Regular compositional steps generated in GaAs1− x P x VPE layers

For GaAs VPE layers grown on Ge substrates, it is found that a one-dimensional misfit-dislocation density 1/p is proportional to lattice constant mismatch, just as for compositionally graded GaAs1−xPx layers grown on GaAs substrates. On the basis of these experimental results, lattice constant variation due to compositional change is given in a form explicitly including a compositional step. The compositional step width, calculated from this relationship and observed dislocation density, is about 500 atomic layers. Regular compositional steps, which are unintentionally formed, are actually observed on A-B etched cleavage planes of the graded layers. The observed width of this step is about 0.3 μm. This value roughly agrees with the calculated one. Presumably, the discontinuous change in composition is caused by competition between the following two processes which alternately dominate GaAs1−x Px epitaxial growth: a free-energy increase due to a decrease in arsenic supersaturation in vapor phase and a free-energy increase due to an increase in formation enthalpy of the crystal. Furthermore, in final compositional layers equidistant striae about 3.5 μm wide are observed on A-B etched cleavage planes. The origin of these striae is considered to be misfit dislocations which are generated in graded layers and climb as a result of association with Ga vacancies. Some of the striae are regularly bent in the direction perpendicular to the growth axis by shear stress due to sample bending.