Using GaP Avalanche Photodiodes for Photon Detection

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Using GaP Avalanche Photodiodes for Photon
Detection

• Abigail Lubow
• EE Senior Project
• Fall 2001 and Spring 2002
• Advisor Professor Woodall

Pictures from http//www.roithner-laser.com/UV-PD.
html
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Background

• Avalanche photodiodes (APDs) are high gain
  photodetectors.
• Nonavalanching p-n and p-i-n photodetectors have
  unity gain. By contrast, APDs use the avalanching
  process to produce a higher gain.
• This device addresses the need for solar blind UV
  detectors.
• Potential applications High density optical
  storage and detection of tryptophan flourescence
  (348nm).
• This project is the first time GaP has been used
  to create an APD.

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Photodiodes
Figure 1.

• Incident photons are absorbed in the photodiode
  giving rise to electron-hole pairs.
• Gain 1. Impact ionization will increase the
  gain above 1.

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Impact Ionization

• Definition a snowballing creation of carriers
  very similar to an avalanche of snow on a
  mountain side.from Pierrets Semiconductor
  Device Fundamentals
• Conditions Applied voltage (VA) is negative and
  VA ? Vbreakdown.
• Figure 2.

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Why use GaP?

• It is more commonly available and less expensive
  than other wide bandgap materials such as SiC and
  GaN.
• For GaP, ni ? 1 /cm3 and for Si, ni 1 x
  1010/cm3. GaP p-n junction will have a smaller
  reverse current
• Large bandgaps correspond to small wavelengths
  Eph 1.24/?. GaP bandgap 2.26 eV, Si
  bandgap 1.12 eV
• Figure 3.

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P-I-N GaP Sample
Figure 4.
Figure 5.

• Fig. 4. P-I-N structure Electric field in
  graded p-layer separates electrons and holes.
• Fig. 5. Electrochemical CV measurement. for
  n type and o for p type.

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Measurements Current vs. Voltage
Figure 6.
Avalanching occurs at about -20V. Dark current is
1 10-¹³ A.
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Measurements Quantum efficiency vs. wavelength
Figure 7.
QE is the ratio of the number of carriers
generated to the number of photons incident upon
the active region. www.seas.gwu.edu/ecelabs/appn
otes/PDF/LED/LEDterms.pdf
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Surface Band Bending
Figure 8.

• Surface band bending due to the Fermi level
  pinning.
• Electron loss due to the surface recombination.

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Measurements Gain vs. Voltage
Figure 9.

• Gain results from the impact ionization process.
• Large gains start at 20V for UV and white
  light.
• Gains reach as high as 1000.

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Measurements Photocurrent vs. Wavelength
Figure 10.

• Current decreases at small wavelengths due to
  surface pinning.
• Current decreases at large wavelengths due to
  low absorption coefficients.

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Schottky Device
Figure 11.

• SAM structure Schottky device improves
  absorption efficiency and reduces noise.
• Processing Issue Choice of metal with low light
  loss

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Summary

• GaP APD has been chosen for use as a solar blind
  UV detector.
• P-I-N device showed promising performance with
  high gain, low dark current, and high QE at
  medium wavelengths.
• UV QE needed improvement. A Schottky device
  structure was proposed.