Electrical Transport Properties of GaAs and Alas Mixed Crystals for Semiconductor Devices

Abstract

For alloy compositions x≳O.25, a deep level dominates the electrical characteristics of the Ga_1-xAl_xAs alloys. The properties of the alloys with compositions in the neighbourhood of direct-indirect transition composition, are strongly controlled by the deep level. The activation energy of this level with respect to the Γ minimum increases with x, being 0.170 eV at x = 0.44 but with respect to the L minima remains almost constant with a value (0.200±0.005) eV for 0.19 ≤ x ≤ 0.44 For x > 0.44, the energy of this level below the X minima decreases monotonically reaching a value of 0.106 eV at x = 0.78. Similar measurements on samples with compositions in the range 0.23 ≤ x ≤ 0.32 and under hydrostatic pressure have shown that the deep level is mainly associated with the higher energy subsidiary L and X minima even for the alloy compositions with the Γ minimum to be the lowest in energy. The indirect nature of the deep level has resulted in the photoconductivity storage in the crystals (0 < x < 0.40) at low temperatures.

The lowest energy indirect minima have been found to be the L in Ga_1-Al_xAs alloys with conmpositions in the range 0 ≤x≤ 0.37 The variation in the energy separations ΔEΓ_L between the Γ and L minima and ΔEΓ_x between the Γ and X minima are best described by the equations Δ_L?. = (0.285–0.595x) eV for 0≤x≤0.47 and ΔEr_x = (0.485–1.12 x) eV for 0 ≤ x ≤ 0.38, respectively. The Γ–L and T–X minima cross-over compositions are, thus, determined to be x = 0.43 and 0.47, respectively.

The electron mobilities in the alloys have been found to be considerably reduced due to intervalley scatterings occurring due to the close proximities of the various minima near the direct-indirect cross-over composition. The relative magnitudes of the mobilities due to various scattering mechanisms and the various band parameters involved in the intervalley scatterings have been derived and most of them are found to increase with the alloy composition x.

Indexing Terms:

• Transport Properties
• Ga_1-x Al_xAs Alloys
• Scattering Mechanisms