Charge transport and mobility mapping in CdTe P.J. Sellin1, A.W. Davies1, A. Lohstroh1, M.E. Özsan1, J. Parkin1, P. Siffert2, M. Sowinska2, A.Simon3,4 1Department

of Physics, University of Surrey, Guildford GU2 7XH, UK 23 Rue du Loess, Strasbourg 67037, France 3Surrey Centre from Ion Beam Applications, University of Surrey, Guildford GU2 7XH, UK 4Institute of Nuclear Research of the Hungarian Academy of Sciences, P.O. Box 51. H-4001 Debrecen, Hungary 2Eurorad,

www.ph.surrey.ac.uk/cnrp

Paul Sellin, Centre for Nuclear and Radiation Physics

Introduction Motivation for this Work: r THM-grown CdTe supplied by Eurorad - investigation of uniformity of: mobility, µτ, and lifetime r What is the role of Te precipitates in degrading signal response? r Pulse shape analysis can identify regions of trapping or reduced mobility r Does CdTe exhibit non-uniformity in the same way as CdZnTe?

Mechanisms for reduced signal amplitude

reduced electron lifetime

r Alpha particle TOF measurements are used to characterise CdTe mobility and µτ as a function of temperature r High resolution ion beam (IBIC) studies map charge transport processes close to precipitates r Digital time-resolved IBIC produces maps of mobility

partial trapping

reduced mobility or field

reduced initial charge

Paulal, Sellin, Centre Nuclear and Radiation Physics M. Amman et JAP 92for(2002) 3198-3206

Electron and Hole Mobility in CdTe Poor signal amplitude and low resolution is caused by low electron and hole mobilitylifetime (µτ) products: Typical mobility and lifetime values for CdTe: µe (300K)

µh (300K)

τe

τh

800-1100 cm2/Vs

60-90 cm2/Vs

~1 µs

~1 µs

Temperature dependent mobility ⇒ µe increases at lower temperature, µh decreases. Scattering mechanisms alone cannot describe the temperature variations – need a trap-controlled mobility model: µ0 – scattering-limited mobility −1

 NT ET    µ = µ 0 1 + exp    kT    NC

ET, NT – trap energy and concentration NC – effective density of states at bend edge

Electron trapping normally associated with a complex defect: 2 Cl donors on Te site + Cd vacancy [VCd 2ClTe ]0 Hole trapping often associated with shallow Cd-vacancies (VCd) and A-Centers (VCddonor complex), acting as single and double acceptors. Paul Sellin, Centre for Nuclear and Radiation Physics

IR microscopy – imaging Te precipitates IR imaging used to identify the distribution of Te precipitates: r are Te precipitates a cause of non-uniform signal response in CdTe, as seen in CdZnTe?

2nd sample shows very low precipitate concentration away from the wafer edges

25mm diameter CdTe wafer, scribed with locating grid lines prior to metal deposition

Paul Sellin, Centre for Nuclear and Radiation Physics

IR imaging and X-ray topography Lang X-ray topography is an imaging method that identifies changes in the nearsurface crystalline orientation – scribed sample allows correlation to IR image:

IR scribed image goes here

IR image

Lang X-ray topograph

Paul Sellin, Centre for Nuclear and Radiation Physics

Alpha particle electron and hole µτ in CdTe 0.9

Alpha particle µτe and µτh data were obtained as a function of temperature: r

CCE pulse height spectra at various bias voltages

-4

2

-1

e e

the single-carrier Hecht equation: 2    − d   µτ V Q ( x) µ eτ eV  CCE = 1 − e e e   = 2  Q0 d   

0.5

Electron Mu-tau fig at single temp goes µ τ = 3.75*10 cm Vhere

0.7

CCE

r

0.8

0.6

0.5

Electrons @ 295K: µτe = 7.8 x 10-4 cm2/V

0.4

T=296K

0.3

Holes at 295K: µ τ = 3.49*10 cm V µτh = 7.3 x 10-5 cm2/V -5

0.4

2

5

-1

10

15

20

25

30

35

40

Voltage (V)

h h

• Sample thickness 1.3mm CCE

0.3

• Electron and hole drift times