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P.J. Sellin, S. Rath, M. Breese, A. Hossain, E.J. Morton, M. Ozsan

Optical and Electrical Characterisation of Defects and Charge Transport in CdZnTe radiation detectors. P.J. Sellin, S. Rath, M. Breese, A. Hossain, E.J. Morton, M. Ozsan Department of Physics, University of Surrey, Guildford GU2 7XH, U.K.

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P.J. Sellin, S. Rath, M. Breese, A. Hossain, E.J. Morton, M. Ozsan

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  1. Optical and Electrical Characterisation of Defects and Charge Transport in CdZnTe radiation detectors P.J. Sellin, S. Rath, M. Breese, A. Hossain, E.J. Morton, M. Ozsan Department of Physics, University of Surrey, Guildford GU2 7XH, U.K.

  2. Material characterisation aims to answer many questions: material homogenity: local variations in alloy compositions, bandgap, resistivity defects: intrinsic, extrinsic, extended and stoichiometric variations metal-semiconductor interface: ohmic vs. rectifying contact behaviour - locally enhanced field strength charge transport properties: charge collection efficiency-determined by carrier mobility-lifetime products and electrically active defects spectroscopic resolution: limited by material variations, electronic noise, leakage current CdZnTe material issues:

  3. Optical characterisation of uniformity: Photoluminescence microscopy/mapping secondary electron microscopy Transient spectroscopy for deep level identification: Photo-induced current transient spectroscopy Electrical measurements: CV for carrier concentration Nuclear spectroscopy for charge transport: -particle,  spectroscopy ion-beam-induced-charge microscopy/mapping Outline:

  4. NBEL defect-activated emission Photoluminescence microscopy on a 2mm Pt-contact CdZnTe detector Excitation - 514.5 nm Ar-ion laser Detection- CCD Spectrometer-Renishaw 2000 Laser spot size – 1-8 um Optical images showing local defects Intensity of defect band/ intensity of near band-edge-luminescence indicator of material quality PL emission suppressed at faults

  5. bulk a c interface b Cd Te (b) (c) Te Cd Pt Pt Secondary electron microscopy (SEM) of metal/semiconductor interface in Pt-contact 2mm CdZnTe • Intense defect-activated emission overwhelms the near-band-edge PL near the interface Te rich precipitates (region c)

  6. Determination of alloy composition from PL spectra EPL (eV) = 1.510 +(0.606  0.010) x + (0.139 0.01) x2 Appl. Phys. Lett. 47 1172 (1985) • Zn comp = 5% • FWHM= 34 meV • Zn comp = 11 % • FWHM = 40 meV PL widths indicate good material quality

  7. Photoluminescence mapping of CdZnTe electrode electrode PL spectrum Distance (m) X scan Distance (m) PL intensity • PL intensity variations are a signature of inhomogenities • In CdZnTe a shift in the peak emission line indicates changes in alloy composition • PL intensity drops by a factor of 20 near the electrodes Y scan Distance (m)

  8. Photo-induced current transient spectroscopy of deep-level defects PICTS study of Au-Au contact commercial CdZnTe detector: 10x10x3 mm PICTS identifies deep levels, against strong background signals:

  9. I-V characterisation of Schottky contacts on CdZnTe CdZnTe pad detectors have been fabricated at Surrey with Au-Au and Au-In contacts (5x5x5mm): Schottky contact shows strong rectifying behaviour  enhanced E-field within the bulk

  10. C-V characterisation to determine carrier concentration in p-i-n structure p-i-n device from Au and Indium diffusion, with intrinsic bulk region Region A corresponds to the bulk intrinsic region Region B probes close to the compensated region

  11. 59.6 keV gamma spectra from 241Am Two devices fabricated from the same material - high room temperature leakage current on Au-Au device causes poor spectrum. Schottky device shows good response at relatively low bias voltage.

  12. Alpha particle spectroscopy gives mobility-lifetime products for electrons and holes Irradiation of anode or cathode gives sensitivity to holes or electrons Mobility-lifetime product 5x5x5 mm Pt-contact detector counts CCE (%) energy resolution (%) channel number electric field (V/cm) 3 mm thick : ()e = 3.5x10-4 cm2/V; ()h = 8.6x10-6 cm2/V 5 mm thick: ()e = 9.6x10-4 cm2/V Hecht approximation assumes a uniform E-field and exponential charge distribution

  13. The effect of ‘hole tailing’ in a 5mm thick CdZnTe detector Poor hole transport causes position-dependent charge collection efficiency  ‘hole tailing’ characteristic of higher energy gamma rays in CdZnTe GF Knoll, Radiation Detection and Measurement, Ed. 3

  14. H He E Signal Output _ + Depletion region _ _ + + _ + _ _ _ + + _ _ + + E Signal Output + (X,Y) _ + + + + + 2. Ion Beam Induced Charge(With depletion region) E Signal Output 1. ConventionalRBS/PIXE/Channeling/(NRA) _ + _ _ _ + + + + _ _ _ _ + + + + _ _ + 3. Ion Beam Induced Charge(Without depletion region) Imaging Methods with Ion Beam Analysis High Beam Current Low Beam Current

  15. Ion-beam induced charge microscopy/mapping (a) Planar detector Excitation-2 MeV proton beam focussed to 2 m penetration depth - 37 m detection 2 MeV Detection: Pre- amp amp MCA (b) Pixel detector detection 2 MeV Spatial variation in charge transport related to material inhomogenities and electric-field profiles

  16. CCE profiles in planar 2mm Pt-contact detector 200 V CCE (%) cathode 400 V Position (m) anode cathode

  17. Bias dependence of CCE for interelectrode irradiation of a 2mm Pt-contact detector 400 V -400 V cathode cathode +400 V -400V Pulse height spectra as a function of depth

  18. Comparison of a PL and an IBIC map on 2 mm Pt-contact detector PL map IBIC map

  19. Time resolved analysis of ion beam induced pulses Digitisation and analysis of ion-beam induced pulses in CdZnTe allows separation of electron and hole components. IBIC imaging can then be extended to directly map electron and hole mu-tau products G. Vizkelethy et al, NIM A458 (2001) 563-567

  20. Summary • PL microscopy/mapping is a useful non-invasive room temperature metrology for investigating material homogenity • PL excitation / SEM in a lateral geometry is a useful probe of the metal/semiconductor interface • Carrier mobility-lifetime products: ()= 3.5-9.6 x10-4 cm2/V; • ()h = 8.6x10-6 cm2/V • Ion-beam-induced charge microscopy used to investigate spatial variations in charge transport and material quality. Can be extended to study charge sharing effects in pixel detectors. • Schottky contacts can be fabricated on CdZnTe with enhanced E-field strengths. • Ongoing improvements in both CdZnTe and CdTe material quality continue to extend the performance of these devices.

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