IEEE Tuun4actiZou on Nucte.a Science, Vot.NS-22, No.6, Deceribet 1975

MECHANISMS OF CHARGE BUILDUP IN MOS INSULATORS

Walter C. Johnson

Department of Electrical EngineeringPrinceton University

Princeton, New Jersey 08540

Abstract

The radiation sensitivity of MOS devices has beenrecognized to be the result of a charge buildupcaused by the sweepout of electrons and the trappingof holes following hole-electron pair production byionizing radiation. Holes have been shown to have afinite, although small, mobility in thermally grownSiO2. Trapping of holes takes place near the Si-SiO2interface, possibly by oxygen vacancies in the oxide.Normal thermally grown SiO2 possesses only small con-centrations of electron traps. Electron traps havebeen shown to be generated, however, in the oxide byion implantation, by irradiation with nonpenetratingelectrons, and by exposure of the surface of the oxideto negative iorns from a corona discharge. AlthoughNa+ and Li+ ions have been shown to be mobile at roomtemperature in SiO2, contamination can be kept tolevels where ionic charge buildup is negligible. Therole of contaminants in the formation of hole traps,however, remains to be determined.

I. Introduction

The publication in 1964 of the paper by H. L.Hughes and R. R. Girouxl concerning the radiationsensitivity of MOS devices started a wave of interestin charge transport and charge buildup in MOSdevices that has continued unabated to the presenttime. Some of the early workers in this area wereKooi,2 Szedon and Sandor,3 Zaininger,4 Grove andSnow,5 and Collins and Sah.6 They observed a buildupof positive charge in HOS structures that were sub-jected to ionizing radiation while under bias, andthis was interpreted in terms of the formation ofelectron-hole pairs in the oxide with the subsequentsweep-out of electrons and the capture of holes.4,5An increase in the density of interface states wasalso observed.3,6

Williams,7 in 1965, showed that normal thermallygrown SiO2 possesses only small concentrations ofelectron traps, about 3 x 1014 cmW3 in his steam-grown oxides. In 1967, Goodman8 published the resultsof Hall-effect measurements which indicated an electronmobility of 29 cm2/V-sec in thermally grown SiO2. Withelectrons so mobile and so lightly trapped, attentionturned to the behavior of holes in SiO2. Section IIof this paper discusses the transport and trapping ofholes, Section III presents evidence concerning thegeneration of electron traps by ion implantation,electron irradiation, and exposure to corona, andSection IV is concerned with the effects of ionic con-taminants in SiO2.

II. Transport and Trapping of Holes

In any study of holes, the valence band structureis of primary interest. DiStefano and Eastman9 usedphotoemission spectroscopy to examine the valence-bandstructure of thermally grown SiO2 and found a narrowband of states near the valence-band edge which theyattributed to the nonbonding orbitals of the oxygen.

) TWO DIMENSOALREPRESENTAt b) SiO4 TETRAHEDRON

Fig. 1. SiO structure: (a) schematically, in twodimensions; tb) SiO4 tetrahedron in three dimensions.

Figure l(a) shows a two-dimensional representation ofSiO2, in which each silicon atom is bonded to fouroxygen atoms and each oxygen atom is bonded betweentwo silicon atoms. The bonding is partially covalentand partially ionic. The valence electrons are shownas black dots, and of the eight of these about eachoxygen atom, six can be considered to have been broughtby the neutral oxygen atom and one each by the twosilicon atoms on opposite sides of the oxygen. Theresult is an O- ion with four electrons in nonbondingorbitals. The two-dimensional representation of Fig.l(a) is misleading as to the actual three-dimensionalstructure, for the basic three-dimensional unit is anSiO4 tetrahedron as shown in Fig. l(b), and idealstochiometric SiO2 is formed by tetrahedrons joinedat their corners. This structure places an oxygenatom between each two silIcons in a three-dimensionalmanner. A further step toward realsim is shown inFig. 2, which is taken from DiStefano and Eastman.9The orbitals pictured here are based on those of anisolated 0O- ion, which has two electrons in aspherically symmetric s-type orbital (not shownexplicitly), four electrons in nonbonding p-typeorbitals, and two electrons in bonding p-type orbitals.Figure 2(b) shows the transition from the energy levelsof the 0O- ion to the valence bands of the solid.

r

\1 rSH~ ~ ~ ~~~ smm

d ION O IN D3d AMORPHOUS

(b) Si0t

Fig. 2. Bonding and energy levels of SiO2. FromDiStefano and Eastman.9

2144

* Work sponsored by the Defense Nuclear Agency andmonitored by the Naval Research Laboratory.

C,

-

zw

z

z0nA

-27 -24 -21 -18 -15 -12 -9 -6ELECTRON BINDING ENERGY (eV)

-3 O EvIq

Fig. 3. Emission spectra for SiO2. From DiStefanoand Eastman.9

Figure 3, also taken from the work of DiStefano andEastman,9 shows the results of ultraviolet photo-emission spectroscopy (UPS) and corresponding resultsobtained with X-rays (XPS) and soft X-rays (SXS). Theimportant point here is that the nonbonding orbitalscontribute a narrow band.of valence states near thevalence-band edge. Holes in this narrow band willhave a large effective mass and a small mobility, andmight even be trapped in the lattice. The questionimmediately arises whether holes in SiO2 have any

mobility whatever. Experiments on the transport ofholes in SiO2 are difficult to perform for severalreasons: the small mobility, the difficulty of con-

trolled injection, and the strong trapping which hasbeen shown to exist.

In 1971, Powell and Derbenwick 0 presented atthis Conference the results of a set of experimentsin which photons of slightly more than bandgap energy

were used to create hole-electron pairs near thefront surface of an MOS structure, and the holes were

drifted toward the substrate under the influence of a

positive field-plate voltage. They determined a

threshold energy of 8.8 eV, which agrees rather wellwith the bandgap value of 8.9 + 0.2 eV measured byDiStefano and Eastman.11 Powell and Derbenwick'sexperiments10 demonstrated that holes generated nearthe front surface are mobile in the oxide and drifttoward the Si-SiO2 interface. Strong hole trappingtakes place, as was shown by a large negative shiftof flatband voltage, and the location of the trappedholes was shown by etch-off experiments to bemainly near the Si-SiO2 interface. From an enhancedetch rate near the front surface and a reduction inphotocurrent with time, they concluded that positivespace charge also accumulated under the field plate.

Verwey12,13 has demonstrated that holes can beinjected into SiO2 from an avalanche created in a

reverse-biased p-n junction under the oxide. Theinfluence of a layer of Si3N4 on the observedcurrentl3 has not been determined.

Curtis, Srour, and Chiu14 have published resultson hole transport and trapping in which the holeswere obtained from hole-electron pairs generated inthe oxide by an electron beam of low enough energythat very few of the primary electrons reachedthe substrate. Additional results and interpretationwere presented at this Conference in 1974 by Srour,Curtis, and Chiu.15 Their incident electron beam was

applied in short bursts to avoid significantradiation damage. Several possible interpretationsof the results were considered. Their current vs.

0.7 0 -DO3

0.6A,,

0.5 0

b inAl Vb (kV

Q 0.A - A 0.105 um 0.051 4,/'0 o 0.28 0O015 6

0 0.62 0.015 8< 0.3 , SCHUBWEG MODEL

02 6

0

0.1 - r0a

O I8, I,1110 2 3

APPLIED FIELD (V/cml4 5xlo'

Fig. 4. Normalized current vs. applied field forthree thicknesses of SiO2. From Curtis, Jr., Srourand Chiu.14

applied field, reproduced here in Fig. 4, showed a

saturation charzcteristic that was suggestive of a

schubweg modell in which the mean distance that a

free carrier moves before permanent trapping or

recombination is given by PTE where 1i is the carriermobility, T is the mean free time, and E is theelectric field intensity. Saturation of the currentis obtained when the electric field is large enoughto remove essentially all of the carriers. With thismodel, however, thicker samples should require a largerelectric field for a given fraction of charge-carrierremoval, and the data did not show this thicknessdependence. Consequently, the authors proposed thatthe observed electric field dependence of the currentwas the result of either geminate recombination16 or

columnar recombination.17 When an electron-hole pairis formed, the two particles have a finite probabilityof recombining, and the probability of this initialor geminate recombination becomes smaller as theparticles are separated more quickly by application ofa greater electric field. The resulting currentshould increase as the electric field is increased andshould saturate when all carriers are separated.Columnar or track recombination occurs along a columnwhere carriers of opposite sign are concentratedafter generation along a track. Ausman and McLean18have succeeded in fitting a columnar recombinationmodel to the current vs. field data of Curtis a a.14It should also be observed that Kepler and Coppage19have explained the large energy required per electron-hole pair in anthracene on the basis of recombination,and that R. C. Hughes has used geminate recombinationto explain the results of radiation-induced transportin anthracene,20 PVC,21 and fused silica.22

Of particular interest is the ratio of trappedpositive charge to collected charge in the Srour,Curtis, and Chiu experiments,15 as is shown in Fig. 5.The trapped charge is presumably that of holescaptured near the Si-SiC2 interface. These resultswere obtained on oxides prepared by methods developedby Aubuchon for radiation hardening23 and presumablyshow hole trapping that is approximately at the state-of-the-art minimum.

R. C. Hughes24 has studied hole transport in steam-grown SiO2 by using short x-ray pulses to generatehole-electron pairs in the oxide of an MOS structure.He finds that the radiation-induced current can beseparated into two components: a "prompt" portion,presumably caused by the transit of electrons, and a

delayed portion caused by the transit of holes. Eachof these contributes half of the observed charge to

2145

2p 2pNOW-2s ~~~BONDING ~BONDIN

IPS

UPS

sxs

.,A0 0

/0 S /5,.,' ,'-oI

A NORTHROP, NOO * 550'O HUGHES, 500'

- A

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0cP/0

,o

1000°C OXIDESAPPLIED FIELD-10 V/uMr

a 105 rads(Si)

Qt/Qc '&VFBSINTER 0.26 7.5V

°C . 0.22 6.3°C * 0.09 4.0

-9- 1610COLLECTED CHARGE Qc (coulomb)

electron pairs, have reported that more positivecharge is stored at 80'K than at 300'K, and furthermorethat the additional charge stored at low temperature isqualitatively different from that stored at roomtemperature in that the former can be removed by photo-depopulation or by field emission whereas the latter isinsensitive to these stimuli. The storage of additionalcharge at low temperatures is in agreement with theresults of R. C. Hughes,24 who reports an apparentimmobilization of holes at 74°K.

In the interpretation of the results of experimentsin which hole-electron pairs are generated byirradiation with x-rays or high-energy particles, it isoften important to know the average amount of energyrequired for the production of such a pair. Fastelectrons in solids lose energy primarily to plasmons26which subsequently give up their energy in the forma-tion of hole-electron pairs.27 Ausman and McLean18have proposed that in SiO2, three plasmons, eachhaving an energy of 22.4 eV, decay on the averageinto four electron-hole pairs, and they estimate theenergy expended in this to be approximately 17 to 18eV per electron-hole pair. The results of Srourea aZ15 agree with this, and R. C. Hughes24 finds asimilar result when the pairs are formed by x-irradia-tion.

-)7

Fig. 5. Trapped positive charge vs. collected charge.From Srour, Curtis, Jr., and Chiu.15

the external circuit, as will be the case if allcarriers move completely to their respectivecollecting electrodes. The temperature dependence ofhole mobility, as determined from hole transit time,is shown in Fig. 6. The time dependence of the hole

20 2.2 2.4 26 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4

1iT (10 KFig. 6. Hole mobility as2determinedtime. From R. C. Hughes.

D. Y. Yang has conducted experiments in ourlaboratories on a slow instability in MOS structuresin which hole trapping near the negative interfaceapparently plays a key role.28,29 A similar in-stability has been observed by Osburn and Weitzman.30In these experiments we used thin (200-1000 A) fieldplates so that the joule heat evolved in theimmediate vicinity of a localized breakdown wouldvaporize the metal about this point; thus breakdownswere self-quenching. If a voltage of eitherpolarity is applied to the MOS structure just belowthe value which would cause immediate breakdown, asmall current initially flows which can be attributedto the tunneling of electrons from the negativeelectrode.30,31 The current gradually increases overa period of time, and this long-term instabilityeventually results in some self-quenched breakdowns(SQBD) and finally terminates in a series of rapidbreakdowns. The instability is found to be enhancedif the temperature is reduced, as is illustrated inFig. 7. With 190 volts applied to the sample thecurrent stabilizes, but with 195 volts applied(average E = 7.8 x 106 V/cm) cooling of the sampleresults in current instability and SQBD's as indicatedby the spikes. Positive charge is observed in theoxide when the instability is induced with the fieldplate positive. An exactly similar instability occurs

i0

9

(10 9amp)

8from transit

current after x-ray pulsing is qualitatively of theform that one would expect if the holes were movingthrough a wide distribution of shallow traps, but thebehavior cannot be fitted to a simple model.

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cooli a

_4 < _,,; ~~~~~AI(+) 195 V

/ - 50-C -75°C

' Cooliu - 50°C - 70'C-15°

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20 40 60 80Time min

100 120 140 160

2

The temperature dependence of positive chargestorage in SiO2 is of interest. Harari, Wang, andRoyce ,25 using x-ray excitation to produce hole- Fig. 7. Illustrating the slow instability.

2 146

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z 9E 100

a

r-

LA

C- 160*1cr:

10110'

-o5-1

-1 .,I-

Fi I

III. Generation of Electron Traps

Si(NEG)

SiO2

Al(POS)

Fig. 8. Interpretation of the slow instability.

when the field plate is negative, but no chargestorage is observed, presumably because the storedpositive charge is immediately under the field plate.

The observed results can be explained as shownin Fig. 8, where the field plate is positive forpurposes of illustration. Application of asufficiently large voltage causes Fowler-Nordheimtunneling of electrons from the silicon substrateinto the oxide. Mose of the electrons are thermalizednear the bottom of the conduction band, but a fewacquire sufficient kinetic energy to cause hole-elec-tron pairs by impact ionization. The holes whichescape recombination drift to the Si-SiO2 interfacewhere some of them are trapped. The resulting positivespace charge enhances the tunneling of electronswhich have the following competing effects: they (a)tend to recombine with the trapped holes, and (b) pro-duce more hole-electron pairs. Instability ariseswhen the production and trapping of holes dominatesover recombination. A similar process has beenproposed by DiStefano and Shatzkes32 for dielectricbreakdown. Powell,33 in investigating the transportof photo-generated holes in SiO2, finds that atsufficiently large fields the total current is con-siderably in excess of the hole current, and he alsointerprets this in terms of the trapping of holes andthe resulting enhancement of Fowler-Nordheim tunnelingof electrons. From the similarity between the break-downs that we produce with the two polarities ofvoltage we deduce the existence of hole traps underthe field plate, and Powell33 also finds evidence forhole trapping in this region from a reduction in holephotocurrent at low electric fields as time goes on.The increased instability that we observe as thetemperature is lowered may be the result of greaterhole trapping,24'25 or larger electron mobility22 atreduced temperatures.

The atomic nature of the hole traps at the Si-SiO2 interface has not been determined. As Fowkes34has pointed out, oxygen vacancies in SiO2 should havethe proper electronic activity to act as hole traps,and one would expect the vicinity of the Si-SiO2interface to be a region of oxygen deficiency, result-ing in the production of E' type defect centers.35One may speculate that an oxygen deficiency might alsobe present in the oxide under an aluminum field platebecause of chemical bonding between aluminum andoxygen atoms, thus giving rise to hole traps in thatregion also.

Although normal thermally grown SiO2 has onlysmall concentrations of electron traps7 and electrontrapping is ordinarily neglected, nevertheless thereare various conditions of use, or one might saymisuse, which can generate large densities of electrontraps in SiO2, and the effects of the resultingnegative space charge can be considerable. Nicollian,Berglund, Schmidt, and Andrews36 have reported onelectron traps which appear when water is diffusedinto dry thermal SiO2. Here we describe observationsof the generation of electron traps in SiO2 by ionimplantation, by irradiation with a nonpenetratingelectron beam, and by exposure to ions from a negativecorona discharge.

In a study done in our laboratories, N. M.Johnson37 used photoelectric and MOS capacitance-voltage techniques to investigate electron trappingin 1400-A dry-grown silicon dioxide films that hadbeen implanted with aluminum ions at 20 keV to afluence of 1014 cm2. Photoinjection of electronsfrom the substrate into the implanted oxide resultedin the buildup of negative space charge, as is shownby the C-V curves of Fig. 9. Here the flatbandvoltage has shifted in the positive direction until itis approximately equal to the sample voltage that hadbeen applied during photoinjection (10 V in thisexample), indicating that the negative space chargeof the trapped electrons had been sufficient to bringthe electric field at the Si-SiO2 interface approx-imately to zero during the photoinjection. Corres-ponding to this, the photocurrent fell to a smallvalue which could be attributed to stray photoemissionfrom other surfaces in the sample chamber, as shown inFig. 10. Data analysis indicated that essentially allinjected electrons had been trapped in the oxide andthat the transient response of the current could beexplained by the electric-field dependence of thephotoinjection, as given by Berglund and Powell.38 Incontrast with the heavy electron trapping found inthe AZ-implanted oxides, the electron trapping inunimplanted control samples was found to be negligible.The negative space charge in the implanted samplescould be annealed either optically by photons ofenergy exceeding 4 eV or thermally at 350°C. A 600°Canneal for 30 min substantially reduced the concentra-tion of electron traps, indicating that a substantialfraction of the traps was associated with displacementdamage introduced by the ion implantation.

C. T. Shih39electron beam tooxides and finds

C(pF)T°i°0TIME (min) =O

-2

has used a low energy (1-5 kV)irradiate rather thick dry-grownthat electron traps are generated

STEAD2Y2 3 4 6 8 10 12 16 STATE

i,

2 4 6FIELD-PLATE BIAS(volts)

Fig. 9. Voltage shifts of C-V curves during electronphotoinjection, showing electron trapping in the AQ-implanted oxide. From Reference 37.

2147

0-1.2 8

i~~~~J ~~~10

10.8 2C) ~~~~~~~~12

160.4 STEADY STATE

- BACKGROUND LEVEL " -

0 5 10 15 20 35 40 45 50TIME (min)

Fig. 10. Current vs. time corresponding to Fig. 9.From Reference 37.

in the oxide beyond the range of the beam itself.Previous reports of negative charge trapping causedby electron-beam irradiation of SiO2 have been madeby Wells40 and by Thomas, Butler, Goldstein andParry.41 A typical result obtained by Shih is shownin Fig. 11 for a 4700 A oxide. Curve 1 is theinitial C-V curve taken at 1 MHz. Curve 2 shows theeffect of irradiation with an electron beam that 0

deposited most of its energy within the first 2000 Aof the oxide. The stretch-out of the curve indicatesthe presence of interface states. Curve 3 shows theresult of an anneal which removed most of the inter-face states. The effect of negative charge storage,presumably in electron traps, is now shown clearly.Curve 4 shows the result obtained by photoinjectingelectrons from the substrate into the SiO2. Some ofthe electrons were trapped, increasing the negativecharge storage. Curve 5, showing additional negativecharge storage, was obtained by an additional photo-injection of electrons, this time from the metalfield plate. As is shown by Curve 6, most of thenegative trapped charge could be annealed out at3500C. The traps themselves remained, however, andthey could be recharged by photoinjecting electronsagain, thus restoring the positive flatband shift.The structure of steady-state I-V curves and thevoltage dependence of electron photoinjection currentsindicated that the electron trapping was in the bulkof the oxide and that a substantial portion of itwas located well ahead of the range of the irradiatingelectrons. The saturated concentration of the elec-tron traps was found to be in excess of 1017 cm-3.

A convenient technique for studying high-fieldphenomena in insulators in a nondestructive way isto use ions extracted from a corona discharge tocharge the unmetallized surface of the insulator andin this way induce the injection and transport ofcarriers.42-44 The first observation to be noted isthat when the surface of an SiO2 film is charged withpositive ions obtained from a corona discharge in air,a current is observed to flow through the oxide whichcan be attributed to the tunneling of electrons fromthe silicon substrate into the oxide, subsequenttransport of the electrons through the oxide, andneutralization of the ionic charge at thesurface.30'43'44 Storage of charge in the oxide isfound to be negligible under the conditions which wedescribe here. The second observation is that whenthe Si02 surface is charged with negative ions, acurrent flows through the oxide and, in addition, abuildup of positive charge is found, presumablycaused by holes trapped near the Si-SiO2 interface.44This is displayed by the shift from Curve 1 to Curve 2in Fig. 12. H. S. Lee of our laboratories has madethe following additional observation: If negativecharging is followed by positive charging, the flat-band voltage actually shifts to a positive value, asshown by Curve 3 of Fig. 12. Apparently electronsare now trapped in the oxide. We take this toindicate that exposure to negative corona hasgenerated electron traps in the oxide. Negativecharge in these centers was not observed until thepositive charge (as shown by Curve 2 of Fig. 12) hadbeen neutralized by electrons injected by thepositive corona, in addition to which the trapsmight have been further populated from the electronsso injected. Insofar as the appearance of electrontraps after negative charging but not after positivecharging is concerned, it should be noted thatnegative charging produces a greater electric fieldintensity in the SiO2 than positive charging.43'44Woods and Williams have also seen evidence of negativecharge trapping in SiO2 after negative coronacharging of the surface.45

It is not surprising that ion implantation, withits displacement damage and introduction of impurityatoms, results in the production of charge-trappingcenters in SiO2. The generation of electron trapsahead of an electron irradiating beam and by surfacecharging with negative ions is, however, anothermatter. The nature of the traps so produced has not

400

C(pf)

300F

SAMPLE HIH,81#24700 SiO ,15OAA

(D INITIAL CURVE

(AFTER VACUUM ANNEALAT 350°C, 2hrsLEADS SHORTED(POST - IRRADIATION)

®DAFTER VACUUM ANNEALAT 300°C,IhrLEADS SHORTED(POST- IRRADIATION)

_30 -20

/I

POST- IRRADIATIONE,=4.5 KVVg 150 volts

| | / - 7X10"ELECTRONS/cM2_ INTERNAL PHOTOINJECTION

AT ROOM TEMP. FOR 30 min,Vg =+20 volt, hV= 5eV

//// INTERNAL PHOTOINJECTION

AT ROOM TEMP. FOR 5 min.// // Vg= 220 volt. hV = 5eV

10 0 10 20 30 40Vg (volts)

50 60

VOLTS

Fig. 12. Evidence concerning the generationof electron traps in SiO2 when the surface ischarged by negative ions from a corona discharge.The positive corona merely causes an injectionof electrons into the SiO2 and brings the electrontrapping into evidence.

2148

Fig. 11. Evidence of electron trapping in SiO2irradiated by a nonpenetrating electron beam.

Il -I

been determined, and questions which immediately cometo mind are whether the two are similar in physicalnature and whether they can be created by still othermeans. The steady-state concentration of electrontraps observed after electron irradiation,t 1017 cmn3, is great enough to have a major influenceon radiation charging and on breakdown characteristics.

IV. Ionic Contaminants

Although the sodium contamination of SiO2 filmsis generally kept under good control in the fabricationof devices, the effects of ionic contaminants cannotalways be ignored. Woods and Williams46 have observedthe transport of Na+ and Li+ ions through SiO2 evenat room temperature. McCaughan and Murphy,47McCaughan, Kushner and Murphy,48 and H. L. Hughes,Baxter and Phillips49 have shown that bombardment ofthe surface of thermally grown SiO2 with low-energypositive ions can release sodium ions and cause themto migrate through the oxide toward the Si-Si02interface. Williams and Woods50 have demonstratedthe mobility of fluoride ions in SiO2 at room temper-ature. DiStefano51 and Williams and Woods52 have usedscanning internal photoemission measurements to showthat sodium ions near the Si-SiO2 interface aredistributed in a laterally nonuniform manner, andDiStefano53 has shown that the nonuniform distributionaffects the breakdown properties of the insulator. Adistinguishing characteristic of Na+ ions near theSi-SiO2 interface is that their positive charge cannotbe annealed out by an injection of electrons into theregion, whereas a positive charge caused by trappedholes can be annealed in this manner.54

Thermal SiO2 films with superior radiationhardness, such as those reported by Aubuchon,23 canbe fabricated by ultra-clean growth in a restrictedrange of temperatures followed by proper annealing.These oxides show a comparatively small amount ofhole trapping near the Si-SiO2 interface.15 The roleof contaminants in the formation of hole traps inSiO2 remains to be determined.

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17.

18.

G. Jaffe, Annalen der Physik 42, 303 (1913).

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29. D. Y. Yang, W. C. Johnson, and M. A. Lampert,"A Study of the Dielectric Breakdown of ThermallyGrown Si02 by the Self-Quenching Technique,"1975 IEEE International Reliability PhysicsSymposium. To be published in the 13th AnnualProceedings on Reliability Physics.

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2 1 49

37. N. M. Johnson, W. C. Johnson, and M. A. Lampert,J. Appl. Phys. 46, 1216 (1975).

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44. Z. A. Weinberg, "High-Field Transport in SiOFilms on Si Induced by Corona Charging," Ph3.Dissertation, Department of Electrical Engineering,Princeton University (Sept. 1974).

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