Properties of glow-discharge deposited amorphous germanium and silicon

JOURNAL OF NON-CRYSTALLINESOLIDS3 (1970) 255-270 © North-Holland Publishing Co.,

PROPERTIES

OF GLOW-DISCHARGE

AMORPHOUS

GERMANIUM

DEPOSITED

AND SILICON

ROBERT C. CHITTICK Standard Telecommunication Laboratories, Harlow, Essex, England Received 25 August 1969; revised manuscript received 24 November 1969 Films of silicon and germanium are deposited on glass using the radio-frequency glowdischarge decomposition of silane and germane gases respectively. When grown on a substrate at room temperature the films are amorphous, with a short range order of about 20 A. The resistivities of these films, as deposited, are typically 10s ~ cm for silicon and 7 × 10a K2cm for germanium, measured at 294°K. Thermal activation energies for conduction decrease continuously below the deposition temperature, and at low temperatures germanium follows the relation logtr=A/T ~, where A is a constant. This would seem to indicate that a hopping process in an impurity band is responsible for conduction at low temperatures. Photoconductivity has been observed in silicon but not in germanium. The threshold energy for this effect decreases with increasing deposition or annealing temperatures. This is also true of the high temperature thermal activation energy. It is suggested that this is due to the de-localisation of states in the valence and conduction bands as the short range order increases. The optical absorption coefficients of germanium and silicon have an exponential dependence on photon energy and the considerable absorption below the fundamental absorption edge of the crystalline form may indicate the presence of localised states in the band gap.

I. Introduction Electrical a n d optical p r o p e r t i e s o f a m o r p h o u s silicon a n d g e r m a n i u m d e p o s i t e d b y e v a p o r a t i o n techniques have been r e p o r t e d by several authors1-12). In c o m p a r i s o n with the crystalline material, resistivities are higher, densities are lower, a n d there is no definite a b s o r p t i o n edge. The a m o r p h o u s state however is n o t well defined, a n d the degree o f d i s o r d e r is very d e p e n d e n t on the m e t h o d o f deposition. The resistivity o f e v a p o r a t e d material, especially a m o r p h o u s silicon, is greatly affected b y dep o s i t i o n rates a n d residual gas pressure. It has been suggested t h a t the high values associated with low d e p o s i t i o n rates a n d high residual pressure are due to large c o n c e n t r a t i o n s o f oxide in the films 12). The r a d i o - f r e q u e n c y glow discharge m e t h o d o f d e p o s i t i o n 13-16) is relatively free f r o m c o n t a m i n a n t s . A n electrodeless discharge is e m p l o y e d a n d the reaction takes place at a pressure o f a b o u t 0.1 T o r r o f silane or g e r m a n e gas, which is high c o m p a r e d with those used in e v a p o r a t i o n 255

256

R.C. el-liT'riCK

systems. Small leaks, therefore, of the order of a Torr over a period of days in the closed system will be of little consequence. It is estimated from the leak rates that the amount of oxygen in the reacting gas is less than 10 ppm. The absence of oxide is evident from the infra-red absorption spectrum which does not show the strong silicon-oxygen vibration absorptions. The purpose of this paper is to report some of the optical and electrical properties of R.F. glow discharge deposited germanium and silicon and compare the results with the predictions of current theoretical models of amorphous materials.

2. Experimental 2.1. DEPOSITION Films of germanium and silicon were deposited by an electrodeless radiofrequency glow discharge (1 MHz) in germane and silane gases respectively la-16) at rates of 2-12 A~/sec. The films were generally deposited at room t~mperature with the substrate on a glass pedestal. With a molybdenum susceptor on the pedestal, depositions could be carried out at substrate temperatures up to about 900 °K. Electrical measurements were made on films grown on Corning 7059 glass substrates with aluminium or gold planar electrodes formed by evaporation. Parallel sided, optically fiat glass substrates were used for the optical measurements near the absorption edge and cleaved single crystal potassium bromide for the infra-red studies. The films grown by this technique are amorphous, having a short range order of about 20 A (ref. 17). 2.2. ELECTRICALMEASUREMENTS A cryostat with a heater incorporated enabled resistivity measurements to be made over the range 77-450°K. The current was measured on a Keithley 610B electrometer and film thickness on a Talystep. The properties of the silicon and germanium films were found to be independent of deposition rate, although admittedly this was restricted to 2-12 A/sec in order not to affect the glow discharge. Work carried out at our laboratories on the resistivities and thermal activation energies of amorphous silicon at various substrate temperatures has been reported la). When deposited at 294°K the resistivity lies between 108 gJcm and 109 f2cm with a continuously varying activation energy below 294°K (fig. 1). (Throughout this paper the field across the planar electrodes is 100 V cm-1 unless otherwise stated). On being heated above the deposition temperature, the resistivity measured at 294°K increases irreversibly until at 353 °K it stabilises at about 10 l° f2 cm and the activation

GLOW-DISCHARGE DEPOSITED AMORPHOUS GERMANIUM AND SILICON

257

energy is constant at 0.83 eV from 273 °K to 450 °K. Unfortunately because of the high resistivity it was not possible to go to lower temperatures. At higher annealing or deposition temperatures the resistivity (fig. 2) and activation energy decrease, roughly following the empirical relation (see discussion) O = ~c e x p [ ( E -

Eg/2)/kT)],

(1)

where Oc is the intrinsic single crystal resistivity, about 3 x 105 ~ cm, E is the thermal activation energy of the amorphous material, and E~ is the band gap of the crystalline material. Above deposition temperatures of 820°K the reT°K 10"5

400

300

250

200

I

I

I

I

1 Film is cooled b e l o w deposition t e m n e r o t ure

106

2 Annealed a t 353°K

10.7

10-8 )osition t e m p e r a t u r e

off~,~I

10.9

~6 ~o

lo -1'

2

I 3

1000

1

I

4

5

-T-'-R-

Fig.

1.

Annealing of room-temperature deposited amorphous silicon from 294°K to 353°K, film thickness I am.

258

g.c. CFLrrrlCK

1010

,o g

10 ~

107 ~-P21°C ncrn

lo 5-

10 4

103

--

\

\ |

,02o Fig. 2.

I I

3OO

I

400

I

I

I

I

I

5O0 ~00 7OO 8O0 gO0 Depositibn t e m l : ) e r a t u r e TD°K

I

1000

1100

Resistivities of amorphous silicon and germanium as a function of deposition temperature. Film thickness 0.3/Jm to 3 am.

GLOW-DISCHARGE DEPOSITED AMORPHOUS GERMANIUM AND SILICON

259

TABLE 1

Deposition temp.

p(expt) (294 °K)

TD (°K)

(12 cm)

E -- Eg/z (eV)

353 573 773

N 1.5 x 10 t0 ~ 2x108 105 --106

0.27 0.15 0

p from eq. (1) (12 cm) 2 x 1010 1.2x108 3 × 105

sistivity at 294 °K is 4 x 1 0 4 ~ cm with a high temperature activation energy 0.25 eV and relation (1) does not hold. The activation energy is not constant but varies continuously with decreasing temperature to 0.05 eV at 77°K (fig. 3). 1

3 "24 '25 '26 "27 '28 "29 '30 '31

"32 33

.34 "35 '36 .37 -38 .39 4.0

10-3 Silicon deposited at

BSO'K

I

• Log o v 7.~

1 x Log OV "~

10-4

10-5

10"6

IO-7

lo-e

0

1

2

3

4

5

6

7

8

9

10

11

12

13 14

15

16 17 16

tO'TO • K ' I

Fig. 3.

Thermal activation energies of silicon deposited at 850°K and curve showing a dependence of loga on 1/T~ at low temperatures. Film thickness 2/tm.

260

R.C. CStTT~CK

As deposited, at room temperature (294 °K) the germanium films have a resistivity of 7 x 103 f2 cm. The activation energy varies continuously between 300°K and 77°K (fig. 4). As can be seen in the log/versus lIT curve there are two main regions, (1) a low activation energy region below about 180°K tending to 0.05 eV at 77°K and (2) a high activation energy tending to 0.28 eV at 300°K. When these films are heated above their deposition temperature, the resistivities at 294°K and the high temperature activation energy increase. At the same time the low temperature activation energy remains constant. This annealing process continues until about 470°K when the activation energy and resistivity measured at 294°K are 0.45 eV and 5 x 104f2 cm respectively. Between 670°K and 770°K the resistivity drops rapidly to 4x 10-3~ cm, indicating the recrystallisation of the films. If a value of 60 f2 cm is taken for the intrinsic crystal resistivity the relation (1) is out by an order of magnitude between 470 °K and 720°K (fig. 2). The annealing process appears to be associated with the removal of impurity levels, probably acceptors due to dangling bonds.

~2t

I\ I I I

400 300 250 200

10"3

\~

3

\\\

....

~4".

"'"-.4.

°"I Fig. 4.

....

Sample G P 28 Deported at room temp. thickneSS

10-7

10-g

1(i)0

Sample GP15 Deported at room temg thickness 1. Cooled to 77"K from 2gS*K. ~lS~tm 2. Heot~l to 336*K then cooled.

'~ ~k

~4/~-

T'K

1~0

I

I

I

I

I

I

I

1 2 3 4 5 6 1 7 Amorphous Germanium Log. OV T

I

8

I

I

g 10 IO0~T. K

I

11

i

12

I I

13

14

15

Reduction of impurity concentration in amorphous germanium with heat treatment.

261

GLOW-DISCHARGE DEPOSITED AMORPHOUS GERMANIUM AND SILICON

2.3. OPTICAL

Films of germanium and silicon were deposited at 21 °C with thicknesses from 800 A to 16/~m on glass and potassium bromide substrates. Optical transmission was measured on a Perkin-Elmer 450 Spectrophotometer, and infra-red transmission on a Grubb-Parsons GS3 Infra-red Spectrometer. The optical constants, refractive index n and absorption coefficient ~ were calculated using the method reported by Wales et al. la) in which the maxima and minima of the transmitted interference fringes are compared with those calculated by a computer program 19) designed to calculate the transmission of a multilayer film. The experimental refractive indices for silicon and germanium, as a function of photon energy, are shown (fig. 5) with the corresponding values for the crystalline material 22, 2~, ~4). By plotting 1/(n 2 1) against 1/~2 (ref. 25) the zero frequency values of the refractive indices are obtained -

Si

3.39 + 0.03,

Ge

-

3 . 9 2 _ 0.03.

4'3 4.21 G ~ I I I I

4'1

(Philipp & Toft) 22

4'0

3-9 3"8

,J

/I

.,/

3"7 3"6

3"5

(SolzborO & vi.?~ =" ~ s i , ¢ o .

--

(Philipp

x. Taft)=-, ~

-

c~ . ~ 3"4

3,a Fig. 5.

.

I

0 -1

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

-2 -3 -4 "5 "6 "7 '8 -9 1"0 1-1 1-2 13 1"4 1"5 1"6 17 1"8 1'9 hY eV

2-0

Refractive indices of amorphous germanium and silicon deposited at room temperature (294 °K), compared with crystalline material.

262

R.C. CHITrICK

These are very similar to the values obtained for crystalline material. The absorption coefficient a has an exponential dependence on the p h o t o n energy (fig. 6). a = ~o exp [~(hv - Eo)/kT], where ao, ~ and E o are constants. In the case o f germanium this relation holds in the energy range 0.5 eV to 1.3 eV and the slope kT/~, = 0 . 1 4 eV [cf. ClarkT)]. Saturation occurs for a - 3 . 5 × l0 5 at about 2.1 eV. The silicon absorption spectrum is exponential in the range 1.3 eV to 2.2 eV with a slope 0.17 eV and saturates with a - 1.8 x l0 s at about 2.8 eV.

10 ~

10 6

m

105 (I ¢m

-1

10 4

m

lO 3 -

lO 2 -

1o 1 o

I •2

I .4

I .6

I .15

I 1.0

I I I 1.2 14, 1.6 hv eV

I 18

I I 2,0 2.2

I 2.4

I 2.6

I ~6

3.0

Fig. 6. Absorption coefficients of amorphous silicon and germanium at 294°K.

263

GLOW-DISCHARGE DEPOSITED AMORPHOUS GERMANIUM AND SILICON

In both cases there is considerable absorption below the crystal fundamental absorption edge, showing the presence of states in the band gap. 2.4. PHOTOCONDUCTIVITY

A marked photo-effect has been observed in amorphous silicon films 16). The magnitude and threshold energy are strongly dependent on deposition temperature, and most noticeable in films grown at temperatures around 570°K. For this deposition temperature the change in conductivity is about 3 orders of magnitude under white illumination of 220 Ix. A Fowler plot28) of (spectral response) ~ as a function of photon energy (fig. 7) gave threshold energies Eph for films grown at different temperatures TD, as given in table 2. These are compared with the thermal gaps 2E (assuming intrinsic conduction these are twice the thermal activation energies). As can be seen there is a rough correlation between these energies. In the case of the 770°K films the Fowler plot gives an energy 1.4 eV

20 10

-- lg

g

- - 18

Fowler

Plot

- - 17

7

16

6

15

5

14

P3

13

c .o

st 0

U 2 o o.

:i

.u o

0

•]6 )7

.18 ~

1"0 1-1 1-2 1"3 1"4 1'5 1 , hv .v "

1"7 1"8 l'g 2"0 0 0 ~ : ~ ' /

~ I '~1

5

a $

z i 0

"1

"2

"3

'4

'5

"6

-7

"8

"g

1"0 1"1

hv eV

12

1"3

1"4 1'5

1'6

1"7 l-e,

1"9 2-0

Fig. 7. Spectral response of photo-current for different deposition temperatures of amorphous silicon, showing the reduction of the optical gap with increasing deposition temperatures.

264

R.C,CHITTICK

and there is a tail at lower energies converging to between 0.8 eV and 0.9 eV. The thermal activation energy at temperatures above 400°K is 0.7 eV, giving a gap of 1.4 eV. Below 400°K the activation energy decreases, and at 294°K this is 0.52 eV. No photocurrents were observed in germanium films measured from 300 °K down to 77 °K. TABLE 2

TD (°K)

Eph (eV)

2E (eV)

770 580 460

< 0.9-1.4 1.48 1.63

1.04-1.4 1.4 1.65

2.5. INFRA-RED ABSORPTION The infra-red absorption spectrum of amorphous germanium, deposited at 21 °C on potassium bromide and on single crystal germanium, has very strong peaks at 0.23 eV and at 0.07 eV (fig. 8). These bands did not change 2 x 1 03

F i l m t h i c k n e s s 4"llJ.m

lx10 3

cm

•1

"2 hV e V

I

I

I

I

"3

4

"5

'6

Fig. 8. Infrared absorption of amorphous germanium.

GLOW-DISCHARGE DEPOSITED AMORPHOUS GERMANIUM AND SILICON

265

in position or in strength when the films were cooled to 77 °K. When a film on a single crystal substrate is heated to 780°K for about an hour, the amorphous film crystallises and both absorptions disappear. They would therefore seem to be associated with the amorphous state and possibly due to hole transitions from acceptor levels formed by dangling bonds. The spectrum of silicon is completely fiat. 2.6. DENSITY Density measurements were made on thick films, about 20 pm, of silicon and germanium by successive weighings of a number of samples. The results were Si

1 . 9 _ 0 . 1 5 g / c m -3,

Ge

4.6 ___0.2 g/cm -3.

As expected they are smaller than the crystalline densities 2.33 and 5.32 respectively 20, 21). The accuracy in the density determination is greatly dependent on film uniformity. The variation in thickness from the outer edge to the centre of the films was found to be about 5%. 3. Discussion

In a recent paper Mott 27) suggests that (1) at the bottom of an impurity band, localisation of states can be important due to large fluctuations in interatomic distance and to random fields [Anderson 28)], and (2) when the wave functions are p- or d-like at the band edge localisation can occur in the band itself, due only to the lack of long range order [Banyai 2a) and Gubanov 30)]. This gives rise to (1) a strongly localised tail in the density of states, within the band gap, and (2) a range of weakly localised states at the top of the valence band and at the bottom of the conduction band. From our results on amorphous germanium there would appear to be a definite acceptor band characterised by the low 0.05 eV activation energy, due to dangling bonds. This is also shown in the work of Grigorovici et al. 5) on evaporated material. Mott 27) suggests that in this region a thermally activated hopping process in this band is responsible for conduction at low temperatures, and that loga against 1/T~should give a straight line. This is found to b~ true for temperatures below 155 °K (fig. 9). As the material is heated above its deposition temperature, these dangling bonds tend to disappear and the onset of intrinsic conduction occurs at lower temperatures. A similar region of impurity conduction probably exists in the silicon although as mentioned

Fig. 9.

10°g" 23

0-14 ¢m

~6 e

I

'2(5

I "27

I "11 -! T4 ('K) 4

"28

I 29

I 30

I

I 32

J_,

Log c ~ T~-

"31

I 33

~ -34

I "35

1 -35

I 37

The conductivity of amorphous germanium deposited at 295°K plotted as a function o£ 1/T~. Film thickness 1.6/~m.

I

'25

I

-24

~"o~

"38

C~

GLOW-DISCHARGE DEPOSITED AMORPHOUS GERMANIUM AND SILICON

267

below the resistivity at sufficiently low temperatures was too high for any measurements to be made. At higher temperatures the conduction process is intrinsic in both germanium and silicon. The carriers are excited into a high mobility region beyond the localised states. According to Gubanov 3°) phonon scattering is mainly due to long-wavelength acoustic phonons, which average over a large number of atoms and are thus fairly insensitive to disorder. Therefore it would be expected that the mobility in the non-localised regions of the valence and conduction bands will be high. If we assume that O = Oo exp(E/kT), (2) where 0o is the extrapolated resistivity on the log0 against 1/T curve at 1/T= O, then 0 = 0o exp [(½Eg + AE)/kT] (3) for a conduction process in which carriers are excited beyond a localised region AE, within the band. From the results for amorphous silicon, we find that, between 350°K and 750°K depositions, 0o is similar to the value for crystalline silicon, i.e. about 10 -4 ~2 cm (fig. 10). Therefore by substituting Oo = Oo exp ( - Eg/2kT)

(4)

0 = 0¢ exp [(E - ½Eg)/kT],

(5)

in (2), we have which is plotted for room temperature resistivities in fig. 2. Also from the change in the thermal and optical gaps of amorphous silicon, in this range of depositions, it can be seen that the effect of heat treatment is to de-localise the states within the bands, the depth of localisation dropping from 0.27 eV for the room temperature deposited material to 0.15 eV for 570 °K. From 294 °K to 350 °K depositions the pre-exponential factor 0o decreases, indicating a change in mobility and impurity concentration (fig. 10). In this range the activation energy increases rapidly to 0.83 eV, showing the transition to intrinsic conduction. Above 750°K depositions, 0o increases to 2 f2 cm and E decreases to 0.25 eV. This is interpreted as an intermediate stage, before complete recrystallisation, in which crystalline islands exist in a matrix of disordered material, the conduction being determined by the barriers between these islands. Crystallisation of our amorphous silicon has been observed by Coleman and Thomas 17) to occur rapidly at temperatures in excess of 780 °K. In fig. 11, Oo and E are plotted for amorphous germanium. Although Po is a factor of ten greater than the crystalline value, in the range 470 OK to

268

R.C. CHITTICK

10-

12

OE

X 13 o

11

,oc -

lo-1 -

--

I'0

--

D'9

--

0"8 EeV

Po~Cm 0.7

10-2

06 sto

10-3 - -

0.4

0"3

10-4 ~ P o

x

for crystal

-- 0-2

0"1

ld

0

I

I

I

I

I

I

I

I

I

De~osltlon temNi-oture TgK

Fig. 10.

Qo and thermal activation energy E for amorphous silicon plotted as functions

of deposition temperature.

670°K, the results are qualitatively similar to those for silicon and the same explanations can be employed. However, above 700°K depositions, recrystallisation occurs without the intermediate stage observed in silicon. 4. Conclusion

The evidence shown in this paper tends to support the concept of localised states in the band gap and in the valence and conduction bands of silicon and germanium. At temperatures up to around 300°K electrical conduction is achieved by thermally activated hopping in a band of acceptor states. At higher temperatures, when carriers are excited beyond the localised states into the high mobility region, the conduction is intrinsic.

269

GLOW-DISCHARGE DEPOSITED AMORPHOUS GERMANIUM AND SILICON

(>4

10-1®

Egl2 for crysttll - - :)'3

PoQcm x

EeV

X

10-3--

--

10.4 ~'-- Po for

0.2

0.1

crystal

QE

xPo

4%

I

100

t

2O0

I

I

I

I

I

300 400 500 800 700 Deposition temperature T~K

I

800

I

gO0

o

1000

Fig. 11. iT0 and thermal activation energy E for amorphous germanium plotted as functions of deposition temperature. The glow discharge system provides a contamination free method for the deposition of non-crystalline films. Many of the properties of amorphous silicon and germanium grown by this process are qualitatively similar to those of evaporated material, but it is impossible to compare any quantitative results, since the two different methods of deposition may produce films with widely differing structure and degree of disorder. Acknowledgement

I wish to thank my colleagues at STL for helpful discussion. Thanks are also due to Mr. A. H. Truelove for the optical and infra-red absorption measurements, and to the management of STL for permission to publish this work.

270

R.C. CHI'VI'ICK

References 1) R. Grigorovici, N. Croitoru and A. Devenyi, Phys. Status Solidi 23 (1967) 621. 2) R. Grigorovici, N. Croitoru, A. Devenyi, L. Vescan and P. Barna, Rev. Roum. Phys. 10 (1965) 649. 3) R. Grigorovici and A. Vancu, Thin Solid Films 2 (1968) 105. 4) R. Grigorovici, Mater. Res. Bull. 3 (1968) 13. 5) R. Grigorovici, N. Croitoru, A. Devenyi and E. Teleman, in: Physique des Semiconducteurs (Dunod, Paris, 1964) p. 423. 6) N. Croitoru and N. Marinescu, Rev. Roum. Phys. 9 (1964) 202. 7) A. H. Clark, Phys. Rev. 154 (1967) 750. 8) R. Grigorovici, N. Croitoru and A. Devenyi, Rev. Roum. Phys. 11 (1966) 869. 9) J. Tauc, Abraham, L. Pajasova, R. Grigorovici and A. Vancu, in" Proc. Intern. Conf. Phys. of Non-Crystalline Solids, Delft, 1964, Ed. J. A. Prins (North-Holland, Amsterdam, 1965) p. 606. 10) J. Tauc, R. Grigorovici and A. Vancu, Phys. Status Solidi 15 (1966) 627. 11) P. A. Walley and A. K. Jonscher, Thin Solid Films 1 (1968) 367. 12) P. A. Walley, Thin Solid Films 2 (1968) 327. 13) H. F. Sterling and R. C. G. Swann, Solid State Electron. 8 (1965) 653. 14) R. J. Joyce, H. F. Sterling and J. H. Alexander, Thin Solid Films 1 (1967/68) 481. 15) H. F. Sterling, J. H. Alexander and R. J. Joyce, Vide Special, A.V.I. Sem. (Oct. 1966). 16) R. C. Chittick, J. H. Alexander and H. F. Sterling, J. Electrochem. Soc. 116 (1969) 77. 17) M. V. Coleman and D. J. D. Thomas, Phys. Status Solidi 24 (1967) K i l l . 18) J. Wales, G. J. Lovitt and R. A. Hill, Thin Solid Films 1 (1967) 137. 19) G. H. Walker, STL Techn. Memo. No. 528, 1966. 20) H. K. Henisch, Electroluminescence (Pergamon, Oxford, 1962). 21) S. F. Sun, Electronics 37, No. 3 (1964) 30. 22) H. R. Philipp and E. A. Taft, Phys. Rev. 113 (1959) 000. 23) H. R. Philipp and E. A. Taft, Phys. Rev. 120 (1960) 37, 1002. 24) C. Salzberg and J. Villa, J. Opt. Soc. Am. 47 (1957) 244. 25) T. S. Moss, Optical Properties of Semiconductors (Butterworth, London, 1959) p. 17. 26) R. H. Fowler, Phys. Rev. 38 (1931) 45. 27) N. F. Mott, Phil. Mag. 19 (1969) 835. 28) P. W. Anderson, Phys. Rev. 109 (1958) 1492. 29) L. Banyai, in: Physique des Semiconducteurs (Dunod, Paris, 1964) p. 417. 30) A. I. Gubanov, Quantum-Electron Theory of Amorphous Conductors (Consultants Bureau, New York, 1965).