Fabrication Of GaN-based High Power Light-emitting Diode Grown On Si Substrate

High power thin-film GaN-based LED has been widely used in street lighting,mobile phones flash lamps,car headlamps,torch lights and other fields.Compared with conventional thin-film LED,via-thin-film LED has better current spreading and no finger-like n-contact metal lines on chip top surface.It fundamentally alleviates the contradiction between current spreading and light extraction.Low cost,large size and easily removed silicon substrate is an ideal substrate for GaN-based thin-film high-power LED.This dissertation is aimed at providing practical technical solutions to realize the mass production of GaN-based LEDs on silicon substrates?LED-on-Si?.After developing an effective epitaxy structure to release the lattice mismatch and thermal mismatch between GaN epilayer and silicon substrate,we performed a systematical study of the high-efficiency thin-film LED-on-Si chip structure,greatly enhanced the thermal stability of metal/GaN Ohmic contact and remarkably improved the light extraction efficiency of LED-on-Si chips.These optimized chip fabrication technologies have been successfully applied on LED-on-Si mass production.The main results are as follows:1.Two low cost metals Ni?high melting point?and Sn?low melting point?are used for low temperature?260°C?silicon substrate GaN wafer bonding by transient liquid phase diffusion bonding process.By adjusting the thickness of Ni layer,we not only controlled the thermal mismatch during wafer bonding process,but also achieved necessary strength and stability of the bonding layers.When the ratio of Ni thickness and Sn thickness was in the region of 0.25⁰.3,flat LED wafers after silicon substrate removal were obtained with good thermal stability.2.The effect of the remaining thickness of AlGaN buffer layer on the roughening behavior of N polar GaN is studied.For the roughening rate of AlGaN is slower than that of GaN,AlGaN is used as mask for GaN roughening.The presence of the mask makes the GaN pyramid become larger and sharper,which is beneficial to the light escaping from GaN.However,there is an optimum range for the remaining thickness of AlGaN.Too much thickness leads to inhomogeneous roughening surface,while too less thickness has no mask effect.For the epitaxy structure in this dissertation,about 200 nm remaining AlGaN thickness results in the best roughening n-GaN surface.3.Pt is used as cover metal for p-GaN Mg activation.The result shows that longer time Mg activation can decrease the forward voltage of LED,but it also degrades the reverse characteristics of LED especially the ability of anti-ESD.There is a correspondence degradation between the reverse characteristics and the anti-ESD ability,suggesting that the two degradations may come from the same reason.4.The effect of Mg activation atmosphere,temperature and time on the photoelectric characteristics of LED is investigated.It is shown that the impact factors of Mg activation from high to low are O₂ atmosphere,temperature and time.However,the better the Mg activation is,the worse the reverse characteristic of the LED is?lower reverse voltage?.ESD test shows that the LED's anti-ESD ability is related to the reverse characteristic and Ohmic contact.Low reverse and high forward voltage will decrease the anti-ESD ability of LED.5.The effect of the thickness of Ni in p-side Ni/Ag bi-layer reflector on the photoelectric characteristics of LED is investigated.It is found that with the increase of Ni thickness,the forward voltage of LED increases and the output power decreases gradually,however,the reverse characteristics and the anti-ESD ability of LED are improved.To reduce the thickness of Ni and improve the crystal quality of GaN,is an effective way to obtain high efficiency and high reliability LED.6.The effect of plasma treatment of the n-GaN surface on the forward voltage of GaN-based LEDs grown on Si substrate is studied in detail.It is shown that the process window of the n-GaN surface after the Ar plasma treatment is broader.X-ray photoelectron spectroscopy?XPS?was used to help elucidate the mechanism.It is found that Ar plasma treatment can increase the concentration of N-vacancy?V_N?at the n-GaN surface.V_N acts as donors,and more V_N helps improve the thermal stability of n-contact because it alleviates the degradation of the n-contact characteristics caused by the high temperature wafer bonding process.It is also found that the O content increases slightly after the Ar plasma treatment and HCl cleaning.O atoms are mainly present in the dielectric GaO_x film before the Ar plasma treatment and the HCl cleaning,and they exist almost equivalently both in the conductive GaO_xN_1-x film and the dielectric GaO_x film after Ar treatment and HCl cleaning.The conductive GaO_xN_1-x film and the V_N donors formed during the plasma treatment can reduce the contact resistance and the LED forward voltage.7.A significant efficiency improvement by optimizing the via-like n-electrode architecture design of GaN-based thin-film LED grown on silicon substrate is obtained.The external quantum efficiency of the as-fabricated 1.1 mm×1.1 mm via-thin-film LED chip at 350 mA was increased by 11.3%as compared to that of vertical thin-film LED chip with conventional finger-like n-electrode.Detailed analysis of encapsulation gain and false colour emission patterns illustrated that the significantly improved LED performance was due to enhanced light extraction efficiency and more uniform current spreading,both of which can be attributed to the optimized via-thin-film chip structure.Minimizing the light loss at the periphery of Ag mirror was demonstrated to be a critical factor for improving light exaction,rather than simply replacing the finger-like n-electrodes with via-like ones.After encapsulation,the median blue lamp power and the wall-plug efficiency of the via-thin-film LED at 350 mAreached 659 mW and 63.7%,respectively.8.Via-thin-film LED chips on silicon substrate with a size of 1.35 mm×1.35mm were packaged on ceramic substrates as LED lamps.12 lamps are aged for1000h at 55°C environment and 1.5 A current.High temperature and large current aging does not cause significant degradation of LED performance,which proves the via-thin-film LED chip on silicon substrate has good reliability.