Research On Interfacial Modulation Of BTO/GaAs Heterojunction And It’s Optoelectronic Effects

Functional oxide/semiconductor integrated devices can meet the demand for multifunctional integration of modern electronic information thin film devices.The epitaxial integration of Ba Ti O₃（BTO）ferroelectric films and Ga As semiconductor materials,with ferroelectric polarization characteristics,can regulate ferroelectric polarization by external electric fields,thus achieving the purpose of fine control of the magnitude,directions and values of both short-circuit current and open-circuit voltage of integrated devices.Therefore,BTO/Ga As heterojunction integration provides a new idea for the design of multifunctional,high-performance,tunable electronic devices.However,BTO/Ga As heterojunctions suffer from interface diffusion and lattice mismatch in the fabrication process,making it difficult to obtain BTO ferroelectric materials with high epitaxial quality by surface oxidation of Ga As semiconductors,which in turn leads to insufficient performance of integrated devices.To address above-mentioned problems,Sr Ti O₃（STO）,Ti O₂ and non-stoichiometric Ti O_2-δwere induced to modify the BTO/Ga As heterojunction interface in this dissertation.The effects of three kinds of buffer layers on the regulation of ion diffusion and lattice mismatch at the heterojunction interface are comparatively analyzed,and their relative mechanisms of different buffer layers on the diffusion and epitaxial growth of the BTO/Ga As interface are systematically investigated as well.The electrical and optoelectronic response properties of the heterostructured devices with different buff layers were characterized.The effects of the buffer layers on the optoelectronic response output of the heterojunctions were analyzed,and the modulating effects of iron electrolysis on the optoelectronic response properties of the devices were investigated as follows:1.To address the problem that BTO cannot be epitaxially grown on Ga As substrates due to severe ion diffusion at the BTO/Ga As heterostructure interface,STO and Ti O₂buffer layers were designed to be inserted into BTO/Ga As to achieve the purpose of blocking interfacial ion diffusion and reducing lattice mismatch.The epitaxial growth and interfacial diffusion behaviours of the STO and Ti O₂ buffer layer films on Ga As substrates were compared and analysed.The study has shown that the STO films were grown in island mode,while the Ti O₂ films changed from island to layer growth mode as the thickness increases.On the other hand,quantification of the interfacial diffusion response using XPS spectra showed that the introduction of the STO buffer layer reduced the Ga-O peak area percentage from 17.00%to 16.17%and 21.11%to 2.64%for that of the As-O peak compared to the original Ga As,indicating that the STO film had a weak blocking effect on the diffusion of ions from the Ga As surface to the interface.In contrast,the introduction of Ti O₂ buffer layer film,compared to the original Ga As,reduced the values to 0.16%and 0.87%for the area percentage of Ga-O and As-O peak respectively,indicating that the Ti O₂ buffer layer film inhibited the diffusion of ions from the Ga As surface to the interface to a certain extent.2.The epitaxial growth of BTO films on STO/Ga As and Ti O₂/Ga As templates have been realized,and the photoresponse performance of the heterostructured devices and the relative modulation effect of iron electrolysis have been investigated.The growth behaviours of the BTO films on STO/Ga As and Ti O₂/Ga As templates were monitored by in situ Reflection High-Energy Electron Diffraction（RHEED）,and their structures were characterized by X-ray Diffraction（XRD）and Transmission Electron Microscope（TEM）,while the surface topography was characterized by Atomic Force Microscope（AFM）.The epitaxial matching relationships were proved to be（110）Ba Ti O₃//（110）Sr Ti O₃//（001）Ga As,（110）Ba Ti O₃//（110）Ti O₂//（001）Ga As and（110）Ba Ti O₃//（110）Ti O₂//（001）Ga As,with the surface roughness of 1.9 and 1.5 nm,respectively.The photoelectric output current densities of the BTO/STO/Ga As and BTO/Ti O₂/Ga As devices were 6.55 and 6.82 m A/cm²,which increased to 11.4 and 11.88m A/cm² respectively when a forward polarization voltage was applied to the devices,indicating that the ferroelectric polarization had a modulating effect on the photoelectric response of the devices.However,the performance of the BTO/Ti O₂/Ga As devices was still not acceptable.The reason is the weakened effect of the Ti O₂ film on the diffusion barrier of the interfacial ions under high-temperature conditions.3.The non-chemo-ratio Ti O_2-δfilm was proposed as an"indirect"layer for BTO/Ga As,and a low-temperature growth mode withδof 0.75-0.80 was established.HR-TEM results show that the Ti O_2-δ/Ga As interface is clear and flat,and the Ti O_2-δ/Ga As lattice mismatch is 0.75%.The Ti O_2-δ/Ga As out-of-plane matching relationship is（010）Ti O_2-δ//（001）Ga As and in-plane matching relationships are[001]Ti O_2-δ//[-110]Ga As and[100]Ti O_2-δ//[110]Ga As.XPS results demonstrated that the Ti O_2-δfilms have a titanium oxidation stoichiometry ratio of 2.5:3 meaningδ=0.75,and the simultaneous presence of Ti⁴⁺,Ti³⁺,Ti²⁺and Ti⁺within the Ti O_2-δfilm,and the presence of oxygen vacancy defects as well.The Hall Effect system demonstrated that the main carriers of the Ti O_2-δfilm are electrons,indicating its N-type semiconductor nature.The UV photoelectron spectroscopy（UPS）results show that the forbidden band width of Ti O_2-δis about 1.65 e V.Following XPS results show that Ti O_2-δeffectively blocked the ion diffusion on the Ga As surface.The BTO/Ti O_2-δ/Ga As device has an opto-responsive output current density of 8.66 m A/cm²,which is increased to 14.7 m A/cm² by external positive polarisation voltage modulation.The improvement of the device’s optoelectronic performance was mainly due to three reasons:first,Ti O_2-δdecreased the interfacial lattice mismatch;second,the Gibbs free energy of Ti O_2-δis larger than that of Ti O₂,furtherly reduced the ion diffusion reaction between the interfaces;third,due to the iron electrodes of BTO the space charge region is created in the weak semiconductor Ti O_2-δlayer,the built-in electric field in the space charge region facilitates the rapid separation of photogenerated carriers at the interfaces.The above study shows that the STO thin film,Ti O₂ thin film and Ti O_2-δthin film studied in this dissertation as buffer layers solved the problems of interfacial defects and large interfacial lattice mismatch in BTO/Ga As heterojunctions.The excellent performance of the Ti O_2-δbuffer layer and the regulation of ferroelectric polarization have improved the optoelectronic output current density of the ferroelectric/Ga As heterojunction devices,providing a new idea for the integration of inorganic ferroelectric oxide materials on semiconductor substrates to achieve tunable optoelectronic performance.