Nuclear Profiling Of Aluminum In GaAlAs/GaAs Heterostructures

We present here a direct measurement of proton energy-loss straggling in Gal-xAlxAs/GaAs, enabling us to take maximum advantage of the 27Al(p, γ)28Si nuclear reaction as a powerful technique for measuring Al profiles in these structures without the removal of sample material. As an illustration of the usefulness of the technique, we have observed and determined quantitatively an effect of substrate preparation on the Al concentration gradient at the GaAlAs/GaAs interface. Results were obtained using samples produced by molecular beam epitaxy and fabricated to have step-function Al concentration distributions to prescribed depths. The exact straggling width was obtained by a least squares comparison of the experimental spectra with curves calculated using a parameterized straggling distribution. Profiling measurements can now be made, using these straggling results, to give the Al concentration fall-off at the interface region in GaAlAs/GaAs to about 4% and epilayer thickness determination to about 2%. These results are also applicable to the profiling of structures such as graded band gap GaAlAs/GaAs solar cells which are constructed to have continuously varying Al concentration gradients. The technique can also be extended to other related materials such as GaAlSb/GaSb.