Full text of DOI:10.1557/JMR.2000.0191

Unit cell indexing of luminescent tantalum zinc oxide
Santosh K. Kurineca)
Microelectronic Engineering, Rochester Institute of Technology, Rochester, New York 14623
Philip D. Rack and Michael D. Potter
Advanced Vision Technolgies, Inc., West Henrietta, New York 14586
Thomas N. Blanton
Eastman Kodak Company, Rochester, New York 14650
(Received 8 September 1999; accepted 8 March 2000)
Tantalum zinc oxide (TZO) with the chemical formula Ta2Zn3O8 can be generated
from the reaction of 3 mol ZnO and 1 mol Ta2O5 at elevated temperatures. This
phase has been shown to exhibit blue cathodoluminescence at low electron beam
voltages. It has also been realized in thin film form on silicon substrate, making TZO
an important material for monolithic field emission display devices. The structure type
of TZO has been investigated using powder x-ray diffraction techniques. The unit cell
of this phase has been determined and found to be monoclinic. These results allow for
indexing of the powder pattern first reported by Kasper in 1967 and correct for a
discrepancy in the single-crystal structure lattice constants reported by Waburg and
Muller-Buschbaum in 1984.
I. INTRODUCTION
ported as being monoclinic (a ס 9.499 Å, b ס 8.411 Å,
c ס 8.881 Å, ␤ ס 116.08°, Space Group C2/c). Though
Waburg and Muller-Buschbaum did not report a powder
diffraction pattern, the International Center for Diffrac-
tion Data recently calculated powder XRD patterns based
on the 40,000+ crystal structures reported in the Inor-
ganic Crystal Structure Database (ICSD).8 A powder
pattern based on the TZO structure of Waburg and
Muller-Buschbaum was calculated (PDF 77-1172) and
found to have enough differences when compared
to Kasper’s TZO data to be considered a different crys-
talline phase. As part of this study, indexing of the ob-
served TZO powder diffraction patterns was performed
to address the discrepancies in the reported TZO refer-
ence patterns.
Bulk Ta2Zn3O8 (TZO) has been synthesized from ZnO
and Ta2O5 powders using conventional ceramic proc-
essing techniques.1 It has also been prepared in thin-film
form by reacting a thin-film stack composite of Ta over
ZnO at a temperature range of 900–1200 °C.2 TZO has
been shown to luminesce at a peak wavelength of
385 nm, under photo and electron beam excitation.1–3 It
is believed that the luminescence mechanism is the result
of a metal-to-ligand radiative transition, where tantalum
is the metal and oxygen is the ligand. The excited state is
the 5d0 state of the tantalum metal whereas the ground
state is the 2p6 state associated with the oxygen ligand.3
Doping TZO with a suitable concentration of manga-
nese (Mn) has been shown to excite green and red lumi-
nescence3–4 giving a full color capability for emissive
displays. To better understand the mechanism of lumi-
nescence of intrinsic and doped TZO, it is essential to
have knowledge of the structure of this material. Kasper5
reported a crystalline Ta2Zn3O8 phase formed by 700 °C
processing of ZnO and Ta2O5 powders. However, he was
unable to obtain a single crystal and the powder x-ray
diffraction (XRD) pattern he obtained was not indexed.
His diffraction pattern is referenced in the Powder Dif-
fraction File, PDF 20-1237, with d-spacings and intensi-
ties listed.6 Waburg and Muller-Buschbaum7 reported a
Ta2Zn3O8 phase, which was generated by high-
temperature (1800 °C) solid-state reaction of ZnO and
Ta2O5 using a CO2 laser. Their work did result in pro-
ducing single crystals and the crystal structure was re-
II. EXPERIMENTAL
Specimens of TZO were generated in powder and thin-
film form. Powder samples were prepared by wet mixing
ZnO:Ta2O5 in a 3:1 molar ratio, respectively, followed
by drying. The mixture was then pressed and prefired at
900 °C in air. After prefiring, dry ball milling was per-
formed using zirconia balls, and the resulting milled
powder was sintered at 1100 °C in air for 4 h. The den-
sity of the powder sample was measured with a Micro-
metrics helium pycnometer (Micromeritics Instrument
Corporation, Norcross, GA) and elemental analysis was
performed using a Perkin-Elmer Optima3000 inductively
coupled plasma-atomic emission spectrometer (ICP-
AES, Perkin-Elmer Instruments, Norwalk, CT). Thin
film samples were fabricated by sputter deposition of
a)Address all correspondence to this author.
e-mail: skkemc@rit.edu
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S.K. Kurinec et al.: Unit cell indexing of luminescent tantalum zinc oxide
400 nm of ZnO from a ZnO target (99.999% pure) on
Ta2Zn3O8 theoretical). The x-ray density (7.147 g/cm3)
and measured density (7.162 g/cm3) are in good agree-
ment with each other. Kasper reported a measured den-
sity of 7.15 g/cm3 for his TZO. All observed diffraction
peaks, including the low intensity peaks mentioned pre-
viously, can be indexed using the derived unit cell data.
Table I summarizes the indexing results for the diffrac-
tion pattern shown in Fig. 2. These results provide con-
clusive evidence that a single-phase sample of TZO
powder was produced using this sintering method.
In Table II, a comparison is made between the powder
TZO unit cell from this study, and the single crystal TZO
unit cell reported by Waburg and Muller-Buschbaum.
Based on the unit cell data it would appear that the two
cells might be considered equivalent, differing only in
the choice of a and c axes. Based on both sets of unit cell
data and the powder XRD results, it is now possible to
explain the differences in the powder pattern reported in
the Powder Diffraction File. The original unit cell re-
ported by Waburg and Muller-Buschbaum has the a and
silicon substrates. Subsequently, 10 nm of Ta (99.999%
pure) was sputtered over the ZnO film. This stack was
rapid thermally annealed at 1100 °C in nitrogen. Low
voltage cathodoluminescence (CL) spectra were acquired
using a Plasma Scan PSS-2 spectrometer (Plasma-
Therm, Kresson, NJ).
The XRD data were collected using a Rigaku RU-300
Bragg-Brentano diffractometer (Rigaku International
Corp., Tokyo, Japan) using Cu radiation with a diffracted
beam monochromator tuned to Cu K␣ radiation, and a
scintillation detector. Data processing was accomplished
using Jade 3.1 (Molecular Dynamics Inc., Livermore,
CA) and indexing was performed using DICVOL91
(Molecular Simulations Ltd., Cambridge, England).
Samples for cross-sectional transmission electron micro-
scope (TEM) were prepared using FEI620 focused ion
beam (FIB) and JEOL 2000FX TEM (Tokyo, Japan) was
used at a beam voltage of 200 kV.
III. RESULTS AND DISCUSSION
Figure 1 shows a CL emission spectrum obtained from
a powder sample of Ta2Zn3O8 taken at an electron beam
voltage of 1000 V. Visible luminescence is observed by
unaided eye at a beam voltage as low as 10 V with cur-
rent density 10 ␮A/cm2. Similar spectra are obtained
from thin film samples prepared as described in the ex-
perimental section. The unfiltered spectrum shows a peak
intensity at a wavelength of 385 nm. While a significant
portion of the broad band emission occurs in the ultra-
violet (UV), efficient blue luminescence with a peak
420 nm and Commission Internationale de I’Eclairage
(CIE) coordinates (1931) of x ס 0.160 and y ס 0.075 are
observed through an attenuated UV filter. It has a satu-
rated blue color highly suitable for phosphor emissive
displays, and is chemically stable during electron beam
irradiation.
FIG. 1. CL spectrum of TZO powder sample at a beam voltage of
1 kV.
The XRD pattern of the sintered TZO powder speci-
men is shown in Fig. 2. This material is highly crystalline
and is identified as TZO, matching the phase reported by
Kasper. There are some low intensity diffraction peaks
observed in our sample, not reported by Kasper, which
initially suggested that there might be some impurity
phase(s) present. Indexing of the powder XRD data using
DICVOL91 allowed for determination of the unit cell
dimensions. The indexing results indicate that this TZO
material is monoclinic, with lattice constants of a ס
8.884(1) Å, b ס 8.420(1) Å, c ס 9.450(1) Å, and ␤ ס
116.08(1)°. Indexing the unit cell as C-centered with ex-
tinctions for c-glide plane symmetry indicates that the
space groups if Cc or C2/c. The centrosymmetry (as in
C2/c) cannot be unequivocally determined by indexing
the powder XRD pattern. Elemental analysis found the
Ta wt% to be 55.65 (52.85% Ta in Ta2Zn3O8 theoreti-
cal), and the Zn wt% to be 27.3 (28.4% Zn in
FIG. 2. XRD pattern for TZO powder.
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S.K. Kurinec et al.: Unit cell indexing of luminescent tantalum zinc oxide
c axes switched or the centering should have been re-
served to be amorphous. Upon annealing the sample of
Figure 3, at 1100 °C for 1 min, XRD results show a
change in microstructure and phase content (Fig. 4). The
Si wafer, as expected, remains (100) oriented. The ZnO
phase exhibits crystallite growth (peak narrowing) and a
higher degree of (001) preferred orientation (peak height
increase). There were no amorphous phases detected.
TZO was found to be present indicating that thin-film
fabrication of this luminescent phase on Si can be ac-
complished. A Zn2SiO4 phase was also found to be pres-
ent, which is due to the reaction of ZnO with the Si
wafer. Figure 5 shows a cross-sectional transmission
electron micrograph (X-TEM) of this annealed sample
that correlates very well with the XRD information. A
thin layer of TZO ( 50 nm) can be observed over the
crystalline ZnO film. Based on the relative densities of
Ta and TZO, 10 nm of Ta should form 44 nm of TZO.
An interfacial layer of zinc silicate can also be observed
between ZnO film and Si substrate. The CL, however, is
obtained from the top surface of the stack. At low volt-
ages, the electron beam penetration is essentially con-
ported as an A-centered monoclinic cell. Utilizing the
unit cell indexing generated using the powder XRD data
of this study, coupled with the single crystal data of
Waburg and Muller-Buschbaum, one can assign the cen-
tric space group C2/c to Ta2Zn3O8. Bond-length calcu-
lations using the switched a and c axes show the original
atomic coordinates to be correct. There is tetrahedral co-
ordination of Zn2+ and octahedral coordination of Ta5+ in
TZO. Octahedrally coordinated (TaO6)−7 and other oc-
tahedrally coordinated transition metals such as MoO6,
WO6, and TiO6 are known to luminesce rather efficiently
via metal-to-ligand transition.10
In order to investigate the ability of fabricating a field
emission device using TZO as the phosphor, attempts
were made to form TZO thin films on silicon (Si) wafers.
In Fig. 3, the XRD pattern for an as-deposited Ta/ZnO/Si
stack is shown. Phase analysis found the Si wafer to be
(100) oriented, ZnO is polycrystalline showing some
(001) preferred orientation, and the Ta component is ob-
TABLE I. Indexed diffraction peak data for TZO powder sample.
TABLE II. Comparison of unit cell data for TZO; this study versus
Waburg and Muller-Buschbaum.
2␪
d-spacing Relative
2␪
d-spacing Relative
observed observed intensity
observed observed intensity
(hkl)
(°)
(Å)
(%)
(hkl)
(°)
(Å)
(%)
TZO Waburg and
Properties
Lattice type
Space group
a (Å)
b (Å)
c (Å)
TZO this study
Muller-Buschbaum
110
−111
002
020
−112
200
−202
021
−221
022
−113
220
−222
−311
−312
130
15.24
15.46
20.80
21.06
21.52
22.24
22.80
23.50
29.18
29.76
30.22
30.82
31.26
32.28
32.60
33.80
35.34
35.74
36.20
36.50
36.96
37.20
38.14
39.48
40.08
40.60
41.12
42.36
42.72
42.92
43.90
44.30
44.90
45.42
46.18
46.66
47.17
47.76
5.809
5.728
4.267
4.215
4.126
3.994
3.896
3.782
3.058
3.000
2.955
2.899
2.859
2.771
2.745
2.650
2.538
2.510
2.479
2.460
2.430
2.415
2.358
2.281
2.248
2.220
2.193
2.132
2.115
2.105
2.061
2.043
2.017
1.995
1.964
1.945
1.926
1.903
69
4
8
67
2
240
−242
114
48.86
49.26
50.02
50.34
50.70
50.86
51.74
52.78
52.92
54.17
54.71
55.76
57.76
57.90
58.36
58.48
59.06
59.40
60.56
61.16
61.30
61.88
62.50
63.68
64.16
64.59
66.62
67.04
67.45
67.62
68.26
68.98
69.44
1.862
1.848
1.822
1.811
1.799
1.794
1.765
1.733
6
15
9
4
4
9
8
6
9
Monoclinic
C2/c
Monoclinic
C2/c
133
8.884 (1)
8.413 (1)
9.500 (1)
116.08 (1)
637.74
4
9.499
8.411
8.881
116.03
637.04
4
−115
−134
−424
−334
−243
043
−511
150
−425
151
42
24
94
13
52
100
19
2
44
71
72
13
26
28
13
36
14
19
4
21
13
3
11
4
11
3
2
␤ (°)
Volume (Å3)
Z (formula units)
X-ray density (g/cm3)
1.729
1.6919
1.6764
1.6473
1.5948
1.5913
1.5798
1.5769
1.5628
1.5547
1.5276
1.5141
1.5110
1.4982
1.4849
1.4602
1.4503
1.4418
1.4027
1.3949
1.3875
1.3843
1.3729
1.3603
1.3524
1
18
4
7.147
7.152
16
28
15
17
11
4
20
12
12
27
3
3
10
5
1
3
332
−206
402
310
221
134
−313
−223
131
−132
023
−442
−406
−115
−443
−153
−351
−604
−534
333
−516
530
−622
−336
−535
351
113
−114
−402
311
004
132
040
−224
041
−332
400
−422
−404
−421
−423
2
4
10
1
5
4
3
1
2
20
8
FIG. 3. XRD pattern for as-deposited Ta/ZnO/Si stack thin film
sample. [Z, ZnO; S, Si wafer; T, amorphous Ta phase].
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S.K. Kurinec et al.: Unit cell indexing of luminescent tantalum zinc oxide
fined to the near-surface region. The penetration depth of
substrate, subjected to the same annealing condition did
not show luminescence. Low voltage CL is essentially a
surface phenomenon and high integrity of the surface
must be maintained for efficient luminescence.
the electron beam is on the order of a few tens of nanom-
eters at 1 kV.2 Therefore the observed luminescence is
from the top TZO layer. This conclusion is also con-
firmed from the observation that the ZnO films on Si
IV. SUMMARY
The unit cell of TZO (Ta2Zn3O8) has been success-
fully determined. Differences in previously reported ob-
served and calculated powder XRD data have been
explained and corrected. Based on the indexing of the
powder XRD data, single-phase material has been gen-
erated in a bulk powder sintering process. Rapid thermal
annealing of a Ta/ZnO/Si stack has resulted in the fab-
rication of luminescent Ta2Zn3O8 on Si wafers. Success
in this venture opens up the opportunity for a new gen-
eration of phosphor integrated monolithic devices. The
understanding of the crystal structure of Ta2Zn3O8 offers
insight into the physical properties and mechanism of
luminescence observed in this material.
ACKNOWLEDGMENTS
The authors express appreciation to Dr. Moi Leong
(Kodak) for elemental analysis, Ms. Letha McClary (Ko-
dak) for density measurements, and Dr. Mary Moore
(Kodak) for TEM. TNB would like to thank Dr. Charlie
Torardi (DuPont) for review of this manuscript and not-
ing the switched a and c axes reported in the original
TZO crystal structure. RIT and AVT would like to ac-
knowledge the support of National Science Foundation
through the Small Business Technology Transfer Grant
No. DMI-9712200.
FIG. 4. XRD pattern for rapid thermally annealed (1100 °C, 1 min)
Ta/ZnO/Si stack thin film sample. [Z, ZnO; S, Si wafer; X, Ta2Zn3O8;
O, Zn2SiO4].
REFERENCES
1. S.K. Kurinec and M.D. Potter, Society for Information Display
Digest, May 1997 (Boston, MA), p. 1077.
2. R.J. Langley, G.F. Pettis, S.K. Kurinec, and M.D. Potter, J. Soci-
ety for Information Display 6/3, 181 (1998).
3. P.D. Rack, M.D. Potter, S.K. Kurinec, W. Park, J. Penczek, B.K.
Wagner, and C.J. Summers, J. Appl. Phys. 84, 4466 (1998).
4. S.K. Kurinec, A.R. Woodard, P.D. Rack, and M.D. Potter, Proc.
Thirteenth Biennial University/Government/Industry Microelec-
tronics Symposium, Minneapolis, MN, 1999, p. 73.
5. V.H. Kasper, Z. Anorg. Allge. Chem. 355, 1 (1967).
6. Powder Diffraction File, Card No. 20-1237, International Center
for Diffraction Data (1999).
7. M. Waburg and H. Muller-Buschbaum, Monatshefte fur Chem.
115, 15 (1984).
8. Inorganic Crystal Structure Database, Fachinformationzentrum
(FIZ) Karlsruhe, (Gesellschaft fu¨r Wissenschaftlich-technische In-
formation mbH, Karlsruhe, Germany).
9. D. Louer, DICVOL91, included in the software package CERIUS
(Molecular Simulations, Cambridge, United Kingdom, 1999).
10. G. Blasse, Structure and Bonding (Springer Verlag, Heidelberg,
Germany, 1980), Vol. 42, p. 1–42.
FIG. 5. Cross-sectional TEM micrograph of the rapid thermally an-
nealed (1100 °C, 1 min) Ta/ZnO/Si stack thin film sample.
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