Study Of Monolithic Phosphor-free White LEDs Grown On GaN Micro Arrays

The commercial white LEDs are manufactured primarily by employing a yellow phosphor coating over a GaN-based blue LED to convert a part of the blue light to a longer wavelength,due to its efficiency,simplicity and low-cost.However,this method suffers from the problems of stability due to ageing of the phosphor coating,deterioration of the light-conversion efficiency of phosphor and variation of chromaticity during operation at high currents and temperatures.More importantly,these white LEDs have low color rendering index(CRI)owing to green emission deficiency in the visible spectrum,and comprise a large portion of blue wavelengths which may disturb the body’s biological clock..Therefore,extensive research efforts were put forward to realize a monolithic,highly CRI phosphor-free white LEDs.It is known that,by adjusting the In content,the band gap of InGaN material is tunable from 0.7 eV to 3.4 eV,and light is emitted almost the whole visible range.Basically,InGaN/GaN multiple quantum well(MQW)with appropriate In contents and well widths can be used for achieving the whole visible light.Fortunately,the growth rate,In atom incorporation efficiency and polarization field of InGaN QWs grown on polar plane differ from semipolar or nonpolar planes,causing the variation of local emission wavelength along the planes.With these distinct features,multi-facets InGaN QWs were proposed to grown for realizing the monolithic phosphor-free white LEDs.In this study,the GaN micro-hole and micro-truncated pyramids arrays with multi-facets were fabricated by etching technology and selective growth technology,respectively.Then multi-facets InGaN MQWs were grown by metal organic chemical vapor deposition(MOCVD)to investigate the possibility for realization of white light emission as well as monolithic phosphor-free white LEDs.The major results are as follows:Firstly,hexagonal GaN micro-hole arrays with non-polar,semi-polar and polar faces were fabricated using a combination of dry etching and wet etching techniques,with 5-μm-diameter and 50-μm-sapcing.Then multi-facets InGaN MQWs were epitaxy by MOCVD.X-ray diffraction {0002} plane ω/2θ scanning results indicate that good periodic InGaN QW structure has been achieved.The room temperature PL spectra show that broadband emissions t covering the wavelength range of 450 nm to 750 nm can be obtained in the samples with 500 nm and 1000 nm hole-depth.However,the large hole spacing and the low ratio of semi-polar surface area results in the proportion of yellow-green light in the spectrum is too high.Therefore,we further decreased the hole spacing from 50 μm to 8 μm,and grown two blue QWs plus three green QWs instead of 5 green QWs to ensure the blue emission from 400 nm to 450 nm.The PL spectrum illustrated that white light emission covering the wavelength range from 400 nm to 700 nm is achieved,and the color coordinates are(0.2859,0.2906),which is close to the standard white light spectrum.On the other hand,by using the selective growth technique,GaN micro-truncated pyramids array with {10 11} or {11 22} semi-polar plane and {0001} polar plane was epitaxially grown on the sapphire substrate,followed by the epitaxy of 5-period InGaN MQWs.SEM image shows that the diameter of the bottom of the micro-truncated pyramids is about 6～8 μm,and the height is about 4～6 μm.The room temperature PL spectrum demonstrates that multicolored-light emission covering the wavelength range from 400 nm to 700 nm has been obtained,in which the color coordinates are(0.3378,0.3786).Furthermore,the light extraction efficiency is found can be greatly improved in such micro-truncated pyramid structure by performing FDTD simulation..These results suggest that micro-truncated pyramids array can be used to fabricate high performance monolithic phosphor-free white LEDs.