ELECTRONIC PROPERTIES OF Si-INVERSION LAYER WITH MODULATED BAND STRUCTURE INDUCED BY PERIODIC ELECTRON BEAM IRRADIATION OF GATE OXIDE

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ELECTRONIC PROPERTIES OF Si-INVERSION LAYER WITH MODULATED BAND STRUCTURE

INDUCED BY PERIODIC ELECTRON BEAM IRRADIATION OF GATE OXIDE

F. Vettese, J. Sicart, J. Robert, G. Vincent, A. Vareille

To cite this version:

F. Vettese, J. Sicart, J. Robert, G. Vincent, A. Vareille. ELECTRONIC PROPERTIES OF

Si-INVERSION LAYER WITH MODULATED BAND STRUCTURE INDUCED BY PERIODIC

ELECTRON BEAM IRRADIATION OF GATE OXIDE. Journal de Physique Colloques, 1987, 48

(C5), pp.C5-195-C5-198. �10.1051/jphyscol:1987539�. �jpa-00226743�

ELECTRONIC PROPERTIES OF Si-INVERSION LAYER WITH MODULATED BAND STRUCTURE INDUCED BY PERIODIC ELECTRON BEAM IRRADIATION OF GATE OXIDE

F. VETTESE, J. SICART, J.L. ROBERT, G. V I N C E N T * and A. V A R E I L L E * Groupe dlEtudes des Semiconducteurs, Universite des Sciences et Techniques du Languedoc-UA 357, Place Eugene Bataillon,

F-34060 Montpellier Cedex, France

"centre National d'Etudes des TB1~communications. Chemin du Vieux-ChBne, BP 98, F-38243 Meylan Cedex, France

Nous presentons une analyse des proprietes electriques d'une couche d'inversion dont le potentiel electrostatique est module en amplitude grace A l'irradiation de l'oxyde de grille par un faisceau d'electrons. .L1irradiation a ete effectuee suivant un reseau parallele ou perpendiculaire au courant avec des pas differents. L'ktude de la conduction dans ces structures de type rbseaux de surface est faite A partir de l'etude des tensions seuil et de la mobilite entre 4.2 K et 300 K.

L'interpretation est basee sur l'existence d'une queue de densite d'etats et permet une approche quantitative des syst&mes 2D modules ou desordonn8s.

Abstract :

--

We report on electrical properties of a modulated potential of a MOS inversion layer. Lines oriented parallel and perpendicular to the current flow have been grated by using periodic electron beam irradiation of the gate oxide.

Structures of different periodicity and dose have been made to study the electronic properties of the 2D electron gas.

The modulation of the charge density inside the gate oxide leads to a modulation of the band structure near the Si-Si02 interface. As a result, the electronic density in the channel is periodically varying and can be controlled by the gate voltage. Therefore the density of states must be modified to account for the

spatial distribution of potential and this leads to an apparent tail in the states distribution.

We interpret the experimental results using a model based on modulation of the in- plane potential and on the existence of an energetic conduction threshold.

INTRODUCTION

It is well known that random distribution of fixed charges located in the oxide near the Si/Si02 interface strongly modifies the inversion layer properties in MOS structures [I].

Measurements performed at low temperatures have been interpreted by means of band- tailing models and the existence of a mobility edge [2,3].

A potential fluctuations (PF) model has been proposed assuming a Gaussian distribution of standard deviation a for the surface potential such that the classical 2D density of states D(E)

-

Do must be changed in the new expression

D(E)

-

^Erfc

^(-E/uG)

^{with Do}

- ^-

m*

nti2

if the energy is measured from the edge of the unperturbed band Eo. Figure 1 shows the shape of the new density of states and the band-tail in the PF model.

Potential fluctuations can be induced in the MOST channel by a second embedded grating gate [ 5 ] or by a modulated insulator thickness [6].

For these microstructured surface superlattices the potential is easily controlable but the gate geometry is fixed and the fabrication requires sophisticated lithography. To avoid this drawback we induce PF by electrical charges in the dioxide. These fixed charges are created via a classical

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987539

JOURNAL

DE

PHYSIQUE

irradiation using an Electron Beam Micro Fabricator such that on the same wafer the chips were subjected to different irradiation treatments. Then a spatially modulated in-plane potential resulting from this electron beam irradiation allows us to investigate the electrical properties of these MOS structures as a first approach of a quantitative study of the conduction in 2D disordered semiconductors.

_Figiie 1 :

..lomalized d e n s i t y of s t a s e s i n t k e Eand-tail v e r s u s energy.

_'ke &s&?& l i n e resreae?cs 3 < n $;.a Zoljess subband u i $ h E = 0 Ofor

;he c 5 a s s t c s l tkreshoyd.

EXPERIMENT

N long-channel (1000pm x 100pm) MOST with aluminium gate have been processed on <loo> p-type boron doped (NA

-

¹⁰¹⁷ silicon substrate. A thermal gate oxide (do,

-

^{72 nm)} was grown and the electron beam irradiation was performed before gate metallisation. In order to avoid any annealing of the fixed charges in the oxide, the wafers must be keeped at moderate temperatures between the electron beam irradiation and the end of the process.

The modulation of the in-plane potential inside the inversion layer was induced by a scanning electron beam (20 keV) with three irradiation doses D (0.3-8-200 p~cm'~) and four grating periods P (0.5-1-5-10 pm). The scanning lines

0.3 .urn broad were grated in the longitudinal (L) direction (parallel to the Drain-

Source axis) or in the transverse (T) direction (perpendicular to Drain-Source axis).

We notice that samples L or T irradiated with the same dose D and with the same grating period have the same total charge Qox fixed in the gate oxide. These two series are closer to longitudinal (LSSL) or transverse (TSSL) microstructured surface superlattices using holographic lithography.

Then a shift of the threshold voltages VT1 on the transconductance curves (figure 2a) and VT2 on the transfer characteristics (figure 2b) was observed for the two LSSL and TSSL geometries.

gm ( a r b ~ t r a r y u n ~ t s l fd Cmlcrohl

, * $ .

1 i10

T I 0

0.3.

L I

0 . 2 . . TL

0

Firare 2 : a ) Typicol transconduczance curve w i ~ h YTl a s thresFsld u o i t c g s .

b / -Transfer c h c c c f e r i s t i c s ( V - 5 0

rnq

u i t h VTTZ as zhe e z t - r ~ o i a ~ 8 6 t h r e s b ld vo l t c c e . d s -

for the same samples, while Figure 3 shows the maximum effective mobility as function of the temperature. We have reported only the data for the most irradiated serie which shows the largest variations.

Table

1 -

Electrical data at T

-

^{295 K}

I

^Sample

I L I O

^{LS LI} L . S ( T I O TS TI ~ . 5 Ref.

-0.60 -0.29

576

-4 ..+ Figure 3 :

n

0

E 1000 The e f f e c t i v e mobi--

t i t y versus temp--

Q)

2 rature for t h e two

.rl

c, I L or TI s t r u c t w % s .

u 500

Q) '+A 44 U

0

0 5 0 100 150 200 250 300 Temperature ( K ) INTERPRETATION

AND

DISCUSSION

As mentionned above, the modulation of the surface potential

6,

is correlated with a modulation of the band banding resulting in a band-tail of the density of states (figure 1).

The conduction in this band-tail differs from the classical hopping conduction in localized or extended states below or above the mobility edge [ 2 ] .

Indeed, the irradiation induce in our structures long-range potential modulation and not short-range disorder as in mobility edge models [ 3 ] .

Conduction is dominated by channeling effects in LSSL structures whereas barrier limited conduction is the main mechanism in TSSL structures.

This behaviour can be understood in terms of conduction above a conduction threshold located in the band-tail below or above the Eo energy. In the L-case, the electrons in the inversion layer are conducting in the potential valleys where the inversion condition is easily verified. Thus the conduction threshold is located near the band-tail edge.

On the contrary, in the T-structures the potential baeriers are distribuGed perpendicular to the current flow then impeding percolation paths. Therefore the electrons located in the potential valleys in the inversion regime must jump over the potential hills.

As a result, the conduction threshold raises up and the inversion condition must be obtained for gate voltages higher than for L-structures having the same total fixed charge QOx.

It can be seen that data for samples P = .5 pm and P

-

1 pm are similar. For these

JOURNAL DE PHYSIQUE

samples, the irradiation conditions are quite similar to an uniform irradiation of the whole sample.

In order to estimate the energetic extent of the band-tail we compare the L10 and T10 samples. Assuming that Qss (interface charges) and Qox are the same for the two samples, the difference between the two V T ~ voltages can be converted in a A+, variation of the surface potential $s.

For

AVT- -

410 mV a simple calculation gives at T

-

^{295K A$,}

-

164 mV using do, and NA in the classical expressions 171 and considering that near the threshold V T ~ the MOST is in weak inversion.

In the present configuration, Ads can be attributed only to PF between the bottom and the top of the band-tail. With this crude approximation, Ads

-

^2(u

^fi)

^(figure

I), the standard deviation of the Gaussian distribution is estimated to u

-

⁵⁸

meV. The same derivation for samples Lg and Tg (AVT~

-

170 mV) leads to o

-

²⁴

meV. These calculations are in good agreement with theoretical calculations of the electrostatic potential distribution in the inversion layer.

Figure 3 shows that the mobilities for T-structures are higher than for L- structures at low temperatures.

In this temperature range, the mobility p(E) increases when the energy increases.

Thus for T-structures, only the most energetic carriers are conducting and their mobility is higher than the mobility of L-structures in which all the electrons must be taken into account.

A mobility higher than the mobility of the unirradiated sample has been even measured on sample Tg.

We notice that for L-structures the mobility is lower than the mobility of the unirradiated sample (Ref) because of channeling effects.

Near room temperature, conduction is controlled by phonon scattering. For such a process, the mobility p(E) decreases when the energy increases.

This explains why the mobility of T-samples was lower than the mobility of L or unirradiated samples.

CONCLUSION :

Electrical measurements on MOSFET irradiated with longitudinal and transverse lines using an electron beam have been performed to support a first approach of 2D disordered systems.

Fluctuations of the in-plane potential are induced which control the current flow.

The electrical behaviour has been interpreted by means of a PF model based on the existence of a band-tailing of the density of states and the location of a conduction threshold energy.

Aknowledpements

The authors are grateful co the pilot production line (CNET-CNS Meylan) for supplying the MOSFET devices. They would like to acknowledge D. Bois (CXET-CNS) for his collaboration in this study.

References :

[I] E.H. Nicollian, J.R. Brews, MOS Physics and Technology, (Wiley, N.Y.), (1982).

121 N.F. Mott, E.A. Davies, Electronic processes in non-crystalline materials (Clarendon-Press, Oxford) (1979).

131 M. Pepper, Proc. R. Soc, Lond. A.353, 225, (1977).

[4] E. Arnold, Appl. Phys. lett. 25, 705, (1974).

(51 A.C. Warren, D.A. Antoniadis, H.I. Smith, J. Melngailis, IEEE Elect. Dev.

Lett. EDL6, 294, (1985).

16) D. Heitmann, Two Dimensional Systems : Physics and New Devices (Ed. G. Bauer.

F. Kucher, H. Heinrich) Springer-Verlag, 285, (1986).

[7] S.M. Sze, Physics of Semiconductor Devices (2nd Edition),(Wiley, N.Y.)(1981).