Development of Ultrahigh Efficiency Compound Semiconductor Quantum Wire Light-Emitting Diodes

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High-efficiency semiconductor light-emitting diodes(LEDs) have attracted great current interests as an energy-efficient and long-lifetime general lighting device of the 21st century. However, in conventional LEDs which are usually grown on flat substrates, it's very difficult to extract light generated in semiconductors to the outside free space with a high efficiency due to the existence of total internal reflection at the semiconductor-air interface, blocking of light by opaque metal electrodes, and light absorption by substrates. The objective of the present project is to realize semiconductor LEDs with light-extraction efficiencies much higher than those of conventional devices using spontaneous emission control technique based on selective growth on pre-patterned, nonplanar substrates. The main achievements are summarized as following.1. Proposal and demonstration of "LED with separated current-injection and light-emitting areas" : We proposed and demonstrated a new type of semico … More nductor LED with separated current-injection and light-emitting areas, aimed at greatly suppressing light blocking by opaque metal electrodes, utilizing the fact that bandgap energies of compound semiconductor layers grown on patterned substrates usually change with crystalline facets. The basic idea of this new device is to form the Ohmic electrodes selectively on the high-bandgap-energy facets of the epitaxial layers. In this device, carriers injected from the high-bandgap-energy facets will first diffuse, through barrier and/or quantum well(QWL) active layers, to the adjacent low-bandgap-energy facets and then recombine there. In this way, we can spatially separate the current-injection area from the light-emitting area and thus greatly suppress the blocking of light by metal electrodes.2. Discovery of a new high-efficiency spontaneous emission control phenomenon: We found that both the light-extraction efficiency and the internal quantum efficiency of the (001) flat QWL grown on the narrow ridge-top region (lateral width < 1μm) of a V-grooved GaAs substrate can be dramatically enhanced just by surrounding the (001) QWL with high Al-composition layers in both the growth and the lateral directions. For example, we have shown by photoluminescence study that more than 60% of the spontaneous emission from an Al_<0.3>Ga_<0.7>AS/GaAS/Al_<0.3>Ga_<0.7>As(001) QWL can be extracted to the outside space by surrounding the above QWL structure with Al_<0.65>Ga_<0.35>As barrier layers. Moreover, the internal quantum efficiency of the above Al_<0.3>Ga_<0.7>AS/GaAS/Al_<0.3>Ga_<0.7>As(001) QWL structure was found to be 100% even at room temperature, which is more than 10 times higher compared with that of sample without the Al_<0.65>Ga_<0.35>As barrier layers. High-efficiency LEDs are being developed currently using this unique spontaneous emission control technique. Less