Upload
others
View
7
Download
0
Embed Size (px)
Citation preview
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