Enhancement of the superconducting transition temperatures in ion-implanted aluminium alloys

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Submitted on 1 Jan 1976

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Enhancement of the superconducting transition

temperatures in ion-implanted aluminium alloys

A.M. Lamoise, J. Chaumont, F. Lalu, Frédéric Meunier, H. Bernas

To cite this version:

L-287

ENHANCEMENT

OF THE

SUPERCONDUCTING

TRANSITION TEMPERATURES

IN

ION-IMPLANTED ALUMINIUM ALLOYS

_(*)

A. M.

LAMOISE,

J. CHAUMONT and F. LALU

Laboratoire René-Bernas du Centre de

Spectrométrie

Nucléaire et de

_{Spectrométrie}

de

_Masse,

91406

Orsay,

France F. MEUNIER

Laboratoire de

_Physique

des Solides

_(**),

Université

_Paris-Sud,

91405

_Orsay,

France

and

H. BERNAS

Institut de

_Physique

Nucléaire,

Université

Paris-Sud,

91406

_Orsay,

France

(Re~~u

le

_{2 juillet}

1976,

le

15

_{septembre 1976)}

Résumé. 2014 La

température critique Tc d’alliages Al-C, d’Al-Ge et _{Al-Si, préparés par}

implan-tation ionique à très basse température atteint 4,2 K _{(C), 7,35} K _(Ge) et _8,35 K (Si). Cette dernière valeur de _Tc est la _plus forte valeur obtenue à _partir de l’aluminium. Toutes ces valeurs de _Tc

corres-pondent à des concentrations d’éléments covalents de l’ordre de 25 _%. Le désordre ne _peut, à lui _seul,

expliquer ces _{augmentations} de _{températures critiques.} Abstract. 2014 Random

alloys of C, Si, and Ge with Al, prepared by ion _implantation at _liquid

helium temperatures, have maximum superconducting transition _{temperatures Tc} of 4.2 K _(C), 7.35 K _(Ge) and 8.35 K _(Si). The latter value is the _{highest yet} obtained for an Al-based _alloy. All

these values are obtained for covalent element concentrations of ~ 25 _%. The effect of lattice disorder

is not found sufficient to account for the observed _Tc enhancements.

LE JOURNAL DE PHYSIQUE - LETTRES

TOME _37, NOVEMBRE _1976,

Classification Physics Abstracts

7.188 - 8.362

One of the crucial

_problems

in

_{superconductivity}

relates to the mechanisms

_by

which the

superconduct-ing

transition _temperature

_(Td

of a metal may be

modified. It is well known that

_alloying

and

_disordering

may

produce

drastic variations in

_Tc.

The latter effect

has been often studied on thin films

_{produced by}

evaporation techniques [1-3]

or

_by

_{sputtering [4].}

In

_{superconductors}

such as

_Al,

_large

increases in

_Tc

have been found for films

_produced

_by

quench-condensation

₍₈₀

K or 4.2

_K)

of the metal with

non-metals such as

_{O2, H2,}

rare _gases, Ge or Si on cold substrates

_{[2, 3, 5].}

For _pure bulk

Al,

7~

= 1.2

K ;

typical

values found in this _{way range} from 2.0 to 4.5

_K,

_except for Al-Si and Al-Ge which reach 6.05 and 6.6 K

_respectively

_[3]

within the frame-work of Macmillan’s

_{theory [6],}

this effect has been

variously

ascribed to

_changes

in the

_phonon

_spectrum

(*) Work partially supported by DGRST under Contract Nb. 74.7.1083.

(**) Associe au C.N.R.S.

or in the electronic

_density

of states at the Fermi

_level,

or to

_changes

in the

_amplitude

of the

_{dcctron-phonon}

interaction. Alternative

_explanations

based on

elec-tron-electron interactions via other

_elementary

exci-tations have also been discussed

[7-10].

At

_present,

no

_overwhelming

evidence favours _{any one} of these

interpretations.

The

_microscopic

structure and the overall

homo-geneity

of the thin film

alloys

are basic features in these

experiments.

The recent use of low _temperature

implantation

techniques

is an

_{important development}

here,

since

_they

_may

_{produce inherently}

random

phases

under well-controlled conditions

_{[11, 1.2].}

We have

_previously

_reported

_Tc

increases for Al films

implanted

with

Al, 0,

He and H

_[12].

While

Tc-enhan-cements were related to lattice disorder in Al-0 and

Al-He,

the effect of

_{H-implantation}

was

_particularly

striking

because of the

_{high resulting Tc}

value

_{(6.75 K)}

and the

_possibility

that an ordered

_compound

had

been

_produced

_by

ion

_{implantation.}

The

_resistivity

annealing properties

were also

reported [13].

In the

L-288 JOURNAL DE PHYSIQUE - LETTRES

present work,

we have studied the critical

tempera-tures of Al based thin film

_alloys

with

_varying

concen-trations of the covalent elements

C,

Si and Ge. We

report

(i)

results on the

_{highest 7~}

values obtained for

these

_systems;

_(ii)

a

study

of the correlation between

Tc

and the

_annealing

_temperature for the Al-Si

system; and

_(iii)

a

study

of the influence of lattice

disorder on the

high Tc

samples.

Implantation,

resistivity

measurements, and film

preparation techniques

are the same as in our

previous

work

_[12].

Thin films

₍₈

x 0.6

_mm2,

thickness 500-1 000

_A)

were

prepared

by

evaporation

on

crys-talline _quartz substrates from a _tungsten crucible in a

bell-jar

vacuum.

_During

the

implantation

the target

temperature

was below 10 K.

Implantation

energies

were

ajusted

in order to obtain

_fairly

_uniform

_samples

(e.g.,

30 keV and 15 keV Si+

_{implantations}

were

performed

in a 500

A

thick aluminium

film).

Since

_heavy

ions

_sputter

_varying

amounts of the

targets, the film

_composition

after ion

_implantation

had to be determined. The

_composition

of the Al-Ge

implanted

alloys

was estimated from a Rutherford

backscattering

study

of the

_{appropriate sputtering}

coefficients

_[14].

For the Si and C

_{implanted samples,}

the

_alloy

concentration was estimated

_{by assuming}

that the

_sputtering

ratio of Si on Al was

equal

to that of Al on

Al,

and that the

_sputtering

of Al

_by

C was

negligible.

Resistivity

measurements were

_performed

_using

a

standard

_four-point

_{probe technique [13].}

_Implanted

samples

were

_repeatedly

measured after a succession

of

_Ge,

Si and C

_{implantations}

at

_varying

doses in order to correlate the

_resistivity

_just

above

_Tc

and the

superconducting

transition _temperature with the

concentration of

_implanted

atoms

_[14].

FIG. 1. -

Superconducting transitions in Al-C, Al-Si and Al-Ge. The curve indexed _Al(Ge) was obtained after Al _implantation

(dose 7 x 1017 at . cm - 2) into a _{room-temperature co-evaporated}

Alo.3oGeo.7o film. The curve indexed Ge was obtained after Ge implantation (dose 1017 at . cm - 2) into a _{pure Al film.}

The

_{highest superCCL -’ ~~ .ing}

transition tempera-ture curves obtained for different

_alloys

are

_displayed

in

_figure

1.

_They

all

_correspond

to estimated _average

Ge,

Si or C concentrations of about 25

_%.

Note that

Al-implanted

pure Ge or

_Al-implanted

_{co-evaporated}

AlGe films

_{produce higher}

_Tc

values than

Ge-implant-ed Al. This could be due to

_{recoil-implanted}

surface

impurities

(the

forward collision cross-section increases

for

_heavy

_{incoming ions) :}

the increased concentra-tions of for

_example,

_{0, N,}

or C

in

the

_Ge-implanted

sample

may reduce the maximum

Tc

value. It is

_quite

possible

that the observed value of 7.35 K is still below the maximum obtainable value for Al-Ge.

Three results demonstrate the effect of lattice disorder on the

_{superconducting properties}

of these

alloys :

(i)

a dose of

1016

Si

at.cm-2

was

_implanted

through

the

_highest-Tc

Al-Si

_sample.

The

correspond-ing

change

in Si concentration was

negligible,

but the

implantation produced -

10

_{displacements}

per atom in the

_implanted

_layer.

This had no effect on

_Tc.

(ii)

The critical _temperature of an Al-Si

alloy

_produced

at 6 K was studied after

_annealing

at

_increasing

temperatures

(Fig.

2).

No drastic variations were

found _up to about 80

K,

but

_Tc

fell to 3.3 K after

annealing

at room _temperature.

_(iii)

The same

_sample

was then

_Si-implanted

at 6 K :

_Tc

was found to increase

_{sharply (Fig.}

₃₎

until the _average number of

displaced

atoms in the

_implanted

_layer

was about one.

FIG. 2. - Effect of

annealing on the superconducting critical

temperature 7~ of an _implanted Al-Si film. All transitions are less than 0.1 K wide.

FIG. 3. - Effect of low

temperature Si-implantation into Al-Si film. This film was _{produced by low-temperature implantation,}

and then annealed at _{room-temperature.} Note _negligible composi-tion change : main effect is lattice _disordering.

The first result is evidence that the Al-Si

_alloy

produced

at 6 K is disordered. Since Al and Si

_{(or Ge)}

L-289 SUPERCONDUCTIVITY OF ALUMINIUM ALLOYS

samples

is

_expected

to result in

_{segregation and}

crys-tallite

_{growth :}

our second result _suggests that such a process could take

place

between 80 K and 300 K

[15].

This is corroborated

_by

the third

_result,

since the

_only

possible

effect of the low-dose Si

_implantation

is to randomize the atomic distribution in the Al-Si

_alloy.

The effect of disorder was also witnessed in the

Al-Ge system. In several

_experiments,

Al was

implant-ed at 6 K into _{room-temperature}

_{co-evaporated}

Al-Ge films of

_{varying composition.}

The

_implanted

Al dose varied from 10 " to 1018 at . cm - 2 : in all cases,

disordering

increased

_~

Lattice disorder is thus found to favour

_high

7~-values

in Al-Si and Al-Ge.

_However,

a

simple

interpretation

in terms of a

_softening

of the Al

_phonon

spectrum is not

_satisfying,

since the

_highest

_Tc-value

measured for the

_equally

disordered

_Al-C,

Al-He and

Al-0

_implanted

_systems

_[12]

is

_only

4.2 K. From

recent

_experiments

on

_implanted

Ge-Cu

super-conducting alloys [18],

it was

_{conjectured that}

super-conductivity

is due to a

liquid-like

metallic

_phase

of

the

_{semiconducting}

_component. This

_suggestion

was in line with the fact that

_{superconductivity}

is obtained

on the Ge-rich side of the

_{Ge 1 _ x CuX}

_{phase diagram}

(the

highest

7~

was found around x ~

_0.3).

In our

experiments,

however,

only

Al-rich

_{alloys (x > 0.75)}

are

_{superconducting.}

It is difficult to

_explain

the difference between our results and those of

_[18]

if

superconductivity

is

_only

due to Ge

_{(or Si).}

Alter-natively,

it is

_interesting

to relate the _{appearance of}

superconductivity

to the metal-insulator transition. This will be discussed for the Al-based

_alloys

in a

forthcoming

paper

[14].

’

We wish to thank M. Salome for his

_help

in these

experiments.

The Rutherford

_{backscattering}

measure-ments were carried out at the

_Groupe

de

_Physique

des

Solides de 1’Ecole Normale

_Superieure

in

collabora-tion with C. Cohen : it is a

_pleasure

to

_acknowledge

his

enthusiastic _support and

_expertise.

We are

_grateful

to

B. Stritzker and H. Wiihl for a

_preprint

of their _paper.

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