Epitaxial Structure Design And Device Fabrication Of High-power GaN-based LED On Si Substrate

GaN,a compound semiconductor with excellent properties such as wide band gap,high electron mobility and good thermal conductivity,is the cirtical material for blue light-emitting diodes(LEDs),which leads the third-generation revolution in lighting technology.However,to realize the further penertraion of LED,it is necessary to further improve the light output power and drecrease manufacturing cost of LEDs.Based on this consideration,single-crystal Si as the substrate for the growth of GaN-based LEDs becomes the spot because the Si substrate has the advantages of mature preparation,large size,low manufacturing cost and excellent thermal conductivity.So far,the technology of GaN-based LED on Si substrate has made a breakthrough,but there are still a series of unsolved issues about epitaxial structure design,which limits the further improvment of the performance of LEDs.How to systematically design a new epitaxial structure and restrain the efficiency droop effect on the condition of applying high operation current is most critical on the research of high-power LED on Si substrate.In this thesis,the epitaxial structure design of GaN-based LEDs on Si substrates and the preparation of LED chips are carried out.The main achievements are as follows:Firstly,we have established a theorectical calculation model for GaN-based LEDs on Si substrates with careful modification to the stress state of the underlying epitaxial layers to reflect the strain-related effects on the phycial processes inside LEDs.We have found that applying a proper tensile stress in the underlying n-GaN layer not only reduces the polarization charge densities of hetero-interfaces,making the whole energy band of multiple quantum wells(MQWs)become flat and decreasing the injection potential barriers for both electrons and holes,but also weakens the quantum confined Satrk effect(QCSE),increasing the radiative recombination coefficient of carriers in MQWs.The optimal stress is also obtained,and by modifying the stress state of underlying n-GaN layer to the optimal value,the light output power and forward voltage of LED can be improved.Moreover,we have designed the structure of buffer layers to realize a residual stress of 1 GPa in GaN-based epitaixial layers without cracks on Si substrate,which achieves a 21.96%improvement of light output power.Secondly,the theorectial calculation model is adopted to investigate the influence from theIn content of InGaN barriers on the properties of LED.It is found that proper In incorporation in GaN barriers can lowen the potential barriers of MQWs,resulting in a 32%enhancement of carrier injection in MQWs.In order to further release the compressive stress of quantum wells,InGaN barriers are grown at low temperature.In-depth studies reveal that the optimal low-temperature barriers remarkably promote the strain relaxation in wells without forming large density of crystalline defects.The optimal growth parameter contributes an extra improvement of 19.69%to the light output power of LEDs.Thirdly,to overcome the disadvantage brought from the increase of In content of MQWs and avoid the larger polarization charge density aroud the hetero-interfaces of the last quantum barrier(LQB)and electron blocking layer(EBL),we use AlInGaN EBL to replace the conventional AlGaN EBL and optimize the composition of AlInGaN EBL.It is demonstrated by simualtion and experimental methods that tuning the In content of AlInGaN EBL can result in polarization mathch for LQB and EBL and thus contribute to flat energy band and the enhancement of hole injection.At the same time,we propose a new structure of p-GaN/InGaN heterojunction.The band discontinuity of p-GaN/InGaN heterojunction is fully investigated and the hole crowding effect is thus reveled to promote hole injection into MQWs.Through these two structures,the hole concenctraion in MQWs is largely enhanced and the asymmetry of electron and hole concentrations in MQWs is weakened,which are advantageous to alleviate efficiency droop.Finally,a full LED epitaxial wafer with all the structure designs mentioned above is grown on a Si substrate,and a vertical LED chip is then prepared by removing the substrate to solve the problem of light absorption by Si substrate.The results show that the the chip with the size of 1mm2 is operated with the volage of 2.97 V at the current density of 70 A/cm2,decreasing by 0.49 V with comparsion to the conventional LED.And the light output power of the chip is 988.70 mW at the same current density,showing a reamarkable improvement of 35%.More importantly,the wall-plug efficiency of the chip is 47.5%at the current density of 70 A/cm2 and its droop value within this range of current density is only 33.7%,which decrease by 1/3 with comparsion to the LED with convetional epitaxial structure.These results prove the superiority of the structure designs for high-power LED on Si substrate.In this paper,the control of stress and its related influence are investigated in detail and the structures of buffer layer,InGaN barreirs,AlInGaN EBL and p-GaN/InGaN heterojunction are carefully designed by the guidance of theoretical studies.Throguh these strucure designs,the carrier injection into MQWs and the radiative recombination coefficient of carrers in MQWs are enhanced.And finally,the LED chips of vertical structure are prepared,providing important guidance for the high-power GaN-based LED on Si substrate.