THE FILMS ON T. G. S. CLEAVED IN U. H. V.

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THE FILMS ON T. G. S. CLEAVED IN U. H. V.

Dr. Ziebert

To cite this version:

Dr. Ziebert. THE FILMS ON T. G. S. CLEAVED IN U. H. V.. Journal de Physique Colloques, 1972,

33 (C2), pp.C2-223-C2-225. �10.1051/jphyscol:1972277�. �jpa-00215013�

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JOURNAL DE PHYSIQUE Colloque C2, supplkment au no 4, Tome 33, Avril 1972, page C2-223

THE FILMS ON T. G. S. CLEAVED IN U. H. V.

Dr. V. ZIEBERT

Institut fiir Experimentalphysik 11, 66, Saarbriicken, Germany

RbsumB.

-

Nous avons observe la surface du T. G. S. en mesurant la resistance, la mobilitk due A l'effet Hall et la mobilite due a l'effet de champ quand on evapore une klectrode semi-conductrice.

Sur des surfaces du T. G. S. normalement preparkes, la mobilitk due I'effet de champ est jusqu'a 20 fois plus petite que celle due a l'effet Hall, tandis que sur des surfaces du T. G. S. clivkes dans un vide Blevk (moins de 5 x 10-9 torr) ces deux mobilitks sont pratiquement egales. Ce comportement diffkrent est une consequence de l'absence d'ions sur les surfaces de T. G. S. clivkes sous vide eleve.

Abstract.

-

Tha surface of T. G. S. is observed by measuring the resistance, the Hall effect mobility, and the field effect mobility of an evaporated semiconducting electrode. On the normaly preparated T. G. S. surface the field effect mobility is up to twenty times smaller than the Hall mobility, on in U. H. V.-cleaved T. G . S. (pressure less than 5 x ^10-9torr) against that the two mobilities are praktically equal. This different behaviour is a consequence of the ion absence at the U. H. V.-cleaved T. G. S. surface.

Measurements of the permittivity E of T. G. S. near the Curiepoint (Tatsuya [I], Unruh [2], Ziebert [3]) showed a dependence of the maximum value of E on the thickness and on the surface preparation of the sample. This surface of an insulator can also be observed by measuring the resistance, the Hall effect, and the field effect mobility of an evaporated semiconducting electrode.

The field effect mobility can be described in the following manner making use of the specific conductivity. The specific conductivity of a semiconductor layer of thickness t is

an

resp. pp are the electron and the hole mobilities resp., and n(x) resp. p(x) are the electron and the hole concentrations resp. of the semiconductor. In figure 1 is given a schematic diagram of the electrode arrangement of the sample which is nearly equal to a thin film transistor (T. F. T.). There are two metal electrodes

S semi- D

(source) conductor (drain)

G

I

( insulator) (gate)

l%. 1. - Schematic diagram of a T. F. T.

(source and drain), between these is a semiconducting electrode, and at the opposite surface of the insulator oncemore is a metal electrode (gate). All electrodes are evaporated in a vacuum with at least torr.

Applying an electrical voltage between gate and semiconductor yields a surface charge resp. a surface field at the boundary insulator-semiconductor. This caused a space charge in the semiconductor which

-

with the Poisson equation -

leads to a band bending in the semiconductor (Fig. 2).

This means, that the integrals in eq. (1) becomes dependent on the charge of the gate. This means also, that the specific conductivity depends on the gate charge. With a slight variation A Q about Q also a is

Inversion

h

Ex haustion 0

Log ( 4 )

-

ⁿ

FIG. 2.

-

Band bending in the semiconductor.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972277

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C2-224 DR. V. ZIEBERT

changed at

Ao.

Of this manner is definable an effective mobility

with

and

In the case of accumulation, that is for example an increase of the electron concentration of a n-type semiconductor caused by Q, the eq. (3) simplifies considerable. It is than

With a space charge of the opposite sign - that is in the case of exhaustion and inversion - the effective field effect mobility can be become remarkably smaller than p, or p, caused by an effect of compensation of electrons and holes close by the boundary. This is

-

as long as t is grater than the ⁽⁽Schottky ⁾⁾- exhaustion length (for example 300 for Te) - nearly indepen- dent of the thickness of the semiconductor layer.

In a similar manner, the electrical field of the boundary influences the measurable Hall effect mobility. Here also by arising of a band bending and of a space charge an effect of compensation results from a shunt connection o f p and n-zones in the semiconductor near the surface. But this effect is very much smaller and dependent on the semiconductor thickness ; therefore the effective Hall mobility is a function of p,, p,, Q and t. The influence of Q is reduced with increasing semiconductor thickness.

Measurements of the specific resistance and of the Hall m3bility were made from Heyman [4], and Buchmsn [5] under atm3spheric pressure, but no measurements were made of the field effect mobility and of the temperature and polarization dependence of the two mobilities. From our measurements at Te on normaly cleaned T. G. S. - that is T. G. S., which was on air cleaved, etched and on this in HV electrodes were evaporated - results in air and in HV at 10- torr a discrepance between p,, and pH that is that pF, was up to twenty times smaller than p, The Hall mobility was hereby measured in the arrangement of figure 3.

The two voltages are so choosed, that without a magnetic field B the current between the T. F. T. 1 and the T. F. T. 2 is exactly zero. With an applied magnetic field 3 then iH is strictly proportional to pH.

The field effect mobility was calculated by measuring

Ao

and

A Q

as mentioned above.

For understanding this large difference we made experiments with two other insulators, namely with -

in U. H. V. very good outgassed - glass and with in

I 1 IH

'

²

FIG. 3. - Arrangement of measuring the Hall mobility.

U. H. V. evaporated SiO (this is a thin film insulator).

The total pressure by the experiments with these insulators was better than 5 x lo-" torr. On these substrates the two measurable effective mobilities was nearly identically, especially considering the difference of approximately 20

%

coming from the different averaging of the scattering times by the Hall-resp. the field effect mobility. Therefore must be concluded, that the above mentioned discrepance between pFE and pH can not come primary from the Te, but that the discrepance is somehow characteristically of theT. G. S.- substrate resp. for the preparation of the T. G. S.

surface.

As next therefore was attempt to find a total new matter of the preparation of the T. G. S. surface, because with the normal preparation on air always ions and smut were absorbed at the surface. Further-

FIG. 4. - Here are to shown the various mobilities ( P F ~ , P H )

of Te on normally preparated

(V,

V) and on U. H. V.-cleaved (+, @) T. G. S, on outgassed glass

(a),

and on SiO ( x , 8).

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THE FILMS ON T. G. S. CLEAVED IN U. H. V. C2-225

more because of the always existing humidity of the air the T. G. S. is little etched and so one has a unwanted roughness of the surface. Therefore it was attempt with success to cleave the T. G. S. vertical to the b-axis in the U. H. V. The obtained pressure of 5 x lo-' torr (the gassing rate of T. G. S. in U. H. V. is very high) was yet sufficient to get essentially other results as in H. V., because immediatly after the cleaving on the very even and practically total clean T. G. S, the Te was evaporated. The probe was measured in the U. H. V, directly after producing.

The very interesting results are that y,, and y, are the same ones and that they have the same magnitude as in Te on SiO and on outgassed glass (Fig. 4).

An interpretation of this effect can be given possibly in the following manner. The Te condenses nearly undistrubed on outgassed glass, SiO, and on U. H. V.-

cleaved T. G. S., but on normally preparated T. G. S.

the always present ions and smut makes a bias band bending in the semiconductor and therefore a great reduction of y, and a smaller reduction y,. This can also be demonstrated by an other distribution of the semiconductor surface, namely by experiments, in which very nead gases are let in the U. H. V. and in this way are absorbed from the semiconductor (Zie- bert [6]). Moreover y,, can also be reduced by the greater roughness of the normaly preparated T. G. S.

surface.

Acknowledgments. - The author thanks Prof.

Dr. H. E. Miiser for his encouragement of this work and also for the useful discussions. The author is also indebted to the Deutsche Forschungsgemeinschaft for a support of his investigations.

References

[I] TATSUYA (S.) and TOSHIO (M.), J. Phys. Chem. Solids, [4] HEYMAN (P. M.) and HEILMEIER (G. H.), Proc. I. E. E. E.,

1967, 28, 967. 1966, 54, 842.

[2] UNRUH (H. G.) and CHINCHOLKAR (V. S.), Phys. stat.

sol., 1968,29,669. [5] BUCHMAN (P.), Solid State Electronics, 1968, 11, 767.

131 MusER (H. E.1 and ZIEBERT (V.1, Czech. J. Phys.7 1969,

[6] (V.), to be published in Thin Solid Films (1972).

B 19, 1400.