Optical and electrical properties of amorphous hydrogenated carbon layers deposited by dc plasma decomposition

Diamond and Related Materials, 1 (1992) 434-439

434

Elsevier Science Publishers B.V., Amsterdam

Optical and electrical properties of amorphous hydrogenated carbon layers deposited by dc plasma decomposition O. Stenzel, G. S c h a a r s c h m i d t , M. Vogel, R. Petrich, F. Wolf, T. W a l l e n d o r f , F. Scholze a n d W. S c h a r f f Department of Physics, University of Technology, 0-9010 Chemnitz, Reichenhainer Str. 70 (FRG)

Abstract The optical properties of hard dc plasma deposited amorphous carbon layers have been investigated from the middle IR up to the visible spectral region. Refractive indices and absorption coefficient behaviour have been determined by means of spectral transmittance and reflectance measurements. The layers show a transparency window in the middle IR in the wavenumber range 1700 c m - ~ < v < 2 8 0 0 cm ~. The refractive indices in the "window" region are between 2.1 and 2.3 with weak dispersion. IR-absorption losses have been discussed in terms of a dispersion model, considering the absorption detected to be caused by a superposition of Lorentz-lines and an Urbach-part. Planar and transverse conductivities of the layers have been determined at room temperature. In addition, the conductivity of representative samples has been investigated in dependence of the temperature in the range of 80 K up to 350 K. A correlation between the thus-obtained activation energies for thermally-activated hopping in the band tails and the Urbach parameters of the layers could be established.

1. Introduction

Many investigations have been carried out to examine infrared absorption behaviour of a-C : H layers. It is well known that optical properties of these layers are strongly dependent on deposition techniques and conditions, and serious differences have been observed for differently deposited layers [1]. Figure 1 presents typical middle infrared (MIR) absorption pictures for layers deposited by different techniques. Typically, MIR-transparent layers (absorption coefficient ~ < 1000 cm-1) show little absorption in the wavenumber region 1700 2800 cm 1 where no C - H or C - C vibrational frequencies occur. Considerably strong absorption lines are caused by CH x stretching and deformation vibrations. A reduction of these absorption losses would be necessary if these layers should find application as hard infrared coatings. In this connection it is useful to investigate the properties of a-C:H layers with comparably low hydrogen contents ( N . < 2 5 at.%) as achieved by dc plasma deposition [2]. However, investigations of the IR absorption behaviour of these layers have shown that the decrease in absorption losses due to C - H vibrations (cf. Fig. 1 - - absorptance of dc plasma deposited layers) is accompanied by an increase in a background absorption which is thought to be due to an Urbach edge as shown by NIR-investigations [2]. In the present study, Urbach parameters of dc plasma deposited layers

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....... - . . . . . .....

dc plasma d e p o s i t i o n rf sputtering e-beam evaporation microwave plasma deposition

10s -4

E.-O E.~O,2 eV n w2,3... 2,9

104

,"

,c 103

¢.

"i'.~,\

" "'~.

V

"

1 o2

E," 0,9 e Y

\

. . . . p_.r

,,

, "'"

,,gev

1,9 ... 2,3

10

0 f10 0

I 2000

I 3000

c m "4

! 4000

V

Fig. 1. Typical middle infrared absorption behaviour of a-C:H, deposited by different techniques.

O. Stenzel et al.

Properties (~]'dc plasma deposited a-C : II layers

TABLE I. Deposition conditions Sputter cleaning of the substrate Residual pressure (mPa) Working pressure (Pa) Ion current density (mA cm 2) Bias voltage (kV) Working gas Deposition temperature ( C ) Deposition rate (nm min ~)

Film deposition

0.1 0.15 0.25 2.0 Ar

0.03 0.2 0.15-0.2 0.6 2.4 Ca Ha < 70 30-50

have been determined by means of NIR spectroscopy and discussed in connection with electrical properties of the layers.

435

regions was discussed by a modified envelope method to obtain the film thickness d. As the second step we applied Phillips' method [3] to determine n(v) and ~(v) for every wavenumber independently, by solving the system of equations: Texp(V)-- T~,lc(n(v), ~(v), v, ...) = 0

(1)

Rexp(V) -- R~l~(n(v), ~(v), v, ...) = 0

Such single wavelength methods suffer from the disadvantage that the ambiguity of solutions n(v) requires a special discussion of multiple solution contours which, in particular, may be discontinuous. Recently, we have therefore attempted to complete the method by a third refinement step, in which a merit function M

F= ~ j

{WT(Vj)[T~xp(Vj)--Tcalc(n(vj), ~(vj), d ) ] 2 1

+WR(Vj)[Rcxp(Vj) R~.lc(n(vi),

2(Vi), d ) ] 2 1

2. Experimental + 2.1. Deposition technique For the present study, films were deposited by a dc plasma deposition technique described elsewhere [2]. Silicon wavers and fused quartz were used as substrate materials. The main deposition parameters are listed in Table 1. 2.2. Electrical measurements Room temperature dc conductivity of the layers was determined in both lateral and sandwich geometries. No noticeable difference between lateral and sandwich conductivities could be established. For selected samples, lateral conductivity was determined in the temperature range 80 K 350 K. The temperature dependence of the conductivities was fitted by the least square method. 2.3. Optical measurements and calculation of optical constants The refractive index n(v) and the absorption coefficient :~(v) were determined from measurements of directed spectral transmittance T(v) and reflectance R(v) of the samples from the middle IR up to the visible spectral regions. Spectra were recorded on double-beam dispersive spectrophotometers. Aluminium mirrors, silicon wavers and quartz glass were used as reflectance standards. 2.3.1. Determination of optical constants in the M I R In general, spectra are discussed in terms of the homogeneous and isotropic layer model. The determination of optical constants from the spectra was achieved in two steps. As the first step, the interference pattern of the reflectance in weakly absorbing spectral

w(v)~dv/ dv

(2)

'1

is minimized in the In, ~, d'~-space. Values xT, wR and w are weight functions, M is the number of wavenumber points considered. The last term of eqn. (2) becomes large if n(v) is discontinuous or strongly dispersive, so that such solutions may be excluded during minimization of F. The procedure leads to unambiguous and continuous solutions and increases significantly the accuracy of n- and d-determination [4]. However, the number of sample spectra discussed by this refinement step is still small, so that most of the results discussed below have been found from eqn. (1) without final refinement. 2.3.2. Determination (~['optical constants in the N I R In the NIR no vibrational absorption lines occur, and the expected absorption behaviour is thus less involved than in the MIR. The determination of optical constants was achieved in the same way as in the MIR except for the refinement step. In the NIR the refinement step was replaced by minimizing a usual quadratic merit function for direct search of Sellmeier and Urbach parameters of the layers and the film thickness.

3. Results 3.1. Optical properties 3.1.1. Visible and near infrared All samples were strongly absorbing in the VIS. Optical gaps Eo determined from standard Tauc's plots have been found between 0.4 and 1.1 eV depending on deposition conditions. The NIR absorptance is given by an Urbach tail :% = :~uoexp [v/Vuo]

(3)

436

O. Stenzel et al. / Properties of dc plasma deposited a-C : H layers

Table 2 presents o p t i c a l gaps a n d U r b a c h p a r a m e t e r s for s a m p l e s d e p o s i t e d with different ion energies.

F o r w a v e n u m b e r s a b o v e 3200 c m - 1 the a b s o r p t i o n increases a n d passes c o n t i n u o u s l y into the U r b a c h tail established in the N I R (see Fig. 2).

3.1.2. M I R The M I R a b s o r p t a n c e of the layers is essentially involved because of a multiplicity of I R active v i b r a t i o n a l excitations. H o w e v e r , due to the low h y d r o g e n content, the typical h y d r o c a r b o n a b s o r p t i o n lines are n o t very strong, so that the a b s o r p t i o n picture is s m o o t h e r t h a n often r e p o r t e d in the literature. T y p i c a l l y little a b s o r p tion is o b t a i n e d in the w a v e n u m b e r region 17002800 cm

3.2. Electrical properties R o o m t e m p e r a t u r e c o n d u c t i v i t y values of the layers are listed in Table 2. F i g u r e 3 presents results of measurements of lateral c o n d u c t i v i t y as a function of temperature. In the t e m p e r a t u r e range investigated, the c o n d u c t i v i t y satisfies the e x p o n e n t i a l law

-1

(4)

a = a o exp [ - E a / k T ]

TABLE 2. Range of selected film properties as a function of kinetic energy Ec per carbon atom during deposition. Typical values are underlined" E~ (eV)

Mass density (g cm 3)

Room temperature conductivity (f~ cm -1 )

E, (eV)

Eo (eV)

Ct,o (cm i)

V,o (cm-1)

80b 110 280 380

1.80 1.85 2.00 2.05 2.00-2.05 2.00 2.05

3× 10 -7 10-8-10-~-10 -5 10 6 l0 5 10-7_10-____~6_10 3

0.15 ~0.12 ~0.10 0.07 0.12

1.1 0.7 0.9 0.7-0.9 0.5-1.0

109 39-186 ~ 62 24 50-347

3016 1860-2330 ~1700 1700-2100

"The reason for the wide range of optical and electrical parameters obtained at Ec = 380 eV is not yet clear. bOnly one sample.

I

~

~

.

-

7

-

-

-

-

-

-

-

"~'MIO[ i .............. i ...................... i;...................... ~,,.= 347,m" i...................... ~ ,('~ , ""'d...................... ~llSi'/""i t ..... " I

.t

::

i

i

i

i

i

!

!

~'

i

::

i :

! :

i

i

o i ,¢~oj~, -o-~

+

. . . . . . . . 13 . . . -o . . . . . . . . .

.! ...........

................ ! ...................... ~. . . . . . . . . . . . . . .

'+....................

! ......................

i ...................... [

Fig. 2. MIR and NIR absorptance of dc plasma deposited layers.

i ....................... ~. . . . . . .

d~t.~- I

]

i ................. )

:

--

437

O. Stenzel et al. / Properties of dc plasma deposited a-C : H layers 4.

X

(£cm[ 1

I0 6

Eo:0.12

Discussion

Figure 2 indicates that for the samples discussed here the description of M I R absorptance in terms of a pure multi-oscillator model is insufficient. In fact, one has to consider that the contribution of the Urbach tail as extrapolated from the NIR into the MIR may he comparable with the purely vibrational M | R absorption losses. A sufficient reproduction of the intrinsic absorption behaviour may be achieved in terms of a model which considers the absorption losses caused by single vibrational lines (Lorentz lines) as well as the Urbach tail. Here

eV

16 7

1Oe

c~ = 4 n v l m w / ~ m + ~:uo" exp [v/'Voo]

(5)

with I0 9

~IR('V) = A + By 2 +

I ~ j = a voj

1~10 Eo:O 15eV 6~1 I

J

I

3

5

1 7

i 103 K

4

1 9

T-1 _

Fig. 3. Typical lateral conductivity as a function of the temperature. Activation energy values E, were between 0.07 and 0.15 eV (Table 2), while pre-exponential factors varied between 3 x 10 6 and 7 x 10 2 (["2 X c m ) - l . The conductivity mechanism has been assigned to thermally activated hopping in the band tails.

Jj

-

v

--

(6) iF.i

and A, B, {Vojl, {rj}, {Jj} being constants. Voj is the resonance frequency of the corresponding oscillator. Figure 4 shows the result of a fit of the :qv)-picture of comparable weakly absorbing samples in spectral regions where intrinsic absorption of the a - C : H material is expected (v ~ 3000 c m - l : C - H stretching; v < 1600 cm 1: C - C and C - H deformation). The residual absorption near v ~ 2200 cm 1 and 3400 cm cannot be explained by C - C or C - H absorption lines and may be due to contamination (N, O) [2]. On the basis of this model it is thus possible to make an estimation of the relative contribution of different absorption mechanisms (intrinsic vibrational absorption, contaminational vibrational absorption, Urbach absorption) to the full MIR absorptance of the layers.

1000

¢m'1 I

o

O

O

ol

500

o

, k", .,', ~?,,i, "X~ooo 10g / i ', 4 % j eil l O,. l A~ . . . . . . ~ - ~ " I l lXtt'l*~ [ --i- --,'-':-~¥~ "-%~Z , " ~ " ~_ I

1000

2000 9

o

. .../ /',. __~-~~ ~.~-~t .~ x " - . . / .-" ~ " . "" - _ /

•

3000

Fig. 4. Result of a fit of the absorptance of a weakly absorbing sample: - - , ..... , single Lorentz lines: - . Urbach tail.

cm"1

z, O00

as calculation from eqns (5) and (6): ,

experimental values:

O. Stenzel et al. / Properties of dc plasma deposited a-C : H layers

438

2,5 2,4 ck~

2,3

(20 _00

2,2

__

•,.,

° o"

0

u ~.'~

G

0

oC'~-"

0

2,1 2,0

0

[

1

'tO00

I

2000

cm 1

Fig. 5. Dispersion behaviour of dc plasma deposited layers: - -

, as calculated from eqn. (6); ©, experimental values. In addition, Fig. 5 shows a c o m p a r i s o n of the dispersion of refractive index described by our model and refractive index values experimentally obtained. The dispersion graphs are in qualitative agreement, the negligible dispersion for v = 1200-4000 c m - 1 is reproduced as well as the a n o m a l o u s dispersion for v < 1000 c m - 1. The model is described in full detail in ref. 2. The necessity of a contribution of an U r b a c h tail for exploring IR absorption losses of the layers investigated is consistent with other experimental data: - - The films investigated have considerably low optical gaps (Eo ~ 0.5-1.1 eV). Rewriting eqn. (3) to

N(E) /

A EV

EB

EF

EA

I

4000

EC

Fig. 6. Energy band diagram.

c(u(v) = C(uo exp [v/Vuo ] -- const • exp [/~(E - E~)]

10 3

cm-1

l

~,uO 1

02

10 t

[

i

03

I

O.Z. Eo/hCVuo - -

=

I

___

05

.,._

Fig. 7. Values of ~(uo as a function of the ratio Ea/hcvuo: 0 , experimentally obtained; - - ,

calculated from eqn. (8).

(7)

O. Stenzel et al. ,' Properties qf dc plasma deposited a-C : H layers

Fig. 6). In fact, 2 x E a corresponds to typical MIR photon energies. As a consequence, transitions between localized band tail states may contribute to the MIR absorption picture which could be an explanation for the observed Urbach tail. This hypothesis is consistent with the experimental fact that an increase in Ea is accompanied by a decrease in Urbach absorption losses. Moreover, for the samples investigated, a correlation between Urbach parameters and activation energy Ea could be established (Fig. 7). As seen from Fig. 7, E~ is related to :~uo and Vuo via the rough relationship

0,3

eV

0.2

Ea ~ hcvu° In 104 cm ~

I[ a

10.8

0.1

439

,i!

n

Relation (8) shows that the activation energy E. is directly proportional to E~ (eqn. ¢7/):

E~,7~E~Eo

I

1.0

(8)

:~uo

L

eV

I 2.0

Fig. 8. Experimentally obtained activation energies E~ as a function of the optical gap: @, own measurements: ± , values taken from ref. 6; (',, values taken from ref. 7: A, values taken from ref. 8: Y], values taken from ref. 9.

(where E is the photon energy, /3 = (hcvuo) 1, Ee is the characteristic energy which is comparable with Eo [5]), it is obvious that a decrease in E o (and consequently in Ee) leads to an increase in Urbach absorption. - - The films investigated have a considerably low hydrogen content NH ~ 22 at-%. Usually films with a lower hydrogen content have a higher density of gap states (cf. a-Si:H and a-Ge:H), so that noticeable Urbach absorption at low photon energies is here expected rather than in the case of fully hydrogenated materials [5]. - - The low activation energies E~ indicate that the energy distance between localized band tail states in the Davis Mort model E A - - E , is also rather small (see

(9)

Figure 8 demonstrates that the proportionality between E~ and Eo following from eqn. (9) is not restricted to the low gap range of E o < 1 eV, but may be obtained for gaps up to 2 eV. It would be useful to study in future whether or not relation (8) is obtained only for dc plasma deposited layers with a hydrogen content NH < 25 at-% or whether it is a more general relationship relating the activation energy to Urbach parameters in amorphous hydrogenated carbon.

References

1 H. Tsai and D. B. Bogy, d. Vat'. Sci. Techn., A5 (19871 3287. 2 0 . Stenzel, G. Schaarschmidt, F. Wolf, M. Vogel and T. WallendorL Thin Solid Films, 203 (1991) 11. 3 R. T. Phillips, d. Phys. D, 16 (1983) 489. 4 0 . Stenzel, R. Petrich, W. Scharff, V. Hopfe and A. V. Tikhonravov, Thin Solid Films, 207, in press. 5 M. H. Brodsky (ed.), Topics in Applied Physics. Vol. 36: Amorphous Semiconductors, Springer Verlag, Berlin, Heidelberg, New York, 1979, p. 73. 6 T. Nakono, S. Koike and Y. Ohki, J. Phys. D. 23 (1990) 71 I. 7 D. A. Anderson, Phil. Mag., 35 (19771 17. 8 E. Staryga, A. Lipinski, S. Mitura and Z. Has, Thin Solid Fihns, 145 (1986) 17. 9 R. Kleber, Diplom. thesis, University of Kaiserslautern, Germany, 1988.