Performances of epitaxial GaAs detectors E. Bréelle, H. Samic, G. C. Sun, J. C. Bourgoin Laboratoire des Milieux Désordonnés et Hétérogènes Université 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 ( 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 techniquesThicknes (µm) Energy resolution References MBE1? VPE8 4.4% fwhm for 60 keV gammaHesse et al. (1972) LPE % fwhm for 60 keV gamma 5% fwhm for 60 keV gamma Alexiev et al. (1992) Gibbons et al. (1972) CVPD (!) % fwhm for 60 keV gammaOwens et al. (2002) LPVPE40 0.2% fwhm for 5 MeV alphaBates et al. (1999) Chemical Reactionup to 500? Epitaxial GaAs detector

What has been achieved by Chemical Reaction method: A. Growth of thick epitaxial GaAs layers: µm thick layers Homogenous electronic properties Electronic properties similar to that of standard epilayers B. Pixel technology: Polishing Ion implantation + annealing Photolithography Chemical etching Si 3 N 4 deposition Metallic alloy deposition ohmic contact (Au, Ge, Ni) p + ion implantation GaAs epilayer n + substrate (Cz GaAs)

Oscilloscope (1M  ) p + /i/n + Bias (10 V) C. Photo current induced by X-ray What has been achieved by Chemical Reaction method:

Results 1. Proton detection GaAs detector : 2 mm 2, 4.3 x cm -3, bias of 100 V (depleted depth 18.4 µm) Retrodiffusion of 1.2 MeV (a) and 1.3 MeV (b) protons

Results 2. Electron detection Si detector: 25 mm 2, depleted depth 100 µm at 50 V GaAs detector: 2 mm 2, depleted depth about 13 µm at 50 V.

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

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

Conclusion 1. Good energy resolution 2. Width of the space charge region, small ! Decrease of the residual doping Work in photocurrent 3. Optimise the technology of the detector.