Surface Science 602 (2008) 1948–1953

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Surface Science

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Complete prevention of reaction at HfO2/Si interfaces by 1 nm silicon nitride layer

Hikaru Kobayashi a,b,*, Kentaro Imamura a,b, Ken-ichi Fukayama a,b, Sung-Soon Im a,b, Osamu Maida a,b,Young-Bae Kim c, Hyun-Chul Kim c, Duck-Kyun Choi c

a Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japanb CREST, Japan Science and Technology, Kawaguchi, Japanc Division of Materials Science and Engineering, Hanyang University, Seongdong Ku, Seoul 133-791, Republic of Korea

a r t i c l e i n f o a b s t r a c t

Article history:Received 21 November 2007Accepted for publication 21 March 2008Available online 29 March 2008

Keywords:X-ray photoelectron spectroscopyHafnium oxideHigh k dielectricsSilicon nitrideMetal–insulator–semiconductor (MIS)structuresSemiconductor–insulator interfacesSputtering

0039-6028/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.susc.2008.03.031

* Corresponding author. Address: Institute of ScienOsaka University, 8-1 Mihogaoka, Ibaraki, Osaka 5676879 8450.

E-mail address: h.kobayashi@sanken.osaka-u.ac.jp

When hafnium oxide (HfO2) is directly deposited on Si by the RF sputtering method, Hf silicide is formedand post-deposition anneal (PDA) at 400 �C transforms Hf silicide to Si suboxide plus Hf suboxide. Theleakage current density for the haluminum (Al)/HfO2/Si(100)i diodes without PDA is high due to the highdensity interface states near the Fermi level (0.86 eV above the Si valence band maximum, VBM) and min-ute conduction channels. PDA at 400 �C eliminates the interface states and the conduction channels, andimproves the characteristics of the HfO2 layer, but interface states are newly formed at 0.53 eV abovethe VBM, resulting in still high leakage current density. Silicon nitride (SiN) layers formed by Si nitridationusing N2-plasma generated by the low energy electron impact method possess a high nitrogen atomicconcentration ratio, N/(N + O) of 0.65. When a 1.0 nm SiN layer is inserted between HfO2 and Si, interfacialreaction is completely prevented, resulting in a smaller effective oxide thickness, EOT of 1.4 nm. In spite ofthe smaller EOT, the leakage current density is nearly the same as that with no SiN layer, possibly due to theprevention of the formation of the conduction channels. PDA at 400 �C improves HfO2 characteristics with-out causing the interfacial reaction, leading to a decrease in the leakage current density.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

Miniaturization of complementary metal–oxide–semiconductor(CMOS) devices requires use of ultrathin gate insulating layerswith maintaining a low leakage current density. Hafnium dioxide(HfO2) is one of the most promising materials for gate insulatinglayers to replace silicon dioxide (SiO2) because of its high dielectricconstant, high thermal stability, wide band-gap, etc. However,when HfO2 is directly deposited on Si, an interfacial layer isformed, resulting in high interface state densities and in an in-crease of the effective oxide thickness (EOT) [1–12]. The composi-tion of the interfacial layers strongly depends on the depositionconditions of HfO2 films, i.e., SiO2 [1–6], Hf silicate [7–9], SiO2 richhafnium silicate [4,10], and Hf silicide [11,12]. To prevent the for-mation of interfacial layers, SiO2 [9,13], silicon oxynitride (SiON)[13–16], and silicon nitride (SiN) [15–17] are inserted betweenHfO2 and Si. However, insertion of a SiO2 layer increases the effec-tive oxide thickness, and moreover the interfacial reaction cannotbe prevented completely by the ultrathin (e.g., 1 nm) SiO2 layer[18]. SiON is more favorable than SiO2 because of its denser struc-ture and higher dielectric constant than that of SiO2. We have

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tific and Industrial Research,-0047, Japan. Tel./fax: +81 6

(H. Kobayashi).

recently found that the energy distribution of interface statesstrongly depends on the kinds of buffer layers [18].

Ultrathin SiON layers can be formed at low temperatures (e.g.,450 �C) by plasma nitridation of Si in NH3, N2O, etc. [14,19,20]. Inthe case of NH3 plasma, high concentration hydrogen atoms areincorporated, resulting in the formation of high density interfacialtrap states [19–21]. Ultrathin SiON buffer layers formed by NH3

and N2O plasma nitridation cannot prevent the interfacial reactioncompletely, leading to the formation of an Hf silicate interfaciallayer [14]. The incomplete prevention of the interfacial reactionis possibly due to a low nitrogen concentration in the SiON layer.

We have developed a low temperature formation method ofSiON (or SiN) layers by use of nitrogen (N2)-plasma generated bythe low energy electron impact method [22–24]. Using this meth-od, SiON (or SiN) layers having a nitrogen atomic concentrationmore than 10% can easily be formed below 400 �C.

In the present study, 1 nm silicon nitride (SiN) layers formed bythe low energy electron impact method are applied to buffer layersbetween HfO2 and Si. It has been found that the 1 nm SiN layerscan completely prevent the interfacial reaction.

2. Experiments

Metal–insulator–semiconductor (MIS) structures were fabri-cated from phosphorus-doped n-type Si(100) wafers having

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�1 X cm resistivity. After cleaning the Si wafers using the RCAmethod and etching with dilute hydrofluoric acid to remove anative oxide layer, Si was nitrided by N2-plasma generated bythe low energy electron impact method. In this method, a tungstenfilament was heated at �1500 �C and 30 V was applied to a gridwith respect to the filament in 1 Pa N2 atmosphere. HfO2 layersof �3 nm thickness monitored by a quartz thickness monitor weredeposited on SiN/Si and bare Si specimens by means of an electron-beam evaporation method using an HfO2 target. No external heat-ing was performed during the deposition. For some specimens,post-deposition annealing (PDA) was performed at 400 �C in N2

atmosphere for 10 min. Aluminum (Al) dots of 0.3 or 0.37 mmdiameter were formed on the HfO2 surfaces, leading to hAl/HfO2/SiN/Si(100)i metal–insulator–semiconductor (MIS) structure.

X-ray photoelectron spectroscopy (XPS) measurements wereperformed using a VG Scientific ESCALAB 220i-XL spectrometerwith a monochromatic Al Ka radiation source. Photoelectrons weredetected in the surface-normal direction.

Current–voltage (I–V) curves were measured using a HP 4140Bpicoammeter. Capacitance–voltage (C–V) and conductance–volt-age (G–V) curves were recorded at 1 MHz using an YHP 4192Aimpedance analyzer.

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Fig. 1. XPS spectra in the Hf 4f region (a) and Si 2p region (b) for the HfO2 layerdeposited on bare Si(100) surfaces without (upper) and with (lower) PDA at 400 �Cin nitrogen.

3. Results

Fig. 1 shows XPS spectra in the Hf 4f region (spectra a) and Si 2pregion (spectra b) for the HfO2 layer directly deposited on theSi(100) substrate. The main peaks at 17.7 and 19.3 eV in the Hf4f spectra are due to Hf 4f7/2 and 4f5/2 levels of HfO2. With noPDA (upper spectrum), weak peaks attributable to Hf silicide[25–27] were observed at 14.6 and 16.2 eV while these peaks dis-appeared after PDA at 400 �C (lower spectrum).

In the Si 2p XPS spectrum for the HfO2/Si(100) specimens withno heat treatment (upper spectrum), sharp doublet peaks due to Si2p3/2 and 2p1/2 levels of the substrate were present at 99.3 and99.9 eV, respectively, but a peak due to Si oxide was not observedin the higher energy region. After the heat treatment at 400 �C innitrogen, on the other hand, a broad peak appeared at 102.2 eV(i.e., 2.9 eV shift from the substrate Si 2p3/2 peak), which wasattributable Si suboxide species [28].

Fig. 2 shows C–V (a), I–V (b), and G–V (c) curves of the hAl/HfO2/Si(100)iMIS diodes with no buffer layer. With no PDA (solid line),the saturation capacitance was �1700 pF and the effective oxidethickness, EOT, was estimated to be 2.2 nm. A hysteresis with themagnitude of �0.3 V was present in the bias region higher than0.7 V. With PDA at 400 �C (dotted line), the saturation capacitancedecreased to �1350 pF, indicating an increase in EOT to 2.7 nm. Inthis case, a hysteresis with the magnitude of 0.3–0.4 V was presentin all the bias regions. The flat-band voltage for the MOS diodeswith no PDA was estimated to be 0.2–0.3 V, while those withPDA to be �0.03 and 0.16 V for the bias sweeps in the positiveand negative voltage directions, respectively.

The leakage current density for the MIS diodes without PDA (so-lid line) was considerably high (0.8 and 10 A/cm2 at the forwardgate bias, VG, of 1 V, for the bias sweeps in the positive and negativevoltage directions, respectively, and 0.015 A/cm2 at VG of �1 V).With PDA at 400 �C (dotted line), the leakage current density con-siderably decreased (0.3 A/cm2 at VG = 1 V, and 8 � 10�5 A/cm2 atVG = �1 V).

Fig. 3 shows the equivalent circuit of the MOS diodes with theultrathin insulating layer having capacitance, CI. CD is the capaci-tance of the semiconductor depletion layer, and its response timeis very short. Thus, the resistance in series to CD is negligibly small.Cis is the capacitance due to interface states and Ris is the corre-sponding resistance which is needed to express that interface

states possess a time constant, sis (sis = CisRis). RF expresses a leak-age current flowing through the ultrathin dielectrics. The conduc-tance, G, of the equivalent circuit is given by

G ¼ 1RFþ x2 Cissis

1þ x2s2is

ð1Þ

where x is the angular frequency. RF can be estimated from theleakage current density in Fig. 2b (from the slope of the I–V curve),and the background due to RF is shown by the dashed lines inFig. 2c. The background is relatively high for the MOS diodes with-out PDA, while it is negligibly low for the MOS diodes with PDA.After subtracting 1/RF from the measured conductance, it is in pro-portion to Cis which is, in turn, proportional to the interface statedensity [29]. The conductance after background subtraction isshown by the solid–dotted line.

The conductance of the MIS diodes with no PDA (solid–dottedline) was low in the gate bias region negative of �0.1 V, but itwas very high near 0 V. This result shows that the density of inter-face states more than �0.1 eV below the Fermi level (i.e., 0.86 eVabove the Si valence band maximum, VBM) is negligibly low, whilethat near the Fermi level is considerably high. With PDA at 400 �C(dotted line), the conductance near 0 V decreased but peaks ap-peared at �0.22 and �0.46 V for the bias sweeps in the negative

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Fig. 2. C–V (a), I–V (b), and G–V (c) curves for the hAl/HfO2/Si(100)i MOS diodeswithout (solid line) and with (dotted line) PDA at 400 �C in nitrogen. The dashedand solid–dotted lines in Fig. 2c show the background due to the leakage currentflowing through the ultrathin dielectrics, and the conductance after subtraction ofthe background, respectively.

Fig. 3. Equivalent circuit for a MOS diode.

1950 H. Kobayashi et al. / Surface Science 602 (2008) 1948–1953

and positive directions, respectively. This result indicates that theinterface states near the Fermi level are eliminated by PDA, but

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Fig. 4. XPS spectra in the following energy regions for the SiN/Si(100) structureformed by N2-plasma generated by the low energy electron impact method (a) N 1sregion (b) O 1s region (c) Si 2p region.

H. Kobayashi et al. / Surface Science 602 (2008) 1948–1953 1951

interface states are newly generated below the Fermi level. Usingthe flat-band voltage and the conductance peak positions, the en-ergy levels of the interface states of the MOS diodes with PDAwas estimated to be 0.53 eV above the Si VBM.

The time constant, sis, can be obtained from the plot of the con-ductance peak intensity, Gpeak, divided by x vs. the frequency, sinceGis/x in Eq. (1) has the maximum value when xsis is equal to one.Gis/x values measured in the frequency range between 10 k and1 MHz for the MOS diodes with PDA decreased only gradually withx. This result is probably due to the presence of interface stateswith various time constants.

A conductance value for MOS diodes includes only contributionfrom interface states (after subtracting the effect due to a leakagecurrent) (cf. Eq. (1)) while capacitance of MOS diodes consists ofsemiconductor capacitance, oxide capacitance, and interface statecapacitance. Since the semiconductor capacitance and oxide capac-itance do not show frequency dispersion, it is highly probable thatconductance of MOS diodes is much more frequency-dependentthan capacitance. Detailed study of the frequency dispersion ofthe C–V and G–V characteristics will be reported elsewhere.

Fig. 4 shows XPS spectra for the SiN layers formed by N2-plasmagenerated by the low energy electron impact method. In the N 1s

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Fig. 5. XPS spectra for the HfO2/SiN/Si(100) structure a) in the Hf 4f region with noPDA b) in the Si 2p region with no PDA (upper spectrum) and with PDA at 400 �C innitrogen (lower spectrum).

region (spectrum a), a peak is present at 397.8 eV, and it is attrib-uted to a nitrogen atom singly bound to three Si atoms having aplaner structure [30]. In the O 1s region (spectrum b), a peak wasobserved at 532.7 eV. From the intensity ratio between thesepeaks, the nitrogen atomic concentration ratio, N/(N + O), is esti-mated to be 0.65. In the Si 2p region (spectrum c), a broad peakwas observed at 2.8 eV with respect to the substrate Si 2p3/2 peak.This binding energy shift is in good agreement with that of SiN[31,32].

Fig. 5 shows XPS spectra for the HfO2/1 nm SiN/Si(100) struc-ture. In the Hf 4f region (spectrum a), only peaks due to HfO2 wereobserved at 17.6 and 19.3 eV. This XPS spectrum was not changedby the heat treatment at 400 �C. These results demonstrate that theinterfacial reaction between HfO2 and Si to form Hf silicide (cf.upper spectrum in Fig. 1a) is completely prevented by the 1 nmSiN layer.

With no heat treatment, the XPS spectrum in the Si 2p region(upper spectrum in Fig. 5b) had a structure nearly identical to thatwithout HfO2 deposition (Fig. 4c), indicating the unchanged thick-ness (i.e., 1.0 nm) and composition of the SiN layer. This result alsoverifies complete prevention of the interfacial reaction. The XPSspectrum in the Si 2p region for the HfO2/1 nm SiN/Si(100) struc-ture was not changed by the heat treatment at 400 �C (lower spec-trum in Fig. 5b), clearly showing that diffusion of oxygen and Hfatoms is completely prevented by the 1 nm SiN layer even at400 �C.

Fig. 6 shows the C–V (Fig. 6a) and I–V (Fig. 6b) curves of the hAl/HfO2/SiN/Si(100)i MIS diodes with the SiN layer formed byN2-plasma generated by low energy electron impact. EOT for both

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Fig. 6. C–V (a) and I–V (b) curves for the hAl/HfO2/1 nm SiN/Si(100)i MOS diodeswith the SiN layer formed by the low energy electron impact method without (solidline) and with (dotted line) PDA at 400 �C in nitrogen.

1952 H. Kobayashi et al. / Surface Science 602 (2008) 1948–1953

the diodes without and with PDA at 400 �C was estimated to be1.4 nm from the saturation capacitance. Without PDA (solid line),the magnitude of the hysteresis in the C–V curves was 0.05 V, whileit increased to 0.3 V after PDA (dotted line). The flat-band voltagesestimated from the center of the two curves measured in the dif-ferent bias sweep directions were 0.05 and �0.48 V, for the MOSdiodes without and with PDA, respectively.

The leakage current density was decreased by PDA at 400 �C for10 min in spite of unchanged EOT.

4. Discussion

When HfO2 is deposited on bare Si, Hf silicide is formed (cf.Fig. 1a). There are two possibilities for the mechanism of the Hf sil-icide formation 1) Hf atoms formed during electron-beam evapora-tion directly react with Si, and 2) deposited HfO2 reacts with Si. It isreported in the previous literature that in the case of HfO2 deposi-tion by means of reactive magnetron sputtering, Hf silicide isformed only at temperatures above 700 �C [33]. This indicates thatdeposited HfO2 does not react with Si at temperatures below700 �C, and thus mechanism 1) is more probable. In the case ofreactive sputtering, the oxygen pressure is sufficiently high to formHfO2 in the gas phase, and consequently, no Hf atoms impinge onSi surfaces, while in the case of the evaporation method, Hf atomsare likely to be incident to the Si surfaces, resulting in the forma-tion of Hf silicide.

Hf silicide induces interface states near the Fermi level (i.e.,0.86 eV above the Si VBM). A leakage current flows via interfacestates, resulting in an increase in its density [34,35]. Another rea-son for the high leakage current density may be the formation ofHf silicide islands which form thin HfO2 regions and/or minuteelectrical contacts between Si and Al.

When PDA at 400 �C is performed, Hf silicide reacts with HfO2,leading to the formation of Si suboxide (cf. Fig. 1b) and Hf subox-ide. No spectral feature due to Hf suboxide was observed inFig. 1a. This is probably because of i) a decrease in the intensityof the suboxide peak by the HfO2 overlayer, ii) small energy shiftof the suboxide peak from the HfO2 peaks [36], and iii) low subox-ide concentration. PDA eliminates the conduction channels due toHf silicide, resulting in a decrease in the leakage current density.However, the formation of suboxides induces interface states be-low the Fermi level (0.53 eV above the Si VBM) and slow stateswhich cause the hysteresis in the C–V curve (cf. dotted line inFig. 2a) [37]. The leakage current is likely to flow via interfacestates, resulting in still high leakage current density.

Considering that each oxygen atom bound to a Si atom causes�1 eV energy shift of the Si 2p peak from the substrate peak[28], the 2.9 eV shift (lower spectrum in Fig. 1b) indicates thatSi3+ (i.e., Si2O3) is the most probable interfacial species formed byPDA at 400 �C. Assuming that the Si suboxide layer is present di-rectly on the Si substrate, its thickness is estimated to be 1.2 nmfrom the intensity ratio between the Si 2p peak due to Si suboxide,Isuboxide, and the Si substrate peak, ISi [28,38]

dsuboxide ¼ ksuboxide lnIsuboxide

ISi

CSirSikSi

csuboxidersuboxideksuboxideþ 1

� �ð2Þ

where k is the mean free path of photoelectrons, c is the con-centration of Si atoms, r is the photoionization cross section, andsubscripts suboxide and Si denote the values for the suboxide layerand the Si substrate, respectively. In the estimation, 3.2 and 2.7 nmwere used for ksuboxide and kSi, respectively, 1.9 for cSi,/csuboxide and1.1 for rsuboxide/rSi [28]. The values for a SiO2 layer are adopted asksuboxide and rsuboxide.

Hf silicide forms interface states near the Fermi level (interfacialFermi level: 0.86 eV above the Si VBM). PDA at 400 �C converts Hfsilicide to Si suboxide plus Hf suboxide, and consequently the

interface states near the Fermi level are eliminated. However, theformation of suboxides induces another interface states at0.53 eV above the Si VBM (interfacial Fermi level: 0.94 eV abovethe Si VBM).

Si can easily be nitrided at low temperatures (400 �C in the pres-ent study) by N2-plasma generated by low energy electron impact.Considering that the atomic ratio, N/(N + O), of 0.65, and assumingthat the SiN layer consists of mixture of Si3N4 and SiO2, the Si atom-ic concentration ratio, DSiN/DSi, is estimated to be 0.65. Assumingthe mean free path of photoelectrons and photoionization crosssection for the SiN layer are the same as those for SiO2 layers[28], the SiN thickness is estimated to be 1.0 nm from the intensityratio between the SiN peak and the substrate Si 2p3/2 peak.

The physical thickness of the HfO2 layer was roughly deter-mined to be �3 nm using a quartz thickness monitor. Using thephysical thickness and the relative dielectric constants (i.e., 7.5for SiN and �20 for HfO2), EOT for the hAl/HfO2/SiN/Si(100)iMOS diodes is estimated to be 1.2 nm, in reasonable agreementwith that obtained from the C–V measurements (i.e., 1.4 nm). Inthe estimation, eSiN is estimated to be 6.2, assuming a linear com-position dependence of eSiN and considering the atomic concentra-tion ratio, N/(N + C), of 0.65.

The nitrogen concentration of the SiN layer (i.e., N/(N + O) =0.65) formed in the present study is much higher than that forSiON layers formed by NH3-plasma and N2O-plasma [13,14,19].The complete prevention of the interfacial reaction even at400 �C (cf. Fig. 5b) probably results from the dense structure ofthe SiN layer with a high nitrogen atomic concentration. It shouldbe noted that a Hf silicate interfacial layer is formed after the RFsputtering of HfO2 on a SiON layer with a low nitrogen concentra-tion of �3% [14], indicating that diffusion of Hf atoms cannot beprevented. It is also noted that when HfO2 layers are depositedon 1.0 nm SiO2 layers, the SiO2 thickness increases to 1.6 nm[18], indicating that oxygen diffusion proceeds. We think that thecapability of the formation of the SiN layers with the high nitrogenatomic concentration and thus with the great effect on preventingdiffusion is an advantage of the present SiN formation methodusing N2-plasma generated by low energy electron impact.

Insertion of the 1.0 nm SiN layer can decrease the leakage cur-rent density due to avoidance of the formation of minute conduc-tion channels and high density interface states resulting from Hfsilicide or suboxides. On the other hand, the 1.0 nm SiN layer com-pletely prevents the interfacial reaction, resulting in a smaller EOTthan that without the SiN layer. On the other hand, the thickness ofthe interfacial Si2O3 layer for the MOS diodes with no SiN layer isestimated to be 1.2 nm from the XPS spectrum (cf. lower spectrumin Fig. 1b). The tunneling probability, PT, through an insulatinglayer is given by [39]

PT ¼ exp½�ð4p=hÞð2mvÞ1=2d�

where v is the mean barrier height, d is the thickness of the insulat-ing layer, and m is the effective mass of a charge carrier. The similarleakage current densities for the hAl/HfO2/SiN/Si(100)i and hAl/HfO2/Si2O3/Si(100)i MOS diodes are probably because the barrierheight for the Si2O3/Si structure is slightly lower (i.e., �30% lower)than that for the SiN/Si structure.

The leakage current densities in the forward bias region for thehAl/HfO2/Si(100)i and hAl/HfO2/SiN/Si(100)i MOS diodes aredecreased to 1/8–1/100 and 1/4–1/10, respectively, by PDA. Thedecrease for the latter diode is attributable to the improvementof the HfO2 layer because no interfacial reaction proceeds. Forthe former diode, on the other hand, the interfacial reaction occurs,resulting in the removal of Hf silicide. Considering that the PDA-in-duced improvement of HfO2 decreases the leakage current densityto 1/4–1/10, it is roughly estimated that the interfacial reaction de-creases it to 1/2–1/10.

H. Kobayashi et al. / Surface Science 602 (2008) 1948–1953 1953

5. Conclusion

We have performed XPS and electrical measurements of HfO2/Si(100) and HfO2/SiN/Si(100) structures and reached the follow-ing conclusions:

1) With no buffer layer, Hf silicide is formed at the HfO2/Sistructure, resulting in the high leakage current density dueto minute channels and/or high density interface states nearthe Fermi level.

2) Heat treatment of the HfO2/Si structure transforms Hf sili-cide to Si suboxide plus Hf suboxide, while the leakage cur-rent density is still high due to the formation of interfacestates at 0.53 eV above the Si VBM.

3) A 1 nm SiN layer formed by N2-plasma generated by lowenergy electron impact possesses a high nitrogen atomicconcentration, N/(N + O), of 65%.

4) Insertion of the 1 nm SiN layer decreases EOT because of thecomplete prevention of the interfacial reaction.

5) The 1 nm SiN layer can completely prevent diffusion of Hfand oxygen atoms, and consequently, PDA at 400 �C doesnot change EOT. In spite of the unchanged EOT, the leakagecurrent density for the hAl/HfO2/SiN/Si(100)i MOS diode isgreatly decreased by PDA due to the improvement in thecharacteristics of the HfO2 layer.

References

[1] R. Puthenkovilakam, Y.-S. Lin, J. Choi, J. Lu, H.-O. Blom, P. Pianetta, D. Devine, M.Sendler, J.P. Chang, J. Appl. Phys. 97 (2005) 023704.

[2] Y. Hoshino, Y. Kido, K. Yamamoto, S. Hayashi, M. Niwa, Appl. Phys. Lett. 81(2002) 2650.

[3] J. Lu, J. Aarik, J. Sundqvist, K. Kukli, A. Hårsta, J.-O. Carlsson, J. Crystal Growth273 (2005) 510.

[4] P.K. Park, J.-S. Roh, B.H. Choi, S.-W. Kang, Electrochem. Solid-State Lett. 9(2006) F34.

[5] Y. Senzaki, S. Park, H. Chatham, L. Bartholomew, W. Nieveen, J. Vac. Sci.Technol. A 22 (2004) 1175.

[6] R.P. Pezzi, J. Morais, S.R. Dahmen, K.P. Bastos, L. Miotti, G.V. Soares, F.L. Freire Jr,J. Vac. Sci. Technol. A 21 (2003) 1424.

[7] R. Tan, Y. Azuma, I. Kojima, Appl. Surf. Sci. 241 (2005) 135.[8] G. He, M. Liu, L.Q. Zhu, M. Chang, Q. Fang, L.D. Zhang, Surf. Sci. 576 (2005) 67.

[9] C.M. Lopez, E.A. Irene, J. Appl. Phys. 99 (2006) 024101.[10] O. Renault, D. Samour, J.-F. Damlencourt, D. Blin, F. Martin, S. Marhon, N.T.

Barrett, P. Besson, Appl. Phys. Lett. 81 (2002) 3627.[11] Y. Nagasato, T. Ueno, Jpn. J. Appl. Phys. 44 (2005) 1665.[12] L. Wang, K. Xue, J.B. Xu, A.P. Huang, P.K. Chu, Appl. Phys. Lett. 88 (2006)

072903.[13] S. Sayan, E. Garfunkel, S. Suzer, Appl. Phys. Lett. 80 (2002) 2135.[14] S. Maikap, J.-H. Lee, R. Mahapatra, S. Pal, Y.S. No, W.-K. Choi, S.K. Ray, D.-Y. Kim,

Solid-State Electron. 49 (2005) 524.[15] Z. Zhang, B. Xia, W.L. Gladfelter, S.A. Campbell, J. Vac. Sci. Technol. A 24 (2006)

418.[16] H. Ota, N. Yasuda, T. Yasuda, Y. Morita, N. Miyata, K. Tominaga, M. Kadoshima,

S. Migita, T. Nabatame, A. Toriumi, Jpn. J. Appl. Phys. 44 (2005) 1698.[17] S. Kamiyama, T. Miura, Y. Nara, J. Electrochem. Soc. 152 (2005) G903.[18] O. Maida, K. Fukayama, M. Takahshi, H. Kobayashi, Y.-K. Kim, H.-C. Kim, D.-K.

Choi, Appl. Phys. Lett. 89 (2006) 122112.[19] C.S. Lai, K.M. Fan, Jpn. J. Appl. Phys. 44 (2005) 5964.[20] T.M. Pan, T.F. Lei, H.C. Wen, T.S. Chao, IEEE Trans. Electron. Dev. 48 (2001) 907.[21] R. Nieh, R. Choi, S. Gopalan, K. Onishi, C.S. Kang, H.-J. Cho, S. Krishnan, J.C. Lee,

Appl. Phys. Lett. 81 (2002) 1663.[22] H. Kobayashi, T. Mizokuro, Y. Nakato, K. Yoneda, Y. Todokoro, Appl. Phys. Lett.

71 (1997) 1978.[23] T. Mizokuro, K. Yoneda, Y. Todokoro, H. Kobayashi, J. Appl. Phys. 85 (1999)

2921.[24] M. Takahashi, T. Mizokuro, Y. Nishioka, H. Kobayashi, Surf. Sci. 518 (2002) 72.[25] G.D. Wilk, R.M. Wallance, J.M. Anthony, J. Appl. Phys. 87 (2000) 484.[26] H. Kato, T. Nango, T. Miyagawa, T. Katagiri, K.S. Seol, Y. Ohki, J. Appl. Phys. 92

(2002) 1106.[27] Q. Fang, J.-Y. Zhang, Z. Wang, M. Modreanu, B.J. O’Sullivan, P.K. Hurley, T.L.

Leedham, D. Hywel, M.A. Audier, C. Jimenez, J.-P. Senateur, I.W. Boyd, ThinSolid Films 453 (2004) 203.

[28] F.J. Himpsel, F.R. McFeely, A. Taleb-Ibrahimi, J.A. Yarmoff, G. Hollinger, Phys.Rev. B 38 (1988) 6084.

[29] E.H. Nicollian, A. Goetzberger, Bell Syst. Tech. J. 46 (1967) 1055.[30] Y.-N. Xu, W.Y. Ching, Phys. Rev. B 51 (1995) 17379.[31] C.H.F. Peden, J.W. Rogers Jr., N.D. Shinn, K.B. Kidd, K.L. Tsang, Phys. Rev. B 47

(1995) 15622.[32] Z.-Q. Yao, J. Appl. Phys. 78 (1995) 2906.[33] H. Takahashi, S. Toyoda, J. Okabayashi, H. Kumigashira, M. Oshima, Y. Sugita,

G.L. Liu, Z. Liu, K. Usuda, Appl. Phys. Lett. 87 (2005) 012903.[34] A. Asano, Asuha, O. Maida, Y. Todokoro, H. Kobayashi, Appl. Phys. Lett. 80

(2002) 4552.[35] T. Sakoda, M. Matsumura, Y. Nishioka, Appl. Surf. Sci. 117/118 (1997) 241.[36] Q. Li, S.J. Wang, P.C. Lim, J.W. Chai, A.C.H. Huan, C.K. Ong, Thin Solid Films 462–

463 (2004) 106.[37] E.H. Nicollian, J.R. Brews, MOS (Metal Oxide Semiconductor) Physics and

Technology, Wiley, New York, 1982 (Chapter 15).[38] H. Kobayashi, T. Ishida, Y. Nakato, H. Tsubomura, J. Appl. Phys. 69 (1991)

1736.[39] H.C. Card, E.H. Rhoderick, J. Phys. D 4 (1971) 1589.