W. Kriegseis

Behind the weak excitonic emission of ZnO quantum dots: ZnO/Zn(OH)2 core-shell structure

The structure of ZnO quantum dots prepared via the wet chemical method was studied. By introducing an annealing treatment (150 °C–500 °C), we also investigated the effect of the change in the structure of the dots on their luminescence properties. Our studies revealed that the surface of the as-prepared dots is passivated by a thin layer of Zn(OH)2, thus, the dots consist of a ZnO/Zn(OH)2 core-shell structure. We present evidence that the weak excitonic transition of ZnO quantum dots is strongly correlated with the presence of the surface shell of Zn(OH)2. When Zn(OH)2 is present, the excitonic transition is quenched.

Tungsten and fluorine co-doping of VO2 films

Abstract Fluorine- and tungsten-doped vanadium dioxide (VO2) is a promising coating material for applications as energy-conserving windows. We prepared VO2 films simultaneously co-doped with fluorine and tungsten and report on the results of optical measurements and photoelectron spectrometry. A comparison is given to single-element doping. Clear evidence for an interaction of fluorine and tungsten in VO2 is found in the switching behaviour at the semiconductor-to-metal phase transition. An explanation is given based on two different effects of fluorine incorporation observed in the ultraviolet photoelectron spectrometry results. Concerning other film properties, the two elements act independently of each other.

Dependence of the electrical and optical behaviour of ITO–silver–ITO multilayers on the silver properties

Abstract ITO–metal–ITO (IMI) multilayers and ITO single layers were prepared by DC-magnetron sputtering with different oxygen concentrations of the sputtering atmosphere during ITO deposition, and different substrate temperatures during a subsequent annealing treatment in vacuum. The intermediate metal layer consisted of 10 or 30 nm silver. Electron concentration and mobility of the Ag partial layer were determined from measurements of the electrical resistivity and by evaluating the complex dielectric function, obtained from optical transmission and reflectance. Microstructure and purity were studied with X-ray diffraction (XRD) and with X-ray photo electron spectroscopy (XPS) depth profiles. With a 10-nm intermediate silver layer an IMI sheet resistance of 4.7 Ω/□. was achieved. The Ag films had almost bulk electron concentrations but reduced electron mobilities. The ITO crystallinity in IMI systems influenced the microstructure and purity of the Ag layer. Minimum Ag resistivity and optimum Ag purity were obtained by ITO deposition under stoichiometric conditions and annealing in vacuum at 300°C. An increase of the ITO lattice constant is indicative that as-deposited IMI multilayers prepared with ITO deposition at high oxygen concentration or under stoichiometric conditions are associated with stress. The presence of the Ag layer impedes the lattice relaxation upon annealing.

Heteroepitaxial growth of CuInS2 thin films on sapphire by radio frequency reactive sputtering

Direct heteroepitaxial growth of uniform stoichiometric CuInS2 (CIS) thin films on sapphire (0001) substrates has been achieved by radio frequency reactive sputtering. X-ray ω–2θ scans reveal that the sputtered layers grow in a (112) orientation with a chalcopyrite structure. A rocking curve full width at half maximum of about 0.05° (180 arc sec) for the (112) peak demonstrates a nearly perfect out-of-plane arrangement of CIS (112)∥sapphire (0001). X-ray diffraction Phi scans further illustrate an excellent in-plane ordering of CIS [110]∥sapphire (1010). The sputtered thin CIS epilayers had a smooth surface with a typical root-mean-square roughness of about 3.3 nm as evaluated by atomic force microscopy. The epitaxial growth of tetragonal CIS on hexagonal sapphire provides evidence that heteroepitaxial growth may be realized between structures of different symmetry, such as films of cubic or tetragonal structures on hexagonal substrates or vice versa.

Structural and optical characterization of RF reactively sputtered CuInS2 thin films

The ternary compound semiconductor CuInS 2 is one of the most attractive materials for high efficiency solar cells due to its bandgap of 1.55 eV which is well matched to the solar spectrum. We deposit CuInS 2 films on float glass substrates by a reactive RF sputter process using a Cu-In alloy target and H 2 S gas. By optimizing the sputter parameters, such as the sputter power, temperature of the substrate, and the flow of H 2 S, high quality films were obtained. The surface morphology, phase structure and composition of the layers were analyzed by atomic force microscopy (AFM), X-ray diffraction (XRD), energy-dispersive X-ray analysis (EDX) and Rutherford backscattering spectroscopy (RBS), respectively. The electrical properties of the films were characterized by Hall effect measurements, photoluminescence (PL) and transmission measurements were performed to examine the optical properties of the films, and the absorption coefficients and the direct hand gap of the films were evaluated by transmission measurements.

Annealing studies on CuIn(Ga)Se2: the influence of gallium

In-rich CuInSe2 (CIS) and CuIn(Ga)Se2 (CIGS) with Ga/(Ga+In)=28% were annealed in air at 400°C. After annealing the Ga-free CIS layer, the broad photoluminescence (PL) spectrum changes to a structured spectrum which is identical to that of a Cu-rich layer. Annealing of In-rich films causes the passivation of donors and the reduction of the high compensation. The change in the PL spectrum and the clear reduction of compensation cannot be seen for the Ga-containing CIGS layers. Only a slight blue shift of the spectrum and an increase of the full width at half maximum is observed after air annealing. However, photoluminescence excitation experiments reveal a decrease of the band-tail character of the absorption edge which can be interpreted as a decrease in compensation. Air-annealing causes a strong reduction of the Ga-concentration in the CIGS layer. The Ga-loss can be detected by X-ray diffraction and secondary ion mass spectroscopy. Ga segregates as oxide phase on top of the crystal and is analysed with X-ray photoelectron spectroscopy. The thickness of that oxide layer was estimated to be 200 nm.

(001)-Textured Cu2S Thin Films Deposited by RF Reactive Sputtering

Cu2S thin films were deposited on float glass substrates by radio frequency reactive sputtering. X-ray diffraction measurements, including grazing incidence geometry, showed that the sputtered Cu2S films had hexagonal structure with a strong (001) fiber texture. The thickness, surface density and roughness of the sputtered layers were characterized by X-ray reflectivity. Scanning electron microscopy from both top view and cross section revealed good morphology and uniform microstructure of the sputtered films. Rutherford backscattering spectroscopy and energy dispersive X-ray analysis were used to examine the composition of the films. By the optical transmission measurements, the indirect and direct band gaps of the films were estimated to be 1.19 and 1.82 eV, respectively.

Dislocation reduction in GaN grown by hydride vapor phase epitaxy via growth interruption modulation

Thick GaN layers have been deposited on c-Al2O3 (0001) substrates using hydride vapor phase epitaxy by modulating the growth process via switching on/off GaN growth. Cathodoluminescence and transmission electron microscopy images of the cross-sectional structure show that there are separated multilayers structure in the GaN films. The dislocations density decreases from 1010 cm−2 in the initial layer to 109–108 cm−2 in the subsequently grown layer and then to 107 cm−2 in the top layer. This shows that the interruption of the growth process is helpful for suppression of structural defects. X-ray diffraction and photoluminescence measurements also demonstrate the high quality of the GaN films.

Modulated growth of thick GaN with hydride vapor phase epitaxy

High-temperature (HT) deposited buffer layer and flow modulation growth are applied as two modulated techniques to the growth of high-quality GaN films by hydride vapor phase epitaxy. The characterization results indicate that the use of HT buffer layer has improved the morphological, structural and optical properties of the GaN films. The improvement is due to the HT buffer layer that supplies both high-density nucleation centers and promotes the lateral growth of epitaxial film with a columnar structure. Moreover, the buffer layer is also helpful to relax the strain in the film. With the flow modulation growth, the dislocation density in the films are greatly reduced and the homogeneity is highly improved. We attribute this promotion to the relaxation of strain and suppression of defects in sublayers and reconstruction of the as-grown surface of sublayers due to the interruption of the growth for a certain time.

Highly (112)-Oriented CuInS2 Thin Films Deposited by a One-Stage RF Reactive Sputtering Process

We demonstrate the first one-stage growth of CuInS2 films by radio frequency (RF) reactive sputtering with a Cu–In alloy target and H2S gas. High quality films of good adhesion can be sputtered either on bare float glass at a minimum substrate temperature of 400°C or on Mo- or ZnO-coated float glass at a relatively low temperature of 200°C. X-ray diffraction results revealed that the films sputtered on the bare, Mo- or ZnO-coated float glass substrates are highly (112) oriented. Typically the as-deposited films are slightly Cu-rich as determined by Rutherford backscattering spectroscopy. The surface morphology and homogeneity of the layers were as well analyzed by atomic force microscopy and secondary ion mass spectroscopy.