Material properties of glow-discharge a-SiSn:H alloys

Journal of Non-Crystalline Solids 66 (1984) 13-18 North-Holland, Amsterdam

13

MATERIAL PROPERTIES OF GLOW-DISCHARGE a-SISn:H AllOYS

B. yon ~@~dern, A.H. Mahan, R. KSnenkamp Sanchez , and A. Madan

, D.L. Willlamson**, A.

Solar Energy Research Institute Golden, CO 80401

In this study, a series of a-SiSn:H alloys is investigated. A transition from n- to p-type in the conduction mechanism is found with Sn incorporation, while the ~ products of electrons and holes decreased drastically at the same time. We attribute this to the creation of additional states in the lower half of the gap. Similar trends can be observed in a-SiGe:H and a-SIC:H. Phosphorous doping recovers the ~z products of the photo-carrlers in a-SiSn:H.

I. INTRODUCTION Hydrogenated

amorphous

sillcon

their potential

applications

offer

opportunity

a unique

based

alloys

in multlJunctlon to surpass

are of

interest

because

of

thin film solar cells, as they

20% conversion

efflciency I.

Much re-

search using a-SiGe:H as the low bandgap material has been recently reported, but a common

result

seems

to be an increase

in the density of states and a

loss in the photoresponse with bandgap varlatlon 2. the bandgap

has been demonstrated

using a-SiSn:H

Another approach to lower alloys I.

Initial

studles 3

using sputtering have indicated a bandgap reduction of 0.056 eV per at. % of added Sn.

To date, the sputterlng 3 and glow dlscharge I studies have provided

scant information about the electronic properties of the material. In this paper, we report the optical, electrical and structural properties of a-SiSn:H films deposited by the r.f. glow discharge technique using SI~4+H 2 mixed with SnCI 4 or Sn(CH3) 4. sample

preparatlon 4.

With

In a recent paper, we described the details of

the addition

of Sn, the samples

show a decreased

band gap and the conduction mechanism changes from n- to p-type and is accompanied by a drastic reduction

in the photoconductivlty;

however,

the conduc-

tion mechanism can be changed back to n-type with P-doping while retaining the narrow band gap nature and is accompanied by a recovery in the photoconduetlvity response.

Further, we report that the n- to p-type transition may well be

a common feature to other alloy systems such as a-SiGe:H and a-SIC:H.

Permanent Addresses: *Department of Physics, Tulane University, New Orleans, LA 70115; **Physics Department, Colorado School of Mines, Golden, CO 80401 *** Instltuto de Investlgaciones en Materlales, Ciudad Unlversitarla, 04510 Mexico, D.F., Mexico

0022-3093/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

14

B. yon Roedern et al. / Glow-discharge a-SiSn:H alloys

2. RESULTS The Sn concentration of the a-SiSn:H samples was determined by electron microprobe anlysls and from 119Sn M~ssbauer measurements 3. of ~i0 at. % Sn, the bandgap E

g which has a competing effect on Eg.

(CI) incorporation Sn

also

contain

indicated

that

addition,

~i0

at.

the vast

substltutionally

% C

(~18 at.

majority

% CI).

(> 95%)

into the a-Si based matrix.

showed

that

the

films

using Sn(CH3) 4 were homogeneous

~-Sn precipitates

10'

on a 30 A scale

~. I~~

(~i-2 ~m size) were

With

~D shows

AE exhibits of

~D

OD,

energy,

decreasing

tion),

~ .~

AE,

Eg

the a

conductivity

function

(increasing

Sn

of

Eg.

incorpora-

a minimum and correspondingly,

a maximum.

changes

and as

to

The activated

hopping

for

0.8 /

~

0.6

~

0.4 0.2

behavior

alloys

with

,I

r , ,Ix-~l, ~ (b)

lO1.0 -U

la and lb show the room temperature

conductivity,

activation

P+ (a)n

C~ 10" x ~ o~ ~ oil '~~x "~ 10.... °~xxxx x~J-!°~ 10-"

Figures

have

incorporated

10~ ~, ,

(TEM)

produced

observed3, 5.

dark

measurements

In

unlike for the sputtered a-SiSn:H films in whlch metallic

Films with ~i0 at. %

MSssbauer

of the Sn atoms were

transmission electron mlcroscope

measurements

With the incorporation

is lowered to ~1.3 eV, despite the sizeable C

Eg

10-,

I i

.

.

.

.

(C)

,

,

1.6 eV since the in(oD) vs. I/T plot exhibits curvature ~z

at low T.

product derived

Figure

Ic illustrates

from the photoconductlvity,

~L' response at 600 nm, using the equation o L = eG~z, where ~ is the majority carrier mobility, ¢ is the recombination electron-hole

generation

lifetime, rate 6.

and

G is the

As shown,

the

photoresponse drops drastically with Sn incorporation

and

Intrinsic

remains

low

as

more

Sn

is

conduction with AE ~0.8 eV, indicating that the position of the Fermi level E F is located above Wlth the initial addition

of Sn, states whose nature has yet to be determined are created above the valence band, which are causing an increase in AE. incorporation,

With further Sn

the conduction mechanism

~

l

lO.g

•

Oro, '

Oo!%

I

~°c)o 10"2 1131.4 1.5 1161.7 1.8 EQleVI

added.

a-SI:H exhibits n-type extended-state

the mldgap position.

10~

the

is then

changed to hole hopping within these states.

FIGURE 1 (a) Dependence of dark conductivity ~D (b) Activation energy AE (c) Photoresponse ~ on a-SISn:H bandgap E . Samples were made w~th SnCI 4 (O,l,~, see ref. 4 for details) or Sn(CH3) 4 (×) source.

B. yon Roedern et al. / Glow-discharge a-SiSn.'H alloys

Eg (eV)

g(EF) (1016cm -3 eV -I)

1.80 1.57 1.42 1.40 1.35 1.32 1.32 1.29

1 4 6 19 I0 22 20 15

(I, no Sn) (I) (i) (2)

15

TABLE I. State Densities at EF, g(EF) , in a-SiSn:H. Values were derived from space charge limited current measurement on samples produced with the Sn(CH3) 4 g ~ . s o u r c e (I) or from a in[o(T- j4)] plot for SnCI 4 produced films (2).

(I) (2) (i) (i)

This interpretation has been confirmed by positive Seebeck coefficients (up to 2000 ~V/K) for a-SiSn:H films exhibiting low bandgaps. We have used a plot of in(~D) vs. T -I/4 4 and space charge limited current measurements 7 (SCLC) to determine g(EF) , the density of states near E F. data

is shown

in Table

i for

samples with decreasing Eg.

The

The increase

in

g(EF) with Sn incorporation is surprisingly low and may be in part due to the saturation of

Sn bonds with H, as is evidenced

peak centered

at

mode.

This

absorption

prepared without associated

1750-1780

peak

C, which

with

C;

cm -I, is

also

observed

eliminates

further,

from an infrared absorption

which we attribute

the

to the Sn-H stretching

in sputtered

the possibility

stretch modes

of

of

the

normalized to the SiH stretch mode at 2000 cm -I, predict to occur at

~1720 cm -I.

a-SISn:H

this

alloys

feature

SnH 4 molecule,

being when

a Sn-H stretch mode

The preferential attachment ratio of H bonding to Sn

vs. Si has been measured and has been found to vary between i/4 to 1/12, and seems to depend on the C content of the films.

Higher C contents corresponded

to lower preference ratios. Further

information about

the localized states was obtained

thermal deflection spectroscopy

(PDS), which measures

from photo-

subbandgap absorption.

A shoulder was observed in the spectrum near 1.3 eV in the 1.75 eV band gap material

(2% Sn) which shifted to 1.15 eV in samples with a band gap of 1.57

eV (7.5% Sn).

The agreement between PDS and the photoconductivity spectrum

suggests that electrons are excited from below E F to a featureless conduction band edge.

Similar shoulders near 1.0 to 1.2 eV in the absorption or photo-

conductivity spectra have been observed in a-SiGe:H alloys 2,8. Charge collection experiments indicate that the ~z products for both types of carriers are strongly reduced with the incorporation of Sn. show a plot of ~

In Fig. 2 we

versus g(EF), as obtained from SCLC measurements,

doped a-Si:H and a-SiSn:H samples.

for un-

The linear relation which is seen to hold

for a-Si:H films is indicative of a common capture cross-sectional area of o = 4.10 -14 cm 2.

For a-SiSn:H samples,

~

for both carriers decrease much more

rapidly with Sn incorporation, while g(EF) increases only moderately.

16

B. yon Roedern et al.

It is possible

to explain

/ Glow-discharge a-SiSn:H alloys

this

10-7

behavior in view of the changes

in 9 Kirby et al.

AE shown in Fig. lb. have

shown

that

the

~

10 4 ~ r ~

products

~ ' O ~

Electro.s

~ 10.o

depend on the position of E F or on the

charged

states.

nature

of

the

In a-SiSn:H,

in ( ~ ) e

could be due to a shift of from

while

the small

conduction

(~)h

to

hopping

Holes

I

10-12 101s

band,

I

[

I

lO'~a

g (EF)

is caused by

conduction.

% \

:H

1017

(cmVeV-')

FIGURE 2 ~ products of electrons and holes vs. density of states at E F

an unfavorable change from extended state

10-1°

the decrease

E F away

the

~-

midgap

An

alternative explanation can be based upon the introduction of a new kind of defect

state

through

the

incorporation

of

Sn, which has a larger

trapping

cross-sectlon for both electrons and holes and, therefore, predominates over the defect states inherent to the a-Si:H films. In order to achieve an improved photoresponse, we have doped the a-SiSn:H alloys with phosphorous.

Thermopower measurements confirmed that the material

was n-type with the addition of 1% PH 3 to the gas phase. the ~L-derived ~ spectively,

while

product

and ~D increased

the narrow bandgap

was

At the same time,

by as much as 10 3 and 10 4 re-

retained.

The D~z products

thus

obtained are similar to the best a-SiGe:H alloys, although ~D is much higher. In a-Si:H, P doping leads to a sharp decrease in the (~z) h product 9, while in compensated

a-Si:H doped with equal amounts of PH 3 and B2H6,

(~)e

and

(F~)h remain reasonably high I0.

Jackson and Amer I! have shown that in compen-

sated

absorption

a-Si:H,

the

sub-bandgap

containing only one of the dopants. sity was observed upon doping 12.

is

decreased

compared

to

a-Si:H

In a-SiC:H, a decrease in the spin den-

If the above was true for the narrow-bandgap

alloys, P doping might improve their photoelectric properties significantly.

4. COMPARISON WITH a-SiGe:H AND a-SiC:H ALLOYS It is appropriate at this time to compare the results so far obtained with a-SiSn:H alloys with other amorphous silicon alloy systems employing group IV additives such as Ge and C.

A survey of the existing literature for a-SiGe:H

and a-SiC:H alloys indicates similarities between the systems,

even though Ge

and Sn lower Eg whereas C tends to increase Eg. In Fig. 3, we have compiled ~D data as a function of Eg (a) or x (b) where

B. yon Roedern et al.

FIGURE 3 Dark conductivity o D vs. (a) bandgap E (b) alloy con~ent x A: a-SiSn:H (present work) B: a-SIGe:H (ref. 13) C: a-SIGe:H (ref. 13) D: a-SiGe:H (ref. 14) E: a-SIGe:H (ref. 8) F: a-SIC:H (ref. 15)

lO.r lO'

/ Glow-discharge a-SiSn.H alloys

\'\A

(a)

•

o '\

~

lO-, 10-,o .

C 10' \\

•

I~ 10 "'°

10,~

1.3

-/ 114

,:s ,le ,I,

17

iI / /\/I Y -¢1,

,o" t, \ ,.8

215 510 Alloy Content

Eg (eV)

715 x (%)

x is the Sn, Ge or C fraction, and note a minimum In oD in many cases.

100

The

existence of these minima along with the fact that AE decreases less than Eg/2 upon

alloying,

indicates

the

introduction

of

acceptor

midEap, and hence can produce an n to p transition.

llke

states

below

In a-SiGe:H, thermopower

measurements have showed an increase in the Seebeck coefficient with Ge content approaching 0 mVK -I at 70 at. % Ge 16.

In the case of a-SIC:H, Anderson

and Spear 15 noted a conductivity change from extended state to hopping behavior but dld not indicate whether an n-p transition took place.

Further, the n

to p transition can also occur in a-Si:H when the deposition temperature Is altered, and was associated wlth changes in the g(E) spectra 17. The trend towards p-type conduction in all these alloys may be caused by a common defect.

One obvious suggestion is the neutral dangling bond (DB) which

is located below midgap 18.

The dangling bond signal, as determined by ESR,

increased sharply when a-Si is alloyed wlth Ge 19 or C 20. al.

have

shown

that

both

(~Z)e and

Si:H 18, which is inconsistent with

(~%)h scale wlth

However, Street et

the DB density

in a-

the sharp decrease of the ~% products of

the a-SISn:H observed in Flg. 2. Another similarlty between the alloys is the change in the SI-H infrared features.

For the a-SiGe:H case, the addition of Ge not only produces a shift

in the SI-H stretch mode towards 2100 cm -I, but also produces 845-890 cm -I doublet indicative of SI-H 2 vibrations.

the familiar

For the case of a-SIC:H,

although some controversy exists, the same trend may occur wlth the addition of C.

In the present case of a-SISn:H, we have seen that the 2000 cm -I Si-H

mode shifts toward 2100 cm -I wlth the addition of Sn, and in some cases have observed the 845-890 cm -I doublet as well.

The implication of these observa-

tions, as regards the creation of similar defect states in different types of alloys

Is not clear at the moment,

but such common trends may also help ex-

18

B. yon Roedern et al. / Glow-discharge a-SiSn:H alloys

plain the degradation takes

place.

from structural in accordance

in photobehavior

The appearance

observed when alloying with C, Ge or Sn

of a Si-H mode near 2100 cm -I might

inhomogeneity 21 indicating

with earlier work 22, although

structural

changes

also arise

in these alloys

to date these structures

have not

been resolved by TEM.

ACKNOWLEDGMENT We

thank

Chris

Walker

Trefny for the thermopower

for

help

with

the

sample

preparation,

Dr.

John

data, and Kim Jones for the TEM measurements.

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2) 3) 4) 5) 6)

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