Piezoelectric Spectroscopy of the Defects States on the Surfaces of Semiconducting Samples

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Piezoelectric Spectroscopy of the Defects States
on the Surfaces of Semiconducting Samples

• M. Malinski1, J. Zakrzewski2, K.
  Strzalkowski2, F. Firszt2
• 1 Department of Electronics and Computer Science,
  Technical University of Koszalin, 2 Sniadeckich
  St, 75328 Koszalin, Poland
• 2 Instytut Fizyki, Uniwersytet Mikolaja
  Kopernika, ul.Grudziadzka 5/7, 87100 Torun,
  Poland

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ABSTRACT

• This presentation shows both theoretical and
  experimental aspects connected with piezoelectric
  detection of defects states located on surfaces
  of semiconducting samples. This kind of states
  can provide absorption bands visible in the
  energy gap region of semiconductors. Theoretical
  considerations presented in this paper comprise
  computations of the piezoelectric spectra for
  defects states located on different surfaces.
  Experimental part of the paper comprises
  numerical analysis of several experimental
  amplitude and phase piezoelectric spectra of a
  group of Zn1-x-yBeyMnxSe mixed crystals for
  different x, y compositional parameters i.e.
  y0.05, x0.05, 0.10, 0.15, 0.20 after different
  surface treatment.

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SAMPLE PREPARATION THE MEASURING METHOD

• Zn1-x-yBexMnySe (x0.05, y 0.05, 0.10, 0.15 and
  0.20) samples were grown from the high purity
  powder with the high pressure Bridgman method.
  The crystal rod was cut into about 1mm thick
  samples which were first grinded, then polished
  with diamond paste and finally chemically etched.
  Solution of H2SO4 (96), K2Cr2O7 and water was
  used for etching the samples. After etching, the
  samples were rinsed in distilled water and then
  put for a few seconds in boiling NaOH. Then the
  samples were rinsed again in cold and next in
  boiling distilled water and finally in ethyl
  alcohol.
• In the piezoelectric photothermal experiment
  samples were illuminated with the intensity
  modulated beam of light of a xenon lamp after
  passing through the prism monochromator. The
  piezoelectric signal was detected with a lock-in
  phase selective amplifier. The characteristics
  were measured at room temperature in the rear
  experimental configurations.

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INTRODUCTION

• Typical optical transmission measurements do not
  bring information on the spatial locations of the
  defects responsible for the absorption bands.
  Another basic problem of this type of
  measurements is the lack of the possibility of
  identification of the type of the optical
  absorption i.e. a bulk or surface one.
• Piezoelectric phase measurements bring
  information on the spatial location of the
  absorbing centers!
• Numerical analysis of the PZE spectra can also
  bring information on the type of absorption a
  volume or a surface one.

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THEORY

• Lets consider the surface type optical
  absorption coefficient spectra connected with the
  presence of surface defects in the form of the
  Gaussian distribution

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THEORY

• The bulk type optical absorption coefficient
  spectra in the Urbach edge energy region, below
  the energy gap value, and in the band to band
  absorption region, above the energy gap value,
  are given by the formulae

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PIEZOELECTRIC SIGNAL

• Piezoelectric signal S is computed according to
  Jackson Amer formula.
• T(x) is the temperature distribution in the sample

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DIAGRAM OF A SAMPLE
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TEMPERATURE DISTRIBUTIONSSURFACE VOLUME
ABSORPTION
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THEORY-surface absorption

• Parameters of the surface absorption band
  Eg2.81 eV, A380 cm-1,?1, E12.18 eV, ?10.13
  eV, A1290 cm-1.

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THEORY- PZE SPECTRA

• Piezoelectric amplitude and phase spectra
  computed for the parameters ?0.03 cm2/s, f36
  Hz and l0.1 cm the rear experimental
  configuration.
• The defect located on the illuminated side of
  the sample

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THEORY- PZE SPECTRA

• The defect located on the dark side of the
  sample.
• Parameters ?0.03 cm2/s, f36 Hz and l0.1 cm

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THEORY- PZE SPECTRA

• The defect located on the illuminated and dark
  side of the sample.
• Parameters ?0.03 cm2/s, f36 Hz and l0.1 cm

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THEORY- PZE SPECTRA

• Piezoelectric spectra - under the assumption of
  the volume absorption type connected with the
  defect center at E2.18 eV .
• Parameters ?0.03 cm2/s, f36 Hz and l0.1 cm

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EXPERIMENTAL RESULTS

• Piezoelectric amplitude a) and phase b) spectra
  of the Zn0.75Be0.05Mn0.20Se sample at f126 Hz in
  the rear configuration. Circles are experimental
  points, solid lines are theoretical curves.

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EXPERIMENTAL RESULTS

• Optical parameters of the center determined
  A1290 cm-1, ?10.13 eV, E12.18 eV, Eg2.81 eV,
  ?31. The center is located on the illuminated
  side of the sample.

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EXPERIMENTAL RESULTS

• Piezoelectric amplitude a) and phase b) spectra
  of the annealed Zn0.85Be0.05Mn0.10Se sample at
  f76Hz in the rear configuration. Dots Circles
  are experimental points, solid lines are
  theoretical curves.

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OPTICAL ABS. COEFF. SPECTRUM

• Parameters of these surface absorption bands
  determined
• A1 350 cm-1, A2 260 cm-1, E1 2.35 eV, E2
  2.60 eV, ?1 0.15 eV, ?2 0.15 eV.
• Both defects are located on the same,
  illuminated, side of the sample.

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EXPERIMENTAL RESULTS

• Experimental and theoretical PZE amplitude a) and
  phase b) spectra of Zn0.75Be0.05Mn0.20Se sample
  measured at f76 Hz
• Circles-exp. results, lines- theoretical curves

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CONCLUSIONS

• Theoretical considerations, presented in the
  paper, indicate that it is possible to determine
  the location of the surface defects, visible in
  the amplitude piezoelectric spectra, on one of
  the surfaces of the sample from the numerical
  analysis of the piezoelectric phase spectra.
• Theoretical and experimental results analysed in
  the paper also proved the possibility of
  distinguishing between the bulk and surface
  absorption.

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