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@INPROCEEDINGS{Br:172209,
      author       = {Bär, M. and Starr, M. and Lambertz, Andreas and
                      Holländer, Bernhard and Alsmeier, B. and Weinhardt, J.-H.
                      and Blum, L. and Gorgoi, M. and Yang, M. and Wilks, W. and
                      Heske, R. G.},
      title        = {{M}icrocrystalline silicon oxides for silicon-based solar
                      cells: impact of the {O}/{S}i ratio on the electronic
                      structure},
      reportid     = {FZJ-2014-05698},
      year         = {2014},
      abstract     = {Hydrogenated microcrystalline silicon oxide (μc-SiOx:H)
                      layers are one alternative approach to ensure sufficient
                      interlayer charge transport while maintaining high
                      transparency and good passivation in Si-based solar cells.
                      We have used a combination of complementary x-ray and
                      electron spectroscopies to study the chemical and electronic
                      structure of the (μc-SiOx:H) material system. With these
                      techniques, we monitor the transition from a purely Si-based
                      crystalline bonding network to a silicon oxide dominated
                      environment, coinciding with a significant decrease of the
                      material’s conductivity. Most Si-based solar cell
                      structures contain emitter/contact/passivation layers.
                      Ideally, these layers fulfill their desired task (i.e.,
                      induce a sufficiently high internal electric field, ensure a
                      good electric contact, and passivate the interfaces of the
                      absorber) without absorbing light. Usually this leads to a
                      trade-off in which a higher transparency can only be
                      realized at the expense of the layer’s ability to properly
                      fulfill its task. One alternative approach is to use
                      hydrogenated microcrystalline silicon oxide (μc-SiOx:H), a
                      mixture of microcrystalline silicon and amorphous silicon
                      (sub)oxide. The crystalline Si regions allow charge
                      transport, while the oxide matrix maintains a high
                      transparency. To date, it is still unclear how in detail the
                      oxygen content influences the electronic structure of the
                      μc-SiOx:H mixed phase material. To address this question,
                      we have studied the chemical and electronic structure of the
                      μc-SiOx:H (0 ≤ x = O/Si ≤1) system with a combination
                      of complementary x-ray and electron spectroscopies. The
                      different surface sensitivities of the employed techniques
                      help to reduce the impact of surface oxides on the spectral
                      interpretation. For all samples, we find the valence band
                      maximum to be located at a similar energy with respect to
                      the Fermi energy. However, for x > 0.5, we observe a
                      pronounced decrease of Si 3s – Si 3p hybridization in
                      favor of Si 3p – O 2p hybridization in the upper valence
                      band. This coincides with a significant increase of the
                      material’s resistivity, possibly indicating the breakdown
                      of the conducting crystalline Si network. Silicon oxide
                      layers with a thickness of several hundred nanometres were
                      deposited in a PECVD (plasma-enhanced chemical vapor
                      deposition) multi chamber system using an excitation
                      frequency of 13.56 MHz with a plasma power density of 0.3
                      W/cm2. Glass (Corning type Eagle) and mono-crystalline
                      silicon wafer substrates were coated in the same run at a
                      substrate temperature of 185°C. The deposition pressure was
                      4 mbar and the substrate-electrode distance 20 mm. Mixtures
                      of silane (SiH4), $1\%$ TMB (B(CH3)3) diluted in helium,
                      hydrogen (H2), and carbon dioxide (CO2) gases were used at
                      flow rates of 1.25 - 0.18/0.32/500/0 – 1.07) sccm
                      (standard cubic centimeters per minute) for the deposition
                      of μc-SiOx:H(B) layers. By changing the CO2/SiH4 gas flow
                      rate ratio from 0 to 6, μc-SiOx:H(B) layers with a
                      composition of 0 ≤ x = O/Si ≤ 1 were prepared
                      using a constant sum of SiH4 and CO2. The TMB flow and the
                      H2 flow were kept constant within the series. For more
                      details see Ref. [1]. The oxygen content in the films was
                      determined using Rutherford Backscattering Spectroscopy
                      (RBS). With RBS, the area-related atomic density of oxygen
                      and silicon can be determined (± $2\%$ [2]), and thus x can
                      be calculated. This quantity considers only the number of
                      silicon / oxygen atoms and not the number of atoms of other
                      elements, such as hydrogen, which is also incorporated to a
                      considerable extent: up to $20\%$ in μc-SiOx:H (measured
                      using the hydrogen effusion method). To avoid charging
                      effects, the measurements were performed on films deposited
                      on a substrate of mono-crystalline silicon wafers. The
                      electrical conductivity was measured in the planar direction
                      of the film in a vacuum cryostat, using voltages from - 100
                      V to + 100 V. For that two co-planar Ag contacts were
                      evaporated on the film with a gap of 0.5 mm  5 mm.},
      month         = {Oct},
      date          = {2014-10-06},
      organization  = {SPIE Optics $\&$ Photonics 2014, San
                       Diego (USA), 6 Oct 2014 - 9 Oct 2014},
      subtyp        = {After Call},
      cin          = {IEK-5},
      cid          = {I:(DE-Juel1)IEK-5-20101013},
      pnm          = {111 - Thin Film Photovoltaics (POF2-111)},
      pid          = {G:(DE-HGF)POF2-111},
      typ          = {PUB:(DE-HGF)6},
      url          = {https://juser.fz-juelich.de/record/172209},
}