A comprehensive study on the optical properties of thin-film CuInSe2 as a function of composition and substrate temperature

Abstract

The absorption coefficient (α) and fundamental transition energies of thin-film CuInSe2 were determined by spectrophotometry in the near-infrared (NIR) and visible wavelength regions from 500 to 2000 nm for a wide range of compositions. The results suggest a relationship between the constituent specie fluxes and substrate temperature, and the resulting polycrystalline nature of the film which dominates the optical properties. Near-stoichiometric and Cu-rich films appear to crystallize in larger grain sizes in comparison with Cu-poor films, with a Cu2−δ Se secondary phase at grain boundaries and free surfaces. Correspondingly, significant variations in the absorption coefficient among different film compositions exist in the neighborhood of the band edge. At energies well above the gap, all films behave similarly with α’s of (1–2)×105 cm−1 at 500 nm. Similarly, continuous dispersion curves for the index of refraction have only been derived for single phase Cu-poor material by an iterative technique. The absorption data are substantiated through spectral response simulations that accurately reproduce measured device data. The range of primary and secondary transition energies, respectively, is 0.95–1.01 and 1.17–1.22 eV. These values indicate a valence-band splitting of 0.20–0.24 eV, in good agreement with single-crystal values.

Near‐edge Optical Properties of CuInSe2 Studied by Photoacoustic Spectroscopy

“…Extrapolation of these straight lines to LX = 0 yields the gap energies which are E, = 1.009 eV for sample 1 and E, = 1.003 eV for sample 2, the error being not larger than AE, = kO.001 for each sample. These values are in good agreement with previous gap energy determinations of CuInSe, single crystals and thin films (NEUMANN 1986;TUTTLE et al). For the constant A in relation (1) we found Fig.…”

“…lo4 -1.2 . lo5 cm-I eVliz (HERRERO, GUILLBN;NEUMANN et al 1982;TUTTLE et al;VARELA et al) although A is a material parameter that is only determined by the momentum matrix element of the corresponding optical transition and by the effective masses of the valence and conduction bands (JOHNSON). Therefore, since absolute values of the absorption coefficient spectrum (1) are required to estimate conversion efficiencies (HOVEL; MOLLER), it is necessary to evaluate the true value of the constant A in CuInSe,.…”

“…It has been established by numerous experimental studies of single crystals ) and polycrystalline thin films (NEUMANN 1986;TUTTLE et al) that CuInSe, is a direct-gap semiconductor with a gap energy of E, = 1.01 & 0.01 eV. Furthermore, it has been proved that at photon energies hv just above the fundamental edge the absorption coefficient a follows the standard relation for direct allowed transitions between parabolic bands, a(hv) = (A/hv) (hv -Eg)l'Z .…”

Photoconductivity Spectra of n‐type CuInSe2 Single Crystals

“…If this value is compared with experimental data for the absorption coefficient spectrum of CuInSe, NEUMANN et al 1982;TUTTLE et al;VARELA et al) it follows that the corresponding photon energy must be very close to the gap energy, kv,,, 6 E , + 0.01 eV. Furthermore it is evident from all absorption studies on thin polycrystalline films NEUMANN et al 1982;TUTTLE et al) that at photon energies above 1.4 eV the absorption coeficient increases monotonically and slowly up to about 2.5 eV and at a slightly faster rate at higher photon energies. Thus, since the spectral dependence of the photoconductivity at hv > hv,,, is given by relation (4) we should expect that do exhibits a continuous and structureless decrease up to about 2.5 eV followed by a slightly faster decrease at kv > 2.5 eV.…”

“…Furthermore, it has been found that band-gap narrowing effects due to high concentrations of free charge carriers or ionized impurities can result in gap energy shifts down to about 0.94 eV (NEUMANN, TOMLINSON 1986). Optical absorption measurements on bulk single crystals (NEUMANN, TOMLINSON 1986) and polycrystalline thin films prepared by different deposition methods (HORIG et al; NEUMANN et al 1982;TUTTLE et al;VARELA et al) have shown that there is a rapid increase of the absorption coefficient immediately above the fundamental edge reaching values close to lo4 cm-' at photon energies hv = E, + 0.02 eV and exceeding lo5 cm-' above about 2 eV. With regard to the electrical properties of CuInSe, it has been established by numerous experimental studies (NEUMANN, TOMLINSON 1990 and references cited therein) that they are essentially governed by intrinsic defects and therefore depend on growth conditions and post-growth annealing treatments influencing the elemental composition of the compound NEUMANN, TOMLINSON 1990) and thus the intrinsic defect equilibrium.…”