Yellow Luminescence Intensity Research Articles

Impact of oxygen plasma power on the performance of Ga2O3 passivated GaN ultraviolet photodetectors

The role of oxygen plasma power on the extent of surface passivation of Gallium Nitride (GaN) epitaxial layers and the photo-response of Au/Ni/GaN Schottky detectors is investigated. The surface morphology is found to be preserved in oxide passivated GaN at intermediate plasma power which significantly deteriorates at higher power. The photoluminescence spectrum shows a fall in the intensity of band edge and defects related yellow luminescence at higher power indicating the generation of non-radiative recombination centres. The chemical and electronic structures of the passivated layer investigated by x-ray photoelectron spectroscopy revealed the formation of stoichiometric Ga2O3 up to intermediate plasma power and partial decomposition of Ga2O3 along with formation of gallium sub-oxides at higher power. This leads to an enhancement in the responsivity of the photodetector after passivation at an intermediate plasma power followed by a considerable reduction at higher power. From the electrical transport measurement across the Au/Ni/GaN Schottky diode, it is observed that the diode manifests more Schottky (Ohmic) character after oxygen plasma treatment at intermediate (higher) power. The present investigations provide a clear guideline on the selection of oxygen plasma power for surface passivation on GaN, which is highly beneficial in optimizing the performance of GaN-based optoelectronic and power devices.

Vertical GaN PIN Structure with Intrinsic AlGaN Drift Layer Grown Using Metal‐Organic Chemical Vapor Deposition

In this study, it is aimed to examine the DC characteristics of a vertical PIN diode featuring AlGaN as the drift layers. In the investigation, observing the changes in DC characteristics with the variation of Al content (0%–4%) in the drift layers as well as its thickness is focused on. By applying AlGaN as the drift layer, the semi‐insulating properties are augmented and the detrimental background carbon impurity is reduced, leading to a reduction in the leakage current and an improvement in the breakdown voltage characteristics of the device. Room‐temperature and low‐temperature photoluminescence measurements confirm a reduction in the intensity of both yellow luminescence and blue luminescence, which can be attributed to the decrease in defect concentration. Furthermore, to minimize the occurrence of premature breakdown voltage, p‐type GaN is meticulously etched into a bevel shape, and subsequent confirmation through the utilization of a scanning electron microscope reveals an inclination of 13°. The outcome of the analysis on the DC characteristic indicates an improvement in reverse‐bias characteristics with the increase in the Al ratio in the drift layer for the 2 μm series. Furthermore, for the 20 μm Al0.04Ga0.96N, a breakdown voltage of 2540 V is recorded.

Investigation of degradation mechanism in GaN-based blue and ultraviolet laser diodes

We have studied the aging-induced degradation effect and the related mechanism of blue and ultraviolet (UV) laser diodes (LDs). First of all, the F parameter value, leakage current, and yellow luminescence intensity of LDs all increase after 24 h of the aging process, indicating that one of the reasons for the degradation of UV LDs may be the increase of the non-radiative recombination center in the material. Second, irreversible damage may be found on the front cavity surface of the UV and blue LDs. Due to the large UV photon energy, water molecules in the environment atmosphere are ionized to form OH− ions, which combine with dust in air to form SiO2 sediments and then attach to the front cavity surface. In addition, a large photon energy may cause damage to the anti-reflection film on the front cavity surface and lead to a too-high local temperature near the cavity surface, resulting in molten Ga droplets. Both sediment and the precipitation of molten GaN on the cavity surface will directly affect the function of the front cavity surface and the output power of the LD. In order to improve the reliability of the GaN-based UV LDs, it is necessary to reduce the density of material defects, select more stable coating materials on cavity facets, and improve the sealing property of the device package.

Hydrogen iodide (HI) neutral beam etching characteristics of InGaN and GaN for micro-LED fabrication

We investigated the etching characteristics of hydrogen iodide (HI) neutral beam etching (NBE) of GaN and InGaN and compared with Cl2 NBE. We showed the advantages of HI NBE versus Cl2 NBE, namely: higher InGaN etch rate, better surface smoothness, and significantly reduced etching residues. Moreover, HI NBE was suppressed of yellow luminescence compared with Cl2 plasma. InCl x is a product of Cl2 NBE. It does not evaporate and remains on the surface as a residue, resulting in a low InGaN etching rate. We found that HI NBE has a higher reactivity with In resulting in InGaN etch rates up to 6.3 nm min−1, and low activation energy for InGaN of approximately 0.015 eV, and a thinner reaction layer than Cl2 NBE due to high volatility of In-I compounds. HI NBE resulted in smoother etching surface with a root mean square average (rms) of 2.9 nm of HI NBE than Cl2 NBE (rms: 4.3 nm) with controlled etching residue. Moreover, the defect generation was suppressed in HI NBE compared to Cl2 plasma, as indicated by lower yellow luminescence intensity increase after etching. Therefore, HI NBE is potentially useful for high throughput fabrication of μLEDs.

Quantitative analysis of carbon impurity concentrations in GaN epilayers by cathodoluminescence

In this work, a technique for quantifying carbon doping concentrations in GaN:C/AlGaN buffer structures using cathodoluminescence (CL) is presented. The method stems from the knowledge that the blue and yellow luminescence intensity in CL spectra of GaN varies with the carbon doping concentration. By calculating the blue and yellow luminescence peak intensities normalised to the peak GaN near-band-edge intensity for GaN layers of known carbon concentrations, calibration curves that show the change in normalised blue and yellow luminescence intensity with carbon concentration in the 1016 − 1019 cm−3 range were derived at both room temperature and 10 K. The utility of such calibration curves was then examined by testing against an unknown sample containing multiple carbon-doped GaN layers. The results obtained from CL using the normalised blue luminescence calibration curves are in close agreement with those from secondary-ion mass spectroscopy (SIMS). However,the method fails when applying calibration curves obtained from the normalised yellow luminescence likely due to the influence of native VGa defects acting in this luminescence region. Although this work shows that indeed CL can be used as a quantitative tool to measure carbon doping concentrations in GaN:C, it is noted that the intrinsic broadening effects innate to CL can make it difficult to differentiate between the intensity variations in thin ( < 500 nm) multilayered GaN:C structures such as the ones studied in this work.

Yellow luminescence and carrier distribution due to polarity-dependent incorporation of carbon impurities in bulk GaN by Na flux

We have grown a high growth rate, and low dislocation density (DD) Ga-polar GaN (DD: 4 × 105/cm2) and N-polar GaN (DD: 8 × 105/cm2) bulk single crystal using Na Flux. Variable temperature PL shows that the yellow luminescence (YL) peak energy of Ga-polar GaN (Ga-GaN) and N-polar GaN (N-GaN) are 2.31–2.30 eV and 2.28–2.26 eV, respectively. It can be found that N-GaN has a more vigorous YL intensity and a lower peak energy of YL than Ga-GaN. The variable-temperature and variable-power PL results suggest that the YL origin may be a donor-acceptor pair defect. Dislocations and defects associated with gallium vacancies that could cause YL behavior were ruled out by dislocation density determination and Raman spectroscopy, respectively. The different concentrations of carbon and oxygen in the Ga-GaN and N-GaN that were associated with the adsorption energy were discussed and proved by SIMS. The Raman results show that the carrier concentrations of Ga-polar and N-polar GaN are comparable, which may be because the higher content of carbon impurities in N-polar GaN can act as a deep acceptor to compensate for its higher content of shallow donor impurity oxygen. Therefore, it is suggested that carbon-related energy level transitions are the primary source of YL in both Ga-polar and N-polar GaN, and the compensating effect of carbon impurities leads to similar carrier concentrations in both polar planes.

Effects of the diameter of thermally generated nanopits on carrier dynamics in AlGaN/GaN heterostructures

The size and density of nanopits, generated at the surface of their top layer, strongly affect the electrical and optical properties of AlGaN-based structures. Therefore, the control of the layer quality evolution as a function of the nanopits size/density is a crucial issue to enhance the device performance. In this paper, the effects of the nanopits diameter observed at the surface of AlGaN on carrier dynamics are systematically investigated. The variation of nanopits diam-eter is achieved through thermal annealing of a set of AlGaN/GaN heterostructures at different temperatures. The samples are characterized using the scanning electron microscope (SEM), energy-dispersive x-ray, high-resolution x-ray diffraction, photoluminescence (PL), and time-resolved PL spectroscopies. SEM images have revealed an increase in the nanopits diameter with increasing annealing temperature. In addition, we observed a linear development in the yellow luminescence intensity, accompanied by a deterioration in the PL decay times due to an increase in the density of point-defect complexes that act as nonradiative recombination centers. We also performed temperature-dependent PL measurements to study the impact of the nanopits diameter on electron–phonon scattering processes. Both electron-acoustic- and electron-longitudinal optical phonon interactions enhance with increasing nanopits diameter.

Large negative thermal quenching of yellow luminescence in non-polar InGaN/GaN quantum wells

Large negative thermal quenching (NTQ) of the yellow luminescence (YL) for a temperature increase from 5 to 300 K is observed in non-polar InGaN/GaN quantum well (QW) samples due to the thermal migration of carriers from the InGaN QW layers to the GaN barrier layers for the first time. Such an unusual phenomenon happens only when the carriers are optically excited inside the QW layers, providing solid evidence for the occurrence of thermal transfer of photoexcited carriers from the QW layers to the GaN barrier layers. A simple model considering the thermal transfer of carriers is proposed to interpret the observed NTQ phenomenon. The thermal activation energy of the carriers is determined by fitting the reciprocal temperature dependence of the YL intensity in the Arrhenius plot with the model.

Impact of growth conditions on intrinsic carbon doping in GaN layers and its effect on blue and yellow luminescence

In this study, the GaN films were deposited in the multilayered structures under different growth conditions. SIMS analysis showed that intrinsic carbon incorporation around two orders can be controlled effectively by varying the growth parameters. Defects analysis by cross-sectional cathodoluminescence (X-CL) is demonstrated as a useful analytical technique for unambiguously understanding the effect of incorporation of the intrinsic carbon concentration on the yellow luminescence (YL) and blue luminescence (BL) band intensities. It was inferred from the experimental data that the observed BL and YL intensities are due to the electronic transitions through the isolated CN and CNON complex, respectively. This finding is in contrast to the theoretically published literature that two charged states of CN are responsible for both BL and YL which sought that the BL emission to be always smaller in comparison to YL emission. Our experimental data, for all different growth conditions showed the prominent BL in comparison to YL except for the growth at sufficiently lower temperature which is considered to be more conducive for higher oxygen incorporation into the stoichiometry. This increases the probability of CNON formation which is manifested in the enhancement of YL intensity over BL as the formation energy of CNON complex is lower than the CN defect. Thus, X-CL analysis method can be key to address the challenges of understanding the impurity character in multi-thin layered GaN structures.

Using Cross-Sectional Cathodoluminescence to Visualize Process-Induced Defects in GaN-Based High Electron Mobility Transistors

We performed cross-sectional cathodoluminescence (CL) for gallium nitride (GaN)-based high electron mobility transistors (HEMTs) to investigate process-induced defects. The cross-sectional CL measurements at room temperature clearly show the intensity distribution for the 25 nm-thick AlGaN layer, even though it is very thin. We also observed that the intensities of the band-edge emissions from the AlGaN and GaN layers fall near the source and drain regions. Those intensity decays demonstrate that ion implantation at the source and drain regions generates many nonradiative recombination centers and that these defects are not eliminated even by activation annealing after ion implantation. We also observed that the intensity of yellow luminescence (YL) in the GaN layer drops not only at the source and drain regions, but also in deeper regions of the 1.4 μm-thick GaN layer. We consider diffusion of the implantation-generated defects and their interactions with point defects in this deep region, which are responsible for the YL, to be the mechanism responsible for the YL intensity decrease. Ion implantation and subsequent annealing not only activates the dopant atoms, but also changes the point defects, which may affect the device characteristics. These findings show that cross-sectional CL spectral mapping can visualize the process-induced defects in GaN HEMTs and can therefore be used for process optimization and failure analysis of these devices.

Praseodymium doped YF3:Pr3+ nanoparticles as optical thermometer based on luminescence intensity ratio (LIR) – Studies in visible and NIR range

Luminescent YF3 nanoparticles doped with Pr3+ activator ion have been successfully synthesized via a facile hydrothermal approach for their application as a multi-range optical sensor of temperature. The product synthesized shows intense yellow luminescence at 457 nm laser excitation, and the emission spectra were recorded from 475 to 1650 nm, showing the presence of sharp emission peaks in the measured spectral range. The intensity of the observed emission bands typical of Pr3+ ion changes with temperature significantly. Hence, the intensity ratios of the thermalized emission bands were determined in the temperature range from 293 to 421 K, and they were calibrated for thermal sensing purposes. High relative sensitivity values, ranging from 0.6 to 1.2 %K−1, were found for the emission bands of the thermally-coupled 3P1 and 3P0 excited states located around 522–537, 586–603 and 700–723 nm. The temperature calibration based on the thermalized 3P1→3F4 and 3P0→3F4 bands located in the first biological optical window (650–950 nm), may be especially useful for potential biological applications.

Evaluation of lattice displacement and electrical property of Zn-ion implanted GaN by Rutherford backscattering spectrometry

The lattice displacement in Zn-ion implanted GaN single crystal wafer is evaluated by Rutherford backscattering spectrometry using proton beam and Photoluminescence measurements. Zn-ion implantation into GaN single crystal wafer is performed with energies of 300 KeV (ion fluence: 2.32 × 1014cm-2), 500 KeV (3.02 × 1014cm-2) and 700 KeV (4.65 × 1014cm-2) at room temperature. The displacement concentration of Ga and N atoms evaluated from minimum yields along <0 0 0 1> direction are 4.8 × 1021 cm−3 and 3.6 × 1021 cm−3 respectively, for as-implanted GaN and decrease to 2.3 × 1021 cm−3 and 4.7 × 1020 cm−3 by annealing in N2 ambient at 700 °C. The lattice displacement of Ga atoms for as-implanted GaN evaluated from the half-value angle of the channeling dip is 0.094 Å for Zn-implanted GaN and recovers to a value of the thermal vibrational amplitude by annealing. The intensity of Yellow Luminescence (YL) observed at around 530 nm for as-implanted GaN increases significantly compared to un-implanted ones, suggesting that gallium vacancy acceptor and nitrogen vacancy donor as origins of YL are induced by Zn-ion implantation. The YL intensity for annealed GaN is almost same as the one for un-implanted GaN. The sheet resistance is 1.04 × 102cm-2 Ω/sq. for un-implanted, 3.98 × 109cm-2 Ω/sq. for as-implanted, and 3.77cm-2 Ω/sq. for annealed GaN. The high resistance for as-implanted GaN is attributed to gallium vacancy as a compensation center. The low resistance and the decrease of YL intensity after annealing is attributed to both a shallow nitrogen vacancy and the decrease of gallium vacancy.

Structural evaluation of ions-implanted GaN films by photothermal deflection spectroscopy

We characterize the behavior caused by thermal annealing for C, O, Si and Mg ions implanted in GaN films by photothermal deflection spectroscopy (PDS) with respect to structural disorder and defect levels related to yellow luminescence. Although the implanted region damaged by ion bombardment is recovered by annealing, the FWHM values of ω(0002) evaluated by x-ray diffraction are almost independent of the temperature. However, the Urbach energy, as an index of structural disorder, evaluated by PDS is improved. Recovery of the structural disorder is likely to depend on the dose quantity rather than the acceleration voltage. Defect states in the band gap are reduced as well, though featured PDS signals related to the kind of implanted ion are hardly detected except for the carbon ion. The intensity of yellow luminescence at room temperature is enhanced according to the improvement of the Urbach energy and reduction of defect states in the band gap. PDS is useful for defect analysis of III-V nitride semiconductors that are electrically and optically inactive, such as ion-implantation samples, especially Mg-implanted GaN for achieving reliable p-type conduction.

The compensation role of deep defects in the electric properties of lightly Si-doped GaN

In this work, we have investigated the compensation effect of defects in lightly Si-doped GaN layer. It is found that the compensation of defects can be enhanced by increasing Si doping concentration and dislocation density in GaN layer. Subsequently, through theoretical analysis it is found that a more effective compensation is activated by increasing Si doping, which is mainly due to the enhanced effect of deep defects in the compensation role rather than by the increase of density of deep defects. Meanwhile, it is found that the yellow luminescence (YL) intensity of lightly Si-doped GaN samples increases with the increasing Si doping concentration, which can also be caused by the enhancement of compensation of deep level defects, while the increase of peak intensity of band edge luminescence with increasing Si doping is mainly caused by the increase of free carriers.

(Invited) Electrochemical Formation and Application of Porous Gallium Nitride

Gallium nitride (GaN), which is a III-V compound semiconductor, has wide direct bandgap (3.4 eV), high breakdown electric field (3.3x106 V/cm) and high saturation electron velocity (2.7 x107 cm/sec). On the basis of excellent optical and electrical properties, significant progress has been achieved in the GaN-based devices, such as ultraviolet laser diodes, white light-emitting diodes, high electron mobility transistors (HEMTs), and high-power FETs. In addition, GaN and its alloys are getting much attention as building block materials for electrochemical (EC) energy conversion systems such as chemical sensors, water splitting, and artificial photosynthesis due to their superior chemical stability, direct transition, and widely tunable bandgap by alloying. Among the various techniques for improving their conversion efficiency, porosification utilizing EC reactions is one of the most powerful because a high-density array of pores exhibits high specific surface area, low reflectance, and high absorptance properties. In addition, this technique is performed at room temperature and does not require any complicated process such as lithography, indicating lower damage and higher productivity than other nanostructure fabrication techniques such as reactive ion etching and selective-area growth. In this presentation, we review our recent work on the fabrication of porous GaN utilizing various EC techniques and their applications [1-3]. One example is the two-step process combining EC etching process with conventional chemical etching process. EC etching was first performed in the dark condition using an aqueous electrolyte consisting of H2SO4 and H3PO4. Wet chemical etching was subsequently conducted in 25 % tetramethylammonium hydroxide (TMAH) at 90°C. The porous structure formed by the first EC etching has straight pores oriented perpendicular to the top surface. Pore depth linearly increased with EC etching time, indicating that superior depth control was achieved. This behavior can be explained as resulting from anisotropy of the EC etching. Namely, the etching reaction proceeded only to the direction belong to the high-electric field induced by the anodic voltage. On the other hand, the pore diameter kept a constant value throughout the EC etching and could not be controlled by the EC etching time. However, the pore diameter could be changed by wet chemical etching utilizing TMAH as subsequent treatment after the EC etching. By applying TMAH etching, pore shape changed from a round to a hexagonal shape surrounded with 6 facets of {1−100}. In addition, the pore diameter estimated from SEM observation increased linearly with the TMAH etching time, whereas the pore depth was unchanged. These results indicate that TMAH etching proceeds anisotropically in the horizontal direction, but not in the vertical direction of the substrate. The optical and photoelectrochemical properties of porous GaN were very sensitive to the structural properties. From the photoluminescence (PL) measurements, the intensity of yellow luminescence (YL) related to high-density point defects was drastically decreased by the formation of porous structures, suggesting that gallium vacancy (V Ga) related defects were preferentially removed by EC etching. Photoreflectance measurement revealed that porous sample had an effective refractive index that could be controlled by TMAH etching time. In photoelectrochemical measurement, the incident-photon-to-current conversion efficiency (IPCE) was dramatically enhanced by the formation of porous structures. A series of experimental results were consistently explained by the change of thickness of pore wall with a width of space charge region. These results suggest that precise structural controlling is crucially important to obtain superior capability for EC energy conversion systems, and the two-step process utilizing anisotropic EC etching and TMAH etching is very promising as a nanostructure fabrication technique. Acknowledgement: This work was supported by JSPS KAKENHI - JP15K13937，JP16H06421，JP17H03224. A. Watanabe et al., ECS Electrochem. Lett., 4, H11 (2015).Y. Kumazaki et al., J. Electrochem. Soc., 164, H477 (2017).M. Toguchi et al., 233rd ECS Meeting, Z01-2550, Seattle, WA, 15 May (2018).

Intense yellow luminescence from Dy3+-doped TeO2–WO3–GeO2 glasses: structural and optical characterization

Different concentrations of Dy3+-doped oxyfluoride tellurite (TWGDy) glasses of composition (85 − x) TeO2 + 5 WO3 + 10 GeO2 + x DyF3, where x = 0, 0.1, 0.5, 1.0 and 2.0 mol% were prepared by melt quenching method. The Judd–Ofelt theory was applied to study the luminescence properties. Three phenomenological Judd–Ofelt (Ωλ=2,4,6) intensity parameters were obtained as Ω2 = 4.90 × 10−20 cm2, Ω4 = 1.73 × 10−20 cm2 and Ω6 = 1.48 × 10−20 cm2 for 0.5 mol% of Dy3+-doped TWGDy glass. When excited at different wavelengths, the TWGDy glasses exhibit three emission bands due to 4F9/2 → 6H15/2 (blue), 4F9/2 → 6H13/2 (yellow) and 4F9/2 → 6H11/2 (red) transitions. The excitation wavelength was optimized as 453 nm for strong and proficient luminescence from TWGDy glasses. Beyond 0.5 mol% of Dy3+ concentration, the TWGDy glasses show luminescence quenching. The laser characteristic parameters such as effective bandwidth, stimulated emission cross-section, gain bandwidth and optical gain were determined using the emission spectra and radiative parameters. The 0.5 mol% of Dy3+-doped TWGDy glass could be potential to design a new class of yellow laser material.

Deep-level traps in lightly Si-doped n-GaN on free-standing m-oriented GaN substrates

In this study, we investigated the deep-level traps in Si-doped GaN epitaxial layers by metal-organic chemical vapor deposition on c-oriented and m-oriented free-standing GaN substrates. The c-oriented and m-oriented epitaxial layers, grown at a temperature of 1000 °C and V/III ratio of 1000, contained carbon atomic concentrations of 1.7×1016 and 4.0×1015 cm–3, respectively. A hole trap was observed at about 0.89 eV above the valence band maximum by minority carrier transient spectroscopy. The trap concentrations in the c-oriented and m-oriented GaN epitaxial layers were consistent with the carbon atomic concentrations from secondary ion mass spectroscopy and the yellow luminescence intensity at 2.21 eV from photoluminescence. The trap concentrations in the m-oriented GaN epitaxial layers were lower than those in the c-oriented GaN. Two electron traps, 0.24 and 0.61 eV below the conduction band (EC) minimum, were observed in the c-oriented GaN epitaxial layer. In contrast, the m-oriented GaN epitaxial layer was free from the electron trap at EC – 0.24 eV, and the trap concentration at EC – 0.61 eV in the m-oriented GaN epitaxial layer was lower than that in the c-oriented GaN epitaxial layer. The m-oriented GaN epitaxial layer exhibited fewer hole and electron traps compared to the c-oriented GaN epitaxial layers.

Influence of highly-charged 209Bi33+ irradiation on structure and optoelectric characteristics of GaN epilayer

The microstructure and optoelectric properties of GaN epilayer irradiated by highly-charged 209Bi33+ to different fluences are investigated by means of atomic force microscopy, X-ray photoelectron spectroscopy, Raman scattering spectroscopy and photoluminescence spectroscopy. After Bi33+ irradiation, AFM observation shows the irradiated GaN surface is a swelling and swelling rate nonlinearly increases with increasing ions fluence. XPS analysis reveals the relative content of Ga-N bond reduces and Ga-O, Ga-Ga bonds have been produced as the fluence increases. Raman scattering spectra display the thickness of surface depletion layer increases, free carrier concentration and its mobility decrease generally with an increase in ions fluence. Furthermore, the length of Ga-N bond shortens and lattices experience compressive stress with increasing ions fluence are observed from Raman spectra. Room temperature PL spectra reflect the intensity of yellow luminescence (YL) emission increases and its peak has a blueshift after 1.061×1012 Bi33+/cm2 irradiation. Moreover, as the temperature rises, the thermal quenching of YL occurs and its peak position first exhibits a blueshift and then a redshift. Results may be served as a useful reference for HCI to be used in semiconductor fields.

Structure-related optical behavior of nanoscale GaN island, tip, tube and cone arrays formed by inductively coupled plasmas etching

We introduce a one-step self-assembled technique to form various nanostructures on N-type GaN film and then present an optical characterization of a series of low-dimensional GaN nanostructures by using low-temperature photoluminescence (LTPL) spectroscopy. Nanoscale GaN island, tip, tube and cone-like structures of diameters ranging from 50 to 190 nm were self-assembled on a c-axis perpendicular to substrate surface with uniform diameter and uniform length by inductively coupled plasmas etching process without lithography. Optical LTPL measurements on nanostructures show consistent variations in the properties of the fabricated GaN structures as a function of surface area of GaN nanostructures. LTPL mapping gives an evidence for defect-induced donors and/or acceptors near the facets of the ICP-etched nanostructure. Our results indicate that a higher concentration of donor-related defects is introduced on the surface of GaN nanotubes. In particular, the nanotubes sample exhibits a conspicuous increased in yellow luminescence intensity compared to the other nanostructure samples. This result may support more information for the application of nanotubes on nanogenerators.

Effect of light Si doping on the properties of GaN

An obvious increase in electron mobility and yellow luminescence (YL) band intensity was found in light Si doping GaN. For a series of GaN samples with different doping concentration, the dislocation density is almost the same. It is inferred that the abrupt increase in mobility and YL intensity does not originate from the change of dislocation density. The mobility behavior is attributed to the screening of scattering by dislocation and increase of ionized impurity scattering with the increase of Si doping concentration. At lower doping level, the screening of dislocation scattering is dominant, which results in the increase in carrier mobility. At higher doping level, the increase in ionized impurity scattering leads to the decrease in carrier mobility. Higher mobility causes longer diffusion length of nonequilibrium carrier. More dislocations will participate in the recombination process which induces stronger YL intensity in light Si doping GaN.