Preface

Today, gallium nitride (GaN) and other related materials (ternary AlGaN and InGaN and quaternary InAlGaN) are widely used for optoelectronics components. In addition, some of these nitrides are also emerging as promising semiconductors for energy-efficient power electronic devices. Hence, the revolution expected in modern electronics and optoelectronics is often regarded, half-jokingly, as "GaNification."

Our main intention in preparing this book was to give, with the valuable contributions of leading specialists, a general overview on the state of the art of GaN-based technologies, covering both fields of power electronics and optoelectronic devices.

Chapter 1 is a general introduction to the properties and applications of GaN and related materials. After an historical background on the relevant milestones of nitrides research, special emphasis will be put on InGaN quantum wells and AlGaN/GaN heterostructures, which are important systems for light-emitting diodes (LEDs), laser diodes (LDs), and high electron mobility transistors (HEMTs). The main applications of nitride materials for both optoelectronic devices and power- and high-frequency electronics are also described, anticipating some of the most critical issues that are illustrated in detail in the rest of the book.

Typical nitride-based devices are made of multilayered heterostructures, which require epitaxial growth on appropriate substrates. Hence, Chapter 2 starts by discussing some recently developed methods to produce GaN wafers. Then, the most popular epitaxial method for GaN growth, i.e. metalorganic vapor-phase epitaxy (MOVPE), is described, discussing the role of growth temperature, the deposition on foreign substrates, and the methods for reducing the high threading dislocation density, as well as the doping difficulties for achieving p-type conductivity. A part of this chapter is dedicated to InGaN quantum wells, which find important applications in light-emitting devices but exhibit serious problems of composition uniformity and thermal stability with respect to decomposition.

Regarding high-frequency electronics, today, the interest for millimeter-wave (mmW) band (30-300 GHz) is continuously increasing because of its reduced wavelength and wide-frequency bands, which enable smaller component with improved performances. Wireless communication systems are extending to higher frequencies. However, several challenges need to be overcome in order to use the mmW spectrum successfully. In this context, Chapter 3 focuses on GaN-based devices for mmW applications. Targeted applications including high-power amplifiers, broadband amplifiers, and 5G wireless networks are introduced. Various GaN-based material designs for mmW applications are described, showing the advantages and limitations for high-frequency applications. Device design and fabrication of mmW GaN devices are analyzed. Finally, an overview of monolithic microwave-integrated circuit (MMIC) power amplifiers is also reported.

GaN is also considered a promising semiconductor for power electronics. Owing to the presence of the two-dimensional electron gas (2DEG), AlGaN/GaN HEMTs are inherently normally-on devices. However, in many power electronic applications, normally-off transistors are desired. Hence, Chapter 4 reviews the current technologies for normally-off GaN-based HEMTs. First, the HEMT "cascode" approach is briefly described, highlighting advantages and limitation of this design. Then, after illustrating the recessed gate HEMT and the fluorinate gate approach, the focus is put on the recessed gate hybrid metal insulator semiconductor high electron mobility transistor (MISHEMT) and on the p-GaN gate HEMT. These are today the most promising and robust approaches for normally-off GaN HEMTs. The most critical issues of these technologies (e.g. heterostructure design, gate dielectrics, and metal gates) are discussed in this chapter.

In power electronics, the vertical device topology would be preferred to the lateral one, in order to reduce the on-resistance and increase the current capability. In this context, Chapter 5 is designed to give an overview on the progress made in vertical devices based on bulk GaN substrates. In GaN technology, vertical devices are now in the forefront showing impact through both two- and three-terminal devices developed over the past decade. In particular, two different types of vertical devices are discussed, namely the current aperture vertical electron transistor (CAVET) and a regrowth-based trench MOSFET called oxide gate interlayer FET (OGFET). Moreover, the latest findings on the coefficients of impact ionization are also reviewed.

The development of GaN-based power amplifiers for RF, microwave, and mm-wave applications would have not been possible without an intense research on the stability and reliability issues. Chapter 6 reviews the most important reliability issues of GaN HEMTs for RF and microwave applications as well as power switching transistors. Failure modes and mechanisms of RF AlGaN/GaN and InAlN/GaN HEMTs are reviewed, focusing on gate-edge, hot electrons, and hot phonons related failure modes and thermal effects. For power switching devices, the effect of GaN buffer carbon doping on dynamic on-state resistance and time-dependent dielectric breakdown of the buffer are described. The gate degradation of normally-off p-GaN gate HEMTs and threshold voltage instabilities in GaN MISHEMTs are also discussed.

GaN-based semiconductors are great materials for optoelectronic devices because of their broad emission wavelength range from ultraviolet to the yellow-green. GaN-based LEDs are discussed in Chapter 7. LEDs have made tremendous progress in the past 15 years and have reached a point where they are reinventing and redefining artificial lighting. Nevertheless, in their own realm, they suffer from decrease in efficiency at higher currents (droop). Moreover, the full potential in terms of light quality, i.e. color rendering index (CRI) and correlated color temperature (CCT) that can be offered by these devices, can still be improved with existing or alternative schemes and device configurations and are discussed in this chapter. AlGaN deep ultraviolet light-emitting diodes (DUV LEDs) have a wide variety of potential applications, including sterilization, water purification, etc. However, the efficiency of AlGaN DUV LEDs is still low in comparison to InGaN blue LEDs. In this chapter, the methods to improve internal quantum efficiency (IQE), electron injection efficiency (EIE), and light extraction efficiency (LEE) for AlGaN DUV LEDs are presented.

Chapter 8 presents the recent advancement in the III-nitride LDs grown by plasma-assisted molecular beam epitaxy (PAMBE). First, the growth fundamentals in PAMBE are discussed. Then, the nature of carrier recombination in wide InGaN quantum wells is studied. A model of a highly efficient transition path through excited states is proposed to explain the experimental observations. Additionally, the reliability of the LDs grown by PAMBE is presented. Finally, the tunnel junction (TJ) and its applications to devices are discussed.

In Chapter 9, after recalling the history of nitride semiconductors LDs, the main technical and scientific challenges related to their development and the perspective for the future are reviewed. In particular, the recently emerged distributed feedback (DFB) nitride LDs and its possible applications are described. The less known, but closely related to LDs, superluminescent diodes and optical amplifiers are also described.

In Chapter 10, the vertical-cavity surface-emitting lasers (VCSELs) are discussed. They have many advantages, including small footprint, circular low-divergent output beams, wafer-level testing, densely packed two-dimensional arrays, good price-performance ratio, and simple optics/alignment for output coupling. Green and blue emitting VCSELs are especially important for applications such as visible light communication, full color display, and microlaser projector. In particular, the current status and the different device design approaches are discussed, presenting different VCSEL concepts, based on InGaN quantum dots (QDs) and quantum wells (QWs).

Finally, Chapter 11 illustrates the recent developments in the integration of 2D materials (graphene and MoS2) with nitride semiconductors for electronics and optoelectronics. The state-of-the-art approaches for the fabrication of heterojunctions between these two classes of materials are discussed, considering advantages and limitations. Examples of electronic devices based on 2D material/nitride heterostructures are presented, such as the hot electron transistor (HET) with a Gr base and Al(Ga)N/GaN emitter for THz electronics and the band-to-band tunneling diode based...