| Recent years, the rapid development of GaN based LEDs starts an age of solidstate lighting, LEDs come into people’s lives and be known by more and morepeople. However, there is still a long way to go for solid state lighting, the currentwhite lighting is based on phosphor conversion which has a relatively low efficiency,high cost and insufficient illumination quality. If we mix high efficient LEDs withmultiple colors to obtain white light, there will be huge potential on efficiencyimprovement with much better illumination quality. The key issue for realization ofcolor mixing is to enhance the efficiency of LEDs in long wavelength range,especially the yellow LEDs. In this study, we choose to research GaN based yellowLED on Si substrate, the purpose is to use the conventional method, utilize currentmature equipment, source materials and epitaxial growth technologies, optimize thedevice structure and growth technology on the basis of blue and green LEDs toenhance the efficiency of GaN based yellow LEDs in a simple way.The paper begins with epitaxial growth, introduces many methods to optimizethe growth technology and epitaxial structure for yellow LEDs：use AlN interlayersto improve crystal quality of GaN and AlGaN step buffers to modulate the stress inGaN; improve the quality of quantum wells by increasing growth temperature,introduce blue quantum wells as prestrained layer to release stress, introduceV-defects to screen dislocations, optimize growth conditions to reduce carboncontamination, and finally, we successfully grow high brightness GaN based yellowLED on Si substrate.The epitaxial properties of as grown wafer were investigated. Dislocationdensities were calculated by rocking curve measurement, the interfaces and defectsof buffer layers, MQWs and V-defects were studied by TEM, the stress within GaNand MQWs were analyzed by XRD reciprocal space mapping, thickness andcomposition of the layers were measured and simulated.We test the device properties of GaN based yellow LEDs on Si substrate, atroom temperature and350mA driven current, the LED emits72mW yellow lightwith a wavelength of567nm, whereas the external quantum efficiency (EQE) is 9.4%, and the EQE reaches22.2%at a small current density, the results are of highlevel around the world. Higher voltage observed at a lower temperature is explainedby the carrier transportation mechanism. We also conclude that the main factoraffecting wavelength shift is screening of polarization field, and a method is set upto evaluate the effects of screening and band filling on wavelength shift. Three sidepeaks are observed on the El spectra which are determined to be originated from theside wall of V-defects, the prestrained blue MQWs and the supper lattice,respectively, a model of hole leakage is introduced to explain the phenomenon.Finally, we study the similarities and differences on droop behavior amongGaN based blue, green and yellow LEDs. Through the study, we conclude that: thedroop of efficiency with current density can be attributed to polarization field, thedroop of efficiency with temperature can be attributed defect related non-radiativerecombination, and the droop of efficiency with wavelength is mainly attributed toincreasing of stress and defects due to increment of indium content. We setup a newmodel to explain the droop mechanism in the view point of energy conversion, thatis any energy transferring process will increase carriers’ recombination life time,which lower the efficiency. Besides, the efficiency droop between AlGaInP and GaNbased LEDs is compared. We observed that the GaN based LEDs’ efficiencystability as well as wavelength stability versus temperature is much better than thoseof AlGaInP based LEDs, thus we predict that GaN will replace AlGaInP and takedominant in yellow emission range.This study reveals that GaN based LEDs on Si substrate have not only highefficiency on blue and green emission range, but also great potential in longwavelength range. With the development of technology, we believe that theefficiency of GaN based yellow LED will be greatly enhance in the near future,whereas the color mixing white lighting will become true. |
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