# Presentation: 660106

## Slide 1

## Proton Radiation Damage in High-Resistivity n-Type Silicon CCDs

- C. Bebek, D. Groom, S. Holland, A. Karcher, **W. Kolbe**, J. Lee, M. Levi, N. Palaio, B. Turko, M. Uslenghi, M. Wagner, G. Wang
*Lawrence Berkeley National Laboratory*

## Outline

- Description of LBNL developed CCDs
- Radiation damage study and measurement of charge transfer efficiency (CTE)
- Identification of radiation induced traps using pocket pumping techniques
- Fitting of CTE data to trap densities
- Summary of results

## LBNL CCD Technology

- LBNL CCDs differ from conventional devices in that they are made from high resistivity, *n*-type silicon which can be fully depleted
- They use buried p-channels (instead of n-channels) for charge storage and transport. Carriers are holes.
- Large photosensitive volume results in a greatly enhanced near IR response.
- They can be operated back illuminated without the need for thinning, resulting in a significant cost savings.

## Radiation Testing of CCDs

- Plans to employ LBNL CCDs in long term space based missions require them to have high radiation tolerance.
- We performed room temperature irradiation study using 12 MeV proton doses of 5x109, 1x1010, 5x1010 and 1x1011 p/cm2.
- Characterized the devices by measuring their CTE and dark current.
- Measurements demonstrated excellent radiation tolerance superior to n-channel CCDs.

## Charge Transfer Efficiency Calculation

- CTE measured by exposing CCD to 55Fe x-rays which deposit 1620 e- per pixel
- X-ray peak heights are plotted vs row or column number transferred.
- Slope of the line fitted to the clustered single pixel events is measure of the CTE

- CCD irradiation dose 1x1010 p/cm2 measured at 125K

- X-ray stacking plot for analysis

- Parallel CTE =0.99997

## Measured CTE vs Radiation Dose

- CTE vs radiation dose measured at 128K

## CTE vs Temperature at 1x1011 p/cm2

- Both serial and parallel CTE exhibit significant temperature dependence due to interactions with radiation induced trapping centers.

## Pocket Pumping Illustrated

- Pre-radiation measurement
- Trap density 0.0021 traps/pixel

- Pocket pumping:
- 5 shifts, 60000 cycles

- Dose 1x1010 protons/cm2
- Trap density 0.096 traps/pixel

## Histogram to Measure Trap Efficiency

**Flatfield level**

**Pocket pumping peak**

**Depletion peak**

**Saturation**

**Pixel Value (adu)**

## Measured Trap Effectiveness

- Clock overlap period = 19.5 s
- Peaks at 160 and 190K.

## Hole Traps Found in n-Type Si

- Sii

- V

- V + V

- VV

- Ci

- CiOi

- CiCs

- Proton Irradiation

- Trap parameters measured using DLTS

## Charge Transfer Efficiency Model

- where

- The effectiveness factor, ***F*** determines the temperature range over which the trap will significantly reduce the CTE.
***F*** should also predict the shape, amplitude and location of each trap as measured in the pocket pumping experiment.
- We use an improved model which includes asymmetrical clocking and finite trap capture time.

***N******t**** *and ***n******e*** are the density of traps and electrons (holes) per pixel
***T*****0** and ***T*****p** are the clock overlap and period
-  ***N*****z** distance between x-rays and ********e** the trap emission time

## Trap Effectiveness vs Temperature

**V-V**
**C****i**
**C-O**

## Trap Effectiveness vs Temperature

- Excellent matching of calculated and pocket pumping measurement of trap effectiveness for clock overlap time of 19.5 s.
- Confirms the existence of the proposed traps.

**V-V**
**C****i**
**C-O**

## Fitting Trap Concentrations to CTI

## Fitted Trap Density

## Summary

- 4 LBNL p-channel CCDs proton irradiated at 12 MeV and doses up to 1x1011 p/cm2.
- Measurements show they are significantly more radiation resistant than n-channel CCDs.
- Pocket pumping experiments identified primary trapping centers impacting CTE.
- Fit of CTE to trap concentrations shows good agreement with radiation dose.
- Results will help us to predict and perhaps enhance long term performance in radiation environment.

## Slide 19