Surface Science 58 (1976) 56-59 0 North-Holland Publishing Company

FINE STRUCTURE IN THE FIELD EFFECT MOBILITY OF MOS

TRANSISTORS *

Teresa COLE, Amir A. LAKHANI and P.J. STILES Department of‘ Physics, Brown University, Providence, Rhode Island 02912, USA

We have studied the fine structure in the field effect mobility of MOS transistors at

4.2 K. With applied substrate bias, the position in gate voltage of the structure is un-

changed and the amplitude decreases. The position of the structure is also unchanged by

normal magnetic fields of up to 150 kgauss. We have studied samples with four equi-

valent source-drain contacts. The structure depends strongly on the contacts. The struc-

ture in the field effect mobility appears to be due to the contacts and not a property

of the inversion layer or surface states.

Several investigators [ 1,2] have reported the appearance of a peaked structure in the field effect mobility of MOS transistors at temperatures 4.2 K and below. This structure which is sample dependent is strongest at low carrier densities and

consists of small random structure superimposed on the usual shape of the field effect mobility. The position in gate voltage of this fine structure is temperature independent and the amplitude grows with decreasing temperature. Pals and Van Heck [ 1 ] found that the structure was more pronounced in p-channels than in n-channels and that increasing the channel area decreased the magnitude of the structure. The structure has been attributed by Pals and Van Heck [I] to trapping and scattering of the carriers by surface states and by Tidey et al. [2] to fluctua- tions in the inversion layer density of states due to localized states, surface strains,

or surface roughness. In this paper we report further studies of the tine structure in the field effect

mobility. In order to determine if the fine structure is correctly attributed to in- homogeneities at the Si-SiO2 interface, we have studied the effects of substrate bias, magnetic field and of the contacts to the inversion layer.

The studies of substrate bias and the effects of changing the contacts were made on n-channel (100) samples with a source-drain distance of -50 pm and a Van der Pauw geometry [3] (see inset to fig. 3). The substrate resistivity was 2 a cm and the oxide thickness 1000 A. The samples used for the magnetic field studies were rectangular samples of dimensions 200 X 600 iirn, oxide thickness 2350 A and re- sistivity 4 Q cm. We measured the transconductance, do/dvg which is proportional to the field effect mobility.

* Supported by Materials Research Program at Brown University funded by the National

Science Foundation.

56

T. Cole et al. /Fine structure in fiefd effect mobility of MOS rransistars 57

Fig. I. Fine structure in the tramconductance of an n-channel (100) surface of si at 4.2 K fox substrate biases of +O.5 (a), 0 (b), -1 (c), -2 Cd), -3 (e), -5 Q”l, -7 k), and -10 V (h).

Applying a reverse substrate bias to the sample increases the width of the deple- tion layer and narrows the potential welI at the interface [4]. This forces the inver- sion layer electrons closer to the surface so they shouId feel the ~~omogeneities at the interface more strongly. If the fine structure in the field effect mobility were due to these inhomogeneities we would thus expect a reverse substrate bias to in-

crease the magnitude of the structure if the screening constant remains unchanged. If the screening constant changes, then according to Stern and Howard [S] the bound state energy of surface states and traps changes. This should result in a shift in the position of the structure in gate voltage.

In fig. I we show the results of applying substrate bias to the samples. The magni- tude of the structure is reduced as the substrate bias is increased, and the position in gate voltage is unchanged. This indicates that the structure is dependent. on the sur- face fieid not on the carrier density in the inversion layer.

We have aIso applied magnetic fiefds from 0 to I SO kgauss normal to the inver- sion layer to see if this has any effect on the structure. The results are shown in fig.

2. We observe no shift in the position of the structure. This is the same as was ob- served at lower magnetic fields by Tidey et al. [2]. Similar structure has also been seen by Voshchenknv [6] in capacitance measurements. The position in gate vol- tage of the structure in the capacitance is also independent of magnetic field.

The po~ibiIity that the structure in the field effect mobility might be associated with the contacts has not previously been discussed or investigated experimentally. Therefore, it seemed appropriate to investigate the fine structure in a sample with four contacts to the inversion layer. Different combinations of the source-drain contacts were used in measuring the transconductance. The results are shown in fig. 3. We observe that both the position and the magnitude of the fine structure

58 T. Cole et al. /Fine structure in field effect mobility of MOS transistors

SV/dlv

VI

I

.5 .5 V/div

“g

Fig. 2. Fine structure in the transconductance of a p-channel device on (811) surface of Si at

4.2 K under normal magnetic fields of 0 (a), 35 (b), 70 (c), and 105 kOe (d).

Fig. 3. Fine structure in the transconductance of a (100) n-channel device for different pairs of source-drain contacts labeled A,B,C, and D. Inset shows the device geometry.

depend strongly on the particular set of contacts used but not on the polarity of the source drain voltage. This leads us to conclude that the tine structure may be

due to the contacts and not the inversion layer. The key to the understanding of this structure is the results of the study with

substrate bias. These indicate that the position of the structure in gate voltage de- pends only on gate voltage and.not on the carrier density or Fermi energy of the inversion layer. Thus all proposed effects appear to be ruled out except for contact effects.

A contact dependent structure could result if part of the contact which is under- neath the gate has a gate voltage dependent resistance. An example is if the doping at the n+-diffusion and the n-inversion layer interface under the contact were later- ally non-uniform to the extent that p-type patches in a n+ contact existed. A second example would be the existence of similar patchy effects in which some areas at the interface were weakly n-type but below their conducting state at a given gate voltage.

As is easily reasoned, both of the above give structure whose position depends only on gate voltage. In addition, the fact that the range in which the structure at low gate voltage disappears, increases with increasing substrate bias is explainable. It is a consequence of the large variation of the inversion layer resistance with sub-

T. Cole et al. / Fine structure in field effect mobility of MOS transistors 59

strate bias compared to the small change in resistance when one of the patches

mentioned above changes its resistance. If there is no conductance in the inversion layer, the spike structure will not be seen.

In summary, we have investigated the effects of substrate bias, magnetic field and of the contacts to the inversion layer on the tine structure in the field-effect mobility. We observe that substrate bias and magnetic field have no effect on the position of the structure in gate voltage, and that applying a substrate bias causes a decrease in the magnitude of the structure. Both the position and the magnitude of the structure depend on which set of contacts to the inversion layer is used. This

leads us to conclude that the fine structure is caused by the contacts. Further con- firmation of this could come from a four probe measurement.

References

[l] J.A. Pals and W.J.J.A. van Heck, Appl. Phys. Letters 23 (1973) 550. [2] R.J. Tidey, R.A. Stradling, and M. Pepper, J. Phys. C7 (1974) L353. [3] L.J. van der Pauw, Philips Res. Rept. 13 (1958) 1. [4] J.A. van Nielen, and O.W. Memelink, Philips Res. Rept. 22 (1967) 55. [S] F. Stern and W.E. Howard, Phys. Rev. 163 (1967) 816. [6] A.M. Voshchenkov and J.N. Zemel, Phys. Rev. B9 (1974) 4410.