AlGaN-based materials own direct transition energy bands and wide bandgap and thus can be used in high-efficiency ultraviolet (UV) emitters and detectors. Over the past two decades, AlGaN-based materials and devices experienced rapid development. Deep ultraviolet AlGaN-based light-emitting diodes (LEDs) with improved efficiency of 20.3% (at 275 nm) have been produced. An electron beam (EB)-pumped AlGaN-based UV light source at 238 nm, output power of 100 mW, and power conversion efficiency (PCE) of 40% has also been fabricated. UV stimulated emission from AlGaN multiple-quantum-wells laser diodes (LDs) using electrical pumping at room temperature has also been achieved at a wavelength of 336 nm. Compared with GaN-based blue and green LEDs and LDs, the efficiency of AlGaN-based UV LEDs and LDs is lower. Further optimization and improvements in both structure and fabrication are required to realize high-performance devices. In AlGaN-based UV photodetectors (PDs), gain as high as 104 orders of magnitude has been reported using the separated absorption and multiplication region avalanche photodiode structure but is still far from detecting the weak signal, and thus UV single-photon detectors with high detectivity is challenging. Recently, there has been extensive work in the nonlinear optical properties of AlGaN and AlGaN-based passive devices, such as waveguides and resonators. However, how to minimize the scattering and defect-related absorption needs to be further studied. In this review, first, approaches used to grow an AlGaN epilayer and p-type doping are introduced. Second, progress in AlGaN-based UV LEDs, EB-pumped light sources, LDs, PDs, passive devices, and the nonlinear optical properties are presented. Finally, an overview of potential future trends in AlGaN-based materials and UV devices is given.

AlGaN photonics: recent advances in materials and ultraviolet devices

Free-standing single crystalline semiconductor membranes have gained intensive attention over the last few years due to their versatile usage in many applications. This material platform possesses a high level of material quality similar to their bulk counterparts because single crystallinity is maintained. Si, Ge, and III–V based membranes have been widely studied for flexible electronic and optoelectronic devices such as thin-film transistors and photodetectors. However, the current status of research and development on free-standing single crystalline wide band-gap membranes is at a relatively early stage compared to IV and III–V based membranes. This review highlights recent advances in free-standing wide band-gap membranes, including GaN, SiC, ZnO, β-Ga2O3, and diamond and their applications. Fabrication techniques of each membrane are presented with material characterization. Some prospects for new research opportunities and challenges are also discussed.

Recent advances in free-standing single crystalline wide band-gap semiconductors and their applications: GaN, SiC, ZnO, β-Ga2O3, and diamond

Progress in metal-organic chemical vapor deposition of high quality N-polar (Al, Ga, In)N films on sapphire, silicon carbide and silicon substrates is reviewed with focus on key process components such as utilization of vicinal substrates, conditions ensuring a high surface mobility of species participating in the growth process, and low impurity incorporation. The high quality of the fabricated films enabled the demonstration of N-polar (Al, Ga, In)N based devices with excellent performance for transistor applications. Challenges related to the growth of high quality N-polar InGaN films are also presented.

Recent progress in metal-organic chemical vapor deposition of $\left( 000\bar{1} \right)$ N-polar group-III nitrides

The demand for improved electronic and optoelectronic devices has fuelled the development of epitaxial growth techniques for single-crystalline semiconductors. However, lattice and thermal expansion coefficient mismatch problems limit the options for growth and integration of high-efficiency electronic and photonic devices on dissimilar materials. Accordingly, advanced epitaxial growth and layer lift-off techniques have been developed to address issues relating to lattice mismatch. Here, we review epitaxial growth and layer-transfer techniques for monolithic integration of dissimilar single-crystalline materials for application in advanced electronic and photonic devices. We also examine emerging epitaxial growth techniques that involve two-dimensional materials as an epitaxial release layer and explore future integrated computing systems that could harness both advanced epitaxial growth and lift-off approaches. This Review Article examines the development of epitaxial growth and layer transfer techniques for monolithic integration of dissimilar single-crystalline materials for application in advanced electronic and photonic devices.

Epitaxial growth and layer-transfer techniques for heterogeneous integration of materials for electronic and photonic devices

A porous medium is a special type of material where voids are created in a solid medium. The introduction of pores into a bulk solid can profoundly affect its physical properties and enable interesting mechanisms. In this paper, we report the use of mesoporous GaN to address a long-standing challenge in GaN devices: tuning the optical index in epitaxial structures without compromising the structural and electrical properties. By controlling the doping and electrochemical etching bias, we are able to control the pore morphology from macro- to meso- and microporous. The meso- and microporous GaN can be considered a new form of GaN with unprecedented optical index tunability. We examine the scattering loss in a porous medium quantitatively using numerical, semiempirical, and experimental methods. It is established that the optical loss due to scattering is well within the acceptable range. While being perfectly lattice-matched to GaN, the porous GaN layers are found to be electrically highly conductive. As a...

Mesoporous GaN for Photonic Engineering-Highly Reflective GaN Mirrors as an Example

Miniaturization of optoelectronic devices offers tremendous performance gain. As the volume of photoactive material decreases, optoelectronic performance improves, including the operation speed, the signal-to-noise ratio, and the internal quantum efficiency. Over the past decades, researchers have managed to reduce the volume of photoactive materials in solar cells and photodetectors by orders of magnitude. However, two issues arise when one continues to thin down the photoactive layers to the nanometer scale (for example, <50 nm). First, light-matter interaction becomes weak, resulting in incomplete photon absorption and low quantum efficiency. Second, it is difficult to obtain ultrathin materials with single-crystalline quality. We introduce a method to overcome these two challenges simultaneously. It uses conventional bulk semiconductor wafers, such as Si, Ge, and GaAs, to realize single-crystalline films on foreign substrates that are designed for enhanced light-matter interaction. We use a high-yield and high-throughput method to demonstrate nanometer-thin photodetectors with significantly enhanced light absorption based on nanocavity interference mechanism. These single-crystalline nanomembrane photodetectors also exhibit unique optoelectronic properties, such as the strong field effect and spectral selectivity.

https://advances.sciencemag.org/content/advances/3/7/e1602783.full.pdf

Single-crystalline germanium nanomembrane photodetectors on foreign nanocavities.

Single crystalline nanomembranes (NMs) represent a new embodiment of semiconductors having a two-dimensional flexural character with comparable crystalline perfection and optoelectronic efficacy. In this Letter, we demonstrate the preparation of GaN NMs with a freestanding thickness between 90 to 300 nm. Large-area (>5 × 5 mm2) GaN NMs can be routinely obtained using a procedure of conductivity-selective electrochemical etching. GaN NM is atomically flat and possesses an optical quality similar to that from bulk GaN. A light-emitting optical heterostructure NM consisting of p-GaN/InGaN quantum wells/GaN is prepared by epitaxy, undercutting etching, and layer transfer. Bright blue light emission from this heterostructure validates the concept of NM-based optoelectronics and points to potentials in flexible applications and heterogeneous integration.

Wide bandgap III-nitride nanomembranes for optoelectronic applications

By using a novel conductivity-based selective electrochemical etching, we have introduced nanometer-sized pores into GaN to significantly decrease its refractive index while maintaining its crystallinity. Such advantages of nanoporous (NP) GaN can thus overcome the epitaxial (strain) and optical (index) limitations of AlGaN, which has been used as the cladding layers in InGaN laser diodes for two decades. Compared to Al0.1Ga0.9GaN cladding, which has a refractive index contrast (Δn) of 0.04 to GaN, the Δn of NP-GaN to GaN can be engineered to be over 0.4 without inducing any tensile strain. The high Δn not only increases the optical confinement factor (Γ) from lower than 3% in the state-of-the-art (SOTA) InGaN laser diodes to 9% but also offers a broad tunability of the Γ and the modal gain as well. We have observed a clear correlation between the Γ and the modal gain by using the variable stripe length method and have demonstrated an over 100% increase of the modal gain by engineering the Γ of the wavegu...

Optical Engineering of Modal Gain in a III-Nitride Laser with Nanoporous GaN

Epitaxial lateral overgrowth (ELO) of nitrogen-polar (0001) GaN (N-polar GaN) by metalorganic vapor deposition has been studied to achieve a high microstructural quality of N-polar GaN. The influence of growth conditions on lateral growth is investigated, and a correlation of growth conditions with the observed inversion of polarity is established. Most of the observed trends for N-polar ELO are contrary to those reported for Ga-polar experiments. Such differences are explained by considering the property of surface reactivity of N-polar GaN with hydrogen species. On the basis of the trends of the occurrence (or absence) of polarity inversion, an atomistic model is proposed to explain the origin of polarity inversion. This model also allows us to control the process and to completely eliminate polarity inversion, resulting in fully coalesced, purely N-polar GaN with improved crystalline quality.

Epitaxial Lateral Overgrowth of Nitrogen-Polar (0001̅) GaN by Metalorganic Chemical Vapor Deposition

Here, we demonstrate a process to produce planar semipolar (20 2¯1) GaN templates on sapphire substrates. We obtain (20 2¯1) oriented GaN by inclined c-plane sidewall growth from etched sapphire, resulting in single crystal material with on-axis x-ray diffraction linewidth below 200 arc sec. The surface, composed of (10 1¯1) and (10 1¯0) facets, is planarized by the chemical-mechanical polishing of full 2 in. wafers, with a final surface root mean square roughness of <0.5 nm. We then analyze facet formation and roughening mechanisms on the (20 2¯1) surface and establish a growth condition in N2 carrier gas to maintain a planar surface for further device layer growth. Finally, the capability of these semipolar (20 2¯1) GaN templates to produce high quality device structures is verified by the growth and characterization of InGaN/GaN multiple quantum well structures. It is expected that the methods shown here can enable the benefits of using semipolar orientations in a scalable and practical process and can...

Kanglin Xiong

Papers

Single-crystalline germanium nanomembrane photodetectors on foreign nanocavities.

Wide bandgap III-nitride nanomembranes for optoelectronic applications

Optical Engineering of Modal Gain in a III-Nitride Laser with Nanoporous GaN

Epitaxial Lateral Overgrowth of Nitrogen-Polar (0001̅) GaN by Metalorganic Chemical Vapor Deposition

Semipolar (20 2¯1) GaN and InGaN quantum wells on sapphire substrates