List of publications, courtesy Altmetrics
High power and high frequency electronics for wireless and communication applications
It is clear that generations beyond 5G will continue to operate at higher frequencies, demanding higher gain and efficiency. There is also a need for efficient solid-state power amplification to replace complex, low-efficiency multi-stage circuits and bulky and fragile vacuum tubes in defense applications. Additionally, in the era of internet of things (IoT), artificial intelligence and autonomous vehicles, there is an urgent need for high-power and high frequency transistors that can facilitate ultra-fast, highly reliable, and low latency wireless networks. To serve these needs, transistors which can provide a combination of high power density and high efficiency at high frequencies are required. In our group, we contribute to this area by (i) developing materials with enhanced functionality and (ii) design and fabricating novel device structures
- Developing relaxed InGaN pseudosubstrates
A combination of higher electron velocity and large bandgap is needed for high-frequency and high-power applications. One way of achieving this is using relaxed InGaN as the channel material which provides a reduced electron effective mass, critical in reducing electron scattering and enhancing electron velocity. Relaxation of the InGaN channel is required since an InGaN channel coherently grown on GaN substrate does not provide an effective mass commensurate with the In mole fraction.
In this project, we are developing relaxed In0.2Ga0.8N buffers on ZnO. They both have Wurtzite crystal structure. Furthermore, based on Vegards law, in-plane lattice constant of In0.2Ga0.8N is perfectly matched to that of ZnO. This allows growth of high quality relaxed In0.2Ga0.8N on ZnO. Also, PAMBE is a relatively low temperature growth technique which should suppress the formation of unwanted In2O3 interlayer because of reaction between O and InGaN at the interface enabling smooth growth on these lattice-matched but chemically dis-similar materials.
Ultra-scaled channel N-polar GaN High Electron Mobility Transistors (HEMTs) on on-axis GaN and SiC substrates
Gallium Nitride has surfaced as a candidate for high power and high frequency applications due to its unique properties, including a large band gap of 3.4 eV, large breakdown electric fields, and high electron mobility and saturation velocity. Additionally, the possibility of epitaxial growth of (Al,Ga,In)N-GaN heterostructures with sharp interfaces allows for suitable band engineering depending on the application. Furthermore, the (Al,Ga.In)N presents spontaneous polarization across the unit cell and piezoelectric polarization when the material is strained, which adds one more knob to engineer the material depending on application requirements.
GaN is most stable in a Wurzite crystal structure. This crystal structure lacks inversion symmetry, with the polarization field in the material depending on the crystal orientation. When the crystal is oriented in the  direction, it is terminated with Ga atoms on the surface and has negative net polarization, which is known as Ga-polar or Ga-face. On the other hand, the [000-1] orientation is terminated with N atoms and has positive net polarization, which is called N-polar or N-face.
GaN-based optoelectronic and electronic devices for commercial applications have been mainly developed on Ga-polar GaN templates due to less complexity of epitaxial growth compared with that on N-polar. Nonetheless, N-polar GaN-based HEMTs have several advantages over Ga-polar GaN-based HEMTs that make them a promising candidate for highly scaled devices. In N-polar GaN-based HEMTs, the two-dimensional electron gas (2DEG) forms on top of the barrier, unlike in Ga-pol
ar HEMTs where the 2DEG is below the barrier due to the lack of inversion symmetry in the wurtzite crystal structure, which results in N-polar and Ga-polar GaN having opposite polarizations. Formation of the 2DEG above the barrier results in the formation of a natural back-barrier in N-polar HEMTs with better confinement of the 2DEG. The enhanced confinement of the 2DEG improves the output resistance and pinch-off of the devices. Furthermore, the charge centroid of the 2DEG is closer to the gate in N-polar GaN-based HEMTs, allowing for better gate control, especially for scaled channels. In addition, as there is no large-bandgap barrier between the free surface and the 2DEG in N-face HEMTs, it is easier to achieve Ohmic contacts with very low resistance.
In this project, using a novel barrier design, we aim to aggressively reduce the channel thickness to 3 nm in N-polar GaN HEMT structures. We will use molecular beam epitaxy for epitaxial growth which allows for growth of this materils on on-axis GaN and SiC substrates.
High power diodes and transistors for power conversion applications
Wide bandgap-based RF and power electronics are going to define many crucial “more-electric” transportation systems and communication systems including RADAR. Underlying this success is the enhanced efficiency in conversion processes afforded by wide bandgap materials from kHz to GHz. The need is currently being served by SiC and GaN, but the requirements are even beyond the performance of these systems. Nonetheless, producing large scale, cost-effective, and high quality GaN and (to a lesser extent) SiC substrates remains the major challenge in the development roadmap of power electronics based on these wide bandgap materials. Gallium Oxide is the most promising material to provide the next step in performance based on availability of cost effective, large area, high quality substrates.
- Epitaxial growth and electron transport of (Al,Ga)2O3 films and heterostructures
The β-Ga2O3 has recently attracted a great deal of attention due to its wide bandgap of ~4.8 eV, and availability of melt growth techniques to produce high quality substrates cost-effectively. Nevertheless, there are still many unknowns about this material system, which need to be addressed before it can be broadly considered for mission-critical applications. A very fundamental question in any emerging semiconductor technology concerns electron transport.
This project aims to deepen our limited understanding of electron transport in Ga2O3, (AlxGa1-x)2O3 and Ga2O3-(AlxGa1-x)2O3 heterostructures via comprehensive pressure-dependent electrical measurements. The material for the above-mentioned studies will be grown epitaxially via a hybrid MBE system equipped with Ga and Ge conventional effusion cells, a custom-built gas delivery system to supply gas sources for Si and Al sources, and an oxygen RF plasma source.
- Ga2O3 vertical diodes and transistors
The need for efficient power generation, distribution, and delivery is quickly expanding in different sectors of industry. This includes transportation, industrial automation, and renewable energy. Power electronics is the heart of this industrial revolution, which can be found in various applications ranging from power amplifiers in servers, solar invertors, electric vehicles, and motor drives/servos in industrial robotics. The power electronics market as a whole was about $20 billion in 2012, and is expected to increase to $41.7 Billion by 2022. In our group, we are working on vertical Ga2O3 diodes and transistors for ultra-high voltage (2kV-10kV) applications.