Optical absorption and photoluminescence studies of thin GaAs layers in GaAs–AlxGa1−xAs double heterostructures

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Abstract
A three‐layered AlxGa1−xAs–GaAs–Alx Ga1−xAs structure has been used to measure the optical absorption and photoluminescence in thin GaAs layers prepared by liquid‐phase epitaxy. The results presented here are for lightly doped n‐type GaAs with free‐carrier concentrations near 1016 cm−3; however, the technique can be used for arbitrarily doped material. The absorption coefficient α was measured between 1.4 and 2.2 eV at 2 and 298 K. The absorption strength at the band gap was found to be (1.15×104)±1000 cm−1 at 2 K and (0.99×104)±1000 cm−1 at 298 K. At 1.96 eV, the energy of the He–Ne laser commonly used for photoexcitation of GaAs, α at 298 K was measured to be 4.4×104 cm−1. A value of 3.8 meV for the room‐temperature exciton binding energy was inferred from the temperature dependence of the interband absorption strength. This value together with previous reflectivity data for high‐purity GaAs gives the energy gap of pure unstrained GaAs at 298 K as 1.424±0.001 eV. The effects of strain due to lattice mismatch in the three‐layered structures were observed in the absorption edge at 2 K. The calculated photoluminescence spectrum obtained through the principle of detailed balance from the absorption data agrees well with the measured photoluminescence at 298 K. A comparison of the photoluminescence from the excited surface and back surface permits assignment of an upper limit of 5×104 cm/sec for the room‐temperature GaAs–AlxGa1−xAs interface recombination velocity for x ≈ 0.5. This comparison of the front and back photoluminescence also shows that the minority‐carrier diffusion length at 298 K for a sample with an electron concentration of 2×1016 cm−3 is at least 2.5 μ. The data presented here can be used to calculate the intrinsic carrier concentration ni, the thermal radiative generation rate G, and the radiative constant B (=Rr/np). The values at 298 K (corrected where necessary to a band‐gap energy Eg =1.424 eV) are as follows: ni =1.8×106 cm−3, G =4500 cm−3/sec, and B=1.4×10−9 cm3/sec.