1. Guojian Wang University of South Dakota
Crystal Growth, characterization and detector performance of High-purity Germanium crystals
Guojian Wang
University of South Dakota

2. Outline Motivation Crystal Growth Crystal properties
Detectors from LBL by Dr. Mark
Super-CDMS type detector
Conclusion

3. Motivation
Preparation for ton-scale experiments - ton scale HP-Ge crystals are needed for dark matter and neutrino experiments (Super-CDMS, GERDA, MAJORANA, CDEX ). The detail information is required to explain the detector performance.
Development of skills for large-size crystal growth – growing large diameter HP-Ge crystals is a challenge.
Study the crystals’ properties and evaluate the crystal quality by detector fabrication

4. Crystal growth 2.1. The processes of crystal growth: Dash process
Shouldering process
Equal diameter growth
Ending process

5. 2.2. Grown crystals
3 Kg Ø7.5 cm
3.5 Kg Ø 9 cm
5 Kg, Ø12.9cm

6. Crystal Properties 3.1 The main impurities
Photo-thermal ionization spectroscopy

7. 3.2 The axial distribution of impurities
About 30% of crystal is detector grade crystal (5Kg) .

8. Recent progress Useful Part: From 6E10 ~3E10 cm-3. Thickness 7 cm
Diameter ~7.5 cm
Weight 1592g

9. 3.3 The radial distribution
No. 34 1 2 3 4 5 6 7
Sample position
Hall effect

10. 3.3 The radial distribution (cont.)
Sent to Tsinghua University

11. 3.3 The radial distribution (cont.)
At the edge of the wafers, the main impurities are Al and Ga. However, at the center of the wafers, the main impurities are Al.
* G. Wang, et.al, Materials Science in Semiconductor Processing, 39, (2015) 54-60

12. 4. Detectors from LBL by Dr. Mark

13. The spectrum of detector A
Net carrier concentration: 1.2x1011 cm-3
Type: P

14. The spectrum of detector B
Net carrier concentration: 7.9x1010 cm-3
Type: P

15. The spectrum of detector C
Net carrier concentration: 1.59x1010 cm-3
Type: P

16. Electronic Noise (keV)
Electronic Noise and Fano Factor *
Detector
Electronic Noise (keV)
Fano Factor A
1.35
0.0915 B
1.43
0.150 C
1.44
0.168
* G.F. Knoll, Radiation Detection and Measurement, Fourth Edition, Wiley (2010) 428

17. 5. Super-CDMS type detector
5.1. No.27 crystal (2014)
Weight 2.3kg
112mm (top) x 115.7mm (bottom) x 41.1mm thickness.
5.9Kg
Fabricated into SuperCDMS type detector by Texas A&M University

18. 5.1. No.27 crystal (con.)
Radial distribution of net carrier at 77K

19. 5.2. Detector fabrication
Four Am sources were installed on a plate above the detector. Pinholes sit on the radial midpoint of each electrode and let decay products through to the crystal. Operation temperature is ~50 mK.
Concentric electrodes were deposited on one side of the crystal
Detector fabricated by Mahapatra group at Texas A&M

20. 5.3 Test results from UMN
USD1 charge response at +13V
G101 charge response at +4V[1]
The peaks due to 60 keV gamma-rays from the 241Am sources .
Detector is characterized by Dr. Mandic’ group from University of Minnesota
[1] Hassan Chagani, IDM 2012

21. 5.3 Test results from UMN (a) (b)
60 keV peaks in USD1 Q4 (the outer ring), at (a) three positive biases, such that the signal is generated by electrons, and at (b) three negative biases, such that the signal is generated by holes

22. 5.3 Test results from UMN
The efficiency falls nearly to zero between -4V and +4V. The lower the bias, the slower the carriers drift, so that they have more time to become trapped before being collected. The DLTS measurement will be used to find the charge trapping centers.
The charge signal does not plateau above some characteristic voltage bias, as is typically the case in most SuperCDMS detectors.
[2]H. Chagani, et al. J. Low Temp. Phys. 167(2012)
G102 collection efficiency, based on the 60 keV Am line[2]

23. 6. Conclusion 7.5~12.9 cm in diameter HPGe crystals have been grown.
The variation of impurities’ level is 5x109 ~ 6x1010 cm-3. .
Three planar detectors were fabricated and show excellent energy resolution.
A super-CDMS type detector was fabricated and show interesting properties.

24. Acknowledgement
Dr. Mark Lawrence Berkeley National Laboratory
Dr. Klaus Irmscher and Mike
Institute for crystal growth, Berlin
Dr. Mahapatra Texas A&M
Dr. Mandic University of Minnesota
This work is supported by DOE grant DE-FG02-10ER46709 and the state of South Dakota

25. Thanks for your Attention