Figure 3.1.1 gives a simple description of the basic light-emission process. Electrons fill up energy states in a valence band, which is separated in energy from a conduction band by an energy gap, with energy Eg (where there are generally no allowed states in which electrons can reside). Providing energy to an electron in the valence band can promote that electron to the higher-energy conduction band, also creating a hole (lack of electron) in the valence band. The electron can subsequently return to its lower-energy state: in radiative recombination, the electron returns to the valence band and releases a photon with the energy of the photon approximately equal to the energy Eg. In an LED, radiative recombination is the desirable outcome for an “energized electron,” but there are also numerous non-radiative recombination processes where the electron or hole may be trapped at defects or imperfections in the material. Such imperfections limit the efficiency of the light generation and, therefore, of the LED.
FIGURE 3.1.1 Light emission process.
nitride (InN)). The bandgaps of these III-nitrides produce light emission across a range of wavelengths spanning the infrared to ultraviolet (UV) parts of the spectrum. The IIInitride LEDs have had an unusually rapid development and huge impact on appearance of SSL. Although the first GaN LED was reported by Pankove et al. (1971), almost two decades transpired before substantial further progress was made by Akasaki and Nakamura. Akasaki demonstrated that high crystal quality GaN could be grown by metal organic chemical vapor deposition (MOCVD) using a novel low-temperature buffer (Amano et al., 1986). In 1992, Nakamura, working at Nichia, developed an industrially robust process for p-doping of GaN that led to the first high-brightness blue LEDs. This provided the understanding of the mechanisms that had limited the conductivity of P-type material and allowed for the first time the fabrication of low-voltage p-n junction LEDs and eventually led to the commercialization of high-brightness blue and white LEDs for SSL. The wider bandgaps of the III-nitrides enabled the development of efficient LEDs that emit light at blue wavelengths, which together with green and red LEDs provided the basis for white light as well as full-color displays. The nitride blue emitters can also be coupled with phosphors to generate white light, which is currently the dominant approach to an SSL technology. The later introduction of blue LEDs, compared to their green and red counterparts, is the result of materials issues that are still of importance today: the lack of a well-matched material (substrate) upon which to form the LED structures and some difficulties in controlling the electrical properties of the material. Nonetheless, the III-nitride materials have been pivotal in the success of inorganic SSL, and thus the committee will focus on LEDs formed from those materials. There are several good reviews of LED device technology (see, for example, Schubert ) as well as III-nitride materials technology (Pankove and Moustakas, 1998)).
The basis of the LED device is a p-n junction diode, shown schematically in Figure 3.2. As the name implies, there is a junction between the N-type material (rich in electrons) and P-type material (rich in holes). Under forward bias (positive voltage applied to the P-region and negative voltage applied to the N region) large numbers of electrons are injected into the N region and large numbers of holes are injected into the P region.
Current flows in the device and the large number of injected electrons and holes can combine radiatively, producing significant light emission. The basic structure is modified in actual LEDs to (1) improve the efficiency of injection of electrons and holes and to (2) “localize” the electrons and