Charge transport, optical transparency, microstructure, and processing relationships in transparent conductive indium-zinc oxide films grown by low-pressure metal-organic chemical vapor deposition

Article abstract of DOI:10.1063/1.121823

Indium-zinc oxide films (Zn_xIn_yO_x+1.5y), with x/y=0.08-12.0, are grown by low-pressure metal-organic chemical vapor deposition using the volatile metal-organic precursors In(TMHD)₃ and Zn(TMHD)₂ (TMHD=2,2,6,6-tetramethyl-3,5-heptanedionato). Films are smooth (rms roughness=40-50A) with complex microstructures which vary with composition. The highest conductivity is found at x/y=0.33, with σ=1000S/cm (n-type; carrier density=3.7×10²⁰cm³; mobility=18.6cm²/Vs; dσ/dT<0). The optical transmission window of such films is broader than Sn-doped In₂O₃, and the absolute transparency rivals or exceeds that of the most transparent conductive oxides. X-ray diffraction, high resolution transmission electron microscopy, microdiffraction, and high resolution energy dispersive X-ray analysis show that such films are composed of a layered Zn_kIn₂O_3+k phase precipitated in a cubic In₂O₃:Zn matrix.

Full text of DOI:10.1063/1.121823

Charge transport, optical transparency, microstructure, and processing relationships in
transparent conductive indium–zinc oxide films grown by low-pressure metal-organic
chemical vapor deposition
Anchuan Wang, Jiyan Dai, Jizhi Cheng, Michael P. Chudzik, Tobin J. Marks, Robert P. H. Chang, and Carl R.
Kannewurf
Citation: Applied Physics Letters 73, 327 (1998); doi: 10.1063/1.121823
View online: http://dx.doi.org/10.1063/1.121823
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/73/3?ver=pdfcov
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APPLIED PHYSICS LETTERS
VOLUME 73, NUMBER 3
20 JULY 1998
Charge transport, optical transparency, microstructure, and processing
relationships in transparent conductive indium–zinc oxide films grown
by low-pressure metal-organic chemical vapor deposition
Anchuan Wang, Jiyan Dai, Jizhi Cheng, Michael P. Chudzik, Tobin J. Marks,a)
Robert P. H. Chang, and Carl R. Kannewurf
The Materials Research Center, Northwestern University, Evanston, Illinois 60208
͑
Received 23 March 1998; accepted for publication 11 May 1998͒
Indium–zinc oxide films (Zn In Oxϩ1.5y), with x/yϭ0.08–12.0, are grown by low-pressure
x
y
metal-organic chemical vapor deposition using the volatile metal–organic precursors In͑TMHD͒3
and Zn͑TMHD͒2 ͑TMHDϭ2,2,6,6–tetramethyl–3,5–heptanedionato͒. Films are smooth ͑rms
roughnessϭ40–50 Å͒ with complex microstructures which vary with composition. The highest
20
3
conductivity is found at x/yϭ0.33, with ␴ϭ1000 S/cm ͑n-type; carrier densityϭ3.7ϫ10 cm ;
2
mobilityϭ18.6 cm /V s; d␴/dTϽ0͒. The optical transmission window of such films is broader than
Sn-doped In O , and the absolute transparency rivals or exceeds that of the most transparent
2
3
conductive oxides. X-ray diffraction, high resolution transmission electron microscopy,
microdiffraction, and high resolution energy dispersive X-ray analysis show that such films are
composed of a layered Zn In O
phase precipitated in a cubic In O :Zn matrix. © 1998
k
2
3ϩk
2 3
American Institute of Physics. ͓S0003-6951͑98͒03028-9͔
Transparent conductive oxide ͑TCO͒ films have been ac-
tively studied for applications in numerous solid-state opto-
electronic devices ͑e.g., displays, solar cells͒. A number of
charge transport properties can be grown over a broad com-
positional range using low pressure MOCVD.
The horizontal, low-pressure reactor described
previously16 was used. The volatile metal–organic precur-
sors In͑TMHD͒ , and Zn͑TMHD͒ were prepared from high-
1
–9
studies
have focused on processing and doping In O ,
2
3
5ϩ
3ϩ
4ϩ
3ϩ
4ϩ
2ϩ
ZnO, and SnO with Ga , Sn , Al , Si , Cd , Sb
,
3
2
2
4
ϩ
10–11
purity ͑99.999%͒ metal nitrates and were triply sublimed.
For indium-zinc oxide film growth, precursor
temperatures/Ar carrier gas flow rates were: Zn͑TMHD͒2,
and Zr as well as on other compositions.
At present,
4ϩ
coatings of In O doped with Sn ͑‘‘ITO’’͒ are employed
2
3
on a massive scale for display devices although neither the
electrical conductivity nor optical transparency window is
truly optimum for current or envisaged applications.
Indium–zinc oxide, another promising TCO candidate, ex-
7
8–85 °C/32 sccm; In͑TMHD͒ , 110–112 °C/32 sccm; with
3
2
00 sccm flowing O as the oxidizer gas. The system pres-
2
sure was 470 Pa and substrate temperature, 450 °C. Although
films were grown over a range of substrate temperatures
hibits transport and optical properties rivaling or exceeding
͑400–550 °C͒, higher temperatures ͑Ͼ450 °C͒ led to no im-
those of many TCOs.1
,6,7,12–14
Recent complementary struc-
provement in the conductivity, while lower temperatures led
to a slight decrease in conductivity and significant carbon
contamination as assessed by scanning electron microscopy
equipped with an energy dispersive x-ray ͑SEM/EDX͒ detec-
tor having a light element window. At 450 °C, film growth
rates on quartz were 20–25 Å/min. Microstructure character-
ization and charge transport analysis procedures were de-
tural, optical, and electrical studies of phase relationships in
bulk In O -ZnO ͑Ref. 15͒ reveal nine homologous layered
2
3
compounds of composition Zn In O ͑kϭ3, 4, 5, 6, 7, 9,
kϩ3
k
2
11, 13 and 15; prepared at 1300 °C͒, with resistivity and
band gap increasing with k. For indium–zinc oxide thin film
growth, several deposition techniques have been utilized in-
1
,7,12,13
pulsed laser deposition,6 spray
17
cluding sputtering,
pyrolysis,
scribed previously. The conductivity of the films was mea-
5
,14
and recently, metal–organic chemical vapor
sured in a four-point-probe configuration. Film patterning for
van der Pauw type Hall effect measurements employed stan-
dard photolithographic techniques and concentrated HCl as
the etchant. For Hall measurements, Bϭ4000 G with a dc
current of 1.0 mA. Metal composition analysis by SEM/EDX
was calibrated using inductively coupled plasma ͑ICP͒ spec-
trometry, and optical transparency measurements were car-
ried out with Cary spectrophotometers. Film thickness was
measured with a Tencor P-10 profilometer after etching a
step in the film.
7
deposition ͑MOCVD͒. Among these, MOCVD offers the
attraction of in situ growth under a variety of atmospheres,
easy control of composition, amenability to large area more
conformal coverage with high throughput, and the possibility
of creating metastable phases. In this letter, we describe the
application of low pressure MOCVD to indium-zinc oxide
films over a wide composition range using volatile ␤-
diketonate precursors, with the goals of defining appropriate
precursors and growth conditions, the role of dopants, and
the relationship of film processing and microstructure to
what has been learned from bulk studies. It is seen that
indium-zinc oxide films with excellent transparency and
Measurements of resistivity for Zn In O films over
x
y
xϩ1.5y
the composition range x/yϭ0.08–12.0 indicate that film re-
sistivity is minimum at x/yϷ0.33, which is similar to
6
results for Sn-doped, physical vapor deposition ͑PVD͒ de-
rived films, where ␳ is minimized at xϭ0.5–0.6. At 298 K,
the conductivity of an undoped MOCVD-derived
a͒Corresponding author. Electronic mail: tjmarks@casbah.acns.nwu.edu
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003-6951/98/73(3)/327/3/$15.00 327 © 1998 American Institute of Physics
55.33.120.167 On: Sun, 23 Nov 2014 21:40:08
0
1

328
Appl. Phys. Lett., Vol. 73, No. 3, 20 July 1998
Wang et al.
FIG. 2. X-ray diffractograms of MOCVD-derived indium–zinc oxide films
͑
a͒ with Zn/Inϭ1.0 having the In O3 crystal structure and ͑b͒ at Zn/In
2
ϭ10.0 having the ZnO crystal structure. Insets are ␻ rocking scans of ͑a͒
In O :Zn ͑222͒ and ͑b͒ ZnO:In ͑002͒ for the corresponding films.
2
3
ZnO at low In levels ͑Fig. 2͒. However, films with Zn/In
ϭ1.5–5.0 exhibit poorly crystalline microstructure with
weak, broad reflections, similar to the results for other
FIG. 1. ͑a͒ Electrical resistivity as a function of temperature for a MOCVD-
1
7
sputtering and MOCVD derived films at this level of char-
acterization. More detailed TEM studies of a present film
having nominal composition Zn In O reveal a polycrystal-
derived Zn0.66In O3.66 film deposited at 450 °C; ͑b͒ transmission optical
2
spectrum of a film sample with the same composition.
2
2
5
line microstructure from plan-view with 1000–2000 Å poly-
crystalline grains ͓Fig. 3͑a͔͒, while electron diffraction ͓Fig.
Zn0.66In O
film is ϳ1000 S/cm, with n-type transport, a
2
3.66
20
Ϫ3
carrier concentration of 3.7ϫ10 cm , and mobility of
3
͑b͔͒ is consistent with poor in-plane alignment and a cubic,
2
1
7.6 cm /V s. At 77 K, the carrier concentration is 3.6
homogeneously Zn-doped In O structure. This is confirmed
2
3
2
0
Ϫ3
2
ϫ10 cm and the mobility 18.5 cm /V s. These results
can be compared with maximum ␴ ͑25 °C͒ values of ϳ2500
S/cm for Sn-doped films grown by pulsed laser deposition,6
ϳ1800 S/cm for Ga-doped films prepared by rf magnetron
by microdiffraction and EDX using a 5 nm electron beam.
High resolution TEM additionally reveals the presence of a
layered phase precipitated in the In O :Zn matrix ͓Fig. 3͑c͔͒,
2
3
although this feature would be difficult to identify by con-
ventional XRD. The layered phase is assigned from the lat-
3
sputtering, and ϳ2900 S/cm for Sn-doped films prepared by
magnetron sputtering. These doped systems have compa-
rable mobilities and carrier concentrations as the present
films, which exhibit metal-like (d␴/dTϽ0) charge transport
1
tice parameters to the kϭ3 member of the Zn In O fam-
k
2
kϩ3
ily, the structure of which consists of alternating (ZnO) and
k
15,18
In O layers.
For films grown in this study, as more Zn is
2
3
͓
Fig. 1͑a͔͒.
Figure 1͑b͒ shows the optical absorption coefficient ͑␣͒
film ͑0.5
doped into the In O structure, the carrier concentration is
2
3
expected to increase, resulting in increased conductivity
͑Zn/In Ͻ0.33͒. However, it is found that when more Zn is
added, the layered Zn In O phase begins to precipitate. Al-
as a function of wavelength ͑␭͒ for a Zn0.66In O
2
3.66
␮m thick͒, having the lowest resistivity. The transmitted
3
2
6
light is normalized to the light through a segment of un-
coated quartz substrate. The transparency window is consid-
though this phase is the most conductive one in the bulk
Zn In O family in the reduced state ͑270 S/cm; com-
k
2
kϩ3
15
erably greater than that of typical ITO films and comparable
pacted polycrystalline sample͒, it is less conductive than
the Zn0.66In O film ͑ϳ1000 S/cm͒. Formation of the
6
to that of PVD-derived Zn In Sn O
films, while
1
.2 1.9
0.1 4.25Ϫ␦
2
3.66
the transmittance of this film ͑Ͼ95%͒ rivals or exceeds that
of the most transmittive TCO films reported to date for this
conductivity level.
As defined by x-ray diffraction ͑XRD Cu K␣͒ and trans-
mission electron microscopy ͑TEM͒ analysis, the microstruc-
modulated structure is likely a reason why the conductivity
of the as-deposited Zn–In oxide films falls for Zn/In atomic
ratios larger than 0.33. Further annealing of the present
Zn In O film under the same growth conditions results in
greater quantities of the modulated phase and correspond-
ingly higher resistivities.
2
2
5
tures of the MOCVD-derived Zn In O films with k
k
2
kϩ3
ϭ0.16–24, are highly composition dependent. The XRD re-
flections with Zn/In ratios р1.0 or at low In levels are sharp
and strong. The films are found to be highly textured, with
Atomic force microscopic analysis reveals that the as-
deposited films are dense with a grained structure The shape
and size of the grains are consistent with those found in the
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structures assignable to cubic In O at low Zn levels and to
plan-view TEM. The surface of the films is smooth, with an
2
3
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Appl. Phys. Lett., Vol. 73, No. 3, 20 July 1998
Wang et al.
329
textured film with a relatively poor out-of-plane texture ͑␻
scan full width at half maximum, ϳ7°͒, scattering of charge
carriers by high-angle grain boundaries may limit the
mobility.20 Better aligned films or even epitaxial ones are
obvious approaches to improve conductivity and light trans-
mittance.
In summary, smooth, transparent conductive indium–
zinc oxide films having a wide composition range have been
prepared by low pressure MOCVD. The as-deposited films
are highly textured, with microstructures assignable to cubic
In O at low Zn levels and to ZnO at low In levels. The
2
3
lowest electrical resistivities are found in films with Zn/In
atomic ratios near 0.33. The transparency window of this
film is considerably greater than that of state-of-the-art ITO,
and the absorption coefficient is lower than previously re-
ported for doped or undoped In–Zn oxide films.
This research was supported by NSF/ONR/DARPA
͑
CHE-94219101/N00014-95-0717͒ and the Northwestern
University Materials Research Center ͑NSF, DMR-
632472͒.
9
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FIG. 3. ͑a͒ Plan-view image and ͑b͒ the corresponding diffraction pattern of
a MOCVD-derived Zn In O film; ͑c͒ HRTEM cross-section image with
2
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1
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