Electrical and optical properties of hydrogenated amorphous carbon films

Journal of Non-Crystalline Solids 35 & 36 (1980) 435-440 ~North-Holland PublishlngCompany

ELECTRICAL AND OPTICAL PROPERTIES OF HYDROGENATED AMORPHOUS CARBON FILMSa B. Meyerson and F.W. Smith Department o f P h y s i c s The C i t y C o l l e g e o f New York New York, New York 10031

We have measured the electrical conductivity (25 to 350C) and optical absorption (1.65 to 4 eV) of a series of hydrogenated amorphous carbon (a-C:H) films prepared via dc glow discharge decomposition of acetylene (C2H2) at deposition temperatures T d between 25 and 375C. The electrical conductivity is not simply activated, and varies by over ii orders of magnitude for the samples studied. Optical energy gaps inferred from optical absorption data lie between 0.9 and 2.1eV, decreasing with increasing T d. It can be inferred from these measurements that the films will be ngraphltic" in their electrical and optical properties for T d greater than ab6ut 425C. INTRODUCTION There is a growing interest in the properties of amorphous semiconductors. In the present drive towards cost-efficient semiconducting devices, notably photovoltaic devices, the thin film technology involved in the production of most amorphous semiconductors makes them of particular interest. From the standpoint of feasibility, recent successes in doping hydrogenated amorphous silicon (a-Si:H) I have further enhanced interest in the field. We report here on studies of the electrical and Optical properties of hydrogenated amorphous carbon (a-C:H) films. Amorphous carbon films produced by ~eans of glow discharge decomposition of o.,ganic yapors form an interesting and technologically important class of thin films 2. Such films, sometimes referred to as organic polymer films, have found applications as dielectrics in the electronics industry and as protective coatings for metals and other reactive surfaces, but there have as yet been only a few systematic attempts to investigate their electrical and optical properties and to determine their potential usefulness for el~3ctronic applications3,q. Such a t t e m p t s seem p a r t i c u l a r l y t i m e l y now s i n c e i t has been demonstrated r e c e n t l y t h a t amorphous carbon f i l m s can be produced w i t h p r o p e r t i e s which a r e c l o s e r to those o f diamond, a l a r g e band gap semiconductor, than to t h o s e o f g r a p h i t e , a semimetal. In p a r t i c u l a r , t h e f i l m s a r e e x t r e m e l y h a r d , n e a r l y t r a n s parent, insoluble in a v a r i e t y of solvents, highly i n s u l a t i n g ~ 0 1 2 £ ~ e m ) , and of high d i e l e c t r i c s t r e n g t h s , ° . Since t h e s e f i l m s c o n t a i n a p p r e c i a b l e amounts of hydrogen, we w i l l r e f e r t o them as hydrogenated amorphous carbon f i l m s (a-C:H). We p r e s e n t here r e s u l t s on the growth o f a-C:H f i l m s , d e t a i l i n g the effects of growth parameters o n the electrical and optical properties of such films. EXPERIMENTAL

The a-C:H samples were prepared i n a f l o w - t h r o u g h dc glow d i s c h a r g e a p p a r a t u s . For o p t i c a l a b s o r p t i o n J t u d i ~ s , g l a s s s l i d e s were used as s u b s t r a t e s . For c o n d u c t i v i t y s t u d i e s ~ a 1000X t h i c k Ho f i l m e v a p o r a t e d onto the s l i d e s

435

436

So Meyerson, F.W. Smith / Hydrogenated Amorphous Carbon Films

(Mo/Glass~ was used. The system was flushed with dry N 2 and then pumped to below 10 -0 Torr, a flow of acetylene (Linde 99.6~) was established at 0.5-1.0 sccm, using a system pressure of 0.9 Torr. On the Mo/Glass substrates, a-C:H was deposited using a screen cathode lcm above the substrate, the growth parameters being deposition temperature Td=25 to 375C, discharge current id=0.8 to 2 ma, with a deposition rate rd=lO~/sec. For deposition on glass, both id and r d dropped by an order of magnitude. Resulting films were from 1500 to 7500 A thick. T h e voltage necessary to maintain the discharge varied from 300 to 400 volts, decreasing with increasing T d. Following deposition, 1.6mmMo dots w~re deposited on the a-C:H films grown on the Mo/Glass substrates, forming a sanO~ich geometry, Mo/a-C:H/Mo dot, for conductivity studies carried out in high vacu~,m. Measuring voltages of 0.I Volt were typically used. The optical absorption ~tudies, from 1.65 to 4.eV, were performed using a GCA/McPherson Spectrophotometer. RESULTS

The a-C:H films prepared on Mo/Glass substrates for Td4200C , were nearly transparent. As T d increased above 200C, the samples grew progressively darker, until they were a glossy black at Td=350C. Films grown on glass were a faint tan, and highly transparent at Td=25C. As T d approached 350C, the films became deep brown, but were still moderately transparent. The a-C:H films were insoluble in a variety of organic solvents and acids, were quite hard, and proved extremely resistant to scratching. The films adhered well to both types of substrates for thicknesses of 4 1 micron. The films grown at Td=25C were unstable when heated to 350C in vacuum, and flaked off the substrate, but all others remained intact during this process. After being heated to 350C and following prolonged exposure to air, those samples with T d , 250C also deteriorated. Representative electrical conductivity data for these samples are shown in Fig. I, where qr is plotted versus I/T on a logarithmic scale. The dependence of r on T d is quite strong, with the room temperature conductivity q'(RT) varying from about 10 -16 to i 0 - 6 ~ -I cm -I as T d is increased from 75 to 350C (see Fig. 2). Except for the Td=350C sample, ~" does not have the simple activated form, O'(T)=U'oexp(-EA/kBT), w i t h ~ a n d EA independent of measuring temperature. As a means of characterizing these curves, the two parameters O~e(250) and EA(250) , obtained from the intercept and slope, respectively, of tangents to the curves at 250C, are presented as functions of T d in Fig. 2. (~e(250) is in the range 10 -4 to lO-2/L-icm -I and is observed to be essentially independent of T d. g A (250), however, is observed to decrease linearly with increasing Td, extrapolating to zero at about Td=425C. Annealing the samples at 350C typically resulted in a decrease in(r(T) by a factor of twQ, indicating that these a-C:H films are thermally stable (in vacuum) up to this temperature. The measured energy dependence of the optical absorption coefficient OC indicates that the optical absor~tio~ "edges" in these a-C:Hfilms are quite broad. For the Td=300C s a m p l e , ~ l O cm- at 1.6 eV, rising to 105cm-I at 3.3eV. The corresponding energies for the 150C sample are 2.25 and 3.geV. Optical energy gaps Eop t have been determined for these samples by plotting (~E)} versus E as a test of the expression (~E)}=A(Eopt-E) where E is the photon energy, and A is a constant. Eopt, displayed in Fig. 3, was obtained from the intercept of the extrapolation of the linear part of the curves to (~E)}=O. Eop t is observed to decrease from 2.1 to 0.9eV as T d increases from 25 to 375C, with most of the decrease occurring for T d greater than 250C. Preliminary infrared absorption studies of these a-C:H films have indicated a strong absorption due to C-H bond-stretching modes near 2900cm -I , with little or no absorption due to C=O bonds observed in the region near 1700 cm-I.

8. Meyerson, F.W. Smith / Hydrogenated Amorphous Carbon Films

10"3/

'

I

'

I

'

I

437

'

Td(C)-

L

-

Fig. 1 Electrical conductivity~ as a function of 1/T for these a-C:H films. Td=deposition temperature.

~

I0°

-__

__

--.%I0 -'s.5

,

I

,

I~7 5 , ~ -

2.0 2.5 3.0 I / T (10 "~ K " )

3.5

DISCUSSION For the semiconducting a-C:H films studied, with T d between 75 and 350C, the curvature of the log Q" versus 1/T plots and the low values of(~.(250) obtained (in the range I0 -q to 10-2~L-Icm -1) would seem to preclude simple conduction via extended states in either the valence or conduction band. In addition, our conductivity data are not consistent with variable range hopping conduction in states near the Fermi energy EF, as plots of logO'vs T'~ are nonlinear7. A possible mechanism for the electrical conductivity in these films might involve thermal excitation into a broad range of localized energy states (tail states) at one of the band edges, where conduction would occur via thermally activated hopping 7. Such a mechanism would explain the low values of~(250) observed experimentally if, as expected, the mobility associated with such localized states was low. I n a d d i t i o n , t h e o b s e r v e d c u r v a t u r e i n l o g ~ ' v s 1/T would r e s u l t i f a d d i t i o n a l e n e r g y l e v e l s , e i t h e r i n i t i a l or f i n a l , a r e a c c e s s i b l e to t h e r m a l a c t i v a t i o n as the temperature is raised. Such a broa d r a n g e o f e n e r g y s t a t e s a t t h e band e d g e s i s not i n c o n s i s t e n t w i t h our o p t i c a l a b s o r p t i o n d a t a which i n d i c a t e t h a t the optical absorption "edges" in these a-C:H films are smeared out. For the case of thermally activated hopping in localized states at the valence band edge, for example, O'(T)=~exp(-(EF-EB + WI)/kBT), where E B is the energy at the valence band e6ge and W I is the activation energy for hopping7. We note that the pre~actor ~ is expected to be 102 to 104 times lower than for extended state conduction, which is consistent with our observations (Fig. 2). For t h e s e a-C:H f i l m s , " t e m p e r a t u r e - d e p e n d e n t " v a l u e s o f ~ and EB (or EA f o r t h e c o n d u c t i o n band edge) may a r i s e dde to t h e e n e r g y dependence o f

438

B. Meyerson, F.W. Smith / Hydrogenated Amorphous Carbon Films

m

'E

'

I

'1"

t o -2

I

0

0

0

0

0

0

0

0

~ 10-4

0

I'

I"

10"" o

I0"°

e

o

I0"" ~-

o 1.0

>

-

e

l

Ol

I

0.8-

OJ

.~ la.l

I

@ 0

0.6 o

0

0.4.-

o @ 0

0.2oo

,~-,6

,

I

,oo

i

-

I

zoo 3oo T,(Cl

4oo

Fig. 2 Parameters obtained from the conductivity data as functions of deposition temperature T d. f(RT)foonductivity at room temperature. Q'o(250) and EA(250) are the intercepts and "activation" energies obtained from straight lines drawn tangent to the conductivity curves (Fig. I) at T=250C.

the m o b i l i t y ~ ( E , T ) , i f t h e energy a t which t h e dominant c o n d u c t i o n p r o c e s s e s take p l a c e v a r i e s with t e m p e r a t u r e in a n o n - n e g l i g i b l e way. That the l a t t e r b e h a v i o r should be c o n s i d e r e d f o r amorphous semiconductors has been s t r e s s e d r e c e n t l y 8. As has been p o i n t e d o u t , measurements o f the t e m p e r a t u r e dependence of th~ t h e r m o e l e c t r i c power a r e e x t r e m e l y u s e f u l i n o r d e r to t e s t t h e s e h y p o t h e s e s 5. We p l a n t o u n d e r t a k e such measurements on t h e s e a-C:H films. The f a c t o r which appears to determine the v a r i a t i o n of (J" w i t h Td in t h e s e samples is the decrease of EA(250) with increasing T d. For Td=75C, the "activation" energy EA(250)=O.98eV is one half of the optleal gap Eopt=2eV, i n f e r r e d from F i g . 3. One may t h u s conclude t h a t t h e Fermi l e v e l EF must l i e near the middle o f the band gap f o r a sample w i t h t h i s Td. As Td i n c r e a s e s up to 250C, EoDt remains f a i r l y c o n s t a n t a t 1.9-2.0eV, while EA(250) i s observed to d e c r e a s e from 0.98 t o about 0.4eV, s u g g e s t i n g a s h i f t

8. ~4eyerson, F.W. Smith / Hydrogenated Amorphous Carbon Films

~.0I F i g . 3 O p t i c a l energy gap Eopt as a f u n c t i o n o f d e p o s i t i o n temperature Td f o r t h e s e a-C:H f i l m s .

i

I

i'

I

,

l

I

'

I

i

I

I

2.0

Q. 0

i,I

1.0

O0

,

I00

200

I

300

I

I

400

T.(C) o f EF by 0.6 eV c l o s e r to one of the band edges. We note t h a t EA and Eop t have not beer determined on the same f i l m s (see e x p e r i m e n t a l d e t a i l s ) , so t h a t t h e s e c o n c l u s i o n s are o f a p r e l i m i n a r y n a t u r e .

As T d increases above 250C, Eop t drops raoidly, signalllng the onset of fundamental changes in the bon~ing and band structure in these a-C:H films. Such changes would result from a decreasing incorporation of hydrogen at h i g h e r Td. With l e s s hydrogen p r e s e n t , the f r a c t i o n of 3 - f o l d as opposed t o q - f o l d c o o r d i n a t e d carbon atoms would be expected to i n c r e a s e . By analogy w i t h the f u l l y 3 - f o l d c o o r d i n a t e d form o f s o l i d carbon ( g r a p h i t e ) , with zero energy gap, and the f u l l y q - f o l d c o o r d i n a t e d form (diamond), w i t h a 5.qeV energy gap, it is not surprlsinE that Eopt should decrease if 3-fold c o o r d i n a t i o n indeed b e g i n s to dominate f o r Td > 250C. It is of interest to compare the electrical and optical properties of these a-C:H films prepared via do glow discharge decomposition of C2H 2 with amorphous carbon films prepared via evaporation or sputtering~. Evaporated and sputtered a-C films, prepared without intentional incorporation of hydrogen, have been found to have, in part, a graphltic structure, indlcating a tendency toward trlgonal (3-fold) coordination of the C atoms. The room temperature conductivity of these films (10-3 - I02~.-Icm -I) is comparable to that of high quallt~ graphlte((Y(a-axis)~lOq.f).'icm"I, O'(c-axis)~igL-icm-l). For the a-C:H films studied here, we note that EA(250) e x t r a p o l a t e s t o zero a t Td=q25C ( F i g . 2 ) , ~ ' ( R T ) e x t r a p o l a t e s to 10-2.0.-lcm-1 at Td=qqOC (Fig. 2), and Eop t extrapolates to zero near q25C (Fig. 3). Thus for Td~q25C, we expect that these a-C:H films would be "graphitlc" as far as their electrical and optlcal properties are concerned, presumably due to the reduced incorporation of hydrogen.

439

440

B. Meyerson, F.W. Smith / Hydrogenated Amorphous Carbon Films

Anderson has undertaken an investigation4of the electrical and optical properties of amorphous carbon films prepared by rf glow discharge decomposition of C2H 2. Our results for EoD t, although consistently (0.4-0.6)eV lower than Anderson's, are similar to his in their dependence on T d. A possible explanation for our lower values of Eopt is that the a-C'H films prepared at a given T d via the rf glow discharge technique used by Anderson contain more hydrogen than those prepared by our dc technique. Further evidence consistent with such an explanation comes from Anderson's electrical conductivity data, which also show curvature on a logO'versus I/T plot. In particular, his~(RT) values are 3 to 5 orders of magnitude lower than ours for films prepared at the same Td, which again is consistent with higher concentration of hydrogen i~ Anderson's samples.

CONCLUSIONS The hydrogenated amorphous carbon films produced and studied here have electrical and optical properties which vary quite strongly with deposition temperature T d. Their properties may be summarized as follows: l)_The room temperature electrical conductivity ~(RT) varies from 10 -16 to lO-b~Q.-icm-I as T d is increased from 75 to 350C. 2) The measured conductivity~(T) is neither simply activated, nor is it consistent with variable range hopping. Instead, activated hopping conduction in a broad region of localized states (tail states) near one of the band edges is likely, with the additional complication that the dominant conduction processes occur at energies which lie further from E F as the temperature is raised. This explanation appears to be consistent with the low values of observed (10 -4 to 10"2.O- -1 cm -1) and also with the curvature observed in the logO'versus 1/T plots. 3) The optical energy gap E o p t decreases from 2 " 1 to 0 . 9 eV as T d is increased from 25 to 375C, with most of the decrease in Eop t occurring for Td >250C. 4) The electrical and optical properties of these a-C:H films appear to extrapolate to those of evaporated or sputtered a-C films, which are primarily "graphitie" in nature, for Td~425C. We conclude from these results that the electrical and optical properties of these a-C:H films depend in a significant way on the amount and bonding of the incorporated hydrogen, and that the variation with T d of these properties is due primarily to a decreasing incorporation of hydrogen as T d is increased.

(a) Research supported ( i n p a r t ) by the PSE-BHE Research Award Program of the City U n i v e r s i t y of New York. (1) W.E. Spear and P.G. LeComber, S o l i d S t a t e Comm. 17, 9 ( 1 9 7 5 ) ; P h i l . Nag. 33, 935 (1976). (2) See M. Millard, "Techniques and Applications of Plasma Chemistry", ed. by J.R. Hollahan and A.T. Bell (Wiley, New York 1974) p. 177-213, for a useful review and for further references. (3) S.M. Ojha and L. Holland, Thin Solid Films 40, L31 (1977). (4) D.A. Anderson, Phil. Mag. 35, 17 (1977). (5) D.S. Whitmell and R. Williamson, Thin Solid Films 35, 255)(1976). (6) L. Holland and S.M. Ojha, Thins Solid Films 38, LI~-(1976 . (7) N.F. Mort and E.A. Davis, "Electronic Process'es in Non-Crystalline Materials", (Clarendon Press, Oxford, 1971) Ch. 2,7. (8) G.H. DShler, Phys. Rev. B19, 2083 (1979). (9) J.J. Hauser, J. Non-Cryst. Solids 2-3, 21 (1977), and references therein.