Influence of interface states on the charge injectionin m.n.o.s. memory devices

L.I. Popova, P.K. Vitanov and B.Z. Antov

Indexing terms: Interface electron states, Semiconductor storage devices, Metal-insulator-semiconductordevices

Abstract: Switching characteristics and charge centroids of m.n.o.s. memory devices with different interfacestate densities were studied at 25° ,C and at low temperatures. Experimental evidence, showing that interfacestates with high density affect the charging process in m.n.o.s. memory devices, is presented.

1 Introduction

There are numerous models describing the charging behav-iour of m.n.o.s. memory structures.1 As to which modeldescribes the switching behaviour of a particular m.n.o.s.device best, it has been said that it depends on the oxidethickness and the value of the applied voltage. It is alsopossible that, as the switching process goes on, the dominantinjection mechanism may shift from one type to another.In all proposed models the influence of interface stateswith high density on the carrier-injection mechanism wasneglected. The effect of interface states was introduced2'3

only in the models for the memory-charge loss.For mji.o.s. structures with a large density of interface

states, a considerable discrepancy between experimentand currently accepted models for the charging mechanismis to be expected, owing to the fact that during the writingpulse the interface-state charge changes the actual oxidefield. On the other hand, it has been well established4

that interface states with high density stimulate the dis-charge process especially in its initial phase. Lundstromand Svensson5 neglected the time dependence of thetunnelling probability and assumed that the charge wasfirmly held practically at the oxide-nitride interface.Recently, using the charge-centroid concept,6 Chang7

was able to include in his model the time dependence ofthe oxide field as well as the memory trap parameters.In spite of these refinements, however, a quantitativeagreement between theory and experiments was not shownexplicitly, because flatband voltage shifts are usuallymeasured after a massive back tunnelling occurring immed-iately after the switching voltage is turned off.

In Reference 8 the strong dependence of VFB on thevariation of the temperature alone was attributed to thecharging/discharging of acceptor-type interface stateswith high density. The interface Si-SiO2 is not able toproduce such an effect and, consequently, the observedeffect was associated with the presence of the oxide-nitride interface. It was experimentally established9 that,when the oxide thickness in the m.n.o.s. structure isincreased, the density and the energetic distribution of theinterface states tend to those of an m.o.s. structure withthick oxide.

The term interface state as used throughout this paperwill involve any state contributing to the low-frequencym.n.o.s. capacitance or to VFB during the corresponding

Paper T 3 5 9 S , received 11th December 1978The authors are with Institute of Microelectronics, Sofia 1113 ,Bulgaria

experiments. States that may exist close to the Si-SiO2

interface are not distinguished from those that may existat, or close to, the SiO2-Si3N4 interface. Neither interfaceis abrupt, and they might be considered to form a kind ofa transition region containing a significant defect densitywhich gives rise to the observed interface state distribution.Similar conclusions were reported in Reference 10 based onexperiments on Auger depth profiling of m.n.o.s. structures.

The objective of the present work was to show experi-mentally that interface states with high density should betaken into account in the modelling of the memory chargestorage.

2 Experimental methods

The measurements were carried out on «-type (111)-oriented m.n.o.s. capacitors and m.n.o.s. transistors with anoxide thickness dox = 22 A and a nitride thickness dn =800—1500A. The nitride was grown by two methods:

(a)pyrolytic reaction of SiCl4 and NH3 at 1000° Cwith N2 as a carrier gas (samples A)

(b) pyrolytic reaction of SiHj and NH3 at 900° C withH2 as a carrier gas (samples B)

Aluminium field plates were evaporated in a ring-dotconfiguration with a dot diameter of 5 x 10~2 cm.

The switching characteristics (VFB against t) weremeasured in the following way: pulses of constant voltage,but of increasing lengths, were successively applied. TheC/V curve for the determination of VFB was quicklyrecorded on an XT-recorder after each pulse; then a puiseof opposite polarity and of appropriate height and lengthto ensure the restoring of the initial condition VFBO of theas-grown capacitor. A similar procedure was followed tomeasure the threshold voltage VT of the m.n.o.s. trans-istors. The interface-state distribution in the silicon band-gap was obtained by the quasistatic C/V technique.9 Thevoltage operation range was from + 6Vto — lOVto ensurethe transition from strong accumulation to strong inversion.The rate of the linearly increasing ramp voltage was 40 mV/s.The leakage current was negligible owing to the relativelylow conductance exhibited by the thick nitride layer.It was assumed that during the tracing of the quasistatic$/V curve the applied fields were sufficiently weak toprevent charge trapping in the deep nitride traps. In fact,no hysteresis in the high-frequency C/V curve was observedwith + 10 V applied on the metal electrode for lOmin.

In order to look for a correlation between the interfacestate density and the penetration of charge into the nitridelayer, the centroid of the memory charge was determinedusing the technique described in Reference 11.

SOLID-STATE AND ELECTRON DEVICES, MAY 1979, Vol. 3, No. 3 610308-6968/79/030061 + 04 $01-50/0

3 Results and discussion

Fig.l shows switching characteristics for different positivepulses Ve at 298° C (sample A). Calculations for VFB

against t using the modified Fowler-Nordheim tunnellingmodel5 are also shown. The following expression was used:

FB- V

-VFBO

dnJox(Eoxo) J'ox(EOxo)t

where VFBO is the initial flatband voltage of the as-grownstructure, y = en/eox, the ratio between the two dielectricconstants and Jox, Jox are the charging tunnel current andits derivatives, respectively, at the initial oxide field Eoxo:

= CFNE\XO exp -

/ ' =

where

aD

2 CFN {2EOXO - a [ 1 -5 doxEoxo xox

(bV2-ciny)-D]}

*ioxo

(2)

(3)

a = -(2qm*ox<nyn

b = ^oxodox)

c - (0! -02 -Eox(^ox)D = 03/2_63/2+c3/aT (4)

Eqn. 1 differs slightly from the similar expressions inReference 5, owing to our different determination ofEoxo, namely

Eoxo = (V, - VFBO)l(dox + djy) (5)

In the calculation, the following parameters were used:en =6-9, eox =3-82, 0i =3-2eV, 02 = l-05eV, m*x =m * = 0 4 2 m o , CFJV = M 5 x 1(T6 A/V2. The large dis-agreement between experiment and theory in Fig. 1 isattributed to the presence of interface states with highdensity. When a strong positive pulse is applied, the siliconconduction-band edge at the surface moves towards theFermi level, resulting in the filling of acceptor-type inter-face states with electrons. This process is similar to thatobtained by means of decreasing only the temperature.8

In order to verify this assumption switching character-istics of the same mji.o.s. structure have been obtained atlow temperature.12 As was noted in Reference 8 when thetemperature is reduced below 140° K, a saturation in VFB

is observed, i.e. the interface-state density near theconduction-band edge is decreasing. Almost all interfacestates are filled with electrons and when a positive chargingpulse is applied the excess charge stored in the interfacestates by the field, is negligible.

CD

X)5 15* «J» X)2 tf1 10°

Fig. 1 Switching characteristics at 298° K

• • • • experimenttheory

zw 1 0

o B 2

o

o

o

• o

o •

-0 6 -OU -02

Fig. 2 Switching characteristics at 103° K

• • • • experimenttheory

62

Fig. 3 Interface-state energy distribution

• • • • B, ; low density of interface stateso o o o B2 ; high density of interface states

SOLID-STA TE AND ELECTRON DE VICES, MAY 19 79, Vol. 3, No. 3

At low temperature the influence of the interface stateson the switching characteristics is reduced in two ways:

(i) Almost all states are occupied, and their integralcharge may be readily obtained from the initial flatbandvoltage.

(ii) There are no available empty interface states tofacilitate the back tunnelling, through an increase of theshort-term decay rate.4

Thus, more realistic values of the injected charge may beobtained, compared with the case of similar measurementsat room temperature. Fig. 2 shows the good agreementbetween the m.f.n. model5 and the experiment at T —103° K.

The energetic distribution of the interface states isshown in Fig. 3 for samples Bi and B2. They are fabricatedin identical conditions except that the first one was annealedat 900° C for 30min in hydrogen ambience. Fig. 4 showsthe switching characteristics of the same samples. It isclearly seen that for Bi (lower density of interface states)the charge stored with same pulse length and height is abouttwice as large compared with B2 (higher density of interfacestates). This result shows again the strong influence theinterface states might exercise on the charging process.

The choice of the model5 is not essential in the presentwork. When applied at room temperature (without anycorrection for Eox due to interface state charge whichaffects the oxide field in a way explicitly unknown atpresent) and at low temperatures (with fixed correction toEox due to completely filled interface states), the model5

24

20

18

i 1 2

8

4

0

-4

-

-

/

//

/t

>

TBv /

//

t

x

* • •

x ^y^" 2Byx ^ ' ^ •

x X *'*'' * x NiSa^ ' • X

X ^ ^ a,*' X

' ^ *

• x ^

is good enough to show in a first approximation that theinterface states do influence the charge-storage process.

It should be expected that when negative writing pulsesare applied on the metal electrode, the interface states willhave no influence on the charging process due to theiracceptor character. In fact, for pulse lengths less than theinversion-layer relaxation time, an insufficient number ofminority carriers (holes) will accumulate at the siliconsurface to be injected into the nitride traps. Fig. 5 shows aswitching characteristic for sample Ai. The measured VFB

is a result of hole injection from silicon into the nitride.The time corresponding to the steep increase in VFB maybe identified as the inversion-layer relaxation time. For allmeasured m.n.o.s. structures it was in the range 2—5 x10"2 s.

The dependence of the threshold voltage on the writingpulse length of a p-channel mji.o.s. transistor fabricated asthe structure Aj is shown in Fig. 6. As can be seen, there isgood agreement between the m.f JI. model5 and the experi-ment for negative polarity. For positive polarity the dis-crepancy is similar to that for an m JI.O.S. structure. It maybe assumed that the acceptor-type interface states areexcluded from the charging process for negative polarity.In fact, during the charging process, the interface states areshifted above the Fermi level, and are unable to facilitatethe eventual hole.back tunnelling, after the writing voltageis turned off.

XT'

Fig. 6 Threshold voltage variation of p-channel m.n.o.s. memorytransistor fabricated as sample A,

KT5 10""-t.s

XT1 10° experimenttheory

Fig. 4 Switching characteristics of Bx and B^ at 298° K

x B, experiment• B2 experiment

theory

• \

O X • X O %

1CT5 XTt.s

10" 10°

200

100

Fig. 5 Switching characteristics of A, for negative polarity

SOLID-STATE AND ELECTRON DEVICES, MA Y1979, Vol. 3, No. 3

Fig. 7 Charge centroid of sample A (high interface state density)and sample B, (low interface state density)

63

It should be expected that the interface states signifi-cantly affect the memory-charge distribution, i.e. thecharge centroid xc. From Fig. 7 it can be seen that thecentroid of the structure A (high interface state density)exhibits considerably smaller values compared with thecentroid of the structure Bi (low interface-state density).The interface-state charge reduces the nitride field and,thus, impedes the penetration of charge mto the nitridebulk.

The results reported in this work show unambiguouslythat interface states with high density should be taken intoaccount in the determination of the magnitude of thestored charge and the charge centroid. In applicationswhere it is important to have a very stable 'window' and arelatively low writing voltage it is necessary to minimisethe interface state density.

4 References

1 CHANG, J.J.: 'Nonvolatile semiconductor memory devices',Proc. IEEE, 1976,64, pp. 1039-1059

2 WHITE, M.H., and CRICCHI, J.R.: 'Characterization of thin-oxide MNOS memory transistors', IEEE Trans., 1972, ED-19,pp. 1280-1288

3 FERRIS-PRABHU, A.V.: 'Charge transfer by direct tunnellingin thin-oxide memory transistors', ibid., 1977, ED-24, pp.524-530

4 NEUGEBAUER, C.A., and BURGESS, J.F.: 'Endurance andmemory decay of MNOS devices', /. Appl. Phys., 1976, 47,pp.3182-3191

5 LUNDSTROM, K.I., and SVENSSON, CM.: 'Properties ofMNOS structures', IEEE Trans., 1972, ED-19, pp. 826-836

6 ARNETT, P.C., and YUN, B.H.: 'Silicon nitride trap propertiesas revealed by charge-centroid measurements on MNOS struc-tures', Appl. Phys. Lett., 1975, 26, pp. 94-96

7 CHANG, J.J.: Theory of MNOS memory transistor', IEEETrans., 1977, ED-24, pp. 511-518

8 POPOVA, L.I., VITANOV, P.K., and ANTOV, B.Z.: Tempera-ture behaviour of MNOS structures', Phys. Lett., 1977, 59A,pp. 481-482

9 POPOVA, L.I., VITANOV, P.K., and ANTOV, B.Z.: 'Interfacestates in MNOS systems', Thin Solid Films, 1978, 51, pp.305-309

10 JOHANNESSEN, J.S., HELMS, C.R., SPICER, W.E., andSTRAUSSER, Y.E.: 'Auger depth profiling of MNOS structuresby ion sputtering', IEEE Trans., 1977, ED-24, pp. 547-551

11 MAES, H., and van OVERSTRAETEN, R.J.: 'Simple techniquefor determination of centroid of nitride charge in MNOS struc-tures', Appl. Phys. Lett., 1975, 27, pp. 282-284

12 POPOVA, L.I., VITANOV, P.K., and ANTOV, B.Z.: 'Influenceof interface states on the write characteristics of MNOS memorystructures', Solid-State Electron., 1979, 22, pp.

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