We present prism coupling measurements on AlxGa1-xAs native oxides showing
the dependence of refractive index on composition (0.3 less than or equal t
o x less than or equal to 0.97), oxidation temperature (400 less than or eq
ual to T less than or equal to 500), and carrier gas purity. Index values r
ange from n = 1.490 (x = 0.9, 400 degrees C) to 1.707 (x = 0.3, 500 degrees
C). The oxides are shown to adsorb moisture, increasing their index by up
to 0.10 (7%). Native oxides of AlxGa1-xAs (x less than or equal to 0.5) hav
e index values up to 0.27 higher and are less hygroscopic when prepared wit
h a small amount of O-2 in the N-2+H2O process gas. The higher index values
are attributed to a transition from a hydroxide to a denser (AlxGa1-x)(2)O
-3 oxide phase. (C) 1999 American Institute of Physics. [S0003-6951(99)0043
4-9].