Efforts to improve carrier mobility in radio frequency sputtered aluminum doped zinc oxide films

Agashe, Chitra; Kluth, O.; Hüpkes, J.; Zastrow, U.; Rech, Bernd; Wuttig, Matthias

doi:10.1063/1.1641524

Cited by 255 publications

(147 citation statements)

References 15 publications

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“…The carrier mobility seems to be maximal at a certain substrate temperature which increases when TAC is reduced. Carrier mobility was further improved by optimized deposition conditions compared to the data of Agashe et al 26 Figure 3 shows transmission and absorption spectra of ZnO:Al films sputtered at a fixed substrate temperature of 375°C using 0.2, 0.5, and 2 wt % TAC. The absorption was calculated using transmission and reflection spectra.…”

Section: A Structural Electrical and Optical Zno:al Film Propertiesmentioning

confidence: 98%

“…In earlier work, Agashe et al studied the influence of the target doping concentration on the electrical, structural, and optical properties of sputter-deposited and asdeposited smooth ZnO:Al films. 26 They found much lower parasitic absorptions with similar conductivity values using sputter targets with a low amount of alumina. Besides the transparency, it is crucial to develop a TCO with a suitable surface texture which scatters the light very efficiently in order to extend the effective path length within the active silicon layers.…”

Section: Introductionmentioning

confidence: 99%

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The effect of front ZnO:Al surface texture and optical transparency on efficient light trapping in silicon thin-film solar cells

Berginski

Hüpkes

Schulte

et al. 2007

Journal of Applied Physics

488

275

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This study addresses the material properties of magnetron-sputtered aluminum-doped zinc oxide (ZnO:Al) films and their application as front contacts in silicon thin-film solar cells. Optimized films exhibit high conductivity and transparency, as well as a surface topography with adapted light-scattering properties to induce efficient light trapping in silicon thin-film solar cells. We investigated the influence on the ZnO:Al properties of the amount of alumina in the target as well as the substrate temperature during sputter deposition. The alumina content in the target influences the carrier concentration leading to different conductivity and free carrier absorption in the near infrared. Additionally, a distinct influence on the film growth of the ZnO:Al layer was found. The latter affects the surface topography which develops during wet-chemical etching in diluted hydrochloric acid. Depending on alumina content in the target and heater temperature, three different regimes of etching behavior have been identified. Low amounts of target doping and low heater temperatures result in small and irregular features in the postetching surface topography, which does not scatter the light efficiently. At higher substrate temperatures and target doping levels, more regularly distributed craters evolve with mean opening angles between 120° and 135° and lateral sizes of 1–3μm. These layers are very effective in light scattering. In the third regime—at very high substrate temperatures and high doping levels—the postetching surface is rather flat and almost no light scattering is observed. We applied the ZnO:Al films as front contacts in thin-film silicon solar cells to study their light-trapping ability. While high transparency is a prerequisite, light trapping was improved by using front contacts with a surface topography consisting of relatively uniformly dispersed craters. We have identified a low amount of target doping (0.5–1wt%) and relatively high substrate temperatures (about 350–450°C as sputter parameters enabling short-circuit current densities as high as 26.8mA∕cm2 in μc-Si:H pin cells with an i-layer thickness of 1.9μm. Limitations on further improvements of light-trapping ability are discussed in comparison with the theoretical limitations and Monte Carlo simulations presented in the literature.

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Section: A Structural Electrical and Optical Zno:al Film Propertiesmentioning

confidence: 98%

Section: Introductionmentioning

confidence: 99%

The effect of front ZnO:Al surface texture and optical transparency on efficient light trapping in silicon thin-film solar cells

Berginski

Hüpkes

Schulte

et al. 2007

Journal of Applied Physics

488

275

View full text Add to dashboard Cite

show abstract

“…13 However, there have also been reports describing its presence, although to a much smaller extent, in the case of the AZO films deposited by PLD and physical sputtering. 16,17 The presence of a gradient in sheet resistance may derive either from specific conditions, i.e., related to the substrate conditioning ͑e.g., deposition temperature, surface pretreatment, or external substrate bias͒, as well as from the plasma chemistry ͑working pressure and gas phase composition͒.…”

Section: Introductionmentioning

confidence: 99%

Evolution of the electrical and structural properties during the growth of Al doped ZnO films by remote plasma-enhanced metalorganic chemical vapor deposition

Volintiru

Creatore

Kniknie

et al. 2007

Journal of Applied Physics

115

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Al-doped zinc oxide (AZO) films were deposited by means of remote plasma-enhanced metalorganic chemical vapor deposition from oxygen/diethylzinc/trimethylaluminum mixtures. The electrical, structural (crystallinity and morphology), and chemical properties of the deposited films were investigated using Hall, four point probe, x-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), electron recoil detection (ERD), Rutherford backscattering (RBS), and time of flight secondary ion mass spectrometry (TOF-SIMS), respectively. We found that the working pressure plays an important role in controlling the sheet resistance Rs and roughness development during film growth. At 1.5 mbar the AZO films are highly conductive (Rs<6 Ω∕□ for a film thickness above 1200 nm) and very rough (>4% of the film thickness), however, they are characterized by a large sheet resistance gradient with increasing film thickness. By decreasing the pressure from 1.5 to 0.38 mbar, the gradient is significantly reduced and the films become smoother, but the sheet resistance increases (Rs≈100 Ω∕□ for a film thickness of 1000 nm). The sheet resistance gradient and the surface roughness development correlate with the grain size evolution, as determined from the AFM and SEM analyses, indicating the transition from pyramid-like at 1.5 mbar to pillar-like growth mode at 0.38 mbar. The change in plasma chemistry/growth precursors caused by the variation in pressure leads to different concentration and activation efficiency of Al dopant in the zinc oxide films. On the basis of the experimental evidence, a valid route for further improving the conductivity of the AZO film is found, i.e., increasing the grain size at the initial stage of film growth.

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“…The measured value of 52.6 cm 2 / V s is close to the theoretical limit proposed by Ellmer,20 and thus higher than the experimental limit found on the sputtering coater used in this study. 18 This indicates two different processes are responsible for the increase of conductivity. The higher mobility for the SPC sample is most likely caused by an improved crystallinity of the ZnO:Al film after the annealing.…”

mentioning

confidence: 99%

“…% Al 2 O 3 at a substrate temperature of 300°C. 18 The thickness of the ZnO:Al films, determined by fitting optical transmission and reflection spectra, was between 690 and 770 nm. These kinds of films are usually applied for the development of state-of-the-art amorphous and microcrystalline Si based single and multijunction solar cells.…”

mentioning

confidence: 99%

Temperature stability of ZnO:Al film properties for poly-Si thin-film devices

Lee

Becker

Muske

et al. 2007

Applied Physics Letters

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The crystallization of thin silicon films at temperatures between 425 and 600°C was investigated on glass substrates coated with Al-doped zinc oxide (ZnO:Al). Bare ZnO:Al layers degrade at the crystallization temperatures used. A silicon layer on top, however, efficiently prevents deterioration. The resistivity was even found to drop from 4.3×10−4Ωcm for the as deposited ZnO:Al to 2.2×10−4Ωcm in the case of aluminium induced crystallization and to 3.4×10−4Ωcm for solid phase crystallization. The temperature-stable conductivity of ZnO:Al films coated with Si opens up appealing options for the production of polycrystalline silicon thin-film solar cells with transparent front contacts.

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Efforts to improve carrier mobility in radio frequency sputtered aluminum doped zinc oxide films

Cited by 255 publications

References 15 publications

The effect of front ZnO:Al surface texture and optical transparency on efficient light trapping in silicon thin-film solar cells

The effect of front ZnO:Al surface texture and optical transparency on efficient light trapping in silicon thin-film solar cells

Evolution of the electrical and structural properties during the growth of Al doped ZnO films by remote plasma-enhanced metalorganic chemical vapor deposition

Temperature stability of ZnO:Al film properties for poly-Si thin-film devices

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