Properties of RF sputtered hydrogenated amorphous germanium-silicon alloys

Journal of Non-Crystalfine Solids 50 (1982) I-11 North-Holland Publishing Company

1

PROPERTIES OF RF SPUT'I'ERED HYDROGENATED AMORPHOUS GERMANIUM-SILICON ALLOYS P.K. B A N E R J E E , R. D U T T A a n d S.S. M I T R A Department of Electrical Engineering, University of Rhode Island, Kingston, RI 02881, USA

D.K. P A U L * Gordon McKay Laboratory, Harvard University, Cambridge, MA 02138, USA

Received 16 June 1981 Revised manuscript received 30 November 1981 Amorphous Ge-Si:H alloys have been prepared by rf sputtering in an argon-hydrogen atmosphere of varying hydrogen partial pressures. Electrical transport properties in the temperature range of 80 K to 373 K and optical and structural properties of these films have been measured. It is found that for a fixed Ge-Si composition, the hydrogen content strongly affects the electrical conductivity. The absorption edge has been seen to suffer an initial blue shift yielding to a small red shift at higher hydrogen concentrations and is consistent with the conductivity data. The optical pseudo-gap obtained approaches closer to twice the corresponding mobility gap at higher hydrogen concentrations. Structural studies by Raman scattering confirm the amorphous nature of the films. Raman spectra also reveals the presence of various hydrogenic and lattice modes attributable to silicon-hydrogen, germanium-hydrogen, Si-Si, Ge-Si and Ge-Ge interactions. The depolarization ratio p, measured for both unannealed and annealed samples, indicates stabilization, due to annealing at selected temperatures. In view of the development of possible material for photovoitaic conversion of solar energy, it is suggested that by an appropriate choice of a fixed Ge-Si composition and varying the hydrogen content it will be possible to optimize the electrical conductivity and optical band gap in sputtered hydrogenated a-Ge-Si alloys.

1. Introduction Several investigations on the properties of a m o r p h o u s n o n h y d r o g e n a t e d G¢~Si~-x thin films have been reported. F o r example, the optical a b s o p t i o n in evaporated films [l] a n d the structure a n d R a m a n scattering in the a m o r p h o u s Ge0.sSi0.0s alloys [2] have been investigated to a limited extent. The temperature d e p e n d e n c e of electrical c o n d u c t i v i t y a n d the effect of a n n e a l i n g temperature on the properties of sputtered a-Ge0.sSi0. 5 films have also been reported [3,4]. However, a few properties only, viz., optical properties of h y d r o g e n a t e d a m o r p h o u s G e ~ S i ~ _ x : H alloys as prepared by glow discharge [5], structure, a b s o r p t i o n edge a n d v i b r a t i o n a l b o n d - s t r e t c h modes in the infrared region have been briefly reported [6,7]. H y d r o g e n a t e d a m o r p h o u s semiconductors * Present address: Comsat Laboratory, 22300 Comsat Drive, Clarksburg, Md. 20734, USA. 0 0 2 2 - 3 0 9 3 / 8 2 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 7 5 © 1982 N o r t h - H o l l a n d

2

P.K. Banerjee et al. / RFsputtered G e - S i : H alloys

have displayed interesting properties. Hydrogen with its single electron and its small covalent radius can compensate individual dangling bonds on the void surfaces of sputtered semiconductor G¢,Si t_ x films. Thus, like the S i : H [8,9] films, a relatively low density of defect states in sputtered G¢,.Si 1 , : H films may be achieved. By incorporating hydrogen therefore, there is a possibility of obtaining optimum absorption edge characteristics and electrical properties of a-Ge-Si : H alloys for use as a suitable material for the photovoltaic conversion of solar energy. To this effect Dong et al. [10] have published some work on the electronic properties of sputtered a-G¢,Sit_,: H films for a number of values of x. In view of solar energy research interests, Dong et al?s work is an important step in stressing the need for new materials to effectively utilize the solar spectrum. However, it is also equally meaningful to understand the effect of varying amounts of hydrogen in a fixed Ge-Si composition on the electronic and optical properties. The work presented here is concerned with the electronic, optical and structural properties of sputtered a - G e - S i : H alloys with various amounts of hydrogen content with a fixed Ge-Si composition, viz., Ge0.sSi0. 5. 2. Film preparation and characterization We have produced amorphous Geo.sSi0.5: H alloys by rf sputtering in an argon-hydrogen atmosphere. Our system (MRC, New York) was maintained at a base vacuum of 1 × 10 -6 Torr. An rf power of 100 W and several partial pressures of hydrogen, PH, from 0.3 to 5 mTorr with fixed argon pressure were chosen as deposition parameters. The substrates used were boro-silicate glass, quartz and silicon wafers. The substrates were water-cooled. The sputtering target (diameter 2 in and thickness 0.25 in).was made of homogeneous polycrystalline and highly pure Ge0.sSi0. 5 alloy. The target to substrate distance was 2.5 in. The deposition rate with these parameters was ~0.75 /~m/h. Other parameters being kept the same, the deposition rate decreased with increased hydrogen. Films were 2-3 /~m thick. The homogeneity and the composition of the flms were checked by electron microprobe which verified the composition within 2% of the target material. This suggests the presence of equal numbers of Ge and Si atoms in the sputtered film. Both in-plane and out-of-plane scanning electron micrographs of the samples indicated a homogeneously smooth region throughout the film thickness. Molybdenum electrodes were sputtered on the films, the separation between the electrodes being 0.6 mm. 3. Results and discussion

3.1. Electrical conductivity The dc conductivity was measured using a conventional two probe method over a temperature range of 375 > T > 80 K. The dc conductivity was indepen-

P.K. Banerjeeet al. / RF sputtered Ge-Si: H alloys

3

dent of the direction of current through the samples. The variation of conductivity with temperature was measured in situ under high vacuum (10 -6 Torr). The voltage source was made up of a 5.4 V mercury cell. A chromel-alumel thermocouple monitored the temperature of the film. Fig. 1 shows the plot of log (o) versus ( 1 0 0 0 / T ) for four samples deposited at different hydrogen pressures PH. From fig. 1, it can be seen that o decreases rapidly at first, goes through a minimum at PH//Ptotal -----0.4 and then increases again at higher PHThe films with a high PH//Ptotal ratio (0.5) display higher conductivity. At temiaeratures higher than room temperature the conductivity curves are approximated by the general expression, o - - o 0 e x p ( A E / k T ) . The mobility gap derived from the slope of the high temperature conductivity goes through a maxima with the increasing hydrogen concentration as shown in fig. 2. The pre-exponential factor lies between 0.5 × 102 and 5 × 102 for all hydrogenated samples. The low temperature conductivity could be explained by hopping between states near the Fermi level and is given by Mott's law [11].

o = - ~ °°

e x p [ - (To/T)l/4],

-3.2

-4.2 IE 0

7 -5.2 E tO

"I0 " -6.2 s (.9

~ -7.2 H = 0 . 3 mT -8.2

H HH-

5.0 mT 1.0 mT 3 . 0 mT

-9.2 I

I

I

I

2.6

4.2

5.8

7.4

1000IT

(°K-D

Fig. 1. Log(o) versus (l/T) for films deposited at 0.3 mT, 1 mT, 3 mT and 5 mT hydrogen pressure and fixed 5 mT argon pressure.

P.K. Banerjee et al. / R F sputtered G e - S i : H alloys 1.6

; 'Optical P s u e d o Gap M o b i l i t y Gap x 2 I

1.4

> 1.2 v O.

< o >,.. 1.0 o tr uJ z

LU 0.8

0.6

0.0

I

I

I

I

1

0.1

0.2

0.3

0.4

0.5

PH / ( P H ÷ PA )

Fig. 2. Optical pseudo-gap and mobility gap of Geo.sSio,5:H films as a function of ratio of hydrogen pressure to total pressure.

-4.0

o 0.3mT o 5.0roT 1.0mT • 3.0mT

•

"-- - 5 . 0 v

,'7 E

O

O

|

C -6.0 t~

o 0 .J

-7.0

-8.0

0.25

I

0.~27

0.26 T-1/4(K-1/4)

Fig. 3. Log(o T | / 2 ) versus T I/4 for Ge0.sSio s : H films.

I

0.28

P.K. Banerjee et aL / RF sputtered Ge- Si : H alloys

5

Mott's parameters are calculated from the straight line fit between log(oT~/2) and T - i/4 (fig. 3). The constants o0 and TO are expressed as 00 = e2a21,phN( E f

)

and

7"0 = X,

3/kN(e, ),

where e is the electronic charge, a is the hopping distance, Vph is a phonon frequency ( ~ 1013 S - 1 ) obtained from Debye temperature, k is Boltzmann's constant, ), is a dimensionless constant [12] ( ~ 18.1). o0 and TO are found to depend on the hydrogen concentration in the films. The density of states N ( E f ) near the Fermi level is calculated according to the equation [4]

N ( E , ) -- 1.996 × 10 48 (ooT]/2)3To,/2 cm_ 3 e V - ' . 3 Pph

The values of the different parameters are presented in table 1. The density of states N(Ef) is somewhat larger ( ~ 10 25) for samples with low hydrogen concentrations and approaches a reasonable limit ( ~ 1018) at higher hydrogen concentration ( P u / Ptotaz ~- 0.4). The constant o0 was obtained by the extrapolation of the straight line fit of conductivity data at low temperatures. This may lead to unreasonably high values [13] of N(Et), one as high as 10 25. The large discrepancy may thus be caused either by the difficulty in identifying the true hopping region or by the uncertainty in the method itself. It is worthwhile to note that such unreasonably high values of the density of states have also been reported previously by other authors [4,13]. In spite of this high value of N(Ef) for different samples, there is a definite trend which indicates that the density of such localized states in the neighborhood of the Fermi level is lowered by the increase of hydrogen concentration. Hydrogen thus satisfies the dangling bonds and makes the random network more complete. At very high concentration of hydrogen, however, the density of states again starts increasing. For example, our films with PH//Ptotal -~ 0.5 has a density of states of 10 20 compared with a value of

Table 1 Mott's parameters for rf sputtered (100 W) a-Ge0.5Si0.5 :H films Hydrogen concentration (mTorr)

TO (107 K)

o0 T l /2 (~2 - I cm - I K l/2)

a (cm - I )

0.3 1.0 3.0 5.0

1.897 1.677 1.296 1.677

1.939X 104 4.347 × 102 7.551 × lO 1 4.347 X 102

4.014X 8.448 X 6.175 × 8.448>(

N( Ef ) (cm-3/eV)

!014 I 012 lO II 1012

6.349× 6.727 × 3.099× 6.727X

1025 1020 lO ~8 10 20

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P.K. Banerjee et a L / R F sputtered G e - S i : H alloys

1018 for films with eH//etotal =0.4. This suggests that addition of hydrogen beyond saturation may have been attained with the films at PH//Ptotal = 0.5. It is worthwhile to point out that for a similar hydrogen concentration we did not observe such an effect in a-SiC: H films [14]. Thus, this effect may be inherent to the particular composition of Ge and Si in the G e - S i alloy and the excess hydrogen possibly forms three center bonds which may introduce new gap states [15,16] in the a-Ge05Si0.5:H alloy.

3.2. Optical absorption The effect of hydrogenation can also be seen in the optical absorption measurements as shown in fig. 4. Optical measurements were obtained using a double beam spectrophotometer. The absorption coefficient was calculated by the optical transmission and reflection measurements near the principal absorption edge. The optical pseudo-gap is obtained by extrapolating the straight line fit between (aht,) i/2 and hu to zero absorption (fig. 5). Our optical gap for

10 s-

H - 0.3 mT H = 1.0 m T H - 3.0 mT H -- 5 . 0 m T

•E

104-

vo

I.-Z W u.

W 0 Z 0

103-

I-n n-

O (/r; 02

,<

102-

0.8

10

1.

i

1.2

114 116 ENERGY (by)

I

1.8

Fig. 4. Optical a b s o r p t i o n versus p h o t o n energy for GeosSio. 5 : H films.

210

P.K. Banerjee et al. / RF sputtered G e - S i : H alloys

7

300 • H= oH=

AH= • H=

0 . 3 mT 1 . 0 mT 3.0 mT

5 . 0 mT

250

200 "•E O ¢,i !

"A ~" 1 5 0

J~

100

•

•

50

0

o"

0 •

0.8

0

I

I

I

I

I

I

1.0

1.2

1.4

1.6

1.8

2.0

ENERGY

(hv) eV

Fig. 5. (e~hv)]/2 versus h v; the optical pseudo-gap is obtained lrom this plot.

Ge05Si0.5:H films at low hydrogen content agrees well with the measurements of Chevallier et al. [5]. The absorption edge suffers an initial blue shift yielding to a small red shift at higher hydrogen concentrations and is consistent with the conductivity data. The optical pseudo-gap obtained is found to approach closer to twice the corresponding mobility gap for films with PH/Ptotal ~ 0.4. 3.3. S t r u c t u r a l p r o p e r t i e s

Vibrational properties of these films were studied by R a m a n scattering measurements. The R a m a n spectra of four samples were recorded: "pure"

P.K. Banerjee et al. / R F sputtered G e - S i : H alloys oo

tr~ I-~

I

I

I

ego

I

I

5

c5 .6 ea

m

e~

m

'o 0 0¢.

0

0 r-

2

£ "0 ¢.

.=_

.~ .o ¢q

0

~ ~,

E e,

r~ ~ . o

8 .=

P.K. Banerjee et al.

/ R F sputtered G e - S i : H alloys

9

a-Geo.s-Si0. 5 prepared by sputtering in argon (5 mTorr) only, and three others sputtered in the same argon pressure with 0.3 to 5 mTorr hydrogen pressures. Additional Raman measurements were performed with one typical hydrogenated sample (1 mT) after annealing at temperatures of 100°C and 200°C. A careful scrutiny of the 500 cm-I to 900 cm -1 frequency range is useful in ascertaining the presence of Si-H, and G e - H , wag and bend modes, and of the 1800 cm -I to 2100 cm -1 range to identify the Si-H, and G e - H , stretch modes. The Raman scattering data were obtained at room temperature in 90 ° scattering configuration with an argon ion laser. The wavelengths used were 514.5 nm and 488.0 nm. Both pure and hydrogenated samples exhibit Ge and Si lattice modes. There is virtually no change of these lattice modes in the annealed samples. The spectrum for the 5 mT (PH) hydrogenated sample appears to have more features than other samples prepared with 0.3 mT through 5 mT hydrogen concentrations. In general the main difference between the spectra of the hydrogenated and the non-hydrogenated samples is the decrease of the TA, LA, and LO structures with increasing hydrogen content. The frequencies of structures noted in our Raman spectra and those reported for samples prepared by glow discharge (Ge-Si:H [91), plasma deposited (2-Si:H [17]),

,of_

200

(a)

140

10 ~c 70

725 770 815 860

,,.-m

i

(~)

~

10 400

I

I

I

I

20 1860

1980

2100

1860

1980

2100

zf

v~ 10 z)~ 7° f ~ ~ j ~770 c 1 816 7 2 860 5

Z <

80

200 Z < 140 < i

<

0 w

80

2O

M.I

A

t-., LU n."

(c)

8O i

0 180

i

,

i

I

320 480 (cm-')

l

1860

1980

2100

(cm-')

Fig. 6. Reduced R a m a n spectra for a typical (GSHI) sample. (a), (b) and (c) correspond to spectra of samples unannealed, annealed at 100°C and annealed at 200°C for the same range of wave n u m b e r ( ~ c m - i ), respectively.

P,K. Banerjee et al. / R F sputtered G e - S i : H alloys

l0

<3.

•

UNANNEALED

o_

•

ANNEALED

•

A N N E A L E D S A M P L E ( 2 0 0 ° C)

I< n-

SAMPLE

S A M P L E ( 1 0 0 e C)

z

~ .9 I--

< .7 _N

o. uJ .1

:::

•: •, 54.0

;:

••

580

"::' "::~:':

-eeeeoo|'••••;

500

""" .=I:

....l=.. I 620

I 770

••anm•m°e

810 G,.} cm -1

• I /( I 850//1870

I 1920

/ 1970

I 2010

Fig. 7. D e p o l a r i z a t i o n r a t i o for u n a n n e a l e d a n d a n n e a l e d Ge0.sSi0. 5 : H films.

sputtered (a-Ge [18] and a-Si [18-21]) are presented in table2. Annealing at 200°C for one hour produced a reduction of the Raman peaks at 800 c m - ] and 1980 cm -1 and a broadening at 2100 cm -n. We also noticed that in the frequency range of 400 era- n to 600 c m - ~, the intensities of bands due to S i - H modes are stronger than those of the G e - H modes. Reduced Raman spectra of these films are shown in fig. 6. In fig. 7, we have presented the depolarization ratio O [ratio of the scattering efficiencies for parallel-perpendicular (HV) to parallel-parallel (HH) polarizer analyzer geometry] of hydrogenic modes of a typical hydrogenated sample (1 roT) both unannealed and annealed at 100°C and 200°C. This ratio p varies from 0.2 to 0.7 in the annealed sample. The unannealed sample shows p between 0.5 and 0.7. The depolarization ratio P of the unannealed sample and that of the sample annealed at 100°C are very similar except that p is somewhat lower for annealed samples than for unannealed samples. This indicates stabilization of the hydrogenic bonds in films due to annealing. The depolarization ratio fluctuates between 0.2 and 0.7 for samples annealed at 200°C. This effect suggests the re-creation of void states due to the possible escape of hydrogen from the films when annealed at higher temperatures. Indeed, in glow discharge films hydrogen has been reported [9] to evolve at 150°C from Ge-sites in a G e - S i : H films. In conclusion, we have shown that a-Ge-Si :H films can be prepared by sputtering in a controlled atmosphere and that for similar deposition conditions the electrical conductivity is strongly dependent on the hydrogen content for a fixed G e - S i composition. At high hydrogen concentrations there is a saturation effect as supported by both conductivity and optical absorption data. Recently, Dong et al. [8] have shown that in a - G e - S i : H alloys for Ge content ranging from 0.18 to about 0.3, there is a critical composition of Ge above which the electronic properties change considerably. On the other hand, we have found that for a fixed G e - S i composition, by varying the hydrogen content, it is possible to optimize the electrical conductivity and absorption edge in sputtered hydrogenated a-Ge-Si alloys. For Ge0. s - S i 0 . 5 : H alloys

P.K. Banerjee et al. / RF sputtered Ge-Si: H alloys

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s p u t t e r e d in a n a r g o n - h y d r o g e n a t m o s p h e r e o u r films s h o w o p t i m u m e l e c t r i c a l a n d o p t i c a l p r o p e r t i e s at Pn/Ptota~ b e t w e e n 0.4 a n d 0.5.

References [1] [2] [3] [4] [5] [6]

[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

D. Beaglehole and M. Zavetova, J. Non-Crystalline solids 4 (1970) 272. N.J. Shevchik, J.S. Lanin and J. Tejeda, Phys. Rev. B4 (1973) 3987. D.K. Paul and S.S. Mitra, J. Non-Crystalline Solids 18 (1975) 407. D.K. Paul and S.S. Mitra, Phys. Rev. lett. 31 (1973) 1000. J. Chevallier, H. Wieder, A. Onton and C.R. Guarnieri, Sol. St. Commun. 24 (1977) 867. A. Onton, H. Wieder, J. Chevallier and C.R. Guarnieri, proc. 7th Int. Conf. Amorphous and liquid semiconductors, Edinburgh, 1977, ed., W.E. Spear (G.C. Stevenson, Dundee, 1978) p. 357. D.K. Paul, J. Blake, S. Oguz and W. Paul, J. Non-Crystalline Solids 35-36 (1980) 501. W. Paul and M. Kastner, eds., Proc. 8th Int. Conf. Amorphous and liquid semiconductors, Cambridge, 1979; J. Non-Crystalline Sohds 35-36 (1980). D.K. Paul, B. Von Roedern, S. Oguz, J. Blake and W. Paul, 15th Int. conf. Physics of semiconductors, Kyoto, Japan, September 1980. Ngnyen Van Dong, Tran Hun Danh and J.Y. Leng, J. Appl. Phys. 52 (1) (1981) 338. N.F. Mott, Phil. Mag. 19 (1969) 835. V. Ambegoankar, B.I. Halperin and J.S. Langer, Phys. Rev. 34 (1971) 2612. M.H. Brodsky and R.J. Gambino, J. Non-Crystalline Solids 8-10 (1972) 739. R. Dutta, P.K. Banerjee and S.S. Mitra, Sol. State Commun., in press. R. Fisch and D.C. Licciardello, Phys. Rev. Lett. 15 (1978) 889. H. Matsumura, Y. Nakagome and S. Furukawa, J. Appl. Phys. 52 (1981) 291. G.A.N. Conneil and J.R. Pawlik, Phys. Rev. BI3 (1976) 787. B. Bremejo and M. Cardona, J. Non-Crystalline Solids 32 (1979) 405. B. Bremejo and M. Cardona, J. Non-Crystalline Solids 32 (1979) 421. M.H. Brodsky, M. Cardona and J. Cuomo, Phys. Rev. B16 (1977) 3556. E.C. Freeman and W. Paul, Phys. Rev. B18 (1978) 4288.