H. Neumann

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.

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.

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.

Growth structure and optical properties of LixCu1−xInSe2 thin films

Abstract Epitaxial layers of LixCu1−xInSe2 were deposited onto {111}-oriented GaAs substrates by flash evaporation in the substrate temperature range Ts = 745−845 K. Up to x = 0.5 the chalcopyrite phase predominates. At x ⩾ 0.6 the films have the β-NaFeO2 structure. To determine the fundamental energy gap, layers deposited onto glass substrates were used. It is found that the fundamental absorption edge is due to an allowed direct transition for x ⩽ 0.6 and to an allowed indirect transition for x ⩾ 0.6.

Trends in the high-temperature heat capacities of ternary chalcopyrite semiconductors

The anharmonic contribution to the heat capacity of any chalcopyrite semiconductor AIBIIIC2VI or AIIBIVC2V is evaluated. It is shown that the degree of lattice anharmonicity decreases with increasing atomic weight of the constituent atoms of the compounds, and there is no essential difference in the degree of lattice anharmonicity of the two groups of compounds. Except for CdGeAs2, the trend in the Grüneisen constants is the same.ZusammenfassungDer anharmonische Beitrag zur Wärmekapazität verschiedener Halbleiterverbindungen AIBIIIC2VI und AIIBIVC2V vom Chalkopyrit-Typ wird bestimmt. Es wird gezeigt, dass die Gitteranharmonizität mit steigender relativer Atommasse der Bestandteile der Verbindungen abnimmt und dass keine wesentlichen Unterschiede in der Gitteranharmonizität beider Gruppen von Verbindungen bestehen. Der Trend der Grüneisen-Konstanten ist der gleiche, ausgenommen bei CdGeAs2.РЕЗЮМЕОценен ангармоничес кий вклад в теплоемко сть халькопиритных полу проводников АIВIIIС2VI и АIIВIVС2V. Показа но, что степень решеточной ангармон ичности уменьшается с увеличением увеличением атомного веса элемен тов, составляющих дан ные соединения, и что для о боих групп соединени й нет особого различия в степени решеточной ангармоничности. Пос тоянные Грюнайзена одинаковые для всех с оединений, за исключе нием CdGeAs2.