NSG Found. Mat. Sci. Eng. Rep.

透明酸化物半導体 (Ba, Sr)SnO3 薄膜の真空熱処理による高移動度化
北海道大学

電子科学研究所

ジョ、ヘジュン

Mobility Enhancement in Transparent Oxide Semiconductor (Ba, Sr)SnO3 Films via
Vacuum Annealing
Hai Jun Cho
Research Institute for Electronic Science, Hokkaido University
は、新しい透明酸化物半導La ドープペロブスカイトスズ酸化物（La-ASnO 3 , A = Ba, Sr）体として注目されている。Sn 5 s 軌道からなる伝導帯と、O 2 p 軌道からなる価電子帯の間のバンドギャップは可視光の最大エネルギー（3 . 1 eV）よりも広く、Sn 5 s 軌道の大きな重なり積分により、La-ASnO 3 単結晶は高い電子移動度を示す。しかし、薄膜成長中に構造欠陥が生じるため、La-ASnO 3 薄膜の電子移動度は光電子デバイス応用に必要なレベルに達していない。本研究では、単純な熱処理プロセスにより、La ドープ SrSnO 3 薄膜の電子移動度が向上したことを報告します。4 . 6 eV の広いバンドギャップとともに、約 3000 S
cm − 1 の記録的な高導電率が得られました。以上の結果から、La ドープ SrSnO 3 薄膜はDUV 透明電極としての応用が期待できます。
La-doped perovskite stannates (La-ASnO 3 , A = Ba, Sr) have attracted attention as noveltransparent oxide semiconductors. The bandgap between the conduction band composed of
Sn 5 s orbital and the valence band composed of O 2 p orbital is wider than the maximum
energy of visible light ( 3 . 1 eV), and La-ASnO 3 single crystal exhibits high electron mobility
due to large overlap population of Sn 5 s orbital. However, thin films of La-ASnO 3 exhibit low
electron mobility due to structural defects associated with the film growth, which strongly
limits the full potential of La-ASnO 3 in optoelectronic device applications. Here I reportsuccessful mobility enhancements in La-doped SrSnO 3 (LSSO) films via a simple posttreating process. A record-high electrical conductivity value of ~ 3000 S cm − 1 was obtained
alongside a wide bandgap of 4 . 6 eV. The La-doped SrSnO 3 thin film would be applied as a
DUV transparent electrode.
1．Introduction
Transparent semiconductors are essential materials for building transparent thin filmtransistors, which provide a platform for controlling light with electricity. Therefore, there
are extensive research effort towards improving the optoelectronic properties of transparent
semiconductors to transcend the performance of optoelectronic devices. Two of the mostimportant characteristics in this regard are high optical bandgap (Eg) and electron mobility(μ), which determine the spectral range of operation and the operation speed, respectively.
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（公財）日本板硝子材料工学助成会，40（2022）

This is the greatest challenge in designing the transparent semiconductor for optoelectronic
applications since increasing Eg usually decreases the electrical transpor t proper ties,including μ. For example, Ga 2 O 3 -based materials exhibit a very large bandgap of > 5 eV, but
their electrical conductivities ( σ ) are below 1 S cm- 1 [ 1 , 2 ].
La-doped perovskite stannates (La-ASnO 3 , A = Ba, Sr) can potentially overcome the typicaltradeoff relationship between Eg and μ. Their conduction and valence bands consist of Sn 5 sand O 2 p orbitals, respectively. The large electronegativity difference between Sn and O
provides a large bandgap while the disperse nature of Sn 5 s orbitals allow rapid electrontransportations. One of its kind, La-BaSnO 3 (LBSO), exhibits with a very high mobility of 320
cm 2 V- 1 s- 1 in single crystal form [ 3 ], which is even comparable to traditional semiconductors
like Si [ 4 ]. However, its Eg of ~ 3 . 1 eV limits its optoelectronic operation is to visible range.
Higher E g values of > 4 eV can be achieved La-SrSnO 3 (LSSO), which is suf ficient fortransmitting UV lights. However, their electrical transport properties are poor, especially inform of thin films. The highest σ reported from LSSO to date is 1000 S cm- 1 [ 5 ].
In our previous research, we found that thestructural defects in LBSO films can be eliminatedwith a simple post-annealing process in vacuum,
which greatly enhanced the electrical transpor tproperties [ 6 ]. In this research, we extend the same
approach to LSSO films and repor t the results,including a record-high σ value of ~ 3000 S cm- 1 witha E g of 4 . 6 eV, which can even transmit deep-UV(DUV) lights. The σ observed from the LSSO films
in this work is much higher than that observed fromother oxide conductors at similar Eg (Fig. 1 ) [ 1 , 2 , 7 - 9 ].
2．Experiment
La xSr 1 − xSnO 3 (x = 0 . 005 , 0 . 02 , 0 . 03 and 0 . 05 )Figure 1 – Optoelectronic proper ties oftransparent semiconductors
and the results obtained fromthis work.

epitaxial films were fabricated on ( 001 ) MgO (a =0 . 4212 nm) single cr ystal substrates using pulsed
laser deposition (PLD) technique (KrF excimer). The laser parameters are wavelength (λ)= 248 nm, fluence ~ 2 J cm − 2 pulse − 1 , and repetition rate = 10 Hz. The substrate temperatureand oxygen pressure inside the chamber during the film growth were 790 ° C and 20 Pa,respectively. High- resolution X-ray diffraction (Cu Kα1 , ATX-G, Rigaku Co.) was performed
to investigate the crystalline phase, orientation, lattice parameters, film thickness, and thelateral grain size of the films. Scanning transmission electron microscopy (STEM) wasperformed to observe the microstructures of the films. DC four-probe method in van derPauw configuration was used to measure the σ , carrier concentration (n), and Hall mobility(μHall) of the films. The thermopower (S) was acquired from the thermos-electromotive force(∆V) generated by a temperature difference (∆T) of ~ 4 K across the film using two Peltier− 182 −NSG Found. Mat. Sci. Eng. Rep.

devices. The temperatures at each end of the films were simultaneously measured, and the S
was calculated from the slope of the ∆T − ∆V plot. The characterizations were repeated after
the LSSO films were vacuum annealed at 790°C and 10 - 2 Pa for 30 mins.
3．Results and Discussions
The reciprocal space mappings (RSM) of the as-grown and vacuum annealed LSSO films(x = 0 . 03 ) are shown in Fig. 2 as an example. The 204 diffraction peaks of LSSO and MgOsubstrates can clearly be identified, but the epitaxial growth was not coherent as the qx
values of peaks from the film and the substrate are not the same. The vacuum annealing
sharpened the LSSO peak in the diffraction pattern along qx and decreased its integral width(Figs. 2 a and 2 b). All LSSO films exhibited a substantial increase in the lateral grain size (D)(Fig. 2 c). According to the Scherrer's equation, expressed as D = (Integral width) − 1 , thelargest grain growth was observed at x = 0 . 03 , which changed from ~ 12 nm in the as-grownstate to ~ 28 nm after vacuum annealing.

Figure 2 –（a, b）RSMs around 204 LSSO diffraction spot of（a）as grown and（b）vacuum annealedfilms. The cross-sectional diffraction patterns along qx/ 2 π direction are shown at thebottom of each RSM.（c）Changes in（top）the average lattice parameter and（bottom）
lateral grain size of the as-deposited LSSO films（white）and annealed LSSO films（red）
as a function of La concentration.

The lateral grain growth after vacuum annealing was further confirmed with low-angle
annular dark-field scanning transmission microscopy (LAADF-STEM). As shown in Fig. 3 ,columnar grains perpendicular to the film/substrate interface are observed. The vacuum
annealing significantly increases the D of the LSSO film, which is in excellent agreementwith the RSM results (Fig. 2 ). It is also shown that the strain contrast in the LAADF-STEM
noticeably decreases after the vacuum annealing, suggesting that the lateral grain growthrelaxes the LSSO film.
The resulting electrical transport properties at room temperature are summarized in Fig.
4 . All electron transport properties (σ, n, μHall) substantially increased after the vacuumannealing (Fig. 4 a and 4 b). The absolute value of S decreased after the vacuum annealing,− 183 −（公財）日本板硝子材料工学助成会，40（2022）

Figure 3 – C r o s s - s e c t i o n a l L A A D F - S T E M
micrographs of（a）as-grown and（b）
vacuum annealed LSSO films.

which is consistent with the changes in n. The greatest
enhancement was observed at x = 0 . 05 , but the highest
absolute values were observed at x = 0 . 03 . This is likely
attributed to the fact that 5 % is further away from thesolubility limit of La in SrSnO 3 lattice.
The σ of ~ 3000 S cm- 1 observed from the 3 % LSSO
film after vacuum annealing is a record among allreported values from LSSO. This is largely associatedwith the increase in n, as the change in μHall is notsubstantial at x = 0 . 03 (Fig. 4 d). Considering the shortvacuum annealing time, the amount of oxygen vacancy
is likely small to affect the overall n, and it is reasonable
to assume that the generation of carrier electrons ismostly attributed to the activation of La dopants. The 3 %LSSO film has the highest La dopant activation rate(~ 70 %) in this study, which was also accompanied by
the largest grain size. In this regard, the data suggestthat La dopant activation is the most critical factor forenhancing the electron transport properties of LSSO
films, which is strongly affected by the lateral grain
size.
The optical transmission and reflection spectra ofvacuum annealed 3 % LSSO film (thickness: 112 nm)− 184 −Figure 4 – E l e c t r o n t r a n s p o r t
proper ties of the LSSOfilms as a function of La
content（%）.（a）σ,（b）n,（c）− S,（d）μHall and（e）
Activation rate of La
dopants.

NSG Found. Mat. Sci. Eng. Rep.

gr own of double-side polished ( 001 ) MgO
substrate is shown in Fig. 5 (inset). According tothe (αhν) 2 − hν plot, the estimated Eg value is~ 4 . 6 eV, which agrees well with previously
repor ted values [ 10 ] and is suf ficient enough totransmit DUV lights. The LSSO films examined inthis research have appropriate optoelectronic
properties for UV-based optoelectronic devices.
4．Conclusions
In this research, LSSO films with excellent
optoelectronic properties (~ 3000 S cm − 1 , Eg ~ 4 . 6eV) were successfully fabricated on MgO substrate

Figure 5 – Optical proper ties of vacuum
annealed 3 % LSSO film. The Eg
was ~ 4 . 6 eV, which can transmit
DUV lights.

using PLD and a simple vacuum annealing process. Structural analyses revealed that thecolumnar grains grow in the lateral direction during the vacuum annealing, and the latticestrain significantly gets reduced. The str uctural improvements also considerable
improvements in activation of La dopants, which lead to enhanced the electron transport
properties like high La dopant activation (~ 70 %, 3 . 26 × 1020 cm − 3 ) and high Hall mobility( 55 . 8 cm 2 V − 1 s − 1 ). These are very promising characteristics for expanding the operation
range of optoelectronic devices to UV region, as they are sufficient for UV transmitting
electrodes.
Acknowledgements
The financial support from the Nippon Sheet Glass Foundation for Materials Science andEngineering is greatly appreciated by the author of this work.
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