Micorstructure And Photoelectrical Properties Of GaN/AlGaN Multiple Quantum Well Films

Nowadays, GaN-based Multiple Quantum Well （MQW） films have beenattracted much attention as a candidate for photodetector which could be used inultraviolet and infrared region. Detector performance is highly influenced by thecrystal quality of epitaxial films, therefore, the study on the crystal quality anddefects of epitaxial MQW films is very important. In this letter, GaN/AlGaN MQWfilms are designed, the influence of different intermediate layers on the properties ofepitaxial thin film morphology, defects and interface structures are investigatived.The optical and electrical properties of MQW films are characterized, themid-infrared absorption of MQW films have been obtained. The impact ofmicroscopic defects was researched on the intersubband transition in MQW films.The main content of this thesis including:The energy levels of GaN/AlGaN MQW are calculated through self-consistentsolutions of Schr dinger equation and Poisson equation. When the doping density ofwell layers was between10¹⁹cm-3and10²⁰cm-3，the energy level of MQW areobviously varied, which will caused the change of intersubband absorption. Thechanging of Al content of cap layer on the top will lead to blue-shifted ofintersubband absorption wavelength. When the dislocation density was lower than10¹⁰cm^-2, the intersubband energy level will not vary with the change of dislocationdensity. With the same thickness and Al content, the affected thickness ofGaN/AlGaN MQW films with the AlGaN intermediate layer is thicker than the onewhich with the GaN/AlN short period superlattice intermediate layer. Therefore, itwill decrease the symmetry of quantum well energy levels, and then reduce theintensity of intersubband absorption.We have grown GaN/AlGaN MQW films with different intermediate layers byMetal-Organic Chemical Vapor Deposition （MOCVD）. Surface cracks wereinvestigated through optical microscope and scanning electron microscopy. Cracksmainly along the <11-20> direction, distribution of cracks are not correlation withsurface dislocations, the inserting of intermediate layer release the strain in a certainextent and reduce the crack density. The step flow growth mode was proved throughatomic force microscope, at the same time hexagonal spiral hillocks arised fromscrew dislocations were observed.The crystal quality and interface structure of films were analyzed byhigh-resolution X-ray diffraction and high-resolution transmission electronmicroscopy, it is observed that the growth of epitaxial layers and substrates wasbelong to pseudomorphic growth. The film with the AlGaN intermediate layer shown well periodicity, straight and sharp interface. Z-contrast image analysisindicated that compositions were uniform. No diffusion of Aluminum element wasfound between the GaN layer and the AlGaN layer.Interface microstructures of different layers were examined by high-resolutiontransmission electron microscopy and Fourier transform processing technology. It isfound that a large number of dislocations existed at the interface between the GaNbuffer layer and the α-Al₂O₃substrate, which play a key role to release misfit stress.The AlGaN intermediate layer blocked threading dislocations which origined fromthe GaN buffer layer, however, new misfit dislocations generated at the interface.Misfit dislocations were confined at the interface. So the crystal quality of quantumwell layers was improved, the introduction of GaN/AlN short period superlatticeintermediate layer did not affect dislocation distribution and films stress state.Furthermore, the relationship between V-typed defects and dislocations wasanalyzed.Dislocation density, distribution, and types were investigated through weakbeam dark field image. It is found that dislocations are mainly edged-typedislocations, with a density between10⁸cm^-2and10⁹cm^-2. Threading dislocationsorigined at the interface between the GaN buffer layer and the α-Al₂O₃substrate,and then spreaded upwards. Dislocations were blocked at different interfaces. Fourtypes of dislocation reaction were mannered. Stress field changes were analyzed bygeometric phase analysis technique，strain state of dislocations were recongnizedthrough the light and dark contrast.The effect of Surface cracks, V-typed pits and dislocations on luminescenceproperties of MQW films have been studied by cathodoluminescence spectra andscanning electron microscopy. It is shown that the peak located at550nm originedfrom radiative recombination, which resulted from the defect state caused byabsorpation of donor and point defects. The surface dislocation and V-typed defectsacted as non-radiative recombination centers, decreased the carrier density, and thenreduced the rate of intersubband transition.Infrared absorption properties of GaN/AlGaN MQW films were studied byFourier transform infrared spectroscopy. The result shown the sample with AlGaNmiddle layer had an infrared absorption with a peak wavelength corresponding to3.75μm, it is believe that the absorption arised from the transition between first andsecond subband level. The disappearance of intersubband transition in sample2wasdue to high defect density and reduced polar field caused by nonuniform thicknessof quantum wells.Through the analysis of microstructure of MQW films, the association wasshown between microscopic defects and film growth. According to the furtheranalysis of the intrinsic relationship between the microstructure and the optical-electrical properties of films, some improvement was maded on GaN/AlGaNMQW films to achieve the mid-wave infrared detection.