Two-step Growth Of GaN-based LED Epitaxial Material On Si Substrate

Light-emitting diode?LED?exhibits many merits such as high efficiency,energy conservation,environmental protection,long lifespan,and so on,which make it widely applied in various fields such as traffic signal,architectural ornament,display,illumination,etc.So far,most commercial LEDs have been fabricated from III-nitride,i.e.gallium nitride?GaN?,which is grown on sapphire substrate.However,the thermal conductivity of sapphire substrate is low and large-size sapphire substrates are hard to fabricate,which hinder LEDs from developing into high-performance,high-power and low-cost lightings.In this regard,silicon?Si?has been considered as one of the most promising substrate material for fabricating high-performance,high-power and low-cost GaN-based LEDs,owing to its low cost,large size,high thermal conductivity and mature production process.However,even though the preparation technology of LED on Si has gained important progress,there are some critical problems to realize high-performance GaN-based LEDs on Si substrates.First,as the mismatches of lattice and coefficient of thermal expassion between Si and GaN are as large as 16.9%and 54%,respectively,numerous defects and cracks would be caused in GaN during the epitaixl growth and cooling process.Second,Si would react with active nitrogen?N?to form silicon nitride?SiN_x?interfacial layer under high growth temperature,which greatly deteriorates the quality of as-grown Ga N-based LEDs.Solving these two problems is the key to realizing high-quality GaN-based LEDs wafers and high-perforamance devices on Si substrates.Herein,this work proposes two-step epitaxial growth,i.e.,combining low-temperature epitaxial growth with high-temperature epitaxial growth,to overcome the two problems and realize high-quality GaN-based LED wafers on Si substrates.The growth mechanism,interfacial reaction expression effect and defect control mechanism are studied in depth.Main achievements are shown as follows.Firstly,we employ pulsed laser deposition?PLD?to realize the low-temperature epitaxial growth of AlN and suppress the interfacial reaction between Si substrates and III-nitride films,which helps to obtain low-temperature AlN?LT-AlN?films with abrupt heterointerfaces on Si substrates.Various properties of as-grown LT-AlN such as crystalline quality,surface morphology evolution,interfacial structure,etc.are carefully studied,and the growth mechanism and interfacial reaction suppression effect are revealed.On the one hand,the high energy effect of PLD provides AlN species with enough kinetic energy to overcome the potential energy barrier of migration caused by lattice mismatch,and the pulse effect of PLD offer enough time span to species for sufficiently migrating to its equilibrium position,which is essential to low-temperature epitaxial growth.On the other hand,the optimal growth condition enables the species to collide with N atoms in atmosphere under the most suitable pressure and growth distance and to have moderate kinetic energy when reaching Si substrates.The moderate kinetic energy can prevent not only the formation of rough interface or the interfacial reaction caused by excessive kinetic energy,but also the formation of amorphous interfacial layer caused by insufficient kinetic energy.Therefore,the low-temperature epitaxial growth of LT-AlN templates is realized,and abrupt heterointerfaces of Si/AlN can be obtained.Secondly,LT-AlN films are used as the growth templates for high-temperature epitaxial growth of GaN films and LED wafers.The surface morphology and dislocation evolutions of epitaxial layers in different periods have been studied,and the growth mechanism and dislocation annihilation mechanism of two-step epitaxial growth are revealed.It suggests that the ex situ LT-AlN templates with flat surface can improve the wetting of Al N epitaxial surface,decrease the migration barrier of Al adatoms,and enhance the surface migration of Al adatoms,which helps to facilitate the two-dimensional growth of AlN and realize high-quality AlN buffer layers with smooth surface.The GaN nucleation layers grown on such AlN buffer layers have low-density nuclei,which is beneficial to reduce the dislocations.Meanwhile,when the GaN growth changes from three-dimensional mode into two-dimensional mode,dislocations happen to bend and annihilate,which also benefits to decrease the dislocation density of GaN epitaxial layers.To further improve the quality of GaN,step-graded Al_xGa_1-xN buffer layers with dimishing Al component are introduced into AlN/GaN,and its effects on controlling defects and stress have been explored.For one thing,dislocations would bend and annihilate with each other at each interface,and,therefore,less dislocations extend to GaN layers.For the other thing,the underlying layer would produce compressive stress to upper layer,and eventually increase the compressive stress in GaN layer.Such enhanced compreesive stress can greatly compensate the tensile stress produced by Si susbtrates during the cooling process,and,therefore,the formation of cracks is effectively suppressed.Combining the optimal growth conditions of AlN and step-graded Al_xGa_1-xN buffer layers,crack-free and high-quality GaN epitaxial layers are obtained.The full width at half maximums?FWHMs?of GaN?0002?and GaN?10-12?X-ray rocking curves are 394 and 460 arcsec,repectively.Thirdly,two-step epitaxial is applied to realize high-quality Ga N-based LED wafers on Si substrates,and its effect on the performance of LEDs is studied.Through optimizing the thickness of ex situ LT-AlN templates,crack-free and high-quality GaN-based LED wafers are obtained on 40-70 nm-thick templates.After the epitaxial growth,the as-grown LED wafers are fabricated into LED chips with lateral structures by standard process.The LED grown on 40nm-thick ex situ LT-AlN template exhibits the best performance.Under an injection current of300 mA,its light output power is 70.2 mW.When the current increases from 20 to 300 mA,the droop of its external quantum efficiency is 46.2%.When the injection current increases from 2to 9 mA,electroluminescence spectra show the smallest blue shift,suggesting that the quantum-confined Stark effect in this LED is weakest.Comparing our results with the reported values of the LEDs with identical chip structure,the light output power achieved in this work is higher than those reported values,which suggests that the two-step epitaxial growth proposed in this work is promising to improve the performance of LEDs.This work systematically and comprehensively studies the two-step epitaxial growth combining low-temperature epitaxial growth with high-temperature epitaxial growth.Through investigating the properties of epitaxial layers in different growth periods,the growth evolution,defects and stress controlling mechanisms of GaN on Si are explored,which is instructional for the growth of high-quality III-nitride films on other unconventional substrates by combination methods,and significant for achieveing high-performance GaN-based devices.