Photosensitive nanocrystalline films based on semiconducting lead chalcogenides

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Photosensitive nanocrystalline films based on
semiconducting lead chalcogenides

• Z. Dashevsky

Nanoscale Devices - Fundamentals and Applications
NDFA-2004
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Detectivity vs. Wavelength for Various Infrared
Detectors
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Lead chalcogenides

• Narrow direct band gap
• Eg(PbS) ? 0.4 eV at the room temperature. The
  temperature coefficient dEg/dT is positive.
• High mobility of the charge carriers
• PbS crystals have a carrier mobility m ? 600
  cm2/Vsec at the room temperature due to low
  effective mass and high permitivity. PbS has
  eo175 (at T ? 300 K). The elevated value of eo
  gives rise to the sharply decreased impurity
  scattering of charge carriers in the heavily
  doped state.

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PbS Phase Diagram
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Varying the Carrier Concentration

• Carrier concentration can be varied by
• introducing intrinsic defects during the thin
  film growth
• intrinsic defects are those due to the presence
• of excess Pb (cause S vacancies (VS ),
• behaving like donors) or excess S (cause Pb
• vacancies (VPb--), behaving like acceptors)
• doping with impurities during or after the thin
  film growth
• iodine and indium are donors, sodium are
  acceptors in PbS films

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Thin Film Growth by Thermal Evaporation Technique
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PbS Thin Film Deposition Parameters
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PbS Thin Films Doping from Gaseous Phase
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Structural Investigation of PbS Thin Film
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AFM of PbS film on glass substrate
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Topography of thin film surfacePbS film as
deposited
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Topography of film surfacePbS film after
oxidation
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Dark conductivity vs. temperature
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Photosensitivity vs. temperature
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Carrier life time measurements (PbS film after
oxidation)
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Decay time as a function of temperature
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Schematic view of thin film after oxidation
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Schematic view of fragment of a film
1 quasineutral parts of n-type grains
2, and 3 p-type conducting inversion
channels on grain surfaces 4 region of
intersection of two conducting channels (node)
the arrow shows the current direction.
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As grown p-type PbTe film with excess lead and
sulphur vacancies on grain boundaries
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Absorption of oxygen on grain boundaries
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Energy band diagram of a system consisting of
grains and boundaries
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Influence of oxygen on energy band diagram
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Separation of electron-hole pairs on grain
boundaries
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Separation of electron-hole pairs on grain
boundaries
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Generation of electron-hole pairs by
IR-irradiation
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Diffusion of In along grain boundaries
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Influence of indium on energy band diagram
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Generation and recombination of electron-hole
pairs (P group samples)
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Generation and recombination of electron-hole
pairs (G group samples)
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Decay time for nanocrystalline PbS film at 300 K
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Conclusions

• High decay lifetime (50 msec at 300K) in
  nanocrystalline PbS film (nanocrystals 30 nm)
  has been observed, which is connected to
  persistent photoconductivity effect.
• The effect is attributed to the separation of
  electron hole pairs by potential barriers at
  the grain boundaries, preventing their
  recombination.