Huade Yao

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 ...

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...

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.

Spectroscopic ellipsometry studies of HF treated Si (100) surfaces

Both ex situ and in situ spectroscopic ellipsometry (SE) measurements have been employed to investigate the effects of HF cleaning on Si surfaces. The hydrogen‐terminated (H‐terminated) Si surface was modeled as an equivalent dielectric layer, and monitored in real time by SE measurements. The SE analyses indicate that after a 20‐s 9:1 HF dip without rinse, the Si (100) surface was passivated by the hydrogen termination and remained chemically stable. Roughness of the HF‐etched bare Si (100) surface was observed, in an ultrahigh vacuum (UHV) chamber, and analyzed by the in situ SE. Evidence for desorption of the H‐terminated Si surface‐layer, after being heated to ∼550 °C in the UHV chamber, is presented and discussed. This is the first use of an ex situ and in situ real‐time, nondestructive technique capable of showing state of passivation, the rate of reoxidation, and the surface roughness of the H‐terminated Si surfaces.

Anisotropic optical responses of sapphire (α-Al2O3) single crystals

Anisotropic dielectric functions of sapphire have been determined by both reflection and transmission variable angle spectroscopic ellipsometry. The measurements were made on a-plane (2110) sapphire substrates in the energy range of 0.75–6.5 eV at room temperature. The orientation of the optic axis of the a-plane sapphire sample was determined by polarized Raman scattering based on which the Euler angles are set for the fitting model. Two pairs of Cauchy user-defined functions were constructed to describe the optical constants of both ordinary n⊥(ω) and extraordinary n∥(ω) rays, respectively. As a result the optical constants from the generalized ellipsometry analysis are in good agreement with the reported data. The Kramers–Kronig (KK) relation between the real and the imaginary part have been carefully checked for both ordinary and extraordinary rays. The perfect fitting between the calculated and experimental e1(ω) function indicates that both functions of ordinary and extraordinary rays are KK consi...

Ordinary optical dielectric functions of anisotropic hexagonal GaN film determined by variable angle spectroscopic ellipsometry

Standard variable angle spectroscopic ellipsometry (VASE) has been employed to study the ordinary optical dielectric response of hexagonal gallium nitride (GaN) thin films—an important material for blue and ultraviolet light emitting device applications. The GaN films were grown by molecular beam epitaxy on c-plane sapphire substrates (α-Al2O3). Room temperature isotropic and anisotropic mode VASE measurements were made at angles of incidence between of 20° and 80°. Evidence of anisotropy was observed from the anisotropic mode measurements, reflecting the nature of wurtzite crystal structure of GaN. The sizable off-diagonal elements (Aps and Asp) of the Jones matrix indicate that the optical axis 〈c〉 of the c-plane sample are slightly off from the surface normal due to a small miscut of substrates. VASE data simulations by isotropic and anisotropic models indicate that the anisotropic effect on both diagonal and off-diagonal elements of the Jones matrix can be minimized to a negligible level at small angle of incidence. Thus the ordinary optical dielectric functions (E⊥〈c〉) are precisely determined by the isotropic mode VASE measurements at angles of incidence between 20° and 40° in the range of 0.75–6.5 eV. The VASE data were analyzed by a model dielectric function based on the GaN critical point structure, which allows for a nonzero extinction coefficient k below the band gap. The thicknesses of these GaN films are accurately determined via the analysis as well.Standard variable angle spectroscopic ellipsometry (VASE) has been employed to study the ordinary optical dielectric response of hexagonal gallium nitride (GaN) thin films—an important material for blue and ultraviolet light emitting device applications. The GaN films were grown by molecular beam epitaxy on c-plane sapphire substrates (α-Al2O3). Room temperature isotropic and anisotropic mode VASE measurements were made at angles of incidence between of 20° and 80°. Evidence of anisotropy was observed from the anisotropic mode measurements, reflecting the nature of wurtzite crystal structure of GaN. The sizable off-diagonal elements (Aps and Asp) of the Jones matrix indicate that the optical axis 〈c〉 of the c-plane sample are slightly off from the surface normal due to a small miscut of substrates. VASE data simulations by isotropic and anisotropic models indicate that the anisotropic effect on both diagonal and off-diagonal elements of the Jones matrix can be minimized to a negligible level at small angl...

Spectroscopic ellipsometric characterization of Si/Si1−xGex strained-layer supperlattices

Abstract Spectroscopic ellipsometry (SE) was employed to characterize Si/Si 1− x Ge x strained-layer superlattices. An algorithm was developed, using the available optical constants measured at a number of fixed x values of Ge composition, to compute the dielectric function spectrum of Si 1− x Ge x at an arbitrary x value in the spectral range 1.7 to 5.6 eV. The ellipsometrically determined superlattice thicknesses and alloy compositional fractions were in excellent agreement with results from high-resolution X-ray diffraction studies. The silicon surfaces of the superlattices were subjected to a 9 : 1 HF cleaning prior to the SE measurements. The HF solution removed silicon oxides on the semiconductor surface, and terminated the Si surface with hydrogen-silicon bonds, which were monitored over a period of several weeks, after the HF cleaning, by SE measurements. An equivalent dielectric layer model was established to describe the hydrogen-terminated Si surface layer. The passivated Si surface remained unchanged for > 2 h, and very little surface oxidation took place even over 3 to 4 days.

IN SITU INFRARED AND VISIBLE-LIGHT ELLIPSOMETRIC INVESTIGATIONS OF BORON NITRIDE THIN FILMS AT ELEVATED TEMPERATURES

In situ infrared (IR) spectroscopy and visible-light (VIS) spectroscopic ellipsometry over the spectral range from 700 to 2000 cm−1 and 1.5–3.5 eV, respectively, were used to investigate the optical behavior of boron nitride (BN) thin films at temperatures from room temperature (RT) to 600 °C. The polycrystalline hexagonal (h) and mixed-phase h- and cubic (c)-BN thin films were deposited by magnetron sputtering on [001] silicon. We observe a reversible moisture incorporation process in as-grown h-BN samples. When stored in normal ambient, the h-BN thin films absorb water into thin-film micropores. When annealed in ultrahigh vacuum or a dry nitrogen atmosphere, the samples expel moisture but retain their microstructure. This is observable by reduction of the thin-film refractive indices in accordance with changes in the IR lattice resonance behavior. The optical properties of high c-BN content thin films remain unchanged during annealing. And both intrinsic h- and c-BN thin-film VIS refractive indices are ...

In situ ellipsometric studies of optical and surface properties of GaAs(100) at elevated temperatures

A rotating-polarizer ellipsometer was attached to an ultrahigh vacuum (UHV) chamber. A GaAs(100) sample was introduced into the UHV chamber and heated at a number of fixed elevated temperatures, without arsenic overpressure. In situ spectroscopic ellipsometric (SE) measurements were taken, through a pair of low-strain quartz windows, to monitor the surface changes and measure the pseudodielectric functions at elevated temperatures. Real-time data from a GaAs surface covered with native oxide showed clearly the evolution of oxide desorption at approximately 580°C. In addition, surface degradation was found before and after the oxide desorption. An oxide free and smooth GaAs surface was obtained by depositing an arsenic protective coating onto a molecular beam epitaxy grown GaAs surface. The arsenic coating was evaporated immediately prior to SE measurements. A comparison showed that our room temperature data from this GaAs surface, measured in the UHV, are in good agreement with those in the literature obtained by wet-chemical etching. The surface also remained clean and smooth at higher temperatures, so that reliable temperature-dependent dielectric functions were obtained.

In Situ Spectroscopic Ellipsometry for Real Time Semiconductor Growth Monitor

A modular spectroscopic ellipsometer for in situ and ex situ materials analysis is described, and results for in situ MBE growth of GaAs/AlGaAs are reported.