| The third-generation semiconductor GaN are widely used in short-wavelength optoelectronic devices and high-frequency microwave devices because of their excellent properties.Most of the current GaN devices are hetero-epitaxially grown on a foreign substrates.Due to the large lattice mismatch and thermal mismatch between GaN and substrates,a large dislocation density and residual stress exist in the GaN device,which impair the performance and service life of the GaN devices.Homoepitaxial growing GaN devices on GaN single crystal substrates is a fundamental method to solve this problem.Hydride vapor phase epitaxy(HVPE)method is considered as the most promising method to obtain GaN single crystal.At present,GaN devices are mainly grown on c-plane GaN,and the built-in electric field resulting from the polarization effect causes red shift of the emission wavelength,lowering the luminous efficiency,and greatly reducing the performance of the GaN photoelectric devices.To prepare non-polar GaN optoelectronic devices can eliminate the effects of polarization effects.The most promising method for growing non-polar GaN materials is to grow c-plane GaN bulk crystal with large thickness,and obtain non-polar GaN by directional cutting c-plane GaN bulk crystal.In this work,we optimized the growth process of GaN single crystals by HVPE.A two-step etching method for preparing porous substrate was developed,and high-quality freestanding GaN single crystals wereobtained using the twice-etched substrate.GaN wafers with different crystal faces were obtained by diced the bulk GaN crystal.The crystal quality and properties of GaN wafers with different crystal faces were studied.The main research contents are as follows:(1)The effect of the material gas flow rate on the crystal growth and single crystal properties of the GaN crystal was investigated.It was found that the grown GaN crystal with NH3 flow rate of 2000 sccm had the highest crystal quality and the best optical performance.The NH3 flow rate affected the growth of GaN crystal on the nucleation island density and the lateral growth rate.As NH3 flow rate increased,the density of nucleation islands increased and the lateral growth rate decreased.The grown GaN crystal with NH3 flow rate of 2000 sccm had a larger nucleation island density and a higher lateral growth rate.It was found that when the NH3 flow rate was 2000 sccm,the surface morphology of GaN crystals was the best,and the crystals merged completely,with no obvious hexagonal pits;the FWHMs of GaN(002)and(102)were the smallest,indicating that dislocation density of the GaN crystal grown at this condition was the lowest;the optical properties of the GaN crystal crystal grown at this condition were best,the band-edge emission(NBE)peak of this GaN was the strongest,and the yellow luminescence(YL)band and green luminescence(GL)band were the weakest.According to the E2(high)Raman mode peak,the residual stress of GaN crystals with NH3 flow rates of 1800 seem and 2000 sccm was greater than that of GaN crystals with NH3 flow rate of 2200 seem,indicating that the GaN crystals were mainly grown in 2D mode with the NH3 flow rate of 2000 seem.(2)A method for preparing porous substrates by a two-step etching process combining electrochemical etching and phosphoric acid chemical etching was developed.High-quality 2-inch freestanding GaN single crystals were obtained using the twice-etched substrates.The hot phosphoric acid was transported into the MOCVD-GaN through the etched holes obtained by the electrochemical etching.Since the N-polar face was more active than Ga-polar face,the N-polar face was etched by phosphoric acid at a relatively low temperature.And then,the porosity of the porous substrates was enlarged,and the surface maintained smooth.The twice-etched substrate was employed as a seed crystal for HVPE growth of GaN.Since the twice-etched substrate had a large porosity,the GaN crystal grown by HVPE was weakly connected to the substrate,and the GaN crystal was easily separated from the substrate.The FWHMs of(002)and(102)peaks for the GaN crystal grown on the twice-etched substrate were smaller than that from MOCVD-GaN substrate;according to PL measurement,the optical properties of the the GaN crystal grown on the twice-etched substrate were better;the Raman results showed that the the GaN crystal grown on the twice-etched substrate was stress free;the EBSD mapping showed that the porous buffer layer structure played an important role in reducing the stress in the GaN crystal.(3)Ga-polar,N-polar,and m-plane GaN wafers were obtained from the high-quality,large-thickness,freestanding bulk GaN single crystal grown on twice-etched substrate.The FWHM of the Ga-polar GaN wafer was only 160 arcsec,which was greatly reduced compared with that before machining;the FWHM of the m-plane GaN wafer was only 212 arcsec,indicating that the crystal quality of m-plane GaN wafers was high;Raman results showed that the GaN wafers with different crystal faces had good crystal orientation,and it was found that the GaN crystals had uniform internal stress distribution according to E2(high)Raman mode peak;the PL measurement showed that the GaN wafers with different crystal faces had high luminous intensity.(4)The electrocatalytic performances on different faces of GaN crystal were systematic studied.The m-plane of GaN crystal had the best catalytic activity and stability.Linear polar curve manifested that m-plane GaN wafer possessed the lower overpotential and smaller Tafel slope than Ga-GaN wafer and N-GaN wafer on acidic HER,alkaline HER and alkaline OER.The density functional theory(DFT)calculations indicated the adsorption free energy of H*(?GH*)for the possible adsorption sites of H*on the surfaces.The lower ?GH*of m-plane of GaN crystal proved the corresponding good electrocatalytic HER ability.The time-dependent potential curve and time-dependent current density curve further illustrated the excellent durability.The chemical stability was also verified by the XRD,SEM and Raman patterns of GaN wafers with different crystal faces after stability test. |
PDF Full Text Request