Growth, intermixing, and surface phase formation for zinc tin oxide nanolaminates produced by atomic layer deposition Citation for published version (APA): Hägglund, C., Grehl, T., Tanskanen, J. T., Yee, Y. S., Mullings, M. N., Mackus, A. J. M., ... Bent, S. F. (2016). Growth, intermixing, and surface phase formation for zinc tin oxide nanolaminates produced by atomic layer deposition. Journal of Vacuum Science and Technology A: Vacuum, Surfaces, and Films, 34(2), [021516]. https://doi.org/10.1116/1.4941411 DOI: 10.1116/1.4941411 Document status and date: Published: 08/02/2016 Document Version: Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication: • A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal. If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, please follow below link for the End User Agreement: www.tue.nl/taverne Take down policy If you believe that this document breaches copyright please contact us at: [email protected]providing details and we will investigate your claim. Download date: 18. May. 2020
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Growth, intermixing, and surface phase formation for zinc tinoxide nanolaminates produced by atomic layer depositionCitation for published version (APA):Hägglund, C., Grehl, T., Tanskanen, J. T., Yee, Y. S., Mullings, M. N., Mackus, A. J. M., ... Bent, S. F. (2016).Growth, intermixing, and surface phase formation for zinc tin oxide nanolaminates produced by atomic layerdeposition. Journal of Vacuum Science and Technology A: Vacuum, Surfaces, and Films, 34(2), [021516].https://doi.org/10.1116/1.4941411
DOI:10.1116/1.4941411
Document status and date:Published: 08/02/2016
Document Version:Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers)
Please check the document version of this publication:
• A submitted manuscript is the version of the article upon submission and before peer-review. There can beimportant differences between the submitted version and the official published version of record. Peopleinterested in the research are advised to contact the author for the final version of the publication, or visit theDOI to the publisher's website.• The final author version and the galley proof are versions of the publication after peer review.• The final published version features the final layout of the paper including the volume, issue and pagenumbers.Link to publication
General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.
• Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal.
If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, pleasefollow below link for the End User Agreement:www.tue.nl/taverne
Take down policyIf you believe that this document breaches copyright please contact us at:[email protected] details and we will investigate your claim.
Growth, intermixing, and surface phase formation for zinc tin oxide nanolaminatesproduced by atomic layer depositionCarl Hägglund, Thomas Grehl, Jukka T. Tanskanen, Ye Sheng Yee, Marja N. Mullings, Adriaan J. M. Mackus,Callisto MacIsaac, Bruce M. Clemens, Hidde H. Brongersma, and Stacey F. Bent
Citation: Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 34, 021516 (2016); doi:10.1116/1.4941411View online: http://dx.doi.org/10.1116/1.4941411View Table of Contents: http://avs.scitation.org/toc/jva/34/2Published by the American Vacuum Society
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Growth, intermixing, and surface phase formation for zinc tin oxidenanolaminates produced by atomic layer deposition
Carl H€agglunda)
Department of Chemical Engineering, Stanford University, Stanford, California 94305 and Departmentof Engineering Sciences, Division of Solid State Electronics, Uppsala University, 75121 Uppsala, Sweden
Thomas GrehlION-TOF GmbH, Heisenbergstraße 15, 48149 M€unster, Germany
Jukka T. TanskanenDepartment of Chemical Engineering, Stanford University, Stanford, California 94305
Ye Sheng YeeDepartment of Electrical Engineering, Stanford University, Stanford, California 94305
Marja N. Mullings, Adriaan J. M. Mackus, and Callisto MacIsaacDepartment of Chemical Engineering, Stanford University, Stanford, California 94305
Bruce M. ClemensDepartment of Material Science and Engineering, Stanford University, Stanford, California 94305
Hidde H. BrongersmaION-TOF GmbH, Heisenbergstraße 15, 48149 M€unster, Germany
Stacey F. Bentb)
Department of Chemical Engineering, Stanford University, Stanford, California 94305
(Received 20 November 2015; accepted 25 January 2016; published 8 February 2016)
A broad and expanding range of materials can be produced by atomic layer deposition at relatively
low temperatures, including both oxides and metals. For many applications of interest, however, it
is desirable to grow more tailored and complex materials such as semiconductors with a certain
doping, mixed oxides, and metallic alloys. How well such mixed materials can be accomplished
with atomic layer deposition requires knowledge of the conditions under which the resulting films
will be mixed, solid solutions, or laminated. The growth and lamination of zinc oxide and tin oxide
is studied here by means of the extremely surface sensitive technique of low energy ion scattering,
combined with bulk composition and thickness determination, and x-ray diffraction. At the low
temperatures used for deposition (150 �C), there is little evidence for atomic scale mixing even
with the smallest possible bilayer period, and instead a morphology with small ZnO inclusions in a
SnOx matrix is deduced. Postannealing of such laminates above 400 �C however produces a stable
surface phase with a 30% increased density. From the surface stoichiometry, this is likely the
inverted spinel of zinc stannate, Zn2SnO4. Annealing to 800 �C results in films containing crystal-
line Zn2SnO4, or multilayered films of crystalline ZnO, Zn2SnO4, and SnO2 phases, depending on
the bilayer period. VC 2016 American Vacuum Society. [http://dx.doi.org/10.1116/1.4941411]
I. INTRODUCTION
Zinc tin oxide (ZTO) is a wide gap semiconductor of in-
terest as an earth abundant, nontoxic transparent conductive
oxide.1 It is a promising alternative to cadmium sulfide
buffer layers in thin film solar cells,2,3 as a channel layer in
thin film transistors,4,5 and for photocatalytic applications.6
It is also employed in varistors and gas sensors.7
The deposition of ZTO by atomic layer deposition (ALD)
is desirable as this technique allows for uniform and confor-
mal growth on three dimensional, nanoscale topographies.8
It is further of much interest to be able to produce ZTO at
low temperatures making the processing compatible with a
broader range of substrate materials and applications, and
hence enabling lower production costs. To this end, the
production of ZTO of varying (average) stoichiometry has
previously been investigated by stacking thin ZnO and SnOx
layers by low temperature ALD.9–11 In this way, a homoge-
neous ternary compound has been approached by repeating a
supercycle of the individual Zn and Sn oxide ALD sequen-
ces. The atomic scale control of the oxide sublayer thick-
nesses provided by the design of the supercycle allows for
fine tuning the compound properties toward specific condi-
tions. Critical questions for this approach are to what extent
the ZnO and SnOx sublayers mix, and if a solid solution can
be formed at low temperature.
In this work, stacks of zinc and tin oxide layers were de-
posited using supercycles with bilayer periods ranging from
a few cycles up to 800 cycles. Based on measurements by
which contain the cations in the octahedral sites. For spinels,
the cations in the tetrahedral sites are not in the outer surface
detected by LEIS.26 For Zn2SnO4, this implies a structure
Znt [Sn Zn]o O4 where superscripts t and o indicate tetrahe-
dral and octahedral sites, respectively. This means that the
outer surface should contain equal amounts of Sn and Zn,
explaining the saturated levels observed here.
The high surface mobility of cations has been shown to
promote the formation of a “surface spinel.”26 The low sur-
face energy of the plane with the octahedral sites may in the
present case lead to nucleation of the inverse spinel at the
surface, while the bulk remains amorphous at these tempera-
tures (see below).
Only at much higher temperatures can the entire film be
converted to the ternary spinel phase. This is confirmed by
synchrotron-radiation GIXRD on thicker films (deposited
by 100 supercycles). Figure 6(a) shows XRD patterns of a
44 nm thick sample annealed in air at various temperatures.
The as-deposited material appears to be amorphous up to
400 �C, with a crystalline phase identified as ZnO appear-
ing in samples annealed at 600 �C. In the film annealed at
800 �C, the diffraction patterns changes to that of the spi-
nel zinc stannate Zn2SnO4 phase with some weak lines of
rutile SnO2. This indicates that an annealing temperature
exceeding 600 �C is required to form crystalline ZTO,
which is in agreement with the previous observation of
bulk crystallization of ZTO around 750 �C.4 The Sn/
(SnþZn) fraction is 0.46 as determined by ICP-OES,
while Zn2SnO4 has a Sn fraction of 0.33. This accounts for
the observed SnOx.
The effect of annealing to 800 �C was further analyzed
for films of various bilayer periods. Figure 6(b) shows the
presence of diffraction patterns of ZnO and SnO2 for films
with long bilayer periods but mostly Zn2SnO4 for shorter
bilayer periods, indicative of better sublayer mixing for
shorter bilayer periods. XRD peaks from crystalline ZnO are
only clearly observed for the two longest bilayer periods of
160 and 800 cycles, while rutile SnO2 peaks appear for
FIG. 5. (a) Elemental surface fraction of Zn (mole fraction of Zn relative to
the summed mole fractions of Zn and Sn, on the surface) for varying anneal-
ing time and temperature. (b) LEIS sputtering profile before annealing. (c)
LEIS sputtering profile after annealing.
021516-6 H€agglund et al.: Growth, intermixing, and surface phase formation 021516-6
J. Vac. Sci. Technol. A, Vol. 34, No. 2, Mar/Apr 2016
bilayer periods of 8 and higher. The data may be interpreted
such that for the longest bilayer periods, the sample is com-
prised of thick layers of crystalline ZnO and SnO2 with only
thin layers of crystalline Zn2SnO4 at the sublayer interfaces.
The pattern for the shortest bilayer period of 4 cycles appears
to be “pure” crystalline Zn2SnO4 (without any significant
contribution of crystalline SnO2 or ZnO phases), suggesting
that with thinner bilayers the majority of the deposited ZnO
and SnO2 is converted into crystalline Zn2SnO4. However, it
should be noted that the films deposited using bilayer periods
of 8 and 40 cycles also contain rutile SnO2, which is likely
due an excess of Sn in these films. The film prepared using a
bilayer period of 160 is an outlier as it is more crystalline as
compared to the other films. From the width of the XRD
peaks, it can be concluded that the films consist of relatively
small grains.
The picture emerging from these results is thus that ALD
deposited ZnO/SnOx laminates do not yield ternary oxide
phases until annealed at about 400 �C or above, and then
only in the form of a surface phase in the form of the
inverted spinel Zn2SnO4. In the range of 600–800 �C this
spinel phase evolves predominantly around ZnO/SnOx inter-
faces of the laminate to eventually penetrate throughout the
bulk—when the stoichiometry is right.
IV. SUMMARY AND CONCLUSIONS
A combined analysis of data from spectroscopic ellipsom-
etry, optical emission spectroscopy, x-ray diffraction, and
low energy ion scattering was performed for ZTO thin film
laminates. It is found that ZnO follows a substrate inhibited,
overshooting growth rate behavior on SnOx, indicative of
island type growth. SnOx displays a more conformal layer-
by-layer like growth on ZnO. This results in a morphology
for short bilayer periods (<20 cycles) characterized by ZnO
inclusions (islands, clusters) in a SnOx matrix, in line with
recent TEM observations.11 For bilayer periods exceeding
approximately 20 cycles, more discrete layers are formed.
There are no clear indications of atomic scale mixing and
formation of a solid solution for ZTO stacks grown by ALD
at 150 �C; however, postannealing at 400 �C creates a stable
surface phase with equal amounts of Zn and Sn in the outer-
most atomic layer. We suggest that this corresponds to the
inverted spinel Zn2SnO4. The LEIS data indicate that the
density of the surface phase is 30% higher than the as-
deposited film. The bulk phase of zinc stannate is further
confirmed by XRD to require a much higher annealing tem-
perature (>600 �C) for its formation. This study thus pro-
vides insight into the complex characteristics of mixed metal
oxides at the nanoscale, and points toward the surface
inverted spinel as a first ternary phase developed in the ZnO/
SnOx system prior to the Zn2SnO2 bulk phase forms at
higher temperature.
ACKNOWLEDGMENTS
This work was supported by the Department of Energy
under Award No. DE-SC0004782. C.H. acknowledges
financial support from the Marcus and Amalia Wallenberg
Foundation. A.J.M.M. is grateful for financial support from
the Netherlands Organization for Scientific Research (NWO-
Rubicon 680-50-1309). The GIXRD measurements were
carried out at the SSRL, a Directorate of SLAC National
Accelerator Laboratory and an Office of Science User
Facility operated for the U.S. Department of Energy Office
of Science by Stanford University. The authors would like to
thank beam line engineers Chris Tassone and Chad Miller
for assistance during these measurements.
FIG. 6. (Color online) GIXRD patterns of ZTO films, deposited with a 1:3 ZnO:SnOx cycle ratio. (a) A 44 nm thick sample with a bilayer period of 8, annealed
at different temperatures. (b) A series of films with 800 total cycles and varying bilayer periods, annealed at 800 �C. Film thicknesses decrease with decreasing
bilayer period/increasing number of supercycles (from 97 to 43 nm) due to pronounced nucleation effects when switching from SnOx to ZnO.
021516-7 H€agglund et al.: Growth, intermixing, and surface phase formation 021516-7
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