AlGaN-based ultra-violet light emitting diodes (UV LEDs) are promising for a range of applications, including water purification, air disinfection and medical sensing. However, widespread adoption of UV LEDs is limited by the poor device efficiency. This has been attributed to the strong internal light absorption and poor electrical injection efficiency for the conventional UV LED structures, which typically use an absorbing p-GaN layer for p-type contact. Recent development of ultra-wide banggap AlGaN tunnel junctions enabled a novel UV LED design with the absence of the absorbing p-GaN contact layer. In this presentation, we will discuss recent progress of the AlGaN tunnel junctions and the development of tunnel junction-based UV LEDs, and discuss the challenges and future perspectives for the realization of high power, high efficiency UV LEDs.
The unique material properties of Gallium Oxide make it promising for a range of future applications, but innovative materials and device engineering are needed to translate these ultimate material limits to real technology. This presentation will discuss our recent work on epitaxy, heterostructure design, and electrostatics to achieve high-performance β-Ga2O3 lateral and vertical electronic devices. We will discuss some advances in materials growth and device design for lateral structures which enabled key transistor demonstrations including the first β-(Al,Ga)2O3/β-Ga2O3 modulation-doped structures with excellent transport properties, double-heterostructure modulation-doped structures, scaled delta-doped transistors with cutoff frequency of 27 GHz, and self-aligned lateral field effect transistors with > 900 mA/mm current density. We will discuss the use of a new damage-free epitaxial etching technique using Ga atomic flux that enables highly precise fabrication of 3-dimensional structures. We will also show some applications of atomic Ga-flux etching to realize excellent field termination in vertical diodes, and lateral FINFETs with enhanced performance. Finally, we will discuss promising results using high-permittivity dielectrics integrated with semiconductors that have enabled lateral transistors with > 5.5 MV/cm breakdown field, the highest for a field effect transistor in any material system. We acknowledge funding from DOE/NNSA under Award Number(s) DE-NA000392, AFOSR GAME MURI (Award No. FA9550-18-1-0479, project manager Dr. Ali Sayir), and NSF ECCS-1809682.
GaN/(In,Ga)N heterostructure based visible Light Emitting Diodes (LED) have enabled a wide range of solid-state lighting applications through excellent efficiency and power output in the shorter wavelengths (≤ 475nm) of violet/blue emission. However, the efficiency of emitters in the longer wavelength range (≥ 500nm) drops drastically due to the need to include higher Indium-content in the InGaN quantum wells. Large average polarization fields for high Indium-content quantum wells for conventional P-up structure, opposes the depletion field leading to large electrostatic barriers for both electrons and holes injection. LEDs fabricated along the N-polar direction with a p-up orientation or Ga-polar direction with p-down orientation lower such electrostatic barriers to carrier injection due to alignment of the polarization dipole field and depletion region field. This can therefore theoretically improve the electrical injection efficiency and reduce the forward voltage of operation. Such a Ga-polar p-down LED requires a bottom buried tunnel junction to avoid current spreading issues for a buried p-GaN layer. In this report, we demonstrate for the first time Ga-polar p-down green emitting LEDs using bottom tunnel junctions and having external quantum efficiencies comparable to those of equivalent p-up LEDs grown by Metal Organic Chemical Vapor Deposition (MOCVD).
This presentation will give an overview of our recent work on materials growth and device engineering of 𝛽-Ga2O3 electronic devices. We will discuss the design and properties of advanced modulation-doped (Al,Ga)2O3/Ga2O3 structures with high sheet charge density and excellent transport properties. We will then outline novel strategies for realizing high breakdown fields and low resistance within devices, and outline design, growth, and characteristics of state-of-art Gallium Oxide devices, including scaled transistors with cutoff frequency of 27 GHz, and transistors with a power switching figure of merit of 586 MW/cm2 and breakdown voltage of 660V.
We report on the design, demonstration and current status of tunnel-injected ultra-violet light emitting diodes (UV LEDs). III-Nitride ultraviolet light emitting diodes (UV LEDs) are promising in various applications including sterilization, water purification and medical sensing. However, both the light extraction efficiency and electrical efficiency face fundamental challenges for the conventional UV LED structures. This stems from the poor p-type conductivity and high p-type contact resistance. Hole injection using interband tunneling provides an elegant solution to the fundamental issues of UV LEDs, and can resolve both the hole injection and light extraction issues that have been the primary problems for UV LEDs. In this talk, we will discuss in detail the heterostructure design and demonstration through polarization engineering to realize efficient interband tunneling in ultra-wide band gap AlGaN material. We will then outline some of the growth and fabrication challenges, and discuss our approaches to overcome these. Finally, we will present our results on tunnel-injected UV LEDs that have enabled us to achieve efficient UV light emission in the UVA and UVB wavelength ranges with on-wafer efficiencies comparable to state-of-the-art values [1,2,3].
References:
1. Yuewei Zhang, et al. ''Interband tunneling for hole injection in III-nitride ultraviolet emitters", Appl. Phys. Lett. 106, 141103 (2015);
2. Yuewei Zhang, et al. "Design of p-type cladding layers for tunnel-injected UV-A light emitting diodes", Appl. Phys. Lett. 109, 191105 (2016);
3. Yuewei Zhang, et al. “Tunnel-injected sub-260 nm ultraviolet light emitting diodes”, Appl. Phys. Lett. 110, 201102 (2017).
We discuss the design and demonstration ultra-low resistance III-nitride tunnel junctions, and how tunnel junctions could solve the long-standing problem of efficiency droop in solid state lighting. We have used nanoscale band engineering based on polarization and mid-gap states to reduce tunneling resistance by four orders of magnitude. We will discuss experimental demonstration of highly efficient tunnel junctions (resistivity ~ 0.1 mOhm-cm2) in PN junctions, p-contact free LEDs, and multiple junction structures. Finally we will show how tunneling based carrier regeneration in multiple active region cascade LEDs could help to enable low current, high voltage operation to overcome the efficiency droop problem.
Given the large thermal activation energy of acceptors in high %Al AlGaN, a new approach is needed to control p-type
conductivity in this material. One promising alternative to using impurity doping with thermal activation is using the
intrinsic characteristics of the III-nitrides to activate dopants with polarization-induced charge in graded heterostructures.
In this work polarization-induced activation of dopants is used in graded AlGaN nanowires grown by plasma-assisted
molecular beam epitaxy to form ultraviolet light-emitting diodes. Electrical and optical characterization is provided,
showing clear diode behavior and electroluminescent emission at 336nm. Variable temperature electrical measurements
show little change in device performance at cryogenic temperatures, proving that dopant ionization is polarizationinduced
rather than thermally activated.
This research seeks to develop a novel branch of materials systems called Distributed Intelligent Materials Systems
(DIMS) which incorporate actuation, sensing, electronics and intelligence as inherent parts of the material structure. A
microcantilever optical switch is fabricated as a concept demonstrator with Gallium nitride (GaN) as host material. GaN
has several material characteristics which enable it to outperform other semiconductor materials for electronic
applications. It also displays exceptional chemical inertness, has a relatively high piezoelectric coefficient, good
mechanical strength and toughness and is transparent to wavelengths in the visible spectrum. In this paper we develop
and fabricate a GaN-based, piezoelectrically actuated microcantilever optical switch/waveguide. While the GaN-material
offers the benefits mentioned above, the piezoelectric actuation and the cantilever design provide benefits of lighter
weight, compactness, speed of actuation, reduced structural complexity enabling easier fabrication and low wear and tear
due to minimal moving parts. The proposed design has a conventional unimorph configuration with GaN actuated in d31
mode. In this configuration, a laminar metal electrode and a doped n-type GaN layer are used to apply an electric field in
the top layer to actuate the unimorph. The unimorph is fabricated as a micro-cantilever by using surface micromachining
methods on epitaxial GaN grown on a GaN substrate. The cantilever is then etched partially using conventional
semiconductor processing techniques and using a recent microfabrication technique known as photoelectrochemical
(PEC) etch. PEC etching enables the fabrication of MOEMS structures that are rather difficult to create using conventional methods. Novel modifications and improvements to the current state-of-the art in PEC for GaN are presented and discussed.
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