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. OzsanDepartment 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 ofnear band-edge-luminescence indicator of material quality

1.3 1.4 1.5 1.6 1.7energy (eV)

PL in

tens

ity (

arb.

uni

ts)

Excitation - 514.5 nm Ar-ion laserDetection- CCDSpectrometer-Renishaw 2000Laser spot size – 1-8 um

Optical images showing local defects

PL emission suppressed at faults

1.25 1.30 1.35 1.40 1.45 1.50 1.55 1.60 1.65 1.70 1.75

PL in

tens

ity (a

rb. u

nits

)

Energy (eV)

defect-activatedemission

NBEL

Secondary electron microscopy (SEM) of metal/semiconductor interface in Pt-contact 2mm CdZnTe

bulk

interface

a

b

c

Te rich precipitates (region c)

PtPt

Cd

Cd TeTe (b) (c)

1.3 1.4 1.5 1.6 1.7 1.8

(e) 25 m(d) 20 m

(c) 15 m(b) 10 m

(a) 5 m

PL in

tens

ity (

arb.

uni

ts)

Energy (eV)

•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

1.50 1.55 1.60 1.65 1.70 1.75

1.568 eV1.542 eV

PL in

tens

ity (

arb.

uni

ts)

Energy (eV)

PL widths indicate good material quality

Fractional Zn composition

0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20

Pea

k em

issi

on e

nerg

y (e

V)

1.52

1.54

1.56

1.58

1.60

1.62

1.64

1.66

EPL (eV) = 1.510 +(0.606 0.010) x + (0.139 0.01) x2

Appl. Phys. Lett. 47 1172 (1985)

Photoluminescence mapping of CdZnTe

Distance (m)

PL

inte

nsi t y Distance (m)

Dis

tanc

e (

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 electrode

Y scan

X scan

PL spectrum

Photo-induced current transient spectroscopy of deep-level defects

PICTS identifies deep levels, against strong background signals:

100 150 200 250 3000.010

0.015

0.020

0.025

0.030

0.035

0.040

0.045

0.050

0.055

E

A CZT, pqr16_20V

tref

= 2.89 msec, en= 346 sec

-1

D

C

B

temperature (K)

PIC

TS s

igna

l pqr16_20V

1000/T (K-1)

4 5 6 7

T2 t re

f (K

2 sec)

b[0]1.9240264323b[1]0.4868849588r ²0.9761074883

Peak BEA= 0.097 eV

b[0]-3.4706345338b[1]2.2498398787r ²0.9786526981

Peak DEa=0.47 eV

b[0]-4.2281824569b[1]2.0283617667r ²0.9595388354

Peak CEa=0.40 eV

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):

I-V CHARACTERISTICS OF CZT DEVICES

-5.00E-08

0.00E+00

5.00E-08

1.00E-07

-300 -200 -100 0 100 200 300

APPLIED BIAS (Volts)

CU

RR

ENT

(Am

ps) Au - CZT - Au

Au - CZT - In

C-V characterisation to determine carrier concentration in p-i-n structure

1/C^2 vs. V CURVE FOR Au-CZT-In SCHOTTKY DIODE (ILLUMINATED)

9.80E+22

1.02E+23

1.06E+23

1.10E+23

1.14E+23

-10.0 -8.0 -6.0 -4.0 -2.0 0.0 2.0

APPLIED BIAS (Volts)

1/C

^2

, ( F

^-2

, cm

^4

)

N = (2/qεs) {-dV/d(C^-2)}

(A)

N(A) = 1.6x10^12 cm^-3N(B) = 2.3x10^11 cm^-3

(B)

Region A corresponds to the bulk intrinsic regionRegion 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 241AmTwo 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.

2d

Veτeμdx1

exp12d

Veτeμ

0QQCCE

3 mm thick : ()e = 3.5x10-4 cm2/V; ()h = 8.6x10-6 cm2/V

5 mm thick: ()e = 9.6x10-4 cm2/V

Mobility-lifetime productIrradiation of anode or cathode gives sensitivity to holes or electrons

5x5x5 mmPt-contactdetector

electric field (V/cm)0 400 800 1200 1600 2000

CC

E (

%)

0

10

20

30

40

50

60

70

80

90

ener

gy r

esol

utio

n (%

)

0

2

4

6

8

10

electric field (V/cm)

ener

gy r

esol

utio

n (%

)

channel numberC

CE

(%

)

coun

tsAlpha 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 Current Low Beam Current

_

H He

+

++

+++

___

__

+_ ++

+__

_

E Signal Output

++++

Depletion region

++

++

+

__ __ +

E Signal Output

_

___

__+

++

+

2. Ion Beam Induced Charge(With depletion region)

3. Ion Beam Induced Charge(Without depletion region)

E Signal Output

1. ConventionalRBS/PIXE/Channelin

g/(NRA)

(X,Y)

Ion-beam induced charge microscopy/mapping

Excitation-2 MeV proton beam focussed to 2 mpenetration depth - 37 m

MCA

(a) Planar detector

(b) Pixel detector

detection

detection

2 MeV

2 MeV

Detection: Pre-amp amp

Spatial variation in charge transportrelated to material inhomogenities and electric-field profiles

CCE profiles in planar 2mm Pt-contact detector

200 V

400 V

Position (m)

CC

E (

%)

cath

ode

cathode anode

Bias dependence of CCE for interelectrode irradiation of a 2mm Pt-contact detector

400 V -400 V

cath

ode cat hod e

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 map IBIC 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) 563-567

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.