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Thin Solid Films 472

Ternary phase analysis of interfacial silicates grown in

HfOx/Si and Hf/SiO2/Si systems

Jung-Ho Lee*

Department of Materials and Chemical Engineering, Hanyang University, Ansan, Kyoungki 426-791, Korea

Received 27 January 2004; received in revised form 10 July 2004; accepted 16 July 2004

Available online 8 September 2004

Abstract

The annealing of two different film stacks, i.e., HfOx/Si and Hf/SiO2/Si, was investigated in situ in an ultrahigh vacuum by using scanning

tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Although a kinetic metastability in the incongruently melting

compounds such as HfO2 was reported to form SiO2 and HfO2 separately at the interface with Si, interfacial silicates (Hf�O�Si bonding

units) were grown irrespective of film stacks. Ternary phase consideration in the Hf–Si–O system suggests that many nonstoichiometric

silicates can be formed from the solid solutions of various compositions. The presence of nonstoichiometric silicates is ascertained by the

STM results that show vagueness between oxygen-poor silicates and oxygen-containing silicides.

D 2004 Elsevier B.V. All rights reserved.

PACS: 68.60.Dv; 68.55.Nq; 68.37.Ef; 79.60.-i

Keywords: Hafnium; Scanning tunneling microscopy; X-ray photoelectron spectroscopy; Interfaces

1. Introduction

Upon rapid shrinking of the transistor feature size, the

gate dielectric has emerged as one of the most difficult

challenges in the semiconductor device fabrications. Many

researchers are now pursuing various paths to discern the

most suitable alternative high-permittivity (high-k) gate

material to thermally grown SiO2. Hafnium oxides satisfy

many of the requirements needed to replace SiO2 in the next

generation silicon devices since they have a high dielectric

constant (~25), a high dielectric breakdown field (15–20

MV/cm), a large bandgap (~5 eV) and are calculated to be

thermodynamically stable on silicon [1].

One major concern with high-k materials such as

hafnium oxides is the degradation [1–4] of the channel

carrier mobility along with the gate oxide integrity. This

feature is even observed at the abrupt interface with Si, as

0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.tsf.2004.07.060

* Formerly with Hynix Semiconductor Inc. Tel.: +823 1400 5278;

fax: +823 1419 7203.

E-mail address: jungho@hanyang.ac.kr.

reported in the Al2O3/Si system [2]. Since the SiO2-like

interface can prevent this problem, interfacial reactions that

produce a low-k layer [5–7] (SiO2 or silicate) are reported to

be advantageous for attaining the expected mobility as well

as the oxide integrity in high-k metal oxide systems.

Identifying whether a low-k layer is all based on SiO2 (or

silicate), however, leads to some ambiguities [7–9] even in

the same material system because phase and compositional

analyses are required within only a few monolayers.

Although previous study [8] based on binary-oxide phase

diagrams suggests that the interfacial layers between Si and

Hf(or Zr)O2 tend towards pure SiO2, the formation of silicate-

like layers was reported in some experiments [9–11] utilizing

chemical vapor deposition and/or oxygen-rich ambients.

To clarify the detailed structure and chemistry of the

high-k oxide/Si interfaces, this work investigates the various

combinations of initial film stack that consist of HfOx/Si and

Hf/SiO2/Si. Utilizing scanning tunneling microscopy (STM)

with in situ X-ray photoelectron spectroscopy (XPS),

interfacial silicates (Hf�O�Si bonding units) rather than

SiO2 are found to form irrespective of film stack. Ternary

phase consideration in the Hf–Si–O system explains why

(2005) 317–322

J.-H. Lee / Thin Solid Films 472 (2005) 317–322318

many nonstoichiometric silicates can be formed from the

solid solutions of various compositions. Indeed, our STM

results that do not clearly distinguish between oxygen-poor

silicates and oxygen-containing silicides support the pres-

ence of nonstoichiometric silicates.

Fig. 1. Hf 4f, Si 2p and O 1s core-level photoelectron spectra of the Hf

(~0.4 ML)/SiO2/Si(001) system. At 700 8C annealing, the Hf 4f doublets

(4f5/2 and 4f7/2) position indicates a formation of silicates (4f5/2=~20 eV),

not the metallic (4f5/2=~17 eV) or oxide (4f5/2=~19 eV) ones. At 900 8C,however, phase separation of silicate phase into silicides (4f5/2=~16.7 eV) is

observed and the oxide component of Si 2p core-level drastically weakens

to indicate a disappearance of Si–O bonding units.

2. Experimental

The sample substrates used were cut from Czochralski-

grown n-type (5–10 V cm) Si(001) wafers. Two different

film structures, i.e., Hf/SiO2/Si and HfOx/Si, were made to

compare the interfacial phase and compositions. For the Hf/

SiO2/Si structure, clean Si(001) surfaces were thermally

oxidized to form 1-nm-thick SiO2 by backfilling a STM

chamber with molecular oxygen for 60 min at 740 8C under

5�10�2 Pa [12]. Clean Si(001)-2�1 surfaces were prepared

by a flash at 1200 8C in the chamber. Electron-beam

evaporated 0.4�2-monolayer (ML)-thick Hf (1 ML=2.4 2)was deposited on the underlying SiO2 at room temperature

(RT), and annealed in ~2�10�8 Pa in order to avoid a phase

stability being affected by externally supplied oxygen. For

the HfOx/Si structure, hafnium (0.5�2 nm) was deposited

on the clean Si(001)-2�1 surface at RT and annealed in an

oxygen ambient (~10�2 Pa) to grow a HfOx layer on

Si(001).

The interfacial structure and composition were analyzed

in situ by using XPS, where Mg Ka was adopted for the x-

ray radiation source and the photoelectrons were detected at

a takeoff angle of 608 with respect to the surface normal.

Furthermore, XPS was also used to determine the thick-

nesses of the oxide layers. The SiO2 film thickness was

estimated by using the escape depth value of 3.5 nm for Si

2p photoelectrons through the SiO2 layer. Morphological

variations of ultrathin hafnium oxides upon annealing were

also observed using STM by applying a sample bias of ~5 V

to inject tunneling electrons into the oxide conduction band.

Scanning tunneling spectroscopy (STS) measurements

[11,13–15], which are taken within a sample bias (Vs)

range withinF6 V, were performed in spectroscope mode to

obtain the tunneling current spectra that could clarify the

dielectric property of the silicides precipitated from silicate

phases. The dielectric strength of the silicide surfaces, which

reflects the surface bandgap of the insulator, is measured

from the current–voltage (I–V) spectra. The measured bias

width of a ~0 tunneling current, which is observed near the

Fermi level (=0 V), corresponds to the local dielectric

strength.

3. Results

Fig. 1 shows the Hf 4f, Si 2p and O 1s core-level

photoelectron spectra of the Hf (~0.4 ML)/SiO2/Si(001)

system. Annealing in the 600�800 8C temperature ranges

causes the phase transformation of the SiO2 into the silicate

(Hf�O�Si bonding units) through the reaction and inter-

mixing at the Hf/SiO2 interface. At 700 8C annealing, the Hf

4f doublets (4f5/2 and 4f7/2) position indicates a formation of

silicates (4f5/2=~20 eV), not the metallic (4f5/2=~17 eV) or

oxide (4f5/2=~19 eV) ones [15,16]. Analyzing the Si 2p and

O 1s core-level components also shows that Si�O bonds in

a thin SiO2 layer likely transform into Hf�O�Si bonds, but

an amount of low-energy peak shifts is very limited because

the silicate transformation triggered by a submonolayer

hafnium is localized near the topmost surface. This agrees

well with an early STM work [11] that shows a formation of

the compositionally graded silicate.

Fig. 2 shows the STM topographies of the as-Hf

deposited, 580 and 800 8C annealed Hf (~1 ML)/SiO2/Si

stacks, respectively. Ultrathin metal deposition on oxide

film has previously been found to form cluster-like

morphology like Fig. 2(a) on Al/TiO2 and Pd/MgO

[17,18]. The average diameter of cluster tends to increase

with deposited metal coverage, and the density of state near

the Fermi level also increases with cluster size implying

development of metallic nature. The interesting thing is that

a cluster-like nodular structure transforms into a well-

developed surface crystalline texture at 580 8C.A surface crystalline texture was reported [7,11] to stem

from the polycrystalline HfOx, where a compositional

gradient was observed from the topmost HfOx to the SiO2

at the Si/SiO2 interface. These features were obtained while

Si�O bonds in 1-nm-thick SiO2 transformed into Hf�O�Si

bonds in the annealed silicate phase, which is also consistent

with a clear Hf 4f spin-orbit splitting with a high-energy shift

(see the 700 8Cannealed data in Fig. 1). As shown in Fig. 2(c),

the crystalline structure starts to decompose around 800 8C.A strong propensity for silicate transformation was also

found in the HfOx/Si(001) system. Analyzing the Hf 4f and

O 1s core-level photoelectron spectra shows that the initial

Hf�O bonds in the HfOx system transform into the

Hf�O�Si bonding units in the 600�800 8C temperature

Fig. 2. The STM topographies of the (a) as-Hf deposited, (b) 580 8C and (c) 800 8C annealed Hf (~1 ML)/SiO2/Si(001) stacks. The measurement condition is 5

V for the sample bias and 0.5 nA for the tunneling current. The well-developed surface crystalline texture is shown in (b) and likely decomposes at 800 8C.Scanning area is 80�80 nm2.

J.-H. Lee / Thin Solid Films 472 (2005) 317–322 319

range, as shown in Fig. 3. The HfOx/Si structure starts to

transform into silicate (Hf�O�Si bonding units) around

600 8C, based on a higher energy shift of Hf 4f doublets

position. The bonding structure in the 600�800 8C temper-

ature ranges is consistent with silicate phase (4f5/2=~20 eV)

[15]. This feature agrees well with previous chemical

analysis [19] of an interface region with subnanometer

spatial resolution. Utilizing a scanning high-resolution

Fig. 3. Hf 4f and O 1s core-level photoelectron spectra of a ~1.5-nm-thick

HfOx layer on Si(001). (a) At 600�800 8C annealing, a high-energy shift of

Hf 4f doublets (4f5/2 and 4f7/2) position indicates a formation of silicates

(4f5/2=~20 eV), not the metallic (4f5/2=~17 eV) or oxide (4f5/2=~19 eV)

ones. Above 900 8C, however, the phase separation into silicides (4f5/2=

~16.7 eV, 4f7/2=~14.8 eV) is observed. (b) Upon silicidation, O 1s core-

level spectra reveal that Hf�O�Si bonding units leave some Si�O

bondings (O 1s=~532.5 eV) due to a disappearance of Hf�O bondings (O

1s=~530 eV).

transmission electron microscope attached with an energy-

dispersive X-ray analyzer, the presence of Si ions that

migrated into a hafnium oxide was clearly observed along

with the voids in the substrate close to the interface.

At 1000 8C, however, the phase separation of silicate

phase into silicides (4f7/2=~14.8 eV) is observed and the O

1s core-level analysis reveals the presence of remnant Si�O

Fig. 4. The STM image and tunneling current spectra of silicides

precipitated from the silicate phases. (a) At 900 8C for 2 min, phase

separation of silicate into silicides is observed. The irregularly shaped

morphology of silicides is noted in a silicate matrix. Scanning area is

120�120 nm2. (b) Tunneling current spectra of silicides. The irregularly

shaped silicides show insulating characteristics. The insulating feature

slightly decreases upon further annealing, but still remains even after being

annealed at 1000 8C. In contrast, a metallic character is shown in crystalline

HfSi2 facetbars. The measurement condition is 5 V for the sample bias and

0.5 nA for the tunneling current.

Fig. 5. Binary phase diagram of a ZrO2–SiO2 system. Cub, Mon and

Tet=cubic, monoclinic and tetragonal forms. Crs=cristobalite, Trd=tridymite,

H and L=high- and low-temperature forms. (See Refs. [23,24] for more

details; reproduced with permission from Ref. [23].)

J.-H. Lee / Thin Solid Films 472 (2005) 317–322320

bonds due to a disappearance of Hf�O bonds in the

Hf�O�Si bond structure. Upon silicidation, silicate-bonds

are believed to form Hf�Si and Si�O bonds separately.

Remnant Si�O bonds in Hf silicides are estimated to affect

the morphology of silicides precipitated from the silicates.

In contrast with crystalline hafnium silicides having a

bfacetbarQ shape [15,20], oxygen-containing silicides reveal

the birregularly shapedQ morphology shown in Fig. 4(a).

Note that the irregularly shaped silicides do not clearly

distinguish between oxygen-poor silicates and oxygen-

containing silicides, although the Hf 4f core-level position

shown in Fig. 3(a) indicates a complete silicidation from the

silicate structure.

The irregular morphologies reflect that the local surface

region of silicides contains some oxygen as a form of Si�O

bonds, and hence the atomic structure of these phases could

not be observed by STM because of their insulating

characteristics. To clarify the dielectric property of the local

surface, STM measurements were also performed in a

spectroscope mode to obtain the tunneling current spectra

shown in Fig. 4(b). While the crystalline HfSi2 facetbars

show metallic nature [15], some amounts of the dielectric

strength are observed in the irregularly shaped silicides. The

measured bias width of a ~0 tunneling current, correspond-

ing to the local dielectric strength, decreases but still

remains upon further annealing to 1000 8C.

1 For isothermal section at 2000 8C, ZrSi2 powder and ZrSi2/SiO2

powder mixtures were heated to compare the stability of silica and ZrSi2phases. For 1680 8C, two series of specimens were prepared; pure silicon

powder was the starting material for all preparations. In the first series, Zr

sponge (4.5 at.% O) was used to prepare six compositions. For the second

series, impure Zr (~22 at.% O) was used for a similar compositional range.

Additional specimens were prepared with 15–18% O and b30% Si to

clarify the a-Zr ss�silicide phase relations.

4. Discussion

In an effort to elucidate the interfacial phase between

hafnium oxides and the silicon substrate, silicate phases that

are not SiO2-like appeared clearly in our work based on

annealing the HfOx/Si and Hf/SiO2/Si films. Regarding

previous reports [11,21], this feature would be more evident

in an oxygen-rich ambient which is typical for high-k

deposition.

It is noteworthy that these results contradict the system-

atic study based on the consideration of oxide phase

diagrams. Maria et al. [8] suggested that candidate metal

oxides should be classified as bsilicate formersQ versus

bphase separatorsQ from the consideration of binary-oxide

phase diagrams. The former corresponds to M2O3-type

compounds such as La2O3, Y2O3 and Gd2O3, which form

congruently melting silicates based on the M2O3–SiO2

phase diagram, while the latter is typified as a MO or MO2

set such as Hf(Zr)O2, ThO2 and NiO, which form

incongruently melting silicates in oxide phase diagrams

(see Fig. 5). A congruently melting compound will melt to a

liquid of its own composition when it is heated and no

kinetic limitations are required for the silicate formation.

On the other hand, the incongruently melting compound,

which when heated does not melt to a liquid of its own

composition, melts to a liquid and another solid. Since this

type of reaction requires a solid-state diffusion of Si and

metal atoms to form a equilibrium silicate phase, the

metastability should kinetically lead to a strong propensity

for phase separation, i.e., Hf(Zr)O2+SiO2, in these peritectic

systems. Their conjectures can explain the experimental

results [7,22] reporting that the interfacial layers between Si

and Hf(Zr)O2 are likely towards pure SiO2, not silicate.

After high-temperature annealing, the SiO2-like interfacial

layers should be thicker without a composition change. In

other works [9,11,15,19,21] (including this work) for the

same materials system, however, a strong propensity for

silicate formation has also shown a possibility of various

energetics of elementary reaction steps at the interface

region.

To clarify these elusive characteristics, we consider

ternary phase diagrams1 [23] of O–Si–Zr (Fig. 6(a) and

(b)) in conjunction with binary-oxide phase diagram of

ZrO2–SiO2. We note that the phase diagram of ZrO2–SiO2

shown in Fig. 5 is a study of the stability of zircon (ZrSiO4)

mixed with silica and zirconia, and the materials considered

are not based on the elements but on the oxide powders (or

pellets) [24]. This approach would be inappropriate to

thermodynamically predict the elementary reactions around

Fig. 6. Ternary phase diagrams of Zr–Si–O system showing isothermal

sections at (a) 2000 8C and (b) 1680 8C. SS1=ZrO2 solid solution (ss),

SS2=Zr5Si3 ss, a=a-Zr ss, h=h-Zr ss. (See Refs. [23,25] for more details;

reproduced with permission from Ref. [23].)

Fig. 7. The spinodal system which is unstable against fluctuation in

composition. As the reaction time (t) continues, one part of the system

grows more concentrated at the expense of another one (see Ref. [30] for

more details).

J.-H. Lee / Thin Solid Films 472 (2005) 317–322 321

the interface because the oxygen-free reactions such as

metal-silicon bonds (silicidation) cannot be basically con-

sidered by the oxide end-members such as ZrO2 and SiO2.

For example, at 2000 8C, an oxide diagram predicts a

coexistence of tetragonal-ZrO2 and [(ZrxSi1�x)O2]L, but

considering a ternary diagram based on the elements, the

liquid phase, [(ZrxSi1�x)O2]L, is estimated to be a mixture of

(SiO2)L and (ZrSi)L. A further noticeable difference appears

at 1680 8C (just above the peritectic point), where a two

phase region of tetragonal-zirconia and cristobalite spans the

entire compositional range in the oxide phase diagram.

Since a presence of two solid phase region requires a solid-

state diffusion of Si and Zr atoms through highly refractory

solids to form a equilibrium silicate phase, it is kinetically

estimated that a metastability should make it difficult to

form a silicate at the interface.

On the contrary, the important thing we note at 1680 8Cin consideration of the ternary phase diagram is an

appearance of solid solutions. Upon cooling, zirconia and

Zr5Si3 clearly form a Si-containing ZrO2 solid solution (SS1in Fig. 6(b)) and an oxygen-containing Zr-silicide solid

solution (SS2), respectively. The Zr-rich regions also trans-

form into solid solutions (a and h) and a liquid solution.

The presence of equilibrium solid solutions between

zirconia and other Zr-silicides is also predicted below

1625 8C (not shown) [25]. Considering that the metallo-

graphic distinction between solid solutions and composi-

tionally neighboring compounds is not reliable [23], these

solid solutions are expected to be closely consistent with Si-

poor silicates (for SS1), O-poor silicates (for SS2) and Zr-

rich silicates (for a and h), respectively.The same should be expected in the Hf–O–Si system

based on the chemical similarities between Hf and Zr [1]. In

our STM work, the birregularly shapedQ silicides shown in

Fig. 4(a) did not clearly distinguish between oxygen-poor

silicates and oxygen-containing silicides, although the

author had already identified them as silicides by the XPS

results. A significant insight of these conjectures is that a

peritectoid reaction that requires a solid-state diffusion

(kinetically unfavorable) is not a unique way to form

silicates.

Many nonstoichiometric silicates (or glasses) are known

to be normally phase-separated via bspinodal decom-

positionQ [26–28] triggered by compositional fluctuations,

which are kinetically favorable compared with bnucleationand growthQ. For those cases in which the molar free energy

of mixing has regions of negative curvature, i.e., B2Gmix/

BX2b0, a system is unstable against fluctuations in a

composition, so one part of the system becomes more

concentrated at the expense of another [29,30]. The system

is inherently unstable and phase separation will proceed as

illustrated in Fig. 7. The most characteristic feature of these

systems is that both phases are structurally and composi-

tionally interconnected [26,30], in contrast to the systems

that are phase-separated by nucleation and growth, where

the precipitates are distinctively different from the parents.

J.-H. Lee / Thin Solid Films 472 (2005) 317–322322

Taking the compositional flexibility of Zr(Hf)–Si–O

solid solutions (e.g., SS1, SS2, a, h) into account, kineti-

cally, many nonstoichiometric silicates should form via a

spinodal decomposition which only requires an arbitrarily

small compositional fluctuation in solid solutions. Along

with these features, the dominance of the Zr–ZrO2–silicide

solid solutions at moderate O-activities suggests a possi-

bility of compositionally graded silicates, which has

recently been reported [11], at the high-k interface.

5. Conclusions

The annealing of two different film structures, i.e., HfOx/

Si and Hf/SiO2/Si, resulted in the formation of interfacial

silicates (Hf�O�Si bonding units). Although a metastabil-

ity in the incongruently melting compounds such as HfO2

was reported to form SiO2 and HfO2 separately without

silicate reactions, our ternary phase analysis suggested the

possibility of silicate formation from the solid solutions of

various compositions in the Hf–Si–O system. The

birregularly shapedQ silicides found in the STM work

reflected the presence of nonstoichiometric silicates that

showed vagueness between oxygen-poor silicates and oxy-

gen-containing silicides.

Acknowledgements

The author would like to thank Prof. Masakazu Ichikawa

and Dr. Alexander Shklyaev for helpful discussions and

technical assistance on STM measurement.

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