NMR investigation of hydrogen in amorphous silicon and related materials

Journal of Non-Crystalline Solids 114 (1989) 211-216 North-Holland

211

Section 7: Silicon structure and hydrogen bonding

NMR INVESTIGATIONOF HYDROGEN IN AMORPHOUS SILICONAND RELATED MATERIALS J. B. BOYCE,a S. E. READY,a M. STUTZMANN,a,band R. E. NORBERGc aXerox Palo Alto Research Center, Palo Alto, California 94304, USA b Max-Planck-lnstitut fCir Festk6rperforschung, Stuttgart, Federal Republic of Germany CDepartment of Physics,Washington University, St. Louis, Missouri 63130, USA Hydrogen NMR spectra and spin lattice relaxation times have been obtained as a function of temperature for a-Si:H that had been sequentially annealed up to 600C, for pc-Si:H, and for a-Sil-xGex:H alloys. These three materials differ from dense-grade a-Si: H in that they have more and larger voids containing larger quantities of non-bonded molecular H2. The low-density, intergranular regions in pc-Si:H provide more interconnected, open space than do the voids in annealed a-Si:H and in the Ge rich alloys. 1.

INTRODUCTION

2.

EXPERIMENTALDETAILS

Nuclear magnetic resonance (NMR) measurements

The samples were prepared by the glow discharge

have provided much useful information on the local

deposition of the appropriate mixtures of silane,

atomic bonding and microstructure in hydrogenated

germane and hydrogen gases. Low power density (0.02-

amorphous silicon and related materials. T M Here we

0.1 W/cm 2) and heated substrates (230-250C) were used

describe NMR results on the H distribution and bonding

in order to produce good quality films. The aluminum

in these materials, contrasting the results on a-Si:H,

foil substrates were etched away in dilute HCI. Both the NMR spectrum and the spin-lattice

thermally annealed a-Si:H, microcrystalline Si:H (pcSi:H) and amorphous (Si,Ge):H alloys, tn a-Si:H, most of

relaxation time, T1, of H were measured on these

the hydrogen is bonded to the Si in a dilute phase and in

samples as a function of temperature, T, from 1.4K to

a clustered phase, with a small fraction (41%) of the H

300K and, in some cases, to 470K. The spectrum was

in the form of H2 molecules. A larger fraction of the H is

measured at a Larmor frequency of 92.SMHz (113MHz

found to be H2 in these other three materials systems.

for pc-Si) and was obtained from a Fourier transform of the free induction decay. A two component line was

i

(a)

observed in all samples, and the concentration of hydrogen, n(H), and the full width at half maximum (FWHM) were determined. T1 was determined from the magnetization recovery following a saturation pulse sequence. This recovery was observed to be exponential for most of the samples and temperatures studied.

_t i

(b)

,,

; \j

3.

EXPERIMENTALRESULTS 3.1. a-Si:H The NMR spectrum of H has been extensively

studied.t, 2 The narrow line with FWHM=3-4kHz (see Fig. la) corresponds to isolated Si-H units randomly

200 0 (v-vo) (kHz) Fig. 1. H NMR spectrum of a-Si:H t h a t has been annealed at 500C, showing the Pake doublet. - 200

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

distributed throughout the amorphous Si network. The broad line (FWHM-~25kHz) arises from regions with more clustered H, thought to be hydrogenated internat surfaces, polyhydride groups and polysilane chains. The

212

J.B. Boyce et aJ./NMR investigation of hydrogen in amorphous silicon

total hydrogen content for the samples prepared on

characteristic minimum in the vicinity of 50K (see Fig. 3)

heated substrates is found to be ~ 10 atomic %, with ~ 4

due to the presence of a small number of H2 molecule

at. % in the narrow line and ~ 6 at. % in the broad line.

relaxation centers on the surfaces of voids in the a-Si

When the sample is cooled to low temperatures,

host J5 The H atoms in the molecule relax quickly due to

the NMR spectrum evolves into a three-corDponent line:

interactions between the rotational angular momentum

the central line w i t h t w o w i d t h components, as

of the ortho-H 2 and the phonons of the host. The H2

observed at RT, plus a Pake doubletS,6, shown in Fig. l b

molecules thereby serve as relaxation centers for the H

for T=1.43K, w i t h a splitting of 175 +10kHz. The

atoms bonded to the Si, relaxing them by spin diffusion.

observed doublet is due to orientationally ordering of molecular H2 or a slowing d o w n of the molecular tumbling rate to less than the 175kHz splitting. As T increases, the Pake d o u b l e t collapses due to the averaging by the rapid molecular tumbling and the signal due to H2 is transferred to the broad line (Fig. 2b). As T increases further, the solid H2 melts and the signal becomes motionally narrowed and contributes to the narrow line. This has a measurable effect on the signal fraction in the two central line components in samples containing a large amount of H2 (Fig. 2b) and a small effect for samples with small amounts of H2(Fig. 2a).5 The NMR spin-lattice relaxation time T~ exhibits a I

I

I

I

I

i

I

3.2.

Annealed a-Si:H

Two good quality samples prepared from pure silane were annealed in a sequence of one hour anneals form 300 to 600C. The NMR spectra and T 1 w e r e measured versus T at each anneal step. The spectra versus T evolves from that of Fig. 2a to that of Fig. 2b, which show the results for as-deposited and 500C anneal for another similarly-prepared sample. The quantities of hydrogen in the three components of the NMR line are shown in Fig. 4 as a function of anneal temperature, TA. It is seen that no significant change occurs until TA--> 450C. This is to be contrasted with the charges observed in the hydrogen3 and deuterium8 NMR spectra at about TA~200C for samples deposited on RT substrates, i.e.,

c~B

0.6

poor electronic grade material. These observations are

O_

z

consistent with evolution studies 16 which show a Iow-T

CJ <[ 0.4 14.

(~400C) and high-T evolution peak (~600C) for RTsubstrate films but only a high-T evolution peak for substrate tem peratures above 200C.

~ o.2 5

C~p

i

f , 0.6

(b)l

I

1

c~B

.~'

I ~f~l

,

/T

J

I

~ 0.2 0.1

o~

,} 10

o = Unannealed × = 5 8 0 C anneal

~,',x

0.05

"?,~_

5

,x'

0.5

o.4

2

|

_

ry[4.

~ o.2 - % ?

i

1 ba

Z

i

2 %

_

z O

i

0.02

20 T(K)

50

100

200

500

0.01

I

I

I

I

I

I

I

2

5

10

20

50

100

200

500

T (K)

Fig. 2. The fraction of the NMR signal in the three components of the line versus T for a-Si:H (a) asdeposited and (b) annealed at 500C.

Fig. 3. T1 versus T for a-Si:H, as-deposited and annealed at 580C.

J.B. Boyce et al./ NMR investigation of hydrogen in amorphous silicon

213

other hand, does not change significantly with T. This

T~ versus T for as-deposited and TA=500C is shown above in Fig. 3. Upon annealing, the usual T~ minimum2

narrow line has been observed previouslyg, lo and has

at 54K disappears and another minimum of comparable

been attributed to mobile, bonded H, such as Sill 2

magnitude appears near 23K. In a d d i t i o n , a deep

chains. We find that it is due, in large part, to molecular

m i n i m u m develops near RT, due, most likely, to relaxation by mobile hydrogen. The T~ data at the

•

Hz. The fraction of the various components of the NMR

intermediate sequential annealing steps show the

line vary with T, as shown in Fig. 5. The Pake doublet due

gradual disappearance of the 54K minimum and the

to molecular hydrogen is observed b e l o w - 5 K . Its

gradual growth of the 231< minimum. This trend has

content, which goes from 0 to 25% of the total signal, is

probably been observed previously but with incomplete

approximately equal to the decrease in the narrow line

data so that it was not fully recognized by Carlos and

from 40% to 10% of the signal, as T goes from 300K to

Taylor2 and Bork, et. al.,7 w h o observed a shift of the T~

OK. This indicates that a major component of the narrow

minimum to low temperatures for TA= 500C. Carlos and

line is m o b i l e molecular H 2. The T 1 vs. T shows a

Taylor2, however, observed the a l m o s t c o m p l e t e

minimum of about 0.3sec near 40K, as for good quality

evolution of the H 2 relaxation centers at TA= 530C. This

a-Si:H. This indicates that H2 relaxation centers are present in pc-Si:H in about the same concentration as

does not happen in our samples until TA >600C. 3.3.

for a-Si: H.

pc-Si:H

Three samples of pc-Si:H prepared with different

3.4.

a-(Si,Ge)'H alloys

H2/SiH4 ratios (98/1, 54/1, and 20/1) were investigated.11

A series of a-(Sil.xGex):H alloys was prepared from

These have a total H content of 12, 8, and 6 at. %,

the a p p r o p r i a t e m i x t u r e s of Sill 4 and GeH 4 and

respectively.t1,17 The narrow-line linewidth is quite

deposited on heated substrates (230C). 13 The Si and Ge

narrow, =0.4kHz, to be compared with 3-5kHz for a-

content was determined using electron microprobe. The

Si:H. This narrow line broadens as the sample is cooled,

spectra were measured at RT as a function of Ge content

reaching 5kHz, comparable to the narrow linewidth of

and, for some alloys, down to low temperatures. The

a-Si:H, below 201<. The width of the broad line, on the

total hydrogen content and that in the broad and narrow lines versus x are shown in Fig. 6. The total H

10

'~ I .........................

9

I n(H) Tot A n(H) Broad line _ '~ [] n(H)Narrowllna

content varies in a bowed fashion, being about equal at 1.0

I

I

, × n(H2)

c

J

I

• 0.8

7

I

I

J

98/1 H2/SiH4

•

e e , ~ c~B

A

== 6 0.6

o

4 "'''~r

-r

u. ~g c

.....

E7 - -"

",

3 z~ "b

2

",

~P

", ", 0.2

1 0

200

aN

0.4

'

A ~ '~

"/

-:I

I

300

400

,--~'"

I

500

"x-.~

600

Tannaal(C)

Fig. 4. The hydrogen content in the various components of the NMR line for annealed a-Si:H v e r s u s TA.

j

2

,b~

5

I

I

I

t

I

10

20

50

100

200

500

Temp(K) Fig. 5. The signal fraction versus T of the NMR signal in the three components of the line for pc-5i:H.

J.B. Boyce

214

et

al./ NMR investigation of hydrogen in amorphous silicon

the endpoints and about twice as large at 50:50. This

the w i d t h of the narrow line at RT, being considerably

differs from the results of othersl2,14,1a w h o find that

n a r r o w e r for the three systems discussed here. The

the H content drops w i t h x f o r t h e i r samples. The

w i d t h increases as the material is cooled and splits into a

narrow-line H-content in our materials is essentially

Pake doublet below about 1OK. This indicates that the

constant at = 5 at. %, the a-Si:H value, until the Ge-rich

RT narrow line is due, in large part, to mobile hydrogen

end of the diagram where it doubles t o = 10 at. %. Fig. 7

which is p r e d o m i n a n t l y molecular hydrogen moving

shows the signal fractions versus T for a-Ge:H, where it

about in voids.

is evident that mobile molecular H2 contributes to the RT

substantial changes occur in the H NMR until TA--400C,

narrow line, appearing below 25K as a Pake doublet.

consistent w i t h H - e v o l u t i o n studies16,19 on samples

For

the

annealed

a-Si:H,

no

T 1 vs T for a-Si:H, a-Si0.42Ge0.sa:H and a-Ge:H is

prepared on heated substrates. The H 2 content increases

shown in Fig. 8. The T1 minimum indicates the presence

w i t h TA up to about 500-550C as the total H content

of H2 relaxation centers. The numbers of these centers

drops (see Fig. 4). Above this TA, the H2 in the voids also

are of comparable magnitude for a-Si and a-Ge but

begins to evolve. The RT linewidth of the narrow line

about an order of magnitude smaller in the alloy.

narrows for TA>500C , evidence of the mobility of the H2

4.

in the voids. This implies that the void size grows w i t h TA

DISCUSSION All three of the materials systems discussed here are

similar in that they all contain a large quantity of H (~ 10 at. %) that resides in three distinct phases: a clustered phase exhibiting a broad (~25kHz FWHM) NMR line, a dilute phase exhibiting a narrow NMR line, and nonbonded molecular hydrogen t h a t c o n t r i b u t e t o the narrow NMR line near RT but is split off as a 175kHz

to accommodate the larger a m o u n t of H2 and to provide the space for the H2 to move and motionaily narrow the linewidth. As these samples are cooled, the narrow line broadens as the translational motion of the H2 freezes out. Cooling f u r t h e r b e l o w a b o u t 1OK causes the molecular tumbling motion to freeze out and the Pake d o u b l e t appears in the spectrum.

Pake doublet at low temperatures ( < 10K). In addition,

The T 1 versus T for the various TA'S (Fig. 3) provides

all these materials exhibit a m i n i m u m in the T~ vs. T

interesting complementary information on the charges

curves at l o w temperatures due t o relaxation by H2 molecules on the surfaces of voids. A major difference between a-Si:H and these materials is the H content and 30

I

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a-Sil-x G e x : H 25

,

20

15

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200

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ON

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CtB

0.6

n(H),,0J..

n(H) /

I

; t_,

& o.4

~

I

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'" "

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/

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./

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10~

"r

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a-Ge:H

\ \ "",,,

/

I

-"'f,>n):o,

,,~"

{:n

£

.'"

/"

increasing TA. The T 1 minimum near 54K, that exists in as-deposited a-Si:H, goes away and another minimum

"m.

,'" • .'"

'E

c~ o~j

-.

/

w/ ,

(~

'='m

I

m. ....

in the void structure and on the nature of the H2 with

1.0

x in Solid

Fig. 6. The hydrogen content in the various components of the NMR line versus Ge content, x.

I 2

I 5

{ I 10

I 20

I 50

i 500

T(K)

Fig. 7. The fraction of the NMR signal in the three components of the line for a-Ge:H versusT.

J.B. Boyce et al./ NMR investigation of hydrogen in amorphous silicon

20

I

I ~--

10 --

I I I a-Si 042Ge0 s8: H

"-:~

~

215

interconnected space for H 2 m o t i o n . The second

I

difference is in the amount of H that makes up the

.~"

immobile portion of the narrow line as T - . 0. This is 5

v

bonded hydrogen in the dilute phase and consists of

2

about 10% of the H in pc-Si:H (Fig. 5) and ~30% of the

o.

H in a-Si:H annealed to S00C (Fig. 2b). In both cases, J 0.5

a-Ge:H

•. " ~ • ' *

e-~o

however, the broad line contains about half of the H.

-: ,,~

A possible e x p l a n a t i o n f o r b o t h o f t h e s e observations is that the microcrystalline regions contain

0.2

little or no H and the amorphous matrix regions are

0.1

I 2

I 5

I I I I I ~ I 10 20 50 100 200 500 1000 T (K) Fig. 8. T1 versus T for a-Si:H, a-Ge:H, and a-Si0.42Ge0.ss:H. near 23K appears as T A is increased. The minimum is due to relaxation center H2 molecules on the surfaces of voids and occurs w h e n t h e m o l e c u l a r a n g u l a r momentum relaxation rate, Fm, is about equal to w 0. Since cu0/2n was kept fixed at 92.5MHz,

F(T)

has

changed. The coupling of the molecular t u m b l i n g motion to the a-Si phonons has increased, thereby increasing

['m

and pushing the T~ minimum to lower

temperatures. This implies that, as the void sizes grow and void shapes change with annealing, the relaxation center H2, located at the surfaces of these voids, become more strongly coupled to the phonons of the a-Si. For a T2 functional dependence of I - , a six-fold increase in the coupling to the phonons occurs w i t h a n n e a l i n g , provided the Debye temperature and the EFG symmetry remain unchanged. This is a substantial increase in the phonon coupling constant. The change in the void structure responsible for the charge is, at this point, unclear. For pc-Si:H, the H NMR spectral features are similar to those of a n n e a l e d a-Si:H. A m a j o r d i f f e r e n c e between the two is in the nature of the narrow line where two significant differences are noted. First, for pc-Si:H, the RT width is about 0.4kHz versus about 11.5kHz for a-Si:H annealed to 500C. The motion of the H2 in pc-Si:H has a more substantial effect on the linewidth than does the motion in a-Si:H annealed to S00C. A reason may be t h a t t h e l o w - d e n s i t y , intergranular regions in pc-Si provide more open,

highly defective, containing large interconnected voids in the inter-grain regions. The hydrogen bonded to the Si on the surfaces of these voids accounts for the H in the broad line. These open intergranular regions contain the molecular H2, a small fraction of which is adsorbed on the surfaces of these regions and serves as relaxation centers for the remaining hydrogen and yields the T 1 minimum near 50K. The H2 in the open regions is able to diffuse readily at RT, giving rise to the narrow line, but its translational motion stops as the H2freezes near 20K. The small amount of dilute-phase bonded H can be contained at the surfaces of the crystalline regions or in regions of the amorphous matrix that have l o w H density. The fact that it has the standard narrow-line linewidth of 4-5kHz as T--, O, argues against it being distributed in the microcrystalline regions at, say, defect sites. Rather it appears that the microcrystalline regions are relatively free of H and most of the H, bonded and non-bonded, is contained in the amorphous matrix regions. For the a-Sil_xGex:H alloys, the H content is found to be about the same at ~ 15 at. % for the Si and Ge ends and to increase to about 27 at. % at 50:50 (Fig. 6). The bowed behavior differs from the monotonic decrease in n(H) w i t h increasing Ge c o n t e n t , x, observed by

others.12,14,18,19 This increase in H content t h a t w e observe occurs entirely in the clustered phase since the dilute phase content remains constant at ~5 at. % until the Ge end, where it doubles to ~ 10 at. %. In addition to doubling in c o n t e n t at the Ge-rich end of t h e diagram, the narrow-line linewidth decreases f r o m ~3kHz for x < 0.6 to = l k H z for x > 0.9. Also a significant fraction of this narrow line in the Ge-rich

J.B. Boyce et al./NMR investigation of hydrogen in amorphous silicon

216

alloys is due to molecular Hz. As the a-Ge:H is cooled,

sample preparation. This work was supported in part by

the narrow line broadens quickly and a Pake doublet

the SERI and by NSF DMR 87-01515.

appears at lower temperatures (< 25K) that contains = 2 at. % H2 (Fig. 7). This behavior is similar to that observed

REFERENCES 1.

J.A. Reimer, R. W. Vaughn, and J. C. Knights, Phys. Rev. B 24(1981) 3360.

2.

W.E. Carlos and P. C. Taylor, Phys. Rev. B 26 (1982) 3605.

3

J.A. Reimer, R. W. Vaughan, and J. C. Knights, Sol. State Commun. 37 (1981) 161.

4.

D.J. Leopold, J. B. Boyce, P. A. Fedders, and R. E. Norberg, Phys. Rev. B, 26 (1982) 6053.

5.

J.B. Boyce and M. Stutzmann, Phys. Rev. Lett. 54 (1985) 562.

6.

J. B. Boyce, in Hydrogen in Disordered and Amorphous Silicon, eds. G. Bambakidis and R. C.

in a-Si:H annealed to ~500C. It is also consistent with the conclusions stated above for annealed a-Si:H and drawn from hydrogen evolution studies 19, namely, that the addition of Ge creates alloys possessing a void-rich structure w h i c h becomes more p r o n o u n c e d w i t h increasing Ge content. The NMR shows that these voids contain a significant amount of H2 and that the total H2 content is larger for a-Ge than for a-Si. The T 1 results on our samples indicate that the number of relaxation center H2 molecules are about the

Bowman, (NATO Advanced Study Inst. Proc. Rhodes, Greece, 1985), p. 101.

same for a-Si:H and a-Ge:H (nRc(H2) ~-0.1 at. %), even though the total H2 content is higher for a-Ge: H. But it is

7.

V.P. Bork, P. A. Fedders, R. E. Norberg, J. B. Boyce, and M Stutzmann, J. Non. Cryst. Solids 77&78 (1985) 711.

8.

V. P. 8ork, P. A. Fedders, D. J. Leopold, R. E. Norberg, J. B. Boyce, and J. C. Knights, Phys. Rev. B 36 (1987) 9351.

9.

S. Hayashi, K. Hayamizu, S. Yamasaki, A. Matsuda and K. Tanaka, J. Appl. Phys. 56 (1984) 2658.

lower for a-Si0.42Ge0.58:H by an order of magnitude (nRc(H2)~0.014 at.%). This comparison between x = 0 and x = 1/2 agrees w i t h the results of Vanderheiden, et a1.,14 but is different for x = l , a-Ge:H. The reason for this discrepancy is not known but is most likely related to the d i f f e r e n t d e p o s i t i o n rates and substrate temperatures used in the sample preparation. This may also account for the different total H-content variation with x. 5.

CONCLUSIONS The h y d r o g e n NMR spectra and spin lattice

relaxation times have been measured as a function of T for annealed a-Si:H, pc-Si:H, and a--(Si,Ge):H alloys. The Ge-rich alloys and the annealed a-Si:H (TA~500C) are very similar to one another and different from densegrade a-Si:H in that they have more and larger voids containing larger quantities of non-bonded molecular H2. pc-Si:H has a more open, interconnected structure. Also the amount of bonded H in the dilute phase is less, consistent w i t h the suggestion t h a t the crystalline regions contain little or no hydrogen. ACKNOWLEDGMENTS The authors are pleased to thank C. C. Tsai and N. M. Johnson for helpful discussions and R. Thompson for

10. M. Kumeda, Y. Yonezawa, A. Morimoto, S. Ueda and T. Shimizu, J. Non. Cryst. Solids 59 (1983) 775. 11. S. E. Ready, J. B. Boyce, and C. C. Tsai, Mat. Res. Soc. Symp. Proc. 118 (Materials Research Society, Pittsburgh, 1988), p. 103. 12. T. Shimizu, M. Kumeda, A. Morimoto, Y. Tsujimura, and I. Kobayashi, Mat. Res. Soc. Syrup. Proc. 70 (Materials Research Soc., Pittsburgh, 1986), p. 313. 13. M. Stutzmann, R. A. Street, C. C. Tsai, J. B. Boyce, and S E. Ready, J. Appl. Phys. 66 (1989) 569. 14. E. J. Vanderheiden, G. A. Williams, P. C. Taylor, F. Finger, and W. Fuhs, MRS Proceedings, 1989, to be published. 15. M. S. Conradi and R. E. Norberg, Phys. Rev. 8 24 (1981) 2285. 16. W. Beyer, in Tetrahedrally-Bonded Amorphous Semiconductors, eds. D. Adler and H. Fritzsche, (Plenum, New York, 1985), p. 129. 17. N.M. Johnson, S. E. Ready, J. B. Boyce, C. D. Doland, S. H. Wolff, and J. Walker, Appl. Phys. Lett. 53 (1988) 1626. 18. B. von Roedern, D. K. Paul, J. Blake, R. W. Collins, G. Moddel, and W. Paul, Phys. Rev. B 25 (1982) 7678. 19. W. Beyer, H. Wagner, and F. Finger, J. Non-Cryst. Solids 77&78 (1985) 857.