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Research Focus Areas: Electronic and Photonic Devices

Semiconductor nanostructures for nanoelectronics and optoelectronics
Epitaxial growth and characterization of III-nitride nanostructures
III-V based materials and devices
Light emitting diodes, laser diodes, photodetectors
Solar fuels, solar cells

The Nano-Optoelectronic Materials and Devices laboratory has focused on developing high-performance nanophotonic devices using III-nitride nanostructures and their applications in nanophotonics and nanoelectronics. The direct energy bandgap of III-nitride material system covers a wide energy range from ~ 0.65eV (InN) to 6.4eV (AlN), which encompasses nearly the entire solar spectrum. They have emerged as a powerful platform to effectively scale down the dimensions of future devices and systems. Prof. Nguyen’s research group aims to develop superior quality III-nitride nanostructures wherein we will investigate the epitaxial growth, characterization and devices applications of III-N nanostructures on Si, sapphire, and nano-patterned substrates, providing an ideal materials system and device structure for applications in biological sensors, solid-state lighting (SSL), digital displays, electronic textiles, water purification systems, solar cells, and hydrogen generation/carbon dioxide reduction for future clean, storable and renewable source of energy.

I. Growth and characterization of high efficiency III-nitride nanowire for optoelectronic applications

The performance of conventional GaN-based quantum well light emitting diodes (LEDs) in the green, yellow, and red wavelength ranges has been plagued by the very low efficiency and “efficiency droop”, i.e. the decrease of external quantum efficiency with increasing current, due to the presence of dislocations, polarization fields as well as hot carrier effect. Recently, we have developed self-organized InGaN/GaN dot-in-a-wire LED heterostructures on Si substrates with the goal to obtain nearly dislocation-free material that exhibits high internal quantum efficiency (> 60%) across the entire visible spectral range. We will investigate the molecular beam epitaxial growth and characterization of high efficiency III-nitride nanowires on both Si and sapphire substrates for optoelectronic devices including LEDs, lasers, solar cells and photodetectors. We will also develop the new applications of III-nitride nanowires for high-brightness emissive displays with long life, full color capability, and low power consumption where the quantum dot-in-nanowire structure permits easy tuning of the color emission compared to current liquid-crystal and organic light-emitting diode displays.

a) Scanning electron microscopy image showing the morphology of the InGaN/GaN dot-in-a-wire heterostructures grown on a Si(111) substrate by molecular beam epitaxy. (b) A low magnification bright field scanning transmission electron microscopy image showing the position and vertical alignment of the InGaN dots in a GaN nanowire. (c) Room temperature photoluminescence spectra of various InGaN/GaN dot-in-a-wire heterostructures grown on Si(111). The black curve shows the broad white emission from ten InGaN quantum dots in a GaN nanowire heterostructure. The green (dashed line), yellow (dotted line) and orange (dashed-dotted-line) emission spectra from the dot-in-a-wire heterostructures grown under different conditions are also shown. (d)Room temperature electroluminescence spectra of AlxGa1-xN nanowire-based UV LEDs.

Related publications:

H. P. T. Nguyen, K. Cui , S. Zhang, Saeed Fathololoumi and Z. Mi, "Full-color InGaN/GaN dot-in-a-wire light emitting diodes on silicon", Nanotechnology, 22 (2011) 445202.
H. P. T. Nguyen , S. Zhang, K. Cui, X. Han, S. Fathololoumi, M. Couillard, G. A. Botton, and Z. Mi, "p-type modulation doped InGaN/GaN dot-in-a-wire white-light-emitting diodes monolithically grown on Si(111)", Nano Letters, 11 (2011) 1919.
J. Titus,H. P. T. Nguyen, Z. Mi, and A. G. U. Perera, “Optical phonon modes in InGaN/GaN dot-in-a-wire heterostructures grown by molecular beam epitaxy”, Applied Physics Letters, 102 (2013) 121901.
H. P. T. Nguyen, K. Cui, S. Zhang, M. Djavid and Z. Mi, "Controlling electron overflow in phosphor-free InGaN/GaN nanowire white-light-emitting diodes", Nano Letters, 12 (2012) 1317.
H. P. T. Nguyen, M. Djavid, Kai Cui and Z. Mi, " Temperature-dependent nonradiative recombination processes in GaN-based nanowire white-light-emitting diodes on silicon", Nanotechnology, 23 (2012) 194012.
H. P. T. Nguyen, S. Zhang, K. Cui and Z. Mi, "High efficiency InGaN/GaN dot-in-a-wire red light emitting diodes", IEEE Photonics Technology Letters, 24 (2012) 321.
H. P. T. Nguyen, K. Cui , S. Zhang, Saeed Fathololoumi and Z. Mi, "Full-color InGaN/GaN dot-in-a-wire light emitting diodes on silicon", Nanotechnology, 22 (2011) 445202.
H. P. T. Nguyen, S. Zhang, K. Cui, X. Han, S. Fathololoumi, M. Couillard, G. A. Botton, and Z. Mi, "p-type modulation doped InGaN/GaN dot-in-a-wire white-light-emitting diodes monolithically grown on Si(111)", Nano Letters, 11 (2011) 1919.
H. P. T. Nguyen, S. Zhang, K. Cui, X. Han, and Z. Mi, "Molecular beam epitaxial growth, fabrication, and characterization of high efficiency InGaN/GaN dot-in-a-wire white light emitting diodes on Si(111)", ECS Transactions, 35 (2011) 41

II. High power phosphor-free InGaN/GaN nanowire white light-emitting diodes

Phosphor-free white light emitting diodes, that can be fabricated on low cost, large area substrates and can display high luminous flux, hold immense promise for the emerging solid state lighting. Such devices can be realized monolithically by stacking blue, green and red emitters in a single epitaxial step. They can exhibit much higher quantum efficiency, better color rendering capability, and significantly reduced manufacturing cost and improved reliability, compared to the commercial phosphor-based white LEDs. Although tremendous progress has been made for InGaN/GaN quantum well LEDs, the performance of such devices in the green, yellow, and red wavelength ranges has been plagued by the very low efficiency and “efficiency droop”, i.e. the decrease of the external quantum efficiency with increasing current. The underlying mechanism has been extensively investigated. Defects and carrier delocalization, polarization field, Auger recombination, carrier leakage, and poor hole transport have been identified as some of the most probable causes. However, a significant roadblock for the development of nanowire LEDs is the very low quantum efficiency. In this regard, we are continuously working on improving the nanowire LEDs efficiency and output power. We have reported several key techniques to enhance the device performance including p-type modulation doping in the device active region, the usage of high bandgap material as electron blocking layer to minimize electron overflow, and core-shell structures to reduce nonradiative surface recombination and increase current injection into device active region. We aim to further increase output power form our nanowire LED devices for practical applications.


Related publications:

Q. Wang, X. Liu, M. G. Kibria, H. P. T. Nguyen, S. Zhao, K. H. Li, Z. Mi, "P-type dopant incorporation and surface charge properties of catalyst-free GaN nanowires revealed micro-Raman scattering and X-ray photoelectron spectrocopy", Nanoscale 6 (2014) 9970
J. Titus, H. P. T. Nguyen, Z. Mi, and A. G. U. Perera, “Optical phonon modes in InGaN/GaN dot-in-a-wire heterostructures grown by molecular beam epitaxy”, Applied Physics Letters, 102 (2013) 121901.
H. P. T. Nguyen, K. Cui, S. Zhang, M. Djavid and Z. Mi, "Controlling electron overflow in phosphor-free InGaN/GaN nanowire white-light-emitting diodes", Nano Letters, 12 (2012) 1317.
H. P. T. Nguyen, M. Djavid, Kai Cui and Z. Mi, " Temperature-dependent nonradiative recombination processes in GaN-based nanowire white-light-emitting diodes on silicon", Nanotechnology, 23 (2012) 194012.
H. P. T. Nguyen, S. Zhang, K. Cui and Z. Mi, "High efficiency InGaN/GaN dot-in-a-wire red light emitting diodes", IEEE Photonics Technology Letters, 24 (2012) 321.
H. P. T. Nguyen, K. Cui , S. Zhang, Saeed Fathololoumi and Z. Mi, "Full-color InGaN/GaN dot-in-a-wire light emitting diodes on silicon", Nanotechnology, 22 (2011) 445202.
H. P. T. Nguyen, S. Zhang, K. Cui, X. Han, S. Fathololoumi, M. Couillard, G. A. Botton, and Z. Mi, "p-type modulation doped InGaN/GaN dot-in-a-wire white-light-emitting diodes monolithically grown on Si(111)", Nano Letters, 11 (2011) 1919.
H. P. T. Nguyen, S. Zhang, K. Cui, X. Han, and Z. Mi, "Molecular beam epitaxial growth, fabrication, and characterization of high efficiency InGaN/GaN dot-in-a-wire white light emitting diodes on Si(111)", ECS Transactions, 35 (2011) 41

III. Full-color InGaN/GaN LEDs on transparent substrates for flexible electronics

Beside the applications in SSL illumination where the requirement of high light output power is mostly the main goal, the use of LEDs in telecommunication devices, and decoration displays which can fabricated on a flexible substrate, called flexible electronics devices has also been intensively developeddue to the greatintegration of these LEDs in these electronics devices. In this project,we will develop highly reliable full-color InGaN/GaNnanowire LEDs on flexible substrates.High performance flexible inorganic LEDs will be achieved by transferring the as-grown devices from the Si (or amorphous glass) substrates onto plastic and/or metal substrates by different methods including laser lift-off, mechanically transferring, and selective etching processes, leading to new and unique opportunities for conformal indoor lighting, flexible displays, and integrated water purification systems.

Bendable, wearable phone developed by Samsung Electronics
Related publication:

H. P. T. Nguyen, Q. Wang, and Z. Mi, “Phosphor-free InGaN/GaN dot-in-a-wire white light emitting diodes on Cu substrates”, Journal of Electronic Materials 43 (2014) 868.
H. P. T. Nguyen, M. Djavid, Kai Cui and Z. Mi, " Temperature-dependent nonradiative recombination processes in GaN-based nanowire white-light-emitting diodes on silicon", Nanotechnology, 23 (2012) 194012.
H. P. T. Nguyen, S. Zhang, K. Cui and Z. Mi, "High efficiency InGaN/GaN dot-in-a-wire red light emitting diodes", IEEE Photonics Technology Letters, 24 (2012) 321.

IV. Heterogeneous integration of nanowire ultraviolet LEDs for water purification

LEDs with emission in the ultra-violet to visible wavelength range have been intensively studied for applications in SSL, flat-panel displays, and solar-blind detectors. My research in particular has demonstrated the potential of GaN-based nanowire LEDs for the next generation SSL and full-color displays where improved efficiencies and reduced degradation of the devices are advantages of the nanowire structures. Moreover, we have lead an effort that has realized ultra-violet (UV) emission from this materials system, providing a unique light source that can be used as a disinfectant in water purification systems where the current technology is based on bulky and inefficient mercury lamps. The nanowire form factor is ideal for integrating the light-emitter within the filter fabric for enhancing antibacterial and particulate filtration in water purification systems.

SteriPen with a UV lamp encased in a durable quartz sleeve for water purification.

Related publications:

Q. Wang, A. Connie, H. P. T. Nguyen, M. G. Kibria, S. Zhao, S. Sadaf, and Z. Mi, “High efficiency, spectrally pure 340 nm ultraviolet emission from AlxGa1-xN nanowire based light emitting diodes”, Nanotechnology 24 (2013) 345201.
Q. Wang, H. P. T. Nguyen, K. Cui, and Z. Mi, "High efficiency ultraviolet emission from AlxGa1-xN core-shell nanowire heterostructures grown on Si(111) by molecular beam epitaxy",Applied Physics Letters , 101 (2012) 043115.

V. High efficiency III-V nanowire solar cells

To date, solar cells with the highest efficiencies, ~ 32.0% and >40.7% have been realized in III-V stacked cells and concentrators, respectively. Such devices are generally grown on GaAs, InP or Ge substrates, which are prohibitively expensive and, as a result, their terrestrial applications have been very limited. Additionally, the performance of current multi-junction solar cell devices are generally limited by the poor absorption in the energy ranges of 1.0 – 1.2eV and 2.5 – 3.4eV. The energy band-gap or absorption spectrum of InxGa1-xN alloy can be continuously varied from 0.7to 3.4eV, representing the only semiconductor that can encompass nearly the entire solar spectrum, promising low cost, high efficiency full-solar-spectrum solar cells. By varying the indium and gallium compositions in the nanowire, the nanowire bandgap, or the absorption spectra, can be optimized to achieve solar cells with improved efficiency. Critical to the double-junction device is the formation of n+/p+ tunnel injection, which can be achieved by optimizing the growth conditions, including the substrate temperature, group III/V ratios, and the dopant incorporation.

First demonstration of InN nanowire solar cell (left+center) and proposed high efficiency single (left) and multi-junction (right) solar cells based on InGaN material system.

Related publications:

H. P. T. Nguyen, Y.-L Chang, I. Shih and Z. Mi, "InN p-i-n nanowire solar cells on Si", IEEE Journal of Selected Topics in Quantum Electronics: Nanowires, 17 (2011) 1062. > (IF = 4.078)

VI. High efficiency photoelctrochemicalwater splitting and hydrogen generation using III-nitride nanowire arrays

Solar water splitting has recently been intensively investigated and recognized as one of the key sustainable technologies to provide environmentally friendly solution to the worldwide energy crisissince it directly converts solar energy into hydrogen: a carbon-free, affordable and sustainable source of energy. Typically, solar-to-hydrogen conversion is based on metal-oxidematerials that are responsive to UV light which is just ~4% in solar spectrum. The need to find out alternative material that possesses all the requirements for water splitting such as band-gap, band alignment and corrosion resistant are really necessary. In this regard, compared to the conventional large bandgap metal oxides, the energy bandgap of III-nitrides can encompass nearly the entire solar spectrum, III-nitrides hold great promise for PEC water splitting.

Related publications:

M. G. Kibria, S. Zhao, F. A. Chowdhury, Q. Wang, H.P.T. Nguyen, M. L. Trudeau, H. Guo, and Z. Mi, "Ultrahigh efficiency spontaneous overall water splitting on p-type GaN nanowires", Nature Communications 5 (2014) 3825.
M. G. Kibria, H. P. T. Nguyen, K. Cui, S. Zhao, D. Liu, H. Guo, M. L. Trudeau, S. Paradis, H. Abou-Rachid, and Z. Mi, “One-step overall water splitting under visible light using multi-band InGaN/GaN nanowire heterostructures”, ACS Nano 7 (2013) 7886.
B. AlOtaibi, u>H. P. T. Nguyen, S. Zhao, M. G. Kibria, S. Fan, and Z. Mi, "Highly Stable Photoelectrochemical Water Splitting and Hydrogen Generation Using a Double-Band InGaN/GaN Core/Shell Nanowire Photoanode", Nano Letters 13 (2013) 4356.
B. AlOtaibi, M. Harati, S. Fan, S. Zhao, H. P. T. Nguyen, M. G. Kibria, and Z. Mi, “High efficiency photoelectrochemical water splitting and hydrogen generation using GaN nanowire photoelectrode”, Nanotechnology 24 (2013) 175401.