GaN-based Ultraviolet (UV) Emitters
GaN-based Heterojunction Bipolar Transistors (HBTs)
Omni Directional Reflectors (ODRs)
Polarization Enhanced Ohmic Contacts
Electronic and Optical Properties of AlGaN/GaN Superlattices
Double Active Region Light Emitting Diodes (DARLEDs)
Directional Reflectors (ODRs) in Light Emitting Diodes (LEDs):
Semiconductor light emitting diodes (LEDs) stand at the forefront of a lighting
revolution. The performance of these devices is rapidly increasing, and they are
gradually displacing traditional lighting sources in many applications. The
highest-performance LEDs currently in production are based on AlGaInP compound
semiconductors. Devices fabricated from these materials emit light in the red,
orange, yellow, and yellow-green regions. While the AlGaInP internal quantum
efficiency in the red and orange wavelength range is quite high significant
gains in the external quantum efficiency are still attainable by overcoming
limitations of light extraction from current devices.
There are two ways to attain near-perfect light extraction from a spontaneous light emitter, namely (1) the use of a fully transparent structure or (2) the employment of multi-layer mirrors for broadband light reflection irrespective of the angle of incidence.
Way (1) may be realized by replacing the absorbing substrate (AS) by a transparent substrate (TS), preferably GaP (Krames, 1999; Kish et al., 1994). To date, TS devices with external efficiencies as high as 55 % have been reported (Krames, 1999) exceeding the performance of AS-LEDs by approximately a factor of two (Kish et al., 1994). However, the TS approach suffers from the use of costly GaP substrates and a complicated wafer bonding process (Kish, 1995).
Metallic layers are capable of reflecting light over a wide range of wavelengths and incident angles (Drude, 1904; Lorentz, 1909). The high-reflectivity band, however, is limited to frequencies below the plasma frequency of the free electron gas. Electron oscillations induced by incident light waves (Lorentz, 1909) not only result in reflection but also in undesirable absorption caused by electron–phonon scattering.
Distributed-Bragg reflectors (DBRs) represent a periodic multi-layer structure with a high-reflectivity stop band in a certain wavelength range. The stop band is centered on a certain wavelength usually given by the Bragg wavelength of the DBR for normally incident light. The DBR reflectivity strongly depends on the angle of incidence, q, such that the stop band shifts towards shorter wavelengths for increasing q without changing its spectral width (Macleod, 1986). As a result, the Bragg wavelength usually lies outside the high-reflectivity region at oblique angles q leading to a pronounced reflectivity decrease.
Recently, DBRs with greatly improved wide-angle reflectivity have been achieved, e.g., by using aperiodically stacked layers with thickness gradients or random thickness distributions (Popov et al., 1997; Xu et al., 1998). Truly omni-directional reflection in the wavelength range from 10 to 15 mm was obtained with DBRs using polystyrene and Te layers. Another intriguing work demonstrated omni-directional reflectors (ODRs) based on multi-layer birefringent polymers with two different refractive indices parallel and vertical to the layer planes (Weber et al., 2000).
These ODRs promise interesting lighting applications including light transport tubes (Weber et al., 2000), all-dielectric coaxial wave guides (Ibanescu et al., 2000), and omni-directional mirror fibers (Hart et al., 2002). Unfortunately their applicability to LEDs is limited due to their insulating electrical characteristics. Since their total thickness is in the mm range, they would also present a significant thermal barrier preventing efficient LED heat sinking.
A thin-film omni-directional reflector may be composed of only up to three layers (“triple-layer ODR”), namely the LED semiconductor layer itself, an intermediate low-index dielectric, and a metal. The dielectric layer serves as high-reflection coating for the metal and is about 100 nm thick; its thermal resistance can therefore be neglected.
A related way of improving AlGaInP device performance is the use of substrate-less thin film LEDs (Streubel et al., 2002; Illek et al., 2002). The epitaxially grown semiconductor layers, including the active region, are placed on a highly reflective metal mirror before bonding the wafer to a new carrier. Particular emphasis is being paid to geometrically shaping the LED backside so as to obtain micro-reflectors increasing the light extraction through the device top surface.
Figure 1. Calculated reflectivity (a) versus wavelength and (b) versus angle of incidence for two ODRs and a DBR with a Bragg wavelength of 630nm. GaP is the external medium. The transparent AlGaInP-DBR consists of 35 [(Al0.3Ga0.7)0.5In0.5P/Al0.5In0.5P] quarterwave pairs. The ODRs comprise a 500 nm thick metal layer of either Ag or Au covered by a quarterwave SiO2layer. The solid and dashed lines correspond to TE and TM-polarized waves, respectively.
Figure 2.Schematic structure of the RS (reflective submount) LED. The wafer is grown in the standard “p-side up” mode.
Figure 3. (a)Micrograph of AlGaInP RS-LED without applied voltage. (b)Micrograph of RS-LED under operation (I = 20 mA).
Double Active Region Light Emitting Diodes (DARLEDs): For last few years, there have been a lot of progresses on GaN-based high brightness light emitting diodes (LEDs).
Lots of applications based on this technology have been developed, such as traffic light, large sized display and optical storage system.
As for lighting applications, white light needs to be generated. Currently, the most common way to fabricate white light LEDs is coating phosphor layer on blue or ultraviolet (UV) InGaN/GaN based LEDs. The phosphors are photo-excited by a fraction amount of the blue/UV light and then reemit wide-bandwidth yellowish light. The combination of the yellowish light and the other fraction of the blue/UV light generate the white light. It is a very similar mechanism to fluorescent light. However, more packaging steps are needed and the lifetime of the phosphors is usually short. Therefore, some other technologies need to be developed.
According to Guo, et al, white light can be generated by two complementary wavelengths light. Therefore, a chip incorporated two complementary-wavelength active regions is a candidate to generate white light with longer life time and easier packaging process. In this project, LED wafers are epitaxied by metal-organic chemical vapor deposition (MOCVD).
Figure 1 The profile of the DARLED Figure2 The room temperature photoluminescence plot