Performances of epitaxial GaAs detectors

E. Bréelle, H. Samic, G. C. Sun, J. C. Bourgoin

Laboratoire des Milieux Désordonnés et HétérogènesUniversité Pierre et Marie Curie (Paris 6)

Introduction Material for X-ray imaging

  Bulk Epitaxial  Semi-insulating Thick enough Large defect concentration, Low defect concentration, →Non-uniform electronic properties Residual doping (1013-1014 cm-

3) →Short life time →Small depleted depth 

Imaging at room temperature Bulk CdTe Epitaxial GaAs (InP,

GaP…..) Limited area Large area Bad homogeneity Homogenous No technology Existing technology → Limitation in space charge region

Aim of the work

Growth techniques Thicknes (µm)

Energy resolution References

MBE 1 ?  

VPE 8 4.4% fwhm for 60 keV gamma Hesse et al. (1972)

LPE 20030-120

4.5% fwhm for 60 keV gamma 5% fwhm for 60 keV gamma

Alexiev et al. (1992)Gibbons et al. (1972)

CVPD (!) 40-325 0.9% fwhm for 60 keV gamma Owens et al. (2002)

LPVPE 40 0.2% fwhm for 5 MeV alpha Bates et al. (1999)

Chemical Reaction up to 500 ?  

Epitaxial GaAs detector

What has been achieved by Chemical Reaction

method: A. Growth of thick epitaxial GaAs layers: 100-500 µm thick layers Homogenous electronic properties Electronic properties similar to that of standard epilayers

 

B. Pixel technology: Polishing Ion implantation + annealing Photolithography Chemical etching Si3N4 deposition

Metallic alloy deposition 

ohmic contact (Au, Ge, Ni)

p+ ion implantation

GaAs epilayer

n+ substrate (Cz GaAs)

          

0 2 4 6 8 10 120

2

4

6

8

1: 0.2 nA/mm2

H2O, 60 kV

Telebix, 60 kV Telebrix, 120 kV

Ln

ph

oto

vo

ltag

e(m

V)

Thickness (cm)

Oscilloscope (1M)

p+/i/n+

Bias (10 V)

C. Photo current induced by X-ray

What has been achieved by Chemical Reaction

method:

Results1. Proton detection GaAs detector : 2 mm2, 4.3 x 1014 cm-3, bias of 100 V (depleted depth

18.4 µm)Retrodiffusion of 1.2 MeV (a) and 1.3 MeV (b) protons

0 200 400 600 800 10000

20

40

60

80

100

120

140

160

180

Channel number

Ene

rgy

(MeV

)

(b)

(a)

Co

un

ts

Channel number

0 200 400 600 800 10000.0

0.5

1.0

1.5

2.0

Results 2. Electron detectionSi detector: 25 mm2, depleted depth 100 µm at 50 VGaAs detector: 2 mm2, depleted depth about 13 µm at 50 V.

0 50 100 150 2000

5000

10000

15000

20000

25000

Si GaAs

Co

un

ts

Channel number

Results  3. Alpha detection (241Am)-5.49 MeV  Bias of 80 V

Results  4. Gamma detection (241Am)- 59 keVCooled at –50C, at bias of 70V:  

 Conclusion

1. Good energy resolution2. Width of the space charge region, small

! Decrease of the residual doping Work in photocurrent3. Optimise the technology of the detector.