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Submitted on 1 Jan 1981

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THE EFFECT OF ANNEALING AND ILLUMINATION ON THE FIELD EFFECT CONDUCTANCE OF AMORPHOUS SILICON

M. Powell, B. Easton, D. Nicholls

To cite this version:

M. Powell, B. Easton, D. Nicholls. THE EFFECT OF ANNEALING AND ILLUMINATION ON THE

FIELD EFFECT CONDUCTANCE OF AMORPHOUS SILICON. Journal de Physique Colloques,

1981, 42 (C4), pp.C4-379-C4-382. �10.1051/jphyscol:1981481�. �jpa-00220938�

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CoZZoque C4, suppzdment au nOIO, Tome 42, octobre 1981

THE EFFECT OF ANNEALING AND ILLUMINATION ON THE FIELD EFFECT CONDUC- TANCE OF AMORPHOUS SILICON

M.J. Powell, B.C. Easton and D.H. Nicholls

PhiZips Research Laboratories, RedhiZZ, S m r e y , U.K.

ABSTRACT

-

We have measured the effect of white light illumination and annealing to 180°C on the field effect conductance of glow discharge a-Si:H. Annealing produces a change in the off conductance by up to a factor of 30, and moves the threshold field by 2 x lo4 ~cm-l. These changes are reproducibly reversed upon illuminating with white light.

A major part of the effect of illumination is most likely due to a movement of the Fermi level in the bulk of the material. Annealing to 180°C reverses this effect. Furthermore excess electrons can be trapped at the amorphous silicon- silicon nitride interface by a field assisted trapping mechanism and these are also de-trapped upon annealing to 180°C.

Reversible changes in the dark,conductance of thin films of glow discharge amorphous silicon, due to annealing at T > 150°C and illumination with white light, have been reported by a number of groups. These have been interpreted as due to metastable changes in the density or occupancy of defect states which cause a movement of the bulk Fermi level (1)-(3) or conversely as due to the creation and elimination of space charge regions at one or other of the two surfaces of the film (4),(5).

We have performed similar measurements on field effect devices, where one surface of a 0.5 um film of a-Si:H is separated from a gate electrode by a 0.5 pm layer of plasma deposited silicon nitride and the other surface is covered by a 0.5 um plasma deposited Si02 layer, but has no gate electrode (6). Such devices are currently of considerable interest as thin film transistors (TFTs) for matrix addressing of displays (7).

In our devices, annealing to 180°C (with zero gate volts) produces a

room temperature conductance between 5 and 30 times the room tempera?-ure conductance after illuminating the sample with white light from a 50 Watt quaftz-halogen lamp for 90 minutes. Figure 1, shows an example where a conductance change of 30 is observed. Subsequent re-illumination reproduces the initial condition (indicated by the vertical arrow in figure 1) and further cycles of annealing and illumination reproduce the same results. Although the magnitude of the effect in our samples is much smaller than originally reported by Staebler and Wronski (I), it is very comparable with results observed in other laboratories (5).

In figure 2 we show the field effect conductance (at 4O0C) in each of the two states. State A is after annealing the sample to 180°C and state B after illumin- ation. The field effect conductance is also reproducible on repetitive cycles of annealin and illumination. In this figure the ordinate is the sheet conductance

f

G( $210)- and the abscissa is the surface field in the semiconductor at the

amorphous silicon-silicon nitride interface, given by F = E ~ ~ ~ V ~V / E ~ ~ ~ ~ . is the gate voltage, tins the dielectric constant and dins the thickness of the

insulator and E the dielectric constant of the semiconductor. In figure 2, we plot the same results on both a log and a linear scale.

In both states, the field effect conductance shows little change with negative gate voltages, but for increasing positive gate voltages the conductance first increases approximately exponentially, then at higher gate voltages increases approximately linearly. The onset of the linear region occurs at a threshold field Ft. Clearly the cycle of annealing and illumination has two effects, to change the threshold field Ft and to change the off-conductance Go (the approximately constant

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981481

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C4-380 JOURNAL DE PHYSIQUE

Fig 1. Temperature dependence of Fig 2 . Field effect conductance a t 4 0 ' ~ t h e d a r k conductance of a 0.5pm a f t e r annealing to 1 8 0 ~ ~ ( s t a t e A )

conductance at negative gate voltages). These two parameters Go and Ft are of para- mount importance in the characterisation of TFTs, and changes in these quantities represent instabilities which are highly'undesirable for device applications.

Although the changes are brought about by conditions which are well outside the normal operating conditions of TFTs, an understanding of the origin of the effects is of considerable interest.

[

amorphous silicon film. and a f t e r illumination ( s t q t e B ) 10-

G

Before we consider the interpretation of the changes in the field effect in terms of either a 'bulk effect' or a 'surface effect' model, we must consider the general interpretation of the field effect.

In both the surface space charge model (4,5) and the bulk effect model (1-3).

the state B (after illumination) represents a situation at least close to flat bands.

Thus we will consider that state B in figure 2, corresponds to flat bands at or close to zero gate volts. In our.devices a negative gate voltage produces little change in conductance. This may indicate a high density of bulk or surface states below the Fermi level, thus preventing the upward band-bending and co-,sequent hole conduction at the amorphous silicon-silicon nitride interface, or conversely the absence of hole conduction may only indicate an inability of the source and drain contacts (which are on the opposite side of the semiconductor film from the gate) to inject holes across the n-type film into a surface inversion layer.

The exponential increase in conductance at small gate voltages can be explained by a constant density of bulk gap-states in the a-Si:H (6) (for the results of figure 2 the value would be I x ~ o ~ ~ c I u - ~ ~ v - ~ ) and the linear increase in conductance at higher gate voltages is due to the higher density of electrons in conduction band states (extended states plus tail states). The field effect mobility of electrons in the conduction band lies in the range 0.03-0.15 cm2(vsec)-l at 40°C (the sample of figure 2 has a low mobility of 0.035 cm2(vsec)-I). Assuming for the moment that this is the correct interpretation of the field effect, we can consider the effect of annealing for the proposed bulk effect and surface effect.

In the bulk effect, there is a change in the effective density of gap-states which causes a shift of the Fermi level towards th anduction band. In order to account for the change in off conductance by a &or of 30, the bulk Fermi level must be raised by 0.1 eV. For a bulk density of states of 1 0 1 7 c m - ~ e ~ - ~ , this would

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the density of states would hardly affect the rate of increase in the field effect conductance with applied field, unless there was an associated much larger increase in the total density of states of which only a small fraction were active in shifting the Fermi level.

F i g 3. Calculated field effect Fig 4. Field effect conductance conductance for bulk and after various combinations of

surface effect m o d e l s (see t e x t ) annealing, i l l u m i n a f ion and field assisted electron trapping (see t e x t )

In figure 3, curve (a) shows the calculated field effect when the Fermi level has moved towards the conduction band by 0.1 eV by com arison to curve (b), but where the bulk density of states remains at 101fcm-3eV-P. It can be seen that this results in both an increase in the off conductance Go and a shift of the threshold field Ft, and the agreement between the calculated curves and the'experimental result of figure 2 is very good.

In the surface effect model, the effect of annealing is to leave a certain density of fixed positive charges at one or other of the two interfaces, since electrons, which were trapped at the interface during illumination, are thermally emitted back into the semiconductor. A fixed positive charge density at the silicon nitride interface would result in a change of the threshold field, but with no change in the 'off conductance', even though the conductance at zero gate volts might change by a factor of 30. Conversely a fixed positive charge density at the SiO2 interface would result in a change in the off conductance, but with no change in the threshold field.

Thus we cannot explain the experimental results by a surface effect involving only one interface. However a combination can explain the results reasonably well.

Curve (c) in.fi ure 3 shows the calculated field effect for a surface charge density of +101fcm-2 at the silicon nitride interface and +1.35 x 10llcm-~ at the Si02 interface, but where the bulk Fermi level position has remained unchanged by comparison to curve (b). Although this result does not fit the experimental result quite as well as curve (a), it is not a sufficient discrepancy to eliminate the surface effect model.

In fact we do have evidence that electron trapping can take place at the amorphous silicon-silicon nitride interface, which is shown in figure 4. Curves (a) and (b) represent the usual field effect after annealing and illumination.

Curve (c) represents the field effect after illumination, but followed by heating

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C4-382 JOURNAL DE PHYSIQUE

to 100°C with a large positive applied gate voltage (producing 3 times a larger field than normally applied). This results in a considerable change in the threshold field but with negligible change in the off conductance. Similarly curve (d) shows the field effect after annealing to 180°C, but also followed by heating to lO0OC with a large positive gate voltage. Again by comparison to curve (a) we observe a large change in the threshold field, but no change in the off conductance. In both cases trapping of electrons in the silicon nitride near the amorphous silicon-silicon nitride interface must be taking place. Indeed such field and thermally assisted electron trapping is well known in plasma nitrides (8).

If curve (b) of figure 4 corresponds to flat bands at zero gate volts, then both curves (c) and (d) must have a net negative charge at the interface at zero gate volts and hence the bands must be bent upwards at this interface. In the surface effect model, the effect of illumination is to trap electrons to produce near flat bands. However if the bands are initially bent upwards, one might expect illumination to de-trap electrons, again tending towards flat bands. This would imply that the effect of illumination on a device in a state given by curve (d) of figure 4, would be to lead to curve (b) again. In fact we find that we obtain curve (c) again. Therefore either 4llumination is responsible for the trapping of even more electrons, which seems unlikely, or a major part of the effect of illumination is due to a bulk shift of the Fermi level. Conversely, if the effect of annealing to 180°C was only a bulk effect and did not involve any de-trapping of electrons, the effect of annealing a sample initially in a state given by curve (c) of figure 4 would be to obtain curve (d) again. In fact we actually obtain curve (a) again. Thus if we start from a situation of a large density of trapped electrons, then the effect of annealing does more than reverse the effect of illumination, it also causes the de-trapping of electrons, as suggested by the surface effect model.

Recently it has been suggested that the true bulk density of deep gap states (as opposed to tail states) can be as low as (9), which would imply that a field efect experiment measures fast surface states. It is extremely likely that fast surface states are significant (6), but the bulk density of states is not necessarily as low as l ~ ~ ~ c m - ~ e v - ~ . However if this was the case, then the

interpretation of the field effect would be very complicated. The band-bending would now extend right across the 0.5 um film and the surface states at the opposite interface would also be important. The whole film would be dominated by the surface and the only viable explanation of theeffect of illumination and annealing would be a 'surface effect'. However the distinction between a surface and bulk effect becomes imprecise. For example, a change in the fixed charge density at either interface would lead to a corresponding change in the occupancy of fast surface states and would appear a 'bulk effect' in the sense that the band- bending potential profile across the layer would remain virtually unchanged.

In summary, we can see that it is only meaningful to distin uish between a bulk effect and a surface effect for densities of states nearer to lof7 than 1 0 l ~ c m - ~ e ~ - l , but whatever the true value for the density of states the following conclusions

remain valid. Illumination produces a shift of the bulk Fermi level away from the conduction band. Annealing to 180°C reverses this shift, but also de-traps any excess of electrons trapped at the amorphous silicon-silicon nitride interface.

REFERENCES

(1) STAEBLER D.L. and WRONSKI C.R. J. Appl. Phys.

51

(1980) 3262.

(2) JOUSSE D. BASSET R. DELIONIBUS S. and BOURDON B. Appl. Phys. Lett.

37

(1980) 208.

(3) ELLIOTT S.R. Phil. Mag.

B39

(1979) 349.

(4) SOLOMON I. Proc. Kyoto Summer Instit. Sept 8-11 1980 (Springer-Verlag 1981) 33.

(5) FRITZSCHE H. Solar Cells

2

(1980) 289.

(6) POWELL M.J. EASTON B.C. and HILL O.F. Appl. Phys. Lett.

38

(1981) 794.

(7) SNELL A.J. MACKENZIE K.D. SPEAR W.E. LE COMBER P.G. and HUGHES A.J. Appl. Phys.

24 (1981) 357.

(8) See for exampG

-

SINHA A.K. and SMITH T.E. J. Appl. Phys.

2

(1978) 2756.

(9) COHEN J.D. LANG D.V. and HARBISON J.P. Phy. Rev. Lett.

5

(1980) 197.

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