Omni 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. 

    Fig.1 illustrates the reflection characteristics of triple-layer ODRs compared to a transparent conventional DBR. The best ODR maintains its high reflectivity at virtually all angles of incidence and clearly outperforms the DBR. A numerical analysis shows that the use of the best ODR results in a reflectivity increase by about a factor of two as compared to the DBR.

Figure 2.Schematic structure of the RS (reflective submount) LED. The wafer is grown in the standard “p-side up” mode. 

    The schematic of the proposed ODR and its incorporation in an LED is shown in Fig. 2. The mirror can be deposited on the p-type side of a conventional AlGaInP wafer using standard semiconductor processing steps. In order to achieve electrical conductivity an array of ohmic p-type micro contacts penetrates the dielectric layer thereby contacting the metal with the semiconductor. The total area of these micro contacts consumes only about 1 % of the die area. Assuming a reflectivity of 50 % for the alloyed p-type contacts, an insignificant reduction of the overall reflectivity by only 0.5 % results. The LED wafer can be bonded p-type side down to a low-cost conductive Si submount. The thermal conductivity of Si is superior as compared to GaAs or GaP and will result in more efficient LED heat sinking. Bonding can be accomplished either by using a conductive epoxy glue or by metal-to-metal bonding – in either case the process requirements are much less stringent than for the GaP-wafer bonding process. The original substrate will subsequently be removed by chemo-mechanical polishing.


Figure 3.
(a)Micrograph of AlGaInP RS-LED without applied voltage. (b)Micrograph of RS-LED under operation (I = 20 mA).

    Fig. 3 shows a micrograph of an RS-LED employing a silver mirror as ODR. In the turned-off state (I = 0 A) the small microcontacts are clearly visible as dark spots since theIr reflectivity is smaller than the reflectivity of the surrounding silver. Under operation the microcontacts appear brighter due to current crowding effects in their vicinity. 


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