Single-phase SnxGe1−x alloys with x up to 0.3 have been grown by molecular beam epitaxy. X-ray diffraction measurements indicate the layers to have the diamond crystal structure. The metastability of the alloys is apparent as increases in the growth temperature, layer thickness, or Sn composition cause phase separation of the Sn into a noncubic (white or β-Sn) form. Rutherford backscattering spectrometry and reflection high-energy electron diffraction measurements indicate that the initial stages of growth are complicated. The first several hundred angstroms of growth are compositionally graded, with the Sn incorporation rate increasing with film thickness. Thereafter, the alloy composition remains constant, determined by flux composition, until a critical thickness for phase separation is reached (≂2000 Å for x=0.3).
We have observed Stark shifts of quantum well intersubband transitions in a perpendicular electric field. Two samples consisting of 100 and 120 Å GaAs quantum wells separated by 350 Å AlGaAs barriers showed optical absorption peaks at 11.1 and 13.9 μm, respectively. In an electric field of 36 kV/cm, the 13.9 μm peak shifted to 13.7 μm and the 11.1 μm shifted to 11.0 μm, in good agreement with theoretical calculations. These tunable transitions can be applied to high-speed infrared light modulators.
We present a new technique utilizing computer simulations to determine the exact eigenstates of a quantum well superlattice of GaAs/AlGaAs in a perpendicular electric field. The technique is applied to quantify the tunability of a new infrared detector utilizing an intraconduction-band transition in the quantum well. Solutions found by this technique are compared with those found by a variational method and the Kronig–Penney model. The technique has several advantages over a conventional variational approach. It allows the easy calculation of the excited states in addition to the ground state. It is able to determine the energy levels in a finite period superlattice in addition to those in a single well. It also provides an indication of its own precision.
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