Optical absorption studies were performed to investigate the dependence of
the fundamental band gap of Al-x Ga1-xN epitaxial films on Al content and a
pplied hydrostatic pressure. The results of absorption measurements perform
ed at atmospheric pressure yielded the variation of the band-gap energy E(x
) = 3.43 + 1.44x + 1.33x(2) eV for the AlxGa1-xN system. Optical absorption
edge associated with the direct Gamma band gap shifts linearly towards hig
her energy under applied pressure. By examining the pressure dependence of
the absorption edge in samples with different AlN mole fractions and taking
into account the difference of compressibility between the epitaxial films
and sapphire substrate, the pressure coefficients for the direct Gamma ban
d gaps of AlxGa1-xN were determined. (C) 1999 American Institute of Physics
. [S0021-8979(99)03312-5].