J. Cieslak

Epitaxial growth of CuGaS2 on Si(111)

We demonstrate the direct heteroepitaxial growth of the ternary semiconductor CuGaS2 on Si(111) substrates by means of molecular beam epitaxy. X-ray diffraction data prove the epitaxial growth of the CuGaS2 films in the highly ordered chalcopyrite structure. Using photoluminescence, we are able to detect strong excitonic emissions up to room temperature, while photocurrent spectra reveal the A, B, and C valence-to-conduction-band transitions as they are typical for the tetragonal chalcopyrite structure.

Hetero-epitaxial growth of Cu(In,Ga)S2 on Si substrates

Abstract On the way to perfectly lattice-matched Cu(In,Ga)S 2 , we present the first CuGaS 2 films epitaxially grown on Si substrates and compare their structural properties to epitaxial CuInS 2 on the same type of substrate. Both of these chalcopyrites have been grown on sulfur-terminated Si(111) and showed a six-fold surface symmetry in low energy electron diffraction. Composition was controlled in and ex-situ using Auger electron spectroscopy and Rutherford backscattering, respectively. A detailed film analysis by means of X-ray diffraction methods yielded the lattice parameters of the epitaxial layers, wider rocking curves of the gallium compound as compared to the indium compound, and showed the suppression of twin formation in Ga-rich CuGaS 2 . The results indicate that a perfect lattice match between Cu(In (1− x ) Ga x )S 2 and Si will be attainable with x ≈0.5.

Metastability of CuInS2 and its implications on thin-film growth

Thin layers of the ternary chalcopyrite semiconductor CuInS2 grown epitaxially on Si(001) substrates were investigated with respect to the existing phases and their lattice parameters by means of x-ray diffraction at the European Synchrotron Radiation Facility in Grenoble (France). The predominant parts of the samples exhibit the metastable CuAu-type ordering. The smaller volume fractions of the samples showing the ground-state chalcopyrite ordering are distorted in order to accommodate themselves to the tetragonal CuAu-type structure. It is suggested that this finding is a consequence of the growth mode of CuInS2 thin films.

Excitonic luminescence of polycrystalline CuInS2 solar cell material under the influence of strain

Using molecular beams, polycrystalline CuInS2 (CIS) films were deposited on Mo-covered Si substrates. In order to investigate the influence of growth-induced strain on the optical and structural properties, detailed photoluminescence, photoreflectance and x-ray diffraction (XRD) measurements were performed. The transition energy of the free A-exciton (FXA) transition decreases with (i) decreasing thickness of the CIS layer at a constant thickness of the Mo buffer layer and (ii) increasing thickness of the Mo buffer layer at a constant CIS layer thickness. This appreciable redshift of FXA is accompanied by an increase of the energetic splittings between FXA, FXB, and FXC. When we compare theoretically predicted valence band splittings as a function of the crystal field—obtained from the calculated relative valence band energies—to our experimental values, a completely coherent picture is obtained. We also derived the structure of the conduction band as a function of crystal field, based on the theoreticall...

Epitaxial CuIn(1−x)GaxS2 on Si(111): A perfectly lattice-matched system for x≈0.5

We demonstrate the direct heteroepitaxial growth of the quaternary semiconductor CuIn(1−x)GaxS2 on Si(111) substrates by means of molecular-beam epitaxy. Using Rutherford backscattering, x-ray diffraction, transmission electron microscopy, and photoreflectance, samples of various Ga contents, x, were characterized in detail. Epitaxial growth was achieved in the whole compositional range and perfect lattice match between the epitaxial layer and substrate was obtained for x≈0.5. The epitaxial layers show the coexistence of bulk chalcopyrite and metastable CuAu-type cation ordering. Photoreflectance data reveal a linear increase of the fundamental band gap with increasing Ga content.

Epitaxial CuIn{sub 1-x}Ga{sub x}S{sub 2} on Si(111) (0{<=}x{<=}1): Lattice match and metastability

Epitaxial thin films of CuIn{sub 1-x}Ga{sub x}S{sub 2} (0{<=}x{<=}1) were grown on single-crystalline silicon substrates of (111) orientation by means of molecular-beam epitaxy from elemental sources. Employing x-ray diffraction in transmission and reflection geometries, as well as Rutherford backscattering spectroscopy, the lattice parameters and compositions of these films were determined, respectively. A linear dependence of the lattice constants a and c on the gallium content x was found. Best lattice match with the cubic silicon substrate was observed for x=0.56. Furthermore, the coexistence of the metastable CuAu-type ordering with the ground-state chalcopyrite structure was found to depend on the gallium content. This coexistence of the stable ground-state structure with a metastable ordering is accompanied by a nonlinear dependence of the tetragonal distortion on the gallium content of the thin films.