Paul Sellin, Centre for Nuclear and Radiation Physics

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, UK2Eurorad, 23 Rue du Loess, Strasbourg 67037, France

3Surrey Centre from Ion Beam Applications, University of Surrey, Guildford GU2 7XH, UK4Institute of Nuclear Research of the Hungarian Academy of Sciences, P.O. Box 51. H-4001

Debrecen, Hungary

www.ph.surrey.ac.uk/cnrp

Paul Sellin, Centre for Nuclear and Radiation PhysicsM. Amman et al, JAP 92 (2002) 3198-3206

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?

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

reduced electron lifetime

reduced mobility or field

partial trapping

reduced initial charge

Mechanisms for reduced signal amplitude

Paul Sellin, Centre for Nuclear and Radiation Physics

Typical mobility and lifetime values for CdTe:

µe (300K) µh (300K) τe τh800-1100 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:

Electron trapping normally associated with a complex defect: 2 Cl donors on Te site + Cd vacancy

Hole trapping often associated with shallow Cd-vacancies (VCd) and A-Centers (VCd-donor complex), acting as single and double acceptors.

Electron and Hole Mobility in CdTe

Poor signal amplitude and low resolution is caused by low electron and hole mobility-lifetime (µτ) products:

1

0 exp1−

+=

kTE

NN T

C

Tµµ

µ0 – scattering-limited mobility

ET, NT – trap energy and concentration

NC – effective density of states at bend edge

[ ]02 TeCd ClV

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?

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

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

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 near-surface crystalline orientation – scribed sample allows correlation to IR image:

IR scribed image goes here

Lang X-ray topographIR image

Paul Sellin, Centre for Nuclear and Radiation Physics

Voltage (V)

0 20 40 60 80 100 120

CC

E

0.0

0.1

0.2

0.3

0.4

0.5

µhτh = 3.49*10-5 cm2V-1

Alpha particle electron and hole µτ in CdTe

• Sample thickness 1.3mm

• Electron and hole drift times

Paul Sellin, Centre for Nuclear and Radiation Physics

Electron and Hole mobility vs Temperature

Alpha particle time-of-flight (TOF) was used to measure electron and hole drift mobility in our sample.

Drift mobility µ is calculated from the carrier drift time tdr :

where d is the device thickness, v is the drift velocitydrtV

dEv 2

==µ

Temperature (K)

180 200 220 240 260 280 300

Mob

ility

(cm

2 /V

s)

0

20

40

60

80

100

120

Temperature (K)

180 200 220 240 260

Mob

ility

(cm

2 /V

s)

1000

1050

1100

1150

1200

1250

1300

1350

1400

Electron drift mobility vs T

Hole drift mobility vs T

Paul Sellin, Centre for Nuclear and Radiation Physics

Hole mobility has exponential decrease at temperatures below 220 K.

Trap-controlled mobility gives: ET=140 meV above the VB, NT=3.3x1015 cm-3

Consistent with A-Centres

Drift mobility in Acrorad CdTe

Similar data previously reported for Acrorad THM-grown CdTe: K. Suzuki et al, Trans Nucl Sci 49 (2002) 1287-1291

Electron mobility saturates at low temperature – maximum µe=2000 cm2/Vs

Trap-controlled mobility gives ET=28 meV below CB, NT=1x1016 cm-3

Consistent withcomplex[ ]02 TeCd ClV

Paul Sellin, Centre for Nuclear and Radiation Physics

µτ and lifetime

Knowing µτ and µ, the effective carrier lifetime τ can be calculated:

µτ

lifetime τ

Electron lifetime is 0.6 – 0.8 µs, approximately constant with temperatureDrift time (1.3mm thick sample) ~250 ns @ 50V

µτ

lifetime τ

Hole lifetime is 0.7 µs at RT, increasing slightly at low temperature – similar to hole drift time ⇒ Lifetime limits hole drift lengths at typical device field strengths

Paul Sellin, Centre for Nuclear and Radiation Physics

Digital IBIC - ion beam TOF mapping

amplitude

risetime – carrier drift time

Time Resolved IBIC for TOF mapping:

- A fast waveform digitiser acquires pulse shapes on an event-by-event basis:

⇒ 200 samples per pulse @ >1.5 kHz event rate

⇒ waveform sampling down to 1ns per point

- Quantitative images of carrier drift time, mobility, and lifetime.

IBIC uses the nuclear microbeam at the Surrey Tandetron accelerator:

- 2 MeV protons or alpha particles:- high spatial resolution (

Paul Sellin, Centre for Nuclear and Radiation Physics

Proton IBIC imaging

Energy (keV)

1000 1200 1400 1600 1800 2000 2200 2400

Cou

nts

0

200

400

600

800

1000

1200

1400

-7V

-14V -28V

-34V

-21V

2.05 MeV protonsincident on cathode

Ion Beam Induced Charge (IBIC) imaging was performed with a 2.05 MeV scanning proton beam, focussed to

Paul Sellin, Centre for Nuclear and Radiation Physics

Uniformity of electron µτ in CdTe

Electron µτ Map of electron µτe in CdTe shows µτe ~ 4.2x10-4 cm2/Vs at T=295K

Good uniformity across the majority of the area

Local regions show “25% reduction in µτ“- may be due to reduced field or reduced µτ

⇒ Requires pulse shape information

Horizontal linescan at Y=101

A A

Section A-A

Paul Sellin, Centre for Nuclear and Radiation Physics

Time resolved IBIC maps

Time resolved IBIC captures complete pulse shapes for every pixel:

⇒ maps of electron drift time

No sign of ‘slow’ pulses in the precipitate region

Drift time ~150 ns at -50V

Paul Sellin, Centre for Nuclear and Radiation Physics

Uniformity of electron mobility

Second CdTe sample, 8mm thick:

Amplitude – electron signal

Edge region – lower amplitude

V = 150V, d = 8mm, ⇒ µe ~ 850 cm2/Vs

Electron mobility

Paul Sellin, Centre for Nuclear and Radiation Physics

250 µm

Zoom into precipitate region

Pulses from the precipitate region:

Pulses from a ‘normal’ region:

Drift times remain unaffected: cannot be reduced µ or E.

Pulse shapes require further study – at higher waveform resolution

Extract amplitude and risetime data from the same data set

Paul Sellin, Centre for Nuclear and Radiation Physics

Conclusions

r Temperature dependent alpha particle TOF measurements show a defect-limited mobility for electrons and holes in CdTe

r THM-grown CdTe has a very low concentration of bulk precipitates, except for regions close to the wafer edge

r Hole drift lengths are severely restricted, due to the short hole lifetime and low mobility

r IBIC µτ maps demonstrate excellent uniformity of electron transport over a micrometer length scale

r Time resolved IBIC used to produce maps of electron mobility, showing good uniformity

r Bulk precipitates reduce µτ is an analagous way to CdZnTe -further work is required to identify the exact trapping mechanism

Paul Sellin, Centre for Nuclear and Radiation Physics