Non‐Γ Deep Levels and the Conduction Band Structure of Ga 1− x Al x As Alloys

The electrical properties of the Ga 1− x Al x As alloys are strongly controlled by the emergence of deep levels in crystals which have x values near to the direct—indirect conduction band transition composition. The activation energy of the deep levels and the energy of the L conduction band minima with respect to the lowest energy Γ minimum (0.19 ≦ x ≦ 0.43) and × minima (0.43 < 〈 x ≦ 0.61), are determined from the temperature dependence (77 K ⪅ T ⪅ 750 K) of the Hall electron concentration on high‐purity crystals. It is shown that the lowest energy indirect minima is L in Ga 1− x Al x As for alloy compositions in the range 0.19 ≦ x ≦ 0.37. The variation in the energy separation Δ E ΓL at 300 K between the Γ and L minima is best described by the equation Δ E ΓL = (0.285 — 0.595 x ) eV for 0.19 ≦ x ≦ 0.47. The Γ—L cross‐over composition is, thus, determined to be x = 0.47. For x ⪆ 0.25, a deep level dominates the electrical characteristics of the crystals. The activation energy of the deep level with respect to the Γ minimum increases with x , being (0.170 ± 0.005) eV at x = 0.44, but with respect to the L minima remains almost constant with a value of ≤0.200 ± 0.005≤ eV for 0.19 ⪅ x = 0.44. For x > 0.44, the energy of this level below the × minima decreases monotonically reaching a value of (0.106 ± 0.005) eV at x = 0.78. Heavily compensated shallow donor levels are also present in the alloys. 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 indirect minima L and × although the Γ minimum is the lowest in energy.