Electroreflectance measurements allowed us to determine the I 8 -+L 6, I 8 -+I 6, L4 5 -+L6, I p -+I 6, and L6~L6 AlSb transition energies from 2S to 29S K. At room temperature the I 8~L6 and I 8~I 6 energies are, respectively, 2.211 and 2.300 eV; the spin-orbit splittings I 8 -I 7 and L4 5 -L6 are, respectively, 673 and 427 meV. The L45 -+L6 transition energy is 2.890 eV.Aluminum antimonide is an indirect-edge semiconductor whose theoretical' and experimental ' band-structure determinations yield to dispersive evaluations. The transition energies above the lowest indirect band gap I g~Xs (1. 610 eV at room temperature' ) have not been measured with good precision in a large range of temperatures. Its possible application in the solar energy conversion' and the recent development of the GaA1Sb-based optoelectronic devices in the (1.3 -1.6)-pm wavelength range have given a great interest to the accurate determination of the AlSb band structure and particularly to the indirect I"s~Ls (L) transition energy; its only previous determination by Mead et al. ' agrees with the theoretical k-p calculation by Topol et al. ' (L=1.86 eV at room temperature), but their value was deduced a slight break in the slope of photoresponse curves and was not absolutely convincing. Modulation techniques have been very helpful in elucidating the optical properties of semiconductors. We have applied electroreflectance measurements to p type undoped A1Sb (carrier concentration 10' cm ) prepared by solute diffusion growth from Sb-rich liquid solutions at low temperatures (700'C). The heterojunction barrier 3 which is necessary for the electroreflectance studies is obtained by evaporating a thin layer of Cu2S on the reflective face of the sample. To avoid transmission-back reflection structures, a lack of parallelism is given to the impolished rear face. hR R T=25K
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