The Defect Control And Energy Band Modulation Of GaN-based Near-UV LEDs On Si Substrates

As a new generation of ultraviolet?UV?solid-state light sources,the GaN-based near-UV light-emitting diodes?LEDs?have many advantages,such as non-toxic and harmless,small size and low power consumption,which make it widely applied in various fields such as biomedical,curing,sterilization and anti-counterfeiting detection.To enlarge the market of near-UV LEDs,low cost and high power are important development directions.Thanks to the high thermal conductivity,cheap price and large scale?up to 12-inch?of Si substrates,they are beneficial for the fabrication of low-cost and high-power LEDs.After having developed for decades,some progress of the epitaxial technology of GaN on Si substrates has been achieved.However,some problems need to be solved to realize high-performance GaN-based near-UV LEDs on Si substrates.Firstly,cracks can easily generate on the surface of GaN epitaxial films mainly due to the large coefficient of thermal expansion?CTE?mismatch between GaN and Si.Secondly high-density dislocations are usually found in as grown GaN epitaxial films owing to the large lattice mismatch between GaN and Si.Finally,the polarization field in the active region reduces the carrier transport and radiative recombination efficiency,making it difficult to obtain high-performance UV LEDs.This work is based on the following routs of the preparation of near-UV LEDs with crack-free GaN epitaxial films in order to obtain high-performance near-UV LEDs on Si substrates.The defect control of LED epitaxial films and the energy band modulation of LED epitaxial structure.Main achievements of this work are shown as follows:First,the near-UV LEDs based on crack-free GaN epitaxial films have been fabricated on Si substrates by designing structure of step-graded AlGaN buffer layers.The high-density cracks can easily generate on the surface of GaN epitaxial films directly grown on AlN buffer layer,which are due to the large lattice mismatch between AlN and GaN.In this work,the crack-free GaN epitaxial films is successfully obtained by designing different step-graded AlGaN buffer layers.The mechanisms about the stresses and dislocations of GaN epitaxial films controlled by AlGaN buffer layers are also revealed.The in-plane lattice constant of AlGaN is smaller than that of GaN,which can provide compressive stress compensation effect for the growth GaN.Furthermore,the introduction of step-graded AlGaN buffer layers can bend dislocations at each interface,thereby reducing the dislocation density in GaN epitaxial films.Correspondingly,the compressive stress relaxations in GaN epitaxial films caused by misfit dislocations and dislocation bending are greatly decreased.Therefore,the two-layer step-graded AlGaN buffer layers show a better compressive stress compensation effect.Based on crack-free GaN epitaxial film grown on two-layer step-graded AlGaN buffer layers,the LED epitaxial films with an emission wavelength of 395 nm are realized by adjusting the In composition of the In GaN quantum well,and corresponding vertical structure LED chips are prepared.At an injection current of 350 m A,the light output power?LOP?of the prepared chip is 326 m W and the working voltage is 3.70 V.Second,high-quality AlN buffer layers have been obtained by designing the structures of AlN layer,and thus reducing the defects in LED epitaxial films and further improving the photoelectric performance of the near-UV LEDs.The AlN buffer layers,which acting as the bottom buffer layer,have a great influence on the crystalline quality of the LED epitaxial films.However,high-quality AlN buffer layers are difficult to obtain because of the interface reaction between Si and AlN and the low mobility of Al atoms.On the one hand,this work designs the structure of the AlN nucleation layer to suppress the interdiffusion between Si and AlN,and thereby improving the interfacial properties and the crystalline quality of the AlN buffer layers grown on Si substrates.On the other hand,this work adopts a multi-layer structure with high-and low-V/III ratio alternation technique to release part of the tensile strain formed in AlN buffer layers to prompt the surface of AlN to merge.Accordingly,the AlN buffer layers in 2D growth mode with high crystalline quality and smooth surface have been obtained.Therefore,the defects of the LED epitaxial films are effectively controlled with the dislocation density decreased from 1.4×10⁹ cm^-2 to 3.1×10⁸ cm^-2.At an injection current of 350 m A,the LOP of the corresponding LED chip is increased to 421 m W with a 29%improvement.Last but not least,the high-performance UV LEDs have been achieved by designing the novel structures of electron blocking layer?EBL?and multiple quantum wells?MQWs?based on energy band modulation to suppress the polarization filed in the active region.On the one hand,this work designs an 8-periods Al In GaN/GaN superlattice EBL structure,which can suppress the polarization filed at the interface between the last quantum barrier and EBL,thereby enhancing electron blocking ability and hole injection efficiency at EBL.The corresponding chips test results show that the superlattice EBL structure increases the LOP of the UV LEDs by 17%at an injection current of 350 m A.On the other hand,this work proposes MQW structures of 9-periods In GaN/GaN/AlGaN/GaN to reduce the polarization effect in MQWs,thus increasing the spatial overlap and radiative recombination efficiency of carriers in the MQWs.Moreover,the GaN interlayer barrier grown by two-step temperature control can effectively improve the quality of the MQWs.Eventually,high-performance UV LEDs on Si substrates have been achieved.At an injection current of 350 m A,the LED chip shows a high LOP of 659 m W and a high EQE of 60%.In summary,this work adopts the buffer layer technology to control the stress and defects of the LED epitaxial films on Si substrates,and designs the structure of the active region to enhance the carrier transport and radiative recombination efficiency.These two aspects have realized high-performance GaN-based near-UV LEDs on Si substrates.This work provides important guidance for the high-performance GaN-based microelectronics and optoelectronic devices on Si substrates.