INVESTIGATIONS OF SURFACES OF FERROELECTRICS WITH SEMICONDUCTING ELECTRODES

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INVESTIGATIONS OF SURFACES OF

FERROELECTRICS WITH SEMICONDUCTING ELECTRODES

H. Müser

To cite this version:

H. Müser. INVESTIGATIONS OF SURFACES OF FERROELECTRICS WITH SEMICON- DUCTING ELECTRODES. Journal de Physique Colloques, 1972, 33 (C2), pp.C2-17-C2-19.

�10.1051/jphyscol:1972205�. �jpa-00214941�

JOURNAL DE PHYSIQUE

Colloqz~e C2, suppliment au no 4, Tome 33, Avril 1972, page C2-17

INVESTIGATIONS OF SURFACES

OF FERROELECTRIC S WITH SEMICONDUCTING ELECTRODES

Institut fiir Experimentalphysik 11, Universitat des Saarlandesi D-66 Saarbriicken, Germany

Rksum6. -

Dans un cristal ferroelectrique la conductance d'une Clectrode parallele

la surface depend du signe de la polarisation spontanke, car les charges rklles qui compensent les charges de polarisation peuvent se deplacer. Un moment apres le processus de

surteling

une partie impor- tante de la charge se trouve a la surface du ferrodlectrique et ne contribue plus a la conductance de I'electrode. L'emploi d'electrodes semi-conductrices est tout specialement approprie a l'etude de cet effet. On doit tenir compte du comportement de la couche superficielle du semi-conducteur. Les differences entre les mobilites que I'on definit peuvent atteindre plusieurs ordres de grandeur. Nous decrivons les applications possibles relatives aux ferroelectriques B klectrodes semi-conductrices et aux cristaux semi-conducteurs

films ferroklectriques evapores.

Abstract.

The conductance of an electrode parallel to the surface of a ferroelectric crystal depends on the sign of the spontaneous polarization, because the true charges compensating the polarization charge are movable. Some time after a switching process a considerable part of the charge is in the surface layer of the ferroelectric, no longer contributing to the conductance of the electrode. For an investigation of this effect, semiconducting electrodes are especially appropriate.

The behaviour of the surface layer of the semiconductor must be taken into account. Differently defined mobilities can differ up to orders of magnitude. Possible applications of ferroelectrics with semiconducting electrodes or semiconducting crystals with evaporated ferroelectric films are described.

1. Introduction.

The polarization charge of a ferroelectric crystal normally is completely compen- sated by true charges. They are situated at the ends of the domains, i. e., in a thin surface layer orthogonal to the ferroelectric axis of the crystal, if it is bare. If it is metallized, the charges can be situated either in the surface as in bare crystals or within the electrode. Also it is possible, that the charges immediately after a switching process are in the electrode and from there they are leaking gradually into the surface layer of the crystal.

A long as the charges are within the electrode, they are movable and contributing to the conductivity of the electrode parallel to the surface. Therefore we expect more electrons in a metallic electrode covering the positive end of the spontaneous polarization than in the opposite one. Due to the low carrier concentra- tion in semiconductors these materials are especially appropriate for investigations of the behaviour of the compensating charges. Moreover, the sign of the effect inp-type conductors must be opposite t o that in n-type or metallic electrodes.

2. Experiments.

In figure

an arrangement for measurements of the conductance of semiconducting electrodes is shown. A T. G. S. crystal has been fitted out with electrodes similar to a thin film transistor.

On one side two metallic stripes (source and drain) were evaporated at the ends of the crystal ; they were connected by a semiconducting film (Te, Bi, CdSe,

trigger signal

I ^{A T} i

&

1

osciilo- bridge g ~ p h

semi-

' *

conductor, ⁴

metal

1 '

crystal m e t a l

1

FIG. 1. -

Arrangement

measurements of the conductance of

semiconducting electrode

ferroelectric crystal.

or PbTe). On the opposite side a metallic gate electrode (Au) was evaporated. The resistance between source and drain electrodes was measured by a Wheatstone bridge ; its alteration gives an information on the amount of additional charge in the semiconducting part of the electrode.

Earlier measurements [I], [2] confirmed the beha- viour mentioned above

If the crystal has been swit- ched, the resistance is changed, but then it tends very slowly (during 15 minutes and more) towards its old value. Recently Ziebert studied this effect in detail [3].

Especially, he treated the situation in the surface layer of the semiconductor with aid of the one dimensional

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972205

band model. If the spontaneous polarization is directed to the semiconducting film, the bands near the surface are bended and an accumulation layer arises in the surface of the semiconductor, if it is of the n-type as shall be presumed in the whole discussion. If the pola- rization is directed from the semiconducting electrode away, an exhaustion layer or - for strong polariza- tions or low doping rates in the semiconductor - even an inversion layer is built up. The same is true, if the ferroelectric crystal in figure 1 is replaced by a simple dielectric (e. g. SiO) and a positive or negative gate voltage is applied, resp.

The change of the conductance of the electrode, which is induced by the field effect or the spontaneous polarization, is not directly proportional to the number of additional carriers in the electrode, because the mobility of the carriers in the surface layer can be quite different from the average mobility of the whole film. The field effect mobility is defined by

1

AG

PFE = - _b '

- _{A Q '}

Here I and b are length and breadth of the semi- conducting slit, AG is the measured change of the conductance of the electrode which is caused by the charge A Q having flown into the electrode per square unit. The field effect mobility is rather strictly depen- dent on the band bending. Furthermore, if charges are bound, for instance, in surface levels or in a smut layer between electrode and ferroelectric, the field effect mobility is lowered. Additional scattering phenomena must be considered too, if only a narrow channel near the surface is conducting.

Whilst the field effect mobility is determined only by the situation near the surface, the Hall effect mobility gives an information of the whole semiconducting film.

It is defined by

j,

and

are the current densities orthogonal and parallel to the electric field. Ziebert [3] measured the Hall effect mobility in a four electrode arrangement as described by v. Heek [4]. The Hall effect mobility is influenced especially by inversion layers. I t is possible, that the Hall effects of holes in an inversion layer and of electrons farer away from the ferroelectric elmininate eachother completely. An example of mobility measu- rements in Te ('-type conducting) on T. G. S. is given in figure 2. The field effect mobility is lower in the whole temperature range. In the ferroelectric region the mobilities can be measured both for positive and negative spontaneous polarization. The reduction of the Hall effect mobility due to the electrones near the T. G. S. clearly can be seen. A third field effect mobi- lity can be measured from the change of the conduc- tance as consequence of the full switching charge after eq. (1). The points marked by + and - have been measured by applying a small ac gate voltage in the s of positive and negative remanence, resp.

0 30 40 50 TPcI-

FIG. 2. - Mobilities of holes in Te on T. _{G . S.}

It should be mentioned, that the measurements of figure 2 were made with a sample, which was cleaved and prepared in an ultra high vacuum. In normally prepared T. G. S. (polished, etched, or cleaved in air) the field effect mobility is only of the order 1 cm2/vs, indicating, that in these cases the deposition of the compensation charges mainly is determined by effects of smut or water between the T. G. S. surface and the electrode.

But even if T. G. S. is cleaved and prepared in the ultra high vacuum some after effects, as found in normal prepared samples [I], [2], appear. Ziebert [3]

concluded from a detailed analysis that the main reason for the vanishing of the charge from the elec- trode must be explained by a tunneling of electrones from the surface of the semiconductor into trap levels in the forbidden band of the ferroelectric crystal. If the polarization has been switched, a strong field is present in the surface layer of the ferroelectric. Therefore traps, which had had an energy higher than the Fermi energy before the ferroelectric was switched, now can be occupied. So a space charge is formed near the surface of the ferroelectric, compensating a part of the polari- zation charge. The thicker the space charge layer becomes, the farer the way for the tunneling electrons.

In contrast to earlier similar models, which have been given by several authors [5], [6], [7], [8] for M. 0 . S. and T. F. T. structures, Ziebert [3] assumes quasi continuously distributed energy levels for the traps. With a trap density of some 10'' cm-3 he can derive quantitatively the observed logarithmic law for the change of the electrode resistance after a polariza- tion reversal.

3. Applications. - If it would be possible, t o

produce T. G. S. or another ferroelectric with much

less traps, avoiding after effects in this way, one cotild

INVESTIGATIONS O F SURFACES OF FERROELECTRICS WITH SEMICONDUCTING ELECTRODES C2-19

try to use the arrangement of figure 1 as a new type of memory element. If the semiconducting film is doped so that, e. g., for positive spontaneous polarization it is near the intrinsic case, a considerable change of the resistance - up to several orders of magnitude - could be realized. The two states of the memory would

I m e t a l I I m e t a l I

ferroelectric

I m e t a l I

FIG. 3. - Ferroelectric memory element.

be high and low resistance between source and drain.

With a short positive or negative gate voltage pulse the memory could be switched or switched back. But it seems impossible to avoid completely the leakage of charge from the semiconductor into the ferroelectric.

A similar arrangement, in which some after effects are allowed, is shown in figure

The metallic source and drain electrodes are replaced by p-type semi- conductors with Ohmic contacts. If the ferroelectric is neutral, there is a barrier layer between source and drain electrodes for both signs of applied drain voltage.

If the ferroelectric is positively polarized an accumula-

tion layer is built up in the n-type region. Because it is impossible to adjust the gain electrode quite exactly, in a certain range of the p-type region an inversion layer is formed, as indicated in the middle of figure 3.

Nevertheless, source and drain are separated comple- tely by barrier layers. If the spontaneous polarization is switched by a negative gate voltage pulse, an inver- sion layer is formed connecting source and drain with very low resistance (lower part of Fig. 3). If we have a surely working contact immediately from the inversion layer to a metallic electrode, one can omit one of the two p-type regions, restricting ourselves t o one possible sign of reading voltage.

In figure 3 the after effects are not so critical. We only have to fulfil the condition that, if all after effects, have run, still an inversion layer must exist. A slight modification is shown in figure 4. Here a part of a semiconducting single crystal, as they are used in micro miniature electronics, is shown. Two p-type islands are doped into an n-type basic crystal. Between the p-type islands a ferroelectric film is evaporated and supplied with a gate electrode. Now the arguments are the same as before

Under the ferroelectric film, there is either an accumulation layer, which is unimportant, or an inversion layer connecting the two islands.

Such a memory could find special applications even if the after effects limit the time for which an informa- tion can be storaged. It can be red any times without any restoring as quickly as integrated circuits work.

FIG. 4. - Ferroelectric memory element

in

The energy can be storaged without energy dissipation.

The space demand per bit is very little. A combination with the usual micro miniature electronics is possible in an optimal way. Because it can be switched with a relatively high voltage, also the writing time can be much lower than it was in the old principle of ferro- electric memory.

References

[I] HEYMAN (P. M.) and HEILMEIER (G. H.),

[6] KOELMANS (H.) and

GRAAFF (H.

1966,

842.

1967, 10, 997.

[2]

Mijs~R (H.

and ZIEBERT

1969,

B 19, 1400. [7] PULVER (M.) and DORDA (G.),

1970,

ZIEBERT (V.),

Universitat Saarbriicken, 1971.

65.

[4] v.

(H. F.),

1967, 10, 268. 181 DORDA (G.) and PULVER (M.),

1970, [5] HEIMAN (F. P.) and WARFIELD

1, 71.

electron. Devices,

1965, 12, 167.