We report the first observation of long-range order in a semiconductor III-V ternary alloy. Al x Gai_ x As thin crystals grown by metal-organic chemical vapor deposition or molecular-beam epitaxy have Ga atoms preferentially occupying the 0,0,0 and y,y,0 sites and Al atoms the y>0>T and 0,y,y sites in each unit cell. Our results indicate that this ordered structure is the equilibrium state of Al^Gai-xAs.

%'e report the observation of resonant tunneling of electrons through Landau levels in double-barrier GaA1As-GaAs-GaA1As heterostructures, in the presence of a strong magnetic field perpendicular to the interfaces. This is a t~o-dimensional magnetotunneling effect which manifests itself as periodic structures in the current-voltage characteristics,~ith a period proportional to the electron cyclotron energy in the GaAs quantum~ell, from which the electron mass is determined.

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