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A View of the Past, and a Look into the Future by a Pioneer By Jacques I. Pankove This forword will be a brief review of important developments in the early and recent history of gallium nitride, and also a perspective on the current and future evolution of this exciting field. Gallium nitride (GaN) was syn­ thesized more than 50 years ago by Johnson et al. [1] in 1932, and also by Juza and Hahn [2] in 1938, who passed ammonia over hot gallium. This method produced small needles and platelets. The purpose of Juza and Hahn was to investiagte the crystal structure and lattice constant of GaN as part of a systematic study of many compounds. Two decades later, Grim­ al. [3] in 1959 employed the same technique to produce small cry­ meiss et stals of GaN for the purpose of measuring their photoluminescence spectra. Another decade later Maruska and Tietjen [4] in 1969 used a chloride trans­ port vapor technique to make a large-area layer of GaN on sapphire. All of the GaN made at that time was very conducting n-type even when not deli­ berately doped. The donors were believed to be nitrogen vacancies. Later this model was questioned by Seifert et al. [5] in 1983, and oxygen was pro­ as the donor. Oxygen with its 6 valence electrons on a N site (N has 5 posed valence electrons) would be a single donor.

Inhaltsverzeichnis

Frontmatter

1. Introduction

Abstract
For the last three decades or so, the III–V semiconductor nitride system has been viewed as highly promising for semiconductor device applications for blue and ultraviolet wavelengths in much the same manner that its highly successful As-based and P-based counterparts have been exploited for infrared, red, and yellow wavelengths. The wurtzite polytypes of GaN, AlN and InN form a continuous alloy system whose direct bandgaps range from 1.9 eV for InN, to 3.4 eV for GaN, and to 6.2 eV for AlN. For all practical purposes, the III–V nitrides could potentially be fabricated into optical devices which are active at wavelengths ranging from the green well into the ultraviolet. By using nitride emitters as pumps, all primary and mixed colors can be obtained, too.
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2. General Properties of Nitrides

Abstract
Semiconductor nitrides have excellent material, optical and electrical properties. The respective parameters are informative in determining the utility and applicability of these materials to devices, as will be evident below and throught out the present monograph.
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3. Electronic Band Structure of Bulk and QW Nitrides

Abstract
The band structure of a given semiconductor is pivotal in determining its potential utility. Consequently, an accurate knowledge of the band structure is critical if the semiconductor in question is to be incorporated in the family of materials considered for serious investigations and device applications. The group III-V nitrides are no exception and it is their direct-band-gap nature and the size of the energy gap what spurred the recent activity. A number of researchers have published band-structure calculations for both Wurzite and zincblende GaN, AlN, and InN. The first Wz GaN band structure found through a pseudo-potential method led to a 3.5 eV direct bandgap. The band structure for ZB GaN has been obtained by a first-principles technique within the local-density functional framework with a direct bandgap of 3.40 eV and a lattice constant of 4.50 Å. A treatise of the bad structure in bulk and quantum wells with and without strain will be given below.
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4. Growth of Nitride Semiconductors

Abstract
Although the synthesis of GaN goes back more than a half century, there are several pivotal developments which, in the opinion of the author, are responsible for laying the technological framework and paving the way for the tremendous commercial and scientific interest in nitrides. They are as follows: The synthesis of AlN by Tiede et al. [4.1], the synthesis of GaN through the reaction of Ga and ammonia to produce GaN by Johnson et al. [4.2], the synthesis of InN by Juza and Hahn [4.3], the epitaxial deposition of GaN using the hydride VPE technique by Maruska and Tienjen [4.4], the employment of nucleation buffer layers by Amano et al. [4.5] and Yoshida et al. [4.6], the achievements of p-type GaN by Akasaki et al. [4.7]. A more recent development which paved the way for all the commercial activity is the preparation of high-quality InGaN by Nakamura et al. [4.8] which followed the synthesis of InGaN by Osamura et al. [4.9]. Nearly every crystal-growth technique, substrate-type and orientation, has been tried in an effort to grow high-quality group-III-V nitride thin films. In recent years, various researchers have successfully taken advantage of the Hydride Vapor Phase Epitaxy (HVPE), Metal Organic Vapor Phase Epitaxy (MOVPE), and Molecular Beam Epitaxy (MBE) techniques, which have yielded greatly improved film quality.
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5. Defects and Doping

Abstract
Point defects, unless they are neutral, manifest themselves as background doping or autodoping, and complicate attempts to dope the semiconductor in order to control its conductivity. Moreover, defects influence the radiative-recombination efficiency with adverse impact on the LED and laser performance. Unfortunately, GaN and related materials are rich in structural defects as well as point defects such as vacancies. It is therefore imperative that we discuss doping together with native defects.
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6. Metal Contacts to GaN

Abstract
It is imperative that a semiconductor device be connected to the outside world with no adverse change to its current-voltage characteristics and no additional voltage drop. This can be accomplished only through low-resistance ohmic contacts on the semiconductor. An ideal contact is one where, when combined with the semiconductor, there are no barriers to the carrier flow in either the positive or negative directions. Ideally, this occurs when the semiconductor and the metal work functions are about the same, and there are no appreciable interface states which tend to pin the Fermi level. Since one can not just dial up ideal work functions for the semiconductor-metal system under consideration, particularly, when the work function of the semiconductor varies with doping, it is usually not possible to find just the right combination. In fact, for large-bandgap semiconductors such as GaN, a metal with a large-enough work function to form an ohmic contact to p-type GaN does not exist.
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7. Determination of Impurity and Carrier Concentrations

Abstract
When impurities such as donors and acceptors are introduced into a semiconductor, they produce levels within the energy gap. The energy of a level with respect to the edge of the conduction band in the case of donors, and the valence band in the case of acceptors is called the ionization energy. The simplest calculation of an impurity energy level is based on the hydrogenic model.
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8. Carrier Transport

Abstract
Current conduction, and thus the resistance of a semiconductor material and device, is determined by the ease with which the carriers can traverse through the structure. As the carriers travel through a semiconductor, they undergo a variety interactions with the host material [8.1–3]. In a perfect static crystal, carriers would be accelerated indefinitely by the applied electric field, consistent with the band structure of the crystal. However, the semiconductor crystal contains defects, intentionally added impurities, and even at very low temperatures the semiconductor is in constant motion and far from being static. As free carriers traverse through a semiconductor, they encounter various events referred to as scattering, the most effective of which are by charged impurities and/or centers, and by lattice vibrations. The former manisfests itself as deflections of free carries by the long-range Coulomb potential of the charged centers. This can be thought of as a local perturbation of the band edge, which affects the electron motion. The latter is caused by the interaction of a moving charge with lattice vibrations, contraction and dilation, and can liberally be described as follows: As the atoms moves closer to and farther away from one another, the corresponding undulations on the band edge causes scattering, as will be described in Sects. 8.1–4. An additional scattering mechanism is that due to charged dislocations which can be partially screened at high doping levels. Impurity scattering is eleastic or near eleastic, and conserves energy. However, phonon scattering is inelastic and changes the energy and momentum states. In the scattering process, energy can be gained by phonon absorption or lost by phonon emission.
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9. The p-n Junction

Abstract
As the name suggests, a p-n junction depicts the combination of two semiconductors having n- and p-type conductivities. If the two semiconductors forming the junction are of the same crystal, the term homojunction is used to describe the resulting structure. On the other hand, if two different semiconductors with very similar structural, but varying electrical and optical properties are used, the term heterojunction is applied. In modern LEDs and lasers, heterojunctions are employed for a variety of purposes which include carrier injection, and carrier and light confinement. In fact, before the advent of heterojunctions many optoelectronic and electronic devices were not possible among which was the CW (Continuous Wave) RT (Room Temperature) laser. Being such an integral part of lasers and LEDs, a concise description of the principles of p-n junctions and their characteristics is warranted. Detailed descriptions of heterojunction properties can be found elsewhere [9.1].
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10. Optical Processes in Nitride Semiconductors

Abstract
One of the most amazing properties of semiconductors, particularly direct-bandgap semiconductors, is the light emission, which revolutionized the op-to-electronics field. Light emission can be caused through a variety of stimuli among which electroluminescence has seen the most practical application. When an external voltage is applied across a p-n junction, electrons and holes that are injected into the medium recombine. This annihilation results in the emission of a photon whose energy is equal to the difference in the energies of states occupied by electrons and holes prior to recombination. In indirect semiconductors, phonons are generated predominantly, making them inefficient light emitters unless highly localized centers are utilized such as N in GaP.
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11. Light-Emitting Diodes

Abstract
Light Emitting Diodes (LEDs) convert electrical power to generally visible optical power and are simply p-n-junction devices, when biased in the forward direction. They produce light through spontaneous emission whose wavelength is determined by the bandgap of the semiconductor in which the carrier recombination takes place. Unlike the semiconductor laser, generally the junction is not biased to and beyond transparency. Consequently self absorption occurs and photons are emitted in random directions. A modern LED is generally of a double-heterojunction type with the active layer being the only absorbing layer including the substrate. In addition, a plastic dome to increase the light collection cone and to focus the light is employed. Nitride-based LEDs with InGaN-actice regions span the visible spectrum from yellow to violet, as illustrated in Fig. 11.1.
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12. Semiconductor Lasers

Abstract
Semiconductor lasers cover the respectable wavelength range of about 0.4 to 11μn and are very pivotal in many aspects of human life. As Strite put it, the Holy Grail of GaN research is the realization of an injection laser which would represent the shortest-wavelength semiconductor laser ever demonstrated [12.1]. Although semiconductor lasers have many applications, for example, in communication as pumping sources, and mundane applications such as pointers, the most salient and imminent application of GaN-based lasers is in Digital Versatile Disks (DVD for short). This is a future version of the compact disk where the spot size and therefore the storage density is diffraction limited [12.2]. The present CD players utilize GaAs infrared lasers produced by Molecular Beam Epitaxy (MBE). The interim approach adopted by the industry relies on red lasers with which pit dimensions of about 0.4 μm can be read. Using a two-layer scheme in a DVD, the density can be increased from today’s 1 Gb to about 17 Gb per compact disk [12.3]. The cycle time in the consumer-electronics market is rather short in that even if red-laser-based DVDs are implemented, the blue laser can be introduced some two years after the red lasers. For consumer applications, CW-operation lifetimes on the order of 10,000 hours at 60°C are required. The nitride-based lasers with their inherently short wavelengths, when adopted, offer much increased data storage-capacity possibly in excess of 40 Gb per compact disk. Figure 12.1 presents a photograph of a Nichia InGaN laser emitting near 400 nm, which is intended for such an application.
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Backmatter

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