Characterization of undoped high resistivity CdTe grown by a THM method

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Characterization of undoped high resistivity CdTe

grown by a THM method

R. Stuck, J.C. Muller, P. Siffert

To cite this version:

CHARACTERIZATION

OF

UNDOPED

HIGH

RESISTIVITY

CdTe

GROWN

BY

A

THM METHOD

R.

STUCK,

J. C. MULLER and P. SIFFERT

Centre de Recherches

_Nucléaires,

Laboratoire de

_Physique

des

_Rayonnements

et

_{d’Electronique}

_Nucléaire,

67037

_Strasbourg

Cedex,

France

Résumé. 2014 Des

mesures de _temps de vol des _porteurs dans des échantillons de tellurure de

cadmium purs de haute résistivité ont montré la présence du niveau situé à Ev + 0,15 eV,

généra-lement attribué à l’association impureté donatrice-lacune de cadmium. On a _recherché, par

différentes méthodes _{expérimentales,} la nature de cette _impureté.

Abstract. 2014

Using time of flight technique, it is shown that the level at Ev + 0.15 eV, attributed

generally to the association of a dopant impurity and a cadmium vacancy, is present in undoped

materials. Several methods (TSC, SIMS, nuclear activation) have been used to investigate the

nature of this impurity.

1. Introduction. -

During

the _{past years, the}

progress in the

growth techniques

of cadmium

tellu-ride

_crystals

_(CdTe)

allowed to obtain

_{high resistivity}

crystals

of detector

_quality

_grade

without intentionnal

compensation

with chemical

_dopants

like indium or

chlorine

_{[1, 2].}

The

_procedure

used is as follows : in

the first _step, the vertical zone

melting technique

is

used to

_purify

the material. The obtained

_ingot

is then used as source for the

_travelling

heater method

_(THM)

to _grow the

_crystal.

The

_{crystallisation}

_temperatures range from 700-900 OC at

_speeds

of about 7

_mm/day.

These

_crystals

have resistivities between

106-104

Q. c

and detectors

_prepared

with such material exhibit

interesting

properties : good

energy resolution and

absence of

_{polarization.}

Since it is

_generally

believed that

_crystals

of

_nearly

stoechiometric

_composition

cannot _{be grown without}

compensation,

due to the

_{particular shape}

of the

_phase

diagram

of

_CdTe,

it seemed

_interesting

to characterize

these materials in order to determine wether or not

they

are

_compensated

_by

chemical

_impurities.

For this purpose,

thermally

stimulated current

_(TSC)

measurements, time of

_flight

_studies,

_secondary

ion

mass _spectroscopy

_(SIMS)

and activation

_analyses

have been

_performed.

2. TSC measurements. - In this

method,

a diode

is made on the material under

_{investigation.}

The

device is

cooled,

while reverse

biased,

to low tempera-ture and then illuminated with interband

_light,

in order to fill the _{traps present} in the

_crystal.

When the

temperature of the device is increased

_linearly

as a

function of time

_(at

a rate

_03B2),

the

_trapped

carriers are

reemitted at a _temperature which

_depends

on the

activation _energy of the center. This

_technique

was

successfully

applied

to

_compensated

CdTe

_{[3, 4].}

The

experimental

results,

obtained for two different

undoped

crystals

are

_reported

on

_figure

1. Three

_peaks

are

_{clearly resolved,}

located at

_115,

140 and 195 K.

FIG. 1. - TSC

spectra for two different _{undoped crystals.}

For

_comparison,

on

_figure

_2,

we have

plotted

the TSC curves obtained under identical conditions for

chlo-rine

_{compensated crystals}

of different

_origins.

It is

interesting

to notice that these

_spectra

have all the

same _structure, similar to that

_{reported recently [3, 4].}

The same

_peaks

are _present like in

undoped

materials,

186

FIG. 2. - TSC

curves for chlorine _{doped crystals}

FIG. 3. - TSC

spectra for indium _{doped crystals} of various

origin.

however,

two additionnal

_peaks

can be

observed,

located at 100 and 155 K. These results have also been

compared

to those obtained for indium

_compensated

crystals

of different

_origins.

As shown on

_figure

_3,

the

spectra have

_again

the same structure for the various

crystals.

The basic

_peaks

at

_115,

140 and 155 K are present, the maximum at 100 K is _present like in chlo-rine

_{compensated crystals}

and a _strong

_peak

located at 125 K can be seen.

All the

_experimental

results have been summarized in table I. The

_energies

of the

_peaks

have been

esti-TABLE 1

mated

_{by using}

either different classical methods

_[5]

or

_by

a

_{special procedure [3, 6],}

which allowed us to

identify

the _type of carrier which is

_trapped.

The

centers located at

_{Ec -}

0.23

eV,

0.30 eV and

E,

+ 0.40 eV are _present in all

_crystals.

The center

with an activation energy of 0.19 eV is characteristic

of the

_compensation

either

_by

chlorine or

_by

indium.

The 0.25 and 0.35 eV levels seem to be related

respec-tively

to _{the presence} of indium and chlorine.

These results indicate

_clearly

that

_undoped

material contains less

_{deep impurities}

than

_compensated

crys-tals.

Nevertheless,

they

do not allow to see if the

mate-rial is

_compensated

and the level at

_Ev

+ 0.14 eV

cannot be observed in the _temperature range used here.

However,

with the TSC

_technique

it seems

possible

to

determine the nature of the chemical element used for

compensation.

3. Time

of flight technique.

- To

investigate

shallo-wer

levels,

we used the time of

_flight

method

[7]

and

studied the transit across the detector of carriers

generated by

a

_pulsed

electron _gun.

_Figure

4 shows the

evolution of electron and hole

_mobility

as a function

of _temperature. The most

_interesting

result is the _strong

decrease of the hole

_mobility

at low _temperature. Such a behaviour was also

_reported

for

compensated

crystals

and was attributed to a reduction of the hole

mobility

by multiple

trapping

and

_detrapping

on a

deep

level. In our case, the same

interpretation

can be

given

since,

as shown on

figure

_5,

the

mobility

of the

Ftc. 4. - Electron and hole mobility in

undoped crystals as a

function of temperature.

Mc;. 5. - Variation of hole

mobility with electrical field at

various temperature.

experimental

values of the

_mobility

to that

calculated,

assuming

that the

_mobility

in the absence of

_trapping

is

_given

_by

the

_{semi-empirical}

formula of Yamada

_[10]

and

_using

the tridimensional model of Hartke

_[11]

for the Poole-Frenkel effect. A

_good

_agreement

bet-ween

_experimental

and theoretical results could be

obtained for an activation energy of

ET

= 0.138 eV

and a _trap concentration of 1.1 x

1016 cm-3.

The

investigations performed

on other

undoped

crystals

lead to similar results.

According

to the literature

_{[9, 12]}

a level with the

same activation _energy

_{(0.13-0.15 eV)}

has been found in

_{crystals compensated}

with

chlorine,

bromine or

indium,

which is

_generally

attributed to an association

of one Cd _{vacancy with the} donor used for the

com-pensation.

If this

_{interpretation}

is _correct, our results

would

_clearly

indicate that our

_{nominally undoped}

material is in fact

_{compensated by}

traces _{of group III}

or VII elements. Such a contamination _may occur

since

_{undoped crystals}

are

_exposed

to

_higher

tempera-ture than chlorine

_{doped crystals during}

the zone

refining

process.

Hence,

it is not

_unlikely

that the

_crystal

is

_compensated

_by

aluminium from the _quartz tube as

suggested

in an earlier work

_[13],

or

_by

traces of fluorine

_coming

from the

_etching

of the _quartz or even

residual bromine due to the

_etching

of the

_ingots.

However,

a

_completely

different

_hypothesis

cannot

be ruled out : the level at

_Ev

+ _{0.14 eV may be} related

not to be the association between a cadmium _vacancy

and a group III or VII

_donor,

but rather to an intrinsic

defect,

like interstitial tellurium. In this case the

_high

resistivity

of the

_crystals

could

_only

be

_{explained by}

the presence in the material of a

_deep

level fue to

contamination

_by

other chemical elements like

carbon,

for

_{example. Therefore,}

chemical

_analysis

are _necessary

to

_clarify

this

_point.

4. SIMS studies. - An

attempt was made to

characterize

_{undoped crystals by secondary}

ion mass spectrometry

using

a commercial instrument in which

the ions scattered

_by

a bombardment of 3 keV argon ions are

_analysed

with a

_quadrupole

_{spectrometer.}

This

_technique

is

_basically

a surface

_{analysis method,}

but it may also

_give

informations on the bulk if the

crystal

is bombarded for a

long enough

time.

The

_samples

were

_{prepared by polishing}

the surface

with a diamond _spray

_(grade

0.25

p).

The

measure-ments were

_performed

under vacuum with a residual

pressure below

10-8

mm

Hg.

Since the material was of

high resistivity,

it was necessary to bombard the

_sample

during

the measurement with a beam of low _energy

electrons

_{(3 keV)}

in order to avoid the built _up of a

positive

space

charge

due to the

primary

ion beam.

Figures

6 and 7 show

_{typical positive}

and

_negative

ion _spectra taken after

_removing

a

_layer

of 2 000

A

in

thickness

_(at

that

_depth

the concentration of most

elements was found to be

_constant).

Besides Cd and Te

(Cd

does not _appear as a

_{negative ion) only}

a few ions

are detected :

Ar++, Na+,

Mg+,

Al+, Si+, K+,

Ar+

and

_{C-, 0-,}

F-, CN-, Cl-.

_Surprisingly

_{group III} elements

_(aluminium)

and group VII elements

(chlo-rine,

fluorine and sometimes

bromine)

are

_present.

However, one should be careful before

concluding

that these

_impurities

are

responsible

of the

compensa-tion and _{the presence} of the level at

_Ev

+ 0.15

eV,

188

Fia. 6. - Positive ion SIMS

spectrum in _{undoped crystals.}

FIG. 7. -

Negative ion SIMS _spectrum in _{undoped crystals.}

Finally,

experiments performed

on chlorine

_doped

gave

nearly

identical _{spectra ;} this _{may appear} some-what

_{contradictory}

with the informations obtained

_by

TSC but is

_probably

due to the fact that SIMS is a less

sensitive method than TSC.

5. Activation

_analysis.

- To

overcome this

diffi-culty

we have also

_performed

nuclear activation

ana-lysis

with tritons in the MeV range. Table II shows the

TABLE II

Sample

detection limits of several elements in

_undoped

_samples.

These data confirms the results of the SIMS

measu-rements, since both chlorine and aluminium are

pre-sent in most of the

_crystals.

_Furthermore,

sodium and

magnesium

are found like in SIMS

experiments.

6. Conclusion. - The level at

Ev

+ 0.15 eV is

present both in

_undoped

and chlorine

_compensated

crystals.

This means that either our

_crystals

are

com-pensated by

traces of _{group III} or VII

elements,

or

that the level at

_Ev

+ 0.15 eV is not related to the

association

_{vacancy-donor,}

but rather to an intrinsic

defect,

like tellurium. Chemical

_analysis

indicate that

halogens

and aluminium are _present in our

crystals

which,

in

_principle,

_{may be}

_responsible

of the compen-sation.

_However,

the

_slope

of the TSC curves in both

kind of

_crystals

is

_quite

different,

indicating

a different

chemical or

impurity-defect

structure.

Therefore,

fur-ther measurements are

_requested

before a definite

interpretation

can be

_proposed.

Reterences

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

Nature Phys. Sci. 245 (1973) 12.

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

J. Appl. Phys. 45 ₍₁₉₇₄₎ 2759.

[3] SCHARAGER, C., MULLER, J. C., STUCK, R., SIFFERT, P.,

Phys. Stat. Sol. (a) 31 ₍₁₉₇₅₎ 247.

[4] MARTIN, G. M., BELIN, C., NGO-TICH-PHUOC, FABRE, E., FOGARASSY, E., IEEE Trans. Nucl. Sci. NS-23 1 ₍₁₉₇₆₎ 154.

[5] NICHOLAS, K. H. and _{WOODS, J.,} Br. J. Appl. Phys. 15 (1964) 783.

[6] MULLER, J. C., STUCK, R., BERGER, R., SIFFERT, P., Solid State Elec. 17 (1974) 1293.

[7] MARTINI, M., MAYER, J. W., ZANIO, K. _{R., Appl.} Sol. State Science (Ed. R. Wolf) vol. 3, (Academic Press, New

York) 1972.

[8] FRENKEL, J., Phys. Rev. 54 (1938) 647.

[9] OTTAVIANI, G., CANALI, C., JACOBONI, C., ALBERIGI-QUARANTA, A., ZANIO, K. R., J. Appl. Phys. 44 (1973) 360.

[10] YAMADA, S., J. Phys. Soc. Japan 15 (1960) 1940. [11] HARTKE, J. L., J. Appl. Phys. 39 (1968) 4871.

[12] BELL, R. O., WALD, F. V., CANALI, C., NAVA, F., OTTA-VIANI, G., IEEE Trans. Nucl. Sci. NS-21 (1974) 331.

[13] SIFFERT, P., CORNET, A., STUCK, R., TRIBOULET, R.,