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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

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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?

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Reproducing These Effects Erratic currents have

been seen with every diamond sensor setup.

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

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

We’ve 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.

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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 400°C

Irradiation tests performed with a ~11kRad/H on contact 60Co source

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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 radiation or magnetic field

Bias switched from 500V to 100V

01010510

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Radiation in Rad/Hour

No magnetic field

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High Voltage Stability Before Irradiation

Biased at +500V One pad with

100pA4000pA (after 13 days with no

erratic dark currents) Two pads with ~100pA 9 other pads and two

guard rings showed no erratic dark currents

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

No magnet field

Erratic currents arise in different pads independently

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60Co Irradiation Test

Signal currents become obscured by the onset of erratic currents.

Erratic currents appear in each sensor pad independently.

0 05510 10 151519Rads per Hour of Exposure

No magnet field

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60Co Irradiation Test Sensors exposed to ~2kRad/Hour form 60Co (~140kRad in total)

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.

No magnet field No magnet field

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Effects of 60Co Irradiation with 500V Bias, B=0

Before Irradiation

After 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.

The voltage where the erratic currents go away is unique to each pad.

It takes ~hours before erratic currents return at higher voltage.

No magnet field

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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.