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Past Research Interests

Investigation of high-k metal oxide / III-V system for microelectronic application

Continued scaling of the feature size of metal-oxide-semiconductor (MOS) devices stimulates efforts to introduce both high-k dielectric gate insulators and III-V compound semiconductors as high mobility channel materials. A longstanding problem for structures that include an oxide/III-V interface has been the presence of interfacial defects that can trap charge during device operation. In addition, defects present in the bulk of a high-k oxide layer is of equal importance because of their potential to increase gate leakage current, and to form fixed charge that will scatter carriers in the channel and alter tha threshold voltage of the device. In this project, I worked on the identification of defects present at high-k/III-V interfaces and in the bulk of high-k layers grown by atomic layer deposition (ALD), and developed methods to passivate them.
Click figures below to learn about the details of representative works from this project.

Study of growth kinetics of epitaxial films by MBE and PLD

Thin film growth is an essential part in modern device fabrication. It is desirable to have an ability to control morphology and microstructure to meet functional requirements of devices made by stacking thin film layers. Understanding of growth kinetics is, therefore, crucial in advancing the modern technology. Besides the practical purpose, it has been a challenging fundamental scientific subject. As a method to grow high quality epitaxial films, both molecular beam epitaxy (MBE) and pulsed laser deposition (PLD) are being widely used. There are two well-recognized differences in these two techniques. In PLD the kinetic energy of incident species on a substrate can be as large as a few hundred eV, in contrast to a typical thermal evaporation energy of less than 1 eV in MBE. PLD also occurs in short pulses resulting in an instantaneous flux many orders of magnitude higher than found in MBE for the same average deposition rate. These differences in the nature of the depositional flux are expected to significantly alter kinetic processes during growth, understanding of which was the main goal of this project.
Click figures to learn about the details of representative works from this project.

Atomic-scale study of III-V heterostructures using cross-section STM

Alloying of compound semiconductors enables the synthesis of materials with a wide range of bandgap energies and lattice parameters, useful for the application in electronic and optoelectronic devices. The predicted bandstructures and bandgap energies are often based on the assumption that these alloys contain random substitutions on the cation and anion sublattices. In reality, it has long been reported that most semiconductor alloys deviate from perfect randomness. Macroscopic deviation from randomness appears as either phase separation or long range ordering.혻Microscopic deviations involve non-random atomic arrangements on the scale of a few bond lengths, either a preferential association of like atoms (clustering) or unlike atoms (short-range ordering). Since both macroscopic and microscopic deviations from randomness may significantly alter the electronic and optical properties of semiconductor alloys, I studied phase separation (macroscopic deviation from the randomness) and clustering (microscopic deviation) in III-V semiconductor alloys. Systems investigated include lattice- mismatched heteroepitaxial InAlAs films on GaAs, InAs quantum dots formed on GaAs, InGaAsN/GaAs superlattices, and GaP/InP short-period superlattices. Click figures below to learn about the details of representative works from this project.