TY - JOUR
T1 - Urbach rule in mixed single crystals of ZnxCd1-xSe
AU - Samuel, L.
AU - Brada, Y.
AU - Burger, A.
AU - Roth, M.
PY - 1987
Y1 - 1987
N2 - The dependence of the absorption coefficient on the photon energy and on temperature near the fundamental absorption edge was measured in mixed single crystals of ZnxCd1-xSe, for x=0, 0.04, 0.3, 0.4, and 1. Exponential dependence of that coefficient according to the Urbach rule was observed. The thermal dependence of the slope parameter indicates that the absorption is influenced by the phonon-induced electric microfields and not by compositional or structural potential fluctuations. Up to about 90 K the magnitude of Latin small letter h with stroke0 fits the energy of the piezoelectric LA phonon, and at higher temperatures Latin small letter h with stroke0 fits the energy of the LO phonon. The energy Latin small letter h with stroke0 of the LO phonons varies linearly within the experimental error with x, from 27 meV for CdSe to 31 meV for ZnSe, and is in good agreement with the Raman spectra measured for these amalgamated behavior-type crystals. The energy gap, measured at room temperature, and the value of E0 change linearly with composition for the hexagonal phase crystals. The absorption process can be considered as an internal Franz-Keldysh effect, caused by the phonon-generated electric fields. It is described in the framework of the Dow-Redfield model, which ascribes the Urbach rule to the ionization of the exciton, as an extension of the Stark shift.
AB - The dependence of the absorption coefficient on the photon energy and on temperature near the fundamental absorption edge was measured in mixed single crystals of ZnxCd1-xSe, for x=0, 0.04, 0.3, 0.4, and 1. Exponential dependence of that coefficient according to the Urbach rule was observed. The thermal dependence of the slope parameter indicates that the absorption is influenced by the phonon-induced electric microfields and not by compositional or structural potential fluctuations. Up to about 90 K the magnitude of Latin small letter h with stroke0 fits the energy of the piezoelectric LA phonon, and at higher temperatures Latin small letter h with stroke0 fits the energy of the LO phonon. The energy Latin small letter h with stroke0 of the LO phonons varies linearly within the experimental error with x, from 27 meV for CdSe to 31 meV for ZnSe, and is in good agreement with the Raman spectra measured for these amalgamated behavior-type crystals. The energy gap, measured at room temperature, and the value of E0 change linearly with composition for the hexagonal phase crystals. The absorption process can be considered as an internal Franz-Keldysh effect, caused by the phonon-generated electric fields. It is described in the framework of the Dow-Redfield model, which ascribes the Urbach rule to the ionization of the exciton, as an extension of the Stark shift.
UR - http://www.scopus.com/inward/record.url?scp=0000664708&partnerID=8YFLogxK
U2 - 10.1103/PhysRevB.36.1168
DO - 10.1103/PhysRevB.36.1168
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AN - SCOPUS:0000664708
SN - 0163-1829
VL - 36
SP - 1168
EP - 1173
JO - Physical Review B
JF - Physical Review B
IS - 2
ER -