Study On HVPE Growth And Processing Of Freestanding GaN Crystal

Gallium nitride(GaN)as a third generation material has been widly applied in the short wavelength optoelectronic and high frequency microwave devices thanks to its excellent physicochemical properties.Limited by the lack of homoepitaxial substrates,most GaN-based devices are heteroepitaxially grown on foreign substrates However.heteroepitaxial growth will generate high dislocation density and residual stress caused by lattice mismatch and thermal mismatch,which is bad for the manufacturing of GaN-based devices with reliability and long lifetime.The stability of the device can be improved effectively through reducing its internal stress.At the same time,the GaN single crystal homogeneous substrate can solve these problems fundamentally.HYPE is generally accepted as the most promising growth technique,due to the simple equipment,flexibility of high growth rates and low costIn this work,HVPE method was employed to grow the GaN single crystals.We systematically optimized the growth condition and successfully achieved the uniform stress GaN crystals with a low-stress buffer substrate.The GaN-based device was greatly improved.Freestanding GaN crystals were obtained through successive growth by adjusting growth mode,which provided the possibility of GaN native substrate.What's more,the processing of GaN single crystals was systematically studied for the further preparation of homogeneous growth and the devices fabrication The specific research contents are as follows(1)The effects of different temperature fields including the constant and variable temperature conditions on the qualitis and properties of GaN were investigated.Under constant temperature condition,the crystal quality was relatively good when the reaction space temperature was 1070?.When a temperature gradient of 10? in the reaction space(substrate temperature was 1070? and outlet temperature was 1060 ?)existed,the quality of GaN single crystal was the best.It had a smooth morphology without obvious hexagonal pits,and the FWHM of(002)and(102)were 432 and 486 arcsec,which is the minimal in all samples,and the FWHM were decreased 68 arscec and 26 arcsec compared to the constant temperature process.The results indicated that the GaN single crystal had a good crystallinity and the lowest dislocation density.Strongest band edge emission peaks appeared at this condition which showed a good optical property.As a result,the quality of single crystal can be improved by increasing the temperature and temperature gradient in a certain range.(2)We explored a novel substrate pretreatment technology using the laser decomposition technique and successfully fabricated the buffer substrate with intact surface morphology and low stress,which was regarded as the substrate to grow the low-stress and easily-separated GaN single crystals.The morphology,structure and internal stress condition of treated substrates with different laser energies were systematically investigated.The qualities of GaN crystals with these substrates were also compared.When the laser energy was 7 mJ,the quality of GaN crystal was best.Compared with the traditional substrates,the FWHM values of the(002)and(102)planes decreased about 242 arcsec and 167 arcsec,respectively.Moreover,the photoluminescence spectroscopic analysis showed the band edge emission was enhanced and the luminescent intensity increased more than quadruple.The carrier mobility also increased to 2.230×103 cm2/Vs.Furthermore,high quality UV photodetectors based on the as-obtained GaN with low-stress substrate were fabricated successfully,which showed a stronger photocurrent response and more excellent stability.(3)By tuning ?/? ratio and through the three-step growth process,the growth mode was directly changed through the continuous HYPE growth process,and freestanding GaN single crystals were obtained on the substrates without any pretreatment.Firstly,a high-quality GaN poly porous interlayer structure was obtained by the two-step growth.This structure provided a fracture layer for the self-separated GaN and kept the integrity of GaN single crystal.Meawhile,the structure had a good quality,its FWHM of(002)and(102)was 354 and 397 arcsec,respectively.Which were 466 and 493 arcsec lower than that obtained by only one-step growth process with same ?/?.This also proved that multi-step growth could improve the quality of GaN single crystal.Finally,a 2 inch freestanding GaN was directly obtained during the growth process on the MOCVD-Al2O3 substrate.Compared with others,the obtained GaN through three-step growth had a best crystal and optical quality.And its FWHM of(002)and(102)were 298 and 317 arcsec,the residual stress was 0.24 GPa.This provides a simple,direct and low-cost growth method for obtaining homogeneous substrates.(4)We systematically studied the grinding and polishing method of different crystal planes of the freestanding GaN grown by ourselves.For small size GaN wafers,the grinding efficiency of using diamond or silicon carbide was practical unanimity.But,the advantage of diamond abrasive became very prominent,when the GaN wafer was two inches.It indicated that SiC grinding materials could not meet the grinding requirements of large size GaN single crystal.While in the polishing process,the grinding material hardness had very little effect on the polishing efficiency,and the grinding material with higher hardness made the precision polishing cloth damaged seriously and the cost increased.The SiO2 polishing liquid could polish the GaN single crystal effectively,obtained lower RMS(0.29 nm)and reduced the damage of the polishing cloth which reduced the cost.At this point,diamond is the most suitable grinding material,and SiO2 is the most suitable polishing material.There were obvious differences on GaN grinding and polishing for different crystal facets,among them,the grinding and polishing speed of Ga facet was the slowest,N facet was the second,and m facet was the fastest.