The path of photons in the thin film （TF） light emitting diode （LED） was analyzed. The reflectivity of reflector in AlGaInP TF LED with and without the AlGaInP layer was contrasted. The absorption of the AlGaInP layer was analyzed and then the light extraction was calculated and shown in figure. The TF AlGaInP LED with 8μm and 0.6μm GaP was fabricated. At the driving current of 20 mA, the light output power of the latter is 33% higher. For the 0.6μm GaP LED, the etching of heavily doped GaP except the ohmic contact dot area is advised. The design and optimizing of current spreading between the n-type electrode and the p-type ohmic contact dot need further research.
Three aging experiments were performed for AlGaInP light emitting diodes（LED） with or without indium tin oxide（ITO）,which is used as a current spreading layer.It was found that the voltage of the LED with an ITO film increased at a high current stressing,while there was little change for that of the LED without the ITO.The results of the LEDs with different thicknesses of the ITO film show that the LED with a thicker ITO has a higher reliability.The main reason for the voltage increase of the LED with the ITO film might be the current crowding in the ITO film around the P-type electrode.
The reflectivity versus incident angle of a GaP/Au reflector, a GaP/SiO2/Au triple ODR （omni-directional reflector） and a GaP/ITO/Au triple ODR was calculated. Compared to A1GaInP LEDs with a GaAs absorbing substrate, thin film LEDs with a Au reflector, a SiO20DR and an ITO ODR were fabricated. At a current of 20 mA, the optical output power of four samples was respectively 1.04, 1.14, 2.53 and 2.15 mW. The Au diffusion in the annealing process reduces the reflectivity of the Au/GaP reflector to 9%. The different transmittance of quarter-wave thickness ITO and SiO2 induces different optical output power between the SiO2 and ITO thin film LEDs. The insertion of Zn in the ITO ODR LED does not affect the light output but evidently reduces the voltage.
An optically transparent electrode, indium tin oxide （ITO） film is fabricated by vacuum E-beam evaporation. The thermal annealing effects on the ITO/GaP contact have been investigated by means of the transmission line model method. Under 435 ℃, with rapid thermal annealing for 40 s in N2 ambient, the ITO contact resistance reaches the minimized value of 4.3 × 10^-3 Ω·cm^2 . The results from Hall testing and Auger spectra analysis indicate that the main reasons for the change of the contact resistance are the difference in the concentration of carriers and the diffusion of In, Ga, O. Furthermore, the reliability of AIGalnP LEDs with a 300-nm thickness transparent conducting ITO film is studied. The increase of LED chip voltage results from the degradation of ITO film. Moreover the difference between the thermal expansion coefficient of GaP and ITO results in the invalidation of the LED chip.
