Angelika Polity

Structural properties and bandgap bowing of ZnO1−xSx thin films deposited by reactive sputtering

A series of ZnO1−xSx films with 0⩽x⩽1.0 was deposited by radio-frequency reactive sputtering on different substrates. The structural characterization by x-ray diffraction measurements revealed that the films have wurtzite symmetry and correlated investigations of the layer composition by photoelectron spectroscopy showed that the lattice constant varies linearly with x. The composition dependence of the band gap energy in the ternary system was determined by optical transmission and the optical bowing parameter was found to be about 3eV.

The band alignment of Cu2O/ZnO and Cu2O/GaN heterostructures

Using photoelectron spectroscopy, we investigate the band alignments of the Cu2O/ZnO heterointerface and compare the findings with the corresponding values for Cu2O/GaN. While for Cu2O/ZnO, we find a valence band offset (VBO) of 2.17 eV and a conduction band offset (CBO) of 0.97 eV, both values are considerably reduced for Cu2O/GaN where the numbers are 1.47 eV (VBO) and 0.24 eV (CBO), respectively. The large CBO between ZnO and Cu2O will very likely result in low photovoltaic power conversion efficiencies as is the current status of Cu2O/ZnO solar cells.

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.

Influence of doping with alkaline earth metals on the optical properties of thermochromic VO2

Thin films of doped VO2 were deposited, analyzed, and optimized with regard to their solar energy transmittance (Tsol) and visible/luminous light transmittance (Tlum) which are important parameters in the context of smart window applications in buildings. The doping with alkaline earth metals (AEM) like Mg, Ca, Sr, or Ba increased both Tsol and Tlum due to a bandgap widening and an associated absorption edge blue-shift. Thereby, the brown-yellowish color impression of pure VO2 thin films, which is one major hindrance limiting the usage of VO2 as thermochromic window coating, was overcome. Transparent thin films with excellent switching behavior were prepared by sputtering. Highly doped V1−xMexO2 (Me = Ca, Sr, Ba) kept its excellent thermochromic switching behavior up to x(Me) = Me/(Me + V) = 10 at. % doping level, while the optical bandgap energy was increased from 1.64 eV for undoped VO2 to 2.38 eV for x(Mg) = 7.7 at. %, 1.85 eV for x(Ca) = 7.4 at. %, 1.84 eV for x(Sr) = 6.4 at. % and 1.70 eV for x(Ba) =...

(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.

Post-growth treatment effects on properties of CuInS2 thin films deposited by RF reactive sputtering

Post-growth treatments, such as annealing, sulfurization, etching as well as ageing, were performed on CuInS2 films prepared by RF reactive sputtering. Their effects on the structural, optical and electrical properties of the films were studied by means of x-ray diffraction (XRD) and scanning electron microscopy (SEM), optical transmission, and Hall effect measurement, respectively. Heating under vacuum at 500 °C for a certain duration causes recrystallization of the as-sputtered films. The secondary Cu–In phases coexisting in the films sputtered with an insufficient H2S flow during sputtering can be eliminated by annealing in H2S atmosphere at 500 °C for suitable duration. Meanwhile, the film structural as well as optical properties are enhanced. The electrical properties of the as-grown films changed dramatically with ageing in air, and annealing in vacuum or air. KCN etching removed CuxS segregations on the film surfaces and returned the film electrical property to its initial state.

Influence of the preparation conditions on the properties of CuInS2 films deposited by one-stage RF reactive sputtering

Abstract We investigated systematically the influence of the sputter parameters, such as substrate temperature, H 2 S flow, and sputter power on the structural, optical, and electrical properties of the sputtered CuInS 2 films, to optimize the parameters for the one-stage growth of CuInS 2 films by radio frequency (RF) reactive sputtering. When the H 2 S flow during sputtering is too low, there are mainly Cu–In secondary phases coexisting in the films. With a RF power of 200 W and a H 2 S flow in the range of 20–30 sccm (standard cubic centimeter per minute), high quality films with good adhesion can be sputtered on bare float glass at a substrate temperature of 400 °C or above. A higher substrate temperature (500 °C) resulted in bigger grain sizes of the films. Moreover, the films sputtered at 500 °C showed a sharper absorption edge than those sputtered at 400 °C, and the corresponding band gap shifted from approximately 1.27 to 1.44 eV. Mo- or ZnO-coatings of the substrates can help to reduce the substrate temperature to 200 °C, and work effectively as a barrier to prevent Na diffusion from the substrates into the sputtered CuInS 2 layers.

Characterization of RF reactively sputtered Cu-In-S thin films

Abstract The ternary compound semiconductor CuInS 2 has attracted much attention owing to its potential applications in photovoltaic devices. We deposit CuInS 2 films on float glass substrates by a reactive radio frequency sputter process using a Cu–In inlay target and H 2 S gas in one step. The morphology of the films was studied by Atomic Force Microscopy, X-ray Diffraction was used to check the crystal structure of the films. The composition of the layers was determined by Rutherford Back-scattering Spectroscopy and Energy-Dispersive X-ray Analysis. The electrical properties of the layers, i.e. the carrier concentration, Hall mobility, and specific resistivity and their dependencies on temperature were investigated by Hall effect measurements.