Charge Drift in partially-depleted epitaxial GaAs detectors

1
Charge Drift in partially-depleted epitaxial GaAs
detectors

• P.J. Sellin, H. El-Abbassi, S. Rath
• Department of Physics
• University of Surrey, Guildford, UK
• J.C. Bourgoin
• LMDH, Université Pierre et Marie Curie, Paris,
  France

2
Overview

• Chemical reaction growth of thick epitaxial GaAs
  layers
• Depletion thickness and residual impurity
  concentration
• Performance of partially depleted detectors
• C-V measurements of impurity concentration at low
  temperature
• Optical probing of charge transport using a
  focussed laser

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Potential challenges for epitaxial GaAs

• Strengths of epitaxial GaAs
• intermediate photon detection efficiency between
  Si and CZT/CdTe
• metal-semiconductor contacts and device physics
  are well understood
• epitaxial GaAs has low concentrations of native
  EL2 defect
• source of highly uniform whole wafer material,
  compatible with flip-chip bonding and monolithic
  electronics
• Existing problems
• even high purity epitaxial is compensated due to
  residual impurities- does not exhibit intrinsic
  carrier concentrations
• depletion thickness is severely limited
• charge carrier lifetimes are reduced

4
Chemical Reaction growth of thick epitaxial GaAs

• Epitaxial GaAs material studied in this work was
  grown by a Chemical Reaction Method by Jacques
  Bourgoin (Paris).
• An undoped GaAs wafer is used as the material
  source, which is decomposed in the presence of
  high temperature high pressure water vapour to
  produce volatile species.
• Typically, growth rates of lt10 mm/hr are used to
  achieve EL2 concentrations of 1013 cm-3

L. El Mir, et al, Compound semiconductor growth
by chemical reaction, Current Topics in Crystal
Growth Research 5 (1999) 131-139.
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Whole wafer photoluminescence mapping

• GaAs material uniformity is characterised using
  room temperature photo-luminescence mapping - a
  contact-less, whole wafer technique
• A 25 mW 633 nm HeNe laser is focussed to 50 mm
  on the wafer
• the wafer is mounted on an XY stage, and scanned
• PL intensity maps at peak the band edge emission
  wavelength (870 nm) are acquired

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PL maps of GaAs

• Photoluminescence mapping clearly shows the
  uniformity of epitaxial GaAs compared to
  semi-insulating VGF material

Epitaxial GaAs
Bulk GaAs
H. Samic et al., NIM A 487 (2002) 107-112.
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Calculated depletion thickness

• This material is nominally 1-5 x 1014 cm-3-
  corresponds to a 10-20 mm depletion thickness _at_
  30V, and 15-30 mm _at_ 80V

8
Alpha particle spectra

• 5.48 MeV alpha particles are irradiated through
  the Schottky (cathode) contact - range in GaAs
  20 mm.
• A peltier cooler controlled the device
  temperature in the range 25C to -55C. Shaping
  time 0.5 ms.

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Alpha particle pulse shapes

• Alpha particle pulses at room temperature

time base 1ms per division
fast component
preamplifier
slow component
shaping amplifier
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Alpha particle tracks

• An un-collimated alpha particle source produces a
  characteristic double peak pulse height
  spectrum if the depletion thickness is shallower
  than the particle range

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59.5 keV gamma spectra

• Depth-dependent CCE produces poorly resolved
  gamma spectra

T -50C
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Temperature dependent CV analysis
Allows the doping density ND to be extracted from
the gradient of 1/C2 vs V
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Depletion Thickness vs Bias Voltage
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Impurity Densities
The CV analysis confirm the shallow depletion
thicknesses achieved in these devices, and
correspond to impurity densities of 3 x 1013
cm-3 in sample S16 at low temperature
15
Focussed IR laser scans
Probe the variation in pulse shape as a function
of position from the Schottky contact, and
temperature
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Scanning optical bench
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Laser pulse shapes

• T273K, 20V
• At 60mm from cathode
• no slow component to signal
• At 180mm from cathode
• charge drift times are 350ms
• IR laser spot appears to have significant beam
  waist

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Laser pulse shapes (2)

• T223K, V90V
• At 60mm from cathode
• no slow component to signal
• At 180mm from cathode
• charge drift times are 350ms
• IR laser spot appears to have significant beam
  waist

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Pulse risetime and amplitude vs bias
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Interaction close to the anode - inside depletion
region
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Interaction close to n substrate - in low field
region
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Temperature dependent pulse shapes (1)
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Temperature dependent pulse shapes (2)
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Conclusions

• The epitaxial GaAs layers studied showed
  excellent uniformity, and a residual impurity
  concentration of 1-5 x 1014cm-3
• Long electron lifetimes gt 300 ms were observed in
  the low field regions - confirms the very low EL2
  concentration
• Lateral laser scans show
• good charge transport in the shallow depleted
  region
• long-lived components to the pulse shapes when
  irradiated close to n substrate - consistent
  with slow electron diffusion towards the
  substrate
• significant penetration of the depletion region
  when cooled to -50C
• Future work
• further lateral scanning is required with
  focussed lasers and high resolution proton
  microbeams to quantify these phenomena
• further modest reductions in impurity
  concentration will produce significant
  performance improvements

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Acknowledgements

• This work was partially funded by the UKs
  Engineering and Physics Science Research Council

26
Alpha particle spectra

• Epitaxial GaAs pad detectors were irradiated with
  an uncollimated 241Am alpha particle source. The
  detector was mounted in a vacuum cryostat,
  attached to a peltier cooler to allow operation
  in the temperature range of 25C to -55C. Pulse
  height spectra (figure 3) were acquired using a
  conventional charge integrating preamplifier and
  spectroscopy amplifier (shaping time 0.5ms).

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Interaction in intermediate region
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Temperature dependent pulse shapes (2)