Research

Vertical one-dimensional (1D) semiconductor nanostructures have attracted great interest for fabricating nanoelectronic or optoelectronic devices in a scalable and integrable platform due to their favorable properties such as high surface-to-volume aspect ratio in a small footprint. However, limitation of growth compatibility allows only specific nanomaterial-substrate combinations to support well-controlled vertical growth, significantly limiting the potential applications of the 1D nanostructures. Recently, 1D nanostructures fabricated on two-dimensional (2D) nanomaterials such as graphene have been studied as a new class of material system, where the vertical growth of 1D nanostructures can be performed on 2D nanomaterial that has been transferred onto a desired substrate prior to growth. However, the growth of 1D semiconductor nanostructures on 2D nanomaterials is still at an early stage, restricted to specific combinations of materials in small scale or with limited control of dimensions and positions. We are growing individually dimension and position control of ZnO nanostructures on graphene films and extending to the application-specific substrate with desired properties.

GaN microstructure growth on CVD-graphene films

We grow GaN on 2D layered materials including graphene and hBN using MOCVD system. In general, inorganic compound semiconductors have many advantages over organic materials, including their high carrier mobility and radiative recombination rate as well as their long-term stability and reliability However, problems associated with high-quality inorganic film growth on large or flexible substrates represent one of the major obstacles to the use of inorganic semiconductors in many applications on flexible electronic optoelectronics. To address this problem, we developed growth of high quality GaN on 2D layered materials which exhibit large-scalability and flexibility. We have been interested in not only growth of GaN thin film, but also GaN microstructures including microdisk, pyramid and microrod on the 2D layered materials for their applications on flexible devices.

2. 2-dimensional material growth

Large scale & single crystal 2-dimensional material growth

Our group aims synthesis of large-area, high-quality 2d nanomaterials. Especially, our group has invented centimeter-sized epitaxial h-BN film in which h-BN hexagonal lattice is oriented in single direction. We also provide high-quality graphene films which is basic building block for the 1d-2d hybrid nanomaterials.

Home-made MBE growth of Topological insulators

We built a ultrahigh vacuum molecular beam epitaxy (UHV-MBE) system. It is designed for topological insulator and superconductor van der Waals materials growth and customized for quick operations. Ultrahigh purity Bi, Sb, Se, Te sources are provided by effusion cells. An electron beam gun is installed for Nb evaporation which requires temperature higher than 2000 degree Celsius. We design and engineer heterointerfaces of 2D van der Waals (vdW) materials for the realization of novel quantum electronic states. We employ MBE combined with nanofabrication techniques to form functional nanostructures that can be utilized for new electronic device applications. In particular we focus on making devices based on high quality vdW heterostructures composed of Bi₂Se₃, NbSe₂, and h-BN – a model topological insulator, superconductor, and normal insulator, respectively.

3-dimensional nanoarchitecture microLEDs are attracting tremendous amount of interest as a candidate for next-generation light-emitters, since they offer huge amount of additional light-emitting area compared with planar LEDs and show many unconventional properties which were very hard to achieve with conventional LEDs. We are interested in designing new nanoarchitecture LEDs to create novel device concepts.

4. Flexible inorganic optoelectronics

We also fabricated flexible vertical field-effect transistor (VFET) and resistive random access memory (RRAM) using ZnO nanotube and NiO/GaN microdisk grown on CVD-graphene films, respectively. The flexible inorganic electronics are important component in flexible devices due to their excellent device stability under various substrate bending conditions.

6. Flexible sensors

Wearable sensors are key components for the next-generation electronic devices. Our group has developed an unique process to produce flexible and stretchable sensors. The 1d-2d hybrid nanomaterial system has enabled to fabricate high performance inorganic sensors which can be detached from original substrates and can be used in devices with flexible and stretchable formfactors.

7. Bio-applications

Inorganic semiconductors have been attracted many attentions to in-vitro/in-vivo researches because they enable researchers to study bio-interfaces using conventional electronics/optoelectronics with their excellent bio-compatibility. However, due to the large differences in mechanical properties between the inorganic semiconductors and cells (or brain), acute immune responses have come and restricted good signal recordings. In terms of this issue, our material can be suggested one of promising candidates to record signals with minimum acute immune responses because they are very small and flexible. We have been co-worked with various groups where are pioneers in this field.