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Elecfronies Vol. 38, No. 3, pp. 61 I-614. 1995 Copyright C 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-I 101195 $9.50 + 0.00

EFFECTS OF BULK-IMPURITY AND INTERFACE-CHARGE ON THE ELECTRON MOBILITY IN MOSFETs F. GAMIZ, Departamento

J. BANQUERI,

J. E. CARCELLER

and J. A. LOPEZ-VILLANUEVA

de Electr6nica y Technologia de Computadores, Universidad de Granada, Facultad de Ciencias, Avd. Fuentenueva s/n, 18071 Granada, Spain (Received 25 February 1994; in reoised form

8 June 1994)

Abstract-The effects of interface and bulk-impurity charges on electron mobility in a MOSFET are compared by using Monte Carlo simulation. It has been shown that the increase in bulk-impurity concentration causes a reduction of mobility at low electric fields, in two ways: (i) an increase of Coulomb interaction produced by an increase of charges in the bulk; and (ii) the reduction of screening caused by the loss of charge in inversion layer. Except for high-doping levels, the latter is the most significant cause of mobility degradation

theless, this is not the only effect that the bulk-impurity concentration has on the electron mobility. It is well known that the charges right at the interface or in the oxide also contribute to mobility degradation[7,8]. The way in which these charges affect the mobility strongly depends on the bulk-impurity concentration. For a fixed effective transverse electric field, when the concentration of bulk impurities increases, the depletion charge also increases, thus decreasing the amount of charge in the inversion layer. This involves the loss of screening of the charged centres by mobile carriers when the impurity concentration rises. Coulomb scattering by the oxide and interface charged centres is therefore more effective when the bulk concentration is higher and the electron mobility falls. There already exist in the literature several empirical and semi-empirical models to quantitatively describe deviations from the universal behaviour of mobility curves produced by screened Coulomb scattering due to doping impurities in the channel[9,10]. Although these models describe very well the roll-off of effective mobility in the low field region, the bulk impurities and interface-charge cannot be isolated, making it impossible to differentiate between the two effects. Determining the relative importance of each of these effects is, however, quite interesting from the viewpoint of device reliability. For this purpose it is necessary to use a device numerical simulation.

1. INTRODUCTION

The modelling of an electron device provides obvious advantages for studying its suitability for manufacturing and for predicting its behaviour in a circuit environment. For effective modelling of a MOSFET, it is necessary to accurately understand the behaviour of the inversion-layer mobility. In this paper the influence of the silicon-bulk-impurity concentration and the interface charge on the mobility is studied by applying a Monte Carlo simulation, whose validity we have proved by reproducing experimental results[l-31. A mobility study by Monte Carlo simulation is very useful for obtaining information not available experimentally[ l-31, as is the case when one wishes to study the effects of bulk-impurity concentration and interface charges on the electron mobility. which cannot be isolated experimentally.

2. MOBILITY DEGRADATION

In order to avoid small-size effects, the reduction of the dimensions in scaled MOS devices requires the use of high bulk-impurity concentrations. An immediate consequence of the increase of the bulk concentration is an increase of threshold voltage[4] and an electron mobility decrease[4,5]. This degradation becomes clear from the reduction and deviation of the mobility curves from universal behaviour, experimentally demonstrated by Sabnis and Clemens, when mobility is plotted vs the socalled effective field[6]. The deviation was observed to increase as the bulk-impurity concentration increases, which suggested that the mobility is affected by Coulomb scattering due to bulk-impurities[5]. Never-

3. RESULTS AND DISCUSSION

The strong effect Coulomb scattering has on the mobility curves for different concentrations of interface states, measured at 77 K and 300 K, are shown as a solid line. The results of our Monte Carlo simu611

612

F. Gamiz et al

lation have been plotted with symbols (Fig. 6). The good agreement of simulated and experimental curves can be considered as a validation of our simulation. Note that the increase in scattering-centre density makes the mobility decrease and deviate from universal behaviour. To obtain the universal curve, therefore, all the charged centres in the structure must be eliminated, and this is impossible to accomplish in practice. Nevertheless, it can be done by simulation. Mobility curves for different bulk-impurity concentrations are shown in Fig. I, where Coulomb scattering is assumed not to exist. It can be seen that all the curves coincide, thus reproducing the universal behaviour shown by Sabnis and Clemens. However, when a charge concentration is put in the oxide, or the effects of the impurities distributed in the bulk are taken into account, a deviation from the universal curve can be observed. This fact demonstrates that Coulomb scattering is responsible for the deviation of electron mobility curves from universal behaviour. Figure 2 shows the effects on the mobility of a typical charged-centre concentration of N,, = 5 x 10” cm-* located right at the interface for different bulk-impurity concentrations, neglecting the effect of Coulomb scattering due to bulk impurities. The relative deviation of the curves is thus due to the reduction of screening as the bulk-impurity concentration increases, since the amount of charge responsible for Coulomb scattering is the same for all curves. Figure 3, in contrast, shows the effects of the bulk impurities on the electron mobility assuming no charge in the oxide. To take into account the Coulomb interaction between bulk impurities and electrons in the inversion layer, we have considered only the charges placed at a distance closer than 200 A from the interface, by assuming two charged layers located inside the bulk in the following two situations: (a) A layer located at IO A from the interface with a concentration of 2 x IO-’ x N,cm-*. and a

T=300

A

5 Nt

0

-

x

K

800 600

-

.c .=

400’1

2

200

n

t .* # .l *.fi.*,, .-• :** . . . 1 . i

0-

Ll c

10" Effective

lo6 Field

(V/cm)

Fig. 2. Mobility curves vs effective geld at room temperature for different bulk-impurity concentrations with Coulomb scattering due only to the oxide and interface charged centres. (Nx = (*): lOI cm-j, (*): lOi cm-‘. (M): lOu crnm3, (A): 10”ctn3.) second one at IIOA with a concentration of I.8 x 10e6 x NAcm-*, NA being the bulk-impurity concentration (expressed in cmm3). (b) A layer at 20 w from the interface with a concentration of 4 x lo-’ x N,cm-* and a second one at l20,& with a concentration of 1.6 x 10m6 x NA cm-*. We obtained mobility curves for the two above situations, and no differences were seen between them. We divided the space-charge region into a greater number of sublayers in order to obtain a better approximation to the actual doping profile, but no significant differences were observed in the resulting mobility. Nevertheless, if only one charged layer is considered, the curves obtained depend on the position of this layer. No changes are observed if a third layer is considered in order to take into account the charge located at distances greater than 200A.

1000

1000 800

““’

T=300

A




5

600

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4

200

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K _

l

- +t ***a

-

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l

+a .

( >

-

613

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>3

800

&

600

‘N*=lx;O;‘{ T=300

4

200 0

Effective

/

12

IO5 Field

1

.z? .= 400 n

a

0

K

04 12

105 Effective

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Field

(V/cm)

Fig. 4. Different contributions to the electron mobility, showing the effects of charges in the oxide and silicon bulk, for two bulk-impurity concentrations. (*) Phonon and surface-roughness scattering only. (A) Effect of Coulomb scattering due only to silicon-bulk impurities. (*) Effect of the oxide and interface, neglecting bulk-impurity charges. (m) The whole mobility. The observed deviation in the curves of Fig. 3 is now due to Coulomb scattering from charges in the bulk. These curves prove that the deviation of mobility curves from universal behaviour as the bulk-impurity concentration changes, is due not only to the effect of Coulomb scattering by bulk charges, but also to the change in the screening of oxide charges. Comparing Figs 2 and 3, it can be seen that: (a) for low bulk-impurity concentrations, the main effect of bulk charges on electron mobility is the reduction of screening, thereby making the effect of the charges in the oxide very significant; (b) however, for higher bulk concentrations, Coulomb scattering by bulk impurities becomes more important, thus partially masking the effect of the oxide charges on mobility. This is clearer in Fig. 4, where the different contributions are shown for two bulk-impurity concentrations. In Fig. 4(a), where the bulk-impurity concentration is low, the reduced-screening effect is dominant. However, in Fig. 4(b) where the bulk impurity concentration is higher, the Coulomb interaction due to bulk impurities becomes the main effect the bulkimpurity concentration has on electron mobility. It is important to note that these results are only obtainable by simulation, as Coulomb scattering by bulk impurities cannot be separated from the effects of charges in the oxide experimentally. Until now, we have compared our results for different substrate doping levels at the same effective field. When the effective field is kept fixed for comparison, greater doping concentrations correspond to lower inversion-layer densities. Instead of using the effective field, however, we could represent mobility vs vos - V,h, V,, being the applied voltage between gate and source, and V,h the threshold voltage. The effect of threshold-voltage variation due to modification of the doping level could thus be compensated. This is equivalent to representing mobility vs electron

concentration per unit area in the inversion layer. We have therefore used this representation to avoid the influence of the oxide capacitance. Figure 5 shows mobility curves calculated by Monte Carlo method vs the inversion charge for different substrate-doping levels. The deviation is observed now both at low and intermediate inversion-charge densities due to a double effect: -Surface roughness and phonon scattering strongly depend on the effective transverse field. For a fixed value of Nin,, the effective field increases concomitantly bulk-impurity concentration. Therefore, for a fixed value of the inversion-charge concentration, mobility decreases when bulk-impurity concentration increases, due 1000

7’

T=300 100

10”

1 o12

K

1 o13

Ninv (Cm-2) Fig. 5. Mobility curves vs inversion charge concentration at room temperature for different bulk-impurity concentrations taking into account Coulomb scattering by both oxide and interface charge and silicon-bulk impurities (N,, = 5 x 10’0cm-2). (NA = (*): lOI4cm-‘, (Jr): 10’5cm-r, (m): IOr cm-j 3(A)’. IO” cm-j.)

F. Gdmiz

614

er al.

1000

I

-I

L - * . ’’”

. :_ N; :

-

I

.-

Y 5 6:

V

I

-

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5

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E

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4 100

’ “‘I

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OI(cm:;

(V/cm)

Effective

1 lo6

lo5

lo6 Field

1 , , , ,,,, NA=,9x!

K

Field

(V/cm)

Fig. 6. Experimental mobility curves (-) and calculated ones by Monte Carlo method (W) vs the transverse electric field after successive Fowler-Nordheim tunnelling injection series for different T= 77 K (1): temperatures: N,,: T = 300 K (I): 5 x IO9 cm-‘, (2): 5 x 10” cm-*, (3): I.5 x 10”cm-‘; 1 x IO” cm-‘,

to surface roughness and phonon scattering (Fig. 6). -For a fixed value of Niny, the screening of charged centres (both oxide-charge and bulk impurities) by mobile carriers is the same. This means that Coulomb scattering by oxide and interface charges is the same in all the curves, regardless of substrate doping. However, as substrate doping increases, the number of bulk impurities also increases and, as a consequence, Coulomb scattering by bulk impurities is more effective (the number of charged-centres increases), thus causing the drop in mobility data at low electric fields.

(2): 2 x IO” cm-‘.

the reliability of the device: any modification of the charge in the oxide will modify the electric properties of the device. concentrations, the (iii) For high bulk-impurity effect of Coulomb scattering due to bulk contamination is comparable to and even greater than the effect of oxide charges for state-ofthe-art devices. Hence, the electric properties of the device are much less affected by modification of the oxide charge due to operation at high electric fields. Therefore, a high-impurity concentration makes the device much more reliable since it is then more stress-resistant in operation conditions.

4. CONCLUSIONS

According to the above, the following can be drawn:

(9 The increase in bulk-impurity

REFERENCES

conclusions

concentration involves a reduction of mobility at low electric fields, in two ways: (a) the reduction of screening caused by the growth of the depletion charge and the consequent loss of charge in the inversion layer; and (b) an increase of Coulomb scattering due to an increase of charges in the bulk. concentration, (ii) Except for very high-impurity the effect of the oxide charge on the screening dominates. This means that for low-impurity concentrations, the charge trapped at the interface has a strong influence, and therefore it is extremely important to use high-quality oxides. In addition, the mobility would be strongly affected by any modification of the charge in the oxide, which would detract from

Gamiz, J. A. Lopez-Villanueva, J. A. JimenezTejada and A. Palma, J. appl. Phys. 75, 924 (1994). 2. F. Ggmiz, J. Banqueri, I. Melchor, J. E. Carceller, P. Cartujo and J. A. Lopez-Villanueva. J. appl. Phys. 74, 1. F.

3289 (1993). 3. F. Gbmiz, I. Melchor, A. Palma, P. Cartujo and J. A. Lopez-Villanueva. Semiconductor Sci. Technol. To be published. 4. M. J. Van Dort, P. H. Woerlee, A. J. Walker, A. H. Casper and H. Lifka, IEEE Trans. ED-39, 932 (1991). 5. S. Takagi, M. Iwase and A. Toriumi, IEDM Tech. Dig. 88, 398 (1988). 6. A. G. Sabnis and J. T. Clemens, IEDM Tech. Dig. 79, I8 (1979). 7. S. Manzini, J. appl. Phys. 57, 41 I (1985). 8. J. Banqueri, F. Gamiz, J. E. Carceller, P. Cartujo and J. A. Lopez-Villanueva, J. Elect. Marer. 23, I159 (1993). 9. H. Shin, G. M. Yeric, A. F. Tasch and C. M. Maziar, Solid-St. Electron. 34, 545 (1991). 10. M. Shirahata, H. Kusano, N. Kotani, S. Kusanoki and Y. Akasaka, IEEE Trans. Comp. Aided Design CAD-11, I1 I4 (1992). . I