Growth And Characterization Of GaAs/AlAs Superlattice Alloys

ABSTRACT Recent studies of short period GaAs/AlAs superlattices grown by molecular beam epitaxy have shown that the superlattice bandstrueture resembles that of the AlCaAs alloy system. We have studied indirect gap GaAs/AlAs superlattice alloys using x-ray diffraction and pressure dependent magneto-luminescence. Analysis of the x-ray data shows that the AlAs layers are strained to be pseudomorphic with the GaAs substrate. Because the indirect conduction band ground state derives from the AlAs X-point these superlattice alloys must be treated as strained-layer superlattices. Interpretation of luminescence data using Kronig-Penney and tight-binding analyses of the superlattice bandstructure and optical dipole matrix elements shows that this experiment provides a sensitive measure of the conduction band offset for the GaAs/AlAs system. Quantum size effects lift the three-fold degeneracy of the X-point to yield an in-plane X band quantized according to the transverse electron mass (X, ) and a separate X band along the superlattice axis quantized according to the longitudinal electron mass (X«). Strain shifts at the X-points are of the same magnitude as quantum size effect shifts and drive the X band toward lower energies. The effect of strain in the conduction band can be accurately modeled by assuming GaP deformation potentials for the AlAs layers.IntroductionIn any realistic model of superlattice bandstructure the he-terojunction alignment of conduction and valence band extrema of the same symmetry is a critical input parameter. It is assumed for a given heterojunction system such as GaAs/AlGaAs that one aligns the direct bandgap according to a rule which expresses the conduction or valence band offset as some fraction of the difference in the bandgap energies of the two bulk materials. Evidence has accumulated for a 60/^0 rule in which approximately 60 % of t,he bandgap difference appears in the conduction band and ^40 % in the valence band. ''" An interesting aspect of the 60/^0 alignment rule is that a Kronig-Penney (KP) analysis of GaAs/AlAs superlattices predicts for a sufficiently short period that the superlattice conduction band ground state will be derived from bulk states with X symmetry rather than r symmetry. It also predicts that the degeneracy of the X-point will be broken so that two distinct bands will be observed, one in the plane of the superlattice (X\) and one on the superlattice axis (X-^). °In an earlier paper we reported the negative pressure dependence of a strong photoluminescence signal in a series of GaAs/AlAs and GaAs/InAlAs superlattices providing the first unambiguous demonstration of an X symmetry ground state in a GaAs/AlAs superlattice. In previous work, evidence for indirect GaAs/AlAs superlattices has been derived from the absorption spectra of (GaAs) (AlAs) and (GaAs)-(AlAs)Q supgr^attices and the transport properties of unipolar superlattice barrier diode structures. ' J Very recently, time-resolved photoluminescence studies of QaAs/AlAs superlattices have been interpreted as evidence for X symmetry ground states.0 In this paper we show that, for GaAs/AlAs superlattices which are not removed from a GaAs substrate, most of the strain due to the small lattice mismatch between GaAs and AlAs is accomodated in the AlAs layers. Even very small strains have a large effect on the conduction band structure AlAs. To calculate the bandstructure of GaAs/AlAs superlattice alloys (SLAs) we have used the strain modified AlAs bandstructure in a Kronig-Penney analysis. Optical dipole moments were calculated by a sp s* tight-binding method. Our theoretical predictions agree very well with the experimental luminescence data.ExperimentalSuperlattices of CaAs/In A11 As with x _< 0.04 were grown by molecular beam epitaxy in a Varian MBE 360. Substratis we$e undoped GaAs with an orientation of (100) +_ 0.25 . Growth rates were 0.8 pm/hr^for GaAs an 0.3 pm/hr for AlAs and the growth temperature for the superlattice was 565 C. In some cases small amounts of In were added to the AlAs layer. This was done under the assumption that the In atoms would have an extremely high surface mobility at the superlattice growth temperature and would help maintain a planar surface during the growth of the AlAs. We have noted in the past that clear (3x1) HEED