CONTACT EFFECTS OF METALS ON CHALCOGENIDE AMORPHOUS FILMS

HAL Id: jpa-00220839

https://hal.archives-ouvertes.fr/jpa-00220839

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.

CONTACT EFFECTS OF METALS ON CHALCOGENIDE AMORPHOUS FILMS

S. Okano, M. Suzuki, H. Suzuki

To cite this version:

S. Okano, M. Suzuki, H. Suzuki. CONTACT EFFECTS OF METALS ON CHALCO- GENIDE AMORPHOUS FILMS. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-959-C4-962.

�10.1051/jphyscol:19814210�. �jpa-00220839�

JOURNAL DE PHYSIQUE

CoZZoque C 4 , suppZdment au nOIO, Tome 42, octobre 1981 page C4-959

CONTACT E F F E C T S OF METALS ON CHALCOGENIDE AMORPHOUS F I L M S

S. Okano, !.I. Suzuki and il. Suzuki

Lr'acuZty o f TechnoZogy, Kanazaua University, Kanozaua 920, Japan

Abstract.- Several metals with strong ionization tendency were found to form rectifyins contacts with chalcogenide amorphous films. Eleasurements of fre- quency dependences of parallel capacitance and resistance of sandwich-typc sarrples of Mctal(I)/a~.orphous film/Metal(lI) revealed that rectifying effects wcrc due to the formatioc of a high resistivity layer with large thickness, which was thought to be caused by a chemical reaction at the interface.

Introduction.- Although many interesting phcnomcna such as electronic switching, photo-induced and acousto-induced structural changes etc. have been reported in chalcogenide amorphous semiconductors (1,2,3), conventional contact properties and difficulties in conductivity control by irpurity doping restrict the ways to device applications. These have becn considered to be mainly due to a larqc density of gap state with negative correlation energy ( 4 , 5 ) . For conductivity control additives should be introduced under non-equilibrium conditions ( 6 ) , but the results appear unsatisfactory.

We observed that a layer with different electronic properties from those of the bulk exists in the contact regions of chalcogenide amorphous semiconductors with metals with strong ionization tendency. The formation of such a layer brought about rectifying pnenomena in I-V characteristics (7). In fact, measurements of parallel capacitance and resistance rcvcalcd that a high resistivity layer with large thickness, which should be referred to as the rectifying layer, are formed in the contact regions. The high resistivity layer are considered to bc formed by the diffusion of metal ions into chalcogenide amorphous semiconductors through chemical reactions, which was suggested by the existence of a small short circuit currcnt in the dark. Thus, the formation of the rectifying layer appears to be a result of modification of electronic properties by introducing metal ions into chal.cogenide amorphous semiconductors under non-equilibrium conditions.

Sample preparation and cxperimcnta1s.- Amorphous As2SelTe2 bulk materials were crushed into powder, which was used as the evaporation source for amorphous films.

Sandwich-type sarr.ples of hetal(I)/amorphous As2SelTe2 film/Metal(lI), where metal(I1, thc lower electrode, was evaporated onto glass substrate, and the AszSelTez film and metal (111, the upper electrode, were successively evaporated in a vaccum of 13x10-~ Torr. Elg, Al, Zn, Fe, Bi and Sb were chosen for the contact metal. The details of the preparation processes of the sandwich-type sm.ples have been report- cd clsewhere (7). The special feature of our sample preparations was that thc whole processes were conducted in vacuo.

Measurements of I-V charactcristics and the frequency dependcnces of parallel capacitance and resistance were carried out in the dark at room temperature. The spectral response of the photovoltage of the samples was measured by irradiating the sample with light frorai a monochromator.

Results.- I-V charactcristics of the sandwich-type samples are devided into two groups. One is the ohmic behavior and the other is the rectifying behavior.

Sb/As2SelTe2/Sb, 'e/As2SelTe2/Sb, Ni/As2SelTe2/Sb and Bi/AszSelTe2/Sb exhibited the former one which is common i.n xetal-chalcogenide amorphous semiconductor systems,

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814210

C4-960 JOURNAL DE PHYSICUE

but Mg/AszSelTez/Sb, Al/As2SelTez/Sb and Zn/As2SelTez/Sb showed the latter one, in which a forward current flowed when the Sb electrode was biased positively with respect to the Mq, A1 and Zn electrode. The behaviors of frequency dependences of parallel capacitance C(w) and resistance R(w) of the latter type samples were found

(a) A1/AszSelTe2/Sb

(b) Sb/As2SelTe2/Sb Fig.1. I-V characteristics of the sandwich-type samples.

to be distinct from those of the former type

samples.

-

^o3

In the case of Zn/As2SelTe2/Sb, the rg ? rectification ratio I F / I ~ was rather small as -10, where I F and I, were the forward and

3

the backward current at a bias voltage of 01

0 -

1 V respectively. The largest IF/Is in the 2 present study was observed in Mq/AszSelTez/

- _B

SS and was -3000. The reproduciblity of I-V 0

characteristics and the frequency depend- ^" 0

ences of C(w) and R(w) of Mg/As2SelTez/Sb g l O l -

and Zn/As2SelTe2/Sb were very poor. Moreover, these kinds of samples were fragile, so that the sandwich region of metals and acorphous films partially peeled off from the sub- strate in a few days.

On the other hand, A1/As2SelTez/Sb exhibited fairly good reproduciblity and the sample structure were stable. So, the quan- titative analysis and discussion should be

done for Sb/As2SelTe2/Sb and A1/AszSe lTez/Sb,

loi.O

on1

5

ASb$ 0 0 000000~

o

0 0 0

oO 0

0

oO

0

0

o A A A A A O A

- O A

A

0 A

of which characteristics represent the ohmic ² .O 3.0 4.0 Photon Energy-tiw teV1 and the rectifying behaviors respectively.

Figure l (a) and (b) show the contrast

I-V characteristics of A1/As2SelTe2/Sb and Fig.2. Spectral response of photo- Sb/As2SelTe2/Sb. In the case of Sb/AszSel- voltage of Al/As~SelTez/Sb Tez/Sb, ohm's law holds in the field lower irradiated from A1 side ( 0 )

than -.5x103 V/cm and conductivity of the and from ~b side ( A ) . sandwich-type samples nearly the same as

l

OS-

i o 6 -

a

1 0 1 0 0 1 K 1 0 K lOOK 1M

Frequency Bz]

(a) Sb/As2SelTe2/Sb

1

@ck

18-

LL a

9 -

-

^-

E l & - U

1 0 4 -

1 02

1 1 0 1 0 0 1 K 1 0 K lOOK 1 M

Frequency CHJ

Fig.3. Frequency dependences of the sandwich-type samples.

that of the coplanar-type samples ( 10-l0 a-'cm-'). These results indicate that the ohmic contact was formed between Sb and evaporated AszSelTez films, while a barrier was formed at the interface between A1 and evaporated AszSelTez films.

Photovoltaic effects were observed for the samples exhibiting rectifying phe- nomena. The spectral response of the photovoltage for the A1/AszSelTe2/Sb sample is shown in Fig. 2. The rise of the photovoltaqe corresponds to the optical band gap

(-1.4 eV) in the evaporated AsnSelTe2 film, and the polarity of the photovoltages is consistent with the rectification.

Frequency dependences of parallel capacitance C ( w ) and resistance R(@) of these two kinds of samples are shown in Fig. 3 ( a ) and (b) respectively. The behaviors of

C4-962 JOURNAL DE PHYSIQUE

C(w) and R(W) in Sb/As2SelTe2/Sb suggest that the equivalent circuit consists of a parallel circuit of Cl and R1 connected to a small series resistance r, as shown in the inset of Fig. 3(a). Cl and R] are considered to be the geometrical capacitance and bulk resistance of evaporated AsnSelTet films and r is the contact resistance between the metal and the amorphous film.

Frequency dependences of C(w) and R(w) for Al/As2SelTe2/Sb are different from those for Sb/As2SelTe2/Sb in the low frequency reaions. These behaviors of Clw) and R(w) suggest that another parallel circuit of capacitance and resistance with a large time constant should be added to the circuit in Fig. 3(a). Thus, we obtained the equivalent circuit shown in the inset of Fig. 3(b). The values of circuit elements in these lumped constant networks were chosen to fit the calculated curves of C(w) and R(w) to experimental data, and they are given in fipures.

Discussion.- It is certain that the ohnic contacts were formed between Sb and the evaporated AszSelTe2 films, while a hole barrier should be formed in the contact region between A1 and the AszSelTez films, since the evaporated As2SelTe2 films are p-type materials. The built-in potential of the barrier was estimated to be -0.2 eV from the saturation value of the photovoltage.

It is interesting that the product of C:R~ in the lumped constant networks of Al/As2SelTe~/Sb is nearly equal to CIRl in Sb/As2SelTe2/Sb, but C2R2 is largely different from ClRl as shown in Fig. 3(a) and (b). These results indicate that the systems of Al/As2SelTez/Sb consist of two layers; one is the layer preserving orig- inal properties of the As2SelTe2 film, so that this layer might be called bulk layer. The other is the layer with different properties from the bulk region, which should be referred to as the rectifying layer.

Resistivity, dielectric constant and thickness of the bulk layer and the rec- tifying layer could be estimated fron the values of circuit elements in lumped con- stant networks obtained from frequency dependences of C(w) and R(w). Resistivity of the rectifying layer ( - 5 ~ 1 0 ' ~ ~ c m ) was higher than that of the bulk layer (-1x10'~

Rcm). The thickness of the rectifying layer occupied nearly 80% of film thickness.

A large width of the rectifying layer can not result from a depletion region Sue to a Schottky barrier, if any, it must be very thin since amorphous materials have a large density of localized state (8).

The oxide contacts might be a cause of the rectifyin? and photovoltaic effects.

In the present case, such oxide layers would probably be thin, since all preparation processes are conducted in vacuo.

A chemical reaction at the interface was suggested by a small short circuit current in the dark. So, the high resistivity layer with large thickness, which was responsible for the rectification, is considered to be formed by a chemical reac- tion.

Rectifying effects in Mg/As2SelTe2/Sb and Zn/P.s2SelTe2/Sb are considered to be the same mechanisms as in the case of A1/As2SelTe2/Sb. In those samples, a small short circuit current was also observed in the dark.

It is noticeable that electrode metals with strong ionization tendency such as Mg, A1 and Zn give rectifying contacts. Judging fron the direction of the short circuit current, metal ions such as Mg, A1 and Zn probably diffuse into As2SelTep films and the alloy region is formed. Doping chalcogenide amorphous semiconductors with metal ions through a chemical reaction is likely to cause a shift in the Fermi level as pointed out by Fritzsche et a1.19).

References.

(1) OVSHINSKY S. R., Phys. Rev. L,etters 2_1 (1968) 1450.

(2) KENEMAN S. A., Appl. Phys. Letters

19

(3) OKANO S., NAGAOKA H., SUZUKI M. and HATA T., Solid State Commun.

2

(4) STREET R. A. and MOTT N. F., Phys. Rev. Letters

2

(5) ADLER D. and YOFFA E. J., Phys. Rev. Letters

36

( 6 ) FLASCK R., IZU M., SAPRU K., AtGDERSON T., OVSHIKSKY S. R. and FRITZSCHE H., Proc. of 7th Int. Conf. on Amorphous and Liquid Semiconductors, Edinburgh 1977.

(7) OKANO S., NISHIJIMA K. and SUZUKI M., Japan. J. Appl. Phys.

19

(8) WEY H. Y. and FRITZSCHE H., J. Non-Cryst. Solids

8

(9) FRITZSCHE H. and KASTNER M - , Philos. Mag. B

21