The topic of this book is very timely. Direct-bandgap semiconductors are used for conventional light-emitting diodes (LEDs) and lasers. Although there have been tremendous improvements in terms of efficiency, technical challenges remain in fabrication and in handling toxic and rare compounds, such as arsenic and indium. Silicon, being an indirect-bandgap semiconductor, exhibits poor efficiency because electrons must transition from the conduction band to the valence band to emit light spontaneously by electron–hole recombination. However, the momentum of the electron in the conduction band is different from that in the valence band, requiring a phonon in the process to satisfy the net momentum conservation. Modifications of silicon bulk density using porous Si, a superlattice structure of Si and SiO2, or Er-doped Si have been employed to overcome this efficiency issue with little or no commercial success.
This brings us to the topic of this book: dressed photons (DPs) and dressed photon phonons (DPPs) and their utility to allow silicon bulk crystal into an efficient light-emitting material or device. A DP, unlike a conventional photon, provides a physical picture that illustrates the small size (quantum confinement), meaning that the quasiparticle is created in a nanomaterial or quantum dot material and has short duration, making the quasiparticle a virtual photon. Semiconductor materials have phonon-excited states within the bandgap. Since propagating far-field light cannot excite electrons from the valence band to these phonon-excited states, these transitions are electric-dipole-forbidden; hence, a DPP can be generated in nanometer-sized (quantum confinement) semiconductor materials that can excite multiple modes of coherent phonons around nanometer-sized structures. In an ideal DPP-assisted process, two-step excitation from the valence band to the conduction band is realized via an intermediate phonon state, and the energy required to create electron–hole pairs is therefore smaller than the bandgap energy.
Chapter 1 lays out the strategy and a case for the book, stating the problems with conventional light-emitting devices and their respective solutions. It also defines the DP, DPP, and photon breeding concepts. Conventional optical technology has used propagating light merely as a tool instead of exploring new types of light. In contrast, DP technology was born as a result of exploring a new type of light (i.e., the DP). Since conventional classical and quantum theories of light cannot be directly applied to describe the DP, novel concepts and theoretical bases are required.
Chapter 2 discusses fabrication and operation of visible LEDs using silicon crystals. Discussions include approaches for increasing light extraction efficiency. Chapter 3 describes infrared LEDs using silicon crystals, emphasizing the fabrication and operation of these devices. The chapter also covers spatial distribution of dopants, such as boron, in the device layers, as well as some plans for ways to improve effective polarization and control.
Chapter 4 covers the contribution and control of coherent phonons and evaluates light emission spectra. Chapter 5 illustrates basic device structures for infrared lasers using silicon crystals. The chapter also discusses ways to decrease the threshold current density and evaluation of optical amplification quantities. It provides thoughts on novel devices with high output optical power using indirect-bandgap semiconductors such as silicon-using DPPs. Chapter 6 discusses silicon carbide as green, ultraviolet, or broad spectral width LEDs using the DPP-assisted process in bulk crystals.
Chapter 7 gives examples using other crystals, such as GaP and ZnO. Optimum conditions for DPP-assisted annealing are discussed separately for each semiconductor and are assessed in terms of performance.
Chapter 8 reviews applications of the DPP-assisted technique to other devices, such as oscillators, photodetectors, and polarization rotators. This chapter stresses the utility of this new DPP technique outside of LEDs and laser devices.
This book (considered a monograph) has good flow and in-depth content to target a broad audience, including students, professors, academic researchers, and industry folks. It includes many current references for experimentalists and provides mathematical definitions in the appendices for theoreticians. My only critique is that the book could have been organized a little differently by placing figures and tables in places more relevant to the corresponding text. It is a good read for anybody who wants to learn about DPP-assisted silicon light-emitting devices and lasers.
Reviewer: Sudip Mukhopadhyay is a Honeywell Fellow at Honeywell, Calif., USA.
