NGUYEN VAN DONG and TRAN QUOC HAI: Electronic Transport in a-Ge and a-Si 555

phys. stat. sol. (b) 88, 555 (1978)

Subject classification: 2 and 14.3.1; 22.1.1; 22.1.2

Laboratoire de Physique des Matdrriaux, Service de Chirnie Physique, Centre d'Etudm.Nucldaires de Saclay, Gif-sur- Yvettel)

Electronic Transport in Doped a-Ge and a-Si Prepared by DC Cathodic Sputtering

BY NOUYEN VAN DONG and BAN Qnoc HAI

Doped a-Ge and a-Si films are prepared by the cathodic sputtering technique with Sb and I n im- purities for a-Ge, P and B impurities for a-Si. The electrical conductivity u and thermopower 8 of different samples are measured as a function of absolute temperature between 150 and 500 K. The effect of doping is to change not only the activation energies E, and E s but also other parameters in the expression of d and 8. The considerable reduction in the pre-exponential term of cr is found to be much larger for a-Si than for a-Ge. The temperature shift of the Fermi level is taken into account to explain the variations of these parameters. The activation energy of copductivity is higher than that of thermopower by about 0.10 eV for a-Ge and 0.15 eV for a-Si. Experimental data analysis suggests that in both, a-Ge and a-Si, for high doping concentrations the hopping conduction through an impurity band makes a dominant contribution to the current flow below room temperature.

Des couches de Ge e t de Si amorphes dopes ont 6th preparees par la methode de pulvbisation cathodique avec des impuret6s Sb e t In pour a-Ge, P e t B pour a-Si. La conductivit4 Blectrique u et le pouvoir thermoelectrique 8 des diffbrents 6chantillons ont 6th mesures en fonction de la tem- pbrature absolue entre 150 et 500 K. Le dopage a pour effet de changer non seulement les hergies d'activation E, et E s mais aussi les autres paramhtres dans les expression de G et de 8. Nous avons trouve que la forte diminution du terme pr6-exponentiel de u est beaucoup plus importante pour le Si amorphe que pour le Ge amorphe. On a tenu compte du dbplacement du niveau de Fermi avec la temperature pour expliquer les variations de ces paramhtres. L'Bnergie d'activation E, est plus grande que celle Es d'environ 0,lO eV pour le Ge amorphe et 0,15 eV pour le Si amorphe. L'analyse des donn6es exp6rimentales sugghre que dans le cas d'un fort dopage, le transport dans ces materiaux serait domin6 au-dessous de la temperature ambiante par une conduction par saut dans une bande d'impuret6s.

1. Introduction

The substitutional doping of amorphous Si and Ge, first reported by Spear and Le Comber [l] in recent years has opened new interesting possibilities for the study of these materials. Several research groups investigate now intensively the electronic properties of doped a-Si and a-Ge. The current approach to substitutional doping has been from the gas phase in films prepared by the technique of glow discharge. I n the samples prepared by this procedure, the low density of gap states which allows the doping process appears to be due mostly to the compensation of dangling bonds by hydrogen. The fact that incorporated hydrogen atoms reduce the density of gap states has been demonstrated in the work on a-Ge prepared by rf sputtering [2]. Therefore, it appears of interest to apply this procedure to amorphous semiconductors in an attempt to perform doping experiments.

I n this paper, we investigate the electronic transport properties of doped a-Ge and a-Si films prepared by dc sputtering. Some results obtained for a-Si have been pub-

l) 91 190 Gif-sur-Yvette, France.

556 NGUYEN VAN DONG and "RAN QUOC HAI

lished recently [3]. Similar doping experiments on these aniorphous semiconductors have been performed by the group a t Harvard University [4].

2. Experimental Details

Both doped and undoped films with thicknesses varying from 0.6 to 1 pm were grown by dc sputtering in a Veeco triode sputtering system. A vacuum of lo-' Torr was obtained by dry pumping, before introducing 3 x Torr mixture of argon with 10% hydrogen. The substrates (aluminium oxide) were heated a t 200 "C to ensure a low density of gap states in the films. The targets were supplied with about 30 W dc power a t 1 kV.

Doping of a-Ge was achieved by co-sputtering a small quantity of antimony or indium. It is estimated that about 5 x parts per volume of Sb or I n were in- corporated in the layers. I n the case of a-Si, the undoped films were obtained from a relatively pure crystalline Si target, while those doped with phosphorus or boron were deposited from a series of c-Si targets containing various concentrations of P or B, between 2 x l0ls and l O l e

After deposition of the aniorphous layers, nichrome electrodes, approximately 0.2 pm thick, were sputtered onto the samples. The dc conductivity and thermopower were measured with a Keithley electrometer (type K 616). The upper limit of the temperature range investigated was about 500 K corresponding to the deposition temperature. Because the films were highly resistive a t low temperature, conductivity measurements were confined to temperatures above 100 K, while thermoelectric measurements were restricted to higher temperatures, namely above 150 K.

3. Results and Discussion 3.1 Litdoped amorphous films

The effects of hydrogen on the electrical conductivity and thermopower of a-Ge and a-Si is clearly shown in Fig. 1. I n unhydrogenated films, the density of gap states 1s quite high so that electron transport takes place in localized states near the Fermi level even a t room temperature. This is argued from the conductivity which obeys the Mott law (logo- T-lI4) over a large temperature range investigated. For hydrogenated films, the room temperature conductivity is smaller by several orders of magnitude because of compensation of defect states by atomic hydrogen. This defect compensation is efficient enough to allow electron transport to occur in the

. a -I2 L 4 8

Fig. 1. Effect of hydrogen on the a) electrical con- ductivity and b) thermopower of undoped a-Ge and a-Si. (1) Hydrogenated a-Ge, (1') unhydrogenated a-Ge, (2) hydrogenated a-Si, (2') unhydrogenated a-Si.

Deposition temperature 200 "C

Electronic Transport in Doped a-Ge and a-Si Prepared by DC Sputtering 557

extended states near room temperature as suggested from the activated behaviour of conductivity and thermopower which can be described for T > 300K, respectively, by the usual expressions

s = -;(g + A ) .

The values of E,, Es, C, and A are listed in Table 1. For a-Si, the activation energy E, = 0.78 eV is larger than that obtained from discharge produced films [l].

Table 1 Transport parameters of undoped a-Ge and a-Si

a-Ge 0.40 0.28 6.5 x lo8 3 a-Si I 0.78 1 0.58 1 8.0 x 10s 1 1

The difference E, - Es of 0.12 eV for a-Ge and 0.20 eV for a-Si is similar in magni- tude to that observed by Paul et al. [4] and Anderson et al. [5 ] . The existence of this difference has been Dublished at first bv Bever and Stuke r61. The results are, however,

" Y

quite different for &specimens prepared by glow dischaige decomposition in which E, z E , r7i.

" L 2

The fact that E, is higher than Es can be explained in terms of the model of simul- taneous current paths as used at first by Nagels et al. [ 8 ] : one by electrons excited in the extended states near the mobility edge E,, the other by hopping of electrons in the localized states a t the band edge EA. For undoped a-Ge, the curvature of a and S below and near room temperature may support this model according to which the conductivity and thermoelectric power are expressed as

a = a, + 0, , (3)

(4)

respectively, with

crl = C, exp -

0, = C, exp -

Conductivitv and

[ [

kT kT ,

I thermopower data can be fitted to expressions (3) and (4) up to 200 K. The v iues of. the parameters used are as follows:

C, = 450 S2-l cni-1 , C, = 0.1 Q-1 cm-' ,

E, - EF = 0.40 eV , A, = A 2 = 1 ,

E, - EF = 0.17 eV , W = 0.05 eV .

The difference between the two activation energies for S, and S, gives the range of tail states E , - E, = 0.23 eV, in agreement with the result obtained by Jones et al. [7]. Below 250 K, there is an appreciable deviation from the calculated curves.

558 NGWYEN VAN DONG and %AN Qnoc HAI

Sb-doped = 6 x ppv In-doped z 5 x 10-8 ppv

This fact can be attributed to the remaining defects which would not be compensated by hydrogen.

3.2 Doped amorphous films

Now notice the effect of doping on the transport parameters. For a-Ge, the addition of an amount of antimony x 5 x parts per volume raises the room temperature conductivity by two orders of magnitude (Fig. 2). Above 250 K, the conductivity curve exhibits an activated behaviour with a slope smaller as compared with the undoped case. This film is n-type according to the therniopower which displays a well defined but small activation energy also above room temperature (Fig. 3). On the other hand, the addition of a n equivalent quantity of indium produces an increase in u of less importance. The film turns now to p-type according to the sign of the thermopower which exhibits a t high temperatures an activated behaviour.

Qualitatively similar results are obtained in a-Si, but here the effects of doping on the transport properties are much larger (Fig. 4 and 5).

Turning now to the activation energies E , and E, for conductivity and thermo- power, we see that the difference E, - E , for both doped a-Ge and a-Si is similar in magnitude to that observedin the undoped case. Again this difference can be explained in terms of the model of two current paths similar to that used above in the undoped case. The first current path is in extended states at E , or a t E, and the second is in an impurity band at E , where conduction occurs by hopping with an activation energy WI. Le Comber et al. [lo] have analysed their results on conductivity and Hall effect in highly doped a-Si in terms of this model. E , is taken as the average energy of donor or acceptor states distributed throughout the entire band tail [lo]. For doped a-Ge, we have attempted to fit conductivity and thermopower data to expressions (3) and (4), respectively. The values of different parameters obtained from the best fits are reported in Table 2.

Table 2 Values of transport parameters for doped a-Ge obtained by fitting u and S data

with the model of two conduction mechanisms

0.20 15 0.075 0.6 0.05

0.20 18 0.125 1 0.05

A

4

6.6

One can now deduce the average energy E , where transport takes place by hopping in the impurity band. It is found E , - E , x 0.12 eV for Sb doping and E I - E, x x 0.08 eV for I n doping. The calculations show that this conduction mechanism becomes dominant below room temperature.

At lower temperature (T < 200 K), the measured conductivity deviates from the calculated curve and displays a n activated behaviour with a small activation energy of about 0.07 eV. This fact can be ascribed to the hopping conduction a t the Fermi level, which arises presumably from the association of small defects (multi-vacancies) with some of the impurity atoms.

For undoped a-Si, it is not possible to test the model because the temperature range over which u and S have been measured was confined to temperatures above room temperature.

Electronic Transport in Doped a-Ge and a-Si Prepared by DC Sputtering 559

Fig. 3. Thermopower of doped a-Ge; (1) undoped, (2) Sb-doped, (3) In-doped. Experimental data : solid lines; model of two conduction mechanisms:

dashed lines

Fig. 2. Conductivity of doped a-Ge; (1) undoped, (2) Sb-doped, (3) In-doped. Solid lines represent experimental data; dashed lines are calculated from the model of two conduction mechanisms

Analysis of experimental data shows that for both films, doping produces not only a shift of the Fermi level towards the conduction band or the valence band, but also the variations of C and A. The values of E,, C, Es, and A for a-Ge and a-Si are reported in Table 3. The decrease in C is much stronger for a-Si than for a-Ge. As observed in the case of undoped films, E,is higher than Es by about 0.10 eV for a-Ge and 0.15 to 0.20 eV for a-Si.

In the case of a-Ge, the values of E, obtained with a high doping level would indicate that saturation occurs around this level. This is in agreement with the above estimate of the range of the tail states for the conduction band.

A similar conclusion can be drawn for n-type a-Si with high phosphorus doping. However, for boron doping a much larger impurity concentration would be required to produce saturation.

To explain the drop in C and the increase in A with doping, we suppose the Fermi level to move towards the midgap with increasing temperature, as proposed by Spear 36 physics (b) 88/2

560 N a U Y E N VAN D o N a end AN Quoc HAI

-7 L " - 2'

-2 0 2 4 L d,p, T

Fig. 5. Thermopower of doped a-Si; (1) undoped, (2') P-doped, 3 x l0ls ~ r n - ~ , (3) B-doped,

2 x 10'8 0111-3

f-lK-1- 7$ 1

Fig. 4. Conductivity of doped a-Si; (1) undoped, (2) P-doped, 2 x 1016 ~ r n - ~ ; (2') P-doped, 3 x 10l8 ~ r n - ~ ; (3) B-doped, 2 x 10l8 cm-a; (3') B-doped, 10'8 ~ r n - ~

and Le Comber [l]. If the variation of E , - E, or EF - E , is assumed linear:

E, - E, = ( E , - Ed0 + y T with ( E , - EF)O the value extrapolated to T = 0, the conductivity and thermoelec- tric power for n-type samples can be expressed as, respectively,

o=o,exp(-$exp[- ;Po 1 (5)

The experimentally determined C- and A-values come out t o be

c = Go e=p (--ylk) 9

A = A 0 + Ylk

The constants a. and A, are referred to undoped films for which E, is close to the midgap and y x 0.

The increase in A which arises from doping allows the determination of y. The cal- culated value for y is found to be in the range 3k to 4k. These values account satis- factorily for the observed decrease in C for Sb-doped and In-doped a-Ge. In the case of doped a-Si, the drastic drop in C of three orders of magnitude or more for high impurity contents, which is similar in glow discharge samples [Y], cannot be ascribed only to the temperature shift of the Ferriii level. This difference in C between a-Ge and a-Si is difficult to explain. Perhaps, in highly doped a-Si there could be addi- tionally, as suggested by Itehm et al. [Y], a transition of carriers excited from the im- purity band to more extended states with presumably higher a,,.

Electronic Transport in Doped a-Ge and a-Si Prepared by DC Sputtering 56 1

The case of strongly doped a-Si is qualitatively similar. The conductivity curves show a change in slope at low temperatures which may arise from the predominance of hopping conduction in the impurity band. The activation energy is found to be 0.15 eV for phosphorus doping and 0.20 eV for boron doping. Taking into account the value of the hopping energy W , = 0.08 eV [lo], we obtain E , - El = 0.14 eV for phosphorus impurity and E, - E , = 0.10 eV for boron doping.

4. Conclusions

films prepared by the technique of dc sputtering.

ductivity. This reduction is much larger for a-Si than for a-Ge.

impurity band becomes predominant below room temperature.

1. Substantial doping effects have been observed in hydrogenated a-Ge and a-Si

2. Doping produces a considerable reduction in the pre-exponential term of con-

3. For strongly doped films, it is suggested that hopping conduction through an

Acknowledgement

The authors would like to express their thanks to J. Y. Le S y for technical assist- ance.

References [l] W. E. SPEAR and P. G. LE COMBER, Phil. Mag. 33, 935 (1976). [2] A. J. LEWIS, Phys. Rev. B 14, 658 (1976). [3] NQUYEN VAN DONG and %AN Quoc HAI, Rev. Phys. appl. 1 3 , 7 (1978). [4] W. PAUL, A. J. LEWIS, G. A. N. CONNELL, and T. D. MXJSTAKAS, Solid State Commun. 20,

[5] D. A. ANDERSON, T. D. MOUSTAKAS, and W. PAUL, Proc. 7th Internat. Conf. Amorphous and

[6] W. BEYER and J. STUKE, Proc. 5th Internat. Conf. Amorphous and Liquid Semiconductors,

[7] D. I. JONES, W. E. SPEAR, and P. G. LE COMBER, J. non-crystall. Solids 20,259 (1976). [8] P. NAQELS, R. CALLAERTS, and M. DENAYER, see [6] (p. 867). [9] W. REHM, R. FISCHER, J. STWE, and H. WAQNER, phys. stat. sol. (b) 79, 539 (1977).

969 (1976).

Liquid Semiconductors, Edinburgh 1977.

Garmisch 1973 (p. 251).

[lo] P. G. LE COMBER, D. I. JONES, and W. E. SPEAR, Phil. Mag. 36, 1173 (1976).

(Received May 2, 1978)

36'