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NUS Graduate School for Integrative Sciences and Engineering Research Project Write-up Title of Project : Advanced materials and device structures for nonvolatile magnetic memory applications Name of Supervisor : Teo Kie Leong Contact Details: Dept. of Electrical & Computer Engineering National University of Singapore 4 Engineering Drive 3 Singapore 117576 Phone: (+65)6516-4543 Fax: (+65)6779-1103 Office: E2-03-30 E-mail: [email protected] Short Description Magnetic random access memory (MRAM) has been envisaged to have the capability of the speed of static RAM and the density of dynamic RAM, and is nonvolatile. It could be the “dream memory” since it has the potential to replace all the existing memory devices in a computer, maybe even the hard disk drives, becoming the “universal memory”. MRAM used sub-micron magnetic tunnel junction and the process integration is compatible with standard CMOS allowing it to be easily embedded in System-on-Chip applications. Because the data is stored as a magnetic state, MRAM is inherently nonvolatile as well as having unlimited endurance and fast operation. The project involves the fabrication of epitaxial Heusler-alloys-based and perpendicular MTJ for spin-torque MRAM (ST-MRAM) and investigates their spindependent properties such as tunneling magneto-resistance (TMR) and currentinduced magnetization switching (CIMS). Our ultra-high vacuum deposition cluster tool (UHV-CDT) will be a dedicated backbone to fabricate novel TMR device structures with high MR ratio. The UHV cluster tool allows growth of perfect crystalline structure and atomically sharp interfaces which enable discovery of new material systems with coherent tunneling, control of interface strain and spin dependent tunneling, low defect density and greater device uniformity. Title of Project : Dilute magnetic semiconductors for spintronic applications Supervisor : Teo Kie Leong Contact Details: Dept. of Electrical & Computer Engineering National University of Singapore 4 Engineering Drive 3 Singapore 117576 Phone: (+65)6516-4543 Fax: (+65)6779-1103 Office: E2-03-30 E-mail: [email protected] Short Description The field of dilute magnetic semiconductors (DMS) is currently one of intense activity. These materials are of great interest because of the novelty of their fundamental properties and also due to their potential as the basis of future semiconductor spintronic technologies which promise integration of magnetic, semiconducting and optical properties and a combination of information processing and storage functionalities. They should be compatible with existing microelectronic technologies and have potential as low-dissipation, non-volatile, and nano-scale alternatives to current microelectronics. The project involves molecular beam epitaxy (MBE) growth of DMS materials, characterization, and electrical transport studies. Thin films and spintronic prototype devices, such as the spin-filters will be fabricated to investigate the spin injection efficiency and transport of spin-polaized current in semiconductor using different injector materials (spin sources), spin transfer interfaces or aligners and nanostructures. Title of Project : Gallium Nitride-based materials for high power electronic applications Name of Supervisor : Teo Kie Leong Contact Details: Dept. of Electrical & Computer Engineering National University of Singapore 4 Engineering Drive 3 Singapore 117576 Phone: (+65)6516-4543 Fax: (+65)6779-1103 Office: E2-03-30 E-mail: [email protected] Short Description GaN-based materials have superior properties, such as large critical electric field, wide energy band gap EG, and high electron mobility, making them advantageous as materials of choice for high power, high frequency, and high temperature applications. High electron mobility transistors (HEMTs) employing the AlGaN/GaN heterostructure further exploit the high electron mobility in the two-dimensional electron gas (2-DEG) at the AlGaN/GaN interface. Nonetheless, there are still many issues needed to be solved. Many GaN-based HEMTs employ a Schottky gate which has the shortcoming of a large gate leakage current density JG. In addition, GaN HEMTs suffer from current collapse during large signal operation at high frequency, usually referred as “dc-to-radio frequency (RF) dispersion”. Enhancement mode HEMTs with a positive threshold voltage VTH are desirable for many circuit applications. The VTH of HEMTs depends on the epitaxial structure design as well as surface states. The project involves the design and fabrication of GaN-based high-power HEMTs and focus on several key process modules, some of which are limiting factors for achieving better electrical performance. This includes surface/interface quality, device passivation and encapsulation for operation under harsh environment (high temperature and harsh chemical environment). All process modules will be integrated in GaN high-power devices. Additionally, we will investigate the device physics which involves modeling and simulation work as well as study the carrier transport and scattering mechanisms in the two-dimensional electron gas (2DEG) channel for the novel AlGaN/GaN HEMT using magneto-transport measurement techniques. Title of Project: Solid-state Lighting and Displays. Name of Supervisor : Teo Kie Leong Contact Details: Dept. of Electrical & Computer Engineering National University of Singapore 4 Engineering Drive 3 Singapore 117576 Phone: (+65)6516-4543 Fax: (+65)6779-1103 Office: E2-03-30 E-mail: [email protected] Short Description Today the mainstream solid-state lighting (SSL) still relies on inorganic semiconductor technology; for example, InGaN/GaN quantum well (QW) based LED plus phosphors. III-nitride based LEDs critically require high crystal quality of the epitaxial layers, and therefore are not satisfactory for general lighting in terms of both price and efficiency. We intend to address this problem by adopting a new strategy that makes use of excitonic emission in different low dimensional semiconductors (LDS) such as 2D-QWs, 1D-nanowires (NWs, or quantum wires) and 0 D-nanocrystal quantum dots (QDs). All these low dimensional semiconductor structures (LDSS) serve as basic building blocks for light emitting devices, but possess different characteristics. We will make use of the distinctive characteristics of these LDS to design and demonstrate new device structures and therefore achieve device performance previously unattainable. The project involves design and fabrication of alternative excitonic architectures for LED applications with higher performance/cost ratio, study the fundamental physics underlying the excitonic energy transfer between different component materials and improvement of energy efficiency and spectral quality of LEDs by taking advantage of different characteristics of inorganic QWs, NWs and QDs.