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Photon extraction enhancement of praseodymium ions in gallium nitride nanopillars

The date of: 2022-12-12
viewed: 2
source:nature


Abstract
Lanthanoid-doped Gallium Nitride (GaN) integrated into nanophotonic technologies is a promising candidate for room-temperature quantum photon sources for quantum technology applications. We manufactured praseodymium (Pr)-doped GaN nanopillars of varying size, and showed significantly enhanced room-temperature photon extraction efficiency compared to unstructured Pr-doped GaN. Implanted Pr ions in GaN show two main emission peaks at 650.3 nm and 651.8 nm which are attributed to 3P0-3F2 transition in the 4f-shell. The maximum observed enhancement ratio was 23.5 for 200 nm diameter circular pillars, which can be divided into the emitted photon extraction enhancement by a factor of 4.5 and the photon collection enhancement by a factor of 5.2. The enhancement mechanism is explained by the eigenmode resonance inside the nanopillar. Our study provides a pathway for Lanthanoid-doped GaN nano/micro-scale photon emitters and quantum technology applications.
Introduction
Efficient and ecological small photon emitters, e.g. nano/micro-scale light emitting diode (LED)1,2,3,4,5,6 and monolithic RGB emitters7,8,9, are required for next generation lighting and display applications. Reliable single photon sources (SPS or single photon emitter: SPE)10, which provide on-demand single photons are indispensable in the field of rapidly growing quantum technologies such as quantum computing, quantum sensing, and quantum communications (quantum internet)11,12,13,14. For maximum utility, these sources should be room temperature, compatible with mature fabrication processes, and able to be integrated with other photonic systems. These demands have stimulated studies on gallium nitride (GaN)-based photon emitters and devices, which is a promising photonic platform.
Lanthanoid (Ln)-doped GaN (Ln:GaN) exhibits visible/near infrared (VIS–NIR) emission that is temperature insensitive, sharp and stable, since the energy levels in 4f-shell which are involved in the luminescence transitions are surrounded by filled 5s and 5p orbitals and thus isolated from free carriers in the host material. Additionally, such materials utilize well-developed GaN platform, enabling integration into more complex devices15. The photon emission can be electrically controlled and Ln:GaN LEDs have been demonstrated16,17,18,19,20,21,22. These superior optical and opto-electronic properties are also suitable for SPS applications. Single photon emission from single praseodymium (Pr) ions implanted in a YAG crystal at room temperature (RT)23 and coherent optical and spin control of single Pr ions have been reported24,25,26,27. Although these previous literatures have demonstrated the strong potential of Pr-SPS, insulating materials have been used for the host and the electric control of photon emissions has not been considered. In contrast, electrically controlled SPS operating at RT, which is particularly advantageous for practical applications, is feasible by Pr:GaN. However, there is still a significant limitation resulting from a poor light extraction efficiency due to the high refractive index of GaN (nGaN = 2.3). Therefore, improvement of the photon extraction efficiency is crucial for the realization of reliable Pr:GaN SPS.
An important method for improving photon extraction efficiency is through the creation of surface nanostructures, such as waveguides, cavities, and other resonant structures. Such structures can be used to improve photon extraction efficiency, photon emission rate, and internal quantum efficiency of the photon emitters28. Nanostructure fabrication can be described as bottom-up or top-down. With respect to the bottom-up approach, selective-area growth of nanocolumns with SiNx or Ti masks is employed. Obtained nanocolumns exhibit dislocation-free and strain relaxation properties8,9,29,30,31,32,33. With respect to the top-down approach, lithography and dry/wet etching techniques are mainly used and highly flexible nanostructures are available2,3,34,35. Because of its structural flexibility, the top-down is more advantageous than the bottom-up in terms of integration of luminescent centers into nanostructures. By embedding quantum dots/wells into (In)GaN nanocolumns (or nanopillars, nanorods), significant enhancements of photon extraction efficiency and internal quantum efficiency have been reported35, and also, the feasibility for SPS applications have been demonstrated36,37,38,39.
On the other hand, little is known about the optical coupling of Ln ions to GaN nanostructures and its effectiveness with respect to photon extraction enhancement and photon emission rate, despite the fact that studies on Ln:GaN materials and devices have been conducted for more than two decades. Although previous work has reported Ln-related emission from a nanopatterned GaN40 and its enhancement16,41, the optimization of nanostructures based on the enhancement mechanism has not been performed. One of the greatest advantages of Ln:GaN is the wavelength tunability from VIS to NIR by appropriate choice of Ln elements. Such dopant-dependent wavelength variation is unavailable in InGaN quantum structures. Nanostructures to improve the photon extraction efficiency strongly depend on both the photon emission mechanism and wavelength, and thus some degree of optimization is required for each class of Ln:GaN device.
Here we discuss optical properties of praseodymium (Pr)-implanted GaN nanopillars of varying size and structure, which show different photon extraction enhancement. These nanopillars are fabricated using the top-down approach: electron beam lithography, metal deposition, and dry etching techniques. Room temperature (RT) optical properties of individual nanopillars with different sizes are characterized by using a home-built confocal microscopy. The photon emission saturation shows that both the excitation laser collection and the extraction efficiency of emitted photons from implanted-Pr ions are enhanced by nanopillar structures when compared to the photon emission at a region where no pillars are etched into the structure. To theoretically understand the effects of pillar size and structure, we also establish the model for the photon emission enhancement of GaN nanopillars with different sizes based on the eigenmode analysis, and the theoretical results verifies the measured emission enhancement in experiment.
Experimental
The fabrication procedure of Pr-doped GaN nanopillars is summarized in Fig. 1a. Nanopillar structured surface was formed on an undoped GaN epilayer with thickness of 15 μm grown on n-type GaN (n-GaN) which were implanted with 100 keV-Pr at a fluence of 1.3 × 1014 cm−2 at RT. The ion implantation was performed at Takasaki Advanced Radiation Research Institute, National Institutes for Quantum Science and Technology (QST). The peak implanted region and Pr concentration were estimated to be 24 nm from the top of pillars and 5.3 × 1019 cm−3, respectively, according to the Monte Carlo simulation code TRIM (Fig. 1b)42. After Pr ion implantation, the sample was annealed at 1250 °C for 2 min under N2 atmosphere (1 atm) using an infrared furnace to remove radiation induced defects and to activate implanted Pr ions as luminescence centers. Prior to the thermal annealing treatment, a 50 nm thick SiN cap layer was formed on the surface using a magnetron sputtering method to suppress the deterioration of crystallinity by preferential evaporation of nitrogen43. The furnace temperature rose to the set temperature in 1 min, then kept at the temperature for set time, and was then naturally cooled down to RT for about 15 min. The SiN cap layer on the samples were subsequently removed by hydrofluoric acid treatment (HF:H2O = 1:5, 20 min). Resist film was coated on the GaN epilayer and dot arrays (squares and circles) were formed using electron beam (EB) lithography. The pillar size (side length for squares and diameters for circles) ranged from 100 nm to 2 µm, and the grid pitch of dot arrays was 5 µm or 10 µm. A 120 nm thick Ni layer was then deposited on the sample using electron beam evaporation. Following lift-off process, metal dot arrays are formed on the sample surface. These arrays were used as a mask for etching the GaN epilayer by ICP (Inductively Coupled Plasma) dry etching operating at 2.0 Pa with a Cl2 30 sccm. The ICP and bias powers were 150 W and 30 W. Finally, the metal layer was removed by acid treatment (aqua regia). The fabricated nanopillar structures was investigated by scanning electron microscopy (SEM) as shown in Fig. 1c. All pillars were fabricated mostly within the error of 5%. The pillar length and taper angle were measured to be 510 nm and 4 degrees, which were constant with pillar size. Also, a micro-trench structure was found around the bottom of nanopillars, indicating the dry etching conditions could be further improved for better fabrication, although we do not believe that the micro-trench was significant for our results. A sample implanted into nanoscale square regions was also fabricated for comparison, as shown in Fig. 1d (control sample). The EB lithographic pattern was formed on the resist film and then 100 keV Pr ions were implanted at RT. The implantation fluence was 1.0 × 1014 cm−2. The resist film was removed after implantation and the thermal annealing at 1200 °C for 1 min was performed. The side length of square implantation area ranged from 100 nm to 2 µm. Detailed optical properties of the nanoscale implantation samples have been reported elsewhere
Photon emission properties of the samples were characterized by a home-built laser scanning confocal microscopy (CFM). A Supercontinuum wavelength-tunable pulsed laser (6 ps pulse width, 80 MHz repetition rate) was used for excitation. Photon emission from the Pr-implanted regions was collected with an objective lens (numerical aperture, NA = 0.90) and detected by a Si avalanche photo-diode (APD). The PL spectra were investigated by an imaging spectrometer installed in the CFM. A 650 nm band pass filter (13 nm bandwidth) was placed in front of the APD and the spectrometer to selectively collect photons emitted from the implanted-Pr ions. The PLE spectra ranging from 400 to 610 nm and the luminescence lifetime were also measured using the same measurement setup. For the luminescence lifetime measurement, the repetition rate of pulsed laser was set to be 0.1 MHz. All measurements were performed at RT.
Results and discussion
Optical properties of Pr ions in different GaN nanopillars
A representative CFM image of the Pr-doped GaN nanopillars is shown in Fig. 2a. The obtained CFM image reproduced the nanopillar array patterns. The inset shows the zoom-in CFM image of a 200 nm circular pillar. The observed luminescence spot was larger than the actual size of nanopillar and the image reflected the CFM point spread function (PSF), since the lateral resolution (356 nm according to the Rayleigh limit) of CFM was larger than the 200 nm circular pillar44. Figure 2b shows a CFM image of the nanoscale implantation (control) sample. The implantation area was 100 × 100 nm and the grid interval was 10 µm. The obtained CFM image reproduced the designed EB lithography pattern, indicating the EB lithography pattern was successfully transferred to the Pr implantation pattern. In both CFM images, the implanted-Pr ions were excited with 525 nm laser for the resonant excitation. We revealed two resonant excitation peaks at 506 nm (2.45 eV) and 525 nm (2.36 eV) which are presumably due to the 3H4-3P1 transition22,46 from the analysis of photoluminescence excitation spectrum (see Supplementary Information for more detail). PL spectrum of the 200 nm circular pillar was also characterized as shown in Fig. 2c. Two emission peaks at 650.3 nm and 651.8 nm are attributed to 3P0-3F2 transition in the 4f-shell of Pr3+ (trivalent Pr) ions and the multiple peaks are caused by crystal field splitting47. A similar PL spectrum was obtained from the control sample, indicating that the nanostructuring does not affect the PL spectrum.
Conclusion
We fabricated Pr-implanted GaN nanopillars with different sizes, down to a minimum diameter and side-length of 100 nm for circular-shapes and square-shapes, and investigated their RT photon emission properties. It was confirmed by SEM observation that all pillars were fabricated at their desired size mostly within the error of 5%. We clarified using the home-built CFM that the implanted Pr ions in GaN showed two main emission peaks at 650.3 nm and 651.8 nm, which are attributed to 3P0-3F2 transition in the 4f-shell. We also observed resonant excitation peaks at 506 nm (2.45 eV) and 525 nm (2.36 eV), which are presumably due to the 3H4-3P1 transition. These optical transitions were unaffected by pillar structures. Dependences of the photon emission intensities on pillar sizes and excitation powers were systematically investigated and as a result, we found that the photon emisssion enhancement appeared when the size of nanopillars were less than 1 µm and the highest enhancement ratio was obtained from the 200 nm-sized circular pillar. The maximum value for the enhancement ratio was 23.5 when the excitation power density was lowest (0.67 kW/cm2). This value can be divided into the emitted photon extraction enhancement by a factor of 4.5 and the photon collection enhancement by a factor of 5.2, from the analysis of photon emission saturation behaviors. We established a theoretical model to analyze the enhancement based on coupling to the simulated eigenmodes at both the excitation and emission wavelenth. The change in enhancement ratio with different pillar sizes was explained by the envelope of those eigenmodes. Coupling to a series of eigenmodes generates broadband enhancement and the PL spectral shape of Pr ions remains unchanged by nanopillar structures. Our study paves the way for lanthanoid-doped GaN nano/micro-scale photon emitters and quantum technology applications, although optimizaiton of pillar structures and improvement of fabrication processes could be further explored for their realization.



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