Research%20on%20Erratic%20Dark%20Currents%20in%20pCVD%20Diamonds

1
Research on Erratic Dark Currents in pCVD Diamonds

• Diamonds are Forever

Adam Edwards, Stanford University M. Bruinsma (UC
Irvine), P. Burchat (Stanford), H. Kagan (OSU),
R. Kass (OSU), D. Kirkby (UC Irvine), B.A.
Petersen (Stanford) RD42 Meeting, CERN May 14,
2004
2
Introduction

• In February 2003, the 1.5T magnetic field was
  temporarily lost and erratic dark currents in two
  diamonds were observed.

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Experimental Questions

• Some of the questions that we have been trying to
  answer about these erratic dark currents
• Are these surface/edge currents?
• Is it an effect of the metalization?
• Is it a property of the whole diamond or a
  localized effect?
• Is this common to pCVD diamonds?
• Are these currents caused by radiation?
• How do they depend on bias voltage?
• How does a magnetic field change them?

4
Reproducing These Effects

• Erratic currents are always eliminated by a 1.5T
  magnetic field perpendicular to the electric
  field.

• Erratic currents have been seen with every
  diamond sensor setup.

Example Diamond UT31 with solder connections,
kapton packaging, guard ring and CrAu
metalization.
Example Diamond UT31 placed in 1.5T
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Reproducing These Effects

• Weve worked with various setups.
• Different metalizations
• Different metalization geometries, including
  guard rings
• Different packaging, using wire bonds and
  soldering
• Different DC readout electronics
• These erratic dark currents are real and
  reproducible in the lab.

6
Laboratory Tests

• We have recently used four new pCVD diamond
  sensors made with G10 packaging and wire bonding.

• Two sensors have one pad and two have 4 pads
• Two sensors have AuCr metalization and two have
  AuCr/Al
• All sensors have a guard ring

• CCD measured 200µm
• Exposed to 1kRad from 90Sr for CCD measurement,
  then heat treated at 400C
• Irradiation tests performed with a 11kRad/H on
  contact 60Co source

7
Diamond Sensor Operation

• Both with and without radiation and with no
  applied magnetic field, all four diamonds were
  operated at 100V without seeing any onset of
  erratic currents.

No magnetic field
No radiation or magnetic field
Radiation in Rad/Hour
10
0
10
15
10
5
Bias switched from 500V to 100V
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High Voltage Stability Before Irradiation

• Biased at 500V
• One pad with 100pA?4000pA
• (after 13 days with no erratic dark currents)
• Two pads with 100pA
• 9 other pads and two guard rings showed no
  erratic dark currents

No magnet field

• Biased at -500V
• Two with 200pA
• One with 20pA

Erratic currents arise in different pads
independently
9
60Co Irradiation Test

• Signal currents become obscured by the onset of
  erratic currents.
• Erratic currents appear in each sensor pad
  independently.

0
0
5
5
10
10
15
15
19
Rads per Hour of Exposure
No magnet field
10
60Co Irradiation Test

• Sensors exposed to 2kRad/Hour form 60Co
  (140kRad in total)

No magnet field
No magnet field

• Within minutes there are erratic currents seen
• Some currents are an order of magnitude higher
  than expected from the amount of radiation.
• Every biased sensor in irradiation has some
  erratic current.

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Effects of 60Co Irradiation with 500V Bias, B0

• After Irradiation

• Before Irradiation

Note the different current scales!
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Effects of 60Co Irradiation Tests Without a Bias
Voltage

• One diamond sensor had no voltage bias during the
  irradiation and had no subsequent increase of
  erratic dark currents.
• A later irradiation of this sensor with a bias
  voltage created subsequent erratic dark currents
  of 250pA-15000pA.

No magnet field
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Lowering the Bias Voltage

• We have found the lowering the bias voltage can
  reduce the amount of erratic dark currents.
• No diamond sensors see any erratic currents at a
  100V bias.

No magnet field

• The voltage where the erratic currents go away is
  unique to each pad.
• It takes hours before erratic currents return at
  higher voltage.

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Magnetic Field Test

• All four sensors were placed in a 1.5T magnetic
  field with the same orientation as the diamond
  sensors inside BaBar

• Over 5 orders of magnitude, all pads with erratic
  dark currents showed that they could be stopped
  by the magnetic field.

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Conclusions

• We have reproduced, in the lab, the erratic dark
  currents and magnetic field effects that are seen
  in BaBar.
• Guard rings show that the erratic dark currents
  are not surface currents.
• The erratic dark currents have been seen with 3
  different kinds of metalization.
• Erratic dark currents are localized, affecting
  each pad differently.
• Every pCVD diamond that we have tested has had
  erratic dark currents.
• These currents arise under high bias voltage and
  increase during irradiation at high voltage.
• Erratic dark currents are not present at low bias
  voltages.
• Erratic dark currents are not present at 1.5T
  magnetic fields.