Acta Physica Academiae Scientiarum Hungaricae, Tomus 49 (1--3), pp. 207-214 (1980)

STRUCTURE AND ELECTRICAL PROPERTIES OF AMORPHOUS Ge--Mo FILMS By A . BELU, A. D•201

R . MANAILA, L. M I u , C. R u s u

I N S T I T U T E OF PHYSICS AND MATERIALS TECHNOLOGY, BUCHAREST, ROUMANIA

and 93 BARNA, P. B. BAR/NA, G. RADN£

L. T£

RESEARCH INSTITUTE FOR TECHNICAL PHYSrCS,UUNGAmANACADEMY OF SCIENCES ti-1325 BUDAPEST, HUNGARY

Coevaporated amorphous Ge t xMOx fihns (0.07 < x < 0.32) were investigated by means of electron microscopy, electron and X-ray diffraction and electrical measurements. Inhomogeneity in the Mo concentration has been found only in the samp]es of high metal content. Mo canses drastic changes in the ED and XRD patterns of a-Ge. The experimental interference functions for x < 0.22 are well matched by those calculated on the basis of a structural model f o r a relaxed a-Ge random network in which the M9 atoms fill part of the empty spaee. In the range 80 470 K both samples with x ~ 0.07 and 0.16 display activation energies zlE 6 times less than those of a-Ge and close to k B T, which is indicative of hopping on high density localized states. Below 80 K for the samples of high Mo content 3 E becomes very small suggesting the presence of metallie regions, which dominate conduction at low T.

Introduction C e r m e t s , c o n s i s t i n g o f a m e t a l d i s p e r s e d o n a t o m i c scale i n t o a d i e l e c t r i c m a t r i x , r e p r e s e n t a n e w class o f m a t c r i a l s o f t e c h n i c a l i n t e r e s t , t h e i r p h y s i c a l p r o p e r t i e s b e i n g s t i l l h a r d l y u n d e r s t o o d . C e r m e t s a r e s t a b l e m a t e r i a l s w i t h cont r o l l a b l e e l e c t r i c a l r e s i s t i v i t y ~ a n d s m a l l t h e r m a l coes of the electrical r e s i s t a n e e T C R ( : ~ 5.10 -~ K - l ) . T h e y a r e g o o d c a n d i d a t e s for p a s s i v e r e s i s t o r s w i t h s m a l l T C R in m i c r o e l e c t r o n i c s . A l s o , c c r m e t s w i t h o p t i m i z e d p r o f i l e s o f t h e m e t a l c o n c e n t r a t i o n a r e m u c h e x p l o r e d n o w a d a y s as a b s o r b e r s for s o l a r cells. C e r m e t s a r e u s u a l l y o b t a i n e d as t h i n f i l m s , b y c o - d e p o s i t i o n o f t h e c o m ponents. T h e s y s t e m i n v e s t i g a t e d in t h e p r e s e n t w o r k r e p r e s e n t s a n a l t e r a t i o n o f a m o r p h o u s Ge, w h i c h fs a r e f e r e n c e m a t e r i a l in t h e p h y s i c s o f a m o r p h o u s s e r n i c o n d u c t o r s , b y i n t r o d u c i n g a m e t a l , t h e r e b y m o d i f y i n g its e l e c t r i c a l properties and its structure. Thus, by Iowering the electrical resistivity the r a n g e o f c o n d u c t i o n m e a s u r e m e n t s c a n b e e x t e r M e d b e l o w 10 K . A l s o , t h e r e l a t i v e l y h i g h D e b y e t e m p e r a t u r e o f Mo (360 K ) s u g g e s t s a s l o w d i f f u s i o n o f Mo in Ge, w i t h o u t f o r m a t i o n o f m e t a l l i c Mo i s l a n d s . A t h i g h e r Mo c o n c e n t r a t i o n s a s u p e r c o n d u c t i n g b e h a v i o u r is e x p e c t e d , Mo h a v i n g a c r i t i c a l t e m p e r a t u r e T c o f 0.98 K , w h i l e Ge s h o w s n o s u p e r c o n d u c t i v i t y a t n o r m a l p r e s s u r e . Acta Physica Acaderaiae Scientiarum Hungaricae 49, 1980

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Preparation G%_ xMoxfilms with 0.07 < x < 0.32 were obtained by coevaporation from two independently controlled sources, under a residual pressure of 1.10 -4 Pa. Mo was evaporated from ah eleetron gun while Ge from a W filament. The deposition rate was controlled by means of a calibrated photodiode. The composition parameter x was varied by changing the deposition rate of Mo. The films were deposited on fused silica substrates maintained at room temperature, on which Mo electrodes in planar configuration had been deposited. The deposition rate was 6--11 nm/min. Samples for electrical conduction, thermopower and piezoresistance measurements (thickness 80--100 mm), for electron microscopy and diffraction (thickness 30--80 mm) as well as for X - r a y diffraction (thickness 0.25--0.6 #m) were deposited at the same time.

Electron microscopy Electron micrographs of the samples Mo concentrations x < 0.16 show a microstructure ver y similar to t ha t of pure amorphous Ge (Fig. 1) [1]. This means th at a quasi-periodic density fluctuation is present in the films which causes a weak phase contrast on the micrographs. The mean period length is 10--15 mm without any remarkable change with composition. The amplitude of these fluctuations, however, becomes higher with increasing Mo content. Ir can be supposed t h a t in the samples x > 0.22 fluctuations occur not only in density but also in concentration. Probably Mo-rich regions are formed without any crystalline Mo precipitation, and pointing to increasing separation t e n d e n c y of two different amorphous phases with increasing metal content.

Electrical properties Electrical resistance measurements in the wide temperature range 1,9-760 K showed a semiconducting behaviour (TCR < 0) for x < 0.16. The electrical properties were described by the variation of the activation energy AE with temperature T. The activation energies were calculated from the o(T) data and from the S(T) (thermopower) data by a numerical differentiation program. The samples with x ~-- 0.07 and 0 . 1 6 show activation energies of the conduction AEr smaller by a factor of ~---6 as compared to ah evaporated a-Ge sample (Fig. 2). For T > 80 K AE~ is practicaUy equal to kBT (k~ is the Boltzmann constant) which is indicative of a hopping conduction mechanism on high density localized states [2]. These states should be at t ri but ed to Mo Acta Physica Academiae Scientiarum Hungaricae 49, 1980

STRUCTURE AND ELECTRICAL PROPERTIES OF AMORPHOUS Gr

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dispersed on ah a t o m i e scale in t h e a Ge m a t r i x which seems to be a c h a r a c t e r istie feature of t h e cermets [3]. The Mo c o n c e n t r a t i o n d e t e r m i n e s t h e AE~ values for T < 80 K . F o r x = 0.07, zJE~ is a l m o s t coincident w i t h kBT while with increasing x AE~

C1

b

d

Fig. 1. E l e e t r o n m i c r o g r a p h s of t h e G e t - x M o x s a m p l e s . L e f t : f o c u s s e d . R i g h t : 100 n m u n d e r ~ f o e u s s e d , a) s = 0,07; b) x = 0,16; c) x = 0,22; d) x = 0,32

14

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-II[ogAE[eV] X=O ~~.~~.,s -2

_

Gel-x Mox

f

-~ x.o~,~-~~~ ~ •

+

+

++ -5

.t-

+

)(=0.32 +

Fig. 2.

o 0's 1 ~s i 2s tog T[KI Activation energy of electrical conduction. Full line: log kB T

decreases below k B T (Fig. 2). This fact could be explained b y the f o r m a t i o n o f " f i l a m e n t s " with metallic-type c o n d u e t i v i t y for x ~ 0.16 whieh t a k e over t h e electrical t r a n s p o r t at low T. These " f i l a m e n t s " must be related to the Mo-rich zones observed b y electron microscopy for x > 0,16. The sample with x = 0.32 shows a superconducting t r a n s i t i o n at T n = = 2.9 K, which agrees with its metallic-type c o n d u c t i o n at v e r y low T (TCR N 2 "10 -3 K -1 b e t w e e n 3 and 6 K). Piezoresistance measurements also show t h e metallic c h a r a c t e r of the conduction increasing with x. The EJEs t h e r m o p o w e r a c t i v a t i o n energies ate s y s t e m a t i c a l l y lower t h a n AE~. This fact is eharacteristic for other cermets too [4] and suggests different mobilites for electrons and holes in the hopping conduction.

Structure Coevaporated Gel_~Mo x alloys (0 < x ~ 0.32) have been found to h a v e ah a m o r p h o u s s t r u c t u r e b o t h b y electron and X - r a y diffraction. A eharacteristic change in the electron diffraction p a t t e r n s of the samples with increasing Mo-content are shown in Fig. 3. T h e trends of these changes are v e r y similar to those observed b y NOWAK et al [5] in amorphous G e - - n o b l e metal alloys and suggest changes in the short range order towards metallic coordin a t i o n of atoms. F o r a more exact s t u d y the diffracted electron i n t e n s i t y was recorded directly and the interference function F(k) :

k 9 I(k) --

,

~

k = 4~r sin_____~_~

z

has been ealculated. E x p e r i m e n t a l F ( k ) functions are c o m p a r e d with those calculated from a model in Fig. 4. .4cia Ph~'sica Academiae Scientiarttm Hungaricae 49. ]980

5TRUCTURE AND ELI~CTRICAL PROPERTIES OF AMORPHOUS C,e---Mo

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3 l [og I 6e1_x MOl_x

2

1

5o

~9

s,o

s# nm-~ ~k

Fig. 3. Densitometric traces of e]ectron diffraetion patterns of Gel _xMOx films F(k) 0.5

~!' /

-0.5

i; v~ ~ .,~ li 11 ",.~.~c,- ~ ~

--

x-o.2z experimenta'~

0.5

# ~~i ,b, --,,.o.o, -0.5

;1

t:r

20'

40 '

t,$

0

60 '

8b

k [nm-1

Fig. 4. Reduced interference functions F(k) experimental and model calculated

Structural model for the Gel_ x Mo x system The strueture of a-Ge is correetly represented b y spatial models based upon local satisfaction of hybrid sp a bonds [6, 7]. These models satisfaetorily aecount for the experimental atomic radial distribution ( R D F ) and for the dis i n t e n s i t y distribution. [4~t

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A closer e x a m i n a t i o n of such a model reveals much free space. On the basis of the a-Ge d e n s i t y ( ~~- 4.92 g .cm -3) and covalent radius of Ge a t o m (0.122 nm) this free space can be calculated to comprise a b o u t 7 0 % of the t o t a l volume. A f r a c t i o n of it ( ~ 18%) is c o n c e n t r a t e d in zones large enough t o a c c o m m o d a t e a Mo a t o m (metallic radius 0.139 nm). The filling of the large zones corresponds to an atomic c o n c e n t r a t i o n of Mo x ~ 0.22. This value is r e m a r k a b l y close to the threshold c o n c e n t r a t i o n x = 0.16--0.22 at which m a r k e d changes in the diffraction p a t t e r n occur. Threshold concentrations a r o u n d 0.20 were also noticed in the systems a-Ge-Cu [5] and a-Si-Fe [8]. On the other h a n d , the Mo solubility in crystalline Ge is u n m e a s u r a b l y small [9] due p a r t l y to the Mo being unable to form sp 3 h y b r i d bonds and to e n t e r s u b s t i t u t i o n a l l y into the Ge lattice. In a r a t h e r crude image we can assume t h a t during the co-deposition the a-Ge n e t w o r k develops preferentially, enclosing the Mo atoms. On the basis of the above a r g u m e n t s the modelling of the a-Ge-Mo s y s t e m has been s t a r t e d b y placing the Mo atoms in the large free zones of a 155-atoms a-Ge model [6]. In this w a y one can a c c o m m o d a t e up to 22o/0 Mo atoms w i t h o u t Mo-Mo repulsive contacts. The Mo atoms were initially placed as being t a n g e n t to 3 of the Ge atoms bordering a large free zone. On the other hand, this positioning does n o t correspond to the m a x i m u m i n t e r a c t i o n between Mo and Ge. W e assumed for this interaction a L e n n a r d - J o n e s - t y p e potential Vo M = / { - x 2 _ A R - 6 , where R is the i n t e r a t o m i c distance, A ~ 2R~~ and RoM is the equilibrium distance between Ge and Mo (0.261 nm). The Mo positions were s u b m i t t e d to a minimization p r o g r a m for the b o n d energy which yielded the energetically most f a v o u r a b l e Mo positions. The atomic pairs distributions were obtained up to Rma x ~ 2 n m from the coordinates of the Ge and Mo atoms (Fig. 5). The first-order distances M o - - G e range between 0.2 and 0.7 n m with a strong m a x i m u m at 0.261 n m (sum of the covalent Ge radius and of the metallic Mo radius). The distribution of M o - - G e distance shows relatively sharp peaks at 0.495 and 0.630 n m too. The distribution of Mo Mo distances displays broad m a x i m a centred u p o n 0.49 and 0.75 n m which are characteristic for the t o p o l o g y of the " f r e e space n e t w o r k " c o m p l e m e n t a r y to the n e t w o r k of the Ge atoms. On the basis of the pairs distributions G e - - G e , Ge Mo and M o - - M o the t o t a l interference f u n c t i o n of the model F(k) was c o m p u t e d . The pairs distributions were previously corrected for the limited size of the model and d a m p e d b y a D e b y e - W a l l e r - t y p e factor in order to reduce the cut-off errors. These model interference function were c o m p u t e d for different x ranging between .~cta Physica Academiae Scientiarum

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STRUCTURE AND ELECTRICAL P R O P E R T I E S OF AMORPHOUS Ge---Mo

P(Rp 20

~ o

20- j 00

213

0.2

0.91 06

0.8

1

nm]

Fig. 5. Model pair density distribution (Ge-network not relaxed) 0.07 and 0.22 and checked against the e x p e r i m e n t a l ones as d e t e r m i n e d from electron diffraction [1]. T h e y account for the main changes b r o u g h t a b o u t in the F(k) of a-Ge b y increasing the Mo concentration (Fig. 4). For example the first m a x i m u m at k = 19,5 n m -1 becomes a broad shoulder for x = 0.22. This blurring is due to the smaller c o n c e n t r a t i o n of G e - - G e pairs b u t also to m a x i m a due to the M o - - M o pairs (at k -= 9.5 and 16.2 n m - i ) and G e - - M o pairs at k = 15 n m - 1 ) . This cha~ge in the diffracted i n t e n s i t y distribution is similar to and of the .~ame order of m a g n i t u d e as t h a t noticed in the a-GeAu, Cu [5], a-Ge-l~e [10] and a - S i - - F e [8] systems suggesting the similarity of these st~uctures. Characteristic changes show up with increasing x also in the regions k = 66 and 82 n m -1 in agreement with the e x p e r i m e n t a l F(k)'s. This s t r u c t u r a l model for the a-Ge-Mo system is a r a t h e r crude description of the process of n e t w o r k f o r m a t i o n during the codeposition process. A b e t t e r picture will be obtained b y a general r e l a x a t i o n c o m p u t i n g procedure including all the interactions between the present atoms. This procedure takes into a c c o u n t the bond stretching and bond-]3~nding potentials of the G e - - G e bonds as well as a L e n n a r d - J o n e s potential for the G e - - M o bonds. ] t minimizes the corresponding distortion energies ES, E B and ELj related to the distortion of the bond lengths and angles f r e m their ~quilibrium values. P r e l i m i n a r y results of this r e l a x a t i o n ~how t h a t the G e - - M o interaetions distort r a t h e r heavily the G e - - G e distances, the~eby increasing E s while E B is less enhanced. The effect on the c o m p u t e d F(k) is a shift in the peak at k = 34 n m -1 towards smaller k i m p r o v i n g the a g r e e m e n t with the e x p e r i m e n t in this region. .4cta Physica Academiae Seierttiarum Hungaricae 49, 1980

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It is worth mentioning t h a t the two constants which enter the LennardJones potential are related to the equilibrium length of the Mo--Ge bond and to its relative strength, respectively. A correet assessment of these constants b y ah optimal fitting of the experimental F(k)'s will gire a new and quite important insight into the nature of the metal-semiconductor bond in cermets. This problem is related to the total density and to the energy distribution of the localized levels in the energy gap. The above described model should be generally valid for all amorphous tetrahedral semiconductor--metal systems with low metal solubility, provided an adjustment of the interaction parameters is made.

Acknowledgements The authors are very much indebted to I. POZSGAIfor carrying out the electron microprobe analysis on the samples.

REFERENCES 1. R. MANAILA,C. Rusu, A. D•201 93 BARNA, P. B. BARNA, G. RADN‰ and L. T6TH, p. $5 in Proc. 9th Hungarian Diffractiort Conference, P› 1978. 2. R. M. HILL, Phys. Stat. Sol. (a), 34, 601, 1976; 35, K29, 1976. 3. A. D• R. MANA~LAand R. M. HXLL, Phys. Rey. Lett., 29, 1738, 1972. 4. A. D~v•162 R. MArr and C. Rusu, Thin Solid Films, 41, 143, 1977. 5. II. J. NOWAK, H. LEITZ and W. BUCKEL, Phys. Stat. Sol. (a), 49, 73, 1978. 6. M. POPESCU, Ph. D. Thesis, Bucharest, 1975. 7. P. STEINHARDT, R. ALBEN, ~~[. S. DUFFY and D. E. POLK, Phys. Rey., B18, 6021, 1973. 8. PH. MANOIN, et al., Phil. Mag., 36, 643, 1977. 9. M. HANS~N and K .AND~RKO,Structure of Binary Alloys, Vol. 2. Izd. Nauka, Moscow, 1962. 10. O. UEMURA,Y. SVZUKI and T. SATOW, Phys, Stat. Sol. (a), 41, 417, 1977.

Acta Physica Academiae Scientiarum Hungaricae 49, 1980