Deposition mechanism of hydrogenated amorphous SiGe Films

Thin Sohd Fdms, 163 (1988) 123 130

123 D E P O S I T I O N M E C H A N I S M OF H Y D R O G E N A T E D SI-Ge FILMS*

AMORPHOUS

K. T A N A K A AND A. MATSUDA

Electrotechnwal Laboratory, 1-1-4 Umezono, Tsukuba, lharakt 305 (Japan)

The deposition rates for hydrogenated amorphous slhcon, hydrogenated a m o r p h o u s germanium and hydrogenated a m o r p h o u s Si Ge alloys, prepared by the plasma chemical vapour deposition method, are presented as functions of the substrate temperature On the basis of the experimental results the behaviour of hydrogen on the growing surface is discussed quantitatively using a simplified model The selection of long-lifetime radicals (such as SIH 3 and GeH3) and the hydrogen-coverage factor are stressed as two key factors for controlling not only the deposition process but also the structural and electronic properties of the resultant alloy films

1

INTRODUCTION

Hydrogenated a m o r p h o u s binary alloys such as hydrogenated amorphous S i - G e (a-SlGe'H) and hydrogenated amorphous SI-C (a-SiC:H) have attracted increasing attention as key materials for tandem-type amorphous thin film solar cells, since their optical band gap can be continuously controlled by changing their alloy compositional ratios 1. However, in contrast to hydrogenated amorphous silicon (a-SI:H) deposited from SIH 4 plasma, the structural and electronic properties of a-SIGe:H and a-SiC H alloy materials prepared by plasma chemical vapour deposition are strongly dependent on the deposition conditions 2 4 This may be related to the many possible reaction pathways for film deposition as a result of the presence of at least two different sorts of key precursor in the glow-discharge plasma. For the deposition of a-SI:H, it has been widely recognized that Sill 3 is preferred as the film precursor for device-quality a-S1 H 2. As has been reported by several groups, S1H 3 has a lower sticking probability than has SIH2 when the growing surface is covered by hydrogen, although both species are abundant in SIH 4 plasma z, 5 A lower sticking probability means a larger surface diffusion coefficient D, of adsorbed species, where D, can be described by the relation D, oc exp - - k T

(1)

Paper presented at the 7th International Conferenceon Thin Fdms, New Delhi, India, December7 1987

*

0040-6090/88/$3 50

11,

© ElsevierSequoia/Printed m The Netherlands

124

K TANAKA, A MATSUDA

where E~ is the thermal activation energy for the site-to-site jumping of adsorbed radical on the surface 2. Specifically, E s is considered to be smaller when the radical has a lower sticking probability and therefore Sill 3 can have enough time to migrate to the surface to find an energetically favourable site, resulting in a high density random network. As is clear from eqn. (1), a higher substrate temperature and/or a smaller value of E, causes a larger value of D, However, too high a temperature reduces the hydrogen-coverage factor for the surface because of thermal evolution of the hydrogen and thereby the sticking probability of Sill 3 approaches unity, 1.e E~ increases. As a result of this trade-off relationship between Es and kT, 250-300 ~'C is known to be the best substrate temperature range for producing high density a-Si: H films 2, A similar discussion has been made on the preparation ofa-Sll _xGex: H alloys via the glow-discharge decomposition of Sill4 GeH 4 mixtures 2. In order to achieve a large surface-diffusion coefficient of adsorbed radicals, two different techniques have been employed, one is termed the "triode" method and the other the "hydrogen dilution" method In the triode approach, radicals are produced between the r.f. and the mesh electrodes of the triode. GeH 3 and SIH3, which are considered to be less reactive than the other germanium- and silicon-related species, are selectively guided through the mesh electrode to the growing surface at the substrate electrode by making use of their longer reaction lifetime in a plasma-free space, since the other reactive species react mostly with their parent molecules and are transformed into inert molecules such as Ge2H~ o r SI2H 6 2. In the hydrogen dilution approach, a larger hydrogen-coverage factor can be obtained by diluting the starting GeH4-SIH 4 mixture with hydrogen, since a higher concentration of atomic hydrogen is produced in the plasma. Using these approaches remarkable improvements in photoconductlvity have been made on a-SiGe H and a-SiC:H alloys 3 It should be noted, however, that too large a value of D, is likely to induce a mlcrocrystalhne structure 2 In this paper, new experimental data on the deposition process for a-S1Ge:H as well as for a-SI:H and a-Ge.H using the triode method are presented and a more detailed discussion of the behaviour of hydrogen on the growing surface is given Some differences in the hydrogen-releasing mechanism between a-SIGe:H and a-SiC: H are suggested. Recent improvements in the photoconductlvity of a-SiGe: H are discussed. 2

EXPERIMENTAL DETAILS

Table I summarizes the typical deposition conditions for a-S11 xGex'H films via the r f (4 MHz) glow-discharge technique with three different modifications. (1) from an SiH4-GeH4 mixture using a diode reactor; (2) from the same gas mixture using a triode reactor (the triode method); (3) from a n SIH 4 GeH4 H2 mixture using a diode reactor (hydrogen dilution) method. The compositional ratios [SI]/[SI + Ge] of those films were found to be lower than the gaseous ratio [-SIH4] / [SIH4 + GeH4] as a result of a difference in the dissociation cross section between SIH 4 and GeH,, 2.~ In order to study the deposition process in detail, deposition rates for a-Sl~ xGex.H as well as for a-Ge.H and a-SI:H were systematically measured as

[S1H4]/[SIH 4 + GeH4] [S1H4]/[SIH4 + GeH4] [H2]/[H 2 +SIH4 + GeH4]

Convenhonal diode Triode H 2 ddutlon

25 25 100-400

(mTorr)

(cm a mln l) 5 5 1-15

Pressure

Flo~ rate

FILMS BY THREE DIFFERENT M E T H O D S

S t a r t m g gas

_xGex H

Method

DEPOSITION C O N D I T I O N S FOR f l - S l I

TABLE I

0 1-1 1 03

(Wcm

2)

Po~ er denst 0'

250 20 480 250

(oC)

Ts

bo

I

©

> Z

~r

Z

-]

126

K TANAKA, A MATSUDA

functions of the s u b s t r a t e t e m p e r a t u r e using the triode m e t h o d where the distance between the mesh a n d the s u b s t r a t e electrodes was kept at 40 m m 3

RESULTS A N D DISCUSSION

F i g u r e 1 shows the r e l a t i o n s h i p between the d e p o s i t i o n rate a n d the s u b s t r a t e t e m p e r a t u r e T,, d e t e r m i n e d e x p e r i m e n t a l l y using the triode technique in o r d e r to select radicals with long r e a c t i o n hfetlmes such as G e H a a n d SIH a. The d a t a are shown for a - S I . H , a-Sl 1 ~Ge~.H a n d a - G e ' H films d e p o s i t e d from SIH4, S 1 H 4 - G e H 4 mixtures and G e H 4 s t a r t i n g gases respectively. As shown in the figure, the d e p o s i t i o n rate for each m a t e r i a l stays a l m o s t c o n s t a n t in a low ~ range, a n d then starts to Increase with an increase in T, when T~ exceeds a certain critical value. It is speculated that the h y d r o g e n - c o v e r a g e factor of the g r o w i n g surface tn the low T, range is a l m o s t unity a n d that the effective sticking p r o b a b i l i t y of SIH 3 a n d / o r G e H 3 is m a i n l y d e t e r m m e d by the h y d r o g e n - c o v e r a g e factor at least a b o v e r o o m t e m p e r a t u r e Therefore, the increase in the d e p o s i t i o n rate t o w a r d s high T, values can be a t t r i b u t e d to a decrease in the h y d r o g e n - c o v e r a g e factor of the g r o w i n g surface. Specifically, the sticking p r o b a b l h t y of S i l l 3 a n d G e H 3, k n o w n to be low when the g r o w i n g surface is fully covered by hydrogen, is e n h a n c e d by an increase m h y d r o g e n - d e f i c i e n t sites (l.e reactive free bonds) on the g r o w i n g surface. The d e p o s i t i o n rate tends to be s a t u r a t e d again at higher T, values, while the sticking coefficient is now unity (as a result of c o m p l e t e e l i m i n a t i o n of h y d r o g e n from the surface) The arrival rate of g r o w t h species to the g r o w i n g surface is finite 10 "--

,

/!

,

05

v

•

D~

czI 122

Q /

•

~ID. I

z O

oel_

it

01

001

3

14 '

15 '

16 '

17 18 19 20 IO00/T (K -~) '

'

'

'

21 ~ 2'9 30 '

Fig 1 Deposition rate,, plotted against the reciprocal substrate temperature for various materials O, a-Ge H fromGeH4, I , a - S I G e H from a I 1 ratio ofS1H4 GeH4, A,a-StGe H froma9 I ratloof SIH., GeH4, O, Sl H from SIH4 As is seen in the figure, the curve for a - G e : H starts to rise at a lower t e m p e r a t u r e than that for a-Si H. This result seems to be c o m p a t i b l e wlth the fact that the b o n d i n g energy of G e - - H (3.0 eV) is w e a k e r t h a n that of S i - - H (3.4 eV) 7, as long as the a b o v e a r g u m e n t is valid. A c c o r d i n g l y , as the G e H 4 - t o - S I H 4 ratio increases in the starting gas, the d e p o s i t i o n rate increases g r a d u a l l y as a result of a large dissociation cross-section for G e H 4 6 a n d the h y d r o g e n - r e l e a s i n g t e m p e r a t u r e shifts t o w a r d s lower t e m p e r a t u r e s It s h o u l d be n o t e d that the slopes of the curve for the two a-S11 ~Ge~ H series are very close in m a g n i t u d e to that o f a - G e H

DEPOSITION MECHANISM OF

Si-Ge

FILMS

127

F r o m the data shown in Fig. 1, the activation energy for the hydrogen release process on the growing surface can be estimated as described below. In a simplified model for a-Sl:H (or a-Ge:H) deposition, the hydrogen-coverage factor fH lS determined from the following rate equation: d(gofH) _ IH No(1 - fH) - N o f n Vo exp d~ kT

(2)

where No is the number of available sites for hydrogen atoms per unit area (i e. close to the order of surface atomic density of silicon or germanium), I H is the flux density of incident hydrogen atoms onto the growing surface, vo is the frequency factor and AE is the thermal activation energy for releasing hydrogen atoms by breaking the S i - - H (or G e - - H ) bond. In this model, It lS assumed that the incident hydrogen atoms stick to the surface by forming an S i - - H (or G e - - H ) bond when the surface is hydrogen free and that fH can reach an equilibrium value within a monolayer deposition time for a-SI:H or a-Ge:H. Then d(N°fn) - 0 dt

produces j'. =

1 1 + a exp(-- A E / k T )

(3)

with a = -Y-o In

(4)

The sticking probability 7 of SIH3 (or GeHa) is relatively low when the surface is fully covered by hydrogen atoms, while 7 = 1 for the hydrogen-free surface Therefore ~ is correlated with fH through the relation 7 = 7ofn+(l--fn)

= l--in(1-7o)

(5)

where 7o IS the sticking coefficient of SIH 3 (or OeH3) for fH = 1. From eqns. (3) and (5) the deposition rate D of a-SI'H (or a-Oe:H) at a temperature T can be expressed by the equation

D(T) = 7Do

=Do{1

_,-7o_

(6)

1 + a exp(-- A E / k T ) J

where Do is the deposition rate of a-Si:H (or a-Ge:H) when 7 = 1, namely the quantity directly related to the flux density of Sill3 (or GeHa). Experimentally Do can be estimated from the saturated value Dh In the high T~ range of Fig. 1, because the relation D h =

D(~)

_ D o ( y o + a)

l+a DO

(7)

128

K. TANAKA, A. MATSUDA

holds approximately when a >> 1 is assumed. Meanwhde, the constant value D~ in a low T~plateau gives the deposition rate for f . = 1 and therefore 7o is given by ~'o ~

D1

(8)

Dh

Then, from eqns.(6)-(8), the activation energy AE can be determined by replottmg the data of Fig. 1 into the form

(AE) D(T)--yoDo - Do-D(T)

aexp-~

(9)

D(T) - D~ D h -- D ( T )

as long as the data points close to the value of Dh are eliminated Figure 2 shows the replotted data for a-SI.H and a-Ge:H. As is clearly shown, the data points are on a straight line for each material, indicating the validity of eqn. (9). From the slope of the curves, 2.70 eV and 1 42 eV were obtained for the activation energy for a-Si'H and a-Ge H respectively These values are relatively small compared with the bonding energies (3.4 eV and 3.0 eV for S I - - H and G e - - H bonds respectivelyT). However, values for the bonding energy depend on the method of estimation 7'8 and, moreover, it is not necessarily required that AE expresses the bonding energy itself, because AE might represent some sort of effective activation energy for the release of hydrogen from the surface, which is affected not only by the bonding energy but also by other factors such as chemical reactions on the surface. The above argument leads to a conclusion that our simple model for the deposition mechanism can qualitatively explain the experimental observations of a-SI:H, a-Ge.H and a-SiGe:H alloys, namely that the deposition rate is strongly correlated with the hydrogen-coverage factor JH and fH IS essentially controlled by thermal elimination of hydrogen atoms from the growing surface As is seen m Fig 1, since the sticking probablhty 7 of GeH3 as well as Sill3 m the deposition of a-SiGe:H starts to increase at around 250 °C (1000/T = 1.9) as a result of a decrease m JH, the substrate temperature T~was set at this temperature in order to obtain the largest value of D~ (see eqn. (1)) for all the a-S1Ge'H series Figure 3 shows dark conductivities and photoconductlVltles of the prepared a-Sll _xGex'H films as functions of their optical gap Eo. Eo decreases as the germamum-to-slhcon rauo increases in a-Si I -xGex:H alloys and, at the same Ume, the temperature at which ]H starts to decrease as a result of thermal hydrogen ehmination is also lowered This IS the reason why the photoconductlvlty of the triode films is degraded m the lower Eo range in the figure. Using the hydrogen dilution method, however, the photoconductivlty is remarkably improved probably because a higher inodenthydrogen flux I Hcan partially compensate for the effect of the thermal ehminatlon of the hydrogen atoms. In contrast to a-Six ~Gex.H alloys, the deposition rate for a-Sll ~,Cx'H alloys depends strongly on the substrate temperature 3. This is mainly because the partial deposition rate of carbon decreases with an increase m T~ m the range between 150 and 400 ~C 3. Considering the large bonding energy (4 3 eV) of the C - - H bond 7 the above phenomenon should be interpreted m terms of the chemical process associated wIth CH3. It is tentatively speculated that adsorbed CH 3 radicals are

DEPOSITION MECHANISM OF Si-Ge FILMS

129 T

,

,

r

i0 -4

10

E

\

3v \

i,

~c~

\

10-5

\ 2 \

o ,ff S t ~¢~ t~-~ ~

~'°~

O

10 - 6

10 -7

10 8 t--

ik

o

\

G_

e,t

10-9 •

•

g 10-t0

01

14

15

[

o~

1

13

I

16 17 1'8 10001]- (K -~ )

1'9

20

10-H

I[.

i

t

•

i

12 I'3 14 15 I'6 17 OPTICAL GAP E0 (eV)

8

Fig 2 (D(T)-DO/(D h -D(T)) plotted against the reoprocal substrate temperature replotted from the data of Fig 1 O, a-Ge H, $ , a-Sl H Fig 3 Dark conductlvmes (full symbols) and photoconducUvmes (open symbols) of a-SIGe H films plotted against their opUcal gap A , /k, convenUonal &ode method, O, Q, mode method, "A', "~', hydrogen ddutlon method

transformed to CH 4 via reaction on the hydrogen-covered surface even at a low temperature and can stay for a long time, while CH 4 formed on the surface is thermally desorbed from the surface at a higher temperature. This chemical extraction of hydrogen atoms from the growing surface predominates and reduces Ds, which is considered to be the main cause of the defective network structure of a-Sl I x C x : H prepared not only by the conventional diode but also by the triode method 3. By compensating for the deficiency of hydrogen on the surface and by realizing the full hydrogen coverage, the hydrogen dilution method can improve the photosensitivity of a-Sit _xCx:H alloys by several orders of magnitude a Quite recently, as a more realistic surface process, Shimlzu 9 proposed a model in terms of a "growth zone", which may be defined as a thm intermediate threedimensional zone between the gas phase and the stable bulk network. The present model does not contradict with the "growth zone" concept at all, although we do not discuss it explicitly in this paper. More experimental data are needed for a detailed discussion REFERENCES 1 G N a k a m u r a , K Sato, Y Yuklmoto, K Shlrahata, T Maruhashl and K Fujlwara, Jpn J Appl Phys , 20 Suppl 1 ( 1981 ) 291 2 K T a n a k a a n d A Matsuda, Mater Set Rep,2(1987) 141,andreferencesotedtherem 3 A Matsuda and K Tanaka, in Pro~ 12th Int Conf on Amorphous andLtqutdSemtconductor~, Praha, 1987, to be pubhshed 4 S AIjlshl, V Chu, Z E Smith, D S Shen, J P Conde, D Slobodm, J Kolodzey and S Wagner, m Pro~ 12th lnt Con[ on Amorphous and Ltqutd Seml~ ondu(tors, Praha, 1987, to be pubhshed 5 J Perrln, m Pro~ 12th lnt Conf on Amorphous and Ltquld Semt~ondu~tor~, Praha, 1987, to be pubhshed 6 U Ito, Y Toyosh]ma, H Onukl, W W a s h l d a a n d T Ibukl, J ('hem Phys , 85 (1986) 4867

130 7

8 9

K. TANAKA, A. MATSUDA

G Lucovsky, m F Yonezawa (ed), Fundamental Phystc~ of Amorphous Semtconductora, Springer, Berhn, 1981, p 87 Amertcan lnstztute of Physics Handbook, McGraw-Hill, 1972, pp 7 169 I Shlmfzu, m Proc 12th Int Conf on Amorphous and Ltquld Semiconductors, Praha, 1987, to be pubhshed