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.

CdZnTe material issues: 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

Outline: 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

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

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

Determination of alloy composition from PL spectra Zn comp = 5% FWHM= 34 meV Zn comp = 11 % FWHM = 40 meV PL widths indicate good material quality E PL (eV) = (0.606  0.010) x + (0.139  0.01) x 2 Appl. Phys. Lett (1985)

Photoluminescence mapping of CdZnTe Distance (  m) PL intensity Distance (  m) 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 electrode Y scan X scan PL spectrum

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

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

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

59.6 keV gamma spectra from 241 Am 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.

3 mm thick : (  ) e = 3.5x10 -4 cm 2 /V; (  ) h = 8.6x10 -6 cm 2 /V 5 mm thick: (  ) e = 9.6x10 -4 cm 2 /V Mobility-lifetime product Irradiation of anode or cathode gives sensitivity to holes or electrons 5x5x5 mm Pt-contact detector electric field (V/cm) energy resolution (%) channel number CCE (%) counts Alpha particle spectroscopy gives mobility- lifetime products for electrons and holes Hecht approximation assumes a uniform E-field and exponential charge distribution

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

Imaging Methods with Ion Beam Analysis High Beam CurrentLow Beam Current _ H He _ _ _ _ _ + _ _ _ _ E Signal Output Depletion region _ _ _ _ + E Signal Output _ _ _ _ _ _ Ion Beam Induced Charge (With depletion region) 3. Ion Beam Induced Charge (Without depletion region) E Signal Output 1. Conventional RBS/PIXE/Channelin g/(NRA) (X,Y)

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

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

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

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

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)

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: (  )= x10 -4 cm 2 /V; (  ) h = 8.6x10 -6 cm 2 /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.