Temperature dependence of optical properties of GaAs

Pseudodielectric functions 〈ε〉= 〈ε₁〉+i〈ε₂〉 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 〈ε〉 spectrum was made using the harmonic-oscillator approximation (HOA). It is shown by the HOA that the E₁ and E₁+Δ ₁ 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 arbitrary temperature in the range of 22-610 °C. Therefore, the ellipsometer can be utilized as an optical thermometer to determine the sample surface temperature.