In this paper we report on ZnCoO thin films grown by atomic layer deposition method in reactor F-120 Satellite. ZnCoO films were grown at low temperature (T s = 160 • C) with a new zinc precursor (dimethylzinc -DMZn) and with cobalt (II) acetyloacetonate (Co(acac)2) as a cobalt precursor and deionized water as an oxygen precursor. In this paper we concentrate on the methods of homogenizing Co distribution in ZnCoO films

A Szczepanik

A Wójcik-G Lodowska

E Guziewicz

E Lusakowska

L Wachnicki

M Godlewski

M Kiecana

M Lukasiewicz

M Sawicki

R Jakie La

R Minikayev

T Krajewski

W Paszkowicz

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Vol. 114 (2008) ACTA PHYSICA POLONICA A No. 5
Proc. XXXVII International School of Semiconducting Compounds, Jaszowiec 2008
ZnCoO Films Obtained
at Low Temperature
by Atomic Layer Deposition Using
Organic Zinc and Cobalt Precursors
M. ÃLukasiewicza,∗, A. Wójcik-GÃlodowskab,
M. Godlewskia,b, E. Guziewiczb, R. JakieÃlab,
T. Krajewskib, ÃL. Wachnickia,b, A. Szczepanika,b,
E. ÃLusakowskab, W. Paszkowiczb, R. Minikayevb,
M. Kiecanab and M. Sawickib
aDept. of Mathematics and Natural Sciences College of Science
Cardinal S. Wyszyński University, Dewajtis 5, 01-815 Warsaw, Poland
bInstitute of Physics, Polish Academy of Sciences
al. Lotników 32/46, 02-668 Warsaw, Poland
In this paper we report on ZnCoO thin films grown by atomic layer
deposition method in reactor F-120 Satellite. ZnCoO films were grown at
low temperature (Ts = 160
◦C) with a new zinc precursor (dimethylzinc
— DMZn) and with cobalt (II) acetyloacetonate (Co(acac)2) as a cobalt
precursor and deionized water as an oxygen precursor. In this paper we
concentrate on the methods of homogenizing Co distribution in ZnCoO films.
PACS numbers: 68.55.Ln, 68.55.Nq, 78.66.Hf, 81.15.Kk
1. Introduction
ZnO doped with transition metal (Co, Mn, Cr, Ni) ions is one of the most
promising materials for spintronics [1, 2]. Theoretical calculation has shown that
ferromagnetic state is easily obtained in n-type ZnCoO at room temperature [2, 3].
It was reported that ferromagnetic phases were observed for content of Co in
ZnCoO less than 10% [4]. However, the following experimental observations for
ZnCoO confirmed and other rejected the presence of ferromagnetic ordering in
such samples. The conflicting reports raise questions about the genuineness of the
∗corresponding author; e-mail: gonnia@autograf.pl
(1235)
1236 M. ÃLukasiewicz et al.
observed ferromagnetism in Co-doped ZnO. After gaining some experience with
Mn-doped ZnO we decided to apply growth methods used there. In a series of
papers [5, 6] we have shown that in-plane and depth homogeneous ZnMnO films
can be obtained in the atomic layer deposition (ALD) processes performed at low
temperature. Such films (for low Mn fractions) were paramagnetic.
This paper presents and discusses the results of structural, magnetic, and
electrical measurements made on ZnCoO thin films obtained at low temperature
(LT) growth conditions by the ALD method.
2. Growth method and experimental results
We used ALD method to deposite ZnCoO layers at LT. In ALD method we
can use more reactive precursors than in chemical vapor deposition (CVD), because
the precursors are sequentially introduced to the growth chamber, so they meet
only at a surface of a grown film. This permitted us the use of fairly reactive organic
precursors to grow ZnCoO thin films. In our experiments we applied as a zinc
precursor dimethylzinc (DMZn), as a cobalt precursor cobalt(II) acetyloacetonate
(Co(acac)2) and deionized water as an oxygen precursor. Precursors’ temperatures
were the following: DMZn −30◦C, Co(acac)2 150◦C, and water 30◦C. Substrate
(sapphire, GaAs, Si, glass, ITO-glass) temperature was 160◦C in all processes
discussed in the present work.
Referring to our experience with Mn we tried to reach a better quality of
ZnCoO films by adding a buffer layer of ZnO. Thus, the obtained by us samples
of ZnCoO can be divided into two groups. The first group (group I) are the
samples with introduced buffer layer of ZnO, and the second group (group II)
are the samples without such buffer layer. The second group of the samples was
grown with different Zn-to-Co ratios of ALD cycles, which was aimed to obtain
homogeneous ZnCoO layers. The parameters of the growth for both groups are
given in Table. All samples discussed in the present work were grown on a silicon
substrate.
TABLE
Growth conditions of ZnCoO thin films.
Group Zn precursor O precursor Co precursor O precursor
number
application purging application purging application purging application purging
time [s] time [s] time [s] time [s] time [s] time [s] time [s] time [s]
I 1.1 1.1 2.75 4.4 3.3 1.65 1.65 2.75
I 1.1 1.1 2.75 4.4 1.1 1.65 1.65 2.75
II 1.1 1.1 2.75 4.4 1.1 1.65 1.65 2.75
II∗ 1.1 1.1 2.75 4.4 1.1 1.65 1.65 2.75
II 1.1 1.1 2.75 4.4 3.3 1.65 1.65 2.75
II 1.1 1.1 2.75 4.4 0.55 1.65 1.65 2.75
II 1.1 1.1 2.75 4.4 7.7 1.65 1.65 2.75
∗different Zn-to-Co ratios of ALD cycles
ZnCoO Films Obtained at Low Temperature . . . 1237
Atomic force microscopy (AFM) investigations (see example of the AFM in
Fig. 1a — for the group I and b — for the group II) indicate that roughness of
layers increases with the thickness of ZnCoO samples. Such clear correlation was
not observed by us in the case of ZnMnO films.
Fig. 1. AFM image of ZnCoO film for: (a) group I, (b) group II.
Fig. 2. XRD spectra for: (a) group I, (b) group II.
1238 M. ÃLukasiewicz et al.
Structural properties of ZnCoO films were evaluated with the X-ray diffrac-
tion (XRD) method. The XRD measurements show a polycrystalline nature of
ZnCoO layers. In all the cases we observed the diffraction maxima related to
(10.0), (00.2) and (11.0) crystallographic orientations. Moreover, the XRD spec-
tra indicate a change of a preferential growth orientation (00.2) for samples of the
group I (Fig. 2a). XRD spectra for samples ZnCoO of the group II are shown in
Fig. 2b.
Co introduction to the films was confirmed with secondary ion mass spec-
troscopy (SIMS), electron probe micro-analysis (EPMA) and superconducting
quantum interference device (SQUID). SIMS investigations showed high homo-
geneity of the obtained ZnCoO layers. We tested several sequences of elementary
ALD cycles and times of pulses of cobalt precursor to optimize in-depth homo-
geneity of Co distribution. We observed the best results for a 1.1 s time of cobalt
precursor pulse and sequences of eighty layers of ZnO and five layers of CoO (SIMS
profile visible in Fig. 3a). We observed similar results for sequences of eighty lay-
ers of ZnO and ten layers of CoO (SIMS profile visible in Fig. 3b). Co fraction in
both of these films was below 1% as concluded from the EPMA and SIMS mea-
surements. This means that Co incorporation does not scale with Co-to-Zn ratio.
We achieved the highest Co fraction in our films for process in which sequences
ZnO to CoO were eight to one. For such samples Co fraction was about 3%. SIMS
and SQUID measurements gave similar Co fractions.
Fig. 3. SIMS profiles of Co depth distribution for ZnCoO layers ALD-grown on silicon
with DMZn and Co(acac)2 for time pulse Co 1.1 s using: (a) 80:5 ratio and (b) 80:10
ratio for the zinc to Co and for the sequences of the ALD pulses repeated 70 times.
We obtained information about magnetic properties of our films from mag-
netization investigations performed using a home made SQUID magnetometer.
Films of ZnCoO with homogeneous Co distribution, with Co fraction below 5%
ZnCoO Films Obtained at Low Temperature . . . 1239
Fig. 4. Magnetization of the ZnCoO ALD film as a function of temperature T . The
magnetization curves were collected under both the field cooling (FC) and the zero field
cooling (ZFC) conditions. Curves of ZFC and FC overlap.
and grown at LT by ALD, show paramagnetic magnetic properties at room tem-
perature. Figure 4 shows the temperature dependences of magnetization for such
ZnCoO samples.
The electrical measurements (Hall measurements) conducted on the sam-
ples of ZnCoO layers show that the free carriers’ concentration varied between
1019 cm−3 and 1017 cm−3 depending on the growth temperature, film thickness
and Co fraction. The values of carriers’ Hall mobility were usually about of
3 cm2/(V s). Electrical measurements indicate preferential n-type conductivity
of our films.
3. Conclusions
We have grown at low temperature ZnCoO films by ALD with organic cobalt
and zinc precursors. The highest Co concentration achieved in the present work
was below 5%. For such films we obtained in-plane and depth homogeneous ZnCoO
films, which are paramagnetic for n-type films with free electron concentration
up to 1019 cm−3. We report also that Co doping does not affect free electron
concentration in our films but reduces their mobility.
References
[1] T. Dietl, H. Ohno, F. Matsukura, Science 287, 1019 (2000).
[2] K. Sato, H. Katayama-Yoshida, Jpn. J. Appl. Phys. 39, L555 (2000).
[3] K. Sato, H. Katayama-Yoshida, Jpn. J. Appl. Phys., Part 2 40, L334 (2001).
[4] K. Ueda, H. Tabata, T. Kawai, Appl. Phys. Lett. 79, 988 (2001).
[5] A. Wójcik, K. Kopalko, M. Godlewski, E. Guziewicz, R. JakieÃla, R. Minikayev,
W. Paszkowicz, Appl. Phys. Lett. 89, 051907 (2006).
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[6] A. Wójcik, M. Godlewski, E. Guziewicz, K. Kopalko, R. JakieÃla, M. Kiecana,
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ZnCoO Films Obtained at Low Temperature by Atomic Layer Deposition Using Organic Zinc and Cobalt Precursors

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ZnCoO Films Obtained at Low Temperature by Atomic Layer Deposition Using Organic Zinc and Cobalt Precursors

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