In a quest to explore and push the performance limits of wide bandgap (WBG) materials and devices for new applications, we are are currently working on following projects:

  • Vertical GaN power diodes for >3 kV applications
  • Ultra-wide bandgap AlN devices for power electronics applications
  • GaN electronic devices and systems for high temperature applications
  • InGaN based solar cells for space applications
  • III-nitride photonics: LEDs, lasers, integrated photonics, THz intersubband devices

Vertical GaN Power Diodes for Power Electronics > 3kV

5Power electronics operating at sub-10kV (3kV–7kV) are highly demanded in application of water plants, wind turbine inverters, locomotive traction, and high voltage direct current (HVDC) power transmission, etc. When operating at > 3kV, conventional Si power devices such as insulated gate bipolar transistors (IGBTs) suffer from large chip area, low switching frequency, and poor heat dissipation, which increase their manufacture cost, energy loss (nearly 10%) and device failure risk at high temperatures. In contrast, power devices based on WBG GaN semiconductors are more attractive in this voltage range due to the material advantages such as wide bandgap (3.4 eV), high critical field, high mobility and better thermal conductivity, which may lead to higher performance power devices than Si devices with increased breakdown voltage and reduced on-resistance. At ASU, we are developing vertical GaN p-i-n power diodes on bulk GaN substrate to achieve blocking voltage of > 3kV. In the preliminary efforts, we designed and fabricated both vertical and lateral (for comparison) GaN p-i-n power diodes with very thick intrinsic layers and low background doping density using advanced growth techniques. High breakdown voltage of > 1kV, low specific on-resistance of 3 mΩ⋅cm2, and near unity ideality factor of 1.07 were achieved on vertical GaN p-i-n power diodes, which are comparable to the state-of-the-art. In the future plan, we will develop suitable junction-termination-extension (JTE) technology for the vertical p-i-n diodes to reduce the electric field crowding at the junction edge, a notorious phenomenon leading to significant reduction of blocking voltage. Novel field plate design will be applied to the GaN p-i-n diodes to control the electric field distribution and maximize the blocking capability. 1

Figure: Forward and reverse I–V of GaN vertical power diodes with >1kV breakdown voltage.

AlN Materials and Devices for Next Generation Power Electronics

3Emerging ultra-wide bandgap semiconductors such as AlN (6.2 eV) have unique material properties that promise next generation high performance power applications with the potential to outperform current GaN or SiC devices. Compared to other semiconductors, AlN exhibits the largest bandgap and highest breakdown field. Furthermore, due to its large bandgap and displacement energy, greater radiation hardness is also expected from AlN devices, which are critical for various DoD electronic systems that must operate in radiation environments or survive manmade nuclear events. Despite these appealing properties, research of AlN is still in its infancy, with little knowledge on their materials and device performance. Under the support of DoD DTRA YIP Award, we are developing high performance radiation harden AlN devices for next-generation power electronics. In the preliminary efforts, we fabricated and characterized Schottky diodes based on n-type AlN, which is one of the first reports on the work. A two-dimensional variable range hopping (2D-VRH) conduction model were developed to model the device characteristics, which suggest that surface states may play an important role in determining the reverse breakdown and leakage current of the AlN devices. The devices showed outstanding high temperature and power performance, which has the potential to outperform the current GaN and SiC devices. In the future plan, we will continue to develop vertical AlN power transistors, as well as carry out radiation experiments and fundamental material studies on the AlN devices. A successful outcome of the research will significantly advance the fundamental knowledge in emerging AlN devices, and provide basic guidance for the design and fabrication of next-generation radiation-insensitive ultra-wide bandgap devices, which are critical to various DoD power applications. This work has attracted significant interests in the WGB community. 2

Figure: Temperature-dependent current density and conductivity curves for AlN Schottky diodes with 2DVRH model.

InGaN Solar Cells for Space Applications 

InxGa1-xN semiconductors have unique material properties that promise high-efficiency next-generation thin-film photovoltaic (PV) applications. InxGa1-xN has tunable direct bandgap from ultraviolet (GaN ~ 3.4 eV) to near infrared (InN ~ 0.7 eV) spectral regions derived by changing the In compositions that provide a perfect match to the solar spectrum. This offers a unique opportunity to design high-performance multi-junction (MJ) solar cells using a single ternary alloy system. Furthermore, compared to conventional PV materials such as Si and GaAs, InGaN has much a larger bandgap and lower intrinsic carrier density, which will allow for efficient solar cells working at high temperatures, critical for various applications in harsh environment and space missions. Despite these appealing material properties, InGaN PV solar cells are confronted by unique materials challenges, and their performance are still lower than conventional PV devices (e.g., Si, GaAs, etc.). Under the support of NASA ECF Award, we are developing high temperature InGaN solar cells for space applications. In the preliminary work, we have performed theoretical analysis on the fundamental loss mechanisms in InGaN solar cells. Using advanced materials engineering, we fabricated world’s first nonpolar and semipolar InGaN solar cells, which has the potential to break through the current limit of conventional polar devices. In the future plan, we will continue our effort to advance fundamental knowledge in the PV properties of WBG materials, and to demonstrate new materials engineering techniques for the development of high performance two-junction (2J) InGaN solar cells, which is the first of its kind. A successful outcome of the research will be very beneficial to various NASA missions as power generation is critical to space technology, with potential applications in terrestrial PV applications.6

Figure: We fabricated and characterized world’s first nonpolar and semipolar InGaN solar cells, which has the potential to break the current limit of InGaN solar cells.

Energy Efficient LEDs for Solid State Lighting 

III-nitride LED-based technology provide significant energy savings and important environmental benefits, and has enabled the potential for wide-scale replacement of the traditional lighting with solid state light sources. The most critical challenge for such replacement, however, relates to the fact that the performance of current III-nitride LEDs is tremendous limited at high injection current densities, a phenomenon called “efficiency droop”. The efficiency of a typical “droop” LED peaks at very low current densities (< 2 A/cm2). At high current densities (e.g., > 100 A/cm2) which are required for most high power applications, the overall performance of the LED degrades more than 70%.

2To address the “efficiency droop”, we developed state-of-the-art small-area (0.1 mm2), high-power, and low-efficiency-droop blue LEDs with world record efficiency at extremely high current densities, by combining novel epitaxial techniques with advanced device engineering. These devices are 10 times smaller, yet 10 times brighter, at up to 5 times of the current densities, compared to the conventional LEDs. This work is the first experimental demonstration of InGaN LEDs with low “efficiency droop”.

Material Science and Engineering for “Green Gap” Optoelectronics

Picture51Another urgent challenge for current semiconductor optoelectronics research is the lack of efficient optical materials for the optical devices operating at 500 nm to 600 nm wavelength region, resulting in a “green gap” in the visible spectrum. As the most promising semiconductors to bridge this gap, the efficiency loss for III-nitrides in this particular spectral is mainly attributed to a number of material issues. Among them, the defect generation in the device active area due to the difficulty of indium incorporations and internal polarization-related effects from III-nitride materials are the two most critical factors.

3In this frame of work, we studied the III-nitride materials by comprehensive characterization techniques. We show that properties such as indium incorporations and polarization-related fields are intrinsic properties to different orientated III-nitride materials and have significant impacts on the emission properties of the “green gap” optoelectronic devices. By engineering these material properties, we achieved one of the world’s first direct emission green laser diodes using semipolar InGaN quantum wells (QWs). Up to today, we are the only university-based research group that has direct-emission green laser technology.

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