Influence of Boron doping on the transport properties of A-SI : H films

~

Solid S=ate Communications, Voi.38, pp.891-894. Pergamon Press Ltd. 1981. Printed in Great Bri=ain.

0038-1098/81/220891-04502.00/0

INFLUENCE OF BORON DOPING ON THE TRANSPORT PROPERTIES OF A - S I : H

FILMS

W. Beyer lnstitut for G r e n z f l ~ c h e n und Vakuumphysik K e r n f o r s c h u n g s a n l a g e JNlich, D - 5170 JNlich, Germany and H. M e l l

Fachbereich

Physik,

(Received

Universit~t Marburg, Germany

29 January

D - 3550 Marburg

1981 by M. Cardona)

The temperature dependences of the c o n d u c t i v i t y ~ and thermoelectric power S have been measured for a series of glow discharge deposited amorphous silicon films doped with boron and/or phosphorus. The data are discussed in terms of the dimensionless quantity Q = ino + le/k.S I whose slope in a ]/T-plot, EQ, formally is a m o b i l i t y activation energy. Doping with boron leads to a p r o n o u n c e d increase of EQ (up to 0.24 eV), surprisingly for both p-type and n-type specimens. This behaviour is attributed to two effects: (i) the broadening of the bandtails and (ii) the increase of longrange potential fluctuations caused by charged centers as well as by inhomogeneities of structure and composition of the a-Si:H films.

As discovered by Spear and LeComber l the electrical c o n d u c t i v i t y of hydrogenated amorphous silicon (a-Si:H) can be systematically controlled by suitable incorporation of phosphorus and boron. This property and the potential application to photovoltaic solar energy c o n v e r s i o n 2 have stimulated much interest in a-Si:H films. In addition to the conductivity several other properties are also affected by the doping process 3 . Among these are the drift mobility and the Hall mobility. To explain the changes of these m o b i l i t i e s Spear et al. 3 suggested that doping introduces a second current path in a donor or acceptor band, respectively. On the other hand, our thermoelectric power and c o n d u c t i v i t y data have indicated that there is only one c o n d u c t i o n path (for either kind of carriers) which itself is affected by the doping process~,5 I t has been t h e a i m o f t h e p r e s e n t paper to study this effect in more detail. In particular we wish to address the question: Does a given impurity solely affect the properties of one kind of carriers or of both. To provide an answer to this question we have carried out a systematic investigation of the c o n d u c t i v i t y ~ and thermoelectric power S for a series of a-Si:H films doped with either boron or phosphorus or with both impurities. The films, about ] ~m thick, were

p r e p a r e d by the decomposition of silane mixed with diborane or/and phosphine using an inductively coupled rf glow discharge system. In each run two substrates (fused quartz and sapphire) were used which were held at a temperature of 3OO°C. A deposition rate of ].5 ~/s was achieved at a pressure of ~O.4 mbar, a flow rate of 6 sccm and a rf p o w e r ~ 5 Watts. In order to obtain a good accuracy for the thermopower data, each value of S was determined by a least square fit using about 10 values of thermovoltage AV and temperature gradient AT, all taken at a given average temperature. In this letter we shall present the c o n d u c t i v i t y and thermopower data in terms of the dimensionless quantity

(i) q = in~ ÷ I~ SI where e is the elementary charge and k is Boltzmann's constant. As demonstrated in two preceding papers4, s, for a nondegenerate semiconductor with unipolar conduction, Q is independent of the Fermi level position and, consequently, is solely a property of the current path at the band or m o b i l i t y edge. In terms of the K u b o - G r e e n w o o d formalism Q is determined by the differential conductivity ~(E,T) 4, i.e. by the product of the density of states g(E) and the mobility u which may depend on both energy E and temperature T. If for instance, as has often been assumed for amorphous semi89 1

BORON DOPING ON THE TRANSPORT PROPERTIES OF A-SI:H FILMS

892

conductors 3, conduction takes place in the extended states beyond a sharp mobility edge, Q is a constant, independent of temperature ~. Fig. ] shows a plot of Q as a function of reciprocal temperature for a number of d i f f e r e n t l y doped a-Si:H films. Clearly in all cases Q depends markedly on temperature. Within the experimental accuracy the data points can be fitted by straight lines whose slope

Vo£. 3 8 ,

No. I0

Beyer ~ showed that a n o n v a n i s h i n g value of EQ can also be reconciled with conduction ~n e x t e n d e d states, if one assumes that the m o b i l i t y edge fluctuates in space. Possible reasons for that could be density fluctuations, strain fields or potential fluctuations caused by c h a r g e d centers. Our results discussed below indicate that both mechanisms, shift of Ema x and fluctuations, may contribute to the temperature depencence of

Q. EQ

=

- k ~

(2)

varies between O.I and 0.2 eV. Such behaviour has been reported for a variety of undoped and doped a-Si:H films~, 5. I t is equivalent ~o the earlier observation for evaporated ° and sputtered 7 a-Si:H

The values of EO and of the extrapolated intercept Q o = Q ( / T = O ) for all specimens used in this study are plotted in Fig. 2 and Fig. 3 as a function of

'2

films that the c o n d u c t i v i t y activation energy Eo is greater than the thermopower slope ES, since ~

EQ : E°

ES

Q_

,0

--: J

o

(3) 03 vppm

~,~ "~\~"\ \'X~,_

• IO0O • I000 a IO00

'&.%

L

vpom

,

o [ ~ 200 /- '-, 3CCC! x

r

~0 :COO : 200 000 • 3000 IO00

92 >

k L

~-"

"-.

I

%

/

0

' ~04 C~2~,

L I/T,

Figure

IO-~K - '

\ I/T,

Figure

IO'3K"

'

, 0

L I0 :

:Oc

vppm

C ~. J,

voorn

2: Dependence of the intercept Qo and of the slope EQ on the c o n c e n t r a t i o n of a single dopant (B2H 6 or PH3, respectively).

J

I: The quantity Q = Ino + le/k'Sl as a function of reciprocal temperature. The full (open) data points represent n-type (p-type) a-Si:H films. Dopant concentrations are indicated in the figure.

Formally EQ appears to be the a c t i v a t i o n energy of the m o b i l i t y ~. Therefore, it has been assumed that EQ is a hopping energy which might be associated with transport in localized states at the band edge 6 or with small polaron formation 7. An a l t e r n a t i v e explanation has been p r o p o s e d by DOhler 8 as well as by GrSnewald and Thomas 9. These authors assumed that the states which contribute to the c o n d u c t i v i t y are spread over a r e l a t i v e l y wide energy range either below and above a gradual m o b i l i t y edge s or deeper in the band tail 9. In this case the temperature dependence of Q originates from a strong shift of the energy of m a x i m u m conduction, Emax- The flatter the band tail or ~(E) is, the larger is EQ. Recently Overhof and

I i

~02

_

IQ

20

I

>

\

3

2b

; i°

,.s I

0

iO C~H~ ,

Figure

: ~3

i vppm

!I

$

,i--~ ~111

0

II

K) CpM 3 ,

~0 vppr6

3: D e p e n d e n c e of Qo and E O on the c o n c e n t r a t i o n of one of two dopants (B2H6 or PH3, respectively). The c o n c e n t r a t i o n of the second dopant is ]OOO vppm PH 3 for curves la and ]b and ]OOO ppm B2H 6 for curves 2a and 2b. Dashed curves see Fig.

2

Vol. 38, No. I0

BORON DOPING ON THE TRANSPORT PROPERTIES OF A-SI:H FILMS

the dopant concentration in the silane gas. The full points represent n-type samples whereas the circles refer to ptype specimens. Fig. 2 shows the results for films doped with only one kind of impurity, either B2H 6 or PH 3. Within the experimental accuracy both EQ and Qo are independent of the dopant concentration up to about 30 ppm and increase markedly for higher concentrations. It is important to note that the increase of EQ is appreciably stronger on the diborane side, where most of the samples are p-type. This result raises the question: Does the transport path of the holes react more sensitively on the addition of impurities than the path of electrons, or does doping with B2H 6 cause stronger changes in the a-Si:H films than doping with phosphine? To clarify this point we have investigated films doped with both impurities. The results of EQ and Qo for these compensated films are shown in Fig. 3. Curves la and Ib have been obtained for specimens doped with the same phosphine concentration, namely 1OOO ppm, and various concentrations of B2H 6. The increase of E O with the diborane content (curve Ib)-is very similar to that for singly doped films, shown by the dashed curve, although the compensated films are n-type up to a diborane concentration of 1OOO ppm. This result suggests that doping with B2H 6 does not merely affect the transport path of holes but also that of electrons. Further evidence for this effect is obtained from films doped with ]OOO ppm B~H 6 and various concentrations of PH 3. The data for these specimens are plotted in curves 2a and 2b (Fig. 3). Here EQ O.19 eV and Qo ~ 10.5 independent of the phosphine c o n c e n t r a t i o n and of the sign of the charge carriers. Clearly, with these compensated films the properties of the transport path are determined by some action of diborane which affects the conduction band in the same way as the valence band. This is also revealed by Fig. ]b where Q is plotted as a function of I/T for three of the samples in consideration. Although one of them is n-type and the others are p-type, the data points can be fitted by a single straight line. This result implies that the conduction mechanism of holes is the same as that of electrons. We believe that this statement also applies to undoped and lightly doped films, since the general trend of the curves in Fig. 2 suggests that for these specimens, too, Q is roughly the same for conduction by holes as for conduction by electrons. The uniform influence of boron doping on the transport paths of electrons and holes is at variance with the assumption that doping introduces a second current path in a donor or acceptor

893

band, r e s p e c t i v e l y 3. It also tells against an e x p l a n a t i o n o~ EQ > 0 in terms of a small polarop model . There are two possible effects through which the dopant concentration might influence the transport paths of electrons and holes in the same way: (i) a broadening of the band tails and (ii) an increase of potential fluctuations. Both effects would cause an increase of the apparent mobility activation energy EQ. The first one according to D6hler's model s and the second in terms of the fluctuation model I°, both m e n t i o n e d above. The occurence of effect (i) is suggested by the observation of a strong tail in the optical absorption spectrum of boron doped a-Si:H 11. For phosphorus doped films, on the other hand, this tail is less pronounced, which is consistent with our result in Fig. 2 that, for dopant c o n c e n t r a t i o n s above 1OO ppm, ~ is a p p r e c i a b l y smaller for PH 3 than r B2H 6. The effect (ii) may occur for the following reasons. It is w e l l - k n o ~ that diborane decomposes to form higher boranes 12, in p a r t i c u l a r at temperatures above 2OO°C. Therefore, it is very likely that part of the boron atoms are deposited in form of clusters containing two or more boron atoms (and probably several hydrogen atoms). This may result in a strongly inhonogeneous distribution of charged centers, which should give rise to p a r t i c u l a r l y strong long-range potential fluctuations I°. The second possible reason for increased fluctuations in boron doped a-Si:H films is related to the observation that the hydrogen evolution from these films is appreciably shifted to lower temperatures as compared with undoped or phosphorus doped films 13 This result has been attributed to an increased p o r o s i t y of the boron doped films 13 which might be due to an increased number of voids ~4 or other mic r o s t r u c t u r a l inhomogeneities 15 These will be a s s o c i a t e d with a strongly inhomogeneous d i s t r i b u t i o n of hydrogen and of charged centers which will cause fluctuations of, respectively, the band gap 16 and the electrostatic potential I° and, consequently, will give rise to large values of EQ. From the present experimental data it is not possible to decide whether one or both of the above effects, tailing and long-range fluctuations, are responsible for the pronounced increase of EQ with the boron concentration. Taking into account the large magnitude of E N (up to 0.24 eV according to Fig. 2) wO consider it very probable that both effects do contribute. Further experiments are in progress aimed at separating these two effects on the transport properties of a-Si:H films.

894

BORON DOPING ON Th~ TRANSPORT PROPERTIES OF A-SI:H FILMS

Vol. 38, No. I0

References

I. W.E. Spear and P.G. LeComber, Phil. Mag. 33, 935 (1976) 2. D.E. Carlson and C.R. Wronki, Appl. Phys.Lett. 28, 671 (1976) 3. For a review see: W.E. Spear, Adv. Phys. 26, 811 (1977) 4. W. Beyer, R. Fischer and H. Overhof, Phil.Mag. B 59, 205 (1979) 5. W. Beyer and H. Overhof, Solid State Comm. 31, I (1979) 6. W. Beyer and J. Stuke, Proc. 5th Int. Conf. on Amorphous and Liquid Semiconductors (Eds. J. Stuke and W. Brenig) p. 251 (1974) 7. D.A. Anderson, T.D. Moustakas and W. Paul, Proc. 7th Int. Conf. on Amorphous and Liquid Semiconductors (Ed. W.E. Spear), p. 334 (1977)

S 9 10 11 12 13 14 15 16

G.H. D~hler, P h y s . R e v . B 19, 2085 (1979) M. G r O n e w a i d and P. Th o m as, p h y s . stat.so!. (b) 94, 125 (1979) H. O v e r h o f and W. B e y e r , P h i l . M a g . , in p r e s s J . C . K n i g h t s , AIP C o n f . P r o c . ~1, 296 (1976) L.V. McCarty and P.A. Di Giorgio, J. Am. Chem. Soc. 73, 3138 (1951) IV. Beyer, H. Wagner and H. Mell, to be published P. D'Antonio and J.H. Konnert, Phys. Rev. Lett. 43, 1161 (1979) J.C. Knights, J. Non-tryst. Solids 35 and 36, 159 (1980) M.H. Brodsky, Solid State Comm., in press