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Effects of strain on the exciton spectrum of gallium antimonide Rickards, Bradley Alfred

Abstract

The optical absorption spectra of undoped GaSb, having a carrier concentration of 8.6 x 10¹⁶ cm⁻³, has been studied using an isotropic and an anisotropic biaxial strain applied in the {001} and {111} crystallographic planes. Thin (2 to 5 µm) single crystals of GaSb were mounted on glass substrates which, when cooled to 2°K, would produce an isotropic biaxial strain in the plane of the sample. Using this technique strains ranging from +10⁻³ (tensile) to -4.2 x 10⁻⁴ (compressive) were obtained. The application of a uniaxial or biaxial strain causes a splitting of the degenerate valence band edge at k=0 in the zinc blende type semiconductors, with a corresponding splitting of the free exciton absorption peak (∝- peak). From the splitting of the exciton absorption peak, the valence band shear deformation potentials have been determined to be D[sub u] =+3.5 ±0.3 eV (b = -2.4 ±0.2 eV) and D[sub u] =+4.4 +0.3 eV (d = -5.1 ±0.3 eV). The strain split exciton absorption peaks energies are found to shift linearly over the range of strain covered. From the mean shift of the exciton absorption peaks with strain, the hydrostatic deformation potential (D[c/d] – D[v/d]) has been found to be -10.4 ±0.9 eV per unit strain applied in the {00l} plane and -8.9 ±0.5 eV per unit strain applied in the {111} plane. The signs of the deformation potentials have been determined by observing that, for a tensile strain, the "centre of gravity" of the strain split valence band moves towards the conduction band with the m[sub J] = ±3/2 valence band moving "up" and the m = ±1/2 valence band moving "down" with respect to the "centre of gravity" of the strain split valence bands. Under a compressive strain the "centre of gravity" of the strain split valence bands moves away from the conduction band with the m[sub J] = ±3/2 valence band moving "down" and the m[sub J] = ±1/2 valence band moving "up" with respect to the "centre of gravity" of the strain split valence bands. The m[sub J] = ±1/2 and m[sub J] = ±3/2 valence bands have been identified from the anisotropic biaxial strain measurements using polarized light. From the biaxial strain measurements the ∝ absorption peak has been interpolated to have an energy value of 0.8107 ±0.0002 eV in the unstrained lattice. The β and γ absorption peaks have been observed in thick unstrained samples at photon energies of 0.8055 ±0.0005 eV and 0.7962 ±0.0002 eV, respectively. These two absorption peaks are also observed in the thin biaxially strained samples but are not observed to split as a result of the strain. These two peaks are also observed to have almost an identical energy shift with strain which appears to be independent of the plane of application of the strain. There is also no polarization dependence of the optical absorption. These results regarding the β and γ absorption peaks contradicts the previous identification of these lines as being due to a bound exciton complex. No alternate explanation as to the origin of the β and γ absorption peaks can be given at present.

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