APPLIED PHYSICS LETTERS 89, 262903
2006

1. APPLIED PHYSICS LETTERS 89, 262903 ͑2006͒
Structural properties of SrO thin films grown by molecular beam epitaxy
on LaAlO3 substrates
O. Maksimova͒ and V. D. Heydemann
Electro-Optics Center, Pennsylvania State University, Freeport, Pennsylvania 16229
P. Fisher, M. Skowronski, and P. A. Salvador
Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh,
Pennsylvania 15213
͑Received 27 September 2006; accepted 27 November 2006; published online 27 December 2006͒
SrO films were grown on LaAlO3 substrates by molecular beam epitaxy and characterized using
reflection high-energy electron diffraction ͑RHEED͒ and x-ray diffraction ͑XRD͒. The evolution of
the RHEED pattern is discussed as a function of film thickness. 500 Å thick SrO films were relaxed
and exhibited RHEED patterns indicative of an atomically smooth surface having uniform terrace
heights. Films had the epitaxial relationship ͑001͒SrO ʈ ͑001͒LaAlO3; ͓010͔SrO ʈ ͓110͔LaAlO3. This 45°
in-plane rotation minimizes mismatch and leads to films of high crystalline quality, as verified by
Kikuchi lines in the RHEED patterns and narrow rocking curves of the ͑002͒ XRD peak. © 2006
American Institute of Physics. ͓DOI: 10.1063/1.2424440͔
The unparalleled variety of physical properties exhibited single crystal LaAlO3 substrates. Films were characterized in
by oxides holds tremendous promise for future applications, situ using reflection high-energy electron diffraction
particularly if these materials can be integrated with semi- ͑RHEED͒ to determine the growth orientation and growth
conductors. Oxides exhibit the full spectrum of electronic, mode. By quick transfer of the samples to a diffractometer,
optical, and magnetic behaviors; in fact, high-temperature we were able to characterize them ex situ using x-ray diffrac-
superconductivity, ferroelectricity, piezoelectricity, and tion ͑XRD͒. We report on the results of XRD measurements
ferromagnetism are exhibited by members of the same carried out in ␪-2␪, ␾, and ␻ modes to determine out-of-
structurally compatible family.1 In some oxides, multiple plane and in-plane orientations, as well as out-of-plane mo-
functionalities can be observed, such as ferroelectricity and saic spread. The small structural mismatch between SrO and
ferromagnetism.2 The integration of functional or multifunc- LaAlO3 leads to higher crystalline quality films that exhibit a
tional oxides with semiconductor devices can enhance device smaller mosaic spread than observed for other rocksalt oxide
functionality and can lead to the development of faster, films grown on structurally dissimilar substrates.
smaller, and more power efficient components. All films were grown in a MBE system discussed
As such, there is a broad technological and scientific elsewhere.13 Sr was evaporated from an effusion cell while
interest in the epitaxial growth of oxides on semiconductors. distilled ozone was introduced through a gas injector.
Frequently, thin interfacial layers are required to achieve Growth was carried out on LaAlO3 ͑001͒pc wafers. The sub-
growth of oxides on semiconductors with sufficient structural script pc indicates that the indices are given with respect to
perfection and device performance. Rocksalt oxides are ex- the pseudocubic ͑pc͒ perovskite subcell. The substrates were
cellent candidates to serve as the interfacial layer between etched in a 3:1 HCl: HNO3 solution, which is reported to
semiconductors and functional oxides,3 as the gate dielectric result in a predominantly AlO2 ͑B site͒-terminated surface.14
layer for metal-oxide-semiconductor devices,4 and as the in- Prior to growth, the substrate was annealed at 750 ° C under
sulating layer through which spins are injected into a the deposition ozone flux for 1 h to improve surface mor-
semiconductor.5 phology and remove organic impurities. Figures 1͑a͒ and
Heteroepitaxial films of rocksalt oxides ͑EuO, SrO, and 1͑b͒ show the RHEED patterns taken at the end of this treat-
MgO͒ have been grown by molecular beam epitaxy ͑MBE͒ ment; they correspond respectively to the ͓110͔pc and ͓100͔pc
for over 20 years.5–7 Metastable solid solutions ͑SrxBa1−xO azimuths. These patterns were consistent with those expected
and MgxCa1−xO͒ have also been prepared.3,8 However, most from a 1 ϫ 1 unreconstructed surface; sharp diffraction spots
prior works have focused on the growth of MgO films on lying along the zeroth order ring were clearly evident. In
semiconductors ͓GaAs,7 GaN,4 and Si Ref. 9͔ and oxide10 addition to those diffraction spots, a number of Kikuchi lines
͑LaAlO3 and SrTiO3͒ substrates. SrO, which has been re- were present due to the high crystallinity of the substrate.
cently used as an interfacial layer for oxide growth on Si,11 These patterns indicate that the LaAlO3 substrate was atomi-
has been much less studied. In the few articles that discuss cally flat and the width of the terraces exceeded the instru-
MBE growth of SrO, only in situ techniques were used to mental resolution ͑ϳ1 ␮m͒.
characterize films.3,6,12 Careful ex situ investigations were To optimize growth conditions, a first series of films was
not performed, partially due to the fact that SrO is unstable grown for which the substrate temperature was varied be-
in air. Thus, the structural properties and crystalline quality tween 350 and 750 ° C, while other parameters were kept
of SrO films are not well understood. constant ͓Sr source at 500 ° C and ozone flux at 0.5 SCCM
In this letter we present MBE growth of SrO films on ͑SCCM denotes cubic centimeter per minute at STP͔͒. Using
RHEED, we observed that SrO grew epitaxially when the
a͒
Electronic mail: maksimov@netzero.net substrate temperature was above 400 ° C. Films deposited at
0003-6951/2006/89͑26͒/262903/3/$23.00 89, 262903-1 © 2006 American Institute of Physics
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2. 262903-2 Maksimov et al. Appl. Phys. Lett. 89, 262903 ͑2006͒
1
͓100͔͒ of dSr–O = 2 ars = 2.58 Å. Owing to the large difference
in the lattice constants ͑⌬a / a0 = −26.6% ͒ and the similarity
in bonding along the ͓100͔SrO and ͓110͔LaAlO3 directions, the
observed heteroepitaxy leads to a reduced mismatch of
f = ⌬d / d0 = + 3.9%.
RHEED images were collected as a function of deposi-
tion time to determine the growth mode of SrO on LaAlO3.
Figure 1͑c͒ shows the RHEED pattern immediately after the
SrO growth was initiated, after approximately 5 ML ͑mono-
layer͒. By comparing this pattern to the substrate pattern
shown in Fig. 1͑b͒, one observes that the sharp diffraction
spots of the substrate have transformed into more diffuse
lines whose intensity has decreased substantially. This sug-
gests that a decrease in the terrace size and an increase in the
number of terraces have occurred after the deposition of 5
ML of SrO. The drastic drop in RHEED intensity also im-
plies that the films are not coherently strained to the substrate
even at these small thicknesses. After the deposition of ap-
proximately 10 ML, the pattern given in Fig. 1͑d͒ was col-
lected, which shows that the streaks have a clear modulation
in intensity along their length. This pattern can be interpreted
as a superposition of a streaky pattern from reflected elec-
trons and a spot pattern from transmitted electrons. It indi-
FIG. 1. ͑Color online͒ RHEED images of the LaAlO3 substrate and SrO
film. ͑A͒ and ͑B͒ are taken on the LaAlO3 substrate along the ͓110͔pc and cates that the initially flat SrO layer has transformed into a
͓100͔pc azimuths, respectively. ͑C͒–͑H͒ are taken on SrO film after different layer with an increased three-dimensional surface, consistent
numbers of SrO monolayers ͑ML͒ have been deposited: ͑C͒ 5 ML, ͑D͒ 10 with a layer plus island ͓Stanski-Krastanov ͑SK͔͒ growth
ML, ͑E͒ 50 ML, ͑F͒ 100 ML, ͑G͒ and ͑H͒ 300 ML. The azimuths of the SrO mode. The patterns given in Figs. 1͑e͒ and 1͑f͒ were col-
patterns are indicated in the images. Since epitaxy proceeds through the 45°
lected after deposition of 50 and 100 ML, respectively. These
in-plane rotation, the images taken along the ͓110͔ azimuth of SrO are taken
with the electron beam along the same direction as for images taken along patterns exhibit narrower and more intense diffraction lines
the ͓100͔ azimuth of LaAlO3. ͑streaks and spots͒, indicating an improvement of the surface
morphology due to the coalescence of neighboring islands.
Nevertheless, because the intensity of the diffraction lines
lower temperatures exhibited a ring pattern in RHEED, in- was modulated and the spots did not disappear, steps be-
dicative of randomly oriented polycrystalline growth. The tween neighboring islands were still of various heights.
RHEED patterns became sharper with increasing growth Figures 1͑g͒ and 1͑h͒ show the RHEED patterns collected
temperature, indicating that the epitaxial quality improved from a 300 ML thick SrO film along the ͓100͔SrO and
with temperature. ͓110͔SrO azimuths, respectively. The sharp streaks observed
A second series of films was grown wherein the sub- in these patterns demonstrate that the final SrO surface was
strate temperature was fixed at 750 ° C while the Sr cell tem- atomically flat and consisted of small terraces with mono-
perature was varied between 470 and 520 ° C ͑growth rate of layer step heights. The observation of intense Kikuchi lines
0.6– 2.3 nm/ min͒, and the ozone flow was varied between also attests to the high crystalline quality of these flat SrO
0.25 and 2.0 SCCM. No significant difference was observed films.
neither in the growth mode nor in the final RHEED pattern, These results agree well with the previous observations
indicating that the growth behavior at 750 ° C was not sen- that rocksalt oxides ͑SrO and MgO͒ grow in a SK mode on
sitive to these parameters over the range of values explored. perovskite substrates.15,16 Rocksalt oxides also grow in a
Further structural characterizations were carried out on layer-by-layer ͓Frank–van der Merve ͑FM͔͒ mode on rock-
the films grown epitaxially at 750 ° C with Sr source at salt oxide ͓MgxCa1−xO on MgO ͑Ref. 8͔͒ and closely lattice
500 ° C and ozone flux at 0.5 SCCM. Figures 1͑c͒–1͑f͒ and matched semiconductor ͓Ba0.7Sr0.3O on Si ͑Ref. 3͔͒ sub-
1͑h͒ show the evolution of the RHEED pattern of the SrO strates. In the case of SrO on LaAlO3, growth initiates in SK
film taken along the same incident direction as that shown in mode in which the islands that form on the thin flat wetting
Fig. 1͑b͒, i.e., the LaAlO3͓100͔pc azimuth. Based on the po- layer are relatively small in height, owing to the small mis-
sition of the diffraction lines, it can be determined that the match. However, the drastic decrease in RHEED intensity
pattern corresponds to the SrO͓110͔ azimuth and that the argues that the films are already partially relaxed at this
epitaxial relationship between SrO and LaAlO3 is stage, leading to a large disorder in the RHEED pattern. As
͑001͒SrO ʈ ͑001͒LaAlO3; ͓110͔SrO ʈ ͓100͔LaAlO3. This is the ex- growth proceeds, the surface order improves and the islands
pected orientation relationship between a rocksalt oxide, start coalescing at approximately 30 ML. Further growth,
such as SrO, and a perovskite oxide, such as LaAlO3, owing most probably, continues in FM mode, similar to rocksalt/
to the similarity between the ͑001͒ plane of rocksalt and the rocksalt film growth.
͑001͒pc LaO plane of the perovskite. LaAlO3 is a pseudocu- The ␪-2␪ scan for a 200 nm thick SrO film is shown in
bic perovskite having a lattice constant of apc Ϸ 3.79 Å and a Fig. 2. Strong ͑002͒ and ͑004͒ diffraction peaks from the SrO
La–O bond distance ͑running along the ͓110͔pc͒ of film were observed ͑in addition to the substrate peaks͒, indi-
dLa–O Ϸ 2 ͱ2a p Ϸ 2.68 Å. SrO is a rocksalt oxide having
1
cating that ͑001͒-oriented SrO grows on LaAlO3, in agree-
ars = 5.16 Å and a Sr–O bond distance ͑running along the ment with the RHEED observations. There are no other
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3. 262903-3 Maksimov et al. Appl. Phys. Lett. 89, 262903 ͑2006͒
FIG. 2. XRD ␪-2␪ scan registered for a SrO film grown on LaAlO3͑001͒pc FIG. 3. XRD f scans registered for the ͑111͒pc peak of the LaAlO3 substrate
substrate. the inset demonstrates XRD ␻-rocking curve registered from the ͑solid line͒ and the ͑111͒ peak of SrO film ͑dotted line͒.
͑002͒ peak of the SrO film.
a = b ϳ 5.138± 0.005 Å. Thus, the SrO film is relaxed, has a
peaks in the spectrum, indicating the absence of misoriented cubic structure, and its lattice parameter is very close to the
grains. From the 2␪ position of the ͑00 l͒ peaks, the out-of- literature value.
plane lattice constant of SrO was calculated to be In conclusion, SrO films were grown by MBE on
cSrO ϳ 5.137± 0.005 Å, very close to the literature value of LaAlO3͑001͒pc substrates. Both RHEED and XRD illustrated
5.16 Å. that SrO grew with the epitaxial relationship of
The ␻-rocking curve of ͑002͒ SrO peak, shown in the ͑001͒SrO ʈ ͑001͒LaAlO3; ͓100͔SrO ʈ ͓110͔LaAlO3. Due to the small
inset of Fig. 2, is relatively narrow and has a full width at lattice mismatch and the alignment of structurally similar
half maximum ͑FWHM͒ of ϳ0.42°, or 25 arc min. Due to directions, the films were of higher crystalline quality than
the lack of the published data for SrO films, we compare the rocksalt oxides previously grown on perovskite and semicon-
SrO data with values for MgO films, the most widely studied ductor substrates, as was demonstrated by the sharp RHEED
rocksalt oxide. The rocking curve of the SrO film on LaAlO3 pattern, the narrow rocking curve of the ͑002͒ XRD peak,
is significantly narrower than the rocking curve of the MgO and the off-axis ͑111͒ reflection.
films grown on LaAlO3 and SrTiO3 ͑FWHMϳ 1.2°,
This work was supported by the Office of Naval
f = + 27% and −7.3%, respectively͒,10 GaAs ͑FWHMϳ 1.8°,
Research under Grants Nos. N00014-05-1-0238 and
f = 34%͒,7 and Si ͑FWHMϳ 2.2°, f = 29%͒.11 A strong lattice
N00014-06-1-1018.
mismatch between MgO films and the underlying substrates
is, most probably, responsible for the significant mosaic 1
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3
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14
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