The broadening in the lower photon energy due to the oxygen impurity is still observable whereas the DAP peak disappeared. Compared to the 77 K PL, we observe in the room temperature PL of the as-grown sample a quenching of D 0 X peak while the FX emission became dominant at 3.42 eV (approximately 362 nm). Figure 2b shows a typical room temperature μPL emission spectrum of the as-grown GaN excited at 0.08 kW/cm 2 together with the excitation power-dependent μPL emission spectrum of the GaN NPs. The power-dependent PL measurement was performed on the NPs. This explains the increase in the emission intensity of DAP peaks. The GaN NPs underwent chemical etching, thus resulting in an increase of oxygen and vacancy sites at the surface due to the competition between the formation and dissolution of Ga xO y (Figure 1c).
In the n-type GaN DAP transitions, these acceptor-like sites have been reported by a number of authors to originate from Ga vacancies ( V Ga). The μPL spectrum of the GaN NPs presents approximately 110-meV red shift that could be attributed to the relaxation of the compressive strain, but foremost, we observe a relatively strong/prominent increase of the DAP and I ox peak intensities.
The scanning and transmission electron microscopy (SEM and TEM) investigations were performed using FEI Quanta 600 and FEI Titan G 2 80–300 electron microscopes (FEI Co., Hillsboro, OR, USA), respectively. The optical excitation was produced using a helium-cadmium (HeCd) laser emitting at 325 nm with a <10-μm spot size. The resultant nanostructure layers were later transferred onto a Si wafer at subsequent room temperature and 77 K for PL measurements using Jobin Yvon’s LabRAM ARAMIS microphotoluminescence (μPL) spectroscopy system (HORIBA, Ltd., Minami-ku, Kyoto, Japan). Prior to wet etching in a HF/CH 3OH/H 2O 2 (2:1:2) solution under UV illumination, 10-nm thin strips of platinum (Pt) were sputtered onto the GaN samples at one end of the surface to complete the loop for electron–hole exchange between semiconductor and electrolyte. The estimated dislocation density and measured carrier concentration of the film are 1 × 10 8 cm −2 and 2 × 10 18 cm −3, respectively. The substrate used in this study consisted of a 30-μm-thick Si-doped GaN epitaxy grown on c-plane (0001) sapphire (α-Al 2O 3) substrate with a measured resistivity of less than 0.03 Ω cm. The tunability feature renders these nanoparticles as a good candidate for further development of tunable-color-temperature III-N-phosphor-based white light-emitting diodes (LEDs) which are essential for matching room lighting with human circadian rhythms. We studied the emission mechanism of such novel GaN NPs, which showed controllable red shift of approximately 80 nm (approximately 600 meV) with increased optical excitation power. The resultant GaN NPs are chemically stable, simple to fabricate, and easy to integrate and, most importantly, offer tunable broadband emission. In this paper, we demonstrated the fabrication of a group III nitride-based nanoparticle (NP) using a UV-assisted electroless chemical etching method and explained the switchover in optical emission mechanism from defect-dominated to bulk-dominated PL transitions. However, to our knowledge, a large PL red shift with increasing excitation power was not reported and requires further investigations. On the other hand, in nanostructures having a large specific area, the surface states effect became significant in influencing the carrier recombination mechanism. observed a blueshift with increasing power due to the potential fluctuation in bulk p-type GaN. The localized potential fluctuations within the GaN matrix introduced by the Ga vacancies and impurities are considered in explaining the PL shifts. Despite enormous efforts in studying the GaN defect-related emissions, there is still a research gap in explaining the origins of PL shift with optical power injection. Although such defects can be destructive in a device, a well-engineered inorganic nanoparticle approach can offer many advantages. The blue luminescence at 2.7 to 3 eV peak energy has been extensively studied this peak dominates due to optically active defects and impurities. In n-type GaN, an ultraviolet (UV) peak at approximately 3.42 eV usually dominates the photoluminescence (PL) spectrum. Optical properties of GaN nanostructures are of great current interest because of the potential application in solid-state lighting.
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