High Al content AlxGa1-xAsySb1-y layers (typically x = 0.85-0.90) are used as cladding layers in GaSb-based mid-infrared laser diode structures grown by molecular beam epitaxy (MBE) [1,2]. The low refractive index and the high band gap of this material result in confinement of photons and electrical carriers, respectively. The thickness of each cladding layer is typically 1-2 µm. These cladding layers must be closely lattice-matched to the substrate in order to prevent dislocations. The thermal expansion coefficient of Al0.9Ga0.1AsySb1-y and of the GaSb substrate differ considerably, such that layers which are lattice-matched at room temperature are not lattice-matched at the growth temperature. This so-called "thermal misfit" is of the order 0.8E-3 and it is therefore important to know the critical thickness of these cladding layers as a function of As content.
Many models have been presented on the topic of critical thickness, e.g. [3,4]. The different approaches used in these papers result in very different values for the critical thickness. The best option at present is thus to experimentally determine the critical thickness for the material composition of interest, in our case Al0.9Ga0.1AsySb1-y. Here we present details on the critical thickness of Al0.9Ga0.1AsySb1-y on GaSb. We also show how the strain in the Al0.9Ga0.1AsySb1-y layers are relieved as the layer thickness increases beyond the critical thickness. The layers were investigated with high-resolution x-ray diffraction (XRD), and XRD scans around several (typically 6) pairs of asymmetric reflections were performed.
References:
[1] H.K. Choi and G.W. Turner, Phys. Scripta T69 (1997) 17.
[2] S. Simanowski et al., J. Cryst. Growth 227/228 (2001) 595.
[3] J.W. Matthews and A.E. Blakeslee, J. Cryst. Growth 27 (1974) 118.
[4] R. People and J.C. Bean, Appl. Phys. Lett. 47 (1985) 322.
|