TEMPERATURE-DEPENDENCE OF OPTICAL-PROPERTIES OF GAAS

Pseudodielectric functions <epsilon> = <epsilon-1> + i<epsilon-2> of GaAs were measured by spectroscopic ellipsometry (SE), in the range of 1.6-4.45 eV, at temperatures from room temperature (RT) to approximately 610-degrees-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 approximately 350-degrees-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 <epsilon> spectrum was made using the harmonic-oscillator approximation (HOA). It is shown by the HOA that the E1 and E1 + DELTA-1 energy-band critical points shift downward approximately 300 meV as temperature increases from RT to approximately 610-degrees-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-degrees-C. Therefore, the ellipsometer can be utilized as an optical thermometer to determine the sample surface temperature.