New results on low level phosphorous doping in a-Si:H

Journal of Non-Crystalline Solids 114 (1989) 265-267

265

North-Holland

NEW RESULTS ON LOW LEVEL PHOSPHOROUS DOPING INa-Si:H Dashen SHEN and Pawan K. BHAT G1asstech Solar,

Inc., (GSI), 12441 West 49th Avenue, Wheat Ridge, CO

80033, USA

We report on new -esults of the effects of ppm level phosph~ne doping ~U hydrogenated amorphous silicon. The results show that the dark conductivity changes from 5 x 10 - N to I x I0 -a SOm-" with ppm phosph~ne doping. The results can be explained by the !ow defect state density near mid-gap in aSi:H and the shift of the Fermi level by doping. These new results mean that the e~ectronic properties of a-Si:H can be tuned by very fine doping without introducing large density of defect states. The application of these results in the photoconductive type image sensors is discussed. I.

INTRODUCTION Table I

Doping is an important issue in amorphous silicon the

devices.

effect

However,

There

are many

of doping on a-Si:H due

to

the

nature

reports

(e.g., of

on Dark Conductivity

-5 x 10 -11

(~ cm) -I

AM 1.5 Photoconductivity

-5 x 10 -5

(~ cm) -I

0.85

(eV)

I-4).

amorphous Activation l~nergy

semiconductors, electronic

the doping efficiency and the

properties

of

a-Si:H

films

Band Gap

1.7 - 1.75

(eV)

SeLC Density of States

~5 x 1015

(cm -3 eV -I)

TOF Defect Density

~5 x 1014

(cm -3)

1.4

(cm-2s-Iv -I)

are

closely related to the localized states within the

band

gap

of

the material.

Due

to

the E~ectron Drift Mobility

possible device application type

imaging

sensors,

low

in photoconductive level

doping

has Figure

attracted

much

attention

recently. 5

dark report,

we present

I shows a plot of the photo- and

In this

results on the effects

conductivity

vs.

phospho-ous

doping.

of Here the photoconductivity

was measured

under

!ow level doping on our a-SI:H f~ims. AMI.5 i!lumination. 2.

of

EXPERIMENTS AND RESULTS

the

dark

inset.

The activ~ation energy Ea

conductivity

Note

that

a

is

shown

drastic

change

in

the

in the

Tne films we-e deposited in the state-ofconductivity the-art

commercially

available

GSI

enhanced

CVD system.

The typical

occurs at a level of -I ppm PH 3

plasma in the gas mixture.

electronic Fig.

properties

of

undoped

summarized

in

Table

a-Si:H

films

2

deposition I.

An

shows

the

conductivities

vs.

are temperature,

T.

The

illumination

independent here is 100 lux at 550 nm.

measurement

using time of flight showed a low

density

of

states

film. 6

The source gases used were SiH 4 and 10

of

5 x

1014

cm -3

in our 3.

DISCUSSION From Fig.

ppm

PH 3 in H 2.

The concentration

conductivity the

gas

mixture

was

checked

with

a

mass

The

photoconductivity

was

magnitude spectrometer.

Si~ 4. measured

under

AMI.5

solar

I, it is obvious that the dark

of PH 3 in

simulator

increases

with The

by

only

~I

activation

seven ppm

orders

of

added

to

PH 3

energy

data

suggests

(100 that

mWcm -2) and under filters which gave intensity ~ 100 lux at 550 nm of i!lumination.

due I).

the to

the The

trapping.

0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)

change

of the dark

shift

of

transport T~s

the

conductivity

Fermi

mechanism

level is

is

(Fig.

multiple-

is expected s~nce the doping

D. Shen, P.K. Bhat / Low level phosphorous doping in a-Si:H

266

level is very low. 7'8

I0-I1 1 10-2I 10-5I

//--

]~

I

1 -

l

[

To understand

this

large

change

in dark

conductivity with such low level of doping, we used a simple model of density of states {n aSi:H. 9

10-4 ~-

.~ 10-6

•

~ph (AM 15}

•

Gd

We

used

the

approximation

to

assuming

the extra

that

by the dopants

zero

calculate

filled

the

temperature Fermi

electrons

level,

N d denote~l

up the gap states r~o~

dark Fermi level of the intrinsic a-Si:H which

~ I0-7

is close to the midgap.

o o

was calculated using

10-8

The dark conductivity

od = qHoN c exp [-(Ec-Efo)/kT].

10-9

The

quasf-Fermi

i0-10 i0-11

photoconductivity

2 L// I I ~ _ ~ . I 1 0 I0-I I00 IOI tO 2 IO5 104 I0 5 1

//

0

PH3/SiH4 (ppm) i i ~ _ _ ~ I0 -I I 0 0 I01 I02 IO 5 I04 PHs/SiH 4 (ppm) I

solved

~evel

Efn

was

under

numerieal~y

deduced

light.

using

from

Ern was

Rose's

photo-

conductivity model]0:

n = N c exp [(-Ee-Efn)/kT] = O~ Fig.

I Photoconduetivity (0, AMI.5) and dark conductivity (A) vs. doping. The inset shows the activiation energy of dark conductivity.

~fo Here

n

their

i0-4

~s

l

I

G,

gap

rE ~ 10.7

the

3(a)

states.

of o

free

the

electrons,

capture

photogeneration

shows

the

Fig. 3(b)

rate

assumed shows

~ph O00Lu×)]

~d

i0-10

the

calculated

line

in

Fig.

3(a)

level

I

I

200

this

the

Thus, Efo

~aised to 0.38 eV below E c by adding

is

pJnned

fn

the doping,

With

high

doping

the

band

t a i l and

the

but once the Fermi

peak,

moves

close

slows

down

experimental 2Photoconducttvity (Q, 100 lux, 550 nm) and dark conductivJty (A) vs. deposJtion temperature.

are

upper and ]ower bounds of Efo with

cm -3 dopants.

slowly, 160 T (°C)

of

dashed

decreas{ng

t20

density

The

1016 •

T,

Here G ~s assumed to be 2 x 1018 em -3.

can be

10-9

and

as a function of

N d changing from 1013 to 1016 cm -3.

=o 10- 8 c o (D

v

cross-

Nd.

ca]culated

Fig.

and

dark and photo conductivity

10-6

iO-II

number

the electron life time. Fig.

10-5

the

ve!oeity

seetion, i

I/v of~rn mE)dE

=

it moves to

much

Fermi

dark

Thus,

on

level moves

level drops below faster.

the D+/D ° states,

again.

Fermi

we

When the

Efo

change

can explain the

conductivity

simple density of states model.

dat~

by

a

The plot of

Efo vs. Nd has the same shape as Fig. 3(b),

D. Shen, P.K. Bhat / Low level phosphorous doping in a-Si:tI

22

{3

time

changed

Efo{Nd~lOIGcm 18~Efo(Nd~lOl3cm_3) I J

T~

that

(a)

the

I

l

l

I

II

I

I

of a-SL:H can be

of magnitude

by ppm

implication of th~s new

is that the electronic properties

of the a-Si:H thin film can be tuned by very

L

-09

fine doping, without introducing large amounts

Ec-E (eV)

of defects.

10-2

orders

The positive

observation 1411

conductivity

by seven

doping.

267

j _

I

e.g.,

T

This is useful

imaging sensors.

is that one must systems

(b)

the

for some devices,

The other implication

use mu]tichamber

for device manufacturing

doping

and/or

deposition

to eliminate

contamination

of

the

intrinsic a-Si:H layer.

c o

We thank J. Sandwisch, C. Matovich and A. Benson for their help in this work.

i0-II I013

I014

1016

I019 N d (cm-3)

REFERENCES

Fig. 3 (a) Assumed density off states for ca I eulating eonductivities. (b) Calculated conductivtties vs. electron density in the fi]m. since the

od

is calculated

mid-gap

states

from

Efo.

is low and

Note that

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~s consistent

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important is

that

implication

the

of

conductivities

our

new

of

high

quality a-Si:H films can be fine-tuned by very low

level

doping.

some

This

device

photoconductive

type

sensors.

this

For

photocur~ent desired.

e.g., the

material photo

useful

100

integrated type

nA

of

under

for

0.1

ppm

image

sensor,

~

100

doping

remains

PH 3 mixture

level of high

sensitivity

of

is

very

quality. -104

intensity)

has been achieved

developed

lux

6.

V. Perez-Mendez, S.N. Kaplan, G. Cho, I. Fujieda, S. Qureshi, W. Ward and R.A. Street, Nucl. Inst and Method, A273, (1988), 127.

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H. ~rerhof and W. Beyer, J. Solids 35/36 (1980), 375.

9.

D.S. Shen, J.P. Conde, V. Chu, J.Z. Liu, S. A]jishi, Z ~. Smith, A. Marujama and S. Wagner, Appl. Phys. Lett., 53 (1988), 1542.

a is

for

in SiH 4. low,

the

In fact,

(at

100

photoconductive

type

image

Cryst.

I0. A. Rose, Concepts in Photoconductivity and A11ied Problems (K~eiger, HuntLnton, New York, 1978).

CONCLUSION In conclusion,

Non.

lux

in our materials

senso~s.

5.

1988.

e.g.,

This could be achieved by low level

doping, Since

of

~s very

applications,

we observed for the first