PIEZORESISTIVITY OF THIN SEMICONDUCTING FILMS

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PIEZORESISTIVITY OF THIN SEMICONDUCTING FILMS

G. Weck, V. Ziebert

To cite this version:

G. Weck, V. Ziebert. PIEZORESISTIVITY OF THIN SEMICONDUCTING FILMS. Journal de

Physique Colloques, 1972, 33 (C2), pp.C2-227-C2-228. �10.1051/jphyscol:1972278�. �jpa-00215014�

JOURNAL DE PHYSIQUE Collogue C2, stipplkent aE no 4, Tome 33, Avril 1972, page C2-227

PIEZORE SI STIVITY OF THIN SEMICONDUCTING FILMS

G. WECK and V. ZIEBERT Institut fuer Experimentalphysik I1

D-66 Saarbruecken, Universitaetscampus 4 (W-Germany)

R6sum6.

Nous avons mesure I'effet pi6zor6sistif sur de minces films semi-conducteurs de Te, CdSe et PbTe. Nous interpretons les resultats comme &ant causes par une diffraction mixte des porteurs due aux phonons acoustiques et aux imperfections chargees et neutres. Nous avons Btudie les effets du recuit et de la pression atmospherique et nous comparons la valeur des effets pikzo- rksistifs aux mesures anterieures faites sur des ckramiques de BaTiO

Abstract. - The piezoresistivity effect of thin semiconducting films of Te, CdSe and PbTe is measured. The results are interpreted as being caused by a mixed scattering of the carriers by acoustic phonons and by charged and neutral imperfections. The effects of annealing and subjecting to atmospheric pressure are investigated, and the magnitude of the piezoresistivity effects is compar- ed with earlier measurements on BaTiO 3-ceramic.

Measurements of surface conductivity of thin semi- conducting films on BaTi0,-ceramic showed an evi- dent change in this quantity, when the sample was heated above the Curie-point. This was interpreted as a piezoresistivity effect [I].

It was our aim to measure such piezoresistivity effects in thin films of vaporizable semiconductors, such as p-type Te, n-type CdSe and n-type PbTe, and thus to obtain evidence about the microscopic processes contributing to electric conductivity in the films.

The samples were prepared by successive evaporating of the substances on mica substrates of a thickness of 100 ... 230 pm. The layers were arranged in such a manner that the sample could work as a thin-film- transistor, sort of a flattened MOS-field-effect-transis- tor. At first, we evaporated copper and gold together to form a layer which would act as the gate of the MOS-transistor. Next, we evaporated a layer of silicon monoxide of a thickness of about 200 ... 500 nm acting as the dielectric. This was followed by the evaporation of the semiconductor to a thickness of about 20 ... 200 nm. The procedure was finished by the evaporation of different alloys which formed the source and drain contacts. During the whole process, pressure was of the order of 1 ... 4 x lo-' tors, and it was possible to carry out all measurements without taking the sample out of vacuum.

By bending the mica plate, the semiconductor was subjected to negative strains up to 2 x and resistance, carrier concentration and mobility were measured as functions of strain. All were linear functions of strain. The carrier concentration per unit of area was measured by applying a bias voltage AV, to the gate of the thin-film-transistor and measuring the resulting change AR of the resistance R between source and drain, and proved to be

where Co is the capacity per unit of area of the dielec- tric, and e is the chage of an electron. By using the length L and the width B of the semiconductor, one can compute from (1) the field-effect mobility of the carriers :

The sign of AR shows, whether the sample is p-type or n-type [2].

On the other hand, the amounts of change 6R, 6 N and 6p, due to the strain can be estimated by using the values for p, and N provided by semiconductor theories : According to the f-sum rule, the effective mass m" of an electron or hole may be approximated by

where EG designates the width of the energy gap and a is the interatomic spacing [3]. For computing the mobility

one needs the average relaxation time z of the carriers, which may be obtained for several different scattering mechanisms. Interaction with acoustic phonons leads

to z, , - r n " - 3 I 2 [4]. Interaction with charged imper-

fections gives z,,, - J a n i o n [5], and interaction with neutral im~erfections yields z, - m*'/n, [6], where nion and n, are the densities of the imperfections, respecti- vely.

The carrier concentrations were supposed to be nearly equal to the concentrations of donors or acceptors. resoectivelv. and so we exoected them not to .

vary very much under strain. The fact that this pre- diction came true, as far as the accuracy of the enperi-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972278

C2-228 G. WECK AND V. ZIEBERT

ments allowed us to see, seems to support our line of argument.

By comparing the computed value of the relative change of effective mass with strain, 6m*/m*, with the measured value of 6p,,/pn, we could get from (4) the value of 6z/z. This value was compared to the values computed from the expressions for z,,, z,, and z,.

Table I shows these values, referred to a strain ^E = 1.

6r

%

Z Zph Zch Z n IS

-

- -

Te - 4.7

18.9 + ^7.1 + ^26.2

^18.3

Te, annealed + ^{12.5 - 18.9} + ^7.1 + ^26.2

^1.1

CdSe - 1.2 + ^15.8 - 4.3 - 20.0 + ^8.3

PbTe in vacuo + ^{33.0 - 44.7} + ^15.9 + 60.6 $. 2.2 PbTe in air + ^38.5

^44.7 + ^15.9 + ^60.6 + ^7.7

The results lead to the conclusion that in annealed Te and in PbTe, we have scattering by charged and neutral imperfections, and perhaps some interaction with acoustic phonons, too. In CdSe, scattering by acoustic phonons and charged imperfections seem to play an important role, but we cannot exclude the possibility of interaction with neutral imperfections.

The value of 6z/z for Te changed remarkably when the film was annealed. This leads to the conclusion that the film, which was evaporated on a cold substrate, was originally rather polycrystalline and, by annealing, was changed to a more ordered structure. The CdSe and PbTe films which were evaporated on hot sub- strates showed no change in behaviour, if they were annealed.

PbTe showed a remarkable reduction of mobility (more than one order to magnitude !) when the sample was subjected to atmospheric pressure. It is believed that this effect is due to an adsorption layer at the surface which causes a bending of the energy bands [2].

Furthermore, the scattering by neutral imperfections becomes more dominant.

The last column of table I shows the change of conductivity with strain. The magnitude of this change leads to the conclusion that the measurements on BaTi0,-ceramic cannot be explained by piezoresistivity effects alone. Thus we need further results until these measurements can be explained satisfyingly.

Acknowledgments. - We should like to thank Prof. Dr. Mueser for his generously supporting the work, for his constant encouragement and for many critical discussions.

References

[I] BUCHMAN (P.), Solid State Electronics, 1968, 11, 767. 141 BARDEEN (J.), SHOCKLEY (W.), Phys. Rev., 1950,80,72.

[21 MUE~ER (H. E.1, Invited Paper 1-7, 2nd EMF, Dijon, _[51 MADELUNG (o.), ~ ~ der Halbleiterphysik ~ ~ d l ~ ~ ~ ~ 1971.

[3] HARRISON (W. A.), Solid State Theory (McGraw-Hill, (Springer, Berlin, 1970).

New York, 1970). [6] ERGINSOY (C.), Phys. Rev., 1950,79, 1013.