Electronic properties of ultrathin high-κ dielectrics studied by ballistic electronemission microscopyH. L. Qin, C. Troadec, K. E. J. Goh, K. Kakushima, H. Iwai, M. Bosman, and K. L. Pey Citation: Journal of Vacuum Science & Technology B 29, 052201 (2011); doi: 10.1116/1.3622296 View online: http://dx.doi.org/10.1116/1.3622296 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/29/5?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Epitaxial, well-ordered ceria/lanthana high-k gate dielectrics on silicon J. Vac. Sci. Technol. B 32, 03D124 (2014); 10.1116/1.4876122 Comparative study of tunneling currents through silicon dioxide and high- κ dielectric hafnium oxide partlyembedded with nanocrystals and nanotubes in metal oxide semiconductor structures J. Appl. Phys. 104, 034313 (2008); 10.1063/1.2963705 Hot-electron transport through Au/CaF 2 /Si (111) structure studied by ballistic electron emission spectroscopy J. Appl. Phys. 85, 941 (1999); 10.1063/1.369214 Current oscillations in thin metal–oxide–semiconductor structures observed by ballistic electron emissionmicroscopy J. Vac. Sci. Technol. B 16, 2296 (1998); 10.1116/1.590164 Ballistic electron emission microscopy studies on Au/CaF 2 /n- Si (111) heterostructures J. Vac. Sci. Technol. A 16, 2653 (1998); 10.1116/1.581396

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 129.24.51.181 On: Sun, 23 Nov 2014 03:07:09

Electronic properties of ultrathin high-j dielectrics studied by ballisticelectron emission microscopy

H. L. Qina)

School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue,Singapore 639798, Singapore

C. Troadecb) and K. E. J. GohInstitute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research),3 Research Link, Singapore 117602, Singapore

K. Kakushima and H. IwaiTokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan

M. BosmanInstitute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research),3 Research Link, Singapore 117602, Singapore

K. L. PeySchool of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue,Singapore 639798, Singapore

(Received 20 February 2011; accepted 27 June 2011; published 15 August 2011)

Ballistic electron emission microscopy was employed in order to investigate the electronic

properties of sub-nanometer high-j dielectrics (CeO2 and La2O3). The authors found that such a

thin dielectric sandwiched between Au and n-Si fails to exhibit the same electronic barrier as its

bulk counterpart, but it can still significantly attenuate the ballistic electron transport. The authors

attribute the observed smaller barrier height to quantum tunneling and/or induced gap states. The

results suggest that such ultrathin high-j dielectrics in a metal-dielectric-semiconductor structure

do not show a fully formed electronic barrier. VC 2011 American Vacuum Society.

[DOI: 10.1116/1.3622296]

I. INTRODUCTION

With the continued scaling of metal-oxide-semiconductor

(MOS) field effect transistors, SiO2 and SiON (silicon oxyni-

tride) are now reaching their thickness limits as effective

gate dielectrics. Alternative approaches such as metal gate/

high-j dielectrics have already been successfully demon-

strated.1 In order to understand how well the new dielectrics

can extend the scaling down, it is of both technological and

fundamental interest to study ultrathin high-j dielectrics and

metal/high-j/Si interfaces in the regime of limiting

thickness.

To date, few have studied ultrathin SiO2 and other dielec-

trics of less than 1 nm. Tang et al.2 and Muller et al.3 found

the minimum thickness for an ideal SiO2 gate oxide to be

around 0.7 nm. A few first-principles calculations indicated

that the band edges at the SiO2-Si interface change gradually

instead of abruptly and that the bandgap of SiO2 decreases at

the SiO2 side near the interface.4–7 In ballistic electron emis-

sion microscopy (BEEM) studies, Ludeke et al.8 and Quat-

tropani et al.9 observed a much smaller threshold than the

expected metal/SiO2 barrier at some areas of very thin SiO2

samples, but not with thick SiO2 samples. LaBella et al.10,11

and Sumiya et al.12 both found rather small electron barrier

heights for ultrathin calcium fluoride (CaF2) samples as com-

pared to those of thicker CaF2 films in the same structures.

However, it is still unclear what electronic properties the

dielectric has when its thickness lies in a crossover regime in

which it is losing its bulklike properties.

In this paper, we investigate ultrathin high-j dielectrics a

few angstroms thick in MOS structures in terms of the elec-

tronic barrier height using BEEM. We find that such a thin

dielectric in a MOS structure does not show as high an elec-

tronic barrier as a thick bulk dielectric does.

II. EXPERIMENT

A. BEEM technique

BEEM, a technique based on scanning tunneling micros-

copy (STM), was developed by Kaiser and Bell in 1988.13,14

Its unique advantage is the ability to probe material interfaces

with high spatial resolution (typically a few nanometers). This

technique was applied extensively to the study of metal/

semiconductor Schottky junctions,14 and later to metal-

oxide-semiconductors.15

Figure 1(a) is a typical experimental configuration of a

BEEM study on a metal/n-type semiconductor device. Figure

1(b) illustrates the corresponding energy band diagram. In this

configuration, the thin metal film (usually �10 nm) is

grounded and the STM tip is biased through a current pream-

plifier that measures the tunneling current; the semiconductor

backside is connected to another current preamplifier via an

ohmic contact in order to measure the BEEM current. When

the tip is negatively biased, electrons tunnel from the tip to

a)Electronic mail: hailangqin@gmail.comb)Electronic mail: cedric-t@imre. a-star.edu.sg

052201-1 J. Vac. Sci. Technol. B 29(5), Sep/Oct 2011 1071-1023/2011/29(5)/052201/5/$30.00 VC 2011 American Vacuum Society 052201-1

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 129.24.51.181 On: Sun, 23 Nov 2014 03:07:09

the metal thin film. After transport through the thin film, a

fraction of the electrons will reach the metal–semiconductor

interface without losing energy significantly. The electrons

that have energies greater than the Schottky barrier height will

have a finite probability of transmitting through the barrier

and can then be collected as a BEEM current; electrons with

energies below the barrier will be blocked or will have a neg-

ligible probability for tunneling. The experimental setup and

the corresponding energy band diagram for a MOS device are

shown in Figs. 1(c) and 1(d), respectively. The basic principle

for BEEM measurement is similar to that in the metal/semi-

conductor case, except that the barrier height in this case is

generally defined by the Fermi level of the metal and the con-

duction band minimum of the oxide and might vary depend-

ing on the actual band alignment and charge trapping.16

In BEEM spectroscopy, commonly referred to as ballistic

electron emission spectroscopy (BEES), the BEEM current

is measured as a function of the tip bias. The behavior of the

BEES spectrum near the threshold (i.e., the region in which

the BEEM current starts to appear) is well described by the

following equation17,18:

IBEEM=It ¼ RðeV � UBHÞ2=eV; (1)

where IBEEM is the BEEM current, It is the tunneling current

(typically set to the tunneling current setpoint during fitting

unless otherwise stated), V is the tip bias, e is the elemental

charge, UBH is the interface barrier height, and R is the trans-

mission attenuation factor. The extracted barrier height is a

property of the interface, and the R factor reflects the propor-

tion of electrons that are actually collected as the BEEM

current. The term eV in the denominator is present to com-

pensate for the tunneling gap distance change during the

spectra, as proposed by Bannani et al.17 Analysis with a

power exponent of 5/2 instead of 2 is also applicable and

will not essentially affect the conclusion; however, there

could be a systematic difference in the extracted threshold

values.19–21

The tip can also be positively biased with respect to the

metal ground, and the electrons will flow from the metal film

to the tip, effectively injecting a hole into the metal. In this

case, the BEES is usually called reverse BEES (RBEES).22,23

A phenomenon that could potentially confuse the normal

BEEM measurements is the so-called STM induced photo-

current (STM-PC).24,25 STM-PC is commonly believed to be

due to the electron-hole pairs generated in the semiconductor

substrate by the photons emitted during tunneling at the

STM tunnel junction.26,27

B. Experimental details

For this study, n-type Si(100) (3� 1015 cm�3 phospho-

rous doped) substrates were first cleaned with a solution of

H2SO4 and H2O2 (ratio of 3:1) at 110 �C for 10 min and then

dipped into a 1% HF solution to form a hydrogen-terminated

surface. After the cleaning, CeO2 (0.3 nm and 1.3 nm) or

La2O3 (0.5 nm) was deposited via electron-beam evaporation

at a pressure of �10�6 Pa and a rate of 0.1 nm/min with the

substrate temperature held at 300 �C. After oxide deposition,

the 1.3 nm CeO2 sample only was annealed at 500 �C for 30

min in N2/H2 (97:3) ambient. All oxide thicknesses were

calibrated against the ellipsometry and transmission electron

microscopy measurements of a relatively thick oxide sample

prepared in the same way as the 1.3 nm CeO2 sample. A

layer of 50 nm of aluminum was deposited at the back of all

of the samples via thermal evaporation in order to make a

good ohmic contact. Using thermal evaporation, a layer of 9

nm Au was evaporated onto the top of each sample through

a shadow mask to form 0.5 mm diameter dots. Finally, the

samples were transferred ex situ into a modified RHK STM

for BEEM measurements. All measurements were carried

out at a base pressure of 8� 10�8 mbar using a mechanically

cut PtIr tip unless otherwise stated.

III. RESULTS AND DISCUSSION

A. Ultrathin dielectrics

Typical BEES spectra (IBEEM/It versus tip bias) with the re-

spective theoretical fittings for three different samples are out-

lined in Fig. 2(a): Au/n-Si(100) (blue cross), Au/0.3 nm

CeO2/n-Si(100) (red circle), and Au/0.5 nm La2O3/n-Si(100)

(magenta triangle). Each spectrum was fitted with Eq. (1),

with the tunneling current set to the setpoint current and the

fitting range limited to 0.4 eV above the best-fit barrier height.

The spectra for the 0.3 nm CeO2 and 0.5 nm La2O3 samples

gave almost the same barrier height as the spectrum obtained

for the Au/n-Si(100) sample (�0.81 6 0.02 eV). However, the

BEEM currents from these two oxide samples were signifi-

cantly attenuated compared to that for the Au/n-Si sample. In

order to obtain statistical information from the BEES analysis

of these samples, the dual parameter UBH versus R factor

FIG. 1. (Color online) (a) Schematic of BEEM configuration for metal/semi-

conductor and (b) the corresponding energy band diagram. (c) Schematic of

BEEM configuration for metal/oxide/semiconductor and (d) the correspond-

ing energy band diagram.

052201-2 Qin et al.: Electronic properties of ultrathin high-j dielectrics 052201-2

J. Vac. Sci. Technol. B, Vol. 29, No. 5, Sep/Oct 2011

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 129.24.51.181 On: Sun, 23 Nov 2014 03:07:09

distributions are plotted in Fig. 2(b), with each data point rep-

resenting the fitting results with Eq. (1) for a single spectrum

randomly taken across each sample. Figure 2(b) shows that

the majority of the spectra from these three samples gave a

similar barrier height of �0.8 eV. However, for the 0.3 nm

CeO2 and 0.5 nm La2O3 samples, larger barrier height spreads

and smaller R values (and hence smaller electron transmis-

sions) were obtained.

In particular, for the La2O3 sample, there are three differ-

ent regions: two regions around 0.8 eV with R factor values

peaking around �9� 10�3 (eV)�1 and �2� 10�3 (eV)�1,

and a third region with a barrier height of 1.0 to 1.1 eV and

an R factor value of �2� 10�3 (eV)�1. The STM topogra-

phy and corresponding BEEM image for the La2O3 sample

are shown in Figs. 3(a) and 3(b), respectively, taken at a tip

bias of �1.5 V and a tunneling current of 1.0 nA. The

BEEM image reveals patches of bright and dark areas that

show no clear correlation with the corresponding STM to-

pography, indicating that the contrast was not due to the Au

morphology. Instead it likely stems from nonuniformity in

the oxide film, with the dark areas representing thicker oxide

film. To have a better idea of the nonuniformity, three repre-

sentative BEES spectra (Fig. 4) were taken in the bright and

dark areas, corresponding to the three different regions in the

R factor versus barrier height distribution; each spectrum is

an average of a few spectra. Spectra (1) and (2) show the

same barrier height of 0.81 eV; however, the R value

(1.8� 10�3 (eV)�1) of spectrum (2) is almost 1 order of

magnitude smaller than the R value (1.1� 10�2 (eV)�1) for

spectrum (1), suggesting that spectrum (2) was from an area

with a relatively thicker oxide layer. In contrast, the barrier

(1.00 eV) of spectrum (3) is higher than those from the other

two spectra. This higher barrier for spectrum (3) could arise

from the STM-PC effect; the actual barrier might be even

higher (see the discussion on the contribution of STM-PC in

the next section). However, the fact that spectrum (3) did not

exhibit a barrier as small as the spectra taken from other

regions suggests that the probed area had an even thicker ox-

ide layer.

For Au/oxide/n-Si samples, the barrier height extracted

from BEES typically represents the offset between the Fermi

level of Au and the conduction band minimum of the oxide,

without considering the charge trapping. Others have

reported that the electron barrier height of Au/CeO2 is in the

range of 0.9–1.5 eV,28–30 and a value of �3.1 eV can be

FIG. 2. (Color online) (a) Representative BEES spectra (IBEEM/It vs tip bias)

with corresponding theoretical fittings for three samples: 9 nm Au/n-Si(100),

9 nm Au/0.3 nm CeO2/n-Si(100), and 9 nm Au/0.5 nm La2O3/n-Si(100). (b) Rfactor vs barrier height distributions from three different types of the samples

in (a).

FIG. 3. (Color online) (a) STM topography (color scale: 0–4 nm) of Au film,

and (b) the corresponding BEEM image (color scale: 0–13 pA) from a 0.5

nm La2O3 sample taken at Vtip¼�1.5 V, Itunnel¼ 1.0 nA.

052201-3 Qin et al.: Electronic properties of ultrathin high-j dielectrics 052201-3

JVST B - Microelectronics and Nanometer Structures

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 129.24.51.181 On: Sun, 23 Nov 2014 03:07:09

found for the Au/La2O3 interface based on Refs. 31 and 32.

However, from our BEEM measurements, the barrier heights

for both Au/0.3 nm CeO2/n-Si and Au/0.5 nm La2O3/n-Si

were around 0.8 eV, similar to the Au/n-Si barrier height.

Like most high-j dielectrics grown on silicon,31–33 there

could be an interfacial silicon oxide layer. However, the fact

that we detected a barrier height of 0.8 eV suggests that the

barrier of the interfacial silicon oxide layer was not yet (or

not fully) formed.

One possible reason for the detection of the lower barrier

height is quantum tunneling of the ballistic electrons below

the conduction band minimum of the dielectric. The dielec-

tric films in our samples are extremely thin, nominally 1 to 2

monolayers. The effective barrier within the oxide, if it

exists, is expected to be ultranarrow. Therefore, significant

tunneling through the oxide is expected, and the BEEM cur-

rent would start to increase at an energy corresponding to the

conduction band minimum of the silicon substrate, giving

the barrier height of Au/n-Si. Another possibility is the

induced gap states resulting from the exponential decay of

the silicon conduction band wavefunctions into the oxide, as

described by Muller et al.3 and Neaton et al.5

B. Thicker dielectric

To understand the transition from ultrathin (less than 1 nm)

to bulklike oxide, we studied a slightly thicker oxide film.

Figure 5(a) shows a representative BEES spectrum (negative

tip bias) of the sample of 9 nm Au/1.3 nm CeO2/n-Si(100).

Fitting of this spectrum with Eq. (1) gave a barrier height of

1.06 eV and an R factor of 5.7� 10�4 (eV)�1. The extracted

barrier height value is significantly larger than that of Au/n-Si,

possibly the barrier height of Au/CeO2.28–30 However, it is

too close to the silicon bandgap at room temperature, which

implies that the signal could be STM-PC. To establish the

contribution of STM-PC, we performed RBEES on the same

sample, and a representative spectrum is shown in Fig. 5(a)

on the positive tip bias side. Fitting this spectrum gave a bar-

rier height of 1.11 eV and an R factor of 7.5� 10�4 (eV)�1,

both of which are very close to those extracted from the

BEES spectrum. In addition, the polarity of the signal is in

the same direction as that of BEES. Further confirming that

the BEES and the RBEES spectra are independent of the tip

bias polarity is the almost identical statistical R factor versus

barrier height distribution shown in Fig. 5(b). As discussed by

Heller et al.24 and Li et al.,25 all of these features suggest that

the detected signal actually arose from STM-PC for the sam-

ple with 1.3 nm CeO2.

Our more recent measurement under ultrahigh vacuum

conditions using an etched W tip on a Au/1.3 nm CeO2/n-Si

device with in situ Au deposition showed a 3.6 eV interface

barrier height [as shown in the inset of Fig. 5(a)], which is

much higher than the interface barrier of Au/CeO2 reported

by others28–30 but is close to the interface barrier of Au/SiO2.9

This likely indicates that a significant layer of silicon oxide

had formed between the CeO2 and the silicon substrate. The

existence of the interfacial silicon oxide layer blocks electrons

with energies less than that of the Au/SiO2 barrier, confirming

that the �1.1 eV threshold observed on the sample with 1.3

nm CeO2 is not due to ballistic electrons.

FIG. 4. (Color online) Three different types of BEES spectra (each the aver-

age of a few spectra) with respective fittings (black solid lines) from the 0.5

nm La2O3 sample. The three types of spectra represent the three different

regions in the R factor vs barrier height distribution. Spectra (2) and (3) are

magnified four times (�4) and shifted vertically for a clearer view. Fittings

of spectra (1)–(3) give barrier heights of 0.81 eV, 0.81 eV, and 1.00 eV and

R factor values of 1.1� 10�2 (eV)�1, 1.8� 10�3 (eV)�1, and 1.9� 10�3

(eV)�1, respectively.

FIG. 5. (Color online) (a) Representative BEES and RBEES spectra (IBEEM/

It vs tip bias) with corresponding theoretical fittings for 9 nm Au/1.3 nm

CeO2/n-Si(100). The inset shows a spectrum (an average of 14 spectra) up

to high tip bias for 9 nm Au/1.3 nm CeO2/n-Si(100) sample. (b) R factor vs

barrier height distributions extracted from BEES and RBEES, all from the

9 nm Au/1.3 nm CeO2/n-Si(100) sample.

052201-4 Qin et al.: Electronic properties of ultrathin high-j dielectrics 052201-4

J. Vac. Sci. Technol. B, Vol. 29, No. 5, Sep/Oct 2011

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 129.24.51.181 On: Sun, 23 Nov 2014 03:07:09

IV. CONCLUSION

Using ballistic electron emission spectroscopy, we have

probed the electronic properties of a few angstroms of high-

j dielectrics of CeO2 and La2O3 and observed that such

ultrathin dielectrics sandwiched between Au and n-Si do not

exhibit bulklike electronic barriers. Although these ultrathin

dielectrics generally reduce ballistic electron transmission,

such MOS structures containing ultrathin dielectrics show

the same electron barrier height as a typical Au/n-Si device,

which we attribute to the possible combined effects of quan-

tum tunneling and induced gap states.

ACKNOWLEDGMENTS

One of the authors, H.L. Qin, would like to acknowl-

edge the NTU research scholarship. This work was sup-

ported by the Ministry of Education (MOE), Singapore

(Grant No. T206B1205).

1M. T. Bohr, R. S. Chau, T. Ghani, and K. Mistry, IEEE Spectrum 44, 29

(2007).2S. Tang, R. M. Wallace, A. Seabaugh, and D. King-Smith, Appl. Surf. Sci.

135, 137 (1998).3D. A. Muller, T. Sorsch, S. Moccio, F. H. Baumann, K. Evans-Lutterodt,

and G. Timp, Nature 399, 758 (1999).4S. T. Pantelides, S. N. Rashkeev, R. Buczko, D. M. Fleetwood, and R. D.

Schrimpf, IEEE Trans. Nucl. Sci. 47, 2262 (2000).5J. B. Neaton, D. A. Muller, and N. W. Ashcroft, Phys. Rev. Lett. 85, 1298

(2000).6T. Yamasaki, C. Kaneta, T. Uchiyama, T. Uda, and K. Terakura, Phys.

Rev. B 63, 115314 (2001).7M. Watarai, J. Nakamura, and A. Natori, Phys. Rev. B 69, 035312

(2004).8R. Ludeke, A. Bauer, and E. Cartier, J. Vac. Sci. Technol. B 13, 1830 (1995).

9L. Quattropani, I. Maggio-Aprile, P. Niedermann, and Ø. Fischer. Phys.

Rev. B 57, 6624 (1998).10V. P. LaBella, L. J. Schowalter, and C. A. Ventrice, Jr., J. Vac. Sci. Tech-

nol. B 15, 1191 (1997).11V. P. LaBella, Y. Shusterman, L. J. Schowalter, and C. A. Ventrice, Jr.,

J. Vac. Sci. Technol. A 16, 1692 (1998).12T. Sumiya, H. Fujinuma, T. Miura, and S.-I. Tanaka, Appl. Surf. Sci. 130,

36 (1998).13W. J. Kaiser and L. D. Bell, Phys. Rev. Lett. 60, 1406 (1988).14L. D. Bell and W. J. Kaiser, Phys. Rev. Lett. 61, 2368 (1988).15R. Ludeke, IBM J. Res. Dev. 44, 517 (2000).16W. Cai, K.-B. Park, and J. P. Pelz, Phys. Rev. B 80, 165322 (2009).17A. Bannani, C. Bobisch, R. Moller, Science 315, 1824 (2007).18K. E. J. Goh, A. Bannani, and C. Troadec, Nanotechnology 19, 445718

(2008).19M. Prietsch, Phys. Rep. 253, 163 (1995).20L. D. Bell and W. J. Kaiser, Annu. Rev. Mater. Sci. 26, 189 (1996).21P. L. de Andres, F. J. Garcia-Vidal, K. Reuter, and F. Flores, Prog. Surf.

Sci. 66, 3 (2001).22M. H. Hecht, L. D. Bell, W. J. Kaiser, and L. C. Davis, Phys. Rev. B 42,

7663 (1990).23L. D. Bell, M. H. Hecht, W. J. Kaiser, and L. C. Davis, Phys. Rev. Lett.

64, 2679 (1990).24E. R. Heller and J. P. Pelz, Appl. Phys. Lett. 82, 3919 (2003).25W.-J. Li, K. L. Kavanagh, A. A. Talin, W. M. Clift, C. M. Matzke, and

J. W. P. Hsu, J. Appl. Phys. 102, 013703 (2007).26J. K. Gimzewski, B. Reihl, J. H. Coombs, and R. R. Schlittler, Z. Phys. B:

Condens. Matter 72, 497 (1988).27P. Johansson, R. Monreal, and P. Apell, Phys. Rev. B 42, 9210 (1990).28V. Grosse, R. Bechstein, F. Schmidl, and P. Seidel, J. Phys. D: Appl.

Phys. 40, 1146 (2007).29J. C. Wang, K. C. Chiang, T. F. Lei, and C. L. Lee, Proceedings of 11th

IPFA, Taiwan, 5–8 July 2004.30M. S. Rahman, E. K. Evangelou, I. I. Androulidakis, and A. Dimoulas,

Electrochem. Solid-State Lett. 12, H165 (2009).31J. Robertson, Rep. Prog. Phys. 69, 327 (2006).32T. Hattori et al., Microelectron. Eng. 72, 283 (2004).33J. C. Wang, Y. P. Hung, C. L. Lee, and T. F. Lei, J. Electrochem. Soc.

151, F17 (2004).

052201-5 Qin et al.: Electronic properties of ultrathin high-j dielectrics 052201-5

JVST B - Microelectronics and Nanometer Structures

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 129.24.51.181 On: Sun, 23 Nov 2014 03:07:09