Investigation On Optical Properties Of Single Photon Emission From Single Point Defects In AlGaN

Ⅲ-nitrides,mainly composed of Al N,Ga N,In N,and their alloys,are key materials for the third generation of novel wide bandgap semiconductors.Compared with the previous generation of semiconductors represented by Ga As,Ⅲ-nitrides have higher breakdown field strength,larger bandgap,and higher electron mobility,making them suitable for applications such as high frequency,high voltage,high power,high temperature,and strong irradiation.Therefore,they have been widely used in commercial optoelectronic devices such as blue and white light emitting diodes（LEDs）,and play an important role in microelectronics,optoelectronics,and power electronics.In addition,Ⅲ-nitride materials have many superior characteristics,such as tunable direct bandgap,compatible with the current complementary metal-oxide-semiconductor（CMOS）fabrication technique,which also demonstrate broad application prospects in the field of photonic integrated circuit platforms.Therefore,research and development on Ⅲ-nitride materials are of great significance and will help promote further innovation and development of semiconductor materials and devices.Point defects in Ⅲ-nitride semiconductors have been recognized as one of the most promising quantum emitters due to their unique emission properties at room or even high temperature.These point defects bind electrons to extremely localized areas of 0.1 nm scale and exhibit rich spin and optical properties,which can be used for quantum device fabrication.In addition,the wide bandgap of the Ⅲ-nitrides can suppress the coupling between the bandgap levels and the bulk states,which meets the main material criteria for addressable quantum emitters.Therefore,owing to high efficiency,low cost and powerful operability at room or even high temperature,single-photon sources based on defects in Ⅲ-nitride semiconductors is very promising.AlGaN,as a ternary compound of Ⅲ-nitride semiconductors,can produce a wider range of point defects and have a wider tunable bandgap compared to binary compounds such as Ga N and Al N.Therefore,AlGaN is considered to be a flexible and scalable material platform for manufacturing quantum emitters.This thesis aims to investigate the optical properties and fluorescence transition dynamics of single-photon emitters based on point defects in AlGaN,in order to achieve high-frequency,high-brightness,narrow linewidth,controllable emission,and stable single-photon sources at room or even high temperature.It also aims to promote the application of Ⅲ-nitride semiconductors related to point defects in numerous applications such as quantum sensing,quantum information technology,biomedicine.This thesis investigates the point defects in AlGaN film in different aspects:Firstly,the single-photon emitters with high brightness,high degree of linear polarization,short photoluminescence（PL）lifetime and narrow linewidths based on single point defects in AlGaN films was investigated.At room temperature,the quantum emitter of the point defect can emit light from 720 nm to 930 nm in the near-infrared region,which provides the possibility of tuning the emission wavelength of a single emitter through choosing types of point defects.The average linewidths of these emitters are less than 10 nm,and a total high PL intensity exceeds 0.28×10⁶ cps at saturation.Moreover,absorption and photoluminescence of the emitters have extremely high degree of linear polarization.In addition,the polarization angle between the emission and excitation of emitters is different,with a measured range of 7.5°～61.5°,indicating that relative orientation between the absorption and emission dipoles of each light emitter is different.The excited-state photon emission lifetime of the emitters is around 2.5 ns,which is beneficial for achieving high-speed quantum systems.These properties make point defects in AlGaN a strong candidate for integrated on-chip quantum photonic devices and promote the application of AlGaN in quantum information systems.Secondly,the transition dynamics of single point defects emitting near-infrared single photon in AlGaN films were studied in detail from both theoreticaliy and experimentally.The species,energy level structure and transition dynamics of point defects were theoretically determined using density functional theory（DFT）based first-principles calculations.The key PL parameters were obtained in a three-level system using single defects PL intensity correlation functions at different time scales,and the transition dynamics of efficient emitters were described.An complex defect(V_NN_Ga)was demonstrated to be the most likely origin of the measured emitter since the calculated zero-phonon line（ZPL）and the lifetime of V_NN_Ga in the AlGaN film coincide well with the experimental results.The obtained results provide deeper insights into the optical properties and energy level structure of single point defects in AlGaN films and pave the way for advanced on-chip photonic quantum networks integration.Thirdly,the temperature dependence of single-defect single-photon emitters in AlGaN micropillars was investigated.The PL spectrum,PL intensity,radiative lifetime and second-order autocorrelation function of the single-photon emitters were measured in temperature range of 303 K～373 K.The point defects in AlGaN exhibit strong ZPL in withlength range of 800 nm～900 nm.As temperature increasing,PL intensity of the single-defect emitters gradually decreases,PL spectrum appears redshift,FWHM increases monotonously and the radiation lifetime of ZPL decreases gradually.At temperature of 373 K,point defects still exhibit high antibunching photon emission characteristics and stability.These results reveal a possible mechanism for optical properties of single-photon emitters in AlGaN at high temperature.This offers a possible key to unlocking a variety of solid-state quantum technologies at high temperatures.Finally,the enhancement effect of signal intensity using Ta₂O₅\Ag film as a function of the single-photon emission in AlGaN was investigated.The experimental results showed that the plasmon coupling effect occurs between the single point defects in the AlGaN film and the Ag film,leading to prominent enhanced PL intensity,shortened the excited-state lifetime and reduced the saturation excitation power of single point defects.In addition,the Ta₂O₅ film also acted as an anti-reflection film,which reduces the reflection of the point defect emitters inside the AlGaN film,increases the fluorescence transmission rate,and ultimately improves the photon extraction efficiency.Based on these characteristics,the single-photon intensity of point defects was enhanced by up to about 13.7 times.Then,the spectral jump characteristics of extremely brilliant single emitter in AlGaN film under 532 nm continuous and pulsed laser excitation were studied.By comparing the spectra recorded over time of emitters,a few single-photon emitters exhibited obvious ZPL tuning characteristics,with a maximum tuning range of up to 80 nm,accompanied by wide spectral broadening and intensity variation characteristics.These results reveal a more complex and rich physical mechanism.