| AlGaN material is a potential candidate for ultraviolet?UV?photoelectronic devices with work wavelength from 200nm to 365nm as its tunable direct wide band-gap from 6.2eV to 3.4eV.AlGaN-based solar-blind detectors have the merits of intrinsic detection,tunable working wavelength,small size,radiation resistance,low working voltage,thus the AlGaN-based solar-blind detectors can be applied in many areas.However,up to date,the performances of AlGaN-based solar-blind detectors are far from real application,which is mainly due to the low material quality caused by lattice and thermal mismatch in hetero-epitaxy.Besides,the structure of AlGaN-based solar blind detectors should be optimized further.In this dissertation,we focus on how to improve the AlGaN material quality and the performances of AlGaN-based solar-blind detectors.Aiming at the problem of low material quality in heteroepitaxy,we proposed the method of quasi-homoepitaxy to grow AlN and AlGaN.Aiming at the problem of low detector performances,we proposed to take full advantages of polarization effect in AlGaN hetero-structures to promote the inner quantum efficiency,and thus improve the detector performances.The main achievements of this dissertation are as follows.?1?We improved the quality of AlN by quasi-homoepitaxy,and we proposed and further demonstrated that the interface strain was the driving force of defect evolution.We study the AlN quasi-homoepitaxy on HVPE-AlN substrates.By growing AlN on HVPE-AlN/Sapphire substrate in MOCVD,the full width at half maximum?FWHM?of the AlN?10-12?plane X-ray rocking curve decreased 102 arcsec.Besides,by applying low temperature AlN layer to alter the interface strain states,we studied the defect evolution in AlN quasi-homoepitaxy,and demonstrated the interface strain was the driving force of defect evolution.At the same time,the required driving force for different types of dislocation was also determined.?2?We found compositional nonuniformity would happen in AlGaN grown on substrates with large cutoff angle and explained the reasons,and furthermore we used the method of“mixed metal-organics pretreating the surface of HVPE-AlN substrate”to successfully suppress the nonuniformity and applied the“Carbon-Cluster”theory to explain the suppression mechanism.The formation of compositional nonuniformity was due to the macro-steps in the surface of HVPE-AlN and the migration distance difference between Al and Ga atoms.The macro-steps would block the Al atoms to migrate further while the Ga atoms would enrich at the macro-step areas.When using mixed metal-organics to pretreat the surface of HVPE-AlN substrate,Carbon-Clusters would form on the surface because the metal-organics would decompose at high temperature.The Carbon-Clusters would reduce the Ga atom migration distance and thus suppressed the compositional nonuniformity.?3?We found there were redundant types of excitons and Vcation-defect related luminescence in quasi-homoepitaxial AlGaN,and we effectively used the method of“mixed metal-organics pretreating the surface of HVPE-AlN substrate”to reduce the types of excitons and suppress the Vcation-defect related luminescence.From the results of temperature dependent PL measurements of the quasi-homoepitaxial AlGaN,we found more than three types of excitons in the two Al-content AlGaN areas and there were complex energy transport processes,which induced complex light emissions.This was due to the high alloy potential fluctuation in the materials.The pretreatment could alleviate the alloy potential fluctuation and thus suppress some exciton states.On the other hand,the PL results also implied that there was(Vcation-complex)2--related luminescence and its intensity was evidently reduced by the pretreatment.It was speculated that the mixed metal-organics pretreatment could provide ephemeral metal-rich condition and the formation of(Vcation-complex)2-defects were suppressed.?4?We proposed to take full advantages of the intrinsic polarization effect in AlGaN hetero-structures to optimize the structure of AlGaN-based solar-blind UV detectors,and eventually realized solar-blind UV detectors with high responsivity.We grown AlGaN high electron mobility transistor?HEMT?structures by MOCVD on HVPE-AlN/Sapphire substrates and then fabricated solar-blind UV detectors.The peak responsivity achieved at 1.42 A/W at voltage of 10 V,which was 50 times higher than that of the detector without polarization enhancement.Besides,a further optimization to the polarization enhanced solar-blind UV detectors resulted in a peak responsivity of 3.11 A/W at voltage of 30 V,which demonstrated the polarization effect could promote the performance of AlGaN-based solar-blind UV detectors. |
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