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We evaluated the effects of grid patterns (GPs) realized on 2-inch sapphire substrates by simple laser treatment on the device characteristics of InGaN/GaN light-emitting diodes (LEDs). The degrees of wafer bowing for the LEDs with distances between the GPs of 1 (GP1-LED), 2 (GP2-LED), and 3 mm (GP3-LED) were 100.05, 100.43, and 101.59 µm, respectively, which are significantly improved compared to that (108.06 µm) of a conventional LED (C-LED) without GPs. Consequently, a blue-shift of the emission wavelength for the GP-LEDs was observed compared to the C-LED via alleviation of the quantum-confined stark effect. A comparative study of the fluorescence microscopy images of the C-LED and GP2-LED samples showed a significant reduction of threading dislocations as a result of the GPs. In the electroluminescence mapping results for the entire 2-inch region, the standard deviations of the emission wavelengths were 1.64, 1.49, and 2.55 nm for the GP1-LED, GP2-LED, and GP3-LED samples, respectively, which are smaller than that of the C-LED (2.66 nm). In addition, the average output power of the GP2-LED was 8.5% higher than that of the C-LED.
© 2016 Optical Society of America
1. Introduction
Generally, III-nitride thin films can be applied to optical devices that emit light in the visible wavelength region. Also, electronic elements for high-power, high-temperature, and high-frequency applications such as automobile engines and power distribution systems have been actively studied [1–3]. GaN-based epitaxial layers are normally grown on sapphire or silicon substrates. However, when growing GaN epitaxial layers on a sapphire substrate, there is inherent bowing largely due to the difference of the lattice constants and thermal expansion coefficients between the epitaxial layers and the substrate [4,5]. Wafer bowing may cause problems for some automatic handling tools in the production line for III-nitride devices, including InGaN/GaN light-emitting diodes (LEDs). The quality of various fabrication process steps such as lithography is surely influenced by wafer bowing, consequently affecting the reliability of LED devices [6,7]. In addition, since the uniformity of the growth temperature for an InGaN/GaN quantum well (QW) across the wafer is surely affected by wafer bowing, the bowing should be well-controlled to obtain uniform device characteristics of LEDs [8,9]. One of the most effective ways to reduce the bowing phenomenon of GaN-based epitaxial layers is to use relatively thick sapphire substrates. Amour et al. compared the bowing characteristics of a bulk n-GaN layer with respect to the diameter and thickness of the sapphire substrate [10]. Also, they improved the wavelength uniformity of InGaN/GaN LEDs by increasing the temperature uniformity of a wafer, accomplished by changing the design of wafer carrier pockets. Huang et al. and Ryu et al. achieved a significant reduction of wafer bowing by using dot air-bridged structures and SiO2 patterns formed by a patterning process on a GaN template, respectively [11,12]. Sakai et al. inserted superlattice inter-layers to reduce the strain originated from the GaN layer to reduce wafer bowing [13]. However, most recent approaches require a new design of susceptor pockets and complicated epitaxial growth.
Recently, Aida et al. controlled bowing of the substrate itself by adopting laser treatments on sapphire and Si substrates [14]. As a result, the bowing of GaN-on-sapphire and GaN-on-Si substrates with laser-treated patterns was significantly reduced. They also mentioned that the laser-treated patterns give a larger flexibility in the design engineering of epitaxial and device-fabrication processes. However, there have been no reports on the device performances of InGaN/GaN LEDs fabricated on laser-treated structures.
In this paper, we report the effects of grid patterns (GPs) formed on sapphire substrates realized by laser treatment on the optical and electrical properties of InGaN/GaN LEDs. More specifically, the effects of the bowing characteristics of the epitaxial layers depending on the GP conditions on the performances of LED devices are discussed. Via statistical investigation, the device characteristics of InGaN/GaN LEDs over the entire wafer were measured by photoluminescence (PL) and electroluminescence (EL) mapping methods.
2. Experiment
The GPs on c-plane 2-inch sapphire substrates with a thickness of 430 μm were realized by illuminating a focused krypton fluoride (KrF) laser with the pulse duration of 25 ns. Figure 1(a) shows the schematics of the GPs formed on a substrate by using a focused laser beam. A KrF laser with a wavelength of 248 nm and an output power of 30 mJ was illuminated at a speed of 40 mm/s to define the GPs on sapphire substrates. If we only consider the wavelength of the KrF laser and the energy bandgap of sapphire (above 9 eV), laser beam passes through the sapphire substrate. However, the energy of the focused KrF laser within very short pulse durations can offer a sufficiently significant influence on sapphire substrates. The very short time scale of laser pulses increases the probability of multiple-photon absorption (nonlinear process) [15–17]. That is, the short-pulse laser can simultaneously absorb the energy from multiple photons to exceed the band-gap occurrence. As a result, sapphire surface can be locally modified or patterned by a much lower energy laser [16,17]. Chang et al. reported that the laser patterning process on sapphire substrates by using a laser with the wavelength of 517 nm [16]. Also, Aida et al. reported that the change of the crystallinity of sapphire substrates by illuminating an internally-focused laser with the wavelength of 1045 nm [14,17,18]. Similarly, the GPs on sapphire substrates in this work could be formed by a focused KrF laser with short pulse duration. Figure 1(b) shows a field-emission scanning electron microscopy image of the sapphire substrate after the laser treatment, where the width of the GP was measured to be 3 μm. GPs with inter-distances of 1 (GP1-LED), 2 (GP2-LED), and 3 (GP3-LED) mm between adjacent GPs were fabricated. After realizing the GPs on a substrate, the InGaN/GaN LED structures were grown by Thomas-Swan metal-organic chemical vapor deposition. The LED structures consisted of a GaN buffer layer, Si-doped n-GaN, a 5-period InGaN/GaN multiple QW (MQW) region, and a p-GaN cladding layer. A conventional InGaN/GaN LED (C-LED) without GPs was prepared as a reference. Figure 1(c) shows the amount of bowing for the 2-inch LED wafers measured by an automatic EtaMax PLATO model PL mapping system. The amount of bowing for the GP1-LED, GP2-LED, and GP3-LED samples was measured to be 100.05, 100.43, and 101.59 μm, respectively, which were significantly improved compared to that of the C-LED (108.06 μm). If the laser treatment is carried out on sapphire surface, the crystallinity for the patterned lines should be modified, resulting in a change of the initial conditions for subsequent epitaxial layers. More specifically, the line region subjected to laser treatment was changed to an amorphous structure leading to volume expansion [17,19,20]. The periodic volume expansion depending on the GP structure on a substrate provides a different amount of strain, resulting in a change of the curvature of the surface area of the sapphire substrate.
Fig. 1 (a) Schematics and (b) FE-SEM image of the GP realized by laser processing on a sapphire substrate. (c) The amount of wafer bowing for the LED samples, where the solid line is only guide for the eyes.
In order to assess information related to the local carrier diffusion, surface inhomogeneity, and defect distribution, we evaluated optical images of the InGaN/GaN LEDs obtained by fluorescence microscopy. For the fluorescence microscopy measurements, an Hg lamp with a wavelength 405 nm was used as an excitation source. The fluorescent images were obtained by a complementary metal–oxide–semiconductor camera. For the PL mapping measurements, a He-Cd laser with a wavelength of 375 nm was used as an excitation source. The peak wavelength and intensity for the LED samples were evaluated for the entire 2-inch substrate surface. For the EL measurements, the LED samples were fabricated by using standard photolithography and dry etching techniques with a die size of 600 x 600 μm2. Indium tin oxide with a thickness of 230 nm was deposited as a transparent conducting layer whereas Cr and Au were deposited on p-GaN and n-GaN as p-type and n-type electrodes, respectively.
3. Results and discussion
Figure 2 shows fluorescence microscopy images of the LED samples. As shown in the image of the C-LED in Fig. 2(a), numerous dark spots and features corresponding to threading dislocations were randomly observed.
Fig. 2 Fluorescence microscopy images of the (a) C-LED and (b) GP2-LED samples, where the inset is the low-magnified image.
On the other hand, the dark areas in the image of the GP2-LED in Fig. 2(b) were drastically decreased compared to the C-LED. In addition, a dark line corresponding to the GP is clearly seen. This indicates that the propagation rate of the threading dislocation is relatively high above the GP. The spatial variation in the fluorescent image of the GP2-LED is directly related to the reduction of dislocations through the MQW layer due to the difference of the growth behavior between the crystalline and amorphous sapphire surfaces. In the previous reports on LEDs formed on the patterned sapphire substrate [21–23], the dislocation was initially formed at the boundary between the amorphous patterns and the planar region. The dislocations then converged from the boundary to the top of the pattern region by staircase-upward propagation [24,25]. As a result, the dislocations were mostly observed above the top of the patterns. In this work, the dislocations initially formed at the interface between an epitaxial layer and a substrate could be similarly reduced via amorphous-type GPs.
Figure 3 shows the PL mapping images of the wavelength for the (a) C-LED, (b) GP1-LED, (c) GP2-LED, and (d) GP3-LED samples, respectively, measured at room temperature (RT). In the PL mapping process for each sample, the total number of measured points was 350. The average PL wavelengths for the GP1-LED, GP2-LED, and GP3-LED samples were measured to be 439, 441, and 441 nm, respectively. In order to clearly distinguish the spatial distributions of the wavelengths for the LED samples, we used different colors. The emission wavelengths for all of the GP-LEDs were blue-shifted compared to that of the C-LED (445 nm), which is mainly due to the alleviation of the quantum-confined stark effect (QCSE) due to strain relaxation related to the decrease of wafer bowing. Figure 3(e) shows the summary of the wavelength distributions with respect to the average values for the LED samples. The PL peaks of the C-LED sample were distributed from 438 to 446 nm. For the GP2-LED sample, the PL peaks were observed at wavelengths ranging from 439 to 444 nm, which is relatively narrower than that of the C-LED. The GP1-LED and GP3-LED samples also showed relatively narrow emission distributions compared to that of the C-LED. The improvement of the uniformity of the emission wavelength for the GP-LEDs is closely associated with the increase of uniformity of the temperature distribution over the entire wafer due to the reduction of wafer bowing. As shown in Fig. 3(f), the average PL intensities for the GP1-LED, GP2-LED, and GP3-LED samples were relatively stronger than that of the C-LED. This result can be again explained by the alleviation of the QCSE due to the existence of the GP structures. That is, the overlap integral between the electron and hole wave functions was enhanced because of strain relaxation, resulting in an increase of the PL intensity for the GP-LED samples. In particular, the average intensity of the GP2-LED is 6.2% higher than that of the C-LED. However, although wafer bowing for the GP1-LED sample is relatively low compared to that of the C-LED, the average PL intensity is similar to that of the C-LED. This may be due to the unintentional influence of the GPs with inter-distances which are too short. That is, the GP lines with an amorphous type can be worked as defects, resulting in sources of non-radiative recombination of carriers. Even though there are also GP lines in the GP2-LED sample, the total amorphous area over the entire wafer is reduced by half compared to the GP1-LED sample. As a result, an increase of the PL intensity was observed. For the GP3-LED sample, the average PL intensity was slightly decreased, which can be explained by the slight increase of the bowing parameter, as shown in Fig. 1(c).
Fig. 3 Mapping images for PL wavelength of the (a) C-LED, (b) GP1-LED, (c) GP2-LED, and (d) GP3-LED, where the region with the dimension of 5 mm from edge was excluded. Summary on (e) distribution of PL wavelength and (f) average PL intensity for LED samples, where the solid lines are only guides for the eyes.
Figure 4 shows the EL mapping images of the wavelength distributions of the (a) C-LED, (b) GP1-LED, (c) GP2-LED, and (d) GP3-LED samples obtained at an injection current of 20 mA, where the number of LED chips for each sample was 4,400. The distributions of the EL wavelengths for the LED samples showed a similar trend as the PL results. The average wavelengths for the C-LED, GP1-LED, GP2-LED, and GP3-LED samples were measured to be 445, 439, 442, and 444 nm, respectively. The emission wavelengths for all of the GP-LEDs were blue-shifted compared to that of the C-LED, which can be explained in the same way as the PL discussion. The standard deviations (SD) of the wavelengths with respect to the main peak for the LED samples were evaluated by using the following equation:
Where is each EL emission wavelength value, is the average value of all emission wavelengths, and n is the number of measurement points. The SDs of the EL wavelengths for the C-LED, GP1-LED, GP2-LED, and GP3-LED samples were calculated to be 2.66, 1.64, 1.49, and 2.55 nm, respectively. The SD of the wavelength for the GP2-LED was significantly decreased compared to that of the C-LED. As shown in Fig. 4(e), the number of LED chips contributing to the main wavelength increased for the GP-LED samples compared to the C-LED. From a commercial point of view, the improvement of the EL wavelength uniformity over the entire LED wafer can be regarded as a meaningful result.Fig. 4 Mapping images for EL wavelength of the (a) C-LED, (b) GP1-LED, (c) GP2-LED, and (d) GP3-LED at injection current of 20 mA. (e) Summary on the distribution of EL wavelength for LED samples, where the solid lines are only guides for the eyes.
Figure 5 shows the distributions of the output powers over the entire substrates of the (a) C-LED, (b) GP1-LED, (c) GP2-LED, and (d) GP3-LED samples obtained at an injection current of 20 mA. The average output powers for the GP1-LED, GP2-LED, and GP3-LED samples were measured to be 285.9, 307.2, and 298.9 mW/sr, respectively, which are relatively improved compared to that of the C-LED (281.1 mW/sr). In particular, the average output power for the GP2-LED sample is 8.5% higher than that of the C-LED. As clearly shown in the EL mapping images, the number of LED chips contributing to the average output power increased for the GP2-LED and GP3-LED samples compared to that of the C-LED. Figure 5(e) shows the output power of LED samples depending on the injection current. We first calculated the average power for the LED samples within a wafer. And then, a LED chip having the average power was selected from each wafer to investigate the current dependence of optical power. The increasing rate of output power for the GP-LED samples with respect to the injection current is higher than that of the C-LED. These results agree well with the PL results. From these results, the GPs formed on a sapphire substrate by laser treatment can be effective at improving the uniformity of the emission wavelength and output power over the entire wafer.
Fig. 5 Mapping images for the output power of the (a) C-LED, (b) GP1-LED, (c) GP2-LED, and (d) GP3-LED at injection current of 20 mA. (e) The output powers of LED samples at the injection current from 4 to 200 mA.
4. Conclusion
In conclusion, we evaluated the effects of GPs formed on sapphire substrates realized by laser treatment on the optical and electrical properties of InGaN/GaN LED wafers depending on the bowing characteristics. The amount of bowing for the LED wafers fabricated on GP structures was significantly improved compared to that of the C-LED without GPs. In the PL and EL mapping results, the uniformity of the PL and EL emission wavelengths for the GP-LEDs was enhanced compared to that of the C-LED. Also, the average PL intensity and EL output power for the GP2-LED sample were 6.2 and 8.5% higher than that of the C-LED, respectively. These results demonstrate that the laser-treated patterns on a sapphire substrate can be an effective way to improve the optical and electrical properties of LED devices.
Acknowledgments
This work was supported by a National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (No. 2015042417), and by the Ministry of Education (No. 2015R1D1A1A01060681).
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