Photoluminescence Oscillations in LEDs Arise from Cylinder-like

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Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

Photoluminescence Oscillations in LEDs Arise from Cylinder-like Nanostructures Fabricated by a Femtosecond Laser Yuduo Xie, Jingya Sun,* Lan Jiang, Qingsong Wang, Feifei Wang, and Changji Pan Laser Micro/Nano-Fabrication Laboratory, School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China

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S Supporting Information *

ABSTRACT: Fabry−Pérot interference has been employed in optical fibers, sensors, electron optics, and so on. The photoluminescence (PL) spectra of a gallium nitride (GaN)-based light-emitting diode (LED), which were irradiated using various femtosecond (fs)-laser fluences, were studied. Notably, the interesting oscillation of PL was observed on the LED under irradiation with specific fs-laser fluence. The PL oscillation spectra were well fitted by the product of modulation and unperturbed luminescence. This may be attributed to surface cylinder-like nanostructures. The decrease of PL intensity was caused by the defect created through fs-laser irradiation. In addition, the film thickness and the refractive index could be calculated based on the data extracted from the PL oscillation spectra. Our report demonstrated that the optical properties of a GaNbased LED were engineered based on the surface morphology and modulated by Fabry−Pérot interference.

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in optoelectronic devices, can be obtained. Quantitative fitting shows an excellent agreement with the measured PL spectra, which indicates that the optical properties of the GaN-based LED can be modulated by FP interference.

INTRODUCTION In recent years, GaN-based light-emitting diodes (LEDs) have attracted considerable attention due to their extensive application in optoelectronic devices (e.g. energy-saving lamps, LED panels, and medical cosmetology). The optical properties of GaN-based LEDs are widely reported in numerous investigations of photoluminescence (PL) spectra.1−3 A notably interesting Fabry−Pérot interference (FPI) phenomenon observed in PL spectra has been reported in many materials (e.g., metals,4 graphene,5 GaN,6 aluminum oxide,7 and two-dimensional materials8). For instance, Huang et al.9 discovered the FPI phenomenon through the alteration of surface roughness. Yi et al.10 deposited GaN microrods on graphene, which showed obvious PL oscillatory spectra. This interference phenomenon can be used for improving the sensitivity of sensors.11 However, FPI on GaN-based LEDs irradiated using femtosecond (fs) lasers has seldom been studied. Compared with various methods to change the surface morphology, such as natural lithography,12 wet etching,13 and others, fs-laser manufacture has potential advantages in the change of the surface morphology because of its flexibility, high accuracy, and reduced heat-effect zone.14−16 In this experiment, we studied the effect of changing the surface morphology through fs-laser irradiation on a GaNbased LED. An interesting oscillation phenomenon is found from the PL spectra of the GaN-based LED. This oscillation can be attributed to the surface cylinder-like nanostructures formed by fs-laser irradiation. The decrease in PL intensity is caused by the defect that is created by fs-laser irradiation. After analysis of the interference phenomenon, useful information (film thickness, refractive index17−19), which has a crucial role © XXXX American Chemical Society

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METHODS In this study, a blue-light-emitting 2 in. GaN epitaxial wafer was used as the sample, which was purchased from Focus Lightings tech co, Ltd. As shown in Figure 1, the sample was irradiated by a regenerative amplified Ti:sapphire femtosecond-laser system (Spitfire, Spectra-Physics Inc.) with a central wavelength of 800 nm, pulse duration of 50 fs, and repetition rate of 1 kHz. A neutral density attenuator was used to control the laser energy. A plano-convex lens (f ∼ 100 mm) was used for focusing the laser beam on the sample that was placed on a high-precision six-axis translation stage (M840.5DG, PI, Inc.) with a resolution of 1 μm in the x- and ydirection and 0.5 μm in the z-direction. The process of laser irradiation could be in situ monitored by a CCD. This experiment was conducted at room temperature in air. Ultimately, laser fluences of 40.6, 44.3, 48.0, 73.8, and 92.3 mJ/cm2, a scanning speed of 500 μm/s, and a scanning interval of 1 (or 2) μm were selected to study the interference phenomenon. The fluences used here were obviously smaller than the threshold value of GaN (∼512.0 mJ/cm2 for single pulse20). Thus, the laser power used in the study did not cause considerable material removal in the process of this sample. Received: April 26, 2019 Revised: June 6, 2019 Published: June 24, 2019 A

DOI: 10.1021/acs.jpcc.9b03939 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. Schematic of the setup for the femtosecond-laser process.

To investigate the effect of surface morphology on the formation of an interesting oscillation phenomenon, an optical microscope (BX51, Olympus Inc.), a coaxial laser confocal microscope (VK-X200, Keyence), a scanning electron microscope (SEM) (SU8220, Hitachi), and an atomic force microscope (AFM) (Bruker) were used to characterize the surface morphology of the nonirradiated sample and the laserirradiated samples at 73.8 mJ/cm2. Comparing the optical micrographs of Figure 3a (nonirradiated sample) and Figure 3b (the sample irradiated at the fs-laser fluence of 73.8 mJ/ cm2), the surface morphology was changed from the polished smooth areas to irregular rough areas. As shown in Figure 3c,d, the confocal microscope and its corresponding mapping results revealed that the irregular rough areas caused by fs laser irradiation included irregular nanostructures. Compared with the SEM micrograph (Figure 3e) of the nonirradiated sample, the morphology of these irregular nanostructures was a cylinder-like structure, which can be seen in the SEM image (Figure 3f). AFM micrographs are used to display an intuitive morphology of the nonirradiated sample (Figure 3g) and the irradiated sample (Figure 3h). The height of the cylinder-like nanostructure was about 30 nm after laser irradiation, which was confirmed by the AFM measurement in Figure 3h. Thus, after laser irradiation, the surface morphology was altered from smooth to cylinder-like structures. More optical and AFM micrographs are available in Figures S1−S3 of the Supporting Information. The schematic diagram of the PL interference is shown in Figure 3i, which could be attributed to the optical path difference. To further analyze the origin of this oscillation, the PL spectrum of the sample at 73.8 mJ/cm2, which demonstrates the most obvious oscillation phenomenon, is selected for comparison with the nonirradiated sample’s PL spectra. As shown in Figure 4a, the PL spectrum of the nonirradiated sample is obtained from the black line (FWHM ∼ 17.6 nm). It could be seen that there are two subbands, which locates at

After femtosecond-laser irradiation, the sample was rinsed with ethyl alcohol and deionized water in an ultrasonic bath for approximately 10 min, respectively.

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RESULTS AND DISCUSSION Room-temperature PL spectra which were excited using a He− Cd laser (325 nm) with an exciting power of 0.13 mW were measured in this experiment. All six PL spectra exhibited two main peaks. As depicted in Figure 2a−e, the positions of two

Figure 2. PL spectra with diverse laser-irradiated fluences at (a) the nonirradiated sample, (b) 40.6 mJ/cm2, (c) 44.3 mJ/cm2, (d) 48.0 mJ/cm2, (e) 73.8 mJ/cm2, and (f) 92.3 mJ/cm2.

peaks remained basically unchanged and the intensity of the PL spectra decreased with the increase of laser fluence. As illustrated in Figure 2e, a symmetrically interesting oscillation phenomenon was observed at 73.8 mJ/cm2. Figure 2f shows that interesting oscillation disappeared when the laser fluence was up to 92.3 mJ/cm2. B


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Figure 3. Characterization of the nonirradiated and the laser-irradiated samples at 73.8 mJ/cm2: (a) optical micrograph of the nonirradiated sample; (b) optical micrograph of the laser-irradiated sample at 73.8 mJ/cm2; (c) confocal micrograph of the laser-irradiated sample at 73.8 mJ/ cm2; (d) three-dimensional topography mapping of the laser-irradiated sample at 73.8 mJ/cm2; (e) SEM image of the nonirradiated sample; (f) SEM image of the laser-irradiated sample at 73.8 mJ/cm2; (g) AFM image of the nonirradiated sample; (h) AFM image of the laser-irradiated sample at 73.8 mJ/cm2; and (i) the schematic diagram of the PL interference.

Figure 4. (a) PL spectra of the nonirradiated sample (black) and the sample irradiated at 73.8 mJ/cm2 (red), which are excited by a He−Cd laser with the excitation energy of 0.13 mW. The enlarged details of the first and the second peak are presented in (b) and (c), respectively.

Thus, we assume that the surface morphology is the primary reason for this interference. As shown in Figure 3h, cylinderlike nanostructures are fabricated to form the optical path difference at the laser fluence of 73.8 mJ/cm2. The schematic diagram of the PL interference is shown in Figure 3i. The oscillation disappears with the increase of laser fluence because the processed structures of the GaN-based LED are destroyed. For simplification, only the air/GaN/sapphire layers are considered in this model. The thickness can be obtained using the relations between peak numbers and n(λpeak)/λpeak, where λpeak can be obtained from Figure 4b,c, and n(λ) is the Cauchy dispersion formula of GaN. Through the analysis of the relations mentioned above and shown in Figure 5a,26 the inverse of slope d obtained from eq 1 represents the film thickness that is 4592 nm in this calculation. The calculated data agree well with the actual value provided by the company. The schematics of the GaN-based LED and peak order are sequentially shown in the inset of the top left corner and lower right corner of Figure 5b, respectively.

2.74 eV (453 nm) and 3.09 eV (401 nm). The maximum peak located at 2.74 eV might originate from the donor−acceptor pair recombination. The second sharp peak, 3.09 eV, might be from conduction band-Mg acceptor transition,21−23 which is caused by the doping of Mg on the top layer of the LED. The PL spectrum of the irradiated sample with the laser fluence at 73.8 mJ/cm2 is presented by the red line in Figure 4a. By comparing with the PL spectrum of the nonirradiated sample, it could be found that the peak positions remain the same and the emission intensity ratio of the two peaks is basically unchanged. This obvious interference can be attributed to many reasons, such as the sharp contrast of the refraction index,24 multiple reflection of light between the layers,25 and the difference of the mean surface morphology.9 The reflection and refraction between the different layers in the sample can be excluded, because the PL interference cannot be found in the nonirradiated sample. With the increase of laser fluence, the mean surface roughness increases through the characterization of AFM, whereas the PL oscillatory phenomenon fades away. C


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Figure 5. (a) Calculation of the Fabry−Pérot peak positions based on eq 1; the schematic of the LED is shown in the left corner, and the peak order is marked in the right corner; (b) the undisturbed luminescence spectra B(λ) (blue line), the modulated interference (green line), the calculated PL spectra (red line), and the PL spectra of the irradiated sample with laser fluence at 73.8 mJ/cm2 (black line).

i

2n(λpeak ) 1 = λpeak d

this oscillating phenomenon is considered as the Fabry−Pérot interference phenomenon.

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(1)

CONCLUSIONS In conclusion, an interesting symmetric PL spectrum oscillation appears at the fluence of 73.8 mJ/cm2. This is attributed to the surface cylinder-like nanostructures created by femtosecond-laser irradiation. With the increase in laser fluence, the Fabry−Pérot interference phenomenon fades away, which can be due to that the processed cylinder-like structures are destroyed. The quench of PL spectra occurs because of the increment of defects induced by fs-laser irradiation. Through the analysis of the FPI phenomenon, the film thickness can be calculated and the results show a good agreement with the actual value provided by the company. Furthermore, the calculated spectra fits well with the PL spectrum of the sample irradiated at 73.8 mJ/cm2, which is accompanied by an interesting FPI phenomenon. Our results show that the optical properties of GaN-based LEDs can be modulated by Fabry−Pérot interference based on surface morphology.

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where n(λ) is the Cauchy dispersion formula of GaN, which is described by eq 2, and λpeak is the peak wavelength. A1

n(λ) = A 0 +

λ

2

+

A2 λ4

(2)

where A0, A1, and A2 are the coefficients of the Cauchy dispersion formula. Refractive index is an important parameter in manufacturing optoelectronic devices, which can be obtained from the relations between the interval of the energy/wavelength and the film thickness. According to eq 3,28 the refractive index of the film n ∼2.395 is obtained. λ12

Δλ =

(

dn

2d n−λ1 dλ

)

(3)

where the interval of the energy/wavelength Δλ (∼7.65 nm) is the average measured value acquired from the red line in Figure 4a, λ1 is the wavelength received from the detector, dn/ dλ is the dispersion formula calculated from eq 2, and the film thickness d is obtained from eq 1. Figure 5b shows that the calculated spectra I(λ, θ) (red line) using the film thickness and the refractive index mentioned above are in accordance with the PL spectra of the irradiated sample at 73.8 mJ/cm2 (black line). According to eq 4,26 I(λ, θ) includes the interference modulation (green line) shown in Figure 5b, which is characterized by eqs 5 and 6, and the undisturbed luminescence spectra B(λ) (blue line), which are expressed by the Gaussian spectrum, are shown in Figure 5b. I (λ , θ ) = M (λ , θ ) B ( λ ) M(λ1) =

λ1* =

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b03939. Optical micrographs and AFM micrographs of the samples (Figures S1−S3) (PDF)

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λ vac n(λ)

AUTHOR INFORMATION

Corresponding Author

(4)

*E-mail: [email protected].

2 1 + R 23 + 2R 23 cos(4πd /λ1*) 2 2 1 + R12 R 23 − 2R12R 23 cos(4πd /λ1*)

ASSOCIATED CONTENT

ORCID

Jingya Sun: 0000-0003-1884-6592 Lan Jiang: 0000-0003-0488-1987

(5)

Notes

The authors declare no competing financial interest.

(6)

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where λ is the excited wavelength (λ ∼ 453 nm), θ is the degree between the incident light and the detector (θ ∼ 0°), n(λ) is the Cauchy dispersion formula of GaN, and the refractive index of sapphire is acquired from ref 29. The fitting results are in good agreement with the measured PL spectra, so

ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (Grant No. 11704028) and National Key R&D Program of China (Grant No. 2017YFB1104300). D


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Photoluminescence Oscillations in LEDs Arise from Cylinder-like

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