Charge Drift in partially-depleted epitaxial GaAs detectors

Paul Sellin, Radiation Imaging Group

Charge Drift in partially-depleted epitaxial GaAs detectors

P.J. Sellin, H. El-Abbassi, S. RathDepartment of Physics

University of Surrey, Guildford, UK

J.C. BourgoinLMDH, Université Pierre et Marie Curie, Paris, France


Overview

Chemical reaction growth of thick epitaxial GaAs layers

Depletion thickness and residual impurity concentration

Performance of partially depleted detectors

C-V measurements of impurity concentration at low temperature

Optical probing of charge transport using a focussed laser


Potential challenges for epitaxial GaAs

Strengths of epitaxial GaAs: intermediate photon detection efficiency between Si and

CZT/CdTe metal-semiconductor contacts and device physics are well

understood epitaxial GaAs has low concentrations of native EL2 defect source of highly uniform whole wafer material, compatible with

flip-chip bonding and monolithic electronics

Existing problems: even high purity epitaxial is compensated due to residual

impurities- does not exhibit intrinsic carrier concentrations depletion thickness is severely limited charge carrier lifetimes are reduced


Chemical Reaction growth of thick epitaxial GaAs

Epitaxial GaAs material studied in this work was grown by a Chemical Reaction Method by Jacques Bourgoin (Paris).

• An undoped GaAs wafer is used as the material source, which is decomposed in the presence of high temperature high pressure water vapour to produce volatile species.

•Typically, growth rates of <10 m/hr are used to achieve EL2 concentrations of ~1013 cm-3

L. El Mir, et al, “Compound semiconductor growth by chemical reaction”, Current Topics in Crystal Growth Research 5 (1999) 131-139.


Whole wafer photoluminescence mapping

GaAs material uniformity is characterised using room temperature photo-luminescence mapping - a contact-less, whole wafer technique:

A 25 mW 633 nm HeNe laser is focussed to ~50 m on the wafer

the wafer is mounted on an XY stage, and scanned

PL intensity maps at peak the band edge emission wavelength (870 nm) are acquired


PL maps of GaAs

Photoluminescence mapping clearly shows the uniformity of epitaxial GaAs compared to semi-insulating VGF material:

H. Samic et al., NIM A 487 (2002) 107-112.

Epitaxial GaAs Bulk GaAs


Calculated depletion thickness

This material is nominally 1-5 x 1014 cm-3- corresponds to a 10-20 m depletion thickness @ 30V, and 15-30 m @ 80V

Width of GaAs Space Charge Region vs Reverse Bias Voltage

Reverse bias voltage (V)

0 50 100 150 200

SC

R w

idth

( m

)

0

50

100

150

200

250

0

50

100

150

200

250

N = 5x1012 cm-3

N = 1x1013 cm-3

N = 5x1013 cm-3

N = 1x1014 cm-3

N = 5x1014 cm-3


-particle spectra taken with an applied bias of 30V

Channel no.

0 500 1000 1500 2000 2500

Cou

nts

1

10

100

1000

220C-540CV = 30VV = 80V

Alpha particle spectra

5.48 MeV alpha particles are irradiated through the Schottky (cathode) contact - range in GaAs ~20m.

A peltier cooler controlled the device temperature in the range +25°C to -55°C. Shaping time = 0.5 s.


Alpha particle pulse shapes

Alpha particle pulses at room temperature:

preamplifier

shaping amplifier

time base = 1s per division

slow component

fast component


Alpha particle tracks

An un-collimated alpha particle source produces a characteristic ‘double peak’ pulse height spectrum if the depletion thickness is shallower than the particle

range:


59.5 keV gamma spectra

Depth-dependent CCE produces poorly resolved gamma spectra:

Channel no.

200 400 600 800 1000

log

co

unts

1

10

100

1000

Energy (keV)

0 10 20 30 40 50 60

-15V-30V -50V-70V -90V

T = -50°C


Temperature dependent CV analysis

Allows the doping density ND to be extracted from the gradient of 1/C2 vs V :

dVCdqN

rD )1(

22

0

Voltage(V)

0 5 10 15 20

1/C

2 (F-2

)

0.0

2.0e+20

4.0e+20

6.0e+20

8.0e+20

1.0e+21

1.2e+21

1.4e+21

1.6e+21

220C

100C

-20C

-120C

-210C

-420C

-520C


Depletion Thickness vs Bias Voltage


Impurity Densities

The CV analysis confirm the shallow depletion thicknesses achieved in these devices, and correspond to impurity densities of ~3 x 1013 cm-3 in

sample S16 at low temperature:

Sample Area (mm2) ND (cm-3) Depletionthickness (m)

V = 30V V = 80VT=22C S16 7.1 1.3 x 1014 18 30

S17 3.8 4.3 x 1014 10 16

T=-54C S16 7.1 3.1 x 1013 37 60S17 3.8 2.1 x 1014 14 23


Focussed IR laser scans

Probe the variation in pulse shape as a function of position from the Schottky contact, and

temperature


Scanning optical bench

850nm laser300ns pulse

XY scanning table

cryostat

imaging camera


Laser pulse shapes

T=273K, 20V

At 60m from cathode:

no slow component to signal


charge drift times are ~350s

IR laser spot appears to have significant beam waist


Laser pulse shapes (2)

T=223K, V=90V


no slow component to signal


charge drift times are ~350s

IR laser spot appears to have significant beam waist


T=273K, V=20V

Position from Schottky (m)

0 50 100 150 200 250

Am

plitu

de

0.0

0.2

0.4

0.6

0.8

1.0

Sig

nal R

iset

ime

(s)

0

100

200

300

400

500

T=223K, V=90V

Position from Schottky (m)

0 20 40 60 80 100 120 140 160 180 200 220

Am

plitu

de

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Sig

nal R

iset

ime

(s)

0.0

0.5

1.0

1.5

2.0

Pulse risetime and amplitude vs bias


Interaction close to the anode - inside depletion region


Interaction close to n+ substrate - in low field region


Temperature dependent pulse shapes (1)

Laser pulses 60m from Schottky

Time (s)

-100 0 100 200 300 400 500

Am

plitu

de

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

273K

223K

248K

248K

198K

223K

V = 60V

V = 20V




Time (s)

-100 0 100 200 300 400 500

Am

plitu

de

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

273K

223K

248K

248K198K

223K

V = 60V

V = 20V


Conclusions

The epitaxial GaAs layers studied showed excellent uniformity, and a residual impurity concentration of 1-5 x 1014cm-3

Long electron lifetimes > 300 s were observed in the low field regions - confirms the very low EL2 concentration

Lateral laser scans show: good charge transport in the shallow depleted region long-lived components to the pulse shapes when irradiated close to

n+ substrate - consistent with slow electron diffusion towards the substrate

significant penetration of the depletion region when cooled to -50°C

Future work: further lateral scanning is required with focussed lasers and high

resolution proton microbeams to quantify these phenomena further modest reductions in impurity concentration will produce

significant performance improvements


Acknowledgements

This work was partially funded by the UK’s Engineering and Physics Science Research Council


Alpha particle spectra

Epitaxial GaAs pad detectors were irradiated with an uncollimated 241Am alpha particle source. The detector was mounted in a vacuum cryostat, attached to a peltier cooler to allow operation in the temperature range of +25°C to -55°C. Pulse height spectra (figure 3) were acquired using a conventional charge integrating preamplifier and spectroscopy amplifier (shaping time = 0.5s).

Figure 3: spectra at room temperature, as a function of bias.

S16, -spectra taken at 80V for several temperatures

Channel no.

0 1000 2000 3000

co

unts

1

10

100

1000

10000

Energy deposited (MeV)

0 1 2 3 4 5

-540C

-310C

220C


Interaction in intermediate region



Time (s)

-100 0 100 200 300 400 500

Am

plitu

de

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

273K

223K248K

V = 20V


Time (s)

-100 0 100 200 300 400 500

Am

plitu

de

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

273K

223K248K

248K

198K

223K V = 60V

V = 20V


Charge Drift in partially-depleted epitaxial GaAs detectors

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