Recrystallization of silicon by pulsed lasers

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Recrystallization of silicon by pulsed lasers

J.C. Muller, C. Scharager, M. Toulemonde, P. Siffert

To cite this version:

Recrystallization

of silicon

_{by pulsed}

lasers

_(*)

J. C.

Muller,

C.

_Scharager,

M. Toulemonde and P. Siffert Centre de Recherches Nucléaires

Groupe de _Physique et _Applications des Semiconducteurs _(Phase) 67037 Strasbourg-Cedex, France

Résumé.

_{2014 Dans}

une _{première partie} de ce travail on a calculé l’évolution de la

_température

d’un film de silicium

amorphe déposé sur un cristal de

_même

nature en _fonction de la _{puissance d’irradiation pour} un laser

_pulsé

rubis ou YAG. Les résultats obtenus ont été confrontés à diverses _expériences au cours _desquelles la recristallisation de films _{amorphes par laser} a _{été mesurée par rétrodiffusion Rutherford (RBS).} Le recuit laser conduit à un cristal de bonne _qualité, toutefois des _{dommages subsistent, ainsi que l’ont montré des expériences} de TSC et DLTS. Abstract. 2014 Calculation of the evolution of

temperature

during

pulsed laser

_annealing

has been

_performed.

The results are _presented in _directly

_useful

_figures

for the two kinds of laser _generally used _{(YAG, Ruby).} The results are

compared to various

_experimental

measurements

_performed

by RBS. If the _{crystallographic}

_quality

is _{quite good,} TSC and DLTS measurements have shown that

_electrically

active defects are still _present after laser

_annealing.

1. Introduction. - The

use of

_pulsed

laser beams

to

_{recrystallize}

_{amorphous layers existing}

on

_top

of

single

crystalline

semiconductors like

silicon,

has

recently

been demonstrated in several labora-tories

_[1-5].

In

_fact,

two

_regrowth

mechanisms have been

observed,

_depending

on the

laser

characteristics : either solid or

_liquid

_phase

_epitaxy

_regrowth.

_However,

the exact mechanisms are still under discussion.

Here,

we will restrict ourself to conditions in which

_liquid

phase

epitaxy

occurs. The

_applications

of such a

technique

are

_quite

obvious : solar

cells,

integrated

circuits in

semiconductor,

_doping

of surface

_layers

of solids...

_[6,

_7].

For these

_{applications,}

it is _necessary

to have models

which

allow an exact estimation of the

optimal

regrowth

conditions. This is one of our

_goals.

These models must be verified

_by

the

_experience

both from the

_point

of view of

_crystal

_quality

and from

the residual electrical

_domage

concentration. On this later

_point

much less is available in the literature

_[8-10].

The

_experimental

methods that we

apply

here,

are

Rutherford

_{backscattering}

under

_channeling

condi-tions

_(RBS)

and

_thermally

stimulated current

_(TSC)

measurements.

2. Evaluation of the

_molten

_depth.

- The

dynamic

of the

_physical

evolution is described in term of a

thermal _transport model. A

_complete

_description

of the used

_approach

has been

_published

elsewhere

_[11].

Recently,

other authors

_[12-14]

_published

similar

models,

which are in _agreement in first

_{approximation,}

although

they

do not include

_temperature

_dependence

of thermal and

_optical

_parameters.

_Recently

Surko and al.

_{[15] taking}

into account this

_dependence,

suggest that the difference in the absorbed and

calcu-lated

energy in a

_pulsed

laser

_experiment

on a

_virgin

crystal

can be reduced

_by

_reducing

the

_reflectivity

coefficient from 0.7 to 0.57.

_By

_taking

into account the latest value of

0.57,

we calculated _{the power}

density

as a function of

_{amorphous layer}

thickness for the various

_layers

considered

_(Fig.

_1).

Fig. 1. - Thickness of molten silicon

as a function of laser _(Ruby and YAG) power density for _amorphoùs silicon layers of various thicknesses. The two curves slash doted correspond to the _reflectivity coefficients _(R = 0.65) of the molten silicon.

866

3. Measurement of the

_{recrystallization by}

laser.

-3.1 EXPERIMENTAL CONDITIONS. - 3.1.1 The

samples.

- Two kind of

amorphous

layers

on

_single

crystals

have been used :

-

amorphous

silicon : thin films of

_amorphous

silicon

_(Si(a)

_H)

have been

_{deposited using}

silane

_glow

discharge

on

a ~ 111 >

silicon substrate. The thickness

and concentration of

_{hydrogen ranged}

between 2

000-3 000

Â

and

_{10-18 %.}

The

_{absorption coefficient}

for

photons

of this material is well

_known

_|16].

-

amorphized

silicon : we have

previously

shown

[17]

that the

_{BF3 glow discharge}

ion bom-bardment of silicon

_produces

a

_{completely amorphous}

layer,

due to the

_{high probability}

of nuclear collisions

by

the low

_speed

molecular ions. 25 keV

_BF3

_glow

discharge produced

a 1 000

Â

thick

_{amorphous layer}

on ( 111 ~

CZ material for a total dosis of

3 x

1016 cm-2

at current densities _up to 1

_mA/cm2.

3.1.2 Laser

_annealing.

- The laser illuminations

have been

_performed

in air

_with

a

_ruby

laser

_having

the

following

characteristics :

_spot

diameter at half

power : 6 mm, half width of

pulse

duration 25 ns, power

density

up to 2

J/cm2

and even 3.5

J/cm2

on the

central 2.5 mm in diameter area,

repetition

rate : 10

_pulses/min.

,

3.1.3

_{Crystal quality}

measurement. - Rutherford

backscattering

of 1

MeV 4

He+ ions was used under

channelling

conditions to

_investigate

the amount

and distribution of

_displaced

atoms.

The backscattered Darticles have been detected by

Fig. 2. - Rutherford

backscattering spectra under channelling conditions of thin amorphous silicon films deposited on _single

crystals annealed under various ruby laser _pulses.

means of an electrostatic

_{analyzer having}

a

_depth

resolution of about 25

Á,

which is more than ten times better than the conventional method used in the

literature.

SIMS can also

_help

to determine the

conditions,

where

_melting

occurs, since in the melt

complete

redistribution of

_dopants

_implanted

or

diffused,

occurs.

Finally,

surface

_reflectivity

of a continuous small

power

density

laser allows in a _storage

_oscilloscope

to see the difference between

_{crystalline, amorphous}

and

_liquid

silicon.

3.2 RESULTS. - 3.2.1

Si(a)H

layers.

- After irradiation with a 1.1

J/cm2

_pulse,

RBS has shown

that the

_layers

are _regrown, as

_expected

from the

model.

However,

if the

_{crystalline-amorphous}

inter-face has become of _very

_good

_{crystallinity,}

a surface

layer

is still

_present

which is not

_{single crystalline}

(Fig.

2). Higher

power densities

are not

_noticeably

modifying

this situation.

_Furthermore,

some

_hydrogen

remains in the _regrown zone, as indicated

_by

nuclear

reaction measurements.

3.2.2 Implanted

layers. - Figure

3 shows the RBS

_analysis

of the

_implanted

surface

_layer,

it

appears that a

_high

_quality

_{recrystallisation}

is

_obtained,

but,

as

_before,

some

_{crystal degradation}

is still visible on the _top surface.

Measurements of the

_{implanted layer}

sheet

resis-tance as a function of _power

_density

indicated that

at 0.8

_J/cm2

a _strong reduction occurs,

by

SIMS

we observed that

impurity

redistribution starts at power densities in excess of 0.8

J/cm2.

It is

_interesting

to notice at that

_point,

that the

recrystallisation

by

this laser

_pulse

is much more

efficient than a 30 min. duration thermal

annealing.

at 950 °C

_|5].

In

fact,

when

_reaching

the

_melting

temperature, all the

_preexisting

radiation induced

Fig. 3. - Backscattering

spectra under _channelling conditions and glancing angle geometry of a _damaged layer annealed by a _ruby

Fig. 4. - TSC _curves recorded for N+P

junction realized _by

PFS glow discharge implantation at 15 keV energy at a dosis of

1016 cm-2 followed _by an LI J/cm2 ruby laser annealing.

stable

_complexes

are dissociated and the

_epitaxial

regrowth

starts from a

_{damage and}

contaminent free silicon

_region.

4. Electrical characteristics of the laser annealed

zone. - 4.1 EXPERIMENTAL CONDITIONS. - For the

TSC and DLTS measurements two

_types

of

_samples

have been

_{employed :}

-

Virgin N-type

FZ silicon of 1 000 03A9.cm

resisti-vity

on which a

_gold

_Schottky

diode was realized

after laser irradiation.

- P-N

junctions

prepared

on

_P-type

FZ silicon of 150Q.cm

_{silicon ~ 111 ~}

oriented

_by

a

_glow

discharge

induced

_implantation

from

_PFS.

After the ion bombardment the

_amorphized

_layer

was _regrown

by 1.1

and

_{1.4 J/cm2}

laser

_pulses.

4.2 RESULTS. - A

typical

TSC _spectrum is

_reported

on

_figure

4 : two

_peaks

_appear,

_respectively

around 113

and 145 °K. These

_{peaks disappear only}

after an

annealing

at 450 °C for 30 min.

_(Fig.

_5).

We tried to _{determine the main characteristics of}

these defect

_{peaks :}

the first one at 113 K is an electron

trap as measured

_by

the method we

_published

pre-viously

[18],

its activation energy was calculated

starting

from different methods

_[19]

and found to be

0.18

± 0.04 eV. The identification of the second

_peak

is not

_completely

_done,

its activation _energy lies between 0.19 and 0.26 eV.

DLTS _gave the same levels and on the

_virgin

silicon

Fig. 5. - TSC

curves obtained on the same _{implanted samples}

after 1.4 _J/cm’ laser _annealing. The effect of a further thermal annealing at 450 °C for 30 min. is _clearly visible.

having

been laser irradiated the same results have also been seen

_indicating

that the residual

_damage

is not

related to the

_implanted

ion. It should be mentionned that the first TSC

_peak

has also been observal

_by

other

authors

_[8]

_by

_Q

switched YAG laser

_annealing

on FZ

implanted

silicon.

_They

attributed its

_origin

to the

(V-0)

vacancy-oxygen association at

_Ec-0.17

in the

state (0, - ).

Today,

all the defects due to laser irradiation have

not been

_fully

understood and much more work is necessary to

_clarify

the situation.

5. Conclusion. - The laser induced

regrowth

of

amorphous

or

_damaged

silicon surface

_layers

has

opened

new

possibilities

for the realization of devices.

However,

if the

_{crystal quality}

as seen

_by

_scanning

microscopy (TEM)

or Rutherford

_{backscattering}

(RBS)

is

_{quasi-perfect,}

the electrical measurements shown that

_damages

are still _present at the

_microscopic

scale. Until their

_origin

will be

_clearly

established it will not be

_possible

to assertain that

_they

could be

brought

under control.

Acknowledgments.

-

868

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