Methods to suppress polarization in chlorine compensated cadmium telluride detectors

HAL Id: jpa-00244168

https://hal.archives-ouvertes.fr/jpa-00244168

Submitted on 1 Jan 1977

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of

sci-entific research documents, whether they are

pub-lished or not. The documents may come from

teaching and research institutions in France or

abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est

destinée au dépôt et à la diffusion de documents

scientifiques de niveau recherche, publiés ou non,

émanant des établissements d’enseignement et de

recherche français ou étrangers, des laboratoires

publics ou privés.

Methods to suppress polarization in chlorine

compensated cadmium telluride detectors

P. Siffert, M. Hage-Ali, R. Stuck, A. Cornet

To cite this version:

METHODS

TO SUPPRESS POLARIZATION

IN

CHLORINE

COMPENSATED

CADMIUM TELLURIDE DETECTORS

P.

_SIFFERT,

M.

_HAGE-ALI,

R. STUCK and A. CORNET Centre de Recherches

Nucléaires,

Laboratoire de

_Physique

des

_Rayonnements

et

_{d’Electronique Nucléaire,}

67037

_{Strasbourg-Cedex,}

France

Résumé. 2014 Dans _une

première partie de ce travail on passe en revue les différents modèles de

polarisation publiés dans la littérature. _Puis, on étudie les caractéristiques essentielles du centre

profond responsable de cet effet. _Finalement, trois méthodes sont présentées qui permettent de

s’affranchir de la polarisation : oxydation de la surface _{préalablement décapée chimiquement} dans

une solution de perhydrol puis dépôt de l’électrode _{conductrice, évaporation} d’un film de SiOx, implantation

ionique.

Abstract. 2014 In _a first

part of this paper, the different models of polarization developped in the

literature are _{critically analyzed. Then,} the origin of the responsible center, its location within

the _bandgap and its concentration is _investigated and discussed. Finally, three methods to suppress

this effect are _{presented, mainly} surface oxidation in perhydrol of the etched _{sample prior} to the

metal _{deposition, evaporation} of a thin SiOx _layer or ion _{implantation.}

Introduction. - It is well known that in nuclear

radiation detectors

_prepared

from

_insulating

material

appears a

_polarization

_effect,

which is characterized

_by

a

_progressive

decrease of both

_pulse

amplitude

and

counting

rate with time after the bias

_voltage

is switch-ed on. This effect was

_responsible

of the _poor

perfor-mance of the earliest

(1945-50)

solid state detectors

(see

references

_quoted

in

_[1]).

At that

time,

the exact

mechanism was not

_completely

understood and no

effective

_procedure

was found to overcome this effect. The

_development

of the silicon

_junction

_type nuclear radiation detector

_considerably

reduced the interest on

insulating

materials,

only

the diamond detectors continued to be

_{investgated [2].}

More

_recently,

the

possibility

to use cadmium telluride in

_{manufacturing}

room _{temperature y-ray spectrometers} has been

considered and the

_travelling

heater method

_(THM)

has allowed the

_growth

of

_{high quality}

_p-type

_crystals,

in which the

_{requested high resistivity (-}

108

_Q.cm)

is achieved

_by

means of chlorine

_{compensation.}

However,

the nuclear radiation detectors

_prepared

with this material show the

_typical

decrease of

_pulse

amplitude

and

_counting

rate with time. Several models have been

_{proposed recently}

to

_explain

the

behaviour of

_CdTe(Cl)

counters. These models will first be reviewed

_critically,

then we will describe the

three methods we

_developped

to _suppress

_{polarization.}

1. Models. - To

explain

the time

_dependent

beha-viour observed in these counters, it is necessary to

consider,

as _{done years ago} for

_crystal

counters

_by

Hofstadter

_[3]

that the electric field within the

detec-tor

_{progressively changes, leading}

to the creation of a

region

of _poor

_charge

collection. Several _groups

_{[4, 1 ]}

REVUE DE PHYSIQUE APPLIQUÉE. - T. 12, N° 2, FÉVRIER 1977

have verified that in

_CdTe(Cl)

detectors the initial

constant field within the counter

_(identical

to that

existing

in an

n-i-p structure)

progressively

increases at

the

_positive

biased electrode and diminished at the

opposite

side,

its

_shape

becomes identical to that of an

n-p junction.

This modification in the field distribution

results from a

_change

of the net

_charge

carrier concen-tration

_(by

_trapping

or

detrapping)

due to the

pro-gressive

evolution of the _occupancy of a

_deep

level located either in the bulk or in close surface

_vicinity

of the device.

2. Characteristics of the

_polarization

level. - The

main characteristics have been established

_by

several groups,

using

quite

different

_techniques,

as shown on

table I.

However,

the

_physical

_origin

of the center is

not

_clearly

established

_today.

Since

_polarization

_only

occurs in THM grown

crystals

when chlorine is

TABLE 1

Characteristics

_of

the

_deep

level

responsible for polarization

336

present

[5,

6J,

the

_{simpliest hypothesis}

would be that this

_halogen

introduces the

_deep

level. In

_fact,

chlorine introduces

_only

a shallow level located at

Ec 2013

0.014

_eV,

its concentration in the

_crystals

is

generally

close to 1017 cm-3. No

_study

has been

published

until now about an eventual correlation

between the concentration of chlorine and the amount

of

_{polarization,}

but luminescence measurements

performed

at 4.2 K indicated that the

_intensity

of the line at 1.42 eV

_{(which corresponds}

to a level located

at

_E,

+ 0.14

_eV)

is

_directly

_{proportionnal}

to the

importance

of

_{polarization [1].}

This

level,

which is

generally

assumed to be due to the

_{[Vcd Cl] - complex}

is too shallow to be

_responsible

of the

_effect,

but its

concentration is

_directly

correlated to that of

_{[Vcd]- -,}

1

which introduces an _acceptor level close to the

_midgap.

The effect of chlorine introduction

_{doping during}

the THM _{process is} a _strong enhancement of

1 V,,d 1

-from about

107

to

1012

cm-3 as shown

_by

our

investi-gation

of the

_compensation

in CdTe

_[7].

Therefore,

chlorine

_{plays only}

an indirect role on

_{polarization.}

The concentration of the

_midgap

level becomes sufficient to contribute

_noticeably

to the total

_charge

concentration.

_Following

this

_model,

_polarization

does

not

_just

result from an increase in

resistivity by

compensation

as often assumed.

3. How does this level become active. - The models

proposed

can be classified into two

_categories

depend-ing

whenever

_they

consider that

_polarization

results

from bulk or surface effects.

3.1 BULK. -

Following

Malm et al.

_[4]

when the

bias

_voltage

is switched on, the electric field sweeps

the free carriers out of the interelectrode

_region,

as in

all

detectors,

causing

a _strong diminution of the

charge

carrier

_density.

The

_deep

_acceptor level has to

adjust slowly

its ionization

_probability

to

_correspond

to the lower hole

concentration,

this is achieved

_by

emission

_(detrapping)

of holes which are bound to

deep

acceptors under zero field condition.

_Therefore,

the

_polarization

results from the time needed to

achieve a new

_equilibrium.

At

_equilibrium,

the fraction

f

of ionized

_deep

_acceptor is

_{expressed by :}

where uv is the

_acceptor

_{capture coefficient} for

_holes,

p the concentration of holes and 03C4D the

detrapping

time. The

_{general expression}

of the ionization

proba-bility

of a

_deep

level located at

_{ET, in a}

_space

_charge

region

has been calculated

_{by Shockley}

and Read

_[8]

and is

_{given by :}

in which the

_symbols

have their

_general

_meaning.

Malm et al. considered also the

_possibility

that

polarization

may result from the capture

(trapping)

of

free

carriers,

but

_they

ruled out this

_hypothesis

after

they

performed

an

_experiment

in which

_they

show that

the source

_strength

has no influence on the amount of

polarization.

In a recent _paper

_[1

_]

we

_suggested

that

trapping

can occur.

_Indeed,

we demonstrated that the

carriers

_{participating}

to the

_polarization

are those due to

the space

charge generated

current rather than those

created

_by

the

radiations,

which are,

_{generally, only}

a

negligeable

function of the total

_charge

_carriers,

even

with strong sources. For

example,

a 10 nA

_leakage

current

_corresponds

_{approximatively}

to 101°

_carriers/

sec and a source of 1

pC

strength

of 100 keV

(4

n)

generates less than

109

_{carriers/s. Furthermore,}

the

high

activation energy may

correspond

to the barrier

height

of a

_{negatively charged (repulsive)}

level to

capture a second electron. This idea is confirmed

_by

the fact that the _capture cross section of the level

( N

10-17

cm2)

is close to that

_generally

observed on

repulsive

centers.

It should be mentionned that this bulk model

includes, in

fact,

strong

surface

effects,

since the

concen-tration of holes in relation

₍₁₎

includes thoses

_injected

through

the

_positive

biased contact. When p is

stron-gly

enhanced

_{(strong injection-low}

barrier

_height)

the

fraction of ionized

_deep

_acceptors diminishes and

polarization

vanishes,

as

experimentally

verified

[9, 1].

3.2 SURFACE. - Bell et al.

[9] proposed

that

pola-rization results from the ionization

_{of deep}

_acceptors in the surface

_vicinity

due to band

_bending,

the level at

ET crossing

the Fermi level. Due to the substantial activation energy which is

required,

the

deep

acceptor can

_only

become ionized with a characteristic time

constant. This model

_explains

several observed effects

but fails to show how the

_counting

rate can become

negligeable

in some detectors. Under flat band

condi-tions,

no

_polarization

would appear.

In our

_opinion

_{[1 ] the major}

_parameter in

polariza-tion is constituted

_by

the

_{charge injection through}

the

positively

biased electrode which modifies the location of the Fermi level. Since _strong hole

_injection

is not

compatible

with a low noise

_operation

of the

_detector,

solutions have to be found to control the rate of carrier

injection.

4.

_Suppression

of

polarization.

- The

general

idea in the research of methods to _suppress

_polarization

was to create a small concentration of electrons in the

vicinity

of the

_positively

biased contact which is sufficient to recombine with the ionized _acceptors but which does not affect the detectors

_operation.

Bell et al.

_[10]

have shown that

_{shining strongly}

absorbed

light

on the

positively

biased contact creates sufficient electrons to overcome

_{polarization.}

An ideal MIS

structure on a

_P-type

_material,

when

_positively

biased would also create an excess electron

_density

close to the

FIG. 1. - Reverse characteristic of both M-S and MIS CdTe

detectors.

voltage

is

_applied

the bands bend downwards and the

majority

carriers are _swept out of the zone

_(depletion

structure).

When the

_{positive voltage}

is further

increas-ed,

the bands bend even more, so that the intrinsic

level

_E;

crosses the Fermi

level,

so the number of

minority

carriers

_(electrons)

in the surface

_vicinity

is

larger

than that of holes

_(inversion

_{configuration).}

However,

under ideal MIS

conditions,

no carrier flow

is

_possible

and the

_charges

accumulated

_{progressively}

diminish the electric field in the

_depletion

zone, as in

the structure

_{developped by Eichinger [11, 12].}

In a

real

_integrated

MIS structure some carrier flow is

possible through

the

_{insulating layer.}

If the later is

sufficiently

thin,

charge tunneling

becomes

_possible.

Therefore,

we

investigated

the

possibility

to realize

structures in which some current flow is

_possible.

Three

different methods have been considered on

bromine-methanol etched

_samples.

4.1 SILICON OXIDE LAYER. - Several methods have been

_proposed

to

_{deposit SiO.,}

films

_by

vacuum

evaporation

of SiO. The characteristics of the film

strongly

depend

on the

speed

of

evaporation,

the

residual _{pressure, the} conditions of

_{forming [13].}

The exact mechanism of carrier _transport

_through

these films is not well

known ;

the

_simplest

model

_[14]

considers that some conductive filaments exist

through

the oxide

_{layer through}

which some

_charge

flow is

possible.

Here,

silicon monoxide was

evaporated

with an

electron _gun under a residual pressure of about

10-’

torr at moderate

_{speed (1}

_Â/s)

on the

_sample

at

thick-ness _up to 100

Á.

Under these

_experimental

conditions,

the oxide

_composition,

as deduced from

backscatter-ing

measurements, is about

_SiO1.8.

The MIS structure

is

_finally

achieved

_{by depositing}

either a

_gold

or an

aluminium electrode.

When used as nuclear

detectors,

voltages

_up to

700 V could be

_applied

without any excess noise.

However,

the _{process is} not _yet

_fully

controlled and it appears that more work is

_requested

to

_fully

control the

_charge

flow

_through

the oxide.

_Furthermore,

the

trapping

center

_density

at the oxide-semiconductor

contact has to be

_carefully

controlled to avoid _any

charge

loss on these centers.

5.

_Boiling

in

_{hydrogen peroxide.}

-= By

boiling

the

etched

_{samples during}

a few minutes in

_H202

an

oxidized surface is

formed,

which is rich in

_cadmium,

as shown

_by

SIMS measurements. It is

_probable

that a

semiconductor

_layer

similar to CdO is

_produced.

The

charge

flow may occur either

_by

_hopping

or

Schottky

emission over the

_{interfacial layer.}

As shown on

_figure

_2,

1

PIG. 2. - Energy band diagrams for ideal MIS structures at

low _(a) and _{high (b)} bias _voltage.

this

_procedure

leads to a _{very strong} reduction in

diode

_leakage

current and enhancement of the

break-down

_voltage.

5.1 ION IMPLANTATION. - The bombardment of

compound

semiconductor

_by

_heavy

ions

_(or

even

protons) produces

radiation

_damage

centers which

compensate the carriers

_existing

_previously

in the material.

_Contrarily

to the

predictions

of the theoretical

models,

the

_damage

extends munch

_deeper

into

the material that the range of the

implanted

ions.

Therefore,

by

a _proper choice of

dopant’s

_energy and

dosis it is

_possible

to

_{perform by}

ion

_implantation

into

CdTe both an

_{insulating layer}

covered

_by

a

heavily

doped

surface film. It has been found that

_implantation

of phosphorous,

copper,

gold

or aluminium at

energies

ranging

from 20 to 60 keV and doses around

1014 cm-2

followed

_by

an

_annealing

around 200 OC gave the

338

6. Conclusion. - The results

presented

in this paper show that the

problem

of

polarization

on

chlorine

_doped

THM cadmium telluride

_crystals

can be solved

_by

a correct choice of the surface

treat-ments.

_Therefore,

it becomes now

_possible

to realize

small

_high

_performance

nuclear counters on these

materials.

However,

to

_fully

use the

_{possibilities}

of

CdTe it is still _{necessary to}

_improve

the

_homogeneity

of the THM

_crystals,

in order to increase the active volumes.

References

[1] SIFFERT, P., BERGER, R., SCHARAGER, C., CORNET, A., STUCK, R., IEEE Trans. Nucl. Sci. NS 23 (1976) 159.

[2] KOZLOV, S. R., STUCK, R., HAGE-ALI, M., SIFFERT, P.,

IEEE Trans. Nucl. Sci. NS 22 (1975) 160. [3] HOFSTADTER, R., Nucleonics _2, p. 29 (1949).

[4] MALM, H. L., MARTINI, M., Can. J. Phys. 51 _{(1973) 2336,} id. IEEE Trans. Nucl. Sci. NS 21 (1974) 322.

[5] TRIBOULET, R., CORNET, A., MARFAING, Y., SIFFERT, P.,

J. _{Appl. Phys.} 45 (1974) 2759, id. Nature Phys. Sci. 245 (1973) 12.

[6] ZANIO, K., KRAJENBRINK, F., MONTANO, H., IEEE Trans.

Nucl. Sci. NS 21 (1974) 315.

[7] STUCK, R., CORNET, A., SCHARAGER, C., SIFFERT, P. J. Phys. Chem. Solids 37 ₍₁₉₇₆₎ 989.

[8] SHOCKLEY, W., READ, W. T., Phys. Rev. 87 (1952) 835.

[9] BELL, R. O., ENTINE, G., SERREZE, H. B., Nucl. Instrum.

Meth. 117 ₍₁₉₇₄₎ 267.

[10] BELL, R. O., WALD, F. V., GOLDNER, R. B., IEEE Trans.

Nucl. Sci. NS 22 ₍₁₉₇₅₎ 241.

[11] EICHINGER, P., KALLMANN, H., Z. Naturforsch 28a (1973) 1888, id. Appl. Phys. Lett. 25 (1974) 676.

[12] EICHINGER, P., HALDER, N., KEMMER, J., Nucl. Instrum.

Meth. 117 ₍₁₉₇₄₎ 305.

[13] HOWES, J. H., AwcocK, M. L., Report Harwell AERE-R 6464 (1970).

[14] DEARNALEY, G., Phys. Lett. 25A (1967) 760.

[15] FABRE, E., NGO-TICH-PHUOC, MARTIN, G. M., ORTEGA. F.,

IEEE Trans. Nucl. Sci. NS 23 (1976) 182.