aSi:H produced by double ion-beam sputtering

Journal of Non-Crystalline Solids 59 & 60 (1983) 723-726 North-Holland PublishingCompany

723

a-Si:H PRODUCED BY DOUBI~ ION-BEAM SPUTTERING

C. COLUZZA, D. DELLA SALA, G. FORTUNAT0 ~, S. SCAGLIONE and A. FROVA

Rome,

Dipartimento di Fisica G. Marconi, Universita' La Sapienza, 00185 Italy ZCNR-IESS Laboratory, Via Cineto Romano 42, 00156 Rome, Italy

A novel technique to obtain a-Si:H films by a dual ion-beam sputtering (DIBS) system is described. A beam of argon is used to sputter silicon, while a beam of hydrogen impinges directly onto the growing film. The system has proven to be very efficient in the control of growth parameters. Hydrogen incorporation can be remarkably affected by varying the energy of the H ions. IR spectra indicate that, when the energy is raised from 80 to 800 eV, the density of isolated Sill clusters in the material increases relative to Sill. The samples obtained exhibit promising optical and electrical properties. The optical energy gap is typically 1.7 eV and the room temperature conductivity is in the range I0-9-I0-i~ ~cm) -I . Photoconductivity values of IO-9(~ cm)-lare obtained. I. INTRODUCTION The commonly used techniques for the deposition of a-Si:H do separate

control

of

the various deposition parameters.

DIBS technique presented here has the pressure

(10-4-10-5mbar),

following

not

permit

a

In this respect, the

advantages:

low

deposition

separation of the substrate from the plasma genera-

tion region, controlled deposition angle, independent regulation

of

ion

beam

currents and energies and better control of the hydrogenation process.

2. EXPERIMenTAL The deposition apparatus is shown in Fig.1. type.

During

growth, a beam of Ar

by

a

up

sources

to 15 at%;

are

Kaufman

The latter is simultaneously bom-

beam of hydrogen ions from gun 2 (procedure A).

growth conditions have been used: 2,

ion

ions sputters silicon from a polycrystal-

line target, to a rotating heated substrate. barded

The

(B) Ar

Two alternative

ions are mixed in the flow from

gun

(C) with gun 2 shut off, an equivalent pressure of hydrogen

was maintained in the chamber (typically 3 x 10-4mbar).

The isst procedure was

meant to verify that most of the hydrogenation arises from the presence of the 0022-3093/83/0000-0000/$03.00 © 1983 North-Holland/Physical Society of Japan

C Coluzza et al, / a-Si:H produced by double ion-beam sputtering

724

H ion beam. The films obtained with all three methods appear to be amorphous throughout, as shown by X-ray analysis.

I

[

l

FIGURE I Scheme of the deposition apparatus. Typical growth parameters: beam e n e r ~ = 600 eV and beam current = 20 mA (GUN i); beam energy = 80-800 eV and beam current : 1-10 mA (GUN 2); deposition temperature = 200-350 °C.

3. OPTICAL MEASUR]~ENTS The samples have been investigated by IR and visible spectroscopy. purpose

of

The main

IR absorption is to estimate the hydrogen content C H from the area

of the wagging-mode bands centered at 640 cm -I

i

The estimate based

on

the

area of stretching bands in the region of 2000 cm-i is considered less reliable, because of the strong influence of the atomic surroundings over the strengths of these modes, as pointed out by some authors In almost all samples,

in addition to bands

shoulder is observed at ~2070 cm -i. affected by the e n e r ~ of the 840-890 cm -I doublet

at

640

oscillator

2,3 and

2000

c~ I a

weak

The strength of this shoulder is strongly

hydrogen

beam,

as

shown

in

Fig.2.

As

the

is not seen, the 2070 cm -i shoulder is not likely to repre-

C, Coluzza et al. / a-Si:H produced by double ion-beam sputtering

sent SiH 2 groupin@s

2,4

725

Moreover, it cannot be attributed to the

electrone-

gativity shifts of the SiH stretching bands due to the presence of oxygen which is negligible in our case. clusters

of

SiH

bonds

The shoulder is

probably

related

3 ,

to

small

surrounding silicon vacancies 2 , which are supported

also by a systematic lower value of C H if this is evaluated from the stretching bands using the known oscillator strength of SiH 2

5

Eg (eV)

0

o

~-Q5

17

•

•

•

•

•

o

q~o oo

•

o

-Q3

o#

X

t5 o

%

.0.1 e o

I

I

I

300

IO0

t-4

I

i

l

500

I

700

1.3

I

o

Eb(eV)

FIGURE 2 Relative strength of 2070 and 2000 cm -I bands vs. beam energy of GUN 2.

I

I

5

10

I

15 CH(% )

FIGURE 3 Optical gap E~ vs. hydrogen content CH. Closed circles: samples A; open circles: samples B; crosses: samples C.

Absorption in the visible has been used to obtain values of the optical Eg

by the usual method of the (~hv ]/2 vs hv intercept.

sults for samples of both type A and B. former

case,

where

the

films, large values of Eg

It is

worth

gap

Fig. 3 shows the re-

stressing

that

in

the

refractive index is 5.8 as opposed to 3.2 for type B are already attained at low H content.

4- ELECTRICAL MEASUREMENTS The electrical measurements were performed in four-probe

vacuum

method for temperatures ranging from T=300 K to 450 K.

tivity of almost all samples shows an activated behaviour with

AE

content.

(10-3torr)

ranging from 0.7 to 0.9 eV.

with

the

The conduc-

o =o o exp(- AE/kT),

This happens also for films with low H

C. Coluzza et al. / a-Si:H produced by double ion-beam sputtering

726

The estimated Co tion

due

is in the range 10 3 -105

to electrons in extended states 6

(g~ cm) -i, pointing

GD

material.

conduc-

This corresponds to a room tem-

perature conductivity in the range 10 -9 -10-i i (2 ca)-i, good

to

i.e.

comparable

to

Photoconductivity has values of order 10-9 (;2 cm~lusing a

He-Ne laser with 1015photons/(cm 2 sec).

CONCLUSIONS AND A C K N O W I ~ T S Preliminary optical and transport measurements in a-Si:H ion-beam

produced

by

dual

sputtering indicate that the quality of the material is promising.

more detailed structural characterization is in progress, studies and FIR spectroscopy measurements.

involving

A

annealing

We wish to thank F. Evangelisti for

helpful discussions, A. D'Amico and R. Moretto for technical support.

REFACES I)

H.R.Shanks, F.R. Jeffrey, M.E. Lowry: Proc. and Liquid Semiconductors II (1981) 773.

2)

H. Shanks, C.J. Fang, L. Ley, M. Cardona, F.J. Demond, S. Kalbitzer, Stat. Sol. (b) 100 (1980) 43.

3)

G. Lucovsky, Proc.

Ninth Int. Conf.

Ninth Int.

Conf.

Amorphous and Liquid

Amorphous

Semiconductors

I I (1981) 741. 4)

W. Paul, Solid State Commun.

5)

A. Kasdan and D.P. Goshorn, J.

34 (1980) 283.

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Sci.

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