- Email: [email protected]
Characterization of flexible InGaN LEDs with various curvatures
Materials Letters 165 (2016) 252–256
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Characterization of flexible InGaN LEDs with various curvatures Won-Sik Choi a,b, Hao Cui a, Si-Hyun Park a,n, Sung Oh Cho b,c, June Key Lee c, Tae Soo Kim d, Jung Hoon Song d, Tak Jeong b,n a
Department of Electronic Engineering, Yeungnam University, 280 Daehak-ro, Gyeongsan-si, Gyeongsangbuk-do 38541, Republic of Korea LED Device Research Center, Korea Photonics Technology Institute, Gwangju 500-779, Republic of Korea c Department of Materials Science and Engineering, and Optoelectronics Convergence Research Center, Chonnam National University, Gwangju 500-757, Republic of Korea d Department of Physics, Kongju National University, Kongju 314-701, Republic of Korea b
art ic l e i nf o
a b s t r a c t
Article history: Received 29 September 2015 Received in revised form 18 November 2015 Accepted 1 December 2015 Available online 5 December 2015
We investigated the properties of flexible InGaN LEDs fabricated on a 2-in. polyimide substrate with various radii of curvature R. The output power of the LEDs slightly increased as R decreased for a given input current. The output power decreased and surface cracks appeared in the curved devices at currents larger than a critical current, although this current was smaller for devices with a smaller R. Reversebiased electro-reflectance spectroscopy revealed that both the compressive strain in the InGaN layers and the related piezoelectric field increased as R decreased. & 2015 Elsevier B.V. All rights reserved.
Keywords: LED InGaN Flexible Curvature
1. Introduction Following the first demonstration of InGaN-based light-emitting diodes (LEDs) [1,2], they have become common industrial products, with applications in outdoor displays, liquid crystal displays, and lighting. Emerging applications of InGaN-based LEDs that require flexible devices include biomedical devices and flexible displays, and the development of such devices has recently attracted a great deal of attention. InGaN-based flexible LEDs (FLEDs) have the various advantages and applications including the wearable electronics, artificial skin devices, conformal contact on a curvilinear, cramped biological organs/tissues surface and next-generation deformable displays [3–6]. Some studies have reported on demonstrations of InGaN-based FLEDs, overcoming difficulties in transfer printing of high-quality InGaN epitaxial layers onto flexible substrates [7–12]; however, significant improvement is required in terms of the development of a cost-effective mass production processes to form high-quality and highcurrent devices. We recently reported an array of InGaN-based FLEDs fabricated on a full-scale 2-in. polyimide substrate [13]. A laser liftoff (LLO) process has been known as one of the useful techniques for the n
Corresponding authors. E-mail addresses: [email protected] (S.-H. Park), [email protected] (T. Jeong). http://dx.doi.org/10.1016/j.matlet.2015.12.019 0167-577X/& 2015 Elsevier B.V. All rights reserved.
fabrication of various novel LEDs [14–17]. Using a simple directtransfer process based on a LLO process, we succeeded in fabricating LED chips over the entire 2-inch substrate, demonstrating potential for mass production of high-quality chips with highcurrent operation. Although we observed the basic operating properties of these FLEDs, detailed characterization of the FLEDs with various curvatures has not yet been carried out, which is required to reduce the time for commercialization. Here we describe the electrical, optical, and material properties of InGaN FLEDs fabricated on a 2-in. polyimide substrate with various radii of curvature of the polymer substrate. The output power was investigated as a function of the radius of curvature, and the strain and piezoelectric response of the epitaxial layers were characterized using reverse-biased electro-reflectance (ER) spectroscopy. Ray-tracing simulations were performed to provide a possible explanation for the variation in the output power of the FLEDs with the radius of curvature.
2. Experiment We fabricated arrays of FLEDs on a full-scale 2-in. polyimide substrate using a simple direct transfer process that we developed previously. This method basically employs LLO process that minimized the number of times of the bonding and related transfer processes. Onto InGaN LED epitaxial structures grown onto a sapphire substrate was formed the open window patterns of SiO2
W.-S. Choi et al. / Materials Letters 165 (2016) 252–256
Fig. 1. (a) Electro-luminescence verse input current, and (b) EL spectrum for a few arrays of FLEDs with different bending conditions of substrates.
layer, followed by a p-ohmic metal inside the open window patterns. After a bonding metal deposition, a double-sided polyimide tape was used for the bonding between the sample and a dummy sapphire wafer. The sapphire substrate was then removed from the epitaxial layer by means of a LLO process using a KrF excimer laser. Chip isolation and n-electrode formation were conducted. Then the remove of the dummy sapphire wafer from the polyimide layer by means of another LLO process completed the final arrays of FLEDs. The more details can be found elsewhere [13]. Commercial reagent bottles were prepared whose dimeters were simply measured using conventional vernier calipers and then arrays of FLEDs on the polyimide substrate were simply bonded onto the bottles as shown in the insets of Fig. 1(a), resulting that the radii of curvature of arrays of FLEDs become same with those of the bottles. The output power and emission spectra were measured, and reverse-biased ER spectroscopy was used to characterize piezoelectric properties of the epitaxial layers. 3. Results and discussion Fig. 1(a) shows the electro-luminescence (EL) as a function of the current for FLEDs with various radii of curvature R; i.e.,
253
2R ¼47 mm (low bending), 2R ¼30 mm (middle bending), and 2R ¼15 mm (highly bending), as well as R-1 (i.e., flat or no bending). The output power of the FLED increased slightly as R decreased for a given current up to a critical current whereby degradation of the optoelectronic properties of the device occurred. At 200 mA input current, the output power of the FLED with 2R¼ 15 mm was 4% larger than that with 2R¼ 30 mm, 14% larger than that with 2R¼ 47 mm, and 16% larger than that for R1. The output power of the FLED on the curved substrates decreased at currents larger than a critical current due to electrical and/or mechanical damage. Furthermore, this current was smaller in FLEDs with a small R. For the FLEDs with 2R¼ 30 mm and 2R ¼15 mm, the output power decreased at currents greater than 200 to 300 mA, whereas the output power of FLEDs with 2R ¼47 mm decreased with currents greater than 300 to 400 mA. The flat FLED exhibited no decrease in power until 500 mA. The thermal effect from the highly elevated temperature in the junction area at the high input current operation due to the poor thermal conductivity of the polyimide substrate may cause the degradation of the devices. The mechanical damage from the physical bending seems to accelerate the degradation of the devices. Fig. 1(b) shows EL spectra for FLEDs with currents smaller than that leading to degradation of the device properties. No significant spectral shifts were observed among FLEDs with differing R, and the FLED with a smaller R exhibited larger output power at a given current. In order for confirming that the geometric structure of the chip may one of the reasons for the variation of the output power with various radii of curvature, we investigated the optical outcoupling from the FLED chips with Rs using the ray-tracing simulations [18,19]. Using the Synopsis LightTools software package, we modeled the chips simply and designed a geometric structure of them in which the p- and n-type GaN layers were curved, as well as the emission layer. Light from the emission layer undergoes multiple reflections and transmissions inside the chip before outcoupling into the air. We defined the extraction efficiency as the ratio of the outcoupled power into air to the optical power from the emission layer. The simulation result was that the extraction efficiency of the FLED with 2R¼15 mm was 0.2% larger than that with 2R ¼30 mm, 0.3% larger than that with 2R¼ 47 mm, and 0.5% larger than that for R-1. Although the tendency of the increase in the output power of the FLED with the decrease of R can be confirmed in this kind of the simple geometric modeling and it may be explained with that the bended surface reduces the chance of total internal reflection of the ray in comparison of the flat surface and therefore enhances the extraction of the ray into air, a further research is needed for another reason for the variation of the output power with R. To investigate the durability of the deformed inorganic chips, we characterized the FLEDs using an optical microscope (OM) and a scanning electron microscope (SEM). The dimensions of the chips were 1.3 mm 1.3 mm with the inter-chip spacing of 100 μm. The shape of the n-electrode pattern in the individual chip is 3 by 3 mesh type with each open window of 400 400 μm2 by the metal line with the width of 15 μm as shown in Fig. 2. Fig. 2 (a) shows a plan-view OM image of the FLED with 2R¼47 mm and the corresponding cross-sectional SEM image following operation at 300 mA. No signs of physical damage such as cracks were observed. In (b), we see the images with 2R¼30 mm following operation at 300 mA. No signs of physical damage were still observed even though degradation in the output power however had occurred, as shown in Fig. 1(a). In (c), we see plan-view OM and cross-sectional SEM images of an FLED with 2R ¼15 mm following operation at 300 mA. The out-of-focus region is shown as black at the left- and right-hand edges of the OM image due to the small radius of curvature. A crack was observed along the n-type
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W.-S. Choi et al. / Materials Letters 165 (2016) 252–256
2R = 47 mm
500um
2R = 30 mm GaN Bonding metal
500um
2R = 15 mm
500um Fig. 2. Optical microscope image and the corresponding scanning electron microscope image just after an electrical operation up to input current level of 300 mA for FLEDs (a) with 2R¼ 47 mm, (b) with 2R ¼30 mm, and (c) with 2R¼ 15 mm.
electrode line inside the individual chip. For another FLED with 2R¼15 mm which is only under a physical bending state but undergoes no electrical operation, however, no cracks were observed. (it is not shown here). A small radius of curvature in addition to a large input current is expected to result in cracks, accelerating degradation of the optoelectronic properties of the device. The use of smaller chips (e.g., 100 mm 100 μm) may reduce the occurrence of physical cracks for a given R. We carried out ER spectroscopy by measuring of the modulation in the reflectance following repeated applications of an electrical bias. The strong piezoelectric polarization generates a large macroscopic electric field in the InGaN/GaN quantum well (QW) with the opposite direction of the field to the direction of the built-in electric filed. The external reverse bias in p–n junction reduces the total electric filed in the QW. Under the condition that the reverse bias becomes larger than the compensation voltage Vcomp, the phase inversion in the ER spectra is observed because the modulation bias increases the electric field in the QW. The piezoelectric field in the FLED can be determined by measuring the phase inversion in the ER spectra when the applied reverse bias is offset by the piezoelectric field in the quantum well (QW) [20–22]. It is known that Δn ¼0 selection rule is due to the symmetry of the conduction and valence band structure in QWs. When a strong
electric field exists in the QW, however, the forbidden transitions (Δn≠0) can occur, which can break the symmetry of electron and hole wavefunction [23]. The higher energy transition at lower reverse bias is attributed to a forbidden transition in the QW. As the reverse bias increases, this forbidden transition disappears and the spectra become a single allowed transition. Fig. 3 shows measured ER spectra for FLEDs with 2R¼47 mm, 2R¼30 mm, and R-1. For all cases, the ER spectra changed from forbidden transitions to a single allowed transition as the applied reverse bias increased, and the phase eventually inverted following an applied voltage termed the compensation voltage Vcomp, at which point the modulation bias increased the electric field in the quantum well. The inset of Fig. 3 shows that we can obtain Vcomp when the sign of the difference between the maximum and minimum of the ER signal changes. Capacitance–voltage (C–V) measurements were carried out to determine the width of the depletion layer using C¼ εA/w, where C is the capacitance, ε is the dielectric constant, A is the area of the chip, and w is the width of the depletion layer. The piezo1 electric field in FLED Epiz was calculated as 2 ∙Epiz ∙wcomp=Vbi+Vcomp where wcomp is the w at Vcomp and Vbi is the built-in potential across the device. Table 1 lists a summary of Vcomp, Vbi, wcomp, and Epiz for the FLEDs with the three different radii of curvature. The piezoelectric fields were Epiz ¼1.75 MV/cm for 2 R¼ 30 mm, Epiz
W.-S. Choi et al. / Materials Letters 165 (2016) 252–256
255
¼1.47 MV/cm for 2 R¼47 mm, and Epiz ¼1.46 MV/cm for R-1. The piezoelectric field increased as R decreased, which is consistent with an increase in the compressive strain due to bending.
4. Conclusion We described the electrical, optical, and material properties of FLEDs with various radii of curvature. InGaN-based FLEDs with 2R ¼47 mm, 2R ¼30 mm, 2R ¼15 mm, and R-1 (i.e., flat) were fabricated on a 2-in. polyimide substrate using a simple directtransfer process. The output power of the FLED at a given current was found to increase slightly as R decreased. Ray tracing simulations of the optical outcoupling showed that the geometric structure of the chip may one of the reasons for the variation of the output power with various radii of curvature revealing an increase in the optical extraction efficiency as R decreased due to a reduction in total internal reflection at the curved surface. The current at which degradation of the output power occurred decreased as R increased; this current was 300–400 mA for 2R ¼47 mm and 200–300 mA for 2Rr30 mm. Cracks were observed in the surface of the chips following operation at currents that led to degradation of the optoelectronic properties of the 1.3 mm 1.3 mm FLED chips with 2R¼ 15 mm, whereas no physical damage was detected prior to electrical operation. No significant spectral shifts were identified among FLEDs with differing R. The piezoelectric field in the FLEDs increased as R decreased, which was attributable to an increase in the compressive strain, and we found Epiz ¼1.75 MV/cm for 2R ¼30 mm, Epiz ¼ 1.47 MV/cm for 2R¼47 mm, and Epiz ¼ 1.46 MV/cm for R-1.
Acknowledgments This work was supported in part by the National IT Industry Promotion Program of the Ministry of Science, ICT and Future Planning, Korea, under grant I2201-14-1002.
References
Fig. 3. Reverse-biased electro-reflectance spectrum for FLEDs (a) with flat, (b) with 2R¼ 47 mm, and (b) with 2R ¼30 mm. Inset is graph for the maximum minus the minimum in ER spectrum.
Table 1 The compensation voltage Vcomp, the built-in potential Vbi, the depletion layer width wcomp and the piezoelectric fields Epiz for FLEDs. Bending in FLED
Vcomp (V)
Vbi (V)
wcomp (nm)
Epiz (MV/cm)
R - 1 (flat) 2R ¼ 47 mm 2R ¼ 30 mm
14.7 14.5 18.3
2.4 2.5 2.5
234 232 238
1.46 1.47 1.75
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Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Characterization of flexible InGaN LEDs with various curvatures Won-Sik Choi a,b, Hao Cui a, Si-Hyun Park a,n, Sung Oh Cho b,c, June Key Lee c, Tae Soo Kim d, Jung Hoon Song d, Tak Jeong b,n a
Department of Electronic Engineering, Yeungnam University, 280 Daehak-ro, Gyeongsan-si, Gyeongsangbuk-do 38541, Republic of Korea LED Device Research Center, Korea Photonics Technology Institute, Gwangju 500-779, Republic of Korea c Department of Materials Science and Engineering, and Optoelectronics Convergence Research Center, Chonnam National University, Gwangju 500-757, Republic of Korea d Department of Physics, Kongju National University, Kongju 314-701, Republic of Korea b
art ic l e i nf o
a b s t r a c t
Article history: Received 29 September 2015 Received in revised form 18 November 2015 Accepted 1 December 2015 Available online 5 December 2015
We investigated the properties of flexible InGaN LEDs fabricated on a 2-in. polyimide substrate with various radii of curvature R. The output power of the LEDs slightly increased as R decreased for a given input current. The output power decreased and surface cracks appeared in the curved devices at currents larger than a critical current, although this current was smaller for devices with a smaller R. Reversebiased electro-reflectance spectroscopy revealed that both the compressive strain in the InGaN layers and the related piezoelectric field increased as R decreased. & 2015 Elsevier B.V. All rights reserved.
Keywords: LED InGaN Flexible Curvature
1. Introduction Following the first demonstration of InGaN-based light-emitting diodes (LEDs) [1,2], they have become common industrial products, with applications in outdoor displays, liquid crystal displays, and lighting. Emerging applications of InGaN-based LEDs that require flexible devices include biomedical devices and flexible displays, and the development of such devices has recently attracted a great deal of attention. InGaN-based flexible LEDs (FLEDs) have the various advantages and applications including the wearable electronics, artificial skin devices, conformal contact on a curvilinear, cramped biological organs/tissues surface and next-generation deformable displays [3–6]. Some studies have reported on demonstrations of InGaN-based FLEDs, overcoming difficulties in transfer printing of high-quality InGaN epitaxial layers onto flexible substrates [7–12]; however, significant improvement is required in terms of the development of a cost-effective mass production processes to form high-quality and highcurrent devices. We recently reported an array of InGaN-based FLEDs fabricated on a full-scale 2-in. polyimide substrate [13]. A laser liftoff (LLO) process has been known as one of the useful techniques for the n
Corresponding authors. E-mail addresses: [email protected] (S.-H. Park), [email protected] (T. Jeong). http://dx.doi.org/10.1016/j.matlet.2015.12.019 0167-577X/& 2015 Elsevier B.V. All rights reserved.
fabrication of various novel LEDs [14–17]. Using a simple directtransfer process based on a LLO process, we succeeded in fabricating LED chips over the entire 2-inch substrate, demonstrating potential for mass production of high-quality chips with highcurrent operation. Although we observed the basic operating properties of these FLEDs, detailed characterization of the FLEDs with various curvatures has not yet been carried out, which is required to reduce the time for commercialization. Here we describe the electrical, optical, and material properties of InGaN FLEDs fabricated on a 2-in. polyimide substrate with various radii of curvature of the polymer substrate. The output power was investigated as a function of the radius of curvature, and the strain and piezoelectric response of the epitaxial layers were characterized using reverse-biased electro-reflectance (ER) spectroscopy. Ray-tracing simulations were performed to provide a possible explanation for the variation in the output power of the FLEDs with the radius of curvature.
2. Experiment We fabricated arrays of FLEDs on a full-scale 2-in. polyimide substrate using a simple direct transfer process that we developed previously. This method basically employs LLO process that minimized the number of times of the bonding and related transfer processes. Onto InGaN LED epitaxial structures grown onto a sapphire substrate was formed the open window patterns of SiO2
W.-S. Choi et al. / Materials Letters 165 (2016) 252–256
Fig. 1. (a) Electro-luminescence verse input current, and (b) EL spectrum for a few arrays of FLEDs with different bending conditions of substrates.
layer, followed by a p-ohmic metal inside the open window patterns. After a bonding metal deposition, a double-sided polyimide tape was used for the bonding between the sample and a dummy sapphire wafer. The sapphire substrate was then removed from the epitaxial layer by means of a LLO process using a KrF excimer laser. Chip isolation and n-electrode formation were conducted. Then the remove of the dummy sapphire wafer from the polyimide layer by means of another LLO process completed the final arrays of FLEDs. The more details can be found elsewhere [13]. Commercial reagent bottles were prepared whose dimeters were simply measured using conventional vernier calipers and then arrays of FLEDs on the polyimide substrate were simply bonded onto the bottles as shown in the insets of Fig. 1(a), resulting that the radii of curvature of arrays of FLEDs become same with those of the bottles. The output power and emission spectra were measured, and reverse-biased ER spectroscopy was used to characterize piezoelectric properties of the epitaxial layers. 3. Results and discussion Fig. 1(a) shows the electro-luminescence (EL) as a function of the current for FLEDs with various radii of curvature R; i.e.,
253
2R ¼47 mm (low bending), 2R ¼30 mm (middle bending), and 2R ¼15 mm (highly bending), as well as R-1 (i.e., flat or no bending). The output power of the FLED increased slightly as R decreased for a given current up to a critical current whereby degradation of the optoelectronic properties of the device occurred. At 200 mA input current, the output power of the FLED with 2R¼ 15 mm was 4% larger than that with 2R¼ 30 mm, 14% larger than that with 2R¼ 47 mm, and 16% larger than that for R1. The output power of the FLED on the curved substrates decreased at currents larger than a critical current due to electrical and/or mechanical damage. Furthermore, this current was smaller in FLEDs with a small R. For the FLEDs with 2R¼ 30 mm and 2R ¼15 mm, the output power decreased at currents greater than 200 to 300 mA, whereas the output power of FLEDs with 2R ¼47 mm decreased with currents greater than 300 to 400 mA. The flat FLED exhibited no decrease in power until 500 mA. The thermal effect from the highly elevated temperature in the junction area at the high input current operation due to the poor thermal conductivity of the polyimide substrate may cause the degradation of the devices. The mechanical damage from the physical bending seems to accelerate the degradation of the devices. Fig. 1(b) shows EL spectra for FLEDs with currents smaller than that leading to degradation of the device properties. No significant spectral shifts were observed among FLEDs with differing R, and the FLED with a smaller R exhibited larger output power at a given current. In order for confirming that the geometric structure of the chip may one of the reasons for the variation of the output power with various radii of curvature, we investigated the optical outcoupling from the FLED chips with Rs using the ray-tracing simulations [18,19]. Using the Synopsis LightTools software package, we modeled the chips simply and designed a geometric structure of them in which the p- and n-type GaN layers were curved, as well as the emission layer. Light from the emission layer undergoes multiple reflections and transmissions inside the chip before outcoupling into the air. We defined the extraction efficiency as the ratio of the outcoupled power into air to the optical power from the emission layer. The simulation result was that the extraction efficiency of the FLED with 2R¼15 mm was 0.2% larger than that with 2R ¼30 mm, 0.3% larger than that with 2R¼ 47 mm, and 0.5% larger than that for R-1. Although the tendency of the increase in the output power of the FLED with the decrease of R can be confirmed in this kind of the simple geometric modeling and it may be explained with that the bended surface reduces the chance of total internal reflection of the ray in comparison of the flat surface and therefore enhances the extraction of the ray into air, a further research is needed for another reason for the variation of the output power with R. To investigate the durability of the deformed inorganic chips, we characterized the FLEDs using an optical microscope (OM) and a scanning electron microscope (SEM). The dimensions of the chips were 1.3 mm 1.3 mm with the inter-chip spacing of 100 μm. The shape of the n-electrode pattern in the individual chip is 3 by 3 mesh type with each open window of 400 400 μm2 by the metal line with the width of 15 μm as shown in Fig. 2. Fig. 2 (a) shows a plan-view OM image of the FLED with 2R¼47 mm and the corresponding cross-sectional SEM image following operation at 300 mA. No signs of physical damage such as cracks were observed. In (b), we see the images with 2R¼30 mm following operation at 300 mA. No signs of physical damage were still observed even though degradation in the output power however had occurred, as shown in Fig. 1(a). In (c), we see plan-view OM and cross-sectional SEM images of an FLED with 2R ¼15 mm following operation at 300 mA. The out-of-focus region is shown as black at the left- and right-hand edges of the OM image due to the small radius of curvature. A crack was observed along the n-type
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W.-S. Choi et al. / Materials Letters 165 (2016) 252–256
2R = 47 mm
500um
2R = 30 mm GaN Bonding metal
500um
2R = 15 mm
500um Fig. 2. Optical microscope image and the corresponding scanning electron microscope image just after an electrical operation up to input current level of 300 mA for FLEDs (a) with 2R¼ 47 mm, (b) with 2R ¼30 mm, and (c) with 2R¼ 15 mm.
electrode line inside the individual chip. For another FLED with 2R¼15 mm which is only under a physical bending state but undergoes no electrical operation, however, no cracks were observed. (it is not shown here). A small radius of curvature in addition to a large input current is expected to result in cracks, accelerating degradation of the optoelectronic properties of the device. The use of smaller chips (e.g., 100 mm 100 μm) may reduce the occurrence of physical cracks for a given R. We carried out ER spectroscopy by measuring of the modulation in the reflectance following repeated applications of an electrical bias. The strong piezoelectric polarization generates a large macroscopic electric field in the InGaN/GaN quantum well (QW) with the opposite direction of the field to the direction of the built-in electric filed. The external reverse bias in p–n junction reduces the total electric filed in the QW. Under the condition that the reverse bias becomes larger than the compensation voltage Vcomp, the phase inversion in the ER spectra is observed because the modulation bias increases the electric field in the QW. The piezoelectric field in the FLED can be determined by measuring the phase inversion in the ER spectra when the applied reverse bias is offset by the piezoelectric field in the quantum well (QW) [20–22]. It is known that Δn ¼0 selection rule is due to the symmetry of the conduction and valence band structure in QWs. When a strong
electric field exists in the QW, however, the forbidden transitions (Δn≠0) can occur, which can break the symmetry of electron and hole wavefunction [23]. The higher energy transition at lower reverse bias is attributed to a forbidden transition in the QW. As the reverse bias increases, this forbidden transition disappears and the spectra become a single allowed transition. Fig. 3 shows measured ER spectra for FLEDs with 2R¼47 mm, 2R¼30 mm, and R-1. For all cases, the ER spectra changed from forbidden transitions to a single allowed transition as the applied reverse bias increased, and the phase eventually inverted following an applied voltage termed the compensation voltage Vcomp, at which point the modulation bias increased the electric field in the quantum well. The inset of Fig. 3 shows that we can obtain Vcomp when the sign of the difference between the maximum and minimum of the ER signal changes. Capacitance–voltage (C–V) measurements were carried out to determine the width of the depletion layer using C¼ εA/w, where C is the capacitance, ε is the dielectric constant, A is the area of the chip, and w is the width of the depletion layer. The piezo1 electric field in FLED Epiz was calculated as 2 ∙Epiz ∙wcomp=Vbi+Vcomp where wcomp is the w at Vcomp and Vbi is the built-in potential across the device. Table 1 lists a summary of Vcomp, Vbi, wcomp, and Epiz for the FLEDs with the three different radii of curvature. The piezoelectric fields were Epiz ¼1.75 MV/cm for 2 R¼ 30 mm, Epiz
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¼1.47 MV/cm for 2 R¼47 mm, and Epiz ¼1.46 MV/cm for R-1. The piezoelectric field increased as R decreased, which is consistent with an increase in the compressive strain due to bending.
4. Conclusion We described the electrical, optical, and material properties of FLEDs with various radii of curvature. InGaN-based FLEDs with 2R ¼47 mm, 2R ¼30 mm, 2R ¼15 mm, and R-1 (i.e., flat) were fabricated on a 2-in. polyimide substrate using a simple directtransfer process. The output power of the FLED at a given current was found to increase slightly as R decreased. Ray tracing simulations of the optical outcoupling showed that the geometric structure of the chip may one of the reasons for the variation of the output power with various radii of curvature revealing an increase in the optical extraction efficiency as R decreased due to a reduction in total internal reflection at the curved surface. The current at which degradation of the output power occurred decreased as R increased; this current was 300–400 mA for 2R ¼47 mm and 200–300 mA for 2Rr30 mm. Cracks were observed in the surface of the chips following operation at currents that led to degradation of the optoelectronic properties of the 1.3 mm 1.3 mm FLED chips with 2R¼ 15 mm, whereas no physical damage was detected prior to electrical operation. No significant spectral shifts were identified among FLEDs with differing R. The piezoelectric field in the FLEDs increased as R decreased, which was attributable to an increase in the compressive strain, and we found Epiz ¼1.75 MV/cm for 2R ¼30 mm, Epiz ¼ 1.47 MV/cm for 2R¼47 mm, and Epiz ¼ 1.46 MV/cm for R-1.
Acknowledgments This work was supported in part by the National IT Industry Promotion Program of the Ministry of Science, ICT and Future Planning, Korea, under grant I2201-14-1002.
References
Fig. 3. Reverse-biased electro-reflectance spectrum for FLEDs (a) with flat, (b) with 2R¼ 47 mm, and (b) with 2R ¼30 mm. Inset is graph for the maximum minus the minimum in ER spectrum.
Table 1 The compensation voltage Vcomp, the built-in potential Vbi, the depletion layer width wcomp and the piezoelectric fields Epiz for FLEDs. Bending in FLED
Vcomp (V)
Vbi (V)
wcomp (nm)
Epiz (MV/cm)
R - 1 (flat) 2R ¼ 47 mm 2R ¼ 30 mm
14.7 14.5 18.3
2.4 2.5 2.5
234 232 238
1.46 1.47 1.75
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