Session Tu-C2

Oxide Glasses

Chair: Jin Jang, Kyung Hee University

Tu-C2.1 10:40–11:00

ALD-grown ZnO Layers on a-Si:H: Initial Growth Stages and Band Line-up

Lars Korte (1), Robert Rößler (1), and Christian Pettenkofer (2)

1. Helmholtz-Zentrum Berlin, Institute for Silicon Photovoltaics, Kekuléstr. 5, 12489 Berlin, Germany

2. Helmholtz-Zentrum Berlin, Institute for Silicon Photovoltaics, Albert-Einstein-Str. 15, 12489 Berlin, Germany

ZnO, usually doped n-type using Al, is used as transparent conductive layer in many solar cell concepts, including wafer-based amorphous/crystalline silicon (a-Si:H/c-Si) high efficiency cells. However, as reported recently [1–3], the deposition of ZnO:Al on a (p)a-Si:H/(n)c-Si heterojunction—i.e. the emitter contact in (n)c-Si based solar cells—leads to a reduced a-Si:H/c-Si interface passivation at low injection levels, thus a decrease in (implied) fill factor. We have related this effect to a change of the c-Si band bending upon ZnO:Al deposition [3], and it is the TCO/a-Si:H band line-up that governs this change in band bending. No detailed investigation of the ZnO/a-Si:H interface formation and the band line-up at this heterointerface has been reported.

In our present study, we aim at elucidating the structural and electronic properties of the ZnO/a-Si:H interface. ZnO layers were grown layer-by-layer using atomic layer deposition (ALD) on a-Si:H with a precursor combination of diethylzinc as metal precursor and water (H2O) as oxidant, at temperatures of up to 210°C. The layers were grown in the well-controlled environment of a dedicated UHV growth chamber (base pressure <1x10–8 mbar). The samples were then transferred without vacuum break in UHV conditions to a photoelectron spectroscopy setup, where XPS characterizations were carried out using monochromatized Al Kα radiation. In addition, UPS measurements (He I, hν = 21.2 eV) were used to determine the work function of the sample surface.

Analyses of the photoelectron spectra show that two components contribute to the O 1s core level peak: While the component at lower binding energies clearly belongs to the ZnO bulk phase, the higher energy peak is attributed to –OH hydroxides, which form the surface termination of the growing layer after the H2O step.

Furthermore, the high energy resolution of the monochromatized XPS allows to monitor the valence band edges of both the a-Si:H substrate and the growing ZnO film simultaneously. From these XPS valence band spectra, the a-Si:H/ZnO band offset can be read off directly with a minimum of assumptions, it amounts to 2.5–2.6 eV. Using additional information e.g. from spectral ellipsometry, the complete band diagram of the ZnO:Al heterojunction can be drawn.

The implications of our findings for a-Si:H/c-Si heterojunction solar cells will be discussed.

[1] A. Kanevce and W. K. Metzger, J. Appl. Phys. 105, 094507 (2009)

[2] M. Bivour, C. Reichel, M. Hermle, and S. W. Glunz, Sol. Energy Mater. Sol. Cells 106, 11 (2012)

[3] R. Rößler, C. Leendertz, L. Korte, N. Mingirulli, and B. Rech J. Appl. Phys. 113, 144513 (2013)

Keywords: zinc oxide, atomic layer deposition, amorphous silicon, heterointerface, photoelectron spectroscopy

Tu-C2.2 11:00–11:20

Ohmic and Schottky Contacts to Atomic Layer Deposited ZnO

Sami Bolat and Ali K Okyay

Department of Electrical and Electronics Engineering, Bilkent University, Ankara 06800, Turkey and

UNAM-National Nanotechnology Research Center, Bilkent University, Ankara 06800, Turkey

We report on the contact properties of several metals with atomic layer deposition (ALD) grown ZnO, to obtain high quality ohmic and Schottky contacts. We present Aluminum (Al), and Palladium (Pd) metal contact properties as ohmic-contacts to ALD-ZnO. For this aim, Cross Bridge Kelvin Resistor (CBKR) structures have been designed and fabricated. Four point probe measurement method has been used to characterize the fabricated structures. For the Al-ZnO contacts, measured contact resistivity was in the order of 10–6 Ωcm2, whereas the contact resistivity of Pd-ZnO structures was in the order of 10–3 Ωcm2. As it can also be seen from the results, Al shows much better performance as the ohmic contact material when compared to Pd.

We have designed and fabricated Schottky junctions having planar geometry. In these structures, Al has been used as the ohmic contact material. As the Schottky contact material two metals are investigated, namely, Silver (Ag), and Palladium (Pd). According to the theoretical work function values of the metals and the electron affinity of ZnO, one expects to have the good Schottky characteristics when Pd (work function = 5 eV) is used as the Schottky contact to ZnO. However, Ag-ZnO junctions have been reported to show the best characteristics in terms of rectification and quality factor. This is attributed to Fermi level pinning that prevents the junction to follow the classical Schottky-Mott Barrier Theory. Characterization of the fabricated devices has been performed by using Keithley SCS 4200 Semiconductor Parameter Analyzer. Ag-ZnO shows the most promising characteristics, having a rectification ratio of 35:1.

In summary, we have examined several materials to obtain high quality ohmic and Schottky contacts to ALD-ZnO. Al, as the ohmic contact and Ag as the Schottky contact have shown the best characteristics among the materials used in this study.

Keywords: ALD, ZnO, ohmic, schottky, contact resistance

Tu-C2.3 11:20–11:40

Study on Textured ZnO:Al Thin Films Prepared by RF Magnetron Sputtering with Water Steam

Shuhei Miura , Masaki Tashiro , Kazutoshi Suzuki , and Shuichi Nonomura

Graduate school of Engineering, Gifu University, Gifu 501-1193, Japan

Textured transparent conductive oxide (TCO) thin films are used for front electrode of thin films Si solar cells to increase the optical path length and light absorption in solar cell. Al-doped zinc oxide (AZO) thin film is one of the textured TCO materials. Many researchers tried to make textured morphology for AZO films surface by sputtering method. However, only a few attempts have so far been made at textured morphology of AZO films surface by adding water steam into deposition atmosphere. In this paper, we report the relationship between water steam and the texture morphology of AZO films surface.

Textured AZO films were prepared by RF magnetron sputtering method with a various ratio of Ar sputtering gas and water steam (0–50% for total sputtering gas). In this study, water steam was fed into sputtering chamber by using bubbler unit which connected with sputtering Ar gas line. The bubbler was filled with distilled water and installed into mantle heater. And, the introduced water steam ratio (Dw) with total sputtering gas was controlled by changing water temperature. All AZO films thickness were kept from 900 nm to 1100 nm. These textured morphology and texture size of AZO films were measured by scanning electronic microscopy (SEM) and atomic force microscopy (AFM), respectively. AZO film thickness and light scattering property (haze value) were obtained by utilizing of UV spectrophotometer equipped with integrated sphere. Crystal orientations of AZO films were observed by X-ray diffraction (XRD).

AZO films with water steam (WAZO) showed milky white color because of the strong light scattering. The highest haze value of 28% (for the wavelength of 600 nm) was obtained at Dw = 50%. Furthermore, the round-rock-like textured morphology was observed on the prepared WAZO films surface from the measurements of SEM. This round structure of the surface has a positive impact that leak currents of thin film solar cells will be reduced.

From AFM measurement, we found that increase of Dw enhances the root mean square roughness (RMS) and the grain-diameter. These values reached to ~60 nm and ~500 nm at 50% of Dw, respectively. The obtained values of RMS and the grain diameter are comparable with the reported values of other making method.

From the results of XRD measurements, all AZO films possess a polycrystalline hexagonal wurzite structure with four diffraction peaks, (100), (002), (110) and (101). The (002) is perpendicular to a c-axis. And both (100) and (110) correspond to parallel direction with a c-axis. The (002) peak intensity decreased with Dw. On the other hand, peaks intensity of (100) and (110) increased. Therefore, peak intensity ratios, (100)/(002), (110)/(002) and (101)/(002) were increased with Dw. These obtained results suggest that textured surface morphology of the deposited WAZO films were caused by the crystal growth paralleled with c-axis.

Keywords: transparent conductive oxide, ZnO, texture, water steam