Ralf Hunger

Interactions between gallium and nitrogen dopants in ZnO films grown by radical-source molecular-beam epitaxy

It has been recently predicted that the co-doping of an acceptor (nitrogen) and a donor (aluminum, gallium, indium) in a 2:1 ratio will dope ZnO p-type due to a reduction in the Madelung energy making the nitrogen acceptor energy level more shallow. We have been growing gallium and nitrogen co-doped ZnO films by radical-source molecular-beam epitaxy by use of oxygen and nitrogen radicals supplied via rf radical source cells. Diode-like current–voltage characteristics and donor acceptor pair-like photoluminescence emission were observed for a Ga and N doped ZnO film grown on an undoped ZnO buffer layer. However, Hall measurements revealed that the conductivity was n-type. Formation of a non-ZnO phase in the sample was confirmed by secondary ion mass spectroscopy and x-ray diffraction measurements. Zn and Zn+O secondary ion intensities fell sharply by two orders of magnitude in going from the undoped ZnO layer to the highly co-doped ZnO. X-ray diffraction measurements indicated the formation of ZnGa2O4.

Growth of Undoped ZnO Films with Improved Electrical Properties by Radical Source Molecular Beam Epitaxy

High-quality undoped ZnO epitaxial films with mobilities as high as 120 cm2V-1s-1 and carrier concentrations as low as 7.6 ×1016 cm-3 have been grown on (1120) a-sapphire substrates using low-temperature buffer layers, a slow substrate cooling process and a modified oxygen radical cell. Pole figure measurements reveal that a-plane sapphire substrates are effective for the elimination of 30° rotation domains, which usually appear in the case of ZnO growth on c-sapphire. The low-temperature buffer layers allow high-temperature growth, because initial ZnO growth does not occur with high initial growth temperature. The use of slow substrate cooling prevents the deterioration of the electrical properties of the ZnO films. Use of quartz insulators in the oxygen radical cell eliminates aluminum contamination, which is a serious problem when using conventional alumina insulators.

Heteroepitaxy of CuInS2 on Si(111)

Epitaxial layers of CuInS2 are grown on chemically hydrogen terminated Si(111) surfaces with 4° miscut by molecular beam epitaxy. The morphological and structural properties are determined by scanning electron microscopy, transmission electron microscopy, x‐ray diffraction, and texture analysis. The data show growth in the 〈112〉 direction and substantial twinning of the 75‐nm‐thick films. High‐resolution cross‐sectional micrographs of the interface indicate semicoherent epitaxial growth via an interfacial indium‐rich secondary phase. The pronounced faceting of the film surface is discussed in relation to twin lamellae.

Photoemission study and band alignment of the CuInSe2(001)/CdS heterojunction

The contact formation of thin-film epitaxial CuInSe2(001) with a physical-vapor-deposited CdS layer is presented in this work. Synchrotron-excited photoelectron spectroscopy was used for this investigation. The epitaxial CuInSe2 films contain a surface layer of reduced Cu stoichiometry similar to the ordered defect compound CuIn3Se5. A valence band offset of 0.79±0.15 eV has been determined for this heterojunction. The comparison to literature data indicates that neither surface orientation nor surface copper content have a major impact on the valence band offset of CuIn3Se5, respectively, CuInSe2 with CdS.

Growth and characterization of undoped ZnO films for single crystal based device use by radical source molecular beam epitaxy (RS-MBE)

The integrated use of (1120) α-plane sapphire substrates and high temperature growth with low temperature buffer layers have led to high quality undoped ZnO epitaxial films with mobilities as high as 120 cm 2 V -1 s -1 and residual carrier concentrations as low as 7.6 x 10 16 cm -3 . Pole figure measurements reveal that a-plane sapphire substrates are effective for the elimination of 30 rotation domains, which usually appear using c-plane sapphire substrates. In particular, when using c-plane sapphire substrates annealing at 1000°C in O 2 with c/2-height surface steps, the X-ray diffraction pole figure peak intensity related to these rotation domains increased. The use of low temperature buffer layers allow high temperature ZnO growth on sapphire, as initial ZnO growth does not occur at high initial growth temperature.

Properties of sputtered ZnO films and its interfaces with CdS

The surfaces of sputtered zinc oxide (ZnO) films and its interfaces with CdS are investigated with X-ray photoelectron spectroscopy and soft X-ray photoelectron spectroscopy measured at the synchrotron radiation facility BESSY II. Cadmium sulfide (CdS) films are prepared by thermal evaporation from the compound and the zinc oxide films by DC magnetron sputtering from an undoped zinc oxide target at low power densities with and without addition of oxygen to the sputtering gas. ZnO films deposited at room temperatures show an additional oxygen component located at the surface that can be mostly removed by heat treatment. The electronic interface properties are studied in dependence to ZnO deposition conditions.

Properties of CuInGaSe2 solar cells based upon an improved three-stage process

Abstract Recently, we have developed an improved three-stage growth method by simultaneously using a pyrometer and a thermocouple, to accurately control thickness and composition during growth of CuInGaSe 2 (CIGS). As a result, we have obtained solar cells that show preliminary efficiencies up to 16.4% without anti-reflection coating. Using the technique, we have investigated possible mechanism behind non-optimal cell efficiency, focusing on the effects of the molybdenum contact layer. Some additional advantages of our pyrometer monitoring technique are described. Optimizing the properties of Mo layer leads to suppression of Na segregation and distinctive damage to the CIGS surface, improving both cell performance and reproducibility.

Vibrational and Electronic Characterization of Ethynyl Derivatives Grafted onto Hydrogenated Si(111) Surfaces

Covalent grafting of ethynyl derivatives (-C triple bond C-H, -C triple bond C-CH3, -C triple bond C-aryl) onto H-terminated Si(111) surfaces was performed by a one-step anodic treatment in Grignard electrolytes. The electrochemical grafting of such ethynyl derivatives, which tends to form ultrathin polymeric layers, can be controlled by the current and charge flow passing through the Si electrode. The prepared ultrathin layers cover the Si surface and had a thickness up to 20 nm, as investigated by the scanning electron microscopy (SEM) technique. Exchanging Cl for Br in the ethynyl Grignard reagent leads to very thin layers, even under the same electrochemical conditions. However, for all ethynyl derivatives, high-resolution synchrotron X-ray photoelectron spectroscopy (SXPS) investigations reveal the incorporation of halogen atoms in the organic layers obtained. Moreover, it was observed that the larger the end group of the ethynyl derivative, the thinner the thickness of the ultrathin polymeric layers as measured by both SXPS and SEM techniques after low and high current flow respectively. For the first time, these new types of ultrathin organic layers on Si surfaces were investigated using infrared spectroscopic ellipsometry (IRSE). The different possible reaction pathways are discussed.

Molecule-solid interfaces studied with infrared ellipsometry: Ultrathin nitrobenzene films

This paper aims to demonstrate the high capability of infrared spectroscopic ellipsometry (IRSE) for the characterization of very thin organic films and the organic to inorganic interfaces. It is shown that the detection limit of IRSE facilitates the investigation of ultrathin nitrobenzene (NB) films with monolayer sensitivity. This accounts for substrates from semiconductors to metals. The process of reoxidation of a NB terminated silicon (001) surface is also reflected in the infrared ellipsometric parameters and evidently proceeds despite the organic layer. As a complementary method, x-ray photoelectron spectroscopy (XPS) measurements were performed.

Molecular beam epitaxial growth and characterization of CuInSe2 and CuGaSe2 for device applications

Abstract CuInSe 2 (CIS)-based semiconductors which are strong candidates for high-efficiency low-cost thin-film solar cells, have been investigated for device applications. First, high-quality CIS and CuGaSe 2 (CGS) epitaxial films were grown by molecular beam epitaxy. CIS films showing predominantly free exciton emissions in photoluminescence spectra were grown for the first time by optimizing the growth conditions. Next, native defects, control of which is critical for solar cell performance, have been systematically investigated. A common acceptor-type defect has been identified as Cu-vacancies (V Cu ) from positron lifetime measurements and the Cu/In composition ratio of the CIS films. Unlike films grown under Cu-excess conditions, films grown under an In-excess tend to be heavily compensated, and the origin of the donor-type defects present in these films was found to be Cu–Se divacancies (V Cu–Se ). Reduction of Se-vacancies (V Se ) was found to be critical for improving solar cell performance. Air-annealing, the presence of a Cu–Se second phase and the interdiffusion of Na from the sodalime glass substrate were all found to be effective in suppressing V Se formation. Post-growth annealing has been extensively used to understand the oxidation process of CIS and CGS. Annealing of CGS films at temperatures higher than 450°C resulted in a significant improvement in CGS film quality as well as the formation of a Ga–O second phase. The Cu/Ga ratios of oxygen-annealed films after the selective etching of Ga–O by H 2 SO 4 were found to become nearly unity, suggesting that the formation of the Ga–O phase enhanced Cu outdiffusion leaving Cu-deficient CGS regions.