yield the best representation of e.