The 70th Japan Society of Applied Physics Spring Meeting 2023
Highlighted Presentations Press Release
April 18, 2023

Selective-area-growth of rutile-type SnO2 on TiO2 (110) substrate

Selective-area growth to produce high-quality rutile-typeoxide semiconductors for the development of nextgeneration power devices
Kyoto University1, NIMS2, Ritsumeikan University3,Hitoshi Takane1, Takayoshi Ohshima2, Katsuhisa Tanaka1, Kentaro Kaneko3

Summary• The growth mechanism of rutile-type tin oxide (r-SnO2), one of the rutile-type oxide
semiconductor candidates for next-generation power devices, was investigated in detailusing electron microscopy.
• The dislocation-propagation behavior was clarified, and guidelines for the fabrication ofhigh-quality rutile-type oxide thin films with low dislocation densities were established.
Hitoshi Takane, a Ph.D student of Kyoto University, and co-researchers have clarified the characteristics of selectivearea growth mechanism used to produce higher-quality rutile-type oxide semiconductors. In an experimental study ofgrowing a rutile-type tin oxide (r-SnO2) on a rutile-type titanium oxide (r-TiO2) substrate, the authors succeeded inclarifying the growth dynamics and forming regions with few defects (dislocation-free regions). In the future, this research
will be applied to other rutile-type oxide semiconductors to produce high-quality thin films required for the development of
next-generation power devices.

Detail

Realization of rutile-type oxide semiconductors with fewer defects
Power semiconductors are used in everything from commonplace smart devices, such as personalcomputers, to power circuits in electric vehicles. Development of next-generation power
semiconductor materials is currently underway. In particular, research and development of "ultrawide bandgap (UWBG) semiconductors," which have an extremely wide bandgap (*1) and are
directly linked to the development of innovative power devices, is accelerating. UWBG
semiconductor candidates include gallium oxide (Ga2O3, band gap of 4.4 – 5.6 eV), diamond (5.5eV), aluminum nitride (AlN, 6.0 eV), and cubic boron nitride (c-BN, 6.3 eV).
Hitoshi Takane, a Ph.D student of Kyoto University, and co-researchers focused on rutile-typeoxide semiconductors, particularly rutile-type germanium oxide (r-GeO2). r-GeO2 is an UWBG
semiconductor with a direct bandgap of 4.7 eV and characterized by its both p- and n-type
dopabilities proven via theoretical calculations. The research group has successfully achieved
epitaxial growth of r-GeO2 on rutile-type titanium oxide (r-TiO2) single-crystal substrates, which arecommercially available, and has proposed a semiconductor alloy system with other rutile-typeoxides. Moreover, they have already succeeded in fabrication of alloy films with rutile-type tin oxide(r-SnO2), which has the next largest band gap of 3.6 eV after r-GeO2, over the entire composition
range.
Rutile-type oxide semiconductors exhibit large bandgaps and high functionality; however,determining their growth dynamics remains challenging. r-GeO2 has been found to containdislocations when epitaxially grown on r-TiO2 substrates.
From the viewpoint of power device applications, these dislocations are known to degrade thedevice performance (low electron and hole mobility, increased leakage current, and premature
breakdown). This study aims to reduce these dislocations and provide knowledge that will lead tothe development of high-performance next-generation power devices.
In this study, r-SnO2 was grown on a r-TiO2 substrate with a SiO2 mask and the microstructure wasobserved in detail using two different electron microscopic techniques to clarify the crystal growthmechanism and determine the conditions for realizing low-dislocation regions. In the future, theresearch group plans to apply this research to other rutile-type oxide semiconductors, such as rGeO2, to achieve high crystal quality, which is necessary for developing next-generation powerdevices.
Clarifying the growth mechanism of rutile-type tin oxide (r-SnO2)
In this study, to clarify the growth mechanism of r-SnO2, it was grown on substrates withmicrofabricated circular and stripe-shaped masks. The r-SnO2 selectively grown on window areasof the masks was examined using scanning electron microscopy (SEM). The "mist CVD method"
was used as the growth technique. This method has been used by various research teams to growoxide thin films, like Ga2O3, zinc oxide, and so on. The team chose this method because it issimpler and less expensive than more common growth methods, including metal–organic chemical
vapor deposition (MOCVD).
First, it was confirmed that the crystal island grew on the circular mask pattern by forming ahexagonal facet structure (Figure 1). The experiment revealed that r-SnO2 crystals grow with afacet structure that reflects the “equilibrium form” (*2) of rutile-type structure based on the surface
energy density derived from theoretical calculations, which is crucial for controlling the crystal
growth in the future.

In the stripe-shaped mask pattern, during the initial growth stage, several small crystal islands
were formed and grew three-dimensionally (Volmer-Weber mode) on the window area. Then, they
coalesced each other to form a striped crystal. Finally, it was found that lateral overgrowth started
after the complete coverage of the window (Figure 2).
Based on these observations, the research group was able to assess the growth process of r-SnO2
from the initial stage. Subsequently, they determined that the facet shape of the striped sidewalls
reflects the order of the surface energy density derived from the theoretical calculations, evenwhen grown on a stripe-shaped mask pattern.

Regarding research on techniques to control low-dislocation regions
The group also observed the grown r-SnO2 using cross-sectional transmission electron microscopy(TEM). Dislocations in the laterally grown region propagated perpendicularly to the inclined facet
plane (the image force effect) (Figure 3). The formation of dislocation-free regions (present in thelower part of the laterally grown region on the mask) was also observed (dislocation density ≤ 5 ×
108 cm−2), confirming the effectiveness of the selective-area growth.
Although the dislocation-free region remains small, the present results indicate that facet surfaces
follow an equilibrium shape that reflects the surface energy density derived from theoretical
calculations. This suggests that by appropriately selecting the substrate surface orientation andmask orientation, the facet shape and dislocation behavior can be controlled.
A method for generating the dislocation-free regions is important for the mass production of powerdevices using rutile-type oxide semiconductors. In the future, the research group will investigate
and develop methods for the epitaxial growth with even larger dislocation-free regions byappropriately selecting the substrate orientation and mask pattern shape to improve the precisionof microfabrication.

Annotation*1 Bandgap It denotes the energy width of the forbidden band in the band structure of a crystal. Depending on thebandgap energy, the material can be categorized as a metal, insulator, or semiconductor.
*2 Equilibrium form The form for which interfacial free energy in a crystal is minimized.

Figures

Figure 1. SEM image of r-SnO2 single crystal selectively grown on acircular window.

Figure 2. SEM images demonstrating the growth process of r-SnO2 on the striped
window (left to right).

Figure 3. Cross-sectional TEM image of r-SnO2 selectively grown on the striped
window.