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

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.