Defects Controlling Mechanism In UV LEDs Grown On Sputtered AlN Nucleation Layer/Patterned Substrate Template

Aluminum gallium nitride?AlGaN?based ultraviolet light-emitting diodes?UV LEDs?have attracted considerable attention in a variety of applications such as UV curing,UV printing,photocatalysis,sterilization,optical medical treatment,due to their long life,mercury-free,fast switching response,and concentrated spectrum.However,compared with the internal quantum efficiency?IQE?of gallium nitride?GaN?based blue LEDs?over 80%?,the IQE of UV LEDs is still low.One of the factors that restrict the improvement of IQE in UV LED is the high density of threading dislocations in GaN or AlN film.In this thesis,the dislocation reduction mechanism of hetero-epitaxial III-nitrides by metal-organic chemical vapor deposition?MOCVD?on sputtered AlN nucleation layer/patterned sapphire substrate template was studied.In addition,a patterned silica array substrate with sputtered AlN nucleation layer was developed for the growth of AlGaN-based UV LEDs.Furthermore,the growth of high quality AlN on nano-patterned sapphire substrate with sputtered AlN nucleation layer was conducted.The main contents are as follows:?1?The dislocation reduction mechanism of hetero-epitaxial GaN layer on sputtered AlN nucleation layer/patterned sapphire substrate template was investigated.It was found that the sputtered AlN nucleation layer inhibited the growth of GaN islands on the sidewall of cone-shape pattern of sapphire substrate and eliminated the dislocations origined by the combination of grains with different orientation.Compared with the GaN nucleation layer,the AlGaN nucleation layer with a larger band gap reduced the absorption of UV photons,which improved the light output power of UV LED with a peak wavelength of 375 nm by 18.2%.The sputtered AlN nucleation layer further improved the light output power of UV LED by 11.3%,owing to the reduction of threading dislocation density.Effects of sputtered AlN thickness on the quality and photoelectric properties of UV-LED were also studied.In the sputtered AlN thickness range of 10-25 nm,the LOP of UV LED with 15-nm-thick sputtered AlN NL was the highest,revealing that optimum thickness of the sputtered AlN NL was around 15 nm.?2?Based on sputtered AlN nucleation layer,effects of sapphire pattern size on crystal quality,light output power and light extraction efficiency of UV LED were studied.The transition time from three-dimensional?3D?to two-dimensional?2D?growth of GaN was extended by increasing pattern size.The full widths at half-maximum?FWHMs?of symmetric GaN?002?and asymmetric GaN?102?X-ray diffraction?XRD?rocking curves decreased as the PSS size increased.By cross-sectional transmission electron microscopy?TEM?analysis,it was found that the threading dislocation?TD?density in UV LEDs decreased with increasing pattern size and fill factor of the PSS,thereby resulting in a marked improvement in internal quantum efficiency?IQE?.Finite-difference time-domain?FDTD?simulation showed that the light extraction efficiency decreased as the pattern size increased.However,the light output power of UV LED with a peak wavelength of 373 nm on the largest pattern size was the highest,due to the significant reduction of dislocation density in multiple quantum wells?MQWs?active region,which markedly improved the internal quantum efficiency.?3?The growth of AlGaN on sputtered AlN nucleation layer/patterned silica array substrate template was developed.The low mobility of Al atoms caused the nucleation of AlGaN on the sidewalls of the patterned sapphire substrate,resulting in the increase of dislocation density in AlGaN epilayer.The patterned silica array substrate inhibited the growth of AlGaN on the sidewalls of silica pattern,and thus reduced the dislocation density of AlGaN.FDTD simulation showed that patterned silica array substrate increased the light extraction efficiency of UV LED,due to the large difference of refractive index between SiO₂ and AlGaN,which increased the escape probability of UV photons from the top and bottom of UV LED chip.Owing to the reduced dislocation density and improved light extraction efficiency,the light output power of UV LED with a peak wavelength of 368 nm grown on the patterned silica array substrate was26.1%higher than that on conventional patterned sapphire substrate.?4?Effects of epitaxial structural parameters of deep-UV LED with a peak wavelength around 279 nm on current-voltage,current-internal quantum efficiency,current-peak wavelength and current-FWHM profiles were simulated using SiLENSe software.Simulation results confirmed that the high density of threading dislocations markedly deteriorated the IQE of deep-UV LED.Simulation also revealed that a thinner quantum well?QW?weakened negative effects of dislocations on IQE of deep-UV LED.Effects of quantum barrier?QB?thickness,Al doping in QB,Si doping in QB,periods of MQWs,stress relaxation,and Al doping in electron blocking layer?EBL?on optical and electrical properties of deep-UV LED were also studied by numerical simulation.Simulation results demonstrated that the increase of compressive stress was beneficial to voltage and IQE of deep-UV LED.?5?The growth of high quality AlN on sputtered AlN nucleation layer/nano-patterned sapphire substrate template was studied.By regulating AlN growth temperature and V/III in combination with a pulsed atomic-layer epitaxy,a high quality AlN was obtained in a modified high-temperature MOCVD.FWHMs of symmetric?002?and asymmetric?102?rocking curves on the AlN film were 143 arcsec and 230arcsec,respectively.The PL spectrum of deep-UV LED grown on the high quality AlN showed a peak wavelength of 279 nm,and the PL intensity of the deep-UV LED on nano-patterned sapphire substrate was 70%higher than that on flat sapphire substrate.