Abstract
This dissertation investigates the growth of GaP AlGaInP LED devices on misorientated GaAs substrates by Aixtron 2400 low pressure MOVPE. The doping efficiencies and behaviors of these materials are discussed and researched in detail under various growth conditions such as growth temperature, [dopant]/[III] ratio and substrate misorientation. Morphologies of the GaP windows layers grown on differently tilted angle substrates are studied. The morphology status is found to be strong dependent on growth temperature, V/III and substrate misorientation. Because LEDs under long-term or high-current operating undergo significant performance change and degradation with time, a novel Ga0.65In0.35P TSBR structure is grown between window and cladding layers of MQW-AlGaInP LEDs. The TSBR (~150 Å Ga0.65In0.35P) film is of lattice size and valence band energy intermediate between window and cladding layers, thus reducing band offset. Experimental characterization shows significant decrease in device forward bias, dynamic resistance and junction heating, with strong improvement in power output degradation for the high current region. LEDs with and without TSBR are fabricated, aged at DC 50mA, and tested at non-radiative and radiative current levels. The TSBR layer demonstrates significantly improved power efficiency, reliability and global lifetime behavior. This, together with TSBR low cost and ease of implementation, makes it potentially a very important design for mass-production and commercial applications. The characteristics of GaP window layers grown on AlGaInP misoriented LEDs are investigated in order to clarify variation of AlGaInP LED characteristics for various misorientated substrates. It is found that the GaP buffer growth rate between the GaP/AlInP interfaces can modify and improve GaP quality. However, the intensity of LED light output does not clearly change, i.e. the light output is essentially unaffected within a suitable range of the GaP quality for 2º and 15º substrates. Second, the carrier concentration of the GaP window layer is related to the light output of the LEDs, and a maximum light output exists for a specific [Mg]/[TMGa] ratio. When this specific value is exceeded, light output decreases gradually as [Mg]/[TMGa] increases. It is conjectured that the reason may be inactive dopant. Third, the GaP crystal of the 15 º-off LED is sensitive to growth temperature and improves with growth temperature, but the 2 º-off LED is almost unchanged with changing growth temperature. Fourth, the stability of light output is superior for the 15 º-off AlGaInP LEDs over a large growth temperature ranges, i.e. it achieves a large growth temperature window by using high tilt angle. In sum, these experimental results will be useful both theoretically and practically for applications using high-tilt substrates, and especially for improved LED designs.