An EELS-Study of the Chemical Homogeneity in Cu-In-S Films

The development of high-efficiency, low-price photovoltaic devices relies on epitaxially grown layers of ternary chalcopyrites. The question addressed in this study is to which extent the near stoichimetric composition of CuInS2 grown on hydrogen terminated Silicon, Si (111), varies cross the sample area. The method usually employed [1] for such analysis is Argon-Rutherford Backscattering (RBS), whose spatial resolution, however, is limited to the order of millimeters. This limit can readily be overcome by electron microscopic and electron spectroscopic means.For observation and recording of the energy loss spectra in the transmission electron microscope, samples were carefully prepared by mechanic polishing and ion-milling. The thinning began with polishing the Si substrate on ALLED diamond lapping films, down to 10 μm and a final ion-milling in a Gatan ion-thinning equipment from the Si-side (Ar+ 3 kV, the incident angle less than 7°).Spectra were taken by means of a Gatan image filter (GIF - 100) in a Philips CM200 FEG electron microscope operated at 200 kV.

Native Defects in the Ternary Chalcopyrites

Ternary-chalcopyrite crystals contain a variety of point defects—the most common of which are vacancies, antisite ions, and impurities. Usually these defects are isolated, but they can also appear as complexes involving two or more of the simple defects. Depending on the material, the concentrations of these defects may vary from a few hundred parts per billion to a few hundred parts per million. Many of the point defects in the ternary chalcopyrites have associated optical-absorption bands with significant oscillator strengths. It is these absorption features that become important when the crystals are exposed to intense laser beams during device operation. Even a small amount of absorption will seriously degrade the performance of the device if any of the wavelengths of the various propagating beams happen to overlap an absorption band. This phenomenon can be a problem for both second-harmonic-generator and optical-parametric-oscillator applications. In general the absorption leads to heating of the crystal and results in-thermal lensing (due to temperature dependence of the index of refraction) and dephasing of the beams, and it can ultimately lead to thermal fracturing of the crystal. Thus it is important to develop a fundamental understanding of the defect structure of the ternary-chalcopyrite crystals if they are to serve as the critical component in midinfrared frequency-conversion devices. Once the nature and behavior of the point defects are established, processes can be developed to remove the defects from the crystals either during the growth itself or during post-growth treatments.

Determination of the Recombination Processes in Copper Ternary Chalcopyrites by Phototransport Measurements

We have measured the phototransport properties of CuGaSe2 films as a function of temperature. The simplest model which is consistent with all the experimental results consists of two recombination levels, one of which is donor-like and the other is acceptor-like. This model is similar to the symmetrical two-level model, which we have recently suggested for CuInS2 films. We thus conclude that this model, with slight variations, represents the general recombination level structure in all copper ternary chalcopyrites.