Study On The Growth Of Nitrogen-Polar GaN Films And Its Related Light-Emitting Devices On SiC Substrates By MOCVD

Group-? nitride semiconductor material is widely used in the field of optoelectronic,due to its tunable direct bandgap ranging from 0.67 eV to 6.2 eV,and excellent and stable physical and chemical characteristics.At present,most of the GaN-based LEDs in the world are gallium-polarity,i.e.the LED structures are epitaxially grown along the+c axis?<0001>direction?.While the nitrogen-polar LEDs epitaxially grown along the-c axis?<0001>direction?are few reported.The main reason is that it is difficult to obtain nitrogen-polar GaN films with high crystalline quality and smooth surface morphology.However,compared with gallium-polar material,nitrogen-polar material has its unique characteristics.The nitrogen-polar InGaN can be grown at a much higher temperature,which means that the nitrogen-polar InGaN has a higher In incorporation efficiency.Then,we can increase the growth temperature of GaN barrier layer and p-GaN layer,due to the higher growth temperature of the nitrogen-polar InGaN quantum well layer,which leads to a high crystalline quality of InGaN quantum well layer,GaN barrier layer,and p-GaN layer.In addition,the polarization field in the nitrogen-polar LEDs is opposite to that in gallium-polar LEDs,which can improve the carrier injection to the active region and suppress the carrier overflow from the active region.As a result,the internal quantum efficiency of GaN-based LEDs can be greatly enhanced with nitrogen-polar structure designation.Therefore,the study and development of nitrogen-polar GaN-based material have great significance for promoting the development of group-? nitride semiconductor material-based optoelectronic devices.Focusing on the difficulty of the epitaxial growth of nitrogen-polar GaN material,this thesis has conducted the following two aspect works on SiC substrates using metal-organic chemical vapor deposition?MOCVD?:?1?controllable growth of nitrogen-polar GaN material;?2?the fabrication of nitrogen-polar InGaN/GaN-based LEDs.The detailed research contents are as follows:1.The fabrication and study of nitrogen-polar GaN films on SiC substrates.The influences of misorientation angle of SiC substrates,low and high temperature two-step growth technique,and V/? ratio on the surface morphology of nitrogen-polar GaN films are investigated.We find that high density of hexagonal hillocks are formed on the surface of nitrogen-polar GaN films grown on SiC substrates with a misorientation angle of 0^o.However,the hexagonal hillocks are disappeared when nitrogen-polar GaN films are grown on the vicinal SiC with a misorientation angle of 4^o.As a consequence,the surface roughness of nitrogen-polar GaN is greatly decreased.We explained the mechanism of the surface morphology transformation from the view that the vicinal substrates can increase the nucleation density of the epilayers.Even though the hexagonal hillocks on nitrogen-polar GaN surface are suppressed by using vicinal SiC substrates,the surface of nitrogen-polar GaN is still very rough,for which the atomic force microscopy?AFM?root mean square?rms?roughness over an area of 20×20?m² is 34.6 nm.Based on the vicinal SiC substrates,low and high temperature two-step growth technique is used for the epitaxial growth of nitrogen-polar GaN films.And growth temperature of low-temperature GaN and high-temperature GaN are optimized.At the optimum growth conditions,the rms roughness of nitrogen-polar GaN over an area of 20×20?m² is 8.13 nm,which is decreased by³ times compared with the nitrogen-polar GaN grown by one-step growth temperature.To further improve the surface morphology of nitrogen-polar GaN films and obtain nitrogen-polar GaN film with smooth surface,we optimized its V/? ratio.The AFM measurement results show that the rms roughness of nitrogen-polar GaN over an area of 20×20?m² is decreased from 8.13 nm to 2.78 nm when the V/? ratio is decreased from 1500 to 125.2.The growth processes study of SiN_x interlayer in nitrogen-polar GaN films on SiC substrates.After the realization of nitrogen-polar GaN film with smooth surface,we need to improve its crystalline quality,which is important for enhancing the light-emitting property of nitrogen-polar InGaN/GaN quantum well?QW?LEDs.Therefore,the SiN_x interlayer is introduced to the growth of nitrogen-polar GaN films,and the deposition time of SiN_x interlayer is optimized.As a result,the optimum deposition time of 60 s is obtained.At the optimum deposition time,the full width at half maximum of?002?and?102?planes X-ray diffraction?XRD?are 400 and 335arcsec,respectively,for the nitrogen-polar GaN,and the corresponding threading dislocation density of nitrogen-polar GaN is⁶.2×10⁸ cm^-2.We can see that the crystalline quality of the obtained nitrogen-polar GaN is comparable to that of gallium-polar GaN.In addition,according to the observed porous feature of the SiN_x interlayer,we proposed a model to illustrate the epitaxial growth mechanism of nitrogen-polar GaN films grown on the top of the porous SiN_x interlayer.3.The fabrication and study of nitrogen-polar InGaN/GaN QW LEDs on SiC substrates.The influence of the nitrogen-polar GaN template surface roughness and the InGaN/GaN QW number on the light-emitting property of nitrogen-polar blue-green?⁴80 nm?LEDs is investigated.We find that the light-emitting intensity and uniformity of nitrogen blue-green LEDs can be improved effectively by reducing the surface roughness of nitrogen-polar GaN templates.And the light-emitting intensity can be further improved by the optimization of InGaN/GaN QW number.Then,we explained the performance difference of the LEDs with different number of InGaN/GaN QW in the view of epitaxial growth of InGaN/GaN quantum well structures:the more number of InGaN/GaN QW,the higher defect density can be generated at the InGaN/GaN interface.Based on the optimized quantum well number,nitrogen-polar InGaN/GaN based green?⁵11 nm?LED are fabricated,and its optical and electrical properties are analyzed in detail.The green LED presents a good rectification effect with a turn-on voltage of 4 V.At the current value of 30 mA,the green-light emission can be observed obviously.