Front Matter

The authors of Chapters 1–6 and 8 acknowledge the assistance of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) through “Research and Development of Next Generation Semiconductors to Realize an Energy-saving Society” program (Grant No. JPJ005357).

The authors of Chapters 1 and 3 acknowledge funding from the Council for Science, Technology, and Innovation (CSTI), under the Cross-ministerial Strategic Innovation Promotion program (SIP) “Next Generation Power Electronics – Research and Development of Fundamental Technologies for GaN Vertical Power Devices” (funding agency: NEDO).

The authors of Chapter 3 (Dr. Narita and Professor Tokuda) thank Dr. Horita of Nagoya University for discussions concerning the deep levels created by electron irradiation.

The author of Chapter 4 (Professor Ikarashi) would like to thank Professor T. Arakawa of Shimane University and Professor N. Tanaka of Nagoya University for valuable discussions regarding TEM analyses. The work described in Chapter 4 was partly supported by the Nagoya University Microstructural Characterization Platform as a part of the MEXT Nanotechnology Platform. A part of the research presented in Chapter 4 was also supported by the Polish National Science Centre (Project No. 2018/29/B/ST5/00338).

The data in Chapter 6 (written by Dr. Sakata and Dr. Kim) were acquired at the SPring-8 facility (Proposal Nos. 2016B1027, 2016B1028, 2016B1029, 2016B1030, 2016B1031, 2017A1030, 2017A1033, 2017A4504, 2017B1029, 2017B1030, 2017B4505, 2018B1010, 2018B4505, 2019A1001, 2019A1012, 2019A1351, 2019A4504, 2019B1007, 2019B4501, and 2020A1002).

The author of Chapter 7 (Dr. Tanikawa) would like to thank Dr. T. Yoshida of SCIOCS, Ltd. for providing GaN wafers. The work described in Chapter 7 was partly supported by JSPS KAKENHI (Grant Nos. JP20H02640, JP19H04532, and JP19K22043).

The success of GaN-based optical devices is reflected in the award of the Nobel Prize for Physics in 2014 to Professor Isamu Akasaki, Professor Hiroshi Amano, and Professor Shuji Nakamura. Surprisingly, InGaN-based light emitting diodes have been obtained using mosaic crystals with high dislocation densities (typically around 10⁸ cm⁻²) as a result of the strong quantum confinement in these materials as well as carrier localization. Consequently, the physical properties of GaN and the effects of defects in this material have been widely studied. Early investigations established the exceptional physical properties of GaN, but the effects of residual strain, impurities, and threading dislocations have sometimes made it difficult to fully evaluate the intrinsic properties of this compound.

GaN also shows promise as component of power devices because of its high breakdown electrical field. In particular, the fabrication of high-quality freestanding GaN substrates that provide laser diodes with increased lifetimes has indicated the viability of vertical GaN power devices. For these reasons, those studying power devices (including the authors) require details regarding the intrinsic material properties and defects in GaN, because these can greatly affect device performance. The purpose of this book is, therefore, to examine the basic nature and effects of defects in GaN with regard to power device applications. Since such defects can involve a wide range of characteristics and size scales, this discussion must be approached from many directions.

In Chapter 1, Dr. Narita and Professor Kachi summarize the background work related to the study of defects in GaN as well as the most reliable data regarding the physical properties of GaN. These values are important as inputs for simulations and so are provided as a basis for the subsequent discussion herein.

A deep level is a charged state formed by a defect and is directly related to the electrical properties of a device. In the case of GaN, which has a large bandgap, it is important to pay special attention to methods for assessment of deep levels. Deep-level transient spectroscopy (DLTS), which is based on thermal emission processes, allows deep levels to be detected within a range of approximately 1 eV below the conduction band minimum or above the valence band maximum. It should also be noted that, when assessing minority carrier traps, the injection of minority carriers via bias pulses or light pulses partly fills these traps, and this phenomenon must be taken into account when performing a quantitative characterization of the traps. When performing a DLTS analysis of a p-type GaN layer, the relatively large ionization energy for magnesium (Mg) acceptors in the material makes it difficult to detect deep levels near the valence band maximum, and this complication sometimes leads to incorrect interpretation of data. In Chapter 2, Dr. Narita and Professor Tokuda summarize the analytical methods used to assess deep levels in GaN and examine the applicable energy range and precision while paying attention to the measurement condition for each technique.

In Chapter 3, Dr. Narita and Professor Tokuda present an overview of the deep levels that have been reported and discuss the origins of these levels. It is important to distinguish between deep levels associated with isolated point defects and those related to threading dislocations, and recently reported data regarding homoepitaxial layers with low threading dislocation densities have demonstrated the ability to separate these defects. In discussing the identification of deep levels, Dr. Narita and Professor Tokuda carefully compare the results obtained using DLTS, the parameters extracted from luminescence bands (generated by deep levels), and the results of first-principles calculations using a hybrid functional approach.

In the case that a defect is formed by several atoms, it can be observed as an extended defect. These structural defects are sometimes seen in GaN layers doped with Mg or implanted with Mg ions. Chapter 4 describes an examination of the atomic structures of extended defects in p-type GaN layers using transmission electron microscopy (TEM) and the correspondence of these structures to the energetically stable configurations predicted by first-principles calculations. Professor Ikarashi also discusses the behavior of extended defects in GaN implanted with Mg atoms and then annealed, which is a key process in the fabrication of power devices. These defect structures have been shown to be transformed by post-implantation annealing due to the diffusion of interstitials and vacancies.

Both nano-scale defects and threading dislocations are sometimes formed along with the segregation of impurities such as Mg atoms. In addition, III-nitride alloys can exhibit compositional fluctuations that affect device performance. In such cases, atom probe tomography (APT) combined with scanning-TEM (STEM) can be used to characterize the three-dimensional structure. In Chapter 5, Dr. Kumar and Dr. Ohkubo examine the Mg-segregated defects that can result from ion implantation and subsequent annealing. Investigations of compositional variations in III-nitride alloys are also reviewed in this chapter.

Threading dislocations present in a freestanding GaN substrate, even at a low density, can significantly impact the performance of a high-power device. A vertical power device typically requires a large active area of greater than 1 mm² due to the high current capability, and therefore several mosaic structures are involved in such a device. In addition, these units are fabricated on wafers having large diameters, and so the dislocations within this large area must be characterized. However, GaN epitaxial layers with threading dislocation densities of less than 10⁶ cm² cannot be suitably characterized using conventional x-ray rocking curves acquired on the macroscale or by local assessment of dislocations using atomic force microscopy. In Chapter 6, Dr. Sakata and Dr. Kim explain the basis of synchrotron x-ray diffraction topography (XRDT), which is an emerging technology for evaluating the orientational distribution of axes in a GaN layer on the wafer scale. Although this technology does not directly observe defects, the lattice distortion resulting from dislocations can be clearly visualized.

Multiphoton microscopy is discussed in Chapter 7 as a means of characterizing the propagation of threading dislocations. This technology allows two-dimensional imaging at specific depths by focusing a laser beam with a lens, resulting in the three-dimensional imaging of dislocations. Dr. Tanikawa provides an overview of the imaging of defects in both silicon carbide (SiC) and GaN. In the case of SiC, threading dislocations, stacking faults, and basal plane dislocations in an epitaxial layer can be clearly distinguished by imaging at different wavelengths. The bending of threading dislocations in GaN via the facet-initiated epitaxial lateral overgrowth technology can also be visualized. The direct and non-destructive observation of dislocations is of increasing importance when assessing the electrical properties of dislocations in GaN.

In Chapter 8, Dr. Narita and Professor Kachi present an overview of the relationship between the performance of a power device and the presence of certain defects. Such defects will not only be present in the epitaxial material but can also be generated during the processes used to fabricate a power device. Therefore, it is important to correlate the electrical properties of such devices with defects introduced during each fabrication step. The key processes in this regard include the fabrication of a freestanding substrate, the introduction of an epitaxial growth layer, the formation of a metal-oxide-semiconductor (MOS) structure, ion implantation, and dry etching.

Although the crystal quality of GaN specimens, the processes used to fabricate devices, and the analytical methods employed to assess these materials are all continually evolving, the present text summarizes the state of the art of defect investigation in GaN as of 2020.

Tetsuo Narita

Tetsu Kachi

Editors

Nobuyuki Ikarashi

Institute of Materials and Systems for Sustainability (IMaSS), Nagoya University, Nagoya, Japan

Tetsu Kachi

Toyota Advanced Power Electronics Funded Research Division, Institute of Materials and Systems for Sustainability (IMaSS), Nagoya University, Nagoya, Japan

Jaemyung Kim

Center for GaN Characterization and Analysis, National Institute for Materials Science (NIMS), Tsukuba, Japan

Ashutosh Kumar

Center for GaN Characterization and Analysis, National Institute for Materials Science, Tsukuba, Japan, and Research Center for Magnetic and Spintronic Materials, National Institute for Materials Science, Tsukuba, Japan

Tetsuo Narita

System Electronics Division II, Power Device Lab., Toyota Central R&D Labs., Inc., Aichi, Japan

Tadakatsu Ohkubo

Center for GaN Characterization and Analysis, National Institute for Materials Science, Tsukuba, Japan, and Research Center for Magnetic and Spintronic Materials, National Institute for Materials Science, Tsukuba, Japan

Osami Sakata

Center for Synchrotron Radiation Research, Japan Synchrotron Radiation Research Institute (JASRI), Sayo, Hyogo, Japan, and Center for GaN Characterization and Analysis, National Institute for Materials Science (NIMS), Tsukuba, Ibaraki, Japan

Tomoyuki Tanikawa

Graduate School of Engineering, Osaka University, Osaka, Japan

Yutaka Tokuda

Department of Electrical and Electronics Engineering, Aichi Institute of Technology (AIT) Faculty of Engineering, Toyota, Aichi, Japan