Real Time and In-Situ Spectroscopic Ellipsometry of Cu_yIn_1−xGa_xSe₂ for Complex Dielectric Function Determination and Parameterization

Abstract

Real time spectroscopic ellipsometry (RTSE) has been applied to characterize the structural evolution and final structural properties of ~50–60 nm thin film Cu_y(In_1−xGa_x)Se₂ (CIGS) solar cell absorber layers deposited by single stage co-evaporation onto crystalline silicon wafer substrates. Two series of depositions were explored; the first spans the range of copper atomic fraction y = [Cu]/([In] + [Ga]) of 0.45 < y < 1.20 for fixed gallium atomic fraction x = [Ga]/([In] + [Ga]) = 0.30 and the second spans the range of 0 ≤ x < 0.50 with fixed y ~ 0.90, as measured by energy dispersive X-ray spectroscopy . Systematic variations in the structural evolution with y reveal that near stoichiometric films undergo significant roughening typically associated with crystallite nucleation and growth whereas films with low and high Cu contents undergo significant smoothening during coalescence typically associated with disordered films or surface regions. The final film structural parameters determined from RTSE enable accurate determination of complex dielectric functions at the deposition temperature and at room temperature based on in-situ SE measurements performed immediately after the deposition process and after film cooling, respectively. Critical point (CP) analysis applying a standard lineshape was performed by fitting twice differentiated dielectric functions . Thus, the resulting CP resonance energies were obtained in accordance with the standardized procedures developed for research on the optical properties of semiconductors. An analytical expression describing the complex dielectric functions of the CIGS films over the range 0.75–3.8 eV was developed that incorporates photon energy independent parameters associated with four CP resonances, a modified Lorentz oscillator as a broad background between CPs, and a sub-bandgap Urbach tail. The procedure for fitting the dielectric functions by this expression was stabilized by fixing the CP energies deduced in the CP analysis. Polynomials describing the dependence on the Cu content y and the Ga content x for each of the photon energy independent parameters were obtained by fitting the plots of these parameter values as functions of y and x. The utility of the dielectric function expression and associated polynomials has been demonstrated through ex-situ spectroscopic ellipsometry (SE) applications in which the compositional parameters of x and y for a ~450 nm CIGS film have been mapped over a 10 cm × 10 cm sample area. Although the dielectric functions have been deduced from ~50–60 nm films on ideal smooth substrates, they have been effective in serving as a database for compositional analysis of much thicker films, as well as films on Mo coated glass substrates in the solar cell configuration.