AMORPHOUS SEMICONDUCTING Ag-Te FILMS

HAL Id: jpa-00220834

https://hal.archives-ouvertes.fr/jpa-00220834

Submitted on 1 Jan 1981

HAL

is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire

HAL, est

destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

AMORPHOUS SEMICONDUCTING Ag-Te FILMS

J. Hauser

To cite this version:

J. Hauser. AMORPHOUS SEMICONDUCTING Ag-Te FILMS. Journal de Physique Colloques, 1981,

42 (C4), pp.C4-943-C4-946. �10.1051/jphyscol:19814206�. �jpa-00220834�

JOURNAL DE PHYSIQUE

CoZZoque C4, supple'ment au noiO, Tome 4 2 , octobre 1981 page C4-943

AMORPHOUS SEMICONDUCTING Ag-Te FILMS

J . J . Hauser

B e l l Laboratories, Murray H i Z Z , New Jersey 07974, U.S.A.

Abstract.- .rlmorphous Aq-Te films were obtained by diffusing Ag atoms at 203K from an already d.epositec?. cr!?stalline Ag film into the oncoming Te atom vapor or by diffusing Ag atoms into an amor- phous Te film. When the Ag fi1r.1 is deposited. first, ~.onitoring in-situ the resistance of the Ag fi1.n while depositing Te esta- blishes unanbiguously that it is Ag that diffuses into Te. The Ag diffusion rate at 300K is 4x105 R2 sec-l with an activation energy of 0.3 eV. The temperature of 203K is high enough to allow sufficiently rapis diffusion and low enough to result in an amorphous film. All amorphous Ag-Te films displa variable range hopping (resistivity proportional to exp(To/T)g).

Introduction.- Bolotov and Kozhin (1) reported a phase transformation in thin silver-tellurium films. Actually, they observed that if Ag was evaporated onto an already deposited polycrystalline Te film, one is left with a mixture of crystalline Ag and Ag2Te. On the other hand, if Te is evaporated onto an already deposited polycrystalline Ag film, one obtains a homogeneous amorphous film. This result was explained by assuming that Te has a high diffusion coefficient in the atomic state and is therefore able to diffuse into the Ag before forming a continu- ous film. The present experiments will establish that on the contrary, it is the Ag which diffuses and that amorphous Ag-Te films can be ob- tained irrespective of the order of deposition. Furthermore, the measured Ag diffusion coefficient is very high, sug esting an ionic Ag diffusion in agreement with a diffusion study of Ag9 ions in crystal- line Ag2Te (2).

Experimental Procedure.- Most of the films of the present study were aepcsited by getter-sputtering on sapphire substrates at temperatures ranging from 77 to 300K. The in-situ resistance was measured by first attaching 4 leads to In contacts made on the sapphire substrate with an ultrasonic soldering iron. A chromel-alumel thermocouple was mounted in a similar way on the substrate to monitor the actual deposition temperature. The substrate was mounted on a copper table with Air Pro- ducts thermal grease; nitrogen gas was circulated through liquid nitro- gen and then through the copper table at various rates to achieve the different deposition temperatures. The highest Te sputtering rate was 900i/min. and the fastest Te deposition was achieved by evaporation

(50i/sec). The amorphous state of the films was established by x-ray and electron diffraction.

Ex erimental Discussion and Results.- The bulk of the experiments were coKducted by first depositing a 1000i Ag film. One then records in- situ the resistance of this Ag film as a function of time. This resis- tance (which is typically 0.55R) remains constant while sputtering the Te target with a shutter over the Ag film, but increases the moment

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814206

C4-944 JOURNAL DE P H Y S I Q U E

2 0

-

the shutter is remov d to reach

1000% ~g - ~ e (150A/rntn)

a value of 103 to 10

2

R after de-

T~ = 2 7 3 s position of about 1000A of Te.

The resistance stops increasing

o 1000% AQ -Te (61Alrnln) if one stops the Te deposition,

TD = 3 0 0 K

thus indicating that whatever diffusion is taking place reaches completion during deposition.

Whether it is Te which diffuses into Ag or vice versa, the resis- tance increase simply reflects the decrease in the thickness of

0 8

-

the pure Ag film as a result of

its conversion into the Ag-Te

0 6 -

alloy. Since the resistance of

0 4 - the resulting amorphous Ag-Te

alloy is four orders of magnitude

0 2 - higher than that of the Ag film,

it is a good approximation to assume that the reciprocal of the

o I 2 3 4 5 6 7 8 9 resistance is proportional to the

TIME (mm) remainins pure As film thickness.

A plot of inverse resistance as a Ffg. 1 Inverse resistance as a func- of time is shown in tion of the Te sputtering time.

Fig. 1 for two deposition temper- atures (TD) and two Te sputtering rates.. The first point to be made about the data shown in Fig. 1 is that the decrease in the Ag film thickness is linear in tive and that at 300K this linear behavior per- sists throughout the lOOOA thick Ag film. This in itself suggests that it is not Te which diffuses, since if this were the case one would ex- pect a parabolic time dependence. It is also clear from Fig. 1 that the departure grom linearity occurs at a smaller film thickness for lower TD ( 2 4 5 0 ~ for 273K)

.

This fact will be used later on to estimate the activation energy. The slopes of the linear portions of the curves shown in Fig. 1 which are equal to the rate at which the Ag film thick- ness diminishes are plotted in Fig. 2 as a function of the Te sputter- ing rate and of TD. The fact that the rate of consumption of the Ag

film is equal to the rate of Te deposition clearly establishes

9 0 0

-

that it is Ag which diffuses into

Te and not the other way around

8 0 0 -

(l). Furthermore, at 300K the

- .E

^{7 0 0} - rate of Ag consumption remains

t equal to the rate of Te deposi-

oa

-

^{6 0 0} - tion up to 50A/sec which was

z achieved by evaporation. It is

also clear that one is not deal- ing with a crystalline-amorphous phase transition as previously suggested (l), but that instead

o TD = 3 0 0 K the Ag atoms are diffusing as

T~ - 2 2 3 K fast as the Te is deposited into

the condensing Te atoms thus forming an amorphous film. The result that it is Ag which dif- fuses is also in aareement with _.,

,&

^{2& 3&}

,k0

^7&

&,

microscopic observations on

Te DEPOSITK)N RATE (A/minl partially overlapping crystalline

Ag-Te films (3).

Fig. 2 Rate of Ag consumption as a At lower temperatures (below function of the Te sputtering rate 223K) the Ag diffusion slows down

and of TD. and the rate of consumption of

the Ag film hecomes indepen- dent of the rate of Te deposi-

* ^- tion for sufficiently high

a rates, allowing the determina-

W tion of the diffusion constant

which is plotted as a function of temperature in Fig. 3. Ex- trapolation of the diffusion data shown in Fig. 3 yields a diffusion constant of 4x105

x2

sec-l at 300K. This diffusion rate at 300K corresponds to a speeg of 400i sec-l through a lOOOA film which exceeds the fastest Te deposition rate available. At higher tempera- tures the activation energy can also be determined by measuring the thickness at which the Ag consumption rate deviates from

\

linearity (Fig. 1)

.

^{Such mea-}

DIFFUSON COEFFICIENT surements are difficult below

0 DEVIATION FROM

LINEARITY 253K because the linear portion

LL o becomes too small. Neverthe-

less, as shown in Fig. 3 the

'%.h

^{5 4 5 4 5 . 5 5 5 6} activation energies obtained by

I O ~ / T ( K ) the two methods are in good

Fig. 3 Ag diffusion rate and devia- agreement. The high value of tion from linearity as a function of the diffusion constant suggests

temperature. an ionic Ag diffusion similar

to that reported in crystalline a-Ag2Te (2). The lower value of the diffusion constant and the higher value of the activation energy measured in the present experiments could be caused by the amorphous nature of the Ag2Te film formed during Te deposition.

When Ag is deposited first, amorphous Ag-Te films are obtained by sputtering Te at 203K. This temperature of 203K is high enough to allow sufficiently rapid Ag diffusion (kl20fl/min) and low enough to result in an amorphous film. The maximum Te concentration attainable in this case corresponds to Ag2Te (because of the stability of this compound (3)) and such films usually recrystallize before reaching 300K. On the other hand, deposition of Te first at 77K followed by the deposition of Ag at 203K allows the formation of amorphous films with the composition AgTe which remain amorphous at 300K. The reason why previous experiments (1) failed to obtain an amorphous film when Ag was deposited last is most probably due to the fact that such experiments used evaporation and the evaporation of Ag requires much more power than that of Te and causes the film to crystallize. Finally, all amor- phous Ag-Te films irrespective of the order of deposition ?isplay var- iable range hopping (resistivity proportional to exp(~,/~)~) with To ranging between 3x108 and 3 x 1 0 9 ~ . An example of variable range hop- ping is shown in Fig. 4 for each order of deposition. When Ag is de- posited first followed by the deposition of Te at 203K the film is measured in the as-deposited state, i.e., it is cooled from 203K to liquid helium temperatures and the resistance is measured without- anneal up to TD = 203K. The rapid decrease of the resistance close to room temperature corresponds to the partial microcrystallization of the sample mentioned above. On the other hand, when Te is deposited first, one can obtain amorphous films with the composition AgTe which remain amorphous at room temperature. As shown in Fig. 4 such films have a lower density of localized states at the Fermi level (higher value of To). Furthermore, the values of To shown in Fig. 4 are close to those

C4-946 J O U R N A L DE PHYSIQUE

measured on Ag Te sputtered at 203K (8.4x108Kf and on AgTe de- posited at 300K (3 .7x109~)

.

Acknowledgment.- I would like to thank R. J. Felder for his tech- nical assistance and U. El-Hanany for many helpful discussions and pointing out the pertinent references on Ag diffusion.

lo3 0 2 4

0

^{026 028} 0 3 0 0 3 2 0 3 4 036 038

T - V 4 (K.1/41

Fig. 4 Temperature dependence of the resistance for two Ag-Te films: 10008 Ag (TD=300K) followed by 1000i Te

(TDY203K) and 600A Te (TDC77K) follow- ed by 3001 Ag (T~=203K).

References

(1) Bolotov, I. Ye. and Kozhin, A. V., Fiz. metal. metalloved.

21

(1973) 383 [Phys. of Metals and Metallog. 35 (1973) 1461.

(2) Okazaki, H., J. of the Phys. Soc. of ~ a p a n 7 3 (1977) 213.

(3) Terao, N., Comptes Rendus Acad. Sciences, ~ E i s 284 (1977) 299.