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Atomic and Electronic Structures of Traps in SiliconOxide and Silicon OxynitrideVladimir Gritsenko a & Hei Wong ba Institute of Semiconductor Physics, Siberian Branch of Russian Academy of Sciences,Novosibirsk, Russiab Department of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong

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To cite this article: Vladimir Gritsenko & Hei Wong (2011): Atomic and Electronic Structures of Traps in Silicon Oxide andSilicon Oxynitride, Critical Reviews in Solid State and Materials Sciences, 36:3, 129-147

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Critical Reviews in Solid State and Materials Sciences, 36:129–147, 2011Copyright c© Taylor and Francis Group, LLCISSN: 1040-8436 print / 1547-6561 onlineDOI: 10.1080/10408436.2011.592622

Atomic and Electronic Structures of Traps in SiliconOxide and Silicon Oxynitride

Vladimir Gritsenko1 and Hei Wong2,∗1Institute of Semiconductor Physics, Siberian Branch of Russian Academy of Sciences,Novosibirsk, Russia2Department of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong

Silicon oxide (SiO2) and silicon oxynitride (SiOxNy) are two key dielectrics used in silicon devices.The excellent interface properties of these dielectrics with silicon have enabled the tremendousadvancement of metal-oxide-semiconductor (MOS) technology. However, these dielectrics arestill found to have pronounced amount of localized states which act as electron or hole trapsand lead to the performance and reliability degradations of the MOS integrated circuits. Abetter understanding of the nature of these states will help to understand the constraints andlifetime performance of the MOS devices. Recently, due to the available of ab initio quantum-mechanical calculations and some synchrotron radiation experiments, substantial progresshas been achieved in understanding the atomic and electronic nature of the defects in thesedielectrics. In this review, the properties, formation and removal mechanisms of various defectsin silicon oxide and silicon oxynitride films will be critically discussed. Some remarks on thethermal ionization energies in connection with the optical ionization energies of electron andhole traps, as well as some of the unsolved issues in these materials will be highlighted.

Keywords dielectric traps, electronic structures, silicon oxide, silicon oxynitride, gate dielectrics

Table of Contents

1. INTRODUCTION .............................................................................................................................................. 130

2. THE MOTT OCTAHEDRAL RULE IN TETRAHEDRAL SILICON-BASED COMPOUNDS ........................... 131

3. OXYGEN VACANCY ......................................................................................................................................... 1313.1. Atomic and Electronic Structures of Neutral Oxygen Vacancy .......................................................................... 1313.2. Formation Mechanism of Si-Si Bonds in Silicon Oxide .................................................................................... 1323.3. Neutral Oxygen Vacancy as a Hole Trap ......................................................................................................... 1343.4. Formation of Si-Si Bonds in Re-oxidized Silicon Oxynitride ............................................................................ 1383.5. Positively Charged Oxygen Vacancy as an Electron Trap ................................................................................. 1393.6. Neutral Oxygen Vacancy as an Electron Trap .................................................................................................. 139

4. OXYGEN DIVACANCY ..................................................................................................................................... 1404.1. Atomic and Electronic Structure of Neutral Oxygen Vacancy, or ≡Si-Si-Si≡ Defect .......................................... 1404.2. Oxygen Divacancy ≡Si-Si-Si≡ as a Hole Trap ................................................................................................ 1404.3. Positively Charged Oxygen Divacancy as an Electron Trap .............................................................................. 141

5. SILICON CLUSTERS IN SiO2 AS ELECTRON AND HOLE TRAPS ............................................................... 141

∗E-mail: [email protected]

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130 V. GRITSENKO AND H. WONG

6. DOUBLY-COORDINATED SILICON ATOM (=Si:) .......................................................................................... 143

7. OXYGEN ATOM WITH AN UNPAIRED ELECTRON (≡Si-O·) AS AN ELECTRON TRAP ............................ 143

8. DOUBLY-COORDINATED NITROGEN ATOM WITH AN UNPAIRED ELECTRON, ≡Si2N· ......................... 144

9. SUMMARY ........................................................................................................................................................ 145

ACKNOWLEDGMENTS .......................................................................................................................................... 145

REFERENCES ......................................................................................................................................................... 145

1. INTRODUCTIONSilicon oxide (SiO2) and silicon oxynitride (SiOxNy) had

been the key dielectrics in silicon integrated circuit technology(IC) technology for many decades. Because of their low densityof states and large band offset energies when interfacing withthe silicon substrate together with the high dielectric breakdownfield, they had been used as the gate dielectrics for metal-oxide-semiconductor (MOS) transistors for many decades. However,the low densities of oxide charge and interface trap still causedsome pronounced performance and reliability degradations ofthe MOS devices and had attracted much attention in the pastfew decades. The charges injected into these dielectrics wouldresult in the charge trapping and causes threshold voltage shiftin the MOS transistors. The requirement for operational sta-bility of a MOS transistor is rather stringent in the modernMOS technology because of the small device size, large sys-tem scale, and long (10 years) operation lifetime. For instance,the threshold voltage shift of MOS transistors in a micropro-cessor should not exceed 10 mV for a 10-year continuous op-eration. Yet the significance of using these dielectrics in theMOS devices has been reduced recently, but the instability is-sues of these dielectric films become even more important. Inthe state-of-the-art nanoscale MOS technology, the gate dielec-tric has been replaced by a higher dielectric constant (high-k)material such as Al2O3 (k ≈ 10), HfSiOx (k ≈15), HfOxNy(k ≈15), HfAlOx (k ≈15), HfO2 (k ≈ 25), and ZrO2 (k ≈ 25)in order to have better control of the channel current with thedesirable gate voltages and to suppress the gate leakage cur-rent.3–6 However, high-k materials are often found to have poorinterface properties with silicon substrate.6 With this connec-tion, an ultrathin layer (about 0.5 nm thick) of thermal oxide oroxynitride was grown before the high-k deposition. Thus, thequality of this ultrathin silicon oxide or oxynitride laryer wouldhave even greater impact on the reliability of MOS integratedcircuits.

A better understanding of the electronic properties of siliconoxide is also important to the IC fabrication process. One of theimportant methods for making the advanced silicon-on-insulator(SOI) wafers is the SIMOX (separation by implantation of oxy-gen) process. In this process, the initial Si wafer was implanted

with a high dose (>1018 cm−2) of high-energy (>100 keV)oxygen ions so as to from a thick buried oxide layer.7,8 Tore-crystallize the implanted surface, high-temperature post-implantation annealing needed to be conducted. As will bediscussed later, this process would result in the generation ofoxygen vacancies and give rise to some instability issues fordevices and circuits fabricated on it.

On the other hand, silicon oxide and oxynitride are stillwidely used as tunneling oxide in memory devices. A flashmemory element comprises a MOSFET whose threshold volt-age can be changed in a write cycle by injecting electrons orholes over the tunneling oxide into the storage medium suchas floating polysilicon9 or silicon nitride with high amount ofdeep traps.1,2 However, the charges accumulated on the electronor hole traps of the tunneling oxide can cause an undesirablethreshold shift of the memory transistors and results in the per-formance degradation of the flash memory. In addition, the highfield writing process also results in the generation of new elec-tron and hole traps in the tunneling oxide. This trap generationnormally involve some weak bonds or electrically inactive de-fect precursors in the dielectrics produced during the fabricationprocess. Finally, silicon oxide and silicon oxynitride are alsoused as core and cladding material for glass fibers and microwave guiding devices.10,11 The charge trapping causes opticalloss and needs to be minimized.11

Hence, a better understanding of the properties, the forma-tion and the removal mechanisms of the dielectric traps willhelp the process engineers to characterize and to improve thefabrication of these dielectric films. It will also help the de-vice engineers to understand the constraints, the performance,and probably to predict the degradation behaviors and then thelifetime, of devices as far as charge trapping events are in-volved. A systematic survey on the defect properties of oxyni-tride was presented.12 In recent years, substantial progress inthe understanding the atomic and electronic structure, and for-mation mechanisms, of defects in silicon oxide and oxynitridehad been achieved with the help of theoretical non-empirical abinitio quantum-mechanical calculations and some synchrotronexperiments. The purpose of this review is to provide a read-ily accessible archive that highlights the important results, in

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ATOMIC AND ELECTRONIC STRUCTURES OF TRAPS IN SILICON OXIDE AND SILICON OXYNITRIDE 131

relation to the atomic and electronic structure of intrinsic de-fects in silicon oxide and silicon oxynitride, reported recently.

2. THE MOTT OCTAHEDRAL RULE IN TETRAHEDRALSILICON-BASED COMPOUNDSThe atomic bonding in a solid is governed by the Mott rule

which relates the coordination number (CN) in a solid with itsnumber of valence electrons (n), i. e.,

CN = 8 − n [1]As a Si atom has four valence electrons given by s2p2, eachSi atom in the Si crystal, SiO2, or Si3N4 would occupy onesite in the tetrahedral coordination and is coordinated to foursilicon, four oxygen, or four nitrogen atoms, respectively, inthese substances. An oxygen atom has six valence electrons(s2p4), thus each oxygen atom bridges with two silicon atomsin SiO2. A nitrogen atom has five valence electrons (s2p3) andhence it is connected to three Si atoms in the Si3N4. Siliconoxynitride consists of both Si-O and Si-N bonding. It was shownthat in a broad range of compositions, from SiO2 to Si3N4,Si-O and Si-N bonds in silicon oxynitride form five types oftetrahedral (SiOνN4-ν , ν = 0, 1, 2, 3, 4) and the distributionof these tetrahedral is governed by the binomial distributionfunction.13,14 In line with the Mott octahedral rule, in SiOxNy,each oxygen atom bridges with two Si atoms, similar to that inSiO2, and each nitrogen atom is coordinated by three Si atomsas similar to Si3N4. The Mott rule provides a clue to understandthe atomic structures and the formation mechanisms of defectsor traps in silicon-based tetrahedral compounds. Based on theMott rule, Gritsenko and coworkers13 proposed a definition ofa perfect material. That is, if a compound that did not containany defect deviating from regular coordination numbers of thecontaining atoms, then the material is said to be defect-less.13

According to this definition, defects in SiO2, Si3N4, and SiOxNycan be exemplified by (a) paramagnetic centers such as ≡SiO·,≡SiOO·, and ≡SiN·; (b) diamagnetic centers such as ≡Si-Si≡,= Si:, and ≡Si2NH; (c) neutral defects include ≡SiOH, ≡Si-Si≡, and ≡Si·; (d) charged defects ≡SiO: and ≡Si+·Si≡; (e)intrinsic defects ≡SiOO·,≡SiN·, and ≡Si-Si≡; and (f) impuritydefects ≡Si2NH, ≡SiOH, and ≡SiH. In the above designations,the notation (–) denotes a regular chemical bond formed bytwo electrons with opposite spin directions, and the notation (·)denotes an unpaired electron. Detail structures, characteristicsand formation mechanisms of these defects will be discussed inSections 3 to 8. A summary together will some further remarkswill be given in Section 9.

3. OXYGEN VACANCY

3.1. Atomic and Electronic Structures of NeutralOxygen Vacancy

The oxygen vacancy, ≡Si-Si≡, or Si-Si bond in short, insilicon oxide can be formed on the removal of an oxygen atom

from the Si-O-Si bridge according to the following reaction:

≡Si-O-Si≡→≡Si-Si≡ +O [2]An oxygen vacancy can also be formed when an oxygen atomjumped to the neighbor site during the formation of peroxideradical ≡Si-O-O-Si≡, i.e.

≡Si-O-Si-O-Si≡→≡Si-Si-O-O-Si≡ [3]It was reported that the inter-atomic distance between the Siatoms in the Si-O-Si bond in amorphous SiO2 is 3.05 Å14 andthe equilibrium length of the neutral ≡Si-Si≡ bond in SiO2is about 2.5 Å15–17 which is slightly larger than the length ofthe Si-Si bond (2.35 Å) in amorphous silicon. Mukhopadhyavyet al.18 examined the length distribution of neutral Si-Si bondsin amorphous silicon oxide and found that it varies from 2. 3 to2.7 Å.

The oxygen vacancy in silicon oxide is a neutral diamagneticcenter. The electronic structure of the neutral O3≡Si-Si≡O3bond in SiO2 was examined by quantum-mechanical calcula-tions.15–20 The binding σ -orbital in the Si-Si bond are found tolie close to the top of the valence band, whereas the antibindingσ ∗-orbital are 7.6 eV above the energy position of the bindingorbital. A binding σ -orbit is occupied by two electrons with op-posite spins whereas the antibinding σ ∗-orbit is empty. Similarabsorption peak at 7.56 eV was also found in disilane moleculeSi2H6 which has a Si-Si bond in it.21

According to the quantum-mechanical calculations, the Si-Si defect in SiO2 gives rise to an absorption band at 7.6 eV.By performing thermal annealing of quartz glass in a hydro-gen ambient, Hosono, Abe, and Imagawa et al. suggested thatthis band should be due to the Si-Si bonds.22 Figure 1 showsthe absorption spectra of SiO2 samples annealed in hydrogen

FIG. 1. Observation of 7.6 eV absorption peak for SiO2 withoutannealing (solid curve), with 1000◦C in hydrogen for 1 hour(dash curve), and with 800◦C annealing in hydrogen for 1 hour(dotted curve). (Reprinted with permission from Hosono et al.22

Copyright 1991: American Physical Society.)

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FIG. 2. The reduction of 7.6 eV absorption coefficient in-creases linearly with the increase of the absorption coefficientfor 2260 cm−1 band due to Si-H vibration. (Reprinted withpermission from Hosono et al.22 Copyright 1991: AmericanPhysical Society.)

ambient. The absorption at 7.6 eV became weaker after hydro-gen annealing. However, the reduction in the absorption at 7.6eV was found to be less pronounced at an annealing temperatureof 800◦C than the one annealed at 1000◦C. An explanation tothis behavior was proposed.22 It was found that the reduction ofthe 7.6-eV absorption band was accompanied by an increase ofthe IR absorption at 2260 cm−1 due to vibrations of Si-H bonds(see Figure 2). Electron paramagnetic resonance (EPR) studyshowed that no EPR signal due to S-Si bonds was observed inboth non-annealed and hydrogen-annealed samples. These ex-perimental findings suggest that the reduction in the absorptionintensity at 7.6 eV was due to the broken of Si-Si bonds forhydrogen incorporation via the following reaction:

≡Si-Si≡ +H2 → ≡Si-H + H-Si≡ [4]

This reaction explains the increase of the absorption at2260 cm−1 because of the increase in Si-H bond density. Basedon the quantitative analysis on the absorption due to Si-Si andSi-H bonds, a linear correlation between the 7.6 eV absorptionand 2260 cm−1 band was obtained.22 And this correlation furthersuggests that the 7.6-eV absorption band in SiO2 is due to the ex-citation of electron from the σ binding orbital to σ ∗ antibindingorbital of the Si-Si bond. This conjecture is further confirmed byexperiments conducted in oxygen-rich oxide film.23 As shownin Figure 3, for samples being annealed in oxygen, the intensityof the 7.6-eV absorption band was reduced. This behavior canbe attributed to the reduction of Si-Si bond concentration as aresult of oxidation, i.e.,

2≡Si-Si≡ + O2 → ≡2Si-O-Si≡ [5]

By exciting a quartz glass with a light source within the 7.6eV absorption band, a photoluminescence (PL) band peakingat about 4.4 eV emerges.24–26 Figure 4 shows the PL spectrumof a quartz sample and the suggested the configuration diagramfor explaining the optical transitions with 7.6 eV PL excitationis shown in Figure 5. The large Stokes shift suggests a strongelectron-phonon coupling during the optical transitions in the Si-Si bonds. According to the configuration diagram, the Franck-Condon shift, or the polaron energy, amounts to Wp = (7.6 −4.4)/2 = 1.6 eV. As will be discussed later on, this polaronenergy coincides well, within the experimental accuracy, withthe energy of the electron and hole traps in SiO2.

3.2. Formation Mechanism of Si-Si Bonds in Silicon OxideThe oxygen vacancy in silicon oxide was observed in samples

irradiated with ions, neutrons, photons, or γ quanta.27 Hosonoand Matsunami28 showed that Si-Si bonds formed in SiO2 lay-ers with Li+, N+, O+, F+, Si+, or P+ ions implantation. Si-Sibonds are formed in SiO2 with ion irradiation via reaction (2).The existence of the Si-Si bonds can be detected through theobservation of the 7.6 eV optical absorption.28

Garrido et al. used x-ray photoelectron spectroscopy (XPS)to investigate the radiation defects in SiO2 films by using Ar+

ion irradiation.29 In the irradiated samples, a broadening of the Si2p atomic level towards the lower binding energy side, accom-panied with a decrease of the signal intensity, was observed (seeFigure 6). A similar phenomenon was also observed in Si-richSiOx films.30,31 SiOx consists of five type of tetrahedral: SiO4,SiSiO3, SiSi2O2, SiSi3O, and SiSi4.31 The central Si atoms inSiOυSi4-υ tetrahedral for υ = 4, 3, 2, 1, and 0 are, respectively,in Si4+, Si3+, Si2+, Si+, and Si0 bonding state and each oxygenatom in the SiOx bridges with two Si atoms. In SiO2, the oxygenand silicon atoms carry -1.0e and +2.0e charges, respectively.Thus the net charge transfer per one Si-O bond is equal to 0.5e.32

The broadening of the Si 2p peak in irradiated silicon oxide witha large irradiation dose (see Figure 6) can be attributed to theincrease of number of tetrahedral with larger amount of Si-Sibonds.

It was found in a 230 nm thick wet oxide with 50 keV B+

ion irradiation that the IR vibrational spectra exhibited severalsignificant changes.31 By increasing the irradiation dose, a redshift and peak broadening the absorption were observed. Thered shift of the absorption maximum was due to the replace-ment of Si atoms in the SiOυSi4-υ tetrahedral with O atoms. Inquartz samples with B+ ion implantation, red shift of the fun-damental absorption edge was also found33 A similar behaviorwas also observed in Si-rich SiOx films.31,34 The red shift of thefundamental absorption edge in SiOx can also be attributed tothe increase of Si-Si bond concentration. That is, the IR spec-troscopy, the photoelectron spectroscopy, as well as the opticalabsorption spectra all together confirmed that oxygen vacancieswill be formed in ion-irradiated silicon oxide films.

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FIG. 3. Infrared absorption spectra of SiO1.98 samples annealed in oxygen at different temperatures. (Reprinted with permissionfrom Hikmott,23 Copyright 1971: American Institute of Physics.)

FIG. 4. Photoluminescence spectra due to 7. 6-eV excitationand the photoluminescence excitation spectra (PLE) of quartzglasses irradiated with neutrons. The spectra were measuredat 15 K. (Reprinted with permission from Gee and Kastner,24

Copyright: American Physical Society.)

Figure 7 shows the XPS spectra of a 6 Å thick SiO2 layergrown on a (100) Si substrate. The excitation source was syn-chrotron radiation with an energy of 130 eV. The spectra wereregistered at two synchrotron radiation angles: 0◦ and 60◦.35

FIG. 5. Configuration diagram for optical transitions of defectsin SiO2 as a result of photoluminescence excitation with energyin the 7.6 eV absorption band.

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FIG. 6. Photoelectron spectra of the Si 2p level in as-grown (trace a) and 130-keV Ar+ ion irradiated with dose of 3.2 ×1013 cm−3(trace b), 3. 2 × 1014 cm−3 (trace c), and 3.2 × 1016 cm−3 (trace d). The vertical line shows the position of the Si 2plevel in Si. (Reprinted with permission from Garrido et al.29 Copyright: Elsevier.)

This thickness is in the range of thinnest bulk silicon oxidethat often used as inter-layer for other kinds of dielectrics de-posited on silicon.6 In the spectra given in Figure 7, the energyis reckoned from the position of the atomic Si 2p peak in the Sisubstrate (SiSi4 tetrahedron). The peak at the energy of 26.5 eVis due to the SiSi4 tetrahedral in the Si substrate and the shapeof this peak is defined by the spin-doublet splitting. The Si4+

peak at 23 eV is due to the SiO4 tetrahedral in the stoichiometricSiO2. In addition to these peaks, three peaks, due to Si3+, Si2+,and Si1+ were observed at intermediate energies which wereresulted, respectively, from the suboxides, SiSiO3, SiSi2O2, andSi3SiO tetrahedral presented at the Si/SiO2 interface. These ex-periments unequivocally imply the presence of excess silicon inthermal oxide in the vicinity of Si/SiO2 interface. The observa-tion of SiSiO3 tetrahedral implies the presence of O3≡Si-Si≡O3bonds (oxygen vacancies) in the samples, while the existenceof SiSi2O2 tetrahedral is an indication of the presence of oxy-gen divacancies O3≡Si-Si-Si≡O3. That is, an oxide film shouldcontain some excess silicon atoms in the vicinity of Si/SiO2interface. An independent confirmation for the existence of ex-cess silicon in the form of O3≡Si-Si≡O3 and O3≡Si-Si-Si≡O3defects was done by Naga, Miyata, and Moriki et al.36 Forthermally grown oxide films with 14–45 Å thick, synchrotronradiation tests on the films yield some similar optical absorptionbands. As mentioned earlier, the O3≡Si-Si≡O3 defects in SiO2exhibit an absorption band at 7.6 eV.15–17 Figure 8 shows the op-

tical absorption spectra of some thin thermal oxides on siliconextracted from the measured reflectance spectra. The absorp-tion at 7.8 eV due to Si-Si defects is obvious and the estimatedconcentration for this defect is about 8×1014 cm−2. Additionalabsorption at 6.5 eV, related to the oxygen divacancy, is alsoobserved.

3.3. Neutral Oxygen Vacancy as a Hole TrapWhen a hole is captured by a ≡Si-Si≡ bond, an E′ center

forms:

≡Si-Si≡ + h → ≡Si·+Si≡ . [6]This positively-charged paramagnetic defect, ≡Si·+Si≡, has

an unpaired electron. There exists a whole family of E′ cen-ters.37 Note that the above definition for the E′ center is notunique. Sometimes a threefold-coordinated Si atom with an un-paired electron, ≡Si·, is also considered as an. E′ center. Theelectronic structure of E′ has been a hot subject for theoreticalstudies.15,17,19,38–41 When a hole is captured by a ≡Si-Si≡ bond,the positively charged silicon atom, +Si≡, shifts towards theplane where the three oxygen atoms lie, while the paramagneticsilicon atom with an unpaired electron, ≡Si·, shifts towards thepositively-charged silicon atom side (see Figure 9).17

The hole trapping mechanism of Si-Si oxygen vacancy wasconfirmed experimentally by Withan and Lenahan.42 Figure 10

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FIG. 7. Photoelectron spectra of Si 2p atomic level of a Si/SiO2 structure with oxide layer of 6 Ǻ thick measured at 0◦ and 60◦

incidence angles. The excitation energy was 130 eV. (Reprinted with permission from Himpsel and McFeely,35 Copyright 1988:American Physical Society.)

shows the capacitance-voltage (C-V) characteristics of a MOSstructure. It demonstrated that the hole injection caused the C-Vcharacteristics being shifted to the negative side. The positivecharges accumulated in the oxide results in an EPR signal withgyromagnetic factor g = 2.0004. This EPR signal was identifiedas arising from the hole trapping at Si-Si bonds according toreaction (6).

Electron injection into oxide leads to the neutralization of thepositive charges and makes the EPR signal to be disappeared(see Figure 10). Figure 11 shows the surface density of E′ cen-ters as a function of trapped hole surface density.43 The slope ofthe curve indicates that the positive charge of E′ centers. Layer-by-layer etching experiments showed that most of the E′ centerswere localized in the oxide near the Si/SiO2 interface. Figure 12

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FIG. 8. Optical absorption spectrum of thermal oxide on Sishowing shoulder energies of 6.5 eV, 7.8 eV, and 8.8 eV.(Reprinted with permission from Terada et al.36 Copyright 1992:American Physical Society.)

plots the density of the E′ centers and the flat-band voltage asa function of annealing temperature during isochronal anneal-ing.43 The dotted curve in Figure 12 shows the kinetic data forE′ centers in the annealed samples and the activation energy ofthe process falls in the range of 1.0 to 1.8 eV according to thefirst-order kinetics given below:

N = No exp[−νt exp(−Wt/kT )] [7]where, Wt is the trap activation energy, t is the annealing time,and ν is the frequency factor. Expression (7) yields a stronger de-pendence in comparison with the experiments. The experimentaldata, however, can still be fitted fairly well with expression (7)

FIG. 9. Structure of the oxygen vacancy (Si-Si bond) in a SiO2film with different charge states: (a) neutral, (b) positively-charged, and (c) negatively-charged.

by assuming a Gaussian distribution of activation energies:

ρ(W ) = 1(2π )1/2

exp[−(W − Wt )2/�W 2] [8]

Figure 12 also shows the predicted dependence calculated byusing Wt = 1.6 eV, �W = 0.3 eV, and ν = 1013 sec−1.

The energy spectrum of hole traps in SiO2 was also studied.The activation energy of a hole trap (Wt) was found to be about1.44±0.2 eV which was determined from the threshold volt-age versus temperature plot.44 In this work, we assumed ν =1013 sec−1, the energy of the hole trap of about 1.6 eV can beobtained from Equ. (7). This value coincides with the polaronenergy obtained from luminescent data taken from samples withexcited Si-Si bonds (see Figure 5).

Miller et al.45 examined the energy distribution of hole trapsin a 350 nm thick thermal silicon oxide by using thermallystimulated current measurements. Two peaks were observed inthe hole trap distribution. By assuming ν = 1014 sec−1 in theirwork, the high-energy peak is at about 1.85 eV, whereas thelow-energy peak is at about 1. 2 eV. Figure 13 shows the energydistribution of hole traps in thermal silicon oxide.45 The high-energy peak due to hole emission from oxygen vacancies isat about 1.5 eV and the low-energy peak which is tentativelyattributed to the emission of holes from oxygen divacancies isat about 0.9 eV.

Manzini and Modelli examined the tunneling ionization ofhole traps in silicon oxide.46 By assuming the hole mass ofmh∗ = 0.42m0, the energy of the hole trap in thermal siliconoxide (Whopt) was estimated to be 3.1 eV. According to themulti-phonon trap ionization theory, the energy for tunnelingionization should be equal to the optical energy, Whopt.47 Thusthe optical ionization energy should be about twice of the ther-mal ionization energy, i.e., Whopt ≈ 2Wt. A similar relation foroptical and thermal ionization energies of hole traps was alsoobserved in amorphous silicon nitride Si3N4.48 Figure 14 showsthe field dependence of the hole capture cross-section of neutraltraps (Si-Si bonds).49 As the electric field in the oxide increasesfrom 105 to 5 × 106 V/cm, the capture cross-section decreasesfrom 10−12 cm2 to 10−13 cm2.

Powell and Berglund50 used the internal photoemissionmethod to investigate the spatial localization of positively-charged traps in oxide layers. It was found that the Si-Si bondswere localized in silicon oxide layers at a distance of about 2nm from the Si-SiO2 interface. It can be deduced that most ofthe oxygen vacancies (Si-Si bonds) in the thermal oxide layersshould be localized within a thin interfacial region near the Sisubstrate. The presence of oxygen vacancies near the interfaceis an intrinsic property of thermal silicon oxide. Oxygen vacan-cies and divacancies, or excess silicon, are formed due to theinsufficient oxidation as a result of less sufficient supply of oxy-gen species which diffuse from the surface during the thermaloxidation of process.

Oxygen vacancies (Si-Si bonds) were also found in SIMOXstructures which were produced by oxygen ion implantation

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FIG. 10. Capacitance-Voltage characteristics of a Si-SiO2-Al structure at (a) initial state, (b) after hole injection, (c) after electroninjection following the hole injection. The insert on the top shows the EPR signals for the corresponding charging states. (Reprintedwith permission from Witham and Lenahan,42 Copyright: American Institute of Physics.)

following with a high-temperature annealing.7,8 Paullet et al.7

examined the charge accumulation in Si/SiO2/Si structures witha 200 nm thick oxide layer being annealed at 1320◦C in inertgas. The oxide layer was then irradiated with an x-ray after thecovering polysilicon layer was removed. By plotting the accu-mulated charge density as a function of the irradiation dose,8

the hole capture cross-section of the neutral trap was determined

and the value is about 3.6 × 10−13 cm2. This result suggestedthat the oxygen vacancy is also an intrinsic defect in the ox-ide of SIMOX structures. The formation of oxygen vacanciescan be attributed to reaction (2) taken place during the high-temperature annealing in inert gases. Note that both electronand hole accumulations were observed in MOS structures withSi+ ion implantation into the gate oxide.51 Figure 15 shows the

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FIG. 11. Surface density of E′ centers plotted as a func-tion of trapped hole density. (Redrawn with permission fromLenahan and Dressendorfer,43 Copyright 1984: American Insti-tute of Physics.)

kinetics of electron and hole accumulation in a MOS structurewith 3 × 1016 cm−2 Si ions being implanted into the gate oxide.Here the threshold voltage of the MOSFET was measured as afunction of the pulse width for charge pumping measurements.The accumulated charge density increases with both the pulsewidth and amplitude, in the similar manner as those in metal-

FIG. 12. Effects of 70 min isochronal annealing on an irradi-ated MOS structure. Markers are data taken from Lenahan andDressendorfer.43 The round markers show the values that areproportional to the EPR signal due to E′ centers, and the squaresshow the values that are proportional to the positive charge. Themaximum values of both quantities are normalized to unity. Thesolid line is the dependence calculated by formulae (6) and (7)with W t = 1.6 eV, �W = 0.3 eV, and υ = 1013 s−1.

FIG. 13. Energy distribution of hole traps in thermal oxide ob-tained by the thermal stimulated current method with the as-sumption of υ ≈ 1013 s−1. (Reprinted with permission fromMiller et al.45 Copyright 1992: American Physical Society.)

oxide-nitride-oxide-metal (MONOS) structures.1 These resultsconfirmed that the Si-Si bonds in silicon oxide act as both holeand electron traps.

3.4. Formation of Si-Si Bonds in Re-oxidizedSilicon Oxynitride

When a silicon oxide film was treated in ammonia, doubly-coordinated nitrogen atoms = SiN· which act as electron traps

FIG. 14. Plot of hole capture cross-section of neutral oxygenvacancy as a function of electric field at room temperature.(Redrawn based on Adamchuk et al.49)

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FIG. 15. Accumulation of electrons and holes in an MOS struc-ture with Si+ ions implantation. (Reprinted with permissionfrom Ohzone and Hori,51 Copyright 1994: Elsevier.)

will be formed (see Section 8). To remove these traps, the samplecan be re-oxidized with process as given in Equ (9). It wasfound that the E′ centers were still found on the outer surfaceof re-oxidized nitrided oxide films.52 This is an indication ofthe formation of Si-Si bonds on the outer surface also. Malliket al.53 showed that some hole traps with an abnormally largecapture cross-section, about 7.9 × 10−12 cm−2, were formedon the surface of re-oxidized nitrided oxide films. On the otherhand, Gritsenko et al.54 showed that in oxidizing silicon nitride,Si-Si bonds can be formed at the Si3N4/SiO2 interface via thefollowing reactions:

2≡Si3N + 2O → 2≡Si2O + 2≡Si· +2N, [9]≡Si· + ·Si≡→≡Si-Si≡ . [10]

It suggests that the Si-Si bonds will be formed by convertingthe Si-N bonds via the oxidation reactions given in Equ. (9) and(10) during the re-oxidation of the nitrided oxide films.

3.5. Positively Charged Oxygen Vacancyas an Electron Trap

The electron capture cross-section for a positively-chargedtrap in an As+ ion irradiated oxide was examined.55 The irra-diation leads to the formation of oxygen vacancies accordingto Equ. (2). Charging of the E′ centers in the irradiated SiO2was obtained by injecting holes from the silicon substrate. Thepositively-charged E′ centers can trap electrons. By injectingelectron with internal photoemission technique, the followingreaction takes place:

≡Si·+Si≡ + e → ≡Si-Si≡ . [11]that is, the electron capture of a positively-charged E′ centerwould lead to the formation of neutral oxygen vacancy, or Si-Si

FIG. 16. Plot of room temperature electron capture cross-section of positively-charged oxygen vacancy, or the E′ center,as a function of electric field. (Reprinted with permission fromBuchanan et al.55 Copyright 1991: American Physical Society.)

bond. Figure 16 shows the electron capture cross-section of E′

centers at room temperature as a function of electric field.55 Thecross-section decreases from about 10−12 cm2 at electric fieldof 2 × 105 V/cm to about 3 × 10−15 cm2 at a field strengthof 2 × 106 V/cm. In Figure 16, it also shows the Monte-Carlosimulation data based on some classical and quantum models.55

3.6. Neutral Oxygen Vacancy as an Electron TrapA neutral oxygen vacancy in SiO2 can trap both electron and

hole. The electron trapping properties of oxygen vacancies insilicon oxide films with As+ ion irradiation was examined.55

Figure 17 shows the field dependence of the electron capturecross-section of Si-Si bonds. Within the experimental accuracy,the capture cross-section is in the order of 10−15 cm2 and doesnot depend on the electric field.55 The hole capturing propertiesof the neutral Si-Si defects were also examined with quantum-chemical calculations.38 It was found that the Si-Si defect actsas a hole trap with energy of 0.7 ± 0.5 eV. DFT method wasalso used to investigate into the electron capture capability ofthe Si-Si bonds.13,52 It was found that the Si-Si defect can alsofunction as an electron trap and the trap energy was reported tobe 1.717,56 and 2.3 eV. On the other hand, it was found experi-mentally that the electron capture cross-section is in the rangeof 10−15 to 10−14 cm2 according to the thermal and optical de-localization experiments of the trapped electrons (presumably,Si-Si bonds).57,58 The thermal ionization energy was found tobe equal to Wt = 1.7 ± 0.2 eV. The trap energy obtained from

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FIG. 17. Field dependence of electron capture cross-section ofneutral Si-Si bonds. (Reprinted with permission from Buchananet al.55 Copyright 1991: American Physical Society.)

thermal stimulation tunneling experiment also falls in the rangeof 1.5 to 1.7 eV.59 Thus, within the experimental accuracy, bothenergies coincide with the polaron energy as determined in Fig-ure 5 from the optical transitions of the Si-Si bond. The reportedoptical photo-ionization energy of electron traps, Wopt, in ther-mal oxide is about 3.0±0.5 eV.57,58 Note that from tunnelingdischarge experiments, Yamabe and Miura60 suggested that theenergy of an electron trap in SiO2 on Si should be equal to 3.1eV. Particular attention is paid to the following relation. Theoptical ionization energy of the electron trap is about twice ofthe thermal ionization energy, namely, Weopt ≈ 2Wet. The samerelation was also observed for the ionization energies of electrontraps in silicon nitride.45 That is, the thermal and optical ioniza-tion energies for electron and hole traps can be approximatedas: Weopt ≈ Whopt and Wet ≈ 2Wht for both silicon oxide andsilicon nitride.

Warren, Lenahan, and Robinson et al. studied the elec-tron trapping in silicon oxide films with slight Si enrich-ment, i.e., SiOx


FIG. 18. EPR spectral of various silicon oxide film annealed at1320◦C in inert atmosphere. Trace a is the result of a quartz withγ -quanta irradiation; trace b is a thermal oxide with γ quantairradiation; and trace c is a thermal oxide with hole injection.(Reprinted with permission from Devine et al.8 Copyright 1995:American Institute of Physics.)

Conley, Lenahan, Evance et al.,66 and Stesmans67 also reportedthe similar values of g factor (2.00246 ± 0.00003) for theEδ center in thermal oxide. Figure 19 depicts the kinetics forannealing-induced transformation of E′ into Eδ centers.8 Basedon expressions (5) and (6), it can be estimated that Wt = 1.65 eV,�W = 0.15 eV for the E′ center, and Wt = 1.33 eV, �W =0. 25 eV for the Eδ center. The trap energy for the E′ center isclose to that extracted from the reported data43 and the result isshown in Figure 12.

The hole trapping in oxygen vacancies and oxygen diva-cancies at room temperature were also investigated.66 Figure20 plots the surface density of trapped holes as a function ofthe number of injected holes per unit area. From these data,the hole capture cross-sections of oxygen vacancy and oxy-gen divacancy were estimated to be about 3 × 10−13 cm2 and10−13 cm2, respectively.

4.3. Positively Charged Oxygen Divacancyas an Electron Trap

Conley et al.66 further suggested that an oxygen divacancywith a trapped hole can capture an electron by the followingreaction:

≡Si-Si·+Si≡ + e → ≡Si-Si-Si≡ [13]Figure 21 shows the EPR signal observed after hole and elec-

tron injection. The accumulation of holes gives rise to an EPRsignal with g = 2.0019; this signal vanishes after a subsequent

FIG. 19. Fitting of temperature dependences of E′ and Eδ cen-ters due to isochronal annealing based on the proposed model.Markers are experimental data taken from Devine et al.8 The an-nealing duration is 15 min. υ = 1013 s−1 is used for the activationenergy calculation.

electron injection. Figure 22 plots the trapped charge densityas a function of electron fluence.66 From this plot, the electroncapture cross-section of oxygen vacancy can be estimated to bein the range of 10−14 -10−13 cm2 and that the oxygen divacancyis in the order of 10−12 cm2.

5. SILICON CLUSTERS IN SiO2 AS ELECTRONAND HOLE TRAPSSilicon clusters embedded in SiO2 are used as a charge stor-

age medium in flash memory cells.68 The injection of elec-trons or holes into these clusters results in the charge localiza-tion. EPR results indicate that these clusters behave similar to

FIG. 20. Plot of trapped hole density as a function of hole in-jection fluence. (Reprinted with permission from Conley et al.66

Copyright 1994: American Institute of Physics.)

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FIG. 21. EPR signals due to oxygen divacancy after hole or elec-tron injection. (Reprinted with permission from Conley et al.66

Copyright 1994: American Institute of Physics.)

some defects in SiO2. By creating silicon nanoclusters with Siimplantation into a thick oxide layer with proper annealing,an EPR signal with g factor of 2.0062 ± 0.0002 and width ofabout 0.65 mT was observed (see trace a in Figure 23). These

FIG. 22. Plot of trapped electron density as a function of elec-tron injection fluence for positively-charged oxygen vacancyand oxygen divacancy. (Reprinted with permission from Conleyet al.66 Copyright 1994: American Institute of Physics.)

FIG. 23. EPR signal of silicon oxide layer with 3 × 1016 cm−2silicon implantation. (a) as-grown silicon oxide; (b) sample withSi implantation and post-implant annealing; (c) the implantedsample with electron injection; (d) the implant sample with holeinjection.

figures indicate the presence of threefold-coordinated siliconatoms with an unpaired electron ≡Si3Si· (D-centers).69 Afterannealing, the radiative defects vanish (see trace b). Injection ofelectron (trace c) or hole (trace d) results in some new signalsresponsible to D centers. Figure 24 shows a model for the elec-tron and hole capture in a silicon cluster at the Si/SiO2 interface.After the hole capture, a D center will be formed whereas anelectron will be localized at the silicon atom coordinated to threeoxygen atoms after an electron capture.

FIG. 24. Model of the silicon cluster/SiO2 interface as a defectfor capturing both electron and hole.

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6. DOUBLY-COORDINATED SILICON ATOM (=Si:)The doubly-coordinated silicon atom with two electrons or

=Si:, is a neutral diamagnetic defect that can be formed whentwo Si-O bonds in a SiO4 tetrahedron are ruptured, i.e.,

2≡Si-O-Si≡→ 2≡Si-O · +2:Si= . [14]

This defect is also called the divalent center. The divalent centerexhibits an absorption band at energy of about 5.0 to 5.2 eV.37

Excitation within this absorption band gives rise to two PL bandsat 2.65 to 2.7 and 4.4 eV (see Figure 25). The 2.7 eV PL bandhas a long lifetime of about 10.2 ms and suggests that the PLis due to an electron transition from the optically excited tripletstate into the singlet state.

The electron and hole capture capabilities of the divalentcenter was also studied.70,71 It was found that the electron capturein the divalent center is unfavorable, whereas the hole capturemay occur via the following reaction:

≡Si: + h → ≡Si· . [15]

Figure 26 illustrates the proposed structure of the divalent centerin the neutral state. However, the magnitude of the hole capturecross-section of the divalent center is still unknown and no anyexperimental value for the hole delocalization energy has beenreported for this center yet.

FIG. 25. Electroluminescence spectra of thermal oxide layersobtained by anodic oxidation (trace 1), dry oxidation (trace 2),1050◦C wet oxidation (trace 3), 950◦C wet oxidation, (trace4), and 900◦C wet oxidation (trace 5). The arrows show the1.9 eV luminescence band due to oxyradical, 2.7 eV band dueto divalent center, and the 4.4-eV band due to oxygen vacancyand divalent center.

FIG. 26. Structure of the divalent center. (Color figure availableonline.)

7. OXYGEN ATOM WITH AN UNPAIRED ELECTRON(≡Si-O·) AS AN ELECTRON TRAPThe oxygen atom with an unpaired electron is also called

oxyradical, or non-bridging oxygen. This defect may arise fol-lowing the rupture of the ≡Si-O-Si≡ bond by the reaction below:

≡Si-O-Si≡→≡Si-O· + ·Si≡ . [16]Alternatively, an oxyradical may also be resulted from the rup-ture of the peroxide bridge ≡SiOOSi≡32 as:

≡SiOOSi≡→≡SiO· + ·OSi≡ . [17]In silicon oxide films, in particular in silicon oxide produced bywet oxidation, hydrogen bonds in the form of ≡SiOH defectsare commonly found.72 The rupture of oxyradical also leads tothe formation of ≡SiO· defect by the following reaction

≡SiOH → ≡SiO· +H. [18]The absorption spectra of the oxyradical has two absorption

bands at energies 4.8 and 2.0 eV, respectively.37 A PL band withenergy of 1.85 to 1.9 eV can be excited for both bands (seeFigure 25). The 2.0 eV absorption band is due to the electrontransitions between the O 2pπ states, which is close to the energyof the top of the valence band of SiO2.

The charge capture capability of the oxyradical defects inSiO2 had been examined by semi-empirical method73 and bythe non-empirical DFT method using cluster approximation.74

Figure 27 illustrates the proposed electronic structure of theoxyradical defect in SiO2. Hole capture of the oxyradical wasshown to be unfavorable from energy point of view. Whereas anelectron capture can be occurred via the following reaction:

≡SiO· + e → ≡SiO: [19]

According to the data obtained from non-empirical DFT cal-culations, the energy gain after an electron capture in the oxyrad-ical amounts to 3.9 eV. However, there is no any experimentalvalue for the electron delocalization energy being reported sofar. The magnitude of the electron capture cross-section of theoxyradical defect is about 1.5 × 10−17 cm2.75

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FIG. 27. Illustration of oxyradical structure in an oxide film.(Color figure available online.)

8. DOUBLY-COORDINATED NITROGEN ATOM WITHAN UNPAIRED ELECTRON, ≡Si2N·As discussed above, in thermal oxide films near the Si-SiO2

interface, there exist some Si-Si bonds. These Si-Si bonds func-tion as hole traps. The Si-Si bonds can be removed by hightemperature annealing in a nitrogen-containing gas, such as am-monia (NH3) or nitrous oxide (NO or N2O).76,77 The nitrogenatom interacts with an oxygen vacancy via the following reac-tion:

3≡Si-Si≡ + 2N → ≡Si3N [20]Thus the amount of hole traps would be reduced by convertingthe Si-Si bonds into Si-N bonds.

Experiments also showed that the nitridation of thermal sil-icon oxide in ammonia also accompanied with the formationof electron traps.78–81 Ammonia annealing also leads to the re-moval of Si-Si bonds according to the following reaction

≡Si-Si≡ + NH3 → ≡Si2NH + H2 [21]EPR experiments indicated that in silicon nitride and oxynitridethere exist doubly- coordinated paramagnetic silicon atoms withan unpaired electron, i.e., ≡Si2N· (see Figure 28).78–81 The un-paired electron in the ≡Si2N· defectis localized at the antibind-ing 2pπ orbital of nitrogen. In the antibinding 2pπ orbital ofnitrogen, 74 ± 6% of the p-type wavefunction and 2% of the s-type wavefunction are localized.82 This kind of defect is formeddue to the decomposition of the ≡Si2NH defect at high nitrida-tion temperatures (700 to 1000◦C) via the following reaction

≡Si2NH → ≡Si2N· +H. [22]

Yount and Lenahan82 showed in oxynitride films with hydro-gen annealing that the EPR signal due to the ≡Si2N· defects

FIG. 28. Model of the doubly-coordinated nitrogen atom withan unpaired electron capturing an electron (a) and capturing ahole (b). (Color figure available online.)

disappears as a result of the hydrogen passivation, i.e.

≡Si2N· + H → ≡Si2NH [23]Morokov et al.83 examined the electron and hole capturing ca-pability of the ≡Si2N· defect based on quantum-chemical cal-culations and found that this defect would have energy gain onelectron capturing but no energy gain for hole capturing. The

FIG. 29. EPR signal of a silicon oxide being annealed in ammo-nia: (a) before electron injection and (b) after electron injection.(Reprinted with permission from Chaiyasena et al.84 Copyright1991: American Institute of Physics.)

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TABLE 1Summary of major defects found in silicon oxide and silicon oxynitride films

Absorption Electron Capture Hole Capture GyromagneticDefect Type Band (eV) cross-section (cm2) cross-section (cm2) factor

≡Si-Si≡ 7.56-7.6 10−14–10−13 3 × 10−13 2.0010 (for E′ center)≡Si-Si-Si≡ 6.5 ∼10−12 10−13 2.0014 ± 0.0005≡Si3Si· unavailable unavailable unavailable 2.0062 ± 0.0002≡Si: 5.0-5.2 NA unavailable unavailable≡SiO· 2.0, 4.8 1.5×10−17 NA unavailable≡Si2N· unavailable 10−15 NA unavailable

electron capture can be described by the following reaction:

≡Si2N· + e → ≡Si2N : [24]The electron capture cross-section of the doubly-coordinatednitrogen atom is about 10−15 cm2.82 The electron capture resultsin transforming the defect from the paramagnetic state into thediamagnetic state and the intensity of the EPR signal decreases(see Figure 29).84 The hole capture in the paramagnetic defect≡Si2N: can be described by:

≡Si2N: + h → ≡Si2N· [25]The ≡Si2N· defect can be further removed from silicon oxyni-tride or nitride film by re-oxidizing the samples in oxygen am-bient via the following reaction:

≡Si2N · + O → ≡Si-O-Si≡ +N [26]

9. SUMMARYIn this work, the formation and the removal mechanisms of

various defects, which are defined from the view point of theMott octahedral rule, in silicon oxide and silicon oxynitridefilms are critically reviewed. The charge trapping capabilities ofthese defects are also analyzed in detail. The major propertiesof these defects are summarized in Table I.

First, the Si-Si bond is a fundamental defect in thermal sili-con oxide and has several significant impacts on the operationsof MOS devices using silicon oxide or oxynitride as the gatedielectrics. This neutral diamagnetic defect can be formed dur-ing the thermal oxide growth or after particle irradiation. TheSi-Si bonds can capture both holes and electrons, but the holecapture cross-section of a Si-Si bond is much larger than thatof the electron ones. That is why only positive charge accu-mulation is reported in silicon oxides that were subjected toionizing irradiation or other treatments giving rise to the gen-eration of hole-electron pairs. This inherent property limits theradiation hardness of silicon oxide films. Here we have pre-sented a detailed discussion on this issue together with somerecently reported EPR results. Interestingly, nitridation of sili-con oxide in ammonia was found to cause the removal of theinterfacial Si-Si bonds and a subsequence re-oxidation of the

nitrided oxide can give rise to the formation of Si-Si bonds onthe outer surface of the dielectric. A further insight from thisreview is on the thermal ionization energies of the electron andhole traps of Si-Si bonds in silicon oxide, Wet and Wht. Theyare equal and coincide with the Franck-Condon (polaron) shiftWp, i.e. Wet = Wht = Wp = 1.6±0.2 eV. The optical ionizationenergies of electron and hole traps, Weopt and Whopt, are equaltoo, i.e., Weopt = Whopt = 3.0±0.5 eV and their values are abouttwice of the thermal ionization energy, i.e. Weopt ≈ Whopt ≈2Wht. Interestingly, these relations also hold for amorphous sil-icon nitride.49,55

Second, according to some quantum-chemical calculations,the doubly-coordinated silicon atom with two electrons, or thedivalent center, should act as a hole trap in SiO2. However, noexperimental confirmation has been reported so far. This may bedue to the small hole capture cross-section of the divalent centerand make the probability of hole trapping at divalent centersless significant. This is still an interesting topic worth of furtherinvestigation.

Finally, a silicon atom with an unpaired electron or peroxideradical also acts as a hole trap in SiO2. This center was observedin oxide films grown by wet oxidation. Unlike the dry oxides,wet oxides are more robust against radiation. This property wasexplained by the partial compensation of the positive chargestrapped in the oxygen vacancies and divacancies at the Si/SiO2interface with the negative charges trapped at oxygen atomswith an unpaired electron in the bulk of the oxide.

ACKNOWLEDGMENTSThis work was supported by the project No. 70 of Siberian

Branch of Russian Academy of Sciences and RFBR grantsNo. 10-02-01221-a and No. 10-07-00531-a and the Strate-gic Research Grant of City University of Hong Kong (ProjectNo. 7008103).

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atomic and electronic structures of traps in silicon oxide and silicon...

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