There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

GaN and related III-nitrides have attracted considerable attention as promising materials for application in optoelectronic devices, in particular, light-emitting diodes (LEDs). At present, sapphire is still the most popular commercial substrate for epitaxial growth of GaN-based LEDs. However, due to its relatively large lattice mismatch with GaN and low thermal conductivity, sapphire is not the most ideal substrate for GaN-based LEDs. Therefore, in order to obtain high-performance and high-power LEDs with relatively low cost, unconventional substrates, which are of low lattice mismatch with GaN, high thermal conductivity and low cost, have been tried as substitutes for sapphire. As a matter of fact, it is not easy to obtain high-quality III-nitride films on those substrates for various reasons. However, by developing a variety of techniques, distincts progress has been made during the past decade, with high-performance LEDs being successfully achieved on these unconventional substrates. This review focuses on state-of-the-art high-performance GaN-based LED materials and devices on unconventional substrates. The issues involved in the growth of GaN-based LED structures on each type of unconventional substrate are outlined, and the fundamental physics behind these issues is detailed. The corresponding solutions for III-nitride growth, defect control, and chip processing for each type of unconventional substrate are discussed in depth, together with a brief introduction to some newly developed techniques in order to realize LED structures on unconventional substrates. This is very useful for understanding the progress in this field of physics. In this review, we also speculate on the prospects for LEDs on unconventional substrates.

https://iopscience.iop.org/article/10.1088/0034-4885/79/5/056501/pdf

GaN-based light-emitting diodes on various substrates: a critical review.

GaN-based light-emitting devices have the potential to realize all visible emissions with the same material system. These emitters are expected to be next-generation RGB displays and illumination tools. These emitting devices have been realized with highly efficient blue and green light-emitting diodes (LEDs) and laser diodes (LDs). Extending them to longer wavelength emissions remains challenging from an efficiency perspective. In the emerging research field of micro-LED displays, III-nitride red LEDs are in high demand to establish highly efficient devices like conventional blue and green systems. In this review, we describe fundamental issues in the development of red LEDs by III-nitrides. We also focus on the key role of growth techniques such as higher temperature growth, strain engineering, nanostructures, and Eu doping. The recent progress and prospect of developing III-nitride-based red light-emitting devices will be presented.

Recent progress in red light-emitting diodes by III-nitride materials

The performance of AlGaN-based mid and deep ultraviolet light emitting diodes (LEDs) is severely limited by electron overflow and by the poor hole injection into the device active region. We have studied the effect of various electron blocking layers on the performance of AlGaN LEDs operating at ~280 nm. It is observed that, compared to conventional p-type electron blocking layer, the incorporation of an n-type AlN/AlGaN superlattice electron blocking layer before the active region can significantly improve the device performance by reducing electron overflow without compromising hole injection. Direct on-wafer measurement showed an external quantum efficiency ~4.4% and wall-plug efficiency ~2.8% by optimizing the design of n-type AlN/AlGaN superlattice electron blocking layer, which is nearly a factor of five to ten times better than identical devices but with the incorporation of a conventional p-type electron blocking layer.

Effect of electron blocking layer on the efficiency of AlGaN mid-ultraviolet light emitting diodes.

In this work, the origin of electron blocking effect of n-type Al0.25Ga0.75N electron blocking layer (EBL) for c+ InGaN/GaN light-emitting diodes has been investigated through dual-wavelength emission method. It is found that the strong polarization induced electric field within the n-EBL reduces the thermal velocity and correspondingly the mean free path of the hot electrons. As a result, the electron capture efficiency of the multiple quantum wells is enhanced, which significantly reduces the electron overflow from the active region and increases the radiative recombination rate with holes.

/pdf/on-the-origin-of-the-electron-blocking-effect-by-an-n-type-a1z7cnff5r.pdf

On the origin of the electron blocking effect by an n-type AlGaN electron blocking layer

A perpendicular InGaN/GaN multiple-quantum- wells structure on ZnO substrate for blue light emitting diode (LED) was successfully fabricated by use of Metal-organic Chemical Vapor Deposition (MOCVD). During the growing process of GaN-based materials on ZnO substrates, the low-temperature-grown GaN buffer layer, inserted between ZnO substrate and undoped GaN layer, prevented the Zn and O from diffusing from ZnO substrate into the n-GaN layer. This thin GaN buffer layer, mainly as a insulating layer, was grown at relatively low temperature of 530 $^{\circ}{\hbox{C}}$ . By using our method, an integrated LED with ZnO substrate can be fabricated with a crack-free GaN film on (0001) ZnO substrate by MOCVD using this method. The epilayer crystalline structure has been measured by atomic force microscopy (AFM), and the optical properties of the LED were also characterized by photoluminescence and Current-Voltage characteristic curve.

A GaN-Based LED With Perpendicular Structure Fabricated on a ZnO Substrate by MOCVD

A design approach is proposed to improve the performances of blue InGaN light-emitting diodes (LEDs) at high current by using of the polarization-matched n-type AlGaInN electron-blocking layer (EBL) instead of conventional EBLs, owing to the more uniform carrier distribution across the multiply quantum well (MQW) active regions. And the response parameters of the blue InGaN LEDs, such as the radiative recombination rate, the internal quantum efficiency and output power, are calculated by the advanced physical model of semiconductor devices (APSYS) software. The simulation results reveal that the total power and the radiative recombination rates of the blue InGaN LEDs with the n-type AlGaInN EBL have been greatly promoted, and their efficiency droop has also been evidently moderated, compared with that of the LEDs with three types of EBL which are p-type AlGaN EBL, p-type AlGaInN EBL and n-type AlGaN EBL.

Effect of Polarization-Matched n-Type AlGaInN Electron-Blocking Layer on the Optoelectronic Properties of Blue InGaN Light-Emitting Diodes

We use a simple and controllable method to fabricate GaN-based light-emitting diodes (LEDs) with 22° undercut sidewalls by the successful implementation of the inductively coupled plasma reactive ion etching (ICP-RIE). Our experiment results show that the output powers of the LEDs with 22° undercut sidewalls are 34.8 mW under a 20-mA current injection, 6.75% higher than 32.6 mW, the output powers of the conventional LEDs under the same current injection.

https://iopscience.iop.org/article/10.1088/1674-1056/22/10/106802/pdf

Fabrication of GaN-based LEDs with 22° undercut sidewalls by inductively coupled plasma reactive ion etching

Two kinds of short-period type II superlattices (SLs) InAs (6 monolayers (MLs))/GaSb (3 MLs) and InAs (8 MLs)/GaSb (8 MLs) which can serve for mid-infrared (MIR) detection have been grown by molecular beam epitaxy (MBE) on p-type GaSb (100) substrates. The cutoff wavelength for the two superlattices (SLs) was found to be around 4.8 µm at 300 K. The high resolution X-ray diffraction (HRXRD) measurements indicated that the InAs (8 MLs)/GaSb (8 MLs) SLs have better crystalline quality than that of the InAs (6 MLs)/GaSb (3 MLs) SLs. However, compared with infrared absorption in the 2.5–4.3 µm range, the optical absorption of InAs (6 MLs)/GaSb (3 MLs) SLs was more excellent. This can be attributed to increase probability of the electron and hole wave function overlap in the thinner period thickness.

MBE growth of high absorption mid-IR type-II InAs/GaSb superlattices

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Papers

A GaN-Based LED With Perpendicular Structure Fabricated on a ZnO Substrate by MOCVD

Effect of Polarization-Matched n-Type AlGaInN Electron-Blocking Layer on the Optoelectronic Properties of Blue InGaN Light-Emitting Diodes

Fabrication of GaN-based LEDs with 22° undercut sidewalls by inductively coupled plasma reactive ion etching

MBE growth of high absorption mid-IR type-II InAs/GaSb superlattices