Photoluminescence of a High Quality CuInSe₂ Single Crystal

Abstract

A CuInSe2 (CIS) bulk single crystal grown by the directional freezing method from the melt at 1250°C shows superior crystal quality, as evidenced by its (p-type) electrical properties. Photoluminescence at 1.5 K shows a strong and well resolved spectrum with fine structures never before observed from this compound. Free exciton emission exhibits a doublet structure which can either be described as a splitting due to the uniaxial crystal field or as a polariton. The energy gap of the CIS semiconductor as determined from the temperature dependence of the free exciton line is 1.058 eV. Two moderate free-to-bound transitions are assigned to VCu and CuIn acceptors. A weak PL peak corresponding to a deep level is interpreted as arising from the Sei acceptor. Strong phonon replicas are also observed for the first time in a CIS bulk single crystal. The phonon wavelength of 218–237 cm-1 is in good agreement with the reported Raman result of 233 cm-1 for the LO phonon.

Excited states of the free excitons in CuInSe2 single crystals

“…3 Despite many studies of the optical properties of CuInSe 2 dating back nearly 40 years 4 only very few reports suggest resolution of the A and B free excitonic states. [5][6][7][8] No reliable information on their excited states has yet been reported and therefore the band-gap cannot be considered to be accurately known.…”

“…Previous experimental attempts to determine the free exciton binding energies E b in CuInSe 2 have been carried out by analyzing either the temperature quenching of the excitonic photoluminescence ͑PL͒ emission intensity 7,9 or diamagnetic shifts under the influence of magnetic fields. 10 The quenching characteristics of the A and B excitons are rather complex due to the small energy splitting and proximity of shallow bound excitons and thus values of E b determined by this method cannot be considered as reliable.…”

Effects of magnetic fields on free excitons in CuInSe2

“…1(a). These spectra exhibit several sharp and well resolved lines associated earlier with excitons [3,4].…”

“…In this report we are not discussing the C free exciton. The bound exciton (BE) lines were assigned to excitons bound to shallow defects [2,3,6].…”

“…However in the case of CIS only a few studies [2][3][4][5] on excitonic states, where excitons in the PL spectra are well-resolved, and probably only one attempt to use magnetic fields for studying excitons [6] can be found in the literature to date.…”