Available online at www.sciencedirect.com

Journal of Non-Crystalline Solids 354 (2008) 2796–2800
www.elsevier.com/locate/jnoncrysol

Factors controlling electron transport properties
in transparent amorphous oxide semiconductors
Hideo Hosono a,b,c,*, Kenji Nomura b, Youichi Ogo b,c, Tomoya Uruga d, Toshio Kamiya b,cbaFrontier Research Center, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
ERATO-SORST, Japan Science and Technology Agency, Frontier Research Center, Tokyo Institute of Technology,
Nagatsuta, Midori-ku, Yokohama 226-8503, Japanc
Materials and Structures Laboratory, Tokyo Institute of Technology, Midori-ku, Yokohama 226-8503, Japand
SPring-8, Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Mikazuki-cho, Hyogo 679-5198, Japan

Available online 20 February 2008

Abstract
Metal cation species and post-annealing treatments were extracted as the primary factors aﬀecting the electron transport properties intransparent amorphous oxide semiconductors (TAOS). Amorphous (a-) In–Ga–Zn–O (a-IGZO) thin ﬁlms were taken as an example toexamine the eﬀects of deposition conditions and thermal annealing. Two groups of a-In–GZO ﬁlms (deposited at the ‘optimized’ and‘unoptimized’ conditions) were deposited by changing laser pulse energy used for pulsed laser deposition. The electron transport properties of the as-deposited ﬁlms fabricated at the unoptimalized condition were improved by thermal annealing to a level which is almost
the same as that in the ﬁlms deposited at the optimized condition. The temperature range eﬀective for the improvement is P300 °C,
which is fairly lower than the on-set temperature (500 °C) for crystallization. To extract the eﬀects of constituent metal cations, transport properties of a-Sn–Ga–Zn–O (a-SGZO) ﬁlms and their TFTs were compared with those of the a-IGZO. Comparison of the TFT
performances between the a-IGZO and a-SGZO channels revealed that the In-based system has much better performances when thedevice is fabricated without intentional annealing. Much stronger absorption tail was seen for the as-deposited a-SGZO thin ﬁlms, implying the formation of a low valence state, Sn2+, and relevant defect states locating near the conduction band minimum deteriorate the
TFT performances.
Ó 2008 Elsevier B.V. All rights reserved.
PACS: 71.20.Nr; 73.61.Jc
Keywords: Amorphous semiconductor; Thin ﬁlm transistors

1. Introduction
Transparent amorphous oxide semiconductors (TAOS)
have been attracting attention as channel layers of thin ﬁlm
transistors (TFTs) since the report of a ﬂexible, high performance TFT fabricated on a plastic substrate employing
an amorphous In–Ga–Zn–O (a-IGZO) thin ﬁlms by pulsed
laser deposition (PLD) in the end of 2004 [1,2]. The TAOS*
Corresponding author. Address: Frontier Research Center, Tokyo
Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8503,
Japan. Tel.: +81 45 924 5359; fax: +81 45 924 5339.
E-mail address: hosono@msl.titech.ac.jp (H. Hosono).

0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jnoncrysol.2007.10.071

TFTs have several advantages over TFTs based on othersemiconductors; they can be fabricated on plastic substrates at a low temperature (below 150 °C) by physicalvapor deposition methods such as conventional sputtering[3], and ﬁeld eﬀect motilities of >10 cm2 (V s) 1 are
obtained at a high reproducibility with a high uniformity.
These excellent performances originate from the uniquecharacteristics of TAOS which is the ﬁrst amorphous semiconductor composed of highly ionic chemical bonds [4,5].
The Hall mobility in TAOS is comparable to that in thecorresponding crystalline phases and no Hall voltage sign
anomaly, which is commonly observed in the conventional
amorphous semiconductors, has been observed [5,6]. TheseH. Hosono et al. / Journal of Non-Crystalline Solids 354 (2008) 2796–2800

electron transport properties come from (i) the large overlap between vacant s-orbitals of neighboring heavy metal
cations at the conduction band bottom and (ii) the small
tail state density [4,5]. Since the tail state density is low,
one can lift the Fermi level to above the mobility gap bycarrier doping, realizing the degenerate state at > the carrier concentration 4  1018 cm 3. The leveling oﬀ ofFermi level by doping observed in a-Si:H is not observedin TAOS, and this observation may be understood byremembering that validity of the Mott–Street model [7] isbased on covalent bonded amorphous materials.
Formation of ionic amorphous materials such as TAOS
needs a extremely high cooling rate, i.e., they cannot be
obtained in the form of bulk, but can be fabricated as athin ﬁlm by using physical vapor deposition techniques.
It is thus considered natural that a chemical species and/or a structure, which is stable at higher temperatures but
is unstable at RT, is frozen in the as-deposited thin ﬁlm
and relaxes to a more stable structure upon thermal annealing, leading to an appreciable change in the electron transport properties. Clarifying the thermal annealing eﬀect isessentially important for understanding the intrinsic natureof TAOS toward a device application. In this paper, we
report the preliminarily experimental results on the eﬀects
of the thermal annealing and the role of constituent heavy
metal cation species. The TAOS thin ﬁlms used were a-In–
Ga–Zn–O (a-IGZO) and a-Sn–Ga–Zn–O (a-SGZO).
According to the working hypothesis [4], candidate materials for high mobility TAOS are required to have heavy
metal cations with (n 1)d10ns0 electronic conﬁgurations(where the principal quantum number n P 5) as a major
constituent. Although both of In3+ and Sn4+ having thesame electronic conﬁguration [Kr](4d)10(5s)0 meet thisrequirement, no comparison in TFT/electronic transport
properties between them has been reported to date.
2. Experimental
a-IGZO and a-SGZO thin ﬁlms were prepared by PLD(laser source KrF) using ceramic targets of crystalline InGaZnO3 and a composite of SnO2, Ga2O3, and ZnO with thechemical composition of SnGaZnO3.5, respectively. The
partial O2 pressures during the deposition were chosen tobe 1.0–7.0 Pa to control carrier concentration from <1015
to 1020 cm 3. No intentional heating of the substrate was
performed. Thin ﬁlms (100–150 nm thick) were depositedon silica glasses for Hall eﬀect and optical measurements.
Hall voltage measurements were carried out using thevan der Pauw conﬁguration with an AC modulation of amagnetic ﬁeld at 0.5 T at RT. Sputtered gold electrodes
were used for ohmic contacts. Films with 40 nm thickness
were deposited on thermally oxidized silicon wafers aschannels for TFTs. Bottom gate TFTs were fabricated on
SiO2 (gate insulator, 150 nmt)/Si substrates (gate electrode), and Au/ITO layers were used as the source anddrain contacts. The TFT structures were deﬁned by photolithography and lift-oﬀ. The channel width (W) and length

2797(L) were 300 and 50 lm, respectively. Output and transfer
characteristics were measured using a Keithley 4200 semiconductor characterization system.
The as-deposited ﬁlms and the as-fabricated TFTs wereannealed at varied temperatures up to 500 °C. The amorphous structure of each sample was conﬁrmed by glancing-incidence X-ray diﬀraction using a high power X-ray
source (50 kV  300 mA), and cross-sectional TEM observations and selected area electron diﬀraction were carriedout for some representative ﬁlms. X-ray absorption measurements were performed at a synchrotron radiation facility (SPring-8) and extended X-ray absorption ﬁne
structures (EXAFS) were extracted by a standard method
and analyzed with a code REX 2000 (Rigaku, Japan). Corresponding crystalline ﬁlms were used as references todetermine the phase-shift parameters. The chemical stateof constituent cation in some samples was measured by
X-ray photoelectron spectroscopy.
3. Results
The a-IGZO ﬁlms deposited by diﬀerent laser power
energy densities showed a large variation in Hall mobility(lHall), i.e., the Hall mobility of the ﬁlm (carrier densityof 1017 cm 3) deposited at a laser energy of 2 J cm 2 was0.9 cm2 (V s) 1, while that of the ﬁlm deposited at the
optimized laser energy of 9 J cm 2 with the same carrier
density was 9 cm2 (V s). Fig. 1(a) shows optical absorption spectra of a-IGZO ﬁlms with diﬀerent thermal histories. The as-deposited ﬁlm shows an absorption tail in the
2.0–3.7 eV region. When the ﬁlm is annealed at 6200 °C,
the absorption tail remains unchanged, but abruptly
decreases when annealed at 300 °C. Further gradualdecrease is observed by annealing at higher temperatures.
The lHall value is conspicuously improved from 0.9 to 4
by annealing at 300 °C and increases up to 8 cm2 (V s) 1
at P500 °C (Fig. 1(b)). These results appear that reductionin the absorption tail is parallel to the improvement oflHall, i.e., the lHall is improved by the reduction of theabsorption tail and the critical temperature at which thelarge change occurs is 300 °C.
Fig. 2 shows the TFT characteristics of a-IGZO TFTs
with diﬀerent thermal histories. Although the ﬁeld eﬀect
mobility (lsat) estimated from the saturation current andS value (sub-threshold voltage swing) of the TFT using
the as-deposited channel are rather poor (Fig. 2(b)), thedevice thermally annealed at 300 °C exhibits a good performance (Fig. 2(c)), which is comparable to that using theﬁlm deposited at the optimized condition as the channel(Fig. 2(a)).
Fig. 3(a)–(c) shows the EXAFS radial distribution function for each metal cation in the a-IGZO thin ﬁlms asdeposited and annealed at 400 °C. Structural parameters
extracted from the EXAFS analyses are listed in Table 1.
No signiﬁcant diﬀerence is seen in the coordination number
around a cation, the cation–oxygen distances, or the cation–cation distances between the as-deposited and the2798

H. Hosono et al. / Journal of Non-Crystalline Solids 354 (2008) 2796–2800

Fig. 1. Eﬀects of thermal annealing on (a) the optical absorption and (b) Hall mobility of a-IGZO thin ﬁlms deposited at an unoptimized condition.
Annealing was performed in air and the holding time at each temperature was 30 min. The carrier concentration before annealing was 2  1019 cm 3.

Fig. 2. Comparison of TFT performances using (a) as-deposited, optimized channel, (b) as-deposited, unoptimized channel, and (c) annealed (b) at 300 °C
for 0.5 h in air.

Fig. 3. Radial distribution function in a-In–GZO ﬁlms as-deposited (solid lines) and annealed at 400 °C (dotted lines) for (a) In, (b) Ga, and (c) Zn
obtained from EXAFS measurements.

annealed ﬁlms. This means that neither the change in thecoordination number around each cation nor distance
between a metal and an oxygen is involved in the structural
relaxation occurring during the annealing.
Next, we compared the optical absorption spectra and
TFT characteristics of the optimized a-IGZO with thoseof a-SGZO. The deposition condition for the a-SGZO ﬁlms

was optimized at room temperature using a lsat as the measure. A distinct diﬀerence was observed in the absorption
spectra and TFT performances. The a-IGZO TFT exhibited lsat = 8 cm2 (V s) 1 and S = 0.2 V (decade) 1, whereas
the a-SGZO TFT did not exhibit a clear pinch-oﬀ. Fig. 4
shows the optical absorption spectra of the as-depositedand annealed ﬁlms plotted in a logarithmic scale. The

H. Hosono et al. / Journal of Non-Crystalline Solids 354 (2008) 2796–2800
Table 1The local structural parameters for as-deposited and annealed a-In–GZO
ﬁlms evaluated by EXAFS measurements
In–O

As-deposited
Annealed

Ga–O

Zn–O

r (nm)Nr (nm)Nr (nm)

N

0.212
0.212

5.0
5.6

0.193
0.193

4.2
4.3

0.191
0.191

5.3
5.1

Annealing was performed at 400 °C for 0.5 h.

Fig. 4. Optical absorption spectra of (a) a-SGZO and (b) a-IGZO ﬁlms
with diﬀerent thermal histories. Samples used were deposited at the
optimized condition for each sample. Annealing was carried out in an
ambient atmosphere.

absorption tail intensity in the SGZO is larger by an orderof magnitude than that in the IGZO for the as-deposited
ﬁlms. The tail intensity in the SGZO is reduced by annealing, but the intensity in the best ﬁlm annealed at 500 °C is
still larger than that in the as-deposited a-IGZO ﬁlm. It
was also observed that the performance of the a-SGZO
TFTs was much improved by annealing at temperatures
P400 °C as reported in a previous paper [8].
4. Discussion
In a previous paper [9], we evaluated the energy width ofthe conduction band tail states from conductivity-temperature data on the a-IGZO ﬁlms with diﬀerent carrier concentration, reporting that the energy width in a-IGZO is 7–
20 meV, depending on the deposition condition (that is,laser power density for ablation) and the magnitude is distinctly smaller than that reported in a-Si:H (55 meV in
Urbach tail width). The present work examined the eﬀectof thermal annealing on the electron transport properties
and optical absorption tails. Hall and ﬁeld eﬀect motilities
in the ﬁlms deposited at the unoptimized condition is much
improved by thermal annealing at temperatures P300 °C,
which is lower by 200 °C than the on-set temperature of
crystallization (500 °C) [2]. An excellent correlation was
found among the Hall mobility, ﬁeld eﬀect mobility, andthe reduction in the absorption tail intensity for the unoptimized a-IGZO ﬁlms with increasing the annealing temperature, and these properties approach to the level thatis attained for the as-deposited, optimized a-IGZO. The

2799

EXAFS results suggested that a local rearrangement ofions which does not involve change in the coordination
number around each cation or metal–oxygen bonding separation is the primary origin for the observed changes. In
the a-IGZO system, the conduction band bottom, whichworks as electron path way, is primarily composed ofthree-dimensionally connected In 5s-orbitals [10]. Thus, aplausible structural relaxation is required to resulting in
the enhancement of connectivity and/or overlap among
the In 5s-orbitals, It was observed that the activation energies of the Hall mobilities in the as-deposited, unoptimized
ﬁlms had larger values than the annealed ﬁlms[9], indicating that the thermal annealing reduces the density of theshallow localized states beneath the conduction band minimum. At the present stage, we cannot identify the entity ofabsorption tail appearing in the as-deposited ﬁlm and theenergy levels of the gap states. However, the tail states controlling the Hall mobility should be located near the mobility gap. Heavily distorted In–O polyhedra and a speciﬁc
oxygen vacancy are plausible candidates. Improvement ofelectron transport properties observed here is similar tothat in amorphous metals and glassy semiconductors.
Structural relaxation upon thermal annealing at a temperature much below the on-set of crystallization favors theelectronic conduction. Further eﬀort employing the technique used for a-Si:H is obviously needed to get more
information about the tail states.
Finally, comparison between a-IGZO and a-SGZO isconsidered. The absorption tail state intensity in the aSGZO is larger by two orders of magnitude than that in
the a-In-GZO compared at the as-deposited state. This
conspicuous diﬀerence may be tentatively attributed tothe diﬀerence in the thermodynamic stability of the valence
states of the constituting metal cations, i.e., the diﬀerence
between a lower valence state (2+) and a higher valence
state (4+) is minimal for Sn among the constituting metals.
XPS revealed that a small peak responsible for Sn2+ was
observed at the position of 3 eV from the Fermi level(theconduction band bottom), i.e., the above of the valenceband top, for the as-deposited sample and the intensityof this peak was distinctly reduced by annealing at
600 °C [11]. It is well known that a Sn2+ having[Kr](5d10)(5s2) conﬁguration gives absorption at an energy
range much lower than that of Sn4+. In general, the coordination number around a Sn2+ ion is lower than that of aSn4+. So, we may suppose that thermal annealing induces
atomic rearrangements involving a displacement of an oxygen ion from a neighboring coordination sphere to that ofa Sn2+ ion, which accompanies the oxidation to a Sn4+
state. The electro-neutrality is kept by incorporating O2ions from O2 molecules adsorbed on the surfaces. Anyway,
we may suggest that valence-changeable heavy metal cations are not appropriate for ﬂexible TFT applications
because thermal annealing at a temperature >300 °C is
requisite to have improved performances.
A transparent TFT is a heart of transparent electronics[12,13] and TAOS is being expected as a promising candi-

2800

H. Hosono et al. / Journal of Non-Crystalline Solids 354 (2008) 2796–2800

date of channel semiconductor in high performance TFTs
[14–17] for OLEDs and e-papers. We consider the understanding and control of the tail states in TAOS is a key
for realizing these applications. Since the TAOS is a novel
class of semiconductors (i.e., ionic amorphous materials), it
is obvious that accumulation of basic data and introduction of approach unique to this system are needed.
5. Summary
We examined the eﬀects of thermal annealing on theelectron mobility in a-IGZO and a-SGZO ﬁlms depositedby pulsed laser deposition. The results obtained are summarized as follows:(1) The Hall mobility and ﬁeld eﬀect mobility in a-IGZO
thin ﬁlms deposited at the unoptimized condition
were much improved by annealing at temperatures
P300 °C and approached to the values of the asdeposited optimized ﬁlms.
(2) The threshold annealing temperature (300 °C) for
the improvements in the transport properties for theunoptimized a-IGZO ﬁlms was much lower thanthe on-set temperature (500 °C) of crystallization.
(3) The intensity of absorption tail appearing in 2.0–
3.7 eV in a-In-GZO was distinctly reduced by annealing at temperatures P300 °C and the extent of thisreduction was parallel to the improvement of themobility.
(4) No signiﬁcant change in the coordination number
around each cation or cation–oxygen bond distanceby annealing was observed in EXAFS measurements.
(5) The performances of a-SGZO TFTs are much lower
than those of a-IGZO, especially for the a-SGZO
channels as-deposited and annealed at 6400 °C.
The absorption tail intensity in the SGZO was larger
by an order of magnitude than that in the IGZO. Thisdiﬀerence is attributed to the formation of Sn2+ andthe relevant defects. It is considered that valencechangeable heavy metal cation-based TFTs are notappropriate for ﬂexible TFT applications.

Acknowledgement
T.K. is supported by Industrial Technology Research
Grant Program (Project ID: 06A12203d) from the NEDO
of Japan.
References[1] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, H. Hosono,
Nature 432 (2004) 488.
[2] K. Nomura, A. Takagi, H. Ohta, T. Kamiya, M. Hirano, H. Hosono,
Jpn. J. Appl. Phys. 45 (2006) 4303.
[3] H. Yabuta, M. Sano, K. Abe, T. Aiba, T. Den, H. Kumomi, K.
Nomura, T. Kamiya, H. Hosono, Appl. Phys. Lett. 89 (2006)
112123.
[4] H. Hosono, M. Yasukawa, H. Kawazoe, J. Non-Cryst. Solids 203(1996) 334;
H. Hosono, J. Non-Cryst. Solids 352 (2006) 851.
[5] S. Narushima, M. Orita, M. Hirano, H. Hosono, Phys. Rev. B 66
(2002) 035203;
S. Narushima, H. Hosono, J. Jisun, T. Yoko, K. Shimakawa, J. NonCryst. Solids 274 (2000) 313.
[6] K. Shimakawa, S. Narushima, H. Hosono, H. Kawazoe, Philos. Mag.
Lett. 79 (1999) 755;
K. Shimakawa, H. Hosono, N. Kikuchi, H. Kawazoe, J. Non-Cryst.
Solids 227–230 (1998) 513.
[7] R.A. Street, Phys. Rev. Lett. 49 (1982) 1187;
J. Robertson, Phys. Rev. B 31 (1985) 3827.
[8] Y. Ogo, K. Nomura, H. Yanagi, T. Kamiya, M. Hirano, H. Hosono,
Phys. Status Solidi (a), accepted for publication.
[9] K. Nomura, T. Kamiya, H. Ohta, K. Shimizu, M. Hirano, H.
Hosono, Phys. Status Soldi (a), accepted for publication.
[10] K. Nomura, T. Kamiya, H. Ohta, T. Uruga, M. Hirano, H. Hosono,
Phys. Rev. B 75 (2007) 035212.
[11] Detailed discussion based on these data will be published in a separate
paper.
[12] R.L. Hoﬀman, B.J. Norrism, J.F. Wager, Appl. Phys. Lett. 82 (2003)
1117;
R.E. Presley, D. Hong, H.Q. Ching, C.M. Hung, R.L. Hoﬀman, J.F.
Wager, Solid State Electron. 50 (2006) 500.
[13] H. Hosono, Thin Solid Films 615 (2007) 6000.
[14] R. Martin et al., J. Non-Cryst. Solids 352 (2006) 1471;
E. Fortunate et al., Adv. Mater. 17 (2005) 590.
[15] P. Görrn et al., Adv. Mater. 18 (2006) 738;
P. Görrn et al., Appl. Phys. Lett. 90 (2007) 063502.
[16] H.-N. Lee et al., SID 07 Digest 38 (2007) 1826.
[17] M. Ito et al., IEICE Trans. Electron. E90-c (2007) 2105.