Band gap energies of bulk, thin-film, and epitaxial layers of CuInSe2 and CuGaSe2

Abstract

Band gap and excitonic resonance energies of high-quality bulk single crystals, polycrystalline thin films, and epitaxial layers of CuInSe2 and CuGaSe2 were determined as a function of temperature by means of photoreflectance, optical absorption (OA), and photoluminescence measurements. OA spectra were fit including excitonic absorption from low temperature up to room temperature (RT). The band gap energy of 1.032 eV and free exciton (FE) resonance energy of 1.024 eV were obtained at RT for strain-free CuInSe2 giving an exciton binding energy of 7.5 meV. The band gap energy of both CuInSe2 and CuGaSe2 was found to be essentially independent of the molar ratio of Cu to group-III atom (Cu/III) for near-stoichiometric and Cu-rich samples. The disappearance of the FE absorption in the In-rich (Cu/In<0.88) CuInSe2 thin films was explained by plasma screening of Coulomb interactions. A slight decrease in the band gap energy of the In-rich films was attributed to a degradation of film quality such as high-density defects, grains, and structural disordering. The fundamental band gap energy in strained CuInSe2 and CuGaSe2 epilayers was shown to decrease due to in-plane biaxial tensile strain.

Temperature dependence of excitonic emission in CuInSe2

“…The band-gap of E g = 1.049 eV in CuInSe 2 at 8.5 K can be found by adding this energy to the spectral position of the A free exciton. This value of the A free exciton binding energy is in a good agreement with 7.0 meV derived from the diamagnetic shifts of the A exciton under the influence of magnetic fields in [13], as well as with earlier experimentally measured values of 7.0 [14] and 7.5 meV [15]. It is smaller than the activation energy of 12 meV reported from the temperature quenching of the A exciton in Ref.…”

Effects of magnetic fields on free excitons in CuInSe2

“…However our value Table 1 The binding energies (Ry*) and Bohr radius (a B ) of the A and B excitons as well as their reduced masses μ calculated from the rates of the diamagnetic shifts c d using expression (2). of the A exciton binding energy Ry* = 7.0 meV, seems to be in a better agreement with the experimental values of 7.0 meV [11] and 7.5 meV [12] than the binding energy of 5.9 meV, which can be calculated using the hydrogenic model from the reduced mass µ = 0.08m 0 and ε = 13. , estimated from the effective mass of the electron m e = 0.09m 0 and hole m h = 0.092m 0 [16], is considerably smaller than our value of 0.098m 0 .…”

“…The excitonic Rydberg energy is the binding energy of the exciton. For the A free exciton Ry* was experimentally measured as 7.0 meV [11] and 7.5 meV [12]. Taking the effective masses of the electron m e = 0.09m 0 and hole m h = 0.71m 0 [13] we can find that at the highest strength of the magnetic field γ does not satisfy the weak field condition (γ Ӷ 1).…”

Deep level emission in polycrystalline CuGaSe2 thin-films observed by micro-photoluminescence

“…1) Quaternary CIGS is a binary compound of ternary CuInSe 2 and CuGaSe 2 . By incorporating Ga into the CuInSe 2 crystal, band-gap can be tuned from 1.04 eV for CuInSe 2 2) to 1.68 eV 3) for its CuGaSe 2 counterpart. The ideal band-gap of the absorber layer to achieve the highest conversion efficiency in single junction solar cell under AM 1.5 sunlight is speculated theoretically to be around 1.4 eV.…”