Analysis of Urbach-like absorption tails in thermally treated ZnO:Al thin films

Schönau, S.; Ruske, F.; Neubert, S.; Rech, Bernd

doi:10.1063/1.4829999

Cited by 14 publications

(20 citation statements)

References 28 publications

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“…A slight change in the slope of the curves, however, hints to additional sub band gap absorptance probably related to additional defects caused by the lower T bulk . 43 In general, the absorptance of samples employing a seed layer in combination with low deposition temperatures was higher in the wavelength range between 350 nm and 600 nm in comparison to the hightemperature reference (T bulk ¼ 430 C) without seed layer. Yet, the seed layer induced lower absorptance compared to films deposited at the same T bulk but without seed layer (see Fig.…”

Section: A Bulk Layer: Temperature Variationmentioning

confidence: 99%

Role of the dopant aluminum for the growth of sputtered ZnO:Al investigated by means of a seed layer concept

Sommer

Stanley

Köhler

et al. 2015

Journal of Applied Physics

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This work elucidates the effect of the dopant aluminum on the growth of magnetron-sputtered aluminum-doped zinc oxide (ZnO:Al) films by means of a seed layer concept. Thin (<100 nm), highly doped seed layers and subsequently grown thick (∼800 nm), lowly doped bulk films were deposited using a ZnO:Al2O3 target with 2 wt. % and 1 wt. % Al2O3, respectively. We investigated the effect of bulk and seed layer deposition temperature as well as seed layer thickness on electrical, optical, and structural properties of ZnO:Al films. A reduction of deposition temperature by 100 °C was achieved without deteriorating conductivity, transparency, and etching morphology which renders these low-temperature films applicable as light-scattering front contact for thin-film silicon solar cells. Lowly doped bulk layers on highly doped seed layers showed smaller grains and lower surface roughness than their counterpart without seed layer. We attributed this observation to the beneficial role of the dopant aluminum that induces an enhanced surface diffusion length via a surfactant effect. The enhanced surface diffusion length promotes 2D-growth of the highly doped seed layer, which is then adopted by the subsequently grown and lowly doped bulk layer. Furthermore, we explained the seed layer induced increase of tensile stress on the basis of the grain boundary relaxation model. The model relates the grain size reduction to the tensile stress increase within the ZnO:Al films. Finally, temperature-dependent conductivity measurements, optical fits, and etching characteristics revealed that seed layers reduced grain boundary scattering. Thus, seed layers induced optimized grain boundary morphology with the result of a higher charge carrier mobility and more suitable etching characteristics. It is particularly compelling that we observed smaller grains to correlate with an enhanced charge carrier mobility. A seed layer thickness of 5 nm was sufficient to induce the beneficial effects.

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Section: A Bulk Layer: Temperature Variationmentioning

confidence: 99%

Role of the dopant aluminum for the growth of sputtered ZnO:Al investigated by means of a seed layer concept

Sommer

Stanley

Köhler

et al. 2015

Journal of Applied Physics

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“…The value of E 0 is 50 to 150 meV lower than the bandgap E g . The slight changes introduced in comparison to [8] resulted in slightly different absolute values for the parameters (E U changing by a few meV for some samples) but leads to the same overall picture.…”

Section: Effect Of Annealing Procedures On Zno:al Filmsmentioning

confidence: 93%

“…Most effects of the various annealing procedures on the optical and electrical properties of ZnO:Al have been reported earlier [7][8][9]11]. In general annealing under a protective layer strongly increases the carrier mobility.…”

Section: Effect Of Annealing Procedures On Zno:al Filmsmentioning

confidence: 99%

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Material properties of high-mobility TCOs and application to solar cells

et al. 2014

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The benefit of achieving high electron mobilities in transparent conducting oxides (TCOs) is twofold: they first exhibit superior optical properties, especially in the NIR spectral range, and secondly their low resistivity enables the usage of thinner films. Remarkably high mobilities can be obtained in Al-doped zinc oxide by post-deposition annealing under a protective layer. The procedure has not only shown to increase mobility, but also strongly reduces sub-bandgap absorption.Extensive optical, electrical and structural characterization is carried out in the films in order to clarify the microscopic origins of the changes in material properties. While the annealing of defect states, most likely deep acceptors, seems clear, earlier results also suggest some influence of grain boundaries. Tailing, on the contrary, seems to be linked to extended defects.In application to a-Si:H/µc-Si:H thin film solar cells the films have already shown to increase spectral response. When reducing the film thickness, the main challenge is to provide a suitable light trapping scheme. Normally this is achieved by a wet chemical etching step in diluted HCl, which provides a surface structure with suitable light scattering properties. Therefore a TCO-independent light scattering approach using textures glass was applied in conjunction with the high mobility zinc oxide. The substrate enables the use of very thin TCO layers with a strongly reduced parasitic absorption.

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“…A suitable way to adjust electrical and optical properties, at least for sputtered, doped ZnO, is the application of thermal post-deposition treatments including at least one step where the ZnO:Al is protected by a thin protective layer. This method leads not just to a considerably enhanced mobility but to significantly reduced sub-band gap absorption, as well 1,2,3 . However, although absorption tails below the band gap are well known for ZnO:Al prepared by various deposition techniques, a deep understanding of the underlying mechanisms is still missing.…”

Section: Introductionmentioning

confidence: 99%

Investigation of Band Tailing in Sputtered ZnO:Al Thin Films Regarding Structural Properties and Impurities

et al. 2014

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Thin films of pure aluminum doped ZnO and with addition of nitrogen, oxygen and hydrogen have been prepared by magnetron sputtering. The spectral absorption coefficient close to the band gap energy has been determined by spectrophotometry and analyzed regarding band tailing and creation of defect bands. We found, that an improvement of Raman crystallinity under O 2 -rich growth conditions is not accompanied by a suppression of band tailing as expected. An additional absorption feature evolves for layers grown in N 2 containing atmosphere. Doping with hydrogen attenuates sub-band gap absorption.

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Analysis of Urbach-like absorption tails in thermally treated ZnO:Al thin films

Cited by 14 publications

References 28 publications

Role of the dopant aluminum for the growth of sputtered ZnO:Al investigated by means of a seed layer concept

Role of the dopant aluminum for the growth of sputtered ZnO:Al investigated by means of a seed layer concept

Material properties of high-mobility TCOs and application to solar cells

Investigation of Band Tailing in Sputtered ZnO:Al Thin Films Regarding Structural Properties and Impurities

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