G. Kühn

Growth and optical properties of CuGaTe2 thin films

Abstract CuGaTe2 thin films with thicknesses in the range of 1000–2000 A have been prepared by flash evaporation of pre-reacted material onto glass and silicon substrates. The films were single phase, polycrystalline and stoichiometric within 1%, as shown by transmission electron diffraction measurements and ion backscattering spectrometry. Four characteristic energy gaps of 1.227, 1.280, 1.97 and 2.67 eV were obtained from an analysis of the optical transmission spectra in the photon energy range from 1 to about 3 eV. A model is proposed for the band structure of CuGaTe2 near the point Γ of the Brillouin zone. Transmission measurements in the far infrared yielded three transverse optical mode frequencies of 166.4 cm−1, 201.4 cm−1 and 209.2 cm−1.

Epitaxial layers of CuInSe2 on GaAs

Abstract CuInSe 2 single-crystal films with (112) orientation and with thicknesses in the range 800–1200 A were deposited onto semi-insulating (111)A-oriented GaAs substrates by flash evaporation under controlled growth conditions. Epitaxial growth began at a substrate temperature T sub = 720 K and twins in the 〈221〉 direction were detected in reflection high energy electron diffraction investigations. The twin concentration decreased with increasing growth temperature. At T sub = 870 K a change of the structure from chalcopyrite to sphalerite was observed. Films produced at T sub ⩽ 720 K showed n-type conductivity whereas at higher growth temperatures the films were always p type and showed a rapid increase in hole concentration with increasing substrate temperature. Two different acceptor levels with ionization energies of 92 meV and about meV were found.

Electrical properties of n-type CuInSe2 single crystals

Abstract The electrical properties of as-grown n-type CuInSe2 single crystals were investigated in the temperature from 80 to about 700 K. The electron mobility data could be analysed accounting for scattering by acoustic, polar optical and nonpolar optical phonons and by ionized and neutral impurities. The crystals are found to be highly compensated with n « NA ≈ ND. Besides a shallow donor level with an ionization energy of some 10-3 eV there are indications of a deep donor level with an ionization energy of (0.22 ± 0.03) eV.

The optical properties of CuInS2 thin films

Abstract The optical absorption in flash-evaporated CuInS 2 thin films was studied in the photon energy range from 0.5 to about 4.2 eV. CuInS 2 was found to be a direct gap semiconductor with a gap energy of 1.524±0.005 eV at room temperature. The ground state energy of the free exciton was found to be about 8 meV. An indirect allowed transition was observed at 1.565±0.005 eV and was ascribed to an optical transition from the valence band maxima at the boundary of the Brillouin zone to the lowest conduction band minimum at the zone centre. Three further optical transitions which were probably due to the copper d states in the valence band were found at energies well above the fundamental edge.

Structural and electrical properties of CuInSe2 epitaxial layers prepared by single-source evaporation

Abstract CuInSe2 thin films with thicknesses in the range 1–5 μm were deposited onto glass and semi-insulating (111)A-oriented GaAs substrates by single-source evaporation. Source temperatures above 1400 K were found to be necessary to prepare single-phase and nearly stoichiometric CuInSe2 films. Epitaxial growth on GaAs was observed in the substrate temperature range 720–870 K. From the electrical properties it follows that the films are characterized by high acceptor concentrations and high degrees of compensation.

Heteroepitaxy of CuInSe2 on {111}-oriented germanium

Abstract Thin films of CuInSe2 were deposited onto {111}-oriented germanium substrates by flash evaporation and were investigated by reflection high energy electron diffraction. Epitaxial growth was found in the substrate temperature range 720–820 K. In all cases the epitaxial layers had the chalcopyrite structure except at growth temperatures higher than 795 K where the layers were cubic. Deposition of CuInSe2 onto {111}-oriented germanium is characterized by one-dimensional epitaxy, and the epitaxial relationship for the chalcopyrite phase is given by {111}Ge⧹⧹{112}CuInSe2

The electrical properties of CuInSe2 thin films deposited onto CaF2 substrates

Abstract CuInSe 2 thin films were deposited onto (111)-oriented CaF 2 substrates by flash evaporation in the substrate temperature range T s = 650–890 K . Epitaxial growth was found at T s = 770–800 K ; at lower and higher substrate temperatures the films were partly polycrystalline. Films produced at T s ⩽ 675 K showed n-type conductivity due to a donor with an ionization energy of 78 ± 5 meV ascribed to indium interstitials. At T s ⩾ 725 K the films exhibited p-type conductivity due to a shallow acceptor with an ionization energy characteristic of indium vacancies. The important role of the substrate material in establishing the electrical parameters of CuInSe 2 thin films is proved by a comparative study of thin films deposited onto GaAs and CaF 2 substrates.

LiInSe2 thin epitaxial films on {111} a-oriented GaAs

LiInSe2 thin epitaxial layers were prepared by flash evaporation on {111}a-oriented GaAs substrates and investigated by reflection high energy electron diffraction. The overgrowth is characterized by one-dimensional epitaxy with preferred azimuthal orientation. In general, the LiInSe2 thin films crystallize in the β-NaFeO2 structure. In the substrate temperature range from 620 to 670 K a second phase with chalcopyrite structure was observed besides the β-NaFeO2 structure.

The optical properties of CuInTe2 epitaxial layers

Abstract The optical absorption in flash-evaporated CuInTe 2 epitaxial layers was studied in the photon energy range from 0.6 to 3.0 eV. CuInTe 2 was found to be a direct gap semiconductor with a gap energy of 1.06±0.01 eV and a spin-orbit splitting of the valence band of 0.60±0.02 eV at room temperature. An optical transition found at about 2.2 eV is ascribed to transitions from the copper d states in the valence band.

Growth and structural properties of AgGaSe2 epitaxial films

Abstract Epitaxial layers of AgGaSe 2 on {111}-, {100}- and {110}-oriented GaAs substrates were prepared by flash evaporation. Epitaxial growth was obtained at substrate temperatures T s ⩾ 845 K for all substrate orientations. In general, the epitaxial relationships found experimentally can be explained by geometrical considerations which account for the lattice misfit between deposit and substrate. Furthermore, compared with the results for CuIIIVI 2 epitaxial layers some unusual structural features are found which are attributed to the non-parallelism of corresponding significant planes and directions in the deposit and the substrate.