Study Of Substrate Transfer And Reliability Of AlGaInP LED

The LED (light emitting diode) has advantages of energy saving, environment-friendly and long life. The application of LED include sight lighting, back light source, indoor and outdoor illumination, full color outdoor display, automobile lamp, agriculture lighting, medical lighting, and stage lighting. However, the light extraction of AlGaInP LED is still relatively low because of small light output cone, absorbing of electrode and substrate. The study of this paper focuses on the research of substrate transfer AlGaInP LED with Si substrate, ODR (Omni-directional reflector), Au-Au metal bonding and roughened surface. The reliability of independent RD high brightness AlGaInP LED was investigated. The main contents of the study are as following:First,various influencing light output factors in substrate transfer AlGaInP LED were analyzed. The path of photon in substrate transfer AlGaInP LED was analyzed in both situations with and without scatter centers. Supposing the active layer scattered all photons, the light extraction was calculated as the changing of the output facet extraction efficiency, the reflectivity and the epi-layer transmittance. The doping concentration and thickness of n-AlGaInP layer in substrate transfer AlGaInP LED is finite and cannot meet the needs of current spreading. The electrode shape can be designed to improve current spreading.Second, the wafer bonding process in substrate transfer AlGaInP LED was analyzed and studied. The Au-Au metal diffusion bonding condition was investigated in substrate transfer AlGaInP LED. The graphite sheet in bonding process can guarantee the whole wafer contact. The crackle can be restrained under small temperature cooling rate. Under different bonding temperature 270℃, 280℃, 290℃, 300℃, substrate transfer AlGaInP LED with SiO₂ ODR were fabricated. With the temperature increasing, the light output decreases and the voltage increases. The main reasons were the diffusion at the reflector interface and the over-annealing of p type ohmic contact.Third, substrate transfer AlGaInP LED with Au/ITO/GaP and Au/SiO₂/GaP ODR were studied. The reflectivity versus incident angle of GaP/Au reflector, GaP/SiO₂/Au ODR and GaP/ITO/Au ODR were calculated. Compared with the AlGaInP LEDs with GaAs absorbing substrate, LEDs with Au reflector, Au/SiO₂ ODR and AuZnAu/ITO ODR were fabricated. At current of 20mA, the optical output powers of four samples were 1.04mW, 1.14mW, 2.53mW and 2.15mW respectively. The Au diffusion at the annealing process reduces the reflectivity of 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 ODR LEDs.The ITO/Au ODR and ITO/AuZnAu ODR were designed and fabricated. The insertion of Zn in ITO ODR LED does not affect the light output but evidently reduces the voltage from 2.378V to 2.033V at 20mA. The AuZnAu/ITO ODR LED with three kinds of ITO thickness which were 65nm, 90nm and 270nm, were fabricated and measured. The LED with 65nm ITO showed the highest brightness and lowest voltage. The main reason was that the diffusion of Zn passed through ITO and reach p-GaP and good ohmic contact was formed.SiO₂ ODR AlGaInP LED with 8μm and 0.6μm GaP was designed and fabricated. At the 20mA, the light intensity was 133 mcd, 180 mcd. The reflectivity of GaP/SiO₂/Au ODR on the LED lighting spectrum of 0.6μm GaP was greatly higher than that of 8μm GaP. The voltage of LED with 0.6μm GaP was higher because of the higher series resistance introduced by the lack of current spreading.0.6μm GaP AlGaInP LEDs with and without roughened surface were made and measured. The light intensities were 315mcd and 173mcd respectively. The current spreading layer n-AlGaInP were etched using the 1HCl : 2.5H₂O solution at 25℃for 20s of etching time to form a patterned surface. The morphology of the etched surface exhibits a pyramid-like feature, which introduced the increasing light output area and the change of photon direction.Finally, the reliability of independent RD high brightness AlGaInP LED was studied. The voltage increasing of AlGaInP LED at high current was studied. The high current density was the main reason and 3λ/4 thickness ITO was suggested. With the new process, the current accelerated aging experiment was conducted. The driving currents were 50mA, 60mA, 70mA, 80mA and 90mA respectively. Using the extrapolation of the current density, the index n in Eyring Model is 3.43 and the theoretical lifetime at 20mA is 999 thousand hours.