NANOscientific

Assistant Professor Munho Kim, Nanyang Technological University,

Singapore Adapted from Presentation, Edited by NanoScientific

In the fast-moving realm of advanced optoelectronic devices, control of material properties at the nanoscale is a decisive factor for performance and innovation. Assistant Professor Munho Kim of Nanyang Technological University, Singapore, is pushing these boundaries through the development of unconventional semiconductor structures and device concepts. His research focuses on inorganic semiconductor nanomembranes and nanostructured architectures, with Atomic Force Microscopy (AFM) serving as a critical tool in characterizing and refining these nanoscale innovations.
This feature explores the two key areas of his research: ultra-thin crystalline
semiconductor nanomembranes and high-aspect-ratio nanostructures fabricated via metal-assisted chemical etching (MACE). Both highlight AFM’s essential role in precision engineering, device optimization, and surface quality validation.

Quasi-2D Semiconductor Nanomembranes
At the core of Prof. Kim’s work lies the creation of quasi-two-dimensional
semiconductor nanomembranes—ultrathin, flexible crystalline layers of materials like silicon, germanium, and gallium nitride. These membranes, with thicknesses ranging from hundreds down to just 10 nanometers, retain exceptional electrical and optoelectronic properties, including high carrier mobility and minimal defect density.
Such membranes can be transferred to a variety of substrates—rigid or flexible—by exploiting oxide sacrificial layers in silicon-on-insulator (SOI) or germanium-on-insulator (GeOI) wafers. Selective etching in hydrofluoric acid releases the top layer, which is then transfer-printed onto the target substrate. This technique combines the mechanical flexibility of a thin film with the high material quality of bulk crystals, paving the way for applications in wearable sensors, flexible electronics, and integrated optoelectronic systems.

Enhancing Light Absorption in Ultra-Thin Layers
Reducing the thickness of semiconductors usually compromises light absorption. Prof. Kim’s lab has addressed this by using optical cavity effects to recycle light within the structure. By placing ultrathin germanium layers—7 to 25 nm—on reflective metals such as silver or gold, light undergoes multiple internal reflections, boosting absorption up to 80% despite the reduced material volume.

However, high absorption alone is not enough; crystallinity matters.
Conventional deposition at these thicknesses often yields amorphous or polycrystalline films, degrading device performance. To overcome this, the team fabricates single-crystalline nanomembranes via smartcut processes and wafer bonding, followed by chemical mechanical polishing (CMP). AFM measurements confirm post-polishing surface roughness as low as 0.6 nm, a key metric for reducing defect densities and improving device efficiency.

Device Applications: Phototransistors with Nanocavity Designs
Using these high-quality membranes, the group has demonstrated devices
such as phototransistors built on aluminum oxide/silver stacks that function as optical nanocavities. The devices exhibit strong wavelength- specific response and high normalized photo-to-dark current ratios (NPDR). Notably, membrane thickness controls the spectral absorption peak, allowing device tuning for specific optical ranges.
At around 20 nm thickness, the depletion width exceeds the active layer,
lowering dark current and enhancing detection sensitivity. AFM’s precision
measurements of membrane step heights and surface topography are crucial in correlating physical structure to device performance.

Nanostructures via Metal-Assisted Chemical Etching (MACE)
In parallel, Prof. Kim’s group develops high-aspect-ratio semiconductor
nanostructures through MACE. This scalable technique uses a thin metal catalyst layer, typically gold or platinum, deposited on semiconductors
like silicon, silicon carbide, or gallium oxide. Immersion in an etchant solution triggers redox reactions at the metal interface, generating holes that oxidize the semiconductor beneath. The oxidized regions are then etched away, leaving pillars, wires, or porous structures with nanoscale precision.

Control of carrier generation and mass transport determines etch geometry and quality. Variants such as forward MACE, inverse MACE, self-anchored MACE, and photo-enhanced MACE (for wide bandgap semiconductors) broaden the design possibilities. In materials like gallium oxide, photo-enhanced MACE uses UV illumination to generate sufficient carriers for efficient etching.

AFM as a Critical Tool in MACE Optimization
AFM’s role in MACE-based nanostructuring is indispensable. It provides:
• Topography Mapping: High-resolution imaging of etched surfaces to verify structure dimensions.
• Surface Roughness Analysis: Ensuring smoothness for reduced defect sites in subsequent device fabrication.
• Depth and Height Profiling: Measuring aspect ratios of nanostructures
with sub-nanometer accuracy.
For example, AFM imaging of MACE-etched silicon carbide reveals nanohole
arrays over 100 nm deep, with etched ridge features corresponding precisely to SEM observations. Similar measurements in gallium oxide quantify nanoridge heights of ~160 nm. These insights enable fine-tuning of etching parameters to achieve desired structure geometries and surface finishes.

Implications for Advanced Optoelectronics
The nanomembranes and nanostructures emerging from Prof. Kim’s lab have wide-ranging implications:
• Flexible and Wearable Electronics: Transferrable nanomembranes enable integration onto diverse substrates.
• High-Sensitivity Photodetectors: Optical nanocavities amplify light absorption in ultrathin active layers.
• Power Electronics: Wide bandgap semiconductor nanostructures promise improved thermal and voltage handling.
• Next-Generation Imaging: Tunable devices span ultraviolet to infrared
detection ranges.
AFM remains at the center of these advances, bridging material synthesis
and device integration by ensuring nanoscale precision in every step.

Conclusion
Prof. Kim’s work exemplifies the synergy between advanced materials engineering and state-of-the-art characterization. By pairing innovative
semiconductor architectures with AFM’s unmatched resolution and surface analysis capabilities, his team is advancing the frontier of optoelectronic
device design. Future directions include expanding the materials palette to carbonitrides and oxide semiconductors, refining MACE selectivity, and integrating nanostructures into full device platforms.
The research underscores a clear lesson for the field: at the nanoscale, precision is everything—and AFM remains one of the most powerful tools to achieve it.

Acknowledgments
Prof. Kim acknowledges the contributions of his research group and support from Nanyang Technological University, A*STAR, and the Ministry of Education, Singapore.