Room temperature operation of epitaxial leadtelluride detectors monolithically integrated on midinfr

1
Room temperature operation of epitaxial
lead-telluride detectors monolithically
integrated on midinfrared filters

• M. Böberl, T. Schwarzl, J. Roither, T. Fromherz,
  G. Springholz and W. Heiss
• Institut für Halbleiter- und Festkörperphysik,
• Universität Linz, Österreich

• Outline
• Introduction and Motivation
• Room temperature photoconductivity from PbTe
  epilayers
• Integrated narrow-band photovoltaic detectors for
  room temperature operation
• Conclusion

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Compact uncooled sensor systems

• Detection of hydrocarbons or CO2 in ambient air
  via IR transmission measurements
• optical prerequisites
• - broad IR source
• 2 broadband detectors and 2 narrow-band
  interference filters
• first filter wavelength coincides with certain
  molecule absorption line, used for the measuring
  detector
• second one is off resonance and is used for the
  reference detector
• c monolithic integration of filter and detector
  for more compact systems

DraegerSensors IR Ex HC-68 08 475
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Applications of IR monitoring systems
e.g. early warning of potentially explosive
hydrocarbon levels. These devices are used in a
wide variety of industrial facilities and are an
integral part of plant safety systems.

• all hydrocarbons have the characteristic C-H
  stretch which occurs between 3100 and 2850 cm-1
• alcohols and amines display strong broad O-H and
  N-H stretching bands in the region 3400 to 3100
  cm-1
• alkene and alkyne C-H bonds display sharp
  stretching absorptions in the region 3100 to 3000
  cm-1
• triple bond stretching absorptions occur in the
  region 2400 to 2200 cm-1

c 3 to 4.5 mm wavelength region favorable
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Polycrystalline PbSe Photodetectors
Room temperature detectivities of 1.8 x 1010
cmHz½W-1 at 3.8 mm
Response times some ms
fabrication film deposition by thermal
evaporation baking process for sensitization to
infrared radiation
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Monolithically Integrated Polycrystalline PbSe
Photodetectors
J. Diezhandino, APL 83, 2751 (2003) Monolithic
integration of spectrally selective uncooled lead
selenide detectors for low cost applications
filter structure 50 layers Ge/SiO
experiment
simulation
c baking for sensitization is limited by thermal
stability of the filter structure
band pass
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Monolithic integration of epitaxial
lead-telluride photodetectors

• PbTe narrow gap semiconductor, high index of
  refraction n,
• EuTe wide gap semicond., low n
• Pb1-xEuxTe completely miscible alloy system
  with very large band gap tunability
• lattice mismatch between
• Pb1-xEuxTe/EuTe lt 2
• good matching of thermal expansion coefficients
  (PbTe 19.8x106K-1, EuTe 13.6x106K-1, BaF2
  19.8x106K-1)
• high refractive index contrast of Dn/n80 c
  efficient interference mirrors and filters

c filter structures of Pb1-xEuxTe/EuTe layers
photosensitive PbTe layer
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Room temperature photoconductivity from PbTe
epilayers grown on l/4 antireflection layers
Experimental setup and sample description
S1 1 mm thick PbTe grown on a l/4 EuTe
antireflection coating R1 single 2.5 mm PbTe
epilayer

• Reflectivity spectra
• S1 shows a pronounced minimum in reflectivity at
  3.1 mm due to l/4 EuTe antireflection layer
• RT photoconductivity spectra measured at a bias
  of 1 mA
• S1 shows a broad maximum at 3.1 mm c complies
  with distinct minimum in reflectivity spectrum
• Spectra obtained by forward biasing (lines) and
  reverse biasing (symbols) match perfectly c pure
  photoconductivity

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Responsivity at room temperature
- measured with a HeNe-laser at 3.36 mm and a
power of 5 mW
at a bias of 1 mA R1 Rl 1.4 V/W S1 -
illumination through EuTe antireflection layer
c Rl 2.9 V/W- direct illumination of the
PbTe layer c Rl 1.29 V/Wc enhancement of
responsivity by 2.2 in agreement with the ratio
of reflectivites (1-Rbackside)/(1-Rfrontside)
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Device characteristics
Noise voltage per band width un/(Df)1/2 22
nV/Hz½ measured with 300 K background radiation
at 10 kHz and a bias of 1 mA
8 ms

• Detectivity calculated from
• DRl(ADf) ½ /un
• room temperature detectivity at a bias of 1 mA
  1.36x107 cmHz½W-1
• small compared to polycrystalline detectors
• studying possibility to obtain room temperature
  photoresponse from photovoltaic devices

4 ms

• Response times
• measured at l 800 nm
• rise times smaller than 8 ms
• comparable with those of polycrystalline lead
  salt detectors

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Integrated PbTe photodetectors on
narrow-bandwidth filters
Design for 3.6 mm and room temperature
filter consisting of - 2 Bragg mirrors with 1
and 1.5 PbEuTe/EuTe mirror pairs, high
refractive index contrast of mirror layers c
reflectivity of mirrors is 83 - in between
l/2 thick PbEuTe layer c cavity resonance at 3.6
mm active layer 500 nm thick p-PbTe with 1x1017
cm-3 p-type contact Pt/Ti/Au on PbTe
n-type contact In on EuTe
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Characteristics of integrated PbTe photodetectors
photosensitive PbTe layer
1 mm
EuTe/Pb0.94Eu0.06Te top Bragg mirror
l/2 cavity
bottom Bragg mirror
mid-IR transparent BaF2 substrate
c one narrow resonance peak
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Photovoltaic response of integrated PbTe
photodetectors

• one single peak at
• 3.66 mm with a linewidth of 100 nm
• Dl/l 2.7
• photovoltage peak coincides with that in
  transmittance in both, position and linewidth
• broad band at l lt 3 mm can be inhibited

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Conclusions

• Demonstration of room temperature
  photoconductivity as well as photovoltaic
  operation of epitaxial PbTe detectors
• Monolithically integration of photosensitive PbTe
  layers with EuTe/PbEuTe midinfrared filters
• PbTe detectors grown on l/4 antireflection layers
  c more than doubling of the responsivity
• Integration of PbTe detectors with microcavities
  acting as efficient narrow band filters c one
  single resonance in the photoresponse with a Dl/l
  of 2.7 in coincidence with OC-H stretching bond

Outlook Improved photovoltaic response by
extrinsically doped EuTe
Acknowledgements
G. Pillwein for SEM, bmbwk, FWF (START-Projekt
Y179 and SFB IR-ON), GME