Dysprosium oxide and dysprosium-oxide-doped titanium oxide thin films grown byatomic layer depositionAile Tamm, Jekaterina Kozlova, Lauri Aarik, Jaan Aarik, Kaupo Kukli, Joosep Link, and Raivo Stern Citation: Journal of Vacuum Science & Technology A 33, 01A127 (2015); doi: 10.1116/1.4902079 View online: http://dx.doi.org/10.1116/1.4902079 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/33/1?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Effect of Dy/Nd double layer on coercivity in Nd-Fe-B thin films J. Appl. Phys. 115, 17A735 (2014); 10.1063/1.4866893 Atomic layer deposition of cobalt oxide thin films using cyclopentadienylcobalt dicarbonyl and ozone at lowtemperatures J. Vac. Sci. Technol. A 31, 01A145 (2013); 10.1116/1.4772461 Atomic layer deposition of zinc indium sulfide films: Mechanistic studies and evidence of surface exchangereactions and diffusion processes J. Vac. Sci. Technol. A 31, 01A131 (2013); 10.1116/1.4768919 Nanostructured resistive memory cells based on 8-nm-thin TiO 2 films deposited by atomic layer deposition J. Vac. Sci. Technol. B 29, 01AD01 (2011); 10.1116/1.3536487 Structural and electrical properties of Ti x Al 1 − x O y thin films grown by atomic layer deposition J. Vac. Sci. Technol. B 29, 01A302 (2011); 10.1116/1.3533763

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Dysprosium oxide and dysprosium-oxide-doped titanium oxide thin filmsgrown by atomic layer deposition

Aile Tamm,a) Jekaterina Kozlova, Lauri Aarik, and Jaan AarikInstitute of Physics, University of Tartu, Ravila 14c, EE-50411 Tartu, Estonia

Kaupo KukliUniversity of Helsinki, FI-00014 Helsinki, Finland

Joosep Link and Raivo SternNational Institute of Chemical Physics and Biophysics, Akadeemia tee 23, EE-12618 Tallinn, Estonia

(Received 22 August 2014; accepted 5 November 2014; published 18 November 2014)

Dysprosium oxide and dysprosium-oxide-doped titanium oxide thin films were grown by atomic

layer deposition on silicon substrates. For depositing dysprosium and titanium oxides Dy(thd)3-O3

and TiCl4-O3 were used as precursors combinations. Appropriate parameters for Dy(thd)3-O3

growth process were obtained by using a quartz crystal microbalance system. The Dy2O3 films

were deposited on planar substrates and on three-dimensional substrates with aspect ratio 1:20. The

Dy/Ti ratio of Dy2O3-doped TiO2 films deposited on a planar silicon substrate ranged from 0.04 to

0.06. Magnetometry studies revealed that saturation of magnetization could not be observed in pla-

nar Dy2O3 films, but it was observable in Dy2O3 films on 3D substrates and in doped TiO2 films

with a Dy/Ti atomic ratio of 0.06. The latter films exhibited saturation magnetization 10�6 A cm2

and coercivity 11 kA/m at room temperature. VC 2014 American Vacuum Society.

[http://dx.doi.org/10.1116/1.4902079]

I. INTRODUCTION

Dysprosium and dysprosium oxide have been materials of

interest for a long time1–9 as dysprosium is a lanthanide

metal with the largest magnetic moment and susceptibility.2

In addition, combination of polymeric architectures with

dysprosium ions can produce hybrid materials with

extremely rich properties. Dysprosium oxide has been con-

sidered as a candidate of high-permittivity (high-k) capacitor

dielectric for nonvolatile memory applications3 and as a

high-k gate dielectric.4–6 Regarding the performance of

Dy2O3 as essentially a paramagnetic oxide, standards in the

form of Dy2O3 powders have been exploited for quantum in-

terference magnetometer,7 vibrating sample magnetometer

(VSM),8 or in the form of small single crystals as additive

seeds for superconducting materials for a susceptometer.9

Dysprosium-doped polymers and glasses can emit white

light,10,11 while Dy:PbGa2S4 compound has been shown to

be a solid state laser material emitting at 4.32 lm when

pumped by 1.7 lm laser diodes.12 It is known from literature,

that ternary compounds containing Dy may have a large

magnetocaloric effect while DyTiO3 (material with perov-

skite structure) has been studied as a material that might find

application in devices for magnetization and heat capacity

measurements.13 Pyrochlore Dy2Ti2O7 has demonstrated

weak ferromagnetic coupling, which may be largely dipolar

in origin. Theory shows its spin ice behavior,14,15 the so-

called frustrated rare earth magnetism in spin glasses.16

Little is known about the magnetic performance of Dy-

doped TiO2, particularly in the form of thin films. At the

same time, there have been claims of room temperature fer-

romagnetism in TiO2 doped with various transition metals.17

Room temperature magnetism has also been observed, e.g.,

in cobalt-oxide-doped TiO2 films18 or Ho2O3 doped TiO2

films19 prepared by atomic layer deposition (ALD) and

nickel-doped sol-gel TiO2.20

This work is devoted to the investigation of dysprosium

oxide and dysprosium-doped titanium oxide thin films, in

order to gain knowledge about the possibilities of preparing

such material by low-temperature ALD. This study involves

also the characterization of the basic structure, comformality,

and magnetometric properties of the thin films prepared. For

deposition of the films, we used ozone-based ALD processes

described earlier.21,22 We deposited TiO2 by using titanium

tetrachloride (TiCl4) as titanium precursor and ozone as oxy-

gen precursor,21 and Dy2O3 by using Dy(thd)3 (thd¼ 2,2,6,6-

tetramethyl-3,5-heptanedionato) and ozone as the precursors.

II. EXPERIMENT

A. Deposition of films

The films were grown in a flow-type in-house built hot-

wall ALD reactor.23 Nitrogen of 99.999% purity was used as

a carrier gas. The reactor pressure ranged from 225 to 235 Pa

in the experiments. Dy2O3 was deposited from Dy(thd)3 and

O3 at a substrate temperature of 300 �C. TiO2 films were

grown from titanium chloride and ozone.21 The Dy2O3-doped

TiO2 films were grown using Dy(thd)3 and TiCl4 as metal pre-

cursors and O3 as the oxygen precursor. The TiCl4 source was

held at room temperature while Dy(thd)3 was evaporated at

140 �C. The Dy2O3 films were grown by using 1000 deposi-

tion cycles with cycle times 5-2-3-5 s at 250, 300, and 350 �C.

The cycle times denote metal precursor pulse, purge, and

ozone pulse and purge, respectively. In order to characterize

the film growth in real time and optimize the deposition pa-

rameters, we used quartz crystal microbalance (QCM) dataa)Electronic mail: aile.tamm@ut.ee

01A127-1 J. Vac. Sci. Technol. A 33(1), Jan/Feb 2015 0734-2101/2015/33(1)/01A127/5/$30.00 VC 2014 American Vacuum Society 01A127-1

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acquired by Q-pod quartz crystal monitor (Inficon). During

deposition of films for postgrowth characterization the

Dy(thd)3, TiCl4 and O3 pulse lengths were chosen a short as

possible to minimize the total length of the growth process,

and, at the same time, long enough to provide coverage over

the substrate surface area used in the experiments. Also, the

growth conformity of Dy2O3 on 3D stacked Si substrates with

aspect ratio of 20:1 was studied at 300 �C.

The samples prepared for characterization of material

properties are listed in Table I. Dy2O3 doped TiO2 films

were grown as alternating layers of TiO2 and Dy2O3 by

using sequences 50� (5�Dy2O3þ 10�TiO2) and

50� (10�Dy2O3þ 10�TiO2). The cycle times used for

TiO2 were 2-2-5-5 s and those for Dy2O3 were 10-2-5-5 s.

For reference, an undoped TiO2 film was deposited using

also the cycle times of 2-2-5-5 s. The selected samples were

annealed at 800 �C under N2 flow for 30 min.

B. Characterization of material properties

The thicknesses of the films were measured by x-ray

reflection (XRR) and the crystal structure evaluated by graz-

ing incidence x-ray diffraction (GIXRD) by using a

PANalytical X’Pert PRO x-ray diffractometer. In the GIXRD

measurements, the incidence angle was 1�. Information on the

mass thickness and impurity content in the films was acquired

from x-ray fluorescence spectroscopy (XRF) measurements

performed by using Rigaku ZSX 400 spectrometer. The mor-

phology on planar and three-dimensional (stack) substrate

was investigated by scanning electron microscopy (SEM)

using a Dual BeamVR

equipment Helios NanoLab 600 (FEI

Company). The films were, in addition, evaluated by Raman

spectroscopy. The Raman spectra were recorded by using a

Renishaw in via micro-Raman spectrometer. The wavelength

of the incident argon-ion laser beam with a power of about 10

mW was 514 nm and the spectral resolution of the measure-

ments was 1.5 cm�1. A silicon reference sample was used for

the calibration of the Raman shift.

C. Magnetic measurements of films

All films were subjected to magnetic measurements. The

measurements were performed by using the VSM option of

the physical property measurement system 14 T (Quantum

Design). Rectangular samples with dimensions of around

7� 4 mm were fixed with General Electric (GE) Varnish on

the commercial quartz sample holders (Quantum Design).

The temperature dependence of magnetization was measured

in the temperature range of 10–350 K, and in the presence of

a magnetic field of 79.6 kA/m parallel to the film surface.

Hysteresis measurements were performed by scanning the

magnetic fields from �4800 to þ4800 kA/m at temperatures

ranging from 10 to 300 K. The diamagnetic signal arising

from the silicon substrate was subtracted from the general

magnetization curve for all the samples in which the ferro-

magneticlike response was detected.

III. RESULTS AND DISCUSSION

A. Film growth

According to our QCM studies, the Dy(thd)3 precursor

source temperature was sufficiently high to obtain stable

growth of thin films from Dy(thd)3 and O3 at reasonably short

precursor pulses (Fig. 1). On planar Si substrates the Dy2O3

films grew with the rates of 0.014, 0.022, and 0.012 nm/cycle

at 250, 300, and 350 �C, respectively (Table I). Comparable

growth rate values measured have earlier been obtained for

similar ALD process by P€aiv€asaari et al.22 As the maximum

growth rate was detected for the films deposited at 300 �C in

our experiments, the thermal decomposition of Dy(thd)3

should be regarded as rather insignificant at the reactor tem-

peratures up to 350 �C. Significant dependence of the growth

rate, the substrate temperature demonstrates, however, that

the surface reactions depend on temperature and the precursor

dosing might need further optimization at these temperatures.

The growth rate measured from undoped TiO2 films de-

posited at 300 �C was 0.065 nm/cycle.21 After including

Dy2O3 layers between TiO2, the growth rate decreased. The

films grown with Dy2O3:TiO2 cycle ratios 10:10 and 5:10

showed growth rates 0.05 and 0.037 nm/cycles, respectively.

B. Structure

In the present study, the Dy2O3 films grown at 300 �C to a

thickness of about 22 nm were x-ray amorphous, but a 12 nm

TABLE I. Growth cycles applied for the deposition of Dy2O3 TiO2 and

Dy2O3 doped TiO2 thin films grown on Si substrate. Indicated are also the

resulting film thicknesses by XRR and the film composition in terms of

Dy:Ti atomic ratio, measured by XRF.

Material (Dy/Ti at. %) Cycles Thickness (nm)

Dy2O3 1000�Dy2O3 grown at 250 �C 14 6 0.4

Dy2O3 1000�Dy2O3 grown at 300 �C 22 6 0.4

Dy2O3/Si 1000�Dy2O3 grown at 350 �C 12 6 0.3

Dy2O3:TiO2 80� (5�Dy2O3þ 10�TiO2) 45 6 0.5

(Dy/Ti 0.04) Grown at 300 �C

Dy2O3:TiO2 50� (10�Dy2O3þ 10�TiO2) 50 6 1.0

(Dy/Ti 0.06) Grown at 300 �C

TiO2 800 � TiO2 grown at 300 �C 53 6 0.5 FIG. 1. QCM frequency change recorded during ten ALD cycles with time

parameters of 3-2-2-5 s at 300 �C.

01A127-2 Tamm et al.: Dysprosium oxide and dysprosium-oxide-doped titanium oxide thin films 01A127-2

J. Vac. Sci. Technol. A, Vol. 33, No. 1, Jan/Feb 2015

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thick Dy2O3 film deposited at 350 �C was crystallized in the

as-deposited state already [Fig. 2(a)]. The main reflections in

the GIXRD patterns taken from the films deposited at 350 �Cas well as from the same films annealed at 800 �C matched

with the ones observed earlier in the case of Dy2O3 ALD

films,22 corresponding to the cubic phase of Dy2O3.24 Also, the

Dy2O3 doped TiO2 films were crystallized in the as-deposited

state, although only one, but main, reflection apparent at 25.2�

indicated the presence of anatase phase [Fig. 2(b)].

Raman spectroscopy studies carried out on undoped TiO2

and Dy2O3 doped TiO2 films were able to reveal only the anatase

phase of TiO2 in the films (Fig. 3). This was an expected result

because the concentration of Dy2O3 was evidently too small for

formation of a separate crystal phase. In addition, Dy2O3 should

demonstrate peaks (e.g., at 326, 375, and 469 cm�1)24 which

overlap with the Raman bands of the Si-SiO2 substrate and ana-

tase phase of TiO2.25,26 The anatase phase was unambiguously

identified in the films on the basis of a characteristic 144 cm�1

peak (Fig. 3) assigned to the Eg phonon modes.26

C. Morphology and conformity

SEM images allowed conclusion that the surface of

Dy2O3 film was quite smooth [Fig. 4(a)]. No significant

features, which could refer to intense polycrystal growth,

appeared. However, pinholelike features were observed after

annealing at 800 �C [Fig. 4(b)]. SEM studies also demon-

strated that good conformity was achieved for the films

grown onto 3D substrates [Fig. 4(c)].

By contrast, the surface morphology of Dy2O3 doped

TiO2 films with Dy:Ti atomic ratios of 0.04 and 0.06

[Fig. 5(a)] indicated the polycrystalline growth of the

films. The surfaces of the films were quite similar regard-

less of the doping level and did not change even during

annealing.

FIG. 2. Grazing incidence diffraction patterns of as-deposited Dy2O3 depos-

ited at 300 and 350 �C and Dy2O3 films deposited at 300 �C and annealed at

800 �C (a). GIXRD patterns of Dy2O3 doped TiO2 films (b). Deposition tem-

perature and annealing conditions are given by labels.

FIG. 3. (Color online) Raman spectra of TiO2 and Dy2O3 doped TiO2 films.

Film compositions are indicated by labels. The modeled mean peak of

Dy2O3 is depicted by inset.

FIG. 4. Representative SEM images from 22 nm thick Dy2O3 film deposited

at 300 �C on Si substrate (a) and after annealing at 800 �C (b). On the right

panel, the same Dy2O3 film grown on a 3D substrate (stack) with an aspect

ratio of 1:20 is shown (c).

01A127-3 Tamm et al.: Dysprosium oxide and dysprosium-oxide-doped titanium oxide thin films 01A127-3

JVST A - Vacuum, Surfaces, and Films

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D. Magnetic behavior

Planar Dy2O3 films alone could not show ferromagnetic-

like behavior and the response to magnetometry was typical

to dominantly paramagnetic material. In the measurements

carried out on the same 22 nm thick Dy2O3 films deposited

on 3D substrates [Fig. 4(c)], magnetization versus field

(M-H) curves, largely characteristic of paramagnetic mate-

rial (Fig. 6, depicted by inset), were recorded. According to

temperature-dependent magnetization measurements (Fig. 6),

the magnetization was measurable in the whole temperature

range examined, and remained clearly above zero at room

temperature.

The film grown with Dy2O3:TiO2 cycle ratio 5:10 and

with Dy/Ti atomic ratio of 0.04 did not show saturation mag-

netization and coercivity values even after annealing, and

only paramagnetic behavior was detected (not shown). The

film grown with Dy2O3:TiO2 cycle ratio 10:10 and showing

Dy/Ti atomic ratio of 0.06 in the films could exhibit certain

saturation magnetization and coercivity values characteristic

of ferro- or ferrimagnetic materials after annealing [Fig. 7(a)].

The coercivity was 11 kA/m and the hysteresis was shifted a

little too negative magnetic fields when measured at room

temperature. This result refers to the creation of very

soft ferromagnetics. At the same time, certain tendency to-

ward the saturation of magnetization was registered at all

measurement temperatures. At the room temperature, the

saturation levels above 10�6 A cm2 (0.001 emu) were

recorded. From temperature-dependent measurements of

susceptibility, it revealed that Curie temperature of that ferri-

magnet is 50 K [Fig. 7(b)].

IV. SUMMARY AND CONCLUSIONS

Dysprosium oxide films were grown by atomic layer dep-

osition at 250, 300, and 350 �C on SiO2/Si(100) and 3D sub-

strates from Dy(thd)3 and O3. Dy2O3 grown at 250 and

300 �C was not crystalline in the as-deposited state, but the

films deposited at 350 �C as well as the films annealed at

800 �C contained the cubic phase of Dy2O3.

The dysprosium-doped titanium oxide films were grown

at 300 �C on nondoped SiO2/Si(100) substrates from

Dy(thd)3, Ti(Cl)4, and O3 process by alternate deposition of

Dy2O3 and TiO2 layers. The films were crystalline already in

the as-deposited state and annealing the films at 800 �C did

not change the phase composition of films.

Magnetometry revealed the ferromagneticlike behavior in

both constituent and doped films, no saturation magnetiza-

tion could be observed in the Dy2O3 films at room tempera-

ture. The film that was grown with Dy2O3:TiO2 cycle ratio

10:10 and had a Dy/Ti atomic ratio of 0.06 exhibited satura-

tion magnetization 10�6 A cm2 and coercivity 11 kA/m,

characteristic of ferro- or ferrimagnetic materials measured

FIG. 5. Bird-eye views taken by SEM on Dy2O3 doped TiO2 films grown

using cycle sequences of (10:10) (a) and (5:10) (b) at 300 �C on Si substrate.

FIG. 6. (Color online) Magnetic moment vs measurement temperature curves

measured from an annealed at 800 �C Dy2O3 film. Representative magnetic

moment vs external magnetic field strength curves measured from as-

deposited Dy2O3 film deposited in 3D substrates are depicted by inset. The

Dy2O3 film thickness was 22 nm.

FIG. 7. (Color online) Representative magnetic moment vs external magnetic

field strength curves measured from an annealed Dy2O3 doped TiO2 film

(a). Representative moment vs measurement temperature curves measured

from an annealed Dy2O3 doped TiO2 (b). Film thickness is 45 nm.

01A127-4 Tamm et al.: Dysprosium oxide and dysprosium-oxide-doped titanium oxide thin films 01A127-4

J. Vac. Sci. Technol. A, Vol. 33, No. 1, Jan/Feb 2015

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at room temperature. Thus, the study proved prospects for

atomic layer deposition of thin solid films demonstrating soft

magnetic polarization under external magnetic field.

Consequently, the method might be of interest in case of

practical applications of these films, especially in the cases

when very thin films with uniform thickness and doping lev-

els are needed on large-area substrates.

ACKNOWLEDGMENTS

The study was partially supported by Estonian Ministry

of Education and Research and Estonian Research Council

(Project IUT2-24 and PUT170) and by Finnish Centre of

Excellence in Atomic Layer Deposition.

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