Electrically conductive high-reflectivity omnidirectional reflectors are highly desirable. With air as the outside medium, omnidirectional reflection characteristics can be demonstrated by using high-contrast DBRs. The high-contrast materials Si (n «3.5 at X = 1 |jm) and SiO2 (n «1.46) are natural candidates for such ODRs. The optical properties of Si / SiO2 distributed Bragg omnidirectional […]

Total internal reflection is a fascinating phenomenon occurring at the boundary between two dielectric media with different refractive indices. Total internal reflection was first discovered by Johannes Kepler in the early 1600s (Kepler, 1611). Kepler attempted to explain the apparent bending of objects partially submersed in water. Kepler discovered that for rays near normal incidence, […]

Having been used by humankind for several millenniums, metal-air reflectors are the oldest type of reflector with high reflectivity. Metal reflectors are characterized by a broad spectral reflectivity band and a weak angular dependence of the reflectivity. The first high-quality metallic reflectors were used in reflection telescopes for astronomy applications (Bell, 1922). An experimental reflectance […]

Ideal reflectors incorporated into a device structure should have (i) high reflectivity, (ii) a sufficiently broad spectral range of the high-reflectivity band, (iii) omnidirectional characteristics, and (iv) low resistivity (provided current flows through the reflector). It is a question of great practical and intellectual interest what type of reflector best meets these multiple requirements. Different […]

For LEDs with two top contacts, such as GaInN/GaN LEDs grown on sapphire substrates, both regular packaging (epi-side up) and flip-chip packaging is used. Flip-chip packaging using solder-bump bonding is a more expensive packaging process compared with regular packaging where the LED top contact pads are contacted by wire bonding. An advantage of flip-chip packaging […]

Anti-reflection (AR) coatings are frequently used in communication LEDs to reduce the Fresnel reflection at the semiconductor-air interface. For normal incidence, the intensity reflection coefficient for normal incidence is given by R = n — nair)2 (9.10) (ns + najr) where ns and najr are the refractive indices of the semiconductor and of air, respectively. […]

Visible-spectrum (AlxGa1-x)05In05P LEDs with typical operating wavelengths of 560-660 nm are grown lattice matched on GaAs substrates. Since the energy gap of GaAs is Eg = 1.424 eV(Xg = 870 nm) at room temperature, GaAs substrates are absorbing at these emission wavelengths. As a result, the light emitted towards the substrate will be absorbed by […]

Different requirements need to be satisfied by the top contact. In regular LEDs, the top contact provides a pad for the bonding wire. The pad is usually circular with a typical diameter of 100 ^m. The top contact pad also provides a low-resistance ohmic contact to the current — spreading layer. Typical top contact geometries […]

Virtually all LED structures employ double heterostructures. They consist of two confinement layers and the active region. The band diagram of a double heterostructure is shown in Fig. 9.2. The active region has a smaller bandgap energy than the two confinement regions. As a result, the confinement regions are transparent to the light emitted by […]

To obtain high light-extraction efficiency and avoid absorption of light, all semiconductor layers other than the active region should have a bandgap energy larger than the photon energy. This can be done in different ways, for example by using double heterostructures, window layers, and other structures that will be discussed below. In this section, we […]